A far infrared reflection-absorption investigation of SnCl4 on silica and Na modified silica surfaces using the buried metal layer approach

One of the key reactions in the CVD growth of SnO2 on glass is that between SnCl4 and H2O. Exploiting the buried metal layer approach, we have used far-infrared RAIRS at the Daresbury synchrotron, to study the initial steps in this process on model glass surfaces, consisting of thin (similar to 500-1000 Angstrom) SiO2 films and Na covered SiO2 films grown on a tungsten substrate. The adsorption of SnCl4 and interaction with H2O have been probed by monitoring the changes in the Sn-Cl stretching frequencies. At low temperature adsorption of SnCl4 gives rise to three vibrational features, a band at 417 cm(-1), one at 370 and another centred at 350 cm(-1). The first of these is due to a SnCl4 multilayer, the second due to a strongly bound cm chemisorbed species and the third is believed to be due to SnCl4 interacting with molecular water on the surface. The chemisorption of SnCl4 at 300 K, results in two bands at 379 cm(-1) and 360 cm(-1), indicating significant interaction with the surface. Coadsorption with H2O gives rise to the same two bands at 300 K but the intensity of both is diminished. This suggests that high coverages of water inhibit the formation of the strongly chemisorbed species. The presence of Na impurities on the model glass surface has a profound effect on the surface chemistry observed. With increasing Na coverage, the two bands at 369 cm(-1) and 379 cm(-1) diminish and a new broad band appears between 275 cm(-1) and 325 cm(-1) depending on Na coverage. These bands are consistent with the formation of NanCLn clusters.