Copper Vacancies Research Articles

Phase Transformation of Metastable Cu2ZnGeO4 with a Wurtz-Kesterite Structure at Elevated Temperatures.

The thermal stability and high-temperature phase transformation of metastable Cu2ZnGeO4 were investigated in an Ar atmosphere by thermogravimetry, differential thermal analysis, and high-temperature X-ray diffraction. Three Cu-deficient CuI2-xZnGeO4-2/x phases with a wurtzite-related structure were observed, with varying amounts of copper deficiency. The metastable Cu2ZnGeO4 was stable at approximately 275 °C and transformed into intermediate phases. The intermediate phases had a wurtz-kesterite structure with a small number of copper and oxygen vacancies, which later transformed into a high-temperature phase at approximately 425 °C. The crystal structure of the high-temperature phase was assumed to be a deficient wurtzite-related structure with hexagonal closely packed oxygen and deficient copper sites on the order of tens of a percent. The high-temperature phase decomposed into stable Cu2O, GeO2, and Zn2GeO4 phases above 550 °C. The mechanism for the formation of the phase with a large amount of copper deficiency is discussed, leading to an understanding of the formation process for the copper-deficient phase of complex compounds containing monovalent copper.

First-Principles Study of Cu-Based Inorganic Hole Transport Materials for Solar Cell Applications.

Perovskite solar cells (PSCs) and dye-sensitized solar cells (DSCs) both represent promising strategies for the sustainable conversion of sunlight into electricity and fuels. However, a few flaws of current devices hinder the large-scale establishment of such technologies. On one hand, PSCs suffer from instabilities and undesired phenomena mostly linked to the perovskite/hole transport layer (HTL) interface. Most of the currently employed organic HTL (e.g., Spiro-OMeTAD) are supposed to contribute to the perovskite decomposition and to be responsible for charge recombination processes and polarization barriers. On the other hand, power conversion efficiencies (PCEs) of DSCs are still too low to compete with other conversion technologies. Tandem cells are built by assembling p-type and n-type DSCs in a cascade architecture and, since each dye absorbs on a different portion of the solar spectrum, the harvesting window is increased and the theoretical efficiency limit for a single chromophore (i.e., the Shockley–Queisser limit) is overcome. However, such a strategy is hindered by the lack of a p-type semiconductor with optimal photocathode features. Nickel oxide has been, by far, the first-choice inorganic p-type semiconductor for both PV technologies, but its toxicity and non-optimal features (e.g., too low open circuit voltage and the presence of trap states) call for alternatives. Herein, we study of three p-type semiconductors as possible alternative to NiO, namely CuI, CuSCN and Cu2O. To this aim, we compare the structural and electronic features of the three materials by means of a unified theoretical approach based on the state-of-the art density functional theory (DFT). We focus on the calculation of their valence band edge energies and compare such values with those of two widely employed photo-absorbers, i.e., methylammonium lead iodide (MAPI) and the triple cation MAFACsPbBrI in PSCs and P1 and Y123 dyes in DSCs, given that the band alignment and the energy offset are crucial for the charge transport at the interfaces and have direct implications on the final efficiency. We dissect the effect a copper vacancy (i.e., intrinsic p-type doping) on the alignment pattern and rationalize it from both a structural and an electronic perspective. Our data show how defects can represent a crucial degree of freedom to control the driving force for hole injection in these devices.

Spectroscopic study and thermoelectric properties of copper sulfide thin films prepared by the flash evaporation method

Different phases of copper sulfide were synthesized by the Flash Evaporation method, followed by annealing at various temperatures. Pure and mixed crystal structures of the covellite, digenite and djurleite phases were identified by the X-ray diffraction patterns and Raman spectroscopy. Scanning Electron Microscopy revealed the effect of annealing on the surface morphology. The carrier concentration was found to decrease with increasing the annealing temperature due to the disappearance of copper vacancies. This results in an increase in the Seebeck coefficient and a decrease in the electrical conductivity. The thermoelectric power factor is better when the digenite phase dominates and doping may be a possible solution towards the integration of this material in the thermoelectric technology. Finally, annealing provided access to the Density Of States engineering, which is useful for future investigations.

Strong anharmonicity induced low lattice thermal conductivity and high thermoelectric performance in (CuInTe2)1−x(AgSbTe2)x system

The suppression of lattice thermal conductivity of thermoelectric CuInTe2 was often realized by complex micrometer-structure control. Herein, we demonstrate that simple alloying AgSbTe2 in CuInTe2 is capable of strongly strengthening lattice anharmonicity, which gives rise to an extremely low lattice thermal conductivity of ∼ 0.62 W m−1 K−1 at 823 K for (CuInTe2)0.95(AgSbTe2)0.05. In addition, the intentionally introduced copper vacancies can effectively increase the carrier concentration and dramatically improve the electrical performance of (CuInTe2)0.95(AgSbTe2)0.05. As a result, the sample with nominal composition of (Cu0.95InTe2)0.95(AgSbTe2)0.05 reaches a maximum zT value of ∼ 1.3 at 823 K. This study provides some insight into designing thermoelectric compounds in the diamond-like semiconductor family by inducing strong anharmonicity through a small amount of alloying.

Djurleite Copper Sulfide-Coupled Cobalt Sulfide Interface for a Stable and Efficient Electrocatalyst.

Transition metal sulfides (TMS) exhibit proliferated edge sites, facile electrode kinetics, and improved intrinsic electrical conductivity, which demand low potential requirements for total water splitting application. Here, we have propounded copper sulfide-coupled cobalt sulfide nanosheets grown on 3D nickel as an electrocatalyst for hydrogen (HER) and oxygen evolution (OER) reactions. The formation of djurleite copper sulfide with a Cu vacancy enables faster H+ ion transport and shows improved HER activity with a remarkably lower overpotential of 164 mV at 10 mA/cm2, whereas cobalt-incorporated copper sulfide undergoes cation exchange during synthesis and shows elevated OER activity with a lower overpotential of 240 mV at 10 mA/cm2 for the OER. Moreover, Cu2-xS/Co is said to have a hybrid CoS-CoS2 interface and provide Co2+ active sites on the surface and enable the fast adsorption of intermediate species (OH*, O*, and OOH*), which lowers the potential requirement. The copper vacancy and cation exchange with a hybrid CoS-CoS2 structure are helpful in supplying more surface reactive species and faster ion transport for the HER and OER, respectively. The full-cell electrolyzer requires a very low potential of 1.58 V to attain a current density of 10 mA/cm2, and it shows excellent stability for 50 h at 100 mA/cm2 as confirmed by the chronopotentiometry test.

Phase Engineering of Defective Copper Selenide toward Robust Lithium-Sulfur Batteries.

The shuttling of soluble lithium polysulfides (LiPS) and the sluggish Li-S conversion kinetics are two main barriers toward the practical application of lithium-sulfur batteries (LSBs). Herein, we propose the addition of copper selenide nanoparticles at the cathode to trap LiPS and accelerate the Li-S reaction kinetics. Using both computational and experimental results, we demonstrate the crystal phase and concentration of copper vacancies to control the electronic structure of the copper selenide, its affinity toward LiPS chemisorption, and its electrical conductivity. The adjustment of the defect density also allows for tuning the electrochemically active sites for the catalytic conversion of polysulfide. The optimized S/Cu1.8Se cathode efficiently promotes and stabilizes the sulfur electrochemistry, thus improving significantly the LSB performance, including an outstanding cyclability over 1000 cycles at 3 C with a capacity fading rate of just 0.029% per cycle, a superb rate capability up to 5 C, and a high areal capacity of 6.07 mAh cm-2 under high sulfur loading. Overall, the present work proposes a crystal phase and defect engineering strategy toward fast and durable sulfur electrochemistry, demonstrating great potential in developing practical LSBs.

Manipulating Cation Exchange Reactions in Cu2-xS Nanoparticles via Crystal Structure Transformation.

Copper-deficient Cu2-xS nanoparticles (NPs) are extensively exploited as a superior cation exchange (CE) template to yield sophisticated nanostructures. Recently, it has been discovered that their CE reactions can be facilely manipulated by copper vacancy density, morphology, and NP size. However, the structural similarity of usually utilized Cu2-xS somewhat limits the manipulation of the CE reactions through the factor of crystal structure because it can strongly influence the process of the reaction. Herein, we report a methodology of crystal structure transformation to manipulate the CE reactions. Particularly, roxbyite Cu1.8S nanodisks (NDs) were converted into solid wurtzite CdS NDs and Janus-type Cu1.94S/CdS NDs by a "full"/partial CE reaction with Cd2+. Afterward, the roxbyite Cu1.8S were pseudomorphically transformed into covellite CuS NDs. Unlike Cu1.8S, the CuS was scarcely exchanged because of the unique disulfide (S-S) bonds and converted into hollow wurtzite CdS under a more reactive condition. The S-S bonds were gradually split and CuS@CdS core@shell-type NDs were generated. Therefore, our findings in the present study provide not only a versatile technique to manipulate CE reactions in Cu2-xS NPs but also a better comprehension of their reaction dynamics and pathways.

Spectrally Resolved Single Particle Photoluminescence Microscopy Reveals Heterogeneous Photocorrosion Activity of Cuprous Oxide Microcrystals

Photocorrosion of cuprous oxide (Cu2O) has notably limited its application as an efficient photocatalyst. We report a facile approach to visualize in situ formation of copper and oxygen vacancies on the Cu2O surface under ambient condition. By imaging photoexcited single Cu2O particles, the resultant photoluminescence generated at Cu2O surface enable effective localization of copper and oxygen vacancies. Single particle photoluminescence imaging showed substantial heterogeneity in the rate of defect formation at different facets with the truncated corners achieving the fastest initial rate of photooxidation before subsequently changing to the face and edge sites as the photocorrosion proceeds. The generation of copper or oxygen vacancy is proportional to the photoexcitation power, while pH-dependent studies rationalized alkaline conditions for the formation of copper vacancy. Reaction in an electron-hole scavenger system showed that photooxidation and photoreduction will simultaneously occur, yet heterogeneously on the surface of Cu2O, with rate of copper vacancy formation being fastest.

Intrinsic defects and the influences on electrical transport properties in quaternary diamond-like compounds: Cd2Cu3In3Te8 as an example

Over the years, the fact that the quaternary diamond-like thermoelectric materials show much lower carrier mobilities than ternary compounds remains mysterious. In this work, by adopting first-principles defect chemistry and electrical transport calculations, the fundamental origin of the difference on carrier mobility between quaternary and ternary diamond-like compounds is addressed, exemplified by Cd2Cu3In3Te8. The results of defect chemistry show that the main intrinsic defects in quaternary compound Cd2Cu3In3Te8 are substitutional defects, i.e., CdIn and CdCu, differing from the copper vacancy defect in ternary Cu-based compound such as CuInTe2. The low defect formation energies in Cd2Cu3In3Te8 result in high defect concentrations, which is caused by the similar atomic radii and electronegativities between CdIn and CdCu. Further calculations show that the low-energy defects are mainly located around the valence band maximum in Cd2Cu3In3Te8. The electrical transport calculations, considering both the acoustic phonon scattering and ionized impurity scattering, demonstrate that mainly due to the higher concentration of the ionized defects, the mobility of the quaternary Cd2Cu3In3Te8 is much lower than that of ternary CuInTe2. Our work sheds light on the intrinsic defects in quaternary diamond-like compounds and their influence on charge transport.

Double-defect-induced polarization enhanced OV-BiOBr/Cu2−xS high-low junction for boosted photoelectrochemical hydrogen evolution

In order to improve the carrier transfer driving force and separation efficiency, OV-BiOBr/Cu2−xS high-low junction with double defects (Oxygen vacancy and Copper vacancy) was successfully prepared by a facile two-step solvothermal in situ growth technique. OV-BiOBr/Cu2−xS-0.2 showed the highest hydrogen evolution rate (509.75 μmol·cm−2·h−1), which was 12.45, 6.94 and 3.44 times that of pure BiOBr, OV-BiOBr and BiOBr/Cu2−xS composite, respectively. Obvious free radicals (·OH and ·O2-) were observed in the dark state for the first time. Theoretical and experimental results demonstrate that the charge imbalance within OV-BiOBr/Cu2−xS intensifies after the introduction of the double defects, resulting in enhanced interfacial polarization phenomena. Boosted photoelectrochemical hydrogen evolution activities were achieved with the synergistic effect of the internal electric field and polarization electric field. This study will lay a scientific and experimental foundation for the preparation of high-performance photoelectrochemical anode materials with double defect heterojunction.

Design of Experiments and Theoretical Investigation for Photoluminescence Optimization of Copper Aluminum Sulfide Nanoparticles through Controlling Crystalline Defects

Copper aluminum sulfide has been explored as an interesting low-toxicity material for application in optoelectronics and biological labeling. Herein, we have applied design of experiments (DOE) and density functional theory simulations (DFT) to investigate the effect of experimental conditions on the optical properties of copper aluminum sulfide nanoparticles synthesized through the heat-up method. The resulting material was characterized by UV–vis-NIR absorption, photoluminescence spectroscopy, X-ray diffractometry (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). XRD and Raman spectroscopy revealed the formation of copper aluminum sulfide alongside byproducts such as covellite, atacamite, paratacamite, and clinoatacamite. DOE revealed the concentration of aluminum precursor as the most important parameter to affect the photoluminescence (PL) intensity. This finding can be correlated to a donor–acceptor pair formed by a copper vacancy and copper substituted by aluminum as the responsible for PL emission. DFT simulations performed for different possible defects reproduced the role of the donor–acceptor pair and provided an insight on their role on PL. The potential application of the nanoparticles for fluorescence biological labeling was confirmed by confocal microscopy assay showing strong fluorescence under excitation at 405, 473, and 559 nm.

Growth-Promoting Mechanism of Bismuth-Doped Cu(In,Ga)Se2 Solar Cells Fabricated at 400 °C.

The classical high-temperature synthesis process of Cu(In,Ga)Se2 (CIGS) solar cells limits their applications on high-temperature intolerant substrates. In this study, a novel low-temperature (400 °C) fabrication strategy of CIGS solar cells is reported using the bismuth (Bi)-doping method, and its growth-promoting mechanism is systematically studied. Different concentrations of Bi are incorporated into pure chalcopyrite quaternary target sputtered-CIGS films by controlling the thickness of the Bi layer. Bi induces considerable grain growth improvement, and an average of approximately 3% absolute efficiency enhancement is achieved for Bi-doped solar cells in comparison with the Bi-free samples. Solar cells doped with a 50 nm Bi layer yield the highest efficiency of 13.04% (without any antireflective coating) using the low-temperature technology. The copper-bismuth-selenium compounds (Cu-Bi-Se, mainly Cu1.6Bi4.8Se8) are crucial in improving the crystallinity of absorbers during the annealing process. These Bi-containing compounds are conclusively observed at the grain boundaries and top and bottom interfaces of CIGS films. The growth promotion is found to be associated with the superior diffusion capacity of Cu-Bi-Se compounds in CIGS films, and these liquid compounds function as carriers to facilitate crystallization. Bi atoms do not enter the CIGS lattices, and the band gaps (Eg) of absorbers remain unchanged. Bi doping reduces the number of CIGS grain boundaries and increases the copper vacancy content in CIGS films, thereby boosting the carrier concentrations. Cu-Bi-Se compounds in grain boundaries significantly enhance the conductivity of grain boundaries and serve as channels for carrier transport. The valence band, Fermi energy level (EF), and conduction band of Bi-doped CIGS films all move downward. This band shift strengthens the band bending of the CdS/CIGS heterojunction and eventually improves the open circuit voltage (Voc) of solar cells. An effective doping method and a novel mechanism can facilitate the low-temperature preparation of CIGS solar cells.

Role of oxygen partial pressure of synthesis environment in tuning the dielectric properties of Nd2CuTiO6

Dielectric studies were carried out on Nd2CuTiO6 samples prepared by high temperature solid-state reaction followed by annealing under different oxygen partial pressures. X-ray diffraction studies revealed that the orthorhombic (Pnma) structure of Nd2CuTiO6 is retained in all the samples. The room temperature permittivity at 100Hz was found to be about 200 in the air-annealed sample, and it increased considerably to about 3000 upon annealing in argon atmosphere. The electrical conductivity in the argon-annealed compound was found to be an order of magnitude higher than that for the air-annealed sample. On annealing in oxygen, both permittivity and electrical conductivity of the sample were found to be significantly reduced. The variation in dielectric properties could be attributed to the induction or annihilation of mixed valence state of copper and oxygen vacancies, which was supported by X-ray photoelectron and diffuse reflectance spectroscopy data. The analysis of temperature and frequency dependent conductivity and dielectric data showed that the overlapping large polaron tunnelling (OLPT) mechanism is responsible for the observed dielectric relaxations and conduction. The observed systematic dependence of properties on the oxygen partial pressure of the annealing environment suggests a simple method to tune the electrical properties of Nd2CuTiO6.

Understanding the Role of Copper Vacancies in Photoelectrochemical CO2 Reduction on Cuprous Oxide.

Controlling the electronic and photoexcited properties of cuprous oxide (Cu2O) through slight modifications of the synthesis method can impact a wide range of emerging technologies. Herein, we consider copper vacancies in Cu2O as a prototype of a p-type oxide semiconductor for studying the impact of crystal and electronic structure on carbon dioxide photoreduction. Oriented films of copper vacancy modulated Cu2O consisting of nano twin structures are electrodeposited by changing the potential in an aqueous alkaline copper(II)-lactate solution. The copper vacancies introduce tail states inside the band gap, improving the hole concentration and facilitating the charge separation and transfer in the Cu2O photocathode. This study gives an in-depth view of how a cation-deficient structure regulates and promotes photoelectrochemical activity toward CO2 reduction.

Binding and energetics of oxygen at the CuInSe2 chalcopyrite and the CuInSe2/CdS interface

The introduction of oxygen in thin-film solar cells based on the CuInSe2 compound and related CuInSe2/CdS devices has been known to affect their electrical properties, with a tendency of neutralizing part of the donor density and favoring a p-type behavior for the CuInSe2 (CIS) absorber material. The present study employed calculations based on density-functional theory supplemented with a hybrid-functional approach to determine the energetics of oxygen incorporation in the bulk CIS compound and the CIS/CdS heterojunction interface. The latter was represented by two distinct faceted interface variants. Oxygen atoms were assumed to exist both as interstitial and substitutional impurities, in the latter case occupying vacant selenium sites. The calculations identified the structural relaxation patterns and examined the thermodynamic stability of the impurity as a function of the electron and the elemental chemical potentials. Oxygen was found to incorporate favourably at the core of the CIS/CdS interfaces, in most cases by taking up a bridging position within the nearest In–In pair. The sites of the lowest-energy oxygen configurations were found to be associated with a copper-poor local environment, owing to the presence of copper vacancies or the relaxation-induced breaking of a copper-oxygen bond. The electronic structures of the CIS/CdS interfaces were also studied by analyzing the site-projected and layer-resolved densities of states for several layers within the interfacial cores. Oxygen introduced deep-lying nonbonding levels and impurity-host bonding states in the valence-energy region.

Intergranular erosion corrosion of pure copper tube in flowing NaCl solution

We investigated the erosion corrosion behavior of a commercial copper tube in 1 wt% NaCl solution using a self-built loop apparatus, and revealed effects of flow velocity on corrosion layer evolution that explains the flow velocity dependent corrosion rate of copper tubes in application. Furthermore, the intergranular erosion corrosion forms the detached and partially oxidized copper grains with nut structure. From core to shell, the compositions are supposed to be copper, copper-based oxygen solid solution, Cu2O with oxygen vacancies, Cu2O, and Cu2O with copper vacancies. These findings provide deepened understanding of erosion corrosion behavior of copper and copper alloys.

Structural, morphological and thermoelectric properties of copper deficient and excessive Cu2-xS nanoparticles with (x=0-0.3)

Cu2-xS emerged as a potential thermoelectric material for use in various temperature ranges. The abundance, low price, the safety of its elements, and strong performance at relatively high temperatures are among its benefits. In this study, Cu2-xS (x= 0, 0.1, 0.2, and 0.3) nanoparticles are synthesized using a hydrothermal method. The effect of copper content on the structural and thermoelectric properties has been investigated. The copper atoms have changed the phase of the Cu2-xS crystal structure due to changes in the kinetic release rate of copper. The lower Seebeck coefficient and high electrical conductivity with increasing x value or decreasing copper concentration in the Cu2-xS matrix were observed. The copper vacancies created more charge carrier concentration. The maximum Seebeck coefficient value in the Cu2.0S sample is achieved by carrier localization. The Cu1.9S sample was found to have the best thermoelectric power generation performance, with a maximum power factor value of 1.72 × 10−3 Wm−1C−2.

Multi-scale kinetic study of the oxygen uncoupling of CuO oxygen carrier in chemical looping

As the kinetic influencing factors were not considered properly in the previous studies, the obtained oxygen uncoupling kinetics of CuO oxygen carrier varies greatly. In this paper, a multi-scale methodology is proposed to explore the oxygen uncoupling kinetics of CuO oxygen carrier through well-designed thermogravimetric experiments and multi-scale modelling. In order to minimize the effects of gas diffusion, a self-made thermogravimetric analyzer (TGA) is built firstly to measure the oxygen uncoupling rates of CuO samples. A multi-scale kinetic analysis model in which many influencing factors are considered is then established to analyze the experimental results. The kinetic parameters are thus extracted. Finally, based on the obtained intrinsic reaction kinetics, the effects of the physical structure of CuO oxygen carrier particle on its oxygen uncoupling rate are analyzed. It is found from this study that under typical CLOU conditions, neutral and singly charged copper vacancies are the dominant point defects in the product layer of CuO, and the chemical diffusions of these defects are NOT the rate limiting step. The gas diffusion in TGA crucible, the gas diffusion in sample layer, the polydispersity of the samples and the sintering of the samples are all important to describe the conversion process of the CuO samples. Among these, the effect of gas diffusion in the sample layer is less significant than that in the crucible. It is found that there is an optimal grain size (that is, 245 nm) of CuO oxygen carrier particle to maximize the averaged conversion rate of the particle, and the intrinsic activation energy and the pre-exponential factor of the oxygen uncoupling of CuO are determined to be 281.23 kJ/mol and 5.679 × 1016 mol/(m2s), respectively.

Cu x S nanosheets with controllable morphology and alignment for memristor devices

In electrochemical metallization memristor, the performance of resistive switching (RS) is influenced by the forming and fusing of conductive filaments within the dielectric layer. However, the growth of filaments, mostly, is unpredictable and uncontrollable. For this reason, to optimize ions migration paths in the dielectric layer itself in the Al/Cu x S/Cu structure, uniform Cu x S nanosheets films have been synthesized using anodization for various time spans. And the Al/Cu x S/Cu devices show a low operating voltage of less than 0.3 V and stable RS performance. At the same time, a reversible negative differential resistance (NDR) behavior is also demonstrated. And then, the mechanism of repeatable coexistence of RS effect and NDR phenomenon is investigated exhaustively. Analyses suggest that the combined physical model of space-charge limited conduction mechanism and conductive filaments bias-induced migration of Cu ions within the Cu x S dielectric layer is responsible for the RS operation, meanwhile, a Schottky barrier caused by copper vacancy at the Cu x S/Cu interface is demonstrated to explain the NDR phenomenon. This work will develop a new way to optimize the performance of non-volatile memory with multiple physical attributes in the future.

Scandium–copper–indides deriving from the ZrNiAl and MnCu2Al type structures

Abstract Phase analytical studies in the Sc–Cu–In system led to samples of the solid solutions ScCu1–x–y In1+x and ScCu2–x In which were studied by X-ray powder diffraction. At room temperature the compounds ScCu1–x–y In1+x crystallize with the ZrNiAl type, space group P 6 ‾ $\overline{6}$ 2m. Exemplarily, the structure of ScCu0.76In1.17 was refined from single crystal X-ray diffractometer data, revealing strong anisotropic displacements for the scandium atoms and a mixed occupied Cu/In site. Superstructure formation is observed at low temperatures. The ScCu0.78In1.14 and ScCu0.76In1.16 structures were refined from diffraction data recorded at 90 K. Both compounds adopt the HfRhSn type, space group P 6 ‾ $\overline{6}$ 2c, a klassengleiche subgroup of index 2; doubling of the subcell c axis. The Cu/In filled trigonal Sc6 prisms are strongly distorted in the superstructure, resulting from pairwise dislocation of the Cu/In atoms from ideal positions within an equidistant chain to shorter (311.0 pm) and longer (392.8 pm) Cu/In–Cu/In distances. Single crystal data of the Heusler phases ScCu1.95In and ScCu1.94In show small degrees of copper vacancies.