Kinetics and Reaction Mechanisms of Thiophenol Adsorption on Gold Studied by Surface-Enhanced Raman Spectroscopy

Thiophenol is commonly used as a model system for surface-enhanced Raman scattering (SERS) of molecules due to the strong affinity of the -SH group toward noble metal surfaces. By performing time-, temperature-, and pH-dependent measurements of thiophenol adsorption on commercial nanostructured gold SERS substrates, we have observed both physisorption and chemisorption processes. These two distinct adsorption regimes were found dependent on the pH which controlled the ionization state of thiophenol in an aqueous medium. At low pH the sulfhydryl proton remains bound, and the kinetic adsorption profile obtained from the SERS intensity follows a sigmoid-shaped curve with an initially slow adsorption rate that deviates from a Langmuir profile. In addition, from temperature-dependent measurements, a near zero value for the activation energy is obtained, indicating that physisorption is the rate-limiting step. At high pH, where the sulfhydryl proton becomes detached, the kinetic adsorption profile follows a classical Langmuir profile, and the activation energy is significantly higher than at low pH, indicating that chemisorption is the rate-limiting step.