Evaluating the Purity of Carbon Nanotube Materials

Considering the multitude of methods used to produce and purify nanotubes, the question arises of how to evaluate the quality of the purified materials. There are several analytical procedures, yet each of them features certain drawbacks. What 

Further techniques must be applied that include larger portions of the material into examination. These may be thermogravimetry as well as spectroscopic methods like infrared or Raman spectroscopy. They provide valuable data to evaluate sample quality and purity, but still they are insufficient to serve as stand-alone procedures. 

The thermogravimetric analysis (TGA) provides information especially on the content of metallic catalyst particles in the sample. When performed in air, the carbon species will be oxidized to give volatile gaseous products way below 1000 C, whereas the metal portion is-in parts or completely-transformed into its oxides and remains in the crucible as such. Analyzing this residue thus allows for quite an accurate assay of the metal in a nanotube sample. It may be up to 30% (m/m) depending on the quality of the material under test. The portion of amorphous carbon, on the other hand, can only be insufficiently determined by TGA. 

Raman spectra as well provide information on sample purity. The D- and G-band of the spectrum are analyzed to this purpose. The D-band, situated at about 1200-1400cm, arises from unordered, sp -hybridized carbon, whereas the G-band at ca. 1500-1600cm originates from the C-C-stretching modes in the nanotubes (Section 3.4.5.1). Due to the lack of a standard, the integration of the G-band alone does not supply information on the amount of nanotubes present in a sample, but the ratio of D- to G-band suits well to determine the portion of amorphous carbon. Yet the low sensitivity of the D-band causes small areas under its signal, which results in considerable uncertainty and scatter of the values. 

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