Surface Enhanced Infrared Studies of 4-Methoxypyridine Adsorption on Gold Film Electrodes

Abstract

This work uses electrochemical surface sensitive vibrational spectroscopy to characterize the adsorption of a known metal nanoparticle stabilizer and growth director, 4-methoxypyridine (MOP). Surface enhanced infrared absorption spectroscopy (SEIRAS) is employed to study the adsorption of 4-methoxypyridine on gold films. Experiments are performed under electrochemical control and in different electrolyte acidities to identify both the extent of protonation of the adsorbed species as well as its orientation with respect to the electrode surface. No evidence of adsorbed conjugated acid is found even when the electrolyte pH is considerably lower than the pKa. Through an analysis of the transition dipole moments, determined from DFT calculations, the SEIRA spectra support an adsorption configuration through the ring nitrogen which is particularly dominant in neutral pH conditions. Adsorption is dependent on both the electrical state of the Au film electrode as well as the presence of ions in the electrolyte that compete for adsorption sites at positive potentials. Combined differential capacitance measurements and spectroscopic data demonstrate that both a horizontal adsorption geometry and a vertical adsorption phase can be induced, with the former being found on negatively charged surfaces in acidic media and the latter over a wide range of polarizations in neutral solutions.

Surface sensitive infrared spectroelectrochemistry using palladium electrodeposited on ITO-modified internal reflection elements

“…MOP, like its parent pyridine compound, can on metal surfaces in different orientations depending on the electrolyte acidity, and the electronic state and crystallography of the metal surface. 42 The two limiting MOP adsorption motifs on gold are end-on (s bonding through the ring N atom) and flat-lying (bonding with the metal through the molecular p-orbitals) as confirmed through ATR-SEIRAS 41 and SERS 31 experiments. Previous surface enhanced vibrational spectroscopy studies have shown intact, edge-tilted orientations for pyridine adsorption on transition metals such as Ru 51 and Rh, 51,52 whereas strong evidence exists for a-pyridyl on Pt 51,[53][54][55][56] (see Fig.…”

“…40 MOP (4-methoxypyridine) was chosen as a test adsorbate to assess the dependence of SEIRA spectra on the amount of electrodeposited Pd. Both the electrochemistry and ATR-SEIRAS behavior of adsorbed MOP have been extensively studied on gold 41,42 but there are no reports of the adsorption behavior of MOP on palladium electrodes. A reference single beam measurement, S OCP , was made at the open circuit potential (B0.35 V vs. Ag/AgCl) of a cell containing only the 0.1 M KClO 4 supporting electrolyte.…”

“…6. Peak assignments, based on previous DFT calculations, 41 are provided in Table 1. These calculations correspond to the free molecule rather than MOP adsorbed on either Pd or Au surfaces.…”

“…However, the intensity of the u 8b mode is much more pronounced on palladium than on polycrystalline gold, where the adsorbed MOP molecule has been shown to be Nbonded to the surface. 41 This implies that the molecular orientation is not identical on the two metals. Determining the molecular orientation on the basis of SEIRAS selection rules is complicated by the fact that the electrodeposited metal islands are more discrete than the ellipsoidal particles used in physical vapour or chemically deposited SEIRAS layers 14 and provide a larger fraction of surface area perpendicular to the IRE surface.…”

Infrared metamaterial for surface-enhanced infrared absorption spectroscopy: pushing the frontier of ultrasensitive on-chip sensing

“…Therefore, continuous improvement of the vibrational signal enhancement through SEIRA optimization is critical, which is also the developing main thread of SEIRA technology. [50,[63][64][65][66] Based on the existing literature researches, there are three main methods for SEIRA performance optimization, ranging from structural design, material choice, to combining machine learning algorithms.…”