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.