Changes in the local structure of nanocrystalline electrochromic films of hydrated nickel vanadium oxide upon ozone-induced coloration

Hydrated thin films of nickel vanadium oxide (Ni_1-xV _xO_y), made by reactive DC magnetron sputtering, were studied by x-ray absorption spectroscopy at the Ni and V K-edges using synchrotron radiation. The XANES signals were analysed within the full-multiple-scattering formalism, whereas EXAFS data were modelled within the multi-shell multiple-scattering approach. We found that transparent films exhibit a nanocrystalline NiO-type structure with homogeneous distribution of V ions substituting Ni ions. Exposure of the films to ozone resulted in dark brown coloration associated with an appearance of Ni³⁺ ions and accompanied by a modification of the local electronic and atomic structures of the V and Ni ions. The largest changes occurred in the environment of the V ions, which presumably change their valence state from V⁴⁺ to V ⁵⁺ and displace into off-centre positions by ∼0.4 Å.