Stable and Efficient CuO Based Photocathode through Oxygen-Rich Composition and Au-Pd Nanostructure Incorporation for Solar-Hydrogen Production

Efficient photoactivated hydrogen evolution promoted by CuxO-gCN-TiO2-Au (x = 1,2) nanoarchitectures

In this work, we propose an original and potentially scalable synthetic route for the fabrication of CuxO-gCN-TiO2-Au (x = 1,2) nanoarchitectures, based on Cu foam anodization, graphitic carbon nitride liquid-phase deposition, and TiO2/Au sputtering. A thorough chemico-physical characterization by complementary analytical tools revealed the formation of nanoarchitectures featuring an intimate contact between the system components and a high dispersion of gold nanoparticles. Modulation of single component interplay yielded excellent functional performances in photoactivated hydrogen evolution, corresponding to a photocurrent of ≈-5.7 mA cm-2 at 0.0 V vs. the reversible hydrogen electrode (RHE). These features, along with the very good service life, represent a cornerstone for the conversion of natural resources, as water and largely available sunlight, into added-value solar fuels.

The prospect of CuxO-based catalysts in photocatalysis: From pollutant degradation, CO2 reduction, and H2 production to N2 fixation

The topic of photocatalysis and CuxO-based materials has been intertwined for quite a long time. Its relatively high abundance in the earth's crust makes it an important target for researchers around the globe. One of the properties exploited by researchers is its ability to exist in different oxidation states (Cu0, Cu+, Cu2+, and Cu3+) and its implications on photocatalytic efficiency improvement. Recently, they have been extensively used as photocatalytic materials for dye and pollutant degradation. However, it has almost reached saturation levels, therefore, currently, they are being mostly utilized for CO2 reduction and H2 evolution. Hence, this review will discuss the evolution (in application) of CuxO-based photocatalysts, relating to their past, present, and future. Moreover, photocatalytic efficiency improvement strategies such as doping, heterojunction formation, and carbonaceous construction with other materials will also be touched upon. Finally, the prospect of Cu2O-based photocatalysts will be discussed in the field of photocatalytic N2 fixation to ammonia. The significance of N2 chemisorption on photocatalysts to maximize ammonia production will also be given importance.

Fabrication of Z-Type TiN@(A,R)TiO2 Plasmonic Photocatalyst with Enhanced Photocatalytic Activity

Plasmonic effect-enhanced Z-type heterojunction photocatalysts comprise a promising solution to the two fundamental problems of current TiO₂-based photocatalysis concerning low-charge carrier separation efficiency and low utilization of solar illumination. A plasmonic effect-enhanced TiN@anatase-TiO₂/rutile-TiO₂ Z-type heterojunction photocatalyst with the strong interface of the N-O chemical bond was synthesized by hydrothermal oxidation of TiN. The prepared photocatalyst shows desirable visible light absorption and good visible-light-photocatalytic activity. The enhancement in photocatalytic activities contribute to the plasma resonance effect of TiN, the N-O bond-connected charge transfer channel at the TiO₂/TiN heterointerface, and the synergistically Z-type charge transfer pathway between the anatase TiO₂ (A-TiO₂) and rutile TiO₂ (R-TiO₂). The optimization study shows that the catalyst with a weight ratio of A-TiO₂/R-TiO₂/TiN of approximately 15:1:1 achieved the best visible light photodegradation activity. This work demonstrates the effectiveness of fabricating plasmonic effect-enhanced Z-type heterostructure semiconductor photocatalysts with enhanced visible-light-photocatalytic activities.

Recent innovations in the technology and applications of low-dimensional CuO nanostructures for sensing, energy and catalysis

Low-dimensional copper oxide nanostructures are very promising building blocks for various functional materials targeting high-demanded applications, including energy harvesting and transformation systems, sensing and catalysis. Featuring a very high surface-to-volume ratio and high chemical reactivity, these materials have attracted wide interest from researchers. Currently, extensive research on the fabrication and applications of copper oxide nanostructures ensures the fast progression of this technology. In this article we briefly outline some of the most recent, mostly within the past two years, innovations in well-established fabrication technologies, including oxygen plasma-based methods, self-assembly and electric-field assisted growth, electrospinning and thermal oxidation approaches. Recent progress in several key types of leading-edge applications of CuO nanostructures, mostly for energy, sensing and catalysis, is also reviewed. Besides, we briefly outline and stress novel insights into the effect of various process parameters on the growth of low-dimensional copper oxide nanostructures, such as the heating rate, oxygen flow, and roughness of the substrates. These insights play a key role in establishing links between the structure, properties and performance of the nanomaterials, as well as finding the cost-and-benefit balance for techniques that are capable of fabricating low-dimensional CuO with the desired properties and facilitating their integration into more intricate material architectures and devices without the loss of original properties and function.

Hydroxyl radical and carbonate radical facilitate chlortetracycline degradation in the bio-photoelectrochemical system with a bioanode and a Bi2O3/CuO photocathode using bicarbonate buffer

The single-chamber bio-photoelectrochemical system (BPES) with a bioanode and a Bi₂O₃/CuO photocathode is developed for chlortetracycline (CTC) degradation under simulated solar irradiation, using phosphate buffer solution (PBS) or NaHCO₃ as buffer solution. The optimized Bi₂O₃/CuO photocathode possesses rich vacancies, great photoresponse capability, and exhibits great photocatalytic activity toward CTC degradation due to its Z-scheme structure. Electron spin-resonance spectroscopy (ESR) and reactive species trapping experiments reveal that superoxide radicals/hydroxyl radicals are both the main radicals contributing to CTC degradation. Moreover, carbonate radical plays a more effective role toward CTC degradation, resulting in 40% improvement for CTC degradation in the BPES within 2 h. Higher current density (maximum of 137.6 A m^-2) and more negative cathode potential are obtained from the illuminated BPES with NaHCO₃ buffer. Possible mechanism and pathways of CTC degradation are proposed. This study contributes to the development of BPESs for antibiotics degradation.

Construction of a Hierarchical Structure of Bimetallic Oxide Derived from Metal-Organic Frameworks

Bimetallic oxides are a class of promising advanced functional metal nanomaterials, especially in terms of the sophisticated hierarchical structure of bimetallic oxide, which not only is in favor of enhancing their intrinsic physiochemical properties because of more accessible actives sites but also is capable of integrating the synergistic effect between two metals. Herein, we report a novel strategy to controllably construct bimetallic CuO/ZnO nanomaterials with sophisticated hierarchical structure through a pseudomorphic transformation and subsequent calcination process. The resulting unique hierarchical structure of ZnO/CuO is primarily constituted of a nanosphere and a rod grafted in a microscale cube with multidimensional size, which thus results in excellent dispersion, superior charge-transport capability, and abundant accessible active sites. Impressively, the optimized hierarchical structure product of CuO/ZnO (4:1) demonstrates an excellent glucose detection performance with a rapid response time, a wide linear range, a low detection limit, and strong antiinterference ability, realizing more advantages than commercial CuO or ZnO materials and shedding light on the positive correlation of the structure and performance. This study provides a new strategy for the controllable fabrication of the sophisticated hierarchical structure of bimetallic oxide nanomaterials.

Photocatalytic Fuel Cells for Simultaneous Wastewater Treatment and Power Generation: Mechanisms, Challenges, and Future Prospects

Technological advancement is accompanied by excessive consumption of fossil fuels and affluent uses of chemical substances in many sectors, including transportation and manufacturing companies, and so on. Being an exhaustible resource, the excessive use of fossil fuels and of chemical substances may lead to a serious energy crisis in the long run, and it may additionally impose environmental pollution. Attempts have been made in the solution of such serious issues from every nook and corner. Nonetheless, no method has been found to be a panacea in waste water treatment and subsequent beneficiaries. One of the attempts in the solution to such issues is the application of photocatalytic technology, which could serve as a dual function in environmental remediation and clean energy production. A photocatalytic fuel cell is a tool developed for the recovery of energy from organic wastes. A rational cell construction needs the fabrication of photoelectrodes, the design of a photoanode and a photocathode chamber, in addition to an ion-transport membrane for pollution treatment and electricity generation. In this review, comprehensive fundamental assessments and recent developments in the design of photocatalytic fuel cells, their applications, future prospects, and challenges are covered.

Copper Oxide-Based Photocatalysts and Photocathodes: Fundamentals and Recent Advances

This work aims at reviewing the most impactful results obtained on the development of Cu-based photocathodes. The need of a sustainable exploitation of renewable energy sources and the parallel request of reducing pollutant emissions in airborne streams and in waters call for new technologies based on the use of efficient, abundant, low-toxicity and low-cost materials. Photoelectrochemical devices that adopts abundant element-based photoelectrodes might respond to these requests being an enabling technology for the direct use of sunlight to the production of energy fuels form water electrolysis (H2) and CO2 reduction (to alcohols, light hydrocarbons), as well as for the degradation of pollutants. This review analyses the physical chemical properties of Cu2O (and CuO) and the possible strategies to tune them (doping, lattice strain). Combining Cu with other elements in multinary oxides or in composite photoelectrodes is also discussed in detail. Finally, a short overview on the possible applications of these materials is presented.

Photoelctrochemically Fabricated and Heated Cu2O/CuO Bilayers with Enhanced Photovoltaic Characteristics

Cu2O/CuO bilayers were fabricated by electrodeposition of the CuO layer in a copper(II)-ammonia complex aqueous solution, followed by photoelectrochemical deposition of the Cu2O layer at potentials ranging from -0.3 to -1.0 V referenced to a Ag/AgCl electrode in a copper(II)-lactate complex aqueous solution under light irradiation, and the effects of varied potentials of the photoelectrochemical Cu2O depositions and post-heating conditions on their structural, optical, and photovoltaic characteristics were investigated with X-ray diffraction, field emission-scanning electron microscopy, optical absorption measurements, and external quantum efficiency (EQE) measurements with and without applied bias voltage. The Cu2O layers with a characteristic 2.1 eV band gap energy were adhesively stacked on the thorn-like grains of the CuO layers possessing a characteristic 1.5 eV band gap energy, and dense and defect-free Cu2O/CuO bilayers could be fabricated at the potentials of -0.4 and -0.5 V, but the grain size of Cu2O decreased at -0.5 V. In addition, the metallic Cu was deposited simultaneously at potentials less than -0.7 V. The Cu2O/CuO bilayer fabricated at -0.4 V revealed photovoltaic features at wavelengths ranging from 350 nm to approximately 900 nm, and a maximum EQE value of 56.8% was achieved at 510 nm in wavelength with a bias voltage of -0.1 V. The maximum EQE value, however, decreased to 1.2% accompanied with the peak wavelength shift to 580 nm, and no photovoltaic feature was observed at potentials of -0.3, -0.7, and -1.0 V. The photovoltaic performance for the Cu2O/CuO bilayer fabricated at -0.4 V was ameliorated by heating at 423 K, and the maximum EQE values were enhanced to 87.7% at 550 nm and 89.8% at 530 nm in an ambient atmosphere and vacuum. Both the Cu2O and CuO layers acted as photovoltaic layers in the Cu2O/CuO bilayer fabricated at -0.4 V and heated at 423 K, and the electrical characteristic including the carrier mobility affected the photovoltaic performance. The photovoltaic feature, however, disappeared by heating above 523 K due to the formation of nanopores inside the CuO layer and near the CuO heterointerface to the Cu2O and fluorine-doped tin oxide substrate.

Photoelectrochemical Water-Splitting Using CuO-Based Electrodes for Hydrogen Production: A Review

The cost-effective, robust, and efficient electrocatalysts for photoelectrochemical (PEC) water-splitting has been extensively studied over the past decade to address a solution for the energy crisis. The interesting physicochemical properties of CuO have introduced this promising photocathodic material among the few photocatalysts with a narrow bandgap. This photocatalyst has a high activity for the PEC hydrogen evolution reaction (HER) under simulated sunlight irradiation. Here, the recent advancements of CuO-based photoelectrodes, including undoped CuO, doped CuO, and CuO composites, in the PEC water-splitting field, are comprehensively studied. Moreover, the synthesis methods, characterization, and fundamental factors of each classification are discussed in detail. Apart from the exclusive characteristics of CuO-based photoelectrodes, the PEC properties of CuO/2D materials, as groups of the growing nanocomposites in photocurrent-generating devices, are discussed in separate sections. Regarding the particular attention paid to the CuO heterostructure photocathodes, the PEC water splitting application is reviewed and the properties of each group such as electronic structures, defects, bandgap, and hierarchical structures are critically assessed.

Role of a Solution-Processed V2O5 Hole Extracting Layer on the Performance of CuO-ZnO-Based Solar Cells

In this research, a heterostructure of the CuO-ZnO-based solar cells has been fabricated using low-cost, earth-abundant, non-toxic metal oxides by a low-cost, low-temperature spin coating technique. The device based on CuO-ZnO without a hole transport layer (HTL) suffers from poor power conversion efficiency due to carrier recombination on the surface of CuO and bad ohmic contact between the metal electrode and the CuO absorber layer. The main focus of this research is to minimize the mentioned shortcomings by a novel idea of introducing a solution-processed vanadium pentoxide (V2O5) HTL in the heterostructure of the CuO-ZnO-based solar cells. A simple and low-cost spin coating technique has been investigated to deposit V2O5 onto the absorber layer of the solar cell. The influence of the V2O5 HTL on the performance of CuO-ZnO-based solar cells has been investigated. The photovoltaic parameters of the CuO-ZnO-based solar cells were dramatically enhanced after insertion of the V2O5 HTL. V2O5 was found to enhance the open-circuit voltage of the CuO-ZnO-based solar cells up to 231 mV. A detailed study on the effect of defect properties of the CuO absorber layer on the device performance was theoretically accomplished to provide future guidelines for the performance enhancement of the CuO-ZnO-based solar cells. The experimental results indicate that solution-processed V2O5 could be a promising HTL for the low-cost, environment-friendly CuO-ZnO-based solar cells.

Preparation of thin layer CuO from Cu2O using the Spin Coating Method at Various Annealing Temperature and Number of Dripping for Photoelectrochemical Water Splitting

A thin layer preparation of CuO from Cu2O powder using Fehling's solution for photoelectrochemical applications has been performed. The research was focused on studying the effect of annealing temperature and the number of drops on the performance of CuO thin layer semiconductors from Cu2O powder prepared by spin coating with a rotation rate of 500 rpm for 15 seconds. The thin layers were treated with annealing with temperature variations of 300°C, 400°C, and 500°C for 1 hour and variations in the number of drops of 10, 20, and 50 drops. The CuO thin layer was tested in a photoelectrochemical process as a photocathode to split water with a simulated light of 1.5 AM (100 mW/cm2). The process of splitting water as a method of producing hydrogen energy by photoelectrochemistry is assisted by semiconductors, such as CuO, in an electrolyte solution to capture photons and drive the water-splitting reactions. Copper (II) Oxide (CuO) is a p-type semiconductor with a band gap of 1.2-2.5 eV, which can be used as a photocathode. The optimum photoelectrochemical measurement results were obtained at an annealing temperature of 400°C and 50 drops with a current density of 0.584 mA/cm2 at a potential of 0.2 V versus the Reversible Hydrogen Electrode (RHE). The results of the Scanning Electron Microscopy (SEM) analysis show that the morphology of the oxide is spherical. Energy dispersive X-ray (EDX) analysis displays that the sample contained 51.46% and 48.54% of Cu and O, respectively. The X-ray diffraction pattern (XRD) analysis shows that the oxide grain size is 44.137 nm.

Solution-Processed Synthesis of Copper Oxide (Cu x O) Thin Films for Efficient Photocatalytic Solar Water Splitting

This article reports a solution-processed synthesis of copper oxide (Cu x O) to be used as a potential photocathode for solar hydrogen production in the solar water-splitting system. Cu x O thin films were synthesized through the reduction of copper iodide (CuI) thin films by sodium hydroxide (NaOH), which were deposited by the spin coating method from CuI solution in a polar aprotic solvent (acetonitrile). The phase and crystalline quality of the synthesized Cu x O thin films prepared at various annealing temperatures were investigated using various techniques. The X-ray diffraction and energy dispersive X-ray spectroscopy studies confirm the presence of Cu2O, CuO/Cu2O mixed phase, and pure CuO phase at annealing temperatures of 250, 300, and 350 °C, respectively. It is revealed from the experimental findings that the synthesized Cu x O thin films with an annealing temperature of 350 °C possess the highest crystallinity, smooth surface morphology, and higher carrier density. The highest photocurrent density of -19.12 mA/cm2 at -1 V versus RHE was achieved in the photoelectrochemical solar hydrogen production system with the use of the Cu x O photocathode annealed at a temperature of 350 °C. Therefore, it can be concluded that Cu x O synthesized by the spin coating method through the acetonitrile solvent route can be used as an efficient photocathode in the solar water-splitting system.

Greywater and bacteria removal with synchronized energy production in photocatalytic fuel cell based on anodic TiO2/ZnO/Zn and cathodic CuO/Cu

An approach that can recuperate of energy from wastewater treatment process is highly necessitate and would help to surmount the both environmental pollution and energy crisis issues. A photocatalytic fuel cell (PFC) employing an anodic TiO₂/ZnO/Zn and a cathodic CuO/Cu has been applied to degrade the raw greywater, which realized advanced organics destruction, bacteria disinfection, and synchronously electricity production. The improved photocatalytic performance has been observed when the cell was incorporated with anodic TiO₂/ZnO/Zn under UV and sunlight irradiation due to the enhanced electric field conductivity of the catalysts and heterojunction interface of TiO₂. In the constructed UV-activated PFC system, the electricity production capability was observed with the measured voltage and power density of 868 mV and 0.0172 mW cm^-2, respectively. Advanced chemical oxygen demand (COD) removal efficiency of greywater achieved a 100% completion within 60 min of light irradiation. The Escherichia coli (E. coli) colonies decreased significantly and accounted ∼99% disinfection efficiency. Moreover, the photoelectrochemical and photoluminescence (PL) experiments elucidated that the charge carrier separation efficiency were higher when TiO₂ was coupled to ZnO. The organic matter elimination principle was assessed by radical trapping experiment, and the findings indicated that the hydroxyl (OH) radical and hole (h⁺) appeared as major functions in the reaction. The stable cycle operation of the cell has been also obtained owing to the stable and film-type materials of anodic material. This performance was among the highest documented for PFC using real wastewater effluent as the fuel source.

Semiconducting p-Type Copper Iron Oxide Thin Films Deposited by Hybrid Reactive-HiPIMS + ECWR and Reactive-HiPIMS Magnetron Plasma System

A reactive high-power impulse magnetron sputtering (r-HiPIMS) and a reactive high-power impulse magnetron sputtering combined with electron cyclotron wave resonance plasma source (r-HiPIMS + ECWR) were used for the deposition of p-type CuFexOy thin films on glass with SnO2F conductive layer (FTO). The aim of this work was to deposit CuFexOy films with different atomic ratio of Cu and Fe atoms contained in the films by these two reactive sputtering methods and find deposition conditions that lead to growth of films with maximum amount of delafossite phase CuFeO2. Deposited copper iron oxide films were subjected to photoelectrochemical measurement in cathodic region in order to test the possibility of application of these films as photocathodes in solar hydrogen production. The time stability of the deposited films during photoelectrochemical measurement was evaluated. In the system r-HiPIMS + ECWR, an additional plasma source based on special modification of inductively coupled plasma, which works with an electron cyclotron wave resonance ECWR, was used for further enhancement of plasma density ne and electron temperature Te at the substrate during the reactive sputtering deposition process. A radio frequency (RF) planar probe was used for the determination of time evolution of ion flux density iionflux at the position of the substrate during the discharge pulses. Special modification of this probe to fast sweep the probe system made it possible to determine the time evolution of the tail electron temperature Te at energies around floating potential Vfl and the time evolution of ion concentration ni. This plasma diagnostics was done at particular deposition conditions in pure r-HiPIMS plasma and in r-HiPIMS with additional ECWR plasma. Generally, it was found that the obtained ion flux density iionflux and the tail electron temperature Te were systematically higher in case of r-HiPIMS + ECWR plasma than in pure r-HiPIMS during the active part of discharge pulses. Furthermore, in case of hybrid discharge plasma excitation, r-HiPIMS + ECWR plasma has also constant plasma density all the time between active discharge pulses ni ≈ 7 × 1016 m−3 and electron temperature Te ≈ 4 eV, on the contrary in pure r-HiPIMS ni and Te were negligible during the “OFF” time between active discharge pulses. CuFexOy thin films with different atomic ration of Cu/Fe were deposited at different conditions and various crystal structures were achieved after annealing in air, in argon and in vacuum. Photocurrents in cathodic region for different achieved crystal structures were observed by chopped light linear voltammetry and material stability by chronoamperometry under simulated solar light and X-ray diffraction (XRD). Optimization of depositions conditions results in the desired Cu/Fe ratio in deposited films. Optimized r-HiPIMS and r-HiPIMS + ECWR plasma deposition at 500 °C together with post deposition heat treatment at 650 °C in vacuum is essential for the formation of stable and photoactive CuFeO2 phase.

Nanoengineered Advanced Materials for Enabling Hydrogen Economy: Functionalized Graphene-Incorporated Cupric Oxide Catalyst for Efficient Solar Hydrogen Production

Cupric oxide (CuO) is a promising candidate as a photocathode for visible-light-driven photo-electrochemical (PEC) water splitting. However, the stability of the CuO photocathode against photo-corrosion is crucial for developing CuO-based PEC cells. This study demonstrates a stable and efficient photocathode through the introduction of graphene into CuO film (CuO:G). The CuO:G composite electrodes are prepared using graphene-incorporated CuO sol-gel solution via spin-coating techniques. The graphene is modified with two different types of functional groups, such as amine (-NH2) and carboxylic acid (-COOH). The -COOH-functionalized graphene incorporation into CuO photocathode exhibits better stability and also improves the photocurrent generation compare to control CuO electrode. In addition, -COOH-functionalized graphene reduces the conversion of CuO phase into cuprous oxide (Cu2O) during photo-electrochemical reaction due to effective charge transfer and leads to a more stable photocathode. The reduction of CuO to Cu2O phase is significantly lesser in CuO:G-COOH as compared to CuO and CuO:G-NH2 photocathodes. The photocatalytic degradation of methylene blue (MB) by CuO, CuO:G-NH2 and CuO:G-COOH is also investigated. By integrating CuO:G-COOH photocathode with a sol-gel-deposited TiO2 protecting layer and Au-Pd nanostructure, stable and efficient photocathode are developed for solar hydrogen generation.

Light-Irradiated Electrochemical Direct Construction of Cu2O/CuO Bilayers by Switching Cathodic/Anodic Polarization in Copper(II)-Tartrate Complex Aqueous Solution

p-CuO with a band gap energy of 1.5 eV, p-Cu₂O with a band gap energy of 2.05 eV, and their bilayers were prepared by controlling the potential of anodic and cathodic polarization in a copper(II)-tartrate complex aqueous solution containing copper(II) sulfate hydrate and tartaric acid in the dark and under light irradiation. Electrochemical characteristics of the electrodeposition and the resultant CuO and Cu₂O layers were investigated with cyclic voltammetry, chronoamperometry, and Mott-Schottky plots, and the structural and optical characterizations were performed with X-ray diffraction, scanning electron microscopy, and optical absorption spectra measurements. The CuO layer prepared at 0.4-0.7 V was composed of aggregates of granular grains with the monoclinic lattice, and the Cu₂O layer composed of coarse grains with the cubic lattice was deposited at -0.4 to 0.6 V. The flat-band potentials were estimated to be 0.145 and -0.1 V (vs Ag/AgCl) for the CuO and Cu₂O layers, respectively. The 0.4 μm CuO/1.1 μm Cu₂O bilayers could be prepared by switching the electrodeposition potentials of 0.4 and -0.4 V, irrespective of the presence of light irradiation. The photoelectrodeposition under light irradiation enabled the preparation of continuous and dense 1.1 μm Cu₂O/0.4 μm CuO bilayer by controlling the potential, while electrodeposition in the dark led to sparse, isolated, and coarse Cu₂O grains being deposited. The mechanism for the photoelectrodeposition of the bilayers was discussed based on the energy band alignment at the heterointerface to the Cu-tartrate complex solution.

Superior supercapacitive performance of grass-like CuO thin films deposited by liquid phase deposition

First report on deposition and supercapacitive performance of grass-like CuO thin films by liquid phase deposition on flat and mesh stainless steel (SS). The maximum specific capacitances on flat and mesh SS are 552 and 849 F g⁻¹.

The Effect of Cu Ohmic Contact on Photoelectrochemical Property of S-CuO Thin Film Photocathodes

The development of semiconductor materials as photocathodes that have excellent performance is significant for the photoelectrochemical reaction of hydrogen evolution. The thin film of sulfur-doped Copper (II) oxide (S-CuO)  was successfully synthesized using the cyclic voltammetry method. Investigation of photoelectrochemical properties of S-CuO photocathodes, including current density, onset potential, applied photon to current efficiency (ABPE), and bandgap had been carried out. It was reported that the Cu ohmic contact affected the photoelectrochemical properties and the stability of the thin film. The presence of Cu ohmic contact can improve the performance of S-CuO thin film photocathodes. The S-CuO TU 20 mM thin film has the best response with a current density of -0.923 mA/cm2, an onset potential of 0.59 V, and ABPE of 0.21%. Stability occurred at pH 7 in 0.2M NaH2PO4. The optical analysis showed S-CuO TU 20 mM bandgap of 1.7 eV.

The Effect of Cu Ohmic Contact on Photoelectrochemical Property of S-CuO Thin Film Photocathodes

The development of semiconductor materials as photocathodes that have excellent performance is significant for the photoelectrochemical reaction of hydrogen evolution. The thin film of sulfur-doped Copper (II) oxide (S-CuO)  was successfully synthesized using the cyclic voltammetry method. Investigation of photoelectrochemical properties of S-CuO photocathodes, including current density, onset potential, applied photon to current efficiency (ABPE), and bandgap had been carried out. It was reported that the Cu ohmic contact affected the photoelectrochemical properties and the stability of the thin film. The presence of Cu ohmic contact can improve the performance of S-CuO thin film photocathodes. The S-CuO TU 20 mM thin film has the best response with a current density of -0.923 mA/cm2, an onset potential of 0.59 V, and ABPE of 0.21%. Stability occurred at pH 7 in 0.2M NaH2PO4. The optical analysis showed S-CuO TU 20 mM bandgap of 1.7 eV.

Ternary Mixed Spinel Oxides as Oxygen Carriers for Chemical Looping Hydrogen Production Operating at 550 °C

Operating chemical looping at moderate temperatures circumvents the issue that the sintering of oxygen carrier materials is serious at typical operating conditions, 800-950 °C. However, lower temperatures can lead to deterioration on the reaction kinetics and thereby the low H₂ production rate and yield. Here, we present several doped spinel oxides consisting of earth-abundant elements for chemical looping water splitting. By virtue of the ability of the Cu dopant to improve the reduction of the Co-based binary spinel, the high reducibility of the dopants in the reduction period, as well as the phase reversibility in the water splitting period, Cu_0.25Co_0.25Fe_2.5O_y shows a high hydrogen yield (∼11.9 mmol g^-1) and an average hydrogen production rate (∼137.7 μmol g^-1 min^-1) at 550 °C, with negligible decays in repetitive redox cycles. The performance of this material is comparable to that of the state-of-the-art perovskites which usually contain rare-earth metals, enabling its potential in industrial implementation.

Electronic Integration and Thin Film Aspects of Au-Pd/rGO/TiO2 for Improved Solar Hydrogen Generation

In the present work, we have synthesized noble bimetallic nanoparticles (Au-Pd NPs) on a carbon-based support and integrated with titania to obtain Au-Pd/C/TiO₂ and Au-Pd/rGO/TiO₂ nanocomposites using an ecofriendly hydrothermal method. Here, a 1:1 (w/w) Au-Pd bimetallic composition was dispersed on (a) high-surface-area (3000 m² g^-1) activated carbon (Au-Pd/C), prepared from a locally available plant source (in Assam, India), and (b) reduced graphene oxide (rGO) (Au-Pd/rGO); subsequently, they were integrated with TiO₂. The shift observed in Raman spectroscopy demonstrates the electronic integration of the bimetal with titania. The photocatalytic activity of the above materials for the hydrogen evolution reaction was studied under 1 sun conditions using methanol as a sacrificial agent in a powder form. The photocatalysts were also employed to prepare a thin film by the drop-casting method. Au-Pd/rGO/TiO₂ exhibits 43 times higher hydrogen (H₂) yield in the thin film form (21.50 mmol h^-1 g^-1) compared to the powder form (0.50 mmol h^-1 g^-1). On the other hand, Au-Pd/C/TiO₂ shows 13 times higher hydrogen (H₂) yield in the thin film form (6.42 mmol h^-1 g^-1) compared to the powder form (0.48 mmol h^-1 g^-1). While powder forms of both catalysts show comparable activity, the Au-Pd/rGO/TiO₂ thin film shows 3.4 times higher activity than that of Au-Pd/C/TiO₂. This can be ascribed to (a) an effective separation of photogenerated electron-hole pairs at the interface of Au-Pd/rGO/TiO₂ and (b) the better field effect due to plasmon resonance of the bimetal in the thin film form. The catalytic influence of the carbon-based support is highly pronounced due to synergistic binding interaction of bimetallic nanoparticles. Further, a large amount of hydrogen evolution in the film form with both catalysts (Au-Pd/C/TiO₂ and Au-Pd/rGO/TiO₂) reiterates that charge utilization should be better compared to that in powder catalysts.

Visible Light Driven Heterojunction Photocatalyst of CuO-Cu2O Thin Films for Photocatalytic Degradation of Organic Pollutants

A high recombination rate and low charge collection are the main limiting factors of copper oxides (cupric and cuprous oxide) for the photocatalytic degradation of organic pollutants. In this paper, a high performance copper oxide photocatalyst was developed by integrating cupric oxide (CuO) and cuprous oxide (Cu₂O) thin films, which showed superior performance for the photocatalytic degradation of methylene blue (MB) compared to the control CuO and Cu₂O photocatalyst. Our results show that a heterojunction photocatalyst of CuO–Cu₂O thin films could significantly increase the charge collection, reduce the recombination rate, and improve the photocatalytic activity.

Flexible cupric oxide photocathode with enhanced stability for renewable hydrogen energy production from solar water splitting

CuO is a promising but unstable photocathode in solar water splitting. Herein, a flexible CuO photocathode is prepared and its degradation mechanisms and stabilization strategies have been discussed. Briefly, we find alkali environment and low light intensity are the critical factors in the stabilization of the CuO photocathode. For practical usage, a composite semiconductor layer, composed of TiO2, La2O3 and NiO, is deposited on the CuO photocathode, which is proved to be effective for enhancing the stabilization of the CuO photocathode. 100% of the photocurrent density has been retained after 20 minutes of continuous illumination. The optimized stable photocurrent density is measured as 0.3 mA cm-2 at 0.5 VRHE.

Hierarchical SnS2/CuInS2 Nanosheet Heterostructure Films Decorated with C60 for Remarkable Photoelectrochemical Water Splitting

Rational architectural design and catalyst components are beneficial to improve the photoelectrochemical (PEC) performance. Herein, hierarchical SnS₂/CuInS₂ nanosheet heterostructure porous films were fabricated and decorated with C₆₀ to form photocathodes for PEC water reduction. Large-size CuInS₂ nanosheet films were first grown on transparent conducting glass to form substrate films. Then, small-size SnS₂ nanosheets were epitaxially grown on both sides of the CuInS₂ nanosheets to form uniform hierarchical porous laminar films. The addition of C₆₀ on the surface of the SnS₂/CuInS₂ porous nanosheets effectively increased visible light absorption of the composite photocathode. Photoluminescence spectroscopy and impedance spectroscopy analyses indicated that the formation of a SnS₂/CuInS₂ heterojunction and decoration of C₆₀ significantly increased the photocurrent density by promoting the electron-hole separation and decreasing the resistance to the transport of charge carriers. The hierarchical SnS₂/CuInS₂ nanosheet heterostructure porous films containing multiscale nanosheets and pore configurations can enlarge the surface area and enhance visible light utilization. These beneficial factors make the optimized C₆₀-decorated SnS₂/CuInS₂ photocathode exhibit much higher photocathodic current (4.51 mA cm^-2 at applied potential -0.45 V vs reversible hydrogen electrode ) and stability than the individual CuInS₂ (2.58 mA cm^-2) and SnS₂ (1.92 mA cm^-2) nanosheet film photocathodes. This study not only reveals the promise of C₆₀-decorated hierarchical SnS₂/CuInS₂ nanosheet heterostructure porous film photocathodes for efficient solar energy harvesting and conversion but also provides rational guidelines in designing high-efficiency photoelectrodes from earth-abundant and low-cost materials allowing widely practical applications.

Review of Anodic Catalysts for SO2 Depolarized Electrolysis for “Green Hydrogen” Production

In the near future, primary energy from fossil fuels should be gradually replaced with renewable and clean energy sources. To succeed in this goal, hydrogen has proven to be a very suitable energy carrier, because it can be easily produced by water electrolysis using renewable energy sources. After storage, it can be fed to a fuel cell, again producing electricity. There are many ways to improve the efficiency of this process, some of them based on the combination of the electrolytic process with other non-electrochemical processes. One of the most promising is the thermochemical hybrid sulphur cycle (also known as Westinghouse cycle). This cycle combines a thermochemical step (H2SO4 decomposition) with an electrochemical one, where the hydrogen is produced from the oxidation of SO2 and H2O (SO2 depolarization electrolysis, carried out at a considerably lower cell voltage compared to conventional electrolysis). This review summarizes the different catalysts that have been tested for the oxidation of SO2 in the anode of the electrolysis cell. Their advantages and disadvantages, the effect of platinum (Pt) loading, and new tendencies in their use are presented. This is expected to shed light on future development of new catalysts for this interesting process.

Photocatalytic 4-nitrophenol degradation and oxygen evolution reaction in CuO/g-C3N4 composites prepared by deep eutectic solvent-assisted chlorine doping

Heterostructured Cl-CuO/g-C₃N₄ composite for OER and photocatalytic 4-nitrophenol degradation.

A high-efficiency and stable cupric oxide photocathode coupled with Al surface plasmon resonance and Al2O3 self-passivation

A high-efficiency and stable CuO/Al/Al₂O₃ photocathode for photoelectrochemical water splitting has been successfully synthesized by a facile magnetron sputtering combined with spontaneous oxidation method.

Evaluation of photocatalytic fuel cell (PFC) for electricity production and simultaneous degradation of methyl green in synthetic and real greywater effluents

Recycling of alternative water sources particularly greywater and recovery of energy from wastewater are gaining momentum due to clean water scarcity and energy crisis. In this study, the photocatalytic fuel cell (PFC) employing ZnO/Zn photoanode and CuO/Cu photocathode was successfully designed for effective greywater recycling as well as energy recovery. The photoelectrodes were analyzed using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and fourier transform infrared (FTIR) spectroscopy. The PFC performance in terms of electricity generation and parallel methyl green (MG) degradation were evaluated under operating parameters such as electrolyte type, initial MG concentration and solution pH. The results showed that the addition of Na₂SO₄ electrolyte, MG concentration of 40 mg L^-1 and solution pH of 5.2 improved the short circuit current density (J_sc) and power density (P_max) in the as-constructed PFC. Such a system also afforded highest MG and chemical oxygen demand (COD) removal efficiencies after 4 h of irradiation. The photoanodes used in this study demonstrated great recyclability after four repetition tests. The COD removal was reduced to some extents when the PFC treatment was tested in the real greywater under optimal conditions. Various greywater quality parameters including ammoniacal nitrogen (NH₃-N), turbidity, pH and biochemical oxygen demand (BOD₅) were also monitored. The phytotoxicity experiments via Vigna radiate seeds indicated a reduction in the phytotoxicity.

Photocatalytic Hydrogen Evolution via Water Splitting: A Short Review

Photocatalytic H2 generation via water splitting is increasingly gaining attention as a viable alternative for improving the performance of H2 production for solar energy conversion. Many methods were developed to enhance photocatalyst efficiency, primarily by modifying its morphology, crystallization, and electrical properties. Here, we summarize recent achievements in the synthesis and application of various photocatalysts. The rational design of novel photocatalysts was achieved using various strategies, and the applications of novel materials for H2 production are displayed herein. Meanwhile, the challenges and prospects for the future development of H2-producing photocatalysts are also summarized.

Different Annealing Atmosphere Gases on the Growth and Photocurrent Performance of CuO Films Grown on FTO Substrate

Improvement in photocurrent performance remains the key subject to prepare a stable and efficient photocathode in photoelectrochemical cell (PEC) water splitting. Different to the ordinary methods, various annealing atmosphere gases were used to study the growth of CuO films on fluorine-doped tin oxide substrate; then, the photocurrent performance was studied when those CuO films were used as photocathodes in PEC. The scanning electron microscopy images indicate that all of the CuO films are composed of vertically arrayed CuO nanosheets, each individual nanosheet with a thickness of 100-500 nm. Those hierarchical CuO photoelectrodes in the PEC exhibit quite different photoelectrochemical activities in visible light, where the air-annealed CuO film has nearly 6 times enhancement in photocurrent (108 μA) at 0 V compared to that of film under oxygen atmosphere, and 34 times of argon. It has an acceptor concentration of 2.9 × 10²¹ cm^-3 from Mott-Schottky analysis, which is more than 2 times larger than that of the oxygen-annealed CuO film, and 37 times larger than that of the argon-annealed film. Ultraviolet photoelectron spectroscopy measurements were carried out to explain the improved photocurrent performance of the air-annealed CuO films, where the obtained valence band of 0.44 eV and work function of 4.92 eV well match the reduction reaction of electrolyte (H₂O).

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron

Cupric oxide (CuO) is considered as a promising photocathode material for photo(electro)chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core-shell structure of the nanowires.

ZnO/CuO/M (M = Ag, Au) Hierarchical Nanostructure by Successive Photoreduction Process for Solar Hydrogen Generation

To date, solar energy generation devices have been widely studied to meet a clean and sustainable energy source. Among them, water splitting photoelectrochemical cell is regarded as a promising energy generation way for splitting water molecules and generating hydrogen by sunlight. While many nanostructured metal oxides are considered as a candidate, most of them have an improper bandgap structure lowering energy transition efficiency. Herein, we introduce a novel wet-based, successive photoreduction process that can improve charge transfer efficiency by surface plasmon effect for a solar-driven water splitting device. The proposed process enables to fabricate ZnO/CuO/Ag or ZnO/CuO/Au hierarchical nanostructure, having an enhanced electrical, optical, photoelectrochemical property. The fabricated hierarchical nanostructures are demonstrated as a photocathode in the photoelectrochemical cell and characterized by using various analytic tools.