Phosphorus and Nitrogen Centers in Doped Graphene and Carbon Nanotubes Analyzed through Solid-State NMR

Graphene and carbon nanotubes (CNTs) have been investigated closely in recent years because of their apparent positive effect on the electrochemical performance of new fuel cell and battery systems as catalyst stabilizers, supports, or metal-free catalysts. This is particularly true for doped graphene and CNTs, where only a small amount of doping with nitrogen and/or phosphorus can have a remarkable effect on the material performance. A direct link between structure and function in these materials is, as of yet, unclear. Doped graphene and CNTs were synthesized using varied chemical vapor deposition-based methods, and ssNMR was used to unambiguously identify dopant atom sites, revealing that these particular synthesis methods result in highly homogeneous populations of installed phosphorus and nitrogen atoms. We present the first experimental N-15 spectrum for graphitic nitrogen in N-doped graphene. N-15-labeled nitrogen-doped graphene synthesized as reported here produces mainly graphitic nitrogen sites located on the edges of sheets and around defect sites. H-1-H-1 and H-1-N-15 correlations were also used to probe dopant nitrogen sites in labeled and unlabeled N-doped graphene. A nearly homogeneous population of phosphorus in P-doped graphene is found, with an overwhelming majority of graphitic phosphorus and a small amount of phosphate oligomer. Similar findings are noted for the phosphorus sites in phosphorus and nitrogen codoped CNTs with a minor change in chemical shift, as would be expected from two chemically similar phosphorus sites in carbon allotropes (CNTs vs graphene sheets) with significantly different electronic structures.