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Simultaneous work function change and conduction measurements have been performed on undoped SnO 2 thick film sensors in order to investigate the surface reactions of CO under dry and humid conditions. On their basis, changes in the electron affinity have been observed for the reaction with water and the reaction with CO in the background of water. Additionally, by comparing the results for the changes in band bending, one can conclude that both water and CO compete for the same reaction sites, i.e. preadsorbed oxygen species. The kinetics of work function and band bending changes show that in the case of humid air more than one dipole species is involved in the reaction with CO.

The paper investigates the gas response of nanocrystalline SnO2 based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H2O) and oxygen (O2) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO2. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R0 and the resistance in the presence of the target gases, R, namely R0/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O2 and H2O related surface species and, as a result, different sensing mechanisms can be observed for CO.

The surface interaction between differently prepared tin oxide samples and CO was studied by Diffuse Reflectance Infrared Fourier Transform (DRIFT)-spectroscopy at room temperature. As samples, commercial powders (Merck) and home-made powders obtained by a sol–gel preparation were used. Several bands corresponding to isolated and rooted OH groups, coordinated water and condensed water in two forms: ordered layers of water molecules and water in the liquid form, were found. Their appearance and intensity depended on the sample origin and storing conditions. Moreover, the spectra of all samples show bands corresponding to different hydrated protons (H3O+, H5O2+) and carbonate ions of different intensities. To get information about the interaction between SnO2 powders and CO, in-situ measurements were conducted. We found that differently bound water and specific surface OH-groups react with CO and that surface carbonate ions dissociate because of acidic intermediate products. As a result, different bands corresponding to physisorbed CO appear at approximately 2200 cm−1, together with changes in the region corresponding to covalent bound chelating and bridging bidentate carbonates and carboxylate (between 1700 and 1000 cm−1).