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

Abstract

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.

Flame-coating of titania particles with silica

“…In contrast to the 40 or 60 wt% TiO 2 powders, the segregation of the two oxides occurs also on the surface and not solely within the particles containing 80 wt% TiO 2 . This is consistent with Powell et al 19 who observed that decreasing the silica concentration resulted in SiO 2 coatings on TiO 2 rather than separate SiO 2 particles.…”

“…9(d)] as it would be expected by a mass balance. This is consistent again with Powell et al 19 who formed denser and smoother silica coatings on titania in a hot-wall reactor by decreasing the silica precursor concentration. A decrease in HDMSO concentration should result in the formation of smaller silica particles, which have a better chance to collide with the newly formed TiO 2 rather than by themselves.…”

“…17 Kodas et al 18 synthesized titania by TiCl 4 oxidation in a hot-wall aerosol flow reactor and introduced Si-, Al-, or Zrcontaining precursors downstream from the particle formation zone to produce oxide coatings on the titania nanoparticles. Powell et al 19 produced titania particles with an average size of 0.8 m from TiCl 4 Despite its significance, silica coating of TiO 2 remains a challenge, in particular, the formation of smooth coatings during flame synthesis of TiO 2 , which is an established industrial process. Here, co-flow diffusion flame reactors, which are commonly used for nanoparticle production, 23 are used in synthesis of silica/titania nanoparticles over a range of titania mass fractions.…”

Gas-Phase Synthesis of Nanostructured Particulate Films

“…Most research in the field has focused on obtaining a powder rather than a film, though. The studies of Kodas and co-workers give examples showing how the particle composition and morphology can be changed by in-situ gas-phase coating processes in hot-wall reactors [45][46][47] . The formation of functional coatings on oxide nanoparticles can also be achieved by introduction of multiple precursors into spray flames 48) .…”

Aerosole und ihre technische Bedeutung

“…Durch stufenweise Zugabe verschiedener Precursoren können sich beschichtete Partikel bilden. P O W E L L et al [65] zeigen dies am Beispiel von TiO 2 -Partikeln, die sie mit SiO 2 beschichtet haben. [71,72].…”