Electrochromic and colorimetric properties of anodic NiO thin films: Uncovering electrochromic mechanism of NiO

In this work anodic NiO electrochromism is investigated under two potential cycling modes: i) the response between coloration and bleaching in increasing potential steps (switching); ii) the stepwise modulation to coloration with increasing potential and back to bleaching (modulation). The films' response is investigated by optical spectroscopy, colorimetry studies, Raman spectroscopy, XRD, cyclic voltammetry and chronoamperometry. Using the CIE (Commisson International de l'Éclairage) system of colorimetry the color stimuli of the films and the changes that take place upon reversible switching/modulation are recorded under various potentials. As shown by the CIE 1931 xy chromaticity coordinates, when NiO film is oxidized, a sharp decrease in luminance is observed. Optical measurements, performed as a function of applied potential, show excellent contrast between colored and bleached states. Up to now, for NiO, the coloration mechanism was considered as a surface phenomenon. Here, for the first time we show that upon ion insertion/extraction the lattice constant of NiO changes, proving that the EC coloration mechanism involves the unit cells in the bulk material. Furthermore, the fraction ratio of Li to Ni atoms (LiNi) is proposed. Considering the actual film fabrication conditions, the x value in LixNiO is found to be equal to 0.0046.