A one-step method for continuous large-scale synthesis of well-defined hollow titania spheres was established by feeding titanium tetrachloride mixed with ethanol vapor to a facile diffusion flame. A mixture of TiCl4 and C2H5OH vapor was transported at 100 m/s into a flame reactor and condensed into mesoscale droplets due to Joule-Thomson cooling and the entrainment of cool gases into the expanding high-speed jet. Hollow crystalline TiO2 spheres with good thermal stability were formed after the hydrolysis of TiCl4 in the H2/air flame at about 1500℃. Structural characterization indicates that the hollow spheres, with uniform diameter of 300 nm and shell thickness of 35 rim, consist of 20-30 nm TiO2 nanocrystallites. A formation mechanism of the hollow spheres was proposed, involving the competition between chemical reaction and diffusion during the flame process. The present study provides a new pathway for continuous and large-scale engineering of hollow nanomaterials.
