Stochastic models of dense or hollow nanoparticles and their scattering properties

peer reviewedA family of stochastic models of disordered particles is proposed, obtained by clipping a Gaussian random field with a function that is space dependent. Depending on the shape of the clipping function, dense or hollow particles can be modelled. General expressions are derived for the form factor of the particles, for their average volume and surface area, and for their density and surface-area distributions against the distance to the particle centre. A general approximation for the form factor is also introduced, based on the density and surface-area distributions, which coincides with the Guinier and Porod expressions in the limits of low and high scattering vector magnitude q. The models are illustrated with the fitting of small-angle X-ray scattering (SAXS) data measured on Pt/Ni hollow nanoparticles. The SAXS analysis and modelling notably capture the collapse of the particles' porosity after being used as oxygen-reduction catalysts

Probing Exfoliated Graphene Layers and Their Lithiation with Microfocused X-rays

X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 μm × 10 μm, individual flakes are probed by specular X-ray reflectivity. By deploying a recursive Parratt algorithm to model the experimental data, we gain access to characteristic crystallographic parameters of the samples. Notably, it is possible to directly extract the bi/multilayer graphene c-axis lattice parameter. The latter is found to increase upon lithiation, which we control using an on-chip peripheral electrochemical cell layout. These experiments demonstrate the feasibility of in situ X-ray diffraction on individual, micron-sized single crystallites of few- and bilayer two-dimensional materials

Electronic and structural reconstructions of the polar (111) SrTiO3 surface

Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) SrTiO3 single crystals. We find that the polar instability of the 1 x 1 surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of SrTiO3 layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of 3 x 3 (111) SrTiO3 surfaces consisting in an incomplete Ti (surface)-O-2 (subsurface) layer covering the 1 x 1 Ti-terminated (111) SrTiO3 truncated crystal. In both cases, the TiO6 octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) SrTiO3 surface can lead to the formation of quasi two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions

Electrochemical Formation of Germanene: pH 4.5

Germanene is a single layer allotrope of Ge, with a honeycomb structure similar to graphene. This report concerns the electrochemical formation of germanene in a pH 4.5 solution. The studies were performed using in situ Electrochemical Scanning Tunneling Microscopy (EC-STM), voltammetry, coulometry, surface X-ray diffraction (SXRD) and Raman spectroscopy to study germanene electrodeposition on Au(111) terraces. The deposition of Ge is kinetically slow and stops after 2–3 monolayers. EC-STM revealed a honeycomb (HC) structure with a rhombic unit cell, 0.44 ± 0.02 nm on a side, very close to that predicted for germanene in the literature. Ideally the HC structure is a continuous sheet, with six Ge atoms around each hole. However, only small domains, surrounded by defects, of this structure were observed in this study. The small coherence length and multiple rotations domains made direct observation with surface X-ray diffraction difficult. Raman spectroscopy was used to investigate the multi-layer Ge deposits. A peak near 290 cm^(−1), predicted to correspond to germanene, was observed on one particular area of the sample, while the rest resembled amorphous germanium. Electrochemical studies of germanene showed limited stability when exposed to oxygen

Structural and Electronic Reconstructions at the LaAlO3/SrTiO3 Interface

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

5D operando tomographic diffraction imaging of a catalyst bed

We report the results from the first 5D tomographic diffraction imaging experiment of a complex Ni-Pd/CeO2-ZrO2/Al2O3 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 × 106 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 CeO2-ZrO2 promoters and the reasons behind the partial deactivation of the catalyst during partial oxidation of methane

In Situ Transmission X-ray Micro-Diffraction from Thin Metal Films Electrodeposited in Microfluidic Channels

International audienceHigh-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

Templating effects of α-sexithiophene in donor-acceptor organic thin films

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.by C. Lorch, R. Banerjee, J. Dieterle, A. Hinderhofer, A. Gerlach, J. Drnec and F. Schreibe