Effects Of Surface Modification On The Gas-sensing Properties Of Nanocrystalline SnO₂

Semiconductor oxides are widely used as gas-sensing materials due to high sensitivity to flammable gas and toxic gases. One of the most studied materials is n-type tin dioxide (SnO2) both in thick and thin film forms. SnO2-based gas sensors have bad thermal stability and selectivity. However, improvement of thermal stability, sensitivity and selectivity of SnO2-based gas sensors is one of the most important subjects to meet increasing demands for more intelligent gas sensors. The thermal stability, sensitivity and selectivity of SnO2-based sensors can be improved by several factors like decreasing crystallite size, surface modification et.al.The SnO2 power is prepared by chemical precipitation method and oxidizing metallic tin with heated nitric acid, The SEM images shown that the average particle size of SnO2 power by oxidizing metallic tin with heated nitric acid is 19 nm, and the shape of SnO2 power is ball. The results show that hydroxy groups are sticking on the surface of SnO2 power.The sensitivity and selectivity to H2 of a new SnO2-based gas sensor were improved significantly by low temperature evaporation. A layer of organic silane near the surface of the SnO2 film was formed by low temperature evaporation. Finally, a layer of SiO2 was obtained after high-temperature calcinations. The sensitivities of modificated SnO2-based sensors to 1000 ppm H2 and 100 ppm C2H5OH gas at the operating temperature of 260℃is 53.8 and 4.5,respectively. The SiO2 layer functioned as a molecular sieve, thereby the diffusion of gases with large molecular diameters, except for H2, was effectively controlled, resulting in a prominent selectivity and high sensitivity for H2. The working mechanism of the sensor was also presented. The unmodified and modified gas sensors were characterized by FT-IR, XRD and SEM.Nanocrystalline SnO2 was subjected to surface chemical modification with small molecule silane through condensation reaction between the ethoxyl and hydroxyl groups on the surface SnO2. The Si remains strongly bonded at the SnO2 surface through Sn-O-Si bonds. The sensitivities of modificated SnO2 to 1000 ppm H2 and 100ppm C2H5OH gas at the operating temperature of 260℃is 23.1 and 2,respectively. The modified SnO2 was characterized by FT-IR, XRD and SEM. The results show that the sensitivity and selectivity of SnO2 for H2 were improved. The SEM pictures show that the thermal stability of modificated nanocrystalline SnO2 was effectively enhanced.Finally this paper inspects the CO gas sensivities of Pd-SnO2 thick film gas sensors prepared by the modified SnO2. The sensitivitiy of modificated SnO2-based sensors to 100ppm CO gas at the operating temperature of 260℃is 350. The gas sensitivity to CO was found to be markedly enhanced. The CO gas as low as 5 ppm can be effectively detected by the modified SnO2-based sensors. At the same time, the modified Pd-SnO2 sensor has good selectivity and response properties.