2.13 Using 13-c nmr to study carbon nanomaterials  (Page 4/4)

Analyzing annealing process using ¹³ C nmr

¹³ C 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 ¹³ C enriched carbon source (99%). There were two peaks in the ¹³ C NMR spectrum at about 69 and 142 ppm which were assigned to sp ³ and sp ² carbons, respectively ( [link] ). 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 [link] b), the full width at half maximum (FWHM) did change ( [link] a). The latter suggested that the structure became more ordered, i.e., the distribution of sp ² and sp ³ 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 sp ² and sp ³ carbons remained unchanged.

Aside from the reported results from the paper, it can be concluded that ¹³ C 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

¹³ C NMR can reveal important information about the structure of SWNTs and graphene. ¹³ C 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 ¹³ C it might be necessary to synthesize ¹³ C-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. C ₆₀ will not be discussed herein.

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