Carbon nanotubes (CNTs) are one of the first major nanoscale manufactured products to enter the market. Therefore, reliable and reproducible quantitative measurement and characterization of CNT samples are important for progress in understanding these materials and the development of new applications incorporating them. An additional value is the development of an information base for CNT toxicology. NIST has obtained a single-walled carbon nanotube (SWCNT) material and prepared about 700 units of the candidate SRM 2483 SWCNT soot. Certified mass fraction values for catalyst and contaminant elements may be obtained by neutron activation analysis and mass spectrometry; collaborative information values may be obtained by thermo-gravimetric analysis, scanning electron microscopy, resonant Raman scattering, and ultraviolet, visible, and near-infrared absorbance, and fluorescence spectrometry.
CNT materials produced by current methods are notorious for their poor overall quality. Typical samples are composed of a multitude of subtypes that differ by length, diameter, and chirality, as well as containing residual catalyst and non-nanotube carbonaceous impurities. Although nanotubes are theoretically expected to have unique optical, thermal, and mechanical properties, the bundled morphology and mixture of nanotubes, catalyst, and impurities in raw soot have substantially hindered the achievement of these properties. Thus, the development of technologies based upon the projected capabilities is stymied. Furthermore, the assessment of the health risks stemming from nanotubes is difficult to quantify if the nanotube purity in a batch is uncertain. To identify the full potential and risk of carbon nanotube technology, there is a critical need for nanotube materials not only of known composition, but also processed for high purity and separated by physical characteristics such as length, diameter, or chirality. Physical properties of CNTs are known to be strongly influenced by their treatment history: method of their fabrication, sample pre-treatment, and subsequent handling. These influences on the characteristics of a CNT sample can pose challenges when attempting to compare measurements performed on different batches, or measurements made on the same sample by different operators or laboratories. The broad characterization of a SWCNT material as an SRM will serve as a commonly established platform for testing equivalence of measurements and for comparison of properties, which underpin our understanding of CNT samples. This work on SWCNT samples may also useful for the characterization of multi-walled carbon nanotube (MWCNT) samples in industrial productions.
Additional Technical Details:
A suitable raw material has been obtained and processed to yield the required units of the candidate SRM. Collaborative efforts on characterization of the physical parameters to provide information values are underway. Primary measurements on the soot follow the ACD approach for value assignment of the chemical composition of SRMs. Neutron activation analysis (NAA) will be used as a nondestructive primary quantification method, and inductively coupled plasma mass spectrometry (ICPMS) will be used as a practical second method for the characterization of the first CNT containing SRM. The first stage of this project will implement neutron capture prompt gamma activation analysis (PGAA) and instrumental NAA (INAA) for compositional analyses of the proposed SRM 2483 CNT soot. These results will serve as reference for development of ICPMS sample preparation methods including conventional microwave digestion and sealed pressure-vessel digestion methods. ICPMS measurements will complement the determinations by NAA.
Critical in all stages is the determination of a reference mass for CNT samples. Investigations are planned to determine true matrix mass. Initial desiccator drying experiments indicated “adhered” moisture. An in-depth study of the various drying procedures such as classical oven and vacuum drying techniques, desiccators, evolved gas analysis during TGA, and also Karl Fischer and H2 determination by PGAA will resolve the question of matrix mass.
- Candidate SRM 2483 has been bottled, tested for homogeneity, and distributed to analysts using all techniques described above.
- Sample preparation for NAA has been established allowing the assay of total carbon.
- Initial measurements by INAA and PGAA have established an analytical approach for the characterization of the residual catalyst and contaminant trace elements.
- Initial gravimetric determinations of drying losses and comparison with H2 content in the material are underway.
May 1, 2008
Lead Organizational Unit:
Rick E. Paul
Rabia O. Spatz
Related Programs and Projects:
Lindstrom, R.M., Zeisler, R., Greenberg, R.R., “Accuracy and Uncertainty in Radioactivity Measurement for NAA,” J. Radioanal. Nucl. Chem., 2007, 271, 311-315
J. E. Decker, A. R. Hight Walker, K. Bosnick, C. A. Clifford, L. Dai, J. Fagan, S. Hooker, Z. Jakubek, C. Kingston, J. Makar, M. T. Postek, B. Simard, R. Sturgeon, S. A. Wise, A. E. Vladar, L. Yang, R. Zeisler, “Sample Preparation Protocols for Realization of Reproducible Characterization of Single-walled Carbon Nanotubes”, J. Res. Nat. Instit. Stand. Technol., 2009, submitted