XRD and XPS characterization of electrochromic nickel oxide thin films prepared by electrolysis–chemical deposition

Abstract

Electrochromic (EC) behaviours of nickel oxide (NiO_x) thin films, grown by chemical process in a nickel ammine complex solution [Ni(NH₃)_x²⁺] after electrolysis of this solution (EL–C method), were investigated using various alkaline electrolyte solutions; potassium borate buffer (pH 12), KOH (pH 12.9), (CH₃)₄NOH and (C₂H₅)₄NOH. The fully oxidized films in all adopted electrolytes commonly showed broadened UV–VIS spectra in the visible and near IR region. However, voltammetry studies associated with photoabsorption of the films revealed an apparent difference in the EC dynamics among the electrolytes caused by the variation of cation sizes and electrolyte solution pH. Characterization of as-prepared, oxidized and reduced films in the electrolytes by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were performed to explain the mechanism of the films based on the conversion between layered α-Ni(OH)₂ and γ₂-2NiO₂·NiOOH. A model according to the extraction/insertion of cations or water molecules from/into the interlayer was proposed.