Thermodynamic stability of nitrogen functionalities and defects in graphene and graphene nanoribbons from first principles

Nitrogen functionalization of graphene significantly enhances the physical and chemical properties of graphitic materials, increasing their applicability as sorbents, heterogeneous catalysts, and electronic components. Being able to selectively induce different nitrogen functionalities via treatment conditions is key to the design and optimization of such materials. Here, we use density functional theory to study the thermodynamic stability of nitrogen functionalities in three graphene structures as a function of temperature and pressure, providing atomistic insight into the most favorable functionalized configurations. Phase diagrams show that nitrogen incorporation is most exergonic at graphene edges, with pyridinic groups dominating under the majority of conditions studied. For all nitrogen functionalities, lower temperatures and higher pressures result in the greater incorporation of nitrogen into the graphene structures. A density of states analysis shows that the stable pyridinic nitrogen structures induce new electronic states just below the Fermi level whose energy is tunable via nitrogen concentration and hence treatment temperature and pressure. Overall, we have characterized the thermodynamic stability of nitrogen functionalities within graphene and graphene nanoribbons, allowing for the directed tuning of such nitrogen groups experimentally and enabling the construction of more realistic models of nitrogenated graphene structures. (C) 2019 Elsevier Ltd. All rights reserved.