Methylthiolate on Au(111): adsorption and desorption kinetics

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.     

Nondissociative chemisorption of short chain alkanethiols on Au(111)

The adsorption of methanethiol and n-propanethiol on the Au(111) surface has been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-temperature scanning tunneling microscopy (LT-STM). Methanethiol desorbs molecularly from the chemisorbed monolayer at temperatures below 220 K in three overlapping desorption processes. No evidence for S-H or C-S bond cleavage has been found on the basis of three types of observations: (1) A mixture of chemisorbed CH3SD and CD3SH does not yield CD3SD, (2) no sulfur remains after desorption, and (3) no residual surface species remain when the adsorbed layer is heated to 300 K as measured by STM. On the other hand, when defects are introduced on the surface by ion bombardment, the desorption temperature of CH3SH is extended to 300 K and a small amount of dimethyl disulfide is observed to desorb at 410 K, indicating that S-H bond scission occurs on defect sites on Au(111) followed by dimerization of CH3S(a) species. Propanethiol also adsorbs nondissociatively on the Au(111) surface and desorbs from the surface below 250 K.     

Model of the dense part of the double layer in the absence of specific adsorption

A model is suggested for the dense part of the double layer in the absence of specific adsorption, which is based on the following assumptions: (1) There are two kinds of adsorbed solvent particles on the electrode surface: associates freely oriented in the double layer field and separate chemisorbed water dipoles; 92) the total number of solvent molecules on the surface is determined by the geometric dimensions of associates and chemisorbed dipoles; (3) account is taken of the electrostatic interaction of associates and chemisorbed dipoles in the surface layer; (4) a discreteness coefficient is introduced, accounting for the degree of influence of the field due to all the other dipoles and associates in the surface layer on the dipole or associate being considered; (5) the dipole moment of a chemisorbed water molecule exceeds the value of μH₂O in the bulk of the solution; (6) account is taken of the decrease with increasing temperature of the average number of water molecules in surface associate.The calculations on the basis of this model describe satisfactorily the shape of the differential capacity curves of the dense layer and their temperature dependence for the system Hg-aqueous NaF solution.     

Evidence for molecularly chemisorbed oxygen on TiO2 supported gold nanoclusters and Au(111)

In this study, we present evidence for the existence of a molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst and a Au(111) single crystal following exposure of these samples to an oxygen plasma-jet molecular beam. We present evidence for the molecularly chemisorbed oxygen species from thermal desorption, collision-induced desorption, and heat of adsorption/reaction-induced desorption measurements. Thermal desorption measurements reveal a peak desorption temperature at approximately 145 K which corresponds to an activation energy for desorption of approximately 0.35 eV.     

Efficient CO oxidation at low temperature on Au(111)

The rate of CO oxidation to CO2 depends strongly on the reaction temperature and characteristics of the oxygen overlayer on Au(111). The factors that contribute to the temperature dependence in the oxidation rate are (1) the residence time of CO on the surface, (2) the island size containing Au-O complexes, and (3) the local properties, including the degree of order of the oxygen layer. Three different types of oxygen--defined as chemisorbed oxygen, a surface oxide, and a bulk oxide--are identified and shown to have different reactivity. The relative populations of the various oxygen species depend on the preparation temperature and the oxygen coverage. The highest rate of CO oxidation was observed for an initial oxygen coverage of 0.5 monolayers that was deposited at 200 K where the density of chemisorbed oxygen is maximized. The rate decreases when two-dimensional islands of the surface oxide are populated and further decreases when three-dimensional bulk gold oxide forms. Our results are significant for designing catalytic processes that use Au for CO oxidation, because they suggest that the most efficient oxidation of CO occurs at low temperature--even below room temperature--as long as oxygen could be adsorbed on the surface.     

Quaternary ammonium bromide surfactant adsorption on low-index surfaces of gold. 1. Au(111)

The coadsorption of the anionic and cationic components of a model quaternary ammonium bromide surfactant on Au(111) has been measured using the thermodynamics of an ideally polarized electrode. The results indicate that both bromide and trimethyloctylammonium (OTA(+)) ions are coadsorbed over a broad range of the electrical state of the gold surface. At negative polarizations, the Gibbs surface excess of the cationic surfactant is largely unperturbed by the presence of bromide ions in solution. However, when the Au(111) surface is weakly charged the existence of a low-coverage, gaslike phase of adsorbed halide induces an appreciable (~25%) enhancement of the interfacial concentration of the cationic surfactant ion. At more positive polarizations, the coadsorbed OTA(+)/Br(-) layer undergoes at least one phase transition which appears to be concomitant with the lifting of the Au(111) reconstruction and the formation of a densely packed bromide adlayer. In the absence of coadsorbed halide, the OTA(+) ions are completely desorbed from the Au(111) surface at the most positive electrode polarizations studied. However, with NaBr present in the electrolyte, a high surface excess of bromide species leads to the stabilization of adsorbed OTA(+) at such positive potentials (or equivalent charge densities).     

The interaction of C6H6 and C6H12 with noble metal surfaces: electronic level alignment and the origin of the interface dipole

The electronic interaction of two molecules, the aromatic benzene (C6H6) and the saturated hydrocarbon cyclohexane (C6H12) with a Cu(111) surface, have been determined using precise, ab initio electronic structure calculations. For the interaction of these adsorbates with the substrate, we present a detailed analysis and decomposition of various individual chemical mechanisms that contribute. A novel aspect of this analysis is the use of charge-density difference contour plots to graphically display the chemistry. A wave-function-based approach was used in order to avoid problems when the presently most commonly employed approach, density-functional theory, is applied to weakly chemisorbed molecules, where the interaction is dominated by van der Waals forces. The present information are not only relevant with regard to understanding the chemistry going on when molecules are adsorbed on a Cu surface but also have important consequences with regard to charge injection in molecular electronic devices, e.g., organic field-effect transistors and organic light-emitting diodes.     

Electrochemical and FTIR studies of -phenylalanine adsorption at the Au(111) electrode

The adsorption of -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.     

Reactivity of molecularly chemisorbed oxygen on a Au/TiO2 model catalyst

We present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalyst sample to a radio-frequency-generated plasma jet of oxygen results in co-population of both atomically and molecularly chemisorbed oxygen species on the sample. We tested the reactivity of the molecularly chemisorbed oxygen by comparing the CO2 produced from a sample populated with both species to the CO2 produced from a sample that has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. Samples that are populated with both species consistently result in greater CO2 produced than samples with only atomic oxygen. We interpret this result to indicate that molecularly chemisorbed oxygen on the sample can directly participate in the CO oxidation reaction. The reactivity of molecularly chemisorbed oxygen has been investigated for five different gold coverages (0.5, 0.75, 1, 1.25, and 2 ML), and we observe that there is a greater fractional difference in the CO2 produced (difference between sample populated with both molecularly and atomically adsorbed oxygen and sample populated solely with atomically adsorbed oxygen) for the 1 ML Au coverage than for the other coverages for equivalent oxygen plasma-jet exposures. However, it is not possible to unambiguously conclude that this observation is directly related to a particle size effect on the chemistry since the absolute O(2,a) and O(a) content on the various surfaces is different for all the coverages studied because of the plasma-jet technique that we employed for populating the surfaces with oxygen. Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gold coverage and hence particle size.     

The mechanism of emerging catalytic activity of gold nano-clusters on rutile TiO2(110) in CO oxidation reaction

This paper reveals the fact that the O adatoms (O(ad)) adsorbed on the 5-fold Ti rows of rutile TiO(2)(110) react with CO to form CO(2) at room temperature and the oxidation reaction is pronouncedly enhanced by Au nano-clusters deposited on the above O-rich TiO(2)(110) surfaces. The optimum activity is obtained for 2D clusters with a lateral size of ～1.5 nm and two-atomic layer height corresponding to ～50 Au atoms∕cluster. This strong activity emerging is attributed to an electronic charge transfer from Au clusters to O-rich TiO(2)(110) supports observed clearly by work function measurement, which results in an interface dipole. The interface dipoles lower the potential barrier for dissociative O(2) adsorption on the surface and also enhance the reaction of CO with the O(ad) atoms to form CO(2) owing to the electric field of the interface dipoles, which generate an attractive force upon polar CO molecules and thus prolong the duration time on the Au nano-clusters. This electric field is screened by the valence electrons of Au clusters except near the perimeter interfaces, thereby the activity is diminished for three-dimensional clusters with a larger size.     

Stereochemistry of 1,2-dichloroethane adsorbed on Pt(111)

The rotational isomerism of 1,2-dichloroethane (DCE, CH2ClCH2Cl) adsorbed on Pt(111) was studied in the temperature range of 35-100 K using high-resolution electron energy loss spectroscopy and metastable atom electron spectroscopy. In the coverage below monolayer the physisorbed and chemisorbed species coexist at 35 K in the gauche and slightly distorted trans form, respectively. Owing to the direct Pt-Cl interactions, the nonbonding Cl 3p states of the chemisorbed DCE are split off, giving rise to degradation in symmetry from the pure trans form (C2h). The physisorbed gauche conformers are arranged with the C2 axis parallel (or heavily tilted) to the substrate and converted irreversibly to the pseudo-trans form by heating at 70 K. In the multilayer, the trans and gauche conformers exist at 35 K, where the former population is increased with increasing layer thickness. Upon annealing the bilayer at 80 K, the irreversible conversion takes place to yield a higher population of the gauche conformer in the topmost layer. The conformational stabilities and mutual changes of DCE adsorbed on a metal surface are discussed in terms of intramolecular rotational potential.     

Transient hydroxyl formation from water on oxygen-covered Au(111)

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed (16)O with H(2)(18)O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable--up to 70%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H(2)(18)O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water.     

Simulation of SNIFTIRS experiments

We present a simulation of subtractively normalized interfacial Fourier transform infrared spectra (SNIFTIRS) for the case of pyridine adsorbed at the Au(111) electrode. From experimental data and SNIFTIRS simulation the tilt angles can be evaluated as a function of the electrode potential for pyridine molecules adsorbed on terrace and defect sites, providing a detailed view of the site and potential dependent molecular orientation.     

The Use of Thin‐Layer Electroanalysis in the Study of the Chemisorption and Anodic Oxidation of Aromatic Molecules at Smooth Polycrystalline Palladium

Thin‐layer electroanalytical chemistry has been used in the study of the chemisorption and anodic oxidation of hydroquinone and benzoquinone at smooth polycrystalline palladium electrode surfaces in aqueous sulfuric acid solutions. The results were reminiscent of those obtained previously on smooth polycrystalline platinum: i) At low aqueous‐solution concentrations, the diphenol is oxidatively chemisorbed to form surface‐coordinated benzoquinone oriented parallel to the surface. ii) At higher concentrations, the oxidative chemisorption occurs via C? H activation to yield an edge‐oriented diphenolic species. iii) Chemisorption from benzoquinone solutions leads to species identical to those from hydroquinone solutions. iv) The extent of anodic oxidation of the chemisorbed organic depends upon the initial adsorbed‐molecule orientation: the flat‐adsorbed species are oxidized completely to carbon dioxide, whereas oxidation of the edge‐chemisorbed species yields other (unidentified) products that are chemisorbed upon regeneration of the oxide‐free surface.     

TiCl4(THF)2 impregnation on a flat SiOx/Si(1 0 0) and on polycrystalline Au foil: determination of surface species using XPS

TiCl₄(THF)₂ was impregnated by spin-coating on a Si(1 0 0) wafer covered with a thin SiO_x layer and on a polycrystalline Au foil. The nature of the surface species was determined at room temperature and after annealing, by X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). A mixed Si:O:Ti interfacial layer was formed on the silicon substrate while in the case of Au, TiOCl_x and TiO_x were the main surface species at room temperature. Annealing at 723 K leads to the total desorption of the Cl atoms, and in both cases a significant amount of Ti atoms was reduced to the Ti³⁺ state. AFM measurements revealed a homogenous distribution of nano-sized TiO_x clusters with semi-ellipsoid shape and increased contact area with the underlying silica.     

Formation of sulfhydryl (SH) species on the clean and (2 × 2)-S covered Pt(111) surfaces by H2S decomposition

H₂S decomposition on the clean and (2 × 2)-S covered Pt(111) surfaces has been characterized using high-resolution electron energy loss (HREELS) and temperature-programmed desorption (TPD) spectroscopies. On the Pt(111)-(2 × 2)-S surface, a mixture of molecular H₂S and sulfhydryl (SH) species forms following H₂S adsorption at 110 K. The molecular H₂S desorbs at 140 K leaving a (2 × 2)-S overlayer saturated with SH; the SH is stable up to 190 K. On the clean Pt(111) surface, a mixture of atomic sulfur, SH and chemisorbed molecular H₂S is formed following H₂S adsorption at 110 K. On the clean surface, adsorbed SH decomposes near 150 K. We report here the first definitive observation of an adsorbed sulfhydryl species on a metal surface.     

Adsorption of polyamides and polyamide–silane mixtures at glass surfaces

The individual and combined adsorption behavior of polyamides and two different silanes to multicomponent glass surfaces was probed with a combination of x‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and static contact angle measurements. Samples were analyzed with and without solvent rinsing to separate the weakly bound (physisorbed) and more strongly bound (chemisorbed) species. High‐resolution XPS on N 1s revealed that the polyamides adsorb to the glass surfaces via protonated amine species at the acidic (OH) sites on the glass surface. Angle‐resolved XPS confirmed this by showing that the alkyl portion of the polymer is oriented away from the glass interface. In competitive coadsorption studies it was found that amino‐terminated silanes preferentially adsorb to the glass surface, relegating the polyamide to a physisorbed outer layer. When mercaptoterminated silanes were competitively coadsorbed the polyamide was preferentially adsorbed even when present at a concentration twenty times less than that of the silane. Altogether, this work reveals the strong interaction between surface silanols and amino‐functionalized organics. Copyright © 2003 John Wiley & Sons, Ltd.     

Molecular tunnel junctions based on π-conjugated oligoacene thiols and dithiols between Ag, Au, and Pt contacts: effect of surface linking group and metal work function

The tunneling resistance and electronic structure of metal-molecule-metal junctions based on oligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (~10 nm(2)) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (<0.2 V) junction resistance (R) increased exponentially with molecular length (s), i.e., R = R(0) exp(βs), where R(0) is the contact resistance and β is the tunneling attenuation factor. The R(0) values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the β value was 0.5 per ring (0.2 ?(-1)) for the dithiol series and 1.0 per ring (0.5 ?(-1)) for the monothiol series, demonstrating that β is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal-S) contacts. R(0) decreased strongly as the contact work function (Φ) increased for both monothiol and dithiol junctions, whereas β was independent of Φ within error. This divergent behavior was explained in terms of the metal-S bond dipoles and the electronic structure of the junction; namely, β is independent of contact type because of weak Fermi level pinning (UPS revealed E(F) - E(HOMO) varied only weakly with Φ), but R(0) varies strongly with contact type because of the strong metal-S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R(0) is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and β is determined by the bridge barrier, E(F) - E(HOMO). Current-voltage (I-V) characteristics obtained over a larger voltage range 0-1 V revealed a characteristic transition voltage V(trans) at which the current increased more sharply with voltage. V(trans) values were generally >0.5 V and were well correlated with the bridge barrier E(F) - E(HOMO). Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes.     

Infrared studies of the potential controlled adsorption of sodium dodecyl sulfate at the Au(111) electrode surface

Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) 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.     

Chemisorbed Oxygen Species over the (110) Face of SnO2

The electronic states of chemisorbed oxygen species on the (110) face of SnO₂ and their variations caused by heat treatments and/or O₂ exposure have been investigated. The reactivities of the chemisorbed oxygen species for methane oxidations were also examined.Four different chemisorbed oxygen species (O₂ ^2-, O_2-, O^-, O_b) were observed, in addition to the lattice oxygen (O^2-), on the surface of the stabilized (110) surface of SnO₂ after O₂ exposure. The O_b species was assumed to be the bridging oxygen at the topmost layer of the SnO₂ (110) surface having no neighboring oxygen vacancies. The electronic state of O_b was converted to O^- upon CH₄ exposure at 473 K by coupling with newly produced vacancies at the bridging site of the SnO₂ (110) surface.