Mo-doped BiVO4 Research Articles

Simultaneous Structural and Electronic Engineering on Bi- and Rh-co-doped SrTiO3 for Promoting Photocatalytic Water Splitting.

Activating metal ion-doped oxides as visible-light-responsive photocatalysts requires intricate structural and electronic engineering, a task with inherent challenges. In this study, we employed a solid (template)-molten (dopants) reaction to synthesize Bi- and Rh-codoped SrTiO3 (SrTiO3 : Bi,Rh) particles. Our investigation reveals that SrTiO3 : Bi,Rh manifests as single-crystalline particles in a core (undoped)/shell (doped) structure. Furthermore, it exhibits a well-stabilized Rh3+ energy state for visible-light response without introducing undesirable trapping states. This precisely engineered structure and electronic configuration promoted the generation of high-concentration and long-lived free electrons, as well as facilitated their transfer to cocatalysts for H2 evolution. Impressively, SrTiO3 : Bi,Rh achieved an exceptional apparent quantum yield (AQY) of 18.9 % at 420 nm, setting a new benchmark among Rh-doped-based SrTiO3 materials. Furthermore, when integrated into an all-solid-state Z-Scheme system with Mo-doped BiVO4 and reduced graphene oxide, SrTiO3 : Bi,Rh enabled water splitting with an AQY of 7.1 % at 420 nm. This work underscores the significance of simultaneous structural and electronic engineering and introduces the solid-molten reaction as a viable approach for this purpose.

Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO4 and Mo-doped BiVO4

Hole transfer at the semiconductor-electrolyte interface is a key elementary process in (photo)electrochemical (PEC) water oxidation. However, up to now, a detailed understanding of the hole transfer and the influence of surface hole density on PEC water oxidation kinetics is lacking. In this work, we propose a model for the first time in which the surface accumulated hole density in BiVO4 and Mo-doped BiVO4 samples during water oxidation can be acquired via employing illumination-dependent Mott-Schottky measurements. Based on this model, some results are demonstrated as below: (1) Although the surface hole density increases when increasing light intensity and applied potential, the hole transfer rate remains linearly proportional to surface hole density on a log-log scale. (2) Both water oxidation on BiVO4 and Mo-doped BiVO4 follow first-order reaction kinetics at low surface hole densities, which is in good agreement with literature. (3) We find that water oxidation active sites in both BiVO4 and Mo-doped BiVO4 are very likely to be Bi5+, which are produced by photoexcited or/and electro-induced surface holes, rather than VOx species or Mo6+ due to their insufficient redox potential for water oxidation. (4) Introduction of Mo doping brings about higher OER activity of BiVO4, as it suppresses the recombination rate of surface holes and increases formation of Bi5+. This surface hole model offers a general approach for the quantification of surface hole density in the field of semiconductor photoelectrocatalysis.

Rational tailoring of spin-polarized photoelectrode for magnetic-assisted overall water splitting

Photoelectrochemical (PEC) devices could play a significant role in efficient solar-to-fuel production for a sustainable economy. Herein, we report a significant enhancement of PEC water splitting via rational tailoring of ferromagnetic ordered oxygen evolution cocatalysts (OECs) iron-cobalt oxide on the Mo-doped BiVO4 (Mo:BiVO4) surface. The Fe3+ and Co2+ tend to form an optimal electron-filled arrangement, contributing to lower barriers for the oxygen evolution reaction (OER). Benefiting from the Fe/Co dual-site as the catalytic center, the prepared FeCoOx/Mo:BiVO4 photoanode achieves 4.55 mA·cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) with a lower starting potential for OER, outperforming the FeOx/Mo:BiVO4 (2.71 mA·cm−2) and CoOx/Mo:BiVO4 (3.48 mA·cm−2). Importantly, ferromagnetic iron-cobalt oxide as the spin polarizer can optimize the OER kinetics. Under spin-related effects, a magnetic stimulation strategy can further enhance the spin alignment. The corresponding photocurrent of FeCoOx/Mo:BiVO4 is increased up to 5.15 mA·cm−2 under a magnetic field, with ηSep and ηTrans reaching 89 % and 91 %, respectively. Specifically, the spin-polarization process will be helpful for the production of O(↓)O(↓)H intermediates, preferring to produce O2. Hence, this study provides a new pathway for designing highly efficient PEC water splitting systems on magnetic-assisted photoelectrodes.

BiVO4-Based Heterojunction Photocathode for High-Performance Photoelectrochemical Hydrogen Peroxide Production.

Photoelectrochemical (PEC) cells provide a promising solution for the synthesis of hydrogen peroxide (H2O2). Herein, an integrated photocathode of p-type BiVO4 (p-BVO) array with tetragonal zircon structure coupled with different metal oxide (MOx, M = Sn, Ti, Ni, and Zn) heterostructure and NiNC cocatalyst (p-BVO/MOx/NiNC) was synthesized for the PEC oxygen reduction reaction (ORR) in production of H2O2. The p-BVO/SnO2/NiNC array achieves the production rate 65.46 μmol L-1 h-1 of H2O2 with a Faraday efficiency (FE) of 76.12%. Combined with the H2O2 generation of water oxidation from the n-type Mo-doped BiVO4 (n-Mo:BVO) photoanode, the unbiased photoelectrochemical cell composed of a p-BVO/SnO2/NiNC photocathode and n-Mo:BVO photoanode achieves a total FE of 97.67% for H2O2 generation. The large area BiVO4-based tandem cell of 3 × 3 cm2 can reach a total H2O2 production yield of 338.84 μmol L-1. This work paves the way for the rational design and fabrication of artificial photosynthetic cells for the production of liquid solar fuel.

Nanoplate structured BiVO4 homojunction photoanode for boosting photoelectrochemical water splitting

Modulation of two-dimensional (2D) morphology structure is an effective strategy to improve the electron transport properties of BiVO4 photoanodes. However, up to now, the application of 2D BiVO4 in photoelectrochemical water splitting has been extremely scarce reported. Here, a simple method to prepare nanoplate BiVO4 photoanodes is reported. Mo-BiVO4 nanoplates were grafted on the conventional BiVO4 particles by the mild method. Due to the special structure, the specific surface area of the photoanode is increased, which enhances the light absorption and utilization efficiency. Moreover, the homojunction formed between BiVO4 substrate and Mo-doped BiVO4 nanoplate promotes the separation of photogenerated carriers and the transfer of photogenerated holes to the electrolyte interface. The abundant specific surface area also provides more support for the co-catalyst. As a result, under multiple facilitation, the photocurrent density of the BiVO4/Mo-BiVO4/CoPi photoanode is enhanced to 5.1 mA/cm2 at 1.23 V vs. RHE.

Characterization of the electronic structure of sputter-deposited Mo-doped BiVO4 thin-film photoanodes

Molybdenum (Mo) doping is a pivotal strategy to enhance the performance of bismuth vanadate (BiVO4) photoanodes in photoelectrochemical (PEC) devices. This research explores the effects of Mo-doping on BiVO4’s electronic properties, uncovering mechanisms behind improved PEC behavior. Mo-doped BiVO4 was produced via single-target RF sputtering, leading to films with increased photocurrent density. Optimal results were achieved with a 3% atomic ratio of Mo and 15% oxygen partial pressure during deposition. Analysis of the local structure revealed Mo6+ substituting V in the BiVO4 host. Mo doping introduced defect states within the VB, partially occupying the d-band of V4+ and creating additional electron states, causing the fermi level to shift from 1.75 to 2.19 eV from the VB edge. This study underscores the adaptability of Mo-doping in shaping BiVO4’s electronic characteristics, opening new pathways in advanced energy conversion technologies.

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

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

The aerosol-assisted chemical vapour deposition of Mo-doped BiVO4 photoanodes for solar water splitting: an experimental and computational study

A series of Mo-doped BiVO4 photoanodes are studied using experimental and DFT methods. Mo doping replaces V sites, increasing electronic conductivity and improving solar water splitting performance.

Oxysulfide-Based Photocatalyst Sheet Modified with Silica Layer for Z-Scheme Overall Water Splitting at Atmospheric Pressure

Photocatalytic water splitting using particulate photocatalysts is considered as an ideal route to produce economically cheap renewable, and transportable H2 utilizing sunlight [1]. Photocatalyst sheets are directly scalable and have recently gained interest because a benchmark STH conversion efficiency of 1.0% was achieved at near ambient pressure using the one composed of Rh, La-codoped SrTiO3 and Mo-doped BiVO4 (BVO:Mo) as the H2- and O2- evolution photocatalysts (HEP and OEP), respectively. However, such systems absorb only a little portion of the sunlight due to the weak visible light absorption of the doped SrTiO3 [2]. To utilize longer wavelength visible light, our group has recently studied photocatalyst sheets composed of Ga-doped La5Ti2Cu0.9Ag0.1O7S5 (LTCA:Ga) (HEP) and BVO:Mo (OEP) with Au as conducting layer. The combination of LTCA:Ga/Au/BVO:Mo modified with Rh/Cr2O3 and CoO x as reduction and oxidation cocatalysts, respectively, achieved an STH efficiency of 0.4% under reduced pressure, but this was far lower than the oxide-based sheet system and the activity was decreased sharply against background pressure [3]. Considering intense visible light absorption of LTCA (λabs ~ 700 nm) and the associated maximum theoretical energy conversion efficiency for this combination, improvements in the net STH efficiency and activation under atmospheric pressure is highly indispensable.In this study, LTCA photocatalysts with different dopants were applied to the sheet system in combination with hydrothermally synthesized BVO:Mo and the resulting sheets were modified with CoO x /Cr2O3/Rh cocatalysts in an attempt to upgrade the water splitting activity. Moreover, surface modification onto LTCA-based photocatalyst sheets with a thin amorphous TiO2 and SiO2 layer were evaluated at elevated pressure and high temperature.Doped LTCA were synthesized by a conventional solid-state reaction in sealed evacuated quartz tubes similar to previous studies [3]. Precursors were blended inside a glove box. BVO:Mo (Mo 0.1 mol%) was synthesized by a hydrothermal method [4]. Photocatalyst sheet was fabricated by the particle transfer method [3]. SiO2 and TiO2 layers functioning as molecular sieves were deposited onto the cocatalyst-modified sheet from titanium tetraisopropoxide (TTIP) or tetraethyl orthosilicate (TEOS) by photo-decomposition and hydrolysis method, respectively, to prevent backward reactions at elevated pressure [5]. Photocatalytic water splitting reaction was carried out at different temperatures and pressures using a closed gas circulation system.Rh cocatalyst was first adsorbed and photo-deposited onto the doped LTCA/Au/BVO:Mo(h) sheet [3]. Gas evolution rates were significantly enhanced upon deposition of Cr2O3 layer as the oxygen reduction reaction (ORR), a backward reaction, was majorly suppressed promoted by uncovered Rh surface under the measured condition. Photocatalyst sheet composed of doped LTCA/Au/BVO:Mo(h) with optimal dopant amounts and cocatalysts loadings showed the H2 and O2 evolution rates of 110 and 54 µmol/h, respectively, under visible light irradiation and yielded an STH of 0.67% at 4 kPa and temperature 301 K [6]. The water splitting activity was improved by 1.6 times under reduced pressure compared to the previously studied LTCAGa/Au/BVO:Mo. However, this activity was highly sensitive against background pressure as nearly 90% reduced upon increasing pressure from 2 to 90 kPa owing to the predominating ORR occurring on LTCA and exposed Au surface. To suppress this backward reaction, TiO2 layer was coated to suppress ORR but the sheet failed to work at a temperature higher than 318 K due to the condensation of such layer. However, SiO2 layer coated sheet effectively suppressed ORR backward reaction and stably work under hot water and elevated pressure [6]. Consequently, under optimized condition SiO2 deposited sheet achieved an STH of 0.4% at 90 kPa at temperature 333 K.Doped LTCA materials were applied to the photocatalyst sheet systems as HEP along with BVO:Mo and Au layer as OEP and conductive layer, respectively, for Z-scheme pure water splitting under visible light. This combination achieved an STH of 0.67% at reduced pressure. This STH is considered as one of the highest values recorded for OWS systems involving non-oxide photocatalysts. The activity of the photocatalyst sheet was found negatively dependent onto background pressure due to the ORR on LTCA and Au layer. Surface modifications with SiO2 layer activated the sheet under atmospheric pressure. These findings will contribute to the improvement in the STH for other systems under realistic condition where OWS is likely to be conducted at an ambient pressure. References Nandy et al. Chem. Sci. 2021, 12, 9866-9884.Wang et al. Nat. Mater. 2016, 15, 611-615.Chen et al. ACS Catal. 2023, 13, 3285-3294.Li et al. Energy Environ. Sci., 2014, 7, 1369-1376.Pan et al. Angew. Chem. Int. Ed. 2015, 54, 2955-2959.Nandy et al. Joule, 2023, under revision.

Molybdenum-doped BiVO4 thin films: Facile preparation via hot-spin coating method and the relationship between surface statistical parameters and photoelectrochemical activity

Molybdenum-doped BiVO4 thin films were uniformly coated on indium-doped tin oxide (ITO) substrates via a facile modified hot spin coating (HSC) technique. The prepared layers were used as photoanode in a photoelectrochemical (PEC) cell. Different percentage of Mo dopant was examined to maximize the photo-current density (J) of the layers. The highest J value (872 ± 8 μA/cm2) was obtained by 5 atomic% of Mo doping. After that, the surface topographies of these samples were changed by varying the initial precursor concentration from 27 to 80 mM. The relation between surface topographies and the PEC activity of Mo-doped BiVO4 thin films was investigated from microscopic point of view by calculating the surface roughness exponent of α, and a mechanism for the PEC activity of Mo-doped BiVO4 photoanodes was proposed accordingly. The sample with a small roughness exponent provided a surface with jagged microscopic fluctuations which may trap the air molecules between the electrolyte and sample surface, hindering the fine atomic interaction for photo-generated electron-hole transition. Therefore, the layer with the highest roughness exponent (2α = 0.48 ± 0.03), which means the smoother microscopic surface and better interfacial contact with the electrolyte, exhibited the best PEC activity.

Defect-designed Mo-doped BiVO4 photoanode for efficient photoelectrochemical degradation of phenol

The monoclinic scheelite BiVO4 has impressive properties such as a narrow energy band gap, exceptional stability, and extended absorption in visible light, making it a suitable photoanode. Nevertheless, the BiVO4 material encounters challenges such as the high recombination rate of photogenerated electron-hole pairs and poor photoelectron conductivity, which limits photocatalytic activity. To address this problem, we developed Mo-doped BiVO4 films on FTO substrates for photoelectrocatalytic degradation of phenol. When exposed to visible light, the Mo-BiVO4 film attained a 70% degradation of phenol in 120 min with a 1.2 V vs. Ag/AgCl bias—a 3.7 times improvement from pristine BiVO4. Mo-doping facilitates better migration and separation of electron-hole pairs and increases the concentration of photogenerated carriers, leading to an upward shift of the valence band potential direction, and an improvement in oxidation capacity. Furthermore, density-functional theory (DFT) calculations were used to explain how Mo-doping with BiVO4 improves the adsorption energy to phenol degradation intermediates, emphasizing its effectiveness in promoting phenol degradation. Therefore, with the inclusion of DFT calculations, this work provides a more comprehensive understanding of the mechanism underlying the enhancement of photocatalytic activity by Mo-doped BiVO4, which is crucial information for the further development of effective and efficient photoanodes.

Photoanodic H2O2 synthesis and in-situ tetracycline degradation using transition-metal phosphide co-catalysts

H2O2 as an essential chemical can be produced by photoanodic water oxidation reaction but is restricted by low production rate and poor selectivity. Herein, three kinds of transition metal phosphide, namely Fe2P, Co2P, and Ni5P4, have been loaded on Mo doped BiVO4 (MB) photoanodes to promote H2O2 production. Among them, the optimal performance has been achieved by Ni5P4/MB photoanode with the yield of 20.8 µmol h−1 cm−2 and the selectivity of 41.9 % at the applied voltage of 1.7 V (vs. RHE) under AM 1.5 G illumination. By loading Ni5P4 on MB, the production rate of OH• radicals and the desorption of the as-formed H2O2 have been promoted. The intermediates, OH• radicals, have been in-situ applied for tetracycline degradation, and Ni5P4/MB photoanode presents the best performance. This work has demonstrated the developments of metal phosphide as co-catalysts for photoanodic H2O2 production from water and meanwhile paved the pathway for its in-situ use in water disinfectant.

Investigation of BiVO4-based advanced oxidation system for decomposition of organic compounds and production of reactive sulfate species

Growth of population and expansion of industries lead to increasing contamination of environment with various organic pollutants. If not properly cleaned, wastewater contaminates freshwater resources, aquatic environment and has huge negative impact on ecosystems, quality of drinking water and human health, therefore new and effective purification systems are in demand. In this work bismuth vanadate-based advanced oxidation system (AOS) for the decomposition of organic compounds and production of reactive sulfate species (RSS) was investigated. Pure and Mo-doped BiVO4 coatings were synthesized using sol-gel process. Composition and morphology of coatings were characterized using X-ray diffraction and scanning electron microscopy techniques. Optical properties were analyzed using UV–vis spectrometry. Photoelectrochemical performance was studied using linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. It was shown that increase in Mo content affects the morphology of BiVO4 films, reduces charge transfer resistance and enhances the photocurrent in the solutions of sodium borate buffer (with and without glucose) and Na2SO4. Mo-doping of 5–10 at.% leads to 2- to 3-fold increase in photocurrents. Faradaic efficiencies of RSS formation ranged between 70 and 90 % for all samples irrespective of Mo content. All studied coatings demonstrated high stability in long-lasting photoelectrolysis. In addition, effective light-assisted bactericidal performance of the films in deactivation of Gram positive Bacillus sp. bacteria was demonstrated. Advanced oxidation system designed in this work can be applied in sustainable and environmentally friendly water purification systems.

Enhanced Photocatalytic Performance of Visible-Light-Driven BiVO4 Nanoparticles through W and Mo Substituting

Bismuth vanadate (BiVO4), W-doped BiVO4 (BiVO4:W), and Mo-doped BiVO4 (BiVO4:Mo) nanoparticles were synthesized at pH = 4 using a green hydrothermal method. The effects of 2 at% W or Mo doping on the microstructural and optical characteristics of as-prepared BiVO4 nanoparticles and the effect of combining particle morphology modification and impurity dopant incorporation on the visible-light-derived photocatalytic degradation of dilute Rhodamine B (RhB) solution are studied. XRD examination revealed that these obtained BiVO4-based nanoparticles had a highly crystalline and single monoclinic phase. SEM and TEM observations showed that impurity doping could modify the surface morphology, change the particle shape, and reduce the particle diameter to enlarge their specific surface area, increasing the reactive sites of the photocatalytic process. XPS and FL measurements indicated that W- and Mo-doped nanoparticles possessed higher concentrations of oxygen vacancies, which could promote the n-type semiconductor property. It was found that the BiVO4:W and BiVO4:Mo powder samples exhibited better photocatalytic activity for efficient RhB removal than that shown by pristine BiVO4 powder samples under visible light illumination. That feature can be ascribed to the larger surface area and improved concentration of photogenerated charge carriers of the former.

Elucidating the Role of Surface Energetics on Charge Separation during Photoelectrochemical Water Splitting

Efficient photoelectrochemical (PEC) water splitting requires charge separation and extraction from a photoactive semiconductor. Such a charge transport process is widely believed to be dictated by the bulk energetics of the semiconductor. However, its dependence on surface energetics along the semiconductor/electrolyte interface remains an open question. Here, we elucidate the influence of surface energetics on the performance of a well-established Mo-doped BiVO4 photoanode whose surface energetics are regulated by the facet-selective cocatalyst loading. Surprisingly, photodeposition of RhOx and CoOx cocatalysts onto the {010} and {110} facets, respectively, strongly enhanced the charge-separation efficiency, in addition to improving the injection efficiency for water oxidation. Detailed optoelectrical simulations confirm that the synergistic enhancement of charge separation originates from the distinct effects of the cocatalyst loading on the surface energetics. This insight into the fundamental charge-separation mechanism in PEC cells provides a perspective for cell design and operation.

Selective site doping impact on the phase transition in bismuth vanadate

Doping in bismuth vanadate (BiVO4) is widely used to enhance the efficiency of photoelectrochemical water splitting, but the fundamental mechanism is not fully understood due to its correlation with structure distortion and phase transition. Here, we investigate the selective site doping effect on the phase transition behavior of BiVO4 using in-situ high-temperature X-ray diffraction. BiVO4 powders with A-site doping by Gd3+ and B-site doing by Mo6+ are prepared, and transition from monoclinic to tetragonal in these powders is observed in real time with increasing temperature from 30 ℃ to 450 ℃. Both dopants are found to assist the tetragonal structure distortion and lower the transition temperature. The lowest transition temperature of 210 ℃ is observed in Mo-doped BiVO4, followed by 220 ℃ in Gd-doped BiVO4 and 235 ℃ in pristine BiVO4. This study delivers mechanistic insights into the doping impact on the structural transition in BiVO4 for efficient photoelectrode design.

Influence of Excess Charge on Water Adsorption on the BiVO4(010) Surface.

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO4(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO4(010) surface stabilizes the formation of a small electron polaron on the VO4 tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V4+ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Synergism of Mo-Dopant and Ag Nanoparticles for Methanol Oxidation on Photoanodes

Photoelectrocatalytic methanol oxidation to formaldehyde by using electric/solar energy is a promising technology. In this work, Ag nanoparticles were decorated on Mo-doped BiVO4 to synthesize Ag/Mo: BiVO4 catalyst. As expected, benefitting from Mo-dopant and plasmonic effect of Ag nanoparticles, formaldehyde production of Ag/Mo: BiVO4 photoanode was 2.2 times higher than pristine BiVO4 photoanode. Faradaic efficiency increased from 19% to 49% in comparison with the pure BiVO4 photoanode.

Electrochemical creation of surface charge transfer channels on photoanodes for efficient solar water splitting

Electrochemical treatment is a popular and efficient method for improving the photoelectrochemical performance of water-splitting photoelectrodes. In our previous study, the electrochemical activation of Mo-doped BiVO4 electrodes was ascribed to the removal of MoOx segregations, which are considered to be surface recombination centers for photoinduced electrons and holes. However, this proposed mechanism cannot explain why activated Mo-doped BiVO4 electrodes gradually lose their activity when exposed to air. In this study, based on various characterizations, it is suggested that electrochemical treatment not only removes partial MoOx segregations but also initiates the formation of HyMoOx surface defects, which provide charge transfer channels for photogenerated holes. The charge separation of the Mo-doped BiVO4 electrode was significantly enhanced by these charge transfer channels. This study offers a new insight into the electrochemical activation of Mo-doped BiVO4 photoanodes, and the new concept of surface charge transfer channels, a long overlooked factor, will be valuable for the development of other (photo)electrocatalytic systems.

(Invited) Solar Hydrogen Production with Particulate Photocatalysts

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen on a large scale. Both the efficiency and scalability of water splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of the solar energy. In this context, overall water splitting using particulate photocatalysts has been attracting growing interest, because such systems can be spread over wide areas by inexpensive processes potentially [1], although it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems. In my talk, recent progress in photocatalytic materials and reaction systems will be presented.The author’s group has studied various semiconductor oxides, (oxy)nitrides, and (oxy)chalcogenides as photocatalysts for water splitting [2]. SrTiO3 is an oxide photocatalyst that has been known to be active in overall water splitting under ultraviolet irradiation since 1980 [3]. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [4]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY has been improved to more than 90% at 365 nm [5], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.The author's group has also been developing panel reactors for large-scale applications. Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultraviolet irradiation even when the water depth is merely 1 mm [6]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [7]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. More recently, a solar hydrogen production system based on 100-m2 arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built, and its performance and system characteristics including safety issues was reported [8].For practical solar energy harvesting, it is essential to develop photocatalysts that are active under visible light irradiation. Ta3N5 and Y2Ti2O5S2 photocatalysts are active in overall water splitting via one-step excitation under visible light irradiation [9,10]. Particulate photocatalyst sheets efficiently split water into hydrogen and oxygen via two-step excitation, referred to as Z-scheme, regardless of size. In particular, a photocatalyst sheet consisting of La- and Rh-codoped SrTiO3 and Mo-doped BiVO4 splits water into hydrogen and oxygen via the Z-scheme, showing a STH exceeding 1.0% [11,12]. Some other (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths can also be applied to Z-scheme photocatalyst sheets. Hisatomi et al., Nat. Catal. 2019, 2, 387.Chen et al., Nat. Rev. Mater. 2017, 1, 17050.Domen et al., J. Chem. Soc., Chem. Commun. 1980, 543Ham et al., J. Mater. Chem. A 2016, 4, 3027.Takata et al., Nature 2020, 581, 411.Goto et al., Joule 2018, 2, 509.Lyu et al., Chem. Sci. 2019, 10, 3196.Nishiyama et al., Nature 2021, 598, 304.Wang et al., Nat. Catal. 2018, 1, 756.Wang et al., Nat. Mater. 2019, 18, 827.Wang et al., Nat. Mater. 2016, 15, 611.Wang et al., J. Am. Chem. Soc. 2017, 139, 1675.