AN IR AND NMR-STUDY OF THE CHEMISORPTION OF TICL4 ON SILICA

The atomic layer epitaxy (ALE) reactions, i.e., the saturating gas-solid reactions of TiCl4 at 175 and 450-degrees-C with silica preheated at 200-820-degrees-C, were studied by nuclear magnetic resonance and Fourier transform infrared spectroscopy, while etching experiments with sulfuric acid were carried out to determine the amount of amorphous titanium species. At 175-degrees-C TiCl4 reacted directly with the OH groups of silica either monofunctionally or bifunctionally depending on the preheat temperature of silica. A bifunctional reaction took place with strongly H-bonded OH groups as verified by using hexamethyldisilazane (HMDS) to prevent TiCl4 from reacting with isolated OH groups. The titanium species bound at 175-degrees-C was amorphous in nature, and anatase-like Ti-OH groups formed upon water treatment. At 450-degrees-C, agglomeration into TiO2 having both anatase and rutile crystal structures occurred, together with the formation of amorphous titanium species. A constant Cl/Ti ratio of 2 was obtained independent of the preheat temperature of silica, which in this case, because of the agglomeration, did not indicate a bifunctional reaction. After water treatment only Ti-OH due to amorphous titanium species was observed. The agglomeration into TiO2 is proposed to be due either to the interaction of HCI with the OH groups of silica or direct chlorination of the OH groups with TiCl4.