Effect of orthorhombic phase on hydrogen gas sensing property of thick-film sensors fabricated by nanophase tin dioxide

Nanophase metallic tin powder was synthesized by an inert gas condensation method and subsequently oxidized into two types of SnO2 by two heat-treatment routes: (1) after annealing up to 900 °C in air, the synthesized powder was oxidized to form a mixture of tetragonal and orthorhombic SnO2, and (2) the synthesized powder was transformed into an amorphous tin oxide by intermediate annealing at 225 °C. The amorphous tin nano-particles were oxidized into a single phase of tetragonal SnO2 during heat treatment up to 900 °C in air. The sensitivity of the sensors fabricated from two types of nanophase SnO2 powders was evaluated by means of electrical resistance measurements. The thick-film sensors fabricated by the mixed SnO2 powder exhibited a lower sensitivity to hydrogen than those fabricated by the single tetragonal SnO2 powder. TEM observations and EPR measurements were carried out in order to interpret the role of the orthorhombic phase in gas sensing. The detrimental effect of orthorhombic SnO2 on the sensitivity was attributed to an increased number of planar defects in the microstructure and a change in species of oxygen absorbents on the surface layer of the SnO2.