Andreas Stierle

High-Energy Surface X-ray Diffraction for Fast Surface Structure Determination

High-energy x-rays incident at grazing angles allow for rapid collection of surface diffraction beams. [Also see Perspective by Nicklin] Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields, such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction–based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a palladium surface in situ during catalytic carbon monoxide oxidation and follow dynamic restructuring of the surface with subsecond time resolution. Speeding Up Surface Diffraction Surface diffraction methods can determine the atomic structure of the topmost layer of a crystal and also subsurface structures. However, many surface diffraction methods either require ultrahigh vacuum conditions, which limits the reaction conditions that can be studied, or require long data acquisition times, which limits temporal resolution. Using high x-ray energies, Gustafson et al. (p. 758, published online 30 January; see the Perspective by Nicklin) were able to measure the intensities of surface-diffracted beams to follow the surface oxidation that accompanies the changes in a palladium surface during the catalytic oxidation of CO with O2.

Growth of fcc(111) on bcc(110): new type of epitaxial transition observed for Pd on Cr

Abstract Depending on film thickness, t , and growth temperature, T , LEED and in-plane X-ray measurements show Kurdjumov—Sachs (KS) as well as Nishiyama-Wassermann (NW) epitaxial relationships for Pd(111) on Cr(110). This is a surprising result, because theoretical models definitely expect the KS orientation for this system. In a systematic study of this phenomenon we determined areas of different epitaxial orientations in a t T diagram. During Pd growth, we observed a continuous transition from the KS to the NW epitaxial relationship, which is accompanied by an enormous reorientation of material. Qualitative considerations, concerning the Pd island structure developing during growth, help to explain the observed behaviour.

Temperature and thickness dependent epitaxial relationship of Pd (111) on Cr (110)

Abstract Considering only the atomic diameter ratio, a Kurdjumov–Sachs (KS) in-plane epitaxial relationship between Pd(111) and Cr(110) appears to be favored. However, depending up on growth temperature and film thickness we obtain both Nishiyama–Wassermann (NW) and KS orientations. This was found by surface X-ray scattering as well as from LEED experiments. The two orientations can be distinguished by the number of in-plane domains which is one for NW and two for KS. The samples were grown by MBE. During the Pd growth, we observe a continuous transition from the KS to the NW relationship, which is accompanied by a major reorientation of material. Qualitative considerations concerning the Pd island structure, which develops during growth, explain the observed behavior. The thickness–temperature dependence of the different epitaxial orientations has been determined in a phase diagram.

Single orientation graphene synthesized on iridium thin films grown by molecular beam epitaxy

Heteroepitaxial iridium thin films were deposited on (0001) sapphire substrates by means of molecular beam epitaxy, and subsequently, one monolayer of graphene was synthesized by chemical vapor deposition. The influence of the growth parameters on the quality of the Ir films, as well as of graphene, was investigated systematically by means of low energy electron diffraction, x-ray reflectivity, x-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. Our study reveals (111) oriented iridium films with high crystalline quality and extremely low surface roughness, on which the formation of large-area epitaxial graphene is achieved. The presence of defects, like dislocations, twins, and 30° rotated domains in the iridium films is also discussed. The coverage of graphene was found to be influenced by the presence of 30° rotated domains in the Ir films. Low iridium deposition rates suppress these rotated domains and an almost complete coverage of graphene was o...

Correlation between stoichiometry and surface structure of the polar MgAl2O4(100) surface as a function of annealing temperature

The correlation between surface structure, stoichiometry and atomic occupancy of the polar MgAl2O4(100) surface has been studied with an interplay of noncontact atomic force microscopy, X-ray photoelectron spectroscopy and surface X-ray diffraction under ultrahigh vacuum conditions. The Al/Mg ratio is found to significantly increase as the surface is sputtered and annealed in oxygen at intermediate temperatures ranging from 1073-1273 K. The Al excess is explained by the observed surface structure, where the formation of nanometer-sized pits and elongated patches with Al terminated step edges contribute to stabilizing the structure by compensating surface polarity. Surface X-ray diffraction reveals a reduced occupancy in the top two surface layers for both Mg, Al, and O and, moreover, vacancies are preferably located in octahedral sites, indicating that Al and Mg ions interchange sites. The excess of Al and high concentration of octahedral vacancies, very interestingly, indicates that the top few surface layers of the MgAl2O4(100) adopts a surface structure similar to that of a spinel-like transition Al2O3 film. However, after annealing at a high temperature of 1473 K, the Al/Mg ratio restores to its initial value, the occupancy of all elements increases, and the surface transforms into a well-defined structure with large flat terraces and straight step edges, indicating a restoration of the surface stoichiometry. It is proposed that the tetrahedral vacancies at these high temperatures are filled by Mg from the bulk, due to the increased mobility at high annealing temperatures.

A New Synthesis Approach for Carbon Nitrides: Poly(triazine imide) and Its Photocatalytic Properties

Poly(triazine imide) (PTI) is a material belonging to the group of carbon nitrides and has shown to have competitive properties compared to melon or g-C3N4, especially in photocatalysis. As most of the carbon nitrides, PTI is usually synthesized by thermal or hydrothermal approaches. We present and discuss an alternative synthesis for PTI which exhibits a pH-dependent solubility in aqueous solutions. This synthesis is based on the formation of radicals during electrolysis of an aqueous melamine solution, coupling of resulting melamine radicals and the final formation of PTI. We applied different characterization techniques to identify PTI as the product of this reaction and report the first liquid state NMR experiments on a triazine-based carbon nitride. We show that PTI has a relatively high specific surface area and a pH-dependent adsorption of charged molecules. This tunable adsorption has a significant influence on the photocatalytic properties of PTI, which we investigated in dye degradation experiments.

Model Catalytic Studies of Novel Liquid Organic Hydrogen Carriers: Indole, Indoline and Octahydroindole on Pt(111)

Indole derivatives were recently proposed as potential liquid organic hydrogen carriers (LOHC) for storage of renewable energies. In this work, we have investigated the adsorption, dehydrogenation and degradation mechanisms in the indole/indoline/octahydroindole system on Pt(111). We have combined infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and DFT calculations. Indole multilayers show a crystallization transition at 200 K, in which the molecules adopt a strongly tilted orientation, before the multilayer desorbs at 220 K. For indoline, a less pronounced restructuring transition occurs at 150 K and multilayer desorption is observed at 200 K. Octahydroindole multilayers desorb already at 185 K, without any indication for restructuring. Adsorbed monolayers of all three compounds are stable up to room temperature and undergo deprotonation at the NH bond above 300 K. For indoline, the reaction is followed by partial dehydrogenation at the 5-membered ring, leading to the formation of a flat-lying di-σ-indolide in the temperature range from 330-390 K. Noteworthy, the same surface intermediate is formed from indole. In contrast, the reaction of octahydroindole with Pt(111) leads to the formation of a different intermediate, which originates from partial dehydrogenation of the 6-membered ring. Above 390 K, all three compounds again form the same strongly dehydrogenated and partially decomposed surface species.

Toward Optimization of Centrifugal Barrel Polishing P rocedure for Treatment of Niobium Cavities

Centrifugal barrel polishing (CBP) is a simple and environmentally friendly method that can be applied for mechanical abrasion of the cavity interior in order to remove the mechanically damaged surface after its production. The CBP recipes described in the literature, however, require CBP to be performed in many stages, require long processing times and nevertheless are unable to provide good cavity RF performance without additional chemical processing. Here, we report new results on characterization of cavity surfaces treated with a typical CBP recipe, including the contamination with abrasive particles, plastic deformation and hydrogen contamination, and critically evaluate it. Methods to reduce the depth of significant plastic deformation as well as the modified commercially viable CBP procedure followed by final electropolishing are proposed and tested on samples.

Hard X-ray Resonant Ptychography for Chemical Imaging at the Sensitivity Limit

1 Australian Synchrotron and La Trobe University, Dept. of Chemistry and Physics, Melbourne, Australia 2 Australian Research Council, Centre of Excellence for Advanced Molecular Imaging, Australia 3 Deutsches Elektronen-Synchrotron, Hamburg, Germany 4 University of Hamburg, Department Physik, Hamburg, Germany 5 Technical University Wien, Institute for Chemical Technologies and Analytics, Vienna, Austria ∗ Corresponding author: juliane.reinhardt@synchrotron.org.au