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

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

TiO2 ALD decorated CuO/BiVO4 p-n heterojunction for improved photoelectrochemical water splitting

Bismuth vanadate (BiVO₄) is a promising photoanode material owing to the narrow bandgap, appropriate band position, and excellent resistance against photocorrosion, however, the performance of photoelectrochemical (PEC) water splitting is largely limited by the poor carrier separation and transport ability. To address these issues, for the first time, we fabricate BiVO₄ film/CuO nanocone p-n junctions as photoanodes by combing a facile spin-coating process and water bath reaction. This structure strengthens the light harvesting and promotes the charge separation and transport ability. The surface defects states are passivated by coating conformally ultrathin TiO₂ onto CuO surface through atomic layer deposition (ALD) technique. Benefiting from the favorable morphology, energy band, and surface treatment, the BiVO₄/CuO/TiO₂ heterojunction generates an improved photocurrent that is much higher than pure BiVO₄. The detailed mechanism investigations indicate that the synergetic optimization of charge separation and injection efficiency in the bulk and surface of photoelectrodes can significantly improve the performance of PEC cells.     

Room-temperature photodeposition of conformal transition metal based cocatalysts on BiVO4 for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using semiconductors offers a promising way to convert renewable solar energy to clean hydrogen fuels. However, due to the sluggish reaction kinetics of water oxidation, significant charge recombination occurred at the photoanode/electrolyte interface and cause decrease of its PEC performance. To reduce the surface recombination, we deposit different transition metal complexes on BiVO₄ nanocone arrays by a versatile light driven in-situ two electrode photodeposition approach without applied bias. Conformal cobalt phosphate “Co-Pi”, nickel borate “Ni-Bi” and manganese phosphate “Mn-Pi” complexes were deposited on BiVO₄ nanocone arrays to form core-shell structure photoanode, all of which lead to enhanced photoelectrochemical performance. The photocurrent of the Co-Pi/BiVO₄ photoanode under front-side illumination for 5 min is increased by 4 folds comparing to that of bare BiVO₄ photoanode at 0.6 V vs. RHE, reaching a hole transfer efficiency as high as 94.5% at 1.23 V vs. RHE. The proposed photodeposition strategy is simple and efficient, and can be extended to deposite cocatalyst on other semiconductors with a valence band edge located at a potential more positive than the oxidation potential of transition metal ion in the cocatalyst.

     

3D Brochosomes‐Like TiO2/WO3/BiVO4 Arrays as Photoanode for Photoelectrochemical Hydrogen Production

An ideal photoelectrochemical (PEC) anode should process effective light absorption, charge transport, and separation efficiency. Here, a novel 3D brochosomes‐like TiO₂/WO₃/BiVO₄ array as an efficient photoanode by combining a colloid polystyrene sphere template and electrochemical deposition routes for PEC hydrogen generation is reported. The as‐fabricated 3D TiO₂/WO₃/BiVO₄ brochosomes photoanode yields excellent PEC performance with photocurrent densities of ≈3.13 and ≈4.27 mA cm^?2 with FeOOH/NiOOH catalyst, respectively, measured in 0.5 m Na₂SO₄ solution with 0.1 m Na₂SO₃ at 1.23 V versus reversible hydrogen electrode (RHE) under simulated AM1.5 light illumination, which is ≈6 times the reference sample of a planar WO₃/BiVO₄ film electrode. The significantly improved performance could be benefited from the ordered hollow porous structure that provides enhanced light absorption and efficient charge transport as well as improved charge separation efficiency by WO₃/BiVO₄ “host–guest” heterojunctions.     

Plasmonic Enhancement in BiVO4 Photonic Crystals for Efficient Water Splitting

Photo‐electrochemical water splitting is a very promising and environmentally friendly route for the conversion of solar energy into hydrogen. However, the solar‐to‐H₂ conversion efficiency is still very low due to rapid bulk recombination of charge carriers. Here, a photonic nano‐architecture is developed to improve charge carrier generation and separation by manipulating and confining light absorption in a visible‐light‐active photoanode constructed from BiVO₄ photonic crystal and plasmonic nanostructures. Synergistic effects of photonic crystal stop bands and plasmonic absorption are observed to operate in this photonic nanostructure. Within the scaffold of an inverse opal photonic crystal, the surface plasmon resonance is significantly enhanced by the photonic Bragg resonance. Nanophotonic photoanodes show AM 1.5 photocurrent densities of 3.1 ± 0.1 mA cm^?2 at 1.23 V versus RHE, which is among the highest for oxide‐based photoanodes and over 4 times higher than the unstructured planar photoanode.     

Efficient Solar Energy Harvesting and Storage through a Robust Photocatalyst Driving Reversible Redox Reactions

Simultaneous solar energy conversion and storage is receiving increasing interest for better utilization of the abundant yet intermittently available sunlight. Photoelectrodes driving nonspontaneous reversible redox reactions in solar‐powered redox cells (SPRCs), which can deliver energy via the corresponding reverse reactions, present a cost‐effective and promising approach for direct solar energy harvesting and storage. However, the lack of photoelectrodes having both high conversion efficiency and high durability becomes a bottleneck that hampers practical applications of SPRCs. Here, it is shown that a WO₃‐decorated BiVO₄ photoanode, without the need of extra electrocatalysts, can enable a single‐photocatalyst‐driven SPRC with a solar‐to‐output energy conversion efficiency as high as 1.25%. This SPRC presents stable performance over 20 solar energy storage/delivery cycles. The high efficiency and stability are attributed to the rapid redox reactions, the well‐matched energy level, and the efficient light harvesting and charge separation of the prepared BiVO₄. This demonstrated device system represents a potential alternative toward the development of low‐cost, durable, and easy‐to‐implement solar energy technologies.     

Thin Zinc Oxide Layer Passivating Bismuth Vanadate for Selective Photoelectrochemical Water Oxidation to Hydrogen Peroxide (Small 33/2023)

Selective photoelectrochemical (PEC) water oxidation to hydrogen peroxide is an underexplored option as opposed to the mainstream oxygen reduction reaction. Albeit interesting, selective H₂O₂ production via oxidative pathway is plagued by the noncontrollable two-electron transfer reaction and the overoxidation of the thus-formed H₂O₂ to O₂. Here, ZnO passivator-coated BiVO₄ photoanode is reported for selective PEC H₂O₂ production. Both the H₂O₂ selectivity and production rate increase in the range of 1.0–2.0 V versus RHE under simulated sunlight irradiation. The photoelectrochemical impedance spectra and open-circuit potentials suggest a flattened band bending and positively shifted quasi-Fermi level of BiVO₄ upon ZnO coating, facilitating H₂O₂ generation and suppressing the competitive reaction of O₂ evolution. The ZnO overlayer also inhibits H₂O₂ decomposition, accelerates charge extraction from BiVO₄, and serves as a hole reservoir under photoexcitation. This work offers insights into surface states and the role of the coating layer in manipulating two/four-electron transfer for selective H₂O₂ synthesis from PEC water oxidation.     

Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting

Nanostructured photoanodes based on well‐separated and vertically oriented WO₃ nanorods capped with extremely thin BiVO₄ absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO₃‐NRs/BiVO₄ photoanode modified with Co‐Pi oxygen evolution co‐catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm² at 1.23 V versus a reversible hydrogen electrode in a stable Na₂SO₄ electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO₄ layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO₃‐NRs/BiVO₄ photoanode effectively increases the optical thickness of the BiVO₄ layer and results in efficient absorption of the incident light.     

The Development of Cocatalysts for Photoelectrochemical CO2 Reduction

The ever‐increasing anthropogenic consumption of fossil fuels and the resulting large emission of CO₂ have led to a severe energy crisis and climate change. Photocatalytic reduction of CO₂ into fuels using solar energy is considered as a promising way to address these two problems. In particular, photoelectrochemical (PEC) reduction of CO₂ can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In addition to the charge generation and separation, the efficient reduction of CO₂ on the surface of a semiconductor‐based photoelectrode remains a scientifically critical challenge, which can be greatly enhanced by the surface modification of cocatalysts. Herein, the recent developments of cocatalysts in PEC CO₂ reduction over semiconductor‐based photoelectrodes are described, and the basic principles of PEC CO₂ reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between the photoelectrodes and the cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder charge transfer between them. Furthermore, the challenges and perspectives for PEC reduction of CO₂ are also presented.     

Structural Engineering of BiVO4/CoFe MOF Heterostructures Boosting Charge Transfer for Efficient Photoelectrochemical Water Splitting

Boosting charge separation and transfer of photoanodes is crucial for providing high viability of photoelectrochemical hydrogen (H₂) generation. Here, a structural engineering strategy is designed and synthesized for uniformly coating an ultrathin CoFe bimetal-organic framework (CoFe MOF) layer over a BiVO₄ photoanode for boosted charge separation and transfer. The photocurrent density of the optimized BiVO₄/CoFe MOF(NA) photoanode reaches a value of 3.92 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE), up to 6.03 times that of pristine BiVO₄, due to the greatly increased efficiency of charge transfer and separation. In addition, this photoanode records one onset potential that is considerably shifted negatively when compared to BiVO₄. Transient absorption spectroscopy reveals that the CoFe MOF(NA) prolongs charge recombination lifetime by blocking the hole-transfer pathway from the BiVO₄ to its surface trap states. This work sheds light on boosting charge separation and transfer through structural engineering to enhance the photocurrent of photoanodes for solar H₂ production.     

GaP/GaNP Heterojunctions for Efficient Solar‐Driven Water Oxidation

The growth and characterization of an n‐GaP/i‐GaNP/p⁺‐GaP thin film heterojunction synthesized using a gas‐source molecular beam epitaxy (MBE) method, and its application for efficient solar‐driven water oxidation is reported. The TiO₂/Ni passivated n‐GaP/i‐GaNP/p⁺‐GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO₂/Ni‐coated n‐GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm^?2, leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm^?2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm^?2 for the coated n‐GaP and n‐GaP/i‐GaNP/p⁺‐GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon‐to‐current efficiency (ABPE) of 1.9% while the ABPE of the coated n‐GaP sample is almost zero. Furthermore, the coated n‐GaP/i‐GaNP/p⁺‐GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon‐to‐current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low‐bias performance and broad absorption of the wide‐bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.     

Defect engineering of nanostructures: Insights into photoelectrochemical water splitting

Development of long-term and sustainable energy economy is one of the most significant technical challenges facing humanity. Photoelectrochemical (PEC) water splitting is regarded as the most attractive approach for conversion of solar energy to chemical energy, with H₂ and O₂ as the energy carriers. Defect engineering of photocatalytic materials has been proved effective in improving their performances in PEC water splitting process involving three basic steps, i.e., light absorption, charge transfer/separation, and surface catalytic reaction. In this paper, recent developments in using various techniques to introduce, characterize and regulate defects are summarized, based on which the important roles played by defects are highlighted in the development of high-performance defect engineered photoelectrodes for PEC water splitting application. Moreover, current challenges and future perspectives in the field of defect engineering of nanostructures for photoelectrodes are discussed.     

3D FTO/FTO‐Nanocrystal/TiO2 Composite Inverse Opal Photoanode for Efficient Photoelectrochemical Water Splitting

A 3D fluorine‐doped SnO₂ (FTO)/FTO‐nanocrystal (NC)/TiO₂ inverse opal (IO) structure is designed and fabricated as a new “host and guest” type of composite photoanode for efficient photoelectrochemical (PEC) water splitting. In this novel photoanode design, the highly conductive and porous FTO/FTO‐NC IO acts as the “host” skeleton, which provides direct pathways for faster electron transport, while the conformally coated TiO₂ layer acts as the “guest” absorber layer. The unique composite IO structure is fabricated through self‐assembly of colloidal spheres template, a hydrothermal method and atomic layer deposition (ALD). Owing to its large surface area and efficient charge collection, the FTO/FTO‐NC/TiO₂ composite IO photoanode shows excellent photocatalytic properties for PEC water splitting. With optimized dimensions of the SnO₂ nanocrystals and the thickness of the ALD TiO₂ absorber layers, the 3D FTO/FTO‐NC/TiO₂ composite IO photoanode yields a photocurrent density of 1.0 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5 illumination, which is four times higher than that of the FTO/TiO₂ IO reference photoanode.     

Hierarchical growth of a novel Mn-Bi coupled BiVO4 arrays for enhanced photoelectrochemical water splitting

Oxygen evolving catalyst (OEC) is a critical determinant for the efficiency of photoelectrochemical (PEC) water splitting. Here we report an approach to depositing a novel manganese borate (Mn-Bi) OER catalyst on BiVO₄ nanocone photoanode by photodeposition in sodium borate buffer solution containing Mn(II) ions. Due to the spontaneous photo-electric-field-enhancement effect at the vertically oriented BiVO₄ nanocone structure, spherical Mn-Bi nanoparticle was selectively photodeposited at the apex of BiVO₄ nanocone. Significant improvement of photocurrent was observed for the obtained hierarchical Mn-Bi/BiVO₄ photoanode which could be ascribed to enhanced hole injection efficiency, especially in low bias region. It was observed that the injection efficiency of Mn-Bi/BiVO₄ is 98% which gave a photocurrent of 0.94 mA/cm² at 1.5 V vs. RHE.

     

Spinel Structural Disorder Influences Solar‐Water‐Splitting Performance of ZnFe2O4 Nanorod Photoanodes

Zinc spinel ferrite, ZnFe₂O₄ (ZFO), is an emerging photoanode material for photoelectrochemical (PEC) solar fuel production. However, a lack of fundamental insight into the factors limiting the photocurrent has prevented substantial advance in its performance. Herein, it is found that ZFO nanorod array photoelectrodes with varying crystallinity exhibit vastly different PEC properties. Using a sacrificial hole scavenger (H₂O₂), spatially defined carrier generation, and electrochemical impedance spectroscopy, it is shown that ZFO with a relatively poor crystallinity but a higher spinel inversion degree (due to cation disorder) exhibits superior photogenerated charge separation efficiency and improved majority charge carrier transport compared to ZFO with higher crystallinity and a lower inversion degree. Conversely, the latter condition leads to better charge injection efficiency. Optimization of these factors, and the addition of a nickel–iron oxide cocatalyst overlayer, leads to a new benchmark solar photocurrent for ZFO of 1.0 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE) and 1.7 mA cm^?2 at 1.6 V versus RHE. Importantly, the observed correlation between the cation disorder and the PEC performance represents a new insight into the factors important to the PEC performance of the spinel ferrites and suggests a path to further improvement.     

Oxygen‐Vacancy‐Introduced BaSnO3−δ Photoanodes with Tunable Band Structures for Efficient Solar‐Driven Water Splitting

To achieve excellent photoelectrochemical water‐splitting activity, photoanode materials with high light absorption and good charge‐separation efficiency are essential. One effective strategy for the production of materials satisfying these requirements is to adjust their band structure and corresponding bandgap energy by introducing oxygen vacancies. A simple chemical reduction method that can systematically generate oxygen vacancies in barium stannate (BaSnO₃ (BSO)) crystal is introduced, which thus allows for precise control of the bandgap energy. A BSO photoanode with optimum oxygen‐vacancy concentration (8.7%) exhibits high light‐absorption and good charge‐separation capabilities. After deposition of FeOOH/NiOOH oxygen evolution cocatalysts on its surface, this photoanode shows a remarkable photocurrent density of 7.32 mA cm^?2 at a potential of 1.23 V versus a reversible hydrogen electrode under AM1.5G simulated sunlight. Moreover, a tandem device constructed with a perovskite solar cell exhibits an operating photocurrent density of 6.84 mA cm^?2 and stable gas production with an average solar‐to‐hydrogen conversion efficiency of 7.92% for 100 h, thus functioning as an outstanding unbiased water‐splitting system.     

RGO nanosheets coupled NaNbO3 nanorods based nanocomposite for enhanced photocatalytic and photoelectrochemical water splitting activity

In this present work, reduced graphene oxide (RGO) coupled with hydrothermally grown sodium niobate nanorods (NaNbO₃-NRs) have been successfully synthesized. The photocatalytic performance of RGO/NaNbO₃-NRs photocatalyst demonstrated faster photodegradation of organic methylene blue (MB) dye than bare NaNbO₃-NRs. A ∼6 fold enhancement in the photocatalytic activity of RGO/NaNbO₃-NRs nanocomposite than that of NaNbO₃-NRs has been demonstrated towards the degradation of MB dye under similar light illumination. Furthermore, the potentiality of the fabricated NaNbO₃-NRs and RGO/NaNbO₃-NRs nanocomposite photoanodes have been investigated for photoelectrochemical (PEC) water splitting. The fabricated RGO/NaNbO₃-NRs nanocomposite photoanode showed ∼4 times higher photocurrent density than the NaNbO₃-NRs photoanode. The electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) measurements demonstrated that coupling of RGO nanosheets in the RGO/NaNbO₃-NRs nanocomposite reduced the charge transfer resistance (R_ct) at the photoanode/electrolyte interface, increased the donor density (N_d), and reduced flat band potential (V_fb) of the RGO/NaNbO₃-NRs, thus significantly improving the PEC performance of the RGO/NaNbO₃-NRs nanocomposite. The enhancement in the PEC measurements of RGO/NaNbO₃-NRs nanocomposite is attributed to the extended absorption of the visible portion of the solar spectra and increased mobility of the photogenerated charge transport in the RGO nanosheets, which improve the separation efficiency and reduce the recombination process. The possible charge transfer mechanism has been proposed responsible for the enhanced photocatalytic and PEC water splitting performance.     

Transition‐Metal‐Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost‐effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition‐metal‐based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co‐based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe‐based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self‐biased PEC devices with high solar‐to‐hydrogen efficiency based on earth‐abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting.     

Influence of the physical, structural and chemical properties on the photoresponse property of magnetron sputtered TiO2 for the application of water splitting

The production of hydrogen from water (called "water splitting"), utilises sunlight as an energy source (solar-hydrogen) in a photoelectrochemical (PEC) solar cell, is a promising source of green energy. In this work, a PEC was used, for evaluating the photoactivity of a thin film TiO2 based photoanode by measuring photocurrent (which is comparable to hydrogen production rate by water splitting process in PEC). The main focus of this work is to study the effect of the TiO2 nanosurface and bulk properties on the photoresponse properties of the photoanode. The TiO2 coatings (360-400 nm) were deposited using a closed field reactive magnetron sputtering system. The structure and morphology of the TiO2 coatings were systematically altered by varying the deposition pressure between 5 x 10(-4) to 1 x 10(-2) mbar. The properties of the deposited nano-coatings were determined using Ellipsometry, SEM, AFM, profilometry, XPS, Raman and X-ray diffraction (XRD). Coating properties were correlated with the light absorption and photocurrent performance which were evaluated using UV-Vis spectroscopy and tri-electrode potentiostat measurements respectively. It was concluded from this study that the coating deposition pressure has a pronounced effect on the TiO2 photoanode properties leading to a significant enhancement in the photoactivity in PEC cell. Over a six fold increase in photocurrent at applied potential 0 V was observed for TiO2 photoanode prepared at 4 x 10(-3) mbar as compared to 5 x 10(-4) mbar. A correlation has been established between the deposition pressure, nano surface morphology and bulk properties, UV-Vis light absorbance and bandgap value, and the consequently higher (i) photocurrent density, (ii) negative flat band, and (iii) open circuit potential measured in Photoelectrochemical (PEC) cell.     

Photoelectrochemical properties of metal-cluster oxide compounds,A2Mo3O8 and (LiY)Mo3O8

PEC studies on the single crystals of the metal-cluster oxide compounds. A₂Mo₃O₈ (A = Zn, Mg, Fe), and polycrystalline LiYMo₃O₈ are reported. The photoresponse behaviour is attributed to the Mod-d transition. The photopotential, the photocurrent vs applied voltage and the wavelength data indicate thatn-Zn₂Mo₃O₈ is stable and possesses a small and indirect band gap of 1·55 eV and a direct band gap of 1·9 eV. With change in A ions in A₂Mo₃O₈, there is no significant change in the PEC properties. LiYMo₃O₈ is found to be ofp-type. PEC studies show that excepting for poor electronic conductivity, A₂Mo₃O₈ possesses all the requisitie characteristics of an ideal photoanode for PAE of water for trapping solar energy.