Mechanism of formation of titanium dioxide crystallites in the reaction of titanium tetrachloride with magnesium hydrosilicate nanotubes

Abstract

Explored herein is the first cycle of atomic layer deposition of titanium dioxide from titanium tetrachloride and water vapours on the nanotubular magnesium hydrosilicate Mg3Si2O5(OH)4, a synthetic analogue of chrysotile. The structure and morphology of the products are characterised by X-ray diffraction, infrared spectroscopy, electron microscopy and nitrogen adsorption measurements. A side reaction between chrysotile and hydrogen chloride is followed by in situ gravimetry. At 150 °C, titanium tetrachloride chemisorption is shown to produce a monomolecular layer of titanium–chlorine groups on the surface of the chrysotile nanotube preserving its original crystal structure. By contrast, at higher temperatures, 300–400 °C, the process is complicated by a secondary interaction including dehydroxylation of the substrate and a reaction of released water with titanium tetrachloride to form titanium dioxide crystallites in the free space between nanotubes. Structural transformations of chrysotile lowering its thermal stability and leading to its dehydroxylation are caused by the action of hydrogen chloride, a by-product of titanium tetrachloride chemisorption and titanium dioxide formation.