Progress on ternary oxide-based photoanodes for use in photoelectrochemical cells for solar water splitting

Heavy-Metal-Free Colloidal Quantum Dots: Progress and Opportunities in Solar Technologies

Colloidal quantum dots (QDs) hold great promise as building blocks in solar technologies owing to their remarkable photostability and adjustable properties through the rationale involving size, atomic composition of core and shell, shapes, and surface states. However, most high-performing QDs in solar conversion contain hazardous metal elements, including Cd and Pb, posing significant environmental risks. Here, a comprehensive review of heavy-metal-free colloidal QDs for solar technologies, including photovoltaic (PV) devices, solar-to-chemical fuel conversion, and luminescent solar concentrators (LSCs), is presented. Emerging synthetic strategies to optimize the optical properties by tuning the energy band structure and manipulating charge dynamics within the QDs and at the QDs/charge acceptors interfaces, are analyzed. A comparative analysis of different synthetic methods is provided, structure-property relationships in these materials are discussed, and they are correlated with the performance of solar devices. This work is concluded with an outlook on challenges and opportunities for future work, including machine learning-based design, sustainable synthesis, and new surface/interface engineering.

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

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

Dual Functionality of Surface States in Dictating Photocurrent Generation of Au Nanocluster-Sensitized TiO2 Electrodes

Gold nanoclusters (Au NCs), composed of only a few atoms, exhibit molecule-like behavior due to their distinct electronic structures arising from quantum confinement effects. Unlike their plasmonic nanoparticle counterparts, these nonplasmonic Au NCs possess unique properties with significant potential for photosensitizer applications. While traditional and NC-based electrodes share architectural similarities, the photoelectrochemical (PEC) behavior of the latter diverges significantly. Sensitizing TiO2 with Au NCs introduces additional surface trap states. In contrast to conventional photosensitizers, where surface states typically have a negligible impact on hole transfer, these trap states actively mediate the charge transfer process in Au NC-sensitized TiO2 electrodes. In this study, we employed impedance spectroscopy to elucidate the role of surface trap states in photocurrent generation. Our investigation revealed that these states are critical in determining PEC performance, presenting a dichotomy: they facilitate charge transfer (enhancing PEC performance) while simultaneously promoting carrier recombination (limiting efficiency). We demonstrated that the judicious control of otherwise deleterious surface trap states can significantly boost photocurrent. Our findings highlight that the dual nature of surface trap states demands a comprehensive investigation to fully understand their intricate impact on PEC performance.

Oxygen vacancy engineering of ultra-small CuWO4 nanoparticles for boosting photocatalytic organic pollutant degradation

Nanomaterials have attracted great interest in the field of photocatalytic degradation due to their larger specific surface area and efficient charge/mass transfer ability, which are beneficial for enhancing photocatalytic activity. However, the bandgap of photocatalysts would increase with the size reduction, weakening the photoabsorption ability. Thus the relationship between the size of catalysts and photoactivity should be balanced to achieve optimal photocatalytic performance. Herein, ultra-small CuWO4 nanoparticles (ca. 39 nm) with moderate oxygen vacancies (CuWO4-OVs) were synthesized by the cascade strategy (ligand confinement@fast calcination). The introduction of oxygen vacancies offset the deficiency of light absorption ability caused by the small size effect. Besides, oxygen vacancies could provide more reaction active sites, conducive to the adsorption and activation of dye molecules and H2O. Degradation experiments reveal that the optimized photocatalyst CuWO4-OVs 350 shows outstanding photocatalytic activity, and the removal ratio of methylene blue (MB) reaches over 90.26% in 70 min, exceeding that of pure CuWO4-air (37.66%). Additionally, the degradation performance of CuWO4-OVs 350 surpasses most of the other CuWO4-based photocatalytic systems. More importantly, the photocatalytic degradation activity of CuWO4-OVs 350 could remain at 88.26% even after five cycles, and high photostability was achieved. This work affords constructive inspiration for synergistic photoactivity enhancement and increase of catalyst reaction active sites to achieve eminent photocatalytic degradation performance.

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.

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.

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.

Cocatalysts-Photoanode Interface in Photoelectrochemical Water Splitting: Understanding and Insights

Sluggish oxygen evolution reactions on photoanode surfaces severely limit the application of photoelectrochemical (PEC) water splitting. The loading of cocatalysts on photoanodes has been recognized as the simplest and most efficient optimization scheme, which can reduce the surface barrier, provide more active sites, and accelerate the surface catalytic reaction kinetics. Nevertheless, the introduction of cocatalysts inevitably generates interfaces between photoanodes and oxygen evolution cocatalysts (Ph/OEC), which causes severe interfacial recombination and hinders the carrier transfer. Recently, many researchers have focused on cocatalyst engineering, while few have investigated the effect of the Ph/OEC interface. Hence, to maximize the advantages of cocatalysts, interfacial problems for designing efficient cocatalysts are systematically introduced. In this review, the interrelationship between the Ph/OEC and PEC performance is classified and some methods for characterizing Ph/OEC interfaces are investigated. Additionally, common interfacial optimization strategies are summarized. This review details cocatalyst-design-based interfacial problems, provides ideas for designing efficient cocatalysts, and offers references for solving interfacial problems.

C-C Bond Formation Coupled with C-C Bond Cleavage during Oxidative Upgrading of Glycerol on a Nanoporous BiVO4 Photoanode

Production of biodiesel generates glycerol as a 10 wt% byproduct. Therefore, efficient and selective glycerol upgrading is critical for the sustainable production of biodiesel as well as for the production of chemicals from renewable feedstocks. In this study, the photoelectrochemical glycerol oxidation reaction (GOR) was investigated using a nanoporous BiVO4 photoanode in pH 9.3 and pH 2 buffer solutions. In both solutions, glycolaldehyde (GCAD), a C2 species, was the major product, which has never been the major product in any previous electrochemical or photoelectrochemical GOR study. To produce GCAD from the C3 species glycerol, C-C cleavage should occur to produce C2 and C1 species with a 1:1 ratio. Intriguingly, our results show that, during photoelectrochemical GOR on BiVO4, more GCAD is produced than can be explained by simple C-C cleavage, meaning that GCAD is also produced from C-C coupling of two C1 species produced from C-C cleavage. This is equivalent to converting two glycerol molecules to three GCAD molecules, which offers an extraordinary way to maximize GCAD production. To gain further insight into the nature of this unprecedented C-C coupling during GOR, photoelectrochemical oxidation of intermediate oxidation products (glyceraldehyde and 1,3-dihydroxyacetone) and glycerol-1,3-13C2 was compared to that of standard glycerol. Photoelectrochemical GOR was also compared with electrochemical GOR on BiVO4 to interrogate whether light is critical for the observed C-C coupling. Results obtained from comprehensive control experiments revealed critical information about C-C cleavage and C-C coupling during GOR on BiVO4.

Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Photoanodes via WO3 Nanostructuring

WO3/BiVO4 heterojunction photoanodes can be efficiently employed in photoelectrochemical (PEC) cells for the conversion of water into molecular oxygen, the kinetic bottleneck of water splitting. Composite WO3/BiVO4 photoelectrodes possessing a nanoflake-like morphology have been synthesized through a multistep process and their PEC performance was investigated in comparison to that of WO3/BiVO4 photoelectrodes displaying a planar surface morphology and similar absorption properties and thickness. PEC tests, also in the presence of a sacrificial hole scavenger, electrochemical impedance analysis under simulated solar irradiation, and incident photon to current efficiency measurements highlighted that charge transport and charge recombination issues affecting the performance of the planar composite can be successfully overcome by nanostructuring the WO3 underlayer in nanoflake-like WO3/BiVO4 heterojunction electrodes.

Conformal Coverage of ZnO Nanowire Arrays by ZnMnO3 : Room-temperature Photodeposition from Aqueous Solution

Compositionally and structurally complex semiconductor oxide nanostructures gain importance in many energy-related applications. Simple and robust synthesis routes ideally complying with the principles of modern green chemistry are therefore urgently needed. Here we report on the one-step, room-temperature synthesis of a crystalline-amorphous biphasic ternary metal oxide at the ZnO surface using aqueous precursor solutions. More specifically, conformal and porous ZnMnO3 shells are photodeposited from KMnO4 solution onto immobilized ZnO nanowires acting not only as the substrate but also as the Zn precursor. This water-based, low temperature process yields ZnMnO3 /ZnO composite electrodes featuring in 1 M Na2 SO4 aqueous solution capacitance values of 80-160 F g-1 (as referred to the total mass of the porous film i. e. the electroactive ZnMnO3 phase and the ZnO nanowire array). Our results highlight the suitability of photodeposition as a simple and green route towards complex functional materials.

Perovskite-Solar-Cell-Powered Integrated Fuel Conversion and Energy-Storage Devices

Metal halide hybrid perovskite solar cells (PSCs) have received considerable attention over the past decade owing to their potential for low-cost, solution-processable, earth-abundant, and high-performance superiority, increasing power conversion efficiencies of up to 25.7%. Solar energy conversion into electricity is highly efficient and sustainable, but direct utilization, storage, and poor energy diversity are difficult to achieve, resulting in a potential waste of resources. Considering its convenience and feasibility, converting solar energy into chemical fuels is regarded as a promising pathway for boosting energy diversity and expanding its utilization. In addition, the energy conversion-storage integrated system can efficiently sequentially capture, convert, and store energy in electrochemical energy storage devices. However, a comprehensive overview focusing on PSC-self-driven integrated devices with a discussion of their development and limitations remains lacking. Here, focus is on the development of representative configurations of emerging PSC-based photo-electrochemical devices including self-charging power packs, unassisted solar water splitting/CO2 reduction. The advanced progresses in this field, including configuration design, key parameters, working principles, integration strategies, electrode materials, and their performance evaluations are also summarized. Finally, scientific challenges and future perspectives for ongoing research in this field are presented.

Multifunctional Role of Ag-Substitution in Enhancing the Photoelectrochemical Properties of LaFeO3 Photocathodes

Earth-abundant LaFeO3 is a promising p-type semiconductor for photoelectrochemical cells due to its stable photoresponses, high photovoltages and appropriate band alignments, but the photoelectrochemical properties of LaFeO3 , especially the incident-photon-to-current conversion efficiency, need to be further improved. Herein, we propose to partially substitute La3+ of LaFeO3 with Ag+ to enhance the photoelectrochemical performance of LaFeO3 . The combined experimental and computational studies show that Ag-substitution improves surface charge transfer kinetics through introducing active electronic states and increasing electrochemically active surface areas. Furthermore, Ag-substitution decreases grain boundary number and increases majority carrier density, which promotes bulk charge transports. Ag-substitution also reduces the bandgap energy, increasing the flux of carriers involved in photoelectrochemical reactions. As a result, after 8 % Ag-substitution, the photocurrent density of LaFeO3 is enhanced by more than 6 times (-0.64 mA cm-2 at 0.5 V vs RHE) in the presence of oxygen, which is the highest photocurrent gain compared with other cation substitution or doping. The corresponding photocurrent onset potential also demonstrates a positive shift of 30 mV. This work highlights the versatile effects of Ag-substitution on the photoelectrochemical properties of LaFeO3 , which can provide useful insights into the mechanism of enhanced photoelectrochemical performance by doping or substitution.

Ni(II)-doped CuWO4 photoanodes with enhanced photoelectrocatalytic activity

CuWO4 has emerged in the last years as a ternary metal oxide material for photoanodes application in photoelectrochemical cells, thanks to its relatively narrow band gap, high stability and selectivity toward the oxygen evolution reaction, though largely limited by its poor charge separation efficiency. Aiming at overcoming this limitation, we investigate here the effects that Cu(II) ion substitution has on the photoelectrocatalytic (PEC) performance of copper tungstate. Optically transparent CuWO4 thin-film photoanodes, prepared via spin coating and containing different amounts of Ni(II) ions, were fully characterized via UV-Vis spectroscopy, XRD and SEM analyses, and their PEC performance was tested via linear sweep voltammetry, incident photon to current efficiency and internal quantum efficiency analyses. From tests performed in the presence of a hole scavenger-containing electrolyte, the charge injection and separation efficiencies of the electrodes were also calculated. Pure-phase crystalline and/or heterojunction materials were obtained with higher PEC performance compared to pure CuWO4, mainly due to a significantly enhanced charge separation efficiency in the bulk of the material.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

CuWO4 is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO4 electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO4 thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO4 is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO4 electrode delivers a photocurrent of 0.37 mA cm-2 at 1.23 VSHE, with a 7% absorbed photon to current efficiency over the CuWO4 absorption spectrum.

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

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

Regulating a Zn/Co bimetallic catalyst in a metal-organic framework and oxyhydroxide for improved photoelectrochemical water oxidation

As one of the most popular photoanode materials, hematite (α-Fe2O3) has obvious advantages in the field of photoelectrochemical water splitting (PEC-WS). However, it is difficult to achieve excellent PEC-WS performance without loading a cocatalyst serving as an electron/hole collector to promote photogenerated carrier separation. In this work, FTO/Sn@α-Fe2O3 photoanodes are modified with ZnCo-ZIF and ZnCoOOH bimetallic catalysts to obtain FTO/Sn@α-Fe2O3/Zn0.5Co0.5-ZIF and FTO/Sn@α-Fe2O3/Zn0.46Co0.54OOH photoanodes. Their photocurrent densities reach 2.6 mA cm-2 and 2.3 mA cm-2 at 1.23 VRHE, respectively. The detailed mechanism studies demonstrate that both ZnCoOOH and ZnCo-ZIF can effectively decrease the transfer resistance, increase the Fe2+/Fe3+ ratio and reduce the charge recombination of the α-Fe2O3 film, which synergistically improves the PEC-WS performance. Compared with ZnCoOOH, the ZnCo-ZIF exhibits better photogenerated carrier transfer efficiency and catalytic performance, which mainly can be attributed to the improved binding energy between the ZnCo-ZIF catalyst and the α-Fe2O3 film. This work provides a simple and feasible strategy for constructing bimetallic catalysts and deepens the understanding of different types of bimetallic catalysts for high-performance PEC-WS systems.

Tandem cells for unbiased photoelectrochemical water splitting

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

Photoactive p-Type Spinel CuGa2O4 Nanocrystals

Herein we report the synthesis and characterization of spinel copper gallate (CuGa₂O₄) nanocrystals (NCs) with an average size of 3.7 nm via a heat-up colloidal reaction. CuGa₂O₄ NCs have a band gap of ∼2.5 eV and marked p-type character, in agreement with ab initio simulations. These novel NCs are demonstrated to be photoactive, generating a clear and reproducible photocurrent under blue light irradiation when deposited as thin films. Crucially, the ability to adjust the Cu/Ga ratio within the NCs, and the effect of this on the optical and electronic properties of the NCs, was also demonstrated. These results position CuGa₂O₄ NCs as a novel material for optoelectronic applications, including hole transport and light harvesting.

Facile preparation of nickel phosphide for enhancing the photoelectrochemical water splitting performance of BiVO4 photoanodes

The photoelectrochemical (PEC) water splitting performance of BiVO₄ (BVO), a promising photoanode material, is constrained by its extremely short hole diffusion length and slow water oxidation kinetics. Modification of oxygen evolution cocatalysts (OECs) by appropriate methods is a practical solution to enhance the PEC water splitting performance of BVO. In this work, two different nickel phosphides Ni₂P and Ni₁₂P₅ were prepared by a facile and mild one-step solvothermal method, and used as OECs to modify a BVO photoanode for enhancing the PEC water splitting performance. The BVO/Ni₂P and BVO/Ni₁₂P₅ photoanodes showed impressive photocurrent densities of 3.3 mA cm^-2 and 3.1 mA cm^-2, respectively. In addition, the PEC water splitting stability of the BVO/Ni₂P photoanode was greatly enhanced compared to that of the bare BVO photoanode. Further characterization and photoelectrochemical analysis revealed that the significant improvement of the BVO photoanode performance was attributed to the effective inhibition of surface charge recombination, facilitation of interfacial charge transfer, and acceleration of water oxidation kinetics after Ni₂P and Ni₁₂P₅ modification.

Pristine GaFeO3 Photoanodes with Surface Charge Transfer Efficiency of Almost Unity at 1.23 V for Photoelectrochemical Water Splitting

Oxide-based photoelectrodes commonly generate deep trap states associated with various intrinsic defects such as vacancies, antisites, and dislocations, limiting their photoelectrochemical properties. Herein, it is reported that rhombohedral GaFeO₃ (GFO) thin-film photoanodes exhibit defect-inactive features, which manifest themselves by negligible trap-states-associated charge recombination losses during photoelectrochemical water splitting. Unlike conventional defect-tolerant semiconductors, the origin of the defect-inactivity in GFO is the strongly preferred antisite formation, suppressing the generation of other defects that act as deep traps. In addition, defect-inactive GFO films possess really appropriate oxygen vacancy concentration for the oxygen evolution reaction (OER). As a result, the as-prepared GFO films achieve the surface charge transfer efficiency (η_surface ) of 95.1% for photoelectrochemical water splitting at 1.23 V versus RHE without any further modification, which is the highest η_surface reported of any pristine inorganic photoanodes. The onset potential toward the OER remarkably coincides with the flat band potential of 0.43 V versus RHE. This work not only demonstrates a new benchmark for the surface charge transfer yields of pristine metal oxides for solar water splitting but also enriches the arguments for defect tolerance and highlights the importance of rational tuning of oxygen vacancies.

Linking the Photoinduced Surface Potential Difference to Interfacial Charge Transfer in Photoelectrocatalytic Water Oxidation

Charge transfer at the semiconductor/solution interface is fundamental to photoelectrocatalytic water splitting. Although insights into charge transfer in the electrocatalytic process can be gained from the phenomenological Butler-Volmer theory, there is limited understanding of interfacial charge transfer in the photoelectrocatalytic process, which involves intricate effects of light, bias, and catalysis. Here, using operando surface potential measurements, we decouple the charge transfer and surface reaction processes and find that the surface reaction enhances the photovoltage via a reaction-related photoinduced charge transfer regime as demonstrated on a SrTiO₃ photoanode. We show that the reaction-related charge transfer induces a change in the surface potential that is linearly correlated to the interfacial charge transfer rate of water oxidation. The linear behavior is independent of the applied bias and light intensity and reveals a general rule for interfacial transfer of photogenerated minority carriers. We anticipate the linear rule to be a phenomenological theory for describing interfacial charge transfer in photoelectrocatalysis.

Polymeric carbon nitride as a platform for photoelectrochemical water-splitting cells

Polymeric carbon nitride (CN) materials are promising low-cost photocatalysts that exhibit a combination of chemical and physical properties suitable for converting light into redox activity on their surface. In this perspective, we describe our experience with this family of materials as light absorbers that serve as an anode in photoelectrochemical cells toward water-splitting. We describe some of the CN deposition techniques and procedures established in our lab. The knowledge gained from powder-based photocatalysis is implemented in photoelectrochemical scenarios and is used to determine the merits and shortcomings of resulting layers. We show how the preparation methods are oriented based on these factors and how high photoelectrochemical water-splitting activity develops in photoanodes we developed where CN(s) act as photoabsorbers. Lastly, we present our view on the future prospects of this field.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe₂ O₄ (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm² at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Narrowing the band gap and suppressing electron-hole recombination in β-Fe2O3 by chlorine doping

The effects of halogen (F, Cl, Br, I, and At) doping in the direct-band-gap β-Fe₂O₃ semiconductor on its band structures and electron-hole recombination have been investigated by density functional theory. Doping Br, I, and At in β-Fe₂O₃ leads to transformation from a direct-band-gap semiconductor to an indirect-band-gap semiconductor because their atomic radii are too large; however, F- and Cl-doped β-Fe₂O₃ remain as direct-band-gap semiconductors. Due to the deep impurity states of the F dopant, this study focuses on the effects of the Cl dopant on the band structures of β-Fe₂O₃. Two impurity levels are introduced when Cl is doped into β-Fe₂O₃, which narrows the band gap by approximately 0.3 eV. After doping Cl, the light-absorption edge of β-Fe₂O₃ redshifts from 650 to 776 nm, indicating that its theoretical solar to hydrogen efficiency for solar water splitting increases from 20.6% to 31.4%. In addition, the effective mass of the holes in halogen-doped β-Fe₂O₃ becomes significantly larger than that in undoped β-Fe₂O₃, which may suppress electron-hole recombination.

Flow cell for operando X-ray photon-in-photon-out studies on photo-electrochemical thin film devices

Background: Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics, are not yet fully understood. X-ray absorption spectroscopy (XAS), and particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. It remains a challenge to establish a standardized thin film exchange procedure and concurrently record high-quality photoelectrochemical and X‑ray absorption spectroscopy data that is unperturbed by bubble formation. Here we address and overcome these instrumental limitations for photoelectrochemical operando HERFD-XAS. Methods: We constructed a novel operando photo-electro-chemical cell by computer numerical control milling, guided by the materials' X‑ray and visible light absorption properties to optimize signal detection. To test the cell's functionality, semiconducting thin film photoelectrodes have been fabricated via solution deposition and their photoelectrochemical responses under simulated solar light were studied using a commercial potentiostat in a three-electrode configuration during HERFD-XAS experiments at a synchrotron. Results: We demonstrate the cell's capabilities to measure and control potentiostatically and in open‑circuit, to detect X‑ray signals unperturbed by bubbles and to fluently exchange different thin film samples by collecting high-resolution Fe K-edge spectra of hematite ( α -Fe 2O 3) and ferrite thin film ( MFe 2O 4, M= Zn, Ni) photoelectrodes during water oxidation. Conclusions: Our cell establishes a measurement routine that will provide experimental access of photo-electro-chemical operando HERFD-XAS experiments to a broader scientific community, particularly due to the ease of sample exchange. We believe to enable a broad range of experiments which acquired fundamental insights will spur further photoelectrochemical research and commercialization of water splitting technologies.

Evaluation of the Efficiency of Photoelectrochemical Activity Enhancement for the Nanostructured LaFeO3 Photocathode by Surface Passivation and Co-Catalyst Deposition

Perovskite-type lanthanum iron oxide, LaFeO₃, is a promising photocathode material that can achieve water splitting under visible light. However, the performance of this photoelectrode material is limited by significant electron-hole recombination. In this work, we explore different strategies to optimize the activity of a nanostructured porous LaFeO₃ film, which demonstrates enhanced photoelectrocatalytic activity due to the reduced diffusion length of the charge carriers. We found that surface passivation is not an efficient approach for enhancing the photoelectrochemical performance of LaFeO₃, as it is sufficiently stable under photoelectrocatalytic conditions. Instead, the deposition of a Pt co-catalyst was shown to be essential for maximizing the photoelectrochemical activity both in hydrogen evolution and oxygen reduction reactions. Illumination-induced band edge unpinning was found to be a major challenge for the further development of LaFeO₃ photocathodes for water-splitting applications.

Recent Advances in Self-Supported Semiconductor Heterojunction Nanoarrays as Efficient Photoanodes for Photoelectrochemical Water Splitting

Growth of semiconductor heterojunction nanoarrays directly on conductive substrates represents a promising strategy toward high-performance photoelectrodes for photoelectrochemical (PEC) water splitting. By controlling the growth conditions, heterojunction nanoarrays with different morphologies and semiconductor components can be fabricated, resulting in greatly enhanced light-absorption properties, stabilities, and PEC activities. Herein, recent progress in the development of self-supported heterostructured semiconductor nanoarrays as efficient photoanode catalysts for water oxidation is reviewed. Synthetic methods for the fabrication of heterojunction nanoarrays with specific compositions and structures are first discussed, including templating methods, wet chemical syntheses, electrochemical approaches and chemical vapor deposition (CVD) methods. Then, various heterojunction nanoarrays that have been reported in recent years based on particular core semiconductor scaffolds (e.g., TiO₂ , ZnO, WO₃ , Fe₂ O₃ , etc.) are summarized, placing strong emphasis on the synergies generated at the interface between the semiconductor components that can favorably boost PEC water oxidation. Whilst strong progress has been made in recent years to enhance the visible-light responsiveness, photon-to-O₂ conversion efficiency and stability of photoanodes based on heterojunction nanoarrays, further advancements in all these areas are needed for PEC water splitting to gain any traction alongside photovoltaic-electrochemical (PV-EC) systems as a viable and cost-effective route toward the hydrogen economy.

Boosting the Performance of BiVO4 Photoanodes by the Simultaneous Introduction of Oxygen Vacancies and Cocatalyst via Photoelectrodeposition

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy, but the efficiency is limited by severe charge recombination especially in photoanodes. Herein, to reduce the charge recombination in the bulk phase and at the surface of the BiVO₄ photoanodes, oxygen vacancy introduction and cocatalyst loading were realized simultaneously by one-step photocathode deposition. A unique re-BiVO₄/FeOOH photoanode was obtained by the photocathodic reduction of BiVO₄ in an electrolyte containing Fe³⁺, where the oxygen vacancies were introduced during the reduction process and the deposition of the FeOOH cocatalyst on the surface was induced by the generated OH^-. When used for PEC water oxidation, the obtained re-BiVO₄/FeOOH photoanode achieved an excellent PEC performance with a photocurrent density of 5.35 mA/cm² at 1.23 V versus RHE under AM 1.5G illumination, which was considerably higher than those for the pristine BiVO₄ photoanode (2.88 mA/cm²) and the re-BiVO₄ photoanode obtained by photocathodic reduction without Fe³⁺ (4.32 mA/cm²). After further modification with a cobalt silicate (Co-Sil) cocatalyst, the resultant re-BiVO₄/FeOOH/Co-Sil photoanode exhibited a photocurrent density as high as 6.10 mA/cm² at 1.23 V versus RHE and a remarkable applied bias photon-to-current efficiency of 2.25%. The outstanding performance of the re-BiVO₄/FeOOH/Co-Sil photoanode could be attributed to the coexistence of plenty of oxygen vacancies in BiVO₄ reducing recombination of photogenerated carriers, the FeOOH cocatalyst interlayer as a hole-transport layer, and the outer Co-Sil cocatalyst with a high activity toward oxygen evolution. This work may open a new avenue toward multifunctional modifications of photoanode systems for efficient solar conversion.

Coordination Chemistry Engineered Polymeric Carbon Nitride Photoanode with Ultralow Onset Potential for Water Splitting

Construction of an intimate film/substrate interface is of great importance for a photoelectrode to achieve efficient photoelectrochemical performance. Inspired by coordination chemistry, a polymeric carbon nitride (PCN) film is intimately grown on a Ti‐coated substrate by an in situ thermal condensation process. The as‐prepared PCN photoanode exhibits a record low onset potential (E_onset) of −0.38 V versus the reversible hydrogen electrode (RHE) and a decent photocurrent density of 242 μA cm⁻² at 1.23 V_RHE for water splitting. Detailed characterization confirms that the origin of the ultralow onset potential is mainly attributed to the substantially reduced interfacial resistance between the Ti‐coated substrate and the PCN film benefitting from the constructed interfacial sp² N→Ti coordination bonds. For the first time, the ultralow onset potential enables the PCN photoanode to drive water splitting without external bias with a stable photocurrent density of ≈9 μA cm⁻² up to 1 hour.

Approaches for Modifying Oxide-Semiconductor Materials to Increase the Efficiency of Photocatalytic Water Splitting

The constant increase in the amount of energy consumed and environmental problems associated with the use of fossil fuels determine the relevance of the search for alternative and renewable energy sources. One of these is hydrogen gas, which can be produced by sunlight-driven photocatalytic water splitting. The decisive role in the efficiency of the process is played by the properties of the photocatalyst. Oxide materials are widely used as photocatalysts due to their appropriate band structure, high-enough photochemical stability and corrosion resistance. However, the bandgap, crystallinity and the surface morphology of oxide materials are subject to improvement. Apart from the properties of the photocatalyst, the parameters of the process influence the hydrogen-production efficiency. This paper outlines the key ways to improve the characteristics of oxide-semiconductor photocatalysts with the optimum parameters of photocatalytic water splitting.

A Transparent, High-Performance, and Stable Sb2 S3 Photoanode Enabled by Heterojunction Engineering with Conjugated Polycarbazole Frameworks for Unbiased Photoelectrochemical Overall Water Splitting Devices

Developing low-cost, high-performance, and durable photoanodes is essential in solar-driven photoelectrochemical (PEC) energy conversion. Sb₂ S₃ is a low-bandgap (≈1.7 eV) n-type semiconductor with a maximum theoretical solar conversion efficiency of ≈28% for PEC water splitting. However, bulk Sb₂ S₃ exhibits opaque characteristics and suffers from severe photocorrosion, and thus the use of Sb₂ S₃ as a photoanode material remains underexploited. This study describes the design and fabrication of a transparent Sb₂ S₃ -based photoanode by conformably depositing a thin layer of conjugated polycarbazole frameworks (CPF-TCzB) onto the Sb₂ S₃ film. This structural design creates a type-II heterojunction between the CPF-TCzB and the Sb₂ S₃ with a suitable band-edge energy offset, thereby, greatly enhancing the charge separation efficiency. The CPF-TCzB/Sb₂ S₃ hybrid photoanode exhibits a remarkable photocurrent density of 10.1 mA cm^-2 at 1.23 V vs reversible hydrogen electrode. Moreover, the thin CPF-TCzB overlayer effectively inhibits photocorrosion of the Sb₂ S₃ and enables long-term operation for at least 100 h with ≈10% loss in photocurrent density. Furthermore, a standalone unbiased PEC tandem device comprising a CPF-TCzB/Sb₂ S₃ photoanode and a back-illuminated Si photocathode can achieve a record solar-to-hydrogen conversion efficiency of 5.21%, representing the most efficient PEC water splitting device of its kind.

Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality

We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO₄) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO₄ films are smooth and compact, possessing remarkably high structural quality across complete 2'' wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO₄, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO₄, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.

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

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

Activating a Semiconductor-Liquid Junction via Laser-Derived Dual Interfacial Layers for Boosted Photoelectrochemical Water Splitting

The semiconductor-liquid junction (SCLJ), the dominant place in photoelectrochemical (PEC) catalysis, determines the interfacial activity and stability of photoelectrodes, whcih directly affects the viability of PEC hydrogen generation. Though efforts dedicated in past decades, a challenge remains regarding creating a synchronously active and stable SCLJ, owing to the technical hurdles of simultaneously overlaying the two advantages. The present work demonstrates that creating an SCLJ with a unique configuration of the dual interfacial layers can yield BiVO₄ photoanodes with synchronously boosted photoelectrochemical activity and operational stability, with values located at the top in the records of such photoelectrodes. The bespoke dual interfacial layers, accessed via grafting laser-generated carbon dots with phenolic hydroxyl groups (LGCDs-PHGs), are experimentally verified effective, not only in generating the uniform layer of LGCDs with covalent anchoring for inhibited photocorrosion, but also in activating, respectively, the charge separation and transfer in each layer for boosted charge-carrier kinetics, resulting in FeNiOOH-LGCDs-PHGs-MBVO photoanodes with a dual configuration with the photocurrent density of 6.08 mA cm^-2 @ 1.23 V_RHE , and operational stability up to 120 h @ 1.23 V_RHE . Further work exploring LGCDs-PHGs from catecholic molecules warrants the proposed strategy as being a universal alternative for addressing the interfacial charge-carrier kinetics and operational stability of semiconductor photoelectrodes.

BiVO4/Fe2O3/ZnFe2O4; triple heterojunction for an enhanced PEC performance for hydrogen generation

n/n/n triple heterojunction photoanodes made up of Zr:W-BiVO₄, Fe₂O₃, and ZnFe₂O₄ metal oxides are fabricated through a simplistic spray pyrolysis method. Use of Zr and W as dopants in BiVO₄ plays an important role as Zr increases the carrier density and W reduces the charge recombination. Further, Fe₂O₃ and ZnFe₂O₄ serve as a protective layer for Zr:W-BiVO₄, which augmented the photoelectrochemical performance and achieved a 1.90% conversion efficiency in the triple heterojunction. XRD measurements display the crystalline nature and reduction in particle size due to strain in the sample, UV-vis absorbance shows an extended absorption towards the visible region and the FE-SEM imaging confirms the successful deposition of ZnFe₂O₄ over BiVO₄/Fe₂O₃. By analyzing the band edge position, it was observed that on formation, the triple heterojunction not only suppresses the charge carrier recombination but also utilizes the band edge offset for the water splitting reaction using solar energy.

n/n/n triple heterojunction photoanodes made up of Zr:W-BiVO₄, Fe₂O₃, and ZnFe₂O₄ metal oxides.

Engineering MoOx /MXene Hole Transfer Layers for Unexpected Boosting of Photoelectrochemical Water Oxidation

The development of semiconductor photoanodes is of great practical interest for the realization of photoelectrochemical (PEC) water splitting. Herein, MXene quantum dots (MQD) were grafted on a BiVO₄ substrate, then a MoO_x layer by combining an ultrathin oxyhydroxide oxygen evolution cocatalyst (OEC) was constructed as an integrated photoanode. The OEC/MoO_x /MQD/BiVO₄ array not only achieves a current density of 5.85 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode (vs. RHE), but also enhances photostability. From electrochemical analysis and density functional theory calculations, high PEC performance is ascribed to the incorporation of MoO_x /MQD as hole transfer layers, retarding charge recombination, promoting hole transfer and accelerating water splitting kinetics. This proof-of-principle work not only demonstrates the potential utilization of hole transfer layers, but also sheds light on rational design and fabrication of integrated photoanodes for feasible solar energy conversion.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Modulation of the Bi3+ 6s2 Lone Pair State in Perovskites for High-Mobility p-Type Oxide Semiconductors

Oxide semiconductors are key materials in many technologies from flat-panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p-type oxide semiconductors due to the localized O-2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba2 BiMO6 (M = Bi, Nb, Ta) via the Bi 6s2 lone pair state to achieve p-type oxide semiconductors with high hole mobility up to 21 cm2 V-1 s-1 , and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x-ray photoemission, x-ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba2 BiMO6 . The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4-0.7 me ). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P-N junction diode constructed with p-type Ba2 BiTaO6 and n-type Nb doped SrTiO3 exhibits high rectifying ratio of 1.3 × 104 at ±3 V, showing great potential in fabricating high-quality devices. This work provides deep insight into the electronic structure of Bi3+ based perovskites and guides the development of new p-type oxide semiconductors.

Crystal-Phase Control of Ternary Metal Oxides by Solid-State Synthesis with Nanocrystals

Ternary metal oxides are materials of interest for many applications, from batteries to catalysis. Their crystalline structure and composition determine their properties, and thus it is important to achieve control over these features. Here, we demonstrate that solid-state chemistry among nanocrystalline precursors is a promising approach for their synthesis. We show that the crystalline phase of nanocrystal precursors direct that of the ternary reaction product. The combination of X-ray and electron microscopy techniques reveals that the spinel and rhombohedral phases of copper iron oxide are obtained by reacting copper nanocrystals with spinel γ-Fe₂O₃ and corundum α-Fe₂O₃ nanocrystals, respectively. Considering the available library of nanocrystals with tunable crystal phases, this discovery opens up an alternative pathway toward the synthesis of a wide variety of ternary and quaternary materials, including those with metastable phases.

Effect of Mn2+ incorporation on the photoelectrochemical properties of BiVO4

Mn2+–BiVO4 photoanodes (Mn2+ = 0.2–1%) to improve the charge-carrier separation and electrical conductivity of BiVO4 are reported.

Seed layer-free hydrothermal synthesis of porous tungsten trioxide nanoflake arrays for photoelectrochemical water splitting

The simple preparation of efficient nano-photoanodes has been a key issue in the development of photoelectrochemical water splitting. In this work, a convenient and seed layer-free hydrothermal approach has been developed to synthesize vertically aligned porous WO₃ nanoflakes on a fluorine-doped tin oxide conductive glass substrate. The morphology of WO₃ nanoflakes could be manipulated by changing the annealing time, which further affected the performance of WO₃ nanoflakes as photoanodes. Under optimum conditions, the obtained photoanode can lead to a high photocurrent density of 2.34 mA cm⁻² at 1.4 V vs. Ag/AgCl under one sun irradiation (100 mW cm⁻²) and an incident photon to current conversion efficiency of 60% at 300 nm. The excellent photoelectrochemical performance can be mainly attributed to the larger active surface area, single crystal structure with an optimum thickness and the exposed highly active facets.

SEM image of the WO₃ nanoflakes after 2 hours annealing (inset). Current density vs. potential curves of the photoelectrodes prepared in a Na₂SO₄ solution were scanned from −0.2 V to +1.4 V (vs. Ag/AgCl @ 25 °C) using linear sweep voltammetry.

Bifunctional PDDA-stabilized β-Fe2O3 nanoclusters for improved photoelectrocatalytic and magnetic field enhanced photocatalytic applications

Bifunctional β-Fe2O3@PDDA nanoclusters applied for the efficient photoelectrocatalytic oxygen evolution reaction and magnetic field enhanced photocatalytic degradation of pollutants.