Coating titania aerosol particles with ZrO2, Al2O3/ZrO2, and SiO2/ZrO2 in a gas-phase process

The formation of ZrO2, Al2O3/ZrO2, and SiO2/ZrO2 coatings on TiO2 particles by a continuous gas-phase process was studied. Titania particles were formed by the reaction of TiCl4 vapor with O-2 in a hot-wall tubular reactor at 1300%C and were mixed with ZrCl4 and AlCl3 or SiCl4 vapors near the end of the reactor. The ZrCl4/TiCl4 molar ratio was varied from 6.7 x 10(4) to 5 x 10(3) while the AlCl3/TiCl4 and SiCl4/TiCl4 molar ratios varied in the ranges 8 x 10(3)-8 x 10(2) and 2 x 10(2)-8 x 10(2), respectively. Discrete tetragonal ZrO2 nanoparticles of average diameter 20 nm were formed on the surfaces of the titania particles at a surface concentration that increased with the ZrCl4 gas-phase concentration. The sequential introduction of AlCl3 and ZrCl4 vapors resulted in composite coatings. These consisted of dense, coherent, amorphous, and smooth Al2O3 layers approximately 10 nm thick, on top of which ZrO2 nanocrystalline particles were dispersed in a similar pattern as in the absence of Al2O3. Concentration depth profiles of these powders were obtained by Auger Electron Spectroscopy and supported the TEM observations. Particles coated by sequentially introducing SiCl4 and ZrCl4 vapors had amorphous rough SiO2 coatings, 5-10 nm thick, and ZrO2 particles sporadically dispersed on their surfaces. Increasing the SiCl4/ZrCl4 molar ratio from 5 to 19 increased the thickness and roughness of the silica layers and resulted in encapsulation of the ZrO2 particles inside the silica coatings. The different coating morphologies obtained were attributed to different coating formation mechanisms for the metal oxides used in this study.