Rare earth oxynitrides: promising visible-light-driven photocatalysts for water splitting

Photocatalytic water splitting is a potential solution for the global energy crisis. However, most photocatalysts only respond to ultraviolet light irradiation, which is not the main component of sunlight. Photocatalysis driven by visible light has therefore become a challenging and cutting-edge research topic. In recent years, perovskite-type rare earth oxynitrides with a formula of ABO(x)N(y) (A = rare earth ions; B = Ti4+, Zr4+, Ta5+, W6+, etc.; x + y = 3) have been reported to be promising candidates for visible-light-driven photocatalytic water splitting. Several factors have been considered to be responsible, including (i) the combination of narrower band gap and higher water stability, (ii) flexible perovskite structure allowing the facile modulation of composition by single or multiple ion doping, (iii) the small differences in neighbouring rare earth elements making ultrafine tuning of properties possible, and (iv) the incorporation of additional functionalities arising from rare earth elements. Despite their importance, to the best of our knowledge, there is no review focusing on the progress of rare earth oxynitride photocatalysts yet. This review describes the proposed mechanism for photocatalytic water splitting, the synthetic methods of rare earth oxynitrides, their photocatalytic performances, the theoretically predicted phases, and the challenges and opportunities for future research on rare earth oxynitrides.