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The knowledge of the structure and properties of water adsorbed on metal surfaces is relevant for understanding electrocatalytic reactions. A monolayer of Pd deposited at a gold electrode surface i...

The charge number per adsorbed molecule and surface concentration of 2,3-di- O -phytanyl- sn -glycero-1-octaethyleneglycol-D,L-α-lipoic acid ester (DPOL) at a Au(111) electrode surface was investigated by chronocoulometry using two different film deposition methods. First, a known amount of the thiolipid was transferred from the air-water interface of a Langmuir trough onto the gold electrode surface via a Langmuir-Blodgett (LB) deposition. The charge density measurements for this monolayer system were used to determine the charge number per adsorbed DPOL molecule (electrosorption valency). The observed electrosorption valency values of the adsorbed DPOL film were much lower than the expected number of transferred electrons for a simple reductive desorption process. In the second deposition method, charge densities were measured for the electrode covered by a self-assembled DPOL monolayer. The electrosorption valency values determined from the LB DPOL film were used to calculate the packing density of the DPOL molecules within the self-assembled monolayer. The surface concentration of the molecules within the thiol monolayer with octaethylene glycol chains gave similar results to a related thiolipid with tetraethylene glycol chains (DPTL). This new finding indicates that both of these molecules (DPTL and DPOL) assume a brush conformation in densely packed self-assembled monolayers.

Reduction of oxygen catalyzed by laccase was studied by surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with electrochemical techniques. Laccase molecules were adsorbed on modified carbon nanotubes (CNTs). SEIRAS provided spectra of Amide I band of adsorbed laccase. Fourier self-deconvolution (FSD) and two-dimensional correlation spectroscopy (2D COS) techniques were employed to reveal the sub-band structure of the Amide I band. The analysis demonstrated that laccase adsorbed on CNTs remains its native state both in the oxidized and reduced states. The detailed analysis showed that change of the oxidation state leads to a small change of the secondary structure of the protein. This change is comparable to the change observed for similar blue-copper oxidases in solution during redox titration. The results of this study demonstrated that CNT's immobilized laccase is an excellent catalyst of oxygen reduction. This study illustrates power of quantitative analysis of IR data to provide information about three-dimensional structure of surface immobilized proteins.

Surface-enhanced infrared reflection-absorption spectroscopy (SEIRAS) was used to investigate the structure of water that is incorporated within a film of sodium dodecyl sulfate (SDS) adsorbed at a thin gold nanoparticle film deposited onto a silicon substrate. Previous studies on a Au(111) electrode surface showed that SDS molecules form long-range ordered hemicylindrical hemimicelles (phase I) for potentials -0.2 ≤ E ≤ 0.45 V vs Ag/AgCl and a disordered bilayer (phase II) for potentials E ≥ 0.5 V vs Ag/AgCl. The SEIRA spectra demonstrated that the hemimicellar film is water-rich and contains both a network of hydrogen-bonded water and a disturbed network of hydrogen bonds consisting of monomeric and dimeric water in the hydrophobic region of the film. No network water was observed in phase II of the film. However, SEIRAS data showed that sulfate groups in the disordered bilayer are hydrated. The SEIRAS spectra of the film of SDS were compared to the previously measured spectra obtained using subtractively normalized interfacial Fourier transform IR spectroscopy (SNIFTIRS). The complementarity of the spectroscopic information obtained by these two techniques was demonstrated.

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.

Changes at the gold–solution interface in an alkaline thiosulfate solution under leaching conditions were investigated using surface enhanced Raman spectroscopy (SERS). Oxidation product of thiosulfate, such as tetrathionate, adsorbed thiosulfate and products of thiosulfate decomposition such as adsorbed sulfide or elemental sulfur were identified as the main contributor to the film that forms on the gold surface as a result of direct contact with the thiosulfate solution. The roles of thiourea and ammonia additives on the enhanced leaching kinetics of thiosulfate-based electrolyte solutions were also investigated. The SERS spectra indicate that these two additives form surface complexes with Au(I) and their presence inhibits either the formation or adsorption of the oxygen-containing sulfur species responsible for passivation of the gold surface. Leaching currents were determined from the analysis of current–potential curves and mixed potential theory. The initial leaching rates were higher in the presence of the additives, suggesting that they inhibit the passivating species from interfering with the Au leaching reaction and assist this reaction by forming complexes with the released Au(I) ion.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.langmuir.6b00635 Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was used to determine the molecular orientation and identify the metal–molecular interactions responsible for the adsorption of adenine from the bulk electrolyte solution onto the surface of the Au(111) electrode. The recorded p-polarized IR spectra of the adsorbed species were subtracted from the collected s-polarized IR spectra to remove the IR contributions of the vibrational bands of the desorbed molecules that are located within the thin layer cavity of the spectroelectrochemical cell. The intense IR band around 1640 cm–1, which is assigned to the pyrimidine ring stretching vibrations of the C5–C6 and C6–N10 bonds, and the IR band at 1380 cm–1, which results from a combination of the ring stretching vibration of the C5–C7 bond and the in-plane CH bending vibration, were selected for the quantitative analysis measurements. The transition dipoles of these bands were evaluated by DFT calculations. Their orientations differed by 85 ± 5°. The tilt angles of adsorbed adenine molecules were calculated from the intensity of these two vibrations at different potentials. The results indicate that the molecular plane is tilted at an angle of 40° with respect to the surface normal of the electrode and rotates by 16° around its normal axis with increasing electrode potential. This orientation results from the chemical interaction between the N10 and gold atoms coupled with the π–π parallel stacking interactions between the adjacent adsorbed molecules. Furthermore, the changes in the molecular plane rotation with the electric field suggests that the N1 atom of adenine must also participate in the interaction between the molecule and metal. Spanish Ministry of Science and Technology (CTQ2010- 19823), Spanish Ministry of Economy and Competitiveness (CTQ2014-57515-C2-1-R), Junta de Andalucia (PAI FQM202) and Discovery Grant from Natural Sciences and Engineering Council of Canada

Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIF- TIRS) was used to determine the conformation and orientation of sodium dodecyl sulfate (SDS) molecules adsorbed at the single crystal Au(111) surface. The SDS molecules form a hemimicellar/hemicylindrical (phase I) structure for the range of potentials between −200 ≤ E < 450 mV and condensed (phase II) film for electrode potentials ≥500 mV vs Ag/AgCl. The SNIFTIRS measurements indicate that the alkyl chains within the two adsorbed states of SDS film are in the liquid-crystalline state rather than the gel state. However, the sulfate headgroup is in an oriented state in phase I and is disordered in phase II. The newly acquired SNIFTIR spectroscopy measurements were coupled with previous electrochemical, atomic force microscopy, and neutron reflectivity data to improve the current existing models of the SDS film adsorbed on the Au(111) surface. The IR data support the existence of a hemicylindrical film for SDS molecules adsorbed at the Au(111) surface in phase I and suggest that the structure of the condensed film in phase II can be more accurately modeled by a disordered bilayer.

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.

Highly ordered microporous films of silver-containing regular arrays of spherical pores of differing diameters were prepared by electrochemical deposition into the interstitial spaces of a template formed by nanosphere lithography. These nanostructured electrodes in conjunction with a Raman microprobe spectrometer were used to obtain surface-enhanced Raman spectra (SERS) of beta-thioglucose (beta-TG) under potential control. The SERS results were compared with SERS of beta-TG on an electrochemically roughened silver electrode surface. The bands in the experimental spectra were assigned to particular vibrations with the help of ab initio predictions of the spectra. The results of this study show that beta-TG self-assembled at a silver electrode forms a hydrophilic film that may be used in biomimetic research to enhance interactions between the electrode and the hydrophilic portion of a model membrane, and to prevent interactions of proteins inserted into this membrane with the metal surface. However, in contact with the aqueous electrolyte an anomerization reaction takes place and the beta-TG film is a mixture of the alpha- and beta-anomers of thioglucose. A partial oxidation of the self-assembled molecules was also observed. In addition, the orientation of the adsorbed molecules changes as a function of the applied potential.

By describing studies of three prototypical surfactants with similar hydrophobic tails but differently charged headgroups, this review provides a summary of the rich phase behavior of soluble surfactant molecules at electrified interfaces. With the use of electrochemical, scanning probe microscopy, and neutron scattering techniques we have been able to fully explore the adsorption and surface aggregation of these molecular systems. Furthermore, we have been able to provide compelling evidence of electric field-driven phase transitions in these surfactant films and their aggregated structures. Cumulatively, our results demonstrate that the electrical state of a surface (namely surface charge or applied potential) plays an integral role in determining the morphology of surfactants at solid interfaces. Unlike other aggregate shape determining factors such as the surfactant packing parameter, the electrical parameter can readily be adjusted in situ, providing a tunable means to control films of soft condensed matter.

Electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the structure of a film formed by cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) adsorbed on the Au(111) electrode surface. The film is disordered at potentials corresponding to either large negative charge densities or to positive charge densities. At small negative charge densities, an ordered adlayer of flat-lying DeTATf molecules is formed. High-resolution images of this adlayer reveal that the triflate anion is coadsorbed with the N-decyl-N,N,N-trimethylammonium cation, effectively forming an ion pair at the electrode surface. This is a significant result because it explains why this surfactant behaves like a zwitterionic surfactant at the metal/solution interface.

We have applied a recently developed method (Langmuir 2006, 22, 5509-5519) to determine charge numbers per adsorbed molecule and packing densities in self-assembled monolayers (SAMs) of octadecanethiol (C18SH), a representative long-chain thiol. Our method yields values of area per molecule that are physically reasonable, in contrast to the popular reductive desorption method, which gives molecular areas that are smaller than those determined by the van der Waals radii. In a nonadsorbing electrolyte, we were able to model the dependence of the charge number per adsorbed molecule on the electrode potential, taking into account that the desorption process is a substitution reaction between the solvent and the adsorbate. We have also shown that the charge number per adsorbed thiol is affected by the specific adsorption of the anion of the electrolyte. In the latter case, the thiol competes for adsorption sites at the surface not only with water but also with the anion of the electrolyte, and this competition has an effect on the measured charge number.

Reduction of fermentation-derived lactic acid (LA) offers a renewables-based pathway to propylene glycol (PG), a large-scale commodity chemical, currently manufactured by the oxidation of petroleum-derived propene. Complementing our previously described catalytic hydrogenation of LA to PG, we now report electrocatalytic hydrogenation (ECH) of LA in an aqueous electrolyte using constant current electrolysis. A reticulated vitreous carbon (RVC) electrode serves to agglomerate, support, and supply current to a 5% Ru/C powder catalyst, the same catalyst used in the classical hydrogenations. The ECH conditions are mild (ambient pressure, 70 °C vs 1500 psi H2, 150 °C) relative to the chemical hydrogenation. More surprisingly, the major electrohydrogenation product is lactaldehyde (LAL), with small quantities of PG also formed. Variable current studies in the range of 10–100 mA show an increase in product yields and a shift in selectivity toward PG with increasing current. Experiments carried out with different acids as electrolytes reveal a distinct effect of the anion on the yields of the two products. In situ ATR-FTIR studies of the ECH of LA point to a chelating bidentate carboxylate adsorption mode for lactate on the Ru surface and offer insight into the effects of electrolyte anions on surface adsorption and reactivity.

Electrochemical methods have been used to provide a thermodynamic description of the adsorption of citric acid (H 3 A) and dihydrogen citrate (H 2 A − ) on a Au(1 1 1) surface. We compare the adsorption of citric acid species from an electrolyte (pH 1) corresponding to the fully-protonated tri-acid species and an electrolyte (pH 3) corresponding to an approximately equimolar mixture of H 3 A and H 2 A − . Our results indicate a stronger adsorption of the monovalent ion compared to the neutral citric acid although both sets of experiments reveal similar charge numbers per adsorbed ion species. The latter values are dependent on the electrical variable (electrode potential or charge density) and approach a value of −3 at the most positive potentials (charge densities) studied for both sets of data (pH 1 and pH 3). This indicates that the adsorbed species is the fully de-protonated citrate ion, A −3 . From our chronocoulometry data we determine the outer Helmholtz potential for the citrate covered gold surface which is significantly larger for the system at pH 3 compared to pH 1. The results of this study on a two-dimensional gold surface can be extended to three-dimensional citrate-stabilized gold colloids, and rationalize the pH induced coagulation of colloidal sols.

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the voltammograms of a Pt(1 1 1) electrode in KBr solutions with an excess of a supporting electrolyte (0.1 M HClO4 and 0.1 M KClO4 + 10?3 M HClO4 + xM KBr where x varied between 5 × 10?4 and 1 × 10?2). The surface Gibbs excess, the Gibbs energy of adsorption and the charge number at a constant electrode potential and a constant chemical potential have been determined. The effect of pH on the magnitude of these parameters has been evaluated. The thermodynamic parameters for bromide have been compared to the parameters determined from previous thermodynamic studies of (bi)sulfate, chloride and OH adsorption at the Pt(1 1 1) electrode surface. As expected, bromide adsorption is stronger than for the other anions and halide adsorption seems to be limited by close packing of the adlayer. The calculated charge number values suggest that Br adsorption involves a full charge transfer, as in the case of chloride

A thermodynamic method based on the work done by Frumkin and Petrii [A.N. Frumkin, O.A. Petrii, Electrochim. Acta 20 (1975) 347], to calculate the so-called double layer capacity for a Pt(1 1 1) electrode is proposed. The analysis requires careful measurement of the total charge density versus potential curves for a series of solutions with composition (0.1 − x) M KClO4 + x M HClO4. A method in which the total charge densities are determined by integration of cyclic voltammograms recorded in solutions with or without chloride is described. Following this procedure the double layer capacity curves were calculated. The double layer capacity curves displayed three peaks that were tentatively assigned to the solvent reorientation, onset of OH adsorption and completion of the OH adlayer. In the hydrogen adsorption region, the double layer capacity values were 14 ± 5 μF/cm2, in good agreement with previous estimates reported in the literature by using other approaches.

The description of the monolayer formed at Au(111) by 2-mercaptobenzimidazole (MBI) under potential control has been based on electrochemical data (charge measurements) and spectroscopic information from the subtractively normalized interfacial Fourier transform infrared spectroscopy method (SNIFTIRS). From the quantitative analysis of the SNIFTIR spectra, a surface coverage Γ/Γ max was extracted for each sample potential. The evolution of the coverage with potential was in full agreement with the charge density curve. The shift of the pzc in the presence of MBI indicates that the adsorbed molecules have a nonzero component of the permanent dipole moment in the direction perpendicular to the electrode surface. Thanks to the high quality of the spectra, it was possible to determine the orientation of MBI molecules at the surface in the monolayer and submonolayer range. The angle between the C 2 -axis of the molecule and the direction normal to the surface is close to 64 ± 4° and its small change (

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in NaCl solutions with an excess of an inert electrolyte (0.1 M HClO 4 ). A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The Gibbs excess, Gibbs energy of adsorption, charge numbers at constant electrode potential and constant chemical potential for chloride adsorption at the Pt(1 1 1) surface have been determined. Our results show that the polarity of the chemisorption bond is very small. The highest packing density of chloride determined by the thermodynamic method corresponds to 0.5 ML and is only 10% smaller than the coverage expected for a close packed monolayer of chlorine atoms. We demonstrated that at negative potentials, where Cl and hydrogen adsorption overlaps, the adsorption has a competitive character.

We have synthesized colloidal Pt nanoparticles with a mean particle size of 2.6±0.4 nm by reducing PtCl2 dissolved in N,N-dimethylacetamide with t-BuMe2SiH. The latter compound acted both as a reducing agent and a stabilizer of the Pt nanoparticles. Pt nanoparticles were deposited onto the Au substrate and IR reflectance absorption spectroscopy (IRRAS) was applied to investigate CO adsorption and oxidation at the surface of the catalyst. The reactivity of the catalyst covered with Nafion was compared with the reactivity of the catalyst without Nafion. In addition, the reactivity of the colloidal Pt was compared with the reactivity of bulk polycrystalline Pt. We found that CO oxidation proceeds at lower over-potentials at nanoparticles than at polycrystalline Pt. The IRRAS data indicate that the difference in the reactivity may be explained by a different mechanism of the oxidation reaction; Langmuir–Hinshelwood at Pt nanoparticles and island formation and growth at polycrystalline Pt. We have also observed that a film of Nafion slows down the CO oxidation reaction. The IRRAS spectra for CO adsorbed at Pt nanoparticles covered by Nafion were significantly different from the spectra recorded for the nanoparticles in the absence of Nafion. The spectroscopic features suggest that in the presence of Nafion the nanoparticles experience regions of lower and higher proton concentration.

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the support's surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the support's surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the support's surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.

The adsorption of citrate on Au(1 1 1) from citric acid containing electrolytes has been quantified using in situ subtractively normalised interfacial Fourier transform infrared spectroscopy (SNIFTIRS). These spectra, together with spectral simulations, provide evidence that citric acid, which contains 3 carboxylic acid groups, deprotonates fully upon adsorption. Spectral simulations are consistent with a limiting coverage of about 3 × 10 −10 mol cm −2 and the surface bound carboxylate groups adopting a tilted orientation. A model for the molecular adsorption is proposed.

Adsorption of coumarin on an Au(210) single-crystal electrode was investigated using chronocoulometry and phase-sensitive a.c. voltammetry. The adsorption parameters, such as the relative Gibbs surface excess, the Gibbs energies of adsorption and the electrosorption valencies, were calculated. The results suggest that coumarin molecules assume a flat, π-bonded orientation on the Au(210) surface. The zero coverage Gibbs energy of adsorption at the potential of maximum adsorption is −42 kJ mol−1, which is a value typical of chemisorption. The adsorption of coumarin on the Au(210) surface was compared with adsorption at the (111) and (100) planes of gold. Copyright © 2003 John Wiley & Sons, Ltd.

We employed electrochemical methods together with in situ neutron reflectometry to describe the aggregation of organic surfactant molecules at a solid–liquid interface. The neutron reflectometry allowed us to determine the surface coverage, thickness, roughness and the relative positions of the aggregates. We found that the applied electric field may be used to reversibly manipulate the architecture of the organic molecules: from uniform monolayers to adsorbed hemi-micelles. These studies are expected to provide a new insight into the roles played by entropic and electrostatic forces in complex fluids or biomaterials.

The thermodynamics of the so-called perfectly polarized electrode was employed to analyze the total charge densities for stepped Pt(hkl) electrodes in solutions of H2SO4 with an excess of an inert electrolyte (0.1 M HClO4). Three Pt single-crystal electrodesPt(10,10,9) = 20(111)×(111), Pt(7,7,6) = 14(111)×(111), and Pt(5,5,4) = 10(111)×(111)vicinal to the Pt(111) surface were employed in these studies. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The Gibbs excess, Gibbs energy of adsorption, electrosorption valency, and Esin−Markov coefficients for (bi)sulfate adsorption at these surfaces have been determined. The thermodynamic data display a dependence on the crystallographic structure of the Pt electrode surface. Both the Gibbs excess and the Gibbs energy of adsorption decrease with increasing step density or decreasing terrace length. This result indicates that adsorption of (bi)sulfate is stronger at larger (111) terraces.

Chronocoulometry and the thermodynamic analysis of charge density data were employed to describe the energetics of sodium dodecyl sulfate (SDS) adsorption at the Au(111) electrode surface. Thermodynamic data such as the Gibbs excess, Gibbs energy of adsorption, and the film pressure of adsorbed SDS were determined for a broad range of electrode potentials, charge densities, and bulk SDS concentrations. The present results, combined with our previous scanning probe microscopy (SPM) studies, show that adsorption of SDS at the Au(111) electrode surface has a two-state character. At small or moderate absolute charge densities, the adsorbed SDS molecules aggregate into hemicylindrical stripelike micelles. This state is well-ordered. The unit cell of its two-dimensional lattice consists of two vectors that are 44 and 5.0 A long and are oriented at an angle of 70°. The Gibbs excess data indicate that five SDS molecules are accommodated into the unit cell. At large positive charge densities, the hemimicellar aggr...

The adsorption of l -phenylalanine (Phe) at the Au(111) electrode surface has been studied using electrochemical techniques and subtractively normalized interfacial Fourier transform infrared (SNIFTIR) techniques. The electrochemical measurements of cyclic voltammetry, differential capacity and chronocoulometry were used to determine Gibbs energies of adsorption and the reference (E1) and sample (E2) potentials to be used in the spectroscopic measurements. The vibrational spectra have been used to determine: (i) the orientation of the molecule at the surface as a function of potential; (ii) the dependence of the band intensity on the surface coverage; (iii) the character of surface coordination, and (iv) the oxidation of adsorbed Phe molecules at positive potentials. The adsorption of Phe is characterized by ΔG values ranging from −18 to −37 kJ mol−1 that are characteristic for a weak chemisorption of small aromatic molecules. The electrochemical and SNIFTIR measurements indicated that adsorbed Phe molecules change orientation as a function of applied potential. At the negatively charged surface Phe is predominantly adsorbed in the neutral form of the amino acid. At potentials positive to the pzc, adsorption occurs predominantly in the zwitterionic form with the COO− group directed towards the surface and the ammonium group towards the solution. At more positive potentials electrocatalytic oxidation of Phe occurs and is marked by the appearance of the CO2 asymmetric stretch band in the FTIR spectrum. Thus, relative to pzc, Phe is weakly chemisorbed at negative potentials, changes orientation at potentials close to the pzc and is oxidized at positive potentials.

Chronocoulometry and a thermodynamic analysis of charge density data were used to describe chloride adsorption at the Pt(111) electrode surface. The Gibbs excess, the Gibbs energy of adsorption, the number of electrons flowing to the interface per adsorbed chloride ion at a constant potential (electrosorption valency) and at constant chloride concentration (reciprocal of the Esin–Markov coefficient) were determined. Chloride forms a chemisorption bond with platinum. The charge numbers at constant potential and at constant chloride concentration are close to unity. This result suggests that the polarity of the chemisorption bond is very small. The highest packing density of chloride determined by the thermodynamic method corresponds to 0.43 ML. This number agrees well with coverage determined in recent surface X-ray scattering experiments [1] . Adsorption of chloride at Pt(111) is potential and pH dependent. We have demonstrated that at negative potentials, co-adsorption of Cl and hydrogen atoms has a synergistic character. In contrast the adsorption of chloride and OH at positive potentials has a competitive nature.

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the conformation of 2,2‘-bipyridine (22BPY) adsorbed at the Au(111) electrode surf...

Cyclic voltammetry (CV), differential capacity (DC), and charge densitymeasurements have been employed to study the benzoate (BZ) adsorption at the Au(111)electrode surface. Thermodynamic analysis of charge density (σM) data has beenperformed to describe the properties of the adsorbed benzoate ion. The Gibbsexcess Γ, Gibbs energy of adsorption ΔG, and the number of electrons flowingto the interface per adsorbed benzoate ion at a constant potential (electrosorptionvalency) and at a constant bulk concentration of the benzoate (reciprocal of theEsin—Markov coefficient) have been determined. The results demonstrate thatalthough benzoate adsorption starts at negative charge densities, it takes placepredominantly at a positively charged surface. At the most positive potentials,the surface concentration of benzoate attains a limiting value of about 7.3×10−10mol-cm−2, which is independent of the bulk benzoate concentration. This valueis consistent with packing density corresponding to a closed-packed monolayerof vertically adsorbed benzoate molecules. At negative charge densities, benzoateassumes a flat (π-bonded) surface coordination. The surface coordination ofbenzoate changes, by moving from a negatively to positively charged surface.At the negatively charged surface, the electrosorption bond is quite polar. Thepolarity of the chemisorption bond is significantly reduced due either to a chargetransfer or a screening of the charge on the anion by the charge on the metal.

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been used to examine (i) the orientation and coordination of benzoate on the Au(111) electrode; (ii) the dependence of the IR band intensity on the surface coverage; and (iii) the chemical stability of benzoate at the Au(111) surface. The results indicate that orientation of adsorbed benzoate changes from a nearly flat co-ordination at a negatively charged surface to a tilted position at positive charge densities. The transition in the orientation of the adsorbed molecule occurs in the vicinity of the potential of zero charge in the solution investigated. The integrated IR band intensities correlate very well with the surface concentration of benzoate obtained from chronocoulometric measurements. An overall agreement between the spectroscopic and electrochemical results is excellent. The spectroscopic data demonstrate that no decomposition or oxidation of benzoate occurred at the Au(111) electrode surface over the potential range investigated.

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been applied to study coordination of the pyridine molecules to the Au(111) electrode surface. The IR spectra have been recorded using both p- and s-polarized radiation. The ratio of the integrated band intensities for the spectra recorded with p- and s-polarized light was then used to study changes in the surface coordination of pyridine molecules. We have derived an expression describing the dependence of this ratio on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface, and: (i) C2v axis of the pyridine molecule, (ii) the direction in plane of the molecule and normal to the C2v axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface.

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at the Au(111) electrode surface. The vibrational spectra have been used to study the character of surface coordination and the stability of adsorbed 4-CNPy molecules. Our studies show that 4-CNPy molecules are totally desorbed from the Au(111) surface at potentials lower than −0.7 V versus SCE and that they adsorb at the gold surface at more positive potentials. At potentials lower than 0.05 V versus SCE, the adsorption has a non-dissociative character. The 4-CNPy molecules are initially oriented flat (π-bonded) on the electrode surface and reorient from the flat to a vertical state when the electrode potential approaches 0 V (SCE). When the potential is greater than 0.05 V versus SCE, the character of 4-CNPy adsorption becomes dissociative and the adsorbed molecules partially hydrolyze to form isonicotinamide (INA). The IR spectra acquired at very positive potentials indicate that 4-CNPy is oxidized at E>0.6 V versus SCE.

To determine the relationship between the electrochemical nature of the solid–liquid interface and its surface structure and composition, a transfer system coupling an electrochemical cell with a UHV analysis chamber was constructed. In this study we report the results of complimentary electrochemical and AES/LEED measurements of bromide ions adsorbed on an Au(100) electrode at various potentials. Our findings highlight the relationship between changes in surface composition and specific features observed in cyclic voltammetry (CV). In addition, the ability of electrochemistry to prepare unique surface structures that are inaccessible to conventional UHV deposition techniques is discussed.

This lecture gives a review of thermodynamic, spectroscopic, STM and AFM imaging, and X-ray diffraction studies of molecular and ionic adsorption at Au(111) electrodes. In the first part, thermodynamics of adsorption of simple ions such as sulfate, chloride, bromide, and iodide will be discussed. At high coverages, anions adsorbed at a single crystal surface form ordered 2D adlayers. We show that the structure of these adlayers can be studied by STM and surface X-ray diffraction techniques. Next, the information concerning adsorption of simple ions is used to describe mixed adlayers formed by coadsorption of anions and metal adatoms. We demonstrate how to combine electrochemical experiments with in situ polarization-dependent Cu K-edge X-ray absorption spectroscopy to determine the composition and the structure of mixed films formed by deposition of Cu on Au(111) in the presence of SO42-. In the last part we review our efforts to describe coordination of organic molecules to gold electrodes. First, we will discuss adsorption of benzonitrile at the Au(111) electrode surface. We combine electrochemical methods with in situ infrared spectroscopy to describe (i) the energetics of molecular adsorption at the gold electrode surface, (ii) the character of the interaction of the adsorbed molecule with the metal substrate, and (iii) the influence of the electric field on the orientation of the adsorbed molecule. In the last section we describe surface aggregation phenomena. We apply AFM and STM to determine the structure of hemimicelles formed at the Au(111) electrode surface by adsorbed molecules of sodium dodecyl sulfate and we discuss the potential-controlled transformation of these hemimicelles into a condensed monolayer.Key words: surface electrochemistry, electrosorption, surface aggregation, gold electrodes, molecular adsorption, ionic adsorption.

Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to present the first direct images of the potential-controlled phase transition between the hemimicellar and condensed states of a dodecyl sulfate (SDS) film at the Au(111) electrode surface. The adsorbed SDS forms stripe-shaped hemimicellar aggregates at small or moderate charge densities at the electrode. High-resolution STM images of these aggregates revealed that adsorbed SDS molecules are ordered and form a long-range two-dimensional lattice. A unit cell of this lattice consists of two vectors that are 4.4 and 0.5 nm long and are oriented at an angle of 70°. We propose that each unit cell contains two flat-laying SDS molecules stretched out along the longer axis of the cell with the hydrocarbon tails directed toward the interior of the cell. The remaining SDS molecules in the hemimicelle assume a tilted orientation. This long-range structure is stabilized by the interactions of sulfate groups b...

The adsorption of hydroxide ions at a Au(111) single-crystal electrode has been investigated quantitatively using chronocoulometry and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). By thermodynamic analysis of the charge density data, the Gibbs excess, Gibbs energy of adsorption, and number of electrons flowing to the interface per one adsorbed hydroxide ion at a constant electrode potential (electrosorption valency) were determined. The electrosorption data indicate that the adsorption of OH- has a three-state character. The adsorbed OH- forms quite a polar surface bond at a negatively charged surface, while the polarity of the surface bond is significantly decreased at positive charge densities. Oxide formation begins at higher charge densities. Infrared spectroscopy shows that oxide formation takes place when the surface concentration of hydroxide ions exceeds one-third of a monolayer. The integrated infrared intensity of the O−H stretching band correlates ver...

The adsorption of benzonitrile (BN) on the Au(111), Au(100), and Au(110) electrodes in aqueous solutions was investigated. The chronocoulometric technique was used to measure the charge density at the metal surface as a function of the electrode potential for BN concentrations ranging from 0 to 0.030 M. The adsorption parameters such as the film pressure, the surface concentration of the organic adsorbate, the standard Gibbs energy of adsorption (ΔGads0), and the charge flowing to the interface per one adsorbed molecule, were determined from the thermodynamic analysis of the charge density data. The analysis was carried out at constant electrode potential and at constant charge density. In addition, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was employed to determine the orientation of BN molecules at the Au(111) surfaces. The thermodynamic and spectroscopic data indicate that BN molecules assume a flat (π-bonded) state at the negatively charged surface and progressively reorient from the flat to the tilted (N-bonded) state as the charge at the metal surface becomes more positive. At very positive potentials the character of BN adsorption becomes reactive and the adsorbed molecules partially hydrolyze to form benzamide (BA).

This paper reviews thermodynamic, spectroscopic and X-ray diffraction studies of SO 4 2− , Cl − , Br − and I − adsorption at the Au(111) electrode surface. The thermodynamic experiments provided information about surface concentrations of specifically adsorbed anions, Gibbs energies of adsorption and the numbers of unit charge flowing to the interface per one adsorbed anion. Second harmonic generation and surface X-ray scattering revealed the affect of ionic adsorption on the electronic and crystallographic structure of the interface. The atomistic information derived from these microscopic studies was used to discuss the thermodynamic data and to verify the theoretical models of the interface. It has been shown that complete description of ionic adsorption at metal electrodes requires a concerted use of the macroscopic and microscopic techniques.

In situ X-ray scattering and chronocoulometric experiments were performed to assess the influence of adsorption of pyridine, 2,2′-bipyridine and uracil on the driving force for the (p×√3)→(1×1) transition of the Au(111) surface. We have shown that the overall driving force is a combination of the driving force due to charge and the driving force due to the adsorbate. We have estimated the magnitude of the two driving forces and have given the upper and lower limits to this estimate. Our results show that the two driving forces are of comparable magnitude and that the interpretation of the surface reconstruction phenomena given in terms of either purely charge or a purely adsorbate effect is an oversimplification.

Chronocoulometry and Raman spectroscopy have been applied to study pyrazine adsorption at the Au(210) electrode surface. The adsorption isotherms, Gibbs energies of adsorption, and the electrosorption valency for pyrazine adsorption at the Au(210) electrode surface have been determined. Surface-enhanced Raman Scattering (SERS) spectra of pyrazine adsorbed on an "unroughened" Au(210) electrode surface have been obtained for the first time. The thermodynamic data and the SERS spectra indicate that pyrazine adsorbs on the Au(210) electrode via the nitrogen lone pair (N-bonded configuration) over the whole range of electrode potentials investigated. Consequently, the pyrazine molecule assumes only the vertical orientation at this surface of gold. The adsorption of pyrazine at the Au(210) and Au(111) surfaces is compared and the influence of the surface crystallography on the adsorption of this molecule at gold electrodes is discussed. Keywords: adsorption at Au(210), pyrazine, SERS, chronocoulometry.

The subtractively normalized interfacial Fourier transform infrared technique has been employed to study the adsorption of benzonitrile (BN) at the Au(111) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed BN molecules at positive potentials. Our studies show that BN molecules are totally desorbed from the Au(111) surface at potentials more negative than −0.6 V (SCE) and they adsorb at the gold surface at more positive potentials. At potentials more negative than 0.05 V (SCE), the adsorption has an associative character. The BN molecules are initially oriented flat (π-bonded) on the electrode surface and progressively reorient from the flat to a vertical (N-bonded) state when the electrode potential approaches the potential of zero charge. This change of the surface coordination is gradual and apparently involves a progressive change of the tilt...

The adsorption of 4-pentadecyl-pyridine (C15-4-Py) onto the Au(111) electrode was investigated with the help of electrochemical and spectroscopic techniques. Electrochemical studies demonstrated that a film of C15-4-Py spreads onto the metal surface at potentials close to the potential of zero charge (pzc) and desorbs from the electrode surface at potentials which are sufficiently negative. The character of C15–4-Py adsorption resembles adsorption of pyridine molecules at the Au(111) electrode, indicating that the film properties are to a large extent determined by the interaction between the pyridine moiety and the metal surface. Addition of 25 mol%) of 10-decyl-9-[2-(4-pyridyl)ethyl]anthracene(DPEA), a dye molecule, to the C15–4-Py film only slightly modified its properties but allowed the use of spectroscopic methods to study the film properties. Fluorescence spectroscopy and light scattering experiments were employed to study the potential-induced desorption and adsorption of the surfactant molecules. Using spectroelectrochemical techniques we have demonstrated that the desorption of the film and its readsorption involves the formation of micelles in the subsurface region and spreading of the micelles onto the electrode surface. A general mechanism of spreading of insoluble surfactants onto electrode surfaces has been proposed.

In situ electromodulated reflectance Fourier transform infrared (FTIR) spectroscopy has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at an Au(111) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed 4-CNPy molecules. It has been observed that the vibrational bands in the spectra acquired in the electroreflection experiment are significantly broader than the corresponding spectra acquired in a transmission cell. Some weaker bands seen in the spectra recorded in transmission were not observed in the electroreflectance experiment. The electroreflectance spectra were dominated by the two ring deformation bands observed at 1416 cm−1 and 1554 cm−1. The intensities of these bands correlated well with the surface concentrations of 4-CNPy molecules determined from independent electrochemical studies. The integrated absorption intensities of the bands recorded in the electroreflection experiments were larger by a factor of five than the absorption intensities measured in the transmission cell. This indicates that the electric field of the photon acting on a molecule, present in front of the gold electrode, is significantly enhanced by reflection from the electrode. The infrared experiments suggest that at positive potentials the 4-CNPy molecules are coordinated to the metal surface through the nitrogen atom of the aromatic ring. The 4-CNPy molecules are oxidized at the Au electrode at potentials higher than 0.6 V (SCE) and are reduced to form (4-CNPy)−• ion at potentials lower than −1.1 V (SCE). Key words: adsorption, infrared spectroscopy, 4-cyanopyridine, gold electrode.

The adsorption of an insoluble monolayer of 12-(9-anthroyloxy) stearic acid (12-AS) onto a single crystal gold electrode has been described. The spreading of the insoluble surfactant onto the metal | solution interface of the gold electrode was initially investigated with the help of cyclic voltammetry and differential capacity. Electrochemical studies indicate that the film of 12-AS molecules spreads onto the metal surface at potentials close to the potential of zero charge (pzc) and desorbs from the electrode surface at potentials that are sufficiently negative. The spreading of the film and its subsequent desorption are repeatable. The 12-AS molecule is a surfactant dye which may be used to perform spectroelectrochemical experiments. Electroreflectance spectroscopy, fluorescence spectroscopy and measurements of the light scattered by the desorbed surfactant molecules were employed to determine the mechanism by which molecules of the insoluble surfactant spread onto and desorb from the metal | solution interface of the gold electrode. With the help of the spectroelectrochemical experiments we have demonstrated that the repeatable potential-induced desorption and adsorption (spreading) of the insoluble molecules involves formation of micelles (or similar molecular organizations) in the subsurface region and spreading of the micelles onto the electrode surface.

The adsorption of surfactants, which form insoluble monolayers on an aqueous substrate, onto a single crystal gold electrode have been described. Adsorption of this class of surfactants have been characterized using a combination of electrochemistry and Langmuir-Blodgett techniques. We have developed a technique to simultaneously measure the film pressure at the gas-solution (GS) interface and the film pressure of the surfactants that spread to the metal-solution (MS) interface. We have shown that surfactants such as octadecanol and stearic acid, which interact weakly with the metal surface, adsorb at an uncharged MS interface (at the potential of zero charge) and progressively desorb when the electrode surface is charged negatively. The electrode potential (charge density at the metal surface) influences the transfer of the surfactant from the GS interface to the MS interface. The transfer ratio is 1:1 at an uncharged MS interface, and is progressively reduced to zero when the MS interface is charged. We have employed 12-(9-anthroloxy) stearic acid, a surfactant dye molecule, to study the mechanism of potential induced desorption and adsorption of the film of insoluble molecules. With the help of electroreflectance spectroscopy and light scattering measurements, we have shown that if desorbed, the surfactant molecules form micelles (flakes or vesicles) that are trapped under the electrode surface. The micelles spontaneously spread back onto the electrode surface when the charge density at the metal approaches zero. The repeatable desorption and readsorption involve micellisation of the film at negative potentials and spontaneous spreading of the micelles to reform the monolayer at potentials close to pzc.

Thermodynamic analysis of charge density data was performed to describe sulfate adsorption at the Au(111) surface. The Gibbs excess, the Gibbs energy of adsorption, the number of electrons flowing to the interface per one adsorbed sulfate ion and the Esin-Markov coefficients were determined. The thermodynamic data indicate that sulfate adsorption at the Au(111) electrode has an ionic character and is driven by the interaction of the ion with its image charge on the metal. The surface dipole formed by the ion and its image is heavily screened by the metal and surface solvent molecules. The ionic character of sulfate adsorption at the Au(111) electrode is consistent with the results of independent second harmonic generation experiments. The second harmonic generation data indicate that adsorption of sulfate affects little the electronic structure of the metal surface.

The reduction of Fe(CN) 6 3- at the Au(111) electrode coated with a thin film of 4-pentadecylpyridine (C15-4-Py) was investigated. The C15-4-Py molecules form a multilayer at the Au(111) surface which may be repeatedly adsorbed and desorbed by varying the electrode potential. At E > -0.2 V (SCE) the film has a very compact structure. Its capacity is lower than 1 μF cm -2 and it exhibits strong blocking behavior with respect to the reduction of Fe(CN) 6 3- . At potentials -0.6 V (SCE) < E < -0.3 V (SCE) the film is less rigid. Its capacity increases to about 2 μF cm -2 and it is more porous. The rate of Fe(CN) 6 3- reduction at the film covered electrode is characteristic of nonlinear diffusion to active pores separated by inactive areas. The average pore size changes between 10 μm and about 30 μm and the pore separation varies between 200 and 500 μm for -0.6 V (SCE) < E < -0.3 V (SCE). In this range the pore density changes from 100 to 6 x 10 3 pores/cm 2 .

Second harmonic generation (SHG) generation was employed to study adsorption of sulfate at the Au(111) electrode surface. The experiments were carried out using both pp and ss polarisations for the input and output photons. The amplitudes of the isotropic, one-fold and three-fold symmetry elements of the electrode susceptibility were determined. The isotropic term is a complex number. Its real part displays this same linear dependence on the electrode charge density in the absence and presence of adsorbed sulfate. The phase angle of the complex number is also not changed by sulfate adsorption. These features indicate that adsorbed sulfate changes the static electric field at the interface, however it does not affect the electronic structure of the metal. The one-fold symmetry amplitude changes with potential. This change displays lifting of the surface reconstruction. We observed similar effect of sulfate on the structure of the electrode surface to that reported earlier by Magnussen et al. [Faraday Discuss. 94 (1992) 329].