M.G. Ramsey

CO adsorption on Pd(1 1 1): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various CO‐ Pd(1 1 1) bonding configurations have been identified from the decomposition analysis of the adsorbate C 1s and the substrate Pd 3d5=2 core-level photoemission spectra obtained after CO adsorption at 120 K. The cO4 2U structure at 0.5 monolayer (ML) and theO2 2U-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fcc and hcp threefold hollow sites in the cO4 2U structure and of hollow and on-top sites in theO2 2U structure, as reported in the literature. For CO adsorption at 300 K, a diAerent site occupation is evidenced by the presence of two components in the C 1s and Pd 3d5=2 core-level and C‐O stretching vibration lineshapes. At coverages up to 0.1 ML only fcc threefold hollow sites in a O AAA

XPS studies of graphite electrode materials for lithium ion batteries

Abstract Surface pre-treatment of graphitic electrode materials for lithium ion cells has recently been shown to significantly reduce the irreversible consumption of material and charge due to the formation of the so-called solid electrolyte interphase (SEI) during battery charging. In this paper, we compare graphite powders and carbon fibres as model materials for X-ray photoemission spectroscopy (XPS) studies of the effects of surface pre-treatments. For carbon fibres, the surface carbon percentage was found to vary from 70–95% depending on the surface treatment, with corresponding changes in the relative proportion of graphitic compared to CO bonds, as determined from C 1s curve fits. In contrast, results from the graphite powders show very little change in surface chemical composition and an essentially constant C 1s lineshape dominated by graphitic carbon. SEM data show the carbon fibre cross-section to be composed of a radial array of layered graphite, leaving a surface consisting largely of prismatic planes, while the graphite powder consists of graphite platelets with the surface area predominantly of basal planes. We conclude that the chemical modification occurs at the prismatic planes, and that the powders are unsuitable as models for XPS studies of electrode surface modification, while the fibres are very well suited.

Acrylic acid nitrile, a film-forming electrolyte component for lithium-ion batteries, which belongs to the family of additives containing vinyl groups

We present results on the electrolyte additive acrylic acid nitrile (AAN), which allows the use of propylene carbonate (PC)-based electrolytes together with graphitic anodes. This report will focus on the basic electrochemical properties and on XPS results of the films formed in the presence of AAN. Further data on in situ investigations of AAN is presented in another paper of this proceedings. The combination of both reports gives strong evidence, that the initiative step for solid electrolyte interphase (SEI) formation is a cathodic, i.e. by reduction induced electro-polymerisation of the vinyl-group. It is concluded that this electro-polymerisation may also be a main reduction mechanism of other vinyl compounds such as vinylene carbonate (VC), vinylene acetate and others.

Structure and orientation of organic molecules on metal surfaces

Abstract Knowledge of the structure and orientation of organic molecules on solid surfaces is of fundamental relevance in many diverse areas of the physical sciences, ranging from heterogeneous catalysis to tribology and polymer interface science. For example, in catalysis the local orientation of a molecule on a metal surface is of decisive importance for determining the subsequent reaction paths, or in the science of composite materials the adhesion between organic layers and inorganic substrate surfaces is a direct consequence of the local chemical bonding between organic functional groups and substrate atoms at the interface.

Nature, growth, and stability of vanadium oxides on Pd(111)

Thin films of vanadium oxides grown on a Pd(111) single crystal surface have been studied using high resolution x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The vanadium oxides have been prepared by reactive evaporation of vanadium in pO2=2×10−7 mbar at 250 °C from submonolayer to 5 monolayer (ML) coverages. As observed on other substrates, the stoichiometry of the oxide phase varies as a function of the coverage, VO/VO2-like at low coverages to V2O3 for thicker oxide layers as indicated by XPS V 2p core level spectra and the characteristic NEXAFS fingerprints at both V 2p and O 1s edges. The V2O3 oxide phase grows epitaxially on the Pd(111) surface in the form of small three-dimensional (3D) islands as revealed by LEED and STM. The thermal stability of the oxides is also coverage dependent: the decomposition onset temperatures range from 300 °C for submonolayer coverage to ⩾500 ...

The adsorption of aromatics on sp-metals: benzene on Al 111

Abstract We have studied the adsorption of benzene on Al(111) using angle-resolved ultraviolet photoelectron, high-resolution electron energy loss, and thermal desorption spectroscopies (ARUPS, HREELS, and TDS, respectively), work function measurements, and by density functional theory (DFT) calculations using the ab-initio vasp code. The analysis of ARUPS and HREELS spectra of a benzene monolayer unambiguously indicate C 6v symmetry and a weak benzene–Al interaction in an adsorption geometry with the ring plane parallel to the surface. The weak interaction is confirmed by TDS. The DFT calculations indicate an electrostatic bond and yield an average benzene–Al(111) distance of 3.7 A. A weak minimum of the potential energy is observed at the hollow adsorption position.

Ordered overlayers of aniline and phenol on Pd(110): Surface structure and bonding

Abstract The surface structure and bonding of aniline and phenol on Pd(110) has been studied by angle resolved UV photoemission (ARUPS) using synchrotron radiation, LEED, and thermal desorption spectroscopy (TDS). Both molecules form ordered c(4 × 2) surface structures on Pd(110), and ARUPS and TDS results suggest that aniline and phenol split off an H atom from the functional group to form C 6 H 5 NH and C 6 H 5 O species in the monolayer phases. The molecules coordinate to the surface via the it electrons of the aromatic rings and via the heteroatoms as indicated by the stabilisation of the π states and of the N or O lone pair electrons in the ARUPS spectra; thus they adopt an adsorption geometry with the ring plane in close proximity to the metal surface. The results do not, however, rule out some tilting of the molecules which would be expected from steric considerations. There is preferential azimuthal orientation in the densely packed adlayers of the monolayer phases, and the molecules align with their functional groups along the [001] azimuth.

Unexpected interplay of bonding height and energy level alignment at heteromolecular hybrid interfaces

Although geometric and electronic properties of any physical or chemical system are always mutually coupled by the rules of quantum mechanics, counterintuitive coincidences between the two are sometimes observed. The coadsorption of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and copper-II-phthalocyanine on Ag(111) represents such a case, since geometric and electronic structures appear to be decoupled: one molecule moves away from the substrate while its electronic structure indicates a stronger chemical interaction, and vice versa for the other. Our comprehensive experimental and ab-initio theoretical study reveals that, mediated by the metal surface, both species mutually amplify their charge-donating and -accepting characters, respectively. This resolves the apparent paradox, and demonstrates with exceptional clarity how geometric and electronic bonding parameters are intertwined at metal-organic interfaces.

A multiplicity of CN bonding configurations on Ni(110)

Abstract High-resolution electron energy-loss spectroscopy (HREELS) has been applied to CN overlayers created by the dissociation of C 2 N 2 on Ni(110). For the well-ordered overlayer formed at room temperature, the lowest CN stretch frequency (190 meV) so far reported is observed, along with distinct frustrated translation and rotation modes. The CN is concluded to be lying in the grooves of the surface with both vertical and lateral π-bonding. When condensed cyanogen is warmed to room temperature, a denser, disordered CN overlayer is formed. HREELS indicates that in addition to the flat-lying in-groove species, CN is adsorbed on the Ni ridges in both carbon- and nitrogen-bound vertical configurations, as evidenced by a band of very high CN stretch frequencies (245–270 meV) and their associated metal-molecule modes.

Band alignment at the organic-inorganic interface

The band alignment of the bithiophene interface with a diverse range of substrates has been determined by a combination of ultraviolet photoemission and work function measurements. Not only is vacuum level alignment clearly shown to be invalid but also any sort of linear relationship between band alignment and substrate work function is shown not to be the case. Rather, the alignment is determined by the interface dipole, which is specific to the interaction at the inorganic-organic interface. The interface dipoles, which always appear, while dominated by the first monolayer interaction, are completed after two to three monolayers. As the ionization potentials of the films are shown to be constant, it is argued that a simple work function measurement, for an organic film on a particular substrate, quantifies the band alignment.