Effect of annealing temperature on anodized nanoporous WO3

Nanoporous tungsten oxide (WO3) was produced on W foil by anodization method. The nanoporous WO3 thickness obtained was 450 nm with pore diameter size of 50–80 nm and pore wall thickness of 15–20 nm. The effect of annealing temperature (200–700 °C) on the morphological, structural, and electrochromic properties of nanoporous WO3 were reported. Nanoporous WO3 transformed from amorphous to monoclinic phase after annealing at 300 °C. The nanoporous structure was retained after annealing; however, the pore wall thickness increased with increasing annealing temperature. A compact oxide layer underneath the nanoporous layer was formed after annealing at 500 °C. The as-anodized oxide showed faster electron and ion intercalation process (with the highest current density of −11.53 and +7.73 mA cm−2) than the annealed oxides. Moreover, the current density decreased with increasing annealing temperature because the annealing process increased the crystallinity and reduced the surface area of the oxide. The as-anodized oxide also exhibited shorter coloring and bleaching times of ~7.4 and ~3.9 s, respectively.