Supercritical fluids are quickly becoming an alternative to costly and dangerous organic solvents in chemical processes. As the boundaries between gas and liquid blur at high temperatures and pressure, a supercritical fluid loses surface tension and can act as an impressive solvent. Readers of Nobel Intent have seen the endless stream of CNT coverage as the nanotechnology boom continues—it was only a matter of time before carbon nanotubes (CNTs) and supercritical fluids were put together.
Supercritical fluid processing of CNTs has generated composite materials that, in some cases, are easier and cheaper to prepare, while in others produced entirely new nanostructures. Advanced Materials has put together a review showing the diversity of the structures prepared with supercritical fluids and CNTs.
CNTs, despite all their glowing press coverage, don’t play well with others. The lock-tight carbon-carbon bonding makes it difficult to bond other molecules to their surface, a process called functionalizing, and the low surface tension makes any attempts at sticking particles to the CNT the rough equivalent of trying to glue something to a Teflon frying pan. The difficulties associated with dispersing CNTs in a solution (past coverage of this problem, and one solution, can be found here) makes solution processing and bulk preparation of CNT composites difficult.
Any CNT composite nanostructures are usually the result of a complicated, low-volume, or otherwise exotic processing route. Supercritical fluids, and their uncanny knack for dissolving nearly anything, give a unique opportunity to put CNTs and other substances in close contact in one another. The elevated temperatures, high pressures, and somewhat tunable properties of supercritical fluids certainly help foster favorable reaction conditions, as well.
The CNT composites described in the review fell into three categories, representing an impressive breadth of potential products from the same general processing principle: metal-CNT composites, metal oxide-CNT composites, and organic molecule-CNT composites. Metal-CNT composites were prepared using primarily supercritical CO2 and a mixture of other solvents and precursor materials that could be decomposed to leave metal behind. Particles of Pt, Ru (or both!) on the order of 2-3nm in size were deposited on the CNTs surface with a high efficiency.
Oxide compounds were also deposited, including zirconia (ZrO2) and titania (TiO2). In some cases, a complete shell was deposited on the CNT; the CNT could then be burnt out, leaving behind a pure oxide nanostructure. Other tests show a synergistic effect between the oxides and the CNTs in applications such as absorbing EM radiation or detecting chemicals.
Polymers (or precursor monomers) could be deposited on the CNTs, and usually resulted in the core of the CNT being filled with the molecule. This could then be reacted further to create a completed polymer or any of a myriad of other options that organic chemistry leaves open.
The advantages of supercritical fluid processing are easy to see: simple substances are used as the solvent (water, CO2 and alcohols), and the whole process can be done in one vessel. This makes it one of only a handful of nanotech processes that are easily scalable for industry.
The CNT composite structures mentioned have an impressive array of potential applications. Pt and Ru are noble metals used to catalyze several important chemical reactions, and are are used in green tech like fuel cells. These reactions are usually driven by surface area and, when you are talking 3nm particle sizes, your surface area to volume ratio skyrockets.
Oxide compounds are useful for a variety of sensor applications, solar cells, and semiconductors. Polymer/CNT composites have a nearly infinite applications as well—in my particular branch of materials (the structural variety), CNTs are less than ideal for use in a structural composite because they just slip out of the epoxy matrix. A functionalized CNT that is chemically bonded to the epoxy matrix would allow one to achieve structures that allow for high performance and energy efficiency.
Maybe even more important than the CNT composites themselves is the potential to make higher-scale ordered nanostructures. For all the amazing properties we see coming out of the labs, getting a bulk device to use those properties is a huge obstacle—being able to attach other compounds to a CNT opens the door for self assembly and the precision ordering that nanomaterials demand, which is, in my opinion, the crux of the nano issue.
Advanced Materials, 2008. DOI: 10.1002/adma.200800609