Fabrication and gas sensing performance of parallel assemblies of metal oxide nanotubes supported by porous aluminum oxide membranes

We describe a conductometric chemical sensor architecture based upon an assembly of parallel metal oxide nanotubes. In this design, an aluminum oxide membrane serves as both a template for growth of the sensing nanotubes and a scaffold to support the nanotubes and the top/bottom electrical contacts for sensing measurements. Important advantages of this sensing architecture are: (1) analyte molecules are detected within the nanotube interior thereby maximizing sensitivity; and (2) metal contacts for resistance measurements are easily and reproducibly established at the ends of the nanotubes. We demonstrate a proof-of-concept for this approach by fabricating and evaluating tungsten trioxide (WO3) nanotube-based conductometric devices. Nanotubes are formed in membrane pores (diameter ≈200 nm, length ≈60 μm) by sol–gel deposition and characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Thin film Au contacts are deposited on the top/bottom of the WO3-coated membranes such that the tube-ends remain open, enabling conductometric sensing along the length of the WO3 nanotubes. The nanotube assemblies detect oxidizing (nitrogen dioxide) and reducing (methanol) gases at 200 °C, and exhibit 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.