Electrochromism of iron oxide films prepared via the sol-gel route by the dip-coating technique

Abstract

Thin iron oxide films (69–500 nm) were deposited by the dip-coating method from iron-ion-containing sols which had been made from an FeCl 3·6H 2O precursor precipitated with ammonium hydroxide. Homogeneous sols were obtained after peptization of precipitates with the addition of acetic acid (∼ 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°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 Fe 2O 3 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 (γ-Fe 2O 3 (maghemite)) with dimensions < 100 Å 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°C when α-Fe 2O 3 (haematite) with a larger grain size (∼ 270Å) 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.