Local geometric and electronic structures of gasochromic VOx films

Abstract

VO_x films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VO_x films were amorphous with mixed V⁴⁺ and V⁵⁺ valences. The amorphous VO_x had a disordered and expanded lamellar structure resembling that of polymer-intercalated V₂O₅ gels. VO_x films were crystallized into orthorhombic V₂O₅ at 300 °C, and the lamellar structure was eliminated at 400 °C. Additionally, the gasochromic reaction reduced the vanadium valence via intervalence transitions between V⁵⁺ and V³⁺. Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V₂O₅ crystalline phase irreversibly into an H_1.43V₂O₅ phase. Based on the results of this study, amorphous VO_x with a lamellar structure is recommended for use in H₂ gas sensors.