Since common materials do not meet all requirements, band engineering is explored intensively to overcome intrinsic materials drawbacks. The principal target for these studies is achieving the maximum generated photocurrent for the minimum energy input. Thus, the onset potential, related to the energy input necessary to initiate the solar water splitting (SWS) reaction, represents one of the key parameters used to evaluate the PEC activity. In this respect the aim is to find ways to modify both the bulk and surface of the semiconductor, which decrease the onset potential. Approaches include surface activation by specific catalysts , partial surface etching , band engineering by doping, annealing or hetero-structuring .