A theoretical study of 4-Mercaptobenzoic acid assembled on Ag for surface-enhanced raman scattering applications

Thiols are widely used in the preparation of self-assembled monolayers (SAMs) on metallic surfaces such as Ag, Cu and Au because of their strong affinity for such surfaces. These SAMs have various applications in nanotechnology and supramolecular chemistry. However, the chemistry of the metal surface and the thiol end is still vaguely understood, yet it fundamentally governs the properties of the SAMs. This paper reports density functional theory (DFT) and corroborating experimental findings focusing on vibrational Raman spectroscopy of Ag-Mercaptobenzoic acid (AgMBA) SAMs. The 4-MBA thiol was self-assembled onto Ag (111) and (100) fcc surfaces via both the COO- and S- ends. The experimental AgMBA vibrational frequencies showed the interaction of the 4-MBA with the Ag surface from both the S- and COO- atoms and the observation was supported by the our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) calculated surface-enhanced Raman spectroscopy spectra and DFT thermodynamic calculations. The Ag(100)MBA self-assembled via the COO- end had the highest enthalpy of adsorption of -310.34 kJ/mol and it was the most stable plane contradicting the sulphur end consensus on previous papers. The adsorption of the 4-MBA moiety on the Ag surface was interplayed by the corrugation potential of the exposed surface and the thiol’s aromatic nature which resulted in the adsorption of 4-MBA predominately via the carbonyl end at slightly acidic pH conditions and a deprotonated thiol end.