Fabrication of nanoporous tungsten oxide by galvanostatic anodization

Nanoporous tungsten oxide (WO₃), with pores of 50 to 100 nm in diameter, has been obtained by galvanostatic (constant-current) anodization of tungsten in a 0.25 M oxalic acid electrolyte. At room temperature, the optimum current density for nanoporous formation is approximately 6.5 to 8 mA/cm². Monitoring of the anodization voltage during the fabrication process reveals a close match with the theoretical model of Parkhutik et al. [V.P. Parkhutik and V.I. Shershulsky, J. Phys. D 25, 1258 (1992)] for growth of nanoporous Al₂O₃. The as-anodized films are amorphous and crystallize upon annealing at 350 °C in an oxygen atmosphere.