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.