Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Platinum (Pt) is currently considered one of the best electrocatalysts for the oxygen reduction reaction (ORR), which occurs at the cathode of a fuel cell and is the key process determining the overall performance.^1–5 However, the high cost and scarcity of Pt limit its wide commercialization in this field. According to the US Department of Energy, the total Pt loading is required to be below 0.125 mg cm⁻², in contrast to a presently used Pt loading of 0.4 mg cm⁻² or more for fuel cell application.⁴ Therefore, reducing the Pt loading without loss or with an improvement of the cathode performance has received significant interest in electrocatalytic research for fuel cell systems.^6–10 In this regard, reducing the size of Pt particles to a nanocluster scale (size < 2 nm) and maximizing the Pt dispersion may offer an efficient way to achieve maximum utilization of the Pt electrocatalyst with appropriate consumption.^4,11–15The size of nanomaterials generally plays a critical role in controlling the physical and chemical properties for catalytic applications.^16–20 With a decrease in the particle size to the nanoscale, quantum size effects are induced, which alter the surface energy of the material due to unsaturated coordination and change in the energy level of the d orbital of metal atoms, leading to spatial localization of the electrons.^17–20 This size-induced effect on the electronic structures at the active sites modifies the capability of binding the reactant molecules in catalytic reactions, thereby altering the activity of the nanocatalyst.²⁰ When the particle contains a few to several dozens of atoms with sizes, ranging from sub-nanometer to 2 nm often termed as nanocluster that bridges nanoparticle and a single atom.²¹ However, the Pt single atom is not an appropriate electrocatalyst for the ORR in a fuel cell system as the fast four-electron (4e⁻) pathway for the reduction of O₂ to H₂O requires at least two neighboring Pt atoms.^22,23 Anderson''s group demonstrated that the ratio between the production of H₂O (product of 4e⁻ process) and H₂O₂ (2e⁻) in the ORR strongly depends on the number of atoms in the Pt cluster. Typically, it requires more than 14 atoms in a Pt cluster to produce H₂O efficiently through the 4e⁻ pathway of the ORR.²⁴ Therefore, Pt nanoclusters having more than a dozen atoms have proven to be highly efficient ORR electrocatalysts for fuel cell systems.^13–15 Upon decreasing the size of the nanoparticles to a nanocluster, the electronic state and structure are known to be changed, leading to an increase of the catalytic activity in the ORR. Therefore, it is highly desirable to synthesize a Pt nanocluster-based material as an ORR electrocatalyst with high catalytic performance. To date, several synthesis strategies, such as wet-chemical, atomic-layer deposition, and photochemical methods, have been applied for the preparation of well-dispersed Pt nanoclusters on different types of support, such as dendrimer, metal oxide, and carbon materials.^{13–15,25–31}An alternative approach to synthesize Pt nanocluster (PtNC) is the encapsulation of the cluster within nanosized pores, for example, by utilizing microporous (diameters less than 2 nm) carbon materials.³² Among the microporous carbons, zeolite-templated carbon (ZTC) has been attractive for supporting Pt clusters due to its ordered microporous structure.^33–37 ZTC is a potentially promising material as catalyst support as it offers the advantages of extremely large surface area and high electrical conductivity of graphene-like carbon frameworks constituting a three-dimensional (3D) interconnected pore structure.³⁶ Moreover, the micropores of ZTC can serve as nanocages for stabilization of the Pt nanoclusters. Coker et al. used Pt²⁺ ion-exchanged zeolite as a carbon template to synthesize Pt nanoparticles in ZTC with size in a range of 1.3 to 2.0 nm.³³ Recently, atomically dispersed Pt ionic species was synthesized via a simple wet-impregnation method on ZTC containing a large amount of sulfur (17 wt%).²³ Itoi et al. synthesized PtNC consisting of 4–5 atoms and a single Pt atom in ZTC using the organoplatinum complex.³⁷ Although these methods produced Pt nanoclusters with narrow size distribution and atomic dispersion, they required multi-step processes and/or high-temperature treatment (>300 °C). High-temperature treatment often induces the sintering of nanoclusters to aggregated clusters. Therefore, it is highly desirable to develop a simple and low-cost method for the preparation of PtNC supported on ZTC (PtNC/ZTC) for use as an efficient ORR electrocatalyst. The electrochemical reduction approach offers an alternate and efficient route for the synthesis of PtNC in the micropores of ZTC. The electrochemical method is one of the popular ways to prepare electrocatalysts because it is a simple single-step procedure and ensures electrical contact between the nanoparticles and the support.^38,39Herein, we report a facile electrochemical method for the formation of PtNC with a narrow size range of 0.8–1.5 nm supported on ZTC. The resultant PtNC/ZTC shows higher electrocatalytic activities towards ORR compared to that of commercial Pt/C. Here, ZTC plays two important roles: (i) it provides nanocages to stabilize the PtNC and (ii) it accelerates the ORR activity by enhancing the accessibility of active sites through its abundant micropores. Fig. 1a shows a schematic representation of the typical electrochemical synthesis of PtNC/ZTC. In the first step, ZTC was impregnated with a Pt-precursor dissolved in a water–ethanol mixture. As ZTC possess ordered micropores (Fig. S1a^†) with high Brunauer–Emmett–Teller (BET) surface area of 3400 m² g⁻¹ (vide infra), the uniform adsorption and anchorage of PtCl₆²⁻ ions into the micropores of ZTC was favored. After impregnating and drying, the resultant ZTC–PtCl₆²⁻ was mixed with water–ethanol and Nafion to make the ink for the preparation of the electrode. Using the prepared electrode, a potential of 0.77 V vs. reversible hydrogen electrode (RHE) (Fig. 1b) was applied followed by potential cycling between 1.12 to −0.02 V vs. RHE until the cyclic voltammogram was stabilized. The Pt content of PtNC/ZTC was determined to be ∼10 wt% (Fig. S2^†) by thermogravimetric analysis (TGA). The obtained PtNC/ZTC was electrochemically characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammogram (Fig. 1c) after potential cycling in fresh KOH electrolyte shows the characteristic Pt peaks corresponding to hydrogen adsorption and desorption. The Nyquist plots (Fig. 1d) demonstrate that PtNC/ZTC has lower electrolyte resistance (42 Ω) than that of ZTC (70 Ω), implying an improvement in the conductivity of ZTC by the presence of PtNC. Due to the increase in the conductivity, PtNC/ZTC could facilitate the electron transfer more effectively than ZTC, enhancing its electrocatalytic activity.Open in a separate windowFig. 1(a) Illustration for the formation of PtNC/ZTC:Pt-precursor was impregnated into ZTC micropores, and then a potential (0.77 V vs. RHE) was exerted on the ZTC–PtCl₆²⁻ composite in a 0.1 M KOH solution to form PtNC/ZTC (b) Chronoamperometric response of ZTC–PtCl₆²⁻ at a constant potential of 0.77 V (vs. RHE) in 0.1 M KOH electrolyte. (c) Cyclic voltammogram of PtNC/ZTC in a fresh 0.1 M KOH at a scan rate of 20 mV s⁻¹. (d) Nyquist plots of ZTC and PtNC/ZTC in 0.1 M KOH. Fig. 2a and b show images from aberration-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF). The HAADF-STEM images exhibit the typical morphology of the final product (PtNC/ZTC) after electrochemical reduction. As shown in Fig. 2a, it is very clear that isolated PtNCs are uniformly dispersed in ZTC. These PtNCs have a homogeneous distribution with a narrow size range (0.8–1.5 nm, Fig. 2b). On further magnification, the STEM image shows a cluster-like structure of Pt (Fig. 2c). The STEM image of selected PtNC (Fig. 2d) reveals that it consists of ∼20 atoms. The number of atom content in PtNC was further determined by matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry using trans-2-[3-(4-test-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix.^40,41 As shown in Fig. S3,^† MALDI-TOF measurement produces a mass spectra with a predominant peak centered at ∼3700 Da corresponding to the Pt₁₉ cluster. The TEM image (Fig. S4 a and b^†) validates the formation of PtNC with an average size of 0.9 nm. In addition, the energy dispersive X-ray spectrometer (EDS) mapping images clearly shows the uniform dispersion of Pt nanocluster in ZTC (Fig. S4c^†). The X-ray powder diffraction (XRD) pattern (Fig. 2e) of PtNC/ZTC showed three broad peaks associated with small size metallic Pt corresponds to (111), (200), and (311) planes (Fig. 2e, inset), along with peaks of ZTC at 2θ = 7.8° and 14.9° corresponding to the ordered microporous structure. Along with the structural analysis, the porous texture of PtNC/ZTC was examined by Ar adsorption (Fig. 2f). PtNC/ZTC had a high BET surface area of 2360 m² g_ZTC⁻¹, which is 1.4 times lower than that of pristine ZTC (3400 m² g_ZTC⁻¹). The decrease in Ar adsorption capacity after the formation of PtNC in ZTC is interpreted as a result of the filling of ZTC micropores by PtNC. This micropore filling was confirmed in the pore size distributions of the pristine ZTC and the metal-loaded carbon (inset of Fig. 2f). The X-ray photoelectron spectroscopy (XPS) results reveal the signature of Pt in ZTC (Fig. S5^†). The elemental survey (Fig. S5a^†) shows the signature of C 1s, O 1s, F 1s (Nafion), and Pt 4f. The chemical nature of Pt in PtNC/ZTC was inspected by a detailed Pt 4f XPS analysis. The deconvoluted Pt 4f XPS spectra (Fig. S5b^†) reveals the presence of both metallic and ionic Pt species. The peaks observed at 71.0 (4f_7/2) and 74.2 (4f_5/2) eV correspond to metallic Pt whereas the other peaks positioned at 72.6 (4f_7/2) and 76.0 (4f_5/2) are attributed to Pt²⁺ and the peaks at 74.9 (4f_7/2) and 77.8 (4f_5/2) eV are attributed to Pt⁴⁺ originating from the surface oxidation of metallic Pt.⁴²Open in a separate windowFig. 2(a–d) Representative spherical aberration-corrected HAADF-STEM images of PtNC/ZTC at various magnifications. (e) XRD pattern of PtNC/ZTC and (f) Ar adsorption–desorption isotherms of ZTC and PtNC/ZTC. Inset in (e) shows a 30 times magnified high-angle region of XRD of PtNC/ZTC. Inset in (f) shows the pore size distributions of the ZTC and PtNC/ZTC.The formation of narrow sized PtNC by the electrochemical method can be ascribed to the stabilization of PtNC in the ZTC micropores, which serve as cages to impose a spatial limitation on the size of the Pt clusters. For comparison, Pt supported on ZTC was also prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction at high temperature (300 °C). The Pt obtained by this incipient wetness impregnation method shows the formation of Pt nanoparticles on the exterior surface of ZTC (PtNP/ZTC) (Fig. S6^†). The formation of larger Pt nanoparticles is due to the sintering at high temperature, showing that even ZTC micropores could not prevent the aggregation of PtNCs at high temperatures. Fig. 3 shows the electrochemical ORR activity of PtNC/ZTC using linear sweep voltammetry (LSV) technique on a rotating disc electrode (RDE) in a 0.1 M KOH solution saturated with O₂ at a scan rate of 5 mV s⁻¹. The ORR activity of ZTC (without PtNC) was measured for comparison as well. As shown in Fig. 3a, PtNC/ZTC exhibited higher diffusion limiting current density and higher positive onset and half-wave potential compared to ZTC alone, indicating that PtNC is the active center for the ORR. To investigate the effect of the Pt loading amount on the ORR activity, PtNC/ZTC with various Pt loadings, 2–20 wt%, was used for the measurement of LSV at 1600 rpm. With an increase in Pt content, both the onset and half-wave potential shifted towards more positive potential up to 10 wt% loading of Pt (Fig. 3a and S7^†). Upon further increase of loading of Pt on ZTC to 20 wt%, both the onset and half-wave potential of PtNC/ZTC shifted towards less positive potential along with a slight decrease in the diffusion limiting current density (Fig. 3a). The decrease in the ORR activity of PtNC/ZTC at high loading of Pt (20 wt%) was attributed to the decrease in the electrochemically active surface area (Fig. S8^†) and decrease in the specific surface area (Fig. S9^†). The STEM image clearly shows that the aggregated Pt clusters were formed on the exterior surface of ZTC at 20 wt% loading of Pt (Fig. S10c^†), blocking the accessibility of active sites. Therefore, PtNC/ZTC with the optimum loading of 10 wt% of Pt leads to superior ORR activity with a high positive onset potential of 0.99 V, which is similar to commercial Tanaka Pt/C (Pt/C-TKK) (Fig. 3b), and a half-wave potential of 0.87 V, which is ∼10 mV more positive than that of commercial Pt/C-TKK (0.86 V) (Fig. 3b). Compared to the case of PtNC/ZTC, both the onset and half-wave potential of PtNP/ZTC prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction with the same loading of Pt exhibited a less positive value (Fig. S11^†). The poorer activity of PtNP/ZTC is due to the blockage of active sites by larger PtNPs formed on the exterior surface of ZTC (Fig. S6^†).Open in a separate windowFig. 3(a) RDE ORR polarization curves of PtNC/ZTC with different mass loading of Pt. (b) Comparison of PtNC/ZTC (PtNC_10%/ZTC) with commercial Pt/C-TKK at the same loading of 40 μg_Pt cm⁻². (c) RDE ORR polarization curves of PtNC/ZTC at different rotation speeds. Inset in (c) shows the corresponding K–L plots at different potentials. (d) Represents the kinetic current density values of Pt/C-TKK and PtNC/ZTC at the potential of 0.8 V vs. RHE.To investigate the kinetics of the ORR activity of PtNC/ZTC, LSV measurements were performed with RDE at different rotating rates (Fig. 3c), and the kinetics was analyzed using a Koutecký–Levich (K–L) plot (Fig. 3c, inset). From Fig. 3c, it was observed that the current density increases with the increasing speed of rotation of the electrode, which is characteristic of a diffusion-controlled reaction. The corresponding linear K–L plots (Fig. 3c, inset) with a similar slope at different potentials reveal that the number of transferred electrons was ∼4, indicating that O₂ is directly reduced to OH⁻ and the ORR is dominated by the H₂O₂-free 4e⁻ pathway. To estimate the amount of produced peroxide ion, rotating ring-disc electrode (RRDE) measurement was performed and the produce peroxide ion calculated from RRDE curve was < 4% (Fig. S12^†). The kinetic current density (J_k) obtained from K–L plot at the potential of 0.8 V (Fig. 3d) for PtNC/ZTC (J_k = 50 mA cm⁻²) is 2.2 times higher than that of commercial Pt/C-TKK (J_k = 22 mA cm⁻²).As Pt-based electrocatalysts are known to be highly active in an acidic medium, the ORR activity of PtNC/ZTC in O₂-saturated 0.1 M HClO₄ was also evaluated by comparing it with that of commercial Pt/C-TKK with the same loading of Pt on the electrode surface using RDE at a scan rate of 5 mV s⁻¹. The PtNC/ZTC-based electrode exhibited ORR activity with an onset potential of 0.96 V (Fig. 4), which is close to that of Pt/C-TKK (0.98 V), and half-wave potentials of 0.84 V, which is 20 mV more positive than that of Pt/C-TKK (0.82 V). PtNC/ZTC showed a slightly higher diffusion-limiting current density of ∼5.9 mA cm⁻² (0.4–0.7 V) compared with that of the Pt/C-TKK catalyst (∼5.6 mA cm⁻²). The kinetics of the ORR in an acidic medium was further analyzed using RDE at different rotation rates (Fig. S13^†) and it was observed that the current density increases with the increasing speed of rotation of the electrode, as in the case of the alkaline medium. The number of electron involved and the amount of produced H₂O₂ estimated by RRDE measurement were ∼4 and < 5%, respectively (Fig. S14^†). The mass activity of PtNC/ZTC obtained using the mass transport corrected kinetic current at 0.8 V is 0.15 A mg⁻¹, which is 3.2 times higher than that of Pt/C-TKK (0.046 A mg⁻¹).Open in a separate windowFig. 4(a) RDE ORR polarization curves at 1600 rpm and (b) mass activity at 0.8 V of PtNC/ZTC and Pt/C-TKK in 0.1 M HClO₄.Furthermore, the methanol tolerance of PtNC/ZTC was assessed by intentionally adding methanol to the oxygen saturated electrolyte solution (both in alkaline and acidic media). The commercial Pt/C-TKK was used for comparison as well. The peak current densities for methanol oxidation with PtNC/ZTC were ∼2.8 and ∼3 times lower than that of Pt/C-TKK in alkaline (Fig. 5a) and acidic (Fig. 5b) media, respectively. These results indicate that PtNC/ZTC has much higher tolerance towards methanol than Pt/C-TKK does. This higher methanol tolerance of PtNC/ZTC can be attributed to the small size of the Pt cluster, which may not be sufficient to catalyze the oxidation of methanol efficiently, as the oxidation of methanol requires Pt ensemble sites.⁴³Open in a separate windowFig. 5ORR polarization curves of PtNC/ZTC and Pt/C-TKK in the absence (solid line) and presence (dotted line) of 0.1 M of CH₃OH at a rotation rate of 1600 rpm in (a) alkaline and (b) acid media.The durability of PtNC/ZTC was also investigated by the amperometric technique. The test was performed at a constant voltage of the half-wave potential in an O₂-saturated alkaline medium and at 0.7 V in an O₂-saturated acidic medium at a rotation rate of 1600 rpm (Fig. S15a and b^†). The durability of the PtNC/ZTC catalyst in the alkaline medium was higher than that of Pt/C-TKK, exhibiting a 30% decrease compared to a 40% decrease of Pt/C-TKK in 5.5 h of ORR operation (Fig. S15a^†). The higher durability of PtNC/ZTC compared to Pt/C-TKK in the alkaline medium may be due to the stabilization of PtNC by pore entrapment. In the acidic medium, however, PtNC/ZTC exhibited a 54% decrease in the initial current after 5.5 h of operation while a 33% decrease was observed in the case of Pt/C-TKK (Fig. S15b^†). The decrease in ORR activity in the acidic medium may be due to the leaching out of tiny Pt nanoclusters in acid electrolyte from the ZTC micropores. To understand the decrease in the ORR durability with time, STEM measurements of PtNC/ZTC after 5.5 h of ORR operation were performed. In the alkaline medium, the STEM image of post-ORR PtNC/ZTC shows a slight change in the size of PtNC (Fig. S15c^†) while the STEM image of PtNC/ZTC after ORR in the acidic medium exhibited sintering of PtNC into large particles with an average size of 30 nm (Fig. S15d^†), resulting in a decrease of the ORR activity. In the alkaline medium, the decrease in ORR activity with time may be due to the oxidation of the ZTC support in KOH.⁴⁴We attributed the excellent ORR activity of PtNC/ZTC to the interplay between the following: (1) the structure of the Pt cluster possessing a high ratio of surface atoms that benefits the surface reactions,^45–47 (2) the microporous 3D graphene-like structure of the ZTC support that enables easy access of O₂ and electrolyte molecules to the active sites,⁴⁸ and (3) the high conductivity and large accessible surface area of ZTC that facilitates the electron transfer.^49–51     

Insight into trifluoromethylation – experimental electron density for Togni reagent I

3,3-Dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole represents a popular reagent for trifluoromethylation. The σ hole on the hypervalent iodine atom in this “Togni reagent” is crucial for adduct formation between the reagent and a nucleophilic substrate. The electronic situation may be probed by high resolution X-ray diffraction: the experimental charge density thus derived shows that the short intermolecular contact of 3.0 Å between the iodine and a neighbouring oxygen atom is associated with a local charge depletion on the heavy halogen in the direction of the nucleophile and visible polarization of the O valence shell towards the iodine atom. In agreement with the expectation for λ3-iodanes, the intermolecular O⋯I–C_aryl halogen bond deviates significantly from linearity.

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.

A “halogen bond” denotes a short contact between a nucleophile acting as electron density donor and a (mostly heavy) halogen atom as electrophile;^1,2 halogen bonds are a special of σ hole interactions.^3–5 Such interactions do not only play an important role in crystal engineering;^6–11 rather, the concept of a nucleophile approaching the σ hole of a neighbouring atom may also prove helpful for understanding chemical reactivity.The title compound provides an example for such a σ hole based reactivity: 3,3-dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole, 1, (Scheme 1) commonly known as “Togni reagent I”, is used for the electrophilic transfer of a trifluoromethyl group by reductive elimination. The original articles in which the application of 1¹² and other closely related “Togni reagents”¹³ were communicated have been and still are highly cited. Trifluoromethylation is not the only application for hypervalent iodine compounds; they have also been used as alkynylating¹⁴ or azide transfer reagents.^15,16 The syntheses of hypervalent iodanes and their application in organofluorine chemistry have been reviewed,¹⁷ and a special issues of the Journal of Organic Chemistry has been dedicated to Hypervalent Iodine Catalysis and Reagents.¹⁸ Recently, Pietrasiak and Togni have expanded the concept of hypervalent reagents to tellurium.¹⁹Open in a separate windowScheme 1Lewis structure of the Togni reagent, 1.Results from theory link σ hole interactions and chemical properties and indicate that the electron density distribution associated with the hypervalent iodine atom in 1 is essential for the reactivity of the molecule in trifluoromethylation.²⁰ Lüthi and coworkers have studied solvent effects and shown that activation entropy and volume play relevant roles for assigning the correct reaction mechanism to trifluoromethylation via1. These authors have confirmed the dominant role of reductive elimination and hence the relevance of the σ hole interaction for the reactivity of 1 in solution by ab initio molecular dynamics (AIMD) simulations.^21,22 An experimental approach to the electron density may complement theoretical calculations: low temperature X-ray diffraction data of sufficient resolution allow to obtain the experimental charge density and associate it with intra- and inter-molecular interactions.^23–25 Such advanced structure models based on aspherical scattering factors have also been applied in the study of halogen bonds.^26–30 In this contribution, we provide direct experimental information for the electronic situation in Togni reagent I, 1; in particular, we analyze the charge distribution around the hypervalent iodine atom.Excellent single crystals of the title compound were grown by sublimation.^§ The so-called independent atom model (IAM), i.e. the structure model based on conventional spherical scattering factors for neutral atoms, confirms the solid state structure reported by the original authors,¹² albeit with increased accuracy. As depicted in Fig. 1, two molecules of 1 interact via a crystallographic center of inversion. The pair of short intermolecular O⋯I contacts thus generated can be perceived as red areas on the interaction-sensitive Hirshfeld surface.³¹Open in a separate windowFig. 1Two neighbouring molecules of 1, related by a crystallographic center of inversion. The short intermolecular I⋯O contacts show up in red on the Hirshfeld surface³² enclosing the left molecule. (90% probability ellipsoids, H atoms omitted, symmetry operator 1 − x, 1 − y, 1 − z).The high resolution of our diffraction data ^¶ for 1 allowed an atom-centered multipole refinement^33,34 and thus an improved model for the experimental electron density which takes features of chemical bonding and lone pairs into account. Fig. 2 shows the deformation density, i.e. the difference electron density between this advanced multipole model and the IAM in the same orientation as Fig. 1.^|| The orientation of an oxygen lone pair (blue arrow) pointing towards the σ hole of the heavy halogen in the inversion-related molecule and the region of positive charge at this iodine atom (red arrow) are clearly visible. Single-bonded terminal halides are associated with one σ hole opposite to the only σ bond, thus resulting in a linear arrangement about the halogen atom. Different geometries and potentially more than a single σ hole are to be expected for λ3-iodanes such as our target molecule, and as a tendency, the resulting halogen bonds are expected to be weaker than those subtended by single-bonded iodine atoms.³⁵ In agreement with these theoretical considerations, the closest I⋯O contacts in 1 amount to 2.9822(9) Å. This distance is significantly shorter than the sum of the van-der-Waals radii (I, 1.98 Å; O, 1.52 Å (ref. 36)) but cannot compete with the shortest halogen bonds between iodine and oxygen^37,38 or iodine and nitrogen.^39–41Fig. 1 and ​and22 show that the C_aryl–I⋯O contacts are not linear; they subtend an angle C10–I1⋯O1′ of 141.23(3)° at the iodine atom. On the basis of theoretical calculations, Kirshenboim and Kozuch³⁵ have suggested that the split σ holes should be situated in the plane of the three substituents of the hypervalent atom and that halogen and covalent bonds should be coplanar. Fig. 3 shows that the C_aryl–I⋯O interaction in 1 closely matches this expectation, with the next oxygen neighbour O1′ only 0.47 Å out of the least-squares plane through the heavy halogen I1 and its three covalently bonded partners C1, O1 and C10.Open in a separate windowFig. 2Deformation density for the pair of neighbouring molecules in 1; the dashed blue and red arrows indicate regions of opposite charge. (Contour interval 0.10e Å⁻³; blue lines positive, red lines negative, green lines zero contours, symmetry operator 1 − x, 1 − y, 1 − z).Open in a separate windowFig. 3A molecule of 1, shown [platon] along O1⋯C1, and its halogen-bonded neighbour O1ⁱ. Symmetry operator 1 − x, 1 − y, 1 − z.The Laplacian, the scalar derivative of the gradient vector field of the electron density, emphasizes local charge accumulations and depletions and it allows to assess the character of intra- and inter-molecular interactions. A detailed analysis of all bonds in 1 according to Bader''s Atoms In Molecules theory⁴² is provided in the ESI.^‡ We here only mention that the electron density in the bond critical point (bcp) of the short intermolecular I⋯O contact amounts to 0.102(5)e Å⁻³; we are not aware of charge density studies on λ3-iodanes, but both the electron density and its small positive Laplacian match values experimentally observed for halogen bonds involving O and terminal I in the same distance range.⁴³The crystal structure of 1 necessarily implies additional contacts beyond the short halogen bond shown in Fig. 1 and ​and2.2. The shortest among these secondary interactions is depicted in Fig. 4: it involved a non-classical C–H⋯F contact with a H⋯F distance of 2.55 Å.Open in a separate windowFig. 4C–H⋯F contact in 1; additional information has been compiled in the ESI.^‡ Symmetry operators i = 1 − x, 1 − y, 1 − z; ii = 1 − x, 1 − y, −z.The topological analysis of the experimental charge density reveals that this non-classical C–H⋯F hydrogen bond and all other secondary contacts are only associated with very small electron densities in the bcps. Table S8 in the ESI^‡ provides a summary of this analysis and confirms that the I⋯O halogen bond discussed in Fig. 1 and ​and22 represents by far the most relevant intermolecular interaction.The relevance of this halogen bond extends beyond the crystal structure of 1: Insight into the spatial disposition of electrophilic and nucleophilic regions and hence into the expected reactivity of a molecule may be gained from another electron-density derived property, the electrostatic potential (ESP). The ESP for the pair of interacting molecules in 1 is depicted in Fig. 5.Open in a separate windowFig. 5Electrostatic potential for a pair of molecules in 1 mapped on an electron density isosurface (ρ = 0.5e Å⁻³; program MoleCoolQt^44,45). Fig. 5 underlines the complementary electrostatic interactions between the positively charged iodine and the negatively charged oxygen atoms. One can easily imagine to “replace” the inversion-related partner molecule in crystalline 1 by an incoming nucleophile.The ESP tentatively obtained for a single molecule in the structure of 1 did not differ significantly from that derived for the inversion-related pair (Fig. 5), and even the results from theoretical calculations in the gas phase for an isolated molecule²⁰ are in good qualitative agreement with our ESP derived from the crystal structure. In the absence of very short contacts, polarization by neighbouring molecules only has a minor influence on the ESP. The experimentally observed electron density matches the proven reactivity for the title compound, and we consider it rewarding to extend our charge density studies on related hypervalent reagents.     

Photoswitchable probe with distinctive characteristics for selective fluorescence imaging and long-term tracing

A photo-switchable and high-contrast bio-imaging indicator 4,4′-(1E,1′E)-(4,4′-(cyclopentene-1,2-diyl)bis(5-methylthiophene-4,2-diyl))bis(methan-1-yl-1-ylidene)bis(azan-1-yl-1-ylidene)bis(2-(benzo[d]thiazol-2-yl)phenol) (BMBT) has been demonstrated, by integrating photochromophore with excited-state intramolecular proton transfer (ESIPT) moiety. The ability of reversible emission switching enables arbitrarily selective labeling or concealing of cells simply by controlling light irradiation. Besides, when the emission was switched on, BMBT is demonstrated to exhibit unique characteristics of aggregation induced emission (AIE), providing a high on–off ratio for favorable bio-imaging. Thus, the non-labeling and easily-controlled selective imaging, as well as good biocompatibility indicates BMBT to be a favorable cell probe with great potentials for functional bio-imaging fluorophore.

In this work, a photoswitchable probe was synthesized by integrating a photochromophore with an excited-state intramolecular proton transfer (ESIPT) moiety. It was explored to be a favorable fluorophore for selective fluorescence imaging and long-term tracing.

Although there are many conventional fluorescent probes used for fluorescent imaging in the past few years,^1–5 such as rhodamine,⁶ cyanine dye,⁷ quantum dots,^8,9 and lanthanide probes,¹⁰ these fluorescent probes can only respond irreversibly to one event.^11,12 In comparison, photochromophores,¹³ which can reversibly response with UV and visible light,¹⁴ are more valuable fluorescent probes for regional optical marking of interested cells.^15,16 Because of their favourable characteristics, such as excellent thermal stability, good fatigue resistance and fast response time, diarylethenes derivatives have drawn wide spread concern of researchers.^17–20 Furthermore, the special optical properties of diarylethenes enable them to be suitable for long time and real time monitoring in bioimaging.The significance of targeted imaging of fluorescence probe for bio-samples in vitro or in vivo is well-known for their high sensitivity, non-invasiveness, real-time detection and especially selectivity.^21–24 However, the synthesis of these targeting materials is usually complicated. Moreover, for the same type of cells, no selectivity is demonstrated by these complex targeting agents.^21–24 Therefore, easy-prepared and easy-controlled non-labeling fluorescence imaging agents have always been pursued by both the industry and scientific communities. Among the above-mentioned photochromophores, diarylethenes are expected to be promising photochromic imaging candidates due to their favorable characteristics described in preceding paragraph. Prospectively, they can be used for selective long-term tracing if the photostability can be ensured.In our previous work, a series of AIE-active excited-state intramolecular proton transfer (ESIPT) complexes have been demonstrated to be good bio-imaging candidate with many advantages such as simple preparation, good biocompatibility, high quantum yields, fast cell staining as well as long-term anti-photobleaching.^25–27 Sometimes, ESIPT compounds exhibit dual emission, originated from keto and enol state, respectively. This caused extremely fast four-level photophysical cycle (E–E*–K*–K–E), mediated by intramolecular H-bonds immediately after photoexcitation, enables two emissions.^28,29 Herein, we have demonstrated a new type of multifunctional bio-imaging materials based on facile synthesis design concept, by introducing a photochromic diarylethene moiety to enable regional emission turn-on, and introducing an ESIPT moiety to allow good photo stability for long-term tracing. To the best of our knowledge, this is the first example with integrating abilities of non-labeling selectively long-term regional tracing.The new diarylethene derivative (BMBT, see the molecular structure in Scheme 1) has been synthesized (Fig. S1^†) via condensation reaction of 5-amino-2-(benzo[d]thiazol-2-yl)phenol with 4,4-(cyclopentene-1,2-yl)-bis(5-methyl-thiophene-2-formaldehyde), according to a previous reported procedure.^30,31 The molecular structures and purities were confirmed by ¹H NMR spectroscopy and mass spectroscopy (Fig. S6 and S7,^† see the synthesis details and full molecular characterizations in the ESI^†).Open in a separate windowScheme 1Structure and photochromic process of BMBT.The photoirradiation-induced changes in absorption and fluorescence spectra at room temperature are investigated in THF/DMEM mixture (1.0 × 10⁻⁵ mol L⁻¹). The open-ring isomer mainly exhibits two absorption peaked at 304 and 380 nm, respectively (Fig. S2^†). This is ascribed to the internal charge transfer and π–π transition of 2-(2′-hydroxy-phenyl)benzothiazole (HBT),^10,30 coupled with the CT inside the HBT unit from the hydroxyphenyl ring to the benzothiazole ring (Scheme 1). BMBT exhibits two emissions; one blue emission peak around 458 nm and a red emission peak around 600 nm, corresponding to the enol and keto emission, respectively (Fig. 1). Upon irradiation with ultraviolet light at 365 nm, the absorption band at longer wavelength region centered at 595 nm increased obviously with irradiation time (Fig. S2^†). This is caused by the formation of the closed-ring isomer (see the photochromic reaction in Scheme 1). Correspondingly, due to spectroscopic overlap between this longer-wavelength absorbance with the red emission ranged from 560–700 nm, the relative intensity of the red emission substantially decreases with the UV irradiation (Fig. 1), because of efficient energy transfer. Upon further visible light (λ = 520 nm) irradiation, the closed-ring isomer transfers back to the initial open-ring isomer, and thus the longer-wavelength absorption decreases and the red emission restores. This indicates good reversibility of the photochromic reaction.Open in a separate windowFig. 1Fluorescence emission changes of BMBT in THF/DMEM (1 : 200, vol : vol) mixture upon irradiation with 365 nm light (λ_ex = 385 nm) and visible light (λ_ex = 520 nm).Excitingly, BMBT exhibit prominent characteristics of Aggregation-Induced Emission (AIE).^31–33 The fluorescence intensity of BMBT in THF solution was relatively weak, while the powder or nanoparticles of the material dispersed in DMEM buffer exhibited a significantly enhanced emission (Fig. 2). When the DMEM buffer fraction was increased gradually from 0% to 20%, the fluorescence intensity only slightly enhanced. When the DMEM fraction was further increased to 40%, the emission exhibits a significant enhancement. The total intensity at the blue enol emission increased more than 10 times at 100% DMEM fraction as compared with that in THF solution. This AIE property enables high signal-to-noise ratios for favorable bioimaging (Fig. S3^†).Open in a separate windowFig. 2Fluorescence spectra of 1 × 10⁻⁵ M BMBT in the THF/DMEM mixture at different water fractions (λ_ex = 385 nm).Based on the good reversibility of light response and AIE characteristics, the practical application of the BMBT as bioprobe was further investigated. The biological imaging of BMBT was observed by using confocal laser scanning microscopy (CLSM). Blue luminescence in the cytoplasm of HeLa cells was observed after incubation with a THF/DMEM (1 : 200, vol : vol) solution of BMBT (20 μM) for 30 min at 37 °C (Fig. 2 inset). The overlay of luminescent images and bright-field images confirmed that BMBT was located mainly in the cytoplasm of cells rather than the membrane and nucleus (Fig. S3^†). Intense intracellular luminescence with a high signal-to-noise ratio (I₁/I₂ > 7) was detected between the cytoplasm (regions 3 and 1) and nucleus (region 2), also implying weak even few nuclear uptake of BMBT (Fig. S4^†). Besides, BMBT has a low cytotoxicity with the cellular viabilities estimated to be greater than 85% after 24 h incubation with the highest cultural concentration of 50 μM BMBT (Fig. S5^†).The luminescence switching of BMBT can also be achieved while alternating UV and visible light illumination in fixed HeLa cells. Cells (shown in red circle, Fig. 3a) were irradiated with 488 nm light (0.5 mW) for 3 min, the blue fluorescence of the irradiated cells was lamped off while the surrounding cells remained almost unchanged. Such fluorescence quenching is likely ascribed to the intramolecular fluorescence resonance energy transfer of BMBT, due to the intensified short-wavelength absorption band (270–450 nm) of the closed-open form of BMBT with the blue emission band (420–560 nm).³⁴ Upon irradiation with 405 nm light (1.25 mW) the fluorescence of all the selected cell was rapidly recovered within 1 min, caused by decrease of the relative intensity of the 270–450 nm absorption. The fluorescence can be repeatedly erased and recovered many rounds without significant fluorescence quenching, which was hardly achieved by conventional fluorophores (Fig. 3b).^35,36Open in a separate windowFig. 3(a) CLSM image (above) and the overlay image (bottom) of fixed HeLa cells incubated with 20 μM BMBT for 30 min at 37 °C (1) and (5) in original state; (2) and (6) irradiated by 488 nm light (0.5 mW) for a single cell; (3) and (7) all cells, and; (4) and (8) recovered by 405 nm light (1.25 mW). (b) Fluorescence switching of fixed HeLa cells by alternating UV (405 nm, 1.25 mW, 10 s/time) and visible (488 nm, 0.5 mW, 3 min/time) light illumination (λ_ex = 405 nm).This characteristic of selectively opto-marking or de-marking of cells may be used for non-invasive and dynamic tracing of the interested objects in vitro. That is, BMBT can arbitrarily opto-label or de-label interested cells without affecting cell proliferation. As shown in Fig. 4, the cell marked in the red circle was treated with visible light illumination (488 nm, 0.5 mW, 3 min), and its fluorescence was effectively quenched. Cells division of the remaining “bright” cells was observed under the microscope field of vision for a long-term tracing with time up to 36 h. Multiple new cells were produced, as indicated by upper white arrow. Even, as indicated by bottom white arrow, the tacked cells were observed to be doubled. The high brightness of the fluorescence in the proliferated cells indicated the good photo stability of BMBT. This indicated that BMBT can be used as a cell marker for arbitrarily selective erasing the fluorescence of the designated cell. It can also be used for selective lighting their emission by making them as the remaining bright cells after selective photo-erasing or selective photo-recovering after full erasing. This long-term tracking with non-labeling and selective optically marking or de-marking is seldom reported by other photochromophore-based bioimaging agents.³⁷ Herein, the excellent anti-photo bleaching characteristic in the long-term tracing is attributed to high photo stability of the HBT moiety.^26,27Open in a separate windowFig. 4Cells was treated with visible (488 nm, 0.5 mW, 3 min) light illumination for erasing the fluorescence. The remaining bright cells were incubated for another 36 hours, which were observed to be amplified normally (λ_ex = 405 nm).     

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO₃)₂ is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.

A simple surfactant-free synthesis of sub 3 nm carbon-supported Pd nanocatalysts is introduced to study size effects in catalysis.

A key achievement in the design of catalytic materials is to optimise the use of resources. This can be done by designing nanomaterials with high surface area due to their nanometre scale. A second achievement is to control and improve catalytic activity, stability and selectivity. These properties are also strongly influenced by size.^1–3 To investigate ‘size effects’ it is then important to develop synthesis routes that ensure well-defined particle size distribution, especially towards smaller sizes (1–10 nm).Metal nanoparticles are widely studied catalysts. In several wet chemical syntheses, NP size can be controlled using surfactants. These additives are, however, undesirable for many applications^4,5 since they can block active sites and impair the catalytic activity. They need to be removed in ‘activation’ steps which can negatively alter the physical and catalytic properties of the as-produced NPs. Surfactant-free syntheses are well suited to design catalysts with optimal catalytic activity⁶ but their widespread use is limited by a challenging size control.³Palladium (Pd) NPs are important catalysts for a range of chemical transformations like selective hydrogenation reactions and energy applications.^7–9 It is however challenging to obtain sub 3 nm Pd NPs, in particular without using surfactants.² Surfactant-free syntheses are nevertheless attracting a growing interest due to the need for catalysts with higher performances.^10–14Promising surfactant-free syntheses of Pd NPs were recently reported.^8,15 The NPs obtained in these approaches are in the size range of 1–2 nm and show enhanced activity for acetylene hydrogenation⁸ and dehydrogenation of formic acid.¹⁵ Enhanced properties are attributed to the absence of capping agents leading to readily active Pd NPs. The reported syntheses consist in mixing palladium acetate, Pd(OAc)₂, in methanol and the reduction of the metal complex to NPs occurs at room temperature. The synthesis is better controlled in anhydrous conditions to achieve a fast reaction in ca. 1 hour. Another drawback is that the synthesis must be stopped to avoid overgrowth of the particles. Therefore, a support material needs to be added after the synthesis has been initiated and no simple control over the NP size is achieved.^8,15In this communication a more straightforward surfactant-free synthesis leading to sub 3 nm carbon-supported Pd NPs in alkaline methanol at ambient conditions is presented. A solution of Pd(OAc)₂ in methanol undergoes a colour change from orange to dark, indicative of a reduction to metallic Pd, after ca. a day. However, only ca. 1 hour is needed with Pd(NO₃)₂, Fig. 1 and UV-vis data in Fig. S1.^† The fast reduction of the Pd(NO₃)₂ complex in non-anhydrous conditions is a first benefit of the synthesis presented as compared to previous approaches.Open in a separate windowFig. 1Pictures of 4 mM Pd metal complexes in methanol without or with a base (as indicated).For particle suspensions prepared with Pd(OAc)₂ or Pd(NO₃)₂ the NPs agglomerate and quickly sediment leading to large ‘flake-like’ materials. When the reduction of Pd(NO₃)₂ in methanol is performed in presence of a carbon support and after reduction the solution is centrifuged and washed in methanol, a clear supernatant is observed indicating that no significant amount of NPs are left in methanol. Transmission electron microscope (TEM) analysis confirms that NPs are formed and well-dispersed on the carbon support surface and no unsupported NPs are observed, Fig. 2a. Likely, the reduction of the NPs proceeds directly on the carbon support. However, the size of the NPs is in the range 5–25 nm, which is still a relatively large particle size and broad size distribution.Open in a separate windowFig. 2TEM micrographs of Pd NPs obtained by stirring 4 mM Pd(NO₃)₂ in methanol and a carbon support for 3 hours, (a) without NaOH and (b) with 20 mM NaOH. Size distribution histograms are reported in Fig. S4.^† The same samples after electrochemical treatments are characterised in (c) and (d) respectively. Size distribution histograms are reported in Fig. S7.^†Assuming a ‘nucleation and growth’ mechanism, the NPs should become larger over time.¹⁶ But the reaction is so fast that by stopping the reaction before completion, size control is not achieved and unreacted precious metal is observed, Fig. S2.^† To achieve a finer size control and more efficient use of the Pd resources, a base was added to the reaction mixture, e.g. NaOH.³ In alkaline media, the formation of Pd NPs is slower; it takes ca. 60 minutes to observe a dark colour for a 5 mM Pd(NO₃)₂ solution with a base/Pd molar ratio of 10 in absence of a support, Fig. 1.Also in alkaline methanol, the NPs agglomerate over time in absence of a support material. However, if the alkaline solution of Pd(NO₃)₂ is left to stir in presence of a carbon support the desired result is achieved, i.e. Pd NPs with a significantly smaller size and size distribution of ca. 2.5 ± 1.0 nm, Fig. 2b. The NPs homogeneously cover the carbon support and no unsupported NPs are observed by TEM suggesting that the NPs nucleate directly on the carbon surface. Furthermore, the supernatant after centrifugation is clear, indicating an efficient conversion of the Pd(NO₃)₂ complex to NPs, Fig. S3.^† Furthermore, there is no need for an extra reducing agent as in other approaches, for instance in alkaline aqueous solutions.⁹The benefits of surfactant-free syntheses of Pd NPs for achieving improved catalytic activity have been demonstrated for heterogeneous catalysis.^8,15 Surfactant-free syntheses are also well suited for electrochemical applications where fully accessible surfaces are required for fast and efficient electron transfer. Several reactions for energy conversion benefit from Pd NPs. An example is the electro-oxidation of alcohols,⁷ in particular ethanol¹⁷ (see also Table S1^†).Previous studies optimised the activity of Pd electrocatalysts by alloying,^18–20 by using different supports^17,21–23 or crystal structures.^24,25 Investigating NPs with a diameter less than 3 nm was challenging.^2,26,27 The surfactant-free synthesis method presented here allows to further study the size effect on Pd NPs supported on carbon (Pd/C) for electrocatalytic reactions.In Fig. 3, results for ethanol electro-oxidation in 1 M ethanol solution mixed with 1 M KOH aqueous electrolyte are reported based on cyclic voltammetry (CV) and chronoamperometry (CA) with Pd/C catalysts exhibiting 2 significantly different size distributions. The electrode preparation, the measurement procedure and the sequence of electrochemical treatments are detailed in the ESI.^† In order to highlight size effects, we compare geometric and Pd mass normalized currents (Fig. 3a and c) as well as the oxidation currents normalized to the Pd surface area (Fig. 3b).Open in a separate windowFig. 3Electrochemical characterisation of carbon supported Pd NPs with 5–25 nm (grey) and 2.5 nm (dark) size in 1 M KOH + 1 M ethanol aqueous electrolyte. (a) 2^nd CV before chronoamperometry (CA), (b) current normalised by the electrochemically active surface area of Pd, (c) CA recorded at 0.71 V vs. RHE after 50 cycles between 0.27 and 1.27 V.It is clearly seen that based on the geometric current density, the smaller Pd NPs exhibit significantly higher currents for ethanol oxidation than the larger NPs. To differentiate if this observation is a sole consequence of the different surface area, the electrochemically active surface area (ECSA) has been estimated based on “blank” CVs (without ethanol) recorded between 0.27 and 1.27 V vs. RHE in pure 1 M KOH aqueous electrolyte and integrating the area of the reduction peak at ca. 0.68 V, Fig. S5.^† As conversion factor, 424 μC cm⁻² was used.²⁸Using this method, the smaller NPs with a size around 2.5 nm exhibit an ECSA of 92 m² g⁻¹ whereas the larger NPs with a size in the range 5–25 nm exhibit an ECSA of 47 m² g⁻¹, consistent with a larger size. Normalising the ethanol electro-oxidation to these ECSA values instead of the geometric surface area, Fig. 3b, still indicates a size effect. It is clearly seen that the smaller Pd NPs exhibit higher surface specific ethanol oxidation currents, in particular at low electrode potentials. Furthermore, a clear difference in the peak ratios in the CVs is observed. The ratio in current density of the forward anodic peak (j_f, around 0.9 V) and the backward cathodic peak (j_b, around 0.7 V vs. SCE) is around one for the smaller NPs, whereas it is about 0.5 for the larger NPs. The forward scan corresponds to the oxidation of chemisorbed species from ethanol adsorption. The backward scan is related to the removal of carbonaceous species not fully oxidised in the forward scan. The higher j_f/j_b ratio therefore confirms that the smaller NPs are more active for ethanol electro-oxidation and less prone to poisoning, e.g. by formation of carbonaceous species that accumulate on the catalyst surface.^29,30 This observation is further supported by a chronoamperometry (CA) experiment, Fig. 3c, at 0.71 V performed after continuous cycling (50 cycles between 0.27 and 1.27 V at a scan rate of 50 mV s⁻¹). In the CA testing of the thus aged catalysts at 0.71 V, the ethanol oxidation current on the two catalysts starts at around the same values, however, its decay rate is significantly different. The Pd mass related oxidation currents for the smaller NPs are after 30 minutes almost twice as high (ca. 200 A g_Pd⁻¹) as for the larger ones (ca. 130 A g_Pd⁻¹), confirming that the small Pd NPs are less prone to poisoning. In particular a factor up to 4 in the Pd mass related ethanol oxidation currents after 1800 s of continuous operation is achieved compared to a recently characterised commercial Pd catalyst on carbon,²⁰ Table S1.^† Despite different testing procedure reported in the literature, it can be concluded from these investigations that the surfactant-free synthesis presented shows promising properties for electrocatalytic ethanol oxidation even after extended cycling.The extended cycling, however, has different consequences for the two catalysts. For the small (2.5 nm) NPs of the Pd/C catalyst, a massive particle loss, but only moderate particle growth is observed as highlighted in Fig. 2 (see also Fig. S6^†). TEM micrographs of the two Pd/C samples recorded before and after the complete testing (CVs and CAs, for details see Fig. S7^†) show that the catalyst with small Pd NPs exhibits a pronounced particle loss as well as a particle growth to ca. 6 nm probably due to sintering. By comparison, for the Pd/C catalyst with the large Pd NPs, no significant influence of the testing on particle size or particle density is apparent.     

Proton transfer network with luminescence display controls OFF/ON catalysis that generates a high-speed slider-on-deck

A three-component network for OFF/ON catalysis was built from a protonated nanoswitch and a luminophore. Its activation by addition of silver(i) triggered the proton-catalyzed formation of a biped and the assembly of a fast slider-on-deck (k₂₉₈ = 540 kHz).

Upon addition/removal of silver(i) ions and due to efficient inter-component communication, a supramolecular multicomponent network acts as an OFF/ON proton relay with luminescence display enabling switchable catalysis.

Proton relays in living systems¹ are often coupled to bio-machinery with enzymatic catalysis.² Frequently, such proton transfer systems comprise intricate networks with information exchange^3,4 amongst multiple components. Using manmade molecules,⁵ however, it is still arduous to create networks that approach the complexity⁶ and functions of biosystems.Although activation via proton transfer plays an eminent role in molecular logic,⁷ switching,^8–10 and machine operation,^11–15 proton transfer networks in combination with the regulation of catalysis remain scarce.^16,17 Beyond that, we present the in situ catalytic synthesis of a highly dynamic supramolecular device by a new proton transfer network requiring interference-free information handling,¹⁸ self-sorting^19–21 and communication²² among multiple components.^23–25Recently, our group has developed various chemically actuated networks of communicating components and used them for the ON/OFF regulation of transition metal catalysis.^26–28 Herein, we describe a proton relay system that reversibly toggles between two networked states and is useful for regulation of acid catalysis. It catalyzes an amine deprotection which was utilized to fabricate in situ a high-speed slider-on-deck,^29,30 for the first time based on aniline → zinc-porphyrin³¹ (N_an → ZnPor) interactions.The communicating network consists of nanoswitch 1 (ref. 32) and luminophore 2 (ref. 33) as receptors (Fig. 1). In the self-sorted networked state (NetState-I), the added proton is captured in the cavity of nanoswitch 1 resulting in [H(1)]⁺, while luminophore 2 remains free. NetState-I is catalytically inactive as the encapsulated proton is unable to catalyze the deprotection of the trityl protected slider biped 4. Upon addition of AgBF₄ to NetState-I, the silver(i) coordinates to switch 1 and the proton is released onto luminophore 2 thus leading to formation of [Ag(1)]⁺ and [H(2)]⁺ in NetState-II (see Scheme 1), which may be followed by characteristic ratiometric emission changes. If NetState-II is formed in presence of 3 and 4, the catalytic trityl deprotection of 4 produces the free biped 5 that eventually binds to deck 3 furnishing the slider-on-deck [(3)(5)].Open in a separate windowFig. 1Chemical structures and cartoon representations of the ligands used in the study.Open in a separate windowScheme 1Schematic representation of the reversible proton relay network. Green circle represents catalytic site.The concept of the catalyzed biped formation through a proton transfer network relies on the following considerations: (a) the initial networked state should be catalytically inactive as expected from a self-sorting of the proton inside the nanoswitch leaving the luminophore free. (b) Addition of silver(i) ions should release the proton from the protonated switch thus forming the protonated luminophore which we anticipated to act as catalyst. (c) A di-protected biped should undergo acid catalyzed deprotection by the protonated luminophore and eventually form a slider-on-deck by coordination to a deck. (d) Finally, the product of the catalysis should not intervene in any state of the operation with the main system.As proton receptor we chose the known phenanthroline–terpyridine nanoswitch 1 (ref. 32) in order to tightly capture the proton/silver(i) as HETTAP (HETeroleptic Terpyridine And Phenanthroline) complexes.³⁴ Luminophore 2 was based on the 2,4,6-triarylpyridine scaffold which is known to fluoresce strongly upon protonation due to intramolecular charge transfer (ICT).³⁵To investigate the self-sorting within the network, we mixed nanoswitch 1, luminophore 2 and TFA in 1 : 1 : 1 ratio in CD₂Cl₂ which furnished NetState-I = [H(1)]⁺ + 2. ¹H NMR shifts of the 4/7-H proton signals to 8.43 and 8.69 ppm and of 9-H and e-H proton peaks to 6.60 and 8.48 ppm confirmed formation of [H(1)]⁺. Moreover, proton signals of c′-H and d′-H at 7.07 and 7.92 ppm corroborated presence of the free luminophore 2 (Fig. 2). Addition of one equiv. of AgBF₄ to NetState-I instantaneously generated NetState-II = [Ag(1)]⁺ + [H(2)]⁺. In the ¹H NMR of NetState-II, the 9-H and e-H proton signals shifted to 6.55 and 8.23 ppm, respectively, which substantiated the formation of [Ag(1)]⁺ whereas the shift of proton peaks c′-H and d′-H to 7.12 and 8.00 ppm, respectively, established the protonated luminophore [H(2)]⁺ (Fig. 2). Upon excitation at 300 nm, NetState-I exhibited emissions at λ = 346, 372 and 393 nm which represent an overlap of the emission peaks of the free luminophore 2 at λ = 346 and 372 nm and of [H(1)]⁺ at λ = 376 and 393 nm (Fig. 3c). In NetState-II, the fluorescence spectrum showed a single emission at λ = 461 nm, which is attributed to [H(2)]⁺ (Fig. 3). In contrast, [Ag(1)]⁺ in NetState-II is nonfluorescent as advocated by the full quenching of the emission of [H(1)]⁺ at λ = 376 and 393 nm upon titration with silver(i) ions (ESI, Fig. S18^†). When one equiv. of tetra-n-butylammonium iodide (TBAI) was added to NetState-II, complete restoration of NetState-I was achieved as confirmed from ¹H NMR data. TBAI acted as a scavenger for silver(i) by removing AgI through precipitation thus reversing the translocation sequence. Multiple switching of NetState-I ⇆ NetState-II was readily followed by ¹H NMR (Fig. 2) and luminescence changes. Upon successive addition of AgBF₄ to NetState-I, the emission changed to λ = 461 nm (sky blue emission of [H(2)]⁺) in NetState-II (Fig. 3a and b) in a clean ratiometric manner that allowed monitoring of the amount of liberated protons. Alternating switching between the NetStates was followed by emission spectroscopy over three cycles with a small decline in emission intensity (Fig. 3c and d), which may be due to the progressive accumulation of AgI.Open in a separate windowFig. 2Partial ¹H NMR spectra (500 MHz, CD₂Cl₂, 298 K) showing reliable switching between NetState-I and II over 3 cycles. (a) After mixing of TFA, 1, and 2 in 1 : 1 : 1 ratio; (b) after adding 1.0 equiv. of AgBF₄ furnishing [Ag(1)]⁺ and [H(2)]⁺; (c) after addition of 1.0 equiv. of TBAI; (d) after adding 1.0 equiv. of AgBF₄; (e) after addition of 1.0 equiv. of TBAI, (f) after adding 1.0 equiv. of AgBF₄; (g) after addition of 1.0 equiv. of TBAI. Blue asterisk marked proton signals represent the tetra-butylammonium ion.Open in a separate windowFig. 3(a) Ratiometric emission titration of 2 (2.1 × 10⁻⁵ M) with TFA (5.3 × 10⁻³ M) in CH₂Cl₂. (b) Change of emission upon protonation of 2. (c) Reversibility of the network NetState-I ⇆ II over three cycles upon addition of silver(i) and TBAI (see text, blue arrow assigns direction of switching) monitored by luminescence changes at λ = 461 nm. (d) Multiple cycles monitored by the change of the fluorescence intensity.For probing the catalytic efficiency of each state towards a simple amine deprotection reaction we chose the trityl-protected aniline 6 (Fig. 4) performing two NetState-I ⇆ NetState-II cycles (Fig. 4b). We first prepared NetState-I by mixing 1, 2 and TFA (1 : 1 : 1) in CD₂Cl₂ and then added 6 and TMB (1,3,5-trimethoxybenzene as an internal standard) in a 10 : 10 ratio. After heating for 2 h at 40 °C, the ¹H NMR spectrum revealed no product 7. Clearly, the proton in [H(1)]⁺ is catalytically inactive (ESI, Fig. S10b^†). Addition of one equiv. of AgBF₄ and probing the catalysis again for 2 h at 40 °C revealed formation of product 7 in (30 ± 2)% yield (ESI, Fig. S10c^†). When the catalytic experiment was probed with only [H(2)]⁺ under identical reaction conditions, it provided the same yield (ESI, Fig. S16^†). This suggested that the reversible proton relay network (Scheme 1) functioned reliably also in presence of 6 with complex [H(2)]⁺ as the catalytically active species. Addition of one equiv. of TBAI to the above NetState-II mixture and probing for catalysis, revealed no further product formation. Apparently, addition of TBAI translocated back the proton to nanoswitch 1 thus restoring NetState-I, which resulted in the OFF state for catalysis. Adding one equiv. of AgBF₄ to the mixture and probing again the catalytic activity resulted in formation of further (29 ± 2)% of product 7, which was basically identical to the yield produced in the first cycle of NetState-II (ESI, Fig. S10e^†). Adding TBAI and the consumed amount of substrate showed no further deprotection. In sum, the robust catalytic performance of the proton relay network as reflected in two successive catalytic cycles resulted in no significant decline of the catalytic activity (Fig. 4b).Open in a separate windowFig. 4(a) Representation of the OFF/ON regulation of the trityl deprotection reaction in NetState-I and II. (b) Reversible switching between the network states furnished reproducible amounts of product 7 in NetState-II (two independent runs). Consumed amounts of substrates were added (green asterisk).The OFF–ON switching of the catalytic trityl deprotection of aniline 6 by the proton relay network inspired us to utilize the system to catalytically fabricate a slider-on-deck based on the N_an (= aniline) → ZnPor interaction. Deprotection of the protected biped 4 should afford the bis-aniline biped 5 and enable its attachment to the tris-zinc porphyrin deck 3 thus forming the slider-on-deck [(3)(5)].Addition of one equiv. of biped 5 (for synthesis and characterization, see ESI^†) to deck 3 (ref. 25) in CD₂Cl₂ quantitatively generated the slider-on-deck [(3)(5)], which was unambiguously characterized by ¹H NMR, ¹H DOSY NMR and elemental analysis (ESI, Fig. S5 and S6^†). In the ¹H NMR-spectrum, several diagnostic shifts of biped and deck proton signals attested the binding of biped 5 to the deck. Formation of the slider-on-deck was further confirmed from a single set in the ¹H DOSY in CD₂Cl₂ (ESI, Fig. S9^†).The single set of protons (p-H, r-H, s-H, t-H, u-H, v-H) in the ¹H NMR spectrum for all three-zinc porphyrin (ZnPor) stations of deck 3 unmistakably suggested rapid intrasupramolecular exchange^29a of the aniline biped across all three ZnPor sites requiring fast N_an → ZnPor bond cleavage. At low temperature (−65 °C) the ZnPor p-H proton signals separated into two sets (1 : 2) (ESI, Fig. S7 and S8^†). Kinetic analysis over a wide temperature range provided the exchange frequency (k) with k₂₉₈ = 540 kHz at 298 K (Fig. 5a) and the activation data as ΔH^‡ = 41.5 ± 0.7 kJ mol⁻¹, ΔS^‡ = 2.8 ± 0.7 J mol⁻¹ K⁻¹ and ΔG^‡₂₉₈ = 40.7 kJ mol⁻¹ (Fig. 5).Open in a separate windowFig. 5(a) Experimental and theoretical splitting of p-H proton signal of nanoslider [(3)(5)] in VT ¹H-NMR (600 MHz) furnishing rate data in CD₂Cl₂. (b) Eyring plot providing kinetic parameters.After determining the kinetic parameters of the slider-on-deck [(3)(5)], we investigated its in situ catalyzed formation by the proton relay network. Therefore, we mixed nanoswitch 1, luminophore 2, TFA, trityl-protected biped 4, deck 3 and TMB in 1 : 1 : 1 : 5 : 5 : 5 ratio in CD₂Cl₂ and heated the reaction mixture for 12 h at 40 °C. The ¹H NMR spectrum suggested no formation of any slider-on-deck in NetState-I (ESI, Fig. S13^†). Addition of one equiv. of AgBF₄ to the same mixture (converting NetState-I to NetState-II) and heating it at 40 °C for 12 h revealed full formation of the slider-on-deck as indicated by the disappearance of the 2′-H, 3′-H and 4′-H proton signals of biped 4 at 6.60, 6.31 and 6.51 ppm (ESI, Fig. S11^†), respectively, and the quantitative emergence of 4′-H proton signal of biped 5 bound to deck 3 at 5.84 ppm (ESI, Fig. S12^†). The upfield shift of the deck''s meso t-H protons from 10.36 to 10.29 ppm allowed monitoring of the formation of [(3)(5)], e.g. by the gradually increased chemical shift difference of proton signal t-H (Δδ_t-H) (Fig. 6a; ESI, Fig. S11^†). In sum, the protonated [H(2)]⁺ in NetState-II catalyzed the deprotection of the trityl-protected biped 4 with the effect that the resultant bis-aniline biped 5 would quantitatively bind to deck 3 affording the slider-on-deck [(3)(5)]. Furthermore, the OFF/ON switching of catalysis was probed for shortened time periods of 3 h. As illustrated in Fig. 6b, the process could be readily followed using the proton signal t-H (Δδ_t-H) (ESI, Fig. S14^†) and the growth of proton signal 4′-H at 5.84 ppm of bound biped 5 (ESI, Fig. S15^†). One clearly sees recurring OFF/ON regulation in NetState-I/II.Open in a separate windowFig. 6Plot of the shift difference of proton signal t-H (Δδ_t-H) vs. time for (a) the formation of [(3)(5)] from 3 and 4. Red curve: formation of [(3)(5)] over 12 h catalyzed by NetState-II. Black curve: OFF state of catalysis in NetState-I. (b) OFF/ON catalytic cycles by switching between NetState-I and NetState-II. Addition of Ag⁺, see blue asterisk; addition of TBAI: red asterisk.In conclusion, we have designed a supramolecular multicomponent network³⁶ that acts as an OFF/ON proton relay with a luminescence display³⁷ and is useful for switchable catalysis.³⁸ The network is toggled by chemical input and intercomponent communication^15,16 and as a result is a conceptual complement to photoacids driving networks.^7,8The release and capture of protons is demonstrated by the ON/OFF trityl deprotection of anilines. To demonstrate functioning in a complex setting, the network was utilized to catalyze formation of a high-speed slider-on-deck assembly based on N_an → ZnPor interactions (sliding frequency of k₂₉₈ = 540 kHz). The robust operation of the proton relay furnishing dynamic machinery³⁹ through acid catalysis followed by self-assembly is a valuable step mimicking sophisticated biological proton relays.²     

Electrochemical preparation system for unique mesoporous hemisphere gold nanoparticles using block copolymer micelles

Gold nanoparticles (AuNPs) are widely used in various applications, such as biological delivery, catalysis, and others. In this report, we present a novel synthetic method to prepare mesoporous hemisphere gold nanoparticles (MHAuNPs) via electrochemical reduction reaction with the aid of polymeric micelle assembly as a pore-directing agent.

Mesoporous hemisphere Au nanoparticles using self-assembled micelles, for the first time, are demonstrated by using electrochemical reduction on a Ti substrate.

Gold (Au) is one of the most stable and versatile elements utilized in various fields, including catalysis, optics, and industrial purposes. Consequently, various shapes and sizes of AuNPs have been intensively studied to improve the performance of Au in different applications.^1–6 Previously, nanoporous or dendritic metal nanostructures, including Au nanostructures, have been synthesized by employing different reagents and conditions such as SH-terminated amphiphilic surfactant,⁷ pH controlling,⁸ and hard-templates.^9,10 The reported porous and dendritic Au nanostructures possess high surface areas and rich active sites, which in turn lead to highly enhanced catalytic activities.Recently, a soft-template method using self-assembled micelles or lyotropic liquid crystals as pore-directing agents has allowed the successful synthesis of mesoporous nanoparticles^11–13 and films^14–17 with different metal compositions. The metals with mesoporous structures demonstrate superior catalytic activity per weight or surface area over their nonporous bulk forms. Previously, our group reported a several-fold increase in the catalytic activity of mesoporous metals in reactions such as the methanol oxidation reaction (MOR),^14,15 ethanol oxidation reaction (EOR),^13,15–17 and nitric oxide reduction¹² as compared to their bulk nanoparticles and films. Such improvement in the catalytic activity of mesoporous structures is mainly attributed to their significantly larger surface areas, more exposed catalytically active sites, and increased durability against aggregation.Interestingly, nanoporous or mesoporous Au structures had been successfully synthesized by using a dealloying method¹⁸ and a hard templating method.⁹ Such methods, however, are a little complicated, and pore-directing templates often remain within the pores, thus leading to severe contamination. Using a thiol group is an alternative way to synthesize mesoporous Au nanospheres.⁷ A significant drawback of using a thiol group, however, is its strong chemical bonding with Au, thus becoming unable to be removed. The synthesis of mesoporous structures using self-assembled polymeric micelles as soft-templates, on the other hand, is a more facile method with fewer synthetic steps, and it is also known to be free of contaminations within the pores. Although a soft-templating method using polymeric micelles has been utilized for the preparation of mesoporous Au and Au-based alloy films towards surface-enhanced Raman scattering (SERS) signals,¹⁹ glucose sensing,^20,21 and MOR,²² the obtained morphologies have been limited to only films.Despite such apparent benefits arising from mesoporous structures and their synthesis using soft-templates, the synthesis of mesoporous AuNPs using soft-templates has not been achieved yet. It is mainly due to the physical and chemical properties of Au which make it extremely hard to form mesoporous structures. Herein, we adopt an electrochemical approach and the soft-template method to synthesize MHAuNPs successfully. As discussed above, we expect MHAuNPs to be highly efficient in various applications in medical diagnosis,²³ optical sensing,²⁴etc.In this report, MHAuNPs with different shapes and sizes are for the first time reported by changing various electrochemical deposition conditions such as applied potentials between electrodes and deposition times. Scheme 1 shows the schematic illustrations of the entire process of precursor preparation (Scheme 1a) and the MHAuNPs fabrication process (Scheme 1b), including the deposition and the detachment of the nanoparticles. The characterization methods implemented in this paper are mentioned in ESI.^†Open in a separate windowScheme 1(a) The process of Au precursor solution preparation and (b) fabricating MHAuNPs by electrochemical reduction.In a typical experiment, a p-doped silicon (Si) wafer was cleaned by using acetone, isopropyl alcohol, and deionized water (DIW) with sonication for 5 minutes, followed by nitrogen (N₂) gas blowing to dry the Si wafer. After the wet cleaning process, the Si surface was treated by oxygen (O₂) plasma for 5 minutes (Oxford Instruments PlasmaPro 80 Reactive Ion Etcher) to remove residual organic impurities. Then, 10 nm of titanium (Ti) layer and 100 nm of Au layer were deposited sequentially by electron beam evaporation (Temescal FC-2000 e-beam evaporator) at 10⁻⁶ torr. Commercially available Au etchant (Sigma-Aldrich) was used to etch the Au film to expose the Ti area (the left image in Scheme 1b). During etching, about 20 percent of Au area was left to be connected to the electrochemical work station, as drawn in Scheme 1b. In preparation of the Au precursor solution, 5 mg of poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO, the number of average molecular weight (M_w) for each block is 18 000 for PS and 7500 for PEO, respectively) was mixed in 1.5 ml of tetrahydrofuran (THF) followed by stirring at 300 rpm for 8 hours. Then, 0.75 ml of ethanol, 0.5 ml of HAuCl₄ aqueous solution (40 mM), and 1.25 ml of DIW were added sequentially. The solution was stirred for another 30 minutes at 200 rpm. The existing block copolymer micelles can be confirmed by TEM observation, and the average diameter is 25 nm, as shown in Fig. S1.^† For the electrochemical deposition, an electrochemical workstation (CH Instruments Inc. 660e) with three electrode system was used to deposit MHAuNPs on the Ti/Si substrate. After the deposition, the particles were carefully washed by chloroform, followed by a rinse using DIW to remove the residual micelles completely. To detach and collect MHAuNPs from the Ti/Si substrate, the substrate was soaked in ethanol and strongly sonicated for a few minutes (Scheme 1b).Fig. S2^† shows the details of the growth mechanism of MHAuNPs by different deposition times. At the initial stage (Fig. S2a^†), small nanoparticles are generated by reducing Au ions in the precursor solution throughout the substrate. Then, the seed starts growing and forming MHAuNPs as the deposition time increases (Fig. S2b–e^†). This similar growth mechanism is the same as the previous report.¹⁹ The high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. S2f^†) shows the mesopores inside the MHAuNPs are homogeneously generated. As-obtained MHAuNPs consist of a pure Au element without any impurities, as shown in Fig. S3.^† Fig. 1 and S4^† show scanning electron microscope (SEM) images of MHAuNPs deposited at different voltages from −0.2 V to −0.9 V vs. Ag/AgCl at high magnification and low magnification, respectively. Different deposition voltages lead to significant changes in the particle sizes but slight differences in the particle shapes. The size distributions of MHAuNPs and the plots of the average diameters of MHAuNPs by different deposition voltages are described in Fig. 2. The distribution graphs show the large sizes of particles, such as more than 1 μm in diameter, when the high voltage (−0.2 V vs. Ag/AgCl) is applied (Fig. 2a). The distribution becomes narrower upon the lower applied voltage. The average diameter-applied voltage plots in Fig. 2b show that the average particle size decreases from around 1.1 μm at −0.2 V to about 300 nm at −0.9 V. Thus when the lower deposition voltages are applied (i.e., the deposition rate is higher) (Fig. 1g–h), the smaller particles with a higher degree of size uniformity are obtained. The opposite trend is observed at higher deposition voltages (i.e., the deposition rate is lower) (Fig. 1a and b), at which the particles become larger and their size uniformity decreases. This trend is because the higher voltage allows only a limited number of seed particles to be deposited on the Ti/Si substrate, and each seed individually grows with no additional seed formation. Whereas the lower voltage can allow a higher number of seeds, leading to a uniform supply of electrons from the working Ti/Si electrode (Fig. S5^†). In addition, the lower deposition voltages make the particle shape more hemispherical in Fig. 1f–h.Open in a separate windowFig. 1The SEM images of MHAuNPs electrochemically deposited at (a) −0.2 V, (b) −0.3 V, (c) −0.4 V, (d) −0.5 V, (e) −0.6 V, (f) −0.7 V, (g) −0.8 V, and (h) −0.9 V for 500 s. The scale bars indicate 200 nm.Open in a separate windowFig. 2(a) Size distributions of MHAuNPs generated by different voltages and (b) the average diameter–the applied voltage plots. Fig. 3 shows the SEM images of MHAuNPs deposited at −0.2 V and for different deposition times from 250 s to 1000 s. Although longer deposition time does not change the number of MHAuNPs, it leads to the growth of MHAuNPs in lateral and vertical directions. Although the MHAuNPs grow more than about two or three times larger at long deposition time, the mesoporous formation does not seem to be changed, as shown in insets in Fig. 3. This point indicates that the deposition time is not the main factor affecting the formation of mesoporous structures as well as the number of particles (seeds), but it affects the sizes of particles.Open in a separate windowFig. 3The SEM images of MHAuNPs deposited at −0.2 V (vs. Ag/AgCl) for (a) 250 s, (b) 500 s, and (c) 1000 s. The scale bars indicate 10 μm. The insets in each figure are magnified SEM images of each condition (The scale bars in insets indicate 500 nm).In this report, 10 nm Ti layer on Si wafer plays an important key role in the formation of MHAuNPs, as previously mentioned in the experimental procedure. The use of the Ti substrate with low conductivity (ca. 2.38 × 10⁶ S m⁻¹), which is about only 5.8% in comparison with that of Au (ca. 4.10 × 10⁷ S m⁻¹), is not common in the electrochemical plating research field.^25–30 Most of the papers on mesoporous metal structures synthesized by electrochemical deposition have utilized Au or Pt substrates due to its chemical stability and high electrical conductivity.^14–17 Fig. S6^† shows the amperometry (i–t) curves during the deposition of MHAuNPs (black dots) on a Ti/Si substrate and mesoporous gold films (red dots) on an Au substrate at the same deposition condition. As shown in Fig. S6,^† around 1/7 times less current flows on the Ti/Si substrate throughout the deposition time. This low current density on the Ti/Si substrate is one of the factors for fabricating MHAuNPs. Low current density causes the formation of a few particles (i.e., seeds) at the initial stage of the deposition and leads to seed growth in a few places, as explained in Fig. S2.^† Furthermore, the use of Ti/Si substrates affects the bottom parts of MHAuNPs to become an arch shape. Only edges of MHAuNPs attach onto the Ti/Si substrates, as shown in Fig. 4. This attachment is because the interaction between the deposited MHAuNPs and the Ti substrate surface (probably, the Ti surface can be partially oxidized, forming TiO_x) is very weak. Therefore, the deposited MHAuNPs can be easily detached from the Ti/Si substrates by sonicating the substrates in solvents (Scheme 1b). The collected MHAuNPs in a solvent are obtained as colloidal particles as shown in Fig. S7.^† Such interesting hemispherical mesoporous nanoparticles have advantages to electrocatalytic activities in comparison to spherical mesoporous metals.³¹ The method using a Ti/Si substrate as a working electrode can be repeatedly implemented with one substrate and without change of the precursor solution, thus it can be effective for mass production in the future.Open in a separate windowFig. 4The SEM image of MHAuNPs deposited at −0.6 V. The arrow shows that the bottom of the MHAuNPs is an arch.Finally, surface-enhanced Raman scattering (SERS) effects on MHAuNPs were investigated by using an adsorbate called rhodamine 6G (R6G), as shown in Fig. S8.^† The resulting MHAuNPs at all conditions (−0.3 V, −0.6 V, and −0.9 V) show substantially strong SERS intensity (Fig. S8a^†), while Ti/Si and Si substrates without MHAuNPs show noise level of intensity. To further investigate enhancement factor (EF) and limit of detection (LoD), various concentrations of R6G with MHAuNPs fabricated at −0.9 V were used for the SERS studies (Fig. S8b^†). The main peak of SERS is 1363 cm⁻¹, and it disappears from less than 10⁻⁶ M concentration, while the 1183 cm⁻¹ peak still exists at 10⁻⁸ M (Fig. S8c^†). The maximum EFs at 1363 cm⁻¹ (10⁻⁶ M) and 1183 cm⁻¹ (10⁻⁸ M) are 1.5 × 10⁴ and 3.1 × 10⁶, respectively (Fig. S8d^†). Transmission electron microscope (TEM) images in Fig. S9^† show the detailed particle structures and the electron diffraction (ED) pattern confirmed the crystal structure is the face-center cubic (FCC) structure. The sharp surface structures and the pores on MHAuNPs provide abundant hot spots that have been reported as the origin that enhances SERS intensity owing to the plasmon resonances.^19,32 Besides, the high density of small-sized MHAuNPs (Fig. S5 and S10^†) boosted higher SERS intensity.In conclusion, we have synthesized MHAuNPs by using 10 nm Ti-coated Si substrates as a working electrode on a Si wafer and electrochemical deposition using self-assembled polymeric micelles as pore-directing agents. The low current generates Au seeds at only a few places, and it acts as the points that MHAuNPs start growing. The particle shapes and sizes can be controlled by changed applied voltages and deposition times. The lower voltages make small particles and the great hemispherical AuNPs with mesoporous architecture. The long-time deposition does not affect any mesoporous formation, but the particle shape and size. Besides, the low affinity between Au and Ti (probably, oxidized layer) results in the arch on the bottom of MHAuNPs, which helps the particles detached from the substrates easily. These results indicate that different thicknesses and compositions of working electrodes can provide different metal deposition phenomena, which can bring out unique shaped particles with mesoporous architectures in the future.     

Coumarin-based fluorescent probe for the rapid detection of peroxynitrite ‘AND’ biological thiols

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC was shown to exhibit only a very small fluorescence response upon addition of a single GSH or ONOO⁻ analyte. Exposure to both analytes, however, resulted in a significant fluorescence enhancement.

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols.

Peroxynitrite (ONOO⁻) is a short-lived reactive oxygen and reactive nitrogen species (ROS and RNS) produced intracellularly by the diffusion-controlled reaction of nitric oxide (NO˙) with superoxide (O₂˙⁻).^1–3 Despite playing a key role as a physiological regulator,⁴ it is commonly known for its high reactivity towards most types of biomolecules, causing deleterious effects and irreversible damage to proteins, nucleic acids, and cell membranes.^5,6 ONOO⁻ is therefore a central biological pathogenic factor in a variety of health conditions such as strokes, reperfusion injuries or inflammatory and neurodegenerative diseases (Parkinson''s disease, Alzheimer''s disease).^7–9 Conversely, biothiols such as glutathione and cysteine are endogenous reducing agents, playing a central role in the intracellular antioxidant defence systems.^10–12 Glutathione (GSH), in particular, is the most abundant biothiol in mammalian cells, and exists as both its reduced GSH form, and as the oxidised disulphide form GSSG.^13–15 Peroxynitrite and biothiols such as GSH are intimately linked, as abnormal levels of highly oxidising ONOO⁻ can perturb the delicate GSH/GSSG balance, causing irreversible damage to key processes such as mitochondrial respiration.¹⁶ Thus, abnormal levels of GSH are common in cells undergoing oxidative stress, in which the regulation of and interplay between ONOO⁻ and GSH is closely associated with physiological and pathological processes.^17,18 One such example is drug-induced liver injury (DILI), in which upregulation of ONOO⁻ occurs in hepatotoxicity. Treatment with GSH could be used to remediate this type of cell injury by depletion of ONOO⁻.^19–22One of our core research interests lies in the development of dual analyte chemosensors capable of detecting two distinct analytes such as biological reactive oxygen species and biothiols.^23–26 Although a wide range of single-analyte probes exist for the detection of ROS and thiols separately,^27–30 ‘AND’ logic sensors for their simultaneous detection are still rare.^31–33 We are therefore interested in developing such probes, containing two distinct sensing units, one for each analyte, working simultaneously or in tandem to elicit a fluorescence response.³⁴ This approach allows the monitoring of multiple biomolecular events and factors involved in specific disease pathologies, in order to achieve optimal predictive accuracy for diagnosis and prognostication.³⁵Using these principles, our group has recently focused on developing a range of ‘AND’ logic based sensors exploiting a variety of sensing units and mechanisms of fluorescence. Two such probes are shown below: GSH-ABAH (Fig. 1a), an ESIPT probe with a 4-amino-2-(benzo[d]thiazol-2-yl)phenol (ABAH) core, employing a maleic anhydride thiol-acceptor group;³¹ and JEG-CAB (Fig. 1b), a coumarin-based probe, this time with a salicylaldehyde homocysteine-reactive unit.²⁴ Both of these sensors employ a benzyl boronate ester as their peroxynitrite-reactive unit.Open in a separate windowFig. 1(a) GSH-ABAH, previously reported probe for simultaneous detection of ONOO⁻ and GSH. (b) JEG-CAB, previously reported probe for simultaneous detection of ONOO⁻ and GSH. (c) AHC – a core fluorescent unit that enables the synthesis of ‘AND’ based fluorescent probe for the detection of ONOO⁻ and GSH (d) ROS-AHC, a novel probe detailed in this work for simultaneous detection of ONOO⁻ and GSH.Herein, we set out to develop an ‘AND’ logic gate based fluorescence probe for simultaneous detection of ONOO⁻ and GSH. 3-Amino-7-hydroxy-2H-chromen-2-one (AHC) was chosen as a suitable coumarin fluorophore core for the development of an ‘AND’ logic based sensor, as its free phenol and amine functional groups provided a good opportunity for independent derivatization (Fig. 1).^36–39Previous literature reports show that protection of AHC with a maleic anhydride group results in quenching of the coumarin''s fluorescence intensity due to photoinduced electron transfer (PeT) processes. This fluorescence is rapidly restored in the presence of biological thiols, however, due to their fast addition to this functional group.⁴⁰ Therefore, we suggested that functionalization of the free phenol of this sensor using a benzyl boronic ester should further block the fluorescence, whilst serving as reporter unit for ONOO⁻. The greatly increased reactivity of peroxynitrite over other ROS towards boronate esters^41–43 should allow this functionality to act as a peroxynitrite-selective reporter, leading to an ‘AND’ logic based probe for the detection of ONOO⁻ and biological thiols (Fig. 1, Scheme 1). ROS-AHC was synthesized in 5 steps, starting with a 4-step synthesis of compound 1 adapted from literature procedures,^40,44 followed by the addition of the benzyl boronic pinacol ester (see Scheme S1 ESI^†).Open in a separate windowScheme 1Fluorescence ‘turn on’ mechanism of ROS-AHC in the presence of ONOO⁻ and GSH.The UV-Vis behaviour of ROS-AHC before and after exposure to both GSH and ONOO⁻ was evaluated in pH 7.40 buffer solution, showing a maximum absorption peak at 340 nm for both the unreacted probe and the probe following exposure to GSH, shifting to 350 nm with the addition of ONOO⁻ to the probe and 365 nm after sequential additions of GSH and ONOO⁻ to the probe (Fig. S1 ESI^†). Fluorescence experiments were then carried out. As expected, ROS-AHC was initially non-fluorescent, with a small fluorescence increase upon addition of ONOO⁻ (6 µM) (Fig. 2 and S2 ESI^†). Incremental additions of GSH (0–4.5 µM) resulted in a much larger increase in fluorescence intensity (>69-fold, see Fig. 2 and S3 ESI^†), demonstrating the need for both GSH and ONOO⁻ in order to achieve a significant ‘turn on’ fluorescence response.Open in a separate windowFig. 2Fluorescence spectra of ROS-AHC (5 µM) with addition of ONOO⁻ (6 µM), wait 5 min then incremental addition of GSH (0–4.5 µM), 5 min incubation before measurements in PBS buffer solution (10 mM, pH = 7.40). Fluorescence intensities were measured with λ_ex = 400 nm (bandwidth 8 nm). The green line represents the highest intensity after addition of GSH (4 µM).Similar fluorescence experiments were then carried out in reverse order, with the addition of GSH (6 µM) to ROS-AHC resulting in only a small increase in fluorescence intensity (Fig. 3 and S4 ESI^†). As before, incremental addition of the second analyte, in this case ONOO⁻ (0–5.5 µM), resulted in a large increase in fluorescence intensity (>46-fold, Fig. 3 and S5 ESI^†), confirming that ROS-AHC requires both GSH and ONOO⁻ for a full fluorescence ‘turn on’ response.Open in a separate windowFig. 3Fluorescence spectra of ROS-AHC (5 µM) with addition of GSH (6 µM), wait 5 min then incremental addition of ONOO⁻ (0–5.5 µM) with 5 min incubation before measurements in PBS buffer solution (10 mM, pH = 7.40). Fluorescence intensities were measured with λ_ex = 400 nm (bandwidth 8 nm). The orange line shows the highest intensity after addition of ONOO⁻ (5 µM).Subsequently, the selectivity of this probe towards both analytes was evaluated. A range of amino acids were evaluated (Fig. S6 ESI^†), with only thiol-containing analytes (glutathione, cysteine and homocysteine) eliciting significant fluorescence response, whilst non-thiol amino acids led to no changes in fluorescence intensity. A broad screen of ROS analytes was also carried out, demonstrating excellent selectivity for ONOO⁻, even over H₂O₂ (Fig. S7 ESI^†).The time-dependent response of ROS-AHC with both ONOO⁻ and GSH was also examined (Fig. S8 and S9 ESI^†). After initial addition of GSH or ONOO⁻ to the probe, subsequent addition of the second analyte triggered a rapid and significant increase in fluorescence, achieving maximum fluorescence intensity within 78 s in both cases. Furthermore, LC-MS experiments confirmed the formation of the suggested non-fluorescent intermediates, as well as the final fluorescent species shown in Scheme 1 (Fig S10, S11 and S12^†).In summary, we have developed a coumarin-based dual-analyte ‘AND’ logic fluorescent sensor, ROS-AHC, for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC has shown high sensitivity and selectivity towards both ONOO⁻ and biological thiols.     

Coumarin-based fluorescent ‘AND’ logic gate probes for the detection of homocysteine and a chosen biological analyte

With this research we set out to develop a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys. Probe JEG-CAB was constructed by attaching the ONOO⁻ reactive unit, benzyl boronate ester, to a HCys/Cys reactive fluorescent probe, CAH. Similarly, the core unit CAH was functionalised with the nitroreductase (NTR) reactive p-nitrobenzyl unit to produce probe JEG-CAN. Both, JEG-CAB and JEG-CAN exhibited a significant fluorescence increase when exposed to either HCys and ONOO⁻ (JEG-CAB) or HCys and NTR (JEG-CAN) thus demonstrating their effectiveness to function as AND logic gates for HCys and a chosen analyte.

With this research we set out to develop of a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys.

Homocysteine (HCys) is a non-proteinogenic amino acid, formed from the de-methylation of methionine,¹ which is then converted into cysteine (Cys) via a vitamin B₆ cofactor. Typical physiological concentrations of HCys range between 5–15 μmol L⁻¹.² However, elevated levels of HCys (>15 μmol L⁻¹), which is known as hyperhomocysteinemia (hHCys),³ have been associated with pregnancy disorders, Alzheimer''s disease, cardiovascular disease and neurodegenerative diseases (NDs).^4–6 It is believed that the main cause of HCys induced toxicity is through the non-enzymatic modification of proteins. This is achieved through irreversible covalent attachment of the predominant metabolite of HCys, homocysteine thiolactone (HTL), to lysine residues; a phenomenon known as ‘protein N-homocysteinylation’ that results in the loss of a proteins structural integrity leading to loss of enzymatic function and aggregation.⁷A number of fluorescent sensors have been developed for the detection of HCys to help improve our understanding of its role in biological systems.^8–11 However, these fluorescent probes have focused on the detection of a single biomarker (HCys), however, processes associated with HCys induced toxicity often involve more than one biochemical species. For example, it has been reported that peroxynitrite (ONOO⁻) and nitric oxide (NO˙) play a significant role in HCys-mediated apoptosis in trigeminal sensory neurons¹² and HCys has been reported to induce cardiomyocytes cell death through the generation of ONOO⁻.¹³ The production of ONOO⁻ is believed to be the result of an increased production of superoxide (O₂˙⁻) by HCys activating the enzyme NADPH oxidase.^14–16 This increased production of O₂˙⁻ leads to a reduction in the bioavailability of NO˙ by increasing the formation of ONOO⁻ (NO˙ + O₂˙⁻ → ONOO⁻).¹⁷ The reported ONOO⁻ concentrations in vivo are believed to be approximately 50 μM but, higher concentrations of 500 μM have been found in macrophages.^18,19 Furthermore, hypoxia has been reported to facilitate HCys production in vitamin-deficient diets²⁰ where hypoxia leads to an upregulation of nitroreductase (NTR) – a reductive enzyme upregulated in cells under hypoxic stress.^21,22 Therefore, the development of tools that enable an understanding of the relationship of HCys with these biologically important species would be highly desirable.To achieve this, a number of fluorescent probes have been developed that are capable of detecting two or more analytes.²³ These include AND logic gate based-fluorescence probes, which require both analytes to work in tandem to produce a measurable optical output.^24–28 In our group, we have developed several ‘AND’ reaction-based probes for the detection of ROS/RNS and a second analyte.^29–32 These ‘AND’ logic scaffolds have been used to detect two analytes within the same biological system.^24,33Owing to the pathological role of HCys, we set out to develop the first example of a fluorescent probe for the detection of HCys and biological related analyte. Aiming towards that target, we became interested in a previously reported coumarin-based fluorescent probe developed by Hong et al.CAH, with a salicylaldehyde (Fig. 1).³⁴ Salicylaldehyde is a known reactive unit towards HCys/Cys, therefore we believed CAH could be used as a scaffold for the development of ‘AND’-based systems for the detection of HCys/Cys and a second analyte.³⁴ In the presence of HCys, CAH exhibited a ‘turn-on’ fluorescence response which is attributed to the nucleophilic nature of the nitrogen and sulfur atoms resulting in thiazine ring formation (Scheme S1, Fig. S1 and S2^†).^34–36Open in a separate windowFig. 1(a) CAH – a core fluorescent unit that enables the synthesis of ‘AND’ based fluorescent probe for the detection of HCys/Cys and a second analyte. (b) JEG-CAB enables the detection of HCys/Cys and (ROS/RNS) while (c) JEG-CAN enables the detection of HCys/Cys and NTR.We believed that CAH was a useful core unit that can be used to introduce the chosen reactive chemical trigger on the phenol for the detection of the corresponding analyte with HCys/Cys. Owing to the relationship between HCys and ONOO⁻/NTR, we set out on the development of a HCys AND ONOO⁻ probe and a HCys AND NTR probe.Therefore, we set out to prepare JEG-CAB and JEG-CAN, which are able to detect HCys/Cys and peroxynitrite (ONOO⁻) or nitroreductase (NTR), respectively (Scheme 1). For JEG-CAB, a benzyl boronate ester was introduced as a ONOO⁻ reactive unit.³⁷ For JEG-CAN, a p-nitrobenzyl group was installed as it is known to be an effective substrate for NTR.^38–40Open in a separate windowScheme 1Synthesis of target probe JEG-CAB and JEG-CAN.To afford CAH, compound 2 was synthesized by refluxing umbelliferone and acetic anhydride at 140 °C. Compound 2 was then dissolved in trifluoroacetic acid at 0 °C followed by the addition of hexamethylenetetramine (HMTA). The mixture was heated to reflux overnight and the solvent was then removed. The intermediate was then hydrolyzed in H₂O for 30 min at 60 °C. Upon isolating CAH, it was then alkylated using 4-bromomethylphenylboronic acid pinacol ester and K₂CO₃ in DMF at r.t. to afford JEG-CAB in 51% yield. JEG-CAN was produced by alkylating CAH using 4-nitrobenzyl bromide and K₂CO₃ in DMF at r.t. to give 49% yield (Scheme 1).We then evaluated the ability of JEG-CAB to detect ONOO⁻ ‘AND’ HCys in PBS buffer solution (10 mM, pH 7.40). The maximum absorption of JEG-CAB at 336 nm shifted to 323 nm with the addition of HCys and then slightly shifted to 328 nm following the addition of ONOO⁻ (Fig. S3^†). As shown in Fig. 2, JEG-CAB was initially non-fluorescent, but the addition of HCys (1 mM) led to a small increase in fluorescence intensity, the subsequent additions of ONOO⁻ (0–24 μM) led to a significant increase in fluorescence intensity (>17-fold, see Fig. S5^†). These results demonstrated the requirement for both ONOO⁻ ‘AND’ HCys to obtain a significant turn ‘‘on’’ fluorescence response.Open in a separate windowFig. 2Fluorescence spectra of JEG-CAB (15 μM) with addition of HCys (1 mM) and incubated for 40 min then measured. Followed by incremental additions of ONOO⁻ (0–24 μM). The data was obtained in PBS buffer solution (pH 7.40, 10 mM) at 25 °C. λ_ex = 371 (bandwidth 20) nm. Dashed line represents JEG-CAB and Hcys addition only. Blue line represents highest intensity after addition of ONOO⁻.The addition of HCys and ONOO⁻ were then performed in reverse where JEG-CAB exhibited a negligible increase in fluorescence intensity upon addition of ONOO⁻ (16 μM). However, in a similar manner to that shown in Fig. 2, a large increase in fluorescence intensity was produced after the subsequent addition of HCys (0–5.5 mM) (Fig. 3 and S6^†). LC-MS experiments were carried out to ascertain the reaction mechanism and the results confirmed the sequential formation of the thiazine ring in the presence of HCys followed by boronate ester cleavage in the presence of ONOO⁻ or vice versa (Scheme S2 and Fig. S19–S21^†).Open in a separate windowFig. 3Fluorescence spectra of JEG-CAB (15 μM) with addition of ONOO⁻ (16 μM) and followed by incremental additions of HCys (0–5.5 mM) measurements were taken after 40 min of both additions. The data was obtained in PBS buffer solution (pH 7.40, 10 mM) at 25 °C. λ_ex = 371 (bandwidth 20) nm. Dashed line represents JEG-CAB and ONOO⁻ addition only. Blue line represents highest intensity after addition of HCys.As expected, probe JEG-CAB was shown to have excellent selectivity with ONOO⁻ against other ROS in the presence of HCys (1 mM) (Fig. S9 and S10^†). Furthermore, JEG-CAB exhibited a high degree of selectivity towards a series of amino acids where only HCys and Cys led to a fluorescence response in the presence of ONOO⁻. This is due to the formation of stable five or six-membered thiazine rings (Fig. S7 and S8^†).³⁴We then evaluated the changes in the fluorescence of JEG-CAN with both HCys and NTR in PBS buffer solution (10 mM, pH 7.40, containing 1% DMSO). As shown in Fig. 4, addition of HCys led to a small increase in fluorescence intensity. However, subsequent addition of NTR (4 μg mL⁻¹) led to a large time dependant increase in fluorescence intensity. To ensure both analytes were required, NTR and NADPH was kept constant (4 μg mL⁻¹ and 400 μM respectively) resulting in a 3.4 fold fluorescence increase (Fig. 5). We attribute the large initial increase to background fluorescence of NADPH.⁴¹ NTR then facilitates reduction of the nitro group of JEG-CAN releasing the core probe CAHvia a fragmentation cascade (Scheme S3^†).^38,42 Subsequent addition of HCys (2.0 mM) led to a 2 fold increase in fluorescence intensity. Again, LC-MS experiments confirmed the proposed reaction mechanism (Fig. S22^†).Open in a separate windowFig. 4Fluorescence spectra of JEG-CAN (15 μM) with initial addition of HCys (2 mM) and incubated for 60 min. Followed by addition of nitroreductase (4 μg mL⁻¹) and NADPH (400 μM) and measured over 90 min in PBS buffer solution (pH = 7.40, 10 mM, containing 1% DMSO). λ_ex = 363 nm. Ex slit: 5 nm and em slit: 5 nm. Dashed line represents JEG-CAN and HCys addition only. Blue line represents highest intensity after addition of NTR.Open in a separate windowFig. 5Fluorescence spectra of JEG-CAN (15 μM) with initial addition of nitroreductase (4 μg mL⁻¹) and NADPH (400 μM) and incubated for 60 min. Followed by addition HCys (2 mM) and measured over 90 min in PBS buffer solution (pH = 7.40, 10 mM, containing 1% DMSO). λ_ex = 363 nm. Ex slit: 5 nm and em slit: 5 nm. Dashed line represents JEG-CAN and NTR addition only. Blue line represents highest intensity after addition of HCys.Kinetic studies for JEG-CAN with both NTR and HCys were carried out (Fig. S11–S18^†) where it is clear that JEG-CAN exhibits a dose dependant fluorescence increase in response of both HCys and NTR.Unfortunately, the probes failed to give good data in cells, which could be due to their short excitation wavelengths or the extremely low intracellular HCys concentrations (5–15 μM). We are now pursuing the development ‘AND’ logic fluorescence probes with longer excitation and emission wavelengths suitable for in vitro and in vivo applications.In summary, we have developed two coumarin-based ‘AND’ logic fluorescence probes (JEG-CAB and JEG-CAN) for the detection of HCys/Cys and ONOO⁻ or NTR, respectively. CAH is a useful platform that enables easy modification for the development of ‘AND’-based fluorescent probes for the detection of HCys/Cys and a second analyte. Both JEG-CAB and JEG-CAN were shown to be ‘AND’-based fluorescent probes.     

Highly efficient hydrogen peroxide direct synthesis over a hierarchical TS-1 encapsulated subnano Pd/PdO hybrid

We report a hierarchical TS-1 encapsulated subnano Pd/PdO hybrid catalyst that shows unprecedented activity in H₂O₂ direct synthesis from H₂ and O₂. The macro reaction rate in 30 min is up to 35 010 mmol g_Pd⁻¹ h⁻¹ at ambient temperature. Such high catalytic activity is achieved due to the hierarchical porous structure of TS-1 and the formation of the encapsulated subnano Pd/PdO hybrid after oxidation/reduction/oxidation treatment.

A hierarchical TS-1 encapsulated subnano Pd/PdO hybrid catalyst that shows unprecedented activity in H₂O₂ direct synthesis from H₂ and O₂.

Hydrogen peroxide as a clean and strong oxidant is one of the commonly used chemicals in various fields of chemical industry, such as the pulp and paper industry, the textile industry, wastewater treatment, green chemical synthesis metallurgy, electronics manufacture, propulsion and the food industry.¹ Compared to the traditional anthraquinone process (sequential hydrogenation and oxidation of alkyl anthraquinone), the direct synthesis of hydrogen peroxide (DSHP) from hydrogen and oxygen was recognized as an efficient and environmental alternative process owing to its remarkable adherence to green chemistry perspectives, such as low energy consumption, minimized toxicity and infrastructure investment.^2–5Pd supported catalysts were the most extensively and earliest studied catalysts for the DSHP since 1914.⁶ Both DFT and experimental results indicated that subnano Pd particles were most effective for the selective oxygen hydrogenation to hydrogen peroxide,⁷ and the activity and selectivity are also highly dependent upon the oxidation state of the Pd particles.⁸ However, there were limitations in applying Pd nanoparticles catalyst to the reaction due to the thermal vulnerability in a calcination and reduction activation process.⁹ To solve this problem, many preparation methods have been adopted to stabilise Pd nanoparticles and control the particle size and morphology, such as yolk–shell structure,¹⁰ core–shell structure¹¹ and other encapsulation structure supports. But there were still problems that the size of metal particles is larger than 2.5 nm. Encapsulation of Pd species by mercaptosilane-assisted dry gel conversion (DGC) synthesis method can provide a precise control over the nanoparticle size as well as limitating the aggregation under high temperature during activation.¹² However, active sites deep inside the encapsulated nanoparticles were often hardly accessible since the internal configuration diffusion limitations of reactants and products in micropores, leading to low H₂ conversion and decomposition of the long residence time of synthetic H₂O₂.¹³ So, the role of the porous structured catalyst was essential for encapsulated metal nanoparticles.Titanium silicalite-1 (TS-1) has already been used as an excellent catalyst for a variety of selective oxidation reactions employing hydrogen peroxide as oxidant.^14,15 Moreover, in situ H₂O₂ generation coupled with these selective oxidation reactions leading to the desired products such as propylene,^16,17 benzyl alcohol,¹⁸ cyclohexene¹⁹ was a desirable, green and lower cost route. More importantly, the Ti–OOH species formed on the TS-1 during selective oxidation might improve the stability of OOH, which is a key reaction intermediate during the DSHP.²⁰ Hutchings et al. reported that hierarchical titanium silicalite supported Au–Pd catalysts showed high peroxide production rate and benzaldehyde production rate for oxidation of benzyl alcohol by in situ generated H₂O₂.²¹ In this report, the encapsulation of subnano-sized Pd metal particles within conventional (Pd@TS-1) and hierarchical titanium silicalite-1 (Pd@HTS-1) has been achieved (see Scheme 1). The Pd@HTS-1 catalyst after oxidation–reduction–oxidation pre-treatment showed unprecedented activity in direct synthesis of hydrogen peroxide from hydrogen and oxygen under ambient temperature without any promoter.Open in a separate windowScheme 1Schematic diagram of the preparation method for Pd@HTS-1.The TS-1 and HTS-1 encapsulated Pd sub-nanoparticles were first synthesized via solvent evaporation-assisted dry gel conversion method, where the Pd was encapsulated in situ through hydrothermal crystallization in assistance of 3-mercaptopropyl-trimethoxysilane (Scheme 1). The results of ICP analysis confirmed that total Pd contents in Pd@TS-1 and Pd@HTS-1 were 0.094 and 0.106 wt%, respectively. The characteristic diffraction “finger peak” on the X-ray diffraction in Fig. S1^† proved that the TS-1, Pd@TS-1 and Pd@HTS-1 had a well-crystallized MFI structure,²² which was further confirmed by the asymmetric stretching of Si–O–Ti in the spectra of Fourier Transform Infrared Spectroscopy (FT-IR, see Fig. S2^†). For all of the samples, the diffraction peak at 2θ of 25.4° was not observed. Meanwhile, the diffraction peak of crystalline Pd was also not detected for Pd@HTS-1 and Pd@TS-1, indicating that the Pd particles were well dispersed in the zeolite.⁷ Besides, the diffuse reflectance UV-vis spectra of the TS-1, Pd@TS-1 and Pd@HTS-1 were shown in Fig. S3.^† The band at 210 nm in three samples confirmed the tetrahedral structural geometry of Ti in these silicates, and the weak band at 280 nm was assigned to small amounts of penta/hexacoordinated Ti species.²³ Moreover, the absorption band around 300 nm indicated that the three samples contain anatase TiO₂.²⁴The textural properties of the synthesized Pd@TS-1 and Pd@HTS-1 were characterized by N₂ adsorption/desorption and the results were shown in Fig. 1 and Table S1.^† Notably, typical irreversible type IV adsorption isotherms with an H1 hysteresis loop were observed over the Pd@HTS-1 sample (Fig. 1b), indicating the presence of a mesoporous structure. The mesopore size of Pd@HTS-1, obtained through the BJH method, and the obtained graph peaked at about 7.0 nm. Volume of the micropores was around 0.14 cm³ g⁻¹ for both Pd@TS-1 and Pd@HTS-1, but the surface area of Pd@HTS-1 (509.9 m² g⁻¹) was 48.9 m² g⁻¹ larger than that of Pd@TS-1 (461.0 m² g⁻¹) due to its mesoporous structure, which is beneficial for the diffusion of reactants and products through the catalysts.²⁵Open in a separate windowFig. 1Nitrogen adsorption–desorption isotherms of the synthesized TS-1: (a) Pd@TS-1 and (b) Pd@HTS-1.Comparison between the experimentally obtained results from ammonia temperature-programmed desorption (NH₃-TPD) analysis (Fig. S4^†) and the previously reported data showed that the peaks observed were corresponding to weak acid sites, medium acid sites, and strong acid sites of the catalysts.²⁶ Furthermore, pyridine adsorption peak on the FT-IR spectra of these samples (Fig. S5^†) revealed that titanium silicate (TS-1) was an acidic support with a large number of Lewis acid segments and few Brønsted acid segments. As shown in scanning electron microscopy (SEM) image (Fig. 2), Pd@TS-1 particles were crystallites with a morphology close to cuboids and a mean particle size of about 3–5 μm, while the Pd@HTS-1 has spherical morphology with a particle size of about 1.3 μm.Open in a separate windowFig. 2SEM images of the synthesized Pd-modified TS-1: (a) Pd@TS-1 and (b) Pd@HTS-1.The synthesized TS-1 and HTS-1 encapsulated Pd sub-nanoparticles were then subjected to oxidation/reduction/oxidation treatment to adjust the valence states of Pd.²⁷ Such heat treatment cycle can switch off the sequential hydrogenation and decomposition reactions in the DSHP. However, Ostwald ripening, thus the migration and coalescence of metal clusters, will occur at a higher temperature. Therefore, high temperature treatments was used to emulate the conditions used in the literature mentioned before,^28,29 and the thermal stability of the encapsulated Pd@TS-1 catalysts before and after the treatments were also evaluated and compared to investigate the effect of high temperature and the thermal treatments on the catalysts. The Pd@TS-1 and Pd@HTS-1 samples after an air/H₂/air thermal treatments at 500/400/500 °C for 4/2/6 h were denoted as Pd@TS-1-O, Pd@TS-1-OR, Pd@TS-1-ORO, Pd@HTS-1-O, Pd@HTS-1-OR, Pd@HTS-1-ORO respectively with O denoting oxidation and R denoting reduction. The Pd particle size distribution after such treatments was first released by the high-resolution transmission electron microscopy (HRTEM) image in Fig. 3 and S6.^† The Pd particles encapsulated within microporous TS-1 zeolites were well dispersed and uniformly distributed throughout the zeolite crystals. The average sizes of Pd particles encapsulated in the TS-1 and HTS-1 were in the range of 1–2 nm, which, however, was bigger than those of the MFI topology channels (0.53 × 0.56 nm) and intersectional channels (∼0.9 nm). Nevertheless, the successful encapsulation of the Pd particles in the TS-1 zeolites was verified by comparing the hydrogenation rates of a mixture of nitrobenzene and 1-nitronaphthalene. As shown in Fig. S7,^† the reaction rate for the hydrogenation of nitrobenzene and 1-nitronaphthalene was much higher over the Pd@HTS-1-OR compared to the Pd@TS-1-OR. We anticipated that the slightly larger Pd size than the zeolite channels might reflect the local disruption of the crystal structures near the location of the particles during the in situ synthesis. More detailed size distributions of Pd particles encapsulated in the TS-1 and HTS-1 zeolites after air, Ar/H₂ and air treatments were shown in Fig. 3d–f and j–l, respectively. The particle sizes of most of the Pd species still remain below 2 nm on average, which indicated the absence of metal clusters migration and coalescence by Ostwald ripening even after such higher temperature treatments. The high thermal stability of the Pd subnano particles resulted from the embedding confinement.³⁰Open in a separate windowFig. 3HRTEM images and metal particle size distributions of the Pd@TS-1 and Pd@HTS-1 before and after high-temperature oxidation–reduction–oxidation treatments. (a, d Pd@TS-1-O. b, e Pd@TS-1-OR. c, f Pd@TS-1-ORO. g, j Pd@HTS-1-O. h, k Pd@HTS-1-OR. i, l Pd@HTS-1-ORO.)The Pd dispersion and average Pd nanoparticle size for Pd@TS-1 and Pd@HTS-1 after the air/H₂ treatment were further determined by CO chemisorption measurements (see Table S2^†). The dispersions of Pd in Pd@TS-1 and Pd@HTS-1 are 85% and 81%, respectively. The average Pd particle sizes for Pd@TS-1 and Pd@HTS-1 calculated by CO adsorption measurements are 1.06 nm and 1.17 nm, respectively, which was smaller than that estimated from the TEM analysis. This was probably due to the presence of Pd nanocluster or single atoms, which cannot be directly observed by HRTEM.We now turn to the Pd valence states of the catalysts after the oxidation/reduction/oxidation treatment by the XPS (see Fig. S8^†). The Pd3d spectra signals were hardly observed when the concentration of Pd atoms was low, the binding energy peaks for different oxidation states of Pd atoms were collected after peak fitting by prolonging the scanning time.³¹ The XPS results demonstrated the presence of both metallic Pd and PdO. The binding energy of peaks for Pd⁰3d_5/2 and Pd⁰3d_3/2 correspond to 335.5 and 340.6 eV, respectively, while the binding energy for Pd²⁺3d_5/2 and Pd²⁺3d_3/2 were at 337.8 and 341.9 eV, respectively.³¹ The transformation of valence state could be observed in Fig. S8a–c,^† which was derived from XPS measurements. Moreover, the ratios for Pd⁰ and Pd²⁺ atoms in Pd@TS-1 and Pd@HTS-1 were approximately 2 and 1, respectively. On the basis of these results, we proposed a reaction mechanism for the synthetic process of the catalysts, subnano-sized Pd particles might be oxidated from Pd⁰ to Pd²⁺ to form PdO on the surface of the catalysts during reoxidation.The catalytic performance of the TS-1 and HTS-1 encapsulated subnano-sized Pd/PdO hybrid in the direct synthesis of hydrogen peroxide from H₂ and O₂ were tested at ambient temperature without any promoters. Compared to the Pd supported by the active carbon, the selectivity of hydrogen peroxide was higher, the reason might be the formation of Ti–OOH³² and the confinement effect of the Pd encapsulated in the channel of the zeolite (Scheme 2). Both HTS-1 zeolite and Pd@zeolites showed significant amount of O₂ adsorption according to the O₂-TPD (Fig. S9^†), which might be the reason for high activity/selectivity. The selectivity for hydrogen peroxide on Pd@TS-1-OR is lower than that on Pd@TS-1-O, while the degradation rate of hydrogen peroxide on Pd@TS-1-OR are higher than that on Pd@TS-1-O (Fig. 4 and Table S3^†), which was attributed to the change in oxidation state from Pd²⁺ to Pd⁰ after reductive treatment, in agreement with previous reports.^27,33 The selectivity of hydrogen peroxide over Pd@TS-1 increased after an oxidation/reduction/oxidation cycle, the reason might be the weaker adsorption of O₂ and H₂, the intermediate OOH and the production H₂O₂ and the suppression of H₂O₂ decomposition.²⁰Open in a separate windowScheme 2Schematic of the mechanism for DSHP by Pd@TS-1.Open in a separate windowFig. 4H₂O₂ selectivity of DSHP over Pd@TS-1 with different oxidation states for 5 min reaction. Reaction conditions (same as Fig. 5 and ​and6):6): H₂/Ar (2.9 MPa) and air (1.35 MPa), 8.5 g solvent (2.9 g water, 5.6 g MeOH), 0.02 g catalyst, RT, 1200 rpm.The productivity of DSHP over Pd@TS-1 increased with oxidation, reduction and reoxidation treatment in 30 minutes (Fig. 5 and Table S3^†), demonstrated that PdO layer on monometallic Pd catalysts could suppress oxygen dissociation and H₂O₂ degradation,¹² the appropriate PdO formed on the surface of the catalysts after reoxidation can optimize the H₂O₂ production. The hierarchical Pd@TS-1 (35 010 mmol g_Pd⁻¹ h⁻¹) is remarkably higher than those of conventional Pd@TS-1 (3210 mmol g_Pd⁻¹ h⁻¹), the superior hydrogen peroxide production rate of Pd@HTS-1-ORO indicating that the Pd encapsulated by uniformed topology structure of TS-1 highly limited by the effect of pore-diffusion resistance.¹¹ Compared to Pd@TS-1, it was noteworthy that Pd@HTS-1 with only 0.1 wt% Pd content and subnano size after oxidative treatments showed famous reaction activity without any promoters under mild condition, which could be mainly ascribed to the presence of internal diffusion limitation within encapsulated micropore zeolites. The micropore structure limited the use of Pd metal because a part of the Pd crystal surface was blocked by zeolite supports, the hydrogen and oxygen were restricted by the configurational diffusion of zeolite to the Pd surface. Moreover, the formed and desorption H₂O₂ was also constrained by the micropore and thereby resulted in prolonged residence time of the product leading to degradation of H₂O₂. The intracrystal diffusion no longer limited the mass transport process of the hierarchical zeolite due to the presence of additional porosity. Although the physical and structural properties (including the primary particle size, the properties of the external surface and so on) were different between Pd@HTS-1 and Pd@TS-1, we may still draw a conclusion that the excellent catalytic activity is mainly attributed to the presence of mesopore favours diffusion of both reactants and products to and off the active sites in micropores.Open in a separate windowFig. 5Macro reaction rate for H₂O₂ production over Pd@TS-1 and Pd@HTS-1. ^aPd/C#C&Pd/C#Ex from Young-Min Chung;^34bPd–Sn/TiO₂ from Hutchings.²⁹The TON of H₂O₂ production at different reaction time over the six different Pd@TS-1 and Pd@HTS-1 catalysts were shown in Fig. 6. The TON increases with increasing reaction time, however, the slop of the TON–time curves (d_TON/d_t) seems decreased with increasing time, which revealed that the net productivity rate of hydrogen peroxide synthesis declined slightly with increasing time, especially for the Pd@HTS-1-OR at the reaction period of 30–60 min. The accumulative productivity of hydrogen peroxide slowed down, the reason might be the rapid decrease of hydrogen partial pressure in the medium and the ongoing H₂O₂ degradation.Open in a separate windowFig. 6The TON of H₂O₂ production with different reaction time over Pd@TS-1 and Pd@HTS-1 catalysts. TON (turnover number) = mol (H₂O₂)/mol (surface Pd).In summary, successful encapsulation of subnano-size Pd metal particles within titanium silicate (TS-1) voids was achieved via the mercaptosilane-assisted DGC synthesis method. The subnano-size Pd nanoparticles encapsulated in HTS-1 zeolites exhibited superior thermal stability after the oxidation/reduction/oxidation heat treatment process adjusting Pd/PdO hybrid owing to the embedding confinement. The synthesized high-efficiency Pd@HTS-1-ORO showed the famous hydrogen peroxide synthesis productivity, a hydrogen peroxide production rate as high as about 35 010 mmol H₂O₂ g_Pd⁻¹ h⁻¹. Our strategy brings about a finely tailored method to control particle size down to the subnano level and eliminate the diffusion inside metal encapsulated microporous zeolites, which is advantageous for catalytic activity and selectivity in direct synthesis of hydrogen peroxide. Thus, our approach opens up the possibility that the titanium-containing zeolites encapsulated noble metal catalyst can be extended further to selective oxidation reactions with H₂O₂ generated in situ from H₂ and O₂.     

Green synthesis of crystalline bismuth nanoparticles using lemon juice

Lemon juice effectively served as a reducing and capping agent for an easy, cost-effective, and green synthesis of crystalline bismuth nanoparticles (BiNPs) in basic aqueous media. Spherical BiNPs with a rhombohedral crystalline structure are capped by phytochemicals and stably dispersible in aqueous media. The BiNPs effectively catalyze the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄.

Lemon juice effectively served as a reducing and capping agent for an easy, cost-effective, and green synthesis of crystalline bismuth nanoparticles (BiNPs) in basic aqueous media.

Nanostructures of bismuth, the heaviest element among the ‘safe ones’ earning the status of a ‘green element’,¹ are particularly interesting due to their large magnetoresistance and excellent thermoelectric properties.^2–4 Bismuth nanoparticles (BiNPs) are the most extensively used nanostructures of bismuth. For example, BiNPs are utilized as contrast agents for computed tomography, photoacoustic imaging and infrared thermal imaging,^5,6 catalysts for the reduction of nitroaromatic compounds,^7,8 and removal of NO from air.⁹ Furthermore, BiNPs can act as intermediates for the synthesis of other nanostructures of bismuth, such as thermoelectric Bi₂Te₃ (ref. 10) and seeds for the solution–liquid–solid growth of nanowires.^11,12 Techniques for the synthesis of pure BiNPs could be categorized into (i) thermal decomposition,¹² (ii) mechanochemical processing,¹³ (iii) photochemical reduction,¹⁴ and (iv) solution-phase chemical reduction methods.^6,15–19 The thermal decomposition method requires harsh preparation conditions, expensive organometallic precursors, high temperature, and long reaction time, while producing high-quality monodispersed BiNPs. The mechanochemical processing technique is advantageous in terms of using inexpensive and nontoxic bulk bismuth pellets as precursors. However, the energy consumption and costly instrumentations are the limitations. The photochemical reactions typically require long time for sufficient conversion to bismuth nanoparticles, while they can also produce highly monodispersed BiNPs. The solution-phase chemical reduction methods are most popular due to the facile procedures and accessible reagents. However, stabilizers and toxic reducing agents often used are the limitations. To resolve the above-mentioned limitations, simple and green alternative methods are highly demanded.The use of abundant plant sources, which has been applied for synthesis of various metal nanoparticles such as Ag and Au, is a promising solution.^20,21 Plant sources contain a wide variety of biomolecules potentially serving as reducing and capping agents. Edible plant sources are obviously safest. To the best of our knowledge, there is no report on the synthesis of crystalline BiNPs using plant sources, while P. Poltronieri et al. reported synthesis of amorphous BiNPs using hydroalcoholic extract of Moringa oleifera.²² We focused on lemon, a very common fruit containing abundant antioxidants such as polyphenols, limonoids, citric acid, ascorbic acid, and vitamins potentially reduce ions with high oxidation states. Some of the phytochemicals, namely carbohydrates and proteins bearing ionic moieties, can be capping agents. Accordingly, lemon juice was successfully applied for the formation and in situ stabilization of silver and gold nanoparticles in aqueous media.^23,24 We presumed that a similar mechanism also works for BiNPs.In this communication, we introduce a greener strategy for the synthesis of crystalline BiNPs using lemon juice as a reducing and capping agent. For example, the synthesis was carried out using Bi(NO₃)₃·5H₂O (0.25 mmol) and freshly prepared lemon juice (25 mL) at 80 °C for 2 h under an aerobic basic condition. The X-ray diffraction (XRD) pattern (Fig. 1) of the obtained product was indexed to the pure rhombohedral phase of elemental bismuth (JCPDS no. 44-1246), indicating that the obtained product is BiNPs without detectable oxide phases. The average crystallite size was calculated to be 20 nm applying the Scherrer''s equation on the peaks of the (012), (104) and (110) plane. This result indicates that the phytochemicals present in lemon juice have ability to reduce bismuth salts to form BiNPs. The plausible phytochemicals for reduction are ascorbic acid, citric acid, and sugars. The reduction of Bi³⁺ with glucose⁸ and ascorbic acid¹⁷ was reported, and we accordingly performed the control experiment using possible reducing agents contained in lemon juice, namely ascorbic acid, glucose, and starch without any stabilizing agents. All of them could form elemental bismuth in highly aggregated forms as confirmed by the XRD patterns (ESI, Fig. S9^†) and SEM images (ESI, Fig. S10^†).Open in a separate windowFig. 1XRD pattern of obtained BiNPs synthesized using lemon juice with that of authentic Bi (JCPDS no. 44-1246).Electron microscopy was employed to confirm the size and the morphology of synthesized BiNPs. The scanning electron micrography (SEM) image (Fig. 2a) shows spherical objects having the size in the range of 50 to 100 nm agglomerated and covered with amorphous substances presumably originating from lemon juice. This agglomeration occurred during drying as confirmed by the stable water dispersibility of BiNPs with an average hydrodynamic diameter (D_h) of 255 nm investigated by dynamic light scattering (DLS) (ESI, Fig. S1a^†). The high colloidal dispersibility of the BiNPs and the coating layer observed in the SEM image suggest that phytochemicals present in lemon juice act also as capping agents. The larger D_h measured by DLS than the size observed by SEM can be attributed to the surrounding hydration layer and swelled phytochemicals attached to the surface of the BiNPs. We then performed transmission electron microscopy (TEM) analysis to get the actual size of the Bi cores. The TEM images of the BiNPs (Fig. 2b and c) indicated the presence of heavy elements in the matrix of light elements. The particles are spherical, and the diameter ranges from 8 to 30 nm. The high-resolution TEM image (Fig. 2d) shows the lattice fringes of 0.298 nm, 0.253 nm, and 0.224 nm of the typical crystallite agreeing well with the distance of the (012), (104) and (110) plane, respectively, of the rhombohedral Bi(0).Open in a separate windowFig. 2(a) SEM and (b–d) TEM images of BiNPs synthesized using lemon juice.The EDX spectrum (ESI, Fig. S2^†) reveals that obtained BiNPs comprise bismuth (Bi), carbon (C), and oxygen (O). The strongest signal of bismuth testifies the successful synthesis of BiNPs, whereas the C and O signals demonstrate the presence of the capping layer on the BiNPs.The FT-IR spectrum of the synthesized BiNPs was analyzed to presume the possible components on the BiNPs surface with the comparison with the solid content of lemon juice (Fig. 3). The FT-IR spectrum of the BiNPs shows the following major absorption bands. The broad absorption band at 3050–3500 cm⁻¹ is assignable to the O–H stretching of alcohol moieties. The absorption band around 2840–2940 cm⁻¹ is assignable to the C–H stretching of alkane, and the absence of the sharp absorption around 2950–3050 cm⁻¹ suggests the absence of trace contents of unsaturated C–H groups. The absence of the adsorptions originating from COO–H around 2400–3100 cm⁻¹ and C O around 1700–1730 cm⁻¹ indicates that citric acid^25,26 and other carboxylic acids contained sufficiently in lemon juice are not the major components on the BiNPs, while the presence of the strong absorption around 1600 cm⁻¹ is assignable to carboxylate moieties. The absorption bands around 930–1130 cm⁻¹ assignable to C–C and C–O stretching and around 1200–1420 cm⁻¹ assignable to O–C–H, C–C–H and C–O–H bending suggest the presence of sugars on BiNPs.^25,26 The content of the organic moieties was estimated approximately 14% by thermogravimetric analysis from weight loss that occurred at the temperature range from 130 to 450 °C (ESI, Fig. S3^†), at which the negligible C and O signals were observed in the EDX spectrum of the residue after TGA (ESI, Fig. S2^†).Open in a separate windowFig. 3FT-IR spectra of solid content of lemon juice and obtained BiNPs synthesized using lemon juice.The ¹H and ¹³C NMR and FTIR spectroscopic analysis (ESI, Fig. S4–S8^†) of the ethanol and chloroform extracts of the BiNPs suggests that the major capping phytochemicals are polysaccharides and fatty acid derivatives. Other minor possible components may include amino acids, terpenes and phenolic compounds contained in lemon juice.The successful formation of BiNPs made us enthusiastic to investigate their catalytic performance in the reduction of nitroaromatic compounds, problematic pollutants arising from explosives, analgesic, and antipyretic drugs and dyes.²⁷ Herein, the catalytic performance of our BiNPs was investigated by selecting the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄ at room temperature as a model reaction.^7,8,28 The progress of the reduction was monitored by UV-vis absorption spectroscopy. The colour of the 4-NP aqueous solution changed from light yellow to deep yellow upon the addition of NaBH₄ to produce 4-nitrophenolate ion and an intense absorption peak at 403 nm appeared instead of the original absorption peak of 4-NP at 316 nm in the UV-vis spectrum.⁷ In the absence of BiNPs, the colour of the solution and the intensity of the peak were retained even after 12 h. In the presence of BiNPs, the deep yellow mixture became almost colourless within 180 min (Fig. 4a). The intensity of the absorption peak at 403 nm decreased overtime, and simultaneously a new absorption peak appeared and grew at 299 nm, indicating the formation of 4-AP (Fig. 4a).^8,27,29 The reaction rate almost follows pseudo-first-order kinetics agreeing with the Langmuir–Hinshelwood mechanism; in which two reactants react after adsorption on the solid surface, and then the product desorbs. The rate constant (k) was determined from the plot of ln(A/A₀) vs. time (Fig. 4b) according to the previous reports.^7,8,28,29 The k value of 0.0134 min⁻¹ (activity factor = 0.1 s⁻¹ g⁻¹) is lower than previously reported PVP-coated bismuth nanodots (6.033 s⁻¹ g⁻¹)⁸ and starch coated BiNPs (0.02751 s⁻¹).⁷ The lower catalytic rate of our BiNPs can be attributed to the electrostatic repulsion of 4-nitrophenolate ions with negatively charged BiNPs (zeta potential value = −31.7 mV) in the reaction mixture. The initial rate is slower than the rate after 50 min. Possible factors are (i) the inductive period for the surface activation of the nanoparticles at the initial stage as reported for bismuth nanodots by Liang et al.⁸ and (ii) partial and gradual release of phytochemicals under the basic conditions. After the catalytic reaction, the D_h value measured by DLS (195 nm) became smaller than the original one maintaining the good water dispersibility, suggesting unravelling of primary particles fused by the capping layer (ESI, Fig. S1b^†). In addition, the weight loss in the TGA curve became lower (ESI, Fig. S3^†), and the SEM image implies the partial loss of the coating (ESI, Fig. S11^†). This decrease of the organic moieties is attributable to the partial removal of coating substances such as fatty acids during the catalytic reaction under the basic and reductive conditions, supported by the disappearance of the absorption band around 1600 cm⁻¹ assignable to carboxylate moieties in the FTIR spectrum of BiNPs after the catalytic reaction (ESI, Fig. S12^†). Carboxylic acids and their esters are reported to be reduced to alcohol by NaBH₄ in the presence of electrophiles.^30–33 The initial presence of the amphiphilic and anionic layer plausibly delays the catalytic reaction.Open in a separate windowFig. 4(a) Optical images and absorption spectra of catalytic reduction of 4-NP ([4-NP] = 15 ppm) by NaBH₄ (4.28 × 10⁻⁴ M) in presence of BiNPs (142 mg L⁻¹); (b) pseudo-first order kinetic plot of catalytic reduction.In conclusion, we have demonstrated a green, cost-effective, and successful approach for synthesis of crystalline BiNPs using lemon juice as a reducing as well as capping agent. The obtained BiNPs effectively catalysed the reduction of 4-NP to 4-AP by NaBH₄. The importance of this method lies in the simple synthetic procedure, uses of safe and low-cost lemon, and good dispersibility over conventional chemical approaches.     

Rhodamine-based cyclic hydrazide derivatives as fluorescent probes for selective and rapid detection of formaldehyde

We describe fluorescent probes to detect formaldehyde (FA) in aqueous solutions and cells. The probes rapidly respond to FA in aqueous solutions and have great selectivity toward FA over other biologically relevant analytes. The results of cell studies reveal that probe 1 can be utilized to monitor endogenous and exogenous FA in live cells.

We developed a fluorescent probe that is useful to monitor endogenous and exogenous formaldehyde in live cells.

Reactive carbonyl species (RCS) produced through metabolic processes are highly reactive and, thus, their overproduction causes damage to a variety of organisms.¹ Formaldehyde (FA), the simplest RCS, is a human toxin and carcinogen as a result of its ability to crosslink DNA and proteins.² FA is generated in cells by several metabolic events, including methanol oxidation, histone demethylation and N-methylamine deamination.³ During normal metabolic processes, the concentration of FA is maintained at physiological levels in the range from 100 μM in blood to 200–400 μM in brain,⁴ where it is involved in spatial memory formation and cognition.⁵ However, upregulation of FA-producing enzymes or exposure to exogenous FA (e.g., industrial pollutants, cigarette smoke, and natural products) can lead to abnormal elevation of FA levels up to as much as 800 μM.^4,5 Elevated levels of FA cause memory impairments, cancers, diabetes and neurodegenerative disorders.⁶ Owing to the physiological and pathological significance of FA, selective and sensitive tools to monitor this RCS in cells are in critical demand.Fluorescence imaging is a powerful method to detect intracellular analytes (e.g., ions, reactive species and biomolecules) owing to its advantageous features such as operational simplicity, sensitivity and non-invasive properties.⁷ Several fluorescent probes for detection of FA in cells, which are based on specific chemical reactions including 2-aza-Cope rearrangement, formimine reaction and aminal formation, have been devised thus far.⁸ However, most of these probes have drawbacks such as low selectivities over other aldehydes and/or slow fluorescence responses to FA.To develop FA-responsive fluorescent probes that do not suffer from the limitations described above, we designed rhodamine-based cyclic hydrazide derivatives 1 and 2 (Scheme 1), which should be weakly fluorescent owing to the absence of an appropriate fluorophore. We reasoned that the tethered amine groups in 1 and 2 would react with FA to form iminium ions A, which would then undergo intramolecular addition of the hydrazide NH to generate cyclic aminals B. We also envisaged that rapid opening of spirocyclic moiety in B would take place to generate highly fluorescent xanthenes C.⁹Open in a separate windowScheme 1The proposed mechanism of fluorescence sensing of formaldehyde by rhodamine cyclic hydrazide-based probes 1 and 2.On the basis of this strategy, the new FA-reactive fluorescent probes 1 and 2 were synthesized using reactions of rhodamine B acid chloride with the corresponding amine-appended hydrazines (Schemes S1–S3^†). All new compounds were characterized using standard spectroscopic methods. To assess the design strategy displayed in Scheme 1, we measured time-dependent changes in the intensities of fluorescence arising from the probes following treatment with FA at a physiologically relevant concentration (10 equiv., 100 μM) in PBS buffer (1% DMSO, pH 7.4).⁴ As shown in the spectra and plots (Fig. 1a and S1^†), the secondary amine tethered probe 1 underwent an immediate fluorescence response to FA and the emission intensity reached a maximum within 5 min. In the case of probe 2 containing a primary amine appendage, the fluorescence intensity promoted by treatment with FA reached a plateau after 2 min but a lesser extent than that from 1 (Fig. S2^†). The pseudo-first-order rate constants for the fluorescence-monitored reactions of 1 and 2 with FA were determined to be k = 1.8 × 10⁻² M⁻¹ s⁻¹ and 4.6 × 10⁻² M⁻¹ s⁻¹, respectively (Fig. S3^†).¹⁰ The FA-concentration dependencies of reactions of probes 1 and 2 in aqueous buffer were determined by measuring fluorescence intensities, 10 min after treatment of the probes with 10 equiv. FA. The results showed that 1 and 2 exhibit a respective 15- and 6.5-fold enhancement in fluorescence intensity after addition of 50 equiv. FA (Fig. 1b and S4^†).Open in a separate windowFig. 1(a) Time-dependent change of the fluorescence spectra of 1 (10 μM) promoted by addition of 10 equiv. FA in PBS buffer (1% DMSO, pH 7.4) at 37 °C (λ_ex = 520 nm). Inset is a plot corresponding to the time-dependent increase in fluorescence intensity of 1 following addition of FA (λ_ex/λ_em = 520/583 nm). (b) FA concentration-dependent changes of the fluorescence spectra of 1 (10 μM), 10 min after each addition of FA. Inset is a plot corresponding to the FA concentration-dependent increase in fluorescence intensity of 1 (10 μM). (c) Color and fluorescence (FI) images of probe 1 in the absence and presence of FA.The selectivity of fluorescence responses of the probes toward FA was then assessed by individually treating 1 and 2 (10 μM) with various biologically relevant analytes, including reactive carbonyl species (FA, acetaldehyde, benzaldehyde, 4-hydroxybenzaldehyde, ethyl pyruvate, ethyl glyoxalate, propionaldehyde, glucose), reactive oxygen species (H₂O₂, HOCl, NO˙, ¹O₂, O₂˙⁻, ˙OH) and cations (Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Zn²⁺). The results showed that both probes respond to FA but not to the other analytes (Fig. 2 and S5^†). Taken together, the above findings indicate that probes 1 and 2 respond rapidly and selectively to FA in aqueous buffer.Open in a separate windowFig. 2Change of fluorescence intensity of 1 (10 μM) at 583 nm (λ_ex = 520 nm) promoted by addition of each of biologically relevant analytes (10 equiv.) in PBS buffer (1% DMSO, pH 7.4) at 37 °C. Numbering of the analytes in the graph is as follows: 1, acetaldehyde; 2, benzaldehyde; 3, 4-hydroxybenzaldehyde; 4, ethyl pyruvate; 5, ethyl glyoxalate; 6, propionaldehyde; 7, d-glucose (1 mM); 8, H₂O₂; 9, HOCl; 10, NO˙; 11, ¹O₂; 12, O₂˙⁻; 13, ˙OH; 14, Cu²⁺; 15, Fe²⁺; 16, Fe³⁺; 17, K⁺; 18, Zn²⁺; 19, FA.To shed light on the mechanistic basis for the responses of probes to FA, the product generated by reaction of 1 and FA was isolated (see ESI^† for the detailed procedure) and characterized by using spectroscopic methods. Analysis of the ¹H and ¹³C NMR spectra of the isolated product in CDCl₃ suggests that a 1,2,4-triazinane ring system exists, as judged from chemical shifts that correspond to bridging methylene protons (CH₂) and carbon (3.24 ppm (s, 2H) and 72.5 ppm, respectively) (Scheme 2). Also, the spectral analysis suggests that the isolated product contains a spirocyclic ring system because of the presence of a signal at 61.3 ppm in the ¹³C NMR spectrum, which is expected for a quaternary carbon in a spiro ring. Furthermore, analysis of UV-Vis absorption and fluorescence spectra revealed that the product in CH₂Cl₂ displays very weak absorbance at 560 nm as well as very weak fluorescence at 583 nm (Fig. S6^†). These observations led us to conclude that the product in CH₂Cl₂ has the spirocyclic structure represented by 3 (Scheme 2).Open in a separate windowScheme 2Solvent (nonpolar organic solvent versus aqueous buffer) dependence of the equilibrium between ring-closed (3) and open (4) forms of the product generated by reaction of 1 with FA.In contrast, the isolated product in aqueous buffer (1% DMSO, pH 7.4) had a strong absorbance at 560 nm and intense fluorescence at 583 nm (Fig. S7^†), spectral properties that are quite similar to those of the substance generated by treatment of 1 with FA in aqueous buffer. Collectively, the results suggest that while the product of the reaction of 1 with FA exists in the ring-closed form 3 in nonpolar organic solvents, in aqueous buffer it exists in the ring-opened xanthene containing form 4 (Scheme 2). As a consequence, it is reasonable to conclude that the reaction responsible for fluorescence generation when 1 is treated with FA in aqueous buffer involves formation of 4. In addition, extinction coefficients (ε₅₁₄), quantum yields and fluorescence outputs (quantum yield × ε₅₁₄) of 1, 2 and 3 were determined (Table S1^†). Furthermore, the limits of detection of 1 and 2 for FA were calculated to be 1.24 μM and 0.59 μM, respectively (Fig. S8^†).Next, the utility of 1 to image FA in live cells was evaluated. Because 1 displayed a larger increase in fluorescence intensity upon treatment with FA than does 2, 1 was utilized in the cell studies described below. To determine the optimal conditions for cell imaging, HeLa cells (human cervical cancer cells) were incubated with 10 μM 1 for various times and with various concentrations of 1 for 1 h. Analysis of confocal fluorescence microscopy images showed that cells exposed to 10 μM 1 for 0.5–1 h display the intense fluorescence signal (Fig. S9^†). In addition, based on the results of an MTT assay as well as the observation of an intact nucleus morphology, 1 had negligible cell death activity under these treatment conditions (Fig. S10^†).We also probed the FA concentration-dependence of the fluorescence response of 1. For this purpose, HeLa cells were first treated with 10 μM 1 and then incubated with concentrations of FA (0–1.0 mM) that are in a physiologically relevant concentration range (ca. 400 μM in normal cells and up to 700–800 μM in cancer tissues).^4,11 The results showed that fluorescence signals arising from 1 in cells increase gradually as the FA concentration increases (Fig. 3), indicating the ability of 1 to serve as a probe for FA in live cells.Open in a separate windowFig. 3Detection of FA in cells using probe 1. HeLa cells were incubated with 1 (10 μM) for 1 h followed by treatment with several concentrations of FA for 1 h. Cell images were obtained using confocal fluorescence microscopy (scale bar = 10 μm). The nucleus was stained with Hoechst 33342. The graph shows normalized fluorescence intensity (FI) in the treated cells (mean ± s.d., n = 3).To evaluate the detection of endogenous FA in cells, several cell lines, including HeLa, MRC-5 (human fibroblast cell line derived from normal lung tissue), HaCaT (human keratinocytes), HEK293T (human embryonic kidney cells) and MCF-7 cells (human breast cancer cells), were incubated with 1 for 1 h. Analysis of cell images revealed that whereas the treated HEK293T cells display the low fluorescence intensity,⁸ⁱ the other four cell lines exhibit similarly strong fluorescence (Fig. 4 and S11^†). The findings indicate that while HEK293T cells produce a low level of FA, the other four cells generate high levels of FA.Open in a separate windowFig. 4Detection of endogenous FA in cells using probe 1. MRC-5, HaCaT, HEK293T, HeLa and MCF-7 cells were incubated individually with 1 (10 μM) for 1 h. Cell images were obtained using confocal fluorescence microscopy (scale bar = 10 μm). The nucleus was stained with Hoechst 33342. The graph shows fluorescence intensities in the treated cells (mean ± s.d., n = 3).It is known that FA is generated in cells by the actions of several demethylases and oxidase enzymes.³ The enzyme lysine-specific demethylase 1 (LSD1) catalyzes the removal of one or two methyl groups from modified lysines to produce free lysine and FA.^3,12 Also, it is known that GSK-LSD1 serves as a potent inhibitor of LSD1.¹³ As a result, production of FA by LSD1 in cells was evaluated by incubating MCF-7 cells with 1 in the absence and presence of GSK-LSD1. The results revealed that the intensity of the fluorescence arising from probe 1 in MCF-7 cells is slightly attenuated when GSK-LSD1 is present (Fig. 5 and S12^†).^8b The findings suggest that LSD1-promoted generation of FA in cells does not occur at high levels in comparison to the amounts formed by several other metabolic events. Taken together, the findings demonstrate that the rhodamine-based probe 1 is capable of detecting endogenous and exogenous FA in live cells.Open in a separate windowFig. 5The effect of an inhibitor on LSD1-promoted generation of FA in cells. MCF-7 cells were incubated with 1 μM GSK-LSD1 for 20 h followed by incubation with 1 (10 μM) for 1 h. Cell images were obtained using confocal fluorescence microscopy (scale bar = 10 μm). The nucleus was stained with Hoechst 33342. The graph shows normalized fluorescence intensities in the treated cells (mean ± s.d., n = 3).In conclusion, we have developed novel rhodamine-based cyclic hydrazide derivatives as fluorescent probes for the detection of FA in both aqueous media and live cells. Upon addition of FA to the probes in aqueous buffer, a fluorescence enhancement occurs within a few minutes. In addition, the probes respond to FA but not to other biologically relevant species, indicating that they have a high selectivity toward FA. Furthermore, the results of cell studies demonstrate that probe 1 can be employed to image exogenous and endogenous FA in live cells. As a result, this probe should be useful in efforts aimed at gaining a more detailed understanding of FA-associated biological processes.     

Photocatalytic degradation of organic pollutants through conjugated poly(azomethine) networks based on terthiophene–naphthalimide assemblies

A conjugated poly(azomethine) network based on ambipolar terthiophene–naphthalimide assemblies has been synthesized and its electrochemical and UV-vis absorption properties have been investigated. The network has been found to be a promising candidate for the photocatalytic degradation of organic pollutants in aqueous media.

A conjugated two-dimensional poly(azomethine) network based on ambipolar terthiophene–naphthalimide assemblies has been synthesized and its electrochemical and UV-vis absorption properties have been investigated.

Due to the rapid growth of urbanization and intensive industrialization, pollution has evolved into a serious concern that produces a great negative impact on human health and the environment.^1,2 Therefore, many efforts are currently devoted to addressing environmental remediation through the degradation and removal of hazardous contaminants.^3–5 In this regard, photocatalysis has been identified as a suitable approach for environmental remediation given that it is an energy efficient technique that does not require chemical input and does not produce sludge residue.⁶ In recent years, organic semiconducting polymers have evolved into a new type of metal-free and heterogeneous photocatalyst suitable for solar-energy utilization.⁷ The modularity of organic polymers allows the efficient tunning of their electronic and optical properties by bottom-up organic synthesis through the choice of suitable monomeric building blocks.^8–10 Within this context, there is a growing demand for new organic polymeric semiconductors carefully designed to have suitable energy levels of the frontier orbitals, an appropriate bandgap and good intrinsic charge mobility.¹¹For the design of suitable polymeric semiconductors for photocatalysis, it is not only important that the photocatalysts absorb light in the visible light range but also an efficient dissociation of the photogenerated charge carriers is required. The combination of electron-poor acceptor (A) and electron-rich donor (D) moieties in the polymer structure may prevent a fast recombination process following photoexcitation.¹² In addition, it has been found that polymers networks bearing conjugated moieties may exhibit π-stacked columns that can facilitate charge transport.¹³In this respect, molecular and polymeric materials based on the combination of oligothiophene^14,15 and naphthalimide moieties^16,17 connected through conjugated linkers have shown to be very effective in order to efficiently tune their frontier orbital levels and produce tunable organic semiconductors with good charge transport properties.^18–24 As an example, in Fig. 1 is depicted the structure of NIP-3T, an ambipolar organic semiconductor, for which the one-electron HOMO–LUMO excitation consists of the displacement of the electron density from the HOMO, primarily localized on the oligothiophene fragment, to the LUMO, localized on the naphthalimide unit.²⁵Open in a separate windowFig. 1(a) Monomer containing an electron donor terthiophene system directly conjugated with an electron acceptor naphthalimide moiety through a conjugated pyrazine linker (NIP-3T). (b) HOMO and (c) LUMO computed orbital topologies for NIP-3T.²⁵Among conjugated polymers, poly(azomethine)s have found application as organic semiconductors in heterogeneous photocatalysis because of their π-conjugated system and suitable band levels matching the redox window of water.²⁶ The incorporation of D–A monomeric assemblies into poly(azomethine) networks represents an efficient strategy to obtain ambipolar polymeric networks with tunable frontier orbital levels for photocatalytic applications. Thus, in this communication we report the synthesis of a novel donor–acceptor poly(azomethine) network (NIP3T-ANW, Scheme 1) based on NIP-3T monomers. The potential of this system as photodegrading agent for the elimination of contaminant organic dyes in aqueous media is also explored.Open in a separate windowScheme 1Schematic representation of the synthesis of NIP3T-ANW.The synthesis of the macromolecular poly(azomethine) network NIP3T-ANW is acomplished through Schiff-base reactions between trigonal monomers endowed with amine functionalities (TAPB,²⁷Scheme 1) and linear naphthalimide–thiophene-based monomers endowed with complementary aldehyde functional groups (NIP3T2CHO,²⁴Scheme 1). Typically, both monomers were dissolved in an o-dichlorobenzene/n-butanol/acetic acid (1 : 1 : 0.1) mixture, which was then heated at 120 °C under solvothermal reaction conditions for 72 h. A black solid was obtained which was insoluble in common solvents such as water, acetone, THF, toluene or chlorinated solvents like dichloromethane or chloroform. The obtained solid was washed several times with THF to remove the starting materials and low-molecular weight by-products. After drying under vacuum, a black solid was obtained. The yield, as determined by weight, was 98%.To investigate the chemical nature of the material, as well as to determine the conversion of the functional groups after the reaction, we have employed attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Fig. 2). The bands arising from the NH₂ stretching (3000–3400 cm⁻¹) and NH₂ deformation (1650 cm⁻¹) vibrations of the primary amine group of TAPB and the signals from the aldehyde groups of NIP3T2CHO around 2870 (C–H stretching) and 1663 cm⁻¹ (C O stretching) are virtually absent in the NIP3T-ANW spectrum. In addition, a prominent new band is found at 1573 cm⁻¹, which can be assigned to the C N stretching vibration of the imine linkages within the newly formed poly(azomethine) network.^28–30Open in a separate windowFig. 2(a) IR spectra of NIP3T2CHO (blue), TAPB (red) and NIP3T-ANW (black). (b) Solid-state ¹³C CP-MAS NMR spectrum of NIP3T-ANW.Solid-state ¹³C cross-polarization magic angle spinning NMR (¹³C CP-MAS NMR) spectrum (Fig. 2) reveals the characteristic imide signals of the 1,8-naphthalimide moiety at 164.4 ppm, as well as the signal corresponding to the imine carbon at 154 ppm and a signal at 148 ppm which can be assigned to the aromatic carbon neighbouring the nitrogen of the C N group. The absence of the sp² carbons from the NIP3T2CHO ²⁴ aldehyde functionalities above 180 ppm satisfactorily confirms the condensation between the aldehyde and the amine derivatives.Due to the rigidity and geometry of the building blocks, the imine linkers could be ideally generated in such a way that result in a canonical layered hexagonal structure³¹ as predicted by theoretical calculations (Fig. S1 and S2^†). However, in the actual framework, X-ray diffraction (XRD) measurements indicate that the material is mainly amorphous with only some ordered regions, as indicated by the good agreement between the weak and broad diffraction peaks observed at 2θ values larger than 3 and those predicted by calculations for the ideal canonical layered hexagonal structure (Fig. S3^†). In this regard, NIP3T-ANW was submitted to an exfoliation process following a previously described protocol³² for the exfoliation of two-dimensional polymers (see ESI^† for details). The exfoliated material was analysed by dynamic light scattering (DLS) showing a monomodal size distribution of ca. 400 nm (Fig. S4^†) and transmission electron microscopy (TEM) reveals a sheet-like structural aspect (Fig. S5^†).Thermogravimetric analysis of the poly(azomethine) network NIP3T-ANW shows that the degradation starts at around 450 °C and only 40% weight loss is observed at 700 °C (Fig. S6^†). This thermal stability is significantly higher than that observed for NIP3T (Fig. 1), the analogous molecular system based on terthiophene connected with naphthalimide through pyrazine for which the degradation starts at 200 °C (Fig. S6^†).^22,25In photocatalysis mediated by semiconductors, electron–hole pairs (excitons) are generated after light absorption and afterwards dissociate into free charge carriers that can be utilized for redox reactions^33,34 such as CO₂ fixation,³⁵ water splitting³⁶ or organic mineralization.³⁷ Some of the crucial factors that make the photocatalytic process favourable are the levels of conduction and valence as well as the width of the band gap.^38–40 Thus, in order to characterize these parameters, the electrochemical and optical properties of NIP3T-ANW have been analysed.The electrochemical properties of NIP3T and NIP3T-ANW were studied by cyclic voltammetry (Fig. 3). Both materials show ambipolar redox behaviour in which the reversible reduction processes are characteristic of the naphthalimide unit, while the oxidation processes can be ascribed to the conjugated oligothiophene moiety.^19–22,24 For NIP3T-ANW, the first reversible reduction wave (−1.29 V) is shifted to less negative values in comparison with NIP-3T (−1.41 V). On the other hand, the first oxidation half wave potential for NIP3T is observed at +0.41 V and for NIP3T-ANW at +0.44 V. These shifts agree with the electron acceptor ability of the imine linker.Open in a separate windowFig. 3(a) The UV-vis DRS spectra of NIP3T and NIP3T-ANW. (b) Cyclic voltammetry of the NIP3T monomer and the corresponding polymer.The absorption spectrum of NIP3T-ANW as determined by UV-vis diffuse reflectance spectroscopy (UV-vis DRS, Fig. 3) shows a strong absorption in all the UV-vis range, extending even to the near infrared. This broad absorption is red-shifted in comparison with the one observed for NIP3T (Fig. 3), which reflects the formation of the new polymer network with an extended conjugation through the alpha positions of the terthiophenes.Using the corresponding cut-off wavelengths, the optical band gaps E_g found for NIP3T and NIP3T-ANW are 1.59 and 1.42 eV respectively. This optical result suggests that the incorporation of the NIP3T core into an extended conjugated system efficiently harvests photons from the visible range, even extending into the near IR region.To shed some light into the degree of crystallinity of the synthesized NIP3T-ANW network, we have carried out a battery of density functional theory (DFT)-based calculations with the QUANTUM ESPRESSO plane-wave DFT simulation code⁴¹ (see details in ESI^†). We have considered periodic boundary conditions to obtain a fully-relaxed ground-state crystal structure. Optimization of the cell-shape and size, simultaneously to the relaxation of the structure, reveals a hexagonal 2D lattice with an optimized parameter of 48.46 Å. Different interlayer stacking fashions have been tested, with only one yielding a good agreement with the experimental diffractogram from 2θ values >3°. The most favourable stacking predicted by theory consists in an intermediate configuration between the perfectly eclipsed and staggered configurations, with an interlayer distance of 3.42 Å, and permits an adequate accommodation of the layers profiting adjacent pores. Details on the structure can be found in the ESI.^†Additionally, we have computed the electronic band diagram of the obtained crystal structure along the high-symmetry k-path Γ → K → M → Γ, revealing a wide-gap (1.91 eV) semiconducting character, with rather dispersive valence and conduction bands, mainly resembling the molecular HOMO and LUMO of the molecular building blocks (see Fig. S7^†). Besides, computed time-dependent DFT (TDDFT) UV-vis spectrum manifests an excellent agreement with the experimental UV-vis spectra (Fig. S8^†). A broad and pronounced peak-feature is obtained between 600 and 800 nm, centred at around 720 nm (1.7 eV), which agrees with the optical gap of 1.6 eV found for NIP3T from Fig. 3a. This feature corresponds to electronic transitions between the valence and conduction bands, with an energy difference of around 0.2 eV between the optical and the electronic gap, which indicates that charge relaxation in excited states is not much significative. The good agreement between theoretical predictions on the canonically periodic computed system and the experimental evidences seems to justify the presence of some high-crystallinity regions from the synthesis.The band gap of a semiconductor material and the reduction and oxidation potentials are key parameters which determine its light-harvesting properties and types of reaction that can be conducted and therefore the overall photo-catalytic activity. A shift in the adsorption edge of a semiconductor towards longer wavelengths implies a narrower band gap and the efficient harvesting of a wider photons range.⁴²NIP3T-ANW seems to be an appealing material to be utilized as photocatalyst given (i) the optimal light harvesting properties as shown by the optical characterization, (ii) the efficient generation of electron–hole pairs owing to the insertion of terthiophene moieties, and (iii) the right energy band positions for the material.⁷ We therefore evaluated the photocatalytic activity of NIP3T-ANW under white light for the degradation of a model organic pollutant (Rhodamine B dye, RhB) in aqueous solution.⁴³ In the absence of catalyst, RhB remains stable in solution under illumination (Fig. 4a and S9^†). However, in presence of NIP3T-ANW nearly 90% of RhB in an aqueous solution is degraded after 120 min, showing the enhanced catalytic activity of the material (Fig. 4a and S10^†). Furthermore, a good stability is shown upon 4 straight catalytic cycles (Fig. 4b, S11 and S12^†). In contrast, in the presence of the NIP3T moiety, only a 55% degradation of RhB is observed in the same timeframe (Fig. 4a and S11^†).Open in a separate windowFig. 4RhB degradation curves. (a) Comparison between the degradation effect of NIP3T, NIP3T-ANW and without catalyst. (b) NIP3T-ANW stability after four recycling cycles.In the photocatalytic degradation of organic pollutants, they are typically broken down through the attack of superoxide and hydroxyl species, formed when atmospheric oxygen reacts with photogenerated electrons or when water or OH ions are oxidized by holes, respectively.⁴⁴ Additionally, electron–hole pairs (or excitons) can directly reduce or oxidize organic pollutants in aqueous environments. Consequently, the evaluation of the photodegradation mechanism of organic pollutants, despite challenging, can provide meaningful insights about the nature of a semiconductor photocatalyst.⁴⁵ With the aim of evaluating the photodegradation mechanism we performed the measurements in the presence of different scavengers, namely an aqueous solution of AgNO₃ (100 mg L⁻¹), which captures photogenerated electrons, or triethanolamine (TEOA), which traps photogenerated holes (Fig. S13^†).^46,47 In the presence of Ag⁺ we could observe that the photocatalytic efficiency is enhanced, while the addition of TEOA quenched the performance, therefore suggesting that holes are the active specie in the photodegradation mechanism (Fig. S14^†).In summary, we have presented an approach towards the incorporation of D–A π-conjugated monomeric assemblies into poly(azomethine) networks to yield a purely organic semiconductor for the photocatalytic degradation of organic pollutants in aqueous media. The poly(azomethine) network benefits from a straightforward poly(condensation) approach which favourably competes with the elaborate high-temperature protocols applied for the preparation of inorganic materials. This work enriches the family of donor–acceptor organic semiconductor networks and, given its modular nature, paves the way for the development of a promising family of materials for photocatalytic applications.     

Thermally induced fragmentation of nanoscale calcite

Calcite nanorods ∼50 nm wide are thermally separated into nanoblocks. The fragmentation is ascribed to the ion diffusion on metastable crystal surfaces at temperatures (∼400 °C) much lower than the melting point. The presence of water molecules enhances the surface diffusion and induces deformation of the nanorods even at ∼60 °C.

Calcite nanorods ∼50 nm wide are thermally separated into nanoblocks.

Calcium carbonate is a common industrial material that is used as a micrometric filler for papers, rubbers, plastics, and inks. The shape and size of micrometric grains are important parameters that affect the physical and chemical properties of composite materials.^1–5 In recent years, nanometric particles of calcium carbonate have attracted much attention as basal building blocks of biogenic minerals^6–10 and functional materials with high biocompatibility and low environmental load.^11–14 Since various properties are influenced by the miniaturization of crystal grains below 100 nm, characterization of the carbonate particles is necessary for application in practical fields. However, their properties, including the thermal characteristics of nanometric calcium carbonate in the nanometric region, have not been sufficiently clarified because calcium carbonate is easily decomposed above 550 °C.^15,16 In the present work, we studied thermally induced deformation on nanometric calcite at temperatures lower than the melting point (1597 °C at 3 GPa)¹⁷ of the bulky crystal.Since the melting temperature of metals decreases when their size is decreased below ∼50 nm,^18,19 metallic nanowires fragment into nanospheres at temperatures much below the melting point of bulk metal.^20–22 The fragmentation is ascribed to the Rayleigh instability that is known for liquid. These results suggest that the surface diffusion of ions and atoms occurs easily on the nanometric particles when the surface instability is increased. The cleavage of solid nanowires has been observed only for metallic phases. In the current work, we found the morphological transformation of ion crystal nanorods into faceted nanograins through the surface diffusion at relatively low temperatures.The preparation of bulky calcium carbonate consisting of nanocrystals is required for reinforcing materials^23–25 and as a precursor of biomedical materials.^26,27 In general, however, fabricating large, bulky bodies through conventional sintering techniques is difficult because calcium carbonate is thermally decomposed into calcium oxide and carbon dioxide. Several methods, such as sintering with a flux^16,28 or in a carbon dioxide atmosphere²⁹ and hot-pressing under hydrothermal conditions^30,31 were developed to prepare bulky calcium carbonate materials. On the other hand, the thermal behaviors of pure calcium carbonate have not been sufficiently studied at temperatures below the decomposition and melting points.In nature, bulky calcium carbonate crystals are commonly produced as biominerals, such as shells, eggshells, sea urchins, and foraminiferal skeletons, under mild conditions.^{3,9,10,32–34} The bulky biogenic bodies are composed of nanometric grains 10–100 nm in size that are arranged in the same crystallographic direction.³ The formation of bridged architectures through oriented attachment is generally related to the ion diffusion on specific surfaces at relatively low temperatures. A detailed study on the stability of nanometric surfaces at relatively low temperatures is needed to understand the morphological change of calcium carbonate crystals.In the present report, we discuss the morphological change of calcite nanocrystals below the decomposition and melting temperatures by observing metastable crystal surfaces in the nanometer-scale range. Calcite nanorods elongated in the c direction were utilized as a typical nanometric shape covered with metastable surfaces. Here, thermally induced fragmentation was studied with and without water vapor. The surface diffusion was found to occur on the metastable surface at around 400 °C under a dry condition and at around 60 °C with water vapor. Our findings are important for clarification of the surface property of nanometric calcium carbonate and for the fabrication of bulky bodies through the attachment of nanocrystals.Single-crystal calcite nanorods elongated in the c direction were utilized in the present study (Fig. 1 and S1^†). Calcite nanorods up to ∼500 nm were formed through the combination of the carbonation of calcium hydroxide and the subsequent oriented attachment of resultant calcite nanoblocks ∼50 nm in diameter by stirring. The detailed mechanism was described in our previous study.³⁵ As shown in Fig. S1,^† the calcite nanorods were covered with metastable surfaces having a curvature. Moreover, we observed depressed parts originating from the oriented attachment of the original calcite grains. As shown in the Fig. S2,^† the XRD peaks of the nanorods were found to shift to higher angles than those of standard X-ray diffraction data (ICDD 00-005-0586). Thus, the crystal lattice of the nanorods was suggested to be stressed with the coverage of irregular surfaces.Open in a separate windowFig. 1SEM (a) and TEM (b and c₁), HRTEM (c₂) images, and the FFT pattern (c₃) of the lattice in (c₂) of calcite nanorods in aqueous dispersions at pH 12 and at 25 °C with stirring. (c) Reprinted from ref. 35 published by The Royal Society of Chemistry.As shown in Fig. 2, the calcite nanorods were deposited on a silicon substrate for clear observation of the morphological change. We redispersed the calcite nanorods in ethanol and evaporated the dispersion medium to deposit them on the substrate at 25 °C (Fig. 2a and S3a^†).³⁶ The nanorods were arranged on the solid surface through evaporation-driven self-assembly. Specifically, monolayers of the calcite nanorods were obtained by adding poly(acrylic acid) (PAA, MW: 5000 gmol⁻¹) to the ethanol dispersion. The dispersibility of the nanorods in ethanol was improved by the modifying agent. The organic components of the PAA-modified nanorods were removed through oxidation in air at around 300 °C (Fig. S4^†).Open in a separate windowFig. 2SEM images and schematic illustrations of calcite nanorods deposited on a silicon substrate before treatment (a) and heated at 400 °C for 1 (b) and 2 h (c). The PAA-modified calcite nanorods were used to obtain the monolayers (b and c). We used bare nanorods for take the images of original nanorods because the definite surfaces were not observed due to the presence of PAA (a).The calcite nanorods were deposited on the solid surface to study the morphological change when heated in air. Obvious changes were not found in the shape of the nanorods upon heating to a temperature below 350 °C in air for 24 h (Fig. S2^†). On the other hand, we observed significant deformation upon heating to 400 °C (Fig. 2). The depressed parts on the side surfaces enlarged in 1 h. The fragmentation of the nanorods was finally induced after treatment for 2 h. Calcite grains were formed by the thermally induced cleavage (Fig. 2c). The average size of the cleaved grains was ∼100 nm, which was larger than the average width of the nanorods, ∼50 nm. As shown in Fig. 3, we observed the definite surfaces covering nanograins. Most of the definite facets were assigned to the (104) of calcite by FFT analysis of the lattice fringes of nanograins in the HRTEM images. Some (012) planes were found in the nanograins. On the other hand, the surfaces of the deformed nanorods during fragmentation were curved and irregular. According to XRD patterns, the crystal phase was not changed with the fragmentation (Fig. S2^†). The diffraction peaks were shifted to the standard values and sharpened with the treatments. This suggests that the formation of the stable faces with the fragmentation is associated with the lattice relaxation.Open in a separate windowFig. 3TEM (a), HRTEM (b), and FFT images of nanocrystals before and after heating at 500 °C.We observed the stability of rhombohedral grains that were covered with the stable {104} planes. As shown in Fig. 4, the deformation of the rhombohedral grains was not observed at 400 °C in air for 6 h. These results indicate that the ion diffusion is not induced drastically on the stable surfaces at a temperature lower than the decomposition temperature.Open in a separate windowFig. 4SEM (a_1,2 and b_1,2), TEM (a₃), and SAED (a₄) images of calcite nanoblocks before treatment (a) and heated to 400 °C for 6 h (b). (a₃) A schematic illustration of a calcite rhombohedron covered with {104} faces.The fragmentation of the calcite nanorods was enhanced in the presence of water vapor. As shown in Fig. 5a, we found cleavage of the nanorods even at 60 °C in a closed vessel containing water. The formation of unifaceted rhombohedral nanoblocks with {104} faces was clearly observed at 100 °C for 24 h (Fig. 5b–d). Since the morphological change was similar to that under a dry condition, the ion diffusion on the surface is deduced to be assisted by adsorbed water molecules. The X-ray diffraction signals shifted to the standard values with the fragmentation (Fig. S2^†). Thus, the stable faces were formed with the lattice relaxation with the exposure to water vapor.Open in a separate windowFig. 5SEM (a and b), TEM (c), and SAED (d) images of calcite nanorods deposited on a silicon substrate subjected to high humidity at 60 °C (a) and 100 °C (b) for 24 h. (c) A schematic illustration of a calcite rhombohedron covered with {104} faces.The ion diffusion at a relatively low temperature, below the decomposition temperature, has not been reported for calcium carbonate. In the present work, however, we found the fragmentation of calcite nanorods at around 400 °C under a dry condition and at around 60 °C with water molecules. These results suggest that the ion diffusion occurs on the nanoscale calcite crystals. On the other hand, rhombohedral calcite grains covered with the stable {104} planes were not deformed at those temperatures. Thus, the diffusion at low temperatures is induced only on metastable surfaces that are exhibited on the nanoscale calcite. Moreover, the presence of water molecules enhances the ion diffusion on the metastable surfaces.The fragmentation of the calcite nanorods can be explained by Rayleigh instability. The cleavage by Rayleigh instability is ascribed to the enlargement of tiny perturbations on cylindrical liquids,³⁷ polymers, and metals. In general, the cylindrical bodies evolve into several spheres to decrease the total surface energy. Recently, Rayleigh instability was applied to the thermally induced fragmentation of metal^20–22 and organic³⁸ nanowires. The breakup phenomena were attributed to surface oscillations due to the high surface energy induced by increased surface-to-volume ratios.³⁹ Thus, metal nanowires are cleaved and form isotropic nanoblocks at temperatures well below the melting point. In the present work, we found fragmentation of the calcite nanorods at relatively low temperatures. The ion diffusion is induced on the metastable surfaces that are exhibited on nanoscale crystals. The depressed parts exist as perturbations on the side faces of the original nanorods. The cleavage occurs through enlargement of the depressed parts and formation of the stable faces to reduce the surface energy and relax the lattice strain.Polyhedral grains covered with flat planes were formed instead of spherical particles by the fragmentation of calcite nanorods. Formation of the stable {104} plane is achieved to reduce the surface energy. The {012} plane of calcite is not stable under the ambient temperature. However, the surface energy of {012} decreases with increasing temperature.⁴⁰ Thus, the facets are deduced to also be formed at temperatures around 400 °C.     

Facile one-pot solvothermal preparation of two-dimensional Ni-based metal–organic framework microsheets as a high-performance supercapacitor material

We report a facile one-pot solvothermal way to prepare two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) using only Ni precursor and ligand without any surfactant. The Ni-MOFms exhibit good specific capacities (91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively) and long-term stability in 5000 cycles when used for a supercapacitor electrode.

Two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) were synthesized via a facial one-pot solvothermal approach and exhibited good specific capacities and excellent long-term stability when used for a supercapacitor electrode.

With the continuous growth of energy demand worldwide, high-performance, environmental-friendly, and low-cost energy storage devices have attracted extensive research interest.^1–3 Among them, supercapacitors are considered most promising because of their high power density, long lifespan, and fast charging/discharging speed.^4–6 To date, numerous materials have been explored for fabricating supercapacitors. Carbon materials have been usually used for electrical double-layer capacitors (EDLCs), including carbon fibers, carbon nanotubes, carbon spheres, carbon aerogels, and graphene,^7–12 while conducting redox polymers and transition metal oxides/hydroxides are widely explored as active materials for pseudocapacitance and battery-type electrodes.^13–16Metal–organic frameworks (MOFs), a porous crystalline material composed of metal nodes and organic linkers, have been widely applied in versatile fields including chemical sensors, catalysis, separation, biomedicine, and gained more and more attention in the area of energy storage.^17–25 Recently, two-dimensional (2D) MOFs have aroused great interest as a new kind of 2D materials.^26,27 Compared with traditional bulk MOFs, 2D MOFs possess distinctive properties, such as short ion transport distances, abundant active sites, and high aspect ratios, making them exhibit better performance than their bulk counterparts.^28–32 Bottom-up methods are generally adopted to prepare 2D MOFs with the addition of surfactants to control the growth of MOFs in a specific direction.^33–35 However, the use of surfactants inevitably blocks part of the active sites at the expense of the performance of materials. Therefore, it is highly necessary to explore and develop a direct solvothermal synthesis of 2D MOFs with the advantages of additive-free, simple operation, and easy scale-up.Herein, we report a facile one-pot solvothermal method to synthesize 2D Ni-based MOF microsheets (denoted as Ni-MOFms) by treating nickel chloride hexahydrate (NiCl₂·6H₂O, the metal precursor) together with the trimesic acid (H₃BTC, the ligand) in a mixed solvent of N,N-dimethylformamide (DMF), ethanol (EtOH) and H₂O. During the whole preparation process, only Ni precursor and the ligand are used while no surfactant is added. When used as active materials for a supercapacitor electrode, the obtained Ni-MOFms displayed excellent reversibility and rate performance. It also exhibited specific capacities of 91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively. Besides, they showed a good cycling performance in 5000 cycles with about 70% of the specific capacity and almost 100% of the coulombic efficiency maintained.Morphologies of the Ni-MOFms were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1a and b, the Ni-MOFms were successfully fabricated via the facile one-pot solvothermal method with varying lateral sizes on the micron scale. Energy dispersive spectroscopy (EDS) mapping indicated that the obtained microsheets were mainly composed of C, O, and Ni. A trace amount of N was also observed, which could be attributed to the residual DMF in the mixed solvent (Fig. 1c). These elements were uniformly distributed throughout the whole microsheet. To measure the exact thickness of the Ni-MOFms, atomic force microscopy (AFM) was used. Fig. 1d showed that the thickness of the microsheet was about 58 nm. Considering the large lateral size, even such thickness could produce a relatively high aspect ratio, which is beneficial to the electrochemical performance.Open in a separate windowFig. 1(a) TEM image, (b) SEM image, (c) EDS mapping, and (d) AFM image and the corresponding height profile of the Ni-MOFms.The composition information of the Ni-MOFms was analyzed by X-ray diffraction (XRD) and the resulting diffraction pattern was shown in Fig. 2a. It was clear that the sample was a crystalline material. However, the exact structure was difficult to determine because no matching MOF structure has been found. Therefore, the structure of the Ni-MOFms was further confirmed by Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2b, there was a sharp peak at 1721 cm⁻¹ for H₃BTC, which could be ascribed to the stretching vibration of C O in the nonionized carboxyl group.³⁶ For the Ni-MOFms, the peak at this location disappeared while four new peaks appeared. Bands at 1634 and 1557 cm⁻¹ were related to the asymmetric stretching vibration of carboxylate ions (–COO⁻) and peaks at 1433 and 1371 cm⁻¹ were the characteristic peaks of the symmetric stretching vibration of –COO⁻.^37,38 All these changes indicate that the ligand interacted well with the metal precursor.Open in a separate windowFig. 2(a) XRD pattern of the Ni-MOFms. (b) FT-IR spectra of H₃BTC and the Ni-MOFms.The chemical status and surface composition of the Ni-MOFms were further examined by X-ray photoelectron spectroscopy (XPS). From Fig. S1a^† we could see that the Ni-MOFms were composed of C, O, Ni, and N, which was consistent with the result of EDS mapping. High-resolution spectra of C 1s, Ni 2p, O 1s, and N 1s were shown in Fig. S1b–e.^† Characteristic peaks of C 1s at 288.27, 286.50, 285.85, and 284.80 eV were related to O C–OH, C–O, C–C, and C C, respectively, suggesting the presence of H₃BTC (Fig. S1b^†).³⁹ The Ni 2p spectrum showed two peaks at 873.32 and 855.77 eV, which could be ascribed to Ni 2p_1/2 and Ni 2p_3/2, respectively, together with two satellite peaks at 879.26 and 861.05 eV, verifying the existence of Ni²⁺ (Fig. S1c^†).⁴⁰ In the O 1s region, bands positioned at 532.94 and 531.40 eV could be ascribed to the adsorbed H₂O molecules on the surface of Ni-MOFms and typical metal–oxygen bonds, respectively, further corroborating the coordination between H₃BTC and Ni²⁺ (Fig. S1d^†).³⁹ Finally, the high-resolution spectrum of N 1s was also analyzed (Fig. S1e^†). There were two main peaks at 400.18 and 402.21 eV that could be ascribed to neutral amine and charged nitrogen, respectively,⁴¹ further proving the residual DMF on the Ni-MOFms surface.To explore the crucial factors in the formation process of the Ni-MOFms, the reaction time and temperature, the solvent, the ligand addition amount, and the ligand type were studied. As shown in Fig. S2,^† different crystalline materials were obtained at different reaction times. With the increase of reaction time, the material gradually changed from sphere to sheet. The reaction temperature is another crucial factor. At 120 °C, the material was amorphous and spherical. When the temperature rose, the crystal formed and appeared as microsheets (Fig. S3^†). The effect of solvent was illustrated in Fig. S4.^† Microsheets could not be synthesized in DMF or DMF with a small amount of EtOH. In the mixed solvent of DMF and H₂O, crystals could be prepared, indicating the vital role of H₂O. However, spheres existed in the products. Only when a mixture of DMF, EtOH, and H₂O with a certain proportion was used as the solvent, the Ni-MOFms could be obtained. Furthermore, we investigated the effect of the ligand addition amount. From Fig. 1 and S5^† we can see that the Ni-MOFms crystals formed when the molar ratio of Ni precursor and H₃BTC was 1 : 2 (Fig. 1). We speculated that ligands could simultaneously act as regulators to adjust the morphology of materials, avoiding the use of additional surfactants. When the ligand was replaced with 2-methylimidazole (2-MI) or terephthalic acid (H₂BDC), flower-like crystals rather than microsheets were obtained (Fig. S6^†), indicating the importance of the ligand type. Taking the above factors into account, we could finally determine the suitable conditions for preparing the Ni-MOFms (see the experimental section in ESI^†).The potential application of the Ni-MOFms in supercapacitors was first tested by cyclic voltammetry (CV) in 3 M KOH between 0 and 0.4 V (vs. saturated calomel electrode, SCE). As can be seen from Fig. 3a, all CV curves had similar shapes and the peak currents improved gradually as the scan rate increased, suggesting the good capacitive behavior of the Ni-MOFms electrode.⁴² When the scan rate was as high as 150 mV s⁻¹, redox peaks could still be observed, which indicated the excellent rate performance and kinetic reversibility.⁴³ Besides, as the scan rate went up from 10 to 150 mV s⁻¹, the reduction and oxidation peaks moved towards negative and positive potential, respectively, demonstrating the electrode polarization at large scan rates.⁴⁴Open in a separate windowFig. 3Electrochemical measurements of the Ni-MOFms. (a) CV curves at different scan rates. (b) GCD curves at various current densities and (c) corresponding specific capacities. (d) The EIS Nyquist plot at the bias potential of 0.4 V and the equivalent circuit model with the fitted plots (the red dots).The galvanostatic charge–discharge (GCD) behavior was further investigated to assess the coulombic efficiency and the specific capacity of the Ni-MOFms (see the ESI^† for detailed calculation method).^45,46 As shown in Fig. 3b, the shape of GCD curves was highly symmetric during charging and discharging, indicating that the coulombic efficiency of Ni-MOFms was almost 100% at various current densities. The specific capacities of 91.4, 78.4, 71.4, 64.0, and 60.0 C g⁻¹ were achieved at current densities of 2, 4, 6, 8, and 10 A g⁻¹ (Fig. 3c), respectively, demonstrating the excellent rate capability with about 65.6% of the specific capacity maintained from 2 to 10 A g⁻¹. The specific capacity at 2 A g⁻¹ was comparable with or even superior to that of some MOF materials reported in the literatures (Table S1^†).^47–50The kinetics of the electroanalytical process was then investigated by electrochemical impedance spectroscopy (EIS). Fig. 3d showed the Nyquist plot of Ni-MOFms from 0.01 to 100000 Hz and the corresponding equivalent circuit model (inset) with the fitted plots. CPE was the constant phase element related to the double layer capacity.⁵¹ The equivalent series resistance was denoted by R_s and its value obtained from the x-axis intercept was about 2.1 Ω, indicating the low resistance of the solution.⁴³R_ct represented the charge-transfer resistance at the interface of the electrode and electrolyte.⁵² For Ni-MOFms, the value of R_ct was up to 147.1 Ω, which could be attributed to the poor conductivity of MOF materials.The long-term stability of Ni-MOFms was also explored by charging–discharging at 10 A g⁻¹ for 5000 consecutive cycles. From Fig. 4 we could see that the specific capacity retention remained about 70% after 5000 cycles and the coulombic efficiency was maintained at almost 100% throughout the whole process. Furthermore, the inset in Fig. 4 exhibited that the GCD curves of the last 10 cycles were the same as the first 10 cycles, indicating excellent cycling stability.Open in a separate windowFig. 4Cycle property of Ni-MOFms at 10 A g⁻¹. Inset: GCD curves of the first 10 cycles (left) and the last 10 cycles (right).     

Micro-supercapacitors based on oriented coordination polymer thin films for AC line-filtering

Reported herein is a facile solution-processed substrate-independent approach for preparation of oriented coordination polymer (Co-BTA) thin-film electrodes for on-chip micro-supercapacitors (MSCs). The Co-BTA-MSCs exhibited excellent AC line-filtering performance with an extremely short resistance–capacitance constant, making it capable of replacing aluminum electrolytic capacitors for AC line-filtering applications.

Micro-supercapacitors exhibiting excellent AC line-filtering with oriented coordination polymer thin-film electrodes are fabricated based on a substrate-independent electrode fabrication strategy.

Micro-supercapacitors (MSCs), as important Si-compatible on-chip electrochemical energy storage devices, have attracted rapidly growing attention due to their rapid energy-harvesting features and burst-mode power delivery.^1,2 In the past few years, a variety of materials including carbon nanotubes,³ graphene,⁴ graphene oxide and mesoporous conducting polymers,^5,6 have already been explored to fabricate the electrodes of MSCs for improving their electrochemical performance. Unfortunately, fabrication procedures of most of these active materials suffer from high cost, harsh and complicated processing conditions, as well as easy cracking and delamination of active films,^1,4 extremely limiting their commercial applications. Moreover, their performances are unsatisfactory for alternating current (AC) line-filtering, which is a key parameter to implement high-frequency operation in most line-powered devices.^7–9For AC line-filtering, capacitors need to respond harmonically at 120 Hz to attenuate the leftover AC ripples on direct current voltage busses.¹⁰ Notably, the development of more compact and miniaturized capacitors to replace traditional aluminum electrolytic capacitors (AECs) for AC line-filtering has become one of the major tasks for future electronics.¹¹ However, typical supercapacitors are incapable for AC line-filtering at this frequency due to their limited ion diffusion and charge transfer efficiency, corresponding to the unsuitable architectures and low conductivity of electrode materials.^10–12 Therefore, the design and fabrication of highly conductive electrodes with optimized architectures for facial electron/ion transportation is crucial for improving the performance of MSCs in AC line-filtering.^12,13 It is worth mentioning that great advancements have been achieved by utilizing vertically oriented graphene sheets as well as 3-dimensional graphene/carbon nanotube carpets prepared by chemical vapor deposition (CVD),^7,8 yielding efficient filtering of 120 Hz AC with short resistance–capacitance (RC) time constants of less than 0.2 ms, which is competitive with those of porous carbon-based supercapacitors (RC time constant = 1 s) as well as AECs (RC time constant = 8.3 ms).⁸ However, the CVD method necessitated in the fabrication of graphene/carbon nanotube electrodes suffers from high cost and complicate procedures.Coordination polymers with an unrivalled degree of structural and property tunability which could be realized by facial procedures, are promising candidates for energy storage.^14,15 Recently, a remarkable achievement which demonstrated a facile and low-cost solution-processed method towards on-chip MSCs based on an azulene-bridged coordination polymer framework (PiCBA) on a Si wafer-supported Au surface was reported.¹⁴ Nevertheless, the reported preparation of coordination polymer film exhibited strong dependence on the surface chemistry (functionality) of the substrate and further improvement of their electrochemical stability was needed. Therefore, the development of substrate-independent fabrication strategies of large-scale and uniform coordination polymer films is in great need not only for fundamental studies, but also for technological applications especially in electronics.Herein, we demonstrate a facial solution-based substrate-independent approach to fabricate oriented coordination polymer (Co-BTA) thin-film electrodes. Remarkably, rigid and flexible Co-BTA-based MSCs with excellent electrochemical stability and AC line-filtering performance were realized, indicating great application potential in micro-supercapacitors.As demonstrated in Fig. 1a–c, a large scale and continuous Co-BTA coordination polymer film composed of one dimensional (1D) molecules ([Co(1,2,4,5-bta)]_n) was prepared at the air–liquid interface through a coordination reaction between 1,2,4,5-benzenetetramine tetrahydrochloride (BTA) and cobalt acetate tetrahydrate (Co(CH₃COO)₂·4H₂O). Notably, the preparation of Co-BTA film is basing on mild conditions and independent of any substrates. The resulting film can be transferred onto any supports including rigid silicon (Si) wafer, glass, as well as flexible PET substrate, indicating great substrate-independence and making it practically applicable for various applications. Besides of a brown film formed at the air–liquid interface, a powder product is also obtained at the bottom of the reaction bottle.Open in a separate windowFig. 1(a) Synthesis of Co-BTA through the coordination reaction between BTA and cobalt ions. (b) Illustration of the gas–liquid interface growth of Co-BTA film. (c) Photographs of the reaction system before and after the coordination reaction.To study the morphology of the resulting Co-BTA film, the brown film was transferred onto a SiO₂/Si wafer by immersing the wafer down to the reaction mixture and subsequently lifting the film up. The scanning electron microscopy (SEM) image reveals a highly uniform and large-scale distribution of the obtained film without cracks or wrinkles (Fig. 2a), which is superior to other reported coordination polymer films obtained via a similar method.¹⁶ An average thickness of approximately 60 nm of the Co-BTA film is observed from the cross-sectional SEM image as shown in Fig. 2b. Interestingly, thickness of the obtained coordination polymer film could be well controlled and Co-BTA films with thicknesses up to several hundred nanometers could be well prepared by adjusting the ratio of raw materials (Fig. 2c and d).Open in a separate windowFig. 2(a) Planar SEM image of Co-BTA film. Cross-sectional SEM images of Co-BTA films with a thickness of (b) 60 nm, (c) 160 nm and (d) 260 nm.To investigate the structure information of the resulting Co-BTA and further explore the coordination reaction, characterizations including powder X-ray diffraction measurements (PXRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were carried out. The PXRD pattern of Co-BTA powder shown in Fig. S1a^† is in great agreement with that simulated from the crystal structure of Ni(dhbq)·nH₂O (Fig. S1b^†), suggesting that Co-BTA and Ni(dhbq)·nH₂O is isostructural and forms 1D structures with straight infinite chain extends.¹⁷ More interestingly, PXRD measurements employing two different scattering geometries (Fig. S2^†) on the Co-BTA thin-film demonstrate two quite different diffraction patterns. As observed in Fig. 3a, the diffraction pattern observed for the out-of-plane scattering geometry exhibits three characteristic peaks of the Co-BTA film at ∼12°, 24° and 36°, which are corresponding to (001), (002) and (003), respectively. In contrast, the in-plane PXRD profile employing grazing-incidence XRD (GIXRD) technique at an incident angle (α) of 0.2° demonstrates a main peak at ∼18°, which is assigned to the (110) diffraction peak. Different diffraction peaks observed through these two XRD scattering geometries indicate an orientation nature of the as-prepared Co-BTA film,¹⁸ which exhibits better crystallinity compared with the powder Co-BTA product. In addition, the N 1s core level spectrum for Co-BTA film exhibit one typical peak at 399.1 eV, which is corresponding to the amido coordinated with Co^II, indicating the strong coordination between Co^II and BTA (Fig. 3b). The weak peak at ∼401 eV is assigned to N–O due to the oxidation of ligand BTA in ambient environment before reaction. The atomic ratio of N : Co is calculated to be 3.53 : 1 for Co-BTA film and 3.71 : 1 for Co-BTA powder respectively (Fig. S3 and Table S1^†), which is close to the theoretical stoichiometric ratio (4 : 1) for Co-BTA structure, suggesting a high degree of coordination in the resulting product through one Co cation and two benzenetetramine groups. Moreover, the disappearance of two characteristic N–H stretching modes from –NH₂ after the coordination reaction whereas the phenyl-related vibration still exists, further confirms the existence of –NH– in the product through the loss of one H per –NH₂ (Fig. S4^†).¹⁹Open in a separate windowFig. 3(a) PXRD profiles of out-of-plane XRD, in-plane XRD and simulated PXRD pattern of Ni(dhbq)·nH₂O,¹⁷ respectively. *SiO₂/Si substrate. (b) N 1s core level spectra of the Co-BTA film.On the basis of facial fabrication, substrate independence, highly orientation nature, low band gap (1.68 eV, calculated from Fig. S5^†) and excellent stability in acid environment (Fig. S6^†), the resulting Co-BTA film is considered as a promising candidate for MSCs application. Fig. 4a schematically depicts the stepwise fabrication of a planar Co-BTA film based MSC on a SiO₂/Si wafer and its electrochemical performance is first examined by cyclic voltammetry (CV) with scan rates ranging from 50 mV s⁻¹ to 1000 V s⁻¹ (Fig. 4b and c). At a low scan rate of 50 mV s⁻¹, the 60 nm-thick Co-BTA film based MSC exhibited a pronounced pseudocapacitive effect, implying the occurance of faradaic reaction.²⁰ With the increase of scan rate, a gradual transition of the CV curves from the pseudocapacitive to the typical electrical double-layer capacitive behavior with a nearly rectangular CV shape was observed. Remarkably, the device exhibited a maximum volumetric capacitance of 23.1 F cm⁻³ at 50 mV s⁻¹, which is comparable with those of reported carbon- or graphene-based MSCs (Table S2^†), e.g., onion-like carbon,²¹ vertically oriented graphene,⁸ and carbon nanotubes/graphene.⁷ Even though a trend that C_V decreased gradually with increasing scan rate was observed, the Co-BTA-based electrode still delivered a C_V of 2.7 F cm⁻³ even at a high scan rate of 1000 V s⁻¹, suggesting an excellent capacitive performance of this Co-BTA-based MSC device.⁷Open in a separate windowFig. 4(a) Schematic illustration of the fabrication of MSC device with the Co-BTA film electrode. (b) CV curves of Co-BTA-based MSCs in the H₂SO₄–PVA gel electrolyte at different scan rates. (c) C_V evolution of the MSCs at different scan rates.Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the charge transport properties within the Co-BTA-based MSCs. The Nyquist plot shown in Fig. S7^† indicated the kinetic features of electron transfer/ion diffusion at the electrode, from which an almost straight line especially in the low frequency region is observed. Notably, the plot shows a closed 90° slope without a charge transport semicircle at high frequency which is corresponding to an almost ideal capacitive ion diffusion mechanism, due to the excellent charge transfer property of the oriented Co-BTA electrode film. Moreover, this microdevice exhibited a low equivalent series resistance of 13.48 Ω (Fig. S7^† (inset)), further suggesting the ultrafast ion diffusion characteristic in such a Co-BTA-based-MSC.²² It''s suggested that the unique kinetic feature of fast ion diffusion and charge transfer benefits from the intrinsic characteristics of the oriented polymer film composed of 1D molecules, which can not only facilitate rapid ionic diffusion but also facilitate the interfacial charge transfer and faradaic redox reaction between the electrode material and electrolyte.What''s more, the dependence of the phase angle on frequency shown in Fig. 5a delivered a high characteristic frequency f₀ of 6812 Hz at the phase angle of −45° (the resistance and reactance of the device have equal magnitudes),¹⁰ which is much higher than that of an active carbon supercapacitor (5 Hz),²³ sulfur-doped graphene MSCs (3836 Hz),²² or an azulene-bridged coordination polymer framework based MSCs (PiCBA-MSCs) (3620 Hz) and so on,¹⁴ as summarized in Table S2.^† Moreover, a max phase angle of −80° at a frequency of 18 Hz is observed, indicating the performance of this Co-BTA based MSCs is 89% of that of an ideal capacitor. Importantly, a large impedance phase angle of −78.6° was obtained at a frequency of 120 Hz, which is the largest reported value for coordination polymer based MSCs (Table S2^†), suggesting an excellent AC line-filtering performance of our microdevice.⁷ To further conform the ultrahigh fast ion diffusion in Co-BTA-based-MSCs, the relaxation time constant τ₀ (τ₀ = 1/f₀, the minimum time needed to discharge all the energy from the device with an efficiency of greater than 50% of its max. value) was calculated to be only 0.15 ms (6812 Hz), which is orders of magnitude higher than that of conventional electrical double-layer capacitors (1 s),⁸ activated or onion-like carbon MSCs (<200 ms, <10 ms),^21,23 and much shorter than those of MSCs based on carbon nanotubes/reduced graphene oxide (4.8 ms) as well as azulene-bridged PiCBA coordination polymer framework film (0.27 ms).^14,24 Moreover, a short RC time constant (τ_RC) of 0.32 ms was obtained (Fig. 5b) through a series-RC circuit model, making it capable of replacing AECs for AC line-filtering application. To the best of our knowledge, this is the first report of coordination polymer-based MSCs exhibiting such a small relaxation time constant and RC time constant.Open in a separate windowFig. 5(a) Impedance phase angle on the frequency for the Co-BTA-based microdevices. (b) Plot of capacitance (C_V′ = volumetric real capacitance and C_V′′ = imaginary capacitance) versus the frequency of Co-BTA-based microdevices. (c) Cycling stability of Co-BTA film with 10 000 cycles at the scan rate of 50 V s⁻¹. Inset displays the CV curves at the first, five thousandth and ten thousandth cycle, respectively. (d) Ragone plots for Co-BTA film, compared with commercially applied Li-thin-film batteries,²¹ electrolytic capacitors,² CNT-graphene carpets,²⁴ PiCBA coordination polymer and MXene-reduced graphene oxide.^14,25Impressively, this oriented electrode structure exhibits excellent long-term electrochemical stability with ∼96.3% capacitance retention even after 10 000 cycles of charging/discharging at a scan rate of 50 V s⁻¹ (Fig. 5c), which has also been confirmed by comparing the CV curves before and after testing for 10 000 cycles (inset of Fig. 5c). It''s worth pointing out that the as-made Co-BTA-based MSCs exhibit the best electrochemical stability among reported MSCs with coordination polymer electrodes.¹⁴ On the basis of the above discussion, it is reasonable to conclude that the ultrahigh fast ion diffusion/charge transfer in Co-BTA-based-MSCs attributed to the oriented architecture of Co-BTA thin-film electrodes, the excellent AC line-filtering performance, as well as remarkable electrochemical stability contributes to the excellent performances of Co-BTA-based-MSCs. Moreover, the power density and energy density of the as-made device is calculated and compared with that of MSCs based on other electrode materials to evaluate the energy storage performance of the Co-BTA based MSCs. The Ragone plot in Fig. 5d reveals a high power density of 1056 W cm⁻³ for our device, which is at least five orders of magnitude higher than that of commercial thin-film lithium batteries. What''s more, our device exhibits an energy density of up to 1.6 mW h cm⁻³ at 50 mV s⁻¹, which is at least one order of magnitude higher than that obtained for MSCs based on CNTs-graphene carpet and high-power electrolytic capacitors.^2,24To further demonstrate the substrate independence of this fabrication strategy, flexible Co-BTA-based-MSC device was fabricated and investigated basing on a flexible polyethylene terephthalate (PET) substrate instead of rigid Si substrate in the same way (Fig. S8–S10^†). The as-fabricated device exhibited a maximum volumetric capacitance of 22.0 F cm⁻³ at 50 mV s⁻¹, a short relaxation time constant τ₀ of 0.15 ms and a RC time constant (τ_RC) of 0.42 ms, which are close to the properties of devices with a Si substrate, confirming the substrate independence of this fabrication scheme. As a proof-of-concept application, bending tests were carried out and the bended device (radius = 1 cm) exhibited a small relaxation time constant τ₀ of 0.21 ms and RC time constant (τ_RC) of 0.42 ms, suggesting that the Co-BTA-based MSC with PET substrate in a bended state still delivers a good ion diffusion and AC line-filtering performance.In conclusion, we have demonstrated a facile method that can be used to construct large scale and highly oriented uniform Co-BTA coordination polymer thin films using a very convenient and fast process. With this method, Co-BTA-based MSCs are fabricated without any dependence of the substrate. The as-fabricated MSCs on Si substrate exhibit high specific capacitance, energy density as well as excellent electrochemical stability. Particularly, the fabricated Co-BTA based MSCs deliver excellent AC line-filtering performance with an extremely short RC time of 0.32 ms, attributed to the facilitated ion diffusion beneficial from the oriented architecture of Co-BTA thin film. The high-performance electrochemical properties of Co-BTA-MSCs makes Co-BTA films promising materials to provide more compact AC filtering units for future electronic devices.     

Effect of nanostructured silicon on surface enhanced Raman scattering

Non-metallic materials are often employed in SERS systems by forming composite structures with SERS-active metal materials. However, the role of the non-metallic structures in these composites and the effect of them on the SERS enhancement are still unclear. Herein, we studied the effect of silicon morphology on SERS enhancement on silver nanoparticles-coated different structured silicon surfaces. Our finding will help to further understand the SERS mechanism and pave the way for making more efficient SERS systems.

The surface morphology of non-metallic silicon has a big effect on the SERS enhancement of silver nanoparticle-coated silicon surfaces.

In past decades, increasing attention has been attracted to surface enhanced Raman scattering (SERS) due to the dramatically enhanced detection sensitivity of Raman scattering (down to single-molecule sensitivity). The Raman intensity of the molecules located in the vicinity of SERS nanostructures can be enhanced up to 10¹⁰ to 10¹¹ times,^1,2 largely extending the application of SERS in the fields of physics, chemistry and biology, etc.^3–9 Metal materials are mainly employed in fabricating SERS structures, especially gold or silver ones for visible spectrum excitation.^10,11 To date, many nanostructures have been reported that can enhance Raman scattering enormously, leading to so called Raman hot spots,^12–15 including nanogaps,^16,17 nanostars,¹⁸ nanotriangles and nanorods,¹⁹ mainly due to the introduction of a localized electromagnetic field under illumination. This kind of enhancement is referred to as the electromagnetic mechanism, which dominates the SERS enhancement in most cases.Non-metallic structures can also contribute to Raman enhancement, although the enhancement factor is usually very low. It has been reported that Cu₂O,²⁰ TiO₂ ²¹ and ZnS²² nanoparticles can enhance the Raman intensity of adsorbed molecules. Graphene has also been proved to be an efficient platform for Raman enhancement.²³ Although non-metallic materials can be used directly for SERS applications, they are usually used by forming a composite with SERS-active metal material, where they act as supporting materials or borrow the SERS activity from the metallic Raman hot spots. It has been reported that SERS activity can be borrowed from SERS-active materials through ultrathin SERS-inactive transition metals (e.g., Pt, Ni, Co and Pd)²⁴ or dielectric (e.g., SiO₂, Al₂O₃)²⁵ layer. Tian et al. reported the shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) by using the gold nanoparticles coated with ultra-thin silica or aluminum oxide shell.^26,27 Raman enhancement can be achieved at the silica shell surface by borrowing the SERS activity from the gold core. However, the role of non-metallic structures in enhancing Raman scattering and the interactions between these two kinds of materials are still fuzzy.Silicon nanostructures fabricated by catalytic etching method can be easily metalized with silver or gold by electroless deposition for SERS applications.^28–32 However, up to now, only limited types of silicon nanostructures have been reported for SERS applications.^17,30,31 In addition, the role of the nanostructured silicon surface in Raman enhancement is still unclear.In this report, we fabricated SERS structures on two types of silicon surfaces, flat silicon and nanoporous silicon, by metallizing the silicon structure with silver nanoparticles (AgNPs). Compared to the fabricated SERS structure on flat silicon surface, the one fabricated on nanoporous silicon surface showed obvious enhancement on the Raman spectrum of adsorbed probe molecules. The effect of pore size and depth of nanoporous silicon on Raman enhancement was investigated in detail.To investigate the role of silicon nanostructures in Raman enhancement, we compared the Raman spectra of probe molecules adsorbed on AgNPs-coated flat silicon and nanoporous silicon surfaces, respectively. The nanoporous silicon was fabricated by following a modified reported procedure (see details described in Experimental section and scheme shown in Fig. S1 in (ESI^†)).²⁸ Vertical nanopores were produced on silicon surface, and the pore size and pore depth can be easily tuned by varying the reaction parameters. Then a flat silicon and a nanoporous silicon substrates were both metallized with silver by immersing them into a mixed aqueous solution of AgNO₃ and HF,^29,33 forming AgNPs with size of 60 ± 30 nm. The as-prepared AgNPs-coated silicon surfaces (see scanning electron microscopy (SEM) images shown in Fig. S2 in ESI^†) served as SERS-active substrates. After p-aminothiophenol (PATP) molecules (Raman probe) were adsorbed on the AgNPs-coated silicon structures, both AgNPs-coated substrates showed uniform and strong Raman enhancements (Fig. 1). On AgNPs-coated nanoporous silicon surface, the Raman bands of PATP molecules at 1076 and 1142 cm⁻¹ are 4.2 and 7.4 times stronger compared to those on AgNPs-coated flat silicon surface (Fig. 1), respectively, demonstrating the vital role of silicon morphology in the obtained Raman enhancement. There are two widely accepted mechanism for SERS enhancement, electromagnetic mechanism and charge transfer mechanism.¹ Both flat silicon and nanoporous silicon substrates are composed of same material and the only difference between them is the silicon morphology. Thus the charge transfer mechanism should contribute similar effect in both conditions. Moreover, both flat and nanoporous silicon surfaces were covered with a thin layer of silicon dioxide,^28,34 which limit the charge transfer between AgNPs and silicon surface. This is also confirmed by the XPS measurement on the nanoporous silicon surface (Fig. S3 in ESI^†). Therefore, the observed different enhancement may attribute to the electromagnetic mechanism, which will be discussed latter. In addition, the Raman enhancement is uniform over the whole substrate. This is probably due to the uniform coating of AgNPs on high-density silicon nanopore structures. The enhancement factor (EF) can be calculated by using the following equation, EF = (I_SERS/I_bulk)(N_bulk/N_SERS),³⁵ where I_SERS and I_bulk represent the Raman intensities in SERS and bulk Raman measurements, respectively; N_SERS and N_bulk represent the number of probe molecules located in the excitation volume under these two conditions. For Raman band at 1076 cm⁻¹ (represents a₁ vibration mode of PATP,¹¹ which sits at 1089 cm⁻¹ for bulk,³⁶ Fig. S4 in ESI^†), the average EFs over the whole surface were calculated as 6.7 × 10⁵ and 2.8 × 10⁶ for SERS structures on flat silicon and nanoporous silicon, respectively. The strong Raman band at 1142 cm⁻¹ indicates a chemical conversion from PATP to 4,4′-dimercaptoazobenzene (DMAB) upon light irradiation.¹¹Open in a separate windowFig. 1Raman spectra of PATP molecules adsorbed on the AgNPs-coated (a) flat silicon and (b) nanoporous silicon. A nanoporous silicon with pore depth of 220 nm was used here. The schemes at the bottom right and top right show the structures of the AgNPs-coated flat silicon surface and AgNPs-coated nanoporous silicon surface, respectively. The size of AgNPs was not drawn to scale.As discussed, electromagnetic mechanism dominates the observed SERS enhancement. To confirm the role of silicon morphology, we did numerical simulation using the finite-difference time-domain (FDTD) method to investigate the localized electromagnetic field distributions on AgNPs-coated flat and nanoporous silicon surfaces. Note that, for the AgNPs-coated nanoporous silicon surface, many AgNPs sit on the edge of silicon nanopores (Fig. S2D in ESI^†). In this case, the electromagnetic field around the AgNPs is more localized. Compared with the AgNPs-coated flat silicon, the electromagnetic field is five times more localized on the AgNPs-coated nanoporous silicon surface (Fig. 2), which in principal could introduce 25 times stronger Raman enhancement.³⁷ However, in real case, only a proportional of the AgNPs locates on the edge of silicon nanopores and the shapes of the coated AgNPs are not exactly same with the ones we used in simulation, which explains the smaller SERS enhancement we observed on nanoporous silicon surface.Open in a separate windowFig. 2Schemes (side views) and FDTD simulations on the AgNPs-coated flat silicon (A and B) and nanoporous silicon (C and D). Dashed circles in (D) indicate the positions of silicon pores. The schemes in (A) and (C) were not drawn to scale.As discussed above, the morphology of nanoporous silicon contributes to the enhanced Raman signal. By varying the pore size and pore depth of the nanoporous silicon, different Raman enhancement should be observed.First, we studied the effect of pore depth of nanoporous silicon on Raman enhancement. The depth of silicon nanopores can be easily tuned by varying the period of catalytic etching of silicon. The Raman intensity of the probe molecules increased continuously with increased silicon nanopore depth (from 40 to 220 nm, Fig. 3). When PTAP molecules were adsorbed on the AgNPs-coated nanoporous silicon surface, the Raman intensity measured on silicon with 220 nm pore depth was increased about 2 times compared to that on silicon with 40 nm pore depth. Further increasing the pore depth to 900 nm, the Raman intensity dropped instead (Fig. 3). These results indicate the important role of the pore depth in Raman enhancement. FDTD simulations were carried out to investigate the mechanism behind (Fig. S5 in ESI^†). As the pore depth increases, the electromagnetic field becomes more localized, which is consistent with the experimental data. However, the Raman intensity decreased on surface with very deep silicon nanopores (900 nm), which can be explained by the enhanced light trapping.^38,39 In this case, part of the Raman scattering light cannot escape from the nanopores (confirmed by the dark black color of the sample, Fig. S6 in ESI^†), leading to a weaker Raman signal. This can also be double confirmed by studying the Raman scattering from nanoporous silicon samples with AgNPs located at the bottom of the nanopores (discussed in ESI and Fig. S7^†).Open in a separate windowFig. 3Raman intensity variation (peak at 1076 cm⁻¹) on the AgNPs-coated nanoporous silicon (pore size of ∼40 nm was used) surface with four different depths. The scheme on top was not drawn to scale.Second, the size of silicon nanopores also plays a role in the Raman enhancement. The pore size on silicon surface can be tuned by controlling the size of catalysts (AgNPs) deposited on silicon wafer (Fig. S1B in ESI^†), whose size was replicated by the nanopores in subsequent catalytic etching process (Fig. S1C^†). By varying the deposition time, nanoporous silicon samples with four different pores sizes, 31 ± 10, 41 ± 11, 80 ± 24 and 160 ± 50 (Fig. 4A–D), were fabricated, respectively. For the AgNPs-coated nanoporous silicon samples, the Raman intensity of probe molecules slightly changed while increasing the pore size (Fig. 4E), indicating a weak effect of pore size on Raman enhancement. As aforementioned discussion, the Raman enhancement is mainly contributed by the AgNPs that locate on the nanopore edges. Therefore, the Raman enhancement is strongly dependent on the perimeter of all the nanopores and the number of AgNPs that locate on the edge of silicon nanopores. While increasing the pore size, the perimeter of single pore increases. However, many pores are fused together, compensating the increase of the perimeter of single pore. Thus, the total perimeter of all nanopores does not change much when increasing the pore size. In this case, the amount of the AgNPs locating on the edge of silicon nanopores may not change too much, which may explain the less dependency of the pore size on SERS enhancement. To investigate the structure of the AgNPs-coated nanoporous silicon, we deposited gold nanoparticles (AuNPs) onto it. In this case, however, a stronger Raman scattering was observed due to the formation of AgNP–AuNP nanogaps and the enhancement varied on different sized silicon nanopores. When increasing the pore size from 31 ± 10 to 80 ± 24 nm, the Raman scattering became stronger. Further increasing the pore size to 160 ± 50 nm, the Raman intensity decreased. As known, two particles formed nanogap shows a more localized electromagnetic field when the polarization of incident light is parallel to the center to center axis of the two particles.^16,40 Therefore, horizontal positioned two-particle nanogaps will give much stronger Raman enhancement. If the size of the silicon nanopores is too small, it is difficult for the AuNPs (13 nm in diameter) to enter the pore, limiting the number of AgNP–AuNP nanogaps that are horizontally positioned, and in turn limiting the Raman enhancement. While increasing the nanopore size, we have a better chance to form the ideally positioned AgNP–AuNP nanogaps to improve the Raman enhancement (Fig. S8 in ESI^†). However, when the pore size is too big, only a small part of the nanostructures locates inside the excitation volume during Raman measurement, leading to a weaker Raman signal.Open in a separate windowFig. 4(A–D) Nanoporous silicon with different pore sizes obtained by varying the silver deposition time described in Fig. 1A and B. Scale bars = 200 nm. The pore depth here was set as 220 nm. (E) Raman intensity variation (peak at 1076 cm⁻¹) on the AgNPs-coated nanoporous (four different pore sizes shown in A–D) silicon surface without (magenta bars) and with (cyan bars) the adsorption of AuNPs.The Raman enhancement can also be affected by the size of the AgNPs coated on nanoporous silicon. The AgNP size can be tuned by varying the AgNP deposition time shown in Fig. S1E.^† It has been reported that AgNPs with several tens of nanometers showed optimized plasmon resonance with excitation wavelength of 632.8 nm.⁴¹ In this work, the 40–75 nm AgNPs coated on nanoporous silicon show higher Raman enhancement than those with smaller or bigger AgNPs (Fig. S9^†), since the size of these AgNPs fall into the optimized range for Raman enhancement, which is consistent with the reported work.     

Molecular chains of coordinated dimolybdenum isonicotinate paddlewheel clusters

A growing focus on the use of coordination polymers for active device applications motivates the search for candidate materials with integrated and optimized charge transport modes. We show herein the synthesis of a linear coordination polymer comprised of Mo₂(INA)₄ (INA = isonicotinate) metal–organic clusters. Single-crystal X-ray structure determination shows that this cluster crystallizes into one-dimensional molecular chains, whose INA-linked Mo₂ cores engage in alternate axial and equatorial binding motifs along the chain axis. Electron paramagnetic resonance spectra, absorption spectra, and density functional theory calculations show that the aforementioned linear coordination environment significantly modifies the electronic structure of the clusters. This work expands the synthetic foundation for assembly of coordination polymers with tailorable dimensionalities and charge transport properties.

Linear molecular chains of coordinated dimolybdenum isonicotinate clusters.

Over the past few decades, researchers have developed and investigated coordination polymers (CP) encompassing an extensive range of architectures.^1–3 Interest in CPs most frequently focuses on their large accessible surface areas, especially in the case of porous metal–organic frameworks, and on their significantly tunable geometries. These features have led to the application of CPs in molecular sieving,^4–6 contaminant encapsulation,^7–9 catalysis,^10–12 and sensing.^13,14 With increased attention towards use of CPs as electrochemical and electronic materials,^13,15,16 there is a need to incorporate and optimize charge transport pathways within the framework of these extended solids. The integration of coupled redox-active entities as part of the CP framework can facilitate charge delocalization and thereby engender long-range electronic transport pathways. This strategy has recently been employed to produce electrochemically- and electronically-active CP frameworks.^16,17We drew our attention to di-metallic tetracarboxylate clusters with metal–metal multiple bonds because of their tendency to form Class III Robin-Day mixed-valence complexes.^18–20 These four-fold symmetric paddlewheel clusters exhibit redox behavior, and there is spectroscopic evidence for electronic communication within small supramolecular units of these clusters joined by organic or organometallic linkers.^21–25 By comparison, the assembly of these clusters into extended solids and an interrogation of their properties is less developed. Obtaining well-defined and structurally robust CPs from these clusters can also be challenging.^26–31Motivated by these challenges and opportunities, we have recently focused on Mo₂(INA)₄ as a new building block for CPs. We previously reported the preparation of CP crystals comprised of commensurately stacked two-dimensional lattices of fully coordinated Mo₂(INA)₄ clusters.³² In this work, we were interested in accessing other crystal topologies containing the same molecular cluster. A red crystalline precipitate forms after refluxing a solution of Mo₂(OAc)₄, excess isonicotinic acid (H-INA), and trace acetic acid in dimethylformamide (DMF) for 2 days (Scheme 1). Electrospray ionization mass spectrometry of a solution of this red precipitate in dimethylsulfoxide (DMSO) and acetonitrile reveals peaks centered around m/z = 681 with isotopic patterning (Fig. S1^†). This value matches the theoretical mass of Mo₂(INA)₄. The organic composition of the crystalline product is further verified by nuclear magnetic resonance (NMR) experiments following acid digestion of the precipitate (Fig. S2^†). These data confirm that the crystalline product (1) of the reaction outlined in Scheme 1 is comprised of the Mo₂(INA)₄ cluster.Open in a separate windowScheme 1Ligand exchange followed by polymerization yields product 1 after 2 days.Large single crystals of 1 were obtained by stirring the aforementioned reaction (Scheme 1) at 60 rpm (Fig. S3^†). The structure of these single crystals was determined by X-ray crystallography (Fig. 1 and Table S1^†). The crystal structure of 1 contains one-dimensional (1D) chains comprised of Mo₂(INA)₄ clusters that alternately bind through axial and equatorial binding sites. One DMF solvent molecule is coordinated to each of two Lewis acidic axial Mo sites on every other cluster. This coordination motif is supported by proton NMR data showing a DMF-to-INA ratio of approximately 1 : 4.07 in acid-digested samples of 1 (Fig. S2^†). The Mo–Mo bond lengths in the two clusters are 2.1219(5) Å and 2.1189(5) Å. These distances are within the anticipated range of lengths for Mo–Mo quadruple bonds (2.06–2.17 Å as described by Murillo et al.) and consistent with axial coordination of a strongly donating ligand such as pyridine.^22,33,34 Furthermore, we observe a Raman band at 383 cm⁻¹ (Fig. S4^†), which is relatively weaker than that anticipated for axially under-coordinated Mo₂ complexes, and treat this observation as further evidence for the axial donation effect.^22,34 We note that the Mo₂ centers within each molecular chain are coordinatively saturated and do not bond to pyridines from neighboring chains (Fig. S5^†).Open in a separate windowFig. 1Structure of the 1D coordination polymer (1) showing the paddlewheel units that alternately bond through axial Mo (cyan) and equatorial N (blue) binding sites. Ellipsoids are shown at 50% probability level. Disorder and H atoms have been omitted for clarity.We next characterized the phase purity, stability, and porosity of bulk samples of 1. First, powder X-ray diffraction (pXRD) patterns collected from products precipitated across a range of stir rates (60–300 rpm) match well with the pXRD pattern simulated from the single-crystal XRD data discussed above (Fig. 2 and S3^†). These data suggest that a single common crystal phase is produced regardless of stir rate, though stir rates <100 rpm are conducive to the formation of high-quality single crystals. Second, Fourier transform infrared (FT-IR) spectroscopy data show no evidence of H-INA or Mo₂(OAc)₄ in the bulk crystalline powder (Fig. S6^†), suggesting good purity of our bulk samples. Third, thermogravimetric analysis shows a mass loss of 10.1% between 190 °C and 252 °C that can be attributed to the loss of coordinated DMF molecules, which account for 9.7% of the 1D CP (Fig. S7^†). Upon further heating, the sample rapidly loses mass above 350 °C and likely degrades. Finally, nitrogen gas adsorption measurements on the crystalline powder yield a Brunauer–Emmett–Teller (BET) surface area of 325.8 m² g⁻¹ (Fig. S8^†). We note that this modest BET surface area is reasonable given the tight packing of the 1D chains within 1 (Fig. S5^†).Open in a separate windowFig. 2Experimental pXRD pattern for powder precipitated from a DMF solution stirred at 300 rpm (blue), and simulated pXRD pattern obtained from single-crystal XRD data of 1 (red).Finally, we sought to analyze the electronic structure of 1 with absorption spectroscopy, density-functional theory (DFT) calculations, and electron paramagnetic resonance (EPR) spectroscopy. A diffuse reflectance UV-vis spectrum collected from a powder sample of 1 exhibits a broad absorption feature between ∼400 and 600 nm (Fig. 3a). The UV-vis absorption spectrum of a solution sample of 1 dissolved in DMF exhibits a significant feature centered at ∼470 nm. We ascribe the modest absorption shoulder at ∼570 nm to absorption by remnant 1D CP chains. We used computational methods^35,36 to examine orbital interactions and electronic transitions that may be responsible for the absorption features identified above. Single point energies and molecular orbitals were calculated from coordinates taken directly from the single-crystal structure of 1. The B3LYP functional was used with the 6-31g* basis set for H, C, N, O and the Stuttgart/Dresden effective core potential for Mo. In the ground state, the first four highest-energy occupied molecular orbitals are all metal-centered, with discrete energy levels, localized electron density, and little sharing between the two coordinatively distinct clusters (Fig. S9^†). These computational results suggest that charge (e.g. introduced from an external circuit or generated chemically in situ) would likely remain localized on the individual clusters comprising 1.Open in a separate windowFig. 3(a) UV-vis spectra of 1 in powder form (red) and dissolved in DMF (blue). (b) DFT calculated HOMO and LUMO orbitals of the monomer. (c) DFT calculated HOMO and LUMO orbitals of the dimer.We performed time-dependent DFT (TD-DFT) calculations to identify possible optical transitions in 1. We did so by considering the electronic structure of a single Mo₂(INA)₄ cluster coordinated by a DMF molecule at both axial sites, and of two Mo₂(INA)₄ clusters having the same coordination environment as observed in the crystal structure of 1. Henceforth we will refer to the former as the monomer and the latter as the dimer. For the monomer, calculations identify a 2.44 eV (508 nm) transition primarily between a HOMO of Mo₂δ character and a ligand-based π* LUMO, which is nearly degenerate with several higher lying LUMO orbitals (Fig. 3b and S10^†). The character of this transition is typical of the metal–ligand charge transfer (MLCT) transitions reported in analogous transition-metal complexes. We note that the calculated HOMO–LUMO transition energy for the monomer is in reasonable agreement with the 470 nm feature observed in the absorption spectrum of the solution sample of 1 dissolved in DMF. On the basis of these data we can assign the aforementioned 470 nm feature to absorption by the isolated monomer.Turning our attention to the dimer, TD-DFT calculations identify a 2.19 eV (566 nm) transition primarily between a HOMO of Mo₂δ character and a ligand-based π* LUMO orbital (Fig. 3c and S11^†). We also note two higher energy MLCT transitions primarily from HOMO to LUMO+5 and from HOMO−1 to LUMO+1 with energies of 2.59 eV (479 nm) and 2.61 eV (475 nm), respectively (Fig. S11^†). Notably, while the two higher energy MLCT transitions involve LUMO orbitals located on the INA ligands perpendicular to the dimer axis, the lower energy MLCT transition at 2.19 eV involves a LUMO orbital of the INA ligand along the dimer axis.The calculated dimer transition energies cover much of the broad absorption envelope observed in the spectrum of the powder of 1. In particular, the low energy MLCT transition at 566 nm calculated for the dimer likely accounts for the significant absorption at wavelengths greater than 500 nm (Fig. 3a). These data suggest the dimer is a good simulant of the electronic properties of 1. This is a reasonable expectation given that 1 can be viewed, from a structural standpoint, as a linear polymer chain of the dimers. We can conclude that our 1D CP has an electronic structure comprised of dimer-like MLCT states arising from a lifting of the degeneracy of monomer orbitals due to their altered symmetry in the 1D chain.Our results show that the alternating coordination environment along the backbone of our 1D CP gives rise to a unique chain topology with a distinct electronic structure and function. Further evidence for the functionally distinct nature of 1 over the monomer is provided by X-band electron paramagnetic resonance (EPR) spectroscopy. An EPR spectrum collected from a powder sample of 1 at 20 K reveals an EPR active system with weak hyperfine structure owing to ⁹⁵Mo and ⁹⁷Mo isotopes, which have I = 5/2 nuclear spin (Fig. 4). This spectrum is qualitatively similar to spectra reported for EPR active Mo₂^V species in transition metal alkoxide-bridged dimolybdenum clusters.²³ This suggests the presence of Mo^II and Mo^III centers in 1 that may have formed during partial oxidation of the powder sample. It is anticipated that the hole is equally present on both Mo atoms as expected from the Class III Robin-Day classification of such systems. We simulated the experimental EPR spectrum by modelling our 1D CP system as a single axial paramagnet and obtained g values of g_‖ = 1.895 and g_⊥ = 1.935 and a perpendicular hyperfine coupling constant A_⊥ of 3.9 mT (Fig. 4 and S12^†).³⁷ The hyperfine coupling observed for our 1D CP is consistent with values reported for systems containing isolated Mo₂^V ions.²³ Moreover, the fact that a single paramagnet model was sufficient to simulate the EPR spectrum suggests that the two distinct coordination environments are fairly similar electronically. Though our EPR data provide evidence for charge localization³⁸ on Mo₂ centers, we cannot exclude the possibility for photo-induced intra-chain (or indeed inter-chain) charge transport arising from population of the low energy LUMO orbital on each bridging INA ligand.Open in a separate windowFig. 4Experimental EPR spectrum of 1 collected at 20 K (blue) and 9.44 GHz. In EasySpin, a fifth-order background correction was applied and the system was modeled as an axial paramagnet with g_‖ = 1.895 and g_⊥ = 1.935 (red).In summary, we have synthesized through a one-step ligand substitution reaction a new inorganic cluster comprised of a quadruply bonded dimolybdenum core coordinated by 4 equatorial isonicotinate ligands: Mo₂(INA)₄. Single-crystal XRD data show that recrystallization of this cluster from DMF yields a 1D molecular chain wherein the clusters alternately coordinate to one another through axial and equatorial binding sites (1). pXRD, FT-IR, and NMR data confirm the phase and compositional purity of this 1D CP in the bulk. UV-vis spectroscopy and DFT calculations reveal an electronic structure for 1 that is distinct from an individual solvated Mo₂(INA)₄ monomer owing to the alternating coordination environment present along the chain. This work establishes the synthetic foundation for assembly of CPs from discrete molecular clusters that have the potential to support through-framework charge transport due to mixed-valence mechanisms. We also envision utilizing the molecular building block discussed herein for the assembly of CPs with tailorable dimensionalities or hierarchies.     

Microwave-assisted synthesis of octahedral Rh nanocrystals and their performance for electrocatalytic oxidation of formic acid

Uniform and well-defined octahedral Rh nanocrystals were rapidly synthesized in a domestic microwave oven for only 140 s of irradiation by reducing Rh(acac)₃ with tetraethylene glycol (TEG) as both a solvent and a reducing agent in the presence of an appropriate amount of KI, didecyl dimethyl ammonium chloride (DDAC), ethylene diamine (EDA) and polyvinylpyrrolidone (PVP). KI, DDAC and EDA were essential for the creation of octahedral Rh nanocrystals. Electrochemical measurements showed a significantly enhanced electrocatalytic activity and stability for the as-prepared octahedral Rh nanocrystals compared with commercial Rh black.

Octahedral Rh nanocrystals were rapidly synthesized in a domestic microwave oven for only 140 s of irradiation by reducing Rh(acac)₃ with tetraethylene glycol as both a solvent and a reducing agent.

To date, platinum group metals play an indispensable role as efficient catalysts for some important reactions in industry. However, due to their limited reserves and high prices, a large number of platinum group metal nanoparticles with different particle sizes, morphologies and surface structures have been synthesized by means of various methods to reduce their cost.¹ As a platinum group metal, Rh has good catalytic activity and stability, and is often used as a typical catalyst for some chemical reactions such as hydrogenation,^2–7 nitrogen oxide reduction,⁸ CO oxidation,^9–11 cross coupling,^12–14 hydroformylation,^15–19 in fuel cells^20,21 and other chemical reactions.²² Therefore, controlled syntheses of Rh nanoparticles with different morphologies have attracted much attention. In recent years, people have successfully prepared Rh nanostructures with various morphologies such as sheet,^23–27 flower,⁶ polyhedron,^28–33 porous ball,⁸ multi branches,^34–39 stars,⁴⁰ nanoframes^13,14,41 and nano nail.⁴² These Rh nanoparticles with unique structures effectively improve the atom utilization as well as their catalytic reaction performances. However, similar to other platinum group metals, the difficulty of large-scale preparation of Rh nanomaterials with single morphology and uniform size still greatly restricts their industrial application.Microwave irradiation has been widely used in chemical synthesis because of its simple, rapid and efficient characteristics as well as special heating mode from the inner. We have synthesized many metallic nanoparticles with different shapes by using microwave irradiation for about 80 to 120 seconds. Herein, we report a simple and fast strategy for the synthesis of octahedral Rh nanocrystals under microwave irradiation with using domestic microwave oven. In a typical synthesis, octahedral Rh nanocrystals with uniform and well-defined morphologies were successfully synthesized with Rh(acac)₃ as the precursor, polyvinyl pyrrolidone (PVP) as the stabilizer, triethylene glycol (TEG) as both a solvent and a reducing agent in the presence of didecyl dimethyl ammonium chloride (DDAC), KI and ethylene diamine (EDA) under microwave irradiation in a very short time. Meanwhile, the electrocatalytical performance of the as-prepared octahedral Rh nanocrystals for the electro-oxidation of formic acid was also investigated with commercial Rh black as a contrast.The TEM and SEM images of the representative Rh nanoparticles obtained under the optimal experimental conditions are shown in Fig. 1, S1 and S2.^† Wherein, the prepared Rh nanoparticles demonstrated uniform and well-defined octahedral structure with sharp edges and corners as well as smooth surfaces (Fig. 1a and b), in which the average side length is about 65 nm. The high-resolution TEM (HRTEM) image (Fig. 1c) shows well-resolved continuous fringes clearly. The corresponding fast Fourier transform (FFT) pattern, as the inset shown in Fig. 1c, shows a lattice distance of 0.194 or 0.216 nm, which can be attributed to the {200} and {111} lattice planes of the octahedral Rh with face-centered cubic structure, respectively, confirming its single-crystal nature. Furthermore, the regular octahedral feature of the as-prepared Rh nanoparticles can be well distinguished from SEM images, as shown in Fig. 1d and S2.^† These results show that the octahedral Rh nanocrystals with a single morphology can be rapidly synthesized in a great quantity by irradiation with domestic microwave oven for only 140 s.Open in a separate windowFig. 1TEM and SEM images of the as-prepared octahedral Rh nanocrystals. (a) and (b) Typical TEM images with different scales. The inset in (b) is the schematic illustration; (c) typical HRTEM image. The inset is the corresponding FFT pattern; (d) SEM image. Fig. 2a shows the XRD pattern of the as-prepared typical octahedral Rh nanocrystals. As can be seen, the diffraction peaks at 2θ values of 41.26, 47.95, 70.18 and 84.33° are observed, which can be well indexed to the diffractions of (111), (200), (220) and (311) lattice facets of metallic Rh referring to the standard powder diffraction card (JCPDS card No. 05-0685), respectively. This observation further confirmed their fcc Rh structure. In addition, the narrow and sharp (111) diffraction peak implied that the typical octahedral Rh nanocrystals exhibited a high purity and crystallinity. The XPS spectrum was taken for the as-prepared octahedral Rh nanocrystals and the result was displayed in Fig. 2b, As it can be seen, two peaks corresponding to the electron binding energies of Rh 3d_3/2 and Rh 3d_5/2 were observed at 311.85 eV and 307.10 eV with an interval of 4.75 eV, respectively, which were consistent with the literature values (311.75 and 307.0 eV),⁴³ revealing Rh(0) metallic state of the octahedral nanocrystals.Open in a separate windowFig. 2XRD pattern (a) and XPS spectrogram (b) of octahedral Rh nanocrystals.The dependence of the morphological evolution of Rh nanocrystals upon irradiation time was investigated. When irradiated for 120 s, the octahedral structural Rh nanocrystals with about 65 nm of the side length produced except for unclear edges and corners as well as a shorter side length, as shown in Fig. 3a. As microwave irradiation progressed to 140 s, uniform and well-defined octahedral Rh nanocrystals with smooth surfaces generated (Fig. 3b). While the irradiation time was extended to 160 s, however, the vertices of some octahedral structures were truncated although with no change of the sizes, as shown in Fig. 3c. As the irradiation time reached 180 s, the octahedral structural feature of most particles disappeared with a further truncation of their vertices (Fig. 3d), which should be ascribed to higher surface free energies for the metallic atoms at the apexes and edges as well as a higher internal temperature due to a longer irradiation time. These results indicated that the optimum microwave irradiation time was 140 s for the creation of regular octahedral Rh nanocrystals.Open in a separate windowFig. 3TEM images of Rh nanoparticles prepared at different reaction time. (a) 120 s; (b) 140 s; (c) 160 s; (d) 180 s.It was noteworthy that KI played a crucial role in controlling synthesis of octahedral Rh nanocrystals. When no KI was used, it would produce irregular Rh nanoparticles, as shown in Fig. 4a. While with addition of 0.6 mmol of KI, octahedral Rh nanostructures with blunt vertices and an average side length of about 50 nm were generated (Fig. 4b), implying an incomplete growth relative to the case of 0.8 mmol of KI as in the typical experimental process (Fig. 1). Nevertheless, the amount of KI was increased to 1.2 mmol, only less octahedral structure features could be observed except for few obscure polyhedral outlines (Fig. 4c). These results indicated that the existence of KI was advantageous to the generation of octahedral Rh nanocrystals. Generally, the eight triangular surfaces of metallic Rh octahedron consists of (111) lattice planes. According to the previous report,^44–49 it can be considered that the preferential adsorption of I⁻ anions on Rh (111) planes is one of the main factors driving the formation of octahedral structure. As a result, a growth along 〈111〉 directions was confined and a growth along 〈100〉 directions was facilitated, which created octahedral structures due to anisotropic growth. However, excessive I⁻ ions would adsorb non-selectively on the surfaces of Rh nanoparticles, which resulted in passivation of the edges and corners of polyhedron. In addition, an equivalent amount of KBr or KCl was used instead of KI, respectively, to clarify the role of I⁻ ions under the same other conditions. As can be seen (Fig. S3, ESI^†), no octahedral Rh nanocrystal except for agglomerated irregular nanosheets was observed in these two contrast experiments. This may be ascribed to the change of the precursor. In the presence of a large number of I⁻ ions, the precursor can be transformed to a more stable [RhI₆]³⁻ complex.^44–47 As a result, the reducing rate of Rh(iii) to Rh atom decreased, which may be favourable for the nucleation of Rh nanoparticles and the oriented growth of Rh octahedra.Open in a separate windowFig. 4TEM images of Rh nanoparticles prepared with different amounts of DDAC or KI under the same other conditions. (a) Absence of KI; (b) 0.6 mmol of KI; (c) 1.2 mmol of KI; (d) absence of DDAC; (e) 0.2 mmol of DDAC; (f) 0.6 mmol of DDAC.Meanwhile, the influence of DDAC on the generation of octahedral Rh nanocrystals was also studied under the same other conditions. In the absence of DDAC, only agglomerated irregular Rh nanoparticles were observed (Fig. 4d). When 0.2 mmol of DDAC was added, octahedral Rh nanostructures with an average side length of about 45 nm, a smaller size relative to the case of 0.4 mmol of DDAC as in the typical experiments (Fig. 1), were generated accompanying with a few irregular nanoparticles (Fig. 4e). With increasing the amount of DDAC to 0.6 mmol, agglomerated irregular polyhedral nanostructures formed (Fig. 4f). Thus, the addition of DDAC was also indispensable for the growth of octahedral Rh nanostructures under microwave irradiation. Whereas an excessive amount of DDAC was also unfavourable for creation of the octahedral Rh nanocrystals. Moreover, no octahedral nanostructures generated except for urchin-like Rh hierarchical superstructures when adding an equivalent amount of cetyltrimethylammonium chloride (CTAC) instead of DDAC (Fig. S4a, ESI^†). While didoctyl dimethyl ammonium bromide (DDAB) was used instead of DDAC, the formation of octahedral Rh structures can be still observed although accompanying with other irregular polyhedral (Fig. S4b, ESI^†). These results suggested that the formation of octahedral Rh nanostructures were strongly dependent upon the hydrophobic chains of DDAC or DDAB but nothing to do with Cl⁻ or Br⁻ anions. The effect of other halide ions can be ignored due to the existence of a large number of I⁻ ions. That is because the strength of adsorption of I⁻ ions on metal surfaces is generally stronger than that of Cl⁻ or Br⁻ ions.⁴⁸Accordingly, the generation of octahedral Rh nanocrystals should be ascribed to the synergistic effect of KI and DDAC under the above experimental conditions. We believe that DDAC could enhance the role of I⁻ ions in generating (111) facets of octahedral by adjusting the adsorption selectivity of I⁻ ions on (111), (100) or (110) facets. On the one hand, the amount of KI would manipulate the reducing kinetics to form octahedral Rh nanostructures under microwave irradiation. A slow reducing rate was favourable for the oriented growth of Rh octahedra due to the formation of a more stable coordinated anion [RhI₆]³⁻. On the other hand, the confinement of DDAC induced the selective adsorption of I⁻ ions on Rh {111} facets which restrained the growth along 〈111〉 directions of Rh nuclei and prompted the growth along 〈100〉 directions. In addition, a proper quantity of DDAC confined the deposition of Rh atoms on {111} facets, which may be beneficial to the growth along 〈100〉 directions. However, an excessive amount of DDAC was unfavourable for the formation of shaped Rh nanoparticles since they disturbed the adsorption of I⁻ anions on Rh {111} facets.Furthermore, it was also found that ethylene diamine (EDA) demonstrated an important effect on the creation of octahedral Rh nanostructures. Under keeping the total volume of the reaction system unchanged, the significantly agglomerated irregular polyhedral nanoparticles with sharp horns were observed in absence of EDA (Fig. S5a^†). When 0.5 mL of EDA was added, a few octahedral nanostructures began to generate though accompanying with agglomerated irregular polyhedra (Fig. S5b^†). While the amount of EDA was increased to 1 mL, uniform and well-defined octahedral Rh nanocrystals with flat and smooth surfaces were produced (Fig. S5c^†). However, a more amount of EDA was added, a part of octahedral nanostructures become deformation as well as agglomeration (Fig. S5d^†). In the reaction system, TEG as a solvent was also served as a reducing agent. As can be seen, even though without adding EDA, the rhodium salt was still reduced completely to produce metal Rh nanoparticles. With the addition of EDA, octahedral Rh nanocrystals began to generate, while an excessive amount of EDA resulted in unclear edges and corners of the octahedral structures. Obviously, EDA demonstrated significant effect on the morphology control of octahedral Rh nanocrystals. It should be ascribed to the coordination adsorption of EDA on the surface of metal particles.⁵⁰ Furthermore, no octahedral nanostructures but irregular nanoparticles or Rh dendrites generated with using an equivalent amount of n-butylamine or n-octylamine instead of EDA (Fig. S6a and b^†). Therefore, we suggest that EDA plays a synergistic role together with DDAC in regulating the rate of atomic packing and nanoparticle growth by coordination adsorption. The growth rate of nanoparticles is faster in absence of EDA, while the growth rate slows down with the increase of EDA dosage. An appropriate amount of EDA facilitates the generation of uniform octahedral Rh nanocrystals by adjusting the balance between nucleation rate and growth rate. Nevertheless, excessive EDA makes a slower growth than nucleation due to their extreme adsorption, resulting in obscure appearances of some octahedral Rh nanoparticles.In addition, PVP was also found to be important but not essential for the formation of octahedral Rh nanocrystals. Either without or with a few amount of PVP, octahedral Rh nanocrystals can also produce except for a little agglomeration (Fig. S7a and b^†). An appropriate amount of PVP contributed to uniform and well dispersed octahedral Rh nanocrystals, while excessive PVP caused aggregation (Fig. S7c and d^†). These results indicated that PVP served mainly as a protecting and dispersing agent for the nanocrystals.The catalytic performance of the synthesized octahedral Rh nanocrystals was tested by cyclic voltammetry (CV) and chronoamperometry (CA) with the formic acid electrooxidation reaction as the model reaction system. Fig. 5a exhibits the representative CV curves obtained for the electrochemical oxidation of 0.5 mol L⁻¹ HCOOH over the octahedral Rh nanocrystals and commercial Rh black in 0.5 mol L⁻¹ HClO₄ solution, respectively. CV measurements showed the peak current density for the octahedral Rh nanocrystals was 3.53 mA cm⁻² at 0.544 V, while it was 1.01 mA cm⁻² at 0.609 V for Rh black. The formic acid electrooxidation indicated that the electrocatalytic activity of octahedral Rh nanocrystals was about 3.5 times that of Rh black, demonstrating an obvious morphological dependence for their electrochemical property. The corresponding CA curves of formic acid electro-oxidation at 0.55 V is shown in Fig. 5b. As can be seen, a higher current retention through the whole measuring range were observed over the as-prepared octahedral Rh nanocrystals than Rh black though both of them showed an equivalent attenuation rate in the initial 20 seconds. The CV curve of continuous cycle scanning for octahedral Rh nanocrystals in 0.5 mol L⁻¹ HClO₄ solution showed a decrease of the electrochemical activity only by 9.6% after 2000 cycles (Fig. S8^†). These results reveal that octahedral Rh nanocrystals exhibit a remarkably enhanced electrochemical activity and stability compared with Rh black. Their enhanced catalytic activity should be attributed to the uniform geometric structure with single surface lattice.Open in a separate windowFig. 5The CV (a) and CA (b) curves for the electrochemical oxidation of 0.5 mol L⁻¹ HCOOH over the octahedral Rh nanocrystals and Rh black in 0.5 mol L⁻¹ HClO₄ solution, respectively.Additionally, CO stripping voltammetry measurements were performed. As shown in Fig. S9a,^† no CO electro-oxidation (CO_ox) was observed for the freshly-prepared octahedral Rh nanocrystals in 0.5 M HClO₄ solution. Subsequently, a current peak for CO_ox appeared at 0.550 V (versus SCE) after adorbing CO for the clean octahedral Rh-modified electrode, as shown in Fig. S9b.^† Then CO_ox peak disappeared in the following second potential scanning, as shown in Fig. S9c.^† These results showed that CO adsorbed on Rh surfaces can be easily removed in the process of electrocatalytic oxidation, showing well CO resistence.In summary, uniform octahedral Rh nanocrystals could be rapidly prepared with domestic microwave oven in only 140 s of irradiation by reducing Rh(acac)₃ with TEG as both a solvent and a reducing agent, PVP as a protecting and dispersing agent in the presence of proper quantities of DDAC, KI and EDA. The formation of octahedral Rh nanocrystals was attributed to the synergism of KI, DDAC and EDA. The electrochemical oxidation of formic acid demonstrated higher electrocatalytic activity and stability for the as-prepared octahedral Rh nanocrystals than Rh black, displaying a significant dependence upon their morphologies.     

Viscosity effect on the strategic kinetic overgrowth of molecular crystals in various morphologies: concave and octapod fullerene crystals

A kinetic overgrowth allowing organic molecular crystals in various morphologies is induced by temperature-dependent viscosity change of crystallization solution. By this strategy, concave cube and octapod fullerene C₇₀ crystals were successfully obtained by antisolvent crystallization (ASC). The structural analysis of fullerene C₇₀ crystals indicates that the morphological difference is the result of kinetic processes, which reveals that viscosity, the only variable that can change dynamics of solutes, has a significant influence on determining the morphology of crystals. The effect of solvent viscosity in the stage of crystal growth was investigated through time-dependent control experiments, which led to the proposal of a diffusion rate-based mechanism. Our findings suggest morphology control of organic crystals by diffusion rate control, which is scarcely known compared to inorganic crystals. This strategic method will promote the morphology controls of various organic molecular crystals, and boost the morphology–property relationship study.

A kinetic overgrowth allowing organic molecular crystals in various morphologies is induced by temperature-dependent viscosity change of crystallization solution.

Because morphology directly affects the properties of crystals, morphology control of crystals has been one of the major research subjects in chemistry. While the strategies for the morphology control of inorganic metal crystals have been established quite well in the solution phase, primarily through surface chemistry guiding crystallization to occur only on non-passivated crystal planes or through kinetic overgrowth,^1–3 the absence of such strategic methods controlling the morphology of organic molecular crystals restricts their potential. Thus, enormous efforts have been devoted to exploring new approaches to control the morphology of molecular crystals through solvent^4,5 and temperature controls.^6,7As frequently demonstrated from inorganic metal crystals, kinetic overgrowth can be a prominent method to obtain molecular crystals in various shapes. The kinetic overgrowth induces preferred growth at specific sites resulting in morphologies that are not available thermodynamically, as demonstrated from concave cubes and octapods.^8–13 The kinetic overgrowth usually occurs with a concentration gradient around the seed crystals, which can be achieved by regulating the diffusion rate (V_diff) of solutes and crystal growth rate (V_grow). Therefore, it is important to understand and develop kinetic overgrowth process for organic molecular crystals, which can correspond to the well-established surface chemistry for inorganic metal crystals.Considering the importance of concentration gradient near seed crystals for kinetic overgrowth, among various crystallization methods, antisolvent crystallization (ASC) process in which supersaturation and nucleation are induced by the injection of antisolvent to which target molecules have a low solubility (Scheme 1),^14–18 is an ideal method for kinetic overgrowth of molecular crystals since it has many variables that can change micro-environment near seed crystals. In addition, fullerenes are good target molecules because obvious results are expected when kinetic product is successfully contrasted to well-known thermodynamic morphologies such as tubes, rods, and even polyhedrons,^19–23 as Yang et al. and Ariga et al. showed.^24–26 In this case, kinetically favorable fullerene concave cubes could be formed by involving sonication²⁴ and controlling solvent ratio,^25,26 and clearly contrasted with fullerene cube crystals. However, the origin and detailed mechanism of these kinetic overgrowths are still veiled, which prevents further application.Open in a separate windowScheme 1Schematic illustration of traditional ASC process. Target molecules are effectively crystallized via solvation shell mechanism.In this regard, the influence of temperature on kinetic overgrowth of organic molecular crystals should be investigated not only because of its contribution for thermodynamic versus kinetic reaction control in many branches of chemistry,^27–29 but also because it can alter the behavior of molecules significantly during ASC, especially by regulating solution viscosity. Therefore, in this study, we aim to investigate the effect of viscosity upon temperature change for kinetic overgrowth of fullerene crystals. Herein, we show that kinetic overgrowth can be induced by controlling temperature in ASC. Kinetically overgrown fullerene C₇₀ crystals, in concave cube and octapod shapes, are successfully obtained at low temperature, while only cube crystals are obtained at higher temperature. These results originate from increased solution viscosity, which causes slow diffusion of C₇₀ molecules to seed crystals, and consequent kinetic overgrowth. Diffusion rate-based mechanism of inorganic metal nanocrystals is successfully applied to this strategic morphology control.All the fullerene crystallization has been performed by ASC method. Isopropanol (IPA), an antisolvent, is added to C₇₀ solution in mesitylene to obtain cube-shaped C₇₀ crystals at room temperature. After 3 h, black precipitates have been separated from the solution by filtration for characterization. Well-defined faces, edges, and vertices of C₇₀ cubes are confirmed using a scanning electron microscope (SEM), which agrees well with previous report (Fig. 1a).¹⁹ To investigate the effect of growth temperature on the morphology, ASC of C₇₀ has been performed at lower temperature. When the growth temperature is lowered from RT to −16 °C using a refrigerator and −78 °C using dry ice bath, concave cube-shaped and unprecedented octapod C₇₀ crystals are obtained, respectively (Fig. 1b and c). The average sizes of the C₇₀ cube, concave cube, and octapod are 2.0 μm, 2.5 μm, and 1.7 μm, respectively (Fig. S1^†). The resulting crystals show a high degree of homogeneity in their morhpologies.Open in a separate windowFig. 1C₇₀ crystals prepared by ASC at different conditions. (a) Cubes at 25 °C, (b) concave cubes at −16 °C, and (c) octapods at −78 °C. (d) Crystallization condition and obtained morphology of product (scale bar: 2 μm).Because these morphologies are well-known kinetically overgrown products for inorganic metal nanocrystals,^8–13 we have checked the viscosity of crystallization solution at each temperature, which is directly related to V_diff of C₇₀ molecules by the Stokes–Einstein equation.³⁰ The apparent viscosity of the solution at the shear rate of 1000 Hz increases from 2.05 cp to 6.40 cp at −16 °C and further to 218.36 cp at −78 °C (Fig. 1d), which indicates dramatic decrease of V_diff of C₇₀ molecules may occur. From these results, it can be assumed that this morphology difference comes from the slow diffusion of C₇₀ molecules, which is induced by high viscosity at low temperature.The crystal structures of each product have been examined by powder X-ray diffraction (Fig. 2a). For C₇₀ cube crystals, the overall diffraction pattern including the intense peak from (100) plane with d-spacing of 10.56 Å indicates a simple cubic structure as known from previous results.¹⁹ Importantly, the XRD patterns of C₇₀ concave cube and octapod crystals are the same as cube crystals, which implies that the concave cube and octapod morphologies are the results of kinetic crystal overgrowth from cube crystals.²⁵Open in a separate windowFig. 2(a) X-ray diffraction patterns and (b) Fourier transform infrared spectra of C₇₀ cubes (black), concave cubes (blue), octapods (red), and pristine powder (green).Furthermore, to examine the effect of solvent inclusion on the crystal morphologies,²² the intercalated molecules have been analyzed using Fourier-transform infrared spectroscopy (Fig. 2b). Four representative peaks of mesitylene (2717, 2841, 2901, 3019 cm⁻¹) are observed equally from all three types of C₇₀ crystals.²⁵ This result suggests that the intercalation of solvent is irrelevant to the morphology decision. The influence of intercalated mesitylene on the rotational motion of fullerene is also investigated by Raman spectroscopy, and no peak shift also indicates that the intercalated mesitylene does not affect the motion of fullerene molecules in crystal lattice (Fig. S2^†).²⁵ Therefore, we conclude that morphological diversity of C₇₀ crystals is not originated from common structural difference, but from the crystal overgrowth induced by diffusion rate change induced by viscosity change.The overall mechanism of morphology control during ASC via viscosity control can be proposed as follows (Fig. 3). When antisolvent is injected into C₇₀ solution of mesitylene, emulsion droplets containing C₇₀ and mesitylene are formed instantaneously, followed by the diffusion of mesitylene out to continuous phase consisted of antisolvent (Fig. 3a).³¹ As a result, supersaturation and nucleation occur, and seed crystals are formed. To examine if the morphology decision is made at the nucleation stage or growth stage, the seed crystals obtained at 25 °C, −16 °C, and −78 °C are identified by SEM to confirm their cube-shaped morphology (Fig. 3b–d and Fig. S3^†). Therefore, the morphological difference must be induced at the crystal growth step, rather than the nucleation step, which is also on the line of seed-mediated kinetic overgrowth mechanism.Open in a separate windowFig. 3(a) Schematic illustration of temperature-independent nucleation of C₇₀ immediately after the injection of antisolvent, and the SEM images of seed crystals prepared at (b) 25 °C, (c) −16 °C, and (d) −78 °C (scale bar: 500 nm). (e) Schematic illustration of diffusion rate-dependent growth from cubic seed crystals.After nucleation, the reaction system goes through metastable states, where crystals continue to grow and kinetic overgrowth starts to play. All the mesitylene emulsion droplets are broken and release C₇₀ nucleates. Then, C₇₀ molecules remained in the continuous phase at the stage of nucleation are attached to the nucleates. In this stage, the competition between V_diff of C₇₀ molecules and V_grow plays a key role in the determination of crystal morphology, where V_diff is controlled effectively by viscosity (Fig. 3e). At 25 °C, the viscosity of solution is quite small, so C₇₀ molecules can easily diffuse from the bulk solution to the seed crystals (V_diff ≫ V_grow). In this condition, the concentration of C₇₀ around seed crystals is maintained almost same, hence no preferential overgrowth occurs, resulting in seed crystal morphology-retained cube crystals.¹⁹ Whereas, C₇₀ molecules cannot diffuse rapidly at −78 °C due to the high viscosity of the solution (V_diff ≪ V_grow). In this regime, the concentration gradient of solute around seed crystals is generated because of the fast consumption of C₇₀ molecules near seed crystals with slow refill from the bulk solution. Such a concentration gradient induces preferential attachment of distant solutes to the sites possessing the highest reactivity, vertices for cubic crystals, as known for the inorganic metal crystals.^8–13 Eventually, octapod-shaped C₇₀ crystals are formed. The kinetic overgrowth that frequently results in anisotropic crystal growth at specific sites^32–34 rather than thermodynamic isotropic growth resulting in simply bigger crystals was supported from time-dependent SEM images of C₇₀ crystals at −78 °C (Fig. 4) showing petals that are preferentially and continuously grown out of cube crystals. In contrast, an isotropic growth only with a size increase from cubic seed has been observed in the case of the growth at 25 °C (Fig. S4 and S5^†), which indicates thermodynamic crystal growth at low viscosity.Open in a separate windowFig. 4Time-dependent SEM images of C₇₀ octapod crystals obtained at −78 °C. Growth time for each image is (a) 0 min, (b) 1 min, (c) 5 min, and (d) 30 min, respectively (scale bar: 1 μm).To verify if this morphology control is directly related with viscosity change rather than temperature change itself, control experiments using less viscous solvent at the same temperature have been conducted, finding no morphology changes. When acetone is used as antisolvent instead of IPA, no morphological change is observed even at the growth temperature of −78 °C, (Fig. 5 and S6^†) and this result owes to the low viscosity of acetone even at low temperature.³⁵ In other words, the addition of acetone to mesitylene solution does not cause a dramatic viscosity change, which implies the importance of the selection of good solvent and antisolvent for the successful morphology control by ASC process. On the other hand, the use of other alcohols (ethanol, 1-propanol, and 1-butanol having viscosity of 5.26, 6.52, and 7.53 cp at −16 °C and 30.0, 79.8, and 132.0 cp at −78 °C, respectively) show clear kinetic overgrowth at low temperature (Fig. S7^†). Other than the viscosity change inducing morphology control, there is another important property change, i.e. solubility change upon temperature change (Fig. S8^†) to be considered, which requires further studies in the future.Open in a separate windowFig. 5C₇₀ cubes prepared using acetone as antisolvent at (a) 25 °C, (b) −16 °C, and (c) −78 °C (scale bar: 2 μm).     

Ni(ii)-based coordination polymers for efficient electrocatalytic oxygen evolution reaction

The exploration of highly efficient, stable and cheap water oxidation electrocatalysts using earth-abundant elements is still a great challenge. Herein, alkaline-stable cationic Ni(ii) coordination polymers (Ni-CPs) were successfully obtained under hydrothermal conditions, which could stabilize the incorporation of Fe(iii) to form Fe-immobilized Fe@Ni-CPs. The newly developed Ni-based CPs were used for the first time as an effective electrocatalyst for the oxygen evolution reaction in strong alkaline media.

An alkaline-stable cationic Ni(ii) coordination polymer showed remarkable oxygen evolution reaction (OER) catalytic activity due to capturing Fe ions.

The oxygen evolution reaction (OER) plays a vital role in energy storage and conversion applications due to energy issues and the need for sustainable development.^1–4 Because of the sluggish kinetics of the OER, excellent electrocatalysts are required to work in acidic or strong alkaline environments.^5,6 Noble metal-based catalysts like IrO₂ and RuO₂ with high efficiency showed excellent OER catalytic activity.⁷ However, noble metal with high-cost and scarcity are impractical for scale-up applications. Currently, to substitute these precious metal-based materials, transition-metal-based (Fe, Co, Ni and so on) and metal-free (e.g., N, P and S) hybrid materials have been extensively developed.^8–10 For example, metal–organic frameworks (MOFs) such as ZIF-8/67 derived metal–carbon composite materials exhibit promising electrocatalytic performance.^11,12 Unfortunately, the pyrolysis process destroys the framework completely and causes agglomeration of metals, resulting in a decreased number of active sites. Therefore, to explore highly efficient and low-cost OER catalysts that can be directly used in the OER without calcination are desired, including complexes and MOFs.Recently, much of transition bimetallic materials showed excellent electrocatalytic activity.^13–15 However, a handful of examples such as Fe–Co-MOFs or Co–Ni-MOFs have been explored due to instability, poor conductivity and harsh synthesis conditions.^16–20 Notably, the disadvantages of MOFs have limited their usage in the potential OER. Therefore, it is urgent to develop cheap, stable and active OER catalysts to replace the precious metals. However, this is still a great challenge. Coordination polymers (CPs) with a low-dimensional framework similar to MOFs are constructed by metal ion and organic ligands with potential active sites and functional groups, exhibiting wide applications in sensing, photoluminescence and photocatalysis.^21–26 However, rare examples of CPs have been directly explored in the OER. For Fe/Ni-based bimetal electrocatalysts, a novel strategy involves doping Fe(iii) into a functional Ni-based CPs, which could enhance the electrocatalytic OER activities.Herein, we report the hydrothermal synthesis of a Ni-based CPs as a high-performance OER electrocatalyst in strong alkaline solutions (Scheme 1). The blue crystals of [Ni(bp)₃·(H₂O)₂]·(bp)·(ClO₄)₂ (bp = 4,4′-biprydine) were obtained upon the reaction of bp ligands with NiClO₄·6H₂O under hydrothermal systems. Chemical stability tests displayed that Ni-CPs could retain their original framework in water or even a strong alkaline (pH = 14) solution after 12 hours (Fig. S1^†), which is rarely reported for most transition metal CPs. The thermogravimetric analysis (TGA) showed that there is a significant change at about 110 °C due to the weight loss of the partial guest (Fig. S2^†). Interestingly, the cationic Ni-CPs successfully captured the Mohr''s salt (ammonium iron(ii) sulfate) by taking advantage of the post-synthetic strategy. The obtained Fe@Ni-CPs exhibited a high-efficient OER activity under strong alkaline conditions.Open in a separate windowScheme 1Illustration of the synthesis process for Fe doped Ni coordination polymers for the OER.Single-crystal X-ray diffraction analysis revealed that the Ni-CPs crystallized in the C2/c space group (Table S1^†). The obtained Ni coordination polymer was the isostructural compound reported by Talham,²⁷ but their packing modes were distinctly different (Fig. S3–S5^†). It also had a similar coordination environment to the railroad-like double chains synthesized by Yaghi.²⁸ The parallel chains were occupied by 4,4′-bpy, perchlorate and water molecules. In the Ni-CPs, most of the phenyl rings adopted the face-to-face mode. There were evident π⋯π interactions between the adjacent 4,4′-biprydine (Fig. 1a). In addition, there were strong intermolecular hydrogen bonds between 4,4′-bpy and perchlorate (strong Cl–O⋯C amongst adjacent layers) (Fig. 1a). The weak reaction increased the high density of the framework and protected the coordination bonds against external guest attacking. These chains and guests were further packed with a three-dimensional structure along the c-axis (Fig. 1b).Open in a separate windowFig. 1(a) The weak reaction between chains in Ni CPs, showing the Cl–O⋯C (green and black dashed line) and π⋯π interactions (pink dashed line) between the adjacent perchlorate and 4,4′-biprydine, respectively; (b) the packing view of Ni CPs along the c-axis. Cl in green, Ni in pale blue, N in blue, O in red and C in black. H atoms and partial guest molecules were omitted for clarity.The unique cationic Ni-CPs framework has the potential to immobilize some counterpart ions. To demonstrate this, Mohr''s salts were investigated. Most strikingly, the color slowly changed from blue to green in an aqueous solution, given by the optical image (Fig. 2a and b), which not only indicated that Ni-CPs captured Mohr''s salts via ion-exchange, but also suggested an alteration in the valence of Fe ions. It was possible that Fe²⁺ may have been further oxidized to Fe³⁺ under the O₂ and water environment when we prepared the Fe@Ni-CPs (4Fe²⁺ + 2H₂O + O₂ = 4Fe³⁺ + 4OH⁻). The PXRD pattern showed that the frameworks remained unchanged after doping with Fe ions (Fig. S1^†). From the transmission electron microscopy (TEM) images (Fig. 2c), after immobilization, the morphology of the Fe@Ni-CPs was still level and smooth; no ring-like patterns arose corresponding to the selected area for electron diffraction (SAED) (Fig. 2d), indicating that no bulk Fe particles formed during ion-exchange. This was further demonstrated using high resolution TEM (HRTEM) (Fig. 2e), in which there was no lattice fringe of crystallized Fe. The well distribution of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs was demonstrated by elemental mapping (Fig. 2f–l). Energy-dispersive X-ray spectroscopy (EDX) also agreed well with the above mapping data (Fig. S6^†). In addition, the Fe³⁺ uptake was 4.1 wt%, as determined by inductively coupled plasma atomic emission spectroscopy (ICP). These results showed Fe ions to have been successfully immobilized by the Ni-CPs.Open in a separate windowFig. 2(a–b) The optical image of Ni-CPs and Fe@Ni-CPs; (c) TEM images of Fe@ Ni-CPs; (d–e) the corresponding SAED and HRTEM pattern. (f–l) Element mapping of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs.The X-ray photoelectron spectroscopy (XPS) survey spectrum of the Fe@Ni-CPs also showed the presence of C, N, O, Cl, Fe, and Ni elements (Fig. 3a). The Fe 2p high resolution XPS spectrum exhibited peaks at 725 eV and 711 eV (Fig. 3b), further indicating the presence of the Fe³⁺ oxidation state. This could be explained by the transformations of Fe²⁺ to Fe³⁺ during the ion-exchange process. Similarly, in the Ni 2p spectra (Fig. 3c), two main peaks located at 855.8 eV and 873.5 eV can be ascribed to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively. These peaks are associated with two shakeup satellite peaks, indicating that Ni still remained in a divalent state. The Cl 2p and N 1s region could be corresponded to the ClO₄⁻ and biprydine, respectively (Fig. S7^†). The O 1s spectrum (Fig. 3d) was divided into two peaks at 531.7 eV and 533.2 eV, which could be assigned to the OH group from filled H₂O molecules and partial ClO₄⁻, respectively.Open in a separate windowFig. 3(a) XPS survey spectrum of the Ni-CPs; XPS spectra of the Ni-CPs in the (b) Fe 2p, (c) Ni 2p, and (d) O 1s regions.The above Ni-CPs with Fe doping encouraged us to investigate its electrocatalytic application in oxygen evolution reaction. To study the electrocatalytic activity of Fe@Ni-CPs for the OER, linear sweep voltammetry (LSV) was performed in a strong alkaline solution (pH = 14) for Fe@Ni-CPs@GC (fresh samples coated on glassy carbon electrode with Nafion binder). Fe@Ni-CPs@GC directly acted as working electrodes and showed good OER activity with an onset potential of 1.52 V (Fig. 4a), overpotential of 368 mV at 10 mA cm⁻², and a Tafel slope of 59.3 mV dec⁻¹ (Fig. 4b). These OER performances are close to some reported MOFs catalysts (Table S2^†) and even better than commercial benchmark OER catalysts like RuO₂ working at the same condition (Fig. S8^†). In contrast, the electrocatalytic OER activities of the pristine Ni-CPs@GC without Fe incorporation displayed much worse activity. The onset potential, overpotential (at 10 mA cm⁻²), and the Tafel slope reached 1.62 V, 458 mV, and 96.8 mV dec⁻¹, respectively (Fig. 4a and b). In addition, Fe@Ni-CPs showed a strong durability during the OER process. The chronoamperometric response of Fe@Ni-CPs displayed a slight anodic current attenuation within 12 h due to the peeling of samples during the evolution of a large amount of O₂ gas (Fig. S9^†). Furthermore, LSV of Fe@Ni-CPs showed negligible changes after OER tests for 12 h (Fig. S10^†). These results indicate that the Fe-doped Ni-CPs with more active sites could serve as an excellent candidate for OER in strong alkaline conditions.Open in a separate windowFig. 4(a) OER polarization curves and (b) Tafel plots of various electrocatalysts in a 1 M KOH aqueous solution; (c) linear relationship of the current density at 1.1 V (vs. RHE) vs. scan rates for Fe@Ni-CPs@GC and Ni-CPs@GC; (d) EIS of Fe@Ni-CPs and Ni-CPs electrode.When Fe(iii) was introduced, the resulting Fe@Ni-CPs catalysts greatly improved OER catalytic performance. There were dynamic collisions between Fe³⁺ ions and Ni-CPs, which allowed for more accessible catalytic active sites compared to the Ni-CPs. Particularly, Fe(iii) doping can contribute to the adsorption and reaction of OH⁻ groups in OER process.²⁹ As a result, Fe@Ni-CPs enhanced charge transfer under an apt electronic environment of the mixed Fe⋯Ni systems. In addition, electrochemical impedance spectrum and double-layer capacitance (C_dl) of the Fe@Ni-CP were also studied. The C_dl of Fe@Ni-CPs was confirmed to be 269.7 μF cm⁻² (Fig. 4c and S11^†), which is higher than that of Ni-CPs (C_dl = 174.2 μF cm⁻²) (Fig. 4c and S12^†). The semicircular diameter in EIS of Fe@Ni-PCP was smaller than that of Ni-CPS (Fig. 4d). These results further showed that Fe@Ni-CPs were more effective in enlarging the catalytically active surface area, conductivity and synergistic effects between Fe and Ni in comparison to Ni-CPs coated on electrodes.In conclusion, a new alkaline-stable cationic Ni(ii) coordinated polymers was synthesized under hydrothermal conditions. The Ni CPs could quickly interact with Mohr''s salt. Interestingly, the Ni CPs could act as a unique oxidation matrix to realize the transformation of Fe²⁺ to Fe³⁺ during the ion-exchange process. Furthermore, the resulting Fe@Ni-CPs electrode, for the first time, showed an excellent electrocatalytic activity for OER in strong alkaline media. This study provides a new avenue to explore stable coordinated polymers by incorporating the low-cost and high-activity transition metal, Fe, which will substitute the rare noble metals used in energy-related research.