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Nano SnO2 powderwas p repared with sol2gelmethod. The crystallite size and structure of this?material were characterized with X2ray diffraction (XRD) and atomic force microscope (AFM). The gas?sensorswere made from mixed solid powderwhich contained nano SnO2 base doped with nano TiO2 pow2?der (molar ratio of SnO2 to TiO2 of 9 ∶ 1) and Ag+ ( content ofAg+ of about 012% —014% according to?the molar ratio). The formaldehyde gas sensing p roperties of thismixed solid powder were characterized?with gas sensing measurement. Test results show that SnO2 based gas sensors exhibit good sensitivity to?200 ×10- 6 formaldehyde gas at operating temperature of 300 ℃ and good gas sensing selectivity to form2?aldehyde at different operating temperatures. The result of theoretical calculation shows that difference in?orbital energy of gasmolecular is a qualitative factor for gas sensing selectivity at different operating tem2?peratures.