Density-functional calculation of methane adsorption on graphite (0001)

Abstract

Methane adsorbed on graphite was studied using density-functional theory. The structure was fully optimized with strict energy and force convergence criteria. The methane converged to the preferred adsorption sites giving the vibrational frequencies, the energy of adsorption, charge distributions, and the electronic density of states. Under the two coverages studied ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ and $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}$) and a differing number of graphite layers (1--6), the methane molecules favored atop sites on the graphite surface with the hydrogen tripod down. We found the methane carbon $3.21\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ above the graphite carbon and the adsorption energy to be $118\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for the lower coverage. The independent harmonic oscillator vibrational frequency perpendicular to the surface for the $\mathrm{C}{\mathrm{H}}_{4}$ molecule was computed to be $87\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{-1}$. The graphite surface contracted 5.0% and 4.1% for the first and second layers, respectively, from the spacing relative to their bulk values. To benchmark our frequency calculations, we also used second-order Moller-Plesset theory and the local spin density approximation (LSDA) in GAUSSIAN 03 for the molecule. All of our LDA results are in good agreement with corresponding experiments, while under the generalized gradient approximation approximation we get only qualitatively results.