UHV Techniques in the Study of Electrode Surfaces

Bridging Electrochemistry and Ultrahigh Vacuum: "Unburying" the Electrode-Electrolyte Interface

ConspectusElectrochemistry has a central role in addressing the societal issues of our time, including the United Nations' Sustainable Development Goals (SDGs) and beyond. At a more basic level, however, elucidating the nature of electrode-electrolyte interfaces is an ongoing challenge due to many reasons, but one obvious reason is the fact that the electrode-electrolyte interface is buried by a thick liquid electrolyte layer. This fact would seem to preclude, by default, the use of many traditional characterization techniques in ultrahigh vacuum surface science due to their incompatibility with liquids. However, combined UHV-EC (ultrahigh vacuum-electrochemistry) approaches are an active area of research and provide a means of bridging the liquid environment of electrochemistry to UHV-based techniques. In short, UHV-EC approaches are able to remove the bulk electrolyte layer by performing electrochemistry in the liquid environment of electrochemistry followed by sample removal (referred to as emersion), evacuation, and then transfer into vacuum for analysis.Through this Account, we highlight our group's activities using UHV-EC to bridge electrochemistry with UHV-based X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) and scanning tunneling microscopy (STM). We provide a background and overview of the UHV-EC setup, and through illustrative examples, we convey what sorts of insights and information can be obtained. One notable advance is the use of ferrocene-terminated self-assembled monolayers as a spectroscopic molecular probe, allowing the electrochemical response to be correlated with the potential-dependent electronic and chemical state of the electrode-monolayer-electrolyte interfacial region. With XPS/UPS, we have been able to probe changes in the oxidation state, valence structure, and also the so-called potential drop across the interfacial region. In related work, we have also spectroscopically probed changes in the surface composition and screening of the surface charge of oxygen-terminated boron-doped diamond electrodes emersed from high-pH solutions. Finally, we will give readers a glimpse into our recent progress regarding real-space visualizations of electrodes following electrochemistry and emersion using UHV-based STM. We begin by demonstrating the ability to visualize large-scale morphology changes, including electrochemically induced graphite exfoliation and the surface reconstruction of Au surfaces. Taking this further, we show that in certain instances atomically resolved specifically adsorbed anions on metal electrodes can be imaged. In all, we anticipate that this Account will stimulate readers to advance UHV-EC approaches further, as there is a need to improve our understanding concerning the guidelines that determine applicable electrochemical systems and how to exploit promising extensions to other UHV methods.

Tailored Heterojunction Active Sites for Oxygen Electrocatalyst Promotion in Zinc-Air Batteries

Rechargeable zinc-air batteries (ZABs) are promising energy storage systems due to their low-cost and safety. However, the working principle of ZABs is based on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which display sluggish kinetic and low stability. Herein, this work proposes a novel method to design a heterogeneous CoP/CoO electrocatalyst on mesopore nanobox carbon/carbon nanotube (CoP/CoO@MNC-CNT) that enriched active sites and synergistic effect. Moreover, the well-defined heterointerfaces could lower the energy barrier for intermediate species adsorption and promote OER and ORR electrochemical performances. The CoP/CoO@MNC-CNT electrocatalyst presents a high half-wave potential of 0.838 V for ORR and a small overpotential of 270 mV for OER. The ZABs-based CoP/CoO@MNC-CNT air-cathode shows an open-circuit voltage of 1.409 V, the long-term cycle life of 500 h with a small voltage difference change of 7.7%. Additionally, the flexible ZABs exhibit highly mechanical stability, demonstrating their application potential in wearable electronic devices.

A versatile electrochemical cell for hanging meniscus or flow cell measurement of planar model electrodes characterized with scanning tunneling microscopy and x-ray photoelectron spectroscopy

We demonstrate the development of a portable electrochemistry (EC) cell setup that can be applied to measure relevant electrochemical signals on planar samples in conjunction with pre- and post-characterization by surface science methods, such as scanning tunneling microscopy and x-ray photoelectron spectroscopy. The EC cell setup, including the transfer and EC cell compartments, possesses the advantage of a small size and can be integrated with standard ultra-high vacuum (UHV) systems or synchrotron end-stations by replacing the flange adaptor, sample housing, and transfer arm. It allows a direct transfer of the pre-characterized planar sample from the UHV environment to the EC cell to conduct in situ electrochemical measurements without exposing to ambient air. The EC cell setup can operate in both the hanging meniscus and flow cell mode. As a proof of concept, using a Au(111) single crystal electrode, we demonstrate the application of the EC cell setup in both modes and report on the post-EC structure and chemical surface composition as provided by scanning tunneling microscopy and x-ray photoelectron spectroscopy. To exemplify the advantage of an in situ EC cell, the EC cell performance is further compared to a corresponding experiment on a Au(111) sample measured by transfer at ambient conditions. The EC cell demonstrated here enables a wealth of future electrocatalysis measurements that combine surface science model catalyst approaches to facilitate the understanding of nano- and atomic-scale structures of electrocatalytic interfaces, the crucial role of catalyst stability, and the nature of low-concentration and atomically dispersed metal (single atom) dopants.

Probing consequences of anion-dictated electrochemistry on the electrode/monolayer/electrolyte interfacial properties

Altering electrochemical interfaces by using electrolyte effects or so-called "electrolyte engineering" provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going "beyond cyclic voltammetry" where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing "snapshots" into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc⁺) redox process including ion-pairing with counter anions (Fc⁺-anion) caused by differences in Fc⁺-anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.

Operando X-Ray Spectroscopic Techniques: A Focus on Hydrogen and Oxygen Evolution Reactions

The study of structural as well as chemical properties of an electrocatalyst in its reaction environment is a challenge in electrocatalysis. This is very important for the better understanding of the dynamic changes in the reactivity with respect to the structure of catalysts to give insight into the reaction mechanism. The in situ/operando investigation of electrode/electrolyte interface has been increasingly explored in recent days due to the significant developments in technology. The review focus on operando X-ray spectroscopic techniques to understand the behavior of electrocatalysts in hydrogen evolution and oxygen evolution reactions (HER and OER). Some recent studies on the application of operando X-ray spectroscopic methods to study the dynamic nature as well as the evaluation of structural and chemical changes of the electrocatalysts for HER and OER in different reaction environment are discussed.

Preparation of complex model electrocatalysts in ultra-high vacuum and transfer into the electrolyte for electrochemical IR spectroscopy and other techniques

Model studies at complex, yet well-defined electrodes can provide a better understanding of electrocatalytic reactions. New experimental devices are required to prepare such model electrocatalysts with atomic-level control. In this work, we discuss the design of a new setup, which enables the preparation of well-defined electrocatalysts in ultra-high vacuum (UHV) using the full portfolio of surface science techniques. The setup allows for direct transfer of samples from UHV and the immersion into the electrolyte without contact to air. As a special feature, the single crystal sample is transferred without any sample holder, which makes the system easily compatible with most electrochemical in situ methods, specifically with electrochemical infrared reflection absorption spectroscopy, but also with other characterization methods such as single-crystal cyclic voltammetry, differential electrochemical mass spectrometry, or electrochemical scanning tunneling microscopy. We demonstrate the preparation in UHV, the transfer in inert atmosphere, and the immersion into the electrolyte for a complex model catalyst that requires surface science methods for preparation. Specifically, we study Pt nanoparticles supported on well-ordered Co₃O₄(111) films which are grown on an Ir(100) single crystal. In comparison with reference experiments on Pt(111), the model catalyst shows a remarkably different adsorption and reaction behavior during CO electrooxidation in alkaline environments.

Discerning the Redox-Dependent Electronic and Interfacial Structures in Electroactive Self-Assembled Monolayers

We explore the redox-dependent electronic and structural changes of ferrocene-terminated self-assembled monolayers (Fc SAMs) immersed in aqueous solution. By exploiting X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS), we can electrochemically control the Fc SAMs and spectroscopically probe the induced changes with the ferrocene/ferrocenium (Fc/Fc⁺) redox center (Fe oxidation state), formation of 1:1 Fc⁺-ClO₄^- ion pairs, molecular orientation, and monolayer thickness. We further find the insignificant involvement of interfacial water in the Fc SAMs irrespective of redox state. Electrolyte dependencies could be identified with 0.1 M NaClO₄ and HClO₄ when probing partially oxidized Fc/Fc⁺ SAMs. Corroborating the occurrence of electrochemically induced oxidation, EC-UPS shows that oxidation to Fc⁺ is accompanied by a shift of the highest occupied molecular orbital toward higher binding energy. The oxidation to Fc⁺ is also met with an increase in work function ascribed to the induced negative interfacial dipole caused by the presence of Fc⁺-ClO₄^- ion pairs along with a contribution from the reorientation of the Fc⁺ SAMs. The reversibility of our observations is confirmed upon conversion from Fc⁺ back to the neutral Fc. The approach shown here is beneficial for a broad range of redox-responsive systems to aid in the elucidation of structure-function relationships.

A combined UHV-STM-flow cell set-up for electrochemical/electrocatalytic studies of structurally well-defined UHV prepared model electrodes

Atomic scale structural characterization and electrochemical measurements under controlled electrolyte transport conditions are the basis for structure–activity relationships in nanostructured electrodes.

Unravelling the electrochemical double layer by direct probing of the solid/liquid interface

The electrochemical double layer plays a critical role in electrochemical processes. Whilst there have been many theoretical models predicting structural and electrical organization of the electrochemical double layer, the experimental verification of these models has been challenging due to the limitations of available experimental techniques. The induced potential drop in the electrolyte has never been directly observed and verified experimentally, to the best of our knowledge. In this study, we report the direct probing of the potential drop as well as the potential of zero charge by means of ambient pressure X-ray photoelectron spectroscopy performed under polarization conditions. By analyzing the spectra of the solvent (water) and a spectator neutral molecule with numerical simulations of the electric field, we discern the shape of the electrochemical double layer profile. In addition, we determine how the electrochemical double layer changes as a function of both the electrolyte concentration and applied potential.

Using "Tender" X-ray Ambient Pressure X-Ray Photoelectron Spectroscopy as A Direct Probe of Solid-Liquid Interface

We report a new method to probe the solid-liquid interface through the use of a thin liquid layer on a solid surface. An ambient pressure XPS (AP-XPS) endstation that is capable of detecting high kinetic energy photoelectrons (7 keV) at a pressure up to 110 Torr has been constructed and commissioned. Additionally, we have deployed a "dip &pull" method to create a stable nanometers-thick aqueous electrolyte on platinum working electrode surface. Combining the newly constructed AP-XPS system, "dip &pull" approach, with a "tender" X-ray synchrotron source (2 keV-7 keV), we are able to access the interface between liquid and solid dense phases with photoelectrons and directly probe important phenomena occurring at the narrow solid-liquid interface region in an electrochemical system. Using this approach, we have performed electrochemical oxidation of the Pt electrode at an oxygen evolution reaction (OER) potential. Under this potential, we observe the formation of both Pt(2+) and Pt(4+) interfacial species on the Pt working electrode in situ. We believe this thin-film approach and the use of "tender" AP-XPS highlighted in this study is an innovative new approach to probe this key solid-liquid interface region of electrochemistry.

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.

Reversible Strain in Porous Metals Charged in Electrolytes

Nanoporous metal samples with millimetre size were prepared either by compacting nanocrystalline powders or by dealloying, the dissolution of the less noble metal(s) of an alloy. The samples were immersed in an electrolyte, and their length was measured as a function of the applied potential in-situ in a dilatometer. The results obtained for nanocrystalline platinum, nanoporous gold and for gold platinum alloys show that the length varies in dependence of the surface charge. The strain amplitude of nanocrystalline platinum was 0.15%, and even larger strains have been measured using an Au-Pt alloy. This strain is comparable to commercial piezoceramics, but it is achieved using smaller voltages.

The strain measured for nanoporous gold prepared by dealloying was smaller than that mainly due to the larger structure size (20 nm structure size compared to 6 nm Pt crystallite size), but in the case of gold, it was possible to prepare stable composite structures of a metal foil and of the nanoporous gold. If such a bimetallic foil is charged, it is found to bend. Due to the mechanical amplification of the contraction or expansion of the nanoporous part of the foil, it was possible to observe the effect of electric charges on the surface stress of metals directly with the naked eye for the first time.

These results demonstrate that nanoporous metals might be useful for actuator applications and for the study of surface strain effects. Furthermore, they are the first realization of a general concept that suggests that most of the properties of conducting nanomaterials can be tuned by controlling the surface charge.

XPS at solid-aqueous solution interface

Application of X-ray Photoelectron Spectroscopy (XPS) to study the solid-aqueous solution interface is reviewed. XPS provides complementary physicochemical information about electrical double layer from the perspective of the solid surface. Experimental techniques, such as differential pumping, controlled adsorption/co-adsorption, freeze-drying, and fast-freezing, are discussed for both electrochemical and dielectric solid-solution interfaces. The use of fast-freezing, as applied to wet pastes centrifuged from aqueous suspensions, makes it possible to approach a real solid-solution interface in UHV conditions. XPS data allow estimation of the surface density of counter-ions, surface point of zero charge, and in some cases the measurement of surface potential. Interfacial chemical reactions such as ion pair formation, specific adsorption and ligand exchange can be directly observed. The technique is easy to apply to any suspension including colloids and gels of inorganic or organic nature, and can be adapted for electrochemistry as complementary to traditional "emersed electrode" studies.

Structure and electrochemistry of 4,4'-dithiodipyridine self-assembled monolayers in comparison with 4-mercaptopyridine self-assembled monolayers on Au(111)

4,4'-Dithiodipyridine (PySSPy) monolayers on Au(111) were investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). The studies were performed in solutions of different anions and pHs (0.1 M H2SO4, 0.1 M HClO4, 0.1 and 0.01 M Na2SO4, 0.1 and 0.01 M NaOH). The cyclic current-potential curves in H2SO4 show current peaks at about 0.4 V, which are absent for all other electrolytes at this potential. The XPS data suggest that PySSPy adsorbs via the S endgroup on the gold surface and the S-S bond breaks during adsorption. From the chemical shift of the N(ls) peak, it is concluded that in acidic media the self-assembled monolayer (SAM) is fully protonated, whereas in basic solution it is not. The pKa is estimated to be 5.3. STM studies reveal the existence of highly ordered superstructures for the SAM. In Na2SO4 and H2SO4, a (7 x mean square root of 3) structure is proposed. However, whereas in Na2SO4 solutions the superstructure does not change with potential, in 0.1 M H2SO4 the superstructure is observed only negative of the current peak at +0.4 V. At more positive potentials, the film becomes disordered. The results are compared to those for 4-mercaptopyridine (PyS) SAMs. XPS experiments and current-potential curves indicate that both molecules adsorb in the same manner on Au(111), that is, even in the case of PySSPy the adspecies is PyS. The STM results, however, call for a more subtle interpretation. While in Na2SO4 solutions the observed superstructures are the same for both SAMs, markedly different structures are found for PySSPy and PyS SAMs in 0.1 M H2SO4.