Structural and spectroelectrochemical investigations of tetragonal CeVO4 and Ce/V-oxide sol-gel derived ion-storage films

Thin films of CeVO4 prepared via the sol-gel route, with a tetragonal (I4(1)/amd) crystalline structure, show good ion-storage properties. The sol-gel route combined with the dip-coating deposition technique allowed thin solid films and powders with a specific Ce/V molar ratio (Ce/V = 2:1, 1:1 (CeVO4), 1:2) to be made. The structure of films and powders prepared at 400 degrees C, and at different times of heat-treatment, was determined using X-ray diffractometry (XRD). It is shown that films containing more ceria than vanadia (Ce/V > 1) are nanocrystalline with a CeO2 grain size up to 100 Angstrom, but a tetragonal CeVO4 (Wakefieldite) phase is formed when the Ce/V molar ratio is I:1. By using cyclic voltammetry it was possible to observe the Ce4+/Ce3+ and V5+/V4+ redox changes occurring in the films, while in-situ UV-VIS spectroelectrochemical measurements confirmed the sole presence of V5+/V4+ redox changes in tetragonal CeVO4 films. The small (similar to 0.1-0.5 cm(2)/C) coloration efficiency of CeVO4 films suggests their utilization as counter electrodes in electrochromic (EC) devices. Comparison of the IR transmission spectra of as-deposited CeVO4 with CeVO4 films charged to x congruent to 0.9 revealed that besides A(2u) and E-u modes - attributed to the VO43- stretching vibrations predicted by factor group analysis, the Raman active E-g and B-2g modes appear in IR spectra. This suggests that inserted Li+ ions occupy the accessible sites within the structure resulting in the loss of inversion symmetry and the consequent disappearance of the distinction between g and u modes. Charging to x congruent to 2.3 causes instability of the film structure. (C) 1999 Elsevier Science B.V. All rights reserved.