Cu2O Crystals Research Articles

Facet-dependent electrical and mechanical properties of polyhedral Cu2O under compression

In this paper, both the facet-dependent electrical properties and mechanical stability of submicron-sized polyhedral Cu2O are investigated with in-situ resistivity measurements with pressure up to 25.0 GPa. The pressure-induced morphology changes are characterized by scanning/transmission electron microscopy. X-ray photoelectron spectroscopy measurements are also adopted to rationalize the diversity of the electrical resistivity of polyhedral Cu2O under compression. The electrical properties of cubic, truncated octahedral, and octahedral Cu2O show extremely different pressure dependence, which can be attributed to the preferential adsorption of oxygen on Cu2O (111) facet. The anomalous changes in electrical resistivity at 0.7–2.2, 8.5, 10.3, and 21.6 GPa are due to pressure-induced structural phase transitions of the Cu2O crystals. The dramatic reductions of resistivity in cubes and octahedras at 15.0 GPa and of truncated octahedras at 21.2 GPa are driven by pressure-induced crush and nanocrystallization of Cu2O crystals. The mechanical stability of truncated octahedral Cu2O is better than those of both cubic and octahedral Cu2O. Our results not only elucidate the underlying mechanisms for understanding the two previously reported pressure-dependent electrical resistance change trends difference, but also give a reasonably explanation of the facet-dependent mechanical property of Cu2O.

Se-doped CuO NPs/H2O2/UV as a highly efficient and sustainable photo-Fenton catalytic system for enhanced degradation of 4-bromophenol

Abstract Brominated aromatic compounds including bromophenols are hazardous and are being fast recognized as priority pollutants. We present here a novel photo-Fenton based degradation system using selenium doped copper oxide nanoparticles (Se-doped CuO NPs)/H2O2/UV, which facilitated 99.5% degradation of 4-bromophenol in 90 min at a rate of 0.057 min−1. The Se-doped CuO NPs are prepared by precipitation method, and their structural, morphological and optical properties are characterized. The selenium doping is chosen owing to its favorable photo-absorption property and its doping in CuO NPs improved charge transfer properties and resulted in enhanced in-situ generation of hydroxyl radicals. The selenium doping also restricted the growth of CuO crystals, due to which the surface area of the nanoparticles increased and led to higher rate of photochemical reaction. The degradation profile is measured by high performance liquid chromatography method. The photo-Fenton process is initiated by extraneous H2O2 and the reaction is sustained by the in-situ generation of hydroxyl radicals, as demonstrated from the ROS scavenger studies. The de-bromination, formation of 4-benozoquinone as an intermediate degradation product and production of anionic species in residual solution after degradation due to cleavage of the molecular structure of 4-bromophenol are the key events confirmed by ion chromatography and gas chromatography coupled with mass spectrometry studies. The robustness of the Se-doped CuO NPs as catalyst has been confirmed from the re-usability studies for seven cycles.

Formation of Cu2O and ZnO Crystal Layers by Magnetron Assisted Sputtering and Their Optical Characterization

Copper (I) oxide and zinc oxide films are formed on silicon and glassy quartz substrates by magnetron assisted sputtering. The thickness of the films is tens and hundreds of nanometers. The films are grown at different substrate temperatures and different oxygen pressures in the working chamber. The film samples are studied by the X-ray diffraction technique, scanning electron microscopy, and optical methods. It is established that an increase in the substrate temperature yields a change in the surface morphology of copper (I) oxide films towards the formation of well-pronounced crystallites. The reflectance and Raman spectra suggest that the quality of such films is close to that of bulk Cu2O crystals produced by the oxidation of copper. As concerns ZnO films, an increase in the substrate temperature and an increase in the partial oxygen pressure make it possible to produce films, for which a sharp exciton structure is observed in the reflectance spectra and the emission of excitons bound at donors is observed in the luminescence spectra.

Efficiency enhancement of ZnO/Cu2O solar cells with well oriented and micrometer grain sized Cu2O films

Cu2O is one of the attractive photovoltaic materials for solar cells because of its low cost, nontoxicity, and good mobility. In this paper, an obvious enhancement of power conversion efficiency (PCE) for ZnO/Cu2O solar cells with perfectly oriented and micrometer grain sized Cu2O films was experimentally demonstrated. Cu2O was fabricated using radical oxidation of Cu foils at a low temperature of 500 °C. When followed by a rapid quenching and post annealing treatment, the perfectly oriented and micrometer sized Cu2O crystals (3∼4 μm) could be obtained. The crystal structure and optical properties of Cu2O were investigated in detail. Compared to conventional solar cells without any treatment, the PCE of the solar cells based on Cu2O with treatment was 3.18%, corresponding to a significant PCE improvement of 60.6%.

Template effect of hydrophobically associating polymers on the construction of cuprous oxide micro structure

To determine the template effect of hydrophobically associating copolymers(HACPs) on the morphology of nano/micro structures, six HACPs were synthesized and used as templates to biomimetically synthesize cuprous oxide(Cu2O), an important semiconductor. This experiment showed a clear relationship between the associating state of the HACP molecules and the morphology of the Cu2O particles. Cu2O hollow spheres were preferentially prepared when the HACP molecules were in an intramolecular associating state. Furthermore, a Cu2O hexapod was easily ob-tained when the HACP molecules were in an intermolecular associating state. The morphologies of the Cu2O crystals prepared in the presence of the HACPs also confirmed this result.

Differentiating Ammonia from Other Reducing Gases via Response Reversal Phenomena by Varied ZnO/CuxO Nanorod Arrays

Semiconductor gas sensors have attracted much attention, but poor specificity limits their practical applications. In the current work, a universal design with varied n-p core-shell structures was proposed to distinguish NH3 gas from other reducing gases by a p-n competition mechanism. Acting as a model system, devices made of n-ZnO/p-CuxO core-shell nanorod (NR) arrays with variable shell thicknesses prepared by a two-step route demonstrate this unique gas specificity behavior. The transmission electron microscopy results show the outer CuxO shells on the surface of NRs are comprised of major Cu2O and minor CuO crystals. When the shell thickness is greater than a critical value, the NR devices exhibit a p-type response to all the testing reducing gases. When the shell thickness is less than the critical value, only NH3 among the testing reducing gases results in an unusual n-type response, which demonstrates extraordinary discernment to ammonia gas by only observing the trend in the increase or decrease of resistance. A thickness-dependent band structures upon gas sensing is proposed to explain the improved gas specificity.

Work function: a determining factor of the photodegradation rate of methyl orange via hollow octadecahedron Cu2O crystals.

Currently, photocatalytic degradation of methyl orange by Cu2O nanocrystals with different morphologies is mainly attributed to their narrow bandgap (Eg) and highly active lattice facets. Herein, three different sizes of hollow octadecahedron Cu2O crystals, with different degrees of exposed {110} facets, were developed and exhibited excellent, stable photocatalytic degradation performances. This study has shown that the intermediates in the photocatalytic degradation process are mainly peroxy radicals from the reaction of excited electrons in the Cu2O conduction band with oxygen molecules adsorbed on the surface. In the photodegradation process of the hollow octadecahedron (HO) Cu2O crystals, the excited-state electrons must break away from the work function restriction before participating in the reaction. HO-2 has achieved the best photocatalytic properties due to its smallest work function as a determining factor affecting the photocatalytic activity.

Facet-Dependent Electrical, Photocatalytic, and Optical Properties of Semiconductor Crystals and Their Implications for Applications.

Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as Cu2O, Ag2O, TiO2, PbS, and Ag3PO4 exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of Cu2O crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows good understanding of the observed phenomena. Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, we should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photocatalytic efficiency or design electrical components utilizing the facet effects. There should be many opportunities to advance applications of semiconductor nanocrystals and nanostructures with continued research on the facet-dependent properties of various semiconductor materials.

CTAB-assisted synthesis of eight-horn-shaped Cu2O crystals via a simple solution approach

A novel type of eight-horn-shaped Cu2O microstructures with a high degree of symmetry in structure was prepared via a simple solution route. The copper(II) chloride dehydrate was reduced by glucose as an environmental friendly reductant in the presence of cetyltrimethylammonium bromide (CTAB) surfactant. The size of these Cu2O crystals is about 4 µm, and the total 24 edges of eight horns are truncated. The obtained Cu2O was characterized by XRD, SEM, and HRTEM techniques, and the results showed that the formation of such eight-horn-shaped Cu2O microstructures was strongly depended on the addition of CTAB. This work might provide a low-cost and naturally abundant microstructured material for use in electronic devices and catalysis.

New electrochemiluminescence catalyst: Cu2O semiconductor crystal and the enhanced activity of octahedra synthesized by iodide ions coordination

Cuprous oxide (Cu2O) crystals are usually used as the photocatalysts of water splittings and carbon monoxide oxidations. Herein, we report the new catalytic properties of Cu2O to the electrochemilumenescence (ECL) reactions of luminol and oxygen. Adjusting the shape of Cu2O microcrystals from cube to octahedra also facilities the improved electrocatalytic acticity, where octahedral Cu2O microcrystals possess higher ECL signal by 25% at −0.6 V (versus Ag/AgCl) at pH 7.4. The octahedral Cu2O microcrystals are synthesized by the coordination of iodide ions to the surface, which alters the crystals shapes from cubes to octahedra. Size-distribution of octahedra is improved with this mehod compared to the previous studies with only hydroxide ions as the coordinates. The catalytic activity of octahedral Cu2O crystals is expected to be compared with the noble metal nanomaterials and constructed the high-efficiency and low-cost ECL biosensors.

Morphology-controllable Cu2O supercrystals: Facile synthesis, facet etching mechanism and comparative photocatalytic H2 production

Monodisperse Cu2O-cubes, Cu2O-concave cubes and Cu2O-octapods supercrystals have been synthesized successfully using a self-assembly approach in the presence of dioctyl sulfosuccinate sodium salt (AOT) surfactant. Specifically, the supercrystals can be prepared by adding aqueous Cu(Ac)2 solution into butyl alcohol solution of AOT to first assemble nanoreactors and then perform hydrothermal reactions. The structures of the supercrystals can be tailored by simply varying the amount of Cu (Ac)2 with the same ratio of the Cu(Ac)2 to ascorbic acid. AOT surfactant was crucial to precisely control the supercrystal morphology. It was demonstrated that the ratio of growth rate along different planes varied by adjusting the amount of Cu(Ac)2 and the volume ratio of alcohol to water, thus Cu2O crystals with cubes, concave cubes, and octapods with the size of 1.2–2.2 μm were obtained. The morphology evolution from cubes, concave cubes to octapods was systematically studied and a facet etching mechanism is proposed. Hence, precise control of the amount of Cu(Ac)2 enables this facet etching, novel Cu2O with concave {100} facets can be fabricated. Photocatalysis investigation on the morphology-controlled Cu2O indicates that the Cu2O-octapods supercrystals showed the highest amount of H2 and the best cyclic-stability. This work reveals that morphology-control of supercrystals is important to the development of novel photocatalysis.

Molecular dynamics simulation of the interaction between methyl benzotriazole and Cu2O crystal

In circulating water system, methyl benzotriazole (TTA) is one of the common corrosion inhibitors for copper. But the inhibition mechanisms have not been clearly understood so far. In different number of water molecules, the interaction between TTA and Cu2O (copper surface) was investigated with molecular dynamics (MD) method. The results showed that the MD simulation result with water was more consistent with the experiment results. In different number of water molecules, the sequence of the interaction energies between TTA and Cu2O (001) was ∆E1 (150H2O) > ∆E1(200H2O) > ∆E1(100H2O) > ∆E1(50H2O) > ∆E1(0H2O). The number of water molecules had an important influence on the interaction between corrosion inhibitors and Cu2O crystal. From non‐bond energy and pair correlation functions, the interaction energies of the model system were mainly contributed by the non‐bond interaction. Strong adsorption could be raised by the Coulomb interaction between the negatively charged functional groups in TTA and the positive copper ions in the Cu2O (001) face, and further interaction between aggressive media and copper could be restricted. So, copper corrosion could be avoided. Chemical bonds and non‐bond interactions were formed between TTA and Cu2O (001) in different number of water molecules. Water molecules could not be ignored during the MD simulation, too. The results obtained here may provide theoretical supports for developing new corrosion inhibitors.

Investigation on the shape evolution of Cu2O crystals in the antibacterial process

In this work Cu2O microcrystals with various morphologies including octahedrons, cubes and hollow spheres were employed as the antimicrobial for the devitalization of Escherichia coli (E. coli). It was found that during the devitalization of E. coli, Cu2O microcrystals with various shapes presented similar change trend and finally was converted into flowerlike microstructures constructed by abundant nanosheets. Also, the shape change always started from {100} planes.

Reduced graphene oxide encapsulated Cu2O with controlled crystallographic facets for enhanced visible-light photocatalytic degradation

The Cu2O/reduced graphene oxide (Cu2O/rGO) composites with effective crystallographic facet controlling of Cu2O crystals were fabricated through a simple one-step wet chemistry method. The crystallographic facet-dependent photocatalytic performance of Cu2O was confirmed, favoring the cuboctahedral Cu2O with {100} and {111} facets and a better photocatalytic activity when compared to cubic and octahedral ones. This was attributed to the slight difference of surface energy between {100} and {111} facets which served as a driving force to promote the separation of photogenerated electron–hole pairs. Moreover, the introduction of two-dimensional rGO sheets could accelerate the transfer of photogenerated electrons from Cu2O to rGO, which further promoted the separation of photogenerated electron–hole pairs and the degradation of methyl orange (MO) under visible-light irradiation. The cuboctahedral Cu2O/rGO composite exhibited a superb photocatalytic performance with the degradation percentage of MO about 97.6% after one periodic photocatalysis due to the synergistic effect of cuboctahedral Cu2O and rGO sheets, foreboding its potential application as photocatalyst.

Further Investigation and Analysis on the Origin of the Optical Properties of Visible Hetero-photocatalyst TiO2/CuO

Hetero-photocatalysts TiO2/CuO were prepared by a wet impregnation process and their optical properties were characterized with a variety of techniques. The x-ray diffraction patterns corresponding to the anatase and the rutile phases of TiO2 were noticed for all the samples, but the peaks belonging to CuO crystals were observed only for samples with high CuO content (≥4 wt.%). The UV–Vis spectra show that absorption edges of the modified TiO2 were shifted toward longer wavelengths, indicating a reduction in the energy bandgap upon CuO. For photocatalytic application, the addition of CuO into TiO2 was found to increase the photodegradation of methylene blue and methylene orange with the highest photodegradation observed at CuO content of 3 wt.%. Moreover, using some extra experimental processes, we reveal how the CuO component affects the optical properties of the resulting materials.

A Facile Method Synthesis of {100}-Truncated Rhombic Dodecahedral Cu2O Microcrystals and Its Photocatalytic Activity

In this study, we report a facile solution-based one-step reduction method to synthesis of {100}-truncated rhombic dodecahedral Cu2O crystals. Each {100}-truncated rhombic dodecahedral Cu2O has six square {100} faces in addition to twelve {110} facets. The obtained microstructures were characterized by SEM, TEM, HRTEM, XRD and UV-vis spectrum. The adsorption and photocatalytic activity of as-prepared {100}-truncated rhombic dodecahedral Cu2O polyhedral for decomposition of methylene blue were investigated and compared to that of octahedral. It is found that the {100}-truncated rhombic dodecahedron Cu2O have a much better photocatalytic activity than octahedron. The{100}-truncated rhombic dodecahedra exposing the {110} facets exhibit an exceptionally good photocatalytic activity toward the fast and complete photo degradation of methylene blue due to a high number density of surface copper atoms, demonstrating the importance of their successful preparation.

Metal oxide semiconductor nanomaterial for reductive debromination: Visible light degradation of polybrominated diphenyl ethers by Cu2O@Pd nanostructures

Polybrominated diphenyl ethers (PBDEs), which have found extensive use as flame-retarding additives to many polymer materials, are now environmentally ubiquitous and persistent pollutants that present potential health risks to humans and wildlife. Herein, we report for the first time the use of metal oxide semiconductor nanostructures for photocatalytic reductive debromination of PBDEs using visible light. Well-defined cubic Cu2O crystals, surface-decorated with Pd nanoparticles, were prepared via a hydrothermal approach. The Cu2O@Pd demonstrated light-activated tandem photocatalysis, in which Cu2O produces H2 from H2O under visible light irradiation; the evolved H2 is subsequently activated by Pd to achieve the reductive hydrodehalogenation of the PBDE. Cu2O@Pd demonstrated effective debromination of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), one of the PBDEs of greatest environmental concern, with initial pseudo-first-order rate constant of 0.21 h−1. It is shown that the reaction proceeds via a reductive mechanism with preferential debromination at the para positions. Reaction rates for various monobromo- and dibromo-congeners were evaluated, confirming that the debromination order of preference is para>meta>>ortho, which is opposite to the order reported for direct photolysis. We conclude that Cu2O@Pd is a promising photocatalyst for reductive dehalogenation of halogenated organic compounds.

Experimental and theoretical DFT+D investigations regarding to various morphology of cuprous oxide nanoparticles: Growth mechanism of ionic liquid-assisted synthesis and photocatalytic activities

Under assistance of ionic liquid, the Cu2O crystal was successfully synthesized by a simple solution-phase method with several morphologies, including octahedrals, truncated octahedrals, facet-etched octahedrals, particle-coated octahedrals and aggregated spheres. The morphology of Cu2O crystals can easily be modified by tuning the adding amounts of ionic liquid 1-ethyl-3-methylimidazolium bromide. A possible growth mechanism for the crystal products can be suggested based on the DFT+D calculation and experiments. Under visible-light irradiation (λ>420nm), the obtained Cu2O samples show an outstanding performance during photodegrading methylene blue (MB) dye. The higher photocatalytic activity for MB photodegradation by the Cu2O facet-etched octahedrals comparing to octahedrals can be attributed to the formation of surface heterojunction between (100) and (111) facets. Therefore, the ionic liquid-assisted solution-phase syntheses would be potentially useful in the fields of catalyst production for photodegrading hazardous pollutants.

Polyhedral Cu 2 O crystal: Morphology evolution from meshed nanocube to solid and gas-sensing performance

Previously, the formation of polyhedral Cu2O crystal is usually proposed like that the seed crystals grow outward following the relative growth rates (R values) along different directions (Inside-out Pattern) or that the surrounding building blocks aggregate toward the center followed by ripening (Outside-in Pattern). However, in this study, simply mixing aqueous Cu(CH3COO)2, NaOH, and D-(+)-glucose in the order listed at 70 °C for a period, a distinct growth manner of Cu2O polyhedron was observed based on carefully recording the solution color changes and the elaborate time-dependent SEM/TEM observations, which was referred to All-direction Pattern. More specifically, the initially formed Cu2O nanoshuttles first self-aggregated into porous braided structure with well-defined cubic framework and then condensed into solid cube from all direction via consuming the surrounding fragments everywhere possible, rather than in gradient densification from inside-out or outside-in. After that, the solid cube fell into the common Outside-in Pattern, i.e., adsorbing the surroundings followed by ripening. Although many complex polyhedral submicrostructures have been already reported, to our best knowledge, Cu2O crystal that grows in such manner is barely observed. It is believed that such interesting growth mechanism of polyhedral crystals could be general. What's more, the 30 min-Cu2O powder was made into a gas sensor towards ethanol, the performance of which has reached the level of application.

Interparticle double charge transfer mechanism of heterojunction α-Fe2O3/Cu2O mixed oxide catalysts and its visible light photocatalytic activity

α-Fe2O3/Cu2O mixed oxide photocatalysts were prepared by one step facile hydrothermal method. During the synthesis, the growth of phase pure and highly crystalline polyhedral Cu2O crystals occurred and simultaneously a heterojunction formed between α-Fe2O3 and Cu2O particles. The optical band gap of bare Cu2O was found to be 1.8eV which was less than the reported values and incorporation of 5wt% α-Fe2O3 further reduced the band gap of Cu2O to 1.75eV, indicating an enhanced absorption of visible light by the binary photocatalysts. More importantly, the loading of α-Fe2O3 on Cu2O augmented the charge carriers separation and transfer at the interface, which was evident by the reduced photoluminescence intensity of α-Fe2O3/Cu2O photocatalysts. The influence of different loading amounts of iron oxide (1–10wt%) on Cu2O was evaluated. Among them, 5wt% α-Fe2O3 loaded Cu2O showed the enhanced photodegradation activity due to the formation of an effective p-n heterojunction and exhibited nearly 30% and 95% increase in the photodegradation rate of methyl orange (MO) compare to bare Cu2O and α-Fe2O3, respectively. The photo generated superoxide radicals were found to be the main reactive species responsible for the degradation of MO. The excellent photodegradation activity of α-Fe2O3/Cu2O photocatalyst is mainly attributed to the enhanced visible light absorption, efficient charge carriers separation and transfer. Based on the experimental results, a double charge transfer mechanism was proposed for the separation and transfer of photogenerated charge carriers in the photocatalysts.