Charge imbalance induced oxygen-adsorption enhances the gas-sensing properties of Al-doped SnO2 powders

In order to evaluate the effect of Al on the enhancement of the sensor performance of SnO2 prepared at a low sintering temperature, the Al-doped SnO2 powder was prepared by a chemical co-precipitation method and sintered at a temperature of 400 °C. The gas-sensing response to ppm-level ethanol was investigated. The results showed that the gas-sensing response increased with an increase of the Al/(Al + Sn) molar ratio, and exhibited a maximum response at a molar ratio of 19%, then decreased drastically. A new possible influence mechanism is therefore proposed that, with the increase of Al/(Al + Sn) molar ratio, Al3+ ions form at first an interstitial solid solution in the SnO2 lattice, which causes a charge imbalance of the Al-doped SnO2 grains, and enhances the oxygen-adsorption onto the surface of SnO2 grains, thereby resulting in an improvement in the gas-sensing performance. At the molar ratio greater than 19%, excess Al3+ ions deposit on the surface of the SnO2 grains, which reduces the chance of contact between ethanol and the adsorbed oxygen, thereby reducing the gas-sensing performance.