WO3/BiVO4: impact of charge separation at the timescale of water oxidation

Cocatalyst activity mapping for photocatalytic materials revealed by the pattern-illumination time-resolved phase microscopy

Photocatalytic water-splitting represents a promising avenue for clean hydrogen production, necessitating an in-depth understanding of the photocatalytic reaction mechanism. The majority of the photocatalytic materials need cocatalysts to enhance the photo-oxidation or reduction reactions. However, the working mechanism, such as collecting charge carriers or reducing the reaction barrier, is not clear because they disperse inhomogeneously on a surface, and it is difficult to follow the local charge carrier behavior. This study employs the pattern-illumination time-resolved phase microscopy (PI-PM) method to unravel the spatial charge carrier behavior in photocatalytic systems, utilizing time-resolved microscopic image (refractive index change) sequences and their clustering analyses. This approach is robust for studying the change in local charge carrier behavior. We studied two major cocatalyst effects on photocatalysts: TiO2 with/without Pt and hematite with/without CoPi. The PI-PM method, supported by charge type clustering and the effects of scavengers, allowed for the analysis of local activity influenced by cocatalysts. This approach revealed that the introduction of cocatalysts alters the local distribution of charge carrier behavior and significantly impacts their decay rates. In TiO2 systems, the presence of Pt cocatalysts led to a local electron site on the micron scale, extending the lifetime to a few tens of microseconds from a few microseconds. Similarly, in hematite films with CoPi, we observed a notable accumulation of holes at cocatalyst sites, emphasizing the role of cocatalysts in enhancing photocatalytic efficiency. The study's findings highlight the complexity of charge carrier dynamics in photocatalytic processes and the significant influence of cocatalysts.

Pattern-illumination time-resolved phase microscopy and its applications for photocatalytic and photovoltaic materials

Pattern-illumination time-resolved phase microscopy (PI-PM) is a technique used to study the microscopic charge carrier dynamics in photocatalytic and photovoltaic materials. The method involves illuminating a sample with a pump light pattern, which generates charge carriers and they decay subsequently due to trapping, recombination, and transfer processes. The distribution of photo-excited charge carriers is observed through refractive index changes using phase-contrast imaging. In the PI-PM method, the sensitivity of the refractive index change is enhanced by adjusting the focus position, the method takes advantage of photo-excited charge carriers to observe non-radiative processes, such as charge diffusion, trapping in defect/surface states, and interfacial charge transfer of photocatalytic and photovoltaic reactions. The quality of the image sequence is recovered using various informatics calculations. Categorizing and mapping different types of charge carriers based on their response profiles using clustering analysis provides spatial information on charge carrier types and the identification of local sites for efficient and inefficient photo-induced reactions, providing valuable information for the design and optimization of photocatalytic materials such as the cocatalyst effect.

Dual Shield: Bifurcated Coating Analysis of Multilayered WO3/BiVO4/TiO2/NiOOH Photoanodes for Sustainable Solar-to-Hydrogen Generation from Challenging Waters

The heterostructure WO3/BiVO4-based photoanodes have garnered significant interest for photoelectrochemical (PEC) solar-driven water splitting to produce hydrogen. However, challenges such as inadequate charge separation and photocorrosion significantly hinder their performance, limiting overall solar-to-hydrogen conversion efficiency. The incorporation of cocatalysts has shown promise in improving charge separation at the photoanode, yet mitigating photocorrosion remains a formidable challenge. Amorphous metal oxide-based passivation layers offer a potential solution to safeguard semiconductor catalysts. We examine the structural, surface morphological, and optical properties of two-step-integrated sputter and spray-coated TiO2 thin films and their integration onto WO3/BiVO4, both with and without NiOOH cocatalyst deposition. The J-V experiments reveal that the NiOOH cocatalyst enhances the photocurrent density of the WO3/BiVO4 photoanode in water splitting reactions from 2.81 to 3.87 mA/cm2. However, during prolonged operation, the photocurrent density degrades by 52%. In contrast, integrated sputter and spray-coated TiO2 passivation layer-coated WO3/BiVO4/NiOOH samples demonstrate a ∼88% enhancement in photocurrent density (5.3 mA/cm2) with minimal degradation, emphasizing the importance of a strategic coating protocol to sustain photocurrent generation. We further explore the feasibility of using natural mine wastewater as an electrolyte feedstock in PEC generation. Two-compartment PEC cells, utilizing both fresh water and metal mine wastewater feedstocks exhibit 66.6 and 74.2 μmol/h cm2 hydrogen generation, respectively. Intriguingly, the recovery of zinc (Zn2+) heavy metals on the cathode surface in the mine wastewater electrolyte is confirmed through surface morphology and elemental analysis. This work underscores the significance of passivation layer and cocatalyst coating methodologies in a sequential order to enhance charge separation and protect the photoanode from photocorrosion, contributing to sustainable hydrogen generation. Additionally, it suggests the potential of utilizing wastewater in electrolyzers as an alternative to freshwater resources.

Identification of the Contributing Factors to the Photoelectric Conversion Efficiency for Hematite Photoanodes by Using Machine Learning

Hematite has gained significant attention in the field of photocatalysis as one of the most promising materials for the photoanode of photoelectrochemical (PEC) water splitting due to visible light absorption and the abundance of availability. However, its performance improvement process suffers from a serious bottleneck due to "sample variation" and "inactivity". However, the physical origin of them has not yet been elucidated. To address these issues, we have developed a machine learning (ML) strategy using a combination of various analytical data of hematite photoanodes to discern "active/inactive" and identify the dominant factors. For the demonstration purpose of the ML strategy, we picked up one of the dominant factors, the interfacial resistivity between hematite and FTO, which has not generally been explored as a first candidate in the improvement of photocatalytic materials. The operational parameters for the sample preparation were optimized to modify the selected physical property. Along with the improvement of the selected resistivity, we found that the other dominant descriptors related to the properties of bulk hematite and the surface facet were also modified and help improve the PEC performance.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

CuWO4 is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO4 electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO4 thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO4 is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO4 electrode delivers a photocurrent of 0.37 mA cm-2 at 1.23 VSHE, with a 7% absorbed photon to current efficiency over the CuWO4 absorption spectrum.

Photochemically Etching BiVO4 to Construct Asymmetric Heterojunction of BiVO4 /BiOx Showing Efficient Photoelectrochemical Water Splitting

BiVO₄ as a promising semiconductor candidate of the photoanode for solar driven water oxidation always suffers from poor charge carrier transport property and photo-induced self-corrosion. Herein, by intentionally taking advantage of the photo-induced self-corrosion process, a controllable photochemical etching method is developed to rationally construct a photoanode of BiVO₄ /BiO_x asymmetric heterojunction from faceted BiVO₄ crystal arrays. Compared with the BiVO₄ photoanode, the resulting BiVO₄ /BiO_x photoanode gains over three times enhancement in short-circuit photocurrent density (≈3.2 mA cm^-2 ) and ≈75 mV negative shift of photocurrent onset potential. This is due to the formation of the strong interacted homologous heterojunction, which promotes photo-carrier separation and enlarges photovoltage across the interface. Remarkably, the photocurrent density can remain at ≈2.0 mA cm^-2 even after 12 h consecutive operation, while only ≈0.1 mA cm^-2 is left for the control photoanode of BiVO₄ . Moreover, the Faraday efficiency for water splitting is determined to be nearly 100% for the BiVO₄ /BiO_x photoanode. The controllable photochemical etching process may shed light on the construction of homologous heterojunction on other photoelectrode materials that have similar properties to BiVO₄ .

Elucidation of a Photothermal Energy Conversion Mechanism in Hydrogenated Molybdenum Suboxide: Interplay of Trapped Charges and Their Dielectric Interactions

Hydrogenated molybdenum suboxide (H_xMoO_3-y) is a promising photothermal energy conversion (PEC) material. However, its charge carrier dynamics and underlying mechanisms remain unclear. Utilizing flash-photolysis time-resolved microwave conductivity, we investigated charge carrier-dielectric interactions in the Pt/H_xMoO_3-y composite. The charge recombination of H₂-reduced Pt/H_xMoO_3-y was 2-3 orders of magnitude faster than that of Pt/MoO₃, indicating efficient PEC. A complex photoconductivity study revealed that Pt/H_xMoO_3-y has two types of trapping mechanisms, Drude-Zener (DZ) and negative permittivity effect (NPE) modes, depending on the reduction temperature. Pt/H_xMoO_3-y reduced at 100 °C exhibited a dominant NPE owing to the electrical interaction of trapped charges with the surrounding ions and/or OH base. This polaronic trapped state retarded the PEC process. We found Pt/H_xMoO_3-y reduced at 200 °C to be optimal owing to the balanced suppression of the NPE and charge diffusion. This is the first report revealing the charge dynamics in hydrogenated metal oxides and their impacts on PEC processes.

Photoelectrochemical Water Splitting with ITO/WO3/BiVO4/CoPi Multishell Nanotubes Enabled by a Vacuum and Plasma Soft-Template Synthesis

A common approach for the photoelectrochemical (PEC) splitting of water relies on the application of WO3 porous electrodes sensitized with BiVO4 acting as a visible photoanode semiconductor. In this work, we propose a new architecture of photoelectrodes consisting of supported multishell nanotubes (NTs) fabricated by a soft-template approach. These NTs are formed by a concentric layered structure of indium tin oxide (ITO), WO3, and BiVO4, together with a final thin layer of cobalt phosphate (CoPi) co-catalyst. The photoelectrode manufacturing procedure is easily implementable at a large scale and successively combines the thermal evaporation of single crystalline organic nanowires (ONWs), the magnetron sputtering deposition of ITO and WO3, and the solution dripping and electrochemical deposition of, respectively, BiVO4 and CoPi, plus the annealing in air under mild conditions. The obtained NT electrodes depict a large electrochemically active surface and outperform the efficiency of equivalent planar-layered electrodes by more than one order of magnitude. A thorough electrochemical analysis of the electrodes illuminated with blue and solar lights demonstrates that the characteristics of the WO3/BiVO4 Schottky barrier heterojunction control the NT electrode efficiency, which depended on the BiVO4 outer layer thickness and the incorporation of the CoPi electrocatalyst. These results support the high potential of the proposed soft-template methodology for the large-area fabrication of highly efficient multishell ITO/WO3/BiVO4/CoPi NT electrodes for the PEC splitting of water.

Enhanced Charge Carrier Separation in WO3/BiVO4 Photoanodes Achieved via Light Absorption in the BiVO4 Layer

Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO₃/BiVO₄ heterojunction photoanodes perform much better than the separate oxide components, though internal charge recombination undermines their PEC performance when both oxides absorb light. Here we exploit the BiVO₄ layer to sensitize WO₃ to visible light and shield it from direct photoexcitation to overcome this efficiency loss. PEC experiments and ultrafast transient absorption spectroscopy performed by frontside (through BiVO₄) or backside (through WO₃) irradiating photoanodes with different BiVO₄ layer thickness demonstrate that irradiation through BiVO₄ is beneficial for charge separation. Optimized electrodes irradiated through BiVO₄ show 40% higher photocurrent density compared to backside irradiation.

Laser-Reduced BiVO4 for Enhanced Photoelectrochemical Water Splitting

The present study proposes a laser irradiation method to superficially reduce BiVO4 photoelectrodes and boost their water oxidation reaction performance. The origin of this enhanced performance toward oxygen evolution reaction (OER) was studied using a combination of a suite of structural, chemical, and mechanistic advanced characterization techniques including X-ray photoelectron (XPS), X-ray absorption spectroscopy (XAS), electrochemical impedance spectroscopy (EIS), and transient absorption spectroscopy (TAS), among others. We found that the reduction of the material is localized at the surface of the sample and that this effect creates effective n-type doping and a shift to more favorable energy band positions toward water oxidation. This thermodynamic effect, together with the change in sample morphology to larger and denser domains, results in an extended lifetime of the photogenerated carriers and improved charge extraction. In addition, the stability of the reduced sample in water was also confirmed. All of these effects result in a two-fold increase in the photocurrent density of the laser-treated samples.

Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

Functional oxide materials have become crucial in the continuous development of various fields, including those for energy applications. In this aspect, the synthesis of nanomaterials for low-cost green hydrogen production represents a huge challenge that needs to be overcome to move toward the next generation of efficient systems and devices. This perspective presents a critical assessment of hydrothermal and polymeric precursor methods as potential approaches to designing photoelectrodes for future industrial implementation. The main conditions that can affect the photoanode's physical and chemical characteristics, such as morphology, particle size, defects chemistry, dimensionality, and crystal orientation, and how they influence the photoelectrochemical performance are highlighted in this report. Strategies to tune and engineer photoelectrode and an outlook for developing efficient solar-to-hydrogen conversion using an inexpensive and stable material will also be addressed.

Modifying the Electron-Trapping Process at the BiVO4 Surface States via the TiO2 Overlayer for Enhanced Water Oxidation

BiVO₄ is one of the most promising photoanode candidates to achieve high-efficiency water splitting. However, overwhelming charge recombination at the interface limits its water oxidation activity. In this study, we show that the water oxidation activity of the BiVO₄ photoanode is significantly boosted by the TiO₂ overlayer prepared by atomic layer deposition. With a TiO₂ overlayer of an optimized thickness, the photocurrent at 1.23 V_RHE increased from 0.64 to 1.1 mA·cm^-2 under front illumination corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface-state passivation. We provide direct evidence via transient photocurrent measurements that the TiO₂ overlayer significantly decreases the photogenerated electron-trapping process at the BiVO₄ surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination rather than back illumination. This feature can be particularly useful for wireless tandem devices for water splitting as the higher band gap photoanodes are typically utilized with front illumination in such configurations. Even though the electron-trapping process is eliminated completely at higher TiO₂ overlayer thicknesses, the charge-transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that the ultrathin TiO₂ overlayer can be used to fine tune the surface properties of BiVO₄ and may be used for similar purposes for other photoelectrode systems and other photoelectrocatalytic reactions.

Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes

Nanoscale oxide layer protected semiconductor photoelectrodes show enhanced stability and performance for solar fuels generation, although the mechanism for the performance enhancement remains unclear due to a lack of understanding of the microscopic interfacial field and its effects. Here, we directly probe the interfacial fields at p-GaP electrodes protected by n-TiO₂ and its effect on charge carriers by transient reflectance spectroscopy. Increasing the TiO₂ layer thickness from 0 to 35 nm increases the field in the GaP depletion region, enhancing the rate and efficiency of interfacial electron transfer from the GaP to TiO₂ on the ps time scale as well as retarding interfacial recombination on the microsecond time scale. This study demonstrates a general method for providing a microscopic view of the photogenerated charge carrier's pathway and loss mechanisms from the bulk of the electrode to the long-lived separated charge at the interface that ultimately drives the photoelectrochemical reactions.

WO3/BiVO4 Photoanodes: Facets Matching at the Heterojunction and BiVO4 Layer Thickness Effects

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO₃/BiVO₄ heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO₃/BiVO₄ interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO₄ layer over a WO₃ scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO₄ growth over the WO₃ layer along the BiVO₄ {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO₃/BiVO₄ photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO₄ layer, a 75 nm thick BiVO₄ layer on WO₃ being best performing.

Preparation of multilayer periodic nanopatterned WO3-based photoanode by reverse nanoimprinting for water splitting

Photoelectrochemical (PEC) water splitting has been studied extensively as an environmentally friendly technology for hydrogen production using solar energy. WO₃is considered a promising semiconducting material for photoanodes due to its high electron mobility, good hole diffusion length, and chemical stability. Periodic nanostructures of WO₃have been investigated for enhancing the PEC performance of WO₃-based photoanodes. In this study, facile fabrication of periodic nanostructures of WO₃was achieved using reverse nanoimprint lithography, and the multilayer stacking of nanopatterned WO₃film was also confirmed. The multilayer nanopatterned WO₃films were used as photoanodes for PEC water splitting. The performance of the fabricated photoanode in PEC was 2 times higher than that of planar WO₃film due to its higher light absorbance and lower charge transfer resistance.

Charge carrier mapping for Z-scheme photocatalytic water-splitting sheet via categorization of microscopic time-resolved image sequences

Photocatalytic water splitting system using particulate semiconductor materials is a promising strategy for converting solar energy into hydrogen and oxygen. In particular, visible-light-driven 'Z-scheme' printable photocatalyst sheets are cost-effective and scalable. However, little is known about the fundamental photophysical processes, which are key to explaining and promoting the photoactivity. Here, we applied the pattern-illumination time-resolved phase microscopy for a photocatalyst sheet composed of Mo-doped BiVO₄ and Rh-doped SrTiO₃ with indium tin oxide as the electron mediator to investigate photo-generated charge carrier dynamics. Using this method, we successfully observed the position- and structure-dependent charge carrier behavior and visualized the active/inactive sites in the sheets under the light irradiation via the time sequence images and the clustering analysis. This combination methodology could provide the material/synthesis optimization methods for the maximum performance of the photocatalyst sheets.

Deposition of zinc cobaltite nanoparticles onto bismuth vanadate for enhanced photoelectrochemical water splitting

During the past few decades, photoelectrochemical (PEC) water splitting has attracted significant attention because of the reduced production cost of hydrogen obtained by utilizing solar energy. Significant efforts have been invested by the scientific community to produce stable ternary metal oxide semiconductors, which can enhance the stability and increase the overall production of oxygen. Herein, we present the ternary metal oxide deposition of ZnCo₂O₄ as a route to obtain a novel photocatalyst layer on BiVO₄ to form BiVO₄/ZnCo₂O₄ a novel composite photoanode for PEC water splitting. The structural, topographical, and optical analyses were performed using field emission scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy, and UV-Vis spectroscopy to confirm the structure of the ZnCo₂O₄ grafted over BiVO₄. A remarkable 4.4-fold enhancement of the photocurrent was observed for the BiVO₄/ZnCo₂O₄ composite compared with bare BiVO₄ under visible illumination. The optimum loading of ZnCo₂O₄ over BiVO₄ yields unprecedented stable photocurrent density with an apparent cathodic shift of 0.46 V under 1.5 AM simulated light illumination. This is also evidenced by the flat-band potential change through Mott-Schottky analysis, which reveals the formation of p-ZnCo₂O₄ on n-BiVO₄. The improvement in the PEC performance of the composite with respect to bare BiVO₄ is ascribed to the formation of thin passivating layer of p-ZnCo₂O₄ on n-BiVO₄ which improves the kinetics of interfacial charge transfer. Based on our study, we have gained an in-depth understanding of the BiVO₄/ZnCo₂O₄ composite as high potential in efficient PEC water splitting devices.

Insight into interface charge regulation through the change of the electrolyte temperature toward enhancing photoelectrochemical water oxidation

The desired photoelectrochemical performance can be achieved by temperature regulation, but the nature for this improvement remains a controversial topic. Herein, we employed BiVO₄/CoO_x as a typical model system, and explored the fate of photogenerated holes at the different interfaces among BiVO₄/CoO_x/electrolyte by means of intensity modulated photocurrent spectroscopy (IMPS), scanning photoelectrochemical microscopy (SPECM) and traditional electrocatalysis characterization methods. Systematic quantitative analysis of the kinetics of photogenerated holes transfer at the BiVO₄/CoO_x interface under illumination and surface water oxidation at the CoO_x/electrolyte interface in the dark indicates that increasing temperature could not only enhance the surface catalytic reaction kinetics but also facilitate the interfacial charge transfer. As expected, the integrated system exhibited a remarkable photocurrent density of 3.6 mA cm^-2 (1.23 V_RHE, AM 1.5G, 45 °C), which is approximately 2.1 times higher than that of BiVO₄/CoO_x (15 °C). This work provides a promising strategy for achieving efficient photoelectrochemical water splitting.

BiVO4/Cs2PtI6 Vacancy-Ordered Halide Perovskite Heterojunction for Panchromatic Light Harvesting and Enhanced Charge Separation in Photoelectrochemical Water Oxidation

Photoelectrochemical water oxidation is a challenging reaction in solar water splitting due to the parasitic recombination process, sluggish catalytic activity, and electrode stability. Oxide semiconductors are stable in an aqueous medium but show huge charge carrier recombination. Creation of a heterojunction is found to be effective for extracting the photogenerated electrons/holes before they recombine to the ground state. In this work, we created a heterojunction of BiVO₄ with vacancy-ordered halide perovskite Cs₂PtI₆ and used it as a photoanode in PEC water oxidation. Cs₂PtI₆ is the only halide perovskite that is found to be extremely stable even in strong acids and bases. We utilized the stability of this material and its panchromatic visible light absorption property and made the first unprotected heterojunction dual-absorber photoanode for PEC water oxidation. At 1.23 V (vs RHE), bare BiVO₄ gave 0.6 mA cm^-2 photocurrent density, whereas the BiVO₄/Cs₂PtI₆ heterojunction shows 0.92 mA cm^-2. With the addition of IrO_x cocatalyst, at 1.23 V (vs RHE), the heterojunction gave ∼2 mA cm^-2. To obtain 2 mA cm^-2 photocurrent, pure BiVO₄ requires 560 mV overpotential, whereas the heterojunction requires 250 mV. The increase in the photocurrent arises from the increase in the efficiency of charge separation from BiVO₄ to Cs₂PtI₆ and the complementary absorption offered by the latter.

Charge Transport Phenomena in Heterojunction Photocatalysts: The WO3/TiO2 System as an Archetypical Model

Recent studies have demonstrated the high efficiency through which nanostructured core-shell WO₃/TiO₂ (WT) heterojunctions can photocatalytically degrade model organic pollutants (stearic acid, QE ≈ 18% @ λ = 365 nm), and as such, has varied potential environmental and antimicrobial applications. The key motivation herein is to connect theoretical calculations of charge transport phenomena, with experimental measures of charge carrier behavior using transient absorption spectroscopy (TAS), to develop a fundamental understanding of how such WT heterojunctions achieve high photocatalytic efficiency (in comparison to standalone WO₃ and TiO₂ photocatalysts). This work reveals an order of magnitude enhancement in electron and hole recombination lifetimes, respectively located in the TiO₂ and WO₃ sides, when an optimally designed WT heterojunction photocatalyst operates under UV excitation. This observation is further supported by our computationally captured details of conduction band and valence band processes, identified as (i) dominant electron transfer from WO₃ to TiO₂ via the diffusion of excess electrons; and (ii) dominant hole transfer from TiO₂ to WO₃ via thermionic emission over the valence band edge. Simultaneously, our combined theoretical and experimental study offers a time-resolved understanding of what occurs on the micro- to milliseconds (μs-ms) time scale in this archetypical photocatalytic heterojunction. At the microsecond time scale, a portion of the accumulated holes in WO₃ contribute to the depopulation of W⁵⁺ polaronic states, whereas the remaining accumulated holes in WO₃ are separated from adjacent electrons in TiO₂ up to 3 ms after photoexcitation. The presence of these exceptionally long-lived photogenerated carriers, dynamically separated by the WT heterojunction, is the origin of the superior photocatalytic efficiency displayed by this system (in the degradation of stearic acid). Consequently, our combined computational and experimental approach delivers a robust understanding of the direction of charge separation along with critical time-resolved insights into the evolution of charge transport phenomena in this model heterojunction photocatalyst.

Growth of NiMn layered double hydroxides on nanopyramidal BiVO4 photoanode for enhanced photoelectrochemical performance

Photoelectrochemical water oxidation for hydrogen generation via utilizing sunlight is considered a very promising pathway for generating sustainable energy in an environmental manner. Here, a composite photoanode, consisting of nanopyramidal BiVO₄ arrays and one layered double hydroxide (NiMn-LDH) was designed and fabricated via a facile route. The obtained BiVO₄/NiMn-LDH composite photoelectrode presented a significant enhancement in the photoelectrochemical (PEC) current density, conversion efficiency and stability for solar water oxidation. With 2D NiMn-LDH decoration, an obvious cathodic shift of ∼480 mV in the onset potential can be observed, and more than two times enhancement in photocurrent performance is achieved. The improvement in photoelectrochemical activity for BiVO₄/NiMn-LDH composite photoanode can be attributed to the enhanced water-oxidation kinetics leading to the efficient separation, transfer and collection of charge carriers at the photoanode/electrolyte interface. The result demonstrates NiMn-LDH represents one of the active oxygen evolution catalysts (OECs) to improve the PEC activity of metal oxide photoanode.

Enhanced catalytic reaction at an air-liquid-solid triphase interface

Gaseous reactant involved heterogeneous catalysis is critical to the development of clean energy, environmental management, health monitoring, and chemical synthesis. However, in traditional heterogeneous catalysis with liquid–solid diphase reaction interfaces, the low solubility and slow transport of gaseous reactants strongly restrict the reaction efficiency. In this minireview, we summarize recent advances in tackling these drawbacks by designing catalytic systems with an air–liquid–solid triphase joint interface. At the triphase interface, abundant gaseous reactants can directly transport from the air phase to the reaction centre to overcome the limitations of low solubility and slow transport of the dissolved gas in liquid–solid diphase reaction systems. By constructing a triphase interface, the efficiency and/or selectivity of photocatalytic reactions, enzymatic reactions, and (photo)electrochemical reactions with consumption of gaseous reactants oxygen, carbon dioxide, and nitrogen are significantly improved.

Gaseous reactant involved liquid–solid diphase interface reactions can be significantly enhanced using rationally designed and constructed air–liquid–solid triphase systems.

Heterojunction α-Fe2 O3 /ZnO Films with Enhanced Photocatalytic Properties Grown by Aerosol-Assisted Chemical Vapour Deposition

Type I heterojunction films of α-Fe₂ O₃ /ZnO are reported here as a non-titania based photocatalyst, which shows remarkable enhancement in the photocatalytic properties towards stearic acid degradation under UVA-light exposure (λ=365 nm), with a quantum efficiency of ξ=4.42±1.54×10^-4 molecules degraded/photon, which was about 16 times greater than that of α-Fe₂ O₃ , and 2.5 times greater than that of ZnO. Considering that the degradation of stearic acid requires 104 electron transfers for each molecule, this represents an overall quantum efficiency of 4.60 % for the α-Fe₂ O₃ /ZnO heterojunction. Time-resolved transient absorption spectroscopy (TAS) revealed the charge-carrier behaviour responsible for this increase in activity. Photogenerated electrons, formed in the ZnO layer, were transferred into the α-Fe₂ O₃ layer on the pre-μs timescale, which reduced electron-hole recombination. This increased the lifetime of photogenerated holes formed in ZnO, which oxidise stearic acid. The heterojunction α-Fe₂ O₃ /ZnO films grown herein show potential environmental applications as coatings for self-cleaning windows and surfaces.

Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO3 Heterojunction-Based Two-Dimensional Photoelectrode

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO₃ and (Er,W):BiVO₄ is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er³⁺ is substituted at the Bi³⁺ sites in the BiVO₄ lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO₃/(Er,W):BiVO₄ electrode shows photocurrent densities of 4.1 and 7.2 mA cm^-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K₂HPO₄ electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO₃/(Er,W):BiVO₄/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm^-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm^-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi³⁺ and V⁵⁺ sites to enhance absorption, this study demonstrates for the first time a planar WO₃/BiVO₄ heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.