Effect of annealing temperature on anodized nanoporous WO₃

Abstract

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.