Hydrogen Incorporation and Reduction of Oxidation States in WO<sub>3</sub> Thin Films Irradiated with Low-Energy H<sub>2</sub><sup>+</sup> Ions

Tungsten trioxide (WO 3 ) is widely known for its technological importance in electrochromic sensors and catalytic devices. The incorporation of hydrogen into WO 3 can strongly influence the material's electrical, optical, and structural properties. This study investigates the evolution of different tungsten oxidation states and the mechanism of oxide reduction of polycrystalline WO 3 thin films induced by low-energy H 2 + irradiation at room temperature. The reduction investigation was conducted in situ using X-ray photoelectron spectroscopy (XPS) measurements around W 4 f and O 1 s core levels. The hydrogen-implanted film, which was irradiated with 5 keV H 2 + ions for 180 minutes, was subsequently characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and secondary ion mass spectrometry (SIMS). During the initial phase of H 2 + irradiation, the W 6+ oxidation state in WO 3 is reduced to W 5+ and W 4+ , while the prolonged hydrogen bombardment leads to further reduction and the formation of W 2+ and W 0 states. SEM reveals crystallinity loss in the irradiated WO 3 sample, while Raman and XRD indicate a phase transformation from monoclinic to tetragonal after hydrogen bombardment. Our analysis shows that WO 3 reduction is confined to the surface while hydrogen-tungsten bronze (H x WO 3 ) is formed in the bulk of the material.