Tuning the energy band-gap of crystalline gallium oxide to enhance photocatalytic water splitting: mixed-phase junctions

Abstract

The rational design and fabrication of mixed-phase oxide junctions is an attractive strategy for photocatalytic applications. A new tuneable α–β mixed-phase Ga₂O₃ has recently been discovered to have high activity for photocatalytic water splitting. Here we perform a first-principles study to reveal the nature of the efficient separation of photogenerated carriers achieved by the mixed-phase Ga₂O₃. It is found that the strain and lattice misfit at the interface junctions significantly tune their energy bands. As the interior angles between two components change, the characteristics of the valence band-edge states can be significantly different. Through analysis of the bonding strength of the bonds near the interfaces, and the comparison of calculated and experimentally-observed carrier migration directions, we suggest a favorable junction for the efficient separation of photogenerated carriers. This junction has a type-II band alignment with a valance band of α-Ga₂O₃ that is 0.35 eV higher than that of β-Ga₂O₃, and a conduction band offset of only 0.07 eV. It seems that electron migration across the phase boundary from α- to β-Ga₂O₃ mainly follows an adiabatic electron-transfer mechanism, due to strong orbital coupling between the conduction bands of the two phase materials.