Electrochemical Tip-enhanced Raman Spectroscopy Research Articles

In situ and ex situ approaches for molecular scale understanding of electrochemical interfaces

Abstract Microscopic studies on electrolyte solution / electrode interfaces provide the most fundamental information not only for understanding the electric double layer formed at the interfaces but also for designing sophisticated electrochemical (EC) devices. In this study, we developed tip-enhanced Raman spectroscopy (TERS), which is based on an electrochemical scanning tunneling microscope (EC-STM), and demonstrated electrochemical TERS (EC-TERS) measurements of benzenethiol monolayers on Au(111). A specially-designed cell enables us to carry out reproducible EC, EC-STM, and EC-TERS measurements, which indicates consistent results among these techniques for the oxidative desorption of the benzenethiol monolayers.

Electrochemical Tip-Enhanced Raman Spectroscopy for the Elucidation of Complex Electrochemical Reactions

Electrochemical Tip-Enhanced Raman Spectroscopy for the Elucidation of Complex Electrochemical Reactions

(Invited) In Situ Electrochemical Raman Spectroscopy for Nanoscale to Macroscale Electrochemical Processes

Raman spectroscopy has been increasingly applied in studying the electrochemical processes, including adsorption and reaction, under in situ or operando condition, because Raman spectroscopy can conveniently work in aqueous solution and provide fingerprint molecular information. In this talk, I will present the recent work in my group on developing various electrochemical Raman spectroscopy for in-situ and operando studies of electrochemical systems at the limits of time and spatial resolution and sensitivityWith the help of the plasmonic enhancement at the tip of a plasmonic material, electrochemical tip-enhanced Raman spectroscopy (EC-TERS) offers itself as the only nanospectroscopy for investigating the electrochemical interface with unprecedent spatial resolution and explicit chemical fingerprint information. We developed the electrochemical TERS both based on STM and AFM using the water immersion objective and homemade optical instrument. EC-TERS was applied to obtain the molecular fingerprint information of surface adsorption and electrochemical reaction on single crystal surfaces and monitor the plasmon-driven reaction at the nanoscale.Aiming at addressing the challenge of monitoring the highly dynamic changing electrochemical interfaces under potential control and presence of the electrolyte, a transient electrochemical surface-enhanced Raman spectroscopy (TEC-SERS) was developed to monitor the structural evolution of surface species at a time resolution that equals the transient electrochemical methods (e.g. cyclic voltammetry and chronoamperometry), allowing the precise correlation of Raman signal with the molecular signature information and the electrochemical current signal. He observed an interesting two-step reaction processes that are impossible to observe by conventional steady state SERS methods. A wide-field Raman imaging system was developed to offer a high imaging speed to monitor the spatial and temporal change at every point over the whole electrode surface simultaneously. The heterogeneity of electrocatalytic reaction on the electrode surface was observed. This technique offers the possibility to spatially correlate electrochemical behavior and the particular surface sites where the reaction happens.

Electrochemical Tip-Enhanced Raman Spectroscopy for the In Situ Characterization of Functional Materials

The understanding of the mechanism of operation and failure of functional (nano)materials and surfaces is a prerequisite to the optimization of their performance and design. This can be address with nanoscale-sensitive techniques which can be implemented, in situ/operando under the operating conditions of such functional material. Tip-Enhanced Raman Spectroscopy (TERS) has already demonstrated its ability to characterize functional materials at the nanoscale, though its operation in liquid conditions and under polarization remains challenging.[1] After the first demonstration of liquid TERS in organic solvent in 2016 in our group [2], we could access electrochemical TERS (EC-TERS) mapping with 8-nm resolution in 2019.[3]Our most recent work focuses on the in situ investigation of the reactivity of electrochemically-active molecular architectures immobilized on electrode surfaces by EC-TERS. To this purpose we have developed a configurations enabling EC-TERS implementation in the Scanning Tunneling Microscopy mode (EC-STM-TERS). In this setup using a home-made bipotentiostat synchronized to the CCD detector, relatively high potential sweep rate[4] and broad potential ranges are accessible, enabling real-time assessments of electrochemical processes without compromising the plasmonic coupling at the tip-sample junction. A complex electrochemical mechanism, involving reaction intermediates and multiple reaction paths, could be resolved on electroactive architectures based on nitrobenzene derivatives. Further EC-TERS investigations on nitrobenzene derivatives assembled as mono- or multilayers on the electrode surface emphasized the influence of the structure of the molecular assemblies on their reactivity.In our latest developments, EC-TERS measurements implemented in the Atomic Force Microscopy mode (EC-AFM-TERS) are also accessible, therefore broadening the range of functional materials which can be studied in situ/operando.[1] T Touzalin, S Joiret, E Maisonhaute, IT Lucas “Capturing electrochemical transformations by tip-enhanced Raman spectroscopy”, Current Opinion in Electrochemistry 6 (2017) 46-52[2] T Touzalin, AL Dauphin, S Joiret, IT Lucas, E Maisonhaute “Tip enhanced Raman spectroscopy imaging of opaque samples in organic liquid”, Physical Chemistry Chemical Physics 18 (2016) 15510-15513[3] T Touzalin, S Joiret, IT Lucas, E Maisonhaute “Electrochemical tip-enhanced Raman spectroscopy imaging with 8 nm lateral resolution”, Electrochemistry Communications 108 (2019) 106557[4] M Mattei, G. Kang, G. Goubert, D.V. Chulhai, G.C. Schatz, L. Jensen and R.P. Van Duyne., “Tip-Enhanced Raman Voltammetry: Coverage Dependence and Quantitative Modeling” Nano Lett.(2017) 17, 590−596 Figure 1

Electrochemical Tip-Enhanced Raman Spectroscopy: An In Situ Nanospectroscopy for Electrochemistry.

Revealing the intrinsic relationships between the structure, properties, and performance of the electrochemical interface is a long-term goal in the electrochemistry and surface science communities because it could facilitate the rational design of electrochemical devices. Achieving this goal requires in situ characterization techniques that provide rich chemical information and high spatial resolution. Electrochemical tip-enhanced Raman spectroscopy (EC-TERS), which provides molecular fingerprint information with nanometer-scale spatial resolution, is a promising technique for achieving this goal. Since the first demonstration of this technique in 2015, EC-TERS has been developed for characterizing various electrochemical processes at the nanoscale and molecular level. Here, we review the development of EC-TERS over the past 5 years. We discuss progress in addressing the technical challenges, including optimizing the EC-TERS setup and solving tip-related issues, and provide experimental guidelines. We also survey the important applications of EC-TERS for probing molecular protonation, molecular adsorption, electrochemical reactions, and photoelectrochemical reactions. Finally, we discuss the opportunities and challenges in the future development of this young technique.

Molecular Scale Assessments of Electrochemical Interfaces: In Situ and Ex Situ Approaches

Abstract Microscopic studies on electrolyte solution/electrode interfaces provide the most fundamental information not only for understanding the electric double layer formed at the interfaces but also for designing sophisticated electrochemical devices. Various types of in situ techniques, performed without taking the electrode out of electrolyte solutions, have become indispensable tools. Among them, electrochemical tip-enhanced Raman spectroscopy (EC-TERS) is considered as an ultimate tool because of simultaneous measurements of electrochemical scanning tunneling microscopy (EC-STM) and Raman spectroscopy just underneath the EC-STM tip. On the other hand, ex situ techniques, where the electrode is emersed from the solution to perform precise measurements, have been still useful because the detailed information not easy to obtain by in situ techniques is available just by combining conventional instruments, such as photoelectron spectroscopy (PES) for the analysis of electronic states. In this highlight review, we present our recent progresses with in situ (EC-TERS) and ex situ (PES combined with electrochemistry) experiments for elucidating the microscopic properties of electric double layers. Current issues and future perspective of both techniques are also discussed in detail.

Probing nanoscale spatial distribution of plasmonically excited hot carriers

Surface plasmons (SPs) of metals enable the tight focusing and strong absorption of light to realize an efficient utilization of photons at nanoscale. In particular, the SP-generated hot carriers have emerged as a promising way to efficiently drive photochemical and photoelectric processes under moderate conditions. In situ measuring of the transport process and spatial distribution of hot carriers in real space is crucial to efficiently capture the hot carriers. Here, we use electrochemical tip-enhanced Raman spectroscopy (EC-TERS) to in situ monitor an SP-driven decarboxylation and resolve the spatial distribution of hot carriers with a nanometer spatial resolution. The transport distance of about 20 nm for the reactive hot carriers is obtained from the TERS imaging result. The hot carriers with a higher energy have a shorter transport distance. These conclusions can be guides for the design and arrangement of reactants and devices to efficiently make use of plasmonic hot carriers.

Atomic Force Microscopy Based Top-Illumination Electrochemical Tip-Enhanced Raman Spectroscopy.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) is a powerful technique for the in situ study of the physiochemical properties of the electrochemical solid/liquid interface at the nanoscale and molecular level. To further broaden the potential window of EC-TERS while extending its application to opaque samples, here, we develop a top-illumination atomic force microscopy (AFM) based EC-TERStechnique by using a water-immersion objective of a high numerical aperture to introduce the excitation laser and collect the signal. This technique not only extends the application of EC-TERS but also has a high detection sensitivity and experimental efficiency. We coat a SiO2 protection layer over the AFM-TERS tip to improve both the mechanical and chemical stability of the tip in a liquid TERS experiment. We investigate the influence of liquid on the tip-sample distance to obtain the highest TERS enhancement. We further evaluate the reliability of the as-developed EC-AFM-TERS technique by studying the electrochemical redox reaction of polyaniline. The top-illumination EC-AFM-TERS is promising for broadening the application of EC-TERS to more practical systems, including energy storage and (photo)electrocatalysis.

Development of Electrochemical Tip-Enhanced Raman Spectroscopy for Chemical Analysis at the Electrified Interfaces

Development of Electrochemical Tip-Enhanced Raman Spectroscopy for Chemical Analysis at the Electrified Interfaces

Reactivity mapping of nanoscale defect chemistry under electrochemical reaction conditions

Electrocatalysts often show increased conversion at nanoscale chemical or topographic surface inhomogeneities, resulting in spatially heterogeneous reactivity. Identifying reacting species locally with nanometer precision during chemical conversion is one of the biggest quests in electrochemical surface science to advance (electro)catalysis and related fields. Here, we demonstrate that electrochemical tip-enhanced Raman spectroscopy can be used for combined topography and reactivity imaging of electro-active surface sites under reaction conditions. We map the electrochemical oxidation of Au nanodefects, a showcase energy conversion and corrosion reaction, with a chemical spatial sensitivity of about 10 nm. The results indicate the reversible, concurrent formation of spatially separated Au2O3 and Au2O species at defect-terrace and protrusion sites on the defect, respectively. Active-site chemical nano-imaging under realistic working conditions is expected to be pivotal in a broad range of disciplines where quasi-atomistic reactivity understanding could enable strategic engineering of active sites to rationally tune (electro)chemical device properties.

Operando Characterization of Iron Phthalocyanine Deactivation during Oxygen Reduction Reaction Using Electrochemical Tip-Enhanced Raman Spectroscopy.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) has been implemented to investigate the structure and activity of iron(II) phthalocyanine (FePc)-a model catalyst for the oxygen reduction reaction (ORR). Using EC-TERS, both reversible change and irreversible degradation to FePc have been observed during ORR. The reversible change in the Raman spectrum of FePc can be related to the FePc molecules that adapt a nonplanar geometry during catalysis. In contrast, the irreversible degradation of FePc is a consequence of FePc demetalation, leading to the subsequent formation of free base phthalocyanine. This observation affirms that FePc demetalation during ORR proceeds via a direct loss of Fe2+ and that carbon corrosion is not the operative mechanism. Importantly, the FePc demetalation process can be correlated with a loss of ORR activity suggesting that Fe-containing sites are essential for FePc to achieve high catalytic activity. This study establishes EC-TERS as a promising technique for the operando characterization of electrocatalytic reactions at the molecular scale.

Electrochemical Tip-Enhanced Raman Spectroscopy with Improved Sensitivity Enabled by a Water Immersion Objective.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) appears as a promising in situ nanospectroscopic tool for characterization and understanding of the electrochemical interfacial processes at the nanometer scale and molecular level. However, the wide application of EC-TERS is hampered by its low sensitivity as a result of the optical path distortion due to the refractive index mismatch of the multilayer media (air, glass, and electrolyte). Here, we propose a new side-illumination EC-TERS setup by coupling a water immersion objective with a high numerical aperture to a scanning tunneling microscope scanning head customized with a large open space and a compact spectroelectrochemical cell. It not only effectively eliminates the optical distortion but also increases the sensitivity remarkably, which allows sensitive monitoring of the electrochemical redox processes of anthraquinone molecules. More importantly, EC-TERS is able to independently control the tip position and laser illumination position. By utilizing this feature, we reveal that the irreversible reduction reaction of anthraquinone observed in EC-TERS is induced by the synergistic effect of the negative potential and laser illumination rather than the localized surface plasmon. The highly improved sensitivity and the flexibility to control the tip and laser illumination position on the nanometer scale endows EC-TERS as an important tool for the fundamental understanding of the photo- or plasmon electrochemistry and the interfacial structure-activity relationship of important electrochemical systems.

Investigation of Cobalt Phthalocyanine at the Solid/Liquid Interface by Electrochemical Tip-Enhanced Raman Spectroscopy

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) with three excitation wavelengths in combination with in situ electrochemical scanning tunneling microscopy (EC-STM) and absorption measurements has been employed to provide comprehensive insights into the electrochemical processes of cobalt phthalocyanine (CoPc) at the solid/liquid interface supported on a Au(111) substrate. As the substrate potential becomes more negative, CoPc molecules form a highly ordered monolayer on the Au(111) surface (>0.1 V) until the ordered-to-diffusing phase transition is triggered (<0.1 V). CoPc molecules in the ordered phase are reduced during cathodic scanning, which leads to a redshift in the resonance condition and gives rise to distinct EC-TERS behaviors which depend on excitation wavelengths. The ordered-to-diffusing phase transition of CoPc molecules results in the disappearance of the EC-TERS signal. The catalytic activity of CoPc for the oxygen reduction reaction (ORR) was not visible in the EC-STM and has negligible effect on the EC-TERS measurements. The comprehensive evidence from EC-TERS, EC-STM, and absorption spectroelectrochemistry clearly demonstrates that partially reduced CoPc molecules are the dominant species under steady state measurements during the oxygen reduction reaction.

Systematic Assessment of Benzenethiol Self-Assembled Monolayers on Au(111) as a Standard Sample for Electrochemical Tip-Enhanced Raman Spectroscopy

A molecular-scale understanding of electrolyte/electrode interfaces has long been a challenging issue in electrochemistry. Spectroscopic tools with high spatial resolution are required for advancing beyond conventional electrochemical measurements such as cyclic voltammetry (CV). In this study, we developed tip-enhanced Raman spectroscopy (TERS), which is based on an electrochemical scanning tunneling microscope (EC-STM), and demonstrated electrochemical TERS (EC-TERS) measurements of benzenethiol self-assembled monolayers (SAMs) on Au(111). A specially designed cell enables us to carry out reproducible CV, EC-STM, and EC-TERS measurements, which indicates consistent results among these techniques for the oxidative desorption of the SAMs. We also present direct evidence that the measured EC-TERS signals originate from molecules adsorbed on Au(111) and not from those on the STM tip.

Unfolding the versatile potential of EC-TERS for electrocatalysis

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.

Probing Redox Reactions at the Nanoscale with Electrochemical Tip-Enhanced Raman Spectroscopy.

A fundamental understanding of electrochemical processes at the nanoscale is crucial to solving problems in research areas as diverse as electrocatalysis, energy storage, biological electron transfer, and plasmon-driven chemistry. However, there is currently no technique capable of directly providing chemical information about molecules undergoing heterogeneous charge transfer at the nanoscale. Tip-enhanced Raman spectroscopy (TERS) uniquely offers subnanometer spatial resolution and single-molecule sensitivity, making it the ideal tool for studying nanoscale electrochemical processes with high chemical specificity. In this work, we demonstrate the first electrochemical TERS (EC-TERS) study of the nanoscale redox behavior of Nile Blue (NB), and compare these results with conventional cyclic voltammetry (CV). We successfully monitor the disappearance of the 591 cm(-1) band of NB upon reduction and its reversible reappearance upon oxidation during the CV. Interestingly, we observe a negative shift of more than 100 mV in the onset of the potential response of the TERS intensity of the 591 cm(-1) band, compared to the onset of faradaic current in the CV. We hypothesize that perturbation of the electrical double-layer by the TERS tip locally alters the effective potential experienced by NB molecules in the tip-sample junction. However, we demonstrate that the tip has no effect on the local charge transfer kinetics. Additionally, we observe step-like behavior in some TERS voltammograms corresponding to reduction and oxidation of single or few NB molecules. We also show that the coverage of NB is nonuniform across the ITO surface. We conclude with a discussion of methods to overcome the perturbation of the double-layer and general considerations for using TERS to study nanoscale electrochemical processes.

Electrochemical Tip-Enhanced Raman Spectroscopy.

Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint molecular information at nanometer resolution. We developed an electrochemical tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. We obtained potential-dependent EC-TERS from the adsorbed aromatic molecule on a Au(111) surface and observed a substantial change in the molecule configuration with potential as a result of the protonation and deprotonation of the molecule. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the molecular level and nanoscale analysis of some important electrochemical systems including solar cells, lithium ion batteries, fuel cells, and corrosion.