Decorated Ru/Au(111) and Os/Au(111) surfaces: An in situ STM and electrochemistry study

The structure and reactivity of bimetallic electrodes obtained by spontaneous deposition of Ru and Os on Au(111) single-crystal surfaces are studied. In situ electrochemical STM and cyclic voltammetry are used to characterize a wide range of surface morphologies thus produced. The STM results on Ru/Au(111) demonstrate a pronounced step decoration, while a random distribution of Ru nuclei, quite uniform in size, occurs on terraces. Osmium deposits show a slight preference for deposition on steps, but it also occurs readily on terraces. However, many of the Os islands grow into multilayer heights. The coverage of the Au(111) by the deposited Ru or Os islands for a particular solution concentration depends on the deposition time. Nanostructures of Ru and Os are tested for catalytic behavior and correlated to CO oxidation activity as measured by CO stripping voltammetry. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1385–1392. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

The structure, growth and reactivity of electrodeposited Ru/Au(111) surfaces

In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru/Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H₂SO₄ solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (>0.1 V), (defective) monolayer (≥−0.1 V), and multilayer films (<−0.1 V). At potentials >0.6 V irreversible formation of Ru oxide/hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at ≈0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film.     

Self-assembly of alkanols on Au(111) surfaces

Self-assembled monolayers (SAMs) of alkanols (1-C(N)H(2N+1)OH) with varying carbon-chain lengths (N = 10-30) have been systematically studied by means of scanning tunneling microscopy (STM) at the interfaces between alkanol solutions (or liquids) and Au(111) surfaces. The carbon skeletons were found to lie flat on the surfaces. This orientation is consistent with SAMs of alkanols on highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces, and also with alkanes on reconstructed Au(111) surfaces. This result differs from a prior report, which claimed that 1-decanol molecules (N = 10) stood on their ends with the OH polar groups facing the gold substrate. Compared to alkanes, the replacement of one terminal CH3 group with an OH group introduces new bonding features for alkanols owing to the feasibility of forming hydrogen bonds. While SAMs of long-chain alkanols (N > 18) resemble those of alkanes, in which the aliphatic chains make a greater contribution, hydrogen bonding plays a more important role in the formation of SAMs of short-chain alkanols. Thus, in addition to the titled lamellar structure, a herringbone-like structure, seldom seen in SAMs of alkanes, is dominant in alkanol SAMs for values of N < 18. The odd-even effect present in alkane SAMs is also present in alkanol SAMs. Thus, the odd N alkanols (alkanols with an odd number of carbon atoms) adopt perpendicular lamellar structures owing to the favorable interactions of the CH3 terminal groups, similar to the result observed for odd alkanes. In contrast to alkanes on Au(111) surfaces, for which no SAMs on an unreconstructed gold substrate were observed, alkanols are capable of forming SAMs on either the reconstructed or the unreconstructed gold surfaces. Structural models for the packing of alkanol molecules on Au(111) surfaces have been proposed, which successfully explain these experimental observations.     

Charge transfer interactions of a Ru(II) dye complex and related ligand molecules adsorbed on Au(111)

The interaction of the dye molecule, N3 (cis-bis(isothiocyanato)bis(2,2(')-bipyridyl-4,4(')-dicarboxylato)-ruthenium(II)), and related ligand molecules with a Au(111) surface has been studied using synchrotron radiation-based electron spectroscopy. Resonant photoemission spectroscopy (RPES) and autoionization of the adsorbed molecules have been used to probe the coupling between the molecules and the substrate. Evidence of charge transfer from the states near the Fermi level of the gold substrate into the lowest unoccupied molecular orbital (LUMO) of the molecules is found in the monolayer RPES spectra of both isonicotinic acid and bi-isonicotinic acid (a ligand of N3), but not for the N3 molecule itself. Calibrated x-ray absorption spectroscopy and valence band spectra of the monolayers reveals that the LUMO crosses the Fermi level of the surface in all cases, showing that charge transfer is energetically possible both from and to the molecule. A core-hole clock analysis of the resonant photoemission reveals a charge transfer time of around 4 fs from the LUMO of the N3 dye molecule to the surface. The lack of charge transfer in the opposite direction is understood in terms of the lack of spatial overlap between the π?-orbitals in the aromatic rings of the bi-isonicotinic acid ligands of N3 and the gold surface.     

Chiral structures of 6,12-dibromochrysene on Au(111) and Cu(111) surfaces

Nanoscale low-dimensional chiral architectures are increasingly receiving scientific interest, because of their potential applications in many fields such as chiral recognition, separation and transformation. Using 6,12-dibromochrysene (DBCh), we successfully constructed and characterized the large-area two-dimensional chiral networks on Au(111) and one-dimensional metal-liganded chiral chains on Cu(111) respectively. The reasons and processes of chiral transformation of chiral networks on Au(111) were analyzed. We used scanning tunneling spectroscopy (STS) to analyze the electronic state information of this chiral structure. This work combines scanning tunneling microscopy (STM) with non-contact atomic force microscopy (nc-AFM) techniques to achieve ultra-high-resolution characterization of chiral structures on low-dimensional surfaces, which may be applied to the bond analysis of functional nanofilms. Density functional theory (DFT) was used to simulate the adsorption behavior of the molecular and energy analysis in order to verify the experimental results.     

Dual-scale modeling of benzene adsorption onto Ni(111) and Au(111) surfaces in explicit water.

We present a multiscale modeling approach for studying interactions of organic molecules with metal surfaces in explicit water. The approach is based on combining adsorption energies of isolated molecules on transition metal surfaces calculated by ab initio density functional methods and classical molecular dynamics simulations using atomistically detailed force fields. The interaction of benzene with Ni(111) and Au(111) surfaces was studied. It is shown that a strong affinity of water for the hydrophilic surfaces makes benzene adsorption on Au thermodynamically unfavorable, while on Ni there is no preference. The work presented here serves as a first step in modeling the interactions of larger organic molecules with metal surfaces.     

On surface states of transition metal carbides,nitrides, and oxides: (001), (011), and (111) surfaces

The simple semiquantitative approach to the calculation of the energy of surface states, proposed recently by the authors, is applied to high-symmetry surfaces of selected transition metal carbides, nitrides, and oxides. The results are compared with recent experimental and theoretical data. The necessity to include the changes in the potential at the crystal surface is indicated.     

The chemistry of trimethylamine on Ru(001) and O/Ru(001)

The interaction and reactivity of trimethylamine (TMA) has been studied over clean and oxygen-covered Ru(001) under UHV conditions, as a model for the chemistry of high-density hydrocarbons on a catalytic surface. The molecule adsorbs intact at surface temperature below 100 K with the nitrogen end directed toward the surface, as indicated from work function change measurements. At coverage less than 0.05 ML (relative to the Ru substrate atoms), TMA fully dissociates upon surface heating, with hydrogen as the only evolving molecule following temperature-programmed reaction/desorption (TPR/TPD). At higher coverage, the parent molecule desorbs, and its desorption peak shifts down from 270 K to 115 K upon completion of the first monolayer, indicating a strong repulsion among neighbor molecules. The dipole moment of an adsorbed TMA molecule has been estimated from work function study to be 1.4 D. Oxygen precoverage on the ruthenium surface has shown efficient reactivity with TMA. It shifts the surface chemistry toward the production of various oxygen-containing stable molecules such as H2CO, CO2, and CO that desorb between 200 and 600 K, respectively. TMA at a coverage of 0.5 ML practically cleans off the surface from its oxygen atoms as a result of TPR up to 1650 K, in contrast to CO oxidation on the O/Ru(001) surface. The overall reactivity of TMA on the oxidized ruthenium surface has been described as a multistep reaction mechanism.     

Self-assembly of normal alkanes on the Au (111) surfaces

The self-assembled monolayers (SAMs) of normal alkanes (n-C(n)H(2n+2)) with different carbon chain lengths (n=14-38) in the interfaces between alkane solutions (or liquids), and the reconstructed Au (111) surfaces have been systematically studied by means of scanning tunneling microscopy (STM). In contrast to previous studies, which concluded that some n-alkanes (n=18-26) can not form well-ordered structures on Au (111) surfaces, we observed SAM formations for all these n-alkanes without any exceptions. We find that gold reconstruction plays a critical role in the SAM formation. The alkane monolayers adopt a lamellar structure in which the alkane molecules are packed side-by-side, to form commensurate structures with respect to the reconstructed Au (111) surfaces. The carbon skeletons are found to lie flat on the surfaces, which is consistent with the infrared spectroscopic studies. Interestingly, we find that two-dimensional chiral lamellar structures form for alkanes with an even carbon number due to the specific packing of alkane molecules in a tilted lamella. Furthermore, we find that the orientation of alkane molecules deviates from the exact [011] direction, because of the intermolecular interactions among the terminal methyl groups of neighboring lamellae; this results in differences of molecular orientation between mirror structures of adjacent zigzag alkane lamellae. Structural models have been proposed, that shed new light on monolayer formation.     

离子液体中Au(111)和Pt(111)表面Ge欠电位沉积的现场STM研究

本文研究BMIPF₆离子液体中Au(111)和Pt(111)表面Ge的电沉积行为. 循环伏安法测试结果表明,在含0.1 mol·L^-1 GeCl₄的BMIPF₆溶液Au(111)和Pt(111)表面均有两个与Ge沉积过程相关的还原峰. 第一个还原峰包含了Ge⁴⁺还原成Ge²⁺及Ge的欠电位沉积,第二个还原峰对应Ge的本体沉积. 现场扫描隧道显微镜研究结果表明,Ge在Au(111)和Pt(111)表面均有两层欠电位沉积. 第一层欠电位沉积厚度约为0.25 nm、形貌平整、带有缝隙的亚单层结构. 第二层欠电位沉积形貌相对粗糙的点状团簇结构. 该欠电位沉积过程伴随表面合金化.     

Silicon surfaces as electron acceptors: dative bonding of amines with Si(001) and Si(111) surfaces.

The bonding of the trimethylamine (TMA) and dimethylamine (DMA) with crystalline silicon surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and density-functional computational methods. XPS spectra show that TMA forms stable dative-bonded adducts on both Si(001) and Si(111) surfaces that are characterized by very high N(1s) binding energies of 402.2 eV on Si(001) and 402.4 eV on Si(111). The highly ionic nature of these adducts is further evidenced by comparison with other charge-transfer complexes and through computational chemistry studies. The ability to form these highly ionic charge-transfer complexes between TMA and silicon surfaces stems from the ability to delocalize the donated electron density between different types of chemically distinct atoms within the surface unit cells. Corresponding studies of DMA on Si(001) show only dissociative adsorption via cleavage of the N-H bond. These results show that the unique geometric structures present on silicon surfaces permit silicon atoms to act as excellent electron acceptors.     

Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.     

Complex orientational ordering of C60 molecules on Au(111)

The orientation and adsorption site for C(60) molecules on Au(111) has been studied using low temperature scanning tunneling microscopy. A complex orientational ordering has been observed for molecules inside the "in-phase" (R0°) domain. A 7-molecule cluster consisting a central molecule sitting atop of a gold atom and 6 tilted surrounding molecules is identified as the structural motif. The 2√3 × 2√3-R30° phase consists of molecules bonding to a gold atomic vacancies with a preferred azimuthal orientation. The quasi-periodic R14° phase is composed of groups of similarly oriented molecules with the groups organized into a 4√3 × 4√3-R30° like super-lattice unit cell.     

Electrochemical detection of cytosine and 5-methylcytosine on Au(111) surfaces

In this communication we report a voltammetric study of the adsorption–desorption of cytosine (C) and methylcytosine (mC) on well-defined gold (Au) electrodes. The voltammetric measurements clearly indicate that these processes are extremely sensitive to the Au surface structure and in particular to the presence of (111) surface domains. Interestingly, on Au(111) surfaces, a linear correlation between the C and mC concentrations (logarithm scale) and the peak potential of the main voltammetric feature is found. In addition, in the simultaneous presence of both molecules, mC governs the electrochemical response, which has allowed its accurate quantification in C-mC mixtures. In situ FTIR spectroscopic measurements have been carried out to deepen on this mC electrochemical sensitivity. This research may contribute to the future development of an electrochemical sensor for the determination of the degree of methylation in DNA.     

Electronic and structural properties of oligophenylene ethynylenes on Au(111) surfaces

The interaction of oligophenylene ethynylene (OPE) on the (111) surface of a gold slab resembling a self-assembled monolayer (SAM) is investigated using ab initio density functional theory calculations. The authors performed a full optimization including all atoms in the OPE and in the slab to better understand OPE adsorption on the surface. It is found that OPE has two energetically favorable adsorption sites on the Au surface with relatively different molecular geometries: the nontop site adsorption greatly modifies the (111) surface structure; however, the extensive electron interactions enable a delocalized electron density distribution, implying an improved conductivity between OPE and Au, and the top site which is 0.9 eV higher in energy than the nontop and features weaker Au-S bonds. Interestingly the on top configuration shows a strong spin imbalance along the molecule and the nontop shows a small spin imbalance on the surface. This feature is of strong interest for the development of resonators for the detection of chemical and biological agents. They have also calculated the frequency spectrum of these SAMs, which yield deformations in the gold surface yielding peak frequency shifts specific to each absorption site.     

An SFG and DFG investigation of Au(111), Au(100), Au(110) and Au(210) electrodes in contact with aqueous solutions containing KCN

In this paper, the behaviour of Au(111), (100), (110) and (210) electrodes in contact with pH-neutral aqueous solutions of KCN has been studied as a function of potential by means of in situ sum frequency generation (SFG) and difference frequency generation (DFG) spectroscopies. The contribution of both free and bound electrons has been included. Spectroelectrochemical results were complemented with cyclic voltammetric measurements. The main emphasis in this work has been placed on systematising and quantifying the interaction between the vibrational and electronic structures of the electrodic interfaces studied by the systematic comparison of SFG and DFG spectra measured under the same electrochemical conditions for different crystal orientations.     

An Electron‐Rich Cyclic (Alkyl)(Amino)Carbene on Au(111), Ag(111), and Cu(111) Surfaces

The structural properties and binding motif of a strongly σ‐electron‐donating N‐heterocyclic carbene have been investigated on different transition‐metal surfaces. The examined cyclic (alkyl)(amino)carbene (CAAC) was found to be mobile on surfaces, and molecular islands with short‐range order could be found at high coverage. A combination of scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations highlights how CAACs bind to the surface, which is of tremendous importance to gain an understanding of heterogeneous catalysts bearing CAACs as ligands.     

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.     

Deposition of thermally unstable molecules with the spray-jet technique on Au(111) surface

A porphyrin derivative (5,15-bis(4-ethynylphenyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrin: trans-BETBPP) possessing chemically reactive substituents was successfully deposited on an Au(111) surface with a new molecular beam deposition system with use of a spray-jet technique (Spray-jet-MBD) without denaturing the molecules. The deposited molecular overlayers were observed at 77 K under ultrahigh vacuum condition by scanning tunneling microscopy (STM). They form two different overlayer structures: a linear arrangement and a square lattice structure. In these overlayers, some molecules were accidentally moved by STM tip agitation, which indicates that the molecules were not polymerized during the deposition process.