113 research outputs found
Recommended from our members
The study of chiral adsorption systems using synchrotron- based structural and spectroscopic techniques: stereospecific adsorption of serine on Au-modified chiral Cu{531} surfaces
We apply modern synchrotron-based structural
techniques to the study of serine adsorbed on the pure andAumodified
intrinsically chiral Cu{531} surface. XPS and
NEXAFS data in combination with DFT show that on the
pure surface both enantiomers adsorb in l4 geometries (with
de-protonated b-OH groups) at low coverage and in l3
geometries at saturation coverage. Significantly larger
enantiomeric differences are seen for the l4 geometries,
which involve substrate bonds of three side groups of the
chiral center, i.e. a three-point interaction. The l3 adsorption
geometry, where only the carboxylate and amino groups form
substrate bonds, leads to smaller but still significant enantiomeric
differences, both in geometry and the decomposition
behavior. When Cu{531} is modified by the deposition of 1
and 2ML Au the orientations of serine at saturation coverage
are significantly different from those on the clean surface. In
all cases, however, a l3 bond coordination is found at saturation
involving different numbers of Au atoms, which leads
to relatively small enantiomeric differences
Site-specific probing of charge transfer dynamics in organic photovoltaics
We report the site-specific probing of charge-transfer dynamics in a
prototype system for organic photovoltaics (OPV) by picosecond time-resolved
X-ray photoelectron spectroscopy. A layered system consisting of approximately
two monolayers of C deposited on top of a thin film of
Copper-Phthalocyanine (CuPC) is excited by an optical pump pulse and the
induced electronic dynamics are probed with 590 eV X-ray pulses. Charge
transfer from the electron donor (CuPC) to the acceptor (C) and
subsequent charge carrier dynamics are monitored by recording the
time-dependent C 1 core level photoemission spectrum of the system. The
arrival of electrons in the C layer is readily observed as a completely
reversible, transient shift of the C associated C 1 core level, while
the C 1 level of the CuPC remains unchanged. The capability to probe charge
transfer and recombination dynamics in OPV assemblies directly in the time
domain and from the perspective of well-defined domains is expected to open
additional pathways to better understand and optimize the performance of this
emerging technology
Recommended from our members
Global and local expression of chirality in serine on the Cu{110} surface
Establishing a molecular-level understanding of enantioselectivity and chiral resolution at the organic−inorganic interfaces is a key challenge in the field of heterogeneous catalysis. As a model system, we investigate the adsorption geometry of serine on Cu{110} using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The chirality of enantiopure chemisorbed layers, where serine is in its deprotonated (anionic) state, is expressed at three levels: (i) the molecules form dimers whose orientation with respect to the substrate depends on the molecular chirality, (ii) dimers of l- and d-enantiomers aggregate into superstructures with chiral (−1 2; 4 0) lattices, respectively, which are mirror images of each other, and (iii) small islands have elongated shapes with the dominant direction depending on the chirality of the molecules. Dimer and superlattice formation can be explained in terms of intra- and interdimer bonds involving carboxylate, amino, and β−OH groups. The stability of the layers increases with the size of ordered islands. In racemic mixtures, we observe chiral resolution into small ordered enantiopure islands, which appears to be driven by the formation of homochiral dimer subunits and the directionality of interdimer hydrogen bonds. These islands show the same enantiospecific elongated shapes those as in low-coverage enantiopure layers
Recommended from our members
Oxidation of polycrystalline Ni studied by spectromicroscopy: Phase separation in the early stages of crystallite growth
Low-energy and photoemission electron microscopy enables the determination of facet planes of polycrystalline
surfaces and the study of their chemical composition at the sub-m scale. Using these techniques the
early oxidation stages of nickel were studied. After exposing the surface to 20 L of oxygen at 373 K a uniform
layer of chemisorbed oxygen was found on all facets. After oxygen exposure at 473–673 K, small NiO
crystallites are formed on all facets but not in the vicinity of all grain boundaries. The crystallites are separated by areas of bare Ni without significant oxygen coverage
Recommended from our members
Surface chemistry of glycine on Pt{111} in different aqueous environments
Adsorption of glycine on Ptf111g under UHV conditions and in different aqueous environments was studied by XPS (UHV and ambient pressure) and NEXAFS. Under UHV conditions, glycine adsorbs in its neutral molecular state up to about 0.15 ML. Further deposition leads to the formation of an additional zwitterionic species, which is in direct
contact with the substrate surface, followed by the growth of multilayers, which also consist of zwitterions. The neutral surface species is most stable and decomposes at
360 K through a multi-step process which includes the formation of methylamine and carbon monoxide. When glycine and water are co-adsorbed in UHV at low temperatures
(< 170 K) inter-layer diffusion is inhibited and the surface composition depends on the adsorption sequence. Water adsorbed on top of a glycine layer does not lead to significant changes in its chemical state. When glycine is adsorbed on top of a pre-adsorbed chemisorbed water layer or thick ice layer, however, it is found in its zwitterionic state, even at low coverage. No difference is seen in the chemical state of glycine when the layers
are exposed to ambient water vapor pressure up to 0.2 Torr at temperatures above 300 K. Also the decomposition temperature stays the same, 360 K, irrespective of the
water vapor pressure. Only the reaction path of the decomposition products is affected by ambient water vapor
Recommended from our members
The importance of attractive three-point interaction in enantioselective surface chemistry: stereospecific adsorption of serine on the intrinsically chiral Cu{531} surface
Both enantiomers of serine adsorb on the intrinsically chiral Cu{531} surface in two different adsorption
geometries, depending on the coverage. At saturation, substrate bonds are formed through the two oxygen atoms of the carboxylate group and the amino group (μ3 coordination), whereas at lower coverage, an additional bond is formed through the deprotonated β−OH group (μ4 coordination). The latter adsorption geometry involves substrate bonds through three side groups of the chiral center, respectively, which leads to significantly larger enantiomeric differences in adsorption
geometries and energies compared to the μ3 coordination, which involves only two side groups. This relatively simple model system demonstrates, in direct comparison, that attractive interactions of three side groups with the substrate are much more effective in inducing strong enantiomeric differences in heterogeneous chiral catalyst systems than hydrogen bonds or repulsive interactions
Potentials in Li-Ion Batteries Probed by Operando Ambient Pressure Photoelectron Spectroscopy
The important electrochemical processes in a battery happen at the solid/liquid interfaces. Operando ambient pressure photoelectron spectroscopy (APPES) is one tool to study these processes with chemical specificity. However, accessing this crucial interface and identifying the interface signal are not trivial. Therefore, we present a measurement setup, together with a suggested model, exemplifying how APPES can be used to probe potential differences over the electrode/electrolyte interface, even without direct access to the interface. Both the change in electron electrochemical potential over the solid/liquid interface, and the change in Li chemical potential of the working electrode (WE) surface at Li-ion equilibrium can be probed. Using a Li4Ti5O12 composite as a WE, our results show that the shifts in kinetic energy of the electrolyte measured by APPES can be correlated to the electrochemical reactions occurring at the WE/electrolyte interface. Different shifts in kinetic energy are seen depending on if a phase transition reaction occurs or if a single phase is lithiated. The developed methodology can be used to evaluate charge transfer over the WE/electrolyte interface as well as the lithiation/delithiation mechanism of the WE
Recommended from our members
“Pop-on and pop-off” surface chemistry of alanine on Ni{111} under elevated hydrogen pressures
The co-adsorption of hydrogen with a simple chiral modifier, alanine, on Ni{111} was studied using Density Functional Theory in combination with ambient-pressure X-ray photoelectron spectros opy and X-ray absorption spectroscopy at temperatures of 300 K and above, which are representative of chiral hydrogenation reactions. Depending on the hydrogen pressure, the surface enables protons to ''pop on and off'' the modifier molecules, thus significantly altering the adsorption geometry and chemical nature of alanine from anionic tridentate in ultra-high vacuum to predominantly zwitterionic bidentate at hydrogen pressures above 0.1 Torr. This hydrogen-stabilised modifier geometry allows alternative mechanisms for proton transfer and the creation of enatioselective reaction environments
Recommended from our members
"Pop-On and Pop-Off" Surface Chemistry of Alanine on Ni{111} Under Elevated Hydrogen Pressures
The co-adsorption of hydrogen with a simple chiral modifier, alanine, on Ni{111} was studied using Density Functional Theory in combination with ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at temperatures of 300~K and above, which are representative of chiral hydrogenation reactions. Depending on the hydrogen pressure, the surface enables protons to "pop on and off" the modifier molecules, thus significantly altering the adsorption geometry and chemical nature of alanine from anionic tridentate in ultra-high vacuum to predominantly zwitterionic bidentate at hydrogen pressures above 0.1 Torr. This hydrogen-stabilised modifier geometry allows alternative mechanisms for proton transfer and the creation of enatioselective reaction environments
Probing Electrochemical Potential Differences over the Solid/Liquid Interface in Li-Ion Battery Model Systems
The electrochemical potential difference (Δμ̅) is the driving force for the transfer of a charged species from one phase to another in a redox reaction. In Li-ion batteries (LIBs), Δμ̅ values for both electrons and Li-ions play an important role in the charge-transfer kinetics at the electrode/electrolyte interfaces. Because of the lack of suitable measurement techniques, little is known about how Δμ̅ affects the redox reactions occurring at the solid/liquid interfaces during LIB operation. Herein, we outline the relations between different potentials and show how ambient pressure photoelectron spectroscopy (APPES) can be used to follow changes in Δμ̅e over the solid/liquid interfaces operando by measuring the kinetic energy (KE) shifts of the electrolyte core levels. The KE shift versus applied voltage shows a linear dependence of ∼1 eV/V during charging of the electrical double layer and during solid electrolyte interphase formation. This agrees with the expected results for an ideally polarizable interface. During lithiation, the slope changes drastically. We propose a model to explain this based on charge transfer over the solid/liquid interface
- …