Gas-to-nanotextile: high-performance materials from floating 1D nanoparticles.

Suspended in the gas phase, 1D inorganic nanoparticles (nanotubes and nanowires) grow to hundreds of microns in a second and can be thus directly assembled into freestanding network materials. The corresponding process continuously transforms gas precursors into aerosols into aerogels into macroscopic nanotextiles. By enabling the assembly of very high aspect ratio nanoparticles, this processing route has translated into high-performance structural materials, transparent conductors and battery anodes, amongst other embodiments. This paper reviews progress in the application of such manufacturing process to nanotubes and nanowires. It analyses 1D nanoparticle growth through floating catalyst chemical vapour deposition (FCCVD), in terms of reaction selectivity, scalability and its inherently ultra-fast growth rates (107-108 atoms per second) up to 1000 times faster than for substrate CVD. We summarise emerging descriptions of the formation of aerogels through percolation theory and multi-scale models for the collision and aggregation of 1D nanoparticles. The paper shows that macroscopic ensembles of 1D nanoparticles resemble textiles in their porous network structure, high flexibility and damage-tolerance. Their bulk properties depend strongly on inter-particle properties and are dominated by alignment and volume fraction. Selected examples of nanotextiles that surpass granular and monolithic materials include structural fibres with polymer-like toughness, transparent conductors, and slurry-free composite electrodes for energy storage.

Ultrafast synthesis of SiC nanowire webs by floating catalysts rationalised through in situ measurements and thermodynamic calculations.

This work presents the synthesis of SiC nanowires floating in a gas stream through the vapour-liquid-solid (VLS) mechanism using an aerosol of catalyst nanoparticles. These conditions lead to ultrafast growth at 8.5 µm s-1 (maximum of 50 µm s-1), which is up to 3 orders of magnitude above conventional substrate-based chemical vapour deposition. The high aspect ratio of the nanowires (up to 2200) favours their entanglement and the formation of freestanding network materials consisting entirely of SiCNWs. The floating catalyst chemical vapour deposition growth process is rationalised through in situ sampling of reaction products and catalyst aerosol from the gas phase, and thermodynamic calculations of the bulk ternary Si-C-Fe phase diagram. The phase diagram suggests a description of the mechanistic path for the selective growth of SiCNWs, consistent with the observation that no other types of nanowires (Si or C) are grown by the catalyst. SiCNW growth occurs at 1130 °C, close to the calculated eutectic. According to the calculated phase diagram, upon addition of Si and C, the Fe-rich liquid segregates a carbon shell, and later enrichment of the liquid in Si leads to the formation of SiC. The exceptionally fast growth rate relative to substrate-based processes is attributed to the increased availability of precursors for incorporation into the catalyst due to the high collision rate inherent to this new synthesis mode.