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June 2, 2004

Growth of controlled diameter single wall carbon nanotubes

D. Ciuparu, Y. Chen, S. Lim, G.L. Haller, and L. Pfefferle
Department of Chemical Engineering, Yale University, New Haven, CT

The electronic properties of single wall carbon nanotubes (SWNTs) depend on their diameter and the chiral angle along which a single graphene sheet is rolled in a cylinder. The selective growth of SWNTs of predetermined diameters allows the synthesis of SWNTs with uniform electronic properties, which are required for electronic applications. Scientists at Yale have shown that SWNTs can be produced with a narrow distribution of diameters centered at different discrete values. The average size of the cobalt (Co) clusters used as catalysts for SWNT growth has been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy at beam line X23A2 of the NSLS.

Catalytic processes that provide more than 90% selectivity to single walled carbon nanotubes (SWNTs) have been developed, but catalysts that allow the engineered control of the mean diameter of SWNTs are not available. Since the properties of SWNTs depend on the tube diameter, a catalyst that selectively produces a given, preselected size is technologically important. Here we demonstrate that the cobalt-based catalyst Co-MCM-41 (where MCM stands for “Mobil composition of matter”) can be used to control the cluster size of cobalt metal in the pores. This, in turn, catalytically produces SWNTs with a narrow distribution of diameters (± 0.05 nm) in the 0.5 to 0.8 nm range.

We chose Co as the catalytic material because it has been demonstrated to be a selective catalyst for SWNTs, especially when stabilized against total reduction. We hypothesized that if Co were isomorphously substituted for silicon (Si) in MCM-41, the MCM-41 matrix would stabilize the Co against total reduction and sintering. Since catalyst metal particle size has been hypothesized to control tube diameter, we studied whether catalyst pore size could be used to control Co cluster size and, thus, SWNT diameter. Four catalysts with pore diameters of about 1.9, 2.2, 2.6, and 2.9 nm (as determined by the classical Barrett-Joyner-Halenda method) were prepared hydrothermally using C10, C12, C14 and C16 organic templates, where 10, 12, 14 and 16 denote the number of carbon atoms in the alkyl chain of the template. The diameters of the SWNTs (synthesized by the disproportionation of carbon monoxide, i.e. the Boudouard reaction) were estimated from the nitrogen adsorption isotherms and high-resolution transmission electron microscope (TEM) imaging, with the former occurring after hydrofluoric acid purification and the latter occurring before the removal of the SWNTs from the catalysts. These estimates were consistent with data from UV-visible/near-infrared absorption spectroscopy, multiple excitation wavelength Raman, and fluorescence (the last two techniques were only used for the C16 template).

In Figure 1 the average SWNT diameter, determined from nitrogen isotherms, is plotted against the Co-MCM-41 pore size (also determined from nitrogen desorption isotherms) and compared with the inner diameter of the SWNT, which was estimated from TEM. A remarkable linear correlation between the synthesized SWNT and the Co-MCM-41 pore size is observed. It is apparent, however, that the SWNT diameter control is not a result of simple geometric or mechanical constraint by the pore walls.

We hypothesize that the actual sites for SWNT growth are Co metal clusters, which is consistent with previously proposed growth mechanisms, and that the diameters of the SWNTs are controlled by the Co metal cluster size. The metal cluster size, in turn, is determined by the stability of the Co⁺² ions in the MCM-41 silica wall. Controlling the cluster size is done using the metal precursor as a cation constituent of the MCM-41 pore wall, which stabilizes the metal against reduction and migration/sintering during catalyst preparation. The smaller the pore diameter of the Co-MCM-41, the more resistant is the Co⁺² against reduction, and the smaller is the Co metal cluster that is formed. We have confirmed this hypothesis using the temperature-programmed reduction method. We used EXAFS to measure the Co cluster size in variable-diameter Co-MCMC-41 samples exposed to the Boudouard reaction under identical conditions. The EXAFS results indicate increased Co cluster sizes in MCM-41 with larger pore diameters, as shown in Figure 2.

BEAMLINE
X23A2

FUNDING
Defense Advanced Research Projects Agency, Defense Sciences Office
Department of Energy, Office of Basic Energy Sciences

PUBLICATION
D. Ciuparu et al., “Uniform-Diameter Single-Walled Carbon Nanotubes Catalytically Grown in Cobalt-Incorporated MCM-41,” Journal of Physical Chemistry B, 108(2), 503-507 (2004).

FOR MORE INFORMATION
Dragos Ciuparu
Department of Chemical Engineering
Yale University
New Haven, CT
Email: dragos.ciuparu@yale.edu