Characterizing Nitrogen Sites in Nitrogen-Doped Reduced Graphene Oxide: A Combined Solid-State 15N NMR, XPS and DFT Approach

Despite the potential applications in energy storage and conversion systems such as Li-oxygen batteries and fuel cells, the nature and distribution of doped nitrogen sites in reduced graphene oxides are still not well understood. In this work, we report a combined approach of ¹⁵N solid-state nuclear magnetic resonance (NMR) spectroscopy alongside the predominantly used X-ray photoelectron spectroscopy (XPS) to characterize the nitrogen environments in reduced graphene oxides. Application of ¹H-¹⁵N low-power double quantum cross polarization under fast magic angle spinning with Carr-Purcell-Meiboom-Gill scheme shows selective detection of protonated sites with low-power radiofrequency irradiation. NMR shift calculations of a series of N-containing molecules and a graphene nanoflake model were performed to help interpret the experimental data. This work demonstrates a powerful approach to identify and quantify the different nitrogen environments in doped graphene materials and can also be widely applied to similar graphitic carbon-based materials with other dopants.