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 rutiletitania 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. 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 TiO2nanoparticles. At higher SiO2 contents, SiO2-coated titania exhibited negative ζ potentials at all pH values (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.