We describe a general, simple strategy for fabricating conductometric gas sensors from metal oxide nanotubes formed in nanoporous aluminum oxide membranes. In this approach, the aluminum oxide membrane serves as both a template for growth of the sensing nanotubes and a scaffold to support the nanotubes and electrical contacts made to the ends of ensembles of nanotubes for sensing measurements. Two significant advantages of this sensing architecture are: 1) analyte molecules are constrained to enter the interior of the nanotubes thereby maximizing sensitivity; and 2) metal contacts for conductometric measurements are easily and reproducibly established at the ends of ensembles of nanotubes. We demonstrate a proof-of-concept for this approach with the fabrication and gas-sensor testing of WO3 nanotube-based conductometric devices. To make these devices, WO3 nanotubes were first formed in the pores (diameter ≈ 200 nm, length ≈ 60 μm) of alumina membranes by sol-gel deposition. Thin film Au electrical contacts (≈ 50 nm) were then deposited at an oblique angle on the top and bottom of the WO3-coated membranes so as to maintain the open pore structure of the nanotubes and to enable conductometric sensing measurements along the length of the nanotube assemblies. The resulting nanotube sensor assemblies were able to detect both oxidizing (nitrogen dioxide) and reducing (methanol) gases at sensor operating temperatures ≤ 200 °C, and exhibited electrical sensing responses two to three orders of magnitude greater than a planar WO3-film sensor. The enhanced sensitivity is attributed to the large surface area presented by the interior of the nanotube assemblies.
Citation: Advanced Materials
Pub Type: Journals
anodized aluminum oxide, gas sensor, nanotube, NO2, sol-gel, tungsten-oxide