Stable Cocatalyst-Free BiVO4 Photoanodes with Passivated Surface States for Photocorrosion Inhibition

Improving charge transport and reducing bulk/surface recombination can increase the activity and stability of BiVO₄ for water oxidation. Herein we demonstrate that the photoelectrochemical (PEC) performance of BiVO₄ can be significantly improved by potentiostatic photopolarization. The resulting cocatalyst-free BiVO₄ photoanode exhibited a record-high photocurrent of 4.60 mA cm⁻² at 1.23 V_RHE with an outstanding onset potential of 0.23 V_RHE in borate buffer without a sacrificial agent under AM 1.5G illumination. The most striking characteristic was a strong “self-healing” property of the photoanode, with photostability observed over 100 h under intermittent testing. The synergistic effects of the generated oxygen vacancies and the passivated surface states at the semiconductor–electrolyte interface as a result of potentiostatic photopolarization reduced the substantial carrier recombination and enhanced the water oxidation kinetics, further inhibiting photocorrosion.     

Boosting the Photoactivity of BiVO4 Photoanodes by a ZnCoFe-LDH Thin Layer for Water Oxidation

Monoclinic bismuth vanadate (BiVO₄) has been used as an efficient photoanode material for photoelectrochemical water oxidation owing to its suitable band gap and nontoxicity. Nevertheless, the practical application of BiVO₄ photoanode has been severely limited by the surface charge recombination and sluggish kinetic, which leads to the obtained photoactivity of BiVO₄ is much lower than its theoretical value. In this case, ZnCoFe-LDH thin layer is conformally decorated on the porous BiVO₄ photoanode through a simple electrodeposition process. The results show that a boosted photoactivity and a remarkably enhanced photocurrent density (3.43 mA cm⁻² at 1.23 V_RHE) are attained for BiVO₄/ZnCoFe-LDH. In addition, the optimized BiVO₄/ZnCoFe-LDH photoanode exhibits significant negative shift in the onset potential (0.51 V_RHE to 0.21 V_RHE), promotes charge separation efficiency (49.3% to 60.4% in the bulk, 29.6% to 61.9% on the surface at 1.23 V_RHE) and enhanced IPCE efficiency (25.5% to 54.7% at 425 nm) compared with that of bare BiVO₄ photoanode. It is demonstrated that the boosted photoactivity of BiVO₄/ZnCoFe-LDH photoanode is mainly ascribed to the synergy effects of the formation of p-n heterojunction between ZnCoFe-LDH and BiVO₄ to accelerate the photogenerated charge transfer and separation, broaden light absorption, as well as promote the surface water oxidation kinetics.     

Enhanced Spatial Charge Separation in a Niobium and Tantalum Nitride Core-Shell Photoanode: In Situ Interface Bonding for Efficient Solar Water Splitting

Tantalum nitride (Ta₃N₅) has emerged as a promising photoanode material for photoelectrochemical (PEC) water splitting. However, the inefficient electron-hole separation remains a bottleneck that impedes its solar-to-hydrogen conversion efficiency. Herein, we demonstrate that a core–shell nanoarray photoanode of NbN_x-nanorod@Ta₃N₅ ultrathin layer enhances light harvesting and forms a spatial charge-transfer channel, which leads to the efficient generation and extraction of charge carriers. Consequently, an impressive photocurrent density of 7 mA cm⁻² at 1.23 V_RHE is obtained with an ultrathin Ta₃N₅ shell thickness of less than 30 nm, accompanied by excellent stability and a low onset potential (0.46 V_RHE). Mechanistic studies reveal the enhanced performance is attributed to the high-conductivity NbN_x core, high-crystalline Ta₃N₅ mono-grain shell, and the intimate Ta−N−Nb interface bonds, which accelerate the charge-separation capability of the core–shell photoanode. This study demonstrates the key roles of nanostructure design in improving the efficiency of PEC devices.     

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

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

Surviving High‐Temperature Calcination: ZrO2‐Induced Hematite Nanotubes for Photoelectrochemical Water Oxidation

Nanotubular Fe₂O₃ is a promising photoanode material, and producing morphologies that withstand high‐temperature calcination (HTC) is urgently needed to enhance the photoelectrochemical (PEC) performance. This work describes the design and fabrication of Fe₂O₃ nanotube arrays that survive HTC for the first time. By introducing a ZrO₂ shell on hydrothermal FeOOH nanorods by atomic layer deposition, subsequent high‐temperature solid‐state reaction converts FeOOH‐ZrO₂ nanorods to ZrO₂‐induced Fe₂O₃ nanotubes (Zr‐Fe₂O₃ NTs). The structural evolution of the hematite nanotubes is systematically explored. As a result of the nanostructuring and shortened charge collection distance, the nanotube photoanode shows a greatly improved PEC water oxidation activity, exhibiting a photocurrent density of 1.5 mA cm⁻² at 1.23 V (vs. reversible hydrogen electrode, RHE), which is the highest among hematite nanotube photoanodes without co‐catalysts. Furthermore, a Co‐Pi decorated Zr‐Fe₂O₃ NT photoanode reveals an enhanced onset potential of 0.65 V (vs. RHE) and a photocurrent of 1.87 mA cm⁻² (at 1.23 V vs. RHE).     

Enhanced Photoelectrochemical Water Splitting of Black Silicon Photoanode with pH-Dependent Copper-Bipyridine Catalysts

Since the water oxidation half-reaction requires the transfer of multi-electrons and the formation of O−O bond, it's crucial to investigate the catalytic behaviours of semiconductor photoanodes. In this work, a bio-inspired copper-bipyridine catalyst of Cu(dcbpy) is decorated on the nanoporous Si photoanode (black Si, b-Si). Under AM1.5G illumination, the b-Si/Cu(dcbpy) photoanode exhibits a high photocurrent density of 6.31 mA cm⁻² at 1.5 V_RHE at pH 11.0, which is dramatically improved from the b-Si photoanode (1.03 mA cm⁻²) and f-Si photoanode (0.0087 mA cm⁻²). Mechanism studies demonstrate that b-Si/Cu(dcbpy) has improved light-harvesting, interfacial charge-transfer, and surface area for water splitting. More interestingly, b-Si/Cu(dcbpy) exhibits a pH-dependent water oxidation behaviour with a minimum Tafel slope of 241 mV/dec and the lowest overpotential of 0.19 V at pH 11.0, which is due to the monomer/dimer equilibrium of copper catalyst. At pH ∼11, the formation of dimeric hydroxyl-complex could form O−O bond through a redox isomerization (RI) mechanism, which decreases the required potential for water oxidation. This in-depth understanding of pH-dependent water oxidation catalyst brings insights into the design of dimer water oxidation catalysts and efficient photoanodes for solar energy conversion.     

Fast fabrication of Ta2O5 nanotube arrays and their conversion to Ta3N5 for efficient solar driven water splitting

This work shows that highly ordered and mechanically stable micrometer-long Ta₂O₅ nanotube arrays can be fabricated by galvanostatic anodization in a few seconds. Typically, ~ 7.7 μm long nanotubes can be grown at 1.2 A cm^− 2 in only 2 s. Such nanotubes can be converted to Ta₃N₅ nanotube arrays by nitridation. Photoelectrochemical (PEC) water splitting using AM 1.5G illumination yields for the Ta₃N₅ nanotube photoanode modified with cobalt phosphate (Co–Pi) remarkable photocurrents of 5.9 mA cm ⁻² at 1.23 V_RHE and 12.9 mA cm ⁻² at 1.59 V_RHE and after Ba-doping a value of 7.5 mA cm ⁻² at 1.23 V_RHE is obtained.     

Unveiling the Activity and Stability Origin of BiVO4 Photoanodes with FeNi Oxyhydroxides for Oxygen Evolution

Understanding the origin of formation and active sites of oxygen evolution reaction (OER) cocatalysts is highly required for solar photoelectrochemical (PEC) devices that generate hydrogen efficiently from water. Herein, we employed a simple pH-modulated method for in situ growth of FeNi oxyhydroxide ultrathin layers on BiVO₄ photoanodes, resulting in one of the highest currently known PEC activities of 5.8 mA cm⁻² (1.23 V_RHE, AM 1.5 G) accompanied with an excellent stability. More importantly, both comparative experiments and density functional theory (DFT) studies clearly reveal that the selective formation of Bi−O−Fe interfacial bonds mainly contributes the enhanced OER activities, while the construction of V−O−Ni interfacial bonds effectively restrains the dissolution of V⁵⁺ ions and promotes the OER stability. Thereby, the synergy between iron and nickel of FeNi oxyhydroxides significantly improved the PEC water oxidation properties of BiVO₄ photoanodes.     

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 V_RHE 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.     

Synthesis of mesoporous functional hematite nanofibrous photoanodes by electrospinning

Iron(III) oxide (hematite, Fe₂O₃) nanofibers, as visible light‐induced photoanode for water oxidation reaction of a water splitting process, were fabricated through electrospinning method followed by calcination treatment. The prepared samples were characterized with scanning electron microscopy, and three‐electrode galvanostat/potentiostat for evaluating their photoelectrochemical (PEC) properties. The diameter of the as‐spun fibers is about 300 nm, and calcinated fibers have diameter less than 110 nm with mesoporous structure. Optimized multilayered electrospun α‐Fe₂O₃ nanostructure mats showed photocurrent density of 0.53 mA/cm² under dark and visible illumination conditions at voltage 1.23 V and constant intensity (900 mW/cm²). This photovoltaic performance of nanostructure mats makes it suitable choice for using in the PEC water splitting application as an efficient photoanode. This method, if combined with appropriate flexible conductive substrate, has the potential for producing flexible hematite solar fuel generators. Copyright © 2015 John Wiley & Sons, Ltd.     

In-situ Synthesis of the Thinnest In2Se3/In2S3/In2Se3 Sandwich-Like Heterojunction for Photoelectrocatalytic Water Splitting

The efficient utilization of solar energy for photoelectrocatalytic (PEC) water splitting is a feasible solution for developing clean energy and alleviating environmental issues. However, as the core of PEC technology, the existing photoanode catalysts have disadvantages such as poor photoelectrocatalytic conversion efficiency, low conductivity of photogenerated carriers, and instability. Here, we report the ultrathin two-dimensional sandwich-like (SW) heterojunction of In₂Se₃/In₂S₃/In₂Se₃ (SW In₂S₃@In₂Se₃) for the first time for PEC water splitting. Our findings identify the efficient separation of electrons and holes by constructing SW In₂S₃@In₂Se₃ heterojunction. The in situ synthesis of ultrathin nanosheet arrays by using surface substitution of Se atom to epitaxially grow cell In₂Se₃ maximizes the contact area of heterogeneous interface and accelerates the transmission of charge carrier. Benefitting from the unique structure and composition characteristic, SW In₂S₃@In₂Se₃ displays excellent performance in PEC water splitting. The photocurrent density of SW In₂S₃@In₂Se₃ reaches 8.43 mA cm⁻² at 1.23 V_RHE. Compared with In₂S₃, the SW In₂S₃@In₂Se₃ photoanode has nearly 12 times higher PEC performance, which represents the best performance among the In₂S₃-based photoanode heterojunction reported so far. The evolution rate of O₂ reaches 78.8 μmol cm⁻² h⁻¹, and the photocurrent has no apparent variety within 24 h.     

Enhanced Hydrogen Evolution under Simulated Sunlight from Neutral Electrolytes on (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 Photocathodes Prepared by a Bilayer Method

A (ZnSe)_0.85(CuIn_0.7Ga_0.3Se₂)_0.15 photocathode with a bilayer structure was fabricated and found to exhibit a photocurrent almost twice that of a photocathode with a monolayer structure during hydrogen evolution from water. The cathodic photocurrent reached maximum values of 12 and 4.9 mA cm^?2 at 0 and 0.6 V_RHE in a neutral phosphate buffer under simulated sunlight, while the half‐cell solar‐to‐hydrogen conversion efficiency was 3.0 % at 0.6 V_RHE, with a maximum value of 3.6 % at 0.45 V_RHE. Cross‐sectional mapping of the electron‐beam‐induced current established that the increased photocurrent can be attributed to improved uniformity at the solid–liquid junction in the bilayer sample, which results in enhanced carrier collection.     

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

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

A Titanium‐Doped SiOx Passivation Layer for Greatly Enhanced Performance of a Hematite‐Based Photoelectrochemical System

This study introduces an in situ fabrication of nanoporous hematite with a Ti‐doped SiO_x passivation layer for a high‐performance water‐splitting system. The nanoporous hematite with a Ti‐doped SiO_x layer (Ti‐(SiO_x/np‐Fe₂O₃)) has a photocurrent density of 2.44 mA cm^?2 at 1.23 V_RHE and 3.70 mA cm^?2 at 1.50 V_RHE. When a cobalt phosphate co‐catalyst was applied to Ti‐(SiO_x/np‐Fe₂O₃), the photocurrent density reached 3.19 mA cm^?2 at 1.23 V_RHE with stability, which shows great potential of the use of the Ti‐doped SiO_x layer with a synergistic effect of decreased charge recombination, the increased number of active sites, and the reduced hole‐diffusion pathway from the hematite to the electrolyte.     

Cs2PtI6 Halide Perovskite is Stable to Air,Moisture, and Extreme pH: Application to Photoelectrochemical Solar Water Oxidation

Halide perovskites show incredible photovoltaic power conversion efficiency coupled with several hundreds of hours of device stability. However, their stability is poor in aqueous electrolyte media. Reported here is a vacancy ordered halide perovskite, Cs₂PtI₆, which shows extraordinary stability under ambient conditions (1 year), in aqueous media of extreme acidic (pH 1), basic (pH 13), and under electrochemical reduction conditions. It was employed as an electrocatalyst and photoanode for hydrogen production and water oxidation, respectively. The catalyst remains intact for at least 100 cycles of electrochemical cycling and six hours of hydrogen production at pH 1. Cs₂PtI₆ was employed as a photoanode for PEC water oxidation, and the material displayed a photocurrent of 0.8 mA cm⁻² at 1.23 V (vs. RHE) under simulated AM1.5G sunlight. Using constant voltage measurement, Cs₂PtI₆ exhibited over 12 hours of PEC stability without loss of performance.     

Cs2PtI6 Halide Perovskite is Stable to Air,Moisture, and Extreme pH: Application to Photoelectrochemical Solar Water Oxidation

Halide perovskites show incredible photovoltaic power conversion efficiency coupled with several hundreds of hours of device stability. However, their stability is poor in aqueous electrolyte media. Reported here is a vacancy ordered halide perovskite, Cs₂PtI₆, which shows extraordinary stability under ambient conditions (1 year), in aqueous media of extreme acidic (pH 1), basic (pH 13), and under electrochemical reduction conditions. It was employed as an electrocatalyst and photoanode for hydrogen production and water oxidation, respectively. The catalyst remains intact for at least 100 cycles of electrochemical cycling and six hours of hydrogen production at pH 1. Cs₂PtI₆ was employed as a photoanode for PEC water oxidation, and the material displayed a photocurrent of 0.8 mA cm^?2 at 1.23 V (vs. RHE) under simulated AM1.5G sunlight. Using constant voltage measurement, Cs₂PtI₆ exhibited over 12 hours of PEC stability without loss of performance.     

A co-activation strategy for enhancing the performance of hematite in photoelectrochemical water oxidation

Hematite(α-Fe_2O_3) is a promising photoanode for photoelectrochemical(PEC) water splitting.However,the severe charge recombination and sluggish water oxidation kinetics extremely limit its use in photohydrogen conversion.Herein,a co-activation strategy is proposed,namely through phosphorus(P)doping and the loading of CoAl-layered double hydroxides(CoAl-LDHs) cocatalysts.Unexpectedly,the integrated system,CoAl-LDHs/P-Fe_2O_3 photoanode,exhibits an outstanding photocurrent density of 1.56 mA/cm~2 at 1.23 V(vs.reversible hydrogen electrode,RHE),under AM 1.5 G,which is 2.6 times of pureα-Fe_2O_3.Systematic studies reveal that the remarkable PEC performance is attributed to accelerated surface OER kinetics and enhanced carrier separation efficiency.This work provides a feasible strategy to enhance the PEC performance of hematite photoanodes.     

Dual Oxygen and Tungsten Vacancies on a WO3 Photoanode for Enhanced Water Oxidation

Alleviating charge recombination at the electrode/electrolyte interface by introducing an overlayer is considered an efficient approach to improve photoelectrochemical (PEC) water oxidation. A WO₃ overlayer with dual oxygen and tungsten vacancies was prepared by using a solution‐based reducing agent, LEDA (lithium dissolved in ethylenediamine), which improved the PEC performance of the mesoporous WO₃ photoanode dramatically. In comparison to the pristine samples, the interconnected WO₃ nanoparticles surrounded by a 2–2.5 nm thick overlayer exhibited a photocurrent density approximately 2.4 times higher and a marked cathodic shift of the onset potential, which is mainly attributed to the facilitative effect on interface charge transfer and the improved conductivity by enhanced charge carrier density. This simple and effective strategy may provide a new path to improve the PEC performance of other photoanodes.     

Tailoring a Zinc Oxide Nanorod Surface by Adding an Earth-Abundant Cocatalyst for Induced Sunlight Water Oxidation

Herein, a detailed investigation of the surface modification of a zinc oxide (ZnO) nanorod electrode with FeOOH nanoparticles dispersed in glycine was conducted to improve the water oxidation reaction assisted by sunlight. The results were systematically analysed in terms of the general parameters (light absorption, charge separation, and surface for catalysis) that govern the photocurrent density response of metal oxide as photoanode in a photoelectrochemical (PEC) cell. ZnO electrodes surface were modified with different concentration of FeOOH nanoparticles using the spin-coating deposition method, and it was found that 6-layer deposition of glycine-FeOOH nanoparticles is the optimum condition. The glycine plays an important role decreasing the agglomeration of FeOOH nanoparticles over the ZnO electrode surface and increasing the overall performance. Comparing bare ZnO electrodes with the ones modified with glycine-FeOOH nanoparticles an enhanced photocurrent density can be observed from 0.27 to 0.57 mA/cm² at 1.23 V_RHE under sunlight irradiation. The impedance spectroscopy data aid us to conclude that the higher photocurrent density is an effect associated with more efficient surface for chemical reaction instead of electronic improvement. Nevertheless, the charge separation efficiency remains low for this system. The present discovery shows that the combination of glycine-FeOOH nanoparticle is suitable and environmentally-friend cocatalyst to enhance the ZnO nanorod electrode activity for the oxygen evolution reaction assisted by sunlight irradiation.     

Highly Active GaN‐Stabilized Ta3N5 Thin‐Film Photoanode for Solar Water Oxidation

Ta₃N₅ is a very promising photocatalyst for solar water splitting because of its wide spectrum solar energy utilization up to 600 nm and suitable energy band position straddling the water splitting redox reactions. However, its development has long been impeded by poor compatibility with electrolytes. Herein, we demonstrate a simple sputtering‐nitridation process to fabricate high‐performance Ta₃N₅ film photoanodes owing to successful synthesis of the vital TaO_δ precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta₃N₅ by forming a crystalline nitride‐on‐nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm⁻² was obtained with a CoPi/GaN/Ta₃N₅ photoanode at 1.2 V_RHE under simulated sunlight, with O₂ and H₂ generated at a Faraday efficiency of unity over 12 h. Our vapor‐phase deposition method can be used to fabricate high‐performance (oxy)nitrides for practical photoelectrochemical applications.