Low onset potential on single crystal Ta 3 N 5 polyhedron array photoanode with preferential exposure of {001} facets

•Ta3N5 polyhedron array photoanode with preferential exposure of {001} facets was grown through a bottom-up mechanism in RbCl molten salt.•Ta3N5 polyhedron array photoanode exhibited superior water splitting performance due to enhanced charge separation efficiency.•The high work function of [001] facet and the small hole effective mass along [001] axis are the reasons for anisotropic hole accumulation and enhanced charge separation.