2.13 Using 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 C ₆₀ . 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 ¹³ C NMR in studying graphene and SWNTs. In addition, a discussion of how ¹³ C NMR could be used to analyze a thin film of amorphous carbon during a low-temperature annealing process will be presented.

¹³ C nmr versus ¹ H nmr

Since carbon is found in any organic molecule NMR that can analyze carbon could be very helpful, unfortunately the major isotope, ¹² C, is not NMR active. Fortunately, ¹³ C with a natural abundance of 1.1% is NMR active. This low natural abundance along with lower gyromagnetic ratio for ¹³ C causes sensitivity to decrease. Due to this lower sensitivity, obtaining a ¹³ C 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 ¹ H NMR spectrum. Although it has a lower sensitivity, it is still highly used as it discloses valuable information.

Peaks in a ¹ H 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 ¹³ C NMR is different. First of all, C-C splitting is not observed, because the probability of having two adjacent ¹³ C 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 ¹ H NMR, the integral of the peaks are used for quantitative analysis, whereas this is problematic in ¹³ C 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 ¹³ C NMR and ¹ H 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 ¹ H NMR is narrower than that of ¹³ C NMR.

Solid state nmr

¹³ C 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.