Enhanced photoelectrochemical water oxidation on WO3 nanoflake films by coupling with amorphous TiO2

Recent Developments in Nickel-Based Layered Double Hydroxides for Photo(-/)electrocatalytic Water Oxidation

Layered double hydroxides (LDHs), especially those containing nickel (Ni), are increasingly recognized for their potential in photo(-/)electrocatalytic water oxidation due to the abundant availability of Ni, their corrosion resistance, and their minimal toxicity. This review provides a comprehensive examination of Ni-based LDHs in electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) water oxidation processes. The review delves into the operational principles, highlighting similarities and distinctions as well as the benefits and limitations associated with each method of water oxidation. It includes a detailed discussion on the synthesis of monolayer, ultrathin, and bulk Ni-based LDHs, focusing on the merits and drawbacks inherent to each synthesis approach. Regarding the EC oxygen evolution reaction (OER), strategies to improve catalytic performance and insights into the structural evolution of Ni-based LDHs during the electrocatalytic process are summarized. Furthermore, the review extensively covers the advancements in Ni-based LDHs for PEC OER, including an analysis of semiconductors paired with Ni-based LDHs to form photoanodes, with a focus on their enhanced activity, stability, and underlying mechanisms facilitated by LDHs. The review concludes by addressing the challenges and prospects in the development of innovative Ni-based LDH catalysts for practical applications. The comprehensive insights provided in this paper will not only stimulate further research but also engage the scientific community, thus driving the field of photo(-/)electrocatalytic water oxidation forward.

Heterostructured WO3-TiVO4 thin-film photocatalyst for efficient photoelectrochemical water splitting

Photoelectrochemical water splitting via solar irradiation has garnered significant interest due to its potential in large-scale renewable hydrogen production. Heterostructure materials have emerged as an effective strategy, demonstrating enhanced performance in photoelectrochemical water-splitting applications compared to individual photocatalysts. In this study, to augment the performance of sprayed TiVO4 thin films, a hydrothermally prepared WO3 underlayer was integrated beneath the spray pyrolised TiVO4 film. The consequent heterostructure demonstrated notable enhancements in optical, structural, microstructural attributes, and photocurrent properties. This improvement is attributed to the strategic deposition of WO3 underlayer, forming a heterostructure composite electrode. This led to a marked increase in photocurrent density for the WO3/TiVO4 photoanode, reaching a peak of 740 μA/cm2 at an applied potential of 1.23 V vs RHE, about nine-fold that of standalone TiVO4. Electrochemical impedance spectroscopy revealed a reduced semicircle for the heterostructure, indicating improved charge transfer compared to bare TiVO4. The heterostructure photoelectrode exhibited enhanced charge carrier conductivity at the interface and sustained stability over 3 h. The distinct attributes of heterostructure photoelectrode present significant opportunities for devising highly efficient sunlight-driven water-splitting systems.

Bifunctional PDDA-stabilized β-Fe2O3 nanoclusters for improved photoelectrocatalytic and magnetic field enhanced photocatalytic applications

Bifunctional β-Fe2O3@PDDA nanoclusters applied for the efficient photoelectrocatalytic oxygen evolution reaction and magnetic field enhanced photocatalytic degradation of pollutants.

Boosting light harvesting and charge separation of WO3 via coupling with Cu2O/CuO towards highly efficient tandem photoanodes

Photoanodes based on semiconductor WO₃ have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron–hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO, which is prepared first by hydrothermally growing a layer of WO₃ on the ITO surface, then by electrodepositing an additional layer of Cu₂O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm⁻², which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO₃ and Cu₂O/CuO, (3) better electrocatalytic activity of formed Cu_xO than pure Cu₂O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.

A highly efficient photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO produces a photocurrent about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively.

Crystal Growth and Design of Disk/Filament ZnO-Decorated 1D TiO2 Composite Ceramics for Photoexcited Device Applications

Disk- and filament-like ZnO crystals were decorated on one-dimensional TiO2 nanostructures (TiO2-ZnO) through various integrated physical and chemical synthesis methods. The morphology of the ZnO crystals on TiO2 varied with the chemical synthesis method used. ZnO nanodisks decorated with TiO2 nanorods (TiO2-ZnO-C) were synthesized using the chemical bath deposition method, and ZnO filament-like crystals decorated with TiO2 nanorods (TiO2-ZnO-H) were synthesized through the hydrothermal method. Compared with the pristine TiO2 nanorods, the as-synthesized TiO2-ZnO composites exhibited enhanced photophysiochemical performance. Furthermore, because of their fast electron transportation and abundant surface active sites, the ZnO nanodisks in the TiO2-ZnO-C composite exhibited a higher photoactivity than those in the TiO2-ZnO-H composite. The morphology and crystal quality of the ZnO decoration layer were manipulated using different synthesis methods to realize disk- or filament-like ZnO-decorated TiO2 composites with various photoactive performance levels.

In Situ Induced Crystalline-Amorphous Heterophase Junction by K+ to Improve Photoelectrochemical Water Oxidation of BiVO4

Solar water splitting is one of the most efficient technologies to produce H₂, which is a clean and renewable energy carrier. Photoanodes for water oxidation play the determining roles in solar water splitting, while its photoelectrochemical (PEC) performance is severely limited by the hole injection efficiency at the interface of semiconductor/electrolyte. To address this problem, in this research, by employing BiVO₄ as the model semiconductor for photoanodes, we develop a novel, facile, and efficient method, which simply applies K cations in the preparation process of BiVO₄ photoanodes, to in situ induce a crystalline-amorphous heterophase junction by the formation of an amorphous BiVO₄ layer (a-BiVO₄) on the surface of the crystalline BiVO₄ (c-BiVO₄) film for PEC water oxidation. The K cation is the key to stimulate the formation of the heterophase, but not incorporated in the final photoelectrodes. Without sacrificing the light absorption, the in situ formed a-BiVO₄ layer accelerates the kinetics of the hole transfer at the photoanode/electrolyte interface, leading to the significantly increased efficiency of the surface hole injection to water molecules. Consequently, the BiVO₄ photoanode with the crystalline-amorphous heterophase junction (a-BiVO₄/c-BiVO₄) exhibits almost twice the photocurrent density at 1.23 V (vs reversible hydrogen electrode) for water oxidation than the bare c-BiVO₄ ones. Such advantages from the crystalline-amorphous heterophase junction are still effective even when the a-BiVO₄/c-BiVO₄ is coated by the cocatalyst of FeOOH, reflecting its broad applications in PEC devices. We believe this study can supply an efficient and simple protocol to enhance the PEC water oxidation performance of photoanodes, and provide a new strategy for the potential large-scale application of the solar energy-conversion related devices.

Photoactive Tungsten-Oxide Nanomaterials for Water-Splitting

This review focuses on tungsten oxide (WO3) and its nanocomposites as photoactive nanomaterials for photoelectrochemical cell (PEC) applications since it possesses exceptional properties such as photostability, high electron mobility (~12 cm2 V-1 s-1) and a long hole-diffusion length (~150 nm). Although WO3 has demonstrated oxygen-evolution capability in PEC, further increase of its PEC efficiency is limited by high recombination rate of photogenerated electron/hole carriers and slow charge transfer at the liquid-solid interface. To further increase the PEC efficiency of the WO3 photocatalyst, designing WO3 nanocomposites via surface-interface engineering and doping would be a great strategy to enhance the PEC performance via improving charge separation. This review starts with the basic principle of water-splitting and physical chemistry properties of WO3, that extends to various strategies to produce binary/ternary nanocomposites for PEC, particulate photocatalysts, Z-schemes and tandem-cell applications. The effect of PEC crystalline structure and nanomorphologies on efficiency are included. For both binary and ternary WO3 nanocomposite systems, the PEC performance under different conditions-including synthesis approaches, various electrolytes, morphologies and applied bias-are summarized. At the end of the review, a conclusion and outlook section concluded the WO3 photocatalyst-based system with an overview of WO3 and their nanocomposites for photocatalytic applications and provided the readers with potential research directions.

WO3-x horizontally-grown on TiO2(B) nanosheets for enhanced photo- and electro-chemical activity

WO3-x was deposited on TiO2(B) nanosheets prepared using TiCl4 to form layered heterostructures via a two-step solvothermal synthesis, in which the horizontal growth of WO3-x on TiO2(B) nanosheets was carried out using WCl6 and ascorbic acid as a reducer. Optimized preparation conditions allowed WO3-x/TiO2(B) layered heterostructures to be formed. The photo- and electro-chemical properties of layered heterostructures depended strongly on the amount of WO3-x. The WO3-x/TiO2(B) heterostructures demonstrated perfect catalytic performance in full solar-spectrum light and a fast degradation effect for dye and organic colorless pollutant. All target chemicals were degraded within 10 min using WO3-x/TiO2(B) samples as a photo-catalyst in the full solar-spectrum. The photo-assisted production kinetic of Cr(VI) ions were tested. The results indicate that the reproduction rate of Cr(VI) ions using WO3-x/TiO2(B) sample is three times higher than the initial TiO2 nanosheets. The result of the photo current and Mott-Shottky curve indicates that enhanced catalysis activity is ascribed to the surface of the metastable TiO2(B) with Ti3+ defects and oxygen vacancies as active sites for photocatalytic reaction. In addition, the loading of WO3-x greatly broadened the light absorption range of TiO2(B), meaning that the product responded in the full solar spectrum.

Nanostructured core-shell metal borides-oxides as highly efficient electrocatalysts for photoelectrochemical water oxidation

Oxygen evolution reaction (OER) catalysts are critical components of photoanodes for photoelectrochemical (PEC) water oxidation. Herein, nanostructured metal boride MB (M = Co, Fe) electrocatalysts, which have been synthesized by a Sn/SnCl₂ redox assisted solid-state method, were integrated with WO₃ thin films to build heterojunction photoanodes. As-obtained MB modified WO₃ photoanodes exhibit enhanced charge carrier transport, amended separation of photogenerated electrons and holes, prolonged hole lifetime and increased charge carrier density. Surface modification of CoB and FeB significantly enhances the photocurrent density of WO₃ photoanodes from 0.53 to 0.83 and 0.85 mA cm^-2, respectively, in transient chronoamperometry (CA) at 1.23 V vs. RHE (V_RHE) under interrupted illumination in 0.1 M Na₂SO₄ electrolyte (pH 7), corresponding to an increase of 1.6 relative to pristine WO₃. In contrast, the pristine MB thin film electrodes do not produce noticeable photocurrent during water oxidation. The metal boride catalysts transform in situ to a core-shell structure with a metal boride core and a metal oxide (MO, M = Co, Fe) surface layer. When coupled to WO₃ thin films, the CoB@CoO_x nanostructures exhibit a higher catalytic enhancement than corresponding pure cobalt borate (Co-B_i) and cobalt hydroxide (Co(OH)_x) electrocatalysts. Our results emphasize the role of the semiconductor-electrocatalyst interface for photoelectrodes and their high dependency on materials combination.

Designing of WO3@Co3O4 Heterostructures to Enhance Photoelectrochemical Performances

Heterostructured photocatalysts are superior to single photocatalysts because they offer better charge separation and broaden light harnessing abilities. Although WO₃ is considered an oxygen-evolving photocatalyst with decent stability and proper band gap, its lower photocatalytic efficiency is ascribed to high charge recombination. In this research, a WO₃@Co₃O₄ heterostructure reduced the recombination of photocatalytic charges and extended light absorption abilities, resulting in improved photocatalytic activity. The presence of Co₃O₄ nanoparticles improved light absorption and charge transfer of tungsten oxide films for photoelectrochemical reactions. For photoelectrochemical water oxidation, WO₃@Co₃O₄ nanostructures generated a photocurrent 20 times higher than that of pure WO₃. Both electrodeposition and sol gel techniques were utilized to synthesize the WO₃@Co₃O₄ photoelectrode. Scanning electron microscopy and X-ray diffraction were used to characterize the formation of the above photocatalyst. A photocurrent study was done to investigate the charge separation mechanism to explain the enhanced photocatalytic activity.

Photoelectrocatalytic Hydrogen Production Using a TiO2/WO3 Bilayer Photocatalyst in the Presence of Ethanol as a Fuel

Photoelectrocatalytic hydrogen production was studied by using a photoelectrochemical cell where the photoanode was made by depositing on FTO electrodes either a nanoparticulate WO3 film alone or a bilayer film made of nanoparticulate WO3 at the bottom covered with a nanoparticulate TiO2 film on the top. Both the electric current and the hydrogen produced by the photoelectrocatalysis cell substantially increased by adding the top titania layer. The presence of this layer did not affect the current-voltage characteristics of the cell (besides the increase of the current density). This was an indication that the flow of electrons in the combined semiconductor photoanode was through the WO3 layer. The increase of the current was mainly attributed to the passivation of the surface recombination sites on WO3 contributing to the limitation of charge recombination mechanisms. In addition, the top titania layer may have contributed to photon absorption by back scattering of light and thus by enhancement of light absorption by WO3. Relatively high charge densities were recorded, owing both to the improvement of the photoanode by the combined photocatalyst and to the presence of ethanol as the sacrificial agent (fuel), which affected the recorded current by “current doubling” phenomena. Hydrogen was produced under electric bias using a simple cathode electrode made of carbon paper carrying carbon black as the electrocatalyst. This electrode gave a Faradaic efficiency of 58% for hydrogen production.

Recent electrochemical methods in electrochemical degradation of halogenated organics: a review

Halogenated organics are widely used in modern industry, agriculture, and medicine, and their large-scale emissions have led to soil and water pollution. Electrochemical methods are attractive and promising techniques for wastewater treatment and have been developed for degradation of halogenated organic pollutants under mild conditions. Electrochemical techniques are classified according to main reaction pathways: (i) electrochemical reduction, in which cleavage of C-X (X = F, Cl, Br, I) bonds to release halide ions and produce non-halogenated and non-toxic organics and (ii) electrochemical oxidation, in which halogenated organics are degraded by electrogenerated oxidants. The electrode material is crucial to the degradation efficiency of an electrochemical process. Much research has therefore been devoted to developing appropriate electrode materials for practical applications. This paper reviews recent developments in electrode materials for electrochemical degradation of halogenated organics. And at the end of this paper, the characteristics of new combination methods, such as photocatalysis, nanofiltration, and the use of biochemical method, are discussed.

Amorphous TiO2 nanostructures: synthesis, fundamental properties and photocatalytic applications

In this review, we mainly highlight the advances made in the development of amorphous TiO₂ nanostructures for photocatalysts. Some perspectives on the challenges and new direction are also discussed.

Fabrication of a three-dimensional visible-light-driven Ag–AgBr/TiO2/graphene aerogel composite for enhanced photocatalytic destruction of organic dyes and bacteria

The configuration of Ag–AgBr/TiO₂/GA integrated critical components of solar absorption and charge separation for environmental remediation and antibacterial treatment.

Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes with Multihigh-Index Crystal Facets

The crystal facet of the BiVO₄ photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO₄ polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi₂O₃ template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO₄ film with a {121} monohigh-index facet, the BiVO₄ film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm² was achieved on the multihigh-index facet BiVO₄ photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na₂SO₄, which is about 200% improved over the normal BiVO₄ photoanode (0.61 mA/cm² at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO₄ photoanode (0.22 V vs RHE) relative to the normal BiVO₄ photoanode (0.52 V vs RHE) in 0.1 M Na₂SO₄. Although the UV-vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO₄ film were boosted obviously. Theory calculations reveal that the adsorption of H₂O molecules on BiVO₄{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO₄ is changed to the step of two protons reacting with •O to form •OOH species (•O + H₂O_(l) + 2H⁺ + 2e^- → •OOH + 3H⁺ + 3e^-), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H₂O to •OH (2H₂O_(l) + • → •OH + H₂O_(l) + H⁺ + e^-). In addition, the overpotential of water oxidation limiting step on BiVO₄{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.