Direct Spectroscopic Measurement of Interfacial Electric Fields near an Electrode under Polarizing or Current-Carrying Conditions

Interfacial electric fields and the related molecular polarization are the central quantities that govern charge transfer between an electrode and a molecule. The presence of the interfacial field is often inferred indirectly through transport and capacitance measurements. It is desirable to measure such fields directly via the Stark shift that they induce on molecular vibrations. We report the Stark shift of a well-known vibrational chromophore tethered near an electrochemical interface measured using vibrational sum frequency generation spectroscopy. We have two important findings. First, we observe that the measured local field scales with respect to the ionic concentration in the electrolyte according to a model that combines the Gouy–Chapman theory with the capacitive response of a molecular layer. This behavior holds over 3 orders of magnitude in ionic concentration, therefore lending support to the validity of the model. Our results along with this model allow for estimation of the electric field near the electrode as the potential and ionic concentration are varied. Second, we observe that the mentioned variation of the local field with changing potential only occurs for positive potentials, for which the electrode is polarized but negligible current flows. For negative potentials, a sustained electrochemical current is observed that likely arises due to electron transfer and subsequent reduction of protons in the electrolyte. Interestingly, we observe that, under this condition, the local field does not vary with increasingly negative applied potential, reminiscent of the field within a leaky capacitor. The important consequence of this observation is that an increase in the thermodynamic drive for an electrochemical reaction does not necessarily translate to increased molecular polarization near the surface when a sustained current is passing. This study will serve as a baseline in all areas of chemistry in which understanding the role of local fields near interfaces is important and will provide a new perspective for interfacial charge transfer theories.