Activation of Graphenic Carbon Due to Substitutional Doping by Nitrogen: Mechanistic Understanding from First Principles

Maximally valent orbitals in systems with non-ideal bond-angles: atomic Wannier orbitals guided by the Mayer bond order

In pursuit of a directed minimal set of basis for systems with non-ideal bond angles, in this work we find the exact orientation of the major overlapping orbitals along the nearest neighbouring coordination segments in a given system such that they maximally represent the covalent interactions throughout the system. We compute Mayer's bond order, akin to Wiberg's bond index, on the basis of atomic Wannier orbitals with customizable non-degenerate hybridization leading to variable orientations, constructed from first principles, in a representative variety of molecules and layered systems. We put such orbitals in perspective with unbiased maximally localized descriptions of bonding and non-bonding orbitals, and energetics to tunneling of electrons through them between nearest neighbours, to describe the different physical aspects of covalent interactions, which are not necessarily represented using a single unique set of atomic or bonding orbitals.

Hybrid Atomic Orbital Basis from First Principles: Bottom-Up Mapping of Self-Energy Correction to Large Covalent Systems

Construction of hybrid atomic orbitals is proposed as the approximate common eigenstates of finite first moment matrices. Their hybridization and orientation can be a priori tuned as per their anticipated neighborhood. Their Wannier function counterparts constructed from the Kohn-Sham (KS) single particle states constitute an orthonormal multiorbital tight binding (TB) basis resembling hybrid atomic orbitals locked to their immediate atomic neighborhood, while spanning the subspace of KS states. The proposed basis thus renders predominantly single TB parameters from first principles for each nearest neighbor bond involving no more than two orbitals irrespective of their orientation and also facilitates an easy route for the transfer of such TB parameters across isostructural systems exclusively through mapping of neighborhoods and projection of orbital charge centers. With hybridized 2s, 2p and 3s, 3p valence electrons, the spatial extent of the self-energy correction (SEC) to TB parameters in the proposed basis is found to be localized mostly within the third nearest neighborhood, thus allowing effective transfer of self-energy-corrected TB parameters from smaller reference systems to much larger target systems, with nominal additional computational cost beyond that required for explicit computation of SEC in the reference systems. The proposed approach promises inexpensive estimation of the quasi-particle structures of large covalent systems with workable accuracy.

Structural and Electronic Descriptors of Catalytic Activity of Graphene-Based Materials: First-Principles Theoretical Analysis

Characteristic features of the d-band in electronic structure of transition metals are quite effective as descriptors of their catalytic activity toward oxygen reduction reaction (ORR). With the promise of graphene-based materials to replace precious metal catalysts, descriptors of their chemical activity are much needed. Here, a site-specific electronic descriptor is proposed based on the p_z (π) orbital occupancy and its contribution to electronic states at the Fermi level. Simple structural descriptors are identified, and a linear predictive model is developed to precisely estimate adsorption free energies of OH (ΔG_OH ) at various sites of doped graphene, and it is demonstrated through prediction of the most optimal site for catalysis of ORR. These structural descriptors, essentially the number of ortho, meta, and para sites of N/B-doped graphene sheet, can be extended to other doped sp² hybridized systems, and greatly reduce the computational effort in estimating ΔG_OH and site-specific catalytic activity.

Size effect of oxygen reduction reaction on nitrogen-doped graphene quantum dots

The adsorption strength of ORR intermediates, the reaction free energy of rate-determining step, and the overpotential increase with the increase of the size of N-doped graphene quantum dots.