Transition-metal (oxy)nitride photocatalysts for water splitting

Abstract

Solar-driven water splitting based on particulate semiconductor materials is studied as a technology for green hydrogen production. Transition-metal (oxy)nitride photocatalysts are promising materials for overall water splitting (OWS) via a one- or two-step excitation process because their band structure is suitable for water splitting under visible light. Yet, these materials suffer from low solar-to-hydrogen energy conversion efficiency (STH), mainly because of their high defect density, low charge separation and migration efficiency, sluggish surface redox reactions, and/or side reactions. Their poor thermal stability in air and under the harsh nitridation conditions required to synthesize these materials makes further material improvements difficult. Here, we review key challenges in the two different OWS systems and highlight some strategies recently identified as promising for improving photocatalytic activity. Finally, we discuss opportunities and challenges facing the future development of transition-metal (oxy)nitride-based OWS systems.

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Kaihong Chen

Kaihong Chen obtained his PhD in chemistry (2018) at Zhejiang University under the supervision of Prof. Congmin Wang. He then pursued his postdoctoral research with Prof. Liangnian He at Nankai University. From August 2021, he worked as a JSPS postdoctoral fellow with Prof. Kazunari Domen at Shinshu University. Now, he works in the Institute of Green Chemistry and Engineering at Nanjing University. His current research interests are photocatalytic water splitting and CO2 utilization.

Jiadong Xiao

Jiadong Xiao obtained a PhD in engineering at the University of the Chinese Academy of Sciences under Prof. Hongbin Cao's supervision in 2018 and performed PhD exchange studies at the Leibniz Institute for Catalysis under Prof. Angelika Brückner's supervision from 2016 to 2018. He worked as a postdoctoral researcher at Utrecht University with Prof. Bert M. Weckhuysen in 2018 and 2019. Since then, he has joined Prof. Kazunari Domen's laboratory as a postdoctoral researcher and was promoted to specially appointed assistant professor at Shinshu University in 2022. His main research interests include (photo)catalysis for sustainability and (photo)catalytic in situ/operando studies.

Takashi Hisatomi

Takashi Hisatomi received a PhD in engineering from the University of Tokyo in March 2010. He worked as a postdoctoral fellow in École Polytechnique Fédérale de Lausanne from April 2010 to March 2012. He moved to the University of Tokyo in April 2012 as a postdoctoral fellow and acquired an assistant professor position in August 2012. He moved to Shinshu University as an associate professor in April 2018 and was promoted to professor in April 2023. His major research interests include semiconductor photocatalysts for overall water splitting, kinetics of photoexcited carriers in semiconductors, and reaction systems for renewable solar hydrogen production.

Kazunari Domen

Kazunari Domen is a special contract professor at Shinshu University and university professor at the University of Tokyo, Japan. He received a PhD in science from the University of Tokyo in 1982. He joined the Tokyo Institute of Technology in 1982 as an assistant professor and was promoted to associate professor in 1990 and professor in 1996. He moved to the University of Tokyo as a professor in 2004 and was appointed by Shinshu University as a special contract professor in 2017. His research interests include heterogeneous catalysis and materials chemistry, with a particular focus on photocatalytic water splitting for solar hydrogen production.

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Introduction

The conversion of solar energy into chemical fuels is an intriguing approach to alleviating energy and environmental issues. Given the high gravimetric energy density of hydrogen (H₂), solar-driven overall water splitting (OWS) into H₂ and oxygen (O₂) offers a possibility to take full advantage of solar energy.¹ To date, an apparent quantum yield (AQY) of almost unity has been achieved in OWS using an Al-doped SrTiO₃ particulate photocatalyst facet-selectively modified with Rh/Cr₂O₃ and CoOOH cocatalysts via photodeposition.² Even so, the solar-to-hydrogen energy conversion efficiency (STH) of this system is only 0.65% because Al-doped SrTiO₃ responds solely to ultraviolet light. The STH value necessary to make this process economically competitive with H₂ production from fossil resources has been estimated to be approximately 5–10%.³ Clearly, it is necessary to develop narrow-bandgap photocatalysts that have a longer absorption-edge wavelength to effective utilize visible components in sunlight.^4,5

Compared with metal oxide semiconductors, (oxy)nitride semiconductors have narrower bandgaps because N 2p orbitals have a more negative potential energy than O 2p orbitals and because the valence-band edge is shifted negatively.⁶ As a result, the absorption edges of these (oxy)nitride materials are mostly extended to the visible-light range while band-edge potentials suitable for OWS are maintained.^7–10 For instance, with increasing N content, the absorption-edge wavelengths become longer: from ∼320 nm for Ta₂O₅ to ∼500 nm for TaON and ∼600 nm for Ta₃N₅. Thermodynamically, all of these materials can evolve both H₂ and O₂ from an aqueous solution.¹¹ Note that nitrogen-doped transition-metal oxides have been reviewed elsewhere^12,13 and will not be discussed in this perspective, because N atoms only form impurity (discontinuous) levels in these materials.

Apart from the extension of the absorption edge, charge separation and migration should also be improved and the desired surface redox reactions should be enhanced to increase the OWS activity.¹¹ However, the situation is complicated and often difficult to control when transition-metal (oxy)nitride-based photocatalysts are used. First, anion vacancies and reduced species (e.g., Ta^4+/3+ or Ti³⁺) are inevitably formed during high-temperature nitridation under flowing ammonia (NH₃) because of the decomposition of NH₃ into N₂ and H₂ at high temperatures. Such defects can function as recombination centers, decreasing the number of surviving photoexcited charge carriers and reducing the photocatalytic activity. In addition, a complicated nitridation process (anion exchange and rearrangement) also limits the availability of (oxy)nitrides with well-defined facets. Moreover, the poor thermal stability of (oxy)nitrides at elevated temperatures in air is a large obstacle to the design of effective cocatalyst loading procedures. Even though most oxynitride materials are expensive, the material cost of the photocatalyst is estimated to be insignificant compared with the cost of the photocatalytic reactor due to the small amount of the photocatalyst loaded, and therefore will be allowable.³

Numerous transition-metal (oxy)nitrides have been shown to exhibit activities toward both the H₂ evolution reaction (HER) and the O₂ evolution reaction (OER) in the presence of sacrificial electron donors and acceptors, respectively.⁷ However, very few of them have shown activity in the one-step-excitation or two-step-excitation (i.e., Z-scheme process) OWS reaction. In addition, in the few known cases of OWS-active transition-metal (oxy)nitride photocatalysts, the AQY is relatively low. For example, a BaTaO₂N-based photocatalyst with a bandgap of 1.9 eV was recently reported to achieve OWS via one-step excitation; however, the AQY was less than 0.1% at 420 nm.¹⁴ In fact, the STH value for an OWS process driven solely by (oxy)nitrides has not exceeded 0.3% yet, irrespective of whether one-step or two-step excitation systems are used. This poor photocatalytic performance is partially attributable to the surface reduction or oxidation reactions being much more sluggish in the absence of sacrificial reagents, which allows unreacted electrons and holes to simply recombine.

The development of strategies that can increase the STH value for OWS using (oxy)nitride photocatalysts is critical and urgently needed. In this perspective, we summarize some key strategies developed for enhancing OWS activity by one- or two-step excitation schemes using transition-metal (oxy)nitride photocatalysts. Our goal is to provide a springboard for further research.

Two OWS systems

Before describing challenges and strategies for enhancing the OWS activity of photocatalytic systems based on transition-metal (oxy)nitrides, basic aspects of one- and two-step-excitation OWS systems are introduced because the reaction properties required for the respective systems differ. The one-step-excitation OWS reaction has been studied intensively for decades as a simple and readily scaled-up approach.^15–18 In this scheme, the bandgap of the semiconductor photocatalyst should straddle both the HER and OER potentials (Fig. 1a). In addition, because the HER and OER occur on the same photocatalyst, the photocatalysts need to be carefully designed to promote spatial charge separation and HER/OER processes while suppressing the water-formation reaction. To this end, in most cases, several strategies are applied concurrently to realize the one-step-excitation OWS reaction.

Fig. 1 Schematics of (a) one-step-excitation and (b) two-step-excitation (Z-scheme) overall water-splitting processes.

Inspired by natural photosynthesis, a typical Z-scheme OWS system comprises a H₂-evolution photocatalyst (HEP), an O₂-evolution photocatalyst (OEP), and shuttle redox mediators.^19,20 As shown in Fig. 1b, the HEP and OEP produce H₂ and O₂ separately and the redox mediator functions as an electron mediator between the HEP and OEP. Photocatalysts can be used in Z-scheme OWS as long as they are active either toward the HER or the OER; therefore, the bandgap of the photocatalyst is not required to straddle both the HER and OER potentials. This approach provides opportunities for narrow-bandgap photocatalysts to participate in OWS under visible light. However, this feature utilizing a cascade of electron flow also provides opportunities for backward electron transfer. Undesirable backward reactions involving redox couples, in addition to those involving the water-formation reaction, should therefore be suppressed.

Z-scheme OWS systems

We first describe progress and challenges associated with Z-scheme OWS despite the complexity of the system, because many strategies originally developed to promote half-reactions have been effectively exploited for Z-scheme OWS prior to the realization of one-step-excitation water splitting. In this section, we highlight some representative strategies such as reducing defect densities, promoting charge separation and migration, and loading cocatalysts to promote surface redox reactions.

Reduction of defect densities

The synthesis of (oxy)nitrides by NH₃ nitridation at high temperature tends to generate defects both in the bulk and at the surface. Decreasing the defect density is an important target for transition-metal (oxy)nitrides used in OWS. One reason for the formation of defects is the structure difference between the precursors and the target products. In this context, a radical redesign of the starting materials and synthesis procedure could be beneficial. Xu et al. designed some double-layer Sillén–Aurivillius-type compounds, in which the Aurivillius units contain perovskite-type blocks topotactically similar to (oxy)nitrides (Fig. 2a), as precursors.^21–23 (Bi_0.75La_0.25)₄TaO₈Cl can be transformed into LaTaO₂N (denoted as LaTaO₂N-P) under high-temperature nitridation.²¹ During the nitridation, Bi³⁺ and Cl⁻ ions evaporate and the residual perovskite-type blocks are converted to LaTaO₂N without a drastic rearrangement of the atoms. Meanwhile, the evaporation of Bi³⁺ and Cl⁻ ions can form voids in the particles to facilitate replacement of O by N, which helps suppress the reduction of Ta⁵⁺ (Fig. 2b).²¹ After CoO_x was loaded as an O₂-evolution cocatalyst (OEC), the OER activity of such porous LaTaO₂N was six times greater than that of LaTaO₂N prepared from LaTaO₄. When combined with a Ru/SrTiO₃:Rh HEP and a Fe³⁺/Fe²⁺ redox mediator, the porous LaTaO₂N could split water under visible light via the Z-scheme. Moreover, this topotactic conversion strategy has been extended to the synthesis of SrTaO₂N and SrNbO₂N.^22,23

Fig. 2 (a) Schematic of structural transformation from (Bi_0.75La_0.25)₄TaO₈Cl into LaTaO₂N. (b) Field-emission scanning electron microscopy images of (Bi_0.75La_0.25)₄TaO₈Cl and LaTaON₂-P. Adapted with permission from ref. 21. Copyright 2021 American Chemical Society.

The addition of certain additives to precursors can also afford the desired transition-metal (oxy)nitride photocatalysts. Recently, the flux-assisted nitridation technique has emerged as a powerful method to prepare (oxy)nitrides with controllable crystallinity, morphology, surface features, and particle size. Li et al. discovered that BaTaO₂N prepared via a flux method exhibits weaker background absorption in the 700–800 nm range, indicating a decreased defect density compared with BaTaO₂N nitrided in the absence of a flux.²⁴ Using a LiBa₄Ta₃O₁₂ oxysalt as a precursor, in which the Li⁺ was easily evaporated, they fabricated BaTaO₂N (denoted as BaTaO₂N-flux) with a porous structure and low defect density.²⁴ A Z-scheme OWS reaction using the Pt/BaTaO₂N-flux as the HEP, PtO_x/WO₃ as the OEP, and IO₃⁻/I⁻ as the redox mediator exhibited twofold greater activity than an analogous system based on BaTaO₂N synthesized without the flux.

Another example reported by Zhang and coworkers is the addition of Mg powder during the nitridation of YTaO_4−xN_y from YTaO₄.²⁵ The added Mg increased the N content in the nitridation product and extended the absorption edge toward longer wavelengths. This effect was attributed to Mg functioning as a reducing reagent to weaken the metal–oxygen bond in YTaO₄ to facilitate the replacement of O atoms by N atoms. Using Pt/YTaO_4−xN_y as the HEP, PtO_x/WO₃ as the OEP, and IO₃⁻/I⁻ as the redox mediator, the authors constructed a Z-scheme system that could split water into H₂ and O₂ under visible light.

The second approach is constructing solid solutions between two different semiconductors to tune the bandgap energy. In 2011, Maeda et al. prepared a BaZrO₃–BaTaO₂N solid solution by nitriding a mixture of BaZrO_x and BaTaO_x.^26,27 Because of the incorporation of the BaZrO₃ component, this solid solution had a larger bandgap than pure BaTaO₂N and demonstrated a stronger driving force for the HER and OER. In addition, the defect density in the BaZrO₃–BaTaO₂N solid solution decreased because the background absorption beyond the absorption-edge wavelength (650 nm) decreased. After being modified with Pt as a H₂-evolution cocatalyst (HEC), the BaZrO₃–BaTaO₂N solid solution exhibited enhanced HER activity. It was applicable to Z-scheme OWS; however, the AQY at 420–440 nm was less than 0.1%. Similarly, SrZrO₃ was found to enlarge the bandgap and reduce the defect density of LaTaON₂ by forming solid solutions.²⁸ Compared with pristine LaTaON₂, the LaTaON₂–SrZrO₃ solid solution exhibited enhanced OER activity. Moreover, when coupled with Ru/SrTiO₃:Rh in FeCl₃ solution, the LaTaON₂–SrZrO₃ solid solution split water into stoichiometric H₂ and O₂ under visible light via a Z-scheme process.

Doping is also a prevalent strategy in the preparation of photocatalysts. The introduction of foreign elements enables control of the optical, particle, and semiconducting properties. For example, doping BaTaO₂N with a moderate amount of Zr was shown to enhance the H₂ evolution of Na–Pt/BaTaO₂N.²⁹ Analysis by transient absorption spectroscopy (TAS) showed that doping with 1 mol% Zr could increase the population of shallowly trapped electrons and that increasing the amount of Zr to 10 mol% reduced both the electron and hole densities. When combined with CoO_x/Au/BiVO₄ as the OEP and [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as the redox mediator, Na–Pt/BaTaO₂N:Zr exhibited an AQY of 1.5% at 420 nm and an STH of 2.2 × 10⁻²% in Z-scheme OWS.

Promotion of charge separation and migration

The efficiency of photocatalysis is affected by both carrier recombination and charge separation/migration. The anisotropic nature of some semiconductors, such as SrTiO₃ and BiVO₄, can provide materials with a sufficient driving force for the separation and transportation of photogenerated electrons and holes toward surface reduction and oxidation sites, respectively.^30,31 The facet-selective loading of cocatalysts onto these materials can substantially reduce the charge recombination probability and improve the charge separation efficiency. As previously mentioned, the difficulty in constructing transition-metal (oxy)nitrides with high crystallinity and anisotropic facet exposure limits the applicability of this strategy. Nevertheless, recent studies have suggested that flux-assisted nitridation can overcome this limitation. Luo et al. showed that {100} and {110} facets can be exposed simultaneously on BaTaO₂N when a KCl flux is used.³² In addition, Pt nanoparticles can be selectively photo-deposited onto the {100} facets. This feature enhanced the charge separation of BaTaO₂N, leading to tenfold greater HER activity compared with that of BaTaO₂N with only its {100} facets exposed.³² This observation suggests that facet-selective cocatalyst loading methods can be applied to transition-metal (oxy)nitrides.

A heterostructure system can promote photoexcited electron and hole transfer in different directions. Such systems are also often effective for forming heterojunctions promoting the functionality of transition-metal (oxy)nitride photocatalysts. Chen et al. prepared a MgTa₂O_6−xN_y/TaON heterostructure via the one-pot nitridation of MgTa₂O₆/Ta₂O₅ (Fig. 3a).³³ Because the photodeposition of Pt nanoparticles mainly occurred on the TaON surface (Fig. 3b), the authors suggested that photogenerated electrons migrated toward the conduction-band minimum of TaON while holes migrated to the valence-band maximum of MgTa₂O_6−xN_y. Benefitting from the effective interfacial charge separation and reduced defect density, the AQY for Z-scheme OWS involving MgTa₂O_6−xN_y/TaON reached 6.8% at 420 nm. This strategy has also been extended to similar bi-(oxy)nitride heterojunctions such as BaTaO₂N/Ta₃N₅,^34,35 BaMg_1/3Ta_2/3O_3−xN_y/Ta₃N₅,³⁶ and CaTaO₂N/Ta₃N₅.³⁷ Analyses using Kelvin probe force microscopy (KPFM) and electrochemical impedance spectroscopy (EIS) demonstrated enhanced charge separation in the BaTaO₂N/Ta₃N₅ and CaTaO₂N/Ta₃N₅ heterojunction systems (Fig. 3c and d) according to the authors of the articles.^34,37

Fig. 3 (a) Estimated band positions for the MgTa₂O_6−xN_y/TaON heterostructure. (b) Field-emission scanning electron microscopy images of the Pt–MgTa₂O_6−xN_y/TaON photocatalyst. Adapted with permission from ref. 33. Copyright 2015 Wiley-VCH. (c) Surface potentials of three typical BTON in the dark and under irradiation with 450 nm light. (d) EIS results for three BTON measured at 1.23 V vs. the reversible hydrogen electrode (RHE). Adapted with permission from ref. 34. Copyright 2019 Wiley-VCH.

Cocatalyst design promoting surface redox reactions

The cocatalyst is also an important component of OWS systems. Cocatalysts are expected to promote not only charge separation and transfer but also surface redox reactions. This function will, in turn, necessitate intimate interfacial contact between semiconductors and cocatalysts via a suitable junction. However, (oxy)nitrides might decompose into oxides at high temperatures in air. To avoid the decomposition of (oxy)nitrides during the loading of cocatalysts, Zhang et al. devised a method to load CoO_x as an OEC onto (oxy)nitrides under a high-temperature NH₃ flow.³⁸ Strong interfacial interaction occurred between the Co species and LaTiO₂N, without destruction of the oxynitride. The resultant CoO_x/LaTiO₂N exhibited an excellent AQY in the OER (27.1 ± 2.6% at 440 nm).

Concerning the HER, a sequential cocatalyst loading procedure (i.e., impregnation–reduction, followed by photodeposition) was recently developed to modify Ta₃N₅ and BaTaO₂N with highly dispersed and intimately contacted Pt nanoparticles (Fig. 4A).^39,40 The HER activity of the resultant Pt/BaTaO₂N was enhanced almost threefold compared with the case of BaTaO₂N modified with Pt via an impregnation–reduction procedure.⁴⁰ Z-scheme OWS using Pt/BaTaO₂N as the HEP and PtO_x/WO₃ as the OEP exhibited an AQY of 4.0% at 420 nm and an STH of 0.24%.⁴⁰ Moreover, electron transfer from BaTaO₂N to the Pt nanoparticles was verified via TAS experiments. The absorption intensity at 5000 cm⁻¹ (2000 nm, 0.62 eV) for Pt/BaTaO₂N decayed faster when Pt was loaded via a two-step procedure than when it was loaded via only a single-step impregnation–reduction procedure or via photodeposition. Notably, the HER activity of Pt/BaTaO₂N was well correlated with the OWS activity of the Z-scheme systems, indicating that further improvements in the performance of the HEP based on Pt/BaTaO₂N will improve the OWS activity of this Z-scheme system.⁴⁰ Adding a small amount of Na⁺ during the impregnation of Pt is another method to fabricate well-dispersed Pt nanoparticles on the surface of BaTaO₂N (Fig. 4B), resulting in Pt/BaTaO₂N with improved HER activity.⁴¹

Fig. 4 (A) Schematic of sequential Pt cocatalyst deposition onto BaTaO₂N. Adapted with permission from ref. 40. Copyright 2021 Springer Nature. (B) Scanning transmission electron microscopy images and particle size distributions of (a) Na-containing and (b) Na-free Pt/BaTaO₂N. Adapted with permission from ref. 41. Copyright 2021 The Royal Society of Chemistry.

In an ideal Z-scheme system, ionic redox mediators can transfer electrons and holes between two different photocatalysts. However, backward reactions involving redox mediators generally suppress the OWS activity because these side reactions are thermodynamically more favorable. Moreover, the activity of Z-scheme OWS systems is, in most cases, highly sensitive to the kinds and concentrations of the redox mediators. As an example, IrO₂/TaON can oxidize water to O₂ in the presence of AgNO₃ as a sacrificial electron acceptor; however, it exhibits almost no OER activity in an aqueous NaIO₃ solution, likely because of preferential oxidation of the generated I⁻ ions.⁴² To avoid problems arising from reversible redox mediators, solid electron conductors such as reduced graphene oxide (RGO) and Au have been applied to Z-scheme systems involving transition-metal (oxy)nitrides. Z-scheme photocatalyst sheets consisting of RhCrO_x/ZrO₂/LaMg_1/3Ta_2/3O₂N as the HEP and BiVO₄:Mo as the OEP embedded in a Au layer have been reported to split water under visible-light irradiation; however, the STH is still low (∼1 × 10⁻³%).⁴³ The STH was slightly improved to 3.5 × 10⁻³% when RGO was introduced as an additional solid electron conductor into this system; this improvement was attributed to an enhancement in the efficiency of charge transfer between the photocatalytic particles (Fig. 5).⁴⁴ Moreover, (oxy)nitrides can be used as the OEP in a Z-scheme sheet system.⁴⁵ A photocatalyst sheet based on Ga-doped La₅Ti₂Cu_0.9Ag_0.1O₇S₅ as the HEP and CoO_x/LaTiO₂N as the OEP embedded in a thin Au film split water with an AQY in the order of 10⁻²% at 420 nm. In this case, CoO_x was suggested to promote not only the OER on LaTiO₂N but also electron transfer from LaTiO₂N to Au.⁴⁵

Fig. 5 Schematic of a-TiO₂-coated (RhCrO_x/ZrO₂/LaMg_1/3Ta_2/3O₂N)/(Au, RGO)/BiVO₄:Mo photocatalyst sheet. Adapted with permission from ref. 44. Copyright 2016 Wiley-VCH.

One-step-excitation OWS systems

Compared with the Z-scheme OWS system, the one-step-excitation OWS reaction is a relatively simpler system that uses only one semiconductor material as a photocatalyst. Nevertheless, because of the rigorous requirements for the one-step-excitation OWS procedure, only a limited number of (oxy)nitrides have achieved OWS via such a mode. Herein, we first highlight a typical example that combines band engineering and surface modification to achieve one-step-excitation OWS by using an (oxy)nitride photocatalyst for the first time. We then focus on cases based on a dual-cocatalyst strategy that have recently been widely applied.

Combination of band engineering and surface modification

In 2015, our group reported, for the first time, one-step-excitation OWS under visible light with wavelengths as long as 600 nm using an LaMg_1/3Ta_2/3O₂N photocatalyst (Fig. 6a).⁴⁶ In this case, both band engineering and surface modification were used. Through the formation of solid solutions of two perovskite-type semiconductors (LaTaON₂ and LaMg_2/3Ta_1/3O₃), the absorption-edge wavelength of LaMg_xTa_1−xO_1+3xN_2−3x varied from 525 to 640 nm with a decreasing value of x from 0.6 to 0 because of the increase in the O-to-N ratio in conjunction with the substitution of Mg²⁺ for Ta⁵⁺. According to theoretical calculations and a photoelectron spectroscopy investigation, the valence-band maximum was shifted toward more positive potentials, leading to an improvement in both the OER and OWS reaction (Fig. 6b).⁴⁷ Mg²⁺ was also found to tune the bandgap of LaMg_xNb_1−xO_1+3xN_2−3x; the OWS reaction could only be achieved when a suitable amount of Mg²⁺ was present.⁴⁸

Fig. 6 (a) Schematic of the OWS reaction mechanism on surface-coated RhCrO_x/LaMg_1/3Ta_2/3O₂N. Adapted with permission from ref. 46. Copyright 2015 Wiley-VCH. (b) Band levels for LaMg_xNb_1−xO_1+3xN_2−3x (x = 0 and 0.33) estimated by theoretical calculations (CAL) and photoelectron spectroscopy in air (PESA). Adapted with permission from ref. 47. Copyright 2016 The Royal Society of Chemistry.

Recently, the substitution of Ca²⁺ for La³⁺ in LaTaO₂N was investigated to modulate the band structure and defect concentration via the formation of a (LaTaON₂)_1−x(CaTaO₂N)_x solid solution.⁴⁹ After the optimized La_0.1Ca_0.9TaO_1+yN_2−y was decorated with RhCrO_x as a cocatalyst, it exhibited OWS activity, with an AQY of ∼0.06% at ∼420 nm. In contrast to La_0.1Ca_0.9TaO_1+yN_2-y, neither LaTaO₂N nor CaTaO₂N showed activity toward the OWS reaction when used as a photocatalyst. Our group reported one-step-excitation OWS using CaTaO₂N modified with RhCrO_x; however, the AQY was low (on the order of 10⁻³% at 440 nm).⁵⁰

The surface of LaMg_1/3Ta_2/3O₂N was also modified with a nanolayer of amorphous oxyhydroxide (OXH).^46,51 Photodegradation of the (oxy)nitride and the water-formation reaction were both suppressed after the entire RhCrO_x/LaMg_1/3Ta_2/3O₂N surface was coated with a TiOXH layer. Such a deposited OXH nanolayer can function as a molecular sieve, where H₂ and O₂ evolved on the surface of RhCrO_x/LaMg_1/3Ta_2/3O₂N can migrate to the outer phase, whereas migration in the opposite direction is inhibited. As a result, backward reactions involving the O₂ reduction reaction are effectively suppressed. Moreover, compared with RhCrO_x/LaMg_1/3Ta_2/3O₂N coated only with a TiOXH layer, RhCrO_x/LaMg_1/3Ta_2/3O₂N coated with both TiOXH and SiOXH layers showed enhanced OWS activity.⁵¹ We speculated that the double coating layer was more uniform because SiOXH could increase the hydrophilicity. The AQY at 440 ± 30 nm was increased from ∼3 × 10⁻²% to 0.18% by optimization of the nanolayer precursor and deposition procedure.

Strategies based on dual cocatalysts

A dual-cocatalyst strategy is another prevalent method used to realize one-step-excitation OWS reactions.^52–54 Because the surface HER and OER are sluggish, enhancing both the HER and OER simultaneously is critical, particularly for narrow-bandgap oxynitrides. In this context, the adsorption of colloidal IrO₂ onto the surface of an oxynitride semiconductor surface is a feasible approach. For example, IrO₂/Cr₂O₃/RuO_x/ZrO₂/TaON has been reported to split water into H₂ and O₂ in a stoichiometric ratio under visible-light irradiation; however, the AQY was less than 0.1% at 420 nm (Fig. 7).⁵⁵ In this system, Cr₂O₃/RuO_x can extract photogenerated electrons from ZrO₂/TaON while IrO₂ provides water oxidation sites that consume photogenerated holes. For comparison, ZrO₂/TaON loaded with only the HEC (i.e., Cr₂O₃/RuO_x/ZrO₂/TaON) exhibited relatively lower activity and stability for OWS. Notably, TaON without the ZrO₂ modification could not split water; modifying the surface of TaON with ZrO₂ nanoparticles to suppress the formation of surface defects (Ta³⁺ or Ta⁴⁺) was indispensable for realizing OWS. Recently, Zr-doped TaON (TaON:Zr) was fabricated using small, amorphous Ta₂O₅·3.3H₂O particles as a precursor.⁵⁶ As a result of a substantially reduced particle size by precursor design and a lowered defect density by Zr doping, IrO₂/Cr₂O₃/Ru/TaON:Zr achieved greater AQY (420 nm) and STH values of 0.66% and 9 × 10⁻³%, respectively. Similarly, IrO₂/Cr₂O₃/Rh/BaTaO₂N:Mg, which features combined dual cocatalyst loading and lower-valent-cation doping, achieved one-step-excitation OWS with an AQY of 0.08% at 420 nm and an STH of 4 × 10⁻⁴%.¹⁴ Notably, BaTaO₂N:Mg has the smallest bandgap (∼1.9 eV) among the materials that have achieved one-step-excitation OWS.

Fig. 7 Schematic of the OWS mechanism on IrO₂/Cr₂O₃/RuO_x/ZrO₂/TaON. Adapted with permission from ref. 55. Copyright 2013 Wiley-VCH.

In addition to the use of colloidal IrO₂, other strategies have been developed to fabricate dual cocatalysts. Recently, bimetallic nanoparticle cocatalysts were loaded onto a SrTaO₂N-based photocatalyst using microwave-assisted heating (Fig. 8).⁵⁷ Specifically, highly dispersed IrO₂ nanoparticles were deposited through microwave-assisted heating. Subsequent loading of Ru species by impregnation and H₂ reduction produced bimetallic RuIrO_x nanoparticles. The resultant bimetallic nanoparticles were found to effectively extract electrons from semiconductors and accelerate the HER. Coexisting RuO_x served as an OEC. The modified SrTaO₂N-based photocatalyst exhibited an STH value of 6.3 × 10⁻³% and an AQY (420 ± 30 nm) of 0.34% in the OWS reaction.⁵⁷

Fig. 8 Schematic of dispersion of cocatalysts and dominant charge transfer processes on the SrTaO₂N surface. Adapted with permission from ref. 57. Copyright 2023 American Chemical Society.

Although the dual-cocatalyst strategy can improve OWS activity, the random deposition of HEC and OEC nanoparticles increases the likelihood of charge recombination and the water-formation reaction. The importance of separating the HEC and OEC has been suggested in some earlier studies. For instance, a Ta₃N₅ hollow-sphere photocatalyst whose inner and outer shells were loaded with Pt and IrO₂ or CoO_x, respectively, exhibited enhanced HER and OER activities compared with Ta₃N₅ hollow-sphere particles randomly loaded with the cocatalysts.⁵⁸ Xu et al. constructed ZnTiO_3−xN_y hollow nanospheres using carbon spheres as a template and selectively deposited Pt as the HEC and RhO_x as the OEC onto the inner and outer surfaces, respectively (Fig. 9).⁵⁹ Compared with ZnTiO_3−xN_y hollow nanospheres with randomly decorated cocatalyst nanoparticles, such a selective deposition method was found to improve the AQE of the one-step-excitation OWS reaction eightfold to 0.22% (420 ± 20 nm) and to increase the STH to 0.02%. The improvement in photocatalytic activity was attributed to the core–shell photocatalyst and the spatial separation of the cocatalysts, both of which can enhance the separation of photogenerated electrons and holes as well as inhibit the backward reaction. The use of hollow-sphere photocatalysts offers opportunities to separate the cocatalyst loading sites. However, a key issue in this approach is the difficulty associated with preparing single-crystal shells. If the photocatalyst shell is polycrystalline, it will contain grain boundaries; thus, charge separation might not be promoted even if cocatalysts are loaded with good spatial separation.

Fig. 9 Schematic of charge separation processes on Pt@ZnTiO_3−xN_y@RhO_x. Adapted with permission from ref. 59. Copyright 2021 Wiley-VCH.

Conclusion and outlook

Over the past decade, numerous transition-metal (oxy)nitride semiconductors capable of driving the OWS reaction via the Z-scheme or one-step excitation have been reported. Traditionally, efforts have been devoted to reducing the defect density while suppressing the excessive growth of particles in the development of transition-metal (oxy)nitrides because they often contain defects as a result of harsh nitridation conditions (Fig. 10). To this end, modifications of precursors and refinements of the preparation conditions (e.g., flux-assisted nitridation and aliovalent doping) and post-treatment (e.g., surface modifications) have been applied, partially solving the problem. Additional efforts have been devoted to improving charge separation/migration and surface redox reactions while suppressing side reactions through surface modifications. However, the STH achieved with existing (oxy)nitride photocatalysts falls far short of the goal (>5%). Given the AQY values (<7% for Z-scheme OWS and ≤0.02% for one-step-excitation OWS), only a very small portion of photons are successfully used to split water. Thus, establishing strategies that can enhance charge separation/injection and suppress side reactions is critical.

Fig. 10 Summary of challenges and recent strategies.

Much room remains for further improving the synthesis of transition-metal (oxy)nitrides (Fig. 10). Such improvements will remain an important approach to enhancing the OWS activity to reduce the particle size, because smaller particles shorten the charge carrier migration distance and, in principle, lower the probability of recombination in the bulk of the material if the crystallinity and the defect density are not deteriorated. One approach to balancing small particle size and high crystallinity is to synthesize one-dimensional (rod-like) or two-dimensional (sheet-like) transition-metal (oxy)nitride single-crystal nanoparticles and exploit their anisotropic crystal structures. For example, single-crystalline Ta₃N₅ nanorods generated directly on the edges of KTaO₃ particles have been reported to split water without any sacrificial agent, whereas bulk Ta₃N₅ was inactive.⁶⁰ In addition, given the profound effect of starting materials on the particle and material properties of transition-metal (oxy)nitrides obtained by thermal nitridation, a survey of starting materials and detailed examinations of the nitridation process might provide clues for dramatically improving the synthesis of photocatalyst materials.

Another promising strategy is the site-selective loading of cocatalysts, which has been applied to oxide photocatalysts, resulting in a substantial improvement in their OWS activity. However, this strategy has not been successfully applied to transition-metal (oxy)nitrides. A major challenge is how to prepare (oxy)nitrides with high crystallinity and exposed anisotropic facets.

Regarding the Z-scheme OWS system, when (oxy)nitrides are used as either the HEP or OEP, oxide photocatalysts are used as their counterpart in most cases. Utilizing a wider range of visible light necessitates the development of a Z-scheme OWS system solely involving (oxy)nitride photocatalysts with an absorption-edge wavelength greater than 600 nm. To this end, the design of (oxy)nitrides that can drive the HER and/or OER efficiently and with high selectivity (i.e., without promoting reverse or side reactions) is also desirable. The photocatalyst sheet system based on HEPs and OEPs fixed with conductive materials is a particularly promising approach to realizing efficient Z-scheme OWS, given the relatively high STH and potential scalability demonstrated for some oxide photocatalysts. An STH beyond 1% has been achieved by a SrTiO₃:La,Rh/C/BiVO₄:Mo photocatalyst sheet even under ambient pressure.⁶¹ However, the highest STH achieved with photocatalyst sheets based on transition-metal (oxy)nitrides is 3.5 × 10⁻³% for (RhCrO_x/ZrO₂/LaMg_1/3Ta_2/3O₂N)/(Au,RGO)/BiVO₄:Mo.⁴⁴ A better understanding of the low activity is needed, and the photocatalytic activity should be further improved through appropriate design. Research into solid-state electron conductors is important but is still in progress and immature.^15,62 A photovoltaic-electrochemical (PV-EC) solar hydrogen production system is also a choice for transition-metal (oxy)nitride semiconductors because significant progress has been achieved in such systems.⁶³ However, approaches to scale up the system and reduce manufacturing costs should also be identified.

Overall, transition-metal (oxy)nitrides constitute a promising group of materials for OWS catalysts because their absorption-edge wavelength is sufficiently long to meet the STH target and their band positions can be controlled. We hope this perspective will facilitate progress in this field and bridge the gap between what researchers have achieved and what society needs for practical solar hydrogen production via photocatalytic water splitting.

Author contributions

Kaihong Chen: conceptualization, writing original draft, writing review & editing; Jiadong Xiao: conceptualization, writing review & editing; Takashi Hisatomi: conceptualization, writing review & editing; Kazunari Domen: conceptualization, funding acquisition, writing review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO) and JST-PRESTO (JPMJPR20T9). K. C. acknowledges support from the Japanese Society for Promotion of Sciences (JSPS, grand no. P21038) for Overseas Researchers and the National Natural Science Foundation of China (grant no. 22005154).

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