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.

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

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. . Copyright 2021 American Chemical Society.

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. . 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. . Copyright 2019 Wiley-VCH.

Fig. 4. (A) Schematic of sequential Pt cocatalyst deposition onto BaTaO₂N. Adapted with permission from ref. . 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. . Copyright 2021 The Royal Society of Chemistry.

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. . Copyright 2016 Wiley-VCH.

Fig. 6. (a) Schematic of the OWS reaction mechanism on surface-coated RhCrO_x/LaMg_1/3Ta_2/3O₂N. Adapted with permission from ref. . 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. . Copyright 2016 The Royal Society of Chemistry.

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

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

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

Fig. 10. Summary of challenges and recent strategies.

Kaihong Chen

Jiadong Xiao

Takashi Hisatomi

Kazunari Domen