Oxygen non-stoichiometry, conductivity and gas sensor response of SnO2 pellets

SnO2 pellets lose a very small amount of oxygen at high temperatures to give, for instance, the stoichiometry SnO1.9989(1) at 1200°C in air. The oxygen deficiency, δ can be preserved at ambient temperature in quenched samples. The level of conductivity, which is n-type, depends on oxygen content, 2-δ and varies by several orders of magnitude; activation energies cover the range 1.1 eV for slow-cooled, fully oxidised samples to 0.52 eV for samples quenched from 1200°C. Quenched samples can be readily and reversibly reoxidised and reduced at temperatures as low as 700°C; at lower temperatures, down to ∼350°C, oxidation and reduction is mainly confined to sample surfaces on short timescales but, nevertheless, is sufficient for the conductivity to change by 1 to 2 orders of magnitude. Quenched, oxygen-deficient samples are also moisture-sensitive whereas fully oxidised samples are not. SnO2 shows similar sensitivity to both CO2 and N2, which is attributed to loss of O2 from the sample surface.