Restructuring of Chain Aggregates of Titania Nanoparticles in the Gas Phase

ABSTRACT

The in situ restructuring of chain aggregates of titania nanoparticles by heating was investigated. Titania nanoparticles (dp ≅ 7 nm) were prepared by thermal decomposition of titanium tetraisopropoxide vapor in a N₂ carrier gas and allowed to form aggregates with a count mean mobility diameter of 110 nm. The change in mobility diameter of the aggregates was measured to determine the fractal dimension, D_f, using two differential mobility analyzers and a condensation particle counter. The gas temperature was varied from about 20°C to 1,000°C. Aggregates that restructured at 900°C and a residence time of 5.10 sec reached a close-packed structure as observed in the electron microscope. The value of the D_f increased from 2.33 to 3.00 as the restructuring temperature increased. The activation energy for restructuring estimated from the D_f and the coordination number was 2.28 × 10⁻¹⁹ J, almost an order of magnitude higher than that of metal aggregates such as silver and copper. The effect of carrier gas was studied by making a similar series of experiments using He instead of N₂. A small increase in D_f and the increase of D_f at low restructuring temperature was found with the He atmosphere. Aggregates generated at different temperatures to vary particle bond strengths showed little change in restructuring behavior.