Preparation of electrochromic nickel hydroxide (II) thin film by drying sol obtained by dialysis of solution with precipitate formed from nickel nitrate aqueous solution with ammonia and sorbitol

Nickel hydroxide has two important crystal forms, α-Ni(OH)₂ and β-Ni(OH)₂. α-Ni(OH)₂ is appropriate for application in electrochromic devices due to the reversible and efficient redox process between Ni²⁺ and Ni³⁺. In this research, a stable sol with dispersion of α and β-Ni(OH)₂ nanoparticles was obtained by dialysis of a mixed aqueous solution of Ni(NO₃)₂ and NH₃. The addition of sorbitol into the aqueous solution of Ni(NO₃)₂ enhanced the formation ratio of α-Ni(OH)₂ when the concentration of sorbitol ([sorbitol]) was more than 0.05 mol/L. A stable sol containing single-phase α-Ni(OH)₂ was obtained. α-Ni(OH)₂ has a wide interlayer spacing to enable reversible sorption and desorption of important anions such as hydroxide (OH^-) and nitrate ions (NO₃^-) for redox processes involving Ni ions. Furthermore, the sorbitol molecules adsorbed on the surface of the α-Ni(OH)₂ particles, thus increasing the hydrophilicity of the Ni(OH)₂ nanoparticles to improve the stability of the sol. The optical transmittance of the sol was more than 90% at 500 nm, which indicates that it contained no aggregated particles to cause light scattering. The α-Ni(OH)₂ sol enabled the preparation of thin film by drying the sol on a glass substrate. The resulting α-Ni(OH)₂ thin films showed reversible electrochromic characteristics. Accordingly, the hydrolysis reactions of Ni ions in aqueous solution with sorbitol formed a stable sol of α-Ni(OH)₂ nanoparticles which gave good electrochromic thin films.