Density Functional Theory Studies on the Adsorption of Methanethiol Molecule on Au(111) Surface at Different Coverage

By applying the first-principles method based on density functional theory, we have studied the adsorptions of non-dissociative CH3SH molecule and dissociated CH3S group on Au(111) surface at five different coverage (1/12, 1/8, 1/6, 1/4, 1/3). By performing the total energy calculations for both CH3SH and CH3S at four possible adsorption sites at every studied coverage, we have found the stable non-dissociative adsorption configuration, as well as the dissociated adsorption for CH3SH from low to high coverage. Especially, the effect of van der Waals interaction on the adsorption configurations and energies have been studied by the DFT-D2 method. Our results suggest that the top site is most stable site for non-dissociative CH3SH molecular at all the studied coverage, while the tilt angle of the adsorbed molecule is 70 degrees +/- 2 degrees. The adsorption energy decreases with the increasing of the coverage, that is 0.33 similar to 0.35 eV at low coverage region of 1/12 similar to 1/8. On the other hand, the dissociated CH3S group prefers to adsorb on bri-fcc site at all coverage with the tilt angle of 48.3 degrees similar to 58.5 degrees. For the dissociated CH3S, the adsorption energy also decreases with the increasing of the coverage, that is 2.07 similar to 2.08 eV at low coverage region of 1/12 similar to 1/8. At the saturated coverage of 1/3, the effect of van der Waals interaction was investigated for the adsorption of CH3SH/Au(111) and CH3S/Au(111) systems. The calculation results indicate that the van der Waals force makes the adsorbates closer to the substrate and the adsorption energy larger. The respective adsorption energies for CH3SH and CH3S are about 0.59 eV and 2.27 eV at coverage of 1/3. The calculation result by DFT-D2 method for CH3SH agrees with the experimental conclusion, but that for CH3S is away from the experimental observation. Moreover, our calculation results on the electronic structure show us that charge is transferred from the adsorbed CH3SH and CH3S to the Au substrate. And the amount of the transferred charge decrease with the increase of the coverage for both the CH3SH and CH3S, which imply that the interaction between the adsorbed CH3SH/CH3S and Au substrate decrease with the increasing of the coverage.