13-C NMR to Study Carbon Nanomaterials

Introduction

Carbon nanomaterial

There are several types of carbon nanomaterial. Members of this family are graphene, single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT), and fullerenes such as C₆₀. Nano materials have been subject to various modification and functionalizations, and it has been of interest to develop methods that could observe these changes. Herein we discuss selected applications of ¹³C NMR in studying graphene and SWNTs. In addition, a discussion of how ¹³C NMR could be used to analyze a thin film of amorphous carbon during a low-temperature annealing process will be presented.

¹³C NMR versus ¹H NMR

Since carbon is found in any organic molecule NMR that can analyze carbon could be very helpful, unfortunately the major isotope, ¹²C, is not NMR active. Fortunately, ¹³C with a natural abundance of 1.1% is NMR active. This low natural abundance along with lower gyromagnetic ratio for ¹³C causes sensitivity to decrease. Due to this lower sensitivity, obtaining a ¹³C NMR spectrum with a specific signal-to-noise ratio requires averaging more spectra than the number of spectra that would be required to average in order to get the same signal to noise ratio for a ¹H NMR spectrum. Although it has a lower sensitivity, it is still highly used as it discloses valuable information.

Peaks in a ¹H NMR spectrum are split to n + 1 peak, where n is the number of hydrogen atoms on the adjacent carbon atom. The splitting pattern in ¹³C NMR is different. First of all, C-C splitting is not observed, because the probability of having two adjacent ¹³C is about 0.01%. Observed splitting patterns, which is due to the hydrogen atoms on the same carbon atom not on the adjacent carbon atom, is governed by the same n + 1 rule.

In ¹H NMR, the integral of the peaks are used for quantitative analysis, whereas this is problematic in ¹³C NMR. The long relaxation process for carbon atoms takes longer comparing to that of hydrogen atoms, which also depends on the order of carbon (i.e., 1°, 2°, etc.). This causes the peak heights to not be related to the quantity of the corresponding carbon atoms.

Another difference between ¹³C NMR and ¹H NMR is the chemical shift range. The range of the chemical shifts in a typical NMR represents the different between the minimum and maximum amount of electron density around that specific nucleus. Since hydrogen is surrounded by fewer electrons in comparison to carbon, the maximum change in the electron density for hydrogen is less than that for carbon. Thus, the range of chemical shift in ¹H NMR is narrower than that of ¹³C NMR.

Solid state NMR

¹³C NMR spectra could also be recorded for solid samples. The peaks for solid samples are very broad because the sample, being solid, cannot have all anisotropic, or orientation-dependent, interactions canceled due to rapid random tumbling. However, it is still possible to do high resolution solid state NMR by spinning the sample at 54.74° with respect to the applied magnetic field, which is called the magic angle. In other words, the sample can be spun to artificially cancel the orientation-dependent interaction. In general, the spinning frequency has a considerable effect on the spectrum.

¹³C NMR of carbon nanotubes

Single-walled carbon nanotubes contain sp² carbons. Derivatives of SWNTs contain sp³ carbons in addition. There are several factors that affect the ¹³C NMR spectrum of a SWNT sample, three of which will be reviewed in this module: ¹³C percentage, diameter of the nanotube, and functionalization.

¹³C percentage

For sp² carbons, there is a slight dependence of ¹³C NMR peaks on the percentage of ¹³C in the sample. Samples with lower ¹³C percentage are slighted shifted downfield (higher ppm). Data are shown in Table 1. Please note that these peaks are for the sp² carbons.

TABLE 1: Effects of ¹³C percentage on the sp² peak. Data from S. Hayashi, F. Hoshi, T. Ishikura, M. Yumura, and S. Ohshima, Carbon, 2003, 41, 3047.

Sample	δ (ppm)
SWNTs(100%)	116±1
SWNTs(1%)	118±1

Diameter of the nanotubes

The peak position for SWNTs also depends on the diameter of the nanotubes. It has been reported that the chemical shift for sp² carbons decreases as the diameter of the nanotubes increases. Figure 1 shows this correlation. Since the peak position depends on the diameter of nanotubes, the peak broadening can be related to the diameter distribution. In other words, the narrower the peak is, the smaller the diameter distribution of SWNTs is. This correlation is shown in Figure 2.

Figure 1: Correlation between the chemical shift of the sp² carbon and the diameter of the nanotubes. The diameter of the nanotubes increases from F1 to F4. Image from C. Engtrakul, V. M. Irurzun, E. L. Gjersing, J. M. Holt, B. A. Larsen, D. E. Resasco, and J. L. Blackburn, J. Am. Chem. Soc., 2012, 134, 4850. Copyright: American Chemical Society (2012).

Figure 2: Correlation between FWHM and the standard deviation of the diameter of nanotubes. Image from C. Engtrakul, V. M. Irurzun, E. L. Gjersing, J. M. Holt, B. A. Larsen, D. E. Resasco, and J. L. Blackburn, J. Am. Chem. Soc., 2012, 134, 4850. Copyright: American Chemical Society (2012).

Functionalization

Solid stated ¹³C NMR can also be used to analyze functionalized nanotubes. As a result of functionalizing SWNTs with groups containing a carbonyl group, a slight shift toward higher fields (lower ppm) for the sp² carbons is observed. This shift is explained by the perturbation applied to the electronic structure of the whole nanotube as a result of the modifications on only a fraction of the nanotube. At the same time, a new peak emerges at around 172 ppm, which is assigned to the carboxyl group of the substituent. The peak intensities could also be used to quantify the level of functionalization. Figure 3 shows these changes, in which the substituents are –(CH₂)₃COOH, –(CH₂)₂COOH, and –(CH₂)₂CONH(CH₂)₂NH₂ for the spectra Figure 3b,Figure 3c, and Figure 3d, respectively. Note that the bond between the nanotube and the substituent is a C-C bond. Due to low sensitivity, the peak for the sp³ carbons of the nanotube, which does not have a high quantity, is not detected. There is a small peak around 35 ppm in Figure 3, can be assigned to the aliphatic carbons of the substituent.

Figure 3: ¹³C NMR spectra for (a) pristine SWNT, (b) SWNT functionalized with –(CH₂)₃COOH, (c) SWNT functionalized with –(CH₂)₂COOH, and (d) SWNT functionalized with –(CH₂)₂CONH(CH₂)₂NH₂. Image from H. Peng, L. B. Alemany, J. L. Margrave, and V. N. Khabashesku, J. Am. Chem. Soc., 2003, 125, 15174. Copyright: American Chemical Society (2003).

For substituents containing aliphatic carbons, a new peak around 35 ppm emerges, as was shown in Figure 3, which is due to the aliphatic carbons. Since the quantity for the substituent carbons is low, the peak cannot be detected. Small substituents on the sidewall of SWNTs can be chemically modified to contain more carbons, so the signal due to those carbons could be detected. This idea, as a strategy for enhancing the signal from the substituents, can be used to analyze certain types of sidewall modifications. For example, when Gly (–NH₂CH₂CO₂H) was added to F-SWNTs (fluorinated SWNTs) to substitute the fluorine atoms, the ¹³C NMR spectrum for the Gly-SWNTs was showing one peak for the sp² carbons. When the aliphatic substituent was changed to 6-aminohexanoic acid with five aliphatic carbons, the peak was detectable, and using 11-aminoundecanoic acid (ten aliphatic carbons) the peak intensity was in the order of the size of the peak for sp² carbons. In order to use ¹³C NMR to enhance the substituent peak (for modification quantification purposes as an example), Gly-SWNTs was treated with 1-dodecanol to modify Gly to an amino ester. This modification resulted in enhancing the aliphatic carbon peak at around 30 ppm. Similar to the results in Figure 3, a peak at around 170 emerged which was assigned to the carbonyl carbon. The sp³ carbon of the SWNTs, which was attached to nitrogen, produced a small peak at around 80 ppm, which is detected in a cross-polarization magic angle spinning (CP-MAS) experiment.

F-SWNTs (fluorinated SWNTs) are reported to have a peak at around 90 ppm for the sp³ carbon of nanotube that is attached to the fluorine. The results of this part are summarized inTable 2 (approximate values).

TABLE 2: Chemical shift for different types of carbons in modified SWNTs. Note that the peak for the aliphatic carbons gets stronger if the amino acid is esterified. Data are obtained from: H. Peng, L. B. Alemany, J. L. Margrave, and V. N. Khabashesku,J. Am. Chem. Soc., 2003, 125, 15174; L. Zeng, L. Alemany, C. Edwards, and A. Barron, Nano. Res., 2008, 1, 72; L. B. Alemany, L. Zhang, L. Zeng, C. L. Edwards, and A. R. Barron, Chem. Mater., 2007, 19, 735.

Group	δ (ppm)	Intensity
sp2 carbons of SWNTs	120	Strong
–NH2(CH2)nCO2H (aliphatic carbon, n=1,5, 10)	20-40	Depends on ‘n’
–NH2(CH2)nCO2H (carboxyl carbon, n=1,5, 10)	170	Weak
sp3 carbon attached to nitrogen	80	Weak
sp3 carbon attached to fluorine	90	Weak

The peak intensities that are weak in Table 2 depend on the level of functionalization and for highly functionalized SWNTs, those peaks are not weak. The peak intensity for aliphatic carbons can be enhanced as the substituents get modified by attaching to other molecules with aliphatic carbons. Thus, the peak intensities can be used to quantify the level of functionalization.

¹³C NMR of functionalized graphene

Graphene is a single layer of sp² carbons, which exhibits a benzene-like structure. Functionalization of graphene sheets results in converting some of the sp² carbons to sp³. The peak for the sp²carbons of graphene shows a peak at around 140 ppm. It has been reported that fluorinated graphene produces an sp³ peak at around 82 ppm. It has also been reported for graphite oxide (GO), which contains –OH and epoxy substituents, to have peaks at around 60 and 70 ppm for the epoxy and the –OH substituents, respectively. There are chances for similar peaks to appear for graphene oxide. Table 3 summarizes these results.

TABLE 3: Chemical shifts for functionalized graphene. Data are obtained from: M. Dubois, K. Guérin, J. P. Pinheiro, Z. Fawal, F. Masin, and A. Hamwi, Carbon, 2004, 42, 1931; L. B. Casabianca, M. A. Shaibat, W. W. Cai, S. Park, R. Piner, R. S. Ruoff, and Y. Ishii, J. Am. Chem. Soc., 2010, 132, 5672.

Type of carbon	δ (ppm)
sp2	140
sp3 attached to fluorine	80
sp3 attached to –OH (for GO)	70
sp3 attached to epoxide (for GO)	60

Analyzing annealing process using ¹³C NMR

¹³C NMR spectroscopy has been used to study the effects of low-temperature annealing (at 650 °C) on thin films of amorphous carbon. The thin films were synthesized from a ¹³C enriched carbon source (99%). There were two peaks in the ¹³C NMR spectrum at about 69 and 142 ppm which were assigned to sp³ and sp² carbons, respectively (Figure 4). The intensity of each peak was used to find the percentage of each type of hybridization in the whole sample, and the broadening of the peaks was used to estimate the distribution of different types of carbons in the sample. It was found that while the composition of the sample didn’t change during the annealing process (peak intensities didn’t change, see Figure 4b), the full width at half maximum (FWHM) did change (Figure 4a). The latter suggested that the structure became more ordered, i.e., the distribution of sp² and sp³ carbons within the sample became more homogeneous. Thus, it was concluded that the sample turned into a more homogenous one in terms of the distribution of carbons with different hybridization, while the fraction of sp² and sp³ carbons remained unchanged.

Figure 4: a) Effect of the annealing process on the FWHM, which represents the change in the distribution of sp² and sp³ carbons. b) Fractions of sp² and sp³ carbon during the annealing process. Data are obtained from T. M. Alam, T. A. Friedmann, P. A. Schultz, and D. Sebastiani, Phys. Rev. B., 2003, 67, 245309.

Aside from the reported results from the paper, it can be concluded that ¹³C NMR is a good technique to study annealing, and possibly other similar processes, in real time, if the kinetics of the process is slow enough. For these purposes, the peak intensity and FWHM can be used to find or estimate the fraction and distribution of each type of carbon respectively.

Summary

¹³C NMR can reveal important information about the structure of SWNTs and graphene. ¹³C NMR chemical shifts and FWHM can be used to estimate the diameter size and diameter distribution. Though there are some limitations, it can be used to contain some information about the substituent type, as well as be used to quantify the level of functionalization. Modifications on the substituent can result in enhancing the substituent signal. Similar type of information can be achieved for graphene. It can also be employed to track changes during annealing and possibly during other modifications with similar time scales. Due to low natural abundance of ¹³C it might be necessary to synthesize ¹³C-enhanced samples in order to obtain suitable spectra with a sufficient signal-to-noise ratio. Similar principles could be used to follow the annealing process of carbon nano materials. C₆₀ will not be discussed herein.

Bibliography

• T. M. Alam, T. A. Friedmann, P. A. Schultz, and D. Sebastiani, Phys. Rev. B., 2003, 67, 245309.
• L. B. Alemany, L. Zhang, L. Zeng, C. L. Edwards, and A. R. Barron, Chem. Mater., 2007, 19, 735.
• L. B. Casabianca, M. A. Shaibat, W. W. Cai, S. Park, R. Piner, R. S. Ruoff, and Y. Ishii, J. Am. Chem. Soc., 2010, 132, 5672.
• M. Dubois, K. Guérin, J. P. Pinheiro, Z. Fawal, F. Masin, and A. Hamwi, Carbon, 2004, 42, 1931.
• C. Engtrakul, V. M. Irurzun, E. L. Gjersing, J. M. Holt, B. A. Larsen, D. E. Resasco, and J. L. Blackburn, J. Am. Chem. Soc., 2012, 134, 4850.
• S. Hayashi, F. Hoshi, T. Ishikura, M. Yumura, and S. Ohshima, Carbon, 2003, 41, 3047.
• H. Peng, L. B. Alemany, J. L. Margrave, and V. N. Khabashesku, J. Am. Chem. Soc., 2003, 125, 15174.
• L. Zeng, L. Alemany, C. Edwards, and A. Barron, Nano. Res., 2008, 1, 72.