Identification of Vertical and Horizontal Configurations for BPE Adsorption on Silver Surfaces
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© 2015 American Chemical Society. Adsorption of trans-1,2-bis(4-pyridyl)ethylene (BPE), a molecule with two pyridine rings connected with a C=C double bond, was studied on Ag surfaces with surface-enhanced Raman spectroscopic (SERS) measurements and density functional theory (DFT) calculations. Spectroscopic measurements were collected using well-defined 48 nm monodispersed Ag and Au nanoparticles supported on SiO2. Effects of Ag oxidation were evaluated by varying the duration of an ozone treatment prior to adsorption. Effects of surface coverage were evaluated by exposing unoxidized and oxidized Ag samples to solutions with a variable BPE concentration. Periodic unit-cell DFT calculations were performed using Ag(111), p(4 × 4)-O/Ag(111), and Ag2O(111) surfaces. Two adsorption configurations were identified: vertical and horizontal. In the vertical configuration, BPE adsorbs nearly orthogonal to the surface by binding through one of its N atoms to a single surface Ag atom. In the horizontal configuration, BPE adsorbs nearly parallel to the surface by binding through both of its N atoms to two separate surface Ag atoms. BPE adsorbs initially as a mixture of the vertical and horizontal configurations. As the BPE surface coverage increases, the vertical configuration becomes preferential due to geometric constraints. In contrast, the horizontal configuration becomes preferential with increasing extent of Ag oxidation due to its greater stability on oxidized surfaces. Similarities in spectroscopic results for metallic Ag and Au nanoparticles suggest that the BPE adsorption trends with increasing surface coverage are the same for both metals.
author list (cited authors)
Chen, T., Pal, A., Gao, J., Han, Y., Chen, H., Sukhishvili, S., Du, H., & Podkolzin, S. G.