Abstract
The geometric, thermodynamic and electronic properties of Pd–graphene nanocomposites are comprehensively studied through quantum mechanical methods. Geometries of these clusters are optimized with the well-calibrated Minnesota functional M06-2X. The adsorption energies calculated at the M06-2X/LANL2DZ level show better agreement with those calculated from MP2/ANO-RCC-VDZP. Two different representative models for graphene, coronene and hexabenzocoronene, are used. The adsorption energies analysis reveals that the interaction energies increase with the size of the adsorbed cluster. However, for Pdn/hexabenzocoronene, the interaction energies show a sudden drop at Pd8/hexabenzocoronene. The difference in behavior between the interaction energies of Pdn/hexabenzocoronene and Pdn/coronene is attributed to the edge effect present in coronene. The electronic properties, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), Fermi level, molecular electrostatic potential (MEP), dipole moment, vertical ionization potential (VIP), vertical electron affinity (VEA), chemical hardness (η), softness (S) and chemical potential (μ) are studied. The VIP and VEA reveal that Pdn/coronene clusters are stable in nature with the least reactivity. The HOMO–LUMO energy gaps are reduced with the increase in cluster size. The electronic properties show irregular trends, where the most favorable electronic properties are obtained for Pd7/coronene and Pd10/coronene.
Highlights
Metal clusters are considered as intermediates between solid states, and these molecules generally exhibit unexpected physical properties owing to the quantum size effect.[1]
The Vertical ionization potential (VIP) and vertical electron affinity (VEA) reveal that Pdn/coronene clusters are stable in nature with the least reactivity
The M06-2X functional of DFT is chosen for palladium graphene clusters because it is a validated method for the Pd– graphene cluster based on comparison of the results with those of MP2 and CCSD(T) methods
Summary
Theoretical calculations are efficient tools for studying the various electronic and structural properties of the transition metal clusters.[8,9,10,11] Many reports in the literature illustrate the structure and properties of Pd clusters. The binding energies of coronene metal complexes in water at the M06-2X level are only slightly lower than those in vacuum These investigations were supported by the results of stability of metal nanoparticles ($20 nm) on graphene composites observed by scanning electron microscopy (SEM).[25,26,27]. The interaction energies, geometries and electronic properties of medium-sized clusters on graphene surface are not reported in the literature With this motivation, we studied here the detailed theoretical analysis of geometries, binding energies and electronic properties of small palladium clusters on coronene, a model for graphene.[31] the most stable orientations of Pdn clusters were studied on hexabenzocoronene. This is the rst study of its type where a complete range of palladium clusters Pdn (n 1⁄4 2–10) is studied on the surface of coronene
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