Self-Assembled Monolayers of 2-Mercaptopyridine on Au(111): Insights from in Situ STM

While the S-Au-S motif has been established for thiol molecules adsorbed on a Au(111) substrate, the current in situ scanning tunneling microscopy (STM) study on the adsorption of 2-mercaptopyridine (2-MPY) on a (111)-oriented Au single crystal bead revealed a different adsorption mode under potential control in 0.1 M H2SO4. 2-MPY molecules were adsorbed and self-assembled rapidly into ordered 2D arrays on this Au(111) electrode immersed in 50 mu M 2-MPY ethanol and aqueous dosing solutions for 1 min. The modified Au(111) surface was atomically smooth without vacancy islands (VI). A series of ordered 2-MPY structures were identified by molecular resolution STM imaging, which were stable against the potential modulation between 0.2 and 0.9 V (versus Ag/AgCl). However, the ordered 2-MPY adlayer dissolved rapidly and irreversibly when the potential was made negative to E < 0.1 V, which is presumed to stem from protonation and desorption 2-MPY admolecule from the Au(111) electrode. The 2-MPY spatial structures found in the current study are different from the typical (p x root 3) structures reported for thiols on Au(111). High-quality STM imaging discerned the internal structures of 2-MPY admolecules, from which their binding modes with the Au electrode are inferred. The surface coverage, spatial structure, and degree of ordering for the 2-MPY adlayer changed with the dosage and media.