Electrochemical Scanning Tunneling Microscopy Study Research Articles

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.

Towards Size‐Controlled Deposition of Palladium Nanoparticles from Polyoxometalate Precursors: An Electrochemical Scanning Tunneling Microscopy Study

AbstractWe present a novel in situ electrochemical approach to deposit small size‐controlled palladium(0) clusters from 12‐PdII‐oxo‐metalate precursors. These clusters are formed through the reductive surface polarization of a Au(111) support. Electrochemical scanning tunneling microscopy (EC‐STM) reveals that the electrochemical reduction occurs at much lower potentials than that for simple Pd salt solutions. The resulting particles are one atomic layer high and show a narrow size distribution. Precursor mass transport limitations and preconditioning of the solid‐liquid interface at low potentials influence the obtained morphology. In particular, a concomitant reduction mechanism via formation of molecular hydrogen is discussed. The deposited clusters show the typical behavior of small metallic Pd islands in EC‐STM: The apparent cluster height increases reversibly when lowering the potential close to the onset of hydrogen evolution, which is attributed to hydrogen adsorption.

Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water.

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe3O4(001)-(√2 × √2)R45° surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy and low-energy electron diffraction data show that extensive hydroxylation causes the surface to revert to a bulklike (1 × 1) termination. However, scanning tunneling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM studies leads us to believe that a similar structure exists at the solid-electrolyte interface during immersion at pH 7.

Surface Structure of 4-Mercaptopyridine on Au(111): A New Dense Phase.

4-Mercaptopyridine (4MPy) self-assembled on Au(111) has been studied by in situ electrochemical scanning tunneling microscopy (EC-STM) in HClO4, cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Samples prepared by varying the immersion time at constant concentration named short time (30 s) and long time (3 min) adsorption have been studied. Cyclic voltammetry and XPS showed that the chemistry of the adsorbed molecules does not depend on the adsorption time resulting in a well established chemisorbed thiol self-assembled monolayer on Au(111). EC-STM study of the short time adsorption sample revealed a new self-assembled structure after a cathodic desorption/readsorption sweep, which remains stable only if the potential is kept negative to the Au(111) zero charge potential (EPZC). DFT calculations have shown a correlation between the observed structure and a dense weakly adsorbed phase with a surface coverage of θ = 0.4 and a (5 × √3) lattice configuration. At potentials positive to the EPZC, the weakly adsorbed state becomes unstable, and a different structure is formed due to the chemisorption driven by the electrostatic interaction. Long time adsorption experiments, on the other hand, have shown the typical (5 × √3) structure with θ = 0.2 surface coverage (chemisorbed phase) and are stable over the whole potential range. The difference observed in long time and short time immersion can be explained by the optimization of molecular interactions during the self-assembly process.

In-Situ Scanning Tunneling Microscopy and Electrochemical Quartz Crystal Microbalance Studies of the SEI Formation on Graphite Electrodes

Electric vehicles and portable electronic devices depend on high performance energy storage technology like rechargeable Li ion batteries, whose successful operation requires high energy- and power-densities just as low cost, reliability, safety and long lifetime.1,2 The energy and power density of batteries, as well as their costs are mainly determined by the electrode materials, whereas safety, lifetime and efficiency are strongly affected by the properties of the Solid Electrolyte Interphase (SEI).3 In the past three decades research tried to deepen the understanding of the SEI by a large variety of experimental methods, ranging from basic electrochemistry over microscopic techniques to spectroscopic tools4. Questions regarding its formation mechanism, its morphological appearance and its chemical composition have been tackled, but there is still a lack of understanding on the molecular level. In the mid-1990s M. Inaba et al. carried out a series of experiments about the SEI-formation on model graphite electrodes and the co-intercalation of Li+-ions together with their solvation shell using Electrochemical Scanning Tunneling Microscopy (EC-STM).5 EC-STM is a very powerful technique for surface studies, since it offers the possibility to in-situ image molecular processes at the electrode/electrolyte interface. In the present study, the SEI-formation was examined in depth by in-situ EC-STM on highly oriented pyrolytic graphite (HOPG) model electrodes both in commercial Li+-ion battery electrolytes as well as alternative electrolyte systems (see Figure 1). In addition, the Electrochemical Quartz Crystal Microbalance technique (EQCM) as another in-situ method gave further insights into the SEI-formation. Both in-situ techniques provided valuable information about the electrochemical characteristics of this system, leading to better understanding of the formation mechanism and the morphological appearance of the SEI. Figure 1. In-Situ EC-STM study of the SEI-formation on HOPG in 1M LiPF6 in EC/DMC electrolyte: The surface is imaged while the potential (line) is scanned from 2.0 V vs. Li/Li+ to 1 V, held at this potential, and scanned back to 2 V. The y-axis of the STM image is converted to a time scale to correlate changes in morphology to the applied potential 1M. Armand, J.-M. Tarascon, Nature, 451 (2008) 652 2J. B. Goodenough, J. Solid State Electrochem, 16 (2012) 2019 3P. B. Balbuena, Y. Wang, Imperial College Press (2004), ISBN 1-86094-362-4 4H. Tavassol, J.W. Buthker, G.A. Ferguson, L.A. Curtiss, A.A. Gewirth, J. Electrochem. Soc., 159 (2012) A730-A738. 5M. Inaba, Z. Siroma, A. Funabiki, Z. Ogumi, T. Abe, Y. Mizutani, M. Asano, Langmuir, 19 (1996) 1535 Figure 1

Deposition of copper multilayers on Au(111) in sulfuric acid solution: An electrochemical scanning tunneling microscopy study

The co-adsorption of submono-, mono- and multi-layers of Cu with sulfate anions on a Au(111) electrode surface was investigated in electrochemical environment (0.1 mM CuSO4 + 0.1 M H2SO4) by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). Correlated with the STM investigations the CV measurements indicate co-adsorption/-desorption processes of Cu of submono-, mono- and multi-layer coverages with sulfuric acid species on Au(111). The formation of a quasi-hexagonal Moiré superstructure on terraces of copper multilayers was observed in situ by STM. In detail the observed Moiré-structure is similar but not identical to the one observed on the (111) surface of bulk copper. High resolution STM images show the formation of a 3×7-like sulfate structure on all multilayer copper terraces.

Influence of solvent on the chiral resolution of organic molecules on Au(111): EC-STM study of biphenyl dicarboxylic acid on Au(111) in an aqueous environment

Adsorption-induced chiral resolution of organic molecules is important due to its potential applications in stereo-selective catalysis. We studied the adsorption-induced chiral resolution using a model achiral molecule of 4,4′ biphenyl dicarboxylic acid (BPDA) on Au(111) in 0.1M perchloric acid (HClO4) by electrochemical scanning tunneling microscopy (EC-STM). Our experimental data showed that the BPDA molecules formed island structures with distinctive preferred orientations at the length scale of the molecular size. The molecules did not show any orientational ordering above the length scale, indicating that chiral resolution was absent in the aqueous environment. Previously, the molecules were found to have chiral resolution on Au(111) in ultra-high vacuum conditions (UHV). We calculated angle-dependent binding energy between the substrate and a BPDA molecule, the intermolecular interactions between the BPDA molecules, and their interactions with water molecules. The calculations suggest that the absence of chiral resolution in the aqueous environment originated from the decrease in the intermolecular energy of the BPDA molecules due to their hydrogen bonds with the surrounding water molecules. The strength of the hydrogen bonding between BPDA molecules was sufficient to overcome the energy barrier for chiral resolution through rotational motion in UHV, but not in an aqueous environment.

Comparative Electrochemical Scanning Tunneling Microscopy Study of Nonionic Fluorosurfactant Zonyl FSN Self-Assembled Monolayers on Au(111) and Au(100): A Potential-Induced Structural Transition

We investigate the structure of nonionic fluorosurfactant zonyl FSN self-assembled monolayers on Au(111) and Au(100) in 0.05 M H(2)SO(4) as a function of the electrode potential by electrochemical scanning tunneling microscopy (ECSTM). On Au(111), a (3(1/2) × 3(1/2))R30° arrangement of the FSN SAMs is observed, which remains unchanged in the potential range where the redox reaction of FSN molecules does not occur. On Au(100), some parallel corrugations of the FSN SAMs are observed, which originate from the smaller distance and the repulsive interaction between FSN molecules to make the FSN molecules deviate from the bridging sites, and ECSTM reveals a potential-induced structural transition of the FSN SAMs. The experimental observations are rationalized by the effect of the intermolecular interaction. The smaller distance between molecules on Au(100) results in the repulsive force, which increases the probability of structural change induced by external factors (i.e., the electrode potential). The appropriate distance and interactions of FSN molecules account for the stable structure of FSN SAMs on Au(111). Surface crystallography may influence the intermolecular interaction through changing the molecular arrangements of the SAMs. The results benefit the molecular-scale understanding of the behavior of the FSN SAMs under electrochemical potential control.

ECSTM study of adsorption of C60, C70, C86 and Y@C82 on Au(111)

The adsorption of a variety of fullerenes (C60, C70, C86, Y@C82) on Au(111) electrode surfaces was comprehensively investigated in 0.1 M HClO4 by electrochemical scanning tunneling microscopy (ECSTM). In the ordered C60’s adlayer, C60 molecules formed either (2\( \sqrt 3 \)×2\( \sqrt 3 \)) or “in-phase” structure. The high resolution STM image shows that the C60 cage is not simply round but shows a bifurcated feature. The adsorption orientation of C60 on Au(111) is tentatively suggested. In the ordered C70’s adlayer, the perpendicular fullerene molecules are the main adsorption mode and form (2\( \sqrt 3 \) ×2\( \sqrt 3 \)) structure. However, for C86 and Y@C82, the ordered adlayer could not be obtained on Au(111) under the present condition. These differences may be due to the different molecular shapes and sizes, and the encapsulated metal atom which affects the lattice matches with the substrate. The adsorption of fullerene molecules on Au(111) from disorder to order could be tuned simply by steering the dimensional sizes or shapes of the fullerenes used.

Changes in the structure of model biological membranes in the presence of perfluorooctanesulphonic acid – Electrochemical and EC-STM study

The changes caused by the presence of perfluorooctanesulphonic acid (PFOS) in the structure and electrochemical characteristics of model biological membranes composed of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) supported on gold electrodes were examined using electrochemical techniques and electrochemical scanning tunneling microscopy. Phospholipid mono and bilayers were prepared using the Langmuir–Blodgett (LB) technique alone or combined with of the Langmuir–Schaefer (LB/LS) techniques. Square wave voltammetry was employed to monitor the incorporation of PFOS into already existing model membranes. The accessibility of potassium ferrocyanides to the electrode surface depended on the time of contact of the bilayer with PFOS solutions. Electrostatic attraction between PFOS present in the model membrane and positively charged probes resulted in the adsorption of the probe on the lipid membrane modified electrode. Even highly soluble species, i.e. hexaamineruthenium complex revealed adsorption on the gold electrodes modified with DMPC bilayers containing PFOS. The presence of PFOS in model biological membranes was visualized by means of electrochemical scanning tunneling microscopy (EC-STM). Topographically higher PFOS-enriched domains were clearly distinguishable among lower pure DMPC regions.

ESTM study of nanostructure and surface energetic properties of individual chromium and nickel

Individual chromium and nickel in air and at interfaces to 0.01 N HCl and 0.1 N H2SO4 solutions are studied with the use of electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS) at the controlled potentials of the specimen and tip. The potential range of chromium specimens was 0.1 to 0.9 V in 0.01 N HCl and 0.3 to 1.1 V in 0.1 N H2SO4; the potential range of nickel was −0.01 to 0.64 V in 0.01 N HCl and 0.184 to 0.834 V (N.H.E.) in 0.1 N H2SO4. At the controlled potential, the surfaces in the studied acid solutions are smoother than in air. The potential of the specimen affects the electrophysical properties, which determine the electron tunneling transfer, more noticeably than the nanorelief of the metals. The obtained changes in the It,dZ spectra dependence on the distance in air on a field of 1 nm2 at a step of 0.04 nm enabled us to estimate the effective barrier of tunneling Kef between neighbor atoms, which is in good agreement with the known dependence of Kef on the work function.

EC-STM Study of Potential-Controlled Adsorption of Substituted Pyrimidinethiol on Au(111)

The ability of the pyrimidine derivatives to form numerous complexes and supramolecular assemblies makes them suitable for the construction of new functional surfaces. Therefore, in this paper, the adsorption behavior of 4-hydroxy-6-(trifluoromethyl)pyrimidine-2-thiol (HTPT) on a Au(111) surface has been investigated using electrochemical scanning tunneling microscopy (EC-STM). High-resolution imaging revealed that the HTPT molecules organize on a gold surface producing a highly ordered monolayer consistent with a (4 x radical3)R-30(0) superstructure. It has been observed that the arrangement of the molecules, as well as their orientation with respect to the substrate, remains stable over a relatively broad potential range from -0.40 to 0.55 V. It has been demonstrated that the presence of the functional groups attached to the aromatic ring affects the final structure of the HTPT adlayer, giving rise to the formation of the assembly with a uniform orientation of the molecules on the substrate.

Adsorption of TTF, TCNQ and TTF-TCNQ on Au(111): An in situ ECSTM study

The adsorption and adlayer structures of tetrathiofulvalene (TTF), tetracyanoquinodimethane (TCNQ) and TTF-TCNQ on Au(111) have been systematically investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry in 0.1 mol L−1 HClO4. All the three molecules were found to form well-ordered adlayers in the double-layer potential region of Au(111). For TTF and TCNQ adlayers, (6×3) and (4×7) structures have been observed, respectively. A structural transition was observed on TCNQ adlayer at potential negative of 0.08 V vs. the reversible hydrogen electrode (RHE), and induced a new phase with (3\( \sqrt 3 \) × 12) structure. On the other hand, the charge transfer complex, TTF-TCNQ, self-organized into ordered domains with a lamellar structure different from those of the pure TTF and TCNQ adlayers on Au(111). Its packing arrangement was comparable to surface structures of either single crystal or thin film of TTF-TCNQ.

Well‐ordered structure of methylene blue monolayers on Au(111) surface: Electrochemical scanning tunneling microscopy studies

Well-ordered structure of methylene blue (MB) monolayers on Au(111) surface has been successfully obtained by controlling the substrate potential. Electrochemical scanning tunneling microscopy (ECSTM) examined the monolayers of MB on Au(111) in 0.1 M HClO(4) and showed long-range ordered, interweaved arrays of MB with quadratic symmetry on the substrate in the potential range of double-layer charging. High-resolution ECSTM image further revealed the details of the MB monolayers structure of c(5 x 5 radical 3)rect and the flat-lying orientation of ad-molecules. The dependence of molecular organization on the substrate potential and the formation mechanism of well-ordered structure on Au(111) surface were investigated in detail. The obtained well-ordered structure at the interface between a metal and an aqueous electrolyte might possibly be used as high-density device for signal memory and templates for the advanced nanopatterning of surfaces.

Co-adsorption of CO onto a Ag-modified Pt(111) — Restructuring of a Ag UPD layer monitored by EC-STM

The effect of co-adsorption of CO on an underpotentially deposited (UPD) silver monolayer on a Pt(111) single crystal electrode in 0.05 M sulfuric acid is investigated for the first time by means of electrochemical scanning tunneling microscopy (EC-STM). Pure electrochemical experiments suggest that the co-adsorption of CO onto Pt single crystal electrodes previously modified by a monolayer of Ag, forces Ag atoms of the first UPD monolayer into a second adlayer. The present EC-STM studies reveal the formation of a large-area Ag network after the co-adsorption of CO. The resulting Ag nanostructures formed on wide Pt(111) terraces are approximately 0.5 nm high and 10 nm wide. The desorption of the newly formed second Ag adlayer, the oxidation of CO and the desorption of Ag atoms from the first adlayer are monitored by EC-STM and simultaneously detected in the corresponding CVs in three different oxidation peaks. EC-STM images recorded afterwards show the unchanged Pt surface. The presence of Ag on the surface leads to a downward shift of the onset of oxygen adsorption on the Pt(111) surface.

Phase transition in ordered porphyrin layers on iodide modified Cu(1 1 1): An EC-STM study

The self-assembly of meso-tetra ( N-methyl-4-pyridyl) porphyrine tetratosylat (H 2TMPyP) on an iodide modified Cu(1 1 1) surface has been studied by means of in situ electrochemical-scanning tunneling microscopy (EC-STM). The I/Cu(1 1 1) surface was found to be a good substrate for the self-assembly of highly ordered layers of porphyrin cations. Furthermore, a reversible structural phase transition within the highly ordered porphyrin layer was observed during variation of the electrode potential, as a consequence of changing electronic properties of the substrate.

In Situ EC-STM Studies of MPS, SPS, and Chloride on Cu(100): Structural Studies of Accelerators for Dual Damascene Electrodeposition

Electrochemical, differential capacitance, and in situ electrochemical scanning tunneling microscopy (EC-STM) methods are used to examine the interaction of bis(3-sulfopropyl)-disulfide (SPS) and mercaptopropylsulfonic acid (MPS) with Cu(100) surfaces both in the absence and presence of chloride. Both electrochemical and differential capacitance results are weakly perturbed by the addition of either MPS or SPS in the potential region between -0.2 and -0.5 V versus Ag/AgCl relative to the additive-free case. EC-STM images obtained from solutions of MPS alone exhibit a c(2 x 2) adlattice whereas those from SPS alone yield only the (1 x 1) structure. In the presence of Cl-, both adsorbates evince only a c(2 x 2) adlattice on the Cu(100) surface. The desorption potential of these structure is identical to that found with Cl- alone. These results show that neither MPS nor SPS adsorbs strongly on Cu(100) in the presence of Cl-.

Adlayers of Sb Irreversibly Adsorbed on Pt(111): An Electrochemical Scanning Tunneling Microscopy Study

This work presents an electrochemical scanning tunneling microscopy study of Sb irreversibly adsorbed on Pt(111) at various potentials. At an open circuit potential (0.46 V vs a Ag/AgCl electrode), well-ordered structures of SbO+ were found: four (4 x 3)-3SbO+ structures and one (2 square root(3) x 2 square root(3))R30 degrees-3SbO+ structure. In addition, several unidentifiable transient structures of SbO+ were observed, and their relations to the well-ordered structures of (4 x 3) and (2 square root(3) x 2 square root(3))R30 degrees, regarding structural evolution, were proposed. At a reducing potential (0 V), the Pt(111) surface was covered with irreversibly adsorbed Sb which consisted of three different domains: protruded domain, domain of uniaxially incommensurate (square root(3) x square root(2))-Sb, and domain of bare (1 x 1) Pt(111). During oxidation of elemental Sb at 0.30 V, the Sb domains of the (square root(3) x square root(2)) structure were oxidized, while the protruded domains were not oxidized. After underpotential deposition of additional Sb onto the Pt(111) covered with irreversibly adsorbed Sb, the whole surface was filled with the Sb domains where each Sb atoms were separated by the square root(2a) distance (a = one Pt-Pt distance, 0.277 nm). The observed electrochemical inactivity below 0.3 V was discussed in terms of the protruded domain of a presumable incommensurate (square root(2) x square root(2)) structure.

Adsorption and Electrochemical Activity: An In Situ Electrochemical Scanning Tunneling Microscopy Study of Electrode Reactions and Potential-Induced Adsorption of Porphyrins

The effect of adsorption on molecular properties and reactivity is a central topic in interfacial physical chemistry. At electrochemical interfaces, adsorbed molecules may lose their electrochemical activity. The absence of in situ probes has hindered our understanding of this phenomenon and electrode reactions in general. In this work, classical electrochemistry and electrochemical scanning tunneling microscopy (EC-STM) were combined to provide molecular level insight into electrochemical reactions and the molecular adsorption state at the electrolyte-electrode interface. The metal-free porphyrin 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) adsorbed on Au(111) in 0.1 M H(2)SO(4) solution was chosen as a model system. TPyP is found to irreversibly adsorb on Au(111) over a wide range of potentials, from -0.25 to 0.6 V(SCE). The adsorption state of TPyP has a dramatic effect on its electrochemistry. Preadsorbed, oxidized TPyP displays no well-defined cathodic peaks in cyclic voltammograms in sharp contrast to solution-phase TPyP. Our present work provides direct, molecular level evidence of the electrochemically "invisible" species. Electrochemical activity of absorbed species is recovered by allowing the oxidized molecule sufficient time (tens of minutes) to reduce. The redox state of adsorbed TPyP also affects the nature of the adsorption. Oxidized species can apparently only form monolayers. However, multilayers, stable enough to be imaged by STM, can form when the adsorbed TPyP is in the reduced state. This suggests that by controlling the electrochemistry one can either promote or suppress the formation of multilayers.

EC-STM Studies of Te and CdTe Atomic Layer Formation from a Basic Te Solution

The cyclic voltammetry of Te on Au is markedly affected by pH. This fact can be used to advantage when designing an electrodeposition cycle for CdTe. For instance, if a pH 2 Te solution is used, the underpotential deposition (UPD) potential for Te is 0.8 V positive of that for Cd. However, if a pH 9.2 Te deposition solution is used, the potential for Te UPD coincides with that for Cd, greatly simplifying the development of an electrochemical atomic layer epitaxy (EC-ALE) cycle. This report describes electrochemical scanning tunneling microscopy (EC-STM) studies of Te deposition on Au(111) and Au(100) from basic media. Several structures were observed on Au(111): a tellurite adlayer which spontaneously adsorbed prior to Te formation, a 1/4 coverage and a 1/3 coverage While on Au(111), a 1/3 coverage with light domain walls, and two structures, with coverages of 4/9 and 5/9, were observed. Results of the formation of the first CdTe compound monolayer using an EC-ALE cycle which includes Te deposition from a basic solution are also included. © 2004 The Electrochemical Society. All rights reserved.