We report the results from the first 5D tomographic diffraction imaging experiment of a complex Ni-Pd/CeO
-ZrO
/Al
O
catalyst used for methane reforming. This five-dimensional (three spatial, one ...scattering and one dimension to denote time/imposed state) approach enabled us to track the chemical evolution of many particles across the catalyst bed and relate these changes to the gas environment that the particles experience. Rietveld analysis of some 2 × 10
diffraction patterns allowed us to extract heterogeneities in the catalyst from the Å to the nm and to the μm scale (3D maps corresponding to unit cell lattice parameters, crystallite sizes and phase distribution maps respectively) under different chemical environments. We are able to capture the evolution of the Ni-containing species and gain a more complete insight into the multiple roles of the CeO
-ZrO
promoters and the reasons behind the partial deactivation of the catalyst during partial oxidation of methane.
We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film ...rotating disk electrode screenings (3.43 ± 0.35 A mgPt –1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt –1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core–shell catalysts.
A full understanding of the mechanism of the formation of a two‐dimensional electron gas (2DEG) at the interface between insulating LaAlO3 (LAO) thin films and bulk SrTiO3 (STO) crystals is a ...prerequisite for the full exploitation of this class of materials. Here, by using a combination of advanced X‐ray synchrotron‐based spectroscopic and structural measurements, it is shown that a structural and electronic reconstruction of the interface occurs before the realization of the 2DEG.
High-energy X-ray surface diffraction in transmission geometry is combined with a microfluidic thin layer flow cell for in situ studies of the local structure of electrodeposited epitaxial films. The ...capabilities of this approach are illustrated using Bi films on Au(100) single crystals as an example. We demonstrate that the local film thickness, the strain, and the orientation of the deposits' crystallites can be mapped with a spatial resolution of a few micrometers. The high heterogeneity of the Bi films provides deposits with a wide range of structural properties, allowing to establish correlations between the different parameters as a function of the local thickness.
We experimentally study the influence of the binding energy on nondipole effects in K-shell single-photon ionization of atoms at high photon energies. We find that for each ionization event, as ...expected by momentum conservation, the photon momentum is transferred almost fully to the recoiling ion. The momentum distribution of the electrons becomes asymmetrically deformed along the photon propagation direction with a mean value of 8/(5c)(E_{γ}-I_{P}) confirming an almost 100 year old prediction by Sommerfeld and Schur Ann. Phys. (N.Y.) 396, 409 (1930)10.1002/andp.19303960402. The emission direction of the photoions results from competition between the forward-directed photon momentum and the backward-directed recoil imparted by the photoelectron. Which of the two counteracting effects prevails depends on the binding energy of the emitted electron. As an example, we show that at 20 keV photon energy, Ne^{+} and Ar^{+} photoions are pushed backward towards the radiation source, while Kr^{+} photoions are emitted forward along the light propagation direction.We experimentally study the influence of the binding energy on nondipole effects in K-shell single-photon ionization of atoms at high photon energies. We find that for each ionization event, as expected by momentum conservation, the photon momentum is transferred almost fully to the recoiling ion. The momentum distribution of the electrons becomes asymmetrically deformed along the photon propagation direction with a mean value of 8/(5c)(E_{γ}-I_{P}) confirming an almost 100 year old prediction by Sommerfeld and Schur Ann. Phys. (N.Y.) 396, 409 (1930)10.1002/andp.19303960402. The emission direction of the photoions results from competition between the forward-directed photon momentum and the backward-directed recoil imparted by the photoelectron. Which of the two counteracting effects prevails depends on the binding energy of the emitted electron. As an example, we show that at 20 keV photon energy, Ne^{+} and Ar^{+} photoions are pushed backward towards the radiation source, while Kr^{+} photoions are emitted forward along the light propagation direction.
A new tomographic reconstruction algorithm is presented, termed direct least‐squares reconstruction (DLSR), which solves the well known parallax problem in X‐ray‐scattering‐based experiments. The ...parallax artefact arises from relatively large samples where X‐rays, scattered from a scattering angle 2gθ, arrive at multiple detector elements. This phenomenon leads to loss of physico‐chemical information associated with diffraction peak shape and position (i.e. altering the calculated crystallite size and lattice parameter values, respectively) and is currently the major barrier to investigating samples and devices at the centimetre level (scale‐up problem). The accuracy of the DLSR algorithm has been tested against simulated and experimental X‐ray diffraction computed tomography data using the TOPAS software.
A new reconstruction approach is presented that can directly yield physico‐chemical images and overcome the parallax problem in X‐ray diffraction computed tomography experiments.
We investigate K-shell ionization of N2 at 40 keV photon energy. Using a cold target recoil ion momentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, the ...Auger electron, and both N+ fragments. These fully differential data show that the dissociation process of the N22+ ion is significantly modified not only by the recoil momentum of the photoelectron but also by the photon momentum and the momentum of the emitted Auger electron. We find that the recoil energy introduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately 30∶70 between the Auger electron and fragment ion kinetic energies, respectively. We also observe that the photon momentum induces an additional rotation of the molecular ion.
Small-molecule organic photovoltaic cells often employ a planar heterojunction (PHJ) geometry where the electron donor and acceptor materials are stacked one on top of the other. The thin-film growth ...scenario of such PHJs can be very different from the one of a single compound on a bare substrate. We have investigated the growth of PHJs, consisting of two different donor–acceptor pairs, namely, α-sexithiophene (6T)/C60 and 6T/diindenoperylene (DIP) using real-time in situ X-ray scattering. For both donor–acceptor material combinations, we observe that the coherent in-plane crystalline size of the second material strongly correlates with the one of the bottom one, and hence a strong templating effect of the 6T on the material deposited subsequently, indicating a strong interaction between the two materials in the PHJ. Furthermore, a change in the structure of the 6T film during the deposition of the second material was observed, which shows that the deposition of an additional material on top of a templating layer can partially change the crystal structure of the templating film itself.
A detailed study of the near-surface structure and composition of Nb, the material of choice for superconducting radio-frequency accelerator (SRF) cavities, is of great importance in order to ...understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse x-ray scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex situ during annealing in an ultrahigh vacuum as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to800°Cin a vacuum promotes a partial reduction of the natural surface oxides (Nb2O5,NbO2, and NbO) into NbO. Upon cooling to120°C, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 h. An oxygen enrichment below the Nb-oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides, was observed upon a stepwise annealing up to250°C. Nitrogen introduction to the system at250°Cpromotes neither N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to500°Cin a nitrogen atmosphere, the growth of a new subsurfaceNbxNylayer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.
2D materials with honeycomb lattices are increasingly studied due to their unique electronic and mechanical properties. The typical preparation techniques, such as chemical vapor deposition and ...cleavage of bulk crystals, are either complicated or limited to only certain classes of materials. Here we discuss basic ordering principles in 2D monoatomic layers and show that electrochemical deposition can also result in 2D films with desired honeycomb structures. The principles are first studied on model ionic 2D layers prepared in ultrahigh vacuum. Two ordering possibilities are identified: a bilayer structure for covalently bonded films and a mixed monolayer structure for ionically bonded films. Typically, the most energetically favorable configuration in mixed layers is when ions with one polarity are closely surrounded by ions of the opposite polarity. This translates into a layer with honeycomb structure on substrates with p6/p3 symmetry. This phenomenon can be used to prepare germanene, a layer with such a structure.
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