FORMATION AND PROPERTIES OF AEROCOLLOIDAL TIO2-COATED MICROSPHERES PRODUCED BY ALKOXIDE DROPLET REACTION

The chemical reaction between an electrodynamically levitated microdroplet of titanium tetraethoxide (TTE) and water vapor has been studied by means of elastic and inelastic light scattering. It is shown that the initially rapid rate of reaction leads to the formation of a TiO2 shell which inhibits and eventually stops the reaction. The parameters which govern the simultaneous diffusion and chemical reaction process are elucidated, and experiments were performed to determine the properties of the product. The elastic light-scattering measurements include angular scattering data and morphological resonance spectra from which the size of the microsphere, the refractive index of the core, and the thickness, refractive index and density of the coating are estimated. The inelastic scattering data consist of Raman spectra obtained during the slower part of the reaction and after the product particle was annealed by electromagnetic heating using an infrared laser. The microparticle Raman data, which agree with previously reported spectra for thin films and for bulk ethanol/TTE mixtures, indicate that upon electromagnetic heating the structure of the TiO2 shell changed from a weak rutile/amorphous structure to an anatase crystalline form. © 1994 by Academic Press, Inc.