CHARECTERIZATION OF SWNTs BY C-13 NMR

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CHARECTERIZATION OF SWNTs BY C-13 NMR

• NEELA SEKHAR
• PH06M004

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• Nanotubes have potential applications that range
  from electronic devices to biotechnology.
• The synthesis and preparation of nanotubes with a
  well-defined structure is required.

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• A SWNT(Single-Walled Carbon Nanotube) can be
  viewed as a rolled graphene(a single plane of
  graphite sheet) and is classified by a vector
  connecting the two points that meet upon rolling.
• This vector is called Chiral Vector.
• There are three classes of nanotubes
• armchair (n,n)
• zig-zag (n,o)
• chiral (n,m)

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Rolling a graphene sheet to get SWNT
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Different types of SWNTs
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• The sheet's hexagonal rows of carbon molecules
  run along the nanotube axis in armchair nanotubes
  and around it's circumference in Zigzag
  nanotubes.
• Zig-zag nanotubes contain energy gaps that stop
  current from flowing freely. They are
  semiconductors.
• armchair carbon nanotubes are metallic.
• Chiral nanotubes have intermediate properties.

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J. W. Mintmire, B. I. Dunlap, C. T. White, Phys.
Rev.Lett. 68, 631 (1992).R. Saito, M. Fujita,
G. Dresselhaus, M. S. Dresselhaus,Appl. Phys.
Lett. 60, 2204 (1992).

• Depending on how a nanotube is wrapped up from a
  single plane of graphite (graphene) it may be
  semiconducting or metallic.
• Their physical and chemical properties, depend on
  structural parameters such as their width and
  helicity.

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Arc discharge method to produce SWNTs
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MWNT
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Laser ablation method to produce SWNTs
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• A variety of techniques are used to characterize
  nanotube samples.
• scanning tunneling microscopy
• optical absorption
• fluorescence spectroscopy
• Raman scattering.
• However, these methods even in combination
• do not provide a full characterization.
• One alternative technique is
• C-13 NMR spectroscopy.

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C-13 NMR spectroscopy

• An application of NMR with respect to carbon-13
  that allows the identification of carbon atoms in
  a sample.
• C-13 NMR is much less sensitive than proton NMR
  since the C-13 is only 1.1 abundant naturally.

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Problems with observing NMR spectra of nanotubes

• A very large broadening of the NMR signal
• residual ferromagnetic impurities.
• Insolubility of nanotubes in ordinary solvents

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Solid State NMR

• In general solid-state NMR spectra are very broad
  compared to liquid state NMR, as the anisotropic
  or orientation dependent interactions does not
  average to zero.
• Special techniques, such as magic-angle spinning,
  cross polarization are required.

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Magic angle spinning(MAS)

• By spinning the sample (usually at 1-70 kHz) at
  the magic angle , ?m ( 54.7) with respect to the
  direction of the magnetic field, the normally
  wide lines become narrow, thereby increasing the
  resolution.
• The dipolar interaction,averages to zero.
• The chemical shift anisotropy,averages to a
  non-zero value.
• The quadrupolar interaction is only partially
  averaged by MAS leaving a residual secondary
  quadrupolar interaction.

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Tang, X.-P. Kleinhammes, A. Shimoda, H.
Fleming, L. Bennoune, K.Y. Sinha, S. Bower,
C. Zhou, O. Wu, Y. Science 2000, 288, 492-494.

• Tang et al. reported static and magic angle
  spinning(MAS) 13C NMR spectra of SWNTs.
• A broad signal was obtained with an isotropic
  shift of 124 ppm with respect to
  tetramethylsilane (TMS).
• Two types of nanotube were identified through
  differences in 13C spin-lattice relaxation time.
• A fast-relaxing component was assigned to
  metallic tubes, while semiconducting tubes have a
  slow relaxation time.

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Fig. 2. The observed static and MAS (spun at 11.7
kHz)13C spectra . The chemical shift is124 ppm
based on the MAS spectrum.
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Goze Bac, C. Latil, S. Vaccarini, P. Bernier,
P. Gaveau, P. Tahir, S.Micholet, V. Aznar,
R. Rubio, A. Metenier, K. Beguin, F. Phys.
ReV. B2001, 63, 100302.

• In another study baking at high temperature was
  used to remove the ferromagnetic impurities.
• A band at 126 ppm with a FWHM of 50 ppm was
  obtained. The broadening was attributed to the
  distribution of isotropic shifts of inequivalent
  carbons.

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• Such a baking process can modify the sample.
• Consequently, nanotubes were prepared with a non
  ferromagnetic catalyst such as Pt.
• A MAS spectrum with peak at 126 ppm was
  obtained.
• Only a weak dependence of line shape on both tube
  radius and chirality was found, and it was
  postulated that NMR could resolve electronic
  properties but not structural properties.

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• The manipulation and processing of single-wall
  carbon nanotubes (SWNTs) is limited by their
  insolubility in most common solvents. The strong
  van der Waals interactions between SWNTs in the
  formation of bundles of tubes, called ropes,
  which are not easily broken up into individual
  tubes by either temperature or by solvents.
• Covalent sidewall functionalization of SWNTs
  provides an excellent route to solubilize the
  nanotubes in common solvents.

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13C NMR of functionalized nanotubes in solution
phase

• Ferromagnetic impurities were removed via
  repeated magnetic separation.
• Although a broad signal centered at 132 ppm
  remained, it was resolved into two overlapping
  components at 128 and 144 ppm.
• These were assigned to semiconducting and
  metallic nanotubes.
• Kitaygorodskiy, A. Wang, W. Xie, S.-Y. Lin,
  Y. Shiral Fernando, A. Wang, X. Qu, L. Chen,
  B. Sun, Y.-P. J. Am. Chem. Soc. 2005,
  127,7517-7520

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The 13C NMR spectrum of Poly Ethylene
Glylated-13C-SWNT in D2O solution Shown in the
inset is a deconvolution based on two Lorentzian
peaks (reproduced curve, - - -). The 70 ppm
signal is due to nanotube-attached PEG functional
groups.
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• Theoretical Study of the 13C NMR Spectroscopy of
  Single-Walled Carbon Nanotubes
• Nicholas A. Besley, Jeremy J. Titman, and Matthew
  D. Wright,
• Journal of the American Chemical Society ,2005.

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• The 13C NMR spectroscopy of armchair and zigzag
  single-walled carbon nanotubes has been
  investigated theoretically.
• Spectra for (4,4), (5,5), (6,6), (6,0), (9,0),
  and (10,0) nanotubes have been simulated.
• The calculations predict a dominant band arising
  from the carbon atoms in the tube with smaller
  peaks at higher chemical shifts arising from the
  carbon atoms of the caps.

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• The dominant band lies in the range of 128 and
  138 ppm.
• Its position depends weakly on the length, width,
  and chirality of the tubes.
• The calculations demonstrate how structural
  information may be gleaned from relatively
  low-resolution nanotube 13C NMR spectra.

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• 13C nuclear shieldings were computed with the
  gauge-including atomic orbitals methodology.
• They used Hartree-Fock (HF) theory to determine
  the shieldings of the Carbon

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Dimensions of the Nanotubes Studied

• Tube no. of atoms length (Å)
  diameter (Å)
• (4,4) 116 17.40
  5.27
• (5,5) 140 16.52
  6.82
• (6,6) 128 13.07
  8.28
• (6,0) 108 18.08
  4.74
• (9,0) 150 17.18
  7.01
• (10,0) 160 16.44
  7.83

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Conclusions

• Exploitation of the chemical and physical
  properties of nanotubes requires understanding
  and control of their structure during synthesis.
• While several techniques are used to probe
  nanotube structure, full characterization is not
  possible.
• 13C NMR spectroscopy has the potential to provide
  a more detailed structural characterization.

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