Preparation and Properties of Spin-Coated Nb₂O₅ Films by the Sol-Gel Process for Electrochemical Applications

The preparation and properties of Nb₂O₅ coatings made by the sol-gel process were investigated. The films were deposited by spin coating on In₂O₃:Sn/glass and quartz substrates from a polymeric solutions of niobia derived from niobium ethoxide. The films were characterized by investigation of the stoichiometry, refractive index, optical transmission, electrochemical behavior, and the microstructure. X-ray diffraction studies showed the films to be amorphous for heat treatments below 450 °C. X-ray photoelectron spectroscopy (XPS) measurement revealed the O:Nb atomic stoichiometry to be 5:2. Cyclic voltammetric measurements showed that the Nb₂O₅/1 M LiClO₄-propylene carbonate system exhibits electrochemical reversibility beyond 1200 cycles without change in performance. In situ UV-Vis-NIR spectroelectrochemical measurement revealed that Nb₂O₅ films exhibit an electrochromic effect in the spectral range 300<λ<2100 nm and remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for a 250 nm thick electrode. XPS spectra indicate that Nb(V) is reduced to a lower valence state Nb(IV) in a colored state with injected Li⁺. The bronze coloration is due to a simultaneous injection of electrons and Li⁺ ions into Nb₂O₅. The sol-gel-deposited Nb₂O₅ films are useful for cathodically coloring electrochromic electrodes in electrochromic devices.