Self-weaving WO3 nanoflake films with greatly enhanced electrochromic performance

Vertically oriented WO3 nanoflakes woven from nanowires were obtained using a crystal-seed-assisted hydrothermal technique on a glass substrate coated with fluorine-doped tin oxide (FTO). Investigation of the growth process revealed that two types of nanowire, along the (100) and (002) planes, respectively, were formed in the early stages. In the presence of oxalic acid and urea, these nanowires were then interwoven to give a flake-like nanofabric. This process is similar to weaving on a loom, in which the two types of nanowire act as warp and weft threads and the oxalic acid and urea act as the shuttle. The as-prepared films exhibit tunable transmittance modulation under different voltages, and repeated cycling between the coloration and bleaching states has no deleterious effect on their electrochromic performance after 1000 cycles. A larger optical modulation of 68% at 632.8 nm at a potential of -3.0 V, faster switching speeds of t(c(90%)) = 9.3 s and t(b(90%)) = 5.7 s for coloration and bleaching, respectively, and a higher coloration efficiency of 134.4 cm(2) C-1 than those previously reported for the electrochromic performances of nanostructured films were achieved for this self-weaving nanoflake film.