Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Chemistry of Materials Underpinning Photoelectrochemical Solar Fuel Production

Since its inception, photoelectrochemistry has sought to power the generation of fuels, particularly hydrogen, using energy from sunlight. Efficient and durable photoelectrodes, however, remain elusive. Here we review the current state of the art, focusing our discussion on advances in photoelectrodes made in the past decade. We open by briefly discussing fundamental photoelectrochemical concepts and implications for photoelectrode function. We next review a broad range of semiconductor photoelectrodes broken down by material class (oxides, nitrides, chalcogenides, and mature photovoltaic semiconductors), identifying intrinsic properties and discussing their influence on performance. We then identify innovative in situ and operando techniques to directly probe the photoelectrode|electrolyte interface, enabling direct assessment of structure-property relationships for catalytic surfaces in active reaction environments. We close by considering more complex photoelectrochemical fuel-forming reactions (carbon dioxide and nitrogen reduction, as well as alternative oxidation reactions), where product selectivity imposes additional criteria on electrochemical driving force and photoelectrode architecture. By contextualizing recent literature within a fundamental framework, we seek to provide direction for continued progress toward achieving efficient and stable fuel-forming photoelectrodes.

Pt-free MoS2 co-catalyst enables record photocurrent density in Sb2Se3 photocathodes for highly efficient solar hydrogen production

Antimony selenide (Sb2Se3) emerges as a potential light-absorbing material for thin film photovoltaics and photoelectrochemical (PEC) water-splitting devices, due to its earth-abundant constituents and excellent photoelectric properties. However, losses caused by corrosion and sluggish charge transfer at the semiconductor/electrolyte interface require a co-catalyst to enhance these kinetic factors. In this study, MoS2 is employed as a cost-effective, noble-metal-free catalyst to enhance the photocurrent density (J ph), half-cell solar-to-hydrogen (HC-STH) conversion efficiency and stability of Sb2Se3-based photocathodes. Optimized thermodynamic/kinetic physical vapor deposition of MoS2 substantially improves PEC performance, resulting champion Mo/Sb2Se3/CdS/MoS2 photocathode that achieves a record J ph of 31.03 mA cm-2 at 0 V RHE and the highest HC-STH efficiency of 3.08%, along with stability for over 5 hours in an acidic (pH 1) buffer solution. It is systematically revealed that MoS2 reduces the photo-corrosion effect, decreases electron-hole recombination, and provides a significant increase in charge transfer efficiency at the semiconductor/electrolyte interface. This work highlights the potential of cost-effective, high-performance Sb2Se3-based photocathodes in advancing efficient PEC devices for solar hydrogen production.

Strengthening Bonding Interaction of a (Co0.91V0.09)3(BTC)2 Metal-Organic Framework with BiVO4 Photoanodes Enabling Ultrastable Photoelectrochemical Water Oxidation

Although the oxygen evolution reaction (OER) activity of BiVO4 photoanodes has been significantly enhanced, achieving long-term photostability is still challenging due to the gradual dissolution of V5+ during photoelectrochemical (PEC) water splitting. Herein, we deliberately generate ligand defects in a (Co0.91V0.09)3(BTC)2 metal-organic framework (CoV-MOF) that creates more undercoordinated sites, forming strong chemical bonds with BiVO4. Consequently, the dissolution of V5+ from BiVO4 during PEC water splitting can be effectively suppressed, leading to significantly enhanced stability. The optimized Co3O4/CoV-MOF/BiVO4 photoanode exhibits a high photocurrent density of 6.0 mA cm-2 at 1.23 V vs the reversible hydrogen electrode (RHE). Impressively, the photoanode can stably operate for 500 h at 0.6 V vs RHE under AM 1.5 G illumination. This work demonstrates the proof-of-concept of anchoring V5+ in BiVO4 photoanodes achieving ultrastable PEC water splitting.

Preparation of a Z-scheme BiVO4/Cu2O/PPy heterojunction and studying its CO2 reducing properties

Herein, a new Z-scheme BiVO4/Cu2O/PPy heterostructure photocatalyst was developed with bismuth nitrate and ammonium vanadate as the precursors and sodium dodecyl benzyl sulfonate as the soft template. Through the spatial confinement effect of the sodium dodecyl benzyl sulfonate soft template, peanut-like BiVO4 and the BiVO4/Cu2O/PPy heterojunction were synthesized. The best performance was observed for BiVO4/Cu2O/PPy (5%), and its photodegradation rate was 6.57 times higher than that of pure BiVO4. The mechanism study showed that a light hole (h+), superoxide radical (·O2 -), and hydroxyl radical (·OH) participated in the CO2 reduction process, which was different from the pure BiVO4 reaction system. Therefore, the proposed approach provides a new method for applying BiVO4/Cu2O/PPy photocatalysts and developing the same type of heterojunction photocatalyst, and they have effective practical application for environmental remediation.

Thickness-dependent performance of antimony sulfide thin films as a photoanode for enhanced photoelectrochemical water splitting

A two-step synthesis approach is employed for antimony sulfide thin films, which includes thermal evaporation followed by annealing in a sulfur atmosphere using chemical vapor deposition (CVD). The thickness of the films is systematically varied to study its impact on the material's properties. The orthorhombic crystal structure of each film is verified by Grazing Incidence X-ray Diffraction (GIXRD) analysis. Raman spectroscopy reveals thickness-dependent changes in the vibrational properties. Surface morphology and roughness are examined using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM), with findings indicating that layer thickness significantly affects these surface characteristics. Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) demonstrate that variations in film thickness influence the surface chemical composition and oxidation states. The Sb2S3 thin film with a thickness of 450 nm exhibited a band gap of 1.75 eV, indicating its potential for efficient light absorption. It also demonstrated a conductivity of 0.006 mA at an applied voltage of 1 V, reflecting its electrical transport properties. Furthermore, the film achieved a current density of 0.70 mA cm-2, signifying enhanced charge transfer efficiency. These findings suggest that the 450 nm thick film offers an optimal balance of band gap, light absorption, and photocurrent density, making it the most suitable candidate for photoelectrochemical water-splitting applications.

Enhancing Photoelectrochemical Seawater Splitting Efficiency by a Dual-Strategy Approach of W Doping and CoOOH Layer Deposition on BiVO4 Photoanodes

Photoelectrochemical (PEC) seawater splitting offers a sustainable pathway for hydrogen production, yet its practical application is hindered by sluggish reaction kinetics and severe photocorrosion in chloride-rich environments. This study presents a dual-strategy modification of BiVO4 photoanodes through tungsten (W) doping and cobalt oxyhydroxide (CoOOH) nanolayer deposition to synergistically enhance the PEC performance and stability in natural seawater. W doping optimizes the electronic structure of BiVO4 by reducing the bandgap from 2.4 to 2.35 eV and increasing carrier concentration from 1.41 × 1021 to 3.31 × 1021 cm-3, while CoOOH acts as a dual-functional layer that suppresses surface recombination via oxygen vacancy formation and protects against chloride-induced corrosion. The optimized CoOOH/W-BVO photoanode achieves a photocurrent density of 3.77 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE) with 96 h stability in natural seawater, outperforming pristine BiVO4 by 150% and single-modified counterparts by 40-60%. Mechanistic analyses reveal that W6+ substitution elongates V-O bonds, thereby enhancing the bulk charge separation. Concurrently, CoOOH facilitates hole extraction through oxygen vacancies, with oxygen vacancy content increasing from 3.9% to 24.3%. The dual modification also reduces interfacial charge-transfer resistance to 94.44 Ω and shifts the flat-band potential negatively to 0.15 V vs RHE, improving light absorption and charge utilization efficiency (applied bias photocurrent efficiency (ABPE) of 0.95% at 0.77 V). This work provides a robust strategy for designing efficient and durable photoanodes, advancing marine-resource-utilized renewable energy technologies.

A standalone bismuth vanadate-silicon artificial leaf achieving 8.4% efficiency for hydrogen production

The development of scalable photoelectrochemical water splitting with high solar-to-hydrogen efficiency and long-term stability is essential while challenging for practical application. Here, we design a BiVO4 photoanode with gradient distributed oxygen vacancies, which induces strong dipole fields to promote charge separation. Growing sea-urchin-like FeOOH cocatalyst on the photoanode leads to a photocurrent density of 7.0 mA cm-2 at 1.23 V versus the reversible hydrogen electrode and is stable for over 520 h under AM 1.5 G illumination. By integrating with a silicon photovoltaic cell, the standalone artificial leaf achieves a solar-to-hydrogen efficiency of 8.4%. The scale-up of these artificial leaves up to 441 cm2 in size can deliver a solar-to-hydrogen efficiency of 2.7% under natural sunlight. Life cycle assessment analysis shows that solar water splitting has little environmental footprint for hydrogen production. Our study demonstrates the possibility of designing metal oxide-based artificial leaves for scalable solar hydrogen production.

On-Demand Catalytic Platform for Glycerol Upgrade and Utilization

Surplus byproducts generated during biomass exploitation, such as glycerol from biodiesel manufacturing, seriously undermine the credibility of renewable energy policies. Here, we establish an on-demand catalytic platform for the upgrade and utilization of glycerol via photoelectro-bioelectro-heterogeneous coupling catalysis. Combining theoretical descriptors, specifically the highest occupied molecular orbital energy levels and dual local softness values, along with systematic experimental validation, we demonstrated the reaction activity of glycerol and its upgraded products on BiVO4 photoelectrodes. Glyceric acid was identified as the optimal biofuel candidate through monohydroxyl oxidation of glycerol. Coupling the preferential upgrading of glycerol to glyceric acid by night and its reuse as biofuel by day, a hybrid biophotoelectrochemical system delivered an open-circuit voltage of 0.89 ± 0.02 V and a maximum power density of 0.41 ± 0.03 mW cm-2 with stable diurnal operation for over 10 days. This successful model construction provides valuable insights into the strategic integration of multiple energy sources and the exploration of coupling-catalytic platforms, charting new territory for the next-generation sustainable energy systems.

Surface Coverage Tuning for Suppressing Over-Oxidation: A Case of Photoelectrochemical Alcohol-to-Aldehyde/Ketone Conversion

Suppressing over-oxidation is a crucial challenge for various chemical intermediate synthesis in heterogeneous catalysis. The distribution of oxidative species and the substrate coverage, governed by the direction of electron transfer, are believed to influence the oxidation extent. In this study, we presented an experimental realization of surface coverage modulation on a photoelectrode using a photo-induced charge activation method. Through the surface coverage modulation, both pre-oxidized alcohol substrates and surface coverage were increased, which not only improved the reaction kinetics but also suppressed the over-oxidation of the generated aldehydes/ketones. As a demonstration, the Faradaic efficiency for the conversion of glycerol to dihydroxyacetone increased from 31.8 % to 46.8 % (with selectivity rising from 47.6 % to 71.3 %), from 73.4 % to 87.8 % for benzyl alcohol to benzyl aldehyde (selectivity increasing from 76.7 % to 92.4 %) and from 4.2 % to 53.6 % for ethylene glycol to glycolaldehyde (selectivity increasing from 6.2 % to 62.7 %). Our findings offer a promising strategy for the production of high-value carbon products in heterogeneous catalysis.

Enhanced Charge Transportation in Type II WO3/ZnWO4 Nanoflakes for Boosting Saline Water-splitting Reaction

Photoelectrochemical (PEC) water-splitting is an energy-efficient and eco-friendly technique to produce green hydrogen (H2). Here, WO3 is synthesized for saline water-splitting reaction. Initially, the activity of WO3 is enhanced through morphology tuning. Nanoparticles (NPs), thick nanosheets (TSs), and nanoflakes (NFs) of WO3 are synthesized, and their PEC activity is determined. The NFs show a photocurrent density of 1.53 mA/cm2 at 1.2 V vs. Ag/AgCl, whereas TSs and NPs can generate 1.17 mA/cm2 and 1.07 mA/cm2 at 1.2 V vs. Ag/AgCl, respectively. The low charge transportation rate inhibits the PEC performance of these NFs in water-splitting reactions. To mitigate this problem, the type-II heterojunction is constructed with optimized deposition of ZnWO4 on WO3, which favors the migration of charge-carriers in opposite directions, facilitating the charge-carrier separation and eventually enhancing the PEC activity. The optimized heterojunction shows a photocurrent density 1.5 times greater than bare WO3 and 2.4 times enhanced carrier density, 2.16×1021 cm-3. The heterostructure's rapid OCP decay and higher charge injection efficiency indicate an improved charge transport capability, the primary driving force for enhanced PEC activity. The stability of WO3/ZnWO4 is studied for one hour.

Interface engineering for photoelectrochemical oxygen evolution reaction

Photoelectrochemical (PEC) water splitting provides a promising approach for solving sustainable energy challenges and achieving carbon neutrality goals. The oxygen evolution reaction (OER), a key bottleneck in the PEC water-splitting system occurring at the photoanode/electrolyte interface, plays a fundamental role in sustainable solar fuel production. Proper surface or interface engineering strategies have been proven to be necessary to achieve efficient and stable PEC water oxidation. This review summarizes the recent advances in interface engineering, including junction formation, surface doping, surface passivation or protection, surface sensitization, and OER cocatalyst modification, while highlighting the remarkable research achievements in the field of PEC water splitting. The benefits of each interface engineering strategy and how it enhances the device performance are critically analyzed and compared. Finally, the outlook for the development of interface engineering for efficient PEC water splitting is briefly discussed. This review illustrates the importance of employing rational interface engineering in realizing efficient and stable PEC water splitting devices.

Preparation and Gas-Sensitive Properties of Square-Star-Shaped Leaf-Like BiVO4 Nanomaterials

In this study, square-star-shaped leaf-like BiVO4 nanomaterials were successfully synthesized using a conventional hydrothermal method. The microstructure, elemental composition, and gas-sensing performance of the materials were thoroughly investigated. Morphological analysis revealed that BiVO4 prepared at different reaction temperatures exhibited square-star-shaped leaf-like structures, with the most regular and dense structures formed at 150 °C, exhibiting a large specific surface area of 2.84 m2/g. The response performance of the BiVO4 gas sensors to different target gases was evaluated, and the results showed that the prepared BiVO4 gas sensor exhibited a strong response to NH3. At the optimal operating temperature of 300 °C, its sensitivity to 5 ppm NH3 reached 13.3, with a response time of 28 s and a recovery time of 16 s. Moreover, the gas sensor exhibited excellent repeatability and anti-interference performance. These findings indicate that square-star-shaped leaf-like BiVO4 holds great potential in environmental monitoring and industrial safety detection, offering new insights for the development of high-performance gas sensors.

"Double-Use" Strategy for Improving the Photoelectrochemical Performance of BiVO4 Photoanodes Using a Cobalt-Functionalized Polyoxotungstate

Doping and surface-modification are well-established strategies for the performance enhancement of bismuth vanadate (BiVO4) photoanodes in photoelectrochemical (PEC) water splitting devices. Herein, a "double-use" strategy for the development of high-performance BiVO4 photoanodes for solar water splitting is reported, where a molecular cobalt-phosphotungstate (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) is used both as a bulk doping agent as well as a surface-deposited water oxidation cocatalyst. The use of CoPOM for bulk doping of BiVO4 is shown to enhance the electrical conductivity and improve the charge separation efficiency, resulting in the enhancement of the maximum applied-bias photoconversion efficiency (ABPE) by a factor of ∼18 to 0.54% at 0.87 V vs. RHE, as compared to pristine BiVO4 (0.03% at 1.04 V vs. RHE). The ratio of W/Co on the surface of the photoanode is related to the activity and stability. In addition, modification of CoPOM-doped BiVO4 with CoPOM as a surface cocatalyst enhances the hole extraction and improves the water oxidation kinetics, resulting in the overall enhancement of the ABPE to 0.79% (at 0.82 V vs. RHE), i.e., by a factor of ∼26 with respect to pristine BiVO4. This study establishes the "double-use" strategy involving CoPOMs as an effective, straightforward, and easily scalable approach for the development of high-quality photoanodes for solar water splitting and highlights the future potential of utilizing well-designed polyoxometalates as precursors for the synthesis of energy materials.

Bilayer TiO2/Mo-BiVO4 Photoelectrocatalysts for Ibuprofen Degradation

Heterojunction formation between BiVO4 nanomaterials and benchmark semiconductor photocatalysts has been keenly pursued as a promising approach to improve charge transport and charge separation via interfacial electron transfer for the photoelectrocatalytic degradation of recalcitrant pharmaceutical pollutants. In this work, a heterostructured TiO2/Mo-BiVO4 bilayer photoanode was fabricated by the deposition of a mesoporous TiO2 overlayer using the benchmark P25 titania catalyst on top of Mo-doped BiVO4 inverse opal films as the supporting layer, which intrinsically absorbs visible light below 490 nm, while offering improved charge transport. A porous P25/Mo-BiVO4 bilayer structure was produced from the densification of the inverse opal underlayer after post-thermal annealing, which was evaluated on photocurrent generation in aqueous electrolyte and the photoelectrocatalytic degradation of the refractory anti-inflammatory drug ibuprofen under back-side illumination by visible and UV-Vis light. Significantly enhanced photoelectrochemical performance on both photocurrent density and pharmaceutical degradation was achieved for the bilayer structure with respect to the additive effect of the constituent layers, which was related to the improved light harvesting arising from the backscattering by the mesoporous TiO2 layer in combination with the favorable charge transfer at the TiO2/Mo-BiVO4 interface.

Failure Mechanism Analysis and Emerging Strategies for Enhancing the Photoelectrochemical Stability of Photoanodes

The development of efficient and stable photoanode materials is essential for driving the possible practical application of photoelectrochemical water splitting. This article begins with a basic understanding of the fundamentals of photoelectrochemical devices and photoanodes. State-of-the-art strategies for designing photoanodes with long-term stability are highlighted, including insertion of hole transport layers, construction of protective/passivation layers, loading of co-catalysts, construction of heterojunctions, and modification of the electrolyte. Based on the insights gained from these effective strategies, we present an outlook for addressing key aspects of the challenges of stabilizing photoanodes development in the future work. Widespread adoption of stability assessment criteria will facilitate reliable comparisons of results from different laboratories. In addition, deactivation of photoanode is defined as a 50 % reduction in productivity. An in-depth understanding of the deactivation mechanism is essential for the design and development of efficient and stable photoanodes. This work will provide insights into the stability assessment of photoanode and facilitate the production of practical solar fuels.

Solar-Driven Hydrogen Peroxide Production via BiVO4-Based Photocatalysts

Solar hydrogen peroxide (H2O2) production has garnered increased research interest owing to its safety, cost-effectiveness, environmental friendliness, and sustainability. The synthesis of H2O2 relies mainly on renewable resources such as water, oxygen, and solar energy, resulting in minimal waste. Bismuth vanadate (BiVO4) stands out among various oxide semiconductors for selective H2O2 production under visible light via direct two-electron oxygen reduction reaction (ORR) and two-electron water oxidation reaction (WOR) pathways. Significant advancements have been achieved using BiVO4-based materials in solar H2O2 production over the last decade. This review explores advancements in BiVO4-based photocatalysts for H2O2 production, focusing on photocatalytic powder suspension (PS) and photoelectrochemical (PEC) systems, representing the main approaches for heterogenous artificial photosynthesis. An overview of fundamental principles, performance assessment methodologies, photocatalyst and photoelectrode development, and optimization of reaction conditions is provided. While diverse strategies, such as heterojunction, doping, crystal facet engineering, cocatalyst loading, and surface passivation, have proven effective in enhancing H2O2 generation, this review offers insights into their similar and distinct implementations within the PS and PEC systems. The challenges and future prospects in this field are also discussed to facilitate the rational design of high-performing BiVO4-based photocatalysts and photoelectrodes for H2O2 generation under visible light.

Enhancing Photoelectrocatalytic Efficiency of BiVO4 Photoanodes by Crystal Orientation Control

Bismuth Vanadate (BiVO4) is a promising photoanode material due to its stability and suitable bandgap, making it effective for visible light absorption. However, its photoelectrocatalytic efficiency is often limited by the poor transport dynamics of photogenerated carriers. Recent research found that varying the atomic arrangement in crystals and Fermi levels across different crystal orientations can lead to significant differences in carrier mobility, charge recombination rates, and overall performance. In this work, we optimized the atomic arrangement by controlling the crystal growth direction to improve carrier separation efficiency using a wet chemical method. Systematic investigations revealed that the preferential [010]-oriented BiVO4 film exhibits the highest carrier mobility and photocurrent density. Under an applied bias of 1.21 V (vs. RHE) in a 0.5 M Na2SO4 electrolyte, it achieved a photocurrent density of 0.2 mA cm-2 under AM 1.5 G illumination, significantly higher than that of the [121]-oriented (0.056 mA cm-2) and randomly oriented films (0.11 mA cm-2). This study provides a deeper understanding of the role of crystal orientation in enhancing photoelectrocatalytic efficiency.

Visualizing Ultrafast Photogenerated Electron and Hole Separation in Facet-Engineered Bismuth Vanadate Crystals

Photogenerated charge separation is pivotal for effecting efficient photocatalytic reactions. Understanding this process with spatiotemporal resolution is vital for devising highly efficient photocatalysts. Here, we employed pump-probe transient reflection microscopy to directly observe the temporal and spatial evolution of photogenerated electrons and holes on the surface of facet-engineered bismuth vanadate (BiVO4) crystals. The findings suggest that the anisotropic built-in field of BiVO4 crystals propels the separation of photogenerated electrons and holes toward different facets through a two-step process across varying time scales. Photogenerated electrons and holes undergo ultrafast separation within ∼6 ps, with electrons transforming into localized small polarons toward the {010} facets of truncated BiVO4 octahedral crystals. However, the photogenerated holes prolong their separation up to ∼2000 ps in a drift-diffusion manner before ultimately accumulating on the {120} facets. This work provides a comprehensive visualization of spatiotemporal charge separation at the nano/microscale on semiconductor photocatalysts, which is beneficial for understanding the photocatalysis mechanism.

Oxygen vacancy induced defect dipoles in BiVO4 for photoelectrocatalytic partial oxidation of methane

A strong driving force for charge separation and transfer in semiconductors is essential for designing effective photoelectrodes for solar energy conversion. While defect engineering and polarization alignment can enhance this process, their potential interference within a photoelectrode remains unclear. Here we show that oxygen vacancies in bismuth vanadate (BiVO4) can create defect dipoles due to a disruption of symmetry. The modified photoelectrodes exhibit a strong correlation between charge separation and transfer capability and external electrical poling, which is not seen in unmodified samples. Applying poling at -150 Volt boosts charge separation and transfer efficiency to over 90%. A photocurrent density of 6.3 mA cm-2 is achieved on the photoelectrode after loading with a nickel-iron oxide-based cocatalyst. Furthermore, using generated holes for methane partial oxidation can produce methanol with a Faradaic efficiency of approximately 6%. These findings provide valuable insights into the photoelectrocatalytic conversion of greenhouse gases into valuable chemical products.

Enhancing Photoelectrochemical Water Oxidation on WO3 via Electrochromic Modulation: Universal Effects and Mechanistic Insights

Although WO3 exhibits both electrochromic and photoelectrochemical (PEC) properties, there is no research conducted to investigate the correlation between them. The study herein reports the electrochromic enhancement of PEC activity on WO3. The electrochromic WO3 (e-WO3) exhibits a significantly enhanced activity for PEC water oxidation compared to raw WO3 (r-WO3), with a limiting photocurrent density three times that of r-WO3. The electrochromic enhancement of PEC activity is universal and independent of the type of cations inserted during electrochromism. Decoloring reduces the PEC activity but a simple re-coloring restores the activity to its maximum value. Electrochromism induces large amounts of oxygen vacancies and surface states, the former improving the electron density of WO3 and the latter facilitating the hole transfer across e-WO3/electrolyte interface. It is proved that the electrochromic enhancement effect is due to the significantly improved electron-hole separation efficiency and the charge transfer efficiency across the WO3/electrolyte interface.

Rational Design of ZnO/Sc2CF2 Heterostructure with Tunable Electronic Structure for Water Splitting: A First-Principles Study

Heterostructures are highly promising photocatalyst candidates for water splitting due to their advanced properties than those of pristine components. The ZnO/Sc2CF2 heterostructure was designed in this work, and its electronic structure was investigated to explore its potential for water splitting. The assessments of binding energy, phonon spectrum, ab initio molecular dynamics, and elastic constants provide strong evidence for its stability. The ZnO/Sc2CF2 heterostructure has an indirect band gap of 1.93 eV with a type-Ⅰ band alignment. The electronic structure can be modified with strain, leading to a transition in band alignment from type-Ⅰ to type-Ⅱ. The heterostructure is suitable for water splitting since its VBM and CBM stride over the redox potential. The energy barrier and built-in electric field, resulting from the charge transfer, facilitate the spatial separation of photogenerated carriers, enhancing their utilization efficiency for redox processes. The photogenerated carriers in the heterostructures with lattice compression greater than 6% follow the direct-Z transfer mechanism. The ZnO/Sc2CF2 heterostructure is confirmed with high photocatalytic activity by a Gibbs free energy change of HER, which is 0.89 eV and decreases to -0.52 eV under an 8% compressive strain. The heterostructure exhibits a remarkable enhancement in both absorption range and intensity, which can be further improved with strains. All these findings suggest that the ZnO/Sc2CF2 heterostructure is an appreciated catalyst for efficient photocatalytic water splitting.

BiVO4 Film Coupling with CoAl2O4 Nanoparticles for Photoelectrochemical Water Splitting Utilizing Broad Solar Spectrum through p-n Heterojunction, Photothermal, and Cocatalytic Synergism

Water oxidation is an endothermic and kinetics-sluggish reaction; the research of photoanodes with photothermal and cocatalytic properties is of great significance. Herein, BiVO4/CoAl2O4 film photoanodes were studied for solar water splitting through coupling spinel p-type CoAl2O4 nanoparticles on n-type BiVO4 films. Compared to the BiVO4 photoanode, better performance was observed on the BiVO4/CoAl2O4 photoanode during water oxidation. A photocurrent of 3.47 mA/cm2 was produced on the BiVO4/CoAl2O4 photoanode at 1.23 V vs RHE, which is two-fold to the BiVO4 photoanode (1.70 mA/cm2). Additionally, the BiVO4/CoAl2O4 photoanodes showed an acceptable stability for water oxidation. The BiVO4/CoAl2O4 photoanode being of higher water oxidation performance could be attributed to the presence of p-n heterojunction, cocatalytic, and photothermal effects. In specific, under the excitation of λ < 520 nm light, the holes produced in/on BiVO4 can be transferred to CoAl2O4 owing to the p-n heterojunctions of BiVO4/CoAl2O4. Meanwhile, the temperature on the BiVO4/CoAl2O4 photoanode rises quickly up to ∼53 °C under AM 1.5 G irradiation due to the photothermal property of CoAl2O4 through capturing the 520 < λ < 720 nm light. The temperature rising on the BiVO4/CoAl2O4 photoanode improves the cocatalytic activity of CoAl2O4 and modifies the wettability of BiVO4/CoAl2O4 for effective water oxidation.

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

Heterogeneous CuS QDs/BiVO4@Y2O2S Nanoreactor for Monitorable Photocatalysis

Exploration of multifunctional integrated catalysts is of great significance for photocatalysis toward practical application. Herein, a 1D confined nanoreactor with a heterogeneous core-shell structure is designed for synergies of efficient catalysis and temperature monitoring by custom encapsulation of Z-scheme heterojunction CuS quantum dots/BiVO4 (CuS QDs/BiVO4) and Y2O2S-Er, Yb. The dispersed active sites created by the QDs with high surface energy improve the mass transfer efficiency, and the efficient electron transport channels at the heterogeneous interface extend the carrier lifetime, which endows the nanoreactor with excellent catalytic performance. Meanwhile, real-time temperature monitoring is realized based on the thermally coupled levels 2H11/2/4S3/2→4I15/2 of Er3+ using fluorescence intensity ratio, which enables the monitorable photocatalysis. Furthermore, the nanoreactor with a multidimensional structure increases effective intermolecular collisions to facilitate the catalytic process by restricting the reaction within distinct enclosed spaces and circumvents potential unknown interaction effects. The design of multi-space nanoconfined reactors opens up a new avenue to modulate catalyst function, providing a unique perspective for photocatalytic applications in the mineralization of organic pollutants, hydrogen production, and nitrogen fixation.

Revisiting the photocharging effect on the BiVO4 (010) surface by identifying the charge reaction kinetics

The alteration of intermediates on the (010) facet of BiVO4 leads to variations in charge accumulation numbers before overcoming the rate-determining step, which accounts for the enhanced charge transfer for oxygen evolution. This discovery provides insights into the photocharging effect and the photoelectrochemical reaction mechanism.

Enhanced photoelectrochemical water splitting performance of α-Fe2O3 photoanodes through Co-modification with Co single atoms and g-C3N4

The practical application of α-Fe2O3 in water splitting is hindered by significant charge recombination and slow water oxidation. To address this issue, a CoSAs-g-C3N4/Fe2O3 (CoSAs: cobalt single atoms) photoanode was fabricated in this study through the co-modification of CoSAs and g-C3N4 to enhance photoelectrochemical (PEC) water splitting. The coupling between g-C3N4 and α-Fe2O3 resulted in the formation of a heterojunction, which provided a strong built-in electric field and an additional driving force to mitigate charge recombination. Moreover, g-C3N4 served as a suitable carrier for single atoms, which effectively anchored CoSAs through N/C coordination. The highly dispersed CoSAs provided abundant active sites, which further promoted surface holes extraction and oxidation kinetics, resulting in higher PEC performance and photostability. This study indicates the benefits of these collaborative strategies and provides more efficient designs for solar energy conversion in PEC systems.

Photoelectrocatalytic Hydrogen Generation: Current Advances in Materials and Operando Characterization

Photoelectrochemical (PEC) hydrogen generation is a promising technology for green hydrogen production yet faces difficulties in achieving stability and efficiency. The scientific community is pushing toward the development of new electrode materials and a better understanding of the underlying reactions and degradation mechanisms. Advances in photocatalytic materials are being pursued through the development of heterojunctions, tailored crystal nanostructures, doping, and modification of solid-solid and solid-electrolyte interfaces. Operando and in situ techniques are utilized to deconvolute the charge transfer mechanisms and degradation pathways. In this review, both materials development and Operando characterization are covered for advancing PEC technologies. The recent advances made in the PEC materials are first reviewed including the applied improvement strategies for transition metal oxides, nitrites, chalcogenides, Si, and group III-V semiconductor materials. The efficiency, stability, scalability, and electrical conductivity of the aforementioned materials along with the improvement strategies are compared. Next, the Operando characterization methods and cite selected studies applied for PEC electrodes are described. Operando studies are very successful in elucidating the reaction mechanisms, degradation pathways, and charge transfer phenomena in PEC electrodes. Finally, the standing challenges and the potential opportunities are discussed by providing recommendations for designing more efficient and electrochemically stable PEC electrodes.

First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures

Heterostructure catalysts are highly anticipated in the field of photocatalytic water splitting. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and the electronic structures were revealed with the first-principles method to explore their photocatalytic properties for water splitting. The results found that the thermodynamically stable AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with reduced band gaps of 1.75 eV and 1.84 eV, respectively. These two heterostructures have been confirmed to have type-Ⅰ band alignments, with both VBM and CBM contributed to by the Sc2CF2 layer. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures exhibit the potential for photocatalytic water splitting as their VBM and CBM stride over the redox potential of water. Gibbs free energy changes in HER occurring on AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are as low as -0.31 eV and -0.59 eV, respectively. The Gibbs free energy change in HER on the AlN (GaN) layer is much lower than that on the Sc2CF2 surface, owing to the stronger adsorption of H on AlN (GaN). The AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to monolayered AlN, GaN, and Sc2CF2. In addition, the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures can be effectively tuned with strains. All the results indicate that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable catalysts for photocatalytic water splitting.

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Enhanced Label-Free Photoelectrochemical Strategy for Pollutant Detection: Using Surface Oxygen Vacancies-Enriched BiVO4 Photoanode

Label-free photoelectrochemical sensors have the advantages of high sensitivity and a simple electrode structure. However, its performance is greatly limited due to the photoactive materials' weak photoactivity and poor stability. Herein, a robust homogeneous photoelectrochemical (PEC) aptasensor has been constructed for atrazine (ATZ) based on photoetching (PE) surface oxygen vacancies (Ov)-enriched Bismuth vanadate (BiVO4) (PE-BVO). The surface of the Ov improves the carrier separation ability of BiVO4, thus providing a superior signal substrate for the sensor. A thiol molecular layer self-assembled on PE-BVO acts as a blocker, while 2D graphene acts as a signal-on probe after release from the aptamer-graphene complex. The fabricated sensor has a wide linear detection range of 0.5 pM to 10.0 nM and a low detection limit of 0.34 pM (S/N = 3) for ATZ. In addition, it can efficiently work in a wide pH range (3-13) and high ionic strength (∼6 M Na+), which provides promising opportunities for detecting environmental pollutants under complex conditions.

Unveiling the effect of the structural transformation of CoZn-MOF on BiVO4 photoanode for efficient photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting has been widely investigated for solar-to-hydrogen conversion. However, issues like high charge recombination rate and slow surface water oxidation kinetics severely hinder its (PEC) conversion efficiency. Herein, we constructed MOF-derived CoOOH cocatalyst on BiVO4 photoanode, using a feasible electrochemical activation strategy. The BiVO4-based photoanode obtained shows a high photocurrent density of 3.15 mA/cm2 at 1.23 VRHE and low onset potential. Detailed experiments and theoretical calculations show that during the activation of CoZn-MOFs, there was a partial breakage of 2-methylimidazole (mIM) linker, an increase in the oxidation state of Cobalt ion (Co), and increased O2-. The high PEC performance is mainly attributed to the MOF-derived CoOOH, which provides rich active sites for hole extraction and reduces the overpotential for oxygen evolution reaction. Furthermore, when CoZnNiFe-LDHs were decorated on BiVO4 using the ions exchange method, the photocurrent density of BiVO4/CoZnNiFe-LDHs photoanode got to 4.0 mA/cm2 at 1.23 VRHE, accompanied with high stability. This study provides insights into understanding the key role played by the structural transformation of MOF cocatalyst in PEC water splitting processes.

Alternating 3rd- to 2nd-Order Charge Reaction Kinetics on Bismuth Vanadate Photoanodes with Ultrathin Bismuth Metal-Organic-Frameworks

The most challenging obstacle for photocatalysts to efficiently harvest solar energy is the sluggish surface redox reaction (e. g., oxygen evolution reaction, OER) kinetics, which is believed to originate from interface catalysis rather than the semiconductor photophysics. In this work, we developed a light-modulated transient photocurrent (LMTPC) method for investigating surface charge accumulation and reaction on the W-doped bismuth vanadate (W : BiVO4) photoanodes during photoelectrochemical water oxidation. Under illuminating conditions, the steady photocurrent corresponds to the charge transfer rate/kinetics, while the integration of photocurrent (I~t) spikes during the dark period is regarded as the charge density under illumination. Quantitative analysis of the surface hole densities and photocurrents at 0.6 V vs. reversible hydrogen electrode results in an interesting rate-law kinetics switch: a 3rd-order charge reaction behavior appeared on W : BiVO4, but a 2nd-order charge reaction occurred on W : BiVO4 surface modified with ultrathin Bi metal-organic-framework (Bi-MOF). Consequently, the photocurrent for water oxidation on W : BiVO4/Bi-MOF displayed a 50 % increment. The reaction kinetics alternation with new interface reconstruction is proposed for new mechanism understanding and/or high-performance photocatalytic applications.

Light concentration and electron transfer in plasmonic-photonic Ag,Au modified Mo-BiVO4 inverse opal photoelectrocatalysts

Plasmonic photocatalysis based on metal-semiconductor heterojunctions is considered a key strategy to evade the inherent limitations of poor light harvesting and charge separation of semiconductor photocatalysts. It can be profitably combined with three-dimensional photonic crystals (PCs) that offer an ideal scaffold for loading plasmonic nanoparticles and a unique architecture to intensify photon capture. In this work, Mo-doped BiVO4 inverse opals were applied as visible light-responsive photonic hosts of Ag and/or Au plasmonic nanoparticles in order to exploit the synergy of plasmonic and photonic amplification effects with interfacial charge transfer for the photoelectrocatalytic degradation of recalcitrant pharmaceutical contaminants under visible light. Photoelectrochemical evaluation indicated a major contribution from hot spot-assisted local field enhancement, most pronounced for Ag/Mo-BiVO4 PCs due to the spectral overlap of the localized surface plasmon resonance with the electronic absorption and blue-edge slow photon region of Mo-BiVO4 PCs, in contrast to weak plasmonic sensitization effects for the Au-modified PCs. The diverse band alignment at the metal-semiconductor interfaces resulted in the enhanced photoelectrocatalytic degradation of tetracycline broad spectrum antibiotic by Ag/Mo-BiVO4 and the refractory ibuprofen drug by (Ag,Au)/Mo-BiVO4, attributed to the enhanced charge separation by electron transfer toward Ag nanoparticles. Combination of visible light activated semiconductor PCs and plasmonic nanoparticles with suitable band alignment and photonic band gap may provide a versatile approach for the rational design of efficient plasmonic-photonic photoeletrocatalysts.

An MgO passivation layer and hydrotalcite derived spinel Co2AlO4 synergically promote photoelectrochemical water oxidation conducted using BiVO4-based photoanodes

MxCo3-xO4 co-catalysed photoanodes with high potential for improvement in PEC water-oxidizing properties are reported. However, it is difficult to control the recombination of photogenerated carriers at the interface between the catalyst and cocatalyst. Here, an ultra-thin MgO passivation layer was introduced into the MxCo3-xO4/BiVO4 coupling system to construct a ternary composite photoanode Co2AlO4/MgO/BiVO4. The photocurrent density of the electrode is 3.52 mA cm-2, which is 3.2 times that of BiVO4 (at 1.23 V vs. RHE). The photocurrent is practically increased by 0.86 mA cm-2 and 1.56 mA cm-2 in comparison with that of Co2AlO4/BiVO4 and MgO/BiVO4 electrodes, respectively. Meanwhile, the Co2AlO4/MgO/BiVO4 electrode has the highest charge separation efficiency, the lowest charge transfer resistance (Rct) and best stability. The excellent PEC performance could be attributed to the inhibitive effect provided by the MgO passivation layer that efficaciously suppresses the electron-hole recombination at the interface and drives the hole transfer outward, which is induced by Co2AlO4 to capture the electrode/electrolyte interface for efficient water oxidation reaction. In order to understand the origin of this improvement, first-principles calculations with density functional theory (DFT) were performed. The theoretical investigation converges to our experimental results. This work proposes a novel idea for restraining the recombination of photogenerated carriers between interfaces and the rational design of efficient photoanodes.

Unveiling Enhanced PEC Water Oxidation: Morphology Tuning and Interfacial Phase Change in α-Fe2O3@K-OMS-2 Branched Core-Shell Nanoarrays

A simple fabrication method that involves two steps of hydrothermal reaction has been demonstrated for the growth of α-Fe2O3@K-OMS-2 branched core-shell nanoarrays. Different reactant concentrations in the shell-forming step led to different morphologies in the resultant composites, denoted as 0.25 OC, 0.5 OC, and 1.0 OC. Both 0.25 OC and 0.5 OC formed perfect branched core-shell structures, with 0.5 OC possessing longer branches, which were observed by SEM and TEM. The core K-OMS-2 and shell α-Fe2O3 were confirmed by grazing incidence X-ray diffraction (GIXRD), EDS mapping, and atomic alignment from high-resolution STEM images. Further investigation with high-resolution HAADF-STEM, EELS, and XPS indicated the existence of an ultrathin layer of Mn3O4 sandwiched at the interface. All composite materials offered greatly enhanced photocurrent density at 1.23 VRHE, compared to the pristine Fe2O3 photoanode (0.33 mA/cm2), and sample 0.5 OC showed the highest photocurrent density of 2.81 mA/cm2. Photoelectrochemical (PEC) performance was evaluated for the samples by conducting linear sweep voltammetry (LSV), applied bias photo-to-current efficiency (ABPE), electrochemical impedance spectroscopy (EIS), incident-photo-to-current efficiency (IPCE), transient photocurrent responses, and stability tests. The charge separation and transfer efficiencies, together with the electrochemically active surface area, were also investigated. The significant enhancement in sample 0.5 OC is ascribed to the synergetic effect brought by the longer branches in the core-shell structure, the conductive K-OMS-2 core, and the formation of the Mn3O4 thin layer formed between the core and shell.

Photoilluminated Redox-Processed Rh2P Nanoparticles on Photocathodes for Stable Hydrogen Production in Acidic Environments

While photoelectrochemical (PEC) cells show promise for solar-driven green hydrogen production, exploration of various light-absorbing multilayer coatings has yet to significantly enhance their hydrogen generation efficiency. Acidic conditions can enhance the hydrogen evolution reaction (HER) kinetics and reduce overpotential losses. However, prolonged acidic exposure deactivates noble metal electrocatalysts, hindering their long-term stability. Progress requires addressing catalyst degradation to enable stable, efficient, and acidic PEC cells. Here, we proposed a process design based on the photoilluminated redox deposition (PRoD) approach. We use this to grow crystalline Rh2P nanoparticles (NPs) with a size of 5-10 on 30 nm-thick TiO2, without annealing. Atomically precise reaction control was performed by using several cyclic voltammetry cycles coincident with light irradiation to create a system with optimal catalytic activity. The optimized photocathode, composed of Rh2P/TiO2/Al-ZnO/Cu2O/Sb-Cu2O/ITO, achieved an excellent photocurrent density of 8.2 mA cm-2 at 0 VRHE and a durable water-splitting reaction in a strong acidic solution. Specifically, the Rh2P-loaded photocathode exhibited a 5.3-fold enhancement in mass activity compared to that utilizing just a Rh catalyst. Furthermore, in situ scanning transmission electron microscopy (STEM) was performed to observe the real-time growth process of Rh2P NPs in a liquid cell.

Stabilizing BiVO4 Photoanode in Bicarbonate Electrolyte for Efficient Photoelectrocatalytic Alcohol Oxidation

In order to expand the application of bismuth vanadate (BiVO4) to the field of photoelectrochemistry, researchers have explored the potential of BiVO4 in catalyzing or degrading organic substances, potentially presenting a green and eco-friendly solution. A study was conducted to investigate the impact of electrolytes on the photocatalysis of benzyl alcohol by BiVO4. The research discovered that, in an acetonitrile electrolyte (pH 9) with sodium bicarbonate, BiVO4 catalyzed benzyl alcohol by introducing saturated V5+. This innovation addressed the issue of benzyl alcohol being susceptible to catalysis in an alkaline setting, as V5+ was prone to dissolution in pH 9 on BiVO4. The concern of the photocorrosion of BiVO4 was mitigated through two approaches. Firstly, the incorporation of a non-aqueous medium inhibited the formation of active material intermediates, reducing the susceptibility of the electrode surface to photocorrosion. Secondly, the presence of saturated V5+ further deterred the leaching of V5+. Concurrently, the production of carbonate radicals by bicarbonate played a vital role in catalyzing benzyl alcohol. The results show that, in this system, BiVO4 has the potential to oxidize benzyl alcohol by photocatalysis.

Understanding the Internal Conversion Efficiency of BiVO4/SnO2 Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis

This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (<5 nm) SnO2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample's thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott-Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential.

The route for commercial photoelectrochemical water splitting: a review of large-area devices and key upscaling challenges

Green-hydrogen is considered a "key player" in the energy market for the upcoming decades. Among currently available hydrogen (H2) production processes, photoelectrochemical (PEC) water splitting has one of the lowest environmental impacts. However, it still presents prohibitively high production costs compared to more mature technologies, such as steam methane reforming. Therefore, the competitiveness of PEC water splitting must rely on its environmental and functional advantages, which are strongly linked to the reactor design, to the intrinsic properties of its components, and to their successful upscaling. This review gives special attention to the engineering aspects and categorizes PEC devices into four main types, according to the configuration of electrodes and strategies for gas separation: wired back-to-back, wireless back-to-back, wired side-by-side, and wired separated electrode membrane-free. Independently of the device architecture, the use of concentrated sunlight was found to be mandatory for achieving competitive green-H2 production. Additionally, feasible strategies for upscaling the key components of PEC devices, especially photoelectrodes, are urgently needed. In a pragmatic context, the way to move forward is to accept that PEC devices will operate close to their thermodynamic limits at large-scale, which requires a solid convergence between academics and industry. Research efforts must be redirected to: (i) build and demonstrate modular devices with a low-cost and highly recyclable embodiment; (ii) optimize thermal and power management; (iii) reduce ohmic losses; (iv) enhance the chemical stability towards a thousand hours; (v) couple solar concentrators with PEC devices; (vi) boost PEC-H2 production through the use of organic compounds; and (vii) reach consensual standardized methods for evaluating PEC devices, at both environmental and techno-economic levels. If these targets are not met in the next few years, the feasibility of PEC-H2 production and its acceptance by industry and by the general public will be seriously compromised.

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

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

De-Pinning Fermi Level and Accelerating Surface Kinetics with an ALD Finish Boost the Fill Factor of BiVO4 Photoanodes to 44

With the rapid development of performance and long-term stability, bismuth vanadate (BiVO4 ) has emerged as the preferred photoanode in photoelectrochemical tandem devices. Although state-of-the-art BiVO4 photoanodes realize a saturated photocurrent density approaching the theoretical maximum, the fill factor (FF) is still inferior, pulling down the half-cell applied bias photon-to-current efficiency (HC-ABPE). Among the major fundamental limitations are the Fermi level pinning and sluggish surface kinetics at the low applied potentials. This work demonstrates that the plasma-assisted atomic layer deposition technique is capable of addressing these issues by seamlessly installing an angstrom-scale FeNi-layer between BiVO4 and electrolyte. Not only this ultrathin FeNi layer serves as an efficient OER cocatalyst, more importantly, it also effectively passivates the surface states of BiVO4 , de-pins the surface Fermi level, and enlarges the built-in voltage, allowing the photoanode to make optimal use of the photogenerated holes for achieving high FF up to 44% and HC-ABPE to 2.2%. This study offers a new approach for enhancing the FF of photoanodes and provides guidelines for designing efficient unassisted solar fuel devices.

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

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

Highly Selective Ammonia Oxidation on BiVO4 Photoanodes Co-catalyzed by Trace Amounts of Copper Ions

High-efficient photoelectrocatalytic direct ammonia oxidation reaction (AOR) conducted on semiconductor photoanodes remains a substantial challenge. Herein, we develop a strategy of simply introducing ppm levels of Cu ions (0.5-10 mg/L) into NH3 solutions to significantly improve the AOR photocurrent of bare BiVO4 photoanodes from 3.4 to 6.3 mA cm-2 at 1.23 VRHE , being close to the theoretical maximum photocurrent of BiVO4 (7.5 mA cm-2 ). The surface charge-separation efficiency has reached 90 % under a low bias of 0.8 VRHE . This AOR exhibits a high Faradaic efficiency (FE) of 93.8 % with the water oxidation reaction (WOR) being greatly suppressed. N2 is the main AOR product with FEs of 71.1 % in aqueous solutions and FEs of 100 % in non-aqueous solutions. Through mechanistic studies, we find that the formation of Cu-NH3 complexes possesses preferential adsorption on BiVO4 surfaces and efficiently competes with WOR. Meanwhile, the cooperation of BiVO4 surface effect and Cu-induced coordination effect activates N-H bonds and accelerates the first rate-limiting proton-coupled electron transfer for AOR. This simple strategy is further extended to other photoanodes and electrocatalysts.

Subnanometric Bismuth Clusters Confined in Pyrochlore-Bi2 Sn2 O7 Enable Remarkable CO2 Photoreduction

The development of highly efficient photocatalysts for conversion of carbon dioxide (CO2 ) with water (H2 O) into chemical fuels is of great importance for energy sustainability and carbon resource utilization. Herein, we demonstrated a facile hydrothermal method for in situ construction of subnanometric Bi metallic clusters in pyrochlore-Bi2 Sn2 O7 frameworks, leading to the remarkable improvements of photocatalytic performances for CO2 reduction into CO in the absence of sacrificial reagent. More specifically, an outstanding CO evolution activity of 114.1 μmol g-1  h-1 has been achieved, more than 20-fold improvement compared with the pristine Bi2 Sn2 O7 (5.7 μmol g-1  h-1 ). Detailed experiments together with in situ characterizations reveal that the spatially confined Bi clusters could significantly promote charge-separation/electron-enrichment and adsorption/activation of CO2 molecules, which provides highly efficient reaction channels to facilitate the generation of *COOH intermediate as well as the subsequent desorption of *CO towards CO formation. These demonstrations provide an important knowledge for precise design and fabrication of highly efficient photocatalysts for CO2 conversion into solar fuels.

Preparation of Surface Dispersed WO3/BiVO4 Heterojunction Arrays and Their Photoelectrochemical Performance for Water Splitting

In this work, a surface dispersed heterojunction of BiVO4-nanoparticle@WO3-nanoflake was successfully prepared by hydrothermal combined with solvothermal method. We optimized the morphology of the WO3 nanoflakes and BiVO4 nanoparticles by controlling the synthesis conditions to get the uniform BiVO4 loaded on the surface of WO3 arrays. The phase composition and morphology evolution with different reaction precursors were investigated in detail. When used as photoanodes, the WO3/BiVO4 composite exhibits superior activity with photocurrent at 3.53 mA cm-2 for photoelectrochemical (PEC) water oxidation, which is twice that of pure WO3 photoanode. The superior surface dispersion structure of the BiVO4-nanoparticle@WO3-nanoflake heterojunction ensures a large effective heterojunction area and relieves the interfacial hole accumulation at the same time, which contributes to the improved photocurrents together with the stability of the WO3/BiVO4 photoanodes.

Unveiling the Influence of Sulfur Doping on Photoelectrochemical Performance in BiVO4/FeOOH Heterostructures

BiVO4 is a promising photoanode for photoelectrochemical (PEC) water splitting but suffers from high charge carrier recombination and sluggish surface water oxidation kinetics that limit its efficiency. In this work, a model of sulfur-incorporated FeOOH cocatalyst-loaded BiVO4 was constructed. The composite photoanode (BiVO4/S-FeOOH) demonstrates an enhanced photocurrent density of 3.58 mA cm-2, which is 3.7 times higher than that of the pristine BiVO4 photoanode. However, the current explanations for the generation of enhanced photocurrent signals through the incorporation of elements and cocatalyst loading remain unclear and require further in-depth research. In this work, the hole transfer kinetics were investigated by using a scanning photoelectrochemical microscope (SPECM). The results suggest that the incorporation of sulfur can effectively improve the charge transfer capacity of FeOOH. Moreover, the oxygen evolution reaction model provides evidence that S-doping can induce a "fast" surface catalytic reaction at the cocatalyst/solution interface. The work not only presents a promising approach for designing a highly efficient photoanode but also offers valuable insights into the role of element doping in the PEC water-splitting system.

Progress in Promising Semiconductor Materials for Efficient Photoelectrocatalytic Hydrogen Production

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

Dual Heterojunctions and Nanobowl Morphology Engineered BiVO4 Photoanodes for Enhanced Solar Water Splitting

Photoelectrochemical (PEC) water splitting represents an attractive strategy to realize the conversion from solar energy to hydrogen energy, but severe charge recombination in photoanodes significantly limits the conversion efficiency. Herein, a unique BiVO4 (BVO) nanobowl (NB) heterojunction photoanode, which consists of [001]-oriented BiOCl underlayer and BVO nanobowls containing embedded BiOCl nanocrystals, is fabricated by nanosphere lithography followed by in situ transformation. Experimental characterizations and theoretical simulation prove that nanobowl morphology can effectively enhance light absorption while reducing carrier diffusion path. Density functional theory (DFT) calculations show the tendency of electron transfer from BVO to BiOCl. The [001]-oriented BiOCl underlayer forms a compact type II heterojunction with the BVO, favoring electron transfer from BVO through BiOCl to the substrate. Furthermore, the embedded BiOCl nanoparticles form a bulk heterojunction to facilitate bulk electron transfer. Consequently, the dual heterojunctions engineered BVO/BiOCl NB photoanode exhibits attractive PEC performance toward water oxidation with an excellent bulk charge separation efficiency of 95.5%, and a remarkable photocurrent density of 3.38 mA cm-2 at 1.23 V versus reversible hydrogen electrode, a fourfold enhancement compared to the flat BVO counterpart. This work highlights the great potential of integrating dual heterojunctions engineering and morphology engineering in fabricating high-performance photoelectrodes toward efficient solar conversion.

Piezoelectric polarization induced by dual piezoelectric materials ZnO nanosheets/MoS2 heterostructure for enhancing photoelectrochemical water splitting

Zinc oxide (ZnO) has a broad range of applications in piezo-photoelectrochemical water splitting. However, the narrow light absorption range and high photogenerated carrier recombination efficiency make ZnO somewhat limited in applying piezo-photoelectrochemical water splitting. Heterogeneous structure construction is a superior handle to these two drawbacks. Herein, few-layer molybdenum disulfide (MoS2) nanospheres are compounded on ZnO nanosheets (NSs) to form a dual-piezoelectric-material heterojunction of ZnO NSs/MoS2. The photocurrent density of ZnO NSs/MoS2 reaches 0.68 mA/cm2 at 1.23 V vs. RHE under ultrasonic vibrations. It is 2.4 times higher than that of ZnO NSs under ultrasonic vibrations. The efficient piezo-photoelectrochemical performance is attributed to increased absorption range and polarization field. On the one hand, the narrow band gap of the few-layer MoS2 widens the light absorption range of ZnO. On the other hand, compared to pure ZnO NSs, ZnO NSs/MoS2 has an enhanced polarization field under ultrasonic vibrations due to the piezoelectric properties of dual piezoelectric materials, which dramatically accelerates the electron transfer and suppresses the recombination of between electrons and holes. This work provides a new approach to constructing photoelectrodes with effective piezoelectric photocatalytic properties.

Shape-Controlled First-Row Transition Metal Vanadates for Electrochemical and Photoelectrochemical Water Splitting

Transition metal vanadates (MVs) possess abundant electroactive sites, short ion diffusion pathways, and optical properties that make them suitable for various electrochemical (EC) and photoelectrochemical (PEC) applications. While these materials are commonly used in energy storage devices like batteries and capacitors, their shape-controlled 1D and 2D morphologies have gained equal popularity in water splitting (WS) technology in recent times. This review focuses on recent progress made on various first-row (3d, 4 s) transition metal vanadates (t-MVs) having controlled one-dimensional (fiber, wire, or rod) and two-dimensional (layered or sheet) morphologies with a specific emphasis on copper vanadates (CuV), cobalt vanadates (CoV), iron vanadates (FeV), and nickel vanadates (NiV). The review covers different aspects of shape-controlled 1D and 2D t-MVs including optoelectrical properties, wet chemistry synthesis, and electrochemical (EC-WS) and photoelectrochemical water splitting (PEC-WS) performance in terms of onset potential, overpotential, and long-term stability or high cyclic performance. The review concludes by providing some possible thoughts on how to promote the water-splitting attributes of shape-controlled t-MVs more effectively.