As a pristine graphene nanostructure, zigzag graphene nanoribbons (ZGNRs) has received considerable interest over the past decade due to the presence of polarizable edge states¹. It has also been employed as a model system for non-precious metal catalysts since it can host a variety of active sites². Among various classes of catalysts, metal (Me) and nitrogen co-doped graphene has been emerging as promising electrocatalysts in oxygen reduction reaction (ORR). Furthermore, it has also been shown that porphyrin-like Me vacancy embedded in graphene is more stable at edges than far-away from the edges³. In the light of those recent studies, the change of spin states on the edge and in the bulk caused by Fe and N co-doping is an essential factor to tune the electronic and magnetic states on graphene. Therefore, unraveling spin interactions between co-doped metal-nitrogen complex and edge states (illustrated in Fig. 1) can control the electronic conductivity of doped graphene sheets and electron density at metal active sites, which is essential for their applications in electrocatalysts.

In this work, the effect of nitrogen-metal and vacancies in the bulk and edge configurations on the spin polarization on graphene materials will be presented as obtained from two computational methodologies: plane waves and atomic-centered Gaussian functions. There are some drawbacks on each DFT code regarding magnetic calculations. Sensitivity of the value of total magnetic moment in VASP obtained from Bravais lattice depends on k-point sampling, smearing parameters and stress applied to unit cell. On the other hand, the initial spin moment guess on each atom is not implemented on deMon2k to be able to obtain anti-ferromagnetic states of graphene.

Finally, the ORR reactivity as a function of (a) the metal location in graphene and (b) the edge truncations will be discussed.

Figure 1: Suppression of spin polarization in edge modification by FeN₄

Acknowledgement: This work is supported by the LabEx CheMISyst ANR-10-LABX-05-01.

References

1. Son, Y.-W.; Cohen, M. L.; Louie, S. G. Half-metallic graphene nanoribbons. Nature 2006, 444, 347
2. Zitolo, A. et al. Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature Materials 2015,14, 937–942
3. Holby, E. F.; Taylor, C. D. Control of graphene nanoribbon vacancies by Fe and N dopants: Implications for catalysis. Appl. Phys. Lett.2012,101, 064102