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Molecular understanding of the electrochemical oxidation of metals and the electro-reduction of metal oxides is of pivotal importance for the rational design of catalyst-based devices where metal(oxide) electrodes play a crucial role. Operando monitoring and reliable identification of reacting species, however, are challenging tasks because they require surface-molecular sensitive and specific experiments under reaction conditions and sophisticated theoretical calculations. The lack of molecular insight under operating conditions is largely due to the limited availability of operando tools and to date still hinders a quick technological advancement of electrocatalytic devices. Here, we present a combination of advanced density functional theory (DFT) calculations considering implicit solvent contributions and time-resolved electrochemical surface-enhanced Raman spectroscopy (EC-SERS) to identify short-lived reaction intermediates during the showcase electro-reduction of Au oxide (AuOx) in sulfuric acid over several tens of seconds. The EC-SER spectra provide evidence for temporary Au-OH formation and for the asynchronous adsorption of (bi)sulfate ions at the surface during the reduction process. Spectral intensity fluctuations indicate an OH/(bi)sulfate turnover period of 4 s. As such, the presented EC-SERS potential jump approach combined with implicit solvent DFT simulations allows us to propose a reaction mechanism and prove that short-lived Au-OH intermediates also play an active role during the AuOx electro-reduction in acidic media, implying their potential relevance also for other electrocatalytic systems operating at low pH, like metal corrosion, the oxidation of CO, HCOOH, and other small organic molecules, and the oxygen evolution reaction. J.H.K.P. and K.F.D. gratefully acknowledge financial support by the Max Planck Graduate Center with the Johannes Gutenberg University Mainz (MPGC). K.F.D. acknowledges generous support through the Emmy Noether Program of the Deutsche Forschungsgemeinschaft (DO1691/1−1) and through the “Plus 3” Program of the Boehringer Ingelheim Foundation. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreements No. 665667 and No. 798532. This work was supported by a grant from the Swiss National Supercomputing Centre (CSCS) under project ID s836.

The rational design of electrocatalysts demands the in situ experimental identification of specific surface sites that are most active toward certain electrochemical reactions. Therefore, surface-sensitive techniques with high chemical specificity and the ability to resolve nanoscale features are required. Even more challenging is the requirement to operate in aqueous electrolytes under potential control. Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) smartly comprises these prerequisites. To date, EC-TERS has been used to study the potential-dependent protonation/deprotonation, reorientation and redox reactions of self-assembled monolayers in contact with aqueous electrolytes. This review article highlights the pioneering work of EC-TERS and discusses the enormous potential that this spectro-electrochemical tool entails for the field of electrocatalysis.

Black manganese-rich crusts are found worldwide on the facades of historical buildings. In this study, they were studied exemplarily on the facade of the Freiburger Munster (Freiburg Minster), Germany, and measured in-situ by portable X-ray fluorescence (XRF). The XRF was calibrated to allow the conversion from apparent mass fractions to Mn surface density (Mn mass per area), to compensate for the fact that portable XRF mass fraction measurements from thin layers violate the assumption of a homogeneous measurement volume. Additionally, 200-nm femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS) measurements, scanning transmission X-ray microscopy-near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS), Raman spectroscopy, and imaging by light microscopy were conducted to obtain further insight into the crust material, such as potential biogenic contributions, element distributions, trace element compositions, and organic functional groups. While black crusts of various types are present at many places on the minster's facade, crusts rich in Mn (with a Mn surface density >150 μg cm−2) are restricted to a maximum height of about 7 m. The only exceptions are those developed on the Renaissance-Vorhalle (Renaissance Portico) at a height of about 8 m. This part of the facade had been cleaned and treated with a silicon resin as recently as 2003. These crusts thus accumulated over a period of only 12 years. Yet, they are exceptionally Mn-rich with a surface density of 1200 μg cm−2, and therefore require an accumulation rate of about 100 μg cm−2 Mn per year. Trace element analyses support the theory that vehicle emissions are responsible for most of the Mn supply. Lead, barium, and zinc correlate with manganese, indicating that tire material, brake pads, and resuspended road dust are likely to be the element sources. Microscopic investigations show no organisms on or in the Mn-rich crusts. In contrast, Mn-free black crusts sampled at greater heights (>8 m) exhibited fungal and cyanobacterial encrustation. Carbon-rich spots were found by STXM-NEXAFS underneath one of the Mn-rich crusts. However, these carbon occurrences originate from soot and polycyclic aromatic hydrocarbons (PAHs) deposited on top of the crust, rather than from organisms responsible for the crust's formation, as shown by STXM-NEXAFS and Raman spectroscopic measurements. Our results suggest that the crusts develop abiogenically, with vehicle emissions as dominant element sources.

In this work, we evaluate the dependence of tip-enhanced Raman (TER) spectra of a monolayer of thiophenol at a Au(111) electrode on the scanning tunneling microscope’s tunneling current set-point and bias voltage parameters. We find an increase of the TER intensity upon set-point increase or bias decrease as expected from a gap-distance reduction. The relations obtained follow a theoretical model considering a simple gap-distance change when tuning the mentioned parameters. We find that the value of the bias voltage affects the TER intensity to a larger extent than the current set-point. Therefore it is advisable to work in a low-bias regime when aiming for ultrasensitive TER measurements.

Infrared spectroscopy is a widely employed analytical tool in (electrochemical) surface science as the spectra contain a wealth of information about the interaction of interfacial adsorbates with their environment. Separating and quantifying individual contributions, for example, of co-adsorbates, the substrate or electric field effects, on the overall spectral response, however, is often non-trivial as the various interactions manifest themselves in similar spectral behavior. Here, we present an experimental approach to differentiate between and quantify potential-induced coverage dependence and field-related Stark effects observed in a sulfate band shift of 93.5 ± 1.5 cm−1/V in electrochemical infrared spectra of the showcase sulfate/Au(111) interface. In combination with a simple linear model equation used to describe the potential-induced peak shift of the sulfate stretch vibration, we determine the coverage dependence contribution to be 15.6 ± 1.2 cm−1/θSO and the Stark effect to amount to 75.6 ± 2.7 cm−1/V. Our work provides a novel route to gain fundamental insight into interfacial adsorbate interactions in electrochemical surface science. J.H.K.P. and K.F.D. gratefully acknowledge financial support by the Max Planck Graduate Center with the Johannes Gutenberg University Mainz (MPGC). U.E.Z. is grateful for financial support from the Alexander von Humboldt Foundation. J.M.F. thanks MCINN-FEDER (Spain) for support through Project No. CTQ2016-76221-P. K.F.D. acknowledges generous support through the Emmy Noether Program of the Deutsche Forschungsgemeinschaft (No. DO1691/1-1) and through the “Plus 3” Program of the Boehringer Ingelheim Foundation.