Density Functional Studies on Layered Perovskite Oxyhalide Bi4MO8X Photocatalysts (M = Nb and Ta, X = Cl, Br, and I)

Layered perovskite oxyhalides Bi4MO8X (M = Nb and Ta, X = Cl, Br, and I) have recently emerged as suitable photocatalysts for the photocatalytic water splitting reaction and degradation of organics. Here, we present a comparative study on the crystal structure, electronic structure, water adsorption, and oxygen evolution reaction of these systems. The calculated band gaps using hybrid density functional method HSE06 are smaller than 2.75 eV and increase with the increase of X atomic number, which is in excellent agreement with experimental data. All Bi4MO8X systems possess indirect band gaps, which benefits the separation of photogenerated electron-hole pairs. The density of states reveals that, for all the Bi4MO8X cases, the valence band maximum is mostly composed of O 2p states rather than X np states, which can explain the observed stability of these materials against photocorrosion. It is found that the molecular adsorption of water is energetically favorable on Bi4MO8X(001) surfaces. As a result, the computed free energy changes for every step in the oxygen evolution reaction show that the rate determining step is the first step of generating OH* species for all the cases. The computed overpotentials (0.69-0.77 V) of Bi4MO8X for the oxygen evolution reaction are comparable to and even lower than those of widely used photocatalysts for water oxidation, such as TiO2, WO3, BiVO4, and alpha-Fe2O3. The calculations suggest that Bi4MO8X (M = Ta and Nb, X = Cl, Br, and I) are potential photocatalysts for overall water splitting in the visible light region, and we hope that the results reported in this work will stimulate experimental tests of our predictions.