Gas-phase crystallization of titanium dioxide nanoparticles

We have investigated the development of crystal morphology and phase in ultrafine titanium dioxide particles. The particles were produced by a droplet-to-particle method starting from propanolic titanium tetraisopropoxide solution, and calcined in a vertical aerosol reactor in air. Mobility size classified 40 nm diameter particles were conveyed to the aerosol reactor to investigate particle size changes at 20-1200degreesC with 5-1 s residence time. In addition, polydisperse particles were used to study morphology and phase formation by electron microscopy. According to differential mobility analysis, the particle diameter was reduced to 21-23 nm at 600degreesC and above. Precursor decomposition occurred between 20degreesC and 500degreesC. The increased mobility particle size at 700degreesC and above was observed to coincide with irregular particles at 700degreesC and 800degreesC and faceted particles between 900degreesC and 1200degreesC, according to transmission electron microscopy. The faceted anatase particles were observed to approach a minimized surface energy by forming {101} and {001} crystallographic surfaces. Anatase phase was observed at 500-1200degreesC and above 600degreesC the particles were single crystals. Indications of minor rutile formation were observed at 1200degreesC. The relatively stable anatase phase vs. temperature is attributed to the defect free structure of the observed particles and a lack of crystal-crystal attachment points.