Cuprous Oxide Crystals Research Articles

Experimental Revelation of Surface and Bulk Lattices in Faceted Cu2 O Crystals.

Semiconductor crystals have generally shown facet-dependent electrical, photocatalytic, and optical properties. These phenomena have been proposed to result from the presence of a surface layer with bond-level deviations. To provide experimental evidence of this structural feature, synchrotron X-ray sources areused to obtain X-ray diffraction (XRD) patterns of polyhedral cuprous oxide crystals. Cu2 O rhombic dodecahedra display two distinct cell constants from peak splitting. Peak disappearance during slow Cu2 O reduction to Cu with ammonia borane differentiates bulk and surface layer lattices. Cubes and octahedra also show two peak components, while diffraction peaks of cuboctahedra are comprised of three components. Temperature-varying lattice changes in the bulk and surface regions also show shape dependence. From transmission electron microscopy (TEM) images, slight plane spacing deviations in surface and inner crystal regions are measured. Image processing provides visualization of the surface layer with depths of about 1.5-4nm giving dashed lattice points instead of dots from atomic position deviations. Close TEM examination reveals considerable variation in lattice spot size and shape for different particle morphologies, explaining why facet-dependent properties are emerged. Raman spectrum reflects the large bulk and surface lattice difference in rhombic dodecahedra. Surface lattice difference can change the particle bandgap.

Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films

Cuprous oxide (Cu2O) crystals and films of 10–65 nm thickness are investigated via electron diffraction, scanning tunneling microscopy (STM), and photoluminescence (PL) spectroscopy at temperature between 100 to 300 K. While the chemical composition and surface morphology of both systems are identical, large differences are found in the optical response. Bulk Cu2O shows pronounced PL peaks at 620, 730, and 920 nm, compatible with the radiative decay of free and bound excitons, whereas broad and asymmetric peaks at 775 and 850 nm are found for Cu2O films grown on Au(111) and Pt(111) supports. The latter represent the PL signature of VO2+ and VO+ defects, being inserted with substrate‐dependent concentrations due to the oxygen‐poor preparation of the Cu2O films. Despite the strong VO signature in PL, all Cu2O samples show p‐type conductance behavior in STM spectroscopy, indicating an abundance of Cu defects in the lattice. The fact that O vacancies still govern the thin‐film PL is explained by a more efficient recombination via VO‐ than Vcu‐emission channels, as the latter requires exciton formation first. Herein, the high sensitivity of low‐temperature PL to probe the defect landscape of dielectrics, being neither reached by photoelectron spectroscopy nor scanning probe techniques is demonstrated.

Directional growth of quasi-2D Cu2O monocrystals on rGO membranes in aqueous environments.

The preparation technology of unconventional low-dimensional Cu2O monocrystals, which exhibit specific crystal planes and present significantly unique interfacial and physicochemical properties, is attracting increasing attention and interest. Herein, by integrating a high-temperature oxidation process under vacuum and a pure-water incubation process under ambient conditions, we propose the self-assembled growth and synthesis of quasi-two-dimensional Cu2O monocrystals on reduced graphene oxide (rGO) membranes. The prepared Cu2O crystals have a single (110) crystal plane, regular rectangular morphology, and potentially well conductivity. Experimental and theoretical results suggest that this assembly is attributed to the pre-nucleation clusters aggregation and directional attachment of Cu and O on the rGO membranes in aqueous environment and cation-π interactions between the (110) crystal plane of Cu2O and rGO surface. Our findings offer a potential avenue for the discovery and design of advanced low-dimensional single-crystal materials with specific interfacial properties in a pure aqueous environment.

Spectroscopy of nanoantenna-covered Cu2O : Towards enhancing quadrupole transitions in Rydberg excitons

Plasmonic antennas channel incoming light into a confined near field that comes along with strong field gradients at the edges. Transverse field gradients are capable of driving quadrupole transitions in a more effective way than the weak longitudinal field gradient in dipolar light. Cuprous oxide Rydberg excitons with principal quantum number $n$ between 5 and 15 are several tens to a few hundreds of nanometers in diameter and can have different orbital angular momentum quantum numbers. Usually, only $P$- and $F$-excitons are electric dipole allowed, while $S$- and $D$-excitons remain dark. Here, we use 30-nm-wide and 60- to 110-nm-long plasmonic aluminum nanoantennas deposited on the cuprous oxide crystal surface to create transverse field gradients in similar spatial dimensions as the size of the Rydberg excitons. This way, light field and matter wave function overlap spatially and quadrupole transitions of $S$excitons are enhanced. The relative enhancement of the $S$-exciton oscillator strength with respect to the $P$-exciton oscillator accounts for a factor of up to 1.4.

Enhanced Thermoelectric Efficiency of Cement-Based Materials with Cuprous Oxide for Sustainable Buildings

The thermoelectric effect of plain cement paste is usually weak. To improve the thermoelectric performance of cement composites, functional components, such as carbon fibers, steel fibers, carbon nanotubes, and graphene, are often added to cement paste. In view of the advantage of metal oxides with a higher band gap, pure cuprous oxide crystals with different particle sizes were synthesized by a hydrothermal method and incorporated into the cement matrix to improve the thermoelectric efficiency of cement composites in this study. Pure cuprous oxide crystals with different particle sizes (15 μm, 1.5 μm, and 100 nm) were prepared by controlling the reaction temperature and time, pH value, amount of reducing agent, and polyvinylpyrrolidone in the reaction system. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the cement composites with 5.0 wt.% nanostructured Cu2O powder increased to 3966 ± 54 μV/K, (2.68 ± 0.12) × 10−4 S/m, and 0.69 ± 0.007 W/(m·K), respectively. Thereby, a high figure of merit value of 1.93 × 10−6 was obtained for the cement composites, which made future application of cement composites in energy harvesting for buildings possible.

Scrutinizing the Debye plasma model: Rydberg excitons unravel the properties of low-density plasmas in semiconductors

For low-density plasmas, the classical limit described by the Debye-H\"uckel theory is still considered as an appropriate description even though a clear experimental proof of this paradigm is lacking due to the problems in determining the plasma-induced shift of single-particle energies in atomic systems. We show that Rydberg excitons in states with a high principal quantum number are highly sensitive probes for their surrounding making it possible to unravel accurately the basic properties of low-density nondegenerate electron-hole plasmas. To this end, we accurately measure the parameters of Rydberg excitons such as energies and linewidths in absorption spectra of bulk cuprous oxide crystals in which a tailored electron-hole plasma has been generated optically. Since from the absorption spectra exciton energies, as well as the shift of the single-particle energies given by the band edge, can be directly derived, the measurements allow us to determine the plasma density and temperature independently, which has been a notoriously hard problem in semiconductor physics. Our analysis shows unambiguously that the impact of the plasma cannot be described by the classical Debye model, but requires a quantum many-body theory, not only for the semiconductor plasma investigated here, but in general. Furthermore, it reveals an exciton scattering mechanism with coupled plasmon-phonon modes becoming important even at very low plasma densities.

Enabling direct CO2 electrolysis by alkali metal cation substituted membranes in a gas diffusion electrode reactor

Direct gas phase electrolysis using a membrane electrode assembly (MEA) reactor has been recognized as a promising direction for large scale continuous CO2 conversion. The so called “zero-gap” CO2 reduction was however seldom achieved when proton is the main charge carrier in the system due to the overwhelmed competing hydrogen evolution reaction (HER) taking place at the cathode. In this work, the suppression of HER is achieved by regulating the rate of proton transport through the membrane electrolytes and is realized by the modification of the sulfonic acid side chains with alkali metal cations. The MEA’s CO2 reduction reaction (CO2RR) faradaic efficiency (CO2RR-FE) is significantly improved as the alkali cations are varied sequentially from Li to Cs. Such improvement is attributed to the significantly hindered proton transport in the Cs-MEA, as reflected by the substantially larger high frequency resistance (HFR), which leads to enhanced competitive adsorption of CO2 on the copper catalyst. In addition, the Cs-MEA showed the highest robustness for both the membrane cations and the copper cathode against leaching and corrosion. Moreover, the observed phase transition of copper metal into faceted cuprous oxide crystals exposing low index surfaces also favors CO2RR over HER. The collective desirable properties of the Cs-MEA result in the best CO2RR performance, exhibiting a maximum CO2RR-FE of 0.44% at −1.2 V vs RHE and a space time yield (STY) of 1.06 mmol·h−1·gcat-1 for methane (CH4) and 2.02 mmol·h−1·gcat-1 for carbon monoxide (CO).

Cu 4O3 thin films deposited by non-reactive rf-magnetron sputtering from a copper oxide target

Copper oxide thin films deposited by sputtering are frequently formed by using metal copper targets in reactive atmospheres. In this report, paramelaconite (Cu4O3) thin films were deposited by non-reactive rf magnetron sputtering. The target used for sputtering was a copper oxide disk fabricated by oxidation of metal copper at 1000 °C for 24 h in airatmosphere. X-ray diffraction (XRD) results showed that the copper oxide target was mainly composed of cupric oxide (CuO) and cuprous oxide (Cu2O) crystals. Raman analyses suggested that the surface of the copper oxide disk is composed by a (CuO) layer. XRD measurements performed to the copper oxide thin films deposited by non-reactive rf magnetron sputtering showed that the film is composed of (Cu4O3) crystals. However,Raman measurements indicated that the Cu4O3 thin films are also composed by amorphous CuO and Cu2O.

Surface modification of multi-scale cuprous oxide with tunable catalytic activity towards toxic fumes and smoke suppression of rigid polyurethane foam

The development of flame retardant rigid polyurethane foam (RPUF) composites with low smoke toxicity emission, as well as excellent mechanical properties remains a huge challenge. Herein, a facile wet chemical approach was proposed for the preparation of cuprous oxide (Cu2O) crystals with different sizes, i.e. approximately 5 nm, 100 nm and 1 μm in lateral size, which were confirmed by X-ray diffraction and transmission electron microscopy. The effect of Cu2O crystals on the combustion behavior of RPUF was evaluated by cone calorimetry and steady state tube furnace tests. Significant reduction in peak carbon monoxide (CO) production rate (by 41.2%, 67.6% and 27.9%) and total smoke production (21.6%, 16.1% and 12.2%) were realized by the incorporation of 2 wt% Cu2O into the RPUF matrix in addition to a slight reduction in peak heat release rate. Notably, nano-sized Cu2O with high specific surface area is beneficial for the complete combustion of RPUF and the conversion of CO to carbon dioxide (CO2), involving the reduction of Cu2+-Cu+-Cu0 by degraded gases and the oxidation of Cu0-Cu+-Cu2+ by oxygen. This work demonstrated that submicron-sized Cu2O is an effective smoke and toxic gases suppressant for RPUF, which is expected to find practical applications in polymeric materials.

Synthesis, Structure and Catalytic Properties of Faceted Oxide Crystals

AbstractBulk metal oxide catalysts are key components of numerous large‐scale industrial processes that possess a high degree of complexity in bulk and surface structure. Such complexity manifests itself in the form of large uncertainties with respect to the nature, density, and efficacy of active sites responsible for catalytic turnovers. Faceted oxide crystals present the prospect of exposing well‐defined oxide surfaces while also being amenable for investigation at realistic pressures and temperatures. Despite the opportunity faceted oxide crystals present for developing more rigorous relationships between atomic‐level structure and catalytic function, their use remains far less prevalent compared to their metallic counterparts. This review, focused on cuprous oxide and cerium oxide crystals, seeks to examine and highlight challenges in controlling and rationalizing metal oxide crystallization, as well as limitations preventing elucidation of the relationships between atomic‐level structures and catalytic properties. We postulate that a clearer understanding of these challenges will encourage researchers to further pursue the study of catalytic properties of faceted oxide crystals, aiding not only an enhanced molecular‐level knowledge of bulk oxide catalysis, but also the development of improved materials for large‐scale catalytic processes.

Plasma-Enhanced Interaction and Optical Nonlinearities of Cu_{2}O Rydberg Excitons.

We theoretically investigate the nonlinear optical transmission through a cuprous oxide crystal for wavelengths that cover the series of highly excited excitons, observed in recent experiments. Since such Rydberg excitons have strong van der Waals interactions, they can dynamically break the conditions for resonant exciton creation and dramatically modify the refractive index of the material in a nonlinear manner. We explore this mechanism theoretically and determine its effects on the optical properties of a semiconductor for the case of degenerate pair-state asymptotes of Rydberg excitons in Cu_{2}O. Upon analyzing the additional effects of a dilute residual electron-hole plasma, we find quantitative agreement with previous transmission measurements, which provides strong indications for the enhancement of Rydberg-induced nonlinearities by surrounding free charges.

Preparation of zinc and cerium or both doped Cu2O photoelectric material via hydrothermal method

In the paper, the zinc and cerium or both doped cuprous oxide (Cu2O) crystals were successfully synthesized under low temperature by a simple hydrothermal process. The surface morphologies, structure properties, and optical properties of Cu2O crystals were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-visible absorption spectroscopy. The morphology of pure Cu2O crystals, Zn:Cu2O crystals, Ce:Cu2O crystals and Zn/Ce:Cu2O crystals evolved to irregularly cubic structure, exfoliated octahedron, were regular octahedron, and like-spherical aggregate, respectively. The doping of Cu2O crystals not only affected the surface morphologies but also significantly influenced the optoelectronic properties. Compared with undoped cuprous oxide, the doped samples had larger optical band gaps. Finally, the aggregation and diffusion were the most important factors for the morphology evolution of Cu2O crystal.

Photocatalytic conversion of gas phase carbon dioxide by graphitic carbon nitride decorated with cuprous oxide with various morphologies

Submicron cuprous oxide (Cu2O) crystals with various morphologies were successfully fabricated and incorporated with graphitic carbon nitride (gCN) to evaluate their activity for gas phase CO2 photoreduction under visible-light illumination. Both the morphology of Cu2O and the composition of copper species were tunable and were significantly affected by the presence of gCN. The morphology of Cu2O influenced the band structure and optical property, as well as the efficiency of photo-induced charge transfer within each sample. The compositions of Cu2O_gCN before and after illumination were compared to evaluate the photostability of samples. In addition to the majority of Cu2O crystals, other copper species, CuO or metallic Cu, were presented and considered as the assistance for CO2 adsorption or interfacial charge transfer. Improved conversion of CO2 to CO was achieved by combining n-type gCN and p-type Cu2O crystals with an optimum surface composition, and by selecting the Cu2O crystals with higher photostability.

Facile deposition of Cu2O in a UV-enhanced sulfite-mediated glucose fuel cell for photoelectrocatalytic reduction of oxygen

The photoelectrocatalytic oxidation of glucose (Glu) on a TiO2 photoanode promoted by sulfite can drive the cathodic deposition of flower-like cuprous oxide (Cu2O) crystals through an evolution from cube, truncated cube and truncated octahedron on an indium-tin oxide (ITO) electrode in a UV-excited Glu/Cu(II) fuel cell. The cathodic deposition kinetics of Cu2O is investigated by varying Glu concentration, operation pH and temperature under pseudo-first-order conditions. The spontaneous endothermic deposition reaction is controlled by a coupled dehydration conversion from Glu to 5-hydroxymethylfurfural (5-HMF). The proposed cathodic deposition method has the merits of mild reaction condition, environment-friendly procedure, no requirement of applied voltage, and harvesting UV photons (Pin = 55 μW cm−2) and chemical energy from 1.0 mol L−1 Glu/0.1 mol L−1 SO32− (298 K, pH 12) to produce open-circuit photovoltage (Voc) of 1.121 V, short-circuit current density (isc) of 0.25 mA cm−2 and power conversion efficiency (η = Pmax/Pin) of 14.8%. Furthermore, the as-prepared Cu2O can act as cathode catalysts to improve remarkably the performances of photoelectrochemical Glu/O2 fuel cells (298 K, pH 7) based on its photoelectrocatalytic activity towards the reduction of oxygen with the assistance of methylene blue (MB).

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.

Zinc ion mediated synthesis of cuprous oxide crystals for non-enzymatic glucose detection.

Morphology control is expected to be an effective method to enhance the electrochemical properties of materials. In this work, zinc cation-mediated growth of Cu2O crystals was achieved via an aqueous chemical route at room temperature. Thus, by simply increasing the concentration of Zn2+, concave cube-like (C-Cu2O), porous (P-Cu2O), and hierarchical (H-Cu2O) Cu2O crystals were selectively obtained. The morphologies and structures of the as-prepared Cu2O crystals were characterized by SEM, TEM, XRD and XPS. The three materials were subsequently employed as active materials for the non-enzymatic detection of glucose. The H-Cu2O-based electrode exhibited the highest sensitivity (3076 μA mM-1 cm-2) in virtue of its highest surface area, while the P-Cu2O-based electrode showed the widest linear range (up to 24 mM). The reliability of the Cu2O-based glucose sensors was proved by determining their detection limit, response time, selectivity, and stability characteristics on human serum samples. This work provides a novel strategy for the morphology-controlled Zn2+-mediated fabrication of Cu2O crystals with different glucose sensing performances depending on their structures.

One‐step in‐situ fabrication of silver‐modified Cu 2 O crystals with enhanced visible photocatalytic activity

Silver-modified cuprous oxide (Cu2O) crystals (SMCCs) were successfully created by a facile one-step in-situ Ag-loading reaction under room temperature. The samples were studied by X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflectance spectra and transmission electron microscopy. The photocatalytic performance of the SMCCs were estimated by using methyl orange aqueous solution as a standard pollutant under visible light irradiation. As a result, the SMCCs show a higher photocatalytic activity than the pure phase Cu2O. The silver nanoparticles which adhered on the surface of Cu2O of different size and content can greatly effects the photocatalytic efficiency. The enhanced photocatalytic activity owed to the effective charge transfer of excited electron-hole pairs by loading Ag.

Nonlinear optical properties of cubic cuprous oxide with different sizes

Cubic cuprous oxide (Cu2O) crystals with various sizes are successfully synthesized under room temperature by a simple solution-phase route. The surface morphology, structure properties and optical absorption spectrum of Cu2O crystals are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–visible absorption spectroscopy. The optical limiting (OL) properties of Cu2O crystals are investigated using a nanosecond laser at 532nm. The experimental results show that the OL properties of Cu2O crystals do not only come from nonlinear scattering, but also might be due to nonlinear absorption.

Starch-assisted synthesis and photocatalytic activity of monosized cuprous oxide octahedron microcrystals

Octahedral cuprous oxide (Cu2O) crystals with different particle sizes were successfully synthesized by a simple starch-assisted method. The morphology and size of the obtained samples were examined by field-emission scanning electron microscopy and particle size analyzer. X-ray powder diffraction and ultraviolet–visible absorption spectra were also employed to characterize the crystallinity and light response. The photocatalytic activities of the as-prepared octahedral Cu2O have been investigated under ultraviolet light irradiation, and samples of different sizes exhibit excellent photocatalytic activity toward MO degradation. This simple one-pot synthetic route could provide a good starting point for the research of size-dependent catalytic properties of other materials.

Cupric oxide inclusions in cuprous oxide crystals grown by the floating zone method

Phase-pure cuprous oxide (Cu2O) crystals are difficult to grow since cupric oxide can form within the crystal as the crystal is cooled to ambient conditions. Vacancies are the solute which causes precipitation of macroscopic defects. Therefore, even when a mostly phase-pure single crystal is used as a feed rod, cupric oxide inclusions persist in the recrystallized solid. Control of the thermal profile during crystal growth, however, can improve phase-purity; a slow counter-rotation rate of the feed and seed rods results in fewer inclusions. Cupric oxide can be removed by annealing, which produces a factor of 540 ± 70 increase in phase-purity.