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 Ni2+ to Ni3+ states. The PES measurements showed a higher   concentration of Ni3+ in the colored films. Moreover, two peaks were   present in the O 1s spectra of the bleached film and pointed to contributions of Ni(OH)(2) and NiO. The changes in the O 1s spectra   upon coloration treatment indicate the presence of Ni2O3 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 Ni2O3