Nanowire- and Microsphere-Templated Gas Sensors

Novel materials synthesis techniques were used to fabricate nanostructured and macroporous semiconducting metal oxide (SMO) films exhibiting exceptionally high sensitivity to reducing and oxidizing gases, as compared to conventionally prepared specimens. Increased sensitivity resulted from an elevated surface area and reduced specimen cross section. Several processing routes were pursued including electronspinning of semiconducting metal oxide (SMO) nanowires into a highly porous mat structure and microsphere templating followed by pulsed laser deposition (PLD) of macroporous SMO material onto the microsphere templates.

The TiO2/poly(vinyl acetate) composite nanofiber mats were electrospun onto interdigitated Pt electrode arrays, producing a mesh of 200-500 nm sheaths filled with ~10 nm thick single-crystal anatase fibrils. Testing in the presence of NO2 gas at 300°C demonstrated a minimum detection limit (MDL) of below 1 ppb1. Chemical and physical synthesis routes were combined to prepare macroporous CaCu3Ti4O12 and TiO2 thin films by PLD onto PMMA microsphere-templated substrates. Stable quasi-ordered hollow hemispheres with diameter and wall thicknesses of 800 nm and 100 nm, respectively, were obtained. Current- voltage and impedance spectroscopy measurements point to the crucial role played by grain boundary barriers in controlling the electrical properties of these films. The macroporous CaCu- 3Ti4O12 films exhibited a much superior H2 gas sensitivity (55ppm MDL) to non-templated films2, while macroporous TiO2 films exhibit excellent NOx sensitivity. Studies are continuing to more carefully correlate sensor response with SMO microstructure, morphology, and chemistry.