Application of nano-crystalline porous tin oxide for carbon monoxide sensing

The sensitivity and response of conventional tin oxide thin film sensors are limited by their compact film structures. Quantity control of dopants and dopant dispersions are also difficult to achieve for current thin film technologies. To overcome these limitations, a sol gel process has been established in our research to produce nano-size tin oxide particles for thin film sensors. The tin oxide thin film, which is spin-coated from the sol gel solution, has porous interconnected nano-crystalline tin oxide particles. The thin film is deposited onto interdigitated electrodes for the evaluations of electronic properties, CO sensing characteristics and long term stability of the film.; For the undoped thin films calcined at various temperatures (between 500{dollar}spcirc{dollar}C and 800{dollar}spcirc{dollar}C) for 30 min, the particles and pores are in the diameter range of 7-15 nm and of 1.6-9 nm, respectively. The undoped thin film calcined at 700{dollar}spcirc{dollar}C for 30 min shows high CO sensitivity (G{dollar}sp{lcub}rm g{rcub}{dollar}/G{dollar}sp{lcub}rm air{rcub}{dollar} = 9.8 for 95 ppm CO) at 350{dollar}spcirc{dollar}C with relatively fast response ({dollar}sim{dollar}4 min) and recovery time ({dollar}sim{dollar}1 min), respectively. The optimum sensitivity (38 for 95 ppm CO at 300{dollar}spcirc{dollar}C) is obtained from a 5 wt% Pt doped tin oxide film with good CO sensing reproducibility and response/recovery time of less than 1 min. However, particle growth during the long term test leads to the deterioration of sensitivity of the undoped and the Pt-doped tin oxide sensors.; Silicate doping effectively inhibits the growth of tin oxide particles at high temperature. For a 24 hr 700{dollar}spcirc{dollar}C calcination process, the average particle size is 24 nm for an undoped thin film, and is 6-10 nm for the silicate-doped film. Antimony doping increases the conductance and sensitivity to CO of the silicate-doped film.; Our sol-gel derived thin films have shown exceptionally higher sensitivity and faster response compared to those of tin oxide thin film sensors reported in literature. Our experimental results indicate the potential of using nano-crystalline porous materials for the production of thin film sensors. But the thin films that have high surface morphology stability and high CO sensitivity have not yet been achieved, and should be the focus for the future work.