Coverage and charge dependent adsorption of butanethiol on the Au(1 1 1) surface: A density functional theory study

A theoretical study based on periodic DFT calculations of the structure, the surface bonding, and the energetics of butanethiols adsorbed on the Au(1 1 1) surface is reported. Several sites and coverage have been considered, and neutral and charged metal surfaces have been simulated. Whatever the coverage is, the preferred site is a hollow-bridge type-site in which sulfur atom simultaneously binds to two gold atoms. Thiol adsorption parameters are sensitive to the coverage, especially the adsorption energy that shows a clear response to the number of thiols adsorbed, and the thiol-surface interaction decreases when the coverage grows as the lateral repulsion between the alkyl tails weaken the strength of the Au-S bond. The thiol-surface interaction parameters are also sensitive to the charge of the metal. Also, we found that the adsorption becomes more favorable when the metal surface is negatively charged, and less favorable on positive surfaces. Finally, in order to analyze dynamical effects, we performed a molecular dynamics (MD) simulation considering a system with ethanethiol on an Au slab pre-covered by water. The MD simulation shows that the proton transfer occurs within a few femtoseconds and that prior to the transfer itself, the sulfur atom binds to a gold surface atom. This Au atom is clearly pulled out of the surface, what could be interpreted as the onset of the islands and cluster formation already observed in the low coverage regime.