Adsorption of phenol on graphite(0001) and α-Al2O3(0001):: Nature of van der Waals bonds from first-principles calculations

First-principles calculations of phenol adsorbed on two different surfaces, graphite(0001) and alpha-Al2O3(0001), are performed with traditional semilocal density functional theory (DFT) and with a recently presented density functional (vdW-DF) that incorporates the dispersive van der Waals (vdW) interactions [Phys. Rev. Lett. 92, 246401 (2004)]. The vdW-DF is of decisive importance for describing the vdW bond of the phenol-graphite system and gives a secondary but not negligible vdW contribution for phenol on alumina. We find a predominantly covalent bond at the alumina surface. There, adsorption results in a binding separation (distance between surface Al and the O of the inclining phenol molecule) of 1.95 A and a binding energy of 1.00 eV, evaluated within the generalized gradient approximation (GGA) of DFT, i.e., from covalency, with the energy increasing to around 1.2 eV when the contribution from vdW interactions is also accounted for. On graphite, with its pure vdW bond, the adsorption distance (separation between parallel surface and phenol molecule) is found to be 3.47 A and the adsorption strength 0.56 eV. Comparison of the results for alumina and graphite mutually and with published results for nickel reveals significant differences in the adsorption of this model biomolecule.