Electrochemical Scanning Tunneling Microscopy 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.

Maintaining the order: 4,4′-bipyridine self-assembled layers on the Bi(111) | ionic liquid interface

The organised layer of molecules on a solid surface is at the centre of numerous molecular devices. Although the adsorption of molecules from aqueous electrolytes has been thoroughly investigated, only a limited number of studies have focused on the adsorption from ionic liquid electrolytes. This paper characterises the 4,4′-bipyridine adsorption at the Bi(111) | ionic liquid interface. We combined electrochemical impedance spectroscopy and scanning tunnelling microscopy with density functional theory calculations to gain a comprehensive overview of the interfacial processes. The results show a notable influence of electrode potential and the electrolyte on the 4,4′-bipyridine adsorption behaviour. The formation of organic adlayer on the electrode's surface was demonstrated, while highly organised striated adlayer structure was visualised only in a narrow potential range.

Probe the Dynamic Adsorption and Phase Transition of Underpotential Deposition Processes at Electrode-Electrolyte Interfaces.

Electrochemical scanning tunneling microscopy (EC-STM) and electrochemical quartz crystal microbalance (E-QCM) techniques in combination with DFT calculations have been applied to reveal the static phase and the phase transition of copper underpotential deposition (UPD) on a gold electrode surface. EC-STM demonstrated, for the first time, the direct visualization of the disintegration of (√3 × √3)R30° copper UPD adlayer with coadsorbed SO42- while changing sample potential (ES) toward the redox Pa2/Pc2 peaks, which are associated with the phase transition between the Cu UPD (√3 × √3)R30° phase II and disordered randomly adsorbed phase III. DFT calculations show that SO42- binds via three oxygens to the bridge sites of the copper with sulfate being located directly above the copper vacancy in the (√3 × √3)R30° adlayer, whereas the remaining oxygen of the sulfate points away from the surface. E-QCM measurement of the change of the electric charge due to Cu UPD Faradaic processes, the change of the interfacial mass due to the adsorption and desorption of Cu(II) and SO42-, and the formation and stripping of UPD copper on the gold surface provide complementary information that validates the EC-STM and DFT results. This work demonstrated the advantage of using complementary in situ experimental techniques (E-QCM and EC-STM) combined with simulations to obtain an accurate and complete picture of the dynamic interfacial adsorption and UPD processes at the electrode/electrolyte interface.

SHINERS Study of Chloride Order-Disorder Phase Transition and Solvation of Cu(100).

Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and density functional theory (DFT) are used to probe Cl- adsorption and the order-disorder phase transition associated with the c(2 × 2) Cl- adlayer on Cu(100) in acid media. A two-component ν(Cu-Cl) vibrational band centered near 260 ± 1 cm-1 is used to track the potential dependence of Cl- adsorption. The potential dependence of the dominant 260 cm-1 component tracks the coverage of the fluctional c(2 × 2) Cl- phase on terraces in good agreement with the normalized intensity of the c(2 × 2) superstructure rods in prior surface X-ray diffraction (SXRD) studies. As the c(2 × 2) Cl- coverage approaches saturation, a second ν(Cu-Cl) component mode emerges between 290 and 300 cm-1 that coincides with the onset and stiffening of step faceting where Cl- occupies the threefold hollow sites to stabilize the metal kink saturated Cu <100> step edge. The formation of the c(2 × 2) Cl- adlayer is accompanied by the strengthening of ν(O-H) stretching modes in the adjacent non-hydrogen-bonded water at 3600 cm-1 and an increase in hydronium concentration evident in the flanking H2O modes at 3100 cm-1. The polarization of the water molecules and enrichment of hydronium arise from the combination of Cl- anionic character and lateral templating provided by the c(2 × 2) adlayer, consistent with SXRD studies. At negative potentials, Cl- desorption occurs followed by development of a sulfate νs(S═O) band. Below -1.1 V vs Hg/HgSO4, a new 200 cm-1 mode emerges congruent with hydride formation and surface reconstruction reported in electrochemical scanning tunneling microscopy studies.

Development of a Fast in Situ Scanning Tunneling Microscope for Studies of Electrocatalyst Surfaces

The atomic-scale understanding of processes at the interface between solid electrodes and liquid electrolytes is of high importance for electrochemical energy storage and conversion. Electrochemical scanning tunneling microscopy (ECSTM) is a key technique for the investigation of these interfaces and as such, it has seen widespread use. However, the image acquisition in a conventional ECSTM is a rather slow process, requiring tens of seconds or minutes per image. To help understand the precise reaction mechanisms of atomic and molecular species at solid-liquid interfaces, their movement and interactions need to be resolved. For this, much higher imaging rates are necessary. High-speed STMs (video STMs) are capable of operating at rates >10 images per second, which is sufficiently fast to observe and quantitatively study a wide range of surface dynamic processes, e.g., surface diffusion and growth [1]. However, this technique has not been widely employed, mainly because of the instrumental requirements.The development of an STM for fast in situ measurements poses a set of challenges, which require a carefully planned implementation. For operation in electrochemical environment, the potentials of both STM tip and sample need to be controlled and electrochemical currents at the tip need to be kept way below the tunneling current. Fast image acquisition requires a scanner with high mechanical stability to avoid the excitation of uncontrolled tip oscillations at its resonance frequencies. Additionally, high-bandwidth measurement and control electronics and a sophisticated control software with fast scan generation and data processing capabilities are essential. No commercial system that is capable of fulfilling these requirements exists up to now. For this reason, all existing video-rate STM studies have been performed with home-built setups that employ highly specialized hardware that is not easy to reproduce.In this contribution, we present a new ECSTM developed and built in our group. The setup is based on a Nanonis SPM controller by SPECS and a custom scanner, bipotentiostat, and coarse approach control that were integrated into this system. We show example data and images to demonstrate the performance of the STM. Furthermore, we reveal the modifications employed to make it capable of video-rate imaging. These include a novel scanner design with two independent piezo stacks for slow and high-speed movements and a custom high-bandwidth preamplifier integrated into the scan head as close as possible to the tip. Fast data acquisition is realized by an FPGA-based control software, which features a user-friendly frontend and a backend with good performance even at high image acquisition rates. This software is designed to run alongside the commercial control software for the slow STM operation so that switching between the slow and fast imaging modes is as frictionless as possible. We will show preliminary test results of the early implementation of the fast imaging mode.[1] O. M. Magnussen, Chem. Eur. J. 2019, 25, 12865.

In Situ Probing of CO2 Reduction on Cu-Phthalocyanine-Derived Cux O Complex.

An in situ measurement of a CO2 reduction reaction (CO2 RR) in Cu-phthalocyanine (CuPC) molecules adsorbed on an Au(111) surface is performed using electrochemical scanning tunneling microscopy. One intriguing phenomenon monitored in situ during CO2 RR is that a well-ordered CuPC adlayer is formed into an unsuspected nanocluster via molecular restructuring. At an electrode potential of -0.7V versus Ag/AgCl, the Au surface is covered mainly with the clusters, showing restructuring-induced CO2 RR catalytic activity. Using a measurement of X-ray photoelectron spectroscopy, it is revealed that the nanocluster represents a Cu complex with its formation mechanism. This work provides an in situ observation of the restructuring of the electrocatalyst to understand the surface-reactive correlations and suggests the CO2 RR catalyst works at a relatively low potential using the CuPC-derived Cu nanoclusters as active species.

Watching atoms at work during reactions

Abstract The development of new technologies for the current energy and environmental challenges requires the acquisition of a very fundamental knowledge about the structure and activity of catalytic materials at the nanometric scale. As a consequence, in situ and operando methodologies are blossoming, but only a fraction of them really aims at a local vision that would allow watching atoms at work during reactions. In this short report, we want to outline the merits of a new technique based on scanning tunnelling microscopy (Current-roughness electrochemical scanning tunneling microscopy, cr-EC-STM) which can visualize electrocatalytic reactions down at the single atom level. Results of two case studies in the field of hydrogen evolution reaction (HER) are briefly summarized, witnessing the capability of cr-EC-STM to provide critical information about the structure and catalytic performance of the active sites with atomic resolution.

Unraveling the Dynamic Processes of Methanol Electrooxidation at Isolated Rhodium Sites by In Situ Electrochemical Scanning Tunneling Microscopy.

Materials with isolated single-atom Rh-N4 sites are emerging as promising and compelling catalysts for methanol electrooxidation. Herein, we carried out an in situ electrochemical scanning tunneling microscopy (ECSTM) investigation of the dynamic processes of methanol absorption and catalytic conversion in the rhodium octaethylporphyrin (RhOEP)-catalyzed methanol oxidation reaction at the molecular scale. The high-contrast RhOEP-CH3OH complex formed by methanol adsorption was visualized distinctly in the STM images. The Rh-C adsorption configuration of methanol on isolated rhodium sites was identified on the basis of a series of control experiments and theoretical simulation. The adsorption energy of methanol on RhOEP was obtained from quantitative analysis. In situ ECSTM experiments present an explicit description of the transformation of the intermediate species in the catalytic process. By qualitatively evaluating the rate constants of different stages in the reaction at the microscopic level, we considered the CO transformation/desorption as the critical step for determining the reaction dynamics. Methanol adsorption was found to be correlated with RhOEP oxidation in the initial stage of the reaction, and the dynamic information was revealed unambiguously by in situ potential step experiments. This work provides microscopic results for the catalytic mechanism of Rh-N4 sites for methanol electrooxidation, which is instructive for the rational design of the high-performance catalyst.

Elucidating the Active Sites and Synergies in Water Splitting on Manganese Oxide Nanosheets on Graphite Support

AbstractPhotosystem II is nature's solution for driving the oxygen evolution reaction to oxidize water. A manganese‐oxide cluster is this protein's active center for water splitting, while the most efficient man‐made catalysts are costly noble metal‐based oxides. Facing the climate change, research on affordable and abundant electrocatalysts is crucial. To mimic the biological solution, manganese oxide nanosheets are synthesized and deposited on highly‐oriented pyrolytic graphite. This electrocatalyst is then examined with spectroscopic and electrochemical measurements, electrochemical noise scanning tunneling microscopy, and density functional theory calculations. The detailed investigation assigns the origin of its enhanced water‐splitting performance to detected activity at the nanosheet edges which the proposed mechanism explains further. Therefore, the results provide a blueprint for how to design efficient electrocatalysts for water oxidation with abundant materials.

Cu Surface Chemistry in Acid Media: Vibrational Spectroscopy, Mass Spectrometry and ECSTM Measurements

Copper electrodeposition is a central process in the metallization of microelectronics and more recently copper has also found use as an important electrocatalyst in the conversion of CO2 to hydrocarbons. Gaining mechanistic insight into the reactivity of Cu surfaces requires in situ and better still in operando measurements. Herein the utility of the combination of vibrational spectroscopy (SEIRAS, SHINERS), electrochemical mass spectrometry (EC-MS) and scanning tunneling microscopy (ECSTM) to examine the competitive and co-adsorption interactions between potential dependent halide adsorption, molecular adsorption, underpotential metal deposition and hydride formation on low index Cu single crystals surface will be detailed. Further still, the emergent opportunity to study the impact of such interaction on metal deposition reactions will also be explored.1.Raciti and T.P. Moffat, “Quantification of Hydride Coverage on Cu(111) by Electrochemical Mass Spectrometry,” J. Phys. Chem., 126, 18734-18743 (2022).2.Raciti, A.R. Hight Walker, T. P. Moffat, “Mapping Surface Chemistry During Superfilling with Shell- Isolated Nanoparticle Enhanced Raman Spectroscopy and X-ray Photoelectron Spectroscopy,” J. Electrochem. Soc., 169, 082506 (2022).3.B.M. Tackett, D. Raciti, A.R. Hight Walker, T.P. Moffat, “Surface Hydride Formation on Cu (111) and its Decomposition to Form H2 in Acid Electrolytes,” J. Phys. Chem. Lett., 12, 10936-10941 (2021).4. G.Liu, S. Zou, D. Josell, L.J. Richter and T.P. Moffat, “SEIRAS Study of Chloride Mediated Polyether Adsorption on Cu,” J. Phys. Chem. C., 122, 21933-21951, (2018).

Scanning tunneling microscopy under chemical reaction at solid–liquid and solid–gas interfaces

The task of bridging the pressure gap between ideal ultrahigh vacuum conditions and more realistic reaction conditions involving gas and liquid phases is crucial in surface and interfacial chemistry. Scanning tunneling microscopy (STM) has played a key role in addressing this challenge by enabling atomic-scale probing of the interface. STM enabled us to study surface structure, electronic structure, atomic manipulation, dynamics of molecules and atoms, and chemical properties of the surface at the atomic scale. Over the past four decades, the field of STM has undergone explosive growth. This review article focuses on recent advances in operando STM, specifically in the study of solid–liquid and solid–gas interfaces. It highlights the latest works in ambient-pressure STM, which has enabled the observation of atomic features under various gas and reaction conditions. This information sheds light on the surface mobility of adsorbates and atomic structures of reaction intermediates. The review also addresses research on electrochemical STM, which investigates the evolution of surface morphology under electrochemical processes and provides insights into atomic-scale reaction mechanisms. Finally, the article outlines future challenges and perspectives for operando STM techniques.

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions.

The dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first-principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential- and pH-dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions

AbstractThe dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first‐principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential‐ and pH‐dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

In-situ imaging of the electrode surface during electrochemical reactions with a beetle-type electrochemical scanning tunneling microscope

In this paper, we present the design and performances of a beetle-type electrochemical scanning tunneling microscope (EC-STM) which allows horizontal tip motion at millimeter range (5 mm × 5 mm). With its symmetrical scanner design inducing a relatively low thermal drift, the beetle-type EC-STM has the desirable ability to operate in a variety of chemical reactions. Atom-resolved high-resolution STM images of highly oriented pyrolytic graphite (HOPG) and Au(111) surfaces in the liquid phase are presented to confirm the scan performance of the beetle-type EC-STM, which also provides in situ information on the electrode surface during electrochemical reactions, including adsorbed– and desorbed– electrolyte and metal electrodeposition. These systemically obtained STM images on the electrode surface clearly demonstrate the high stability of the newly developed EC‐STM under reaction conditions.

Electrochemical Scanning Tunneling Microscopy as a Tool for the Detection of Active Electrocatalytic Sites

To advance meaningful guidelines in the design of electrocatalytically active catalysts, a knowledge of the nature of active sites is the starting point. However, multiple factors such as material composition, site coordination, electrolyte effects, the support material, surface strain, and others influence catalytic behavior. Therefore, the identification of active sites can be complex. A substantial contributor can be in-situ experiments, which are able to identify active centers in a specific system while the reaction takes place. An example of such a technique is electrochemical scanning tunneling microscopy (EC-STM), which relates locally confined noise features to local electrocatalytic activity. In this work, we spotlight recent achievements of this technique with respect to palladium (Pd) surfaces for the hydrogen reduction reaction, where strain due to hydride formation comes into play in addition to surface coordination. Secondly, we demonstrate the high resolution of the technique on graphite-based surfaces. Here, edge sites are particularly active. Thus, with the EC-STM technique, we take strain effects (like on Pd) or effects of coordination (like on carbon) into account. Therefore, we can determine active sites with great accuracy under reaction conditions.

On the role of applied potential in adsorption of dibenzyl viologen molecules on HOPG surface

Tuning the charge carier concentration of graphene is one of the key challenges in the field of graphene research. An effective solution for this is to dope graphene by organic molecules that physisorb or self-assemble on the graphene surface. Therefore, a comprehensive understanding of their surface structures at the molecular level is realy nessesary. In this contribution, we report on the role of the applied electrode potential in the adsorption/self-assembly of such n-dope molecule, dibenzyl viologen (DBV), on a highly oriented pyrolytic graphte (HOPG) surface (a multi-layer graphene material) determined by using a combination of cyclic voltametry (CV) and electrochemical scanning tunneling microscopy (ECSTM) methods. The obtained results reveal that dibenzyl viologen molecules can exist at three redox states corresponding to three respective adsorbate phases depending on the applied electrode potential. The DBV2+ molecucles physisorb and form disordered phase, whereas DBV·+ and DBV0 moleucles self-assemble forming dimer and stacking phases, respectively, on HOPG surface.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis.

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O2 and H2 and electrochemical conversion of CO2. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Electron Tunneling at Electrocatalytic Interfaces

It was recently proposed that tunneling current fluctuations in electrochemical scanning tunneling microscopy (EC-STM) can be used to map the electrocatalytic activity of surfaces with high spatial resolution. However, the relation between the increased noise in the electron tunneling signal and the local reactivity for such complex electrode/electrolyte interfaces is only explained qualitatively or hypothetically. Herein, we employ electron transport calculations to examine tunneling at Pt surfaces under the conditions of the oxygen reduction reaction as a case study. By computing current–voltage characteristics, we reveal that the tunneling barrier strongly depends on the chemical identity of the adsorbed reaction intermediate as well as on the orientation of the average dipole moment of water species mediating electron tunneling. Our theoretical results combined with EC-STM measurements suggest that detecting reaction intermediates at electrified interfaces in operando conditions is possible based on tunneling noise amplitudes. This study also aims to stimulate further explorations of tunneling-based electron-proton transfers to enable quantum electrocatalysis beyond conventional approaches.

Single-Molecule Study on the Catalytic Role of Co–O2 Binding in ORR by In Situ ECSTM

We report here the in situ electrochemical scanning tunneling microscopy (ECSTM) study of O2 adsorption and reduction on cobalt porphyrin (CoOEP) and phthalocyanine (CoPc). Electrochemical measurements reveal higher ORR activity of CoOEP than CoPc. Self-assembled CoOEP and CoPc monolayers are fabricated on Au(111) substrates and molecularly resolved by ECSTM. The direct molecular imaging demonstrates that O2 adsorbs on CoII sites and barely on CoIII sites. The CoOEP–O2 and CoPc–O2 complexes exhibit high contrast in ECSTM images due to O2 binding. The surface coverage of CoOEP–O2 is ca. 1.4 times higher than that of CoPc–O2. The dynamics of O2 adsorption is measured quantitatively by fitting data from potential step experiments to obtain rate constants for O2 adsorption (kad) and dissociation (kdi) on Co centers. kad and kdi are higher on CoOEP than on CoPc, indicating higher dynamics of O2 binding and lower barriers to Co–O dissociation on CoOEP.

Catalytic Activity of Defect-Engineered Transition Me tal Dichalcogenides Mapped with Atomic-Scale Precision by Electrochemical Scanning Tunneling Microscopy.

Unraveling structure-activity relationships is a key objective of catalysis. Unfortunately, the intrinsic complexity and structural heterogeneity of materials stand in the way of this goal, mainly because the activity measurements are area-averaged and therefore contain information coming from different surface sites. This limitation can be surpassed by the analysis of the noise in the current of electrochemical scanning tunneling microscopy (EC-STM). Herein, we apply this strategy to investigate the catalytic activity toward the hydrogen evolution reaction of monolayer films of MoSe2. Thanks to atomically resolved potentiodynamic experiments, we can evaluate individually the catalytic activity of the MoSe2 basal plane, selenium vacancies, and different point defects produced by the intersections of metallic twin boundaries. The activity trend deduced by EC-STM is independently confirmed by density functional theory calculations, which also indicate that, on the metallic twin boundary crossings, the hydrogen adsorption energy is almost thermoneutral. The micro- and macroscopic measurements are combined to extract the turnover frequency of different sites, obtaining for the most active ones a value of 30 s-1 at -136 mV vs RHE.