Multicolored absorbing nickel oxide films based on anodic electrochromism and structural coloration

Inorganic electrochromic materials are promising for applications in color-based chromogenic technologies. Limited color control in these materials has, however, hitherto hampered their applications. Here, we show that multicolored nickel oxide (NiO) films can be obtained due to the combined effect of the intrinsic color of NiO and the structural color of the inverse opal structures by tailoring anodic oxide NiO films, exhibiting an absorption tail in the visible region into three-dimensional ordered macroporous inverse opal photonic bandgap structures. Various colors were achieved by the synergistic mechanism of structural and electrochromic coloration, thus realizing a wide spectrum of blue, green, yellow, orange, and brown colors depending on pore size, wall thickness, and viewing angle. Importantly, it is shown that the depth of color can be varied by applying an external potential. The electrochromic coloring of the inverse opal NiO films is found to be very different from the typical optical switching of non-structural NiO films. Thus, our work brings insights into the development of inorganic colored electrochromic materials.