Effect of Thermal Treatment on the Structure of Multi-walled Carbon Nanotubes
The effects of vacuum annealing and oxidation in air on the structure of multi-walled carbon nanotubes (MWCNTs) produced by a large-scale catalytic chemical vapor deposition (CCVD) process are studied using Raman spectroscopy and transmission electron microscopy (TEM). A detailed Raman spectroscopic study of as-produced nanotubes has also been conducted. While oxidation in air up to 400°C removes disordered carbon, defects in tube walls are produced at higher temperatures. TEM reveals that MWCNTs annealed at 1,800°C and above become more ordered than as-received tubes, while the tubes annealed at 2,000°C exhibit polygonalization, mass transfer and over growth. The change in structure is observable by the separation of the Raman G and D′ peaks, a lower R-value (I D/I G ratio), and an increase in the intensity of the second order peaks. Using wavelengths from the deep ultraviolet (UV) range (5.08 eV) extending into the visible near infrared (IR) (1.59 eV), the Raman spectra of MWCNTs reveal a dependence of the D-band position proportional to the excitation energy of the incident laser energies.
Keywordsmulti-walled carbon nanotube Raman spectroscopy transmission electron microscopy annealing oxidation polygonalization heat transfer
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The authors are grateful to Dr. Mickael Havel for helpful discussions and Arkema, France, for supplying nanotubes. The vacuum furnace for annealing experiments was donated by Solar Atmospheres. The Renishaw 1000/2000 Raman spectrometer was purchased with an NSF Grant (DMR-0116645) and is operated by the centralized Materials Characterization Facility of the A.J. Drexel Nanotechnology Institute. The authors are also grateful to LRSM at the University of Pennsylvania for using their TEM facilities. K.␣Behler was supported by an NSF-IGERT Fellowship (Grant DGE-0221664) and the Arkema PhD Fellowship. S. Osswald is supported by Arkema PhD Fellowship.
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