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

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).