Optical, spectroelectrochemical and structural properties of sol-gel derived Ni-oxide electrochromic film

Ni-oxide films prepared using the sol-gel route and dip-coating method have been investigated by X-ray diffraction, scanning electron microscopy, FT-IR spectroscopy, cyclic voltammetry and in situ UV-VIS spectroelectrochemical methods. The sols were prepared from a nickel sulphate hexahydrate precursor with formamide and polyvinylalcohol added to improve the abrasive resistance of films and to increase the yield from dip-coating deposition method (100-120 mu m per dipping cycle). The films consisted of 40% NiO crystalline phase with an average grain size of 10 to 30 Angstrom. The crystallites were incorporated into the amorphous phase which was found to resemble the alpha(II)-Ni(OH)(2) phase. A pronounced electrochromic effect (Delta T approximate to 60%) was found by cycling the films in a 0.1 M LiOH electrolyte. This produced a maximum colouration efficiency (lambda = 450 nm) of 35-40 cm(2) C-1. Longitudinal optical (LO) mode of films potentially cycled up to 200 times have been detected by near-grazing incidence angle (NGIA) FT-IR reflection-absorption spectroscopy. The results revealed that the as-deposited films are progressively transformed to the beta(II)-Ni(OH)(2) phase which is then converted to the beta(III)-NiOOH phase during further potential cycling. The influence of foreign ions (SO42-, CO32-, OH-) on the development of persistent electrochromism and stability of films was established and discussed in detail.