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 O-2 in a hat-wall, tubular, aerosol reactor and directly coated in the gas phase via the reaction of O-2 with SiCl4 vapor. Rough SiO2 coatings were obtained at 1300 degrees C while uniform, dense coatings were obtained at all conditions examined for 1500 degrees 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 degrees C. Coating thicknesses could be controlled from 5 nm to roughly 100 nm, corresponding to growth rates on the order of 10-100 nm/s. 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.