Preparation and structural investigations of electrochromic nanosized NiOx films made via the sol–gel route

The main aim of this work was to study the influence of the NiSO4 precursor and the annealing temperature on the electrochemical stability and electrochromic response of dip-coated Ni-oxide films. Nickel hydroxide was precipitated from the precursor solution using lithium hydroxide and the slurry was then peptized with acetic acid. Dynamic thermogravimetric measurements made on thin films showed that the decomposition and the formation of Ni-oxide phase differed for powders and films. The results showed that the films annealed at 270 °C (at which the acetate groups decomposed) consisted of cubic (space group Fm3m) NiO grains (2–3 nm, TEM measurements) having monodentately coordinated sulfate groups (IR spectrum). Electrochemical and in situ UV–visible spectroelectrochemical studies of the Ni-oxide films (NiSO4 precursor, 270 °C) revealed that the photopic transmittance in the colored state (3rd cycle) is 66% and 89% in the bleached state. Cycling (100th cycle) reduced the transmittance of the films in the colored state to 50%, while the bleached films retained their high transmittance (89%). The coloration efficiency of the films was 35–41 cm2 C−1 depending on the number of cycles. The evolution of the Ni3+–O stretching vibration (transversal optical mode) of the films in the charged state was followed by the help of ex situ infrared spectroelectrochemical measurements.