Carbon-coated titania nanostructured particles: Continuous, one-step flame-synthesis
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Concurrent synthesis of titania-carbon nanoparticles (up to 52 wt.% in C) was studied in a diffusion flame aerosol reactor by combustion of titanium tetraisopropoxide and acetylene. These graphitically layered carbon-coated titania particles were characterized by high-resolution transmission electron microscopy (HRTEM), with elemental mapping of C and Ti, x-ray diffraction (XRD), and nitrogen adsorption [Brunauer–Emmett–Teller (BET)]. The specific surface area of the powder was controlled by the acetylene flow rate from 29 to 62 m2/g as the rutile content decreased from 68 to 17 wt.%. Light blue titania suboxides formed at low acetylene flow rates. The average XRD crystal size of TiO2 decreased steadily with increasing carbon content of the composite powders, while the average BET primary particle size calculated from nitrogen adsorption decreased first and then approached a constant value. The latter is attributed to the formation of individual carbon particles next to carbon-coated titania particles as observed by HRTEM and electron spectroscopic imaging. I. INTRODUCTION
Carbon coatings on ceramic particles can improve their dispersibility in polymers, drastically increase the electrical conductivity of ceramic composites,1 and carbon-coated particles could be used as gray pigments. Furthermore, carbon-coated metal oxides are important for fillers in rubber for the so-called “green tires” and as precursors to synthesis of the corresponding carbides. Typically, these products are made by separate synthesis of the oxide and carbon particles followed by cumbersome powder mixing. Flame technology can offer an alternative synthesis route for these products with closely controlled composition without the multiple steps and high liquid volumes associated with wet chemical processes or long processing times associated with solid processing of materials.2 Carbon-coated silica was made in acetylene/SiCl4 premixed flames by Spicer et al.,3 who found a significant increase in the carbon yield (up to three times) in the presence of silica compared with a pure acetylene flame of the same equivalence ratio. This was attributed to the presence of silica particles acting as seed nuclei for carbon surface growth, as well as to the chlorine presence reducing the flame temperature. Silica-carbon particles with carbon contents up to 5 wt.% were synthesized in a ring-shaped double diffusion hexametyl a)
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J. Mater. Res., Vol. 18, No. 11, Nov 2003 Downloaded: 16 Mar 2015
disiloxane (HMDSO)-oxygen flame at production rates up to 130 g/h by Kammler et al.,4 and recently the pilotscale production (up to 700 g/h) of silica-carbon nanostructured particles in a hydrogen-air diffusion flame was demonstrated.5 Carbon-coated titania has been used by Koc and coworkers6–9 as a precursor for TiC powder production. Thereby, they coated commercially available TiO2 powders with carbon by decomposing C3H6 at 300–600 °C batch wise using
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