Efficient electrochromic nickel oxide thin films by electrodeposition

Nickel oxide (NiO) thin films were prepared by electrodeposition technique onto the fluorine doped tin oxide (FTO) coated glass substrates in one step deposition at 20, 30, 40 and 50 min deposition times respectively. The effect of film thickness (thereby microstructural changes) on their structural, morphological, optical and electrochromic properties was investigated. The mass change with potential and cyclic voltammogram was recorded in the range from +0.3 to -0.8 V versus Ag/AgCl. One step deposition of polycrystalline cubic phase NiO was confirmed from X-ray diffraction study. Optical absorption study revealed direct band gap energy of 3.2 eV. The optical transmittance of the film decreased with increase in film thickness. A uniform granular and porous morphology of the films deposited for 20 min was observed. The film becomes more compact and devoid of pores when deposition time was increased to 30 min. Thereafter severe cracks are observed. All the films exhibit anodic electrochromism in OH^- containing electrolyte (0.1 M KOH). The maximum coloration efficiency of 107 cm²/C and electrochemical stability of up to 10⁴ colour/bleach cycles were observed for the films deposited for 20 min (film thickness of 104 nm).