Hydrogen sensing properties of SnO2 subjected to surface chemical modification with ethoxysilanes

Effects of surface chemical modification with triethoxymethylsilane (TEMS) and ethoxy-trimethylsilane (ETMS) on the gas-sensing properties of SnO2 have been investigated, in comparison with the modification with diethoxydimethylsilane (DEMS) in our previous report. The largest amount of SiO2 was incorporated on the SnO2 surface with the modification with TEMS, which had three ethoxyl groups in a molecule. The temperature-programmed desorption (TPD) peaks of O− or O2− adsorbates and surface hydroxyl groups decreased to disappear with the repeated modification. These observations confirm that chemical fixation of the SiO2 component on the SnO2 proceeds via dehydration–condensation reaction between the ethoxyl and hydroxyl groups on the SnO2 surface. Electrical resistance in air of the SnO2 sensors increased straightly with the amount of the incorporated SiO2, suggesting that the Schottky potential barrier heights at the grain boundaries increased due to the suppressed neck growth between SnO2 grains. A significant increase in sensitivity to H2 was also observed with the repeated incorporation of SiO2 on the SnO2 surface, irrespective of the kind of ethoxysilane, though the sensitivity decreased at a larger amount of SiO2 in the case of TEMS-modification. On the other hand, sensitivity to C3H8 and CH4 showed a maximum at a certain amount of the incorporated SiO2 (Si/Sn≈0.004), and decreased at the larger amounts, again irrespective of the kind of ethoxysilane. The sensitivity enhancement at the small amount of SiO2 was thought to be ascribable to the increased potential barrier heights at the grain boundaries, while the decrease at the larger amounts corresponded to the decreased catalytic activity for C3H8 and CH4 oxidation.