Operando reaction cell for high energy surface sensitive x-ray diffraction and reflectometry.

A proof of concept is shown for the design of a high pressure heterogeneous catalysis reaction cell suitable for surface sensitive x-ray diffraction and x-ray reflectometry over planar samples using high energy synchrotron radiation in combination with mass spectrometry. This design enables measurements in a pressure range from several tens to hundreds of bars for surface investigations under realistic industrial conditions in heterogeneous catalysis or gaseous corrosion studies.

Complex Geometric Structure of a Simple Solid-Liquid Interface: GaN(0001)-Ga.

The equilibrium atomic interface structure between Ga and GaN(0001) is shown to contain substrate surface vacancies followed by substrate-induced layering and preferential lateral ordering in the liquid. The uncovered presence of point defects, in the form of vacancies at both sides of the solid-liquid interface, is an important structural feature which governs the local physical properties. Our x-ray diffraction study reveals that the layering is very stable and persists up to a temperature of 1123 K and a nitrogen pressure of 32 bar. The Ga layer spacing agrees remarkably well with the Friedel oscillation period for this system.

Structure and Oxidation Behavior of Nickel Nanoparticles Supported by YSZ(111).

Nickel nanoparticles supported by the yttria-stabilized zirconia (111) surface show several preferential epitaxial relationships, as revealed by in situ X-ray diffraction. The two main nanoparticle orientations are found to have their [111] direction parallel to the substrate surface normal and ∼41.3 degrees tilted from this direction. The former orientation is described by a cube-on-cube stacking at the oxide-metal interface and the latter by a so-called coherent tilt strain-relieving mechanism, which is hitherto unreported for nanoparticles in literature. A modified Wulff construction used for the 111-oriented particles results in a value of the adhesion energy ranging from 1.4 to 2.2 Jm2, whereby the lower end corresponds to more rounded particles and the upper to relatively flat geometries. Upon oxidation at 10-3 Pa of molecular oxygen and 673 K, a NiO shell forms epitaxially on the [111]-oriented particles. Only a monolayer of metallic nickel of the top (111) facets oxidizes, whereas the side facets seem to react more severely. An apparent size increase of the remaining metallic Ni core is discussed in relation to a size-dependent oxidation mechanism, whereby smaller nanoparticles react at a faster rate. We argue that such a preferential oxidation mechanism, which inactivates the smallest and most reactive metal nanoparticles, might play a role for the long-term degradation of solid oxide fuel cells.

Initial structure and growth dynamics of YBa(2)Cu(3)O(7-delta) during pulsed laser deposition.

The initial heteroepitaxial growth of YBa{2}Cu{3}O{7-delta} films on SrTiO3(001) substrates during pulsed laser deposition shows a growth-mode transition and a change of growth unit. The growth starts with two blocks, each two-thirds the size of the complete unit cell. The first of these blocks grows in a step-flow fashion, whereas the second grows in the layer-by-layer mode. Subsequent deposition occurs layer-by-layer of complete unit cells. These results suggest that the surface diffusion in the heteroepitaxial case is strongly influenced by the competition with formation energies, which is important for the fabrication of heteroepitaxial devices on the unit cell scale.