In Situ X‑ray Photoelectron Spectroscopy Study of Initial Stages of Tungsten Trioxide Reduction by Low-Energy Hydrogen Bombardment

The evolution of different W oxidation states and the oxide reduction mechanism of polycrystalline WO₃ thin films, induced by low-energy H₂⁺ bombardment at room temperature, was investigated in situ by X-ray photoelectron spectroscopy around W 4f and O 1s core levels and the valence band. A hydrogen tungsten bronze is formed at the beginning of the reduction process, as is evident from the development of the W⁵⁺ oxidation state and the creation of the O–H bonds. With the higher H implantation dose, reduction proceeds with the creation of H₂O gas molecules, whose evolution with the bombardment time correlates with the cumulative concentration fractions of W⁴⁺, W²⁺, and W⁰ oxidation states. The generation of H₂O molecules removes O atoms from the WO₃ matrix, inducing the reduction of WO₃ to lower oxides and, subsequently, metallic W, which is the dominant phase on the surface after 180 min of bombardment.