Electrochromic nickel oxide-based thin films deposited by chemical bath

Nickel oxide-based thin films were deposited onto indium-tin oxide (ITO) coated glass substrates by the chemical bath method. The synthesis, based on the decomposition of urea in an aqueous nickel nitrate solution, promotes the deposition of the turbostratic α-Ni(OH)₂ phase. The electrochromic behavior of films obtained from low and high urea concentrations was tested after air annealing at temperatures of 250 and 300 °C. Using cyclic voltammetry, chronoamperometry and in-situ single wavelength transmittance it is shown that the films exhibit good reversibility and coloration efficiency. Transmittance spectra in the visible range show the high optical contrast of the films. It was found that in films annealed at 250 °C the bleached-colored switching is between the Ni(OH)₂ and β-NiOOH phases. Films annealed at 300 °C are comprised by a Ni(OH) ₂-NiO mixture and Ni₂O₃ units or the β-NiOOH phase are responsible for coloring depending on urea concentration in solution. The γ-NiOOH phase was clearly identified in over-colored films. X-ray diffraction, Raman scattering, and infrared reflectance were used for phase identification.