Determining intrinsic stark tuning rates of adsorbed CO on copper surfaces

Abstract

The abrupt change in potential between the electrode and the electrolyte, and the resulting interfacial electric field, is the driving force in electrochemical reactions. For surface mediated electrocatalytic reactions, the interfacial electric field is believed to have a key impact on the stability and reactivity of adsorbed intermediates. However, the exact mechanisms remain a topic of discussion. In this context, reliable measurements of the interfacial electric field are a prerequisite in understanding how it influences the rate and product distribution in electrochemical reactions. The vibrational Stark effect of adsorbates, such as CO, offers an accessible means to assess the interfacial electric field strength by determining the shift of vibrational peaks of the adsorbates with potential, i.e., the Stark tuning rate. However, the vibrational Stark effect could be convoluted with the dynamical dipole coupling effect of the adsorbates on weak binding surfaces such as Cu, thus complicating the determination of the intrinsic Stark tuning rate. In this work, we report a general and effective strategy of determining the intrinsic Stark tuning rate by removing the impact of the dynamical coupling of adsorbed CO on the Cu surface with surface enhanced infrared absorption spectroscopy. A similar intrinsic Stark tuning rate of ∼33 cm V⁻¹ was obtained on oxide-derived Cu in different electrolyte pH of 7.2, 10.9 and 12.9, indicating the pH independence of the interfacial electric field. Investigations on different Cu electrodes show that the intrinsic Stark tuning rates on (electro)chemically deposited films are close to 33 cm V⁻¹, while particulate Cu catalysts show a similar value of ∼68 cm V⁻¹. These observations indicate that aggregate morphology, rather than the size and shape of individual catalyst particles, has a more prominent impact on the interfacial electric field.