Fractal Scaling in the Gas-Phase Agglomeration of Nanowires

Assembling 1D nanoparticles (nanowires (NW) or nanotubes) as networks enables bridging multiple scales to form macroscopic materials such as fibers, sheets and electrodes. This can be done directly in the gas phase from 1D nanoparticle aerosols grown by floating catalyst chemical vapor deposition (FCCVD). In FCCVD nanowires/nanotubes grow to high aspect ratios (10²-10⁶) floating in a gas stream and can agglomerate to form an aerogel. This work studies the agglomeration of Si nanowires by scanning electron microscopy of samples taken from the gas downstream of the reaction zone, and through simulations with a Brownian collision algorithm to form agglomerate models. In the experimental analysis of over 312 samples no individualized NWs are found, only agglomerates. This is consistent with the fast binary collision rates of 0.24 s estimated. The agglomerates show "fractal" scaling, with a fractional dimension D_f of 1.8 and agglomerate size increasing with the number of nanowires to the power of 1/D_f, consistent with a diffusion limited cluster aggregation process. Formation of a nanowire aerogel involves percolation of agglomerates, therefore occurring at much lower volume fraction than for individualized particles considering excluded volume theory. Compared to FCCVD carbon nanotubes of higher aspect ratio, these SiNWs require longer residence time for gelation.