Bonding mechanism and atomic geometry of an ordered hydroxyl overlayer on Pt(111)

Exposing water to a (2 x 2)-O precovered Pt(111) surface at 100 K and subsequently annealing at 155 K led to the formation of a well-ordered (square root 3 x square root 3)R30 degrees overlayer. The structure of this overlayer is determined by DFT and full dynamical LEED calculations. There are two O containing groups per (square root 3 x square root 3)R30 degrees unit cell and both occupy near on-top positions with a Pt-O bond length of (2.11 +/- 0.04) A. DFT calculations determined the hydrogen positions of the OH species and clearly indicate hydrogen bonds between the neighboring adsorbed OH groups whose interaction is mainly of electrostatic nature. A theoretical comparison with H(2)O shows the hybridization of OH on Pt(111) to be sp(3).     

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.     

Nanoscale oxidation of Cu100: oxide morphology and surface reactivity

Surface oxidation of Cu(100) in O(2) has been investigated in situ by x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy (XAES), and scanning tunneling microscopy (STM) as a function of surface temperature (T(S)=303-423 K) and O(2) pressure (p(O(2) )=3.7 x 10(-2)-213 mbars). Morphology of the oxide on the surface and in the near surface layers is characterized by utilizing STM and the inelastic electron background of the XAES O KLL signal. Analysis of the peak shape of the XAES Cu LMM facilitates the quantification of Cu, Cu(2)O, and CuO surface concentrations. The authors conclude that the surface oxidation of Cu(100) proceeds in three distinct steps: (1) Dissociative adsorption of O(2) and the onset of Cu-(2 square root 2 x square root 2)R45 degrees -O (theta(O)=0.5 ML) surface reconstruction, (2) initial formation of Cu(2)O and the appearance of 1.8 A high elongated islands that also adopt the Cu-(2 square root 2 x square root 2)R45 degrees -O structure, and (3) formation of highly corrugated Cu-O islands which together with the surface reconstruction strongly enhance the reactivity of the surface towards further oxide formation. Both Cu(2)O and CuO formations are enhanced by increased surface temperature, but no pressure dependence can be seen.     

Mechanism of CO oxidation reaction on O-covered Pd(111) surfaces studied with fast x-ray photoelectron spectroscopy: change of reaction path accompanying phase transition of O domains

We studied the mechanism of CO oxidation on O-precovered Pd(111) surfaces by means of fast x-ray photoelectron spectroscopy (XPS). The oxygen overlayer is compressed upon CO coadsorption from a p(2 x 2) structure into a (square root(3) x square root(3))R30 degrees structure and then into a p(2 x 1) structure with increasing CO coverage. These three O phases exhibit distinctly different reactivities. (1) The p(2 x 2) phase does not react with CO unless the surface temperature is sufficiently high (<290 K). (2) In the square root(3) x square root(3))R30 degrees phase, the reaction occurs exclusively at island peripheries. CO molecules in a high-density phase formed under CO exposure react with oxygen atoms, leading to quite a small apparent activation energy. (3) The reaction proceeds uniformly over the islands in the p(2 x 1) phase.     

Density functional theory investigation of benzenethiol adsorption on Au(111)

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.     

Comparative investigation of underpotential deposition of Ag from aqueous and ionic electrolytes: An electrochemical and in situ STM study

Underpotential deposition (UPD) of Ag on Au(111) has been studied with two different electrolytes: aqueous 0.1 M H2SO4 solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICl + AlCl3. Of particular interest is the distinct behavior of 2D phase formation at both interfaces, which has been investigated by cyclic and linear sweep voltammetry in combination with in situ electrochemical scanning tunneling microscopy (STM). It is found that one monolayer (ML) of Ag is formed in the UPD region in both electrolytes. In aqueous solution, atomically resolved STM images at 500 mV versus Ag/Ag+ show a (3 x 3) adlayer of Ag, whereas after sweeping the potential just before the commencement of the bulk Ag deposition, a transition from expanded (3 x 3) to pseudomorphic ML of Ag on Au(111) occurs. In BMICl-AlCl3, the first UPD process of Ag exhibits two peaks at 410 and 230 mV indicating that two distinct processes on the surface take place. For the first time, STM images with atomic resolution reveal a transition from an inhomogeneous to an ordered phase with a (square root of 3 x square root of 3)R30 degrees structure and an adsorption of AlCl4- anions having a superlattice of (1.65 x square root of 3)R30 degrees preceding the deposition of Ag.     

Atomic arrangements inside Ru and Os nanoislands spontaneously deposited on Pt(111)

The atomic arrangements inside Ru and Os nanoislands spontaneously deposited on Pt(111) electrode surface were observed with electrochemical scanning tunneling microscopy. The surperlattice of the pristine Ru nanodeposits is (square root of 3 x square root of 3)R30 degrees -RuO+. Upon reduction, the Ru nanodeposits are compressed to a uniaxially incommensurate (square root of 3 x square root of 2)R30 degrees -Ru structure, which does not change during the following reoxidation. The atomic arrangement inside the pristine Os nanodeposits is an incommensurate (square root of 2 x square root of 2)-OsO+ structure, which does not transform during the subsequent reduction-oxidation cycles. The structures of the Ru and Os nanodeposits are discussed in terms of removal and insertion of oxygen ions depending on electrode potential.     

Adlayers of Sb irreversibly adsorbed on Pt(111): an electrochemical scanning tunneling microscopy study

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

Combined STM and FTIR characterization of terphenylalkanethiol monolayers on Au(111): effect of alkyl chain length and deposition temperature

Self-assembled monolayers (SAMs) of 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)n-SH (TPn, n = 1-6) on Au (111) substrates were studied using scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRRAS). When the SAMs were prepared at room temperature (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both molecular orientation and packing density. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2 square root(3) x square root(3))R30 degree molecular arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5 square root(3) x 3) rectangular unit cell was found. The packing density for the even-numbered TPn SAMs is 25% lower than that for the odd-numbered TPn SAMs. When the SAMs were prepared at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing density. In the case of TP2, instead of the (2 square root(3) x square root(3))R30 degree structure formed at room temperature, a c(5 square root(3) x 3) structure was observed. For TP6 SAMs, the room-temperature c(5 square root(3) x 3) structure was replaced by a (6 square root(3) x 2 square root(3))R30 degree structure.     

X-ray emission spectroscopy of (2 square root of 3 x 2 square root of 3)R30 degrees CO/Ru(0001): comparison to c(2x2)CO/Ni(100) and c(2x2)CO/Cu(100)

The atom specific electronic structure of (2 square root of 3 x 2 square root of 3)R30 degrees CO on hcp Ru(0001) has been determined with resonantly excited x-ray emission spectroscopy. We find that the general features of the local adsorbate electronic structure are similar to the situation of CO adsorbed on the fcc metals Ni(100) and Cu(100). The interpretation of the surface chemical bond of (2 square root of 3 x 2 square root of 3)R30 degrees CO/Ru(0001) based on the direct application of the local, allylic model from on-top adsorption on the fcc(100) surfaces Ni(100) and Cu(100) explains many aspects of the surface chemical bond. However, also nonlocal contributions like adsorbate-adsorbate interaction and the deviation from upright on-top adsorption on the Ru(0001) surface influence observables like the heat of adsorption and the Me-CO bond strength.     

Adlayers of C60-C60 and C60-C70 fullerene dimers formed on au(111) in benzene solutions studied by STM and LEED

Scanning tunneling microscopy (STM) and low-energy electron diffraction were used to reveal the structures of ordered adlayers of [2+2]-type C60-C60 fullerene dimer (C120) and C60-C70 cross-dimer (C130) formed on Au(111) by immersingit in abenzene solution containing C120 or C130 molecules. High-resolution STM images clearly showed the packing arrangements and the electronic structures of C120 and C130 on the Au(111) surface in ultrahigh vacuum. The (2 square root3 x 4square root3)R30 degrees, (2square root3 x 5square root3)R30 degrees, and (7 x 7) structures were found for the C120 adlayer on the Au(111) surface, whereas C130 molecules were closely packed on the surface. Each C60 or C70 monomer cage was discerned in the STM image of a C130 molecule.     

Reactivity of V2O3(0001) surfaces: molecular vs dissociative adsorption of water

The adsorption of water on V2O3(0001) surfaces has been investigated by thermal desorption spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy with use of synchrotron radiation. The V2O3(0001) surfaces have been generated in epitaxial thin film form on a Rh(111) substrate with three different surface terminations according to the particular preparation conditions. The stable surface in thermodynamic equilibrium with the bulk is formed by a vanadyl (VO) (1x1) surface layer, but an oxygen-rich (radical3xradical3)R30 degrees reconstruction can be prepared under a higher chemical potential of oxygen (microO), whereas a V-terminated surface consisting of a vanadium surface layer requires a low microO, which can be achieved experimentally by the deposition of V atoms onto the (1x1) VO surface. The latter two surfaces have been used to model, in a controlled way, oxygen and vanadium containing defect centres on V2O3. On the (1x1) V=O and (radical3xradical3)R30 degrees surfaces, which expose only oxygen surface sites, the experimental results indicate consistently that the molecular adsorption of water provides the predominant adsorption channel. In contrast, on the V-terminated (1/radical3x1/radical3)R30 degrees surface the dissociation of water and the formation of surface hydroxyl species at 100 K is readily observed. Besides the dissociative adsorption a molecular adsorption channel exists also on the V-terminated V2O3(0001) surface, so that the water monolayer consists of both OH and molecular H2O species. The V surface layer on V2O3 is very reactive and is reoxidised by adsorbed water at 250 K, yielding surface vanadyl species. The results of this study indicate that V surface centres are necessary for the dissociation of water on V2O3 surfaces.     

A novel series of vanadium-sulfite polyoxometalates: synthesis, structural, and physical studies

Reaction of NH4VO3 with sulfur dioxide affords the hexanuclear cluster (NH4)2(Et4N)[(V(IV)O)6(mu4-O)2(mu3-OH)2(mu3-SO3)4(H2O)2]Cl x H2O (1), and the decapentanuclear host-guest compound (Et4N)5{Cl subset [(VO)15(mu3-O)18(mu-O)3]} x 3 H2O (2). Sequential addition of magnesium oxide to an acidic aqueous solution of NH4VO3 (pH approximately 0) followed by (NH4)2SO3 resulted in the formation of either the non-oxo polymeric vanadium(IV) compound trans-(NH4)2[V(IV)(OH)2(mu-SO3)2] (3) or the polymeric oxovanadium(IV) sulfite (NH4)[V(IV)O(SO3)1.5(H2O)] x 2.5 H2O (4) at pH values of 6 and 4, respectively. The decameric vanadium(V) compound {Na4(mu-H2O)8(H2O)6}[Mg(H2O)6][V(V)10(O)8(mu6-O)2(mu3-O)14] x 3 H2O (5) was synthesised by treating an acidic aqueous solution of NH4VO3 with MgO and addition of NaOH to pH approximately 6. All the compounds were characterised by single-crystal X-ray structure analysis. The crystal structure of compound 1 revealed an unprecedented structural motif of a cubane unit [M4(mu4-O)2(mu3-OH)2] connected to two other metal atoms. Compound 3 comprises a rare example of a non-oxo vanadium(IV) species isolated from aqueous solution and in the presence of the reducing agent SO3(2-), while compound 4 represents a rare example of an open-framework species isolated at room temperature (20 degrees C). In addition to the synthesis and crystallographic studies, we report the IR and magnetic properties (for 1, 2 and 3) of these vanadium clusters as well as theoretical studies on compound 3.     

Electrocatalytic oxidation of formic acid and methanol on Pt deposits on Au(111)

This work presents characteristics of Pt deposits on Au(111) obtained by the use of spontaneous deposition and investigated by electrochemical scanning tunneling microscopy (EC-STM). On such prepared and STM characterized Au(111)/Pt surfaces, we studied electrocatalytic oxidation of formic acid and methanol. We show that the first monatomic layer of Pt displays a (square root 3 x square root 3)R30 degrees surface structure, while the second layer is (1 x 1). After prolonged deposition, multilayer Pt deposits are formed selectively on Au(111) surface steps and are 1-20 nm wide and one to five layers thick. On the optimized Au(111)/Pt surface, formic acid oxidation rates are enhanced by a factor of 20 compared to those of pure Pt(111). The (square root 3 x square root 3)R30 degrees-Pt yields very low methanol oxidation rates, but the rates increase significantly with further Pt growth.     

Scanning tunneling microscopy of template-stripped Au surfaces and highly ordered self-assembled monolayers

Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparation method for SAMs deposited from both vapor phase and solution phase.     

Helium atom scattering study of the interaction of water with the BaF2(111) surface

The interaction of water with the BaF2(111) single crystal surface is investigated using the helium atom scattering technique. It is found that H2O forms a long-range ordered two-dimensional (2D) phase with (1 x 1) translational symmetry already after an exposure of 3 L (1 L=10(-6) Torr s) at temperatures below 150 K. The activation energy for desorption of the saturated 2D phase, which is assigned to a bilayer, is estimated to be 46+/-2 kJ mol(-1), corresponding to a desorption temperature of 165 K. The desorption of multilayers was observed at 150 K, consistent with a binding energy of 42+/-2 kJ mol(-1). Before completion and after desorption of the saturated 2D phase, a superstructure consistent with a disordered (square root of 3 x square root of 3)R30 degrees lattice has been observed, which is attributed to the first layer of water with a coverage of one molecule per surface unit cell, in accordance with recent theoretical studies. Desorption of this phase is observed at temperatures above 200 K, consistent with an unexpectedly strong bonding of the molecules to the substrate.     

Square, hexagonal, and row phases of PTCDA and PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees

We have investigated the ordered phases of the perylene derivatives perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and the imide analogue PTCDI on the Ag-Si(111)square root(3) x square root(3)R30 degrees surface using scanning tunneling microscopy. We find that PTCDA forms square, hexagonal, and herringbone phases, which coexist on the surface. The existence of a square phase on a hexagonal surface is of particular interest and is a result of a near commensurability between the molecular dimensions and the surface lattice. Contrast variations across the square islands arise from PTCDA molecules binding to different sites on the surface. PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees forms extended rows, as well as two-dimensional islands, both of which are stabilized by hydrogen bonding mediated by the presence of imide groups. We present models for the molecular arrangements in all these phases and highlight the role of hydrogen bonding in controlling this order.     

Co(II)5(OH)6(SO4)2(H2O)4: the first ferromagnet based on a layered cobalt-hydroxide pillared by inorganic...OSO3-Co(H2O)4-O3SO..

The synthetic mineral Co(II)5(OH)6(SO4)2(H2O)4 (1), obtained by hydrothermal reaction of CoSO4.7H2O and NaOH at 165 degrees C and consisting of brucite-like Co4(OH)6O2 layers pillared by OSO3-Co(H2O)4-O3SO, is a ferromagnet (T(Curie)= 12 K, Hc= 580 Oe).     

First-principles study of K and Cs adsorbed on Pd(111)

The adsorptions of K and Cs on Pd(111) were studied by the density functional calculations within the generalized gradient approximation. The site preference, bonding character, work function, and electron structure of the system were analyzed. For K and Cs adsorption, the hcp hollow site was found to be preferred for all the coverages investigated. The calculated adsorption geometries for (2 x 2) and (square root 3 x square root 3)R30 degrees phases are both in reasonable agreement with the observed results. The decrease of the work function upon the adsorption of K and Cs can be attributed to a dipole moment associated with the polarized adsorbate atom, which is characterized by depletion of the electron charge in the alkali metal layer and a charge accumulation in the interface region. Our results indicate that the bonding of alkali metal with the Pd(111) surface has a mixed ionic and metallic bond character at low coverage and a metallic bond of covalent character at high coverage.     

Role of the anion in the underpotential deposition of cadmium on a Rh(111) electrode: probed by voltammetry and in situ scanning tunneling microscopy

In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were employed to examine the underpotential deposition (UPD) of cadmium on a rhodium(111) electrode in sulfuric and hydrochloric acids. The (bi)sulfate and chloride anions in the electrolytes played a main role in controlling the number and arrangement of Cd adatoms. Deposition of Cd along with hydrogen adsorption occurred near 0.1 V (vs reversible hydrogen electrode) in either 0.05 M H2SO4 or 0.1 M HCl containing 1 mM Cd(ClO4)2. These coupled processes resulted in an erroneous coverage of Cd adatoms. The process of Cd deposition shifted positively to 0.3 V and thus separated from that of hydrogen in 0.05 M H2SO4 containing 0.5 M Cd2+. The amount of charge (80 microC/cm2) for Cd deposition in 0.5 M Cd2+ implied a coverage of 0.17 for the Cd adatoms, which agreed with in situ STM results. Regardless of [Cd2+], in situ STM imaging revealed a highly ordered Rh(111)-(6 x 6)-6Cd + HSO4- or SO42- structure in sulfuric acid,. In hydrochloric acid, in situ STM discerned a (2 x 2)-Cd + Cl structure at potentials where Cd deposition commenced. STM atomic resolution showed roughly one-quarter of a monolayer of Cd adatoms were deposited, ca. 50% more than in sulfuric acid. Dynamic in situ STM imaging showed potential dependent, reversible transformations between the (6 x 6) Cd adlattices and (square root 3 x square root 7)-(bi)sulfate structure, and between (2 x 2) and (square root 7 x square root 7)R19.1 degrees -Cl structures. The fact that different Cd structures observed in H2SO4 and HCl entailed the involvement of anions in Cd deposition, i.e. (bi)sulfate and chloride anions were codeposited with Cd adatoms on Rh(111).