Low substrate temperature and electrochromic property of sputtered NiO_x films

NiO_x films were grown by magnetron sputtering on glass substrates covered by indium tin oxide (ITO) films, and held at different low temperatures. The films were characterized with X-ray diffraction (XRD), scanning electron microacopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and cyclic voltammograms. The results show that low substrate temperature significantly improves the electro-chromic property of the films. There exist more Ni²⁺ ions, Ni²⁺ vacancies and pores in the film, grown at the low temperature, than those at room temperature. The Ni²⁺ ions participate in the electn-chromic reaction and the pores promote injection and ejection of H⁺ ion. We suggest that insertion and extraction of H⁺ ion account for coloration and bleaching of the NiO_x films in KOH solution. Possible mechanism is: Ni(OH)₂ ↔ NiOOH+H⁺+e^-. The coloration efficiency (38.2 cm²/C) of the NiO_x films deposited at 173 K is better than that at room temperature (16.3 cm²/C).