Photoelectrochemical (PEC) water splitting to its constituents, hydrogen and oxygen, is a promising approach to producing chemical fuels. The preparation of metal oxides on a conductive substrate with a well-defined structure has been an important issue for improving the efficiency of PEC water splitting. In this study, We have developed a facile synthetic process for the manufacture of WO3 nanocrystals with full coverage on a fluorine-doped tin oxide (Fro) substrate. The preparation of WO3 as a continuous film on the Fro substrate is indispensable to study charge separation processes in its composite structure. Typically, various ratios of WO3/BiVO4 composite structures were prepared with the WO3 held constant to investigate charge separation processes in WO3 and BiVO4 for efficient PEC water oxidation. The photocutrent initially drops and then rises with growing BiVO4 molar ratios. We attribute this initial decrease in photocurrent to electron -Hole recombination at the semiconductor-semiconductor interface. On the other hand, an improved photocurrent is attributed to enhanced charge separation of BiVO4 on the WO3 electrode. Our work reveals the important relationship between WO3 and BiVO4 in water oxidation reaction. (C) 2017 Elsevier B.V. All rights reserved.