IR absorption spectroscopy as a technique for measuring the content of single-walled carbon nanotubes in carbon nanomaterials

The IR absorption technique can be used to measure the content of single-walled carbon nanotubes (SWCNTs) in nanomaterials (powder and paste) only in the following conditions: (1) the IR absorption signal measured is proportional to the concentration of nanotubes in an appropriate liquid dispersion sample prepared for the measurements and (2) all other parameters which are characteristic of nanotubes and to which the IR absorption value is sensitive in the S22 band are identical in dispersions of both analyzed and reference samples, namely, (i) the distribution over nanotube diameters, (ii) the ratio of semiconductor and metallic nanotube mass fractions, (iii) the degree of nanotube aggregation in bundlelike microcrystals, and (iv) the degree of covalent functionalization of nanotubes. The experimental data show that the extinction coefficient of the nanotube in the IR range crucially grows with the cross-section size of absorbing elements, i.e., the nanotube diameter or the thickness of a nanotube bundle. A method has been developed for measuring the SWCNT content in purified arc-synthesized nanotube products with a high degree of nanotube aggregation in bundlelike microcrystals. The proposed technique is based on a special method of sample preparation for IR measurements which provides for a degree of nanotube aggregation in liquid dispersion which is much lower than that in the initial reference sample and depends weekly on the latter.