ADSORPTION AND REACTIONS OF TETRAETHOXYSILANE (TEOS) ON CLEAN AND WATER-DOSED TITANIUM-DIOXIDE (110)

The adsorption and reactions of tetraethoxysilane (TEOS) vapor on clean and water-predosed rutile TiO2-(110) have been studied using temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). The molecule sticks with a probability near unity at temperatures of 130-300 K, and at low coverage, TEOS dissociates upon heating. At higher coverages, some desorbs. On the water- and hydroxyl-free surface, the dissociation reaction occurs rapidly between 200 and 350 K, with the initial products being Si(OEt)2,s plus 2EtO(s) (Et = C2H5). This is a new mechanism for silane coupling to oxide surfaces which requires neither hydroxyl groups nor surface defects. The EtO ligands, whether attached to Ti or Si atoms, decompose at approximately 650 K via beta-hydrogen elimination to yield ethylene gas and surface-bound hydrogen, which rapidly attaches to another EtO ligand, yielding ethanol gas. By 700 K, the net products evolved are equal amounts of ethylene and ethanol gas (two molecules of each per dissociated TEOS molecule), while SiO2 remains on the surface. With predosed D2O, the initial reaction also leads to Si(OEt)2,s + 2EtO(s), but the EtO(s) thus produced now reacts with the D2O(s) and/or OD(s) to give an EtOD peak in the TPD spectrum at approximately 350 K. The amount of TEOS which dissociates is nearly doubled when D2O is present in high coverages. This is attributed to the fact that the D2O enhances elimination of EtO(s) (as EtOD), thus creating more free sites to accommodate dissociation products. The Si(OEt)2,s species is much less reactive with water than EtO(s) and will remain on the surface to 600 K in TPD, irrespective of water predose.