Electrochromic Durability and Color Variation of Thickness-Controlled Nanosheet-Structured Nickel–Cobalt Oxide Thin Films

Owing to their electrochromic performance, hierarchically structured transition-metal oxides have promising energy-saving device applications. In this study, nanosheet-structured nickel-cobalt (Ni-Co) oxide thin films were prepared via facile processes under simple solution conditions and were used as energy-saving electrochromic materials. As the growth time increased from 1.5 h to 8 h, the film thickness increased from ~ 0.7 μm to ~ 1.3 μm, and the porous nanosheet-like structures were more uniformly distributed throughout the thin film. At the 1000th cyclic voltammetry (CV) cycle (scan rate = 50 m/V), the in situ transmittance variation ( \(\Delta T\) $\mathrm{\Delta }T$ ) and optical density ( \(\Delta {\text{OD}}\) $\mathrm{\Delta }\text{OD}$ ) of an Ni-Co oxide thin film grown for 8 h were ~ 49% and ~ 0.71 at a wavelength of 700 nm, respectively, which were better than those of samples grown for 1.5 h ( \(\Delta T\) $\mathrm{\Delta }T$ of ~ 26%, \(\Delta {\text{OD}}\) $\mathrm{\Delta }\text{OD}$ of ~ 0.19) and 3 h ( \(\Delta T\) $\mathrm{\Delta }T$ of ~ 46%, \(\Delta {\text{OD}}\) $\mathrm{\Delta }\text{OD}$ of ~ 0.61). However, the Ni-Co oxide thin-film samples grown for 1.5 h and 3 h had good electrochromic durability over 1000 cycles, with reversibility of 0.99. These Ni-Co oxide thin films exhibited tunable color modulation in the bleached and colored states, along with fast switching time (< 4 s) and excellent coloration memory effects.