Charge and adsorption height dependence of the self-metalation of porphyrins on ultrathin MgO(001) films.

We have experimentally determined the adsorption structure, charge state, and metalation state of porphin, the fundamental building block of porphyrins, on ultrathin Ag(001)-supported MgO(001) films by scanning tunneling microscopy and photoemission spectroscopy, supported by calculations based on density functional theory. By tuning the substrate work function to values below and above the critical work function for charging, we succeeded in the preparation of 2H-P monolayers which contain negatively charged and uncharged molecules. It is shown that the porphin molecules self-metalate at room temperature, forming the corresponding Mg-porphin, irrespective of their charge state. This is in contrast to self-metalation of tetraphenyl porphyrin (TPP), which occurs on planar MgO(001) only if the molecules are negatively charged. The different reactivity is explained by the reduced molecule-substrate distance of the planar porphin molecule compared to the bulkier TPP. The results of this study shed light on the mechanism of porphyrin self-metalation on oxides and highlight the role of the adsorption geometry on the chemical reactivity.

Charge-Promoted Self-Metalation of Porphyrins on an Oxide Surface.

Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.

Bonding and thermal stability of cysteine on single-crystalline iron oxide surfaces and Pt(111).

Model studies on the adsorption of simple amino acids on single-crystalline substrates provide the basis for understanding the interactions of more complex biomolecules with inorganic materials. In this study, the adsorption and thermal stability of cysteine on Pt(111) and on monolayer FeO(111)/Pt(111) and thick Fe3O4(111)/Pt(111) films have been investigated in ultrahigh vacuum by means of x-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programmed desorption. At low adsorption temperature (160 K), cysteine stays intact on all studied surfaces. However, the thermal stability differs dramatically. Decomposition via dehydrogenation, decarbonylation/decarboxylation, and deamination starts already below room temperature on Pt(111). By contrast, adsorbed cysteine is stable up to 500 K on Fe3O4(111) and then gets oxidized in a redox reaction involving the Fe3O4 substrate. FeO(111)/Pt(111) is a special case, where decarbonylation of cysteine occurs below room temperature, which is most likely initiated by a structural modification and concomitant charge transfer at the Pt-FeO interface induced by the carboxyl group of the adsorbed cysteine. The experimental results suggest that cysteine binds to the surfaces with the carboxyl group on the iron oxides and with the carboxyl and amino groups on Pt(111).

Toward an Understanding of Selective Alkyne Hydrogenation on Ceria: On the Impact of O Vacancies on H2 Interaction with CeO2(111).

Ceria (CeO2) has recently been found to be a promising catalyst in the selective hydrogenation of alkynes to alkenes. This reaction occurs primarily on highly dispersed metal catalysts, but rarely on oxide surfaces. The origin of the outstanding activity and selectivity observed on CeO2 remains unclear. In this work, we show that one key aspect of the hydrogenation reaction-the interaction of hydrogen with the oxide-depends strongly on the presence of O vacancies within CeO2. Through infrared reflection absorption spectroscopy on well-ordered CeO2(111) thin films and density functional theory (DFT) calculations, we show that the preferred heterolytic dissociation of molecular hydrogen on CeO2(111) requires H2 pressures in the mbar regime. Hydrogen depth profiling with nuclear reaction analysis indicates that H species stay on the surface of stoichiometric CeO2(111) films, whereas H incorporates as a volatile species into the volume of partially reduced CeO2-x(111) thin films (x ∼ 1.8-1.9). Complementary DFT calculations demonstrate that oxygen vacancies facilitate H incorporation below the surface and that they are the key to the stabilization of hydridic H species in the volume of reduced ceria.

Nanostructuring of Au(111) during the Adsorption of an Aromatic Isocyanide from Solution.

We present a combined vibrational and morphological characterization of the self-assembly of 1,4-phenylene-diisocyanide (PDI) on Au(111) from methanol solution. Vibrational sum frequency generation (vSFG) and scanning tunneling microscopy (STM) have been applied to determine the adsorption geometry of the PDI-Au adatom complexes as well as the morphological transformations of the Au(111) substrate upon SAM formation from solutions with PDI concentrations in the µM to mM range. At low concentration/coverage, PDI adsorbs in flat adsorption geometry, with both isocyanide groups attached to Au adatoms on the Au(111) surface. Transformation to a standing-up phase is observed with increasing concentration/coverage. In contrast to findings for PDI adsorbed in ultrahigh vacuum, PDI does not form a long-range-ordered monolayer phase when adsorbed from solution. In addition, the Au(111) surface is subjected to structural modifications. Au vacancy islands and ad-islands, which are typical substrate defects formed during the self-assembly of aromatic thiols on Au(111), are also created during PDI adsorption from solution. At low PDI concentration, the Au vacancy islands and ad-islands are found at specific sites mediated by the herringbone reconstruction of the Au(111) surface, giving rise to long-range-ordered structures. These structures do not form during UHV adsorption of PDI on Au(111) nor has a similar ordering effect been observed for any related thiol-SAM system investigated so far.

Electron stimulated hydroxylation of a metal supported silicate film.

Water adsorption on a double-layer silicate film was studied by using infrared reflection-absorption spectroscopy, thermal desorption spectroscopy and scanning tunneling microscopy. Under vacuum conditions, small amounts of silanols (Si-OH) could only be formed upon deposition of an ice-like (amorphous solid water, ASW) film and subsequent heating to room temperature. Silanol coverage is considerably enhanced by low-energy electron irradiation of an ASW pre-covered silicate film. The degree of hydroxylation can be tuned by the irradiation parameters (beam energy, exposure) and the ASW film thickness. The results are consistent with a generally accepted picture that hydroxylation occurs through hydrolysis of siloxane (Si-O-Si) bonds in the silica network. Calculations using density functional theory show that this may happen on Si-O-Si bonds, which are either parallel (i.e., in the topmost silicate layer) or vertical to the film surface (i.e., connecting two silicate layers). In the latter case, the mechanism may additionally involve the reaction with a metal support underneath. The observed vibrational spectra are dominated by terminal silanol groups (ν(OD) band at 2763 cm(-1)) formed by hydrolysis of vertical Si-O-Si linkages. Film dehydroxylation fully occurs only upon heating to very high temperatures (∼ 1200 K) and is accompanied by substantial film restructuring, and even film dewetting upon cycling hydroxylation/dehydroxylation treatment.

Carbon dioxide activation and reaction induced by electron transfer at an oxide-metal interface.

A model system has been created to shuttle electrons through a metal-insulator-metal (MIM) structure to induce the formation of a CO2 anion radical from adsorbed gas-phase carbon dioxide that subsequently reacts to form an oxalate species. The process is completely reversible, and thus allows the elementary steps involved to be studied at the atomic level. The oxalate species at the MIM interface have been identified locally by scanning tunneling microscopy, chemically by IR spectroscopy, and their formation verified by density functional calculations.

Surface core-level binding energy shifts for MgO(100).

Theoretical and experimental results for the surface core-level binding energy, BE, shifts, SCLS, for MgO(100) are presented and the anomalous O(1s) SCLS is interpreted in terms of the surface electronic structure. While the Mg(2p) surface BE shifts to a higher value than bulk by ≈1 eV as expected from the different surface and bulk Madelung potentials, the O(1s) SCLS is almost 0 rather than ≈-1 eV, expected from the Madelung potentials. The distortion of the surface atoms from the spherical symmetry of the bulk Mg and O atoms is examined by a novel theoretical procedure. The anomalous O SCLS is shown to arise from the increase of the effective size of surface O anions.

Measure of surface potential at the aqueous-oxide nanoparticle interface by XPS from a liquid microjet.

We show that the surface potential at a water-oxide nanoparticle (NP) interface, long considered an immeasurable direct quantity, can be measured by X-ray photoelectron spectroscopy (XPS) from a liquid microjet. This new method does not require a priori knowledge of the particles' surface structure or of the ion distribution throughout the electrical double layer for its interpretation and can be applied to any colloidal suspension independent of composition, particle size and shape, and solvent. We demonstrate the application for aqueous suspensions of 9 nm colloidal silica (SiO2) at pH 0.3 and 10.0, where the surface potential changes from positive to negative. The experimental results are compared with calculated surface potentials based on Guoy-Chapman theory and are shown to be in good agreement.

Kinetic Trapping of Charge-Transfer Molecules at Metal Interfaces.

Despite the common expectation that conjugated organic molecules on metals adsorb in a flat-lying layer, several recent studies have found coverage-dependent transitions to upright-standing phases, which exhibit notably different physical properties. In this work, we argue that from an energetic perspective, thermodynamically stable upright-standing phases may be more common than hitherto thought. However, for kinetic reasons, this phase may often not be observed experimentally. Using first-principles kinetic Monte Carlo simulations, we find that the structure with lower molecular density is (almost) always formed first, reminiscent of Ostwald's rule of stages. The phase transitions to the upright-standing phase are likely to be kinetically hindered under the conditions typically used in surface science. The simulation results are experimentally confirmed for the adsorption of tetracyanoethylene on Cu(111) using infrared and X-ray photoemission spectroscopy. Investigating both the role of the growth conditions and the energetics of the interface, we find that the time for the phase transition is determined mostly by the deposition rate and, thus, is mostly independent of the nature of the molecule.

Interplay of Adsorption Geometry and Work Function Evolution at the TCNE/Cu(111) Interface.

The adsorption of organic electron acceptors on metal surfaces is a powerful way to change the effective work function of the substrate through the formation of charge-transfer-induced dipoles. The work function of the interfaces is hence controlled by the redistribution of charges upon adsorption of the organic layer, which depends not only on the electron affinity of the organic material but also on the adsorption geometry. As shown in this work, the latter dependence controls the work function also in the case of adsorbate layers exhibiting a mixture of various adsorption geometries. Based on a combined experimental (core-level and infrared spectroscopy) and theoretical (density functional theory) study for tetracyanoethylene (TCNE) on Cu(111), we find that TCNE adsorbs in at least three different orientations, depending on TCNE coverage. At low coverage, flat lying TCNE dominates, as it possesses the highest adsorption energy. At a higher coverage, additionally, two different standing orientations are found. This is accompanied by a large increase in the work function of almost 3 eV at full monolayer coverage. Our results suggest that the large increase in work function is mainly due to the surface dipole of the free CN groups of the standing molecules and less dependent on the charge-transfer dipole of the differently oriented and charged molecules. This, in turn, opens new opportunities to control the work function of interfaces, e.g., by synthetic modification of the adsorbates, which may allow one to alter the adsorption geometries of the molecules as well as their contributions to the interface dipoles and, hence, the work function.

Corrosion of Passive Aluminum Anodes in a Chloroaluminate Deep Eutectic Solvent for Secondary Batteries: The Bad, the Good, and the Ugly.

The passivity of aluminum is detrimental to its performance as an anode in batteries. Soaking of native oxide-covered aluminum in a chloroaluminate deep eutectic solvent gradually activates the electrode surface, which is reflected in a continuously decreasing open circuit potential. The underlying processes were studied by analyzing the 3 to 7 nm thick layer of native oxide after increasing periods of soaking with secondary neutral mass spectrometry, X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy in a transmission electron microscope. They consistently show permeation of electrolyte species into the layer associated with gradual swelling. After extended periods of soaking at open circuit potentials, local deposits of a range of foreign metals have been found in scanning electron microscopy images of the electrode surface. The pitting corrosion is caused by trace metal ion impurities present in the electrolyte and results in highly nonuniform current density distribution during discharge/charge cycling of battery cells as shown by local deposits of aluminum. The processes during soaking at open circuit potentials have been monitored by electrochemical impedance spectroscopy and could be analyzed by fitting an equivalent circuit model for pitting corrosion.

Impact of Iodine Electrodeposition on Nanoporous Carbon Electrode Determined by EQCM, XPS and In Situ Raman Spectroscopy.

The charging of nanoporous carbon via electrodeposition of solid iodine from iodide-based electrolyte is an efficient and ecofriendly method to produce battery cathodes. Here, the interactions at the carbon/iodine interface from first contact with the aqueous electrolyte to the electrochemical polarization conditions in a hybrid cell are investigated by a combination of in situ and ex situ methods. EQCM investigations confirm the flushing out of water from the pores during iodine formation at the positive electrode. XPS of the carbon surface shows irreversible oxidation at the initial electrolyte immersion and to a larger extent during the first few charge/discharge cycles. This leads to the creation of functional groups at the surface while further reactive sites are consumed by iodine, causing a kind of passivation during a stable cycling regime. Two sources of carbon electrode structural modifications during iodine formation in the nanopores have been revealed by in situ Raman spectroscopy, (i) charge transfer and (ii) mechanical strain, both causing reversible changes and thus preventing performance deterioration during the long-term cycling of energy storage devices that use iodine-charged carbon electrodes.

Orientation, electronic decoupling and band dispersion of heptacene on modified and nanopatterned copper surfaces.

The adsorption of heptacene (7 A) on Cu(110) and Cu(110)-(2 × 1)-O was studied with scanning tunneling microscopy, photoemission orbital tomography and density functional calculations to reveal the influence of surface passivation on the molecular geometry and electronic states. We found that the charge transfer into the 7 A molecules on Cu(110) is completely suppressed for the oxygen-modified Cu surface. The molecules are aligned along the Cu-O rows and uncharged. They are tilted due to the geometry enforced by the substrate and the ability to maximize intermolecular π-π overlap, which leads to strong π-band dispersion. The HOMO-LUMO gap of these decoupled molecules is significantly larger than that reported on weakly interacting metal surfaces. Finally, the Cu-O stripe phase was used as a template for nanostructured molecular growth and to assess possible confinement effects.

Model oxide-supported metal catalysts--comparison of ultrahigh vacuum and solution based preparation of Pd nanoparticles on a single-crystalline oxide substrate.

Using single-crystalline Fe(3)O(4)(111) films grown over Pt(111) in UHV as a model-support, we have characterized the nucleation behaviour and chemical properties of Pd particles grown over the film using different deposition techniques with scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Comparison of Pd/Fe(3)O(4) samples created via Pd evaporation under UHV conditions and those resulting from the solution deposition of Pd-hydroxo complexes reveals that changes in the interfacial functionalization of such samples (i.e. roughening and hydroxylation) govern the differences in Pd nucleation behavior observed over pristine oxides relative to those exposed to alkaline solutions. Furthermore, it appears that other differences in the nature of the Pd precursor state (i.e. gas-phase Pd in UHV vs. [Pd(OH)(2)](n) aqueous complexes) play a negligible role in Pd nucleation and growth behaviour at elevated temperatures in UHV, suggesting facile decomposition of the Pd complexes deposited from the liquid phase. Applying temperature programmed desorption and infrared spectroscopy to probe the CO chemisorption properties of such samples after reduction in different reagents (CO, H(2)) shows the formation of bimetallic PdFe alloys following reduction in H(2), but monometallic Pd particles after CO reduction.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra.

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.

Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography.

Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.

Oxidation of Au by surface OH: nucleation and electronic structure of gold on hydroxylated MgO(001).

The nucleation and electronic structure of vapor-deposited Au on hydroxylated MgO(001) surfaces has been investigated under ultrahigh vacuum conditions. Hydroxylated MgO(001) surfaces with two different hydroxyl coverages, 0.4 and 1 monolayer, respectively, were prepared by exposure to water (D(2)O) at room temperature. Scanning tunneling microscopy experiments show significantly higher gold particle densities and smaller particle sizes on the hydroxylated MgO surface as compared to gold deposited on clean MgO(001). Infrared spectroscopy and X-ray photoelectron spectroscopy experiments were performed to reveal details about the initial nucleation of gold. Gold atoms are found to chemically interact with a specific type of hydroxyl groups on the MgO surface, leading to the formation of oxidized gold particles. The enhanced adhesion of Au particles, which is due to the formation of strong Au-O interfacial bonds, is responsible for the observed higher stability of small Au clusters toward thermal sintering on hydroxylated MgO surfaces. The results are compared to similar studies on Au/TiO(2)(110) model systems and powder samples prepared by the deposition-precipitation route.

Electronic structure of sub-10 nm colloidal silica nanoparticles measured by in situ photoelectron spectroscopy at the aqueous-solid interface.

X-Ray photoelectron spectroscopy has been extended to colloidal nanoparticles in aqueous solution using a liquid microjet in combination with synchrotron radiation, which allowed for depth-dependent measurements. Two distinct electronic structures are evident in the Si 2p photoelectron spectrum of 7 nm SiO(2)-nanoparticles at pH 10. A core-shell model is proposed where only the outermost layer of SiO(2) nanoparticles, which is mainly composed of deprotonated silanol groups, >Si-O(-), interacts with the solution. The core of the nanoparticles is not affected by the solvation process and retains the same electronic structure as measured in vacuum. Future opportunities of this new experiment are also highlighted.