CO2-sensing properties and mechanism of nano-SnO2 thick-film sensor

The sensing response S of SnO2 thick-film sensor based on nano-powders annealed at 600 °C to 2000 ppm CO2 at the operating temperature 240 °C can reach 1.24 for the background of wet air with 14% of room-temperature (23 °C) relative humidity (RH), which is much larger than the corresponding value (1.048) in dry air. With an increase of humidity from 14% to 66% RH, the sensing response of SnO2 to low CO2 concentrations such as 2000-4000 ppm initially increases, peaks at about 34% RH, and then decreases. CO2-sensing response monotonically decreases with an increase of humidity at high CO2 concentrations such as 6000 and 20,000 ppm. Density functional theory calculations show that CO2 molecule cannot be adsorbed onto stoichiometric SnO2 (1 1 0) surface or SnO2 (1 1 0) surface pre-adsorbed by O2− and O− in dry air. When CO2 is introduced to SnO2 surface in wet air, CO2 reacts with O of pre-adsorbed OH−, bringing about the formation of carbonates containing (CO3)2− and the dissociation/movement of surface OH− group, accompanying the releasing of electron from CO2 to SnO2 surface. The appropriate pre-adsorption of OH− on SnO2 (1 1 0) surface promotes the sensing response of CO2.