Electrooxidation of Pd–Cu NP loaded porous carbon derived from a Cu-MOF

The development of new non-platinum catalysts for alcohol electrooxidation is of utmost importance. In this work, a bimetallic Pd–Cu loaded porous carbon material was first synthesized from a Cu-based metal–organic framework (MOF). The Cu loaded porous carbon was pre-synthesized through calcinating the Cu-based MOF under a N₂ atmosphere. After loading Pd onto the precursor and heating, Pd–Cu loaded porous carbon (Pd–Cu/C) was obtained for alcohol electrooxidation. Electrooxidation experiments revealed that this Pd–Cu bimetal loaded porous carbon assisted steady state electrolysis for alcohol oxidation in alkaline media. Moreover, different alcohols were electrooxidated using the present electrocatalyst for the purposes of discussing the oxidation mechanism. This electrooxidation study of Pd–Cu/C derived from a MOF demonstrates a good understanding of the electrooxidation of different alcohols, and provides useful guidance for developing new electrocatalyst materials for energy conversion and electronic devices.

We have synthesized Pd–Cu NP loaded porous carbon through the direct carbonization of a porous Cu based MOF for efficient electrooxidation.

There is an immediate need to develop direct alcohol fuel cells (DAFCs), which have been proven to be a fine source of energy, which could probably replace fossil fuels use to fulfil global energy demand. As one of the most significant electrocatalytic procedures, the electrooxidation of alcohols is an important process in DAFCs, and has gathered much attention and is attractive, due to high power density output and low pollutant emissions. Generally, Pt based materials are the most common electrocatalysts for alcohol electrooxidation reactions. However, the high cost and limited supply of Pt severely restricts its commercial application. Therefore, the development of new efficient and inexpensive non-platinum alternative materials to Pt-based catalysts is of utmost importance.Metal–organic frameworks (MOF) are assembled from metal ions linked by organic ligands, and are used in catalysis, guest molecule storage/separation, fluorescence, sensors and other devices.^1–3 Due to their highly ordered porous structures and large surface areas, MOFs can also be used as templates/precursors for preparing porous carbon materials through thermal treatment.^4–8 Several MOF derived carbon materials with good electrical conductivity are reported to show effective electrocatalytic performance,⁹ such as in the oxygen evolution reaction (OER),¹⁰ hydrogen evolution reaction (HER),¹¹ and oxygen reduction reaction (ORR).¹² Recently, a zeolitic imidazolate framework ZIF-8 was calcinated in order to prepare porous carbon with both micro- and meso-pores to support Pd electrocatalysts for methanol electrooxidation.¹³ Unfortunately, this could not efficiently limit the use of the noble metal, which gives challenges to scientists for further exploration.It has been demonstrated that the alloying of noble metals with transition metals has been used for the enhancement of catalytic activity and reduction of cost,¹⁴ because of the low cost and relatively high abundance of transition metals. The use of alloyed metal, Pd–M (where M is Cu, Co or Ni), binary electrocatalysts has been reported for effectively improving the catalytic properties.^15–17 For this purpose, introducing a non-noble metal into a noble metal bimetallic system will fulfil the demand for a new catalyst and become an area of interest nowadays. The second metal (such as Cu) will behave as a donor, while Pd has an empty d orbital to accept electrons, and it is assumed that its electronic properties will be more similar to Pt.¹⁸ Therefore, Pd–Cu bimetal loaded porous carbon derived from a Cu-MOF can provide a good electrocatalyst for alcohol oxidation.In this work, as shown in Scheme 1, we have synthesized Cu loaded porous carbon through the direct carbonization of a porous Cu based MOF. After loading Pd onto the precursor and heating, Pd–Cu bimetal loaded porous carbon (Pd–Cu/C) was obtained. Chronoamperometric studies revealed that this Pd–Cu bimetal loaded porous carbon assisted steady state electrolysis for alcohol oxidation in alkaline media. In addition, alcohols with different numbers of carbon atoms (such as ethanol, 1-propanol and 2-propanol) were also investigated for electrooxidation. It is important to note that the effectiveness of this bimetallic NP loaded carbon means that it can serve as a catalyst for the electrooxidation of low-molecular weight alcohols, which probably can be used as energy sources in portable electronic devices.Open in a separate windowScheme 1The preparation procedure for Pd–Cu/C derived from HKUST-1 and PdCl₂: (a) MOF HKUST-1; (b) Cu/C calcinated from HKUST-1; (c) PdCl₂ loaded on Cu/C; and (d) the Pd–Cu/C material.Here, a 3-D MOF, HKUST-1 (also called Cu₃(BTC)₂, BTC = 1,3,5-benzenetricarboxylate), was chosen as the precursor for preparing the Cu/C material, due to the structure having high porosity and it being a rich Cu source. The as-synthesized HKUST-1 was calcinated at 700 °C for 5 h under a N₂ atmosphere, and the Cu/C material was obtained. In addition, the guest species PdCl₂ was loaded onto the calcinated HKUST-1 through immersing the pre-calcinated HKUST-1 into a PdCl₂ ethanolic solution (1 mM) for 2 h (Scheme 1). The PdCl₂ loaded Cu/C (PdCl₂@Cu/C) was heated at 300 °C for 1 h under a N₂ atmosphere. Finally, an alloy of Pd and Cu loaded porous carbon material (Pd–Cu/C) was obtained and characterized through powder XRD, BET and XPS analyses.The PXRD data (Fig. 1a) from as-synthesized HKUST-1 powder and the bimetallic Pd–Cu NP loaded carbon porous material derived from HKUST-1 show that the samples contain bimetallic palladium and copper mostly. The XRD peak appearing at 43.3° corresponds to the (fcc) (111) facet plane of Cu. Due to Pd being dispersed homogenously at a low concentration through the sample, the XRD pattern could not display the obvious peak from Pd. However, inductively coupled plasma emission spectroscopy (ICP) data (Table S1^†) from the sample showed 0.76% Pd and 36.68% Cu, indicating the existence of Pd and Cu. The porosity of Pd–Cu/C was demonstrated through BET data, which shows N₂ adsorption of ∼150 cm³ g⁻¹. The Pd XPS spectrum showed two definite peaks at 335.5 and 341 eV, respectively assigned to 3d_5/2 and 3d_3/2 and matching well with Pd⁰. XPS peaks at 932.4 and 952.1 eV indicate the valence states of Cu ions in the Cu 2p_3/2 and Cu 2p_1/2 orbitals in the Pd–Cu/C material. Cu²⁺ is present in the porous carbon material, with respective peaks at 933.7 eV and 934.4 eV from CuO and Cu(OH)₂, with a prominent satellite observed in the 938–946 eV range. A few Pd²⁺ ions also exist in the sample due the easy oxidation of the surface. The Raman spectrum of Pd–Cu/C (Fig. S4^†) shows typical graphitic carbon. The results of the characterization studies clearly reveal that the nanoparticles have a Pd and Cu bimetallic nature.Open in a separate windowFig. 1(a) XRD data from HKUST-1 and Pd–Cu/C; (b) N₂ sorption isotherms for Pd–Cu/C; and XPS data from (c) Pd and (d) Cu in a sample of Pd–Cu/C.SEM images with EDS (Fig. 2a and b) results show that the sample contained much more copper than palladium, which clearly suggests that the presence of copper in the sample would probably be the reason for the expected electrooxidation of alcohols. It could be possible to replace the use of high-cost Pd or Pt based catalysts. The morphology of the Pd–Cu NPs was further characterized via TEM imaging and TEM element mapping (Fig. 2c, d and S5^†), demonstrating that the nano-sized NPs were dispersed homogeneously. The HR-TEM image in Fig. 2c gives insight into the bimetallic nature of the synthesized nanoparticles, with two noticeable lattice fringes (0.225 nm for Pd(111) and 0.202 nm for Cu(111)). The mean size of the Pd–Cu NPs was 7.38 nm, as shown in Fig. 2d. The homogenous distribution, with well-defined bimetallic Pd–Cu based carbon material, was good for the electrooxidation of alcohols. The electrochemical active surface area (ECSA) for Pd–Cu/C was high compared with commercial Pd/C, which suggested that the synthesized Pd–Cu/C has ample available surface area, mainly because of synergistic effects from the Cu-MOF based carbon material and the morphology of the electrocatalyst.Open in a separate windowFig. 2(a) SEM image of and (b) EDS data from Pd–Cu/C; (c) a TEM image of Pd–Cu NPs in the hybrid carbon material; and (d) the size distribution of the Pd–Cu NPs.In Fig. 3, CV profiles for commercial Pd/C and the presented Pd–Cu/C show two distinct peaks (forward (iF) and backward (iB) peaks) during the oxidation of methanol-containing 1 M KOH solution. The peak at −0.37 V indicates the oxidation of aforementioned carbonaceous species, such as Pd–CO_ads, along with newly formed alcohol adsorbates, following the removal of surface intermediates at lower potentials.¹⁹ For the forward peak potential, a shift in the iF value is observed, mainly because of Cu existing with Pd in the material. This results in the oxidation of poisonous species, such as Pd adsorbed CO, at higher potentials,²⁰ leading to such high activity. ATR-IR (Fig. S7^†) and GC analyses (Fig. S8^†) show the methanol oxidation reaction (MOR) pathway during the formation of the final CO₂ product. The catalytic activity of Pd–Cu/C is found to be ∼13 times higher than commercial Pd/C for methanol oxidation, demonstrating that the presence of Cu with Pd in Pd–Cu based catalysts increases CO oxidation because of a strong binding ability. Cu binds to CO more strongly than Pd, as a result of electronic structure differences,^21,22 thus preventing the electrode from undergoing CO poisoning, a major issue for Pd-based catalysts during the methanol oxidation reaction (MOR). The mechanism of methanol oxidation is shown in the ESI (eqn (8)–(10)).^† The high iF value for Pd–Cu/C can be ascribed to the fast formation of reactive intermediates, such as Pd–CH₂OH, Pd–COOH, Pd–H, Pd–(CHO)_ads, and Pd–(COOH)_ads.^23–25 The removal of these intermediates is necessary for a high current density. Furthermore, formaldehyde (HCHO), formic acid (HCOOH) and CO₂ would be the final products in the MOR.^26,27 Pd–Cu/C has good catalytic activity for the MOR, leading to further investigation into the electrooxidation of different alcohols, such as ethanol, 1-propanol, and 2-propanol (Fig. 4 and Table S2^†).Open in a separate windowFig. 3CV curves from Pd/C and Pd–Cu/C electrocatalysts during CH₃OH (1 M) oxidation in 1 M KOH solution, at a scan rate of 50 mV s⁻¹, at room temperature.Open in a separate windowFig. 4(a) CV curves from: the Pd–Cu/C electrocatalyst for C₁–C₃ aliphatic alcohol (1 M) oxidation in KOH (1 M) solution; and (b) Pd–Cu/C in 1 M EtOH, 1-propanol and 2-propanol at a scan rate of 50 mV s⁻¹ at room temperature.The current densities for different alcohol oxidation processes are summarized in Fig. 4a and b. The normalized iF (calculated using Pd mass) for the MOR (∼4643 mA mg⁻¹) was higher than for three other alcohols, i.e., it was ∼139, ∼94 and ∼26.5 mA mg⁻¹ for ethanol, 1-propanol and 2-propanol, respectively. The iF/iB ratio for methanol is ∼12 times higher than that for ethanol, ∼13 times that for 1-propanol and ∼4 times that for 2-propanol. The reactivity order for Pd–Cu/C is methanol > ethanol > 1-propanol > 2-propanol. 2-Propanol electrooxidation showed a lower current density on a Pd–Cu/C electrode in alkaline medium, although iF/iB is ∼5.4. The negative shift in the onset of the ethanol oxidation reaction (EOR) suggested that a high copper content with very low amount of Pd was suitable for EOR kinetics using a Pd–Cu/C catalyst. The ethoxy (CH₃CO)_ads was strongly adsorbed, and blocked hydrogen absorption/adsorption. The current intensity of iF increased due to the formation of fresh Pd–OH, through stripping carbonaceous residue from the electrode (eqn (11)–(14)^†). In addition, the increased current at high potentials sharply reached the largest value then started to decline, because a PdO layer formed on the electrode, blocking the further adsorption of reactive species.²⁸ ATR-IR spectra (Fig. S9^†) show the presence of CO₂ and CO_ads, whereas bands appear at 1670 cm⁻¹ and 1390 cm⁻¹ because of the formation of acetic acid.²⁹ The Pd–Cu/C electrocatalyst has the potential to oxidize the intermediate to the final product, CO₂, during EOR to some extent; Cu promotes oxidation through increasing the production of OH_ads⁻/H₂O to eliminate the intermediate CH₃CO_ads simultaneously on Pd.³⁰ The stability of Pd–Cu/C in all four alcohols (methanol, ethanol, 1-propanol and 2-propanol) was studied using chronoamperometry at a potential of −0.25 V, as shown in Fig. S2.^† The slow current decay showed that the stability of the Pd–Cu/C electrocatalyst in methanol is best. Comparing the results, the current for methanol oxidation was higher than that for the other three alcohols. However, the oxidation currents from ethanol and 1-propanol were larger than that from 2-propanol. This suggested that Pd–Cu/C is less stable and shows lower anti-poisoning ability during 2-propanol oxidation in an alkaline medium.During this oxidation, 1-propanol oxidizes to propanal first, and its further oxidation results in the formation of a stable product, propanoic acid (Scheme S1^†). 1-Propanol is converted, with its carboxylate as the major product, as verified using ATR-IR (Fig. S10^†). 2-Propanol forms acetone as an intermediate product, leading to the poisoning of the electrode.³¹ ATR-IR spectra (Fig. S11^†) of Pd–Cu/C also confirm that the electrocatalyst follows a dual pathway through acetone and propene intermediates to oxidize to CO₂ finally (Scheme S2^†).^32–34 However, acetone formation is kinetically favored.³⁵ The results show that the location of the –OH group in the alcohol influences the electrooxidation reaction kinetics. In contrast methanol oxidation using Pd–Cu/C has much higher catalytic activity than ethanol, 1-propanol and 2-propanol oxidation, which makes it a good candidate for direct methanol fuel cells.In summary, we have first synthesized a bimetallic Pd–Cu NP loaded porous carbon material from a Cu-based MOF for alcohol electrooxidation. The Cu loaded porous carbon was pre-synthesized by calcinating the Cu-based MOF HKUST-1 under a N₂ atmosphere. Afterwards, Pd–Cu NP loaded porous carbon was obtained for alcohol electrooxidation. Electrooxidation experiments revealed that Pd–Cu/C was suitable for steady state electrolysis for alcohol oxidation in alkaline media. In addition, different alcohols were electrooxidated using the present electrocatalyst to discuss the oxidation mechanism. This electrooxidation study of Pd–Cu/C derived from a MOF offers good understanding into the electrooxidation of different alcohols and it could provide useful guidance for the development of new electrocatalyst materials.     

One-dimensional molecular chains formed by Sierpiński triangles on Au(111)

One-dimensional molecular chains with Sierpiński triangles as building blocks were prepared on Au(111) and studied by low-temperature scanning tunneling microscopy. Fractal Sierpiński triangles were stabilized by the coordination interaction between 1,3-bi(4-pyridyl) benzene (BPyB) and Co atoms. At low coverages, Sierpiński triangles of high orders were observed. At high coverages, Sierpiński triangles formed two types of one-dimensional molecular chains. The high coverages and matching between molecular size and surface lattices are attributed to the formation of molecular chains.

One-dimensional molecular chains with Sierpiński triangles as building blocks were prepared on Au(111) and studied by low-temperature scanning tunneling microscopy.

From snowflakes to Saturn''s rings, self-similar fractals exist widely in nature and are of fundamental importance in mathematics, engineering, science, and arts. Fractal structures have been successfully synthesized in solutions and prepared on solid surfaces.^1–4 Various fractal Sierpiński triangles (STs) have been theoretically predicted via Monte Carlo simulations^5,6 and experimentally discovered by self-assembly or chemical reactions.^7–14 The arrays of STs are predicted to have particular optical, electric, and magnetic properties.^15,16 To measure them, high-order STs or regular patterns with STs as building blocks need to be constructed. Due to the limitation of kinetic growth, it is of high challenge to prepare high-order STs. As shown by the scanning tunneling microscopy (STM) experiments, the largest order of obtained STs is only 5.¹⁷ Preparation of ST arrays looks promising because various ways have been found to grow ordered patterns on surfaces. By a templating method, one-dimensional double chains of STs were prepared along the reconstructed rows on Au(100) at low molecular coverages.¹⁸ Without templates, large-scale ordered patterns of STs has not been prepared on Au(111) using the longer 4,4′′-dicyano-1,1′:3′,1′′-terphenyl (C3PC) molecule.Several factors are needed to be taken into account to construct regular crystalline arrays with STs as the building block on the three-fold Au(111) substrate. Firstly, inert surfaces and molecules with smaller sizes than C3PC are needed to get weak molecule–substrate interactions which allow molecular free diffusion. Secondly, using intermediately strong interactions as driving forces can make sure both the self-correction of defects formed during growth and sufficient molecular diffusion. Thirdly, the structure matching between molecules and surface lattices favors the formation of long-range ordered structures.Based on above considerations, 120° V-shaped BPyB molecules and Co atoms are used to build arrays of STs on Au(111). Low-temperature STM was used to characterize the formed structure at the single-molecule level. At low coverage, STs up to fourth order were observed in measurements. At high coverage, two types of large-scale arrays with STs as the repeated units were successfully prepared. As a comparison, the disordered structure formed by Co atoms and C3PC molecules at high molecular coverages was investigated.Experiments were performed by a Unisoku scanning tunneling microscope (STM) operating at 4.3 K with a base pressure of 10⁻¹⁰ torr. The single-crystalline Au(111) surface was cleaned by cycles of Ar⁺ sputtering and annealing. The cut Pt/Ir tip was first annealed in vacuum chamber and then dipped into Au(111) in a controlled way. The BPyB and C3PC molecules were thermally deposited onto the substrate from Ta boats, separately. The Co atoms were evaporated from a Knudsen cell onto Au(111). The ratio between Co and BPyB was slightly larger than 2 : 3 during deposition. The substrate was kept at room temperature when depositing molecules and Co atoms. All STM images were slightly smoothed using software WSxM.¹⁹After sublimating BPyB molecules and Co atoms onto Au(111) and annealing at 400 K for 10 minutes, isolated metal–organic coordinating STs were obtained in experiments at low molecular coverages. Here we denote STs of order n as BPyB–Co-ST-n or ST-n, where n ranges from 0 to 4. Fig. 1(a) and (b) present a BPyB–Co-ST-4 and its chemical structure, respectively. The node in STs is formed by one Co atom and three BPyB molecules through coordinating interaction, as shown in the upper right in Fig. 1(b). There are 81 Co atoms and 123 BPyB molecules in a BPyB–Co-ST-4. The iterative process of ST-n is demonstrated in Fig. 1(c) where the chemical structure of BPyB was simplified to a blue 120° polyline. In a BPyB–Co-ST-n, the number of BPyB molecules (N_BPyB) and Co atoms (N_Co) are: N_BPyB = 3(3ⁿ + 1)/2 and N_Co = 3ⁿ. Close inspection of the BPyB–Co-ST-4 reveals that they contain various pores of different sizes (Fig. 1(b)). The area of largest pore is around 44.6 nm². When increasing the coverage, BPyB molecules would cover the pores and the order n of STs would decrease.Open in a separate windowFig. 1Fractal phase formed at low molecular coverage. (a) STM image of a defect-free ST of 4 order; inset: coordinated trimer acting as basic recursive unit. The white arrow indicates 〈1$\bar{1}$0〉 direction. Scanning parameters: constant-height mode, V_B = 10 mV, I_t = 80 pA. (b) Structural model of the ST in (a). (c) Schematic models revealing the evolution process of fractal phase. Fig. 2(a) shows a large-scale STM image of Co–BPyB structures on Au(111), which was prepared by depositing BPyB molecules and Co atoms at high coverages and annealing at 400 K for 10 minutes. Partial STs are marked by triangles to guide the eye. Ordered packing of STs is found in the figure. One representative packing mode of STs is shown in Fig. 2(b). One-dimensional (1D) molecular ribbons consist of pairs of BPyB–Co-STs-2 marked by white dashed triangles. The high-resolution STM image and chemical structure are displayed in Fig. 2(c) and (d). Due to the space limitation, the marked pyridyl groups in apical molecules of STs are partially lifted from the Au(111) substrate (Fig. 2(d)). As a result, they appear darker than the rest parts of molecules in the STM image (Fig. 2(c)) because of a weaker coupling to the substrate. The molecular models in Fig. 2(e) are used to illustrate the hierarchically self-assembling process of 1D chains. The combination of a Co atom and three BPyB molecules leads to the formation of a threefold heterotactic node or ST-0. After the iterative process, the ST-1 and ST-2 can be obtained. Lateral translating the rhombic pair of STs-2 produces 1D molecular chains. Vertical packing molecular chains finally cause the formation of the experimentally observed chain array.Open in a separate windowFig. 2BPyB–Co-ST-2 arrays formed at high molecular coverage. (a) Large area of molecular ribbons formed by close-packed second order fractals. (b) Close-up image showing four compact rows of 1D molecular ribbons. (c) Details of two interlocking fractals (2 + 2) playing the role of initial repetitive unit. (d) Structural models corresponding to combination 2 + 2. (e) Hierarchical formation of compact 1D molecular ribbons originated from 1.6D fractal moieties. The white arrow points to 〈1$\bar{1}$0〉 direction. Scanning parameters: (a) constant-current mode, V_B = 1 V, I_t = 12 pA; (b) constant-current mode, V_B = 30 mV, I_t = 12 pA; (c) constant-height mode, V_B = 50 mV, I_t = 12 pA.Interestingly, another type of 1D structure was observed when annealing the sample at a lower temperature of 330 K at high coverages (Fig. 3(a)). These double chains are stabilized by mixed three- and four-fold coordination interactions. The structure marked by white dashed frames was magnified in Fig. 3(b). The high-resolution STM image reveals that the basic repetitive unit contains two BPyB–Co-STs-1 connected through a fourfold coordination node. The chemical structures are overlaid on the right side of the image to guide the eye. Similar to Fig. 2(e), the scheme in Fig. 3(c) is used to reveals the forming sequence of hierarchical chains shown in Fig. 3(a). Density Functional Theory (DFT, ESI^†) calculations reveal that the average binding energies of Co(BPyB)₃ and Co(BPyB)₄ are −1.63 and −1.47 eV, respectively. When annealing at 330 K, the insufficient molecular diffusion results in the metastable four-fold coordination. As expected, the surface structure changes to arrays of ribbons shown in Fig. 2(a) when further annealing the structure at 400 K to allow molecular free diffusion. Same to Co, both Fe and Ni can form STs with C3PC through metal–nitrogen coordinated bonds. Because the BPyB molecule is terminated by N atoms, we expect that similar surface structures should be obtained by replacing Co with Fe or Ni atoms.^10,11Open in a separate windowFig. 3BPyB–Co-ST-1 arrays formed at high molecular coverage. (a) STM image of 1D molecular ribbons formed by combination of 1 + 1. (b) Detailed image of one 1 + 1 ribbon superposed by models. The white arrow points to 〈1$\bar{1}$0〉 direction. Scanning parameters: constant-height mode, (a) V_B = 50 mV, I_t = 60 pA; (b) V_B = 10 mV, I_t = 70 pA; (c) schematic models indicate the formation of 1D molecular ribbons based on combination 1 + 1.To investigate the formation mechanism of 1D chains of STs, we repeated the experiments by replacing BPyB molecules by 4′′-dicyano-1,1′:3′,1′′-terphenyl (C3PC) molecules with other parameters unchanged. The longer C3PC molecule is terminated by cyano groups. Long-range ordered chains were not obtained at various annealing temperature and molecular coverages. Fig. 4(a) shows a representative STM image of Co–C3PC-STs on Au(111), where the longest chain is marked by a white rectangle. The magnified image (Fig. 4(b)) indicates that the molecular chain is composed of six Co–C3PC-STs. To be noted, two sides of the chain are not straight, as marked by white dashed curves.Open in a separate windowFig. 4C3PC–Co-STs formed at high molecular coverage. (a) Overview of irregular network formed by C3PC and Co coordination. The molecular model of C3PC is shown in the left inset. (b) Detailed STM image showing a strip of curved C3PC–Co-ST-2 array indicated by the white rectangle in (a). Both sides of the slightly distorted array are highlighted by dashed curves. Scanning parameters: (a) constant-current mode, V_B = 1.8 V, I_t = 17.5 pA; (b) constant-height mode, V_B = 10 mV, I_t = 15.4 pA.The C3PC molecule is slightly longer than BPyB. As a result, the degrees of commensurability between intermolecular lengths in STs and the lattice distance of Au(111) are different for these two molecules, which might account for their different growth behaviors. To quantitatively elucidate the relation between mismatching degrees and forming structures, K-maps with the undermentioned formula was calculated,K = |1 − |mi + nj|/d|.Here, i and j represent two basic vectors of Au(111) as indicated by black arrows in Fig. 5(a) and (b), with m and n as their integer factors. The lattice distances along these two directions are both 0.288 nm. In the formula, d corresponds to the theoretically calculated distance between two neighboring Co atoms, which equals 13.5 Å for Co–BPyB–Co and 16.9 Å for Co–C3PC–Co, respectively. K values at each (m; n) coordinate, representing the degree of commensurability, were calculated and displayed by squares with different brightness. A smaller K value means a better structural match between STs and Au(111). Four smallest K values are numbered in each K-map to highlight the favorable adsorption direction. For Co–BPyB-STs, the smallest mismatch value is 2.4% at the coordinate (4; 1). This explains well that the sides of Co–BPyB-STs are adsorbed along the 〈4$\bar{5}$1〉 directions in measurements. The adsorption energies of Co–BPyB–Co and Co–C3PC–Co motifs at representative coordinates have been calculated to demonstrate the validity of the K map (ESI^†). For Co–BPyB–Co, its adsorption energy at the coordinate of (4; 1) is 0.39 eV larger than that at the coordinate of (5; 0). The structures with smaller K values are more stable. It is clear that the calculated results are consistent with what the K-map indicates. For the longer Co–C3PC–Co motif, the difference of its adsorption energies at the coordinates of (4; 3) and (6; 0) is only around 0.05 eV. The experimentally observed adsorption directions of sides of the STs are along the 〈4$\bar{7}$3〉 directions and one side of an ST adsorbs nearly along the reconstruction row. For this kind of adsorption, the structural matching between STs and the herringbone reconstruction of Au(111) might account for the energy stabilization of STs. Along the 〈4$\bar{7}$3〉 directions, the K value is 3.7%, which is rather large and does not favors the formation the long-range ordered structures.Open in a separate windowFig. 5 K-Map of (a) BPyB–Co coordination and (b) C3PC–Co coordination. Each left panel corresponds to surface lattice models of Au(111) with the best (blue arrows) and second best (orange arrows) matching orientation; each right panel corresponds to relative gray-scale K maps.In summary, by using BPyB molecules and Co atoms, one-dimensional molecular chains with STs as building blocks were prepared on Au(111) and studied by low-temperature scanning tunneling microscopy. STs were stabilized by the coordination interaction between N atoms of BPyB and Co atoms. The chains were only obtained at high molecular coverages and isolated STs up to the fourth order were observed at low coverages. The structure matching between molecular size and surface lattices are attributed to the formation of molecular chains. As a comparison, long-range ordered chains were not obtained for C3PC molecules and Co atoms at various annealing temperature and molecular coverages. To be continued, it is of interest to investigate the surface structures by replacing Co with Fe or the comprehensive result by mixing the two different molecules and novel bicomponent structures are expected to emerge.²⁰     

Hybrid BiOBr/UiO-66-NH2 composite with enhanced visible-light driven photocatalytic activity toward RhB dye degradation

Metal–organic framework (MOFs) based composites have received more research interest for photocatalytic applications during recent years. In this work, a highly active, visible light photocatalyst BiOBr/UiO-66-NH₂ hybrid composite was successfully prepared by introducing various amounts of UiO-66-NH₂ with BiOBr through a co-precipitation method. The composites were applied for the photocatalytic degradation of RhB (rhodamine B) dye. The developed BiOBr/UiO-66-NH₂ composites exhibited higher photocatalytic activity than the pristine material. In RhB degradation experiments the hybrid composite with 15 wt% of UiO-66-NH₂ shows degradation efficiency conversion of 83% within two hours under visible light irradiation. The high photodegradation efficiency of BUN-15 could be ascribed to efficient interfacial charge transfer at the heterojunction and the synergistic effect between BiOBr/UiO-66-NH₂. In addition, an active species trapping experiment confirmed that photo-generated hole⁺ and O₂⁻ radicals are the major species involved in RhB degradation under visible light.

Metal–organic framework (MOFs) based composites have received more research interest for photocatalytic applications during recent years.     

Renewable juglone nanowires with size-dependent charge storage properties

Inspired by the biological metabolic process, some biomolecules with reversible redox functional groups have been used as promising electrode materials for rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, their controllable synthesis and morphology-related properties have been rarely studied. Herein, one dimensional nanostructures based on juglone biomolecules have been successfully fabricated by an antisolvent crystallization and self-assembly method. Moreover, the size effect on their electrochemical charge-storage properties has been investigated. It reveals that the diameters of the one dimensional nanostructure determine their electron/ion transport properties, and the juglone nanowires achieve a higher specific capacitance and rate capability. This work will promote the development of environmentally friendly and high-efficiency energy storage electrode materials.

Renewable juglone nanowires have been successfully fabricated, and their size effect on electrochemical charge-storage properties has been investigated.

Currently, the development of high-performance electrochemical energy-storage materials and devices is attracting intensive interest. Conventional electrode materials involving transition metal compounds,^1–4 elementary substances,^5–8 and conductive polymers,^9–12 with superior charge storage properties have been widely investigated. However, the poor biocompatibility, rising prices and depletion issues limit their sustainable applications due to their intrinsic material properties.¹³ Thus, exploring naturally abundant and renewable charge-storage materials with promising electrochemical performance is of great significance.In the biological system, its metabolic process mainly relies on ions transport and energy exchanges of redox-active biomolecules with special functional groups such as carbonyl groups, carboxyl groups, and pteridine centres.¹⁴ Due to their abundance, sustainability, environmental benignity, these renewable and nature-derivable biomolecules with well-defined charge-storage behaviors are ideal alternatives to conventional electrode materials for the next-generation green energy-storage devices.^15–17 For instance, biomolecules such as lignin,¹⁸ melanin,¹⁹ riboflavin,²⁰ juglone²¹ and humic acid²² have been demonstrated as promising electrode materials for the rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, they are still confronted with several serious problems of poor conductivity and high electrochemical reaction impedance.²³ For some conventional inorganic and organic active electrode materials, decreasing their size has been demonstrated to be effective strategies to enhance the electrochemical reaction kinetics by exposing more active sites to electrolytes and conductive agent.^8,24,25 These results have strongly motivated us to develop biomolecule-based nanostructures, and investigated their size-correlative charge storage behavior.^26,27Herein, one dimensional (1D) nanostructures based on juglone, a renewable redox-active biomolecule which can be derived from matured fruits of black walnut and the green peel of juglandaceae, have been successfully fabricated by an antisolvent crystallization and self-assembly method.^28–30 The size effect on the electrochemical charge-storage properties of these biomolecule-based 1D nanostructures have been investigated. It reveals that the electronic/ionic transport properties and charge-storage performance can be modulated by the size of self-assembled 1D nanostructures, and the samples with smaller diameter realize the higher specific capacitance and rate capability. Our work will provide insights for the development of high-performance biomolecule-based green energy-storage materials and devices.Juglone, also called 5-hydroxy-1,4-naphthalenedione, is a nature-derivable biomolecule, and displays a well-defined redox behavior in acetonitrile due to its quinone groups (Fig. 1a and b).³¹ As organic molecule inherently, it is soluble in organic solvent and difficult to dissolve in water, so its nano-architectures could be condonably fabricated by an antisolvent crystallization strategy (Fig. 1c) and would carry out stably for charge storage in an aqueous electrolyte.^29,30 Firstly, the juglone-biomolecule-based 1D nanostructures with different size were synthesized. The juglone micropillars with a mean diameter around 12 μm were prepared by directly recrystallizing a water/acetonitrile mixed solution of juglone at room temperature (Fig. 2a and d). Compared to commercially available raw juglone materials (Fig. S1^†), the juglone micropillars could increase its charge storage performance, but its relative large size would still confine its contact with electrolyte, and thus remarkably reduce the reaction kinetics.³² To further improve the potential reaction kinetics, the juglone microwires with an average diameter about 1 μm (Fig. 2b and e) and juglone nanowires with a mean diameter about 550 nm (Fig. 2c and f) were fabricated at room temperature using the reprecipitation method.³³ In the specific synthesis process, a high-concentration juglone acetonitrile solution is injected into water, which is a poor solvent for juglone molecules. Under stirring, juglone started to crystallize within a few seconds owing to its poor solubility in the water/acetonitrile mixture solvent and self-assembled into 1D nanostructures, and this procedure could be resulted from the π–π interaction.²⁹ It can be found that the diameter of 1D juglone materials is adjustable by tuning the concentration of juglone in acetonitrile, and the higher concentration of juglone acetonitrile solution yields the smaller size of 1D juglone materials. The insert panels show the percentage of juglone micropillar, microwire, nanowire with different diameter coverage (Fig. 2d–f).Open in a separate windowFig. 1(a) Chemical structural formula of juglone biomolecules which can be derived from the bark of black walnuts. (b) Juglone redox activity verified in a mixed solution of acetonitrile/deionized water by a three-electrode system using Pt foils as both the counter and working electrodes, Ag/AgCl as the reference electrode, and 2.3 M H₂SO₄ as the electrolyte. (c) Schematically illustration of the fabrication of the juglone nanowire/microwire.Open in a separate windowFig. 2SEM images of juglone 1D nanostructures at different magnification. (a and d) Juglone micropillar, (b and e) juglone microwire, (c and f) juglone nanowire. The insert images in panels (d–f) are corresponding mathematical statistic results of juglone samples with different diameter.Generally, the covalent bond, hydrogen bond, van der Waals forces, electrostatic forces, surface tension forces/dewetting, and π–π stacking interactions are considered as the effective factors in the self-assembly procedure of organic compounds.^34–36 The juglone biomolecule has a α-naphthol backbone with two carbonyl groups, which may induce the molecules self-assembly by the π–π interactions.^29,37–39 Fourier Transform Infrared Spectroscopy (FTIR) of such samples was utilized to examine the ingredient of these samples. As shown in Fig. S2,^† all the samples show similar characteristic peaks, the peak at 1640 cm⁻¹ is attributed to the stretching vibration of carbonyl groups, which is the main reversible redox center presented in juglone molecules. Meanwhile, Raman spectra also corroborated the results (Fig. S4^†). The crystal structures of these samples are further characterized by X-ray diffraction (XRD) as shown in Fig. S3.^† No impurity peak is observed in these patterns, demonstrating that these samples have the same crystal structure. And the peak intensity of juglone nanowire is stronger than juglone microwire and micropillar, suggesting that juglone nanowires have high crystallinity and relatively uniform crystal size.³⁰To investigate the redox behavior of 1D juglone micro/nanostructures, the cyclic voltammetry (CV) measurements were firstly performed (Fig. 3a), and the result reveals that these samples show a superior reversible redox performance, implying a potential application as energy storage materials. Meanwhile, compared with the CV curves of raw juglone, juglone micropillar and juglone microwire, the juglone nanowire exhibits the strongest redox peak, suggesting that the juglone nanowire-based electrode has a better charge storage behavior. To further evaluate the electrochemical performance of these samples, galvanostatic charge–discharge (GCD) measurements were carried out (Fig. 3b). First, juglone nanowire shows a higher specific capacity than that of microwire and micropillar. Besides, the plot of juglone 1D-based electrodes exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, and the voltage plateau of juglone micropillar, microwire and nanowire appears at 0.28 V, 0.26 V, 0.22 V, which is roughly consistent with the results of CV curves. Furthermore, it further reveals that the 1D nanostructure with smaller size exhibits higher specific capacity during the scan rates increasing from 10 to 200 mV s⁻¹ (Fig. 3c). In detail, at a low scan rate of 10 mV s⁻¹, the specific capacity delivered by juglone nanowire, juglone microwire, juglone micropillar are 389 F g⁻¹, 375 F g⁻¹, 116 F g⁻¹, respectively, and these samples possess the specific capacities of 232 F g⁻¹, 205 F g⁻¹, 80 F g⁻¹, when the scan rate are enhanced to 200 mV s⁻¹. The general perception is that nanomaterials with higher specific surface areas usually possess better electrochemical performance.⁴⁰ Obviously, the higher specific capacitance and superior rate capability are achieved by juglone nanowire electrode.Open in a separate windowFig. 3Electrochemical performance of the juglone samples with different diameter. (a) CV curves of juglone micropillar, microwire, nanowire electrodes in the potential range of −0.1 to 0.7 V (vs. Ag/AgCl) with a scan of 50 mV s⁻¹, (b) galvanostatic charge–discharge curves of juglone electrodes with different diameter at 2 A g⁻¹. (c) Statistical study of the rate performance conducted at each scan rate based on the cyclic voltammetry capacity of five electrodes as one batch. (d) Impedance phase angle as a function of frequency for juglone micropillar, microwire, nanowire electrodes. Inset is the Nyquist plot of them. (e) Correlation between the diameter and the specific capacity of the juglone samples at various scan rates.Then, the reaction kinetics of juglone with different sizes were investigated by electrochemical impedance spectroscopy (EIS) (Fig. 3d and S5^†). The crooked curve at high frequency denotes the interface resistance, involving contact and charge transfer resistances, while the low-frequency line represents ion diffusion resistance. The interface resistances of juglone nanowire, microwire, micropillar, raw material electrodes are 3.45 Ω, 3.80 Ω, 3.95 Ω, 4.02 Ω. The plot of the phase angle against the frequency reveals the characteristic frequency f₀ at the phase angle of −45° is 7.46 Hz for juglone nanowire-based electrode, and it is relatively higher than that for juglone microwire (7.36 Hz), micropillar (7.25 Hz) and raw material (6.48 Hz). Accordingly, the time constant t₀ (t₀ = 1/f₀) that represents the minimum time to discharge ≥50% of all the energy from the electrode was 0.134 s for juglone nanowire, whereas 0.136 s, 0.138 s, 0.154 s were required for juglone microwire, micropillar and raw material. The low charge transfer resistance, and short time constant validated the excellent charge and discharge capability of the juglone nanowire-based electrode.⁴¹In addition, for solid-state diffusion of H⁺ in electrode materials, the mean diffusion time is proportional to the square of the diffusion path length according to the following equation:t ≈ L²/D_H⁺1where L is the diffusion length and D_H⁺ the diffusion constant.^42,43 It has been found that the diffusion pathway will be shortening by nanostructuring of electrode materials when the diffusion constant D is same,⁴⁴ indicating that the smaller size nanostructures of juglone facilitate rapid ion/electron transport. Thus, 1D juglone nanowires possess higher specific capacitance and show slower capacity decay during the increased scan rates (Fig. 3e).Furthermore, the CV and GCD for juglone nanowire-based electrodes were tested under various scan rates and current densities. The CV profiles show that the redox activity can be maintained very well when increasing the scan rate from 10 to 100 mV s⁻¹ (Fig. 4a). Moreover, the GCD curves collected at different current densities exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, which is similar to the results of CV test (Fig. 4b and c). And the specific capacity is 342 F g⁻¹ at a current density of 2 A g⁻¹, it is approximately equivalent to the 345 F g⁻¹ at a scan rate of 20 mV s⁻¹. Next, the specific capacitance of our juglone nanowire is further compared to those of the reported conventional electrode materials.^45–51 As shown in Fig. 4d, the capacity of N/rGO (reduced graphene oxide doping with nitrogen) is 138 F g⁻¹, and our juglone nanowire have about 2-fold higher specific capacity than N/rGO, 1.5-fold higher than Fe₃O₄/rGO (154 F g⁻¹).Open in a separate windowFig. 4(a) CV curves of the juglone nanowire electrode at different scan rates from 10 mV s⁻¹ to 100 mV s⁻¹. (b) Charge and discharge curves in the current density range from 2 A g⁻¹ to 10 A g⁻¹. (c) Various specific capacity of the juglone nanowire electrode when increasing the scan rate from 10 mV s⁻¹ to 200 mV s⁻¹. (d) Specific capacity of juglone nanowire in comparison with different materials.     

Efficient removal of pentachlorophenol from aqueous solution by 4-tert-butylcalix[8]arene modified thermally sensitive hydrogels

We prepared poly(N-isopropylacrylamide-co-4-tert-butylcalix[8]arene) (PNIPAM-TBCX) hydrogels by copolymerization of N-isopropylacrylamide (NIPAM) with 4-tert-butylcalix[8]arene (TBCX) to capture hazardous pentachlorophenol (PCP) from aqueous solution. Adsorption experiments showed that the adsorption capacities of PNIPAM-TBCX hydrogels reached 1.96, 2.08 and 2.02 mg PCP per 1 g of hydrogel, while the molar percentage ratio of TBCX in the hydrogels was as low as 0.5%, 0.7% and 1%. The equilibrium adsorption of PCP on the hydrogels was studied using different adsorption models. In addition, the PNIPAM-TBCX hydrogel still retained its performance when regenerated several times by immersing in water at 323 K.

We synthesized 4-tert-butylcalix[8]arene modified poly(N-isopropylacrylamide) hydrogels to enhance the adsorption ability for pentachlorophenol in aqueous solutions.     

Electronic properties of phosphorene nanoribbons with nanoholes

Using first-principles calculation based on density-functional theory, the electronic properties of monolayer black phosphorus nanoribbons (PNRs) with and without punched nanoholes (PNRPNHs) and their mechanical stability are studied systematically. We show that while the perfect PNRs and the PNRPNHs have similar properties as semiconductors in both armchair-edge PNR and zigzag-edge PNR structures, the nanoholes can lead to changes in the electronic structure: the zigzag-edge PNRPNH undergoes a direct-to-indirect bandgap transition while the armchair-edge PNRPNH still retains a direct bandgap but with a significant increase in the bandgap as compared to the perfect PNRs. We found also that nanoholes have little influence on the structural stability of PNRs; but the applied external transverse electric field and strain can be more effective in modulating the bandgaps in the PNRPNHs. These new findings show that PNRs are a promising candidate for future nanoelectronic and optoelectronic applications.

Using first-principles calculation based on density-functional theory, the electronic properties of monolayer black phosphorus nanoribbons (PNRs) with and without punched nanoholes (PNRPNHs) and their mechanical stability are studied systematically.     

Facile construction of a hyperbranched poly(acrylamide) bearing tetraphenylethene units: a novel fluorescence probe with a highly selective and sensitive response to Zn2+

Thermo-responsive hyperbranched copoly(bis(N,N-ethyl acrylamide)/(N,N-methylene bisacrylamide)) (HPEAM-MBA) was synthesized by using reversible addition–fragmentation chain-transfer polymerization (RAFT). Interestingly, the zinc ion (Zn²⁺) was found to have a crucial influence on the lowest critical solution temperature (LCST) of the thermo-responsive polymer. The tetraphenylethylene (TPE) unit was then introduced onto the backbone of the as-prepared thermo-responsive polymer, which endows a Zn²⁺-responsive “turn-off” effect on the fluorescence properties. The TPE-bearing polymer shows a highly specific response over other metal ions and the “turn-off” response can even be tracked as the concentration of Zn²⁺ reduces to 2 × 10⁻⁵ M. The decrement of fluorescence intensity was linearly dependent on the concentration of Zn²⁺ in the range of 4–18 μmol L⁻¹. The flexible, versatile and feasible approach, as well as the excellent detection performance, may generate a new type of Zn²⁺ probe without the tedious synthesis of the moiety bearing Zn²⁺ recognition units.

A novel fluorescent HPEAM-TPEAH, possessing a highly selective and sensitive response to Zn²⁺, was synthesized using RAFT.     

Research into the reaction process and the effect of reaction conditions on the simultaneous removal of H2S,COS and CS2 at low temperature

In this work, tobacco stem active carbon (TSAC) catalysts loaded on to CuO and Fe₂O₃ were prepared by a sol–gel method and used for the simultaneous removal of hydrogen sulfide (H₂S), carbonyl sulfide (COS) and carbon disulfide (CS₂). The influences of the operating conditions such as reaction temperature, relative humidity (RH), O₂ concentration, and gas hourly space velocity (GHSV) were discussed. DRIFTS results showed that the deactivation was attributed to the generation of S and sulfates. H₂O promoted the generation of sulfate. The enhancement of the hydrolysis of COS/CS₂ was due to the promotion of H₂S oxidation by O₂. A high GHSV decreased the contact time between the gases and the catalyst. Meanwhile, a high GHSV was not conducive to the adsorption of gases on the surface of the catalyst. XPS results indicated that the deactivation of the catalyst was attributable to the formation of S containing components, such as thiol/thioether, S, –SO– and sulfate. BET results indicated that the adsorptive ability of the catalyst was related to the microporous volume and surface area.

The removal processes of COS, CS₂ and H₂S could be divided into two parts: a catalytic hydrolysis reaction and a catalytic oxidation reaction.     

Correction: Comparative pharmacokinetics of four active components on normal and diabetic rats after oral administration of Gandi capsules

Correction for ‘Comparative pharmacokinetics of four active components on normal and diabetic rats after oral administration of Gandi capsules’ by Renjie Xu et al., RSC Adv., 2018, 8, 6620–6628.

AJing Xu''s name, affiliation and E-mail address were incorrectly reproduced in the published article. The correct versions are shown here.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Preparation of PSF/FEP mixed matrix membrane with super hydrophobic surface for efficient water-in-oil emulsion separation

Polysulfone (PSF)/fluorinated ethylene propylene (FEP) mixed matrix membranes (MMMs) with super hydrophobic surface were successfully fabricated via non-solvent induced phase separation (NIPS) method. The effects of FEP content on the morphology, roughness, wettability, pore size, and mechanical property of PSF/FEP MMMs were characterized by scanning electron microscope, confocal microscopy, contact angle goniometer, mercury porosimetry, and tensile testing instrument, respectively. When the FEP content was 9 wt%, the average roughness of M-4 reached 0.712 μm. Meanwhile, the water contact angle (CA) and the water sliding angle (SA) was 153.3° and 6.1°, respectively. M-4 showed super hydrophobicity with a micro- and nanoscale structure surface. Then, M-4 was used for separating of water-in-oil emulsion, showing high separation efficiency for water-in-kerosene and water-in-diesel emulsions of 99.79% and 99.47%, respectively. The flux and separation efficiency changed slightly after 10 cycles. Therefore, this study indicated that the obtained PSF/FEP MMM with super hydrophobic surface could be used for efficient water-in-oil emulsion separation.

The PSF/FEP membrane with super hydrophobic and super oleophilic surface had an outstanding separation performance for water-in-oil emulsion.