Crystallized V₂O₅ as Oxidized Phase for Unexpected Multicolor Electrochromism in V₂O₃ Thick Film

Our e-connected society is eager to develop devices with tunable colors. Electrochromic materials, able to modify their optical properties under an applied voltage, offer a smart solution. In the present study, we have successfully synthesized two vanadium oxide powders from a polyol-mediated synthesis, and powder suspensions were coated on glass/ITO substrates by doctor blading. The electrochemical and optical properties of the VxOy films are investigated by cyclic voltammetry (CV) coupled with in situ UV–visible spectroscopy. Both V2O5 and V2O3 films exhibit reasonably good cycling stability, significant reflectance modulation, high optical contrast, and good memory effects revealing the unknown EC properties of V2O3. The similar green ⇄ blue ⇄ orange reversible color changes for both vanadium oxides appear suitable for display application. Then, the evolution of the vanadium cation oxidation states and of the structure of V2O5 and V2O3 upon cycling are analyzed by ex situ XPS and ex situ XRD (at grazing incident angle). This work highlights a robust and novel scenario upon cycling, nearly the same whatever the raw film composition that shows, for each cycle, the crystallization of V2O5 upon oxidation, followed by amorphization upon reduction.