Nanocharacterization Challenge: A Standard Method for the Assessment of
Nanotube Material Quality
Leonard Yowell, Sivaram Arepalli
NASA Johnson Space Center
The surge of interest in single wall carbon nanotubes (SWCNT) and their applications
has stretched the limits of nanotube production capacity as well as materials
characterization techniques. Researchers from industry, academia, and government
laboratories have been using SWCNTs from the full range of available production
methods. These methods result in substantially different tubes (diameter, length, etc.) as
well as different amounts and types of impurities (metals, amorphous and graphitic
carbon). Due to the presence of impurities in the raw product, most researchers would
prefer to use purified nanotubes of the highest reasonable quality.
In the course of our work at NASA, it has become necessary to develop a standard
characterization protocol for the evaluation of our material and the validation of our
purification methods. There is no single analytical technique that can characterize the
essential elements of sample quality: purity, homogeneity, thermal stability, and
dispersability. We have performed a systematic evaluation of available characterization
techniques and evaluated their use in analyzing our material. These standard analytical
techniques include scanning electron microscopy (SEM), transmission electron
microscopy (TEM), thermogravimetric analysis (TGA), Raman, and UV-VIS-NIR
spectrometry. Our suggested protocol standardizes measurements using these established
techniques, and consumes < 30 mg of material. Images from SEM are used to give a
rough qualitative assessment of material quality. Higher resolution images from TEM
are used primarily to monitor the surface texture of individual ropes and to establish
diameter distribution of the tubes in the sample. We currently lack an efficient method
for determining the length distribution of individual SWCNTs. Raman spectra are used
to estimate the extent of amorphous carbon as well as damage to the tubes. Analysis of
TGA data is used to quantify the quality of the tubes (decomposition temperature) as well
as the extent of non-carbon impurities in the collected sample. Absorption spectra of
nanotube solutions are obtained by using a UV-VIS-NIR spectrometer and the variation
of optical density with time is used as a measure of SWCNT dispersability. Dispersion,
and the characterization thereof, seems at present to be the single most critical issue in the
development of SWCNT composites.
The establishment and community acceptance of a standard SWCNT characterization
protocol is necessary in the development of reliable, high-performance nanotube-based
materials for a wide range of applications. As a first step toward this goal, a workshop
organized jointly by the National Aeronautics and Space Administration, Lyndon B.
Johnson Space Center (NASA/JSC) and the National Institute of Standards and
Technology (NIST) was held on May 27-29, 2003 at NIST in Gaithersburg, MD to
discuss and prioritize measurement needs relative to SWCNT purity and dispersion. A
follow-on workshop will be held in Gaithersburg during the fall of 2004 by the same
organizers.