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 (approx. 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 cm^-1 and another centred at 350 cm^-1. The first of these is due to a SnCl₄ multilayer, the second due to a strongly bound 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 325cm^-1 depending on Na coverage. These bands are consistent with the formation of Na_nCl_n clusters.