Efficient BiVO4 Photoanodes by Postsynthetic Treatment: Remarkable Improvements in Photoelectrochemical Performance from Facile Borate Modification

Defect-Engineered Tin Disulfide Nanocarriers as "Precision-Guided Projectile" for Intensive Synergistic Therapy

Nanoformulations with endogenous/exogenous stimulus-responsive characteristics show great potential in tumor cell elimination with minimal adverse effects and high precision. Herein, an intelligent nanotheranostic platform (denoted as TPZ@Cu-SnS2-x /PLL) for tumor microenvironment (TME) and near-infrared light (NIR) activated tumor-specific therapy is constructed. Copper (Cu) doping and the resulting sulfur vacancies can not only improve the response range of visible light but also improve the separation efficiency of photogenerated carriers and increase the carrier density, resulting in the ideal photothermal and photodynamic performance. Density functional theory calculations revealed that the introduction of Cu and resulting sulfur vacancies can induce electron redistribution, achieving favorable photogenerated electrons. After entering cells through endocytosis, the TPZ@Cu-SnS2-x /PLL nanocomposites show the pH responsivity property for the release of the TPZ selectively within the acidic TME, and the released Cu2+ can first interact with local glutathione (GSH) to deplete GSH with the production of Cu+ . Subsequently, the Cu+ -mediated Fenton-like reaction can decompose local hydrogen peroxide into hydroxyl radicals, which can also be promoted by hyperthermia derived from the photothermal effect for tumor cell apoptosis. The integration of photoacoustic/computed tomography imaging-guided NIR phototherapy, TPZ-induced chemotherapy, and GSH-elimination/hyperthermia enhanced chemodynamic therapy results in synergistic therapeutic outcomes without obvious systemic toxicity in vivo.

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.

Scaling up BiVO4 Photoanodes on Porous Ti Transport Layers for Solar Hydrogen Production

Commercialization of photoelectrochemical (PEC) water-splitting devices requires the development of large-area, low-cost photoanodes with high efficiency and photostability. Herein, we address these challenges by using scalable fabrication techniques and porous transport layer (PTLs) electrode supports. We demonstrate the deposition of W-doped BiVO4 on Ti PTLs using successive-ionic-layer-adsorption-and-reaction methods followed by boron treatment and chemical bath deposition of NiFeOOH co-catalyst. The use of PTLs that facilitate efficient mass and charge transfer allowed the scaling of the photoanodes (100 cm2 ) while maintaining ~90 % of the performance obtained with 1 cm2 photoanodes for oxygen evolution reaction, that is, 2.10 mA cm-2 at 1.23 V vs. RHE. This is the highest reported performance to date. Integration with a polycrystalline Si PV cell leads to bias-free water splitting with a stable photocurrent of 208 mA for 6 h and 2.2 % solar-to-hydrogen efficiency. Our findings highlight the importance of photoelectrode design towards scalable PEC device development.

Ultrasound-Driven Non-Metallic Fenton-Active Center Construction for Extensive Chemodynamic Therapy

Chemodynamic therapy (CDT) is an emerging tumor microenvironment-responsive cancer therapeutic strategy based on Fenton/Fenton-like reactions. However, the effectiveness of CDT is subject to the slow kinetic rate and non-homogeneous distribution of H2 O2 . In this study, a conceptual non-metallic "Fenton-active" center construction strategy is proposed to enhance CDT efficiency using Bi0.44 Ba0.06 Na0.5 TiO2.97 (BNBT-6) nanocrystals. The separated charge carriers under a piezoelectric-induced electric field synchronize the oxidation of H2 O and reduction of H2 O2 , which consequently increases hydroxyl radical (·OH) yield even under low H2 O2 levels. Moreover, acceptor doping induces electron-rich oxygen vacancies to facilitate the dissociation of H2 O2 and H2 O and further promote ·OH generation. In vitro and in vivo experiments demonstrate that BNBT-6 induces extensive intracellular oxidative stress and enhances cell-killing efficiency by activating necroptosis in addition to the conventional apoptotic pathway. This study proposes a novel design approach for nanomaterials used in CDT and presents a new treatment strategy for apoptosis-resistant tumors.

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.

Pd@Bi2Ru2O7/BiVO4 Z-Scheme Heterojunction Nanocomposite Photocatalyst for the Degradation of Trichloroethylene

Photocatalytic degradation of chlorinated persistent organic pollutants is a very challenging process due to the high redox potential of the C-Cl bond that requires wide band gap catalysts that are activated under UV light. Designing a Z-scheme heterojunction between visible light-activated metal oxides with compatible band gaps enables these redox potentials. Herein, we report the design of a pyrochlore/Aurivillius Z-scheme heterojunction to enhance the photocatalytic activity of BiVO4 for the degradation of trichloroethylene. We prepared Bi2Ru2O7/BiVO4 heterostructured photocatalysts by a controlled hydrothermal approach. Upon optimizing the Bi2Ru2O7 ratio to 1.0 wt %, the heterostructured photocatalyst demonstrated enhanced activity in the degradation of trichloroethylene (TCE) under simulated sunlight irradiation compared to bare BiVO4 and Bi2Ru2O7, respectively. Decorating the surface of the catalyst with palladium nanodomains to create the Pd@Bi2Ru2O7/BiVO4 nanocomposite showed a substantial increase in the photocatalytic degradation of TCE. The material characterization indicated that the architecture of the material provides a synergy of enhancing the redox potential of the photocatalyst and improving the charge carrier dynamics. Furthermore, the photoelectrochemical characterization confirmed that the dual heterojunctions in the Pd@Bi2Ru2O7/BiVO4 nanocomposite resulted in improved interfacial charge carrier transfer and enhanced the electron/hole separation efficiency compared to the nonpalladized catalysts. This work provides a promising approach for band gap engineering of visible light photocatalysts for the degradation of halogenated persistent organic pollutants.

Insight into the Key Restriction of BiVO4 Photoanodes Prepared by Pyrolysis Method for Scalable Preparation

Bismuth Vanadate (BiVO4 ) photoanode has been popularly investigated for promising solar water oxidation, but its intrinsic performance has been greatly retarded by the direct pyrolysis method. Here we insight the key restriction of BiVO4 prepared by metal-organic decomposition (MOD) method. It is found that the evaporation of vanadium during the pyrolysis tends to cause a substantial phase impurity, and the unexpected few tetragonal phase inhibits the charge separation evidently. Consequently, suitably excessive vanadium precursor was adopted to eliminate the phase impurity, based on which the obtained intrinsic BiVO4 photoanode could exhibit photocurrent density of 4.2 mA cm-2 at 1.23 VRHE under AM 1.5 G irradiation, as comparable to the one fabricated by the currently popular two-step electrodeposition method. Furthermore, the excellent performance can be maintained on the enlarged photoanode (25 cm2 ), demonstrating the advantage of MOD method in scalable preparation. Our work provides new insight and highlights the glorious future of MOD method for the design of scale-up efficient BiVO4 photoanode.

Bismuth Vanadate and 3D Graphene Composite Photoanodes for Enhanced Photoelectrochemical Oxidation of Water

Bismuth vanadate (BiVO4) has been one of the most promising photoanodes for the photoelectrochemical (PEC) water oxidation process. Efforts are still on to overcome the drawbacks of this photoanode to enhance the catalytic efficiency and improve the stability. In the present work, three-dimensional graphene (3D-G) was incorporated inside the BiVO4 matrix, primarily to improve the conductivity of the material. The photoanodes are fabricated with the incorporation of a SnO2 heterojunction and application of cobalt borate (Co-Bi) as a cocatalyst. The incorporation of 3D-G has enhanced the photocurrent from 0.72 o 1.21 mA cm-2 in ITO/SnO2/BiVO4 and ITO/SnO2/3D-G-BiVO4 materials; the photocurrent has been improved from 0.89 to 1.52 mA cm-2 in ITO/SnO2/BiVO4/Co-Bi and ITO/SnO2/3D-G-BiVO4. Semiconductor properties are evaluated from the Mott-Schottky measurements, and the charge transfer and transport kinetics of the PEC process are measured from several photoelectrochemical investigations. Both the charge transport and the charge transfer efficiencies are enhanced upon inclusion of 3D-G into the catalyst system. The lifetime of the charge carrier is observed to be increased. The decrease in the decay kinetics of the holes, enhancement in the open-circuit photovoltage (OCPV), and the resulting modulation of the surface states are responsible for the enhancement in the surface charge transfer process due to the inclusion of 3D-G into the catalytic system. Therefore, the additional role of 3D-G in the modulation of the surface states and release of the Fermi level pinning has made the band alignment between the semiconductor and the analyte better, which resulted in enhanced catalytic performance in the photoelectrochemical oxidation of water.

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.

Surface Oxygen Species in Metal Oxide Photoanodes for Solar Energy Conversion

Converting and storing solar energy directly as chemical energy through photoelectrochemical devices are promising strategies to replace fossil fuels. Metal oxides are commonly used as photoanode materials, but they still encounter challenges such as limited light absorption, inefficient charge separation, sluggish surface reactions, and insufficient stability. The regulation of surface oxygen species on metal oxide photoanodes has emerged as a critical strategy to modulate molecular and charge dynamics at the reaction interface. However, the precise role of surface oxygen species in metal oxide photoanodes remains ambiguous. The review focuses on elucidating the formation and regulation mechanisms of various surface oxygen species in metal oxides, their advantages and disadvantages in photoelectrochemical reactions, and the characterization methods employed to investigate them. Additionally, the article discusses emerging opportunities and potential hurdles in the regulation of surface oxygen species. By shedding light on the significance of surface oxygen species, this review aims to advance our understanding of their impact on metal oxide photoanodes, paving the way for the design of more efficient and stable photoelectrochemical devices.

Nanoarchitectonics on Z-scheme and Mott-Schottky heterostructure for photocatalytic water oxidation via dual-cascade charge-transfer pathways

The bottleneck for water splitting to generate hydrogen fuel is the sluggish oxidation of water. Even though the monoclinic-BiVO₄ (m-BiVO₄)-based heterostructure has been widely applied for water oxidation, carrier recombination on dual surfaces of the m-BiVO₄ component have not been fully resolved by a single heterojunction. Inspired by natural photosynthesis, we established an m-BiVO₄/carbon nitride (C₃N₄) Z-scheme heterostructure based on the m-BiVO₄/reduced graphene oxide (rGO) Mott-Schottky heterostructure, constructing the face-contact C₃N₄/m-BiVO₄/rGO (CNBG) ternary composite to remove excessive surface recombination during water oxidation. The rGO can accumulate photogenerated electrons from m-BiVO₄ through a high conductivity region over the heterointerface, with the electrons then prone to diffuse along a highly conductive carbon network. In an internal electric field at the heterointerface of m-BiVO₄/C₃N₄, the low-energy electrons and holes are rapidly consumed under irradiation. Therefore, spatial separation of electron-hole pairs occurs, and strong redox potentials are maintained by the Z-scheme electron transfer. These advantages endow the CNBG ternary composite with over 193% growth in O₂ yield, and a remarkable rise in ·OH and ·O₂^- radicals, compared to the m-BiVO₄/rGO binary composite. This work shows a novel perspective for rationally integrating Z-scheme and Mott-Schottky heterostructures in the water oxidation reaction.

Effect of acid treatment on boosting the photoelectrochemical performance of doped and codoped α-Fe2O3 photoanodes

Acid treatment of Ti-doped α-Fe₂O₃ photoanode can reduce the onset potential and promote the photocurrent density for photoelectrochemical (PEC) water splitting reaction. However, the inner mechanism of how this occurs has not yet been clarified. This report compares the effect of HCl hydrothermal treatment on α-Fe₂O₃ photoanodes doped with Ge, Pt, Ti, and Sn or codoped with TiGe, TiPt, and TiSn. The findings show that the promotion effect of HCl hydrothermal treatment was far less significant on the Ge-, Pt-, and Sn-doped α-Fe₂O₃ than on the Ti-doped one. In contrast, the codoped photoanodes could realize a lift in the photocurrent of up to 39% at 1.23 V_RHE (versus the reversible hydrogen electrode) and a reduction in the potential onset by ∼60 mV after HCl hydrothermal treatment. Anatase TiO₂ was detected by Raman spectroscopy on the Ti-doped α-Fe₂O₃ with adequate treatment in HCl solution. Thus, the performance promotion by acid treatment was ascribed to the surface-concentrated Ti-O bonds acting as a passivation layer that could increase the charge-capture capacity and reduce the charge-transfer resistance, as demonstrated by the potential-modulated electrochemical impedance spectroscopy results. HCl treatment of the in situ-doped α-Fe₂O₃ and an excessive treatment time for the ex situ-doped α-Fe₂O₃ caused an inhibition in the PEC performance, which could be attributed to the adverse effect of lattice defects induced by acid corrosion. The application scope of HCl treatment on the doped α-Fe₂O₃ was determined by revealing its working mechanism.

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.

Photo-Electrochemical Glycerol Conversion over a Mie Scattering Effect Enhanced Porous BiVO4 Photoanode

The photo-electrochemical (PEC) oxidation of glycerol (GLY) to high-value-added dihydroxyacetone (DHA) can be achieved over a BiVO₄ photoanode, while the PEC performance of most BiVO₄ photoanodes is impeded due to the upper limits of the photocurrent density. Here, an enhanced Mie scattering effect of the well-documented porous BiVO₄ photoanode is obtained with less effort by a simple annealing process, which significantly reduces the reflectivity to near zero. The great light absorbability increases the basic photocurrent density by 1.77 times. The selective oxidation of GLY over the BiVO₄ photoanode results in a photocurrent density of 6.04 mA cm^-2 and a DHA production rate of 325.2 mmol m^-2 h^-1 that exceeds all reported values. This work addresses the poor ability of nanostructured BiVO₄ to harvest light, paving the way for further improvements in charge transport and transfer to realize highly efficient PEC conversion.

A Conformal Carbon Nanolayer Coated Fe2 O3 Cocatalyst for the Promoted Activity of Plasma-Sputtered BiVO4 Photoanode

To simultaneously improve the hole extraction ability of the BiVO₄ photoanode and accelerate the surface reaction kinetics, herein, a carbon nanolayer conformally coated Fe₂ O₃ (C-Fe₂ O₃ ) as oxygen evolution catalyst (OEC) is loaded on the H₂ plasma treated nanoporous BiVO₄ (BVO(H₂ )) surface by a hydrothermal reaction. It is found that the H₂ plasma induced vacancies in BVO remarkably increases the conductivity, and the C-Fe₂ O₃ enables hole extraction from the bulk to the surface as well as efficient hole injection to the electrolyte. As a result, the C-Fe₂ O₃ /BVO(H₂ ) photoanode achieves a photocurrent density of 4.4 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and an ABPE value of 1.5 % at 0.68 V vs. RHE, which are 4.8-fold and 13-fold higher than that of BVO photoanode, respectively.

A BiVO4 Photoanode with a VOx Layer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO₄ ) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VO_x ) with enriched oxygen vacancies conformally grown on BiVO₄ photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO₄ /VO_x photoanode exhibits a photocurrent density of 6.29 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VO_x that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.

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.

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.

Efficient Oxygen Evolution Reaction on Polyethylene Glycol-Modified BiVO4 Photoanode by Speeding up Proton Transfer

The rate-determining step of the oxygen evolution reaction based on a semiconductor photoanode is the formation of the OO bond. Herein, polyethylene glycol (PEG)-modified BiVO₄ photoanodes are reported, in which protons can be transferred quickly due to the high proton conductivity of PEG, resulting in the acceleration of the OO bond formation rate. These are fully demonstrated by different kinetic isotope effect values. Moreover, the open-circuit voltage (U_oc ) further illustrates that PEG passivates the surface states and surface charge recombination is reduced. The composite photoanode can achieve a maximum photocurrent density of 3.64 mA cm^-2 at 1.23 V compared to 1.04 mA cm^-2 for pure BiVO₄ , and an onset potential of 170 mV, which is a 230 mV negative shift compared to pure BiVO₄ . This work provides a new strategy to accelerate water oxidation kinetics for photoanodes by speeding up the transfer of the proton and the OO bond formation rate.

Enhanced Photocatalytic Degradation of Tetracycline and Oxytetracycline Antibiotics by BiVO4 Photocatalyst under Visible Light and Solar Light Irradiation

The efficient degradation of a toxic antibiotic from an aqueous solution is essential for environmental protection. Our research aimed to fabricate a bismuth vanadate (BiVO4) catalyst via a facile hydrothermal method. The prepared catalyst exhibited a monoclinic phase with a band gap energy of 2.33 eV, indicating the excellent visible-light-active properties of a semiconductor. The photocatalytic performance of the synthesized BiVO4 catalyst was studied by determining the removal of tetracycline (TC) and oxytetracycline (OTC) antibiotics. After 240 min, under sunlight conditions, a high performance of 72% and 83% degradation of TC and OTC, respectively, was achieved. The photocatalytic degradation of the antibiotics correlates well with a first-order reaction, with a high rate constant of 0.0102 min−1. Photogenerated electrons and holes played an important role in the removal of the pollutant. After photocatalytic study, the structural stability of the prepared bismuth vanadate photocatalyst was confirmed. The photocatalyst provided a promising performance even after five successive runs. The result indicates the excellent cycling ability of the sample. The present work demonstrates a promising route for the preparation of a BiVO4 catalyst for the complete removal of toxic antibiotics in aqueous solutions.

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.

Carnation-like Morphology of BiVO4-7 Enables Sensitive Photoelectrochemical Determination of Cr(VI) in the Food and Environment

Hexavalent chromium, namely, Cr(VI), is a significant threat to ecological and food safety. Current detection methods are not sensitive to Cr(VI). A photoelectrochemical (PEC) sensor based on bismuth vanadate (BiVO₄) was developed for sensitive detection of Cr(VI). First, BiVO₄-X (X: the pH of the reaction precursor solution) was synthesized using a facile surfactant-free hydrothermal method. The BiVO₄-X morphology was well controlled according to pH values, showing rock-like (X = 1), wrinkled bark-like (X = 4), carnation-like (X = 7), and the collapsed sheet-like morphologies (X = 9, 12). BiVO₄-7 exhibited excellent photoelectric performance due to a proper band structure under visible light and a large specific surface area. Then, BiVO₄-7 was used to construct a PEC sensor to detect Cr(VI), which was demonstrated to have a low detection limit (10 nM) and wide detection range (2–210 μM). The BiVO₄-7 PEC sensor had a stable output signal, as well as excellent reproducibility, repeatability, and selectivity. We used the BiVO₄-7 PEC sensor to detect Cr(VI) in real environmental and food samples, resulting in a satisfactory recovery of 90.3–103.0%, as determined by comparison with results obtained using a spectrophotometric method. The BiVO₄-7 PEC sensor is promising for practical application to heavy metal detection in the food and environment.

Suppressing photoinduced charge recombination at the BiVO4||NiOOH junction by sandwiching an oxygen vacancy layer for efficient photoelectrochemical water oxidation

Nickel oxyhydroxide (NiOOH) is regarded as one of the promising cocatalysts to enhance the catalytic performance of photoanodes but suffers from serious interfacial charge-carrier recombination at the photoanode||NiOOH interface. In this work, surface-engineered BiVO₄ photoanodes are fabricated by sandwiching an oxygen vacancy (O_vac) interlayer between BiVO₄ and NiOOH. The surface O_vac interlayer is introduced on BiVO₄ by a chemical reduction treatment using a mild reducing agent, sodium hypophosphite. The induced O_vac can alleviate the interfacial charge-carrier recombination at the BiVO4||NiOOH junction, resulting in efficient charge separation and transfer efficiencies, while an outer NiOOH layer is coated to prevent the O_vac layer from degradation. As a result, the as-prepared NiOOH-P-BiVO₄ photoanode exhibits a high photocurrent density of 3.2 mA cm^-2 at 1.23 V vs. RHE under the irradiation of 100 mW/cm² AM 1.5G simulated sunlight, in comparison to those of bare BiVO₄, P-BiVO_4, and NiOOH-BiVO₄ photoanodes (1.1, 2.1 and 2.3 mA cm^-2, respectively). In addition to the superior photoactivity, the 5-h amperometric measurements illustrate improved stability of the surface-engineered NiOOH-P-BiVO₄ photoanode. Our work showcases the feasibility of combining cocatalysts with O_vac, for improved photoactivity and stability of photoelectrodes.

Enhanced surface kinetics and charge transfer of BiVO4 photoanode by Rh2O3 cocatalyst loading for boosted solar water oxidation

The Rh 2 O 3 /BiVO 4 composite photoanode exhibits enhanced surface reaction kinetics and charge transfer efficiency, enabling a photocurrent density of ca. 3.5 mA/cm 2 at 1.23 V (vs. RHE), which is about 3.89 times higher than that of the pristine BiVO 4 , with a lower onset potential of 0.29 V (vs. RHE).

Construction of organic–inorganic hybrid photoanodes with metal phthalocyanine complexes to improve photoelectrochemical water splitting performance

The modification of cobalt phthalocyanine complexes on BiVO4 could promote the charge carrier migration and accelerate the water oxidation kinetics, thus significantly enhancing the photoelectrochemical water splitting.

Direct complexation of citric acid to synthesize high-efficiency bismuth vanadate through molten polymerization route for the degradation of tetracycline hydrochloride under visible light irradiation

Tetracycline hydrochloride can be efficiently degraded over BiVO4 prepared through direct citric acid complexation.

Engineering Single-Atomic Ni-N4-O Sites on Semiconductor Photoanodes for High-Performance Photoelectrochemical Water Splitting

Direct photoelectrochemical (PEC) water splitting is a promising solution for solar energy conversion; however, there is a pressing bottleneck to address the intrinsic charge transport for the enhancement of PEC performance. Herein, a versatile coupling strategy was developed to engineer atomically dispersed Ni-N₄ sites coordinated with an axial direction oxygen atom (Ni-N₄-O) incorporated between oxygen evolution cocatalyst (OEC) and semiconductor photoanode, boosting the photogenerated electron-hole separation and thus improving PEC activity. This state-of-the-art OEC/Ni-N₄-O/BiVO₄ photoanode exhibits a record high photocurrent density of 6.0 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (vs RHE), over approximately 3.97 times larger than that of BiVO₄, achieving outstanding long-term photostability. From X-ray absorption fine structure analysis and density functional theory calculations, the enhanced PEC performance is attributed to the construction of single-atomic Ni-N₄-O moiety in OEC/BiVO₄, facilitating the holes transfer, decreasing the free energy barriers, and accelerating the reaction kinetics. This work enables us to develop an effective pathway to design and fabricate efficient and stable photoanodes for feasible PEC water splitting application.

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.

Operando Infrared Spectroscopy Reveals the Dynamic Nature of Semiconductor-Electrolyte Interface in Multinary Metal Oxide Photoelectrodes

Detailed knowledge about the semiconductor/electrolyte interface in photoelectrochemical (PEC) systems has been lacking because of the inherent difficulty of studying such interfaces, especially during operation. Current understandings of these interfaces are mostly from the extrapolation of ex situ data or from modeling approaches. Hence, there is a need for operando techniques to study such interfaces to develop a better understanding of PEC systems. Here, we use operando photoelectrochemical attenuated total reflection Fourier transform infrared (PEC-ATR-FTIR) spectroscopy to study the metal oxide/electrolyte interface, choosing BiVO₄ as a model photoanode. We demonstrate that preferential dissolution of vanadium occurs from the BiVO₄/water interface, upon illumination in open-circuit conditions, while both bismuth and vanadium dissolution occurs when an anodic potential is applied under illumination. This dynamic dissolution alters the surface Bi:V ratio over time, which subsequently alters the band bending in the space charge region. This further impacts the overall PEC performance of the photoelectrode, at a time scale very relevant for most lab-scale studies, and therefore has serious implications on the performance analysis and fundamental studies performed on this and other similar photoelectrodes.

BiVO4 Ceramic Photoanode with Enhanced Photoelectrochemical Stability

Monoclinic bismuth vanadate (BiVO₄) is an attractive material with which to fabricate photoanodes due to its suitable band structure and excellent photoelectrochemical (PEC) performance. However, the poor PEC stability originating from its severe photo-corrosion greatly restricts its practical applications. In this paper, pristine and Mo doped BiVO₄ ceramics were prepared using the spark plasma sintering (SPS) method, and their photoelectrochemical properties as photoanodes were investigated. The as-prepared 1% Mo doped BiVO₄ ceramic (Mo-BVO (C)) photoanode exhibited enhanced PEC stability compared to 1% Mo doped BiVO₄ films on fluorine doped Tin Oxide (FTO) coated glass substrates (Mo-BVO). Mo-BVO (C) exhibited a photocurrent density of 0.54 mA/cm² and remained stable for 10 h at 1.23 V vs. reversible hydrogen electrode (RHE), while the photocurrent density of the Mo-BVO decreased from 0.66 mA/cm² to 0.11 mA/cm² at 1.23 V vs. RHE in 4 h. The experimental results indicated that the enhanced PEC stability of the Mo-BVO (C) could be attributed to its higher crystallinity, which could effectively inhibit the dissociation of vanadium in BiVO₄ during the PEC process. This work may illustrate a novel ceramic design for the improvement of the stability of BiVO₄ photoanodes, and might provide a general strategy for the improvement of the PEC stability of metal oxide photoanodes.

Hierarchical Nanoporous BiVO4 Photoanodes with High Charge Separation and Transport Efficiency for Water Oxidation

To fabricate high efficiency photoanodes for water oxidation, it is highly required to engineer their nanoporous architecture and interface to improve the charge separation and transport efficiency. By focusing on this aspect, we developed hierarchical nanoporous BiVO₄ (BV) from solution processed two-dimensional BiOI (BI) crystals. The orientation of the BI crystals was controlled by changing the solvent volume ratios of ethylene glycol (EG) to ethanol (ET), which resulted in different hierarchical and planar BV morphologies through a chemical treatment followed by thermal heating. The morphology with optimal particle dimension, connectivity, and porosity can offer a highly enhanced electrochemically active surface area (ECSA). The hierarchical BV owning a maximum ECSA showed the best photoelectrochemical (PEC) performance in terms of the highest photocurrent density and charge separation efficiency. However, to further improve the performance of the electrode, conformal and ultrathin SnO₂ underlayers were deposited by a powerful atomic layer deposition technique at the interface to effectively block the defect density, which significantly improved the photocurrents as high as 3.25 mA/cm² for sulfite oxidation and 2.55 mA/cm² for water oxidation at 0.6 V versus the reversible hydrogen electrode (RHE). The electrode possessed record charge separation efficiency of 97.1% and charge transfer efficiency of 90.1% at 1.23 V_RHE among to-date reported BiVO₄-based photoanodes for water oxidation. Furthermore, a maximum applied bias photon-to-current efficiency (ABPE) of 1.61% was found at a potential as low as 0.6 V_RHE, which is highly promising to make a tandem cell. These results indicate that the construction of the hierarchical nanoporous photoanode with an enhanced ECSA and its proper interface engineering can significantly improve the PEC performance.

Insight into the Transition-Metal Hydroxide Cover Layer for Enhancing Photoelectrochemical Water Oxidation

Depositing a transition-metal hydroxide (TMH) layer on a photoanode has been demonstrated to enhance photoelectrochemical (PEC) water oxidation. However, the controversial understanding for the improvement origin remains a key challenge to unlock the PEC performance. Herein, by taking BiVO₄ /iron-nickel hydroxide (BVO/F_x N_4-x -H) as a prototype, we decoupled the PEC process into two processes including charge transfer and surface catalytic reaction. The kinetic information at the BVO/F_x N_4-x -H and F_x N_4-x -H/electrolyte interfaces was systematically evaluated by employing scanning photoelectrochemical microscopy (SPECM), intensity modulated photocurrent spectroscopy (IMPS) and oxygen evolution reaction (OER) model. It was found that F_x N_4-x -H acts as a charge transporter rather than a sole electrocatalyst. PEC performance improvement is mainly ascribed to the efficient suppression of charge recombination by fast hole transfer kinetics at BVO/F_x N_4-x -H interface.

Dual-Mode Aptasensor Assembled by a WO3/Fe2O3 Heterojunction for Paper-Based Colorimetric Prediction/Photoelectrochemical Multicomponent Analysis

The programed bimodal photoelectrochemical (PEC)-sensing platform based on DNA structural switching induced by targets binding to aptamers was innovatively designed for the simultaneous detection of mucin 1 (MUC1) and microRNA 21 (miRNA-21). To promote excellent current intensity as well as enhance the sensitivity of aptasensors, the evenly distributed WO₃/Fe₂O₃ heterojunction was prepared as a transducer material, notably reducing the background signal response and extending the absorption of light. The multifunctional paper-based biocathode was assembled to provide a visual colorimetric assay. When introducing the integrated signal probe (ISP) composed of signal probe 1 (sP1) and signal probe 2 (sP2) on paper-based working units modified with gold nanoparticles (AuNPs), recognition sites of two targets were formed. In the presence of MUC1 protein, both sP1 and the target on the working unit were released into the corresponding colorimetric unit because of the DNA specific recognition. The horseradish peroxidase-streptavidin (HRP-SA) carried by free sP1 could oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to turn a blue-colored oxidized TMB (oxTMB) in the presence of hydrogen peroxide (H₂O₂), which ultimately gained a higher photocurrent signal. Furthermore, miRNA-21 was modified on another working unit by binding with sP2, leading to changes in the current signal and thus enabling real-time detection of analytes with the assistance of a digital multimeter. The PEC aptasensor offered a wide dynamic range of 10 fg·mL^-1-100 ng mL^-1 for MUC1 and 0.1 pM-10 nM for miRNA-21, with a low detection limit of 3.4 fg·mL^-1 and 36 fM, respectively. It laid the foundation for synchronous detection of multiple analytes and initiated a new way for the enhancement in modern next-generation disease diagnosis.

BiNV bond: A hole-transfer bridge for high-efficient separation and transfer of carriers

Addressing the inherent holes transport limitation of BiVO₄ photoanode is crucial to achieve efficient photoelectrochemical (PEC) water splitting. The construction of the hole-transfer bridge between co-catalysts and BiVO₄ photoanode could be an effective way to overcome sluggish hole-transfer kinetics of BiVO₄ photoanode. Herein, C_xN_y/BiVO₄ photoanode was prepared by coupling carbon nitride hydrogel (CNH) containing unsaturated N on the BiVO₄ photoanode during annealing. C_xN_y/BiVO₄ photoanode exhibited excellent PEC performance and stability. Photoelectrochemical tests proved that the coupling of C_xN_y accelerated holes transfer and enhanced oxygen evolution kinetics. X-ray photoelectron spectroscopy (XPS) and theoretical calculations confirmed the existence of the BiNV bond between BiVO₄ photoanode and C_xN_y, which could serve as the hole-transfer bridge to significantly accelerate separation and transfer of carriers driven by the interfacial electric field. Moreover, it was found that the coupling of C_xN_y effectively inhibited the dissociation of metal ions through changing their coordination environment, resulting in the excellent stability of C_xN_y/BiVO₄ photoanode. This result provides unique insights into vital roles of the interfacial structure, which might have a significant impact on the construction of PEC devices.

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

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

Efficient Self-Driving Photoelectrocatalytic Reactor for Synergistic Water Purification and H2 Evolution

The photoelectrocatalytic (PEC) technique has attracted much attention to getting clear energy and environmental purification. Simultaneous reactions of solar energy generation could be used to apply for practical applications to maximize the functionality of reactor systems. Herein, we crafted a self-driving photoelectrocatalytic reactor system, comprising platinum (Pt) modified p-Si nanowires (Pt/Si-NWs) as a photocathode and TiO₂ nanotube arrays (TiO₂-NTAs) as a photoanode for synergistic H₂ evolution and water purification, respectively. Hydrogen evolution in the cathode chamber and environmental remediation in the anode chamber were achieved with the aid of appropriate bandgap illumination and self-built bias voltage. The mismatch of Fermi levels between TiO₂-NTAs and Si-NWs reduced the recombination rates of photoinduced electrons and holes through the formation of Z scheme and inner electric filed. The synergistic PEC reactions exhibited much higher activities than those achieved using other systems so far. This basic principal could be applied for fabricating other PEC reactors in photoelectro conversion devices and be established as design guidelines for reactors to maximize the PEC performance.