Flame-coating of titania particles with silica
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F. Krumeich Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology Zurich (ETHZ), CH-8093 Zurich, Switzerland (Received 2 November 2004; accepted 18 February 2005)
Silica/titania composite particles were prepared by co-oxidation of titanium-tetra-isopropoxide and hexamethyldisiloxane in a co-flow diffusion flame reactor. The influence of precursor composition on product powder characteristics was studied by x-ray diffraction, nitrogen adsorption, electron microscopy, elemental mapping, and energy-dispersive x-ray analysis. The flame temperature was measured by Fourier transform infrared spectroscopy. The evolution of composite particle morphology from ramified agglomerates to spot- or fully coated particles was investigated by thermophoretic sampling and transmission/scanning electron microscopy. At 40–60 wt% TiO2, particles with segregated regions of silica and titania were formed, while at 80 wt% TiO2 rough silica coatings were obtained. Rapid flame-quenching with a critical flow nozzle at 5 cm above the burner nearly halved the product particle size, changed its crystallinity from pure anatase to mostly rutile and resulted in smooth silica coatings on particles containing 80 wt% TiO2.
I. INTRODUCTION
Flame technology is used to produce large quantities of oxide nanoparticles and carbon black.1 For instance, half of the global annual production of pigmentary titania (∼2 million tons) is made by TiCl4 oxidation in flame reactors.2 Titania particles, in particular, possess a combination of attractive optical properties such as absorption and scattering of ultraviolet (UV) light and a very high refractive index3 that make them attractive in sunscreens for protection against UVA and UVB.4 Titania embedded in a polymer matrix is of great interest in fabrication of UV filters, coatings for UV-sensitive materials, and lenses.3,5 However, titania is also photocatalytically active when exposed to UV-light. To prevent degradation of the matrix by the embedded titania particles, their surface should be passivated, for instance, by SiO2 coating.6 This is of importance in sunscreen applications as radiation absorption results in reactive species that may even damage the skin tissue.7 Flame-made titania pigments are typically coated in a post-synthesis, wet-phase treatment: nanosized hydrous oxides of Al, Zr, Sn or Si are precipitated onto the surface
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0160 1336
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 5, May 2005 Downloaded: 11 Mar 2015
of titania, according to a process pioneered by DuPont in the 1960s.2,8,9 The applied coatings can improve the degree of pigment dispersion also, as nanosized asperities are formed on the particle surface preventing agglomeration. Silica coating of titania, in particular, yields maximum powder durability but is accompanied by loss of opacity. This is mainly a consequence of agglomeration during wet-pha
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