AbstractWe report on the development of tungsten-oxide-based photoelectrochemical (PEC) water-splitting electrodes using surface modification techniques. The effect of molybdenum incorporation into the WO₃ bulk or the surface region of the film is discussed. Our data indicate that Mo incorporation in the entire film (WO₃:Mo) results in poor PEC performances, most likely due to defects that trap photo-generated charge carriers. However, compared to a pure WO₃ (WO₃:Mo)-based PEC electrode, a 20% (100%) increase of the photocurrent density at 1.6 V vs. SCE is observed if the Mo incorporation is limited to the near-surface region of the WO₃ film. The resulting WO₃:Mo/WO₃ bilayer structure is formed by epitaxial growth of the WO₃:Mo top layer on the WO₃ bottom layer, which allows an optimization of the electronic structure induced by Mo incorporation while maintaining good crystallographic properties.