Color modulation of electrochromic nanosheet-structured nickel–cobalt oxide thin films

Abstract

Nickel–cobalt (Ni–Co) oxide thin films were directly grown on the fluorine-doped tin oxide (FTO)-coated glass substrates via a simple one-pot solution process using various ratios of Ni and Co precursors. All the Ni–Co oxide samples were composed of nanosheet structures that were vertically aligned on the surface of the substrates. The nanosheets of the Ni₁₀–Co₀, Ni₈–Co₂, Ni₆–Co₄, and Ni₄–Co₆ oxide samples uniformly covered the surface, whereas those of the Ni₂–Co₈ and Ni₀–Co₁₀ oxide samples were sparsely distributed. As the ratio of Co to Ni was increased, the width of the nanosheets gradually increased. Cyclic voltammetry (CV) at a scan rate of 50 mV/s showed that the Ni₈–Co₂ and Ni₆–Co₄ oxide samples exhibited better electrochemical performance than the other oxide samples. Furthermore, all Ni–Co oxide samples except Ni₀–Co₁₀ exhibited a reversibly sustainable in situ transmittance change with cycling (1000 cycles, 30,000 s) and a relatively fast switching time of less than 4 s for the colored and bleached states. Herein, the color modulation from transparent to black nanosheet-structured Ni–Co oxide samples was effectively tuned by adjusting the ratios of the Ni and Co precursors.