Molecular Orbital Approach to Chemisorption. II. Atomic H, C, N, O, and F on Graphite

Abstract

The calculation of the chemisorption behavior of atomic H, C, N, O, and F on a graphite basal (0001) surface is examined with the CNDO (complete neglect of differential overlap) molecular-orbital scheme. This approach is a semiempirical approximation to the Hartree-Fock self-consistent field procedure, in which the Hamiltonian explicitly depends upon both atomic and orbital charge distribution. The location of binding sites and changes in relative binding energies and net charges with the identity of the adsorbed species are explored with an extended carbon (0001) surface, simulated by an 18-carbon lattice with appropriate boundary connections. The strength of binding to the simulated graphite substrate increases in the order H, F, O, N, and C. The atoms C and N are most stable when positioned above the center of a hexagonal sixcarbon ring, whereas H, F, and O are most stable above the center of a bond connecting nearest-neighbor carbons. Calculated charge distributions are used to predict a work-function decrease with the adsorption of atomic H or N on graphite, but an increase with the adsorption of C, O, or F. The commonly used electronegativity reasoning is shown to be inadequate for the prediction of adsorbate-charge transfer.