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Enhanced Mass Transport in Carbon Nanotubes

Written by: 
Haitao Liu, PhD.
Researchers experimentally demonstrate fast mass transport through a specially fabricated, sub-2-nanometer carbon nanotube filter. 

Reviewed by Dr. Haitao Liu, Columbia University

Holt, J.K., Park, H.G., Wang, Y., Staderman, M., Artyukhin, A.B.,  Grigoropolous, C.P., Noy, A., and Bakajin, O., "Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes" Science 312 (2006)1034-1037. DOI: 10.1126/science.1126298

Theoreticians have long believed that the carbon nanotube is the perfect water pipe. This might sound counterintuitive, especially considering that the surface of a carbon nanotube is hydrophobic. In fact, the graphene surface of the carbon nanotube is so flat that a water molecule experiences almost no friction as it passes through the nanotube. Numerical simulations have predicted mass transport in carbon nanotubes at significantly faster rates than those of of classical theory.

Fig. 1
Figure 1. Schematic of the fabrication process (A), with SEM images of the membrane (B, C) and photographs of the membrane and 89-window chip (D, E).
Holt et al. are among the first to experimentally demonstrate this unusual phenomena. The authors perform enhanced water and gas transport through a fabricated carbon nanotube filter (Science 2006, 312, 1034). The filter was created in several steps, starting with a silicon wafer with vertically aligned double wall carbon nanotubes. The carbon nanotubes were first encapsulated by a SiN layer deposited by chemical vapor deposition. The SiN layer was then ion milled to expose the tips of the carbon nanotubes, while the silicon substrate was chemically etched to define the area of the filter. Finally, the carbon nanotube tips underwent reactive ion etching, which opened the tubes to both sides of the filter. To characterize the pore size of the filter, the authors carried out size exclusion tests using various aqua solutions. The filter passes species with sizes up to 1.3 nm and blocked 2 nm gold nanocrystals, which is consistent with the average diameter of the carbon nanotubes (~ 1.6 nm). Then the filter was tested for permeability of water and air.

The authors found that the permeability of the carbon nanotube filter was orders of magnitude higher than the conventional polycarbonate filter. More significantly, they showed that the measured air flow rate was several hundred times higher than what classical theory would predict. In the case of water transport, the enhancement was several thousand times. The enhanced water flux, however, compared well with what predicted by the molecular dynamics simulations, which takes into account of the flat, almost frictionless surface of the carbon nanotube interior.

Image from Holt, J.K.,, "Fast Mass Transport through Sub-2-Nanometer Carbon Nanotubes," Science 312 (2006) 1034-1037. Reprinted with permission from AAAS.

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