2.13 Using 13-c nmr to study carbon nanomaterials  (Page 3/4)

For substituents containing aliphatic carbons, a new peak around 35 ppm emerges, as was shown in [link] , 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 (–NH ₂ CH ₂ CO ₂ H) was added to F-SWNTs (fluorinated SWNTs) to substitute the fluorine atoms, the ¹³ C NMR spectrum for the Gly-SWNTs was showing one peak for the sp ² 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 sp ² carbons. In order to use ¹³ C 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 [link] , a peak at around 170 emerged which was assigned to the carbonyl carbon. The sp ³ 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 sp ³ carbon of nanotube that is attached to the fluorine. The results of this part are summarized in [link] (approximate values).

The peak intensities that are weak in [link] 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.

¹³ C nmr of functionalized graphene

Graphene is a single layer of sp ² carbons, which exhibits a benzene-like structure. Functionalization of graphene sheets results in converting some of the sp ² carbons to sp ³ . The peak for the sp ² carbons of graphene shows a peak at around 140 ppm. It has been reported that fluorinated graphene produces an sp ³ 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. [link] summarizes these results.