Fabrication of nanoporous tungsten oxide by galvanostatic anodization

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Oomman K. Varghese, G.K. Mor, and Craig A. Grimesa) Department of Electrical Engineering and Department of Materials Science and Engineering, 217 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 11 June 2003; accepted 21 July 2003)

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.

Tungsten oxide (WO3) has been receiving considerable attention in recent years for its NOx, CO, and H2S gas sensing properties1,2 as well as for use in electrochromic,3–10 photochromic,11,12 and transparent conducting electrode applications.13,14 To date, many different processing methods including sol-gel,3–5 thermal15 and e-beam6 evaporation, sputtering,15,16 spray pyrolysis,17 pulsed laser deposition,1 colloidal chemistry,11 and polymer-templating7 have been used to fabricate tungsten oxide thin films and nanostructures. Sun et al.11 reported nanocrystalline WO3 thin film preparation by a colloidal chemistry route. Both Badilescu et al.3 and Aliev et al.5 attempted fabrication of nanoporous WO3 by sol-gel synthesis using nanoparticle templates; the structures obtained consisted of agglomerations of nanometer-size grains and voids ranging from 200 to 500 nm in size. Cheng et al.7 reported fabrication of WO3 having 4–5 nm pores by block copolymer templating. Despite a less than optimum nanoporous structure in some of the above cases, these studies have reported enhanced electrochromic properties, with high coloration densities and higher

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J. Mater. Res., Vol. 18, No. 10, Oct 2003 Downloaded: 13 May 2014

rates for coloring and bleaching.3,5,7 For photochromic applications, nanometer grain sizes result in a blue shift of the absorption peaks.11 Our interest is in developing a low cost, relatively simple route of fabricating nanoporous tungsten oxide based on anodization. In addition to the promise of nanoporous WO3 for greatly enhancing electro- and photochromic properties, earlier work of the authors has shown that nanostructured Al2O3 and TiO2 made by anodization display significantly enhanced gas sensing properties compared to their bulk counterparts.18–20 Though anodization has been used in the past to fabricate tungsten oxide, the resulting films were not nanoporous.21–23 Specific routes for nanostructure fabrication by anodization have not, to our knowledge, yet been reported. We report herein the fabrication of nanoporous tun

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