Coloration Mechanism in Proton-Intercalated Electrochromic Hydrated NiO[sub y] and Ni[sub 1−x]V[sub x]O[sub y] Thin Films

Electrochromic EC films of nickel oxide, with and without vanadium, were prepared by reactive dc magnetron sputtering. They were characterized by electrochemical and optical measurements and studied by X-ray photoelectron spectroscopy PES using synchrotron radiation. The films were analyzed under as-deposited conditions and after bleaching/coloration by insertion/ extraction of protons from a basic solution and ensuing charge stabilization. Optical measurements were consistent with a coloration process due to charge-transfer transitions from Ni 2+ to Ni 3+ states. The PES measurements showed a higher concentration of Ni 3+ in the colored films. Moreover, two peaks were present in the O 1s spectra of the bleached film and pointed to contributions of NiOH 2 and NiO. The changes in the O 1s spectra upon coloration treatment indicate the presence of Ni 2 O 3 in the colored film and necessitated an extension of the conventional model for the mechanism of EC coloration. The model involves not only proton extraction from nickel hydroxide to form nickel oxyhydroxide but also participation of NiO in the coloration process to form Ni 2 O 3 .