The valence band structures (VBS) of eight transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were investigated by photoelectron spectroscopy (PES) using He I, He II, and monochromatized Al Kα excitation. The influence of final states, photoionization cross-section, and adsorption of residual gas molecules in an ultrahigh vacuum environment are discussed in terms of their impact on the VBS. We find that VBSs recorded with monochromatized Al Kα radiation are most closely comparable to the ground state density of states (DOS) derived from quantum mechanics calculations. We use the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the "true" ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronicproperty/catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center position is not supported by our results (both experimentally and computationally).
To facilitate a less empirical approach
to developing improved
catalysts, it is important to correlate catalytic performance to surrogate
properties that can be measured or predicted accurately and quickly,
allowing experimental synthesis and testing of catalysts to focus
on the most promising cases. Particularly hopeful is correlating catalysis
performance to the electronic density of states (DOS). Indeed, there
has been success in using just the center of the d-electron density,
which in some cases correlates linearly with oxygen atom chemisorption
energy, leading to a volcano plot for catalytic performance versus
“d-band center”. To test such concepts we calculated
the barriers and binding energies for the various reactions and intermediates
involved in the oxygen reduction reaction (ORR) for all 12 transition
metals in groups 8–11 (Fe–Cu columns). Our results show
that the oxygen binding energy can serve as a useful parameter in
describing the catalytic activity for pure metals, but it does not
necessarily correlate with the d-band center. In addition, we find
that the d-band center depends substantially on the calculation method
or the experimental setup, making it a much less reliable indicator
for ORR activity than the oxygen binding energy. We further examine
several surfaces of the same pure metals to evaluate how the d-band
center and oxygen binding energy depend on the surface.
Abstract:We present a novel synchrotron endstation with a flow-through liquid cell designed to study the electronic structure of liquids using soft x-ray spectroscopies. In this cell, the liquid under study is separated from the vacuum by a thin window membrane, such that the sample liquid can be investigated at ambient pressure. The temperature of the probing volume can be varied in a broad range and with a fast temperature response. The optimized design of the cell significantly reduces the amount of required sample liquid and allows the use of different window membrane types necessary to cover a broad energy range. The liquid cell is integrated into the SALSA (Solid And Liquid Spectroscopic Analysis) endstation that includes a high-resolution, high-transmission x-ray spectrometer and a state-of-the-art 2 electron analyzer. The modular design of SALSA also allows the measurement of solid-state samples. The capabilities of the liquid cell and the x-ray spectrometer are demonstrated using a RIXS (resonant inelastic x-ray scattering) map of a 25 wt% NaOD solution.
On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.
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