NiO is one of the most popular electrochromic (EC) materials due to its high EC efficiency, good cyclic reversibility and low material cost. As an anodic EC material, NiO can be used as a complementary electrode with WO₃, where the optical modulation increases due to the simultaneous modulation of both electrodes. We have demonstrated that porous NiO nanostructures film can be grown directly on FTO-coated glass substrates using a template-free hydrothermal technique. A variety of NiO nanostructures, including nanoflake array, nanorod-based networks, hexagonal nanoplates and hierarchical microflakes, could be obtained by tuning the composition of the precursor solution.

Among the obtained NiO nanostructures, hierarchical structure holds the advantages of both nanometre-sized building blocks and microsized assemblies. Thus, the complementary EC device based hierarchical NiO microflake and self-weaving WO₃ nanoflake film shows a high optical modulation (73.2% at 550 nm), large coloration efficiency (146.9 cm² C^-1 at 550 nm) and fast switching responses with a coloring time of 1.8 s and a bleaching time of 3.2 s, making it attractive for practical applications.