Redox reactions in the Pt/TiO2–WO3/SiO2 planar system

• The thermal behavior of the Pt/TiW/SiO2 system in air or vacuum has been studied.
• Oxidation and reduction processes occurred during annealing.
• WO3/Pt/TiO2–WO3/SiO2 planar system was characterized using surface science analyses.
• The reduction process of surface tungsten oxide WO3 in WO3−x species occurs in vacuum.
• The reduction of surface WO3 is strongly linked to the temperature and Pt amount.

The thermal behavior of the titanium–tungsten adhesive layer (30–70 at.%) deposited on a SiO2 substrate followed by a thicker Pt layer was investigated. The resulting Pt/TiW/SiO2 planar system was annealed under air or vacuum. Morphological and chemical characterizations at different stages of the annealing, as a function of several parameters such as treatment atmosphere, annealing temperature and thickness of the Pt film were performed through surface science analyses. When annealing under air, even at mild temperature (773 K), the whole interlayer oxidizes while a low amount of tungsten diffuses through platinum film. This phenomenon is related to tungsten oxidation which acts as the driving force leading to WO3 ultra thin overlayer. On the obtained WO3/Pt/TiO2–WO3/SiO2 system, whatever subsequent vacuum annealing conditions are, the reduction process of surface tungsten oxide is revealed leading to WOx<3 compounds on the top most layer. This reduction process is strongly connected to the temperature as well as the Pt amount, the reduction being more important when these parameters increase. For a 25 nm thick Pt and temperature higher than 773 K, the WO3 reduction phenomenon can be huge reaching WO2 stoichiometry.