Quantum-Chemical Study of the Adsorption of Pb2+ on Au(111)

The interaction between the Pb²⁺ ion and gold is studied using the cluster metal surface model and the density functional method. The geometric and energy characteristics of the interaction between this ion and the gold surface are estimated. The form in which the Pb²⁺ ion exists on the surface is more ad-ionic than ad-atomic. The electron structure of the Au–Pb_ads²⁺ system is analyzed. The participation of the adsorbed lead ion and its neighboring gold atoms in the formation of molecular orbitals in this system is estimated. It is established that the contribution to their formation is predominantly provided by the lead s-orbitals and the gold d-orbitals. The interaction with a solvent decreases the transfer of a charge from an adsorbed lead ion to gold. It is demonstrated that the hydrolyzability of a lead ion decreases upon its transition from the electrolyte phase to the surface.

     

Quantum-Chemical Study of the Adsorption of Bi3+ Ions on Au(111)

Russian Journal of Electrochemistry - The interaction between the Bi3+ ion and gold is studied using the cluster metal surface model and the density functional method. The geometric and energy...     

Interaction between Thallium and the Au(111) Surface. Quantum-Chemical Analysis

Russian Journal of Electrochemistry - The interaction between thallium atoms and the Au(111) surface is studied using the cluster metal surface model and the density functional method. An adsorbed...     

Adsorption of insoluble surfactants at the Au(111)/solution interface

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.     

Adsorption states of dialkyl ditelluride autooxidized monolayers on Au(111)

Organic ditellurides (R2Te2 where R = n-butyl (C4), n-octyl (C8), and n-cetyl (C16)) were synthesized, and their adsorption states after oxidation on Au(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), theoretical analyses, near-edge X-ray absorption fine structure (NEXAFS) measurements, contact angle measurements, and atomic force microscopy (AFM). The obtained results show that dialkyl ditellurides form autooxidized monolayers (AMs) on the surfaces under ambient conditions and that the oxidation process is accelerated by ambient light. XPS, UPS, and theoretical analyses suggest that the autooxidized ditelluride species consist of polymers or oligomers with Te-O-Te-O network structures stabilized by oxygen bridges between tellurium molecules following the cleavage of tellurium-gold bonds. NEXAFS and contact angle measurements indicate that the average tilt angles of the alkyl chains from the surface normal are smaller for the AMs of dialkyl ditellurides having longer alkyl chains. AFM measurements show defects and roughness features around a few angstroms in height on the surfaces of the dialkyl ditelluride AMs.     

Electrochemical Studies of the Benzoate Adsorption on Au (111) Electrode

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 (_M) data has beenperformed to describe the properties of the adsorbed benzoate ion. The Gibbsexcess , Gibbs energy of adsorption G, 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 (-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.     

MP2 study on adsorption of hydrated Na+ and Au+ cations on the Au(111) surface

The interactions of Na+ and Au+ cations with an Au(111) surface in the presence and absence of water molecules were investigated using Au18 and Au22 cluster models and the MP2 method with a triple-zeta valence basis set. The interactions between Na+ ions and the Au(111) surface were found to be primarily electrostatic, contrary to the much stronger Au+-Au(111) interactions that were dominated by orbital contributions. The largest CP-corrected MP2 adsorption energies were -156.9 kJ/mol for Na+ and -478.7 kJ/mol for Au+. When hydrated, Na+ prefers to be completely surrounded by water molecules rather than adsorbed to the surface, whereas Au+ remains adsorbed to the surface as water molecules bond with each other and with the Au surface. CP correction did not change the relative adsorption energy trends of Na+ or Au+ ions, but it had an effect on the interaction energy trends of the hydrated cations because of the weak water-surface and water-water interactions.     

巴豆醛在Au(111)面上的吸附及选择性加氢机理研究

采用密度泛函理论计算了巴豆醛4种构型的稳定性,并选取最优构型进一步研究了其Au(111)面上的吸附及选择性加氢机理.计算结果表明,具有E-(s)-trans构型的巴豆醛稳定性最高.当巴豆醛通过C O吸附于Au(111)面的顶位时,该构型吸附能最大,吸附模型最稳定;巴豆醛向Au(111)表面转移电子0.045e,且其p轨道与金属表面的d轨道发生较强相互作用,使得巴豆醛的键级减弱.此外,通过分析各基元反应的活化能、反应热以及构型变化可知,巴豆醛在Au(111)面上按照2,1-加成机理(部分加氢机理)生成巴豆醇的可能性最大,且降低温度有利于反应转化率的提高.     

Adsorption and charge transfer dynamics of bi-isonicotinic acid on Au(111)

The interaction of bi-isonicotinic acid (4,4(')-dicarboxy-2,2(')-bipyridine) with the Au(111) surface has been investigated using electron spectroscopic techniques. Near edge x-ray absorption fine structure (NEXAFS) spectra show that monolayers of the molecule lie flat to the surface and also reveal that the monolayer is sensitive to the preparation conditions employed. Core level x-ray photoelectron spectroscopy (XPS) shows that the adsorbed molecule does not undergo deprotonation upon adsorption. The "core-hole clock" implementation of resonant photoemission has been used to probe the coupling between molecule and substrate. This technique has revealed the possibility of ultrafast backtransfer from the substrate into the molecule upon resonant excitation of a N 1s core level electron. This is supported by a NEXAFS and XPS investigation of energy level alignments in the system.     

Adsorption and Dissociation of Methanol on Pd(111), Pd/Au(111) and Pd/Rh(111) Surfaces

The adsorption and dissociation behaviors of methanol on Pd(111), Pd/Au(111) and Pd/Rh(111) surfaces were studied using a periodical slab model and the PW91 generalized gradient approximation(GGA) within the framework of first-principles calculations based on density functional theory(DFT). The adsorption energy and geometric parameters for the three surfaces showed that methanol is preferentially adsorbed onto the top-Pd sites and that the adsorption energy of methanol on these surfaces decreases in the order Pd/Au(111) Pd/Rh(111) Pd(111). After adsorption, the C–O, C–H and O–H bonds in methanol adsorbed onto these surfaces are elongated and the vibrational stretching frequency of the O–H bond is obviously redshifted. Furthermore, the first step for the possible dissociation pathway for methanol on these surfaces was calculated. Our results indicate that the O–H bond in methanol decomposes producing methoxy and a hydrogen atom, with the Pd/Au(111) surface exhibiting the smallest dissociation barrier.     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

Electron dynamics at polyacene/Au(111) interfaces

Two-photon photoemission (2PPE) spectroscopy is used to examine the excited electronic structure and dynamics at polyacene/Au(111) interfaces. Image resonances are observed in all cases (benzene, naphthalene, anthrathene, tetracene, and pentacene), as evidenced by the free-electron like dispersions in the surface plane and the dependences of these resonances on the adsorption of nonane overlayers. The binding energies and lifetimes of these resonances are similar for the five interfaces. Adsorption of nonane on top of these films pushes the electron density in the image resonance away from the metal surface, resulting in a decrease in the binding energy (-0.3 eV) and an increase in the lifetime (from <20 to approximately 110 fs). The insensitivity of the image resonances to the size of polyacene molecules and the absence of photoinduced electron transfer from the metal substrate to molecular states both suggest that the unoccupied molecular orbitals are not strongly coupled to the delocalized metal states or image potential resonances.     

Formation of trioctylamine from octylamine on Au(111)

The adsorption of octylamine on Au(111) under ultrahigh vacuum conditions is investigated. The molecules surprisingly undergo a thermally activated chemical reaction, resulting in formation of trioctylamine as confirmed both by X-ray photoelectron spectroscopy (XPS) and by comparison to the scanning tunneling microscopy (STM) signature of trioctylamine deposited directly onto the surface.     

Adsorption geometry variation of 1,4-benzenedimethanethiol self-assembled monolayers on Au(111) grown from the vapor phase

1,4-benzenedimethanethiol was chemisorbed from the vapor phase onto Au(111). The chemisorption geometry, molecular orientation, and bonding properties were studied at different degrees of surface coverage by photoelectron spectroscopy, metastable deexcitation spectroscopy, and near-edge x-ray absorption fine structure spectroscopy at the carbon K edge. Two main chemisorption regimes were identified: at low coverage the molecules adopt a flat configuration, then, as the molecular density of the first layer increases, the reduction of the available chemisorption sites induces the newly bonded molecules to assume a vertical alignment, with only one of the sulphur head groups interacting with the substrate. Experimental results were interpreted on the basis of theoretical calculations that we performed on the free molecule concerning the molecular orbitals' density of states and simulated x-ray absorption.     

Theoretical Investigation on the Adsorption of Ag^＋ and Hydrated Ag^＋ Cations on Clean Si（111） Surface

In this paper, the adsorption of Ag^＋ and hydrated Ag^＋ cations on clean Si（111） surface were investigated by using cluster （Gaussian 03） and periodic （DMol^3） ab initio calculations. Si（111） surface was described with cluster models （Si14H17 and Si22H21） and a four-silicon layer slab with periodic boundary conditions. The effect of basis set superposition error （BSSE） was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Ag^＋ cations and clean Si（111） surface are large, suggesting a strong interaction between hydrated Ag^＋ cations and the semiconductor surface. With the increase of number, water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Ag^＋ cation. The Ag^＋ cation in aqueous solution will safely attach to the clean Si（111） surface.     

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.     

Adsorption modes of cysteine on Au(111): thiolate, amino-thiolate, disulfide

The adsorption of cysteine on the (111) surface of gold has been studied by means of periodic supercell density-functional theory calculations. A number of different adsorption modes are examined, including adsorption through the thiol group in either thiolate or disulfide form, and adsorption through both the thiol and amino functional groups. We find that at intermediate coverage densities the latter mode of adsorption is favored, followed by thiolate adsorption at the bridge (slightly displace toward fcc) site. The N-Au and S-Au bond strengths in the amino-thiolate adsorption are estimated to be of the order of 6 and 47 kcal/mol, respectively. The electronic structure of the different systems is analyzed, with focus on the total and projected density of states, as well as on the detailed character of the electronic states at the interface. States near the Fermi energy are found to have a metal-molecule antibonding character, whereas metal-molecule bonding states mostly occur near the lower edge of the Au-d band.     

Pb deposition on I-coated Au(111). UHV-EC and EC-STM studies

This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30 degrees -I structure. The importance of this study lies in the use of Pb underpotential deposition (UPD) as a sacrificial layer in surface-limited redox replacement (SLRR). SLRR reactions are being applied in the formation of metal nanofilms via electrochemical atomic layer deposition (ALD). Pb UPD is a surface-limited reaction, and if it is placed in a solution of ions of a more noble metal, redox replacement can occur, but limited by the amount of Pb present. Pb UPD is a candidate for use as a sacrificial layer for replacement by any more noble element. It has been used by this group for both Cu and Pt nanofilm formation using electrochemical ALD. The I atom layer was intended to facilitate electrochemical annealing during nanofilm growth. Two distinctly different Pb atomic layer structures are reported, studied using in situ scanning tunneling microscopy (STM) with an electrochemical flow cell and ultrahigh vacuum surface analysis combined directly with electrochemical reactions (UHV-EC). Starting with the initial Au(111)( radical3 x radical3)R30 degrees -I, 1/3 monolayer of I on the Au(111) surface, Pb deposition began at approximately 0.1 V. The first Pb UPD structure was observed just below -0.2 V and displayed a (2 x radical3)-rect unit cell, for a structure composed of 1/4 monolayer each of Pb and I. The I atoms fit in Pb 4-fold sites, on the Au(111) surface. The structure was present in domains rotated by 120 degrees. Deposition to -0.4 V resulted in complete loss of the I atoms and formation of a Pb monolayer on the Au(111), which produced a Moiré pattern, due to the Pb and Au lattice mismatch. These structures represent two well-defined starting points for the growth of nanofilms of other more noble elements. It is apparent from these studies that the adsorption of I- on Pb is weak, and it will rinse away. If Pb is used as a sacrificial metal in an electrochemical ALD cycle and adsorbed I atoms are employed for electrochemical annealing, I atoms will need to be applied each cycle.     

Self-assembly of phospholipid molecules at a Au(111) electrode surface

We described the first scanning tunneling microscopy study of spreading unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) at a Au(111) electrode surface. At the initial stage of the film formation, the molecular resolution images revealed that DMPC molecules are adsorbed flat with the acyl chains oriented parallel to the surface. The molecules assemble into double rows by aligning the acyl chains in the nearest neighbor direction of the reconstructed Au(111) surface and assuming a 90 +/- 10 degrees angle with respect to line of the molecular row. After approximately 30 min, this film is transformed into a hemimicellar state with long rows characteristic for the formation of hemicylindrical surface micelles. At hydrophilic surfaces such as glass, spreading of vesicles involves adsorption, rupture, and sliding of a single bilayer on a lubricating film of the solvent. We have provided the first evidence that a different mechanism is involved in spreading the vesicles at gold. The molecules released by rupture of vesicles self-assemble into an ordered film, and the assembly is controlled by the chain-substrate interaction.     

Rotational polymorphism in 2-naphthalenethiol SAMs on Au(111)

We evidence by STM that 2-naphthalenethiol self-assembled monolayers formed at the n-tetradecane/Au(111) interface coexist as two structural phases which both possess molecules into two different orientations (standing and lying). Such a rotational polymorphism is observed and understood at the molecular level for the first time.