Ridley, Ruth; (1995) A mechanistic study of platinised tin dioxide as a gas-sensitive resistor. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this work, the gas sensing behaviour at room temperature of sensors made by impregnating pressed pellets of tin dioxide with a platinum complex solution was investigated. The complex was decomposed by firing at temperatures between 300 and 800 °C, to yield small platinum particles supported on the tin dioxide surface. The response of these pellets to low carbon monoxide concentrations was measured as a function of platinum concentration and firing temperature. The effect of doping the tin dioxide with antimony was also investigated. Results showed that the gas response increased with increasing platinum concentration. The response decreased with increasing firing temperature, and this was correlated with an increase in particle size with increasing firing temperature, as measured by the adsorption of carbon monoxide using a volumetric method. The response was found to decrease as the antimony dopant concentration was increased. All these effects were explained theoretically. X-ray photoelectron spectra in the platinum 4f region of these samples exhibited a strong effect of platinum firing temperature. Sensors fired at 300 °C exhibited predominantly peaks assigned to platinum (II) hydroxide species, which correlated with mass spectrometry results that showed that combustion was taking place at room temperature on sensors fired at this temperature. Sensors fired at 650 °C and above exhibited predominantly peaks assigned to platinum (IV) oxide species, and in between these two extremes, the spectra became more complex, with mixtures of species present. An overall decrease in peak intensity with increasing firing temperature was observed, which correlated with the spectra in the valence band region, enabling the peaks to be assigned to platinum states. These states were situated in the band gap of the tin dioxide, at approximately 1.2 eV above the valence band edge, and it is proposed that it is these states which are active in gas response.

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