Electrochromic nickel hydroxide thin films chemically deposited

Nickel hydroxide thin films were deposited on ITO-coated glass substrates by the chemical bath deposition method. Two formulations using nickel nitrate were employed to obtain the films. The first one is through coordination compounds by using an ammonia complex producing the well crystallized phase β(II)-Ni(OH)₂. The second formulation is based on the decomposition of urea at temperatures above of 90°C, which promotes the deposition of the turbostratic phase α(II)- Ni(OH)₂. After thermal annealing in air at temperatures of 250-300°C, the β(II) films retain their polycrystalline structure, while the α(II) films are partially transformed to NiO. The electrochromic behavior of the films was tested by cyclic voltammetry in a three electrodes configuration cell. Also, chronoamperometry and in-situ single wavelength transmittance measurements were carried out to evaluate the switching time response. From Raman spectroscopy the structural transformations between colored and blanched states were studied. In the colored state most of the films show a mixture of β(III) and γ(III) phases of NiOOH, and in one case the Ni₂O₃ phase was found. It is shown that the films can exhibit high optical contrast depending on the processing conditions and electrical potential applied.