ELECTROCHROMISM OF IRON-OXIDE FILMS PREPARED VIA THE SOL-GEL ROUTE BY THE DIP-COATING TECHNIQUE

Thin iron oxide films (69 - 1500 nm) were deposited by the dip-coating method from iron-ion-containing sols which had been made from an FeCl3.6H2O precursor precipitated with ammonium hydroxide. Homogeneous sols were obtained after peptization of precipitates with the addition of acetic acid (approximately 60 mol.%) and no organics were added in order to adjust the sol viscosity for depositing the xerogel film. In situ UV-visible spectroelectrochemical measurements revealed that films heat treated in the temperature range 200-350-degrees-C exhibited intensive bleaching in 10(-3) M LiOH, KOH and NaOH electrolytes at cathodic potentials (- 1.45 V) and became coloured again at anodic potentials (+0.6 V). The ability of films to take up Li+ is 0.9-1.1 Li+ per Fe2O3 giving rise to an intercalated charge of about 0.2 mC cm-2 per nanometre of the film thickness. This indicates that the film consists of particles of small grain size (gamma-Fe2O3 (Maghemite)) with dimensions < 100 angstrom and well-developed grain boundaries which contain OH- and other defect bonds originating from residual acetate groups. Consequently, only minor ''activation'' of films in base electrolytes is needed to attain reversible and steady electrochromism. Electrochromism was not observed for films heat treated at 500-degrees-C when alpha-Fe2O3 (haematite) with a larger grain size (approximately 270 angstrom) is formed. The textural and structural properties of films were investigated with scanning electron microscopy, X-ray diffraction and UV-visible spectroscopic methods. The detected transverse optical and longitudinal optical modes determined with the help of Fourier transform IR reflectance measurements confirmed the homogeneous structure of films prepared at different temperatures. It was also found that the sols consist of iron oxohydroxo ions with chelated acetate units in addition to medium strong OH-O bonds, thus contributing to cross-linking of the gel and xerogel structure.