Hydrothermal treatment of tin oxide sol solution for preparation of thin-film sensor with enhanced thermal stability and gas sensitivity

The effects of treating tin oxide gel hydrothermally in an ammonia solution at 200 °C for 3 h were studied. The size of sol particles increased with increasing concentration of the resulting sol solution, i.e., 5, 8, 10 and 32 nm for 1.8, 3.2, 6.1 and 8.6 wt.% tin oxide sol as determined by an optical analyzer, whereas the crystallite size of tin oxide determined by X-ray diffraction analysis remained to be about 5-7 nm for all the solutions. The tin oxide powder collected was found to be resistant to grain growth on calcination, depending on the concentration of the sol solution. This tendency was maintained in the thin-films spin-coated on an alumina substrate from the sol solutions. The grain size of the film derived from 1.8 wt.% tin oxide sol was smaller than 10 nm in diameter after calcination at 600 °C. This particular film exhibited an outstanding by high sensitivity to 800 ppm H₂ at 350 °C, compared with conventional tin oxide elements of a sintered block type.