Spectromicroscopy investigation of nitrogen-doped carbon nanomaterials

The electronic properties of sp2 carbon nanostructures are very sensitive to local perturbations, such as surface charges and adsorbed gas molecules, so that the grafting of functional groups in a controllable way has been proposed as a feasible reproducible solution for band gap engineering and controllable doping, in order to exploit and tailor the extraordinary properties of these materials. In this framework, nitrogen doped graphene and carbon nanotubes have been proposed as very good metal-free catalysts for oxygen reduction reaction (ORR), in substitution of the expensive Pt-C systems1.
Post-synthesis plasma-based functionalization methods have the advantage to be solvent-free, time efficient and flexible.
Within this context, we will discuss the functionalization of vertical aligned carbon nanotubes (v-CNTs) and suspended graphene via nitrogen plasma treatment. Valence band (UPS) and scanning X-ray photoelectron spectromicroscopy (SPEM) measurements were performed at ELETTRA Synchrotron. The creation of defects induced by ions drives the grafting of nitrogen species (pyridinic, pyrrolic and graphitic) on the carbon nanostructures. An intriguing different behaviour of the grafting at the CNT tips with respect to the sidewalls was observed2. This indicates a different reactivity of the CNT tip, where the presence of natural defects may be involved in different bonding formations between carbon and nitrogen, while the sidewalls behave similarly to graphene. The effect of the temperature has been evaluated with post synthesis annealing, showing variations in the ratio between the nitrogen species, thus allowing their tuning and the identification of the most thermal stable species3.
Finally we demonstrated by photoemission experiments that the nitrogen atoms incorporated in the honeycomb carbon structure act as active sites for molecular oxygen dissociation: upon exposure to O2, N-doped graphene is capable to dissociate the O2 molecule and to adsorb oxygen, while any changes were detected on pristine graphene. The role of pyridinic and graphitic nitrogen in this mechanism will be discussed.

1. Gong et al., Science 323 (2009) 760-764.
2. M. Scardamaglia et al., Carbon (2014) in press DOI: 10.1016/j.carbon.2014.05.035.
3. M. Scardamaglia et al., Carbon 73 (2014) 371-381.