The Thermal Desorption of Surface Hydroxyls on Tin(IV) Oxide

Abstract

The structure of the adsorbed layer of water on SnO2 was investigated by means of thermal desorption, water physisorption, and electron diffraction, using four kinds of SnO2 samples which differ in method of preparation. Two distinct peaks appear around 513 and 573 K in the thermal-desorption spectra. The peak around 513 K is due to the condensation dehydration of hydroxyls on the (100) plane of SnO2, on which the two-dimensional condensation of water occurs. The peak around 573 K is associated with that on the (101) plane, on which the physisorption of water is strong and the two-dimensional condensation of water does not occur. The portion of the (100) plane occupied by water molecules is relatively small, and the 1 : 4 ratio (H2O : OH) holds also on the surfaces on which the two-dimensional condensation of water does not occur. In conclusion, the fully hydroxylated surfaces of SnO2 are hydrophobic in the sense that they sparsely adsorb water molecules.