Importance of catalyst–photoabsorber interface design configuration on the performance of Mo-doped BiVO4 water splitting photoanodes

Photoelectrochemical water splitting is mostly impeded by the slow kinetics of the oxygen evolution reaction. The construction of photoanodes that appreciably enhance the efficiency of this process is of vital technological importance towards solar fuel synthesis. In this work, Mo-modified BiVO₄ (Mo:BiVO₄), a promising water splitting photoanode, was modified with various oxygen evolution catalysts in two distinct configurations, with the catalysts either deposited on the surface of Mo:BiVO₄ or embedded inside a Mo:BiVO₄ film. The investigated catalysts included monometallic, bimetallic, and trimetallic oxides with spinel and layered structures, and nickel boride (Ni_xB). In order to follow the influence of the incorporated catalysts and their respective properties, as well as the photoanode architecture on photoelectrochemical water oxidation, the fabricated photoanodes were characterised for their optical, morphological, and structural properties, photoelectrocatalytic activity with respect to evolved oxygen, and recombination rates of the photogenerated charge carriers. The architecture of the catalyst-modified Mo:BiVO₄ photoanode was found to play a more decisive role than the nature of the catalyst on the performance of the photoanode in photoelectrocatalytic water oxidation. Differences in the photoelectrocatalytic activity of the various catalyst-modified Mo:BiVO₄ photoanodes are attributed to the electronic structure of the materials revealed through differences in the Fermi energy levels. This work thus expands on the current knowledge towards the design of future practical photoanodes for photoelectrocatalytic water oxidation.

     

Highly Efficient Photoelectrochemical Water Splitting with an Immobilized Molecular Co4O4 Cubane Catalyst

Molecular Co₄O₄ cubane water oxidation catalysts were combined with BiVO₄ electrodes for photoelectrochemical (PEC) water splitting. The results show that tuning the substituent groups on cobalt cubane allows the PEC properties of the final molecular catalyst/BiVO₄ hybrid photoanodes to be tailored. Upon loading a new cubane complex featuring alkoxy carboxylato bridging ligands ( 1 h ) on BiVO₄, an AM 1.5G photocurrent density of 5 mA cm⁻² at 1.23 V vs. RHE for water oxidation was obtained, the highest photocurrent for undoped BiVO₄ photoanodes. A high solar‐energy conversion efficiency of 1.84 % was obtained for the integrated photoanode, a sixfold enhancement over that of unmodified BiVO₄. These results and the high surface charge separation efficiency support the role of surface‐modified molecular catalysts in improving PEC performance and demonstrate the potential of molecule/semiconductor hybrids for efficient artificial photosynthesis.     

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.     

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

Water‐splitting photoanodes based on semiconductor materials typically require a dopant in the structure and co‐catalysts on the surface to overcome the problems of charge recombination and high catalytic barrier. Unlike these conventional strategies, a simple treatment is reported that involves soaking a sample of pristine BiVO₄ in a borate buffer solution. This modifies the catalytic local environment of BiVO₄ by the introduction of a borate moiety at the molecular level. The self‐anchored borate plays the role of a passivator in reducing the surface charge recombination as well as that of a ligand in modifying the catalytic site to facilitate faster water oxidation. The modified BiVO₄ photoanode, without typical doping or catalyst modification, achieved a photocurrent density of 3.5 mA cm^?2 at 1.23 V and a cathodically shifted onset potential of 250 mV. This work provides an extremely simple method to improve the intrinsic photoelectrochemical performance of BiVO₄ photoanodes.     

An Efficient Strategy for Boosting Photogenerated Charge Separation by Using Porphyrins as Interfacial Charge Mediators

Surface recombination at the photoanode/electrolyte junction seriously impedes photoelectrochemical (PEC) performance. Through coating of photoanodes with oxygen evolution catalysts, the photocurrent can be enhanced; however, current systems for water splitting still suffer from high recombination. We describe herein a novel charge transfer system designed with BiVO₄ as a prototype. In this system, porphyrins act as an interfacial‐charge‐transfer mediator, like a volleyball setter, to efficiently suppress surface recombination through higher hole‐transfer kinetics rather than as a traditional photosensitizer. Furthermore, we found that the introduction of a “setter” can ensure a long lifetime of charge carriers at the photoanode/electrolyte interface. This simple interface charge‐modulation system exhibits increased photocurrent density from 0.68 to 4.75 mA cm^?2 and provides a promising design strategy for efficient photogenerated charge separation to improve PEC performance.     

BiVO4/TiO2 core-shell heterostructure: wide range optical absorption and enhanced photoelectrochemical and photocatalytic performance

In the present study, pristine BiVO₄, TiO₂ and BiVO₄/TiO₂ core-shell heterostructured nanoparticles are prepared by hydrothermal methods and studied for structural, morphological, optical, photoelectrochemical water splitting and photocatalytic degradation of methylene blue as an organic pollutant. Both pristine BiVO₄ and TiO₂ exhibit poor PEC and PC performance under visible light illumination. However, an enhanced PEC and PC activity in BiVO₄/TiO₂ core-shell heterostructure is observed due to high solar energy absorption and superior charge separation properties in core-shell nanoparticles. The photoelectrode prepared using BiVO₄/TiO₂ core-shell nanoparticles exhibit a photocathode behavior and produced cathodic photocurrent, however, the pristine BiVO₄ and TiO₂ photoelectrodes act as photoanode and produced anodic photocurrent. This behavior of change in current direction is also observe in the Mott-Schottky analysis where the BiVO₄/TiO₂ core-shell nanoparticles photoelectrode exhibits the positive slow showing p-type semiconducting behavior. The change in cathodic photoresponse in core-shell nanoparticles in comparison to anodic photoresponse of BiVO₄ and TiO₂ nanoparticles is explained in terms of the variations in the work function values. These results highlight the advantages of core-shell nanoparticle of suitable materials for photocatalytic and photoelectrochemical applications.     

Metal-Organic Framework Glass Catalysts from Melting Glass-Forming Cobalt-Based Zeolitic Imidazolate Framework for Boosting Photoelectrochemical Water Oxidation

Sluggish oxygen evolution kinetics and serious charge recombination restrict the development of photoelectrochemical (PEC) water splitting. The advancement of novel metal–organic frameworks (MOFs) catalysts bears practical significance for improving PEC water splitting performance. Herein, a MOF glass catalyst through melting glass-forming cobalt-based zeolitic imidazolate framework (Co-a_gZIF-62) was introduced on various metal oxide (MO: Fe₂O₃, WO₃ and BiVO₄) semiconductor substrates coupled with NiO hole transport layer, constructing the integrated Co-a_gZIF-62/NiO/MO photoanodes. Owing to the excellent conductivity, stability and open active sites of MOF glass, Co-a_gZIF-62/NiO/MO photoanodes exhibit a significantly enhanced photoelectrochemical water oxidation activity and stability in comparison to pristine MO photoanodes. From experimental analyses and density functional theory calculations, Co-a_gZIF-62 can effectively promote charge transfer and separation, improve carrier mobility, accelerate the kinetics of oxygen evolution reaction (OER), and thus improve PEC performance. This MOF glass not only serves as an excellent OER cocatalyst on tunable photoelectrodes, but also enables promising opportunities for PEC devices for solar energy conversion.     

Enriched Surface Oxygen Vacancies of Photoanodes by Photoetching with Enhanced Charge Separation

A facile photoetching approach is described that alleviates the negative effects from bulk defects by confining the oxygen vacancy (O_vac) at the surface of BiVO₄ photoanode, by 10‐minute photoetching. This strategy could induce enriched O_vac at the surface of BiVO₄, which avoids the formation of excessive bulk defects. A mechanism is proposed to explain the enhanced charge separation at the BiVO₄ /electrolyte interface, which is supported by density functional theory (DFT) calculations. The optimized BiVO₄ with enriched surface O_vac presents the highest photocurrent among undoped BiVO₄ photoanodes. Upon loading FeOOH/NiOOH cocatalysts, photoetched BiVO₄ photoanode reaches a considerable water oxidation photocurrent of 3.0 mA cm^?2 at 0.6 V vs. reversible hydrogen electrode. An unbiased solar‐to‐hydrogen conversion efficiency of 3.5 % is realized by this BiVO₄ photoanode and a Si photocathode under 1 sun illumination.     

Transparent Ta3N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta₃N₅ photoanodes were fabricated on n‐type GaN/sapphire substrates to promote O₂ evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm^?2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta₃N₅ to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe₂ (CIS), which absorbs up to 1100 nm. A stand‐alone tandem cell with a serially‐connected dual‐CIS unit terminated with a Pt/Ni electrode was thus constructed for H₂ evolution. This tandem cell exhibited a solar‐to‐hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.     

A Combinatorial Approach for Optimization of Oxygen Evolution Catalyst Loading on Mo‐doped BiVO4 Photoanodes

The incident photon to current efficiency (IPCE) of a photoactive surface strongly depends on the loading and thickness of the active materials. We present a combinatorial approach based on an optical scanning droplet cell for simultaneous deposition and systematic characterization of co‐catalysts for the oxygen evolution reaction (OER) on Mo‐doped BiVO₄ (Mo?BiVO₄) photoanodes electrochemically pre‐deposited on transparent conductive FTO substrates. The loading and photoelectrochemical characterization of 10 different OER co‐catalysts deposited by three different deposition techniques on FTO‐supported Mo?BiVO₄ were investigated aiming at determination of the suitable deposition parameters affording the highest enhancement of photoelectrochemical oxygen evolution for the different OER/Mo?BiVO₄ films. A comparison of the photoelectrochemical performance of films of various OER co‐catalyst deposited on FTO‐supported Mo?BiVO₄ by electrodeposition, photo‐assisted electrodeposition and photodeposition revealed the necessity of a material specific optimization with respect to co‐catalyst loading and deposition technique to achieve optimal IPCE for each co‐catalysts.     

Visible-light-promoted gas-phase water splitting using porous WO3/BiVO4 photoanodes

We recently described the use of Ti(0) microfibers as an anodization substrate for the preparation of TiO₂ nanotubes arrays as porous photoanodes. Here, we report the use of these fibers as a scaffold to build porous photoanodes based on a WO₃/BiVO₄ heterojunction. The obtained photoelectrodes show promising results under visible light irradiation for water oxidation both in a typical liquid-phase photoelectrochemical setup and in a gas phase reactor (developed in-house) based on a polymeric electrolyte membrane.     

Construction of MXene–BiVO4–FeOOH composite photoanode with ultra-low onset potential: performance,DFT calculation and mechanism

Synergistic enhancement of carrier separation-transport and acceleration of water oxidation kinetics is an effective way to boost the current density of photoanode. Therefore, in this work, we constructed Ti₃C₂T_x MXene conductive framework by cyclic voltammetric (CV) deposition within BiVO₄ to force carrier separation-transfer while also depositing O-vacancied FeOOH by in situ self-hydrolysis to accelerate water oxidation kinetics. Predictably, the constructed MXene–BiVO₄–FeOOH composite photoanode exhibits an impressive current density of 4.95 mA/cm² (1.23 V vs. RHE) and 4.1 times higher than that of pristine BiVO₄, as well as an ultra-low onset potential and high stability. This fantastic enhancement is attributed to the MXene conductive framework avoiding carrier recombination, tightening the interface of BiVO₄–FeOOH, moreover O-vacancied FeOOH improves the adsorption of H₂O to accelerate the water oxidation kinetics. This work demonstrates a successful method for constructing efficient and stable photoanodes as well as diversifying the application of MXene in photoelectrocatalysis.     

Introducing Bidirectional Axial Coordination into BiVO4@Metal Phthalocyanine Core–Shell Photoanodes for Efficient Water Oxidation

Core-shell photoanodes have shown great potential for photoelectrochemical (PEC) water oxidation. However, the construction of a high-quality interface between the core and shell, as well as a highly catalytic surface, remains a challenge. Herein, guided by computation, we present a BiVO₄ photoanode coated with ZnCoFe polyphthalocyanine using pyrazine as a coordination agent. The bidirectional axial coordination of pyrazine plays a dual role by facilitating intimate interfacial contact between BiVO₄ and ZnCoFe polyphthalocyanine, as well as regulating the electron density and spin configuration of metal sites in ZnCoFe phthalocyanine, thereby promoting the potential-limiting step of *OOH desorption. The resulting photoanode displayed a high photocurrent density of 5.7±0.1 mA cm⁻² at 1.23 V_RHE. This study introduces a new approach for constructing core–shell photoanodes, and uncovers the key role of pyrazine axial coordination in modulating the catalytic activity of metal phthalocyanine.     

Promoted photoelectrochemical activity of BiVO<Subscript>4</Subscript> coupled with LaFeO<Subscript>3</Subscript> and LaCoO<Subscript>3</Subscript>

BiVO₄ has emerged as a promising material for solar water splitting. The poor ability of charge transport and separation always limits its performance for photoelectrochemical water splitting. Herein, we coupled n-type BiVO₄ with p-type LaFeO₃ and LaCoO₃, achieving a photocurrent more than two times as high as bare BiVO₄ at 1.23 V versus a reversible hydrogen electrode. Also, the onset potential was negatively shifted about 260 mV. The promotion of performance is mainly because the space charge layer in BiVO₄ is broadened, and the band bend is enhanced, which facilitates the separation and transport of photo-generated charges.     

Curing BiVO4 Photoanodes with Ultraviolet Light Enhances Photoelectrocatalysis

Exposure of BiVO₄ photoanodes to ultraviolet (UV) radiation for extended time periods (e.g., 20 h) produces a morphological change and concomitant improvement in photo‐electrocatalytic (PEC) efficiency for driving water splitting directly by sunlight. The ～230 mV cathodic shift in onset potential and doubling of the photocurrent at 1.23 V vs. RHE after UV curing are comparable to the effects engendered by the presence of a secondary catalyst layer. PEC measurements and absorption spectra indicate that the cathodic shift after UV curing corresponds to a suppression of charge recombination and a greater photovoltage generation caused by the shift of the flat‐band potential, and not an improvement in electrocatalytic activity or light absorption. Spectroscopic surface analysis suggests that surface defect sites, which are eliminated by UV curing, for the differences in observed charge recombination.     

Fuel‐Free Bio‐photoelectrochemical Cells Based on a Water/Oxygen Circulation System with a Ni:FeOOH/BiVO4 Photoanode

A bio‐photoelectrochemical cell (BPEC) based on a fuel‐free self‐circulation water–oxygen–water system was fabricated. It consists of Ni:FeOOH modified n‐type bismuth vanadate (BiVO₄) photoanode and laccase catalyzed biocathode. In this BPEC, irradiation of the photoanode generates photocurrent for photo‐oxidation of water to oxygen, which is reduced to water again at the laccase biocathode. Of note, the by‐products of two electrode reactions could continue to be reacted, which means the H₂O and O₂ molecules are retained in an infinite loop of water–oxygen–water without any sacrificial chemical components. As a result, the assembled fuel‐free BPEC exhibits good performance with an open‐circuit potential of 0.97 V and a maximum power density of 205 μW cm^?2 at 0.44 V. This BPEC based on a self‐circulation system offers a fuel‐free model to enhance multiple energy conversion and application in reality.     

Solar‐Assisted eBiorefinery: Photoelectrochemical Pairing of Oxyfunctionalization and Hydrogenation Reactions

Inspired by natural photosynthesis, biocatalytic photoelectrochemical (PEC) platforms are gaining prominence for the conversion of solar energy into useful chemicals by combining redox biocatalysis and photoelectrocatalysis. Herein, we report a dual biocatalytic PEC platform consisting of a molybdenum (Mo)‐doped BiVO₄ (Mo:BiVO₄) photoanode and an inverse opal ITO (IO‐ITO) cathode that gives rise to the coupling of peroxygenase and ene‐reductase‐mediated catalysis, respectively. In the PEC cell, the photoexcited electrons generated from the Mo:BiVO₄ are transferred to the IO‐ITO and regenerate reduced flavin mononucleotides to drive ene‐reductase‐catalyzed trans‐hydrogenation of ketoisophrone to (R)‐levodione. Meanwhile, the photoactivated Mo:BiVO₄ evolves H₂O₂ in situ via a two‐electron water‐oxidation process with the aid of an applied bias, which simultaneously supplies peroxygenases to drive selective hydroxylation of ethylbenzene into enantiopure (R)‐1‐phenyl‐1‐hydroxyethane. Thus, the deliberate integration of PEC systems with redox biocatalytic reactions can simultaneously produce valuable chemicals on both electrodes using solar‐powered electrons and water.     

Z‐Scheme Photocatalytic Water Splitting on a 2D Heterostructure of Black Phosphorus/Bismuth Vanadate Using Visible Light

Spontaneously solar‐driven water splitting to produce H₂ and O₂, that is, the conversion of solar energy to chemical energy is a dream of mankind. However, it is difficult to make overall water splitting feasible without using any sacrificial agents and external bias. Drawing inspiration from nature, a new artificial Z‐scheme photocatalytic system has been designed herein based on the two‐dimensional (2D) heterostructure of black phosphorus (BP)/bismuth vanadate (BiVO₄). An effective charge separation makes possible the reduction and oxidation of water on BP and BiVO₄, respectively. The optimum H₂ and O₂ production rates on BP/BiVO₄ were approximately 160 and 102 μmol g^?1 h^?1 under irradiation of light with a wavelength longer than 420 nm, without using any sacrificial agents or external bias.     

Interim Anatase Coating Layer Stabilizes Rutile@CrxOy Photoanode for Visible‐Light‐Driven Water Oxidation

Ternary core–shell heterostructured rutile@anatase@Cr_xO_y nanorod arrays were elaborately designed as photoanodes for efficient photoelectrochemical water splitting under visible‐light illumination. The four‐fold enhanced and stabilized visible‐light photocurrent highlights the unique role of the interim anatase layer in accelerating the interfacial charge transfer from the Cr_xO_y chromophore to rutile nanorods.     

Concentration specific and tunable photoresponse of bismuth vanadate functionalized hexagonal ZnO nanocrystals based photoanodes for photoelectrochemical application

In the present work, dual layer BiVO₄/ZnO photoanode is instigated for photo-electrochemical (PEC) water splitting applications. Two different photocatalytic layers ZnO and BiVO₄, reduces charge carrier recombination and charge transfer resistance at photoanode/electrolyte junction. The concentration-specific, tunable and without ‘spike and overshoot’ features, photocurrent density response is originated by varying BiVO₄ concentration in the BiVO₄/ZnO photoanode. The crystal structure of ZnO (hexagonal wurtzite structure) and BiVO₄ (monoclinic scheelite structure) is confirmed by X-ray diffraction studies. The band gap of BiVO₄/ZnO was estimated to be ca. 2.42 eV through Kubler-Munk function F(R_∞) using diffuse reflectance spectroscopy. Electrochemical behavior of samples was analyzed with photocurrent measurements, electrochemical impedance, Mott-Schottky plots, bulk separation efficiency and surface transfer efficiency. The maximum photocurrent density of BiVO₄/ZnO photoanode was found to be 2.3 times higher than pristine ZnO sample.0.038 M BiVO₄/ZnO exhibited the highest separation efficiency of 72% and surface transfer efficiency of 64.7% at +1.23 V vs. RHE. Mott-Schottky study revealed the maximum charge carrier density in the same sample.