Investigation of the Adsorption Mechanism of Heterocyclic Molecules on Copper using Potentiodynamic ORP-EIS and In-situ Ramans pectroscopy

The adsorption mechanism of 2-amino-5-mercapto-1,3,4-thiadiazole (AMTD) and 2-methyl-5-mercapto-1,3,4-thiadiazole (MMTD) was investigated by a combined electrochemical and surface analysis approach. The link with the adsorption kinetics was maintained during AC voltammetry as well as during potentiodynamic broadband multisine (ORP-EIS) measurements. A discrepancy between the thickness of the adsorbate layers determined by both techniques was identified as a result of the frequency dispersion of the system. The use of a broad frequency range and the statistical evaluation with ORP-EIS deemed this the preferential technique for layer thickness determination. The surface-molecular complex was further investigated through in-situ surface enhanced Raman spectroscopy (SERS), collected under identical electrochemical conditions. The adsorption regions defined by the AC voltammetry measurements corresponded to the SERS response. The proposed adsorption orientation for AMTD and MMTD was through N4-S-Cu σ-bonding. A layer thickness of 0.76 nm and 0.85 nm respectively indicated the formation of multiple adsorbed molecular layers. Application of the in-situ approach on small heterocyclic molecules derived from AMTD and MMTD indicated the importance of adjacent nitrogen atoms in a five membered heterocyclic ring for the adsorbate-substrate complex.