Ultrathin FeOOH Nanolayers with Abundant Oxygen Vacancies on BiVO4 Photoanodes for Efficient Water Oxidation

Photoelectrochemical (PEC) water splitting is a promising method for storing solar energy in the form of hydrogen fuel, but it is greatly hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Herein, a facile solution impregnation method is developed for growing ultrathin (2 nm) highly crystalline β‐FeOOH nanolayers with abundant oxygen vacancies on BiVO₄ photoanodes. These exhibited a remarkable photocurrent density of 4.3 mA cm^?2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G), which is approximately two times higher than that of amorphous FeOOH fabricated by electrodeposition. Systematic studies reveal that the excellent PEC activity should be attributed to their ultrathin crystalline structure and abundant oxygen vacancies, which could effectively facilitate the hole transport/trapping and provide more active sites for water oxidation.     

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

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

Enhanced Photoelectrochemical Water Splitting through Bismuth Vanadate with a Photon Upconversion Luminescent Reflector

As the performance of photoanodes for solar water splitting steadily improves, the extension of the absorption wavelength in the photoanodes is highly necessary to substantially improve the water splitting. We use a luminescent back reflector (LBR) capable of photon upconversion (UC) to improve the light harvesting capabilities of Mo:BiVO₄ photoelectrodes. The LBR is prepared by dispersing the organic dye pair meso‐tetraphenyltetrabenzoporphine palladium and perylene capable of triplet–triplet annhilation‐based UC in a polymer film. The LBR converts the wavelengths of 600–650 nm corresponding to the sub‐band gap of Mo:BiVO₄ and the wavelengths of 350–450 nm that are not sufficiently absorbed in Mo:BiVO₄ to a wavelength that can be absorbed by a Mo:BiVO₄ photoelectrode. The LBR improves the water splitting reaction of Mo:BiVO₄ photoelectrodes by 17 %, and consequently, the Mo:BiVO₄/LBR exhibits a photocurrent density of 5.25 mA cm^?2 at 1.23 V versus the reversible hydrogen electrode. The Mo:BiVO₄/LBR exhibits hydrogen/oxygen evolution corresponding to the increased photocurrent density and long‐term operational stability for the water splitting reaction.     

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.     

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

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

Activation of α-Fe2O3 for Photoelectrochemical Water Splitting Strongly Enhanced by Low Temperature Annealing in Low Oxygen Containing Ambient

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α-Fe₂O₃) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α-Fe₂O₃ is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O₂ in Ar). It was found that low oxygen annealing can activate a significant PEC response of α-Fe₂O₃ even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α-Fe₂O₃ annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm⁻², being 20 times higher than that of annealing in air. The obtained results show that the α-Fe₂O₃ annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α-Fe₂O₃. Additionally, we demonstrate the photocurrent of α-Fe₂O₃ annealed in low oxygen ambient can be further enhanced by Zn-Co LDH, which is a co-catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α-Fe₂O₃ photoanodes.     

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

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

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

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

Freeing the Polarons to Facilitate Charge Transport in BiVO4 from Oxygen Vacancies with an Oxidative 2D Precursor

The BiVO₄ photoelectrochemical (PEC) electrode in tandem with a photovoltaic (PV) cell has shown great potential to become a compact and cost‐efficient device for solar hydrogen generation. However, the PEC part is still facing problems such as the poor charge transport efficiency owing to the drag of oxygen vacancy bound polarons. In the present work, to effectively suppress oxygen vacancy formation, a new route has been developed to synthesize BiVO₄ photoanodes by using a highly oxidative two‐dimensional (2D) precursor, bismuth oxyiodate (BiOIO₃), as an internal oxidant. With the reduced defects, namely the oxygen vacancies, the bound polarons were released, enabling a fast charge transport inside BiVO₄ and doubling the performance in tandem devices based on the oxygen vacancy eliminated BiVO₄. This work is a new avenue for elaborately designing the precursor and breaking the limitation of charge transport for highly efficient PEC‐PV solar fuel devices.     

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

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

     

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.     

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

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

Morphology Engineering of BiVO4 with CoOx Derived from Cobalt-containing Polyoxometalate as Co-catalyst for Oxygen Evolution

Bismuth vanadate (BiVO₄) as a metal oxidation semiconductor has stimulated extensive attention in the photocatalytic water splitting field. However, the poor transport ability and easy recombination of charge carriers limit photocatalytic water oxidation activity of pure BiVO₄. Herein, the photocatalytic activity of BiVO₄ is enhanced via adjusting its morphology and combination co-catalyst. First, the Cu-BiVO₄ was synthesized by copper doping to control the growth of {110} facet of BiVO₄, which is regarded for the separation of photo-generated charge carriers. Then the CoO_x in-situ generated from K₆[SiCo^II(H₂O)W₁₁O₃₉] ⋅ 16H₂O was photo-deposited on Cu-BiVO₄ surface as co-catalyst to speed up reaction kinetics. Cu-BiVO₄@CoO_x hybrid catalyst shows highest photocatalytic activity and best stability among all the prepared catalysts. Oxygen evolution is about 34.6 μmol in pH 4 acetic acid buffer under 420 nm LED irradiation, which is nearly 20 times higher than that of pure BiVO₄. Apparent quantum efficiency (AQE) in 1 h and O₂ yield are 1.83% and 23.1%, respectively. O₂ evolution amount nearly maintains the original value even after 5 cycles.     

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.     

Curing BiVO4 Photoanodes with Ultraviolet Light Enhances Photoelectrocatalysis

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

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.     

硫氧键合BiVO4光阳极与FeNi催化剂实现高效水氧化

光电催化分解水可以将充足的太阳能直接转化存储为绿色清洁的氢能,然而光阳极表面缓慢的析氧反应动力学严重限制了太阳能到氢能的转化效率。我们通过一种简单的S-O键合策略实现BiVO4光阳极与FeNi催化剂的界面耦合（S:BiVO4-FeNi）,其光电催化分解水的光电流达到6.43 mA/cm2（1.23 VRHE, AM 1.5G）。进一步研究结果表明：界面S-O键合能够有效实现BiVO4光阳极光生电荷分离并促进空穴向FeNi催化剂表面迁移。同时,S-O键合可以进一步调控FeNi催化剂表面的电荷分布,从而有效提高光电化学分解水析氧活性和稳定性。该工作为设计构建具有高效、稳定的太阳能光电催化分解水体系提供了一种新的研究策略。     

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

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

电解液成分、厚度及表面改性对旋涂法制备的BiVO4膜层光电化学性能的影响

采用旋涂法在FTO（SnO₂∶F）导电玻璃衬底上沉积得到BiVO₄多孔薄膜用以光解水,改变前驱体的浓度和旋涂次数以调控薄膜的厚度。研究了电解液成分、膜层厚度及表面改性等因素对刚经历过退火处理的BiVO₄薄膜光电化学（PEC）性能的影响。结果表明：通过在电解液中添加适量的空穴吞噬剂Na₂SO₃,或对表面进行Co-Pi改性均能有效改善BiVO₄薄膜的PEC活性。这些措施均能有效抑制固液界面处的载流子复合反应。经Co-Pi改性的BiVO₄薄膜在0.6 V（vs SCE）偏压下,0.1 mol·L^-1 Na₂SO₄+0.1 mol·L^-1 Na₂SO₃的电解液中展现出最高的光电流密度（4.3 mA·cm^-2）。此外,选用一个代表性BiVO₄薄膜作为光阳极制备了一个PEC生物传感器,在检测谷胱甘肽（GSH）上表现出比较高的灵敏度。本研究证实了BiVO₄薄膜的PEC性能严重依赖着光俘获效率和载流子输运过程。     

Towards Long‐Term Photostability of Nickel Hydroxide/BiVO4 Photoanodes for Oxygen Evolution Catalysts via In Situ Catalyst Tuning

Increasing long‐term photostability of BiVO₄ photoelectrode is an important issue for solar water splitting. The NiOOH oxygen evolution catalyst (OEC) has fast water oxidation kinetics compared to the FeOOH OEC. However, it generally shows a lower photoresponse and poor stability because of the more substantial interface recombination at the NiOOH/BiVO₄ junction. Herein, we utilize a plasma etching approach to reduce both interface/surface recombination at NiOOH/BiVO₄ and NiOOH/electrolyte junctions. Further, adding Fe²⁺ into the borate buffer electrolyte alleviates the active but unstable character of etched‐NiOOH/BiVO₄, leading to an outstanding oxygen evolution over 200 h. The improved charge transfer and photostability can be attributed to the active defects and a mixture of NiOOH/NiO/Ni in OEC induced by plasma etching. Metallic Ni acts as the ion source for the in situ generation of the NiFe OEC over long‐term durability.