Pergamon NanoStructured Materials. Vol. 4. No. 5. pp. 537-544. 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0965-9773/94 $6.00 + .00 0965-9773(94)00026-3 THE ROLE OF ELECTROLYTES DURING AEROSOL SYNTHESIS OF TiO2 M.K. Akhtar, S. Vemury,and S.E. Pratsinis Department of Chemical Engineering, Center for Aerosol Processes, University of Cincinnati, Cincinnati, OH 45221-0171, USA (Accepted May 1994) Abstract -- Titania powders are synthesized by gas phase oxidation of TiCl4 in an electrically heated hot wall aerosolflow reactor. Aqueous solutions of ionic additfi:es are sprayed into the process stream and the effect of NaCl, KCI and CsCl (in the presence of AlCl3 or SiCl4) on titania particle characteristics is studied as a function of temperature, residence time, water vapor and ion concentration. Reduction of the aggregate size is obtained with alkali halides only in the presence of either AICl 3 or SiCl4 at 1673 K. Of all the alkali halides, CsC l is the most effecth, e in reducing aggregate size while NaCl is the least effective. These changes are explained on the basis of complex ion formation, particle charging and the subsequent retardation of coagulation. The ionic radii of the alkali metals is too large to form solid solution with titania and hence they do not affect titania morphology and phase composition. INTRODUCTION Vapor phase synthesis of ceramic powders leads to high purity products with controlled particle size. These processes do not involve the multiple steps, high liquid volumes and surfactants of wet chemical processes (1). However, a disadvantage of vapor phase processes is that the employed high temperatures often lead to formation of hard aggregates (2). For efficient powder processing it is necessary to break up these aggregates which, in turn, leads to consumption of energy and possible introduction of impurities. In situ control of aggregation would lead to efficient processing of ceramic powders (titania, silica and zinc oxide) which are traditionally produced by flame processes (3). Bulewicz et al. (4) found that soot formation could be suppressed in acetylene-oxygen and propane-oxygen diffusion flames on adding metallic salts ofLi, Na, K and Cs. They ascribed this behavior to the reduction in hydrocarbon fragment ions obtained on the introduction of alkali cations. Haynes et al. (5,6) found that spraying aqueous solutions of alkali halides into premixed hydrocarbon flames increased the number concentration of soot particles and decreased the particle size. Metals with low ionization potential had the most dramatic impact on soot particle size and number concentration. Haynes et al. (5,6) concluded that an electrostatic mechanism led to alkali salts inhibiting soot formation and growth by coagulation. 537