Nitrogen Doping Enables Covalent-Like π-π Bonding between Graphenes.

The neighboring layers in bilayer (and few-layer) graphenes of both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) π-π bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by up to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the π-π overlap across the vdW gap while the individual layers maintain their in-plane π-conjugation and are accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.

Non-Transition-Metal Catalytic System for N2 Reduction to NH3: A Density Functional Theory Study of Al-Doped Graphene.

The prevalent catalysts for natural and artificial N2 fixation are known to hinge upon transition-metal (TM) elements. Herein, we demonstrate by density functional theory that Al-doped graphene is a potential non-TM catalyst to convert N2 to NH3 in the presence of relatively mild proton/electron sources. In the integrated structure of the catalyst, the Al atom serves as a binding site and catalytic center while the graphene framework serves as an electron buffer during the successive proton/electron additions to N2 and its various downstream NxHy intermediates. The initial hydrogenation of N2 can readily take place via an internal H-transfer process with the assistance of a Li+ ion as an additive. In view of the recurrence of H transfer in the first step of N2 reduction observed in biological nitrogenases and other synthetic catalysts, this finding highlights the significance of heteroatom-assisted H transfer in the design of synthetic catalysts for N2 fixation.