Photoelectrochemical Water Splitting with p-Type Metal Oxide Semiconductor Photocathodes

Effective prediction of SnO2 conduction band edge potential: The key role of surface oxygen vacancies

Several theoretical studies at different levels of theory have attempted to calculate the absolute position of the SnO2 conduction band, whose knowledge is key for its effective application in optoelectronic devices such us, for example, perovskite solar cells. However, the predicted band edges fall outside the experimentally measured range. In this work, we introduce a computational scheme designed to calculate the conduction band minimum values of SnO2, yielding results aligned with experiments. Our analysis points out the fundamental role of encompassing surface oxygen vacancies to properly describe the electronic profile of this material. We explore the impact of both bridge and in-plane oxygen vacancy defects on the structural and electronic properties of SnO2, explaining from an atomistic perspective the experimental observables. The results underscore the importance of simulating both types of defects to accurately predict SnO2 features and provide new fundamental insights that can guide future studies concerning design and optimization of SnO2-based materials and functional interfaces.

A Visible Light-Responsive TiO2 Photocathode Achieved by a Rh Dopant

Developing an efficient and stable photocathode material for photoelectrochemical solar water splitting remains challenging. Herein, we demonstrate the potential of rutile TiO2 as a photocathode by Rh doping with visible light absorption up to 640 nm and an onset potential of 0.9 V versus the reversible hydrogen electrode. The dopant transforms the rutile host from an n-type semiconductor to a p-type one, as confirmed by the Mott-Schottky curve and kelvin probe force microscopy. Physical and photoelectrochemical analyses further suggest that the doping mechanism is dependent on concentration. Lower levels of dopants generate localized Rh3+, while higher levels favor Rh4+ that interacts more strongly with the O 2p orbitals. The latter is found not only to extend the visible light absorption range but also to facilitate charge transport. This work elucidates the role of the Rh dopant in adjusting the photoelectrochemical behavior of TiO2, and it provides a promising photocathode material for solar energy conversion.

Rhodium-Doped Barium Titanate Perovskite as a Stable p-Type Photocathode in Solar Water Splitting

Solar water splitting from a p-n-conjugated photoelectrochemical (PEC) system is a promising way to produce hydrogen sustainably. At present, finding a compatible p-type photocathode material for the p-n system remains a great challenge in consideration of the photocurrent and stability. This paper highlighted a promising candidate, Rh/BaTiO3, by switching BaTiO3 from an n-type photoanode to a p-type photocathode upon Rh doping. The dopant activated visible light absorption up to 550 nm and an onset potential as high as 1.0 V (vs RHE). Using surface photovoltage spectroscopy as a powerful characterization tool, the n- to p-type transition of the semiconductor was studied and explained microscopically by which we quantitatively isolated the cathodic contribution caused by the Rh dopant. Unbiased overall solar water splitting was accomplished by serially connecting the Pt/Rh/BaTiO3 photocathode to a CoOx/Mo/BiVO4 photoanode, which produced a solar to hydrogen conversion efficiency of 0.1% and an excellent stability over 100 h of operation at ambient pressure. This work revealed the key role that the Rh dopant played in the n- to p-type adjustment of titanate semiconductors and demonstrated its great potential for application in PEC water splitting.

Surface Engineering of Cu2O Photocathodes via Facile Graphene Oxide Decoration for Improved Photoelectrochemical Water Splitting

Copper oxide (Cu2O) has attracted significant interest as an efficient photocathode for photoelectrochemical (PEC) water splitting owing to its abundance, suitable band gap, and band-edge potential. Nevertheless, a high charge recombination rate restricts its practical photoconversion efficiency and reduces the PEC water-splitting performance. To address this challenge, we present the facile electrodeposition of graphene oxide (GO) on the Cu2O photocathode surface. To determine the effect of varying GO weight percentages on PEC performance, varying amounts of GO were deposited on the Cu2O photocathode surface. The optimally deposited GO-Cu2O photocathode exhibited a photocurrent density of -0.39 to -1.20 mA/cm2, which was three times that of a photocathode composed of pristine Cu2O. The surface decoration of Cu2O with GO reduced charge recombination and improved the PEC water-splitting performance. These composites can be utilized in strategies designed to address the challenges associated with low-efficiency Cu2O photocathodes. The physicochemical properties of the prepared samples were comprehensively characterized by field-emission scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, UV-visible spectroscopy, and X-ray photoelectron spectroscopy. We believe that this research will pave the way for developing efficient Cu2O-based photocathodes for PEC water splitting.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

The Structural and Electrochemical Properties of CuCoO2 Crystalline Nanopowders and Thin Films: Conductivity Experimental Analysis and Insights from Density Functional Theory Calculations

A novel manufacturing process is presented for producing nanopowders and thin films of CuCoO2 (CCO) material. This process utilizes three cost-effective synthesis methods: hydrothermal, sol-gel, and solid-state reactions. The resulting delafossite CuCoO2 samples were deposited onto transparent substrates through spray pyrolysis, forming innovative thin films with a nanocrystal powder structure. Prior to the transformation into thin films, CuCoO2 powder was first produced using a low-cost approach. The precursors for both powders and thin films were deposited onto glass surfaces using a spray pyrolysis process, and their characteristics were examined through X-ray diffraction, scanning electron microscopy, HR-TEM, UV-visible spectrophotometry, and electrochemical impedance spectroscopy (EIS) analyses were conducted to determine the conductivity in the transversal direction of this groundbreaking material for solar cell applications. On the other hand, the sheet resistance of the samples was investigated using the four-probe method to obtain the sheet resistivity and then calculate the in-plane conductivity of the samples. We also investigated the aging characteristics of different precursors with varying durations. The functional properties of CuCoO2 samples were explored by studying chelating agent and precursor solution aging periods using Density Functional Theory calculations (DFT). A complementary Density Functional Theory study was also performed in order to evaluate the electronic structure of this compound. Resuming, this study thoroughly discusses the synthesis of delafossite powders and their conversion into thin films, which hold potential as hole transport layers in transparent optoelectronic devices.

Tandem cells for unbiased photoelectrochemical water splitting

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

Effects of activation overpotential in photoelectrochemical cells considering electrical and optical configurations

Photoelectrochemical cells (PECs) are a promising option for directly converting solar energy into chemical energy by producing hydrogen (H₂) gas, thus providing a clean alternative to consuming fossil fuels. H₂ as fuel is free from any carbon footprints and negative environmental impacts. Therefore, the H₂ production, especially directly using sunlight in PECs, is critically important for the rapidly growing energy demand of the world. Although promising, PECs are inefficient and must overcome a few inherent losses in producing H₂-the most important being the activation overpotential (ηa) required for splitting water. This work analyzes the impact of ηa on solar-to-fuel efficiency (ηSTF) and H₂ production rate (HPR). This work also discusses choosing appropriate photo-absorbing materials based on their energy bandgaps and suitable electrode pairs to achieve desired ηSTF and HPR for different electrical and optical PEC configurations. Significant changes are observed in ηSTF and HPR when ηa is considered in water splitting.

Understanding systematic growth mechanism of porous Zn1-xCdxSe/TiO2 nanorod heterojunction from ZnSe(en)0.5/TiO2 photoanodes for bias-free solar hydrogen evolution

Herein, a porous Zn_1-xCd_xSe structure was developed on TiO₂ nanorod (NR) array for photoelectrochemical (PEC) application. Firstly, TiO₂ NR and ZnO/TiO₂ NR photoanode were synthesized via a series of hydrothermal methods on FTO. Next, the solvothermal synthesis method was adopted to develop inorganic-organic hybrid ZnSe(en)_0.5 on ZnO /TiO₂ NR-based electrode using different concentrations of the selenium (Se). We found that the ZnO NR acts as a mother material for the formation of inorganic-organic hybrid ZnSe(en)_0.5, whereas TiO₂ NR acts as a building block. In order to further improve the PEC charge transfer performance, inorganic-organic hybrid ZnSe(en)_0.5/TiO₂ NR electrode was transferred into a porous Zn_1-xCd_xSe/TiO₂ NR photoanode using the Cd²⁺ ion-exchange method. The optimized porous Zn_1-xCd_xSe/TiO₂ NR -(2) photoanode converted from ZnSe(en)_0.5 -(2) electrode (optimized Se concentration) showed a higher photocurrent density of 6.6 mA·cm^-2 at applied potential 0 V vs. Ag/AgCl. The enhanced photocurrent density was owing to the effective light absorption, enhanced charge separation, delay the charge recombination, and porous structure of Zn_1-xCd_xSe. This work highlights the promising strategy for the synthesis of porous Zn_1-xCd_xSe/TiO₂ NR from inorganic-organic ZnSe(en)_0.5/TiO₂ NR for effective charge separation and prolonging the lifetime during the photoelectrochemical reaction.

Efficient CuO/Ag2WO4 photoelectrodes for photoelectrochemical water splitting using solar visible radiation

Water splitting energy production relies heavily on the development of high-performance photoelectrochemical cells (PECs). Among the most highly regarded semiconductor materials, cupric oxide (CuO) is an excellent photocathode material. Pristine CuO does not perform well as a photocathode due to its tendency to recombine electrons and holes rapidly. Photocathodes with high efficiency can be produced by developing CuO-based composite systems. The aim of our research is to develop an Ag₂WO₄/CuO composite by incorporating silver tungstate (Ag₂WO₄) nanoparticles onto hydrothermally grown CuO nanoleaves (NLs) by successive ionic layer adsorption and reaction (SILAR). To prepare CuO/Ag₂WO₄ composites, SILAR was used in conjunction with different Ag₂WO₄ nanoparticle deposition cycles. Physicochemical characterization reveals well-defined nanoleaves morphologies with tailored surface compositions. Composite CuO/Ag₂WO₄ crystal structures are governed by the monoclinic phase of CuO and the hexagonal phase of Ag₂WO₄. It has been demonstrated that the CuO/Ag₂WO₄ composite has outstanding performance in the PEC water splitting process when used with five cycles. In the CuO/Ag₂WO₄ photocathode, water splitting activity is observed at low overpotential and high photocurrent density, indicating that the reaction takes place at low energy barriers. Several factors contribute to PEC performance in composites. These factors include the high density of surface active sites, the high charge separation rate, the presence of favourable surface defects, and the synergy of CuO and Ag₂WO₄ photoreaction. By using SILAR, silver tungstate can be deposited onto semiconducting materials with strong visible absorption, enabling the development of energy-efficient photocathodes.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe₂ O₄ (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm² at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Bismuth-Based Multi-Component Heterostructured Nanocatalysts for Hydrogen Generation

Developing a unique catalytic system with enhanced activity is the topmost priority in the science of H2 energy to reduce costs in large-scale applications, such as automobiles and domestic sectors. Researchers are striving to design an effective catalytic system capable of significantly accelerating H2 production efficiency through green pathways, such as photochemical, electrochemical, and photoelectrochemical routes. Bi-based nanocatalysts are relatively cost-effective and environmentally benign materials which possess advanced optoelectronic properties. However, these nanocatalysts suffer back recombination reactions during photochemical and photoelectrochemical operations which impede their catalytic efficiency. However, heterojunction formation allows the separation of electron–hole pairs to avoid recombination via interfacial charge transfer. Thus, synergetic effects between the Bi-based heterostructured nanocatalysts largely improves the course of H2 generation. Here, we propose the systematic review of Bi-based heterostructured nanocatalysts, highlighting an in-depth discussion of various exceptional heterostructures, such as TiO2/BiWO6, BiWO6/Bi2S3, Bi2WO6/BiVO4, Bi2O3/Bi2WO6, ZnIn2S4/BiVO4, Bi2O3/Bi2MoO6, etc. The reviewed heterostructures exhibit excellent H2 evolution efficiency, ascribed to their higher stability, more exposed active sites, controlled morphology, and remarkable band-gap tunability. We adopted a slightly different approach for reviewing Bi-based heterostructures, compiling them according to their applicability in H2 energy and discussing challenges, prospects, and guidance to develop better and more efficient nanocatalytic systems.

Evaluation of the Efficiency of Photoelectrochemical Activity Enhancement for the Nanostructured LaFeO3 Photocathode by Surface Passivation and Co-Catalyst Deposition

Perovskite-type lanthanum iron oxide, LaFeO₃, is a promising photocathode material that can achieve water splitting under visible light. However, the performance of this photoelectrode material is limited by significant electron-hole recombination. In this work, we explore different strategies to optimize the activity of a nanostructured porous LaFeO₃ film, which demonstrates enhanced photoelectrocatalytic activity due to the reduced diffusion length of the charge carriers. We found that surface passivation is not an efficient approach for enhancing the photoelectrochemical performance of LaFeO₃, as it is sufficiently stable under photoelectrocatalytic conditions. Instead, the deposition of a Pt co-catalyst was shown to be essential for maximizing the photoelectrochemical activity both in hydrogen evolution and oxygen reduction reactions. Illumination-induced band edge unpinning was found to be a major challenge for the further development of LaFeO₃ photocathodes for water-splitting applications.

Optoelectrical Regulation of CuBi2O4 Photocathode via Photonic Crystal Structure for Solar-Fuel Conversion

Metal oxide semiconductors have been regarded as ideal candidates for photoelectrochemical (PEC) CO₂ reduction if the contradiction between photon harvesting and photocarrier collection can be resolved. The novel three-dimensional structure provides an available approach to balancing the above-mentioned contradiction. In this work, CuBi₂O₄ photonic crystal photocathodes with different feature sizes were developed to realize the regulation of optoelectrical properties. The resulted photocathode displays promoted PEC activity as the enhanced photocurrent and CO₂ reduction activity. Such an excellent performance was attributed to the improved efficiency of charge carrier generation and collection through extending the optical path and shortening the carrier transport distance inside films. COMSOL simulations and PEC spectroscopy analysis confirmed the promoted photon harvesting capacity and carrier dynamics. This work demonstrates a feasible strategy for developing novel photocathodes with modulated microstructures in solar-fuel conversion.

Facile Zn and Ni Co-Doped Hematite Nanorods for Efficient Photocatalytic Water Oxidation

In this work, we report the effect of zinc (Zn) and nickel (Ni) co-doping of hydrothermally synthesized hematite nanorods prepared on fluorine-doped tin oxide (FTO) substrates for enhanced photoelectrochemical (PEC) water splitting. Seeded hematite nanorods (NRs) were facilely doped with a fixed concentration of 3 mM Zn and varied concentrations of 0, 3, 5, 7, and 9 mM Ni. The samples were observed to have a largely uniform morphology of vertically aligned NRs with slight inclinations. The samples showed high photon absorption within the visible spectrum due to their bandgaps, which ranged between 1.9-2.2 eV. The highest photocurrent density of 0.072 mA/cm2 at 1.5 V vs. a reversible hydrogen electrode (RHE) was realized for the 3 mM Zn/7 mM Ni NRs sample. This photocurrent was 279% higher compared to the value observed for pristine hematite NRs. The Mott-Schottky results reveal an increase in donor density values with increasing Ni dopant concentration. The 3 mM Zn/7 mM Ni NRs sample produced the highest donor concentration of 2.89 × 1019 (cm-3), which was 2.1 times higher than that of pristine hematite. This work demonstrates the role of Zn and Ni co-dopants in enhancing the photocatalytic water oxidation of hematite nanorods for the generation of hydrogen.

Solar-Driven Syngas Production Using Al-Doped ZnTe Nanorod Photocathodes

Syngas, traditionally produced from fossil fuels and natural gases at high temperatures and pressures, is an essential precursor for chemicals utilized in industry. Solar-driven syngas production can provide an ideal pathway for reducing energy consumption through simultaneous photoelectrochemical CO2 and water reduction at ambient temperatures and pressures. This study performs photoelectrochemical syngas production using highly developed Al-doped ZnTe nanorod photocathodes (Al:ZnTe), prepared via an all-solution process. The facile photo-generated electrons are transferred by substitutional Al doping on Zn sites in one-dimensional arrays to increase the photocurrent density to -1.1 mA/cm2 at -0.11 VRHE, which is 3.5 times higher than that for the pristine ZnTe. The Al:ZnTe produces a minor CO (FE ≈ 12%) product by CO2 reduction and a major product of H2 (FE ≈ 60%) by water reduction at -0.11 VRHE. Furthermore, the product distribution is perfectly switched by simple modification of Au deposition on photocathodes. The Au coupled Al:ZnTe exhibits dominant CO production (FE ≈ 60%), suppressing H2 evolution (FE ≈ 15%). The strategies developed in this study, nanostructuring, doping, and surface modification of photoelectrodes, can be applied to drive significant developments in solar-driven fuel production.

Heterojunction Nanomedicine

Exogenous stimulation catalytic therapy has received enormous attention as it holds great promise to address global medical issues. However, the therapeutic effect of catalytic therapy is seriously restricted by the fast charge recombination and the limited utilization of exogenous stimulation by catalysts. In the past few decades, many strategies have been developed to overcome the above serious drawbacks, among which heterojunctions are the most widely used and promising strategy. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction nanomedicine, such as semiconductor-semiconductor heterojunctions (including type I, type II, type III, PN, and Z-scheme junctions) and semiconductor-metal heterojunctions (including Schottky, Ohmic, and localized surface plasmon resonance-mediated junctions). The catalytic mechanisms and properties of the above junction systems are also discussed in relation to biomedical applications, especially cancer treatment and sterilization. This review concludes with a summary of the challenges and some perspectives on future directions in this exciting and still evolving field of research.

Room Temperature Electrodeposition of Ready-to-Use TiOx for Uniform p-n Heterojunction Over Nanoarchitecture

The photocathodes are essential in photoelectrochemical systems for harvesting solar energy as green fuels. However, the light-absorbing p-type semiconductor in them usually suffers from carrier recombination issues. An effective strategy to address it is fabricating the p-n heterojunction to create an interfacial electric field. However, plenty of deposition process of the n-type layer for this purpose requires either sophisticated instruments or subsequent treatments, which may damage the vulnerable p-type structure. Herein, we report a mild approach for a ready-to-use n-type layer with full functionality. Structural analyses proved the successful coating of a uniform titania layer (up to 40 nm) over Cu2O without damaging its structure. Owing to the high Ti3+ content, the layer possesses excellent charge transport ability and requires no additional annealing. The heterojunction effectively facilitates the carrier separation and positively shifts the photocurrent onset potential for 0.2 V. The Mott–Schottky plot and the impedance study reveal an enhanced carrier collection with reduced charge transfer resistances. Such a nano-heterojunction can be further loaded with the hydrogen evolution catalyst, which almost doubles the photocurrent with an extended lifetime than that of the pristine Cu2O nanoarray. This approach puts forward a potentially scalable and efficient choice for fabricating photoelectrochemical devices.

Impact factors and novel strategies for improving biohydrogen production in microbial electrolysis cells

Microbial electrolysis cell (MEC) system is an environmentally friendly method for clean biohydrogen production from a wide range of biowastes owing to low greenhouse gas emissions. This approach has relatively higher yields and lower energy costs for biohydrogen production compared to conventional biological technologies and direct water electrolysis, respectively. However, biohydrogen production efficiency and operating costs of MEC still need further optimization to realize its large-scale application.This paper provides a unique review of impact factors influencing biohydrogen production in MECs, such as microorganisms and electrodes. Novel strategies, including inhibition of methanogens, development of novel cathode catalyst, advanced reactor design and integrated systems, to enhance low-cost biohydrogen production, are discussed based on recent publications in terms of their opportunities, bottlenecks and future directions. In addition, the current challenges, and effective future perspectives towards the practical application of MECs are described in this review.

New Concept for the Facile Fabrication of Core-Shell CuO@CuFe2O4 Photocathodes for PEC Application

The CuO@CuFe₂O₄ core–shell structure represents a new family of photocatalysts that can be used as photoelectrodes that are able to produce hydrogen under a broad spectrum of visible light. Herein, we report a novel approach for the production of this active film by the thermal conversion of CuFe Prussian Blue Analogues. The outstanding photoelectrochemical properties of the photocathodes of CuO@CuFe₂O₄ were studied with the use of combinatory photo-electrochemical instrumental techniques which proved that the electrodes were stable over the whole water photolysis run under relatively positive potentials. Their outstanding performance was explained by the coupling of two charge transfer mechanisms occurring in core–shell architectures.

A phase transition-induced photocathodic p-CuFeO2 nanocolumnar film by reactive ballistic deposition

Vertical nanocolumnar Cu–Fe–O electrodes synthesized by the reactive ballistic deposition technique followed by heat treatment in an Ar atmosphere undergo a switch for conductivity at elevated temperatures.

Uplifting the charge carrier separation and migration in Co-doped CuBi2O4/TiO2 p-n heterojunction photocathode for enhanced photoelectrocatalytic water splitting

Here, cobalt-doped copper bismuth oxide (Co-CuBi₂O₄) was synthesized via a facile hydrothermal method for photoelectrocatalytic (PEC) hydrogen production. The results disclosed that the 5% Co-doped CuBi₂O₄ has better PEC activity which is ∼3 fold higher than pristine CuBi₂O₄. The doping of cobalt in CuBi₂O₄ improves the interfacial charge transfer at an electrode/electrolyte interface and reduces the recombination rate of photogenerated electron-hole pairs. This higher performed 5% Co-doped CuBi₂O₄ photocathode further modified with TiO₂-P25 to form a Co-CuBi₂O₄/TiO₂ p-n heterojunction. This Co-CuBi₂O₄/TiO₂ photocathode displayed a photocurrent density of 330 μA cm^-2 at +0.5 V vs. RHE which was ∼2 fold higher than Co-CuBi₂O₄. Because this p-n junction affords inner electric field in the space charge region that helps for further minimization of electron-hole recombination, which facilitate efficient charge separation and transport thereby enhance the PEC water reduction.

Atmosphere-sensitive photoluminescence of Co x Fe3-x O4 metal oxide nanoparticles

In this work the photoluminescence (PL) of Co x Fe3-x O4 spinel oxide nanoparticles under pulsed UV laser irradiation (λ exc = 270 nm) is investigated for varying Co/Fe ratios (x = 0.4⋯2.5). A broad emission in the green spectral range is observed, exhibiting two maxima at around 506 nm, which is dominant for Fe-rich nanoparticles (x = 0.4, 0.9), and at around 530 nm, that is more pronounced for Co-rich nanoparticles (x > 1.6). As examinations in different atmospheres show that the observed emission reacts sensitively to the presence of water, it is proposed that the emission is mainly caused by OH groups with terminal or bridging metal-O bonds on the Co x Fe3-x O4 surface. Raman spectroscopy supports that the emission maximum at 506 nm corresponds to terminal OH groups bound to metal cations on tetrahedral sites (i.e., Fe3+), while the maximum around 530 nm corresponds to terminal OH groups bound to metal cations on octahedral sites (i.e., Co3+). Photoinduced dehydroxylation shows that OH groups can be removed on Fe-rich nanoparticles more easily, leading to a conversion process and the formation of new OH groups with different bonds to the surface. As such behavior is not observed for Co x Fe3-x O4 with x > 1.6, we conclude that the OH groups are more stable against dehydroxylation on Co-rich nanoparticles. The higher OH stability is expected to lead to a higher catalytic activity of Co-rich cobalt ferrites in the electrochemical generation of oxygen.

Photoelectrochemical Hydrogen Production by Screen-Printed Copper Oxide Electrodes

In this work, copper oxides-based photocathodes for photoelectrochemical cells (PEC) were produced for the first time by screen printing. A total 7 × 10−3 g/m2 glycerine trioleate was found as optimum deflocculant amount to assure stable and homogeneous inks, based on CuO nano-powder. The inks were formulated considering different binder amounts and deposited producing films with homogenous thickness, microstructure, and roughness. The as-produced films were thermally treated to obtain Cu2O- and Cu2O/CuO-based electrodes. The increased porosity obtained by adding higher amounts of binder in the ink positively affected the electron transfer from the surface of the electrode to the electrolyte, thus increasing the corresponding photocurrent values. Moreover, the Cu2O/CuO system showed a higher charge carrier and photocurrent density than the Cu2O-based one. The mixed Cu2O/CuO films allowed the most significant hydrogen production, especially in slightly acid reaction conditions.

A Review of Photoelectrocatalytic Reactors for Water and Wastewater Treatment

The photoexcitation of suitable semiconducting materials in aqueous environments can lead to the production of reactive oxygen species (ROS). ROS can inactivate microorganisms and degrade a range of chemical compounds. In the case of heterogeneous photocatalysis, semiconducting materials may suffer from fast recombination of electron–hole pairs and require post-treatment to separate the photocatalyst when a suspension system is used. To reduce recombination and improve the rate of degradation, an externally applied electrical bias can be used where the semiconducting material is immobilised onto an electrically conducive support and connected to a counter electrode. These electrochemically assisted photocatalytic systems have been termed “photoelectrocatalytic” (PEC). This review will explain the fundamental mechanism of PECs, photoelectrodes, the different types of PEC reactors reported in the literature, the (photo)electrodes used, the contaminants degraded, the key findings and prospects in the research area.

Simulating the performance of a highly efficient CuBi2O4-based thin-film solar cell

In this study, copper bismuth oxide (CuBi2O4) absorber-based thin film heterojunction solar cell structure consisting of Al/FTO/CdS/CuBi2O4/Ni has been proposed. The proposed solar cell device structure has been modeled and analyzed by using the solar cell capacitance simulator in one dimension (SCAPS-1D) software program. The performance of the proposed photovoltaic device is evaluated numerically by varying thickness, doping concentrations, defect density, operating temperature, back metal contact work function, series and shunt resistances. The current density–voltage behaviors at dark and under illumination are investigated. To realize the high efficiency CuBi2O4-based solar cell, the thickness, acceptor and donor densities, defect densities of different layers have been optimized. The present work reveals that the power conversion efficiency can be enhanced by increasing the absorber layer thickness. The efficiency of 26.0% with open-circuit voltage of 0.97 V, short-circuit current density of 31.61 mA/cm2, and fill-factor of 84.58% is achieved for the proposed solar cell at the optimum 2.0-μm-thick CuBi2O4 absorber layer. It is suggested that the p-type CuBi2O4 material proposed in the present study can be employed as a promising absorber layer for applications in the low cost and high efficiency thin-film solar cells.

Photoelectrochemical Behavior and Computational Insights for Pristine and Doped NdFeO3 Thin-Film Photocathodes

Among the different strategies that are being developed to solve the current energy challenge, harvesting energy directly from sunlight through a tandem photoelectrochemical cell (water splitting) is most attractive. Its implementation requires the development of stable and efficient photocathodes, NdFeO3 being a suitable candidate among ternary oxides. In this study, transparent NdFeO3 thin-film photocathodes have been successfully prepared by a citric acid-based sol-gel procedure, followed by thermal treatment in air at 640 °C. These electrodes show photocurrents for both the hydrogen evolution and oxygen reduction reactions. Doping with Mg2+ and Zn2+ has been observed to significantly enhance the photoelectrocatalytic performance of NdFeO3 toward oxygen reduction. Magnesium is slightly more efficient as a dopant than Zn, leading to a multiplication of the photocurrent by a factor of 4-5 for a doping level of 5 at % (with respect to iron atoms). This same trend is observed for hydrogen evolution. The beneficial effect of doping is primarily attributed to an increase in the density and a change in the nature of the majority charge carriers. DFT calculations help to rationalize the behavior of NdFeO3 by pointing to the importance of nanostructuring and doping. All in all, NdFeO3 has the potential to be used as a photocathode in photoelectrochemical applications, although efforts should be directed to limit surface recombination.

A review on self-sustainable microbial electrolysis cells for electro-biohydrogen production via coupling with carbon-neutral renewable energy technologies

Microbial electrolysis cell (MEC) technology is a promising bioelectrochemical hydrogen production technology that utilizes anodic bio-catalytic oxidation and cathodic reduction processes. MECs require a lower external energy input than water electrolysis; however, as they also require the application of external power sources, this inevitably renders MEC systems a less sustainable option. This issue is the main obstacle hindering the practical application of MECs. Therefore, this review aims to introduce a self-sustainable MEC technology by combining conventional MECs with advanced carbon-neutral technologies, such as solar-, microbial-, osmotic-, and thermoelectric-powers (and their combinations). Moreover, new approaches to overcome the thermodynamic barriers and attain self-sustaining MECs are discussed in detail, thereby providing a working principle, current challenges, and future perspective in the field. This review provides comprehensive insights into reliable hydrogen production as well as the latest trends towards self-sustainable MECs for practical application.

Nanostructured Anodic Copper Oxides as Catalysts in Electrochemical and Photoelectrochemical Reactions

Recently, nanostructured copper oxides formed via anodizing have been intensively researched due to their potential catalytic applications in emerging issues. The anodic Cu2O and CuO nanowires or nanoneedles are attractive photo- and electrocatalysts since they show wide array of desired electronic and morphological features, such as highly-developed surface area. In CO2 electrochemical reduction reaction (CO2RR) copper and copper-based nanostructures indicate unique adsorption properties to crucial reaction intermediates. Furthermore, anodized copper-based materials enable formation of C2+ hydrocarbons and alcohols with enhanced selectivity. Moreover, anodic copper oxides provide outstanding turnover frequencies in electrochemical methanol oxidation at lowered overpotentials. Therefore, they can be considered as precious metals electrodes substituents in direct methanol fuel cells. Additionally, due to the presence of Cu(III)/Cu(II) redox couple, these materials find application as electrodes for non-enzymatic glucose sensors. In photoelectrochemistry, Cu2O-CuO heterostructures of anodic copper oxides with highly-developed surface area are attractive for water splitting. All the above-mentioned aspects of anodic copper oxides derived catalysts with state-of-the-art background have been reviewed within this paper.

Template Engineering of CuBi2 O4 Single-Crystal Thin Film Photocathodes

To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copper-based oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi₂ O₄ (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on single-crystal SrTiO₃ (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate -0.4 and -0.7 mA cm^-2 at even 0 V_RHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H₂ O₂ as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.

Direct Identification of Antisite Cation Intermixing and Correlation with Electronic Conduction in CuBi2O4 for Photocathodes

Cu-based p-type semiconducting oxides have been sought for water-reduction photocathodes to enhance the energy-conversion efficiency in photoelectrochemical cells. CuBi₂O₄ has recently attracted notable attention as a new family of p-type oxides, based on its adequate band gap. Although the identification of a major defect structure should be the first step toward understanding the electronic conduction behavior, no direct experimental analysis has been carried out yet. Using atomic-scale scanning transmission electron microscopy together with chemical probing, we identify a substantial amount of Bi_Cu-Cu_Bi antisite intermixing as a major point-defect type. Our density functional theory calculations also show that antisite Bi_Cu can seriously hinder the hole-polaron hopping between Cu, in agreement with lower conductivity and a larger thermal activation barrier under a higher degree of intermixing. These findings highlight the value of the direct identification of point defects for a better understanding of electronic properties in complex oxides.

Noble Metal Modification of CdS-Covered CuInS2 Electrodes for Improved Photoelectrochemical Activity and Stability

In this paper, efficient and stable photoelectrochemical (PEC) hydrogen (H2) evolution using copper indium sulfide (CuInS2) thin film electrodes was studied. Modification with a cadmium sulfide (CdS) layer led to improved charge separation at the interface between CuInS2 and CdS; however, the photocorrosive nature of CdS induced poor stability of the photocathode. Further surface coating with an electrodeposited Pt layer over the CdS-covered CuInS2 photocathode prevented the CdS layer from making contact with the electrolyte solution, and enabled efficient PEC H2 evolution without appreciable degradation. This indicates that the Pt layer functioned not only as a reaction site for H2 evolution, but also as a protection layer. In addition, it was found that surface protection using a noble metal layer was also effective for stable PEC carbon dioxide (CO2) reduction when appropriate noble metal cocatalysts were selected. When Au or Ag was used, carbon monoxide was obtained as a product of PEC CO2 reduction.

Generalized Synthesis to Produce Transparent Thin Films of Ternary Metal Oxide Photoelectrodes

Developing facile approaches to prepare non-light-scattering ternary oxide thin film photoelectrodes is an important goal for solar water splitting tandem cells. Herein, a novel synthesis route is reported that employs ethylenediaminetetraacetic acid (EDTA) to enable compatible water solubility of diverse metal cations, which affords transparent films by solution processing. By using BiVO₄ as a model material, a remarkable improvement in transparency is demonstrated, quantified by the direct transmittance at 600 nm of >80 % versus the <10 % observed with state-of-the-art electrodeposited thin films while maintaining reasonable solar-driven oxidation photocurrents (1.75 mA cm^-2 in the presence of a sulfite hole scavenger). Furthermore, it is demonstrated that the synthesis technique can be applied in a general fashion towards the synthesis of diverse n- and p-type metal oxide materials, such as ZnFe₂ O₄ and CuFeO₂ .

Towards sustainable and efficient p-type metal oxide semiconductor materials in dye-sensitised photocathodes for solar energy conversion

In order to meet the ever-growing global energy demand for affordable and clean energy, it is essential to provide this energy by renewable resources and consider the eco-efficiency of the production and abundance of the utilised materials. While this is seldom discussed in the case of technologies still in the research stage, addressing the issue of sustainability is key to push research in the right direction. Here we provide an overview of the current p-type metal oxide semiconductor materials in dye-sensitised photocathodes, considering element abundance, synthetic methods and large scale fabrication as well as the underlying physical properties that are necessary for efficient solar harvesting devices.

Curtailing the Overpotential of Li-CO2 Batteries with Shape-Controlled Cu2 O as Cathode: Effect of Illuminating the Cathode

Li-air batteries are limited to lab-scale research owing to the uninterrupted formation of discharge products. In the case of Li-CO₂ batteries, the increase in overpotential caused by Li₂ CO₃ formation results in cell death. In this study, Cu₂ O crystals having three different types of shapes (i.e., cubic, octahedral, and rhombic) were synthesized to compare their catalytic activity toward CO₂ reactions. The full-cycle and long-term stability test revealed that rhombohedral Cu₂ O facilitates Li₂ CO₃ decomposition more efficiently than that of cubic and octahedral Cu₂ O. The cycle was extended to investigate the photocatalytic activity of the rhombic Cu₂ O by illuminating the cell. The repeated cycles to 1 h showed a maximum overpotential of 1.5 V, which is 0.5 V lower than that of the cell without illumination. A postmortem analysis of the cell after dividing the cycles into segments demonstrated interesting results concerning the role of light and Cu₂ O during the cell cycle.

Assessment of a W:BiVO4-CuBi2O4Tandem Photoelectrochemical Cell for Overall Solar Water Splitting

We assess a tandem photoelectrochemical cell consisting of a W:BiVO₄ photoanode top absorber and a CuBi₂O₄ photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO₄ photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi₂O₄ photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuO_x is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi₂O₄ rather than proton reduction. Protecting the CuBi₂O₄ photocathode with a CdS/TiO₂ heterojunction and adding RuO_x as a cocatalyst prevents photocorrosion and allows for photoelectrochemical production of hydrogen at potentials ≤0.3 V vs RHE. A tandem photoelectrochemical cell composed of a W:BiVO₄/CoPi photoanode and a CuBi₂O₄/CdS/TiO₂/RuO_x photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO₄ and conduction band of CuBi₂O₄ are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes, that is, the relatively low quasi-Fermi level splitting within BiVO₄ and CuBi₂O₄. This work is the first experimental demonstration of hydrogen production from a BiVO₄-CuBi₂O₄-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi₂O₄ photocathodes.

Long-term stabilized high-density CuBi2O4/NiO heterostructure thin film photocathode grown by pulsed laser deposition

Harvesting sustainable hydrogen through water-splitting requires a durable photoelectrode to achieve high efficiency and long lifetime. Dense, uniform CuBi₂O₄/NiO thin film photocathodes grown by pulsed laser deposition achieved photocurrent density over 1.5 mA cm^-2 at 0.4 V_RHE and long-term chronoamperometric stability for over 8 hours.

Strategies for enhancing the photocurrent, photovoltage, and stability of photoelectrodes for photoelectrochemical water splitting

In this review, we survey recent strategies for photoelectrode optimization and advanced characterization methods towards efficient water splitting cells via feedback from these characterization methods.