Separation of valence states in thin films with mixed V₂O₅ and V₇O₁₆ phases

Among the other applications, vanadium oxide thin films are considered to be excellent candidates for gas sensing. To understand the origins of the sensing capability, we carried out X-ray photoelectron and X-ray absorption spectroscopy measurements to determinate the surface valence states of thin films with mixed V₇O₁₆ and V₂O₅ compounds. Thin films were fabricated by pulsed laser deposition, and the crystal structure and symmetry of the deposited films was studied using grazing incidence X-ray diffraction and Raman spectroscopy. These results together with X-ray photoelectron and absorption spectra showed that the thin-film crystal structures varied between orthorhombic V₂O₅ phase and another phase of triclinic V₇O₁₆. X-ray photoelectron spectroscopy was used to quantitatively confirm the high amount of V⁴⁺ ions on surfaces of the films, especially of films with V₇O₁₆ phase present. This result was confirmed in the quantitative analysis of the V2p near-edge X-ray absorption spectra. Through the observed electronic structures, it was found that in addition to unique crystal structure and morphology, the enhanced gas sensitivity of these layers is attributed to the increase in the amount of surface oxygen vacancies.