(Invited) Stable Layered Oxyhalide Photocatalysts for Visible-Light-Induced Water Splitting
Authors: Abe Ryu 
Published: February 2024

Water splitting using 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 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.
Recently, we have revealed that some layered perovskite oxyhalides fulfil the necessary conditions for visible-light-induced water splitting due to their unique VBMs. For example, Bi4NbO8Cl belongs to the n = 1 members of the Sillén–Aurivillius perovskite family [(Bi2O2)2X]3+[A
n

-1BnO3n+1]3, which is composed of fluorite [Bi2O2], perovskite [NbO4], and halogen [Cl] layers as depicted in Figure 1. DFT calculations show that the VBM mainly consists of O-2p orbitals instead of Cl-3p, but its position determined by electrochemical measurement is much more negative (ca. 2.2 V vs. SHE) than those of conventional metal oxides (ca. 3.0 V). The significantly elevated VBMs of Bi4NbO8Cl can be understood by the strong hybridization between electron-filled Bi-6s and O-2p with the help of empty Bi-6p orbitals. In addition, Madelung site potential analysis for Bi4NbO8Cl indicated that the oxygen sites in the fluorite layer were electrostatically destabilized, which also certainly contributed to the elevation of the VBM that mainly consists of O-2p. Consequently, Bi4NbO8Cl possesses both a narrow band gap (2.4 eV) for visible light absorption up to ca. 500 nm and a more negative CBM (ca.–0.2 V vs. SHE) than the water reduction potential, which were testified by the substantial generation of H2 or O2 in the presence of an electron donor or acceptor, respectively. The substantially high durability against self-oxidative deactivation was also confirmed by the long-term water splitting in a Z-scheme system in which Bi4NbO8Cl was employed as the OEP. Since O– anions are known to be relatively stable, photogenerated holes populated at the O-2p orbitals will not lead to self-oxidative decomposition but rather to the oxidation of water.
More oxyhalides with various perovskite layers (n) have been explored. However, reports on the synthesis of multiple-layered (n ≥ 2) perovskites were limited, probably due to the severe conditions required for their synthesis in the conventional solid state reaction. To solve this problem, a new two-step synthesis method including polymerized complex synthesis and a “bricklaying” synthesis were developed; providing a wide variety of new oxyhalides applicable to visible-light-induced water splitting. We also demonstrate that a oxyiodides such as Ba2Bi3Nb2O11I not only has access to a wider range of visible light than its chloride and bromide counterparts, but also exhibit much higher activity due to longer lifetime of photogenerated carriers.These studies on new oxyhalide materials provide new strategies for developing stable photocatalysts for visible-light-induced water splitting by manipulating the interaction between the s orbitals of post-transition metals (e.g., Bi-6s) and O-2p orbitals in the specific crystal structure.