Preparation of anodic nanoporous WO3 film using oxalic acid as electrolyte

Abstract

Abstract In this work, nanoporous WO 3 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 WO 3 were investigated. Uniform nanoporous WO 3 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 WO 3 after annealing in argon (Ar) and nitrogen (N 2 ) gases. N was doped into WO 3 after annealing in N 2 and it suppressed the grain growth. The N-doped WO 3 which possesses thinner pore wall showed higher electrochromic current density value than the film annealed in Ar.