PREPARATION AND PROPERTIES OF SPIN-COATED NB2O5 FILMS BY THE SOL-GEL PROCESS FOR ELECTROCHROMIC APPLICATIONS

The preparation and properties of Nb2O5 coatings made by the sol-gel p rocess were investigated. The films were deposited by spin coating on In2O3Sn/glass and quartz substrates from a polymeric solutions of niob ia derived from niobium ethoxide. The films were characterized by inve stigation 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 degrees C. X-ray photoelectron spectroscopy (XPS) measurement reveale d the O:Nb atomic stoichiometry to be 5:2. Cyclic voltammetric measure ments showed that the Nb2O5/1 M LiClO4-propylene carbonate system exhi bits electrochemical reversibility beyond 1200 cycles without change i n performance. In situ UV-Vis-NIR spectroelectrochemical measurement r evealed that Nb2O5 films exhibit an electrochromic effect in the spect ral range 300 < lambda < 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 lo wer valence state Nb(IV) ina colored state with injected Li+. The bron ze coloration is due to a simultaneous injection of electrons and Liions into Nb2O5. The sol-gel-deposited Nb2O5 films are useful for cath odically coloring electrochromic electrodes in electrochromic devices.