Band Gap Narrowing in Oxychlorosulfide Photocatalysts through Aliovalent Cl-to-S Anion Substitution

Abstract Mixed-anion compounds are promising visible-light-responsive photocatalysts for solar energy conversion because non-oxide anions form valence bands at higher energies than O 2p orbitals. Although substitution with less electronegative anions is a common approach for valence-band engineering, aliovalent Cl-to-S substitution remains largely unexplored despite the similar ionic radii of Cl– and S2–. Here, we demonstrate aliovalent Cl-to-S substitution in the oxychlorosulfide La5Ti6O15S3Cl3 by simultaneously replacing Cl– with S2– and Ti4+ with Ta5+. This substitution elevates the valence-band maximum by increasing the contribution of S 3p orbitals, thereby enhancing visible-light absorption. The obtained compounds function as visible-light-responsive H2-evolution photocatalysts. This study establishes aliovalent anion substitution between Cl– and S2– as a strategy for controlling the optoelectronic structures of (oxy)chlorides and (oxy)sulfides.