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

The rational design and fabrication of mixed-phase oxide junctions is an attractive strategy for photocatalytic applications. A new tuneable alpha - beta mixed-phase Ga sub(2)O sub(3) 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 Ga sub(2)O sub(3). 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 alpha -Ga sub(2)O sub(3) that is 0.35 eV higher than that of beta -Ga sub(2)O sub(3), and a conduction band offset of only 0.07 eV. It seems that electron migration across the phase boundary from alpha - to beta -Ga sub(2)O sub(3) mainly follows an adiabatic electron-transfer mechanism, due to strong orbital coupling between the conduction bands of the two phase materials.