Atomic structure of Cu adlayers on Au(100) and Au(111) electrodes observed by in situ scanning tunneling microscopy

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.

Role of Transition Metals in Pt Alloy Catalysts for the Oxygen Reduction Reaction

In pursuit of higher activity and stability of electrocatalysts toward the oxygen reduction reaction, it has become standard practice to alloy platinum in various structural configurations. Transition metals have been extensively studied for their ability to tune catalyst functionality through strain, ligand, and ensemble effects. The origin of these effects and potential for synergistic application in practical materials have been the subject of many theoretical and experimental analyses in recent years. Here, a comprehensive overview of these phenomena is provided regarding the impact on reaction mechanisms and kinetics through combined experimental and theoretical approaches. Experimental approaches to electrocatalysis are discussed.

Unconventional Electrochemical Behaviors of Cu Underpotential Deposition in a Chloride-Based Deep Eutectic Solvent: High Underpotential Shift and Low Coverage

The (5 × 5) Moiré pattern resulting from coadsorption of Cu atoms and chloride ions on the Au(111) electrode is one of the most classical structures for underpotential deposition (UPD) in electrochemical surface science. Although two models have been proposed to describe the pattern, the details of the structure remain ambiguous and controversial, leading to a question that remains to be answered. In this work, we investigate the UPD behaviors of Cu on the Au(111) electrode in a chloride-based deep eutectic solvent ethaline by in situ scanning tunneling microscopy (STM). Benefiting from the properties of the ultraconcentrated electrolyte, we directly image not only Cu but also Cl adlayers by finely tuning tunneling conditions. The structure is unambiguously determined for both Cu and Cl adlayers, where an incommensurate Cu layer is adsorbed on the Au(111) surface with a Cu coverage of 0.64, while the Cl coverage is 0.32 (only half of the expected value); i.e., the atomic arrangement of the observed (5 × 5) Moiré pattern in ethaline matches neither of the models proposed in the literature. Meanwhile, STM results confirm the origin of the cathodic peak in the cyclic voltammogram, which indicates that the underpotential shift of Cu UPD in ethaline indeed increases by ca. 0.40 V compared to its counterpart in a sulfuric acid solution, resulting in a significant deviation from the linear relation between the underpotential shift and the difference in work functions proposed in the literature. The unconventional electrochemical behaviors of Cu UPD reveal the specialty of both the bulk and the interface in the chloride-based deep eutectic solvent.

Nanoscale electrochemistry in a copper/aqueous/oil three-phase system: surface structure-activity-corrosion potential relationships

Practically important metal electrodes are usually polycrystalline, comprising surface grains of many different crystallographic orientations, as well as grain boundaries. In this study, scanning electrochemical cell microscopy (SECCM) is applied in tandem with co-located electron backscattered diffraction (EBSD) to give a holistic view of the relationship between the surface structure and the electrochemical activity and corrosion susceptibility of polycrystalline Cu. An unusual aqueous nanodroplet/oil (dodecane)/metal three-phase configuration is employed, which opens up new prospects for fundamental studies of multiphase electrochemical systems, and mimics the environment of corrosion in certain industrial and automotive applications. In this configuration, the nanodroplet formed at the end of the SECCM probe (nanopipette) is surrounded by dodecane, which acts as a reservoir for oil-soluble species (e.g., O₂) and can give rise to enhanced flux(es) across the immiscible liquid–liquid interface, as shown by finite element method (FEM) simulations. This unique three-phase configuration is used to fingerprint nanoscale corrosion in a nanodroplet cell, and to analyse the interrelationship between the Cu oxidation, Cu²⁺ deposition and oxygen reduction reaction (ORR) processes, together with nanoscale open circuit (corrosion) potential, in a grain-by-grain manner. Complex patterns of surface reactivity highlight the important role of grains of high-index orientation and microscopic surface defects (e.g., microscratches) in modulating the corrosion-properties of polycrystalline Cu. This work provides a roadmap for in-depth surface structure–function studies in (electro)materials science and highlights how small variations in surface structure (e.g., crystallographic orientation) can give rise to large differences in nanoscale reactivity.

Probing Cu corrosion in an aqueous nanodroplet/oil/metal three-phase environment revealed unique patterns of surface reactivity. The electrochemistry of high-index facets cannot be predicted simply from the low-index {001}, {011} and {111} responses.

Revealing the formation dynamics of the electric double layer by means of in-situ Rutherford backscattering spectrometry

We report on a new versatile experimental setup for in situ Rutherford backscattering spectrometry at solid-liquid interfaces which enables investigations of electric double layers directly and in a quantitative manner. A liquid cell with a three-electrode arrangement is mounted in front of the beam line, and a thin Si₃N₄ window (thickness down to 150 nm) separates the vacuum of the detector chamber from the electrolyte in the cell. By minimizing the contribution of the window to the measurement, a large variety of elements at the solid-liquid interface with sensitivities far below one monolayer can be monitored. The attachment of Ba onto the Si₃N₄ surface as a function of contact time and pH value of the electrolyte solution was chosen as an example system. From our measurement, we can not only follow the evolution of the double layer but also derive limits for the point of zero charge for the Si₃N₄ surface. Our findings of 5.7≤pH_PZC≤6.2 are in good agreement with values found in the literature obtained by other techniques. Despite focusing on a specific system in this work, the presented setup allows for a large variety of in situ investigations at solid-liquid interfaces such as, but not limited to, tracing electrochemical reactions and monitoring segregation, adsorption, and dissolution and corrosion processes.

Probing the Surface of Noble Metals Electrochemically by Underpotential Deposition of Transition Metals

The advances in material science have led to the development of novel and various materials as nanoparticles or thin films. Underpotential deposition (upd) of transition metals appears to be a very sensitive method for probing the surfaces of noble metals, which is a parameter that has an important effect on the activity in heterogeneous catalysis. Underpotential deposition as a surface characterization tool permits researchers to precisely determine the crystallographic orientations of nanoparticles or the real surface area of various surfaces. Among all the work dealing with upd, this review focuses specifically on the main upd systems used to probe surfaces of noble metals in electrocatalysis, from poly‒ and single-crystalline surfaces to nanoparticles. Cuupd is reported as a tool to determine the active surface area of gold‒ and platinum‒based bimetallic electrode materials. Pbupd is the most used system to assess the crystallographic orientations on nanoparticles’ surface. In the case of platinum, Bi and Ge adsorptions are singled out for probing (1 1 1) and (1 0 0) facets, respectively.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Self-Limiting Adsorption of WO3 Oligomers on Oxide Substrates in Solution

Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO₂(110) and magnetite Fe₃O₄(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO₃ to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO₃ spontaneously forms under all conditions of potential and pH at the tungsten-aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials.

Real-time observation of interfacial ions during electrocrystallization

Understanding the electrocrystallization mechanisms of metal cations is of importance for many industrial and scientific fields. We have determined the transitional structures during underpotential deposition (upd) of various metal cations on Au(111) electrode using time–resolved surface X–ray diffraction and step–scan IR spectroscopy. At the initial stage of upd, a characteristic intensity transient appears in the time–resolved crystal truncation rod depending on metal cations. Metal cations with relatively high coordination energies of hydration water are deposited in two steps: first, the hydrated metal cations approached the surface and are metastably located at the outer Helmholtz plane, then they are deposited via the destruction of the hydration shell. However, Tl⁺ and Ag⁺, which have low hydration energy, are rapidly adsorbed on Au(111) electrode without any metastable states of dehydration. Therefore, the deposition rate is strongly related to the coordination energy of the hydration water. Metal cations strongly interacting with the counter coadsorbed anions such as Cu²⁺ in sulfuric acid causes the deposition rate to be slower because of the formation of complexes.

Palladium Electrodeposition onto Pt(100): Two-Layer Underpotential Deposition

Electrodeposition of the first Pd layers onto Pt(100) was investigated using cyclic voltammetry at a low scan rate (0.1 mV·s^-1). Ultrathin films were characterized by cyclic voltammetry in 0.1 M H₂SO₄ solution and with ex situ AFM (atomic force microscopy). For the first time, we evidenced the underpotential character of the deposition of the first two Pd layers, characterized by a two-step mechanism, each step corresponding to the deposition of a complete Pd atomic layer. For thicker deposits, especially above 10 monolayers as equivalent thickness, the electrochemical characterization displays a strong irreversibility and a broadening of the adsorption/desorption peaks, associated with a reduction of long-range ordered flat areas. Ex situ AFM images are in agreement with this description. They show rough thick deposits and the growth of (100)-oriented rectangular shaped islands with their sides aligned with the two [011] and [0-11] perpendicular directions of the (100) Pt surface.

Investigation of surface charge density on solid-liquid interfaces by modulating the electrical double layer

A solid surface in contact with water or aqueous solution usually carries specific electric charges. These surface charges attract counter ions from the liquid side. Since the geometry of opposite charge distribution parallel to the solid-liquid interface is similar to that of a capacitor, it is called an electrical double layer capacitor (EDLC). Therefore, there is an electrical potential difference across an EDLC in equilibrium. When a liquid bridge is formed between two conducting plates, the system behaves as two serially connected EDLCs. In this work, we propose a new method for investigating the surface charge density on solid-liquid interfaces. By mechanically modulating the electrical double layers and simultaneously applying a dc bias voltage across the plates, an ac electric current can be generated. By measuring the voltage drop across a load resistor as a function of bias voltage, we can study the surface charge density on solid-liquid interfaces. Our experimental results agree very well with the simple equivalent electrical circuit model proposed here. Furthermore, using this method, one can determine the polarity of the adsorbed state on the solid surface depending on the material used. We expect this method to aid in the study of electrical phenomena on solid-liquid interfaces.

Stick-slip behaviour on Au(111) with adsorption of copper and sulfate

Several transitions in the friction coefficient with increasing load are found on Au(111) in sulfuric acid electrolyte containing Cu ions when a monolayer (or submonolayer) of Cu is adsorbed. At the corresponding normal loads, a transition to double or multiple slips in stick-slip friction is observed. The stick length in this case corresponds to multiples of the lattice distance of the adsorbed sulfate, which is adsorbed in a √3 × √7 superstructure on the copper monolayer. Stick-slip behaviour for the copper monolayer as well as for 2/3 coverage can be observed at F N ≥ 15 nN. At this normal load, a change from a small to a large friction coefficient occurs. This leads to the interpretation that the tip penetrates the electrochemical double layer at this point. At the potential (or point) of zero charge (pzc), stick-slip resolution persists at all normal forces investigated.

Impacts of electrode potentials and solvents on the electroreduction of CO2: a comparison of theoretical approaches

Investigating in detail the intrinsic reactivity of CO₂ under aprotic conditions highlights the benefit of explicitly including the electrochemical potential into electronic structure computations together with an implicit solvent.

Adsorption and assembly of ions and organic molecules at electrochemical interfaces: nanoscale aspects

We describe the history of electrochemical scanning tunneling microscopy (STM) and advances made in this field during the past 20 years. In situ STM allows one to monitor various electrode processes, such as the underpotential deposition of copper and silver ions; the specific adsorption of iodine and sulfate/bisulfate ions; electrochemical dissolution processes of silicon and gold single-crystal surfaces in electrolyte solutions; and the molecular assembly of metalloporphyrins, metallophthalocyanines, and fullerenes, at atomic and/or molecular resolution. Furthermore, a laser confocal microscope, combined with a differential interference contrast microscope, enables investigation of the dynamics of electrochemical processes at atomic resolution.

The Under Potential Deposition of Cu on Au (111) in Nonaqueous Acetonitrile

The underpotential deposition (UPD) of Copper on Au(111) in nonaqueous acetonitrile/perchlorate electrolyte has been investigated. The deposition mechanism is thereby strongly influenced by coadsorbed acetonitrile molecules and the stability of the solvation shell around the Cu(I) ions. The UPD mechanism obeys a two step mechanism even in presence of only none specifically adsorbing perchlorate ions. Thereby the first sub monolayer of copper is stabilized by coadsorbed acetonitrile molecules. However already 5% vol water content changes the structure within the double layer and consequently degrades the copper sub monolayer. At about 20% vol water content the Cu(I) ion/solvent complex is still stable. The reduction of Cu(I) ions from this complex is much faster compared to the reduction from the respective Cu(II) water complex.

Exploring the first steps in core-shell electrocatalyst preparation: in situ characterization of the underpotential deposition of Cu on supported Au nanoparticles

The underpotential deposition (upd) of a Cu shell on a non-Pt nanoparticle core followed by galvanic displacement of the Cu template shell to form core–shell electrocatalyst materials is one means by which the Pt-based mass activity targets required for commercialization of PEM fuel cells may be reached. In situ EXAFS measurements were conducted at both the Au L₃ and the Cu K absorption edges during deposition of Cu onto a carbon-supported Au electrocatalyst to study the initial stages of formation of such a core–shell electrocatalyst. The Au L₃ EXAFS data obtained in 0.5 mol dm^–3 H₂SO₄ show that the shape of the Au core is potential dependent, from a flattened to a round spherical shape as the Cu upd potential is approached. Following the addition of 2 mmol dm^–3 Cu, the structure was also measured as a function of the applied potential. At +0.2 V vs Hg/Hg₂SO₄, the Cu²⁺ species was found to be a hydrated octahedron. As the potential was made more negative, single-crystal studies predict an ordered bilayer of sulfate anions and partially discharged Cu ions, followed by a complete/uniform layer of Cu atoms. In contrast, the model obtained by fitting the Au L₃ and Cu K EXAFS data corresponds first to partially discharged Cu ions deposited at the defect sites in the outer shell of the Au nanoparticles at −0.42 V, followed by the growth of clusters of Cu atoms at −0.51 V. The absence of a uniform/complete Cu shell, even at the most negative potentials investigated, has implications for the structure, and the activity and/or stability, of the core–shell catalyst that would be subsequently formed following galvanic displacement of the Cu shell.

Architecture-dependent surface chemistry for Pt monolayers on carbon-supported Au

Pt monolayers were grown by surface-limited redox replacement (SLRR) on two types of Au nanostructures. The Au nanostructures were fabricated electrochemically on carbon fiber paper (CFP) by either potentiostatic deposition (PSD) or potential square wave deposition (PSWD). The morphology of the Au/CFP heterostructures, examined using scanning electron microscopy (SEM), was found to depend on the type of Au growth method employed. The properties of the Pt deposit, as studied using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and cyclic voltammetry (CV), were found to depend strongly on the morphology of the support. Specifically, it was found that smaller Au morphologies led to a higher degree of cationicity in the resulting Pt deposit, with Pt(4+) and Pt(2+) species being identified using XPS and XAS. For fuel-cell catalysts, the resistance of ultrathin catalyst deposits to surface area loss through dissolution, poisoning, and agglomeration is critical. This study shows that an equivalent of two monolayers (ML) is the low-loading limit of Pt on Au. At 1 ML or below, the Pt film decreases in activity and durability very rapidly due to presence of cationic Pt.

STM Studies of Electrode/Electrolyte Interfaces and Silicon Surface Reactions in Controlled Atmospheres

We have assembled a scanning tunneling microscope with an inverted sample that allows the sample surface to be contacted by fluid electrolytes in a controlled atmosphere. A hanging meniscus is formed between the sample and a small cup surrounding the tunneling tip. In-situ imaging of the electrode/electrolyte interface is conveniently achieved with clean samples under potentiostatic control. The functioning of the microscope is illustrated by the imaging of the electrodeposition of copper on gold. This microscope has been used to image hydrogen-terminated silicon surfaces and to demonstrate that islands, tentatively assigned as silicon oxide, are formed on rinsing in water but can be avoided if the surface is not rinsed on withdrawal from the ammonium fluoride etching solution. Finally, STM shows that the convenient, gas-phase photochlorination of H-Si(111) produces the simple Cl-Si(111)(1×1) structure with little or no etching of the silicon surface.

In-Situ Stm Studies on the Electrodeposition of Ultrathin Nickel Films

An in-situ STM study of the initial stages of Ni electrodeposition on Au and Cu single-crystals is presented. On reconstructed Au(111) a complex, potential-dependent nucleation and growth process is found, involving selective Ni island formation at specific surface sites and growth of two types (compact and needle-like) of Ni monolayer islands. At higher coverages (1 ML ≤ θ ≤ 5 ML) an almost perfect layer-by-layer growth of a metallic Ni(111)-film was observed. Considerably rougher films were found on Au(100) and Cu(100).

Heat effects upon electrochemical copper deposition on polycrystalline gold

Heat effects upon Cu deposition on polycrystalline Au surfaces from sulphuric acid electrolytes were calorimetrically measured. By combination of pyroelectric temperature detection at the backside of a thin electrode foil with pulsed electrochemistry, sensitivities to electrochemical conversions of a few percent of a Cu monolayer (ML), corresponding to about 1 microJ cm(-2) were achieved. We compared the heat evolution upon Cu under potential deposition (UPD), Cu deposition onto a fully developed Cu UPD layer and bulk Cu deposition onto a 300 ML thick Cu layer on Au. The heat effects were measured dependent on the amplitudes of the applied potential steps, that is, the driving forces of the respective reactions. From the differences of the heat effects among the Cu deposition processes, we deduced implications on the reaction mechanisms. For Cu UPD, the heat effects were explicable by the deposition of 1.3 Cu atoms per two electrons flowing to the electrode accompanied by sulphate coadsorption, similar to Cu UPD from sulphuric acid solutions on Au(111). Upon Cu deposition on a Cu UPD layer the heat effects signal considerable anion coadsorption up to the deposition of about 0.5 ML of Cu. At higher conversions the deposition mechanism gradually changes towards bulk Cu deposition, accompanied by reduction of the sulphate coverage.

Nanotribology under electrochemical conditions: influence of a copper (sub)monolayer deposited on single crystal electrodes on friction forces studied with atomic force microscopy

Friction force measurements performed by means of an atomic force microscope (AFM) under electrochemical conditions on a pure Au(111) electrode surface and one modified with a foreign metal are presented; after deposition of a (sub)monolayer copper on a Au(111) single crystal electrode a large increase of the friction force is observed compared to the pure Au(111) electrode surface; the extent of the increase not only depends on the copper coverage, but also on the normal load and may be explained by a higher energy dissipation due to motion of the sulfate anions adsorbed on the copper atoms induced by the AFM tip.

Copper sulfide nanostripe patterns at the Au(111)/electrolyte interface studied by in situ STM

One monolayer of Cu was prepared on Au(111) by underpotential deposition from CuSO4/H2SO4 solution and, by two electrolyte exchanges for (i) Cu-free H2SO4 and (ii) NaOH/Na2S solution, exposed to bisulfide. This procedure leads to several incommensurate phases with characteristic stripe patterns. These are irreversibly displaced upon cathodic potential sweeps by different structures, which, after returning to the initial potential, transform into the rectangular CuxS phase already known for the sulfidation of a Cu submonolayer on Au(111).