(Keynote) Stable Mixed-Anion Photocatalysts for Visible-Light-Induced Water Splitting

Abstract

Photo-induced water splitting using semiconductor photocatalysts has attracted considerable attention for producing H2 as a clean energy carrier, while the effective utilization of visible light is imperative to achieve the desired efficiency for practical applications.[1, 2] Recently, mixed-anion compounds such as oxynitrides have been intensively studied as promising candidates since one can expect that higher energy p orbitals of non-oxide anions (e.g., N-2p) elevate their valence band maximum (VBM) values. Unfortunately, most of them are subject to facile self-oxidation by photogenerated holes, while highly dispersed cocatalyst particles certainly improve the stability of some oxynitrides.[3] We have recently demonstrated that Sillén–Aurivillius type perovskite oxyhalides such as Bi4NbO8Cl can stably and efficiently oxidize water to O2 under visible light without any surface modifications, and also exhibits a stable Z-scheme water splitting when coupled with a H2-evolving photocatalyst.[4] It was revealed that the VBMs of these materials consist mainly of O-2p orbitals, instead of Cl-3p (or Br-4p), but their positions are much more negative than those of conventional oxides. [5, 6] Thus, they possess narrow bandgaps for visible light absorption as well as sufficiently negative CBMs for water reduction. DFT calculation visualized a fairly strong hybridization between the Bi-6s and O-2p orbitals, which can explain why the O-2p orbitals are elevated in energy, combined with the result on Madelung site potential analysis that can rationalize the origin of high energy of O-2p orbital in these materials. Since O– anions are known to be relatively stable, photogenerated holes populated at the O-2p orbitals will not lead to self-decomposition but to oxidize water. These results could provide new strategies for developing durable photocatalytic materials for water splitting under visible light, by manipulating the interaction between post-transition metal s orbitals and O-2p orbitals. [1] R. Abe, J. Photochem. Photobiol. C: Photochemistry Reviews, 11 (2011) 179. [2] R. Abe, J. Tang et al., Chem. Rev. 118 (2018) 5201. [3] R. Abe, M. Higashi, K. Domen, J. Am. Chem. Soc., 132, (2010) 11828. [4] H. Fujito, H. Kunioku, H. Kageyama, R. Abe et al., J. Am. Chem. Soc., 38 (2016) 2082. [5] D. Kato, H. Kunioku, R. Abe, H. Kageyama et al. J. Am. Chem. Soc., 139 (2017) 18725. [6] H. Kunioku, H. Kageyama, R. Abe et al., J. Mater. Chem. A, 6 (2018) 3100. Key Words: Photocatalyst, Semiconductor, Water splitting, Solar energy