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 SiO
2-coated, mostly rutile titania nanoparticles made in one-step by flame spray pyrolysis (FSP) were compared to FSP-made mixed (co-oxidized) SiO
2/TiO
2 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 SiO
2 and TiO
2, co-oxidized SiO
2/TiO
2 and low SiO
2 content (< 10 wt%) silica-coated TiO
2 nanoparticles. At higher SiO
2 contents, SiO
2-coated titania exhibited negative zeta potentials at all pH (and thus no IEP) indicating hermetic or complete coverage of the TiO
2 surface by SiO
2 as was confirmed by isopropanol chemisorption. Co-oxidized particles containing segregated TiO
2 and SiO
2 domains exhibited Si-O-Ti and Si-O-Si bonds of higher IR intensity than hermetically-coated TiO
2. 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 TiO
2 could distinguish between partial (patchy) and complete (hermetic) coatings of TiO
2 and when combined with FT-IR it might be possible to identify separate SiO
2 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).