Understanding the effect of specific adsorption on the vibrational Stark effect of adsorbates on an electrode surface via surface enhanced spectroscopy

Abstract

Weakly binding organic species have been shown to significantly impact the electrocatalytic activities of metal surfaces; however, molecular level mechanisms remain elusive. The ambiguity of whether these weakly binding species are specifically adsorbed is a key challenge as there are few experimental techniques with sufficient sensitivity. The vibrational Stark effect of interfacial species offers a feasible way to achieve this goal with vibrational spectroscopy techniques, such as surface enhanced infrared absorption spectroscopy (SEIRAS) and surface enhanced Raman spectroscopy (SERS). In this work, we employed a library of bifunctional molecules with an isocyano group designed to anchor on the Au surface and an unbound cyano or isocyano group to investigate the effect of variations in the electric field on the Stark tuning rate of both functional groups. The surface bound isocyano group shows pronounced Stark tuning rates (2.4–4.7 cm⁻¹ (MV cm⁻¹)⁻¹) while the Stark tuning rates of the free functional groups are comparable to or below the spectral resolution (<0.6 cm⁻¹ (MV cm⁻¹)⁻¹). Contributions from variations in the bond force constant, induced by an electric field or a Fermi level shift, and the anharmonicity effect on the observed Stark tuning rates were deconvoluted. A shift in the vibrational peak of a specifically adsorbed functional group is more sensitive to the change in the electrode potential, which could be used to identify surface adsorbates at electrochemical interfaces. Our results suggest that Stark tuning rates above 0.6 cm⁻¹ (MV cm⁻¹)⁻¹ can be considered as strong evidence for specific adsorption on the electrode surface.