(139d) The Quality of SiO2 Coatings on Flame-Made TiO2-Based Nanoparticles

Teleki, A. - Presenter, Particle Technology Laboratory, ETH Zurich
Akhtar, K. M. - Presenter, Millennium Inorganic Chemicals (a Cristal Company)
Pratsinis, S. E. - Presenter, Swiss Federal Institute of Technology, Particle Technology Laboratory, ETH Zurich

Silica-coated titania nanoparticles are important in sunscreens, UV filters and optical nanocomposites. The surface characteristics (extent of coverage, texture and thickness) of in-situ SiO2-coated, mostly rutile titania nanoparticles made in one-step by flame spray pyrolysis (FSP) were compared to FSP-made mixed (co-oxidized) SiO2/TiO2 ones by transmission electron microscopy (TEM), Raman, FT-IR spectroscopy, electrophoretic mobility and isopropanol chemisorption followed by mass spectroscopy [1]. Increasing the silica content shifted the isoelectric point (IEP) toward that of pure silica for externally-mixed SiO2 and TiO2, co-oxidized SiO2/TiO2 and low SiO2 content (< 10 wt%) silica-coated TiO2 nanoparticles. At higher SiO2 contents, SiO2-coated titania exhibited negative zeta potentials at all pH (and thus no IEP) indicating hermetic or complete coverage of the TiO2 surface by SiO2 as was confirmed by isopropanol chemisorption. Co-oxidized particles containing segregated TiO2 and SiO2 domains exhibited Si-O-Ti and Si-O-Si bonds of higher IR intensity than hermetically-coated TiO2. The latter exhibited a peak at 1225 cm-1 attributed to Si-O-Si asymmetric vibrations in contrast to the former. So electrophoretic behavior and isopropanol chemisorption of these silica-coated TiO2 could distinguish between partial (patchy) and complete (hermetic) coatings of TiO2 and when combined with FT-IR it might be possible to identify separate SiO2 domains (rough coatings) or particles.

[1] Teleki, A., M.K. Akhtar, and S.E. Pratsinis, ?The quality of SiO2 coatings on flame-made TiO2-based nanoparticles,? J. Mater.Chem. in press (2008).