Supercritical carbon dioxide combines liquid-like solvation power with gas-like viscosity, diffusivity, and surface tension. Therefore, it is able to facilitate nanostructured materials fabrication in an environmentally friendly manner to access nanopores, narrow tubings, complicated surfaces, high-aspect-ratio structures, and poorly wettable substrates. This paper has demonstrated the power of the supercritical carbon dioxide fabrication process by depositing metals into channels as narrow as 5 nm in diameter.

“This paper shows supercritical carbon dioxide provides a new and efficient medium for synthesizing such nanomaterials.”

This paper also shows that nanometer-sized metal particles can be deposited on surfaces of carbon nanotubes using supercritical carbon dioxide as a medium to achieve high uniformity, high homogeneity, and high dispersion. The supercritical fluid deposition technique appears promising for making nanowires, nanorods, and nanocomposites using different templates, which may have important applications in nanoscale devices and also in catalysis.

Methods of modifying and functionalizing carbon nanotubes in supercritical carbon dioxide are of great interest to scientists for developing carbon nanotube-based new materials. This paper shows supercritical carbon dioxide provides a new and efficient medium for synthesizing such nanomaterials. The supercritical fluid deposition technique is currently being used by a number of research groups; our paper represents one of the pioneering studies in this area, and for that reason it is widely cited.

Recently, we have extended this work for synthesizing carbon nanotube-supported nanocatalysts and demonstrated their high activities for catalyzing chemical reactions and in low-temperature fuel cell applications. (J Amer Chem Soc 127, 17174, 2005; Langmuir 21, 11474, 2005; J Phys Chem B, 109, 14410, 2005)

This is a new method for depositing materials in the interior and also on the surface of carbon nanotube substrates. The carbon nanotubes can be single-walled, multi-walled, randomly oriented, or well-aligned. The method can be extended to deposition of advanced materials onto other nanostructured substrates besides carbon nanotubes.

Furthermore, a variety of materials other than metals can be deposited using the supercritical fluid deposition approach. It represents a viable nanofabrication strategy with minimal disturbance on the original structure and geometry of substrates. The method should be very useful for nanomaterials synthesis and nanodevices fabrication in general.

Deposition of metals and other materials into small pores, features, trenches, or templates is critical in developing miniaturized and more powerful devices, such as next-generation integrated circuits and ultra-high magnetic information storage media. This paper demonstrates that, by using supercritical fluid as a medium, the deposition can fill pores, features, trenches, or templates as small as 5 nm in diameter, effectively—approximately one twenty-thousandth of a single human hair.

Furthermore, the carbon nanotube-supported metal nanoparticles prepared by this method are powerful catalysts for chemical synthesis and for electrochemical reactions, including oxygen reduction and methanol oxidation reactions involved in low-temperature fuel cells for power generation.

Prior to this research, metal deposition onto carbon nanotubes was performed using liquid or gas phase reactions. Due to the high viscosity, high surface tension, and low diffusivity of liquids and the limited concentrations of starting materials allowed in the gas phase, the procedure was often tedious and time-consuming with low efficiency. Through our previous research using supercritical fluid carbon dioxide as a solvent for dissolution of metal species, we learned the factors controlling the solubility of metal chelates and kinetics of supercritical fluid dissolution processes.

We realized that the novel properties of supercritical fluids could also be utilized to reverse the metal dissolution procedure, i.e., for metal deposition. In this paper, we demonstrated experimentally, the advantages of using supercritical carbon dioxide to replace traditional solvents for metal deposition in the interior and on the surfaces of carbon nanotubes and the results were as good as we expected.

The carbon nanotube-supported metal nanoparticles prepared by this technique may be used for developing more efficient and durable low-temperature fuel cells for power generation. The technique may also be used to fill small structures on semiconductor devices for making smaller and more efficient computer chips.