Mesoporous Cu–Cu2O@TiO2 heterojunction photocatalysts derived from metal–organic frameworks

This study reveals a unique Cu–Cu₂O@TiO₂ heterojunction photocatalyst obtained with metal–organic framework as the precursor, which can be utilized in dye photodegradation under visible light irradiation. The composition, structure, morphology, porosity, optical properties and photocatalytic performance of the obtained catalysts were all investigated in detail. The Cu–Cu₂O@TiO₂ nanocomposite is composed of lamellar Cu–Cu₂O microspheres embedded by numerous TiO₂ nanoparticles. Methylene blue, methyl orange and 4-nitrophenol were used as model pollutants to evaluate the photocatalytic activity of the Cu–Cu₂O@TiO₂ nanocomposite for dye degradation under visible light irradiation. Nearly 95% decolourisation efficiency of Methylene blue was achieved by the Cu–Cu₂O@TiO₂ photocatalyst within 3 h, which is much higher than that of TiO₂ or Cu₂O catalysts. The excellent photocatalytic activity was primarily attributed to the unique MOF-based mesoporous structure, the enlarged photo-adsorption range and the efficient separation of the charge carriers in the Cu–Cu₂O@TiO₂ heterojunction.

Cu–Cu₂O@TiO₂ heterojunction photocatalyst derived from a metal–organic framework shows high photocatalytic activity for dye degradation under visible light irradiation.     

Colorimetric determination of ascorbic acid based on carbon quantum dots as peroxidase mimetic enzyme

Carbon quantum dots (CQDs) were synthesized from litchi peel, exhibiting a peroxidase-like activity and enabling the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in association with H₂O₂ to generate blue oxidized TMB (ox-TMB) with a strong absorption peak at 652 nm. Interestingly, the ox-TMB could be further reduced by ascorbic acid (AA) leading to fading of the blue color and an absorbance decrease. Thus, a convenient and sensitive colorimetric method for detection of AA using CQDs as peroxidase mimics was established. Several factors, such as acidity, temperature, incubating time, and TMB concentration, which might influence the response of the analysis signal, were optimized. The results showed that the decrease of absorbance (ΔA) was in good linear agreement with AA concentration in the range of 1.0–105 μM, with a low detection limit of 0.14 μM. The feasibility of this method was also investigated in commercial beverages with the 94.3–110.0% recovery.

Carbon quantum dots (CQDs) were synthesized from litchi peel, exhibiting a peroxidase-like activity and enabling the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in association with H₂O₂ to generate blue oxidized TMB (ox-TMB) with a strong absorption peak at 652 nm.     

Removal of metallic coatings from rare-earth permanent magnets by solutions of bromine in organic solvents

Successful direct recycling routes are known for both Nd–Fe–B permanent magnets and Sm–Co permanent magnets. Often the magnets are coated by a nickel–copper–nickel coating to prevent corrosion of Nd–Fe–B magnets and chipping of Sm–Co magnets. However, this coating does not contribute to the magnetic properties and only ends up as a contamination in the recycled magnet powder, which in turn dilutes the magnet alloy and reduces the magnetic performance. One solution is the addition of virgin magnet alloy to the recycled powder, but this is not the best option from a sustainable point of view. Another option is to remove the coating prior to the magnet recycling. We developed a solvometallurgical process for removal of the metallic coating prior to direct recycling. In particular, a mixture of bromine in organic solvents was found to be very selective in the removal of the nickel–copper–nickel coating from both Nd–Fe–B permanent magnets and Sm–Co permanent magnets, without codissolution of the magnet alloy.

Rare-earth permanent magnets were treated with Br₂ in organic solvents to remove the Ni–Cu–Ni coating prior to direct magnet recycling by hydrogen decrepitation.     

Investigation of the structural,optical and gas sensing properties of PANI coated Cu–ZnS microsphere composite

Polyaniline (PANI)/Cu–ZnS composites with porous microspheres are prepared by a hydrothermal and in situ polymerization method. The structural, optical, and morphological properties are characterized by X-ray powder diffraction, FTIR, UV-vis, scanning electron microscope, transmission electron microscope. The XRD results confirmed that the PANI/Cu–ZnS composite is formed. The morphological analyses exhibited that the PANI/Cu–ZnS composite comprises the porous microspherical structures. The emission peaks obtained in photoluminescence spectra confirm the presence of surface defects in the prepared composite. The UV-DRS study shows that the bandgap of the samples is found to decrease for the PANI/Cu–ZnS composite compared to the pure Cu–ZnS sample. The calculated band gap (E_g) value of PANI/Cu–ZnS composite is 2.47 eV. Furthermore, the fabricated gas sensor based on PANI/Cu–ZnS can perform at room temperature and exhibits good gas sensing performance toward CO₂ gas. In particular, PANI/Cu–ZnS sensor shows good response (31 s) and recovery time (23 s) upon exposure to CO₂ gas. The p/n heterojunction, surface defects, and porous nature of the PANI/Cu–ZnS composite microsphere enhanced sensor performance.

Polyaniline (PANI)/Cu–ZnS composites with porous microspheres are prepared by a hydrothermal and in situ polymerization method.     

Morphology- and pH-dependent peroxidase mimetic activity of nanoceria

The peroxidase mimetic properties of nanoceria have attracted extensive attention in recent years. In this work, the peroxidase mimetic properties of CeO₂ nanocrystals with different morphologies, namely, nanocubes and nanorods, were investigated. Two types of oxidative species, HO˙ radicals and peroxide-like intermediates, were identified in the CeO₂/H₂O₂ systems. The formation of these oxidative species is strongly dependent on the pH value and the morphology of the CeO₂ nanocrystals. The origin of the peroxidase mimetic activity of nanoceria was mainly ascribed to the presence of HO˙ under acidic conditions, whereas the peroxide-like species also played a major role under neutral and basic conditions. CeO₂ nanorods with excellent redox properties and higher concentration of Ce³⁺ and oxygen vacancies were more favorable for the generation of both HO˙ and peroxide-like intermediates than that of CeO₂ nanocubes, exhibiting excellent peroxidase mimetic activity toward 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), methylene blue (MB), and congo red (CR) in the presence of H₂O₂.

The peroxidase mimetic properties of nanoceria have attracted extensive attention in recent years.     

Enhancing the chloramphenicol sensing performance of Cu–MoS2 nanocomposite-based electrochemical nanosensors: roles of phase composition and copper loading amount

The rational design of nanomaterials for electrochemical nanosensors from the perspective of structure–property–performance relationships is a key factor in improving the analytical performance toward residual antibiotics in food. We have investigated the effects of the crystalline phase and copper loading amount on the detection performance of Cu–MoS₂ nanocomposite-based electrochemical sensors for the antibiotic chloramphenicol (CAP). The phase composition and copper loading amount on the MoS₂ nanosheets can be controlled using a facile electrochemical method. Cu and Cu₂O nanoparticle-based electrochemical sensors showed a higher CAP electrochemical sensing performance as compared to CuO nanoparticles due to their higher electrocatalytic activity and conductivity. Moreover, the design of Cu–MoS₂ nanocomposites with appropriate copper loading amounts could significantly improve their electrochemical responses for CAP. Under optimized conditions, Cu–MoS₂ nanocomposite-based electrochemical nanosensor showed a remarkable sensing performance for CAP with an electrochemical sensitivity of 1.74 μA μM⁻¹ cm⁻² and a detection limit of 0.19 μM in the detection range from 0.5–50 μM. These findings provide deeper insight into the effects of nanoelectrode designs on the analytical performance of electrochemical nanosensors.

In this work, we clarify the roles of phase composition and copper loading amount on the CAP sensing performance of Cu–MoS₂ nanocomposite-based electrochemical nanosensors.     

Crystal engineering with copper and melamine

Coordination complexes and polymers are central in inorganic and materials chemistry as a variety of metal centers and coordination geometries lead to a diverse range of interesting properties. Here, size and structure control of gem-like quality monocrystals is demonstrated at room temperature. Using the same set of precursors, the copper-to-melamine molar ratio is adjusted to synthesize either a novel coordination complex of dinuclear copper and melamine (Cu2M1), or a barely-studied coordination polymer of zigzag copper–chlorine chains (Cu4M1). Crystals of the former are dark green and square with a size up to 350 μm across. The latter is light green, octagonal, and as large as 5 mm across. The magnetic properties of both crystals reflect the low-dimensional arrangements of copper. The magnetic susceptibility of Cu2M1 is modelled with a spin-1/2 dimer, and that of Cu4M1 with a spin-1/2 one-dimensional Ising chain. Controlled synthesis of such quality magnetic crystals is a prerequisite for various magnetic and magneto-optical applications.

Size and structure control of gem-like quality monocrystals – copper–melamine coordination complex, and copper–chlorine coordination polymer – is demonstrated at room temperature.

Advanced crystal engineering continues to draw the attention of the scientific community. This discipline is leading to new crystalline materials, and is focusing on finding strategies and logical ways to control their properties. Coordination chemistry plays an important role in crystal engineering as it allows the creation of various coordination compounds, e.g. metal complexes, coordination polymers or metal–organic frameworks, by designing the coordination between ligands and metal ions.^1–3 While the d orbitals of the metal ions promote directional bonding, there has been widespread use of polyamines, carboxylates, pyridyl and cyano groups as ligands.⁴Coordination compounds are considered useful in a great deal of applications, such as energy transfer, gas storage and separation, heterogeneous catalysis, proton conduction, biomedical applications and chemical sensing.^5,6 Molecular magnets based on coordination compounds play an essential role in information storage in quantum computing.⁷ Single crystals based on Mn and Fe can serve as information storage elements in a dynamic random-access memory device in which decoding and reading the information could be realized by fast electron spin resonance pulses.⁸In the present paper, it is demonstrated how molar ratios can affect reactions among precursor solutes and solvents. As an example, copper chloride and melamine are dissolved in a 1 : 1 mixture of methanol and dimethyl sulfoxide (DMSO) at room temperature. The structure of melamine is shown in Fig. 1b. Melamine-based coordination polymers reported previously include a fluorescent coordination polymer based on Cu(i) and melamine, which is highly stable and suitable for detection of nitro aromatic compounds in aqueous media.⁹ Also, a cationic coordination polymer based on Ag(i) and melamine was used for selective anion exchange.¹⁰Open in a separate windowFig. 1Schematic diagram of (A) CuCl₂·2H₂O and (B) melamine. (C) Crystal growth at room temperature with various concentrations of CuCl₂·2H₂O and melamine.It is found that two kinds of large crystals grow at optimized concentrations and molar ratio between copper chloride and melamine. At an optimal concentration of melamine of 0.1 mol L⁻¹, a copper to melamine ratio in the range 1 : 1 to 2 : 1, leads to the formation of a new copper complex composed of all available elements and molecules, i.e. copper, chlorine, melamine, methanol and DMSO. With a copper to melamine ratio in the range 3 : 1 to 4 : 1, both melamine and methanol are passivated, leading to the formation of a coordination polymer with zigzag copper–chlorine chains with each copper coordinated by three chlorines and two DMSO. Both of them are large single crystals with low-dimensional spin structures and are candidate materials for magnetic and sensing applications.All synthesis was carried out at room temperature (around 25 °C) by mixing a methanol solution of CuCl₂·2H₂O and a DMSO solution of melamine at different concentrations and molar ratio. The concentrations of CuCl₂·2H₂O tested are 0.8, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05 and 0.025 mol L⁻¹, and the concentrations of melamine are 0.05, 0.1 and 0.2 mol L⁻¹. See ESI 1^† for more details on the synthesis procedures and optimization. In each case, 2 mL of a methanol solution of CuCl₂ was placed in a 5 mL glass vial, then 2 mL of a DMSO solution of melamine was added to the CuCl₂ solution. The results are summarized qualitatively in Fig. 1 where ‘no crystal’ refers to the condition in which no solids are formed, while ‘not optimized crystal’ refers to the condition in which crystals grow but their quantity and/or size are not as large as the ‘optimized crystal’.The two optimal conditions are (ρ_CuCl2, ρ_Melamine) = (0.2 mol L⁻¹, 0.1 mol L⁻¹) for crystals named Cu2M1, and (0.4 mol L⁻¹, 0.1 mol L⁻¹) for crystals named Cu4M1, where ρ_CuCl2 is the concentration of CuCl₂·2H₂O in methanol and ρ_Melamine (mol L⁻¹) is the concentration of melamine in DMSO. Crystals are formed only within the concentration window 0.05 mol L⁻¹ ≤ ρ_CuCl2 ≤ 0.6 mol L⁻¹ and 0.1 mol L⁻¹ ≤ ρ_Melamine ≤ 0.2 mol L⁻¹. The fact that no crystal growth can be achieved at high concentrations of both cations, Cu(ii) and anions Cl⁻, can be attributed to the high ionic strength that reduces the mobility of the ions in the solution,¹¹ hindering the metal to ligand coordination. On the contrary, when the concentration is low, the mobility is high, but the nucleation of the crystal does not occur since the critical nucleation concentration is surpassed, and the collisions between both ligand and metal are less probable.¹² Fig. 2 shows the micrographs of crystals Cu2M1 and Cu4M1, formed at the optimal conditions. For Cu2M1, dark green rectangular crystals with truncated edges are formed in 5 hours with sizes as large as 350 μm in length (see panels A, B and C). As for Cu4M1, light green octagonal crystals as large as 5 mm across are formed in 48 hours (panels D, E and F). This demonstrates that only doubling the concentration of CuCl₂·2H₂O can drastically alter the morphology of the formed crystal.Open in a separate windowFig. 2(A–C) Optical micrographs of Cu2M1: (ρ_CuCl2, ρ_Melamine) = (0.2 mol L⁻¹, 0.1 mol L⁻¹). (D–F) Optical micrographs of Cu4M1 (ρ_CuCl2, ρ_Melamine) = (0.4 mol L⁻¹, 0.1 mol L⁻¹). Panels C and F show crystals on the membrane with a diameter of about 2.5 cm. (G) The structure of Cu2M1 illustrating a layer of hydrogen-bonded melamine molecules in the (0 1 1) plane. (H) The structure of Cu4M1 illustrating zigzag Cu–Cl chains.X-ray diffraction analysis of a Cu4M1 single crystal reveals the structure of the coordination complex (formula Cu((CH₃)₂SO)₂Cl₂)*, that was previously reported (CCDC deposition number 1142844),¹³ but not much was reported regarding its properties.^14,15 The determined structure of Cu4M1, illustrated in Fig. 2H, consists of copper, chlorine and DMSO and is an orthorhombic system. As for its structure, it consists of serpent-like Cu–Cl chains. Each copper has a trigonal ligand geometry with a crystallographic point group of D_3h symmetry:¹⁶ bonded to two dimethyl sulfoxide molecules through a Cu–O bond with a length of 1.95 Å, forming a O–Cu–O angle of 173.67°, and to three chlorine atoms in the a–c plane. One of the chlorines is out of the zigzag chain and the other two are in the chain. Cl–Cu–Cl angles are 146.46°, 112.22° and 101.32°. The length of the three Cu–Cl bonds (2.75 Å) are longer than the covalent Cu–Cl bond length (2.3 Å) in CuCl₂, indicating the weak covalent bonding.¹³Fig. 3 shows the inter- and intramolecular bonds. The bond lengths [Å] and the bond angles [°] are also given. It highlights that the zigzag chain results in a (red marked) “cap”. In the area marked in red, two other intramolecular bonds have also been detected (shaded green). The cap encloses a neighbouring strain and it is characterised by several intermolecular interactions (shaded yellow).Open in a separate windowFig. 3Inter- and intramolecular bonds visualized in the crystal structure of Cu4M1 drawn with 50% displacement ellipsoid.X-ray diffraction analysis of a single crystal of Cu2M1 reveals that it is a coordination complex consisting of copper, chlorine, melamine, DMSO and methanol, as shown in Fig. 2G. The empirical formula of the coordination complex is (Cu(C₃H₆N₆)(OCH₃)((CH₃)₂SO)Cl)₂. See ESI 2^† for more details on the structural analysis. To the best of our knowledge, this crystalline structure has not been previously reported, only some similar examples as reported by Chen et al., (2006) (CCDC deposition numbers 280091 and 280092),¹⁷ Goodgame et al., (1999) (CCDC deposition number 134810),¹⁸ and Wiles et al., (2006).¹⁹ In our Cu2M1 complex, two copper atoms are bridged by the oxygen atoms of two methoxides, forming an angle O–Cu–O of 77.88°. Each copper has a distorted square pyramidal ligand geometry with a crystallographic point group of C_4v symmetry.¹⁶ At the top of the square base pyramids is a chlorine Cu–Cl bond (2.63 Å). The basal plane of the pyramids consists of two Cu–O bonds (1.93 Å) where each copper coordinates with the oxygen of methoxide, another Cu–O bond (1.95 Å) which links copper with a molecule of DMSO by its oxygen, and a Cu–N bond (1.98 Å) that coordinates copper with a nitrogen of the pyridine ring of melamine. The two melamines are in the same plane which is outside of the basal plane of the two pyramids. Melamines of the adjacent Cu2M1 molecules are hydrogen bonded to one another, constituting a global two-dimensional layer of melamine. The packing view along (1 1 1) in Fig. 4 shows that every dimer (red shaded area) is surrounded by six neighbouring dimers in the plane. The shaded areas within the plane show the seven intermolecular interactions in yellow, and the two intramolecular interactions in green.Open in a separate windowFig. 4Inter- and intramolecular bonds visualized in the crystal structure of Cu2M1 drawn with 50% displacement ellipsoid.Packing of melamine layers along (1 0 0) leads to two types of one-dimensional void accessible by the solvents (DMSO and MeOH). Fig. 5 compares the structure model for Cu2M1 viewed along the “a” axis without co-crystallised solvents in panel (a), with the methanol-filled model pictured in panel (b). The green-shaded void is along the visible radius in panel (a). This sterically influenced void allows the two different types of solvents (DMSO and MeOH) used during the synthesis to co-crystallise in a disordered way. The second type of void (yellow-shaded) is intruded by the coordinated DMSO and limits the available space in which only MeOH can be modelled. See ESI 2 for more details.^†Open in a separate windowFig. 5(A) The structure of Cu2M1 viewed along the a axis without filling. (B) The structure of Cu2M1 viewed along the a axis with methanol filling. The green and yellow-shaded circles represent the two types of one-dimensional void.The crystal structures of Cu2M1 and Cu4M1 accommodate low-dimensional coordinations of copper, namely, the dinuclear copper molecular unit in Cu2M1 and the zigzag copper–chlorine chain in Cu4M1. The low-dimensional nature of the exchange coupling among copper spins are of particular interest for their potential magnetic applications.The temperature dependence of the magnetic moment per copper in units of μ_B for a dinuclear Cu–melamine complex Cu2M1 sample (33.7 mg of Cu2M1 crystals encapsulated in a gelatin capsule) in an applied magnetic field of 1 T is shown in Fig. 6A. As planar crystals are stacked horizontally, the magnetic field is applied normal to the crystal plane for the majority of crystals. The net magnetism of the dinuclear Cu–melamine complex is much smaller than the dashed curve showing Curie’s Law for a paramagnetic 1/2 spin, with an effective magnetic moment of indicating the presence of strong antiferromagnetic coupling. The magnetic susceptibility as a function of temperature can be analysed using the Bleaney–Bowers equation for an exchange-coupled pair of S = 1/2 spins^20,211where, the first, second and third terms are the Bleaney–Bowers equation, Curie–Weiss law taking into account paramagnetic impurities, and temperature-independent constant η. Here, g_S ≃ 2 is the electron spin g-factor, k_B the Boltzmann constant, J the exchange energy between the spins, and ζ corresponds to a concentration of 1/2 paramagnetic impurities, e.g. isolated Cu(ii) on defects and surfaces. Weiss temperature T_w, takes the molecular field due to intermolecular exchange into account. The least-squares fit is satisfactory with J = −0.0542 ± 0.0047 eV (≃−437 cm⁻¹), T_w = −1.98 ± 0.05 [K], ζ = 0.158 ± 0.002 and η = 8.237 × 10⁻⁴ ± 2.11 × 10⁻⁵ [μ_B/Cu]. In Fig. 6A, the Bleaney–Bowers fit magnified 1000 times is plotted. Because of the relatively high exchange constant, the Bleaney–Bowers component is small compared with the Curie–Weiss term in the measured temperature range 2–300 K, and reaches only about a half of the Curie’s component at room temperature. A negative Weiss temperature of T_w = −1.98 ± 0.05 [K] can be attributed to intermolecular anti-ferromagnetic coupling.Open in a separate windowFig. 6Magnetisation versus temperature for (A) dinuclear Cu–melamine complex Cu2M1, and (B) 1D-Cu coordination polymer Cu4M1.As shown in Fig. 6B, the magnetization of a planar single crystal of copper coordination polymer Cu4M1 (24.0 mg) in the magnetic field applied normal to the crystal plane, exhibits a maximum of 0.0285 μ_B at 10 K. Provided that the exchange coupling is anisotropic and strong along the zigzag copper–chlorine–copper chain, the temperature dependence of the magnetic susceptibility can be evaluated based on the exchange Hamiltonian Ĥ = − ∑_ijJ_ijŜ_iŜ_j taking exchange coupling between any two adjacent spins in a one-dimensional spin chain into account.^22–26 The Bonner–Fisher equation derived from the above Hamiltonian for a S = 1/2 Heisenberg chain^27,28 together with a paramagnetic term, is as follows in eqn (2),2where u(K) = coth(K) − (1/K) and K = J/2k_BT, and ζ corresponds to a concentration of 1/2 paramagnetic impurities or the inverse-temperature term that arises from staggered spins.²⁹ The least-squares fit reproduces the experimental temperature dependence very well as shown in Fig. 6b, giving rise to g = 2.331 ± 0.005, J = −3.05 ± 0.02 meV (24.6 cm⁻¹), ζ = 0.056 ± 0.0011. g is larger than 2 for pure spin states, but this value depends largely on the normalization of the data that may contain errors.The structure analysis reveals that with a Cu–melamine ratio of 4 : 1 (0.4 mol L⁻¹ of CuCl₂·2H₂O in methanol and 0.1 mol L⁻¹ of melamine in DMSO) copper coordination polymer Cu4M1 is formed despite the presence of melamine. During their synthesis at room temperature, the pH of the media evolves differently in the 2 : 1 and 4 : 1 mixed solutions.³⁰ Although evaluating pH values of organic and aprotic solvents is a complicated task, their relative changes upon chemical reactions are worth noting. Since there is no OH⁻ in the precursor solvents, the pH is only related to the presence of H⁺. The pH value of 0.2 mol L⁻¹ of CuCl₂·2H₂O in methanol, 0.4 mol L⁻¹ of CuCl₂·2H₂O in methanol and 0.1 mol L⁻¹ of melamine before mixing are 1.03 ± 0.03, 0.62 ± 0.04 and 9.64 ± 0.02, respectively. The pH of the 2 : 1 and 4 : 1 solutions just after mixing are 5.96 ± 0.03 and 5.56 ± 0.02, respectively. After the formation of crystals of the dinuclear copper–melamine complex Cu2M1, the pH of the 2 : 1 solution remains unchanged within the confidence interval (5.94 ± 0.03). (This statistical parameter is determined by a “t of Student” distribution with 95% confidence interval, for which it is considered five simultaneous measurements.) On the contrary, the pH of the 4 : 1 solution is increased to 6.14 ± 0.03 after the formation of crystals of copper coordination polymer Cu4M1. This increase can be attributed to a reduction of H⁺ as a result of protonation of melamine which is initially deprotonated in pure DMSO. This leaves passivated neutral melamine which does not get coordinated with Cu(ii) ions. Likewise, the pH barely changes in the 2 : 1 solution because melamine ions react with Cu(ii) before being protonated. Hence, the proton concentration needs to be optimised for the formation of copper coordination polymer Cu4M1.In order to justify the above-mentioned scenario, the synthesis of copper coordination polymer Cu4M1 has been attempted by mixing a methanol solution of CuCl₂·2H₂O with aprotic DMSO whose pH value is adjusted by adding anhydrous acetic acid. It is found that crystals of the same copper coordination polymer are formed in the solution without melamine when the pH is adjusted to 9.64 ± 0.02, while no crystals are formed without acetic acid. Thus, the concentration of protons plays an important role in the coordination of DMSO and chlorine with copper ions in the presence of methanol.In summary, a novel copper–melamine complex and a copper coordination polymer have been synthesized selectively by adjusting the concentrations of copper(ii) chloride dihydrate and melamine in a mixed solution of methanol and DMSO at room temperature. Crystals of copper–melamine complex Cu2M1 formed with a Cu–melamine ratio of 2 : 1 are green and square shaped with sizes as large as 350 μm across. The hydrogen-bonded two-dimensional lattice is composed of planes of melamine and one-dimensional pores along the a-axis that accommodate solvent molecules. Crystals of copper coordination polymer Cu4M1 formed with a Cu–melamine ratio of 4 : 1 are light green gem-like octagonals and can grow as large as 5 mm across. The lattice is composed of Cu–Cl zigzag chains and has no porosity. Both Cu2M1 and Cu4M1 exhibit low-dimensional magnetic properties. The magnetic susceptibility of Cu2M1 can be modelled well based on the Hamiltonian for paired spins of 1/2, and that of Cu4M1 based on a spin-1/2 anti-ferromagnetic Ising chain. The well-controlled synthesis of the high quality and large monocrystals demonstrated in the present study will pave the way for future research on spintronic applications of inorganic and organic–inorganic hybrid materials.     

Highly efficient selective hydrogenation of levulinic acid to γ-valerolactone over Cu–Re/TiO2 bimetallic catalysts

Highly active and thermally stable Cu–Re bimetallic catalysts supported on TiO₂ with 2.0 wt% loading of Cu were prepared via an incipient wetness impregnation method and were applied for liquid phase selective hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in H₂. The effect of the molar ratios of Cu : Re on the physico-chemical properties and the catalytic performance of the Cu–Re/TiO₂ catalysts was investigated. Moreover, the influence of various reaction parameters on the hydrogenation of LA to GVL was studied. The results showed that the Cu–Re/TiO₂ catalyst with a 1 : 1 molar ratio of Cu to Re (Cu–Re(1 : 1)/TiO₂) exhibited the highest performance for the reaction. Complete conversion of LA with a 100% yield of GVL was achieved in 1,4-dioxane solvent under the reaction conditions of 180 °C, 4.0 MPa H₂ for 4 h, and the catalyst could be reused at least 6 times with only a slight loss of activity. Combined with the characterization results, the high performance of the catalyst was mainly attributed to the well-dispersed Cu–Re nanoparticles with a very fine average size (ca. 0.69 nm) and the co-presence of Cu–Re bimetal and ReO_x on the catalyst surface.

Herein, we report a highly efficient and recyclable Cu–Re(1 : 1)/TiO₂ bimetallic catalyst for liquid phase hydrogenation of levulinic acid to γ-valerolactone.     

A neutral Cu-based MOF for effective quercetin extraction and conversion from natural onion juice

We report herein a new microporous neutral three-dimensional (3D) metal–organic framework [Cu₂(L)(DMF)(H₂O)]·guest (1·guest) composed of copper paddle-wheel and flexible tetracarboxylic acid linkers (DMF = N,N-dimethylformamide, H₄L = tetrakis[(6-carboxynaphthoxy)methyl]methane). Surprisingly, this MOF with neutral cavities can not only extract pure quercetin (QT) but also convert it into Cu–QT during the desorption process. It has been well characterized by UV-vis, IR, ESI-MS and TEM-EDS studies. Moreover, it can efficiently extract natural product QT from fresh QT-rich onion juice and rapidly convert it into Cu–QT with a relatively high conversion rate.

A new neutral metal–organic framework can efficiently extract natural product quercetin (QT) from fresh QT-rich onion juice and rapidly convert it into Cu–QT with a relatively high conversion rate.     

Synergetic effect over flame-made manganese doped CuO–CeO2 nanocatalyst for enhanced CO oxidation performance

CuO–CeO₂ nanocatalysts with different amounts of Mn dopping (Mn/Cu molar ratios of 0.5 : 5, 1 : 5 and 1.5 : 5) were synthesized by flame spray pyrolysis (FSP) method and tested in the catalytic oxidation of CO. The physicochemical properties of the synthesised samples were characterized systematically, including using X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), oxygen-temperature programmed desorption (O₂-TPD), hydrogen-temperature programmed reduction (H₂-TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The results showed that the 1Mn–Cu–Ce sample (Mn/Cu molar ratio of 1 : 5) exhibited superior catalytic activity for CO oxidation, with the temperature of 90% CO oxidation at 131 °C at a high space velocity (SV = 60 000 mL g⁻¹ h⁻¹), which was 56 °C lower than that of the Cu–Ce sample. In addition, the 1Mn–Cu–Ce sample displays excellent stability with prolonged time on CO stream and the resistance to water vapor. The significantly enhanced activity was correlated with strong synergetic effect, leading to fine textual properties, abundant chemically adsorbed oxygen and high lattice oxygen mobility, which further induced more Cu⁺ species and less formation of carbon intermediates during the CO oxidation process detected by in situ DRIFTS analysis. This work will provide in-depth understanding of the synergetic effect on CO oxidation performances over Mn doped CuO–CeO₂ composite catalysts through FSP method.

The synergetic effect is promoted on Mn doped CuO–Ce O₂ catalyst to induce less carbon intermediates to enhance CO oxidation performance.     

In situ fabrication of hierarchical biomass carbon-supported Cu@CuO–Al2O3 composite materials: synthesis,properties and adsorption applications

Hierarchical Cu–Al₂O₃/biomass-activated carbon composites were successfully prepared by entrapping a biomass-activated carbon powder derived from green algae in the Cu–Al₂O₃ frame (H–Cu–Al/BC) for the removal of ammonium nitrogen (NH₄⁺-N) from aqueous solutions. The as-synthesized samples were characterized via XRD, SEM, BET and FTIR spectroscopy. The BET specific surface area of the synthesized H–Cu–Al/BC increased from 175.4 m² g⁻¹ to 302.3 m² g⁻¹ upon the incorporation of the Cu–Al oxide nanoparticles in the BC surface channels. The experimental data indicated that the adsorption isotherms were well described by the Langmuir equilibrium isotherm equation and the adsorption kinetics of NH₄⁺-N obeyed the pseudo-second-order kinetic model. The static maximum adsorption capacity of NH₄⁺-N on H–Cu–Al/BC was 81.54 mg g⁻¹, which was significantly higher than those of raw BC and H–Al/BC. In addition, the presence of K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions had no significant impact on the NH₄⁺-N adsorption, but the presence of Al³⁺ and humic acid (NOM) obviously affected and inhibited the NH₄⁺-N adsorption. The thermodynamic analyses indicated that the adsorption process was endothermic and spontaneous in nature. H–Cu–Al/BC exhibited removal efficiency of more than 80% even after five consecutive cycles according to the recycle studies. These findings suggest that H–Cu–Al/BC can serve as a promising adsorbent for the removal of NH₄⁺-N from aqueous solutions.

Hierarchical Cu–Al₂O₃/biomass-activated carbon composites were successfully prepared by entrapping a biomass-activated carbon powder derived from green algae in the Cu–Al₂O₃ frame (H–Cu–Al/BC) for the removal of ammonium nitrogen (NH₄⁺-N) from aqueous solutions.     

Electrolytic extraction of dysprosium and thermodynamic evaluation of Cu–Dy intermetallic compound in eutectic LiCl–KCl

The electrochemical reduction of dysprosium(iii) was studied on W and Cu electrodes in eutectic LiCl–KCl by transient electrochemical methods. Cyclic voltammogram and current reversal chronopotentiogram results demonstrated that dysprosium(iii) was directly reduced to dysprosium (0) on the W electrode through a single-step process with the transfer of three electrons. Electrochemical measurements on the Cu electrode showed that different Cu–Dy intermetallics are formed. Moreover, the thermodynamic properties of Cu–Dy intermetallic compounds were estimated by open circuit chronopotentiometry in a temperature range of 773–863 K. Using the linear polarization method, the exchange current density (j₀) of dysprosium in eutectic LiCl–KCl on the Cu electrode was estimated, and the temperature dependence of j₀ was studied to estimate the activation energies associated with Dy(iii)/Cu₅Dy and Dy(iii)/Cu_9/2Dy couples. In addition, potentiostatic electrolysis was conducted to extract dysprosium on the Cu electrode, and five Cu–Dy intermetallic compounds, CuDy, Cu₂Dy, Cu_9/2Dy, Cu₅Dy and Cu_0.99Dy_0.01 were identified by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Meanwhile, the change of dysprosium(iii) concentration was monitored using inductively coupled plasma-atomic emission spectrometry, and the maximum extraction efficiency of dysprosium was found to reach 99.2%.

The electrochemical reduction of dysprosium(iii) was studied on W and Cu electrodes in eutectic LiCl–KCl by transient electrochemical methods.     

Cu–Cd–Zn–S/ZnS core/shell quantum dot/polyvinyl alcohol flexible films for white light-emitting diodes

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation. The PL spectra of Cu–Cd–Zn–S/ZnS core/shell quantum dots can cover the whole visible light region in the case of only two ratios of Cu/Cd/Zn. The emission wavelength of Cu–Cd–Zn–S/ZnS QDs can be conveniently tuned from 474 to 515 and 548 to 629 nm by adjusting the pH value when the ratios of Cu/Cd/Zn are fixed at 1 : 5 : 80 and 1 : 5 : 10, respectively. It is worth noting that under the condition of a constant Cu/Cd/Zn ratio, the UV-vis absorption spectra do not change with the fluorescence spectra, indicating that the band gap of QDs remains unchanged during the change of pH value. The photoluminescence (PL) quantum yield of the as-prepared QDs with yellow emission is up to 76%. The QDs also show excellent chemical stability after the deposition of the ZnS shell. Luminescent and flexible films are fabricated by combining Cu–Cd–Zn–S QDs with polyvinyl alcohol (PVA). The QD/PVA flexible hybrid films are successfully applied on top of a conventional blue InGaN chip for remote-type warm-white LEDs. As-fabricated warm-white LEDs exhibit a higher color rendering index (CRI) of about 89.2 and a correlated color temperature (CCT) of 4308 K.

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation.     

Target-induced activation of DNAzyme for highly sensitive colorimetric detection of bleomycin via DNA scission

In this work, a label-free and sensitive colorimetric sensing strategy for the detection of bleomycin (BLM) was developed on the basis of BLM-mediated activation of G-quadruplex DNAzyme via DNA strand scission. A G-quadruplex based hairpin probe (G4HP) containing the scission site (5′-GT-3′) of BLM at the loop region and guanine (G)-rich sequences at its 5′-end was employed in this protocol. In the presence of BLM, it may cleave the 5′-GT-3′ site of the hairpin probe with Fe(ii) as a cofactor, releasing the G-tetrads DNA fragment, which may further bind hemin to form a catalytic G-quadruplex-hemin DNAzyme. The resultant G-quadruplex DNAzyme has notable peroxidase-like activity, which effectively catalyzes the oxidation of 2,2′-azino-bis(3-ethylbenzothiozoline-6-sulfonic acid) (ABTS) by H₂O₂ to produce the blue-green-colored free-radical cation (ABTS^·+). Therefore, the detection of BLM can be achieved by observing the color transition with the naked eye or measuring the absorbance at a wavelength of 420 nm using a UV-Vis spectrophotometer. Attributing to the specific BLM-induced DNA strand scission and the effective locking of G-tetrads in the stem of the G4HP, the colorimetric sensing strategy exhibits high sensitivity and selectivity for detection of BLM in human serum samples, which might hold great promise for BLM assay in biomedical and clinical research.

A label-free and sensitive colorimetric strategy for bleomycin detection was developed based on target-induced activation of DNAzyme via DNA scission.     

Preparation and characterization of squid pen chitooligosaccharide–epigallocatechin gallate conjugates and their antioxidant and antimicrobial activities

Chitooligosaccharide (COS) and epigallocatechin-3-gallate (EGCG) at various concentrations were used for the preparation of COS–EGCG conjugates. The highest total phenolic content (TPC), representing the amount of EGCG conjugated, was obtained for 1 wt% COS together with EGCG at 0.5 wt% (C1-E0.5-conjugate) or 1.0 wt% (C1-E1.0-conjugate) (66.83 and 69.22 mg EGCG per g sample, respectively) (p < 0.05). The 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging activities (DRSA and ARSA, respectively) and ferric reducing antioxidant power (FRAP) of all the samples showed similar trends with TPC. The C1-E0.5-conjugate had higher DRSA, ARSA, FRAP and oxygen radical absorbance capacity (ORAC) values than COS (p < 0.05). Similarly, the antimicrobial activity of COS increased when conjugated with EGCG (p < 0.05). FTIR, ¹H-NMR and ¹³C-NMR analyses confirmed the successful grafting of EGCG with COS. Therefore, 1 wt% COS and 0.5 wt% EGCG were used for the production of a conjugate with augmented antioxidant activity, which could be used to retard lipid oxidation of fatty foods.

Chitooligosaccharide from squid pen showed increases in both antioxidant and antimicrobial activities via conjugation with epigallocatechin-gallate (EGCG).     

Toluene adsorption on porous Cu–BDC@OAC composite at various operating conditions: optimization by response surface methodology

The work presented here describes the synthesis of Cu–BDC MOF (BDC = 1,4-benzenedicarboxylate) based on oxidized activated carbon (microporous Cu–BDC@OAC composite) using an in situ method. The adsorbents (oxidized activated carbon (OAC), Cu–BDC and microporous Cu–BDC@OAC composite) were characterized by XRD, FTIR, SEM, EDS and BET techniques. Optimization of operating parameters affecting the efficiency of adsorption capacity, including adsorbent mass, flow rate, concentration, relative humidity and temperature, was carried out by central composite design (CCD) of the response surface methodology (RSM). An adsorbent mass of 60 mg, a flow rate of 90 mL min⁻¹, the concentration of toluene (500 ppm), the relative humidity of 30% and a temperature of 26 °C were found to be the optimized process conditions. The maximum adsorption capacity for toluene onto Cu–BDC@OAC composite was 222.811 mg g⁻¹, which increased by almost 12% and 50% compared with pure Cu–BDC and oxidized AC, respectively. The presence of micropores enhances the dynamic adsorption capacity of toluene. The regeneration of the composite was still up to 78% after three consecutive adsorption–desorption cycles. According to the obtained adsorbent parameters, microporous Cu–BDC@OAC was shown to be a promising adsorbent for the removal of volatile organic compounds.

The work presented here describes the synthesis of Cu–BDC MOF (BDC = 1,4-benzenedicarboxylate) based on oxidized activated carbon (microporous Cu–BDC@OAC composite) using an in situ method.     

Heterogeneous Cu–Fe oxide catalysts for preferential CO oxidation (PROX) in H2-rich process streams

The influence of Fe loading in Cu–Fe phases and its effect on carbon monoxide (CO) oxidation in H₂-rich reactant streams were investigated with the catalyst material phases characterized by Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction (XRD) studies and Mössbauer Spectroscopy (MS). There was no change in the oxidation state of the Fe ions with copper or iron loading. The catalytic activity was examined in the feed consisting of H₂, H₂O and CO₂ for the preferential CO oxidation (PROX) process. These catalysts showed an optimized performance in converting CO in WGS streams in the temperature range of 80–200 °C. In addition to the formation of the CuFe₂O₄ phase, the Fe and Cu were found to be incorporated into a Cu–Fe supersaturated solid solution which improved CO oxidation activity, with carbon dioxide and water produced selectively with high catalytic activity in depleted hydrogen streams. Relatively high conversion of CO was obtained with high Fe metal loading. In addition to their catalytic efficiency, the employed heterogeneous catalysts are inexpensive to produce and do not contain any critical raw materials such as platinum group metals.

Fe loading in Cu–Fe phases and its effect on carbon monoxide oxidation in H₂-rich reactant streams were investigated with the catalyst material phases characterized by Field Emission Scanning Electron Microscopy, X-ray diffraction studies and Mössbauer Spectroscopy.     

Catalytic decomposition of N2O over Cu–Al–Ox mixed metal oxides

Cu–Al–O_x mixed metal oxides with intended molar ratios of Cu/Al = 85/15, 78/22, 75/25, 60/30, were prepared by thermal decomposition of precursors at 600 °C and tested for the decomposition of nitrous oxide (deN₂O). Techniques such as XRD, ICP-MS, N₂ physisorption, O₂-TPD, H₂-TPR, in situ FT-IR and XAFS were used to characterize the obtained materials. Physico-chemical characterization revealed the formation of mixed metal oxides characterized by different specific surface area and thus, different surface oxygen default sites. The O₂-TPD results gained for Cu–Al–O_x mixed metal oxides conform closely to the catalytic reaction data. In situ FT-IR studies allowed detecting the form of Cu⁺⋯N₂ complexes due to the adsorption of nitrogen, i.e. the product in the reaction between N₂O and copper lattice oxygen. On the other hand, mostly nitrate species and NO were detected but those species were attributed to the residue from catalyst synthesis.

Cu–Al–O_x mixed metal oxides with intended molar ratios of Cu/Al = 85/15, 78/22, 75/25, 60/30, were prepared by thermal decomposition of precursors at 600 °C and tested for the decomposition of nitrous oxide (deN₂O).     

Influence of cerium doping on Cu–Ni/activated carbon low-temperature CO-SCR denitration catalysts

In this study, to evaluate the effects of two methods for activation of nitric acid, air thermal oxidation and Ce doping were applied to a Cu–Ni/activated carbon (AC) low-temperature CO-SCR denitration catalyst. The Cu–Ni–Ce/AC_0,1 catalyst was prepared using the ultrasonic equal volume impregnation method. The physical and chemical structures of Cu–Ni–Ce/AC_0,1 were studied using scanning electron microscopy, Brunauer–Emmett–Teller analysis, Fourier-transform infrared spectroscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, CO-temperature programmed desorption (TPD) and NO-TPD characterisation techniques. It was found that the denitration efficiency of 6Cu–4Ni–5Ce/AC₁ can reach 99.8% at a denitration temperature of 150 °C, a GHSV of 30 000 h⁻¹ and 5% O₂. Although the specific surface area of the AC activated by nitric acid was slightly lower than that activated by air thermal oxidation, the pore structure of the AC activated by nitric acid was more developed, and the number of acidic oxygen-containing functional groups was significantly increased. Ce metal ions were inserted into the graphite microcrystalline structure of AC, splitting it into smaller graphene fragments, whereby the dispersibility of Cu and Ni was improved. In addition, many reaction units were formed on the catalyst surface, which could adsorb more CO and NO reaction gases. With the increase in Ce doping, the relative proportions of Cu²⁺/Cuⁿ⁺, Ni³⁺/Niⁿ⁺ and surface adsorbed oxygen (Oα) in the Cu–Ni–Ce/AC_0,1 catalyst increased. In addition, after the introduction of Ce into Cu–Ni/AC, the amount of weak and medium acids significantly increased. This may be due to the Ce species or its influence on the Cu/Ni species. Further, the active sites of the acid were more exposed. According to the results of the study, a composite metal oxide CO-SCR denitration mechanism is proposed. Through the oxidation–reduction reaction between the metals, the reaction gas of CO and NO is adsorbed and the incoming O₂ is converted into (Oα), which promotes the conversion of NO into NO₂. The CO-SCR reaction is accelerated, and the rate of low-temperature denitration was increased. Overall, the results of this study will provide theoretical support for the research and development of low-temperature denitration catalysts for sintering flue gas in iron and steel enterprises.

In the process of denitrification, the reaction between NO and CO (NO + CO → N₂ + CO₂) occurs. There will be a redox reaction between copper, nickel and cerium (Cu²⁺ + Ce³⁺ → Cu⁺ + Ce⁴⁺, Ni³⁺ + Ce³⁺ → Ni²⁺ + Ce⁴⁺).