Single Crystal Electrode Surfaces Research Articles

Cover Feature: An Interfacial Study of Au(111) Electrodes in Deep Eutectic Solvents (ChemElectroChem 14/2022)

The Cover Feature illustrates the interface between an Au(111) single crystal electrode surface with deep eutectic solvents (DES) of different types and composition. The different systems were studied with cyclic voltammetry as well as electrochemical impedance spectroscopy, revealing important and interesting new insights into the electrode/electrolyte-interfaces. More information can be found in the Research Article by S. J. Zeller et al.

In Situ Raman Study of CO Electrooxidation on Pt(hkl) Single-Crystal Surfaces in Acidic Solution.

The adsorption and electrooxidation of CO molecules at well-defined Pt(hkl) single-crystal electrode surfaces is a key step towards addressing catalyst poisoning mechanisms in fuel cells. Herein, we employed in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) coupled with theoretical calculation to investigate CO electrooxidation on Pt(hkl) surfaces in acidic solution. We obtained the Raman signal of top- and bridge-site adsorbed CO* molecules on Pt(111) and Pt(100). In contrast, on Pt(110) surfaces only top-site adsorbed CO* was detected during the entire electrooxidation process. Direct spectroscopic evidence for OH* and COOH* species forming on Pt(100) and Pt(111) surfaces was afforded and confirmed subsequently via isotope substitution experiments and DFT calculations. In summary, the formation and adsorption of OH* and COOH* species plays a vital role in expediting the electrooxidation process, which relates with the pre-oxidation peak of CO electrooxidation. This work deepens knowledge of the CO electrooxidation process and provides new perspectives for the design of anti-poisoning and highly effective catalysts.

Reprint of "Electrochemical intercalator binding to single- and double-strand DNA- and LNA-based molecules on Au(111)-electrode surfaces"

We have explored and compared interfacial electrochemical electron transfer (ET) of three organic aromatic redox molecules, methylene blue (MB), resorufin (RSF), and anthraquinone monosulfonate (AQMS), non-covalently bound to single- and double-strand (ss and ds) DNA- and LNA-based (“locked nucleic acids”) oligonucleotides (ONs). The redox probes were chosen for their equilibrium potentials and particularly for their different electrostatic charges. The ONs were linked to Au(111)-surfaces via AuS bonds, diluted by mercaptohexanol (MCH) which blocks direct redox probe electrochemical electron transfer and ensures that space is left for in situ ds formation. The study combines nucleic acid synthesis, electrochemistry at single-crystal electrode surfaces, and scanning tunnelling microscopy under electrochemical control (in situ STM).The three redox molecules bind differently to the ONs. Electrostatic binding dominates for positively charged MB. ET through the MB-marked LNA ONs appears but no ss/ds LNA ON difference could be distinguished. Electrostatically neutral RSF binds in π-π stacking or other non-electrostatic, non-covalent modes onto both DNA and LNA ONs. RSF voltammetric signals on MCH-diluted ON adlayers could be associated with “long-range” ET. There is no electrochemical RSF ss/ds difference on LNA ONs, but clear ss/ds distinction on RSF binding to DNA ONs with a strong RSF signal for ds and no signal for ss. Negatively charged AQMS also binds in intercalating, non-electrostatic modes with clear ds voltammetric signals of both DNA and LNA but no ss signal.

A Detrimental Effect of Acetonitrile on the Kinetics of Underpotentially Deposited Hydrogen and Hydrogen Evolution Reaction, Examined on Pt Electrode in H2SO4 and NaOH Solutions

The present paper reports AC impedance spectroscopic/Tafel polarization and cyclic voltammetry study on the influence of acetonitrile concentration on the kinetics of UPD of H (underpotential deposition of hydrogen) and HER (hydrogen evolution reaction), examined on polycrystalline and polyoriented single-crystal sphere Pt electrode surfaces in 0.5 M H2SO4 and 0.1 M NaOH supporting solutions. The resulted data provided confirmation of the destructive role of Pt surface-electrosorbed acetonitrile on the kinetics of underpotentially deposited hydrogen, as well as cathodic hydrogen evolution reaction. The above was exclusively elucidated through evaluation of the associated charge-transfer resistance and capacitance (and complementary exchange current-density and Tafel slope) parameters, derived comparatively on Pt for pure and acetonitrile-modified acidic and alkaline electrolytes.

Electrochemical intercalator binding to single- and double-strand DNA- and LNA-based molecules on Au(111)-electrode surfaces

We have explored and compared interfacial electrochemical electron transfer (ET) of three organic aromatic redox molecules, methylene blue (MB), resorufin (RSF), and anthraquinone monosulfonate (AQMS), non-covalently bound to single- and double-strand (ss and ds) DNA- and LNA-based (“locked nucleic acids”) oligonucleotides (ONs). The redox probes were chosen for their equilibrium potentials and particularly for their different electrostatic charges. The ONs were linked to Au(111)-surfaces via AuS bonds, diluted by mercaptohexanol (MCH) which blocks direct redox probe electrochemical electron transfer and ensures that space is left for in situ ds formation. The study combines nucleic acid synthesis, electrochemistry at single-crystal electrode surfaces, and scanning tunnelling microscopy under electrochemical control (in situ STM).The three redox molecules bind differently to the ONs. Electrostatic binding dominates for positively charged MB. ET through the MB-marked LNA ONs appears but no ss/ds LNA ON difference could be distinguished. Electrostatically neutral RSF binds in π-π stacking or other non-electrostatic, non-covalent modes onto both DNA and LNA ONs. RSF voltammetric signals on MCH-diluted ON adlayers could be associated with “long-range” ET. There is no electrochemical RSF ss/ds difference on LNA ONs, but clear ss/ds distinction on RSF binding to DNA ONs with a strong RSF signal for ds and no signal for ss. Negatively charged AQMS also binds in intercalating, non-electrostatic modes with clear ds voltammetric signals of both DNA and LNA but no ss signal.

In situ probing electrified interfacial water structures at atomically flat surfaces.

Solid/liquid interfaces are ubiquitous in nature and knowledge of their atomic-level structure is essential in elucidating many phenomena in chemistry, physics, materials science and Earth science1. In electrochemistry, in particular, the detailed structure of interfacial water, such as the orientation and hydrogen-bonding network in electric double layers under bias potentials, has a significant impact on the electrochemical performances of electrode materials2-4. To elucidate the structures of electric double layers at electrochemical interfaces, we combine in situ Raman spectroscopy and ab initio molecular dynamics and distinguish two structural transitions of interfacial water at electrified Au single-crystal electrode surfaces. Towards negative potentials, the interfacial water molecules evolve from structurally 'parallel' to 'one-H-down' and then to 'two-H-down'. Concurrently, the number of hydrogen bonds in the interfacial water also undergoes two transitions. Our findings shed light on the fundamental understanding of electric double layers and electrochemical processes at the interfaces.

Initial stages of silver electrodeposition on single crystal electrodes from ionic liquids

We present a comprehensive study on the initial stages of silver electrodeposition on Pt (111), Au (111), and Au (100) single crystal surfaces in low-viscosity ionic liquids (ILs) containing dicyanamide anions: 1-butyl-1-methylpyrrolidinium dicyanamide [BMP][DCA] and 1-butyl-3-methylimidazolium dicyanamide [BMIm][DCA]. Electrochemical methods in combination with in situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) are employed to explore the Ag underpotential (upd) and overpotential (opd) deposition processes as well as the stability of the single crystal electrode surfaces in the absence of Ag+ ions. The substrate material is shown to significantly affect the mechanism of Ag deposit nucleation and growth in the ILs. While no Ag upd is detected on Pt (111), the formation of a Ag upd monolayer on a Au (111) electrode in both ILs is clearly visualized by in situ STM. The Ag adlayer formation on the Au electrodes in the underpotential regime facilitates Ag opd, which starts on Au (111) and Au (100) at much less negative potentials than on Pt (111). There is an excellent agreement between the electrochemical (voltammetry and chronoamperometry), AFM and STM data, demonstrating the nucleation and growth of individual Ag crystallites on Pt (111) according to the Volmer–Weber mechanism and the layer–by–layer growth of Ag deposit on Au (111) and Au (100). Only at high overpotentials, the Ag growth on the gold electrodes switches to the Stranski-Krastanov mode involving the appearance of 3D crystallites.

Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces.

Metalloporphyrins are active sites in metalloproteins and synthetic catalysts. They have also been studied extensively by electrochemistry as well as being prominent targets in electrochemical scanning tunneling microscopy (STM). Previous studies of FePPIX adsorbed on graphite and alkylthiol modified Au electrodes showed a pair of reversible Fe(III/II)PPIX peaks at about -0.41 V (vs NHE) at high solution pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating probe for long-range electrochemical electron transfer through a G-quadruplex oligonucleotide (DNAzyme); this study disclosed two, rather than a single pair of voltammetric peaks with a new and dominating peak, shifted 200 mV positive relative to the ≈-0.4 V peak. Prompted by this unexpected observation, we report here a study of the voltammetry of FePPIX itself on single-crystal Au(111), (100), and (110) and polycrystalline Au electrode surfaces. In all cases the dominating pair of new Fe(III/II)PPIX redox peaks, shifted positively by more than 200 mV compared to those of previous studies appeared. This observation is supported by density functional theory (DFT) which shows that strong dispersion forces in the FePPIX/Au electronic interaction drive the midpoint potential toward positive values. The FePPIX spin states depend on interaction with the Au(111) interface, converting all the Fe(II)/(III)PPIX species into low-spin states. These results support electrochemical evidence for the nature of the electronic coupling between FePPIX and Au-surfaces, and the electronic states of adsorbate molecules, with a bearing also on recent reports of magnetic FePPIX/Au(111) interactions in ultrahigh vacuum (UHV).

In situ SERS and SHINERS study of electrochemical hydrogenation of p-ethynylaniline in nonaqueous solvents

Surface-enhanced Raman spectroscopy (SERS) studies of electrode/solution interfaces are important for understanding electrochemical processes. However, revealing the nature of reactions at well-defined single crystal electrode surfaces, which are SERS-inactive, remains challenging. In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used for the first time to study electrochemical adsorption and hydrogenation reactions at single crystal surfaces in nonaqueous solvents. A roughened Au surface was also studied for comparison. The experimental results show that the hydrogenation of adsorbed p-ethynylaniline (PEAN) on roughened Au electrode surfaces occurred at very negative potentials in methanol because of the catalytic effect of surface plasmon resonance (SPR). However, because “hot electrons” were blocked by the silica shell of Au@SiO2 nanoparticles and aprotic acetonitrile was an ineffective hydrogen source, surface reactions at Au(111) were inhibited in the systems studied. Density functional theory (DFT) calculations revealed that the PEAN triple bond opened, allowing adsorption in a flat configuration on the Au(111) surface via two carbon atoms. This work provides an advanced understanding of electrochemical interfacial processes at single crystal surfaces in nonaqueous systems.

Plasmon‐Enhanced Spectroscopies with Shell‐Isolated Nanoparticles

Shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS), due to its versatility, has been able to break the long‐term limitations of the material‐ and substrate‐specific generalities in the traditional field of surface‐enhanced Raman spectroscopy. With a shell‐isolated work principle, this method provides an opportunity to investigate successfully in surface, biological systems, energetic materials, and environmental sciences. Both the shell material and core morphology are being improved continuously to meet the requirements in diverse systems, such as the electrochemical studies at single crystal electrode surfaces, in situ monitoring of photoinduced reaction processes, practical applications in energy conversion and storage, inspections in food safety, and the surface‐enhanced fluorescence. Predictably, the concept of shell‐isolated nanoparticle‐enhancement could be expanded to the wider range for the performance of plasmon‐enhanced spectral modifications.

Potential dependent thiocyanate adsorption on gold electrodes: a comparison study between SERS and SHINERS

Potential dependent adsorption of target molecules on electrode surface has long been analyzed by several analytical techniques at the electrochemical interfaces. Here, the adsorption of thiocyanate (SCN−) on gold electrodes [Au (111) and Au (poly)] is investigated by electrochemical shell isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) and surface-enhanced Raman spectroscopy. Based on the experimental observation, C − N stretching mode of N-bound SCN− can be observed around 2080 cm−1 throughout the whole potential range. The band corresponding to νC−N of S-bound SCN− appears as a shoulder at more negative potentials, and as a well-defined band are more positive potentials. However, the overlapped bands provoke a negative shift in the frequency of S-bound thiocyanate. Therefore, a change in the calculated Stark slope is observed. Interestingly, SHINERS has been employed to demonstrate the thiocyanate orientation and its effect on Raman spectra. Our results widen the opportunities of SHINERS to unravel the potential-dependent adsorption behavior of target molecules on single-crystal electrode surfaces. Copyright © 2016 John Wiley & Sons, Ltd.

Structure and Stability of Underpotentially Deposited Ag on Au(111) in Alkaline Electrolyte

A combination of in situ surface X-ray scattering (SXS) and cyclic voltammetry (CV) measurements have been performed to understand the structure and behavior of underpotentially deposited (UPD) Ag layers on Au(111) following transfer of the electrode to alkaline electrolyte (0.1 M KOH). UPD of Ag from 0.05 M H2SO4 + 1 mM Ag2SO4 electrolyte was used to form a monolayer and bilayer film of Ag on Au(111) before immersion into 0.1 M KOH electrolyte. Analysis of the SXS data shows that, while the bilayer Ag film is stable upon transfer to the alkaline electrolyte, the Ag monolayer film reorders to a partial bilayer structure upon or during the transfer process. The Ag-modified Au(111) electrode shows a potential response for OH– adsorption that is similar to that of Ag single crystal electrode surfaces.

Explicit Detection of the Mechanism of Platinum Nanoparticle Shape Control by Polyvinylpyrrolidone

The irreversible adsorption of polyvinylpyrrolidone (PVP) on a series of well-defined platinum single crystal electrode surfaces has been investigated using voltammetry, ex situ XPS and DFT calculations. It is found that the adsorption of PVP is highly structure sensitive with strong adsorption exhibited by step and {100} terrace sites with only weak interactions observed between PVP and Pt{111} terraces, at least at low PVP surface concentrations. Subsequent investigations using CO electrooxidation confirmed that blocking of platinum surface sites by PVP toward CO chemisorption was marked for Pt{100} terraces but hardly occurred at all at Pt{111} terraces. Density Functional Theory calculations also confirmed that the monomer of PVP interacts more strongly with Pt{100} compared to Pt{111} sites (−142 and −125 kJ mol–1 respectively). Ex situ XPS studies suggested that the main PVP–Pt interaction is associated with charge transfer from the carbonyl substituent of PVP toward the metal surface in accordance ...

Formation of 2,2′-bipyridine adlayers at Sb(111)|ionic liquid + 2,2′-bipyridine solution interface

Abstract In situ scanning tunneling microscopy (STM), cyclic voltammetry and electrochemical impedance spectroscopy methods have been applied to study the adsorption of 2,2′-bipyridine (2,2′-BP) on Sb(111) single crystal electrode surface from 1-ethyl-3-methylimidazolium tetrafluoroborate + 1% 2,2′-BP solution. Influence of the Sb(111) electrode potential on the adsorption characteristics of 2,2′-BP has been demonstrated. The in situ STM data revealed that 2,2′-BP forms highly ordered patterns on the Sb(111) single crystal electrode surface. The formed dense organic adlayer decreases the dielectrical permittivity of the interface, acting as a dielectric capacitor with significantly lower characteristic relaxation time than the pure ionic liquid system.

Adsorption of 4,4′−bipyridine on the Cd(0001) single crystal electrode surface

In situ scanning tunneling microscopy (STM), electrochemical impedance spectroscopy have been applied to study the adsorption of 4,4′–bipyridine (4,4′–BP) on the Cd(0001) single crystal electrode from 0.1 M Na2SO4+1.0×10−5 M H2SO4+x M 4,4′–BP aqueous solutions (x from 0.0 to 3.0×10−3). Influence of the electrode potential (E) on the adsorption characteristics of 4,4′–BP on Cd(0001) has been demonstrated. Two regions with low capacitance values have been observed in the series capacitance vs. E curve. The in situ STM data revealed that 4,4′–BP forms a stripe pattern on the Cd(0001) single crystal electrode surface. The results of density functional theory (DFT) calculations and surface enhanced infrared adsorption spectroscopy data have been applied in order to establish the adsorption layer structure and possible orientation of the 4,4′–BP molecules on the Cd(0001) surface. DFT calculations, used to compliment the experimental data, demonstrate the importance of the van der Waals interactions for the formation of the stripe pattern.

In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy.

Identifying the intermediate species in an electrocatalytic reaction can provide a great opportunity to understand the reaction mechanism and fabricate a better catalyst. However, the direct observation of intermediate species at a single crystal surface is a daunting challenge for spectroscopic techniques. In this work, electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) is utilized to in situ monitor the electrooxidation processes at atomically flat Au(hkl) single crystal electrode surfaces. We systematically explored the effects of crystallographic orientation, pH value, and anion on electrochemical behavior of intermediate (AuOH/AuO) species. The experimental results are well correlated with our periodic density functional theory calculations and corroborate the long-standing speculation based on theoretical calculations in previous electrochemical studies. The presented in situ electrochemical SHINERS technique offers a unique way for a real-time investigation of an electrocatalytic reaction pathway at various well-defined noble metal surfaces.

In Situ STM Studies of Electrochemically Polished Cd(0001) Electrode in 1-Ethyl-3-Methylimidazolium Tetrafluoroborate

Recently studies of room temperature ionic liquids (RTIL) have had a lot of attention in modern chemical research. Due to their high stability under applied potential, the wide electrochemical window and dual usability as solvent and electrolyte RTILs have become very attractive in the field of applied electrochemistry and modern energetics [1]. The performance of RTIL based devices, i.e. the electrochemical applications of RTILs depend on the interaction between the RTIL and the geometrically rough and energetically heterogeneous interfaces. Hence, a detailed understanding of the RTIL|solid electrode interface structure, thermodynamics and kinetics is crucial for designing modern electrochemical devices. The electrochemical kinetics and adsorption properties of various interfacial charge transfer processes at the solid surfaces depend greatly not only on the chemical composition but also on the geometrical characteristics and morphology of the surface studied. According to our knowledge, there are no data for Cd (0001) plane obtained under the electrochemical in situ STM (scanning tunneling microscopy) conditions in RTIL in the literature. Therefore, the main aim of these investigations was to develop the experimental conditions needed to obtain the atomic resolution electrochemical in situ STM data within the wide electrode potential region of the Cd(0001) electrode similarly to our previous works analyzing atomic resolution level structure of the Cd(0001) single crystal electrode in aqueous surface inactive electrolyte solution [2]. The electrochemically polished Cd (0001) plane has been submerged under cathodic polarization (E = −1.1 V vs. Ag|AgCl) into the EMImBF4, previously saturated with Ar (99.999%) inside the glove box (Ar 99.999% atmosphere). The self-made hermetic three-electrode cell with large Pt counter electrode and Ag|AgCl reference electrode, connected to the in situSTM cell through Luggin capillary, has been used. During the measurements atomically flat areas have been observed on the in situ STM images of cadmium (0001) single crystal electrode surface (Fig. 1). Also in situ STM atomic resolution images were obtained for the electrochemically polished Cd (0001) electrode. According to these data, the regular atomic structure can be observed with interatomic distances d = 2.9 ± 0.1 Å, which are in a good agreement with Cd (0001) crystallographic parameters applying hexagonal close-packed structure model for Cd (0001) plane. It was also determined that the structure of nanometric terraces separated by the steps, is very stable during few hours under the cathodic polarization from –1.6 < E < –0.9 V. Acknowledgements: This study was partially funded by the Estonian Science Foundation Grants Nos.7791 and 8357, Estonian Energy Technology Program Project SLOKT10209T, Estonian Basic Research Project SF0180002s08 and Estonian Centers of Excellence in Science Project: High-technology Materials for Sustainable Development TK117.

Cyclic voltammetry and oxygen reduction activity of the Pt{1 1 0}-(1 × 1) surface

A Pt{110}-(1×1) single-crystal electrode surface was created by flame annealing and cooling of the electrode in gaseous CO. For the first time, the voltammetry of this unreconstructed surface is reported using aqueous perchloric acid and sodium hydroxide electrolytes. The voltammetric response for Pt{110}-(1×1) produces marked differences when compared with the reconstructed, H2- and N2-cooled, disordered Pt{110}-(1×2) surface phases. Pt{110}-(1×1) exhibits as many as 6 individual peaks in the low potential region (0–0.25V vs. Pd/H), a singular sharp oxidation peak at 0.95V (Pd/H) corresponding to the electrosorption of oxide and almost zero current associated with OH formation between 0.6V (Pd/H) and 0.9V (Pd/H). Charge density curves indicate that the total charge passed between 0V (Pd/H) and 1.5V (Pd/H) to be almost identical for both the (1×1) and the disordered (1×2) phases in perchloric acid, sulphuric acid and sodium hydroxide electrolyte. The oxygen reduction reaction (ORR) activity of the (1×1) surface phase was also examined using rotating disc electrode voltammetry in the hanging meniscus configuration. The half-wave ORR peak potential was found to be ∼30mV more negative than for the disordered reconstructed surface. This leads to the conclusion that the activity of the unreconstructed basal planes of platinum towards the ORR follows the order {100}<{110}<{111} when E1/2 is used as a measure of activity and that the higher activity usually ascribed to Pt{110} over Pt{111} is actually a manifestation of the disordered (1×2) surface phase in which step sites and defects promote ORR.

Electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy: correlating structural information and adsorption processes of pyridine at the Au(hkl) single crystal/solution interface.

Electrochemical methods are combined with shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) for a comprehensive study of pyridine adsorption on Au(111), Au(100) and Au(110) single crystal electrode surfaces. The effects of crystallographic orientation, pyridine concentration, and applied potential are elucidated, and the formation of a second pyridine adlayer on Au(111) is observed spectroscopically for the first time. Electrochemical and SHINERS results correlate extremely well throughout this study, and we demonstrate the potential of EC-SHINERS for thorough characterization of processes occurring on single crystal surfaces. Our method is expected to open up many new possibilities in surface science, electrochemistry and catalysis. Analytical figures of merit are discussed.

Interaction of water with methanesulfonic acid on Pt single crystal electrodes

The electrochemical behavior of methanesulfonic acid on platinum single crystal electrode surfaces is investigated by cyclic voltammetry and infrared spectroscopy measurements. The results are compared with the voltammetric profiles of perchloric and trifluoromethanesulfonic acids. The differences are interpreted in terms of the effect of the anion on the structure of water. No adsorbed species are detected by infrared spectroscopy.