[Show abstract][Hide abstract] ABSTRACT: The adsorption dynamics of *OH and *O species at Pt(1 1 1) and Cu/Pt(1 1 1) near-surface alloy (NSA) surfaces in oxygen-free and O2-saturated 0.1 M HClO4 was investigated. Subsurface Cu modifies the electronic structure at the Pt(1 1 1) surface resulting in weaker bonding to adsorbates like *OH, *H or *O. This provides a basis for the high oxygen reduction activity of the NSA, as predicted by density functional theory calculations. The shift in *OH adsorption of around 0.16 V towards more positive potentials can be clearly monitored in absence of O2 and under the oxygen reduction reaction (ORR) conditions for the Cu/Pt(1 1 1) NSA. In both cases, for Pt(1 1 1) and NSA, the *OH(*O) adsorption dynamics is very similar in the absence of oxygen and under ORR conditions. Therefore, theoretical assumptions about the coverage of adsorbates in the absence of oxygen can be reasonably extrapolated to the situation when oxygen reduction takes place at the surface. A ∼5-fold improvement in the ORR activity over the Pt(1 1 1) at 0.9 V (RHE) was measured for the Cu/Pt(1 1 1) near-surface alloy.
[Show abstract][Hide abstract] ABSTRACT: I could never be without Cu: An active site has been designed for the catalysis of CO electroxidation. This was achieved by incorporating submonolayer amounts of Cu (orange shperes) into a single crystal of Pt (gray spheres). The electrochemical reactivity of this surface was highly sensitive to the exact position of Cu.
Angewandte Chemie International Edition 10/2012; 51(47). · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The plasmon resonance of metal nanostructures affects neighboring semiconductors, quenching or enhancing optical transitions depending on various parameters. These plasmonic properties are currently investigated with respect to topics such as photovoltaics and optical detection and could also have important consequences for photocatalysis. Here the effect of silver nanoparticles of a size up to 30 nm and at maximum 0.50 monolayers on the photocatalytic oxidation of ethylene on TiO2 is studied. Since the plasmon resonance energy of silver nanoparticles is comparable with the TiO2 band gap, dipole–dipole interaction converts excitons into heat at the silver nanoparticle. This indicates that plasmonic interaction with TiO2 semiconductor catalysts can reduce the photo catalytic activity considerably.
Journal of Photochemistry and Photobiology A: Chemistry. 02/2012; 230(1):10-14.
[Show abstract][Hide abstract] ABSTRACT: Copper adsorption on Ru(0001) has been studied by synchrotron radiation. The clean Ru 3d5/2 spectra were found to consist of two components with a binding energy shift of 400 meV. The component with the lower binding energy represents the first layer of ruthenium atoms. Adsorption of copper gives rise to core level shifts of the Ru 3d5/2 components, which were studied as a function of Cu coverage. Experiments were carried out with copper coverages varying from the submonolayer range up to two monolayers of copper. The binding energy of the Cu 2p3/2 level was measured by X-ray photoemission spectroscopy.
Surface Review and Letters 01/2012; 09(02). · 0.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This communication examines the effect of the surface morphology of polycrystalline copper on electroreduction of CO(2). We find that a copper nanoparticle covered electrode shows better selectivity towards hydrocarbons compared with the two other studied surfaces, an electropolished copper electrode and an argon sputtered copper electrode. Density functional theory calculations provide insight into the surface morphology effect.
Physical Chemistry Chemical Physics 11/2011; 14(1):76-81. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The production of fuels directly or indirectly from sunlight represents one of the major challenges to the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and while platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for largescale use. We show that bio-inspired molecular clusters based on molybdenum sulfides and tungsten sulfides mimic nature's enzymes for hydrogen evolution, molybdenum sulfides evolve hydrogen at a slightly higher overpotential than platinum when deposited on various supports. It will be demonstrated how this overpotential can be eliminated by depositing the same type of hydrogen evolution catalyst on p-type Si which can harvest the red part of the solar spectrum. Such a system could constitute the cathode part of a tandem dream device where the red part of the spectrum is utilized for hydrogen evolution while the blue part is reserved for the more difficult oxygen evolution. The samples have been illuminated with a simulated red part of the solar spectrum i.e. long wavelength (" > 620 nm) part of simulated AM 1.5G radiation. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by DFT calculations of the Mo3S4 cluster adsorbed on the hydrogen-terminated silicon surface providing insights into the nature of the active site.
Proceedings of SPIE - The International Society for Optical Engineering 09/2011; · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using model catalysts, we demonstrate that CO desorption from Ru surfaces can be switched from that typical of single crystal surfaces to one more characteristic of supported nanoparticles. First, the CO desorption behaviour from Ru nanoparticles supported on highly oriented pyrolytic graphite was studied. Both mass-selected and thermally evaporated nanoparticles were deposited. TPD spectra from the mass-selected nanoparticles exhibit a desorption peak located around 410 K with a broad shoulder extending from around 480 K to 600 K, while spectra obtained from thermally evaporated nanoparticles exhibit a single broad feature from ∼350 K to ∼450 K. A room temperature deposited 50 Å thick Ru film displays a characteristic nanoparticle-like spectrum with a broad desorption feature at ∼420 K and a shoulder extending from ∼450 K to ∼600 K. Subsequent annealing of this film at 900 K produced a polycrystalline morphology of flat Ru(001) terraces separated by monatomic steps. The CO desorption spectrum from this surface resembles that obtained on single crystal Ru(001) with two large desorption features located at 390 K and 450 K due to molecular desorption from terrace sites, and a much smaller peak at ∼530 K due to desorption of dissociatively adsorbed CO at step sites. In a second experiment, ion sputtering was used to create surface defects on a Ru(0 1 54) single crystal surface. A gradual shift away from the desorption spectrum typical of a Ru(001) surface towards one resembling desorption from supported Ru nanoparticles was observed with increasing sputter time.
Physical Chemistry Chemical Physics 06/2011; 13(21):10333-41. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The production of fuels from sunlight represents one of the main challenges in the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and although platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. We show that bioinspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo(3)S(4)) efficiently catalyse the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo(3)S(4) clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.
Nature Material 06/2011; 10(6):434-8. · 36.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.
Journal of the American Chemical Society 03/2011; 133(14):5485-91. · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Supported molybdenum-sulfide nanoparticles are known catalysts for petroleum hydrodesulfurization as well as for electrochemical hydrogen evolution. In this study, we investigate molybdenum-sulfide nanoparticles supported on Au(1 1 1) using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), aiming to correlate spectroscopically determined chemical states with atomically resolved nanostructure. The results of this study allow us to conclude the following: (1) the XPS results from our model system are in good agreement with previously published results on supported MoS2 for industrial applications, validating in part the fidelity of the model system; (2) STM reveals that catalytically active, crystalline MoS2 nanoparticles exhibiting the well-known metallic edge state are only present after a post-deposition annealing step in the synthesis procedure, without which the particles exhibit amorphous shapes and incomplete sulfidation; and (3) the sulfided nanoparticles are found to be stable in air at room temperature.
[Show abstract][Hide abstract] ABSTRACT: We study the structural properties of ultrathin Ag(x)Pd(1-x) films on top of a Ru(0001) substrate. Effective interatomic interactions, obtained from first-principle calculations, have been used in Monte Carlo simulations to derive the distribution of the alloy components in a four-monolayer (4-ML) Ag-Pd film. Though Ag-Pd alloys show complete solubility in the bulk, the thin film geometry leads to a pronounced segregation between Ag and Pd atoms with a strong preference of Ag atoms toward the surface and Pd atoms toward the interface. The theoretical prediction of this double-segregation effect is strongly supported by photoelectron spectroscopy experiments carried out for 4-ML thin films. We also show, in an additional experiment, that even in the case where initially 1 ML Ag is buried under 6 ML Pd, the whole Ag ML segregates to the surface.
Physical Review B 03/2008; 77:125406. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Codeposition of Ag and Pd on the Ru(0001) surface has been studied by photoelectron spectroscopy (PES) by recording of valence band (VB) and 3d5/2 core electron energy distribution curves (EDCs). Two- and three-dimensional total film coverages in the temperature range 300–765K have been characterized. At submonolayer total coverage, a flat, monolayer (ML) thick film with some degree of mixing is formed. A stable surface alloy, with Ag–Pd hybridization states ∼2.5eV binding energy (BE), is found when Pd is deposited after Ag. A 1nm thick Ag52Pd48 film formed by deposition of Pd on a Ag-precovered surface at room temperature exhibits a substantial interdiffusion of Pd and Ag. A stable concentration profile is reached between 550 and 660K.
Journal of Electron Spectroscopy and Related Phenomena 05/2007; 156:102-106. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Core-level binding energy shifts (CLSs) and surface CLS (SCLSs) are determined by experiment and theory for ultrathin Pd as well as for PdCu and PdAg surface alloys which vary in thickness from 1 to 4 monolayers (MLs), supported on Ru(0001). Experimentally, the binding energies of Pd and Ag 3d5∕2 are measured by photoelectron spectroscopy using synchrotron radiation, and in the case of Cu 2p3∕2 by x rays (XPS) from a Mg Kα radiation source. The calculations are based on first-principles techniques together with the complete screening picture, including initial and final state effects directly in the same scheme. Dimensional as well as temperature effects are observed and reproduced theoretically, with a good agreement between the calculated CLS and the experimentally observed values. Further it is demonstrated how the layer composition profile of a 4 ML thick PdAg film can be followed by comparing theoretical layer specific CLSs with the measured ones and combine this with the observed intensities of the Ag 3d5∕2 photoelectrons.
Physical Review B 08/2005; 72(7). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Codeposition of Pd and Cu on Ru(0001) with total coverages varying from submonolayer to nm thickness has been carried out at temperatures of 300 and 550 K. The effects of heating the overlayers up to 1250 K were studied. Besides the characterization of the overlayer composition as a function of temperature and observation of possible ordering, a series of questions is addressed. This includes conditions for alloying, possible effects related to dimensionality (2- or 3-dimensions), and the significance of the order of deposition. Photoelectron spectroscopy (PES) based on application of synchrotron radiation and the Mg Kα X-rays (XPS) was used to obtain information on the electronic structure of the bimetallic overlayers via the Ru 3d, Pd 3d core levels (PES) and Cu 2p core levels (XPS). Furthermore, the valence band (VB) electron energy distribution curves (EDC's) were measured at a photon energy of 40 eV which is the Cooper minimum of Cu 3d electrons. LEED was used to establish the order of the overlayers.The main findings of the bimetallic studies are that (i) the deposited Cux and Pd1−x for 0⩽x⩽1 with a total coverage of 1 ML, are distributed in one layer with temperature dependent intermixing, (ii) the valence bands are very characteristic with the Cu 3d band separated from the Pd 4d band and alloying gives rise to new features with binding energies (BE's) between these bands, (iii) the Cu 2p core level shifts (CLS) are correlated linearly with the composition of the films in 2- and 3-dimensions, an effect which is attributed to the different coordination numbers between Pd and Cu in the different dimensions, (iv) the structure of the overlayers depends only slightly on the order of deposition of the two coadsorbates for T>660 K and (v) (1×1) LEED patterns are obtained for 1 ML Cu+2 ML Pd at room temperature and for 2 ML Cu+2 ML Pd after annealing at 660 K indicating, in combination with the pronounced hybridization of Cu 3d and Pd 4d orbitals, formation of a surface alloy.
[Show abstract][Hide abstract] ABSTRACT: The electronic structures of Pd and (Cu + Pd) overlayers on Ru(0001) have been studied. Photoelectron emission spectroscopy based on synchrotron radiation and radiation from a Mg Kα x-ray source has been used to study the valence band of the surfaces and the Ru 3d, Pd 3d and Cu 2p core levels. The interaction of Pd with Ru(0001) is studied in the coverage range of submonolayer to several monolayers. The core level spectra of the Pd 3d5/2 level show a new component for Pd coverages above 1 ML. In the submonolayer range the binding energy (BE) of the Pd 3d5/2 level is constant at 335.08 eV. A state characterized as an interface state between the Pd adlayer and the Ru substrate is observed at a BE of 1.22 eV. This state, which dominates the valence band for submonolayer Pd coverages, has not been reported earlier and it is seen neither for clean Ru(0001) nor for multilayers of Pd. It is a characteristic that the overlayer d bands are narrower in the low-coverage regime. Deposition of Cu, on a 0.4 ML Pd/Ru(0001) surface up to 1 ML total coverage of Pd and Cu, results in a mixed two-dimensional overlayer.
[Show abstract][Hide abstract] ABSTRACT: Using valence band photoelectron spectroscopy it is demonstrated that CO dissociates on the potassium-modified stepped copper surfaces, Cu(112) and Cu(117). The dissociation process is conditioned by both the presence of steps and an alkali metal coverage above a certain threshold value. Carbonate is formed at 375 K via the process CO+CO→CO2+C and reaction with oxygen. Further heating to 600 K results in decomposition of the carbonate. The threshold alkali metal coverage for dissociation of CO on stepped copper surfaces depends on the step density. It is found that the height of the Fermi edge in the valence band photoelectron spectra is correlated with the CO uptake for a given potassium coverage and the perturbation and dissociation of the CO molecule with increasing temperature.