Band Engineering of Double-Layered Sillén–Aurivillius Perovskite Oxychlorides for Visible-Light-Driven Water Splitting

Recently, Bi4MO8X (M = Nb and Ta; X = Cl and Br), Sillén–Aurivillius-type single-layered perovskite oxyhalides, have been shown to be promising visible-light-responsive photocatalysts with unique valence band structures. Here, we report on the synthesis, structures, and photocatalytic properties of a series of double-layered analogues A4A′M2O11Cl (A, A′ = Bi, Pb, Ba, and Sr; M = Ta, Nb, and Ti). Fourteen compounds including ten unreported compounds were successfully synthesized via a two-step method with various combinations of preliminary-prepared multimetal oxide and oxyhalide precursors. In a marked contrast to Bi4MO8X with an almost unvaried valence band maximum (VBM) and conduction band minimum (CBM), both VBM and CBM of A4A′M2O11X are variable to some degree, with a change of compositions. In Sr2Bi3M2O11Cl, for example, Sr-site replacement by Pb and Ba resulted in a shift of the valence and conduction band edges toward narrowing the band gaps. Structural characterizations and density functional theory/Madelung calculations revealed that the observed band edges are mainly understood in terms of the interaction between the Bi/Pb 6s2 lone pair and O 2p orbitals and Madelung site potentials of Bi3+ cations. Furthermore, compounds with a higher occupancy of Bi3+ in the fluorite layer showed improved conductivity of photoexcited electrons, leading to better photoelectrochemical performance for water oxidation under visible light.