Dynamic nuclear polarization - nuclear magnetic resonance for analyzing surface functional groups on carbonaceous materials

Dynamic nuclear polarizing (DNP) technique in nuclear magnetic resonance (NMR) is a powerful tool for a microanalysis. Nevertheless, it has not been applied to analyses of carbon materials such as graphene oxide (GO) and amorphous carbon effectively because of the electroconductivity and microwave absorption of the carbon, which attenuate the enhancement effect of DNP. For this study, we applied DNP-NMR to analyses of surface functional groups on GO and sucrose-derived carbon to evaluate the method. The 1H-13C cross-polarization magic-angle spinning (CP/MAS) DNP-NMR of a GO sample with AMUPol (polarizing agent) showed 2.2-times -enhanced peaks of 13C in epoxide, bonding to hydroxyl group, and in the graphene plane. Signal enhance-ment was raised by AMUPol radicals neighboring the surface functional groups and the graphene planes on GO particles, although attenuation by temperature rise must be considered. Furthermore, additional new peaks assigned to CH3 group on the GO particle surface were highly enhanced and were observed clearly only by the accumulations of 64 scans. For sucrose-derived carbon, DNP-NMR clearly revealed the -OH group on the carbon surface or carbon edge by heat treatment, which was not possible using conventional CP/MAS experiments. Cross Effect was found to be dominant in signal enhancements of the functional groups on GO and sucrose-derived carbon samples, except for the CH3 groups on GO. The CH3 enhancement is ascribed mainly to the Over-hauser effect or solid effect.

Automated summary

Dynamic nuclear polarizing (DNP) technique in nuclear magnetic resonance (NMR) is a powerful tool for microanalysis, but its application to carbon materials like graphene oxide (GO) and amorphous carbon has been limited by their electroconductivity and microwave absorption. In this study, DNP-NMR was successfully applied to analyze surface functional groups on GO and sucrose-derived carbon, revealing enhanced peaks of 13C and the presence of previously undetectable functional groups. The dominant enhancement mechanism was found to be the cross effect, except for the CH3 groups on GO, which were mainly enhanced through Overhauser effect or solid effect.