Interfacial electric fields are important in several areas of chemistry, materials sciences, and device physics. However, they are poorly understood, partly because they are difficult to measure directly and model accurately. We present both a spectroscopic experimental investigation and a theoretical model for the interfacial field at the junction of a conductor and a dielectric. First, we present vibrational sum frequency generation (VSFG) results of the nitrile (CN) stretch of 4-mercaptobenzonitrile (4-MBN) covalently attached to a gold surface and in contact with a variety of liquid dielectrics. It is found that the CN stretch frequency red-shifts with increasing dielectric constant. Second, we build a model in direct analogy to the well-known Onsager reaction field theory, which has been successful in predicting vibrational frequency shifts in bulk dielectric media. Clearly, due to the asymmetric environment, with metal on one side and a dielectric on the other, the bulk Onsager model is not applicable at the interface. To address this, we apply the Onsager model to the interface accounting for the asymmetry. The model successfully explains the red-shift of the CN stretch as a function of the dielectric constant and is used to estimate the reaction field near the interface. We show the similarities and differences between the conventional bulk Onsager model and the interfacial reaction field model. In particular, the model emphasizes the importance of the metal as part of the solvation environment of the tethered molecules. We anticipate that our work will be of fundamental value to understand the crucial and often elusive electric fields at interfaces.

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.

Bridging the concepts of homogeneous and heterogeneous reactions is an important challenge in modern chemistry. Toward that end, here, we connect the homogeneous chemistry concept of the Hammett parameter, used by organic and organometallic chemists to quantify the electron-withdrawing capability of a functional group, to the electrochemical concept of polarization induced by a biased electrode. Because these two effects share similar origins, a theoretically motivated and experimentally verifiable link between them can be established. A convenient experiment that links the two is measuring the shift of vibrational frequency that is induced by these factors. To achieve this, first, we have measured the vibrational frequency of the nitrile stretch of 4-R-benzonitrile for a series of functional groups R spanning the Hammett parameter range −0.83 ≤ σp ≤ +1.11. Because the nitrile stretch is sensitive to molecular polarization, its frequency depends on the Hammett parameter of the polarizing functional groups. Second, we have measured the nitrile vibrational frequency of 4-mercaptobenzonitrile tethered on a gold electrode and polarized in an electrochemical cell as a function of potential from −1.4 to +0.6 V versus Ag/AgCl. Comparison of the nitrile-stretch frequency between the two experiments allows us to correlate the polarization caused by a functional group to that induced by the electrode. The data suggest equivalence between the Hammett parameter σp and the local electric field at the electrode interface, therefore allowing a polarizing electrode to be treated as a functional group. Computational work supports the experimental results and allows for a quantitative relation between the interfacial electric field and σp. We anticipate the benefits of this correlation, in particular, in linking concepts between homogeneous and heterogeneous reactions.

We report the use of surface-enhanced Raman scattering (SERS) to measure the vibrational Stark shifts of surface-bound thiolated-benzonitrile molecules bound to an electrode surface during hydrogen evolution reactions (HERs). Here, the electrode surface consists of Au nanoislands deposited both with and without an underlying layer of monolayer graphene on a glass substrate. The Stark shifts observed in the nitrile (C-N) stretch frequency (around 2225 cm) are used to report the local electric field strength at the electrode surface under electrochemical working conditions. Under positive (i.e., oxidative) applied potentials [vs normal hydrogen electrode (NHE)], we observe blue shifts of up to 7.6 cm, which correspond to local electric fields of 22 mV/cm. Under negative applied potentials (vs NHE), the C-N stretch frequency is red-shifted by only about 1 cm. This corresponds to a regime in which the electrochemical current increases exponentially in the hydrogen evolution process. Under these finite electrochemical currents, we estimate the voltage drop across the solution ( V = IR). Correcting for this voltage drop results in a highly linear electric field versus applied electrochemical voltage relation. Here, the onset potential for the HER lies around 0.2 V versus NHE and the point of zero charge (PZC) occurs at 0.04 V versus NHE, based on the capacitance-voltage ( C- V) profile. The solution field is obtained by comparing the C-N stretch frequency in solution with that obtained in air. By evaluating the local electric field strength at the PZC and the onset potential, we can separate the solution field from the reaction field (i.e., electrode field), respectively. At the onset of HER, the solution field is -0.8 mV/cm and the electrode field is -1.2 mV/cm. At higher ion concentrations, we observe similar electric field strengths and more linear E-field versus applied potential behavior because of the relatively low resistance of the solution, which results in negligible voltage drops ( V = IR).

Understanding Lewis pair (LP) interactions at heterogeneous environments is important for controlling surface reactions. We report the formation of interfacial Lewis adducts with tris(pentafluorophenyl)borane as the Lewis acid and 4-mercaptobenzonitrile attached to gold as the Lewis base. We use the nitrile vibrational frequency as a probe of adduct strength, with stronger adducts leading to larger frequencies. The vibrational frequency shifts of the surface adducts were measured via sum frequency generation spectroscopy and compared to the frequency shifts of bulk adducts. Our results show a distinctly smaller frequency shift for the surface adducts compared to the bulk, indicating a weaker Lewis acid-base interaction near the surface. We explore three possible origins of this difference: interfacial frustration, surface electric fields, and electronic energy level alignment. We highlight the relevance of each and note that likely more than one of them affect the observed surface LP interactions.