This paper reports the synergetic effects of UV and visible light irradiation on the photocatalytic activity of well-defined nanostructures composed of TiO2 films and Au nanoparticles. New insights into the electronic as well as the chemical processes that drive water decomposition were obtained by varying the position of the NPs on top and at different depths inside the semiconductor film. This work highlights a synergetic effect of UV and visible light on the photocatalytic activity of all the Au containing structures: hydrogen produced under UV+Vis shows an 100 % enhancement compared to the net production obtained only under UV or vis light. The systems where Au NPs are embedded in the TiO2 outperform the one where NPs are positioned on the surface, indicating that water-splitting reaction occurs primarily on the TiO2 surface rather than on the metal. Photocurrent and photocatalytic activity measurements under UV (353-403 nm), visible (400-1100 nm) and UV+Vis (300-1100 nm) light, revealed the synergetic contribution of UV and Vis. Indeed, the plasmonic Au NPs create an intense oscillating electric field at the Au NPs/semiconductor interface (visible light contribution); this mechanism coupled with the 2 Schottky barrier formation generates hot electrons resulting in better photo-excited charge separation. In addition, contrary to what is generally assumed, charges injection by the plasmon from the metal into the semiconductor play a marginal role in the Hydrogen Evolution Reaction (HER). Furthermore, the paper highlights the positive impact of the semiconductor crystallinity surrounding the metal particles to avoid the charge carrier recombination; and the importance of a surface free of oxygen vacancies, whose presence can inhibit the water decomposition.