Mo-doped BiVO4 photoanodes synthesized by reactive sputtering

Steel slag source-derived FeOOH for enhanced BiVO4 photoelectrochemical water splitting

Green, healthy, and sustainable energy development has always been the cornerstone of global energy development. In this study, industrial waste steel slag was utilized as the raw material, and FeOOH was loaded onto a BiVO4 surface using the impregnation method. The optimized photoanode exhibited a lower onset potential and higher surface activity, achieving a current density of 3.08 mA/cm2 at 1.23 V vs. RHE, and dramatically enhancing the oxygen and hydrogen production efficiencies of the entire system. The incorporation of FeOOH from a steel slag source improves the photoelectrochemical (PEC) water splitting capacity and broadens the steel slag utilization pathways for more economical and green energy use. This study combines the high value-added utilization of solid waste with the field of PEC, presenting a novel approach.

Correlation between Dynamics of Polaronic Photocarriers and Photoelectrochemical Performance in Mo-Doped Bismuth Vanadate

Bismuth vanadate (BiVO4) has received intense research interest due to its outstanding performance for solar water splitting, and doping it with molybdenum (Mo) ions can effectively boost photoelectrochemical performance. In this material, highly localized polarons play a key role in the photoconversion process. Herein, we uncovered the influence of Mo dopants on the dynamics of polaronic transient species using transient absorption spectroscopy. We find that the preexisting electron small polarons stemming from the thermal ionization of dopants provide additional centers to capture itinerant holes, which significantly decrease the hole lifetime. However, the introduction of dopants increases the lifetime of self-trapped excitons that arise from the binding of electron polarons and holes. The dependence of the photoelectrochemical performance of BiVO4 photoelectrodes on doping levels can be well explained by combining the dopant effects on the lifetimes of delocalized and self-trapped transient species. Our findings provide guidance for rational optimization of dopant concentration to maximize the PEC efficiency.

Ternary Oxide CuWO4/BiVO4/FeCoOx Films for Photoelectrochemical Water Oxidation: Insights into the Electronic Structure and Interfacial Band Alignment

Photoelectrochemical (PEC) water oxidation using ternary oxide systems has been considered a promising approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxide-based CuWO4/BiVO4/FeCoOx film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO4/BiVO4 photoanode exhibits a significant photocurrent density of 0.82 mA cm-2 at 1.23 V vs RHE under AM 1.5G illumination, which is a record 382% increase compared to that of the bare CuWO4 film. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoOx cocatalyst, resulting in a triple CuWO4/BiVO4/FeCoOx heterojunction with a significant reduction in onset potential and a 500% increase in the photocurrent density of bare CuWO4. Experimental and theoretical approaches were used to provide insights into the interfacial band alignment and photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO4/BiVO4 heterojunction, potentially limiting its application in tandem systems. Conversely, the deposition of the FeCoOx ultrathin layer over the CuWO4/BiVO4 heterojunction induces a p-n junction on the BiVO4/FeCoOx interface, which, when combined with the abundant FeCoOx oxygen vacancies, results in improved charge separation and transport as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunction systems and introduces simple tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO₄) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO₄ photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO₄. In this review article, the fundamental properties of BiVO₄ are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO₄ photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO₄ photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO₄ photoanodes are presented, providing guidelines for the design of efficient BiVO₄ photoanodes for solar fuel production.

Selective and sensitive photoelectrochemical aptasensor for streptomycin detection based on Bi4VO8Br/Ti3C2 nanohybrids

Sensitive detection of streptomycin (STR) has attracted increasing attention worldwide because of the relationship between food security and human health. In this paper, Bi₄VO₈Br/Ti₃C₂ nanohybrids were obtained by one-pot solvent hydrothermal method. It was modified on ITO electrode, and STR aptamer was acted as the recognition element. With excellent photoelectrochemical (PEC) performance of Bi₄VO₈Br/Ti₃C₂ nanohybrids, an "on-off-on" PEC aptasensor for STR detection was effectively developed. Compared with pure Bi₄VO₈Br, the photocurrent intensity of as-prepared Bi₄VO₈Br/Ti₃C₂ nanohybrids was about 9 times higher, which ascribed to the highly conductive of Ti₃C₂, driving the photogenerated electrons transferred to the ITO electrode rapidly, so that the recombination of photogenerated electron and hole pairs was inhibited viably. Furthermore, the constructed "on-off-on" PEC aptasensor accomplished STR detection with high sensitivity, excellent specificity and distinguished repeatability in honey. The photocurrent increased with the increment of STR concentration with the linear range from1 nM to 1000 nM, and the detection limit of 0.3 nM (S/N = 3). Compared with the national standard method (SN/T 1925-2007), the as-constructed PEC sensor showed the consistent results.

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.

Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO4 Thin Films

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO₄ . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V₂ O₅ at grain boundaries of Mo-BiVO₄ thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO₄ . After photo-electrochemical operation, the degraded Mo-BiVO₄ films show similar grain center and grain boundary spectra indicating V₂ O₅ dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.

Zinc and Molybdenum Co-Doped BiVO4 Nanoarray for Photoelectrochemical Diethylstilbestrol Analysis Based on the Dual-Competitive System of Manganese Hexacyanoferrate Hydrate Nanocubes

This study proposes a competitive photoelectrochemical (PEC) immunosensor for detecting diethylstilbestrol (DES). The PEC sensing platform uses a zinc and molybdenum codoped BiVO₄ nanoarray ((Zn,Mo):BiVO₄) as the photoactive matrix and manganese hexacyanoferrate hydrate loading silicon dioxide layer composite nanocubes (MHCF@SiO₂ NCs) as the signal quencher. The (Zn,Mo):BiVO₄ nanoarray demonstrated brilliant PEC behavior, by virtue of the local electric field formed by the codoped Zn and Mo. This doping accelerated the electron transfer and improved the photoelectric conversion efficiency in BiVO₄ nanoarray under visible light. Furthermore, the nanoarray structure with its large surface area provided abundant binding sites for the immune response. As the MHCF@SiO₂ NCs-anti-DES competitively bonded with either free DES or bovine serum albumin conjugated DES (BSA-DES), hydrogen peroxide (H₂O₂) as electron donor was competitively consumed and meanwhile steric resistance blocked electrons transfer. For the above reasons, the photocurrent signal was reduced. Thus, the standard sample free DES was accurately detected, and the fabricated PEC immunosensor displayed an outstanding photocurrent response from 0.1 pg/mL to 50 ng/mL with a detection limit of 0.05 pg/mL. Simultaneously, the acceptable stability, selectivity, and reproducibility of the designed dual-competitive sensing platform suggest its applicability to small molecule detection.

Scheelite-related MBi1-xV1-xMoxO4 (MII- Ca, Sr) solid solution-based photoanodes for enhanced photoelectrochemical water oxidation

The photoelectrochemical properties of scheelite-related MBi_1-xV_1-xMo_xO₄ (M^II = Ca, Sr, x = 0.1 to 0.9) solid solutions deposited on conductive glass (coated with SnO₂, F-doped) have been investigated as photoanodes in photoelectrochemical (PEC) water splitting. The variation of the final annealing temperature during the preparation of the conduction electrodes as well as the value of substitution x have been shown to affect the PEC performance. The micropowders of MBi_1-xV_1-xMo_xO₄ (M^II = Ca, Sr, x = 0.1 to 0.9) samples were first fabricated vi a solid-state method; they were characterised by SEM microscopy and powder and single crystal X-ray diffraction, and the band gap values were estimated using diffusive reflectance data. The value of substitution x = 0.1 in the cases of samples containing calcium and strontium affords the highest PEC performance reported for the whole range of substitution. These results demonstrate a promising approach for the beneficial utilization of BiVO₄-substituted scheelite-related solid solutions in photo-electrochemical cells towards efficient and inexpensive photoanodes.

The use of mixed-metal single source precursors for the synthesis of complex metal oxides

This Feature Article highlights the use of mixed-metal single source precursors to directly access useful complex metal oxide materials.

Photoinduced Carrier Dynamics at the Interface of Black Phosphorus and Bismuth Vanadate

Two-dimensional (2D) heterostructures of black phosphorus (BP)/bismuth vanadate (BVO) have attracted much attention due to their potential uses in photocatalytic water splitting. However, the interfacial photoinduced electron- and hole-transfer dynamics are not explored computationally. Herein, we have used density functional theory (DFT) calculations and DFT-based fewest-switches surface-hopping dynamics simulations to investigate the light-driven electron and hole dynamics taking place at the interface of BP and the BVO(010) surface. Our results show that the BP monolayer is adsorbed on BVO(010) via van der Waals interaction. Upon irradiation, the electron transfer takes place from BP to BVO(010) within 500 fs but with two distinct processes. In the first phase, the electron transfer proceeds adiabatically and is mainly driven by atomic motions. In the second phase, the electron transfer decays very slowly. The hole-transfer dynamics from BVO(010) to BP exhibits a similar ultrafast decay in the first stage followed by a slow decay; however, there is a comparable amount of hole trapped in a BP state due to a large energy gap from its higher state. These insights may be useful for the design of novel photocatalytic water-splitting materials.

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar-energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄ , with a bandgap of 2.4-2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p ) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value-added chemicals are also discussed.

Integration of Molybdenum-Doped, Hydrogen-Annealed BiVO4 with Silicon Microwires for Photoelectrochemical Applications

H-BiVO_4-x :Mo was successfully deposited on microwire-structured silicon substrates, using indium tin oxide (ITO) as an interlayer and BiOI prepared by electrodeposition as precursor. Electrodeposition of BiOI, induced by the electrochemical reduction of p-benzoquinone, appeared to proceed through three stages, being nucleation of particles at the base and bottom of the microwire arrays, followed by rapid (homogeneous) growth, and termination by increasing interfacial resistances. Variations in charge density and morphology as a function of spacing of the microwires are explained by (a) variations in mass transfer limitations, most likely associated with the electrochemical reduction of p-benzoquinone, and (b) inhomogeneity in ITO deposition. Unexpectedly, H-BiVO_4-x :Mo on microwire substrates (4 μm radius, 4 to 20 μm spacing, and 5 to 16 μm length) underperformed compared to H-BiVO_4-x :Mo on flat surfaces in photocatalytic tests employing sulfite (SO₃ ^2-) oxidation in a KPi buffer solution at pH 7.0. While we cannot exclude optical effects, or differences in material properties on the nanoscale, we predominantly attribute this to detrimental diffusion limitations of the redox species within the internal volume of the microwire arrays, in agreement with existing literature and the observations regarding the electrodeposition of BiOI. Our results may assist in developing high-efficiency PEC devices.

Synthesis of nanoporous Mo:BiVO4 thin film photoanodes using the ultrasonic spray technique for visible-light water splitting

The use of bismuth vanadate (BiVO₄) scheelite structures for converting solar energy into fuels and chemicals for fast growth in lab to industrial scale for large-area modules is a key challenge for further development. Herein, we demonstrate a new ultrasonic spray technique as a scalable and versatile coating technique for coating pristine and doped nanoporous BiVO₄ thin film photoanodes directly on FTO-coated glass substrates for water splitting under visible irradiation. The successful Mo doping in BiVO₄ lattice was confirmed by various characterization techniques such as XRD, Raman, EDS and XPS. The Mo:BiVO₄ photoelectrode showed excellent performance with higher stability as compared to pristine BiVO₄ samples.

WO3/BiVO4 Type-II Heterojunction Arrays Decorated with Oxygen-Deficient ZnO Passivation Layer: A Highly Efficient and Stable Photoanode

In the present work, we report a ternary WO₃/BiVO₄/ZnO photoanode with boosted PEC efficiency and stability toward highly efficient water splitting. The type-II WO₃/BiVO₄ heterojunction arrays are firstly prepared by hydrothermal growth of WO₃ nanoplate arrays onto the substrates of fluorine-doped tin oxide (FTO)-coated glass, followed by spin-coating of BiVO₄ layers onto the WO₃ nanoplate surfaces. After that, thin ZnO layers are further introduced onto the WO₃/BiVO₄ heterojunction arrays via atomic layer deposition (ALD), leading to the construction of ternary WO₃/BiVO₄/ZnO photoanodes. It is verified that the ZnO thin layer in the WO₃/BiVO₄/ZnO photoanode contains abundant oxygen vacancies, which could act as an effective passivation layer to enhance the charge separation and surface water oxidation kinetics of photogenerated carriers. The as-prepared WO₃/BiVO₄/ZnO photoanode produces a photocurrent of 2.96 mA cm^-2 under simulated sunlight with an incident photon-to-current conversion efficiency (IPCE) of ∼72.8% at 380 nm at a potential of 1.23 V versus RHE without cocatalysts, both of which are comparable to the state-of-the-art WO₃/BiVO₄ counterparts. Moreover, the photocurrent of the WO₃/BiVO₄/ZnO photoanode shows only 9% decay after 6 h, suggesting its high photoelectrochemical (PEC) stability.

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes

Single-source precursors are used to produce nanostructured BiVO₄ photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO₄ films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO₄ films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm^-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm² Co-BiVO₄ photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.

New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting

Bismuth vanadate (BiVO₄ ) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, owing to the short carrier diffusion length, the trade-off between sufficient light absorption and efficient charge separation often leads to poor PEC performance. Herein, a new electrodeposition process is developed to prepare bismuth oxide precursor films, which can be converted to transparent BiVO₄ films with well-controlled oxygen vacancies via a mild thermal treatment process. The optimized BiVO₄ film exhibits an excellent back illumination charge separation efficiency mainly due to the presence of enriched oxygen vacancies which act as shallow donors. By loading FeOOH/NiOOH as the cocatalysts, the BiVO₄ dual photoanodes exhibit a remarkable and highly stable photocurrent density of 5.87 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination. An artificial leaf composed of the BiVO₄ /FeOOH/NiOOH dual photoanodes and a single sealed perovskite solar cell delivers a solar-to-hydrogen conversion efficiency as high as 6.5% for unbiased water splitting.

Anisotropic Electronic Characteristics, Adsorption, and Stability of Low-Index BiVO4 Surfaces for Photoelectrochemical Applications

Many experimental results reveal different activities among different low-index surfaces of photocatalysts. The current investigation focuses on the theoretical understanding of the electronic characteristics, surface activity, and stability of different low-index surfaces of BiVO₄ toward water splitting using first-principle calculations. The results indicate that BiVO₄ has four types of low-index surfaces, namely, (010)T1, (010)T2, (110)T1, and (1̅11)T1. The different band edge potentials of the surfaces, resulting from the variation of the electrostatic potential, lead to a higher oxidation ability for (010)T1 and (010)T2 than for (110)T1 and (1̅11)T1 surfaces. The electrons prefer to accumulate on (010)T1 and (010)T2 surfaces, whereas holes like to accumulate on (110)T1 and (1̅11)T1 surfaces during a photocatalytic process. Moreover, investigation on the adsorbed intermediates during the water-splitting process indicates that the oxygen evolution reaction on BiVO₄ surfaces is mainly dominated by the reaction OH* ↔ O* + H⁺ + e^-, and (110)T1 and (1̅11)T1 surfaces are energetically more favorable as photoanodes for water splitting than (010)T1 and (010)T2. Furthermore, the BiVO₄ surface as photoanodes tend to be unstable and can easily be corroded with or without the presence of an oxidative environment, however, there is an exception for the BiVO₄ (010)T1 and (010)T2 surfaces, which are thermodynamically stable in the solution when there are no strong oxidative species. These results provide important insights into the anisotropy behaviors among low-index surfaces of BiVO₄ for photocatalytic reactions.

A Green Desulfurization Technique: Utilization of Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process Based on Mo-Doped BiVO4

A green photoelectrochemical (PEC) process with simultaneous SO₂ removal and H₂ production has attracted an increasing attention. The proposed process uses flue gas SO₂ to improve H₂ production. The improvement of the efficiency of this process is necessary before it can become industrial viable. Herein, we reported a Mo modified BiVO₄ photocatalysts for a simultaneous SO₂ removal and H₂ production. And the PEC performance could be significantly improved with doping and flue gas removal. The evolution rate of H₂ and removal of SO₂ could be enhanced by almost three times after Mo doping as compared with pristine BiVO₄. The enhanced H₂ production and SO₂ removal is attributed to the improved bulk charge carrier transportation after Mo doping, and greatly enhanced oxidation reaction kinetics on the photoanode due to the formation of SO32− after SO₂ absorption by the electrolyte. Due to the utilization of SO₂ to improve the production of H₂, the proposed PEC process may become a profitable desulfurization technique.

High-Throughput Preparation of Metal Oxide Nanocrystals by Cathodic Corrosion and Their Use as Active Photocatalysts

Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (H₂WO₄) nanoplates, titanium oxide (TiO₂) nanowires, and symmetric star-shaped bismuth vanadate (BiVO₄) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline H₂WO₄ and BiVO₄ particles and amorphous TiO₂ were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO₄ particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.

Photoanode with Enhanced Performance Achieved by Coating BiVO4 onto ZnO-Templated Sb-Doped SnO2 Nanotube Scaffold

The performance of BiVO₄ photoanodes, especially under front-side illumination, is limited by the modest charge transport properties of BiVO₄. Core/shell nanostructures consisting of BiVO₄ coated onto a conductive scaffold are a promising route to improving the performance of BiVO₄-based photoanodes. Here, we investigate photoanodes composed of thin and uniform layers of BiVO₄ particles coated onto Sb-doped SnO₂ (Sb:SnO₂) nanotube arrays that were synthesized using a sacrificial ZnO template with controllable length and packing density. We demonstrate a new record for the product of light absorption and charge separation efficiencies (η_abs × η_sep) of ∼57.3 and 58.5% under front- and back-side illumination, respectively, at 0.6 V_RHE. Moreover, both of these high η_abs × η_sep efficiencies are achieved without any extra treatment or intentional doping in BiVO_4. These results indicate that integration of Sb:SnO₂ nanotube cores with other successful strategies such as doping and hydrogen treatment can increase the performance of BiVO₄ and related semiconductors closer to their theoretical potential.

Ultrafast fabrication of highly active BiVO4 photoanodes by hybrid microwave annealing for unbiased solar water splitting

Hybrid microwave annealing (HMA) with a silicon susceptor in a household microwave oven produces BiVO₄-based photoanodes of much improved performance in photoelectrochemical water oxidation in only 6 min relative to conventional thermal annealing in a traditional muffle furnace (FA) that needs a much longer time, 300 min. This technique can apply equally effectively to bare as well as modified BiVO₄ by Mo-doping, heterojunction formation with WO₃, and an oxygen evolution co-catalyst. Relative to FA, HMA forms BiVO₄ films of smaller feature sizes, higher porosity, and increased three dimensional roughness, which decrease the diffusion distance of holes to the surface and thereby increase mainly the bulk charge separation efficiency (η_bulk) of the photoanodes. Thus, the HMA-treated BiVO₄/WO₃ film achieves the state-of-the art η_bulk of ∼90% for water oxidation. Combination of a photoanode of NiOOH/FeOOH/BiVO₄/WO₃ (HMA, 6 min) with a 2p c-Si solar cell allows a solar to hydrogen conversion efficiency of ∼5.0% in unbiased overall water splitting, which is also comparable to the state-of-the-art for a similar material combination.

Discovery of Fe-Ce Oxide/BiVO4 Photoanodes through Combinatorial Exploration of Ni-Fe-Co-Ce Oxide Coatings

An efficient photoanode is a prerequisite for a viable solar fuels technology. The challenges to realizing an efficient photoanode include the integration of a semiconductor light absorber and a metal oxide electrocatalyst to optimize corrosion protection, light trapping, hole transport, and photocarrier recombination sites. To efficiently explore metal oxide coatings, we employ a high-throughput methodology wherein a uniform BiVO4 film is coated with 858 unique metal oxide coatings covering a range of metal oxide loadings and the full (Ni-Fe-Co-Ce)Ox pseudoquaternary composition space. Photoelectrochemical characterization of the photoanodes reveals that specific combinations of metal oxide composition and loading provide up to a 13-fold increase in the maximum photoelectrochemical power generation for oxygen evolution in pH 13 electrolyte. Through mining of the high-throughput data we identify composition regions that form improved interfaces with BiVO4. Of particular note, integrated photoanodes with catalyst compositions in the range Fe(0.4-0.6)Ce(0.6-0.4)Ox exhibit high interface quality and excellent photoelectrochemical power conversion. Scaled-up inkjet-printed electrodes and photoanodic electrodeposition of this composition on BiVO4 confirms the discovery and the synthesis-independent interface improvement of (Fe-Ce)Ox coatings on BiVO4.

Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes

Artificial photosynthesis relies on the availability of semiconductors that are chemically stable and can efficiently capture solar energy. Although metal oxide semiconductors have been investigated for their promise to resist oxidative attack, materials in this class can suffer from chemical and photochemical instability. Here we present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. Computational modelling indicates that photoexcited charge carriers accumulated at the surface destabilize the lattice, and that self-passivation by formation of a chemically stable surface phase is kinetically hindered. Although chemical stability of metal oxides cannot be assumed, insight into corrosion mechanisms aids development of protection strategies and discovery of semiconductors with improved stability.

Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells

Bismuth vanadate (BiVO4) has been widely regarded as a promising photoanode material for photoelectrochemical (PEC) water splitting because of its low cost, its high stability against photocorrosion, and its relatively narrow band gap of 2.4 eV. However, the achieved performance of the BiVO4 photoanode remains unsatisfactory to date because its short carrier diffusion length restricts the total thickness of the BiVO4 film required for sufficient light absorption. We addressed the issue by deposition of nanoporous Mo-doped BiVO4 (Mo:BiVO4) on an engineered cone-shaped nanostructure, in which the Mo:BiVO4 layer with a larger effective thickness maintains highly efficient charge separation and high light absorption capability, which can be further enhanced by multiple light scattering in the nanocone structure. As a result, the nanocone/Mo:BiVO4/Fe(Ni)OOH photoanode exhibits a high water-splitting photocurrent of 5.82 ± 0.36 mA cm(-2) at 1.23 V versus the reversible hydrogen electrode under 1-sun illumination. We also demonstrate that the PEC cell in tandem with a single perovskite solar cell exhibits unassisted water splitting with a solar-to-hydrogen conversion efficiency of up to 6.2%.

Photoelectrochemical Water Splitting System--A Study of Interfacial Charge Transfer with Scanning Electrochemical Microscopy

Fast charge transfer kinetics at the photoelectrode/electrolyte interface is critical for efficient photoelectrochemical (PEC) water splitting system. Thus, far, a measurement of kinetics constants for such processes is limited. In this study, scanning electrochemical microscopy (SECM) is employed to investigate the charge transfer kinetics at the photoelectrode/electrolyte interface in the feedback mode in order to simulate the oxygen evolution process in PEC system. The popular photocatalysts BiVO4 and Mo doped BiVO4 (labeled as Mo:BiVO4) are selected as photoanodes and the common redox couple [Fe(CN)6](3-)/[Fe(CN)6](4-) as molecular probe. SECM characterization can directly reveal the surface catalytic reaction kinetics constant of 9.30 × 10(7) mol(-1) cm(3) s(-1) for the BiVO4. Furthermore, we find that after excitation, the ratio of rate constant for photogenerated hole to electron via Mo:BiVO4 reacting with mediator at the electrode/electrolyte interface is about 30 times larger than that of BiVO4. This suggests that introduction of Mo(6+) ion into BiVO4 can possibly facilitate solar to oxygen evolution (hole involved process) and suppress the interfacial back reaction (electron involved process) at photoanode/electrolyte interface. Therefore, the SECM measurement allows us to make a comprehensive analysis of interfacial charge transfer kinetics in PEC system.

Amount of tungsten dopant influencing the photocatalytic water oxidation activity of LaTiO2N crystals grown directly by an NH3-assisted flux method

The effect of tungsten dopant on the photocatalytic water oxidation activity of LaTiO₂N crystals grown by an NH₃-assisted flux method was studied.

Effects of Mo/W codoping on the visible-light photocatalytic activity of monoclinic BiVO4 within the GGA + U framework

Mo or W atom doping on V site can form continuum states above conduction band edge of BiVO₄. Mo/W/Mo and W/Mo/W co-doped BiVO₄ have relatively small formation energies and band gaps, which is particularly suitable for visible-light photocatalysis.

Bandgap Tunability in Sb-Alloyed BiVO₄ Quaternary Oxides as Visible Light Absorbers for Solar Fuel Applications

The challenge of fine compositional tuning and microstructure control in complex oxides is overcome by developing a general two-step synthetic approach. Antimony-alloyed bismuth vanadate, which is identified as a novel light absorber for solar fuel applications, is prepared in a wide compositional range. The bandgap of this quaternary oxide linearly decreases with the Sb content, in agreement with first-principles calculations.

Chemically induced porosity on BiVO4 films produced by double magnetron sputtering to enhance the photo-electrochemical response

An overview of the sputtering procedure, the chemical treatment involved in the production of a porous BiVO₄ film and its activity.