Gas-phase coating of TiO₂ with SiO₂ in a continuous flow hot-wall aerosol reactor

The feasibility of in situ coating of titania particles with silica using a high-temperature, gas-phase process was demonstrated. Titania was produced from the reaction of TiCl₄ and O₂ in a hot-wall, tubular, aerosol reactor and directly coated in the gas phase via the reaction of O₂ with SiCl₄ vapor. Rough SiO₂ coatings were obtained at 1300 °C while uniform, dense coatings were obtained at all conditions examined for 1500 °C. The presence of water in the reactor significantly influenced the morphology of the coatings and resulted in smooth, dense, and uniform coatings at 1300 °C. Coating thicknesses could be controlled from 5 nm to roughly 100 nm, corresponding to growth rates on the order of 10–100 nmys. The characteristics of the coatings depended upon the concentration of SiCl₄ and the coating temperature. These process variables influenced the coating mechanism, growth rate, and densification which directly influenced the coating uniformity and thickness.