Electrochromic and Structural Studies of Nanocrystalline Fe/V (1:2) Oxide and Crystalline Fe[sub 2]V[sub 4]O[sub 13] Films

The sol-gel route was used for the preparation of Fe/V oxide films with a molar ratio Fe:V = 1:2. Transmission electron microscopy (TEM) measurements of films prepared at 300 degrees C showed that the films consist of a FeVO4-II phase with the grains (1 nm) embedded in an amorphous phase. TEM measurements of films heated at 400 degrees C agreed with the X-ray diffraction results, confirming Fe2V4O13 grains with dimensions up to 50 nm. Charge capacities per unit thickness (Q(ins)d(-1)/Q(ext)d(-1)) were determined in a 1 M LiClO4/propylene carbonate (PC) electrolyte using cyclic voltammetry (1.5 to -1.5 V vs. Ag/AgCl, 4.8 to 1.8 V vs. Li). The LR spectroscopic result is the assignment of the bands between 1000 and 600 cm(-1) to V-O (terminal) and V-O-V/V-O-Fe (bridging) stretching modes of VO4 tetrahedra. Ex situ IR spectra of Fe2V4O13 films (400 degrees C) charged/discharged to various extents showed that at low charging (Q(ins)d(-1) (-0.13 mC cm(-2) nm(-1)) terminal and bridging modes are affected by lithium insertion. Higher chargings (Q(ins)d(-1) (-0.33 mC cm(-2) nm(-1)) result in the amorphization of the films and in the appearance of a broad absorption below 600 nm ascribed to the Li+-O modes. The optical response of films is evaluated in terms of photopic transmittance and chromaticity coordinates. Results confirmed that films are suitable for use in electrochromic transmissive (smart) windows. (C) 2000 The Electrochemical Society. S0013-4651(99)08-088-X. All rights reserved.