Carbon nanotubes separation techniques

Functionalization of carbon nanotubes with metals can be achieved by different techniques exploiting either the covalent or the noncovalent approach. This topic, which is important for many applications, will be briefly discussed in a separate section after the description of the two methods. 

Chromatographic approaches have been also used to separate nanoparticles from samples coupled to different detectors, such as ICP-MS, MS, DLS. The best known technique for size separation is size exclusion chromatography (SEC). A size exclusion column is packed with porous beads, as the stationary phase, which retain particles, depending on their size and shape. This method has been applied to the size characterization of quantum dots, single-walled carbon nanotubes, and polystyrene nanoparticles [168, 169]. Another approach is hydro-dynamic chromatography (HDC), which separates particles based on their hydro-dynamic radius. HDC has been connected to the most common UV-Vis detector for the size characterization of nanoparticles, colloidal suspensions, and biomolecules [170-172]. 

A carbon-deposited film was prepared from the alumina film with 30-nm channels by the CVD technique using propylene. Fluorination was carried out by direct reaction of the film with dry fluorine gas (purity 99.7%). The film was placed in a nickel reactor and was allowed to react with 0.1 MPa of fluorine gas for 5 days at a predetennined temperature in the range of 50 to 200°C. Then the fluorinated carbon nanotubes were separated by dissolving the alumina film with HF. A schematic drawing of the fluorination process is given in Fig. 10.1.15. 

The size of impurities such as fullerenes, metal nanoparticles, and polyaromatic carbons are much smaller than the carbon nanotubes. Such impurities can be separated using size exclusion chromatography (SEC). The carbon nanotubes are first dispersed in organic solvent either by functionalization or using smfactants. The dispersed SWNTs in organic solvents at low concentration can be used to separate the impurities by SEC techniques. The conventional styrene-... 

The carbon nanotubes can be useful in the separation of molecules not only with different sizes (monomethyl naphthalenes) but also those with different shapes (dimethyl naphthalenes) as demonstrated with a tube of inner diameter of 7.3A. The above results have clearly indicated that the application of carbon nanotubes for gas separation is in very early stage of a technology with far-reaching consequences. The important point brought out is the fact that computational techniques such as MD and CG methods are efficient for screening and designing of carbon nanotubes for selective adsorption and separation of molecules. 

In the section devoted to CNTs, some details concerning synthesis and purification methods, as well as separation techniques for metalfic and semiconducting nanotubes will be reviewed. Some aspects concerning the interactions of CNTs with reactants used in the synthesis of different composites based on conducting polymers, such as PANI, PPy, PEDOT, PBTh, PNVK, PPV, and polyfluorene (PF) will be discussed in the section devoted to the synthesis of the CP/CNT composites. Preparing a composite with the desired properties requires knowledge of the interaction between the host matrix and the guest carbon nanoparticles. 

Separation Techniques for Metallic and Semiconducting Carbon Nanotubes... 

For example, Collins et al. (2001) proposed a method based on the selective destruction of metallic nanotubes, which could be realized with bursts of electricity. The ropes containing both semiconducting and metallic tubes are deposited on a flat surface of silicon oxide. Then electrodes on the top of the ropes are fabricated by lithography technique. A voltage is applied to the tubes through the electrodes. The metallic CNTs are destroyed by current-induced oxidation, and only the semiconducting tubes remain (carbon nanotubes transistors, http //www.research.ibm.com). However, this technique is only useful for transistor geometries and cannot be extended to bulk separation. 

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