Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting

Solar-driven highly selective conversion of glycerol to dihydroxyacetone using surface atom engineered BiVO4 photoanodes

Dihydroxyacetone is the most desired product in glycerol oxidation reaction because of its highest added value and large market demand among all possible oxidation products. However, selectively oxidative secondary hydroxyl groups of glycerol for highly efficient dihydroxyacetone production still poses a challenge. In this study, we engineer the surface of BiVO4 by introducing bismuth-rich domains and oxygen vacancies (Bi-rich BiVO4-x) to systematically modulate the surface adsorption of secondary hydroxyl groups and enhance photo-induced charge separation for photoelectrochemical glycerol oxidation into dihydroxyacetone conversion. As a result, the Bi-rich BiVO4-x increases the glycerol oxidation photocurrent density of BiVO4 from 1.42 to 4.26 mA cm-2 at 1.23 V vs. reversible hydrogen electrode under AM 1.5 G illumination, as well as the dihydroxyacetone selectivity from 54.0% to 80.3%, finally achieving a dihydroxyacetone production rate of 361.9 mmol m-2 h-1 that outperforms all reported values. The surface atom customization opens a way to regulate the solar-driven organic transformation pathway toward a carbon chain-balanced product.

The Emergence of High-Performance Conjugated Polymer/Inorganic Semiconductor Hybrid Photoelectrodes for Solar-Driven Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) energy conversion holds great potential in converting solar energy into storable and transportable chemicals or fuels, providing a viable route toward a carbon-neutral society. Conjugated polymers are rapidly emerging as a new class of materials for PEC water splitting. They exhibit many intriguing properties including tunable electronic structures through molecular engineering, excellent light harvesting capability with high absorption coefficients, and facile fabrication of large-area thin films via solution processing. Recent advances have indicated that integrating rationally designed conjugated polymers with inorganic semiconductors is a promising strategy for fabricating efficient and stable hybrid photoelectrodes for high-efficiency PEC water splitting. This review introduces the history of developing conjugated polymers for PEC water splitting. Notable examples of utilizing conjugated polymers to broaden the light absorption range, improve stability, and enhance the charge separation efficiency of hybrid photoelectrodes are highlighted. Furthermore, key challenges and future research opportunities for further improvements are also presented. This review provides an up-to-date overview of fabricating stable and high-efficiency PEC devices by integrating conjugated polymers with state-of-the-art semiconductors and would have significant implications for the broad solar-to-chemical energy conversion research.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Enhanced Charge Separation in Nanoporous BiVO4 by External Electron Transport Layer Boosts Solar Water Splitting

The optimization of charge transport with electron-hole separation directed toward specific redox reactions is a crucial mission for artificial photosynthesis. Bismuth vanadate (BiVO4 , BVO) is a popular photoanode material for solar water splitting, but it faces tricky challenges in poor charge separation due to its modest charge transport properties. Here, a concept of the external electron transport layer (ETL) is first proposed and demonstrated its effectiveness in suppressing the charge recombination both in bulk and at surface. Specifically, a conformal carbon capsulation applied on BVO enables a remarkable increase in the charge separation efficiency, thanks to its critical roles in passivating surface charge-trapping sites and building external conductance channels. Through decorated with an oxygen evolution catalyst to accelerate surface charge transfer, the carbon-encased BVO (BVO@C) photoanode manifests durable water splitting over 120 h with a high current density of 5.9 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE) under 1 sun irradiation (100 mW cm-2 , AM 1.5 G), which is an activity-stability trade-off record for single BVO light absorber. This work opens up a new avenue to steer charge separation via external ETL for solar fuel conversion.

Progress in Promising Semiconductor Materials for Efficient Photoelectrocatalytic Hydrogen Production

Photoelectrocatalytic (PEC) water decomposition provides a promising method for converting solar energy into green hydrogen energy. Indeed, significant advances and improvements have been made in various fundamental aspects for cutting-edge applications, such as water splitting and hydrogen production. However, the fairly low PEC efficiency of water decomposition by a semiconductor photoelectrode and photocorrosion seriously restrict the practical application of photoelectrochemistry. In this review, the mechanisms of PEC water decomposition are first introduced to provide a solid understanding of the PEC process and ensure that this review is accessible to a wide range of readers. Afterwards, notable achievements to date are outlined, and unique approaches involving promising semiconductor materials for efficient PEC hydrogen production, including metal oxide, sulfide, and graphite-phase carbon nitride, are described. Finally, four strategies which can effectively improve the hydrogen production rate-morphological control, doping, heterojunction, and surface modification-are discussed.

Prompt Hole Extraction Suppresses V5+ Dissolution and Sustains Large-Area BiVO4 Photoanodes for Over 2100 h Water Oxidation

Nanostructured bismuth vanadate (BiVO4) is at the forefront of emerging photoanodes in photoelectrochemical tandem devices for solar water splitting owing to the suitable band edge position and efficient charge separation capability. However, the (photo)chemical corrosion involving V5+ dissolution limits the long-term stability of BiVO4. Herein, guided by DFT calculations, we introduce an ALD-derived NiOx catalyst layer on BiVO4 to stabilize the surface Bi-O bonds, facilitate hole extraction, and thus suppress the V5+ dissolution. At the same time, the ALD NiOx catalyst layer could efficiently suppress the surface recombination and accelerate the surface OER kinetics, boosting the half-cell applied bias photon-to-current efficiency of BiVO4 to 2.05%, as well as a fill factor of 47.1%. By adding trace NaVO3 to the electrolyte, the NiOx/BiVO4 photoanode with an illumination area of 10.5 cm2 shows a record operational stability of more than 2100 h.

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.

Time-Domain View of Polaron Dynamics in Metal Oxide Photocatalysts

The polaron is a fundamental physical phenomenon in transition metal oxides (TMOs), and it has been studied extensively for decades. However, the implication of a polaron on photochemistry is still ambiguous. As such, understanding the fundamental properties and controlling the dynamics of polarons at the atomistic level is desired. In this Perspective, we seek to highlight the recent advances in studying small polarons in TMOs, with a particular focus on nonadiabatic molecular dynamics at the ab initio level, and discuss the implications for photocatalysis from the aspects of the structure, intrinsic physical properties, formation, migration, and recombination of small polarons. Finally, various methods were proposed to advance our understanding of manipulating the small-polaron dynamics, and strategies to design high-performance TMO-based photoelectrodes were discussed.

C-C Bond Formation Coupled with C-C Bond Cleavage during Oxidative Upgrading of Glycerol on a Nanoporous BiVO4 Photoanode

Production of biodiesel generates glycerol as a 10 wt% byproduct. Therefore, efficient and selective glycerol upgrading is critical for the sustainable production of biodiesel as well as for the production of chemicals from renewable feedstocks. In this study, the photoelectrochemical glycerol oxidation reaction (GOR) was investigated using a nanoporous BiVO4 photoanode in pH 9.3 and pH 2 buffer solutions. In both solutions, glycolaldehyde (GCAD), a C2 species, was the major product, which has never been the major product in any previous electrochemical or photoelectrochemical GOR study. To produce GCAD from the C3 species glycerol, C-C cleavage should occur to produce C2 and C1 species with a 1:1 ratio. Intriguingly, our results show that, during photoelectrochemical GOR on BiVO4, more GCAD is produced than can be explained by simple C-C cleavage, meaning that GCAD is also produced from C-C coupling of two C1 species produced from C-C cleavage. This is equivalent to converting two glycerol molecules to three GCAD molecules, which offers an extraordinary way to maximize GCAD production. To gain further insight into the nature of this unprecedented C-C coupling during GOR, photoelectrochemical oxidation of intermediate oxidation products (glyceraldehyde and 1,3-dihydroxyacetone) and glycerol-1,3-13C2 was compared to that of standard glycerol. Photoelectrochemical GOR was also compared with electrochemical GOR on BiVO4 to interrogate whether light is critical for the observed C-C coupling. Results obtained from comprehensive control experiments revealed critical information about C-C cleavage and C-C coupling during GOR on BiVO4.

A Perovskite Photovoltaic Mini-Module-CsPbBr3 Photoelectrochemical Cell Tandem Device for Solar-Driven Degradation of Organic Compounds

Recently, halide perovskites have been widely explored for high-efficiency photocatalysis or photoelectrochemical (PEC) cells. Here, in order to make an efficient photoanode electrode for the degradation of pollutants, concretely 2-mercaptobenzothiazole (MBT), nanoscale cesium lead bromide (CsPbBr3) perovskite was directly formed on the surface of mesoporous titanium dioxide (meso-TiO2) film using a two-step spin-coating process. This photoelectrode recorded a photocurrent of up to 3.02 ± 0.03 mA/cm2 under standard AM 1.5G (100 mW/cm2) illumination through an optimization process such as introducing a thin aluminum oxide (Al2O3) coating layer. Furthermore, to supply high voltage for efficient oxidation of MBT without an external bias, we developed a new photovoltaic/PEC tandem system using a methylammonium lead iodide (MAPbI3) based mini-module consisting of three solar cells interconnected in series and confirmed its successful operation. This approach looks very promising due to its applicability to various PEC reactions.

Surface Hydroxylation during Water Splitting Promotes the Photoactivity of BiVO4(010) Surface by Suppressing Polaron-Mediated Charge Recombination

Polaron-based electron transport restricts the photoelectrochemical (PEC) water splitting efficiency of BiVO4. However, the location and dynamics of polarons are significantly dependent on the surface hydroxylation. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrated that hydroxylation of BiVO4(010) surface greatly alleviates the detrimental effect of oxygen-vacancy-induced electron polaron (EP). Surface hydroxylation stabilizes the EP at the surface to facilitate water splitting, makes the polaron a shallow localized state, and reduces the intensity of high-frequency V-O bond stretching vibrations. By decreasing the nonadiabatic coupling and decoherence time, the charge carrier lifetimes are extended by 1-3 orders of magnitude depending on the hydroxylation coverage. Our study not only reveals that the surface hydroxylation mitigated detrimental impacts of polarons in metal oxides but also provided valuable insights into the benign effect of intermediate species on the photocatalytic reactivity.

All Printed Photoanode/Photovoltaic Mini-Module for Water Splitting

Printing a large-area bismuth vanadate photoanode offers a promising approach for cost-effective photoelectrochemical (PEC) water splitting. However, the light absorption trade-off with charge transfer, as well as stability issues always lead to poor PEC efficiency. Here, the solution-processed recipe is advanced with BiI3 dopant for the printed deposition with controllable crystal growth. The resultant BiVO4 films prefer (001) orientation with nanorod feature on substrate, allowing a faster charge transfer and improved photocurrent. The BiVO4 photoanode in tandem with perovskite solar module delivers an operating photocurrent density of 5.88 mA cm-2 at zero bias in 3.11 cm2 active area under AM 1.5 G illumination, yielding a solar-to-hydrogen efficiency as high as 7.02% for unbiased water splitting. Equally important, the stability of the aged BiVO4 rods has been addressed to distinguish phase segregation at surface. The photocatalysis degradation composes of vanadium loss and Bi2 O3 enriching at the surface, opening a lid on the long-term stability of BiVO4 photoanodes.

Operando IR Optical Control of Localized Charge Carriers in BiVO4 Photoanodes

In photoelectrochemical cells (PECs) the photon-to-current conversion efficiency is often governed by carrier transport. Most metal oxides used in PECs exhibit thermally activated transport due to charge localization via the formation of polarons or the interaction with defects. This impacts catalysis by restricting the charge accumulation and extraction. To overcome this transport bottleneck nanostructuring, selective doping and photothermal treatments have been employed. Here we demonstrate an alternative approach capable of directly activating localized carriers in bismuth vanadate (BiVO4). We show that IR photons can optically excite localized charges, modulate their kinetics, and enhance the PEC current. Moreover, we track carriers bound to oxygen vacancies and expose their ∼10 ns charge localization, followed by ∼60 μs transport-assisted trapping. Critically, we demonstrate that localization is strongly dependent on the electric field within the device. While optical modulation has still a limited impact on overall PEC performance, we argue it offers a path to control devices on demand and uncover defect-related photophysics.

Surface Reconstruction and Passivation of BiVO4 Photoanodes Depending on the "Structure Breaker" Cs. JACS AU 2023;3:1851-1863. [PMID: 37502161 PMCID: PMC10369408 DOI: 10.1021/jacsau.3c00100] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Monoclinic BiVO₄ is one of the most promising photoanode materials for solar water splitting. The photoelectrochemical performance of a BiVO₄ photoanode could be significantly influenced by the noncovalent interactions of redox-inert metal cations at the photoanode-electrolyte interfaces, but this point has not been well investigated. In this work, we studied the Cs⁺-dependent surface reconstruction and passivation of BiVO₄ photoanodes. Owing to the "structure breaker" nature of Cs⁺, the Cs⁺ at the BiVO₄ photoanode-electrolyte interfaces participated in BiVO₄ surface photocorrosion to form a Cs⁺-doped bismuth vanadium oxide amorphous thin layer, which inhibited the continuous photocorrosion of BiVO₄ and promoted surface charge transfer and water oxidation. The resulting cocatalyst-free BiVO₄ photoanodes achieved 3.3 mA cm^-2 photocurrent for water oxidation. With the modification of FeOOH catalysts, the photocurrent at 1.23 V_RHE reached 5.1 mA cm^-2, and a steady photocurrent of 3.0 mA cm^-2 at 0.8 V_RHE was maintained for 30 h. This work provides new insights into the understanding of Cs⁺ chemistry and the effects of redox-inert cations at the electrode-electrolyte interfaces.

Electronic Characteristics, Stability and Water Oxidation Selectivity of High-Index BiVO4 Facets for Photocatalytic Application: A First Principle Study

Some high-index facets of BiVO₄, such as (012), (210), (115), (511), (121), (132) and (231), exhibit much better photocatalytic performance than conventional (010) and (110) surfaces for water splitting. However, the detailed mechanisms and stability of improved photocatalytic performance for these high-index BiVO₄ surfaces are still not clear, which is important for designing photocatalysts with high efficiency. Here, based on first principle calculation, we carried out a systematic theoretical research on BiVO₄ with different surfaces, especially high-index facets. The results show that all of the high-index facets in our calculated systems show an n-type behavior, and the band edge positions indicate that all of the high-index facets have enough ability to produce O₂ without external bias. Electronic structures, band alignments and formation enthalpy indicate that (012), (115) and (132) could be equivalent to (210), (511) and (231), respectively, in the calculation. Oxidation and reduction potential show that only (132)/(231) is stable without strongly oxidative conditions, and the Gibbs free energy indicates that (012)/(210), (115)/(511), (121) and (132)/(231) have lower overpotential than (010) and (110). Our calculation is able to unveil insights into the effects of the surface, including electronic structures, overpotential and stability during the reaction process.

Efficient BiVO4-Based Photoanode with Chemically Precipitated Hierarchical Cobalt Borate (Co-Bi) Oxygen Evolution Reaction Catalysts

The integration of cobalt borate OER catalysts with electrodeposited BiVO₄-based photoanodes through a simple drop casting technique was shown to provide an improvement of the photoelectrochemical performance of electrodes under simulated solar light. Catalysts were obtained by chemical precipitation mediated by NaBH₄ at room temperature. Scanning electron microscopy (SEM) investigation of precipitates showed a hierarchical structure with globular features covered in nanometric thin sheets providing a large active area, whereas X-ray diffraction (XRD) and Raman spectroscopy highlighted their amorphous structure. The photoelectrochemical behavior of samples was investigated by linear scan voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. The amount of particles loaded onto BiVO₄ absorbers was optimized by variation of the drop cast volume. The enhancement of photocurrent generation by Co-B_i-decorated electrodes with respect to bare BiVO₄ was observed with an increase from 1.83 to 3.65 mA/cm² at 1.23 V vs RHE under AM 1.5 simulated solar light, corresponding to a charge transfer efficiency of 84.6%. The calculated maximum applied bias photon-to-current efficiency (ABPE) value for optimized samples was 1.5% at 0.5 V applied bias. Under constant illumination at 1.23 V vs RHE, a depletion of photoanode performances was observed within an hour, likely due to the detachment of the catalyst from the electrode surface.

Dual modification of BiVO4 photoanode for synergistically boosting photoelectrochemical water splitting

Bismuth vanadate (BiVO₄) is a promising nanomaterial for photoelectrochemical (PEC) water oxidation. However, the serious charge recombination and sluggish water oxidation kinetics limit its performance. Herein, an integrated photoanode was successfully constructed by modifying BiVO₄ (BV) with In₂O₃ (In) layer and further decorating amorphous FeNi hydroxides (FeNi). The BV/In/FeNi photoanode exhibited a remarkable photocurrent density of 4.0 mA cm^-2 at 1.23 V_RHE, which is approximately 3.6 times larger than that of pure BV. And the water oxidation reaction kinetics has an over 200% increased. This improvement was mainly because the formation of BV/In heterojunction inhibited charge recombination, and the decoration of cocatalyst FeNi facilitated the water oxidation reaction kinetics and accelerated hole transfer to electrolyte. Our work provides another possible route to develop high-efficiency photoanodes for practical applications in solar conversion.

BiVO4 As a Sustainable and Emerging Photocatalyst: Synthesis Methodologies, Engineering Properties, and Its Volatile Organic Compounds Degradation Efficiency

Bismuth vanadate (BiVO₄) is one of the best bismuth-based semiconducting materials because of its narrow band gap energy, good visible light absorption, unique physical and chemical characteristics, and non-toxic nature. In addition, BiVO₄ with different morphologies has been synthesized and exhibited excellent visible light photocatalytic efficiency in the degradation of various organic pollutants, including volatile organic compounds (VOCs). Nevertheless, the commercial scale utilization of BiVO₄ is significantly limited because of the poor separation (faster recombination rate) and transport ability of photogenerated electron-hole pairs. So, engineering/modifications of BiVO₄ materials are performed to enhance their structural, electronic, and morphological properties. Thus, this review article aims to provide a critical overview of advanced oxidation processes (AOPs), various semiconducting nanomaterials, BiVO₄ synthesis methodologies, engineering of BiVO₄ properties through making binary and ternary nanocomposites, and coupling with metals/non-metals and metal nanoparticles and the development of Z-scheme type nanocomposites, etc., and their visible light photocatalytic efficiency in VOCs degradation. In addition, future challenges and the way forward for improving the commercial-scale application of BiVO₄-based semiconducting nanomaterials are also discussed. Thus, we hope that this review is a valuable resource for designing BiVO₄-based nanocomposites with superior visible-light-driven photocatalytic efficiency in VOCs degradation.

Boosting photoelectrochemical water splitting of bismuth vanadate photoanode via novel co-catalysts of amorphous manganese oxide with variable valence states

Bismuth vanadate (BVO) is a promising photoanode while suffers from sluggish oxygen evolution kinetics. Herein, an ultra-thin manganese oxide (MnO_x) is selected as co-catalyst to modify the surface of BVO photoanode by a facile spray pyrolysis method. The photoelectrochemical measurements demonstrate that surface charge transport efficiency (η_surface) of MnO_x modified BVO photoanode (BVO/MnO_x) is strikingly increased from 6.7 % to 22.3 % at 1.23 V_RHE (reversible hydrogen electrode (V_RHE)). Moreover, the η_surface can be further boosted to 51.3 % at 1.23 V_RHE after applying Ar plasma on the BVO/MnO_x sample_, which is around 7 times higher comparing with that of pristine BVO samples. Additional characterizations reveal that the remarkable PEC performance of the Ar-plasma treated BVO/MnO_x photoanode (BVO/MnO_x/Ar plasma) could be attributed to the increased charge carrier density, extended carrier lifetime and additional exposed Mn³⁺ active sites on the BVO surface. This investigation could provide a new understanding for the design of BVO photoanode with superior PEC performance based on the modification of MnO_x and plasma surface treatment.

Ni-Doped BiVO4 photoanode for efficient photoelectrochemical water splitting

BiVO₄ (BVO) based photoanode is one of the most mega-potential materials for solar water splitting while suffers from poor charge transfer and separation efficiency limit its practical application. Herein, FeOOH/Ni-BiVO₄ photoanode synthesized by the facile wet chemical method were investigated for improved charge transport and separation efficiency. The photoelectrochemical (PEC) measurements demonstrate that the water oxidation photocurrent density can reach as high as 3.02 mA cm^-2 at 1.23 V vs RHE, and the surface separation efficiency can be boosted to 73.3 %, which increases around 4 times comparing with that of pure sample. Further depth studies showed that the Ni doping can effectively promote hole transport/trapping and introduce more active sites for the oxidation of water, while FeOOH co-catalyst could passivate the Ni-BiVO₄ photoanode surface. This work provides a model for the design of BiVO₄-based photoanodes with combined thermodynamic and kinetic advantages.

The impacts of dopants on the small polaron mobility and conductivity in hematite - the role of disorder

Hematite (α-Fe₂O₃) is a promising transition metal oxide for various energy conversion and storage applications due to its advantages of low cost, high abundance, and good chemical stability. However, its low carrier mobility and electrical conductivity have hindered the wide application of hematite-based devices. Fundamentally, this is mainly caused by the formation of small polarons, which show conduction through thermally activated hopping. Atomic doping is one of the most promising approaches for improving the electrical conductivity in hematite. However, its impact on the carrier mobility and electrical conductivity of hematite at the atomic level remains to be illusive. In this work, through a kinetic Monte-Carlo sampling approach for diffusion coefficients combined with carrier concentrations computed under charge neutrality conditions, we obtained the electrical conductivity of the doped hematite. We considered the contributions from individual Fe-O layers, given that the in-plane carrier transport dominates. We then studied how different dopants impact the carrier mobility in hematite using Sn, Ti, and Nb as prototypical examples. We found that the carrier mobility change is closely correlated with the local distortion of Fe-Fe pairs, i.e. the more stretched the Fe-Fe pairs are compared to the pristine systems, the lower the carrier mobility will be. Therefore, elements which limit the distortion of Fe-Fe pair distances from pristine are more desired for higher carrier mobility in hematite. The calculated local structure and pair distribution functions of the doped systems have remarkable agreement with the experimental EXAFS measurements on hematite nanowires, which further validates our first-principles predictions. Our work revealed how dopants impact the carrier mobility and electrical conductivity of hematite and provided practical guidelines to experimentalists on the choice of dopants for the optimal electrical conductivity of hematite and the performance of hematite-based devices.

Dynamic semiconductor-electrolyte interface for sustainable solar water splitting over 600 hours under neutral conditions

Photoelectrochemical (PEC) water splitting that functions in pH-neutral electrolyte attracts increasing attention to energy demand sustainability. Here, we propose a strategy to in situ form a NiB layer by tuning the composition of the neutral electrolyte with the additions of nickel and borate species, which improves the PEC performance of the BiVO₄ photoanode. The NiB/BiVO₄ exhibits a photocurrent density of 6.0 mA cm^-2 at 1.23 V_RHE with an onset potential of 0.2 V_RHE under 1 sun illumination. The photoanode displays a photostability of over 600 hours in a neutral electrolyte. The additive of Ni²⁺ in the electrolyte, which efficiently inhibits the dissolution of NiB, can accelerate the photogenerated charge transfer and enhance the water oxidation kinetics. The borate species with B─O bonds act as a promoter of catalyst activity by accelerating proton-coupled electron transfer. The synergy effect of both species suppresses the surface charge recombination and inhibits the photocorrosion of BiVO₄.

Flow cell for operando X-ray photon-in-photon-out studies on photo-electrochemical thin film devices

Background: Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics, are not yet fully understood. X-ray absorption spectroscopy (XAS), and particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. It remains a challenge to establish a standardized thin film exchange procedure and concurrently record high-quality photoelectrochemical and X‑ray absorption spectroscopy data that is unperturbed by bubble formation. Here we address and overcome these instrumental limitations for photoelectrochemical operando HERFD-XAS. Methods: We constructed a novel operando photo-electro-chemical cell by computer numerical control milling, guided by the materials' X‑ray and visible light absorption properties to optimize signal detection. To test the cell's functionality, semiconducting thin film photoelectrodes have been fabricated via solution deposition and their photoelectrochemical responses under simulated solar light were studied using a commercial potentiostat in a three-electrode configuration during HERFD-XAS experiments at a synchrotron. Results: We demonstrate the cell's capabilities to measure and control potentiostatically and in open‑circuit, to detect X‑ray signals unperturbed by bubbles and to fluently exchange different thin film samples by collecting high-resolution Fe K-edge spectra of hematite ( α -Fe 2O 3) and ferrite thin film ( MFe 2O 4, M= Zn, Ni) photoelectrodes during water oxidation. Conclusions: Our cell establishes a measurement routine that will provide experimental access of photo-electro-chemical operando HERFD-XAS experiments to a broader scientific community, particularly due to the ease of sample exchange. We believe to enable a broad range of experiments which acquired fundamental insights will spur further photoelectrochemical research and commercialization of water splitting technologies.

Bimolecular Self-Trapped Exciton Formation in Bismuth Vanadate

Bismuth vanadate (BiVO₄) is a promising photoanode material for solar-driven water splitting, and knowledge of the photocarrier dynamics in BiVO₄ could offer guidance to propel the development of the photoanode performance. Herein, we uncovered the nature of various photogenerated transient species in BiVO₄ and extracted their respective dynamics. We found spectral and dynamic evidence that the electrons in the conduction band collapsed into severely localized small electron polarons on a subpicosecond time scale, while the holes in the valence band remained delocalized and accounted for the photoconductivity. In the following tens to hundreds of picoseconds, the electron polaron captured the hole to form a self-trapped exciton via a bimolecular reaction mechanism, and in consequence, the hole was immobilized. Our finding suggests that exciton dissociation strategies should be taken into account in the design of the BiVO₄-based water-splitting applications in order to enhance charge transport and suppress charge recombination.

Vacancy defect engineered BiVO4 with low-index surfaces for photocatalytic application: a first principles study

BiVO₄ has been widely investigated as a photocatalyst material for water splitting due to its outstanding photocatalytic properties. In order to further improve its photocatalytic efficiency, it is necessary to conduct an in-depth study of improvement strategies, such as defect engineering. By focusing on the (001) and (011) surfaces, we carried out a systematic theoretical research on pristine and defective systems, including Bi, V and O vacancies. Based on density functional theory (DFT), the electronic properties, band alignments and Gibbs free energy of pristine and defective BiVO₄ have been analyzed. The electronic structures of the (001) and (011) surfaces show different band gaps, and O vacancies make the BiVO₄ become an n-type semiconductor, while Bi and V vacancies tend to form a p-type semiconductor. Moreover, the band edge positions indicate that holes are indeed easily accumulated on the (011) surface while electrons tend to accumulate on (001). However, the (011) surface with Bi and V vacancies does not have enough oxidation potential to oxidize water. The reaction free energy shows that O and Bi vacancies could lower the overpotential to some extent.

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

Metal oxides are promising for photoelectrochemical (PEC) water splitting due to their robustness and low cost. However, poor charge carrier transport impedes their activity, particularly at low-bias voltage. Here we demonstrate the unusual effectiveness of phosphorus doping into bismuth vanadate (BiVO₄) photoanode for efficient low-bias PEC water splitting. The resulting BiVO₄ photoanode shows a separation efficiency of 80% and 99% at potentials as low as 0.6 and 1.0 V_RHE, respectively. Theoretical simulation and experimental analysis collectively verify that the record performance originates from the unique phosphorus-doped BiVO₄ configuration with concurrently mediated carrier density, trap states, and small polaron hopping. With NiFeO_x cocatalyst, the BiVO₄ photoanode achieves an applied bias photon-to-current efficiency of 2.21% at 0.6 V_RHE. The mechanistic understanding of the enhancement of BiVO₄ properties provides key insights in trap state passivation and polaron hopping for most photoactive metal oxides.

Self-Doped Carbon Dots Decorated TiO2 Nanorods: A Novel Synthesis Route for Enhanced Photoelectrochemical Water Splitting

Herein, we have successfully prepared self-doped carbon dots with nitrogen elements (NCD) in a simple one-pot hydrothermal carbonization method, using L-histidine as a new precursor. The effect of as-prepared carbon dots was studied for photoelectrochemical (PEC) water splitting by decorating NCDs upon TiO2 nanorods systematically by changing the loading time from 2 h to 8 h (TiO2@NCD2h, TiO2@NCD4h, TiO2@NCD6h, and TiO2@NCD8h). The successful decorating of NCDs on TiO2 was confirmed by FE-TEM and Raman spectroscopy. The TiO2@NCD4h has shown a photocurrent density of 2.51 mA.cm−2, 3.4 times higher than the pristine TiO2. Moreover, TiO2@NCD4h exhibited 12% higher applied bias photon-to-current efficiency (ABPE) than the pristine TiO2. The detailed IPCE, Mott–Schottky, and impedance (EIS) analyses have revealed the enhanced light harvesting property, free carrier concentration, charge separation, and transportation upon introduction of the NCDs on TiO2. The obtained results clearly portray the key role of NCDs in improving the PEC performance, providing a new insight into the development of highly competent TiO2 and NCDs based photoanodes for PEC water splitting.

Photoelectrochemical glycerol oxidation on Mo-BiVO4 photoanodes shows high photocharging current density and enhanced H2 evolution

Mo-doped BiVO₄'s lower efficiency can be attributed in part to exciton recombination losses. Recombination losses during photoelectrochemical water oxidation can be eliminated by using glycerol as a hole acceptor. This results in an enhanced photocurrent density. In this research, we present the synthesis of a Mo-doped BiVO₄ photoelectrode with a greater photocurrent density than a traditional pristine photoanode system. Increased photon exposure duration in the presence of glycerol leads to 8 mA cm^-2 increase in photocurrent density due to the creation of a capacitance layer and a decrease in charge transfer resistance on the photoelectrode in a neutral-phosphate buffer solution thus confirming the photo charging effect. Glycerol photooxidation improves the photoelectrode's rate of hydrogen evolution. Research into the effects of electrolyte and electrode potential on photoelectrodes has revealed that when the applied potential increases, the light absorbance behaviour changes following its absorption distribution over the applied potential. Under a transmission electron microscope (TEM), a unique dynamical crystal fringe pattern is found in the nanoparticles scratched from the photoelectrode.

Metal Complex/ZnS-Modified Ni Foam as Magnetically Stirrable Photocatalysts: Roles of Redox Mediators and Carrier Dynamics Monitored by Operando Synchrotron X-ray Spectroscopy

Magnetically stirrable photocatalysts binding the ZnS-decorated Ni foam with the metal complex cocatalyst as a redox mediator and light-absorbing composition were investigated. Loading metal complex can improve light absorption, surface hydrophilicity, interfacial charge migration, and H₂ production activity. The variation of the metal valences of the composite photocatalysts in an operando environment (with sacrificial agent solution) with and without light irradiation was investigated by X-ray absorption near-edge structure (XANES) spectra and Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra to monitor the charge carrier dynamics of photocatalysis and explain how the macrocyclic Cu complex (CuC) acted as a redox mediator better than the Ni complex. The smaller valence difference of copper valence in ZS/CuC for dark and light states revealed that the Cu complex facilitates a reversible electron transfer between the ZnS photocatalyst and H⁺. Loading the Cu complex can improve the separation of photogenerated carriers by the redox couple of complexes, leading to a significantly improved photocatalytic H₂ production activity of 8150 μmol h^-1 g^-1. The reactants can flow through these magnetically stirrable Ni foam-based photocatalysts by magnetic-field-driven stirring, which improves the contact between photocatalysts and the sacrificial agents. The operando synchrotron provides new insights for understanding the roles of redox mediators.

Influence of Excess Charge on Water Adsorption on the BiVO4(010) Surface

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO₄(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO₄(010) surface stabilizes the formation of a small electron polaron on the VO₄ tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V⁴⁺ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Boosting the Performance of BiVO4 Photoanodes by the Simultaneous Introduction of Oxygen Vacancies and Cocatalyst via Photoelectrodeposition

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy, but the efficiency is limited by severe charge recombination especially in photoanodes. Herein, to reduce the charge recombination in the bulk phase and at the surface of the BiVO₄ photoanodes, oxygen vacancy introduction and cocatalyst loading were realized simultaneously by one-step photocathode deposition. A unique re-BiVO₄/FeOOH photoanode was obtained by the photocathodic reduction of BiVO₄ in an electrolyte containing Fe³⁺, where the oxygen vacancies were introduced during the reduction process and the deposition of the FeOOH cocatalyst on the surface was induced by the generated OH^-. When used for PEC water oxidation, the obtained re-BiVO₄/FeOOH photoanode achieved an excellent PEC performance with a photocurrent density of 5.35 mA/cm² at 1.23 V versus RHE under AM 1.5G illumination, which was considerably higher than those for the pristine BiVO₄ photoanode (2.88 mA/cm²) and the re-BiVO₄ photoanode obtained by photocathodic reduction without Fe³⁺ (4.32 mA/cm²). After further modification with a cobalt silicate (Co-Sil) cocatalyst, the resultant re-BiVO₄/FeOOH/Co-Sil photoanode exhibited a photocurrent density as high as 6.10 mA/cm² at 1.23 V versus RHE and a remarkable applied bias photon-to-current efficiency of 2.25%. The outstanding performance of the re-BiVO₄/FeOOH/Co-Sil photoanode could be attributed to the coexistence of plenty of oxygen vacancies in BiVO₄ reducing recombination of photogenerated carriers, the FeOOH cocatalyst interlayer as a hole-transport layer, and the outer Co-Sil cocatalyst with a high activity toward oxygen evolution. This work may open a new avenue toward multifunctional modifications of photoanode systems for efficient solar conversion.

Depleted Oxygen Defect State Enhancing Tungsten Trioxide Photocatalysis: A Quantum Dynamics Perspective

Oxygen vacancies generally create midgap states in transition metal oxides, which are expected to decrease the photoelectrochemical water-splitting efficiency. Recent experiments defy this expectation but leave the mechanism unclear. Focusing on the photoanode WO₃ as a prototypical system, we demonstrate using nonadiabatic molecular dynamics that an oxygen vacancy suppresses nonradiative electron-hole recombination, because the defect acts as an electron reservoir instead of a recombination center. The occupied midgap electrons prefer to be populated a priori compared to the band edge transition because of a larger transition dipole moment, converting to depleted/unoccupied trap states that rapidly accept conduction band electrons and then cause trap-assisted recombination by impeding the bandgap recombination regardless of oxygen vacancy configurations. The reported results provide a fundamental understanding of the "realistic" role of the oxygen vacancies and their influence on charge-phonon dynamics and carrier lifetime. The study generates valuable insights into the design of high-performance transition metal oxide photocatalysts.

Elucidating the Role of Hypophosphite Treatment in Enhancing the Performance of BiVO4 Photoanode for Photoelectrochemical Water Oxidation

Slow water oxidation kinetics and poor charge transport restrict the development of efficient BiVO4 photoanodes for photoelectrochemical (PEC) water splitting. Oxygen vacancy as an effective strategy can significantly enhance charge transport and improve conductivity in semiconductor photoanodes. Herein, we obtained BiVO4 photoanodes with appropriate oxygen vacancy by treating them with hypophosphite, which significantly improved the PEC performance. The synthesized photoanode exhibits a remarkable photocurrent density of 3.37 mA/cm2 at 1.23 V vs reversible hydrogen electrode with excellent stability. Interestingly, the performance improvement mainly originates from the oxygen vacancy rather than P doping. Our study provides insights in understanding the role of oxygen vacancy in PEC water splitting and strategies for designing more efficient photoelectrodes.

Activating a Semiconductor-Liquid Junction via Laser-Derived Dual Interfacial Layers for Boosted Photoelectrochemical Water Splitting

The semiconductor-liquid junction (SCLJ), the dominant place in photoelectrochemical (PEC) catalysis, determines the interfacial activity and stability of photoelectrodes, whcih directly affects the viability of PEC hydrogen generation. Though efforts dedicated in past decades, a challenge remains regarding creating a synchronously active and stable SCLJ, owing to the technical hurdles of simultaneously overlaying the two advantages. The present work demonstrates that creating an SCLJ with a unique configuration of the dual interfacial layers can yield BiVO₄ photoanodes with synchronously boosted photoelectrochemical activity and operational stability, with values located at the top in the records of such photoelectrodes. The bespoke dual interfacial layers, accessed via grafting laser-generated carbon dots with phenolic hydroxyl groups (LGCDs-PHGs), are experimentally verified effective, not only in generating the uniform layer of LGCDs with covalent anchoring for inhibited photocorrosion, but also in activating, respectively, the charge separation and transfer in each layer for boosted charge-carrier kinetics, resulting in FeNiOOH-LGCDs-PHGs-MBVO photoanodes with a dual configuration with the photocurrent density of 6.08 mA cm^-2 @ 1.23 V_RHE , and operational stability up to 120 h @ 1.23 V_RHE . Further work exploring LGCDs-PHGs from catecholic molecules warrants the proposed strategy as being a universal alternative for addressing the interfacial charge-carrier kinetics and operational stability of semiconductor photoelectrodes.

Engineering MoOx /MXene Hole Transfer Layers for Unexpected Boosting of Photoelectrochemical Water Oxidation

The development of semiconductor photoanodes is of great practical interest for the realization of photoelectrochemical (PEC) water splitting. Herein, MXene quantum dots (MQD) were grafted on a BiVO₄ substrate, then a MoO_x layer by combining an ultrathin oxyhydroxide oxygen evolution cocatalyst (OEC) was constructed as an integrated photoanode. The OEC/MoO_x /MQD/BiVO₄ array not only achieves a current density of 5.85 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode (vs. RHE), but also enhances photostability. From electrochemical analysis and density functional theory calculations, high PEC performance is ascribed to the incorporation of MoO_x /MQD as hole transfer layers, retarding charge recombination, promoting hole transfer and accelerating water splitting kinetics. This proof-of-principle work not only demonstrates the potential utilization of hole transfer layers, but also sheds light on rational design and fabrication of integrated photoanodes for feasible solar energy conversion.

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO₄ ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO₄ is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO₄ suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO₄ photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO₄ is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO₄ are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

Inter-facet junction effects on particulate photoelectrodes

Particulate semiconductor photocatalysts are paramount for many solar energy conversion technologies. In anisotropically shaped photocatalyst particles, the different constituent facets may form inter-facet junctions at their adjoining edges, analogous to lateral two-dimensional (2D) heterojunctions or pseudo-2D junctions made of few-layer 2D materials. Using subfacet-level multimodal functional imaging, we uncover inter-facet junction effects on anisotropically shaped bismuth vanadate (BiVO₄) particles and identify the characteristics of near-edge transition zones on the particle surface, which underpin the whole-particle photoelectrochemistry. We further show that chemical doping modulates the widths of such near-edge surface transition zones, consequently altering particles' performance. Decoupled facet-size scaling laws further translate inter-facet junction effects into quantitative particle-size engineering principles, revealing surprising multiphasic size dependences of whole-particle photoelectrode performance. The imaging tools, the analytical framework and the inter-facet junction concept pave new avenues towards understanding, predicting and engineering (opto)electronic and photoelectrochemical properties of faceted semiconducting materials, with broad implications in energy science and semiconductor technology.