Restructuring of Chain Aggregates of Titania Nanoparticles in the Gas Phase

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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, Df, 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 Df increased from 2.33 to 3.00 as the restructuring temperature increased. The activation energy for restructuring estimated from the Df 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 Df and the increase of Df 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.