At the same time, the bulk properties of the semiconductor are equally important in determining the efficiency of a photoelectrochemical reaction. Separation and migration of photo-generated charge carriers first occur in the bulk of the materials. In addition, bulk modification, such as doping and defects, remarkably affect light harvesting ability and band alignment . Nonetheless, bulk defects are commonly regarded as recombination centers for charge carriers, and therefore, unfavorable for photoelectrochemical performance .