A HIGHLY SENSITIVE GAS SENSOR BASED ON A

TUNGSTEN OXIDE NANOTUBE-ARRAY FORMED IN

NANOPORE ALUMINUM OXIDE MEMBRANES

Reit Artzi-Gerlitz^1*, Kurt D. Benkstein¹, David L. Lahr¹, Joshua L. Hertz¹, Christopher B. Montgomery¹, John E. Bonevich², Steve Semancik¹ and Michael J. Tarlov¹

¹Chemical Science and Technology Laboratory, ²Materials Science and Engineering Laboratory,

National Institute of Standards and Technology

100 Bureau Dr., Gaithersburg, MD 20899, USA

Nanostructured materials are promising for sensing applications due to their enhanced surface area. Assembly of nanostructures or high-porosity structures further contributes to increased surface area, facile gas diffusion and, hence, higher sensitivity. We report on a tungsten trioxide (WO₃) nanotube-array gas sensor fabricated using a porous aluminum oxide membrane as the template. Continuous, open-ended WO₃ nanotubes were formed in the pores (d=200 nm, l=60 µm) using a sol-gel process, with multiple coating and sintering steps. The WO₃ coated membranes structure was determined by dissolving the aluminum oxide template, and the exposed nanotubes were thoroughly characterized by scanning electron and transmission electron microscopies, X-ray diffraction and X-ray photoelectron spectroscopy. The measurements confirmed the open-pore structure and the chemical state of the nanotubes, and indicated that the nanotube walls are composed of poly-crystalline grains with a diameter of ≈ 60 nm to 80 nm. For electrical characterization and to enable chemiresistive-sensing measurements, porous Au contacts were deposited by thermal evaporation on the top and the bottom of the WO₃-coated alumina membranes. The nanotube-array sensor readily detected 200 ppb (nmol/mol) of nitrogen dioxide (a hazardous pollutant) in air, at temperatures ranging from 37 °C to 200 °C, with enhanced sensitivity at lower temperatures. The electrical sensing response of the nanotube-array sensor to 20 ppm (µmol/mol) of NO₂ at 200°C was measured to be three orders of magnitude larger than that of a non-porous WO₃ film-based sensor, which was prepared with the same sol-gel process.  We attribute the extraordinary sensitivity to the high surface area of the array (~0.1 m² internal surface area calculated, as compared to 4×10^-4 m² geometric surface area) and the multiple grain boundaries of the poly-crystalline nanotubes. Furthermore, the open-pore structure enables gas to access the interior surfaces of the nanotubes, which results in high gas-sensing responses.

Contact information: Reit Artzi-Gerlitz Mentor: Michael Tarlov NIST-CSTL-836.04 Tel: 301-975-3142 Fax: 301-975-2643

100 Bureau Dr., MS 8362 Bldg. 221, Rm B324 Gaithersburg, MD 20899-8362 reit.artzi-gerlitz@nist.gov Category: materials Not Sigma-Xi members