LiBH4 is a complex hydride and exhibits a high gravimetric hydrogen density of 18.5 wt %. Therefore it is a promising hydrogen storage material for mobile applications. The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium. According to the van 't Hoff equation the following thermodynamic parameters are determined for the desorption: enthalpy of reaction DeltarH = 74 kJ mol-1 H2 and entropy of reaction DeltarS = 115 J K-1 mol-1 H2. LiBH4 decomposes to LiH + B + 3/2H2 and can theoretically release 13.9 wt % hydrogen for this reaction. It is shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar. The formation of LiBH4 was confirmed by XRD (X-ray diffraction). In the rehydrided material 8.3 wt % hydrogen was desorbed in a TPD (temperature-programmed desorption) measurement compared to 10.9 wt % desorbed in the first dehydrogenation.

The interaction of atomic hydrogen and low-energy hydrogen ions with sp 2-bonded carbon is investigated on the surfaces of C 60 multilayer filins, single-walled carbon nanotubes, and graphite (0001). These three materials have been chosen to represent sp 2-bonded carbon networks with different local curvatures and closed surfaces (i.e. no dangling bonds). Chemisorption of hydrogen on these surfaces reduces emission from photoemission features associated with the 71' electrons and leads to a lowering of the work function up to 1.3 eV. It is found that the energy barrier for hydrogen adsorption decreases with increasing local curvature of the carbon surface. Whereas in the case of C 60 and single-walled carbon nanotubes, hydrogen adsorption can be achieved by exposure to atomic hydrogen, the hydrogen adsorption on graphite (0001) requires y+ ions of low kinetic energy (~ 1 e V). On all three materials, the adsorption energy barrier is found to increase with coverage. Accordingly, hydrogen chemisorption saturates at coverages that depend on the local curvature of the sample and the fonn of hydrogen (i.e., atomic or ionic) used for the treatment.

We report on a pronounced redistribution of the local electronic density of states at the graphite surface, which is induced by the presence of low energy hydrogen-ion induced point defects. Scanning tunneling microscopy reveals standing waves in the local density of states, which are due to backscattering of electron wave functions at individual point defects. The superstructure thereby formed is directly related to the pointlike structure of the Fermi surface of graphite. For high defect density interference patterns are observed which sensitively change structure on the relative positions of the defects. These patterns could be reproduced by tight binding simulations of various defect distributions. The presence of defects induce strong modifications on the local electronic properties of cristalline solids. These modifications have been thoroughly investigated on metal surfaces where they are known as Friedel oscillations and manifest as standing waves on the electronic density spanning regions up to ϳ10 nm from the defects. 1-3 The observed standing wave patterns on metals and high temperature superconductors could be linked to the electronic structure even for complex patterns. [4][5][6] The long range character of the electronic density standing waves allows, for instance, for indirect interaction between atoms adsorbed on surfaces, leading to adsorbate overlayer structures showing the same periodicity as the oscillations. 2 Similar long range oscillations have also been observed on graphite surfaces [7][8][9] and in carbon nanostructures 10-12 where they have even been used for the realization of single molecular electronic devices. 12 In this work we report on the scattering of graphite -electrons due to single and multiple point defects as observed by STM. The high density of defects, due to the multiple scattering of the electrons, produces a rich variety of patterns strongly departing from the ones observed from isolated defects. Experimental findings will be compared with two models, one phenomenological, based on the superposition of plane waves and one explicit, based on tight binding ͑TB͒ simulations that will allow us to address different aspects of point defect scattering in graphite.Samples of highly oriented pyrolithic graphite ͑HOPG, quality ZYB͒ were cleaved in situ under ultrahigh vacuum conditions. Exposure of the clean graphite surface to a H 2 electron cyclotron resonance ͑ECR͒ plasma created stochastically distributed point defects. A typical ion dose used for the treatment was of the order of 10 13 ions/ cm −2 . The average ion energy of ϳ2 eV, has been determined with an electrostatical analyzer. After plasma treatment, the sample was transferred to the vacuum-connected atomic force/scanning tunneling microscope ͑AFM/STM, Omicron͒ working under ultrahigh vacuum conditions at room temperature. Measurements were performed in the combined AFM/STM mode using the normal force as feedback signal and applying a fixed bias voltage between the sample and the tip. The recording of the piezo z-p...

We present photoelectron spectroscopy investigations of in-situ prepared AgO. The sample was prepared by room temperature oxidation of Ag in an electron cyclotron resonance 0 2 plasma. In contrast to other measurements based on ex situ prepared AgO powder samples, our investigations show a distinct double peak structure of the 0 1 s signal with a remarkable chemical shift of 2.9 e V between the two 0 1 s components. These two components can not be motivated from a crystallographic point of view as the oxygen sites are all equivalent in the unit cell. We interpret this double peak structure as a characteristic feature of AgO and discuss it in terms of mixed valences.

We report on the growth of supramolecular columns of polyaromatic hydrocarbons on Au͑111͒ and Cu͑111͒ single-crystal surfaces. The lateral separation of the columns was found to depend on the substrate and is determined by the commensurately formed superlattice of the first molecular monolayer. X-ray photoelectron diffraction in combination with low-energy electron diffraction reveals stack growth with small lateral offsets from the column axis but with conservation of the molecular orientation. The mechanism of column growth is explained by simulation results of the intermolecular interaction assuming a Lennard-Jones potential. The size of hexabenzocoronene and its ability to condense into one-dimensional supramolecular structures make it an ideal candidate for the accommodation and the positioning of functional groups to form a functional molecular assembly.For the realization of molecule-based electronic devices the understanding and control of intermolecular and molecule-substrate interactions are crucial. 1-4 Directed supramolecular aggregation is achieved by adjusting intermolecular interactions and choosing appropriate substrates. 3,4 Hexa-peri-hexabenzocoronene ͓C 42 H 18 ͑HBC͔͒ is a polycyclic aromatic hydrocarbon and can be viewed as being discshaped, hydrogen terminated two-dimensional graphite sections. The molecules are large enough to accommodate functional groups and therefore, the study of their growth mode and the control of it are of great importance. For the hexaalkyl-substituted derivatives of HBC high solubility and the ability to self-organize into a columnar mesophase in organic solvents has been shown. 5,6 These structures are onedimensional conductors with a very high charge carrier mobility. 6-8 However, the deposition of similar onedimensional structures at surfaces would be desirable in view of a controlled positioning and orientation of such columns.In this work we report on the growth mode of thin films of the insoluble HBC prepared by vacuum sublimation onto Au͑111͒ and Cu͑111͒. We show that the two-dimensional structural information of the first molecular monolayer is transmitted to molecules of the following layers yielding columnar stacks of HBC with substrate-dependent stack separation. The observed aggregation into columnar structures is completely different from the well-known herringbone bulk structure. 9 The combination of low-energy electron diffraction ͑LEED͒ and x-ray photoelectron diffraction ͑XPD͒ allows the determination of the intermolecular ordering and the orientation of the adsorbed molecules within the columns. XPD in combination with single-scattering cluster ͑SSC͒ calculation has been proven adequate for near-surface and adsorbates structure determination of carbon allotropes 10,11 and is used here for the determination of the intracolumn molecular ordering.Experiments were performed in an OMICRON photoelectron spectrometer modified for motorized sequential anglescanning data acquisition having a base pressure in the low 10 Ϫ11 mbar range. X-ray photoelectron sp...

The catalytic hydrogenation of CO(2) at the surface of a metal hydride and the corresponding surface segregation were investigated. The surface processes on Mg(2)NiH(4) were analyzed by in situ X-ray photoelectron spectroscopy (XPS) combined with thermal desorption spectroscopy (TDS) and mass spectrometry (MS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). CO(2) hydrogenation on the hydride surface during hydrogen desorption was analyzed by catalytic activity measurement with a flow reactor, a gas chromatograph (GC) and MS. We conclude that for the CO(2) methanation reaction, the dissociation of H(2) molecules at the surface is not the rate controlling step but the dissociative adsorption of CO(2) molecules on the hydride surface.

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The surface oxidation behavior of LiBH(4) and NaBH(4) was investigated in view of the formation and structure of the surface oxidation and its effect on the hydrogen desorption kinetics. The sample surfaces were intentionally modified by exposure to oxygen in the pressure range from 10(-10) mbar up to 200 mbar. The induced surface changes were systematically studied by means of X-ray photoelectron spectroscopy. NaBH(4) shows a low reactivity with oxygen, while LiBH(4) oxidizes rapidly, accompanied by surface segregation of Li. The hydrogen desorption kinetics of LiBH(4) were studied by thermal desorption spectroscopy with particular emphasis on the analysis of the desorbed gases, i.e. diborane and hydrogen. The surface oxidation induces the formation of a Li(2)O layer on LiBH(4), significantly reduces the desorption of diborane, and enhances the rate of hydrogen desorption.