Hydrogen sensing properties of SnO 2 subjected to surface chemical modification with ethoxysilanes

Abstract

Effects of surface chemical modification with triethoxymethylsilane (TEMS) and ethoxy-trimethylsilane (ETMS) on the gas-sensing properties of SnO 2 have been investigated, in comparison with the modification with diethoxydimethylsilane (DEMS) in our previous report. The largest amount of SiO 2 was incorporated on the SnO 2 surface with the modification with TEMS, which had three ethoxyl groups in a molecule. The temperature-programmed desorption (TPD) peaks of O − or O 2− adsorbates and surface hydroxyl groups decreased to disappear with the repeated modification. These observations confirm that chemical fixation of the SiO 2 component on the SnO 2 proceeds via dehydration–condensation reaction between the ethoxyl and hydroxyl groups on the SnO 2 surface. Electrical resistance in air of the SnO 2 sensors increased straightly with the amount of the incorporated SiO 2, suggesting that the Schottky potential barrier heights at the grain boundaries increased due to the suppressed neck growth between SnO 2 grains. A significant increase in sensitivity to H 2 was also observed with the repeated incorporation of SiO 2 on the SnO 2 surface, irrespective of the kind of ethoxysilane, though the sensitivity decreased at a larger amount of SiO 2 in the case of TEMS-modification. On the other hand, sensitivity to C 3H 8 and CH 4 showed a maximum at a certain amount of the incorporated SiO 2 (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 SiO 2 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 C 3H 8 and CH 4 oxidation.