An Electrochemically Treated BiVO4 Photoanode for Efficient Photoelectrochemical Water Splitting

The CuSCN layer between BiVO4 and NiFeOx for facilitating photogenerated carrier transfer and water oxidation kinetics

Modification of oxygen evolution co-catalyst (OEC) on the surface of bismuth vanadate (BiVO4) can effectively improve the kinetics of water oxidation, but it is still limited by the small hole extraction driving force at the BiVO4/OEC interface. Modulating the BiVO4/OEC interface with a hole transfer layer (HTL) is expected to facilitate hole transport from BiVO4 to the OEC surface. Herein, a copper(I) thiocyanate (CuSCN) HTL is inserted between BiVO4 and NiFeOx OEC to create BiVO4/CuSCN/NiFeOx photoanode, resulting in a significant enhancement of photoelectrochemical (PEC) water splitting performance. From electrochemical analyses and density functional theory (DFT) simulations, the markedly enhanced PEC performance is attributed to the insertion of CuSCN as an HTL, which promotes the extraction of holes from BiVO4 surface and boosts the water oxidation kinetics. The optimal photoanode achieves a photocurrent density of 5.6 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (vs. RHE) and an impressive charge separation efficiency of 96.2 %. This work offers valuable insights into the development of advanced photoanodes for solar energy conversion and emphasizes the importance of selecting an appropriate HTL to mitigate recombination at the BiVO4/OEC interface.

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.

Advancing BiVO4 Photoanode Activity for Ethylene Glycol Oxidation via Strategic pH Control

The photoelectrochemical (PEC) conversion of organic small molecules offers a dual benefit of synthesizing value-added chemicals and concurrently producing hydrogen (H2). Ethylene glycol, with its dual hydroxyl groups, stands out as a versatile organic substrate capable of yielding various C1 and C2 chemicals. In this study, we demonstrate that pH modulation markedly enhances the photocurrent of BiVO4 photoanodes, thus facilitating the efficient oxidation of ethylene glycol while simultaneously generating H2. Our findings reveal that in a pH = 1 ethylene glycol solution, the photocurrent density at 1.23 V vs. RHE can attain an impressive 7.1 mA cm-2, significantly surpassing the outputs in neutral and highly alkaline environments. The increase in photocurrent is attributed to the augmented adsorption of ethylene glycol on BiVO4 under acidic conditions, which in turn elevates the activity of the oxidation reaction, culminating in the maximal production of formic acid. This investigation sheds light on the pivotal role of electrolyte pH in the PEC oxidation process and underscores the potential of the PEC strategy for biomass valorization into value-added products alongside H2 fuel generation.

Liquid Phase Exfoliation of Few-Layer Non-Van der Waals Chromium Sulfide

Exfoliation of 2D non-Van der Waals (non-vdW) semiconductor nanoplates (NPs) from inorganic analogs presents many challenges ahead for further exploring of their advanced applications on account of the strong bonding energies. In this study, the exfoliation of ultrathin 2D non-vdW chromium sulfide (2D Cr2S3) by means of a combined facile liquid-phase exfoliation (LPE) method is successfully demonstrated. The morphology and structure of the 2D Cr2S3 material are systematically examined. Magnetic studies show an obvious temperature-dependent uncompensated antiferromagnetic behavior of 2D Cr2S3. The material is further loaded on TiO2 nanorod arrays to form an S-scheme heterojunction. Experimental measurements and density functional theory (DFT) calculations confirm that the formed TiO2@Cr2S3 S-scheme heterojunction facilitates the separation and transmission of photo-induced electron/hole pairs, resulting in a significantly enhanced photocatalytic activity in the visible region.

Maximizing Photoelectrochemical Performance in Metal-Oxide Hybrid Composites via Amorphous Exsolution-A New Exsolution Mechanism for Heterogeneous Catalysis

Exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle-perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond's low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

Photocatalytic Degradation of Crystal Violet on Cu, Zn Doped BiVO4 Particles

Fabrication of codoped photocatalysts is a developing area of research. Herein, we explore the visible light photocatalytic properties of Cu, Zn codoped BiVO4 particles. Doping lower valent cations (Cu and Zn) makes the BiVO4 surface more acidic and enables us to target the basic crystal violet (CV) dye. The adopted hydrothermal protocol of synthesis results in the formation of Cu-Zn codoped monoclinic BiVO4 particles. Undoped monoclinic BiVO4, prepared by the same protocol, showed significant formation of oxygen vacancies. XPS analyses confirm the coexistence of Cu2+/Cu+ and Zn2+ dopants. Increased dopant percentage reduced oxygen vacancies. XRD indicates that Cu2+/Cu+ or Zn2+ dopants generally substitute Bi3+ in BiVO4. All photocatalysis activities for CV degradation are reported under near-neutral pH conditions. A typical codoped BiVO4 photocatalyst with 1% Zn and 2% Cu demonstrated the best CV degradation photocatalytic activity. The activity of this Zn, Cu codoped photocatalyst is better than that of pure, Zn-doped, and Cu-doped BiVO4 samples. Active species trapping experiments indicated the possible photocatalysis mechanism. The photocatalysts exhibited appropriate recyclability and photostability.

Nanoconfinement Enables Photoelectrochemical Selective Oxidation of Glycerol via the Microscale Fluid Effect

The glycerol oxidation reaction (GOR) run with photoelectrochemical cells (PECs) is one of the most promising ways to upgrade biomass because it is thermodynamically favorable, while irreversible overoxidation leads to unsatisfactory product selectivities. Herein, a tunable one-dimensional nanoconfined environment was introduced into the GOR process, which accelerated mass transfer of glycerol via the microscale fluid effect and changed the main oxidation product from formic acid (FA) to glyceraldehyde (GLD), which led to retention of the heavier multicarbon products. The rate of glycerol diffusion in the nanochannels increased by a factor of 4.92 with decreasing inner diameters. The main product from the PEC-selective oxidation of glycerol changed from the C1 product FA to the C3 product GLD with a great selectivity of 60.7%. This work provides a favorable approach for inhibiting further oxidation of multicarbon products and illustrates the importance of microenvironmental regulation in biomass oxidation.

Liquid metal-embraced photoactive films for artificial photosynthesis

The practical applications of solar-driven water splitting pivot on significant advances that enable scalable production of robust photoactive films. Here, we propose a proof-of-concept for fabricating robust photoactive films by a particle-implanting technique (PiP) which embeds semiconductor photoabsorbers in the liquid metal. The strong semiconductor/metal interaction enables resulting films efficient collection of photogenerated charges and superior photoactivity. A photoanode of liquid-metal embraced BiVO4 can stably operate over 120 h and retain ~ 70% of activity when scaled from 1 to 64 cm2. Furthermore, a Z-scheme photocatalyst film of liquid-metal embraced BiVO4 and Rh-doped SrTiO3 particles can drive overall water splitting under visible light, delivering an activity 2.9 times higher than that of the control film with gold support and a 110 h stability. These results demonstrate the advantages of the PiP technique in constructing robust and efficient photoactive films for artificial photosynthesis.

Enhancing the PEC Efficiency in the Perspective of Crystal Facet Engineering and Modulation of Surfaces

Generation of hydrogen is one of the most promising routes to harvest solar energy for its sustainable utilization. Among different routes, the photoelectrochemical (PEC) process to split water using solar light to produce hydrogen is the green method to generate hydrogen. The sluggish kinetics through complicated pathways makes the oxygen evolution reaction the rate limiting step of the overall water splitting process. Therefore, development of an efficient photoanode for the sustainable oxidation of water is most challenging in an efficient overall PEC water splitting process. The low solar to hydrogen conversion efficiency arises from the slow surface kinetics, poor hole diffusion, and fast charge recombination processes. There have been strategies to improve catalytic performances through the removal of such detrimental effects. The generation of engineered surfaces is one of the important strategies recently adopted for the enhancement of the catalytic efficiencies. The present review has been focused on the discussion of engineered surfaces using crystal facet engineering, protective surface layer, passivation using the atomic layer deposition (ALD) technique, and cocatalyst modified surfaces to enhance the catalytic efficiency. Some of the important parameters defining catalyst performance are also discussed at the beginning of the review.

Synergistic Effect of Co3(HPO4)2(OH)2 Cocatalyst and Al2O3 Passivation Layer on BiVO4 Photoanode for Enhanced Photoelectrochemical Water Oxidation

Bismuth vanadate (BVO) is regarded as an exceptional photoanode material for photoelectrochemical (PEC) water splitting, but it is restricted by the severe photocorrosion and slow water oxidation kinetics. Herein, a synergistic strategy combined with a Co3(HPO4)2(OH)2 (CoPH) cocatalyst and an Al2O3 (ALO) passivation layer was proposed for enhanced PEC performance. The CoPH/ALO/BVO photoanode exhibits an impressive photocurrent density of 4.9 mA cm-2 at 1.23 VRHE and an applied bias photon-to-current efficiency (ABPE) of 1.47% at 0.76 VRHE. This outstanding PEC performance can be ascribed to the suppressed surface charge recombination, facilitated interfacial charge transfer, and accelerated water oxidation kinetics with the introduction of the CoPH cocatalyst and ALO passivation layer. This work provides a novel and synergistic approach to design an efficient and stable photoanode for PEC applications by combining an oxygen evolution cocatalyst and a passivation layer.

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

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

Iron-optimized oxygen vacancy concentration to strengthen the electrocatalytic ability of the urea oxidation reaction

Iron-modified Ni(OH)2/NiSe2 enhances oxygen vacancies, expanding the electrochemically active surface area, which exhibiting superior selectivity and stability in urea oxidation reaction, outperforming pristine Ni(OH)2@NiSe2. It also demonstrates superior catalytic performance in the oxidation reactions of other small molecules.

Functions of metal-phenolic networks and polyphenol derivatives in photo(electro)catalysis

Phenolic compounds are ubiquitous in nature because of their unique physical and chemical properties and wide applications, which have received extensive research attention. Phenolic compounds represented by tannic acid (TA) play an important role at the nanoscale. TA with a polyphenol hydroxyl structure can chemically react with organic or inorganic materials, among which metal-phenolic networks (MPNs) formed by coordination with metal ions and polyphenol derivatives formed by interactions with organic matter, exhibit specific properties and functions, and play key roles in photo(electro)catalysis. In this paper, we first introduce the fundamental properties of TA, then summarize the factors influencing the properties of MPNs and structural transformation of polyphenol-derived materials. Subsequently, the functions of MPNs and polyphenol derivatives in photo(electro)catalysis reactions are summarized, encompassing improving interfacial charge carrier separation, accelerating surface reaction kinetics, and enhancing light absorption. Finally, this article provides a comprehensive overview of the challenges and outlook associated with MPNs. Additionally, it presents novel insights into their stability, mechanistic analysis, synthesis, and applications in photo(electro)catalysis.

Enhanced Carrier Dynamics of CsPbBr3 Nanocrystals Enabled by Short-Ligand Ethanedithiol for Efficient Photoelectrocatalytic Photoanodes

Photoelectrochemical (PEC) water splitting is a potential solution for a low-carbon society and clean energy storage due to its ability to produce hydrogen and oxygen. However, the slow oxidation half-reaction of the process has limited its overall efficacy, necessitating the development of an efficient photoanode. Colloidal CsPbBr3 nanocrystals (NCs) have been identified as promising candidates due to their high light absorption and valence band position. However, the presence of the electrical insulator, long-chain oleate molecules, on the surface of the CsPbBr3 NCs has hindered efficient charge carrier separation and transport. To solve this problem, short-chain 1,2-ethanedithiol (EDT) ligands were used to replace the oleate ligands on the surface of the CsPbBr3 NCs through a solid-state ligand exchange method. This resulted in a reduction of the nanocrystal spacing and a cross-linking reaction, which improved the photogenerated carrier separation and transport while still passivating the dangling bonds on the CsPbBr3 NC surface. Ultimately, this led to a remarkable photocurrent density of 3.34 mA cm-2 (1.23 VRHE), which was 5.2 times higher than that of the pristine oleate-CsPbBr3 NC (0.64 mA cm-2)-based device. This work presents an efficient way of developing inorganic lead halide perovskite colloidal nanocrystal-based photoanodes through surface ligand engineering.

An A2 B2 O7 -Type High-Entropy Oxide for Efficient Photoelectrochemical Photodetector with Excellent Long-Term Stability

Optoelectronics with excellent long-term stability is meaningful for practical applications. Herein, for the first time, an A2 B2 O7 type high-entropy oxide of (La0.2 Ce0.2 Nd0.2 Gd0.2 Bi0.2 )2 Ti2 O7 (ATO) is synthesized and applied for photoelectrochemical photodetection. The lattice distortion, highly dispersed metal composition, and exposed active sites of ATO are beneficial for the fast separation and transmission of photogenerated electron/hole pairs, endowing ATO-based devices with good photodetection performance. Both the density functional theory calculations and the nondegenerate transient absorption spectroscopy demonstrate the good optoelectronic properties of ATO. The systematic experimental studies reveal the tunable photodetection capability of ATO-based photodetector (PD) in the visible region. A photocurrent of 772.00 nA cm-2 and a responsivity of 4.02 µA W-1 can be achieved as the PD in 1.0 m KOH with the bias potential of 0.6 V. Importantly, the robust and reproducible ON/OFF signals of the PD can be verified and there is only ≈5.00% attenuation in photocurrent even after 6 months, revealing the great potential of high- entropy oxides for practical applications.

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.

Temperature-Controlled Transformation of WO3 Nanowires into Active Facets-Exposed Hexagonal Prisms toward Efficient Visible-Light-Driven Water Oxidation

A unique transformation of WO₃ nanowires (NW-WO₃) into hexagonal prisms (HP-WO₃) was demonstrated by tuning the temperature of the (N₂H₄)WO₃ precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO₃ nanocrystals (ca. <100 nm width, 3-5 μm length) with anisotropic growth of monoclinic WO₃ crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO₃ nanocrystals (ca. 500-1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO₃ nanocrystals with a single-crystalline character. The HP-WO₃ electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO₃ electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO₃ was 3.1-fold higher than 15% for the NW-WO₃ electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO₃ layer with the unidirectional nanowire structure is more efficient than that through the HP-WO₃ layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO₃ electrode is more efficient than the NW-WO₃ electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO₃ electrode. The efficient water oxidation reaction at the surface for the HP-WO₃ electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO₃ to suppress the electron-hole recombination at the surface.

Interfacial elaborating In2O3-decorated ZnO/reduced graphene oxide/ZnS heterostructure with robust internal electric field for efficient solar-driven hydrogen evolution

Solar-driven hydrogen evolution over ZnO-ZnS heterostructures is considered as a promising strategy for sustainable-energy issues. However, the industrialization of this strategy is still constrained by suppressed carrier migration, rapid charge recombination, and the inevitable utilization of noble-metal particles. Herein, we envision a novel strategy of successfully introducing In₂O₃ into the ZnO-ZnS heterostructure. Benefiting from the optimized internal electric field and the charge carrier migration mode based on the direct Z-scheme, the interfacial elaborating In₂O₃-decorated ZnO/reduced graphene oxide (rGO)/ZnS heterostructure manifests smooth charge migration, suppressed electron-hole pair recombination, and increased surface active sites. More importantly, the in situ introduction of In₂O₃ optimizes the construction of the internal electric field, favoring directional light-triggered carrier migration. As a result, the light-induced electrons generated from the heterostructure can be efficiently employed for the hydrogen evolution reaction. Hence, this work would shed light on the in situ fabrication of noble-metal-free photocatalysts for solar-driven water splitting.

Photochemical and electrochemical co-regulation of the BiVO4 photoanode for water splitting

A novel pretreatment strategy that can regulate the amount of oxygen vacancies (O_vac) across the wormlike-BiVO₄ photoanode by photochemical and electrochemical co-processing. Upon decorating NiFeO_x as an oxygen evolution cocatalyst for promoting the surface oxidation kinetics, a record-high photocurrent density of 6.42 mA cm^-2 is obtained at 1.23 vs. RHE (100 mW cm^-2).

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.

Manipulating the surface states of BiVO4 through electrochemical reduction for enhanced PEC water oxidation

Bismuth vanadate (BiVO₄) is a prospective candidate for photoelectrochemical (PEC) water oxidation, but its commercial application is limited due to the serious surface charge recombination. In this work, we propose a novel and effective electrochemical reduction strategy combined with co-catalyst modification to manipulate the surface states of the BiVO₄ photoanode. Specifically, an ultrathin amorphous structure is formed on the surface of BiVO₄ after electrochemical reduction ascribed to the breaking of the surface metal-O bonds. Photoelectrochemical measurements and first-principles calculation show that the electrochemical reduction treatment can effectively reduce the surface energy, thereby passivating the recombined surface states (r-ss) and increasing the mobility of photogenerated holes. In addition, the FeOOH co-catalyst layer further increases the intermediate surface states (i-ss) of BiVO₄, stabilizes the surface structure and enhances its PEC performance. Benefiting from the superior charge transfer efficiency and the excellent water oxidation kinetics, the -0.8/BVO/Fe photoanode achieves 2.02 mA cm^-2 photocurrent at 1.23 V_RHE (2.4 times that of the original BiVO₄); meanwhile, the onset potential shifts 90 mV to the cathode. These results provide a new surface engineering tactic to modify the surface states of semiconductor photoanodes for high-efficiency PEC water oxidation.

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.

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

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

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

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

Process Accumulated 8% Efficient Cu2 ZnSnS4 -BiVO4 Tandem Cell for Solar Hydrogen Evolution with the Dynamic Balance of Solar Energy Storage and Conversion

A process accumulated record solar to hydrogen (STH) conversion efficiency of 8% is achieved on the Cu₂ ZnSnS₄ -BiVO₄ tandem cell by the synergistic coupling effect of solar thermal and photoelectrochemical (PEC) water splitting with the dynamic balance of solar energy storage and conversion of the greenhouse system. This is the first report of a Cu₂ ZnSnS₄ -BiVO₄ tandem cell with a high unbiased STH efficiency of over 8% for solar water splitting due to the greenhouse device system. The greenhouse acts as a solar thermal energy storage cell, which absorbs infrared solar light and storage as thermal energy with the solar light illumination time, while thermoelectric device (TD) converts thermal energy into electric power, electric power is also recycled and added onto Cu₂ ZnSnS₄ -BiVO₄ tandem cell for enhanced overall water splitting. Finally, the solar water splitting properties of the TD-Cu₂ ZnSnS₄ -BiVO₄ integrated tandem cell in pure natural seawater are demonstrated, and a champion STH efficiency of 2.46% is presented, while a large area (25 cm² ) TD-Cu₂ ZnSnS₄ -BiVO₄ integrated tandem device with superior long-term stability is investigated for 1 week, which provides new insight into photoelectrochemical solar water splitting 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.

Defect Engineering and Surface Polarization of TiO2 Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution

The relatively low photo-conversion efficiencies of semiconductors greatly restrict their real-world practices toward photoelectrochemical water splitting. In this work, we demonstrate the fabrication of TiO2-x nanorod arrays enriched with oxygen defects and surface-polarized hydroxyl groups by a facile surface reduction method. The oxygen defects located in the bulk/surface of TiO2-x enable fast charge transport and act as catalytically active sites to accelerate the water oxidation kinetics. Meanwhile, the hydroxyl groups could establish a surface electric field by polarization, for efficient charge separation. The as-optimized TiO2-x nanorod photoanode achieves a high photocurrent density of 2.62 mA cm−2 without any cocatalyst loading at 1.23 VRHE under 100 mW cm−2, which is almost double that of the bare TiO2 counterpart. Notably, the surface charge separation and injection efficiency of the TiO2-x photoanode reach as high as 80% and 97% at 1.23 VRHE, respectively, and the maximum incident photon-to-current efficiency reaches 90% at 400 nm. This work provides a new surface treatment strategy for the development of high-performance photoanodes in photoelectrochemical water splitting.

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.

Intrinsic Mechanism Analyses of Significantly Enhanced Photoelectrochemical Performance of the Bi2MoO6/BiVO4 System

In this paper, the electrodeposition and hydrothermal methods were used to prepare the Bi₂MoO₆/BiVO₄ photoelectrode, and the intrinsic mechanism of significantly enhanced photoelectrochemical performance of the prepared Bi₂MoO₆/BiVO₄ heterojunction system was studied. Work functions of Bi₂MoO₆ and BiVO₄ were analyzed using a scanning Kelvin probe, and the direction of the heterojunction electric field and the transfer direction of photogenerated carriers were finally determined by the relative positions of the energy bands and the Fermi levels of Bi₂MoO₆ and BiVO₄. A type II Bi₂MoO₆/BiVO₄ heterojunction system was finally confirmed to be formed. Formation of the type II Bi₂MoO₆/BiVO₄ heterojunction system reduces the recombination efficiency of photogenerated electrons and holes and thus improves the photoelectrochemical performance. The photogenerated current density of the Bi₂MoO₆/BiVO₄ photoelectrode reaches 1.47 mA·cm^-2, which is 4.9 times that of pure BiVO₄ and thousands of times that of Bi₂MoO₆. The successful application of a scanning Kelvin probe in the verification of the heterojunction type provides theoretical and technical bases for the design and construction of efficient heterojunctions.

Visible-Light Photocatalytic Chlorite Activation Mediated by Oxygen Vacancy Abundant Nd-Doped BiVO4 for Efficient Chlorine Dioxide Generation and Pollutant Degradation

Visible-light photocatalytic chlorite activation has emerged as an efficient oxidation process for micropollutant elimination. However, the in-depth mechanism of chlorite activation is not understood. In this study, using neodymium-doped bismuth vanadate (Nd_xBi_1-xVO_4-δ) as a model catalyst, we describe the oxygen vacancy (OV)-mediated chlorite activation process for efficient ClO₂ generation and cephalexin (CPX) degradation. DFT calculations and in situ DRIFTS suggest that the OV-introduced surface -OH serves as the Brønsted acidic center for chlorite adsorption. The OV-mediated chlorite activation involves multistep reactions that surface hydroxylation and proton transfer from the surface -OH to chlorite, forming metastable chlorous acid (HClO₂) and further disproportionating to ClO₂. As compared with vis-photocatalysis, the vis-photocatalysis coupled with chlorite activation (vis/chlorite) technique exhibits superior performance in antibiotic degradation and achieves efficient microorganism inactivation. This work uncovers the role of OVs on chlorite activation and provides a rational strategy for designing visible-light-driven oxidation techniques in water and wastewater treatment.

Surface engineering of hematite nanorods photoanode towards optimized photoelectrochemical water splitting

Rapid charge recombination in hematite (Fe₂O₃) during photoelectrochemical water splitting is a major obstacle to achieving high-efficiency photoelectrodes. Surface defect engineering is considered as a viable strategy for enhancing photoelectrochemical activity of oxide photoanodes. Herein, a one-dimensional (1D) defective γ-Fe₂O₃ nanorods (DFNRs) photoanode is prepared using solvothermal and high-temperature hydrogenation strategies. The as-prepared DFNRs possess superior visible-light absorption capacity and exhibit excellent photoelectrochemical performance (0.98 mA cm^-2), with approximately three-fold higher photocurrent density than that of pristine Fe₂O₃ (FNRs, 0.32 mA cm^-2). The enhanced activity of the DFNRs results from the moderate formation of oxygen vacancy defects, which promotes spatial charge separation and transfer at the DFNRs/electrolyte interface, as well as the 1D nanorod structure, which favors rapid charge transfer. The surface of γ-Fe₂O₃ with hydroxyl (OH) groups provides sufficient surface-active sites. This result suggests that surface-oxygen deficiency of γ-Fe₂O₃ can not only expand the light absorption range but also facilitating photo-generated charge carriers separation. This surface engineering strategy provides an alternative method for preparing stable and highly active metal oxide photoanodes for photoelectrochemical water splitting.

Recent advances on inactivation of waterborne pathogenic microorganisms by (photo) electrochemical oxidation processes: Design and application strategies

Compared with other conventional water disinfection processes, (photo) electrochemical oxidation (P/ECO) processes have the characteristics of environmental friendliness, convenient installation and operation, easy control and high efficiency of inactivating waterborne pathogenic microorganisms (PMs), so that more and more research work has been focused on this topic, but there is still a huge gap between the research and practical application. Here, the research network of inactivating PMs by P/ECO processes has been comprehensively summarized, and the electrode/reactor/process design strategies based on strengthening direct and indirect oxidation, enhancing mass transfer efficiency and electron transfer efficiency, and improving the effective dose of electrogenerated oxidants are discussed. Furthermore, the factors affecting the inactivation of PMs and the issues regarding to stability and lifetime of the electrode are discussed respectively. Finally, the important research priorities and possible research challenges of P/ECO processes are put forward to make significant progress of this technology.

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution

The photoactivity of nanoporous bismuth vanadate (BiVO₄, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H₂) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H₂ evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.

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.

Photoelectrochemical Oxidation of Amines to Imines and Production of Hydrogen through Mo-Doped BiVO4 Photoanode

Imines are important multifunctional intermediates for the synthesis of pesticides, pharmaceuticals, biologics, and fine chemicals. The direct photoelectrochemical (PEC) oxidation of amines to imines is a highly selective, efficient, green, and gentle method. Interestingly, the constructive merging of the PEC oxidation of amines with the production of hydrogen can accelerate hydrogen evolution due to the less challenging oxidation of amines such as benzylamine (BN) in comparison to sluggish water oxidation. Herein, Mo-doped BiVO₄ photoanodes were prepared and first applied to simultaneously oxide benzylamine (BN) to N-benzylidenebenzylamine (BI) and produce hydrogen in a closed two-chamber, three-electrode PEC cell After illumination at a bias of 1.3 V vs SCE for 3 h, the 3% Mo-doped BiVO₄ photoanode achieved a maximum yield of ∼94 μmol h^-1 at a 1 × 1 cm² area with a BN to BI selectivity of almost 100% and a Faradaic efficiency of 98.4%. Our electrode presented enhanced photocorrosion resistance in acetonitrile solvent. Additionally, the PEC oxidations of benzylamine derivatives with different substituents (-F, -Cl, -Br, -CH₃, -OCH₃) to the corresponding imines were also investigated. The results indicated that the Mo-doped BiVO₄ photoanode exhibited an excellent performance in the oxidation of these benzylamine derivatives with corresponding amine to imine selectivities of almost 100% and Faradaic efficiencies of >95%.

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.

Controlled Band Offsets in Ultrathin Hematite for Enhancing the Photoelectrochemical Water Splitting Performance of Heterostructured Photoanodes

Formation of type II heterojunctions is a promising strategy to enhance the photoelectrochemical performance of water-splitting photoanodes, which has been tremendously studied. However, there have been few studies focusing on the formation of type II heterojunctions depending on the thickness of the overlayer. Here, enhanced photoelectrochemical activities of a Fe₂O₃ film deposited-BiVO₄/WO₃ heterostructure with different thicknesses of the Fe₂O₃ layer have been investigated. The Fe₂O₃ (10 nm)/BiVO₄/WO₃ heterojunction photoanode shows a much higher photocurrent density compared to the Fe₂O₃ (100 nm)/BiVO₄/WO₃ photoanode. The Fe₂O₃ (10 nm)/BiVO₄/WO₃ trilayer heterojunction anodes have sequential type II junctions, while a thick Fe₂O₃ overlayer forms an inverse type II junction between Fe₂O₃ and BiVO₄. Furthermore, the incident-photon-to-current efficiency measured under back-illumination is higher than those measured under front-illumination, demonstrating the importance of the illumination sequence for light absorption and charge transfer and transport. This study shows that the thickness of the oxide overlayer influences the energy band alignment and can be a strategy to improve solar water splitting performance. Based on our findings, we propose a photoanode design strategy for efficient photoelectrochemical water splitting.

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.

Selective reduction of nitrate into N2 by novel Z-scheme NH2-MIL-101(Fe)/BiVO4 heterojunction with enhanced photocatalytic activity

Nitrate and its metabolites as common pollutants in water had attracted widespread attentions. Converting nitrate to nontoxic and harmless nitrogen via photocatalysis was a promising approach. In this study, a novel Z-scheme NH₂-MIL-101(Fe)/BiVO₄ heterojunction was successfully prepared. As-prepared Z-scheme heterojunction along with built-in electric field facilitated the charge separation and enhanced the photocatalytic activity in nitrate reduction. The results showed that 0.10-MBiVO photocatalyst exhibited the highest nitrate removal rate of 94.8% (initial concentration 100 mgN/L) and final selectivity to N₂ of 93.4% in 50 min under ultraviolet irradiation. Moreover, formic acid was proved as better hole scavenger compared with methanol and oxalic acid. And the concentration of formic acid had significant influence on the process of nitrate photocatalytic reduction. 0.10-MBiVO photocatalyst exhibited excellent reusability in the recycling tests, indicating its great potential in practical application of nitrate photocatalytic removal. The mechanism of the enhancement as well as reaction pathways for nitrate photocatalytic reduction on NH₂-MIL-101(Fe)/BiVO₄ were comprehensively explored and described at the end.

Polydopamine assisted transformation of ZnO from nanospheres to nanosheets grown in nanoporous BiVO4 films for improved photocatalytic performance

A BiVO4/ZnO nanosheet heterostructure has been fabricated on stainless steel mesh by a solid-solution drying and calcination method, during which ZnO spheres were converted to nanosheets with the aid of polydopamine.

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.

Advancing Photoelectrochemical Energy Conversion through Atomic Design of Catalysts

Powered by inexhaustible solar energy, photoelectrochemical (PEC) hydrogen/ammonia production and reduction of carbon dioxide to high added-value chemicals in eco-friendly and mild conditions provide a highly attractive solution to carbon neutrality. Recently, substantial advances have been achieved in PEC systems by improving light absorption and charge separation/transfer in PEC devices. However, less attention is given to the atomic design of photoelectrocatalysts to facilitate the final catalytic reactions occurring at photoelectrode surface, which largely limits the overall photo-to-energy conversion of PEC system. Fundamental catalytic mechanisms and recent progress in atomic design of PEC materials are comprehensively reviewed by engineering of defect, dopant, facet, strain, and single atom to enhance the activity and selectivity. Finally, the emerging challenges and research directions in design of PEC systems for future photo-to-energy conversions are proposed.

Vertically Aligned Nanorods Fe2TiO5 and Coupling of NiMoO4/CoMoO4 as A Hole-Transfer Cocatalyst for Enhancing Photoelectrochemical Water Oxidation Performance

Fe2TiO5, a promising photoanode material for photoelectrochemical (PEC) splitting water, was limited by its poor conductivity and short carrier diffusion length. Herein, a novel Fe2TiO5 nanorods with the new cocatalyst...