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. 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. 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. 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. [4, 5] 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.
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