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

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 Ni-10-Co-0, Ni-8-Co-2, Ni-6-Co-4, and Ni-4-Co-6 oxide samples uniformly covered the surface, whereas those of the Ni-2-Co-8 and Ni-0-Co-10 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 Ni-8-Co-2 and Ni-6-Co-4 oxide samples exhibited better electrochemical performance than the other oxide samples. Furthermore, all Ni-Co oxide samples except Ni-0-Co-10 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.