First-Principles Calculations on Narrow-Band Gap d Metal Oxides for Photocatalytic H Production: Role of Unusual In Cations in Band Engineering.

The d metal oxides with low effective mass and high mobility of photoexcited electrons have received much attention in photocatalytic water splitting. However, there are still challenges in practical application due to insufficient visible light absorption. Here, an unusual phenomenon of the In cation in PtIn(GeO)O and PtIn(Ga/InO) with a narrow band gap is systematically investigated using density functional theory calculations. According to chemical bond analysis, the final band edge structure results from the interaction between the empty In-5p orbitals and the occupied antibonding state of the In 5s-O 2p orbitals as well as the further hybridization of adjacent In cations in PtIn octahedrons. The unique bonding characteristic of In cations endows them with a narrow band gap and visible light response ability. Moreover, the occupied antibonding state could weaken the strength of the In-O covalent bond and strengthen the orbital hybridization of the In-In bond, causing the conduction band minimum to be located in the electroactive In cavity. This work reveals the origin of the narrow band gap of PtIn(GeO)O and PtIn(Ga/InO) in view of bond theory and shows that they are promising semiconductors for the application of photocatalytic H generation.