Tin oxide surfaces. Part 19.—Electron microscopy, X-ray diffraction, auger electron and electrical conductance studies of tin(IV) oxide gel

Abstract

Physical and electrical properties of both unsintered and sintered tin(IV) oxide gel of high specific surface area and porosity are reported. Particle sizes range from 3 to ca. 5000 nm, the larger bodies apparently consisting of pressed agglomerations of smaller particles. The 3 :1 O : Sn atomic ratio determined at the surface of the discs was consistent with a fully hydroxylated oxide surface. A slight oxygen deficiency from the ideal SnO₂ composition is observed at all depths. Sintering for extended periods at 1273 K increased the minimum particle size from 3 to ca. 40 nm without causing any appreciable change in morphology on the micron scale.

The electrical conductance of the unsintered oxide in air as a function of temperature was found to be extremely complex and exhibits hysteresis dependent upon the history of the sample. Unsintered discs initially displayed a sigmoid relationship between the conductance G and temperature T, with G typically reaching a maximum at 420 ± 20 K, followed by a minimum at 520 ± 50 K, thereafter increasing monotonically. Subsequent G–T cycles produced a wide range of behaviour. Although in general, the G–T characteristic exhibited a more linear form, the rate of change with temperature varied greatly. The initial sigmoid curve was largely unaffected by strict control of the water content of the test atmosphere, and hysteresis effects generally persisted, despite the adoption of more rigorous experimental methods. The air conductance behaviour of presintered samples was typically less complex, with an increasing tendency towards a monotonically rising G–T characteristic with extended duration of the sintering treatment. Significant differences were observed between the electrical behaviour of the unsintered tin(IV) oxide gel samples and that generally exhibited by ‘commercial’ semiconductor gas sensors. However, these differences could be minimised by presintering of the oxide gel.