“…Gallium oxide (Ga 2 O 3 ) possesses five different crystal types including α-, β-, γ-, δ- and ε-phases, 11,12 and they have been widely used in photocatalytic water splitting, such as in α–β phase junctions, 13 metal doping modification, 14 porous nanotubes, 15 etc. β-Ga 2 O 3 is the only thermodynamically stable phase in all these polymorphs.…”