RESTRUCTURING OF CHAIN AGGREGATES OF TITANIA NANOPARTICLES IN THE GAS-PHASE

The in situ restructuring of chain aggregates of titania nanoparticles by heating was investigated. Titania nanoparticles (dp congruent to 7 nm) were prepared by thermal decomposition of titanium tetraisopropox ide vapor in a N-2 carrier gas and allowed to form aggregates with a c ount 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 partic le counter. The gas temperature was varied from about 20 degrees C to 1,000 degrees C. Aggregates that restructured at 900 degrees C and a r esidence 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 ene rgy for restructuring estimated from the D-f and the coordination numb er was 2.28 x 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 ins tead of N-2. A small increase in D-f and the increase of D-f at low re structuring temperature nas found with the He atmosphere. Aggregates g enerated at different temperatures to vary particle bond strengths sho wed little change in restructuring behavior. (C) 1998 American Associa tion for Aerosol Research