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 (NPs). New insights into electronic as well as 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 the synergetic effect of UV and visible light on the photocatalytic activity of all the Au-containing structures: hydrogen produced under UV + Vis light shows 100% enhancement compared to the net production obtained under either UV or visible light alone. The systems where Au NPs are embedded in TiO2 outperform the one where NPs are positioned on the surface, indicating that the 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 light. 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 Schottky barrier formation generates hot electrons resulting in a better photoexcited charge separation. In addition, contrary to what is generally assumed, charge injection by the plasmon from the metal into the semiconductor plays a marginal role in the hydrogen evolution reaction. Furthermore, this 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.
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