Density Functional Theory Study on the Adsorption of Dodecylthiol on Au(111) Surface

By applying the first-principles methods based on density functional theory and the slab model, we have studied the non-dissociative and dissociated adsorptions of a dodecylthiol (C12H25SH) molecule on Au(111) surface. Based on the calculated results, the fate of the H atom has been analyzed, and the long-chain adsorption and short-chain adsorption have been compared. We have performed structure optimizations for a series of initial structures with the S atom located on different sites with different tilt angles. This structure optimizations gave two surface structures before and after the dissociation of S-H; the standing-up and lying-down adsorption structures. Our calculations indicate that the C12H25SH molecule prefers to stay on the top site, the corresponding adsorption energies are 0.35-0.38 eV. The dissociated C12H25S group prefers to adsorb on the bri-fcc site, with adsorption energies of 2.01-2.09 eV. We have compared the non-dissociative C12H25SH/Au(111) and dissociated C12H25S/Au(111) with the H atom adsorbing onto Au and desorbing as H-2, and found that the non-dissociative adsorption is more stable. The formation energy and the electronic structure showed that the non-dissociative adsorption belongs to the weak chemisorption, whereas the interaction between the S atom and Au surface becomes much stronger following cleavage of the S-H. A comparison of the adsorption of long-chain thiols on Au(111) surface with that of the short-chain thiols, indicates that the adsorption energies of the long-chain thiols are slightly larger, and the distances between the S atom and the surface Au atoms are slightly shorter.