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 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. 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%) silicacoated TiO(2) nanoparticles. At higher SiO(2) contents, SiO(2)-coated titania exhibited negative z potentials at all pH values (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.