Preparation of anodic nanoporous WO3 film using oxalic acid as electrolyte

In this work, nanoporous WO3 was formed by anodization in oxalic acid. Various oxalic acid concentrations (0.01–0.4 M) and anodization times (1–240 min) on the formation of nanoporous WO3 were investigated. Uniform nanoporous WO3 with pore diameter size of 70 nm and pore wall thickness of 19 nm was produced by using 0.05 M oxalic acid and 120 min anodization time. Meanwhile, a partially porous structure or film dissolution occurred on the samples prepared using other oxalic acid concentrations and anodization times. In addition to the nanoporous layer, a compact layer embedded with nanobubbles was formed underneath the nanoporous layer. The as-anodized nanoporous film was amorphous and it transformed to monoclinic WO3 after annealing in argon (Ar) and nitrogen (N2) gases. N was doped into WO3 after annealing in N2 and it suppressed the grain growth. The N-doped WO3 which possesses thinner pore wall showed higher electrochromic current density value than the film annealed in Ar.