Cu2O Crystals Research Articles

Ferromagnetism of Single-Crystalline Cu2O Induced through Poly(N-vinyl-2-pyrrolidone) Interaction Triggering d-Orbital Alteration

Ferromagnetic-like properties of cuprous oxide (Cu2O) are induced through its interaction with chemisorbed surfactant poly(N-vinyl-2-pyrrolidone) (PVP), which alters the intrinsic d10 configuration of Cu ions. Structural and magnetism-related properties of intact Cu2O crystals (i-Cu2O) and those capped with PVP (c-Cu2O) were examined using various analytical instruments. SEM, TEM (corresponding selected area electron diffraction (SAED)), and XRD of i-Cu2O and c-Cu2O showed cubic and hexagonal shapes of single crystallinity with facets of {200} and {111}, respectively, resulting from the differential growth rates of the original identical crystals along the facets over time. Bulk magnetic susceptibility (χ) of i-Cu2O and c-Cu2O at room temperature in field-dependent magnetization and the difference in their magnetic moment in temperature-dependent magnetization showed diamagnetic and ferromagnetic properties, respectively. The difference in the fluorescence mode of X-ray absorption near edge structure (XANES) spectra between i-Cu2O and c-Cu2O, showing no quadruple pre-edge peak for the transition 1s → 3d in Cu(II) ions with d9 electronic configuration, indicates an orbital alteration on the surface of c-Cu2O caused by an interaction with PVP. Two peaks for c-Cu2O at higher binding energies in O 1s X-ray photoelectron spectroscopy may be indicative of the ligand-to-metal charge transfer (LMCT) from O atoms of PVP to Cu ions of Cu2O, generating a chemical interaction through coordination bonding. Large hyperfine splitting constants in electron paramagnetic resonance (EPR) spectra of c-Cu2O support this interpretation, with septet hyperfine splitting suggestive of Cu–Cu interactions on the surface of c-Cu2O via the interaction with O atoms of PVP. These results demonstrate that PVP capping of Cu2O crystal (c-Cu2O) induces ferromagnetism of Cu(I) ions through coordination with O atoms of chemically adsorbed PVP. This may induce LMCT and Cu–Cu interactions that lead to changes in electronic configurations, deriving the ferromagnetic moments of c-Cu2O.

Facile synthesis of CeO2 hollow structures with controllable morphology by template-engaged etching of Cu2O and their visible light photocatalytic performance

The novel ceria (CeO2) hollow structures with uniform cubic, polyhedral and sphere shapes were successfully synthesized by template-engaged coordinating etching of shape-controlled Cu2O crystals. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microcopy (TEM), high resolution of transmission electron microcopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) and photoluminescence spectroscopy (PL). The photocatalytic oxygen evolution via water oxidation was investigated for CeO2 hollow structures with varied shapes under visible light irradiation. The photocatalytic results indicate that polyhedral CeO2 nanocages show the highest photocatalytic activity, in contrast with spherical CeO2 hollow structure and cubic CeO2 hollow structure. The excellent catalytic activity can be attributed to the unique properties of the polyhedral CeO2 nanocages, including efficient light refection through the inner shells, more active sites for enhancing separation efficiency of charge carriers. It is expected that this study could provide helpful results for designing and exploration of novel hollow structures with tunable photocatalytic performance.

Ultrathin willow-like CuO nanoflakes as an efficient catalyst for electro-oxidation of hydrazine

In this paper, preparation of ultrathin willow-like CuO nanoflakes via a one-step process was reported. X-ray diffraction pattern showed the formation of monoclinic CuO crystal, which was also confirmed by result of high resolution transmission electron microscopy. Scanning electron microscopy showed that ultrathin willow-like CuO nanoflakes were formed. Catalytic testing indicated that the ultrathin willow-like CuO nanoflakes exhibited high electrocatalytic activity and durability toward the electro-oxidation of hydrazine in alkaline medium. The results suggested that the as-prepared CuO nanoflakes were potential electrode materials for hydrazine fuel cell.

Studies on formation mechanism of 3D Cu2O nanospheres through self-assembly of 0D nanodots

Cu2O crystals with different morphologies (solid and porous) and sizes (from 25 to 282nm) were synthesized controllably through a facile solvothermal route. The growth mechanism was investigated by SEM and TEM with varying the concentration of poly (vinylpyrrolidone) (PVP, K30), CH3COO− (Ac−) and NO3− acid ions in the precursor solution. The self-assembly of three types of Cu2O nano-structures was observed through a general route of zero-dimensional (0D)→2D→3D. When Cu(Ac)2⋅H2O was used as the copper sources, 0D Cu2O nanodots with size of 2–7nm were firstly assembled to 2D quasi-spherical and bookmark-like structures via Oriented attachment (OA), and then converted into 3D hierarchical Cu2O nanoclusters (a few tens of nm) and porous sub-microspheres with an average size of 282nm, respectively. While Cu(NO3)2⋅3H2O was used instead of Cu(Ac)2⋅H2O, the similar assembly process occurred leading to the formation of Cu2O porous nanospheres of 40–140nm which exhibit better adsorption ability toward methyl orange compared with activated carbon. In addition, we also investigated the dependence of Cu2O crystals on the concentration of acid ions (Ac− and NO3−). Compared with Ac−, the size and morphology of the obtained products were less dependent on the concentration of NO3− acid ions. This study might provide a new insight into the growth mechanism of Cu2O based micro- or nanostructures.

Crystal-facet-controllable synthesis of Cu2O microcrystals, shape evolution and their comparative photocatalytic activity

Single crystal Cu2O materials with morphologies of cube, truncated cube, truncated octahedron, octahedron etc. have been synthesized by a facile glucose reduction method. Polyvinylpyrrolidone concentration dependent, crystal-facet-controllable growth mechanism of the synthesized Cu2O crystals has been proposed in the article. The photocatalytic test results show that the octahedral Cu2O crystals with dominant {111} planes have super photocatalytic activity than the cubic ones packaged by {100} surfaces, which could be explained by that the polar, highly active {111} facets with unsaturated “Cu” have more excellent activity on the methyl orange dye degradation. This article will contribute to promote the morphology construction and crystal-dependent catalytic properties of metal oxide semiconductor materials.

N-type Cu2O doped activated carbon as catalyst for improving power generation of air cathode microbial fuel cells

A novel n-type Cu2O doped activated carbon (AC) air cathode (Cu/AC) was developed as an alternative to Pt electrode for oxygen reduction in microbial fuel cells (MFCs). The maximum power density of MFCs using this novel air cathode was as high as 1390±76mWm−2, almost 59% higher than the bare AC air cathode. Specifically, the resistance including total resistance and charge transfer resistance significantly decreased comparing to the control. Tafel curve also showed the faster electro-transfer kinetics of Cu/AC with exchange current density of 1.03×10−3Acm−2, which was 69% higher than the control. Ribbon-like Cu2O was deposited on the surface of AC with the mesopore surface area increasing. Cubic Cu2O crystals exclusively expose (111) planes with the interplanar crystal spacing of 2.48Å, which was the dominate active sites for oxygen reduction reaction (ORR). N-type Cu2O with oxygen vacancies played crucial roles in electrochemical catalytic activity.

Facet-dependent electrical conductivity properties of Cu2O crystals.

It is interesting to examine facet-dependent electrical properties of single Cu2O crystals, because such study greatly advances our understanding of various facet effects exhibited by semiconductors. We show a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is nonconductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets.

Additive Free Co-Deposition of Nanocrystalline Copper/Cuprous Oxide by Electrodeposition

We report a gradual transition in the deposition product from pure Cu2O to pure Cu, during electrodeposition on Au/Pd sputter-coated silicon wafer substrates in copper sulfate electrolyte with various dc potential. At voltages lower than 0.3 V, only pure Cu in a nanocrystalline form is deposited on the cathode substrate, while at voltages higher than 1.2 V, only pure Cu2O, also in a nano/microcrystalline form, is deposited. At intermediate voltages between 0.3 to 1.2 V, the deposition product comprises a mixture of both Cu and Cu2O nano/micro-crystals. The Cu2O crystals are generally of an octahedral shape with sizes ranging from 30 nm to 100 nm, while Cu nano/microcrystals are of irregular shape ranging from 100 nm to 2 μm. This work provides a method to fabricate nanocrystalline Cu2O, Cu and Cu/Cu2O on substrates in a single step without the use of additives.

Shape-controlled preparation of Cu2O crystals and their growth mechanism

A facile solution-phase method is presented via reducing the copper-citrate complex solution with glucose to synthesis Cu2O crystals with different shapes from cubes, truncated octahedral, and finally to octahedral by adjusting the reaction conditions including Polyvinyl Pyrrolidone (PVP) concentration, reaction time, reaction temperature and so on. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) have been applied to analyze the properties of the crystals with various architectures. The comparison shows that the growth mechanism is possibly based on the variation of the ratio (R) of the Cu2O crystal growth rates along the {100} and {111} direction.

Controllable morphology and conductivity of electrodeposited Cu₂O thin film: effect of surfactants.

Both the morphology and conductivity of Cu2O films are controlled in a facile electrodeposition process by tuning the concentration of surfactants. With the increase of the concentration of sodium dodecyl sulfate (SDS) in the plating solution, the average size of Cu2O crystals increases, and the electrical conductivity of Cu2O films changes from n-type to p-type. When the concentrations of SDS are lower than 0.85 mM, the electrodeposited Cu2O films show n-type conductivity because of the formation of oxygen vacancies or copper atoms. When the concentration of SDS is higher than 1.70 mM, the electrodeposited Cu2O films show p-type conductivity owing to the formation of copper vacancies. The concentrations of both the donors and the acceptors increase with the concentration of SDS. The effects of surfactants on the morphology and conductivity of electrodeposited Cu2O films are attributed to the adsorption of SDS molecules on the electrode substrate occupying the deposition sites of Cu(2+) ions and the adsorption of SDS micelles to Cu(2+) ions hindering the diffusion of Cu(2+) ions to the electrode, which affect the reduction rate of Cu(2+) ions and the formation of oxygen vacancies or copper vacancies during the electrodeposition.

Physicochemical properties of Cu loaded onto core–shell Al-MCM-41: Effect of loading methods

Copper species in the structure of Cu/core–shell Al-MCM-41 catalysts prepared by different techniques of Cu loading including substitution, ion-exchange, and impregnation methods were characterized by using temperature-programmed reduction, surface area and porosity analyzer, diffuse reflectance UV–vis-near-infrared spectroscopy, Fourier transforms infrared spectroscopy. Cu2+ in tetrahedral coordination was mostly found from the substitution method. However, from this method CuO was formed from the excess solution of copper nitrate trapped within pores of Al-MCM-41. For the ion-exchange method, only Cu2+ ions were detected. On the other hand, for the impregnation method with high copper loading, the observed copper species was CuO crystals, while with low copper loading both CuO and Cu2+ ions were detected. The acidity of catalysts was also studied by NH3-TPD. It was found that the distinction of copper species in the structure had an effect on Brønsted acid of pre- and post-reduction catalysts, especially in the impregnation method. This study reveals a perspective in the fundamental understanding of copper species-based catalysis.

Characterisation and Optical Studies of Copper Oxide Nanostructures Doped with Lanthanum Ions

Abstract Copper Oxide is an extensively studied group II-VI semiconductor with optical properties. It exhibits a wide variety of morphologies in the nano regime that can be grown by tuning the growth habit of the CuO crystal. CuO nano materials with an average particle size of 15-27 nm are synthesized by chemical route. XRD, SEM, FTIR UV-Vis and EDS characterize the samples. The percentage of doping material is confirmed from the EDS spectra. The average crystal size of the prepared CuO: La nanopowder is determined by XRD. The UV absorption spectra revealed the absorption edge at wavelength 389 nm indicating the smaller size of CuO:La nano particle. The optical direct band gap energy of doped CuO nanoparticle is found to be in the range 3.149 eV. The increasing red shift with decreasing particle size suggests that the defects responsible for the intra gap states are primarily surface defect. The La doped CuO is highly effective and can significantly enhance the photo catalytic degradation.

Leaf‐like copper oxide nanocrystals synthesized by a gas‐liquid diffusion method at room temperature

The leaf‐like copper oxide (CuO) nanocrystals have been synthesized by a gas‐liquid diffusion method in pure aqueous solution at room temperature. The structure, morphologies and related properties of the as‐prepared crystals were characterized with X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED) and thermogravimetric analysis (TG). The influence of copper concentration was investigated, which plays an important role in the formation of nanostructured CuO crystals. Only when the copper concentration was low enough (0.005 M) that the leaf‐like CuO could be obtained directly. Additionally, a growth mechanism of CuO was also proposed based on the observed results.

Fabrication, morphology formation mechanism and properties of nanometer Cu2O thin film with KCl-doping

With CH3COONa/(CH3COO)2Cu as the electrolyte system, nanometer Cu2O thin film is fabricated by means of electrodeposition at TiO2/ITO electrode under the weak acid environment. The crystal morphology of nanometer Cu2O thin film is characterized by XRD, SEM and XPS. The effects of KCl-doping and annealing treatment upon nanometer Cu2O crystal morphology, thin film response to visible light and open circuit voltage are studied. The mechanism of KCl-doping to control thin film morphology is discussed. The results indicate that with the increase of KCl concentration, making nanometer Cu2O thin film compact, the average grain diameter and the open circuit voltage increase, and the forbidden band width decreases, and exhibited obvious absorbance response in visible light range, the preferred orientation character of Cu2O crystal has changed from the obvious (111) face preferred orientation into the obvious (200) face preferred orientation. The crystal morphology of nanometer Cu2O thin film has changed from regular octahedron into truncated cube when the KCl concentration reaches 7.0 mmol L−1. The transformation of nanometer Cu2O thin film morphology is caused by the preferential adsorption of Cl− at the (200) crystal face of Cu2O crystal.

Synthesis of Cu2O micro/nanocrystals with tunable morphologies using coordinating ligands as structure controlling agents and antimicrobial studies

Amino acid based reduced Schiff base ligands that have often produced intriguing supramolecular coordination polymeric structures with metal ions have been used as structure controlling agents in the synthesis of Cu2O micro/nanocrystals. Cu2O micro/nanocrystals in the form of solid spheres, cubes, rods, flat surfaced powders, spheres with rough surfaces and octahedra were obtained by changing the amino acid structure, hydroxyl functionality and oxidizing agent. The absorption studies revealed that the Cu2O micro/nanocrystals with different morphologies exhibited tunable optical band gaps between 1.71 eV and 2.44 eV. Antibacterial studies of Cu2O with different morphologies showed that Cu2O crystals dispersed in liquid are highly effective against both Gram-positive and Gram-negative bacteria whereas, when tested by the agar diffusion method, they were effective only against Bacillus subtilis. All six morphologies exhibited a similar effect, and the antibacterial effect could possibly be attributed to the altered membrane permeability caused by the Cu2O micro/nanocrystals. Comparatively higher membrane permeability was observed in the case of Gram-negative bacteria relative to Gram-positive bacteria.

Direct synthesis of dimethyl ether as a green fuel from syngas over nanostructured CuO–ZnO–Al2O3/HZSM-5 catalyst: Influence of irradiation time on nanocatalyst properties and catalytic performance

CuO–ZnO–Al2O3/HZSM-5 (CZAZ) nanocatalyst is prepared by ultrasound-assisted co-precipitation method at different irradiation times, characterized and tested for direct synthesis of DME from syngas. Synthesized nanocatalysts were characterized using XRD, FESEM, PDS, EDX and BET techniques. Direct synthesis of DME is carried out at 200–300 °C and 10–40 bar using a mixture of CO:H2 = 2:1. The catalyst with the longest sonication time showed the smallest degree of aggregates, the highest dispersion and surface area. Although the long irradiated nanocatalyst has slightly bigger CuO crystal size but the size of particle aggregates was small and less populated. The sonicated catalyst with longest irradiation presented a high CO conversion of ca. 40%. It seems that, not the CuO crystal size but the size of particle aggregates and nanocatalyst surface had a great effect on the CZAZ nanocatalyst performance. While there was an optimal temperature for CO conversion and DME yield in direct synthesis of DME, both the CO conversion and the DME yield increased with the pressure increase. Long irradiated nanocatalyst yielded more stable CuO–ZnO–Al2O3/HZSM-5 nanocatalyst while conventional co-precipitated nanocatalyst lost its activity ca. 11% and 58% in terms of CO conversion and DME yield, respectively, in 24 h time on stream test.

Dispersed Cu₂O octahedrons on h-BN nanosheets for p-nitrophenol reduction.

We demonstrate here that two-dimensional boron nitride (h-BN) nanosheets can be employed as a robust supporting substrate to incorporate function metal oxides. The Cu2O@h-BN composites are thus obtained by dispersing Cu2O octahedrons on the surfaces of h-BN nanosheets. The -OH and -NH groups on the surfaces of h-BN nanosheets are found to be beneficial for anchoring Cu2O octahedrons. Moreover, the Cu2O@h-BN composites exhibit superior activity for the reduction of p-nitrophenol to pure Cu2O crystals and h-BN nanosheets. The h-BN component in the composites plays a critical role in the formation and adsorbing of the p-nitrophenolate ions, and, at the same time, Cu2O components react with brohydride ions and transfer a surface hydrogen species and electrons, resulting in the reduction of p-nitrophenol into p-aminophenol. Our results provide a new approach for the rational design and development of metal oxides composites and open the way to a range of important applications of h-BN-based materials.

Direct conversion of syngas to DME over CuO-ZnO-Al2O3/HZSM-5 nanocatalyst synthesized via ultrasound-assisted co-precipitation method: New insights into the role of gas injection

In this study, direct synthesis of dimethyl ether (DME) is conducted over a bifunctional CuO–ZnO–Al2O3/H Zeolite Socony Mobil-5 (HZSM-5) nanocatalyst. A hybrid method of ultrasound-assisted co-precipitation is used for the synthesis of catalysts, and the effect of gas injection during sonication is investigated. The physicochemical characteristics of the catalysts are analysed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), particle size distribution (PSD), energy dispersive X-ray (EDX), Brunauer–Emmett–Teller (BET) and Fourier-transformed infrared (FTIR) methods. In the absence of gas injection, the acetate-based catalysts have a better morphology and higher surface area than the nitrate-based catalyst. Gas injection significantly changes the morphology and structural properties of the acetate-based catalyst. High surface area, narrow PSD and better dispersion of small CuO crystals are obtained in a gas-injected synthesized sample. DME synthesis experiments showed that the CO conversion and DME selectivity are correlated with surface area, nanocatalyst particle size and its dispersion. The gas-injected CuO–ZnO–Al2O3/HZSM-5 nanocatalyst that has the highest surface area and the smallest dispersed particles showed more than 70% DME selectivity. The gas-injected CuO–ZnO–Al2O3/HZSM-5 nanocatalyst exhibited high stability in terms of CO conversion and DME yield over 1440-min time on a stream test at 275°C, 40 bar and 18 000 cm3 g.h−1. Copyright © 2014 John Wiley & Sons, Ltd.

Cu2O polyhedral nanowires produced by microwave irradiation

The present work reports the oxidation of copper nanowires via microwave irradiation under wet conditions. Structural characterization of the nanowires has been carried out by X-ray diffraction, transmission electron microscopy and scanning electron microscopy coupled with focused ion beam, electron backscatter diffraction and energy dispersive X-ray spectroscopy. The copper nanowires were essentially monocrystalline with 〈110〉 axes, albeit growth twins have also been observed. Oxidation of the metal nanowires resulted in the growth of polyhedral Cu2O crystals arranged in twinned configurations around the nanowire axis. The crystallographic orientation of the twin variants is compatible with a cube-on-cube orientation relation originating from the epitaxial growth of Cu2O from pure Cu.

Direct formation of small Cu2O nanocubes, octahedra, and octapods for efficient synthesis of triazoles.

In most studies describing the preparation of Cu2O crystals of various morphologies, the particle sizes are normally hundreds of nanometers to micrometers due to rapid particle growth, so they are not exactly nanocrystals. Here we report surfactant-free formation of sub-100 nm Cu2O nanocrystals with systematic shape evolution from cubic to octahedral structures by preparing an aqueous mixture of Cu(OAc)2, NaOH, and N2H4 solution. Adjustment of the hydrazine volume enables the particle shape control. Uniform nanocubes and octahedra were synthesized with edge lengths of 37 and 67 nm, respectively. Novel Cu2O octapods with an edge length of 135 nm were also produced by mixing CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH · HCl reductant solution. All of them are nearly the smallest Cu2O nanocrystals of the same shapes ever reported. These small cubes, octahedra, and octapods were employed as catalysts in the direct synthesis of 1,2,3-triazoles from the reaction of alkynes, organic halides, and NaN3 at 55 °C. All of them displayed high product yields in short reaction times. The octahedra enclosed by the {111} facets are the best catalysts, and can catalyze this cycloaddition reaction with high yields in just 2 h when different alkynes were used to make diverse triazole products. Hence, the small Cu2O particles provide time-saving, energy-efficient, and high product yield benefits to organocatalysis.