Spectroscopic and Electrochemical Studies of Electrochromic Hydrated Nickel Oxide Films

The electrochromic properties of hydrated nickel oxide thin films electrochemically deposited by anodization onto doped tin oxide-coated glass have been studied by transmittance measurements, cyclic voltammetry, Fourier-transform infrared spectroscopy, and ion-backscattering spectrometry. The spectral transmittance is reported for films switched in both the bleached and colored states. The photopic transmittance (T_p) can be switched from T_p (bleached) = 0.77 to T_p (colored) = 0.21, and the solar transmittance (T_s) can be switched from T_s(bleached) = 0.73 to T_s(colored) = 0.35. Also reported is the near-infrared transmittance (T_NIR), which was found to switch from T_NIR(bleached) = 0.72 to T_NIR(colored) = 0.55 for a film thickness of 500 Å. The bleached condition is noted to have very low solar absorption in both the visible and solar regions. Ion-backscattering spectrometry was performed on the hydrated nickel oxide film, yielding a composition of NiO_1.0 (dehydrated). Cyclic voltammetry showed that, for films in the bleached or colored state, the reversible reaction is Ni(OH)₂ ←→ NiOOH + H⁺ + e^-. Voltammetry also showed that the switching of the film is controlled by the diffusion of protons, where OH^- plays a role in the reaction mechanism. Analysis of the hydrated nickel-oxide thin films by Fourier-transform infrared spectroscopy revealed that both the bleached and colored states contain lattice water and hydroxyl groups. The surface hydroxyl groups play an important role in the coloration and bleaching of the anodically deposited nickel oxide thin films.