Sören Selve

Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir-Ni Oxide Catalysts for Electrochemical Water Splitting (OER).

Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir-Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir-Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir-Ni oxide surface, we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for the OER activity and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insights into the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acidic environments.

Impact of food components during in vitro digestion of silver nanoparticles on cellular uptake and cytotoxicity in intestinal cells

Abstract Because of the rising application of nanoparticles in food and food-related products, we investigated the influence of the digestion process on the toxicity and cellular uptake of silver nanoparticles for intestinal cells. The main food components – carbohydrates, proteins and fatty acids – were implemented in an in vitro digestion process to simulate realistic conditions. Digested and undigested silver nanoparticle suspensions were used for uptake studies in the well-established Caco-2 model. Small-angle X-ray scattering was used to estimate particle core size, size distribution and stability in cell culture medium. Particles proved to be stable and showed radii from 3.6 to 16.0 nm. Undigested particles and particles digested in the presence of food components were comparably taken up by Caco-2 cells, whereas the uptake of particles digested without food components was decreased by 60%. Overall, these findings suggest that in vivo ingested poly (acrylic acid)-coated silver nanoparticles may reach the intestine in a nanoscaled form even if enclosed in a food matrix. While appropriate for studies on the uptake into intestinal cells, the Caco-2 model might be less suited for translocation studies. Moreover, we show that nanoparticle digestion protocols lacking food components may lead to misinterpretation of uptake studies and inconclusive results.

It takes more than a coating to get nanoparticles through the intestinal barrier in vitro

Graphical abstract Figure. No caption available. ABSTRACT Size and shape are crucial parameters which have impact on the potential of nanoparticles to penetrate cell membranes and epithelial barriers. Current research in nanotoxicology additionally focuses on particle coating. To distinguish between core‐ and coating‐related effects in nanoparticle uptake and translocation, two nanoparticles equal in size, coating and charge but different in core material were investigated. Silver and iron oxide nanoparticles coated with poly (acrylic acid) were chosen and extensively characterized by small‐angle x‐ray scattering, nanoparticle tracing analysis and transmission electron microscopy (TEM). Uptake and transport were studied in the intestinal Caco‐2 model in a Transwell system with subsequent elemental analysis. TEM and ion beam microscopy were conducted for particle visualization. Although equal in size, charge and coating, the behavior of the two particles in Caco‐2 cells was different: while the internalized amount was comparable, only iron oxide nanoparticles additionally passed the epithelium. Our findings suggest that the coating material influenced only the uptake of the nanoparticles whereas the translocation was determined by the core material. Knowledge about the different roles of the particle coating and core materials in crossing biological barriers will facilitate toxicological risk assessment of nanoparticles and contribute to the optimization of pharmacokinetic properties of nano‐scaled pharmaceuticals.

Automatable sample fabrication process for pump-probe X-ray holographic imaging

Soft X-ray holography is a recently developed imaging technique with sub-50 nm spatial resolution. Key advantages of this technique are magnetic and elemental sensitivity, compatibility with imaging at free electron laser facilities, and immunity to in-situ sample excitations and sample drift, which enables the reliable detection of relative changes between two images with a precision of a few nanometers. In X-ray holography, the main part of the experimental setup is integrated in the sample, which consequently requires a large number of fabrication steps. Here we present a generic design and an automatable fabrication process for samples suitable, and optimized for, efficient high resolution X-ray holographic dynamic imaging. The high efficiency of the design facilitates the acquisition of magnetic images in a few minutes and makes fully automatic image reconstruction possible.

A general approach to obtain soft x-ray transparency for thin films grown on bulk substrates

We present a general approach to thin bulk samples to transparency for experiments in the soft x-ray and extreme ultraviolet spectral range. The method relies on mechanical grinding followed by focused-ion-beam milling. It results in a uniformly thin area of high surface quality, suitable for nanoscale imaging in transmission. In a proof-of-principle experiment, nanoscale magnetic bits on a commercial hard drive glass disk are imaged with a spatial resolution below 30 nm by soft x-ray spectro-holography. Furthermore, we demonstrate imaging of a lithographically patterned test object via absorption contrast. Our approach is suitable for both amorphous and crystalline substrates and has been tested for a variety of common epitaxy growth substrates. Lateral thinning areas in excess of 100 μm2 and a remaining substrate thickness as thin as 150 nm are easily achievable. Our approach allows preserving a previously grown thin film, and from nanofocus electron diffraction, we find no evidence for morphological changes induced by the process, in agreement with numerical simulations of the ion implantation depth distributon. We expect our method to be widely applicable and especially useful for nanoscale imaging of epitaxial thin films.

Manufacturing and application of a 2 µm dark field aperture in TEM

For an entire TEM characterization of many materials, it is necessary to achieve selected area electron diffraction (SAED) patterns of smallest regions with assigning the reflexions to their origins in the real image. In a previous work we showed that we were able to successfully reduce the field of view by a customized SAED aperture to a 15 nm range [1]. Though it gives us very local information about the samples structure, in daily work it is not always satisfying, since the real image is as important to understand the correlation between certain Bragg spots and the real structure, e.g. given by a series of dark field images. Especially for closely neighboured reflexions, commercial objective apertures are too large and do not allow the separated selection of these spots. Since our conventional TECNAI is equipped with the standard aperture-stripe, we are limited to a smallest size of 10 µm (12 µm in reality) which delivers a field of view of ca. 7.5 mrad inside the back focal plane. The smallest commercially available aperture has a diameter of 5 µm. Figure 1a displays a section of a polycrystalline fcc diffraction pattern. The marked large circle represents the standard 10 µm objective aperture, while the smaller one represents our custom made aperture with a diameter of 2 µm or 1.5 mrad inside the back focal plane. This example shows, with standard apertures it is impossible to select the (311) reflexions without overlap of their neighboured (220) or (222). Therefore, we reworked the present PtIr aperture-stripe by focused ion beam (FIB) in two steps [2]. At first an existing hole of the stripe – there are 2 rows of holes, one provides smaller and the other one larger diameters, which are seldom used – was closed by ion beam-induced Pt-deposition. As a second step, a centred opening was sputtered into that layer by using of circular masks up to 2-µm in diameter. To minimize a conical shape of the opening, at low ion beam current (280 pA) with a high aspect ratio is used and the hole is successively milled from both sides. If the Pt-deposition is too thin, there is a high risk that scattered electrons in the TEM will not be entirely blocked by the new aperture and create artefacts and distortions in the images. Therefore, it has a thickness of around 6.5 µm. First investigations with TEM proved that the deposited layer is not transparent for 200 kV electrons anymore and thermally stable as well. Figure 1b-d shows an application of the new objective aperture on a multi-twinned system of polycrystalline diamonds. Although the twinned areas are in the range of 5–10 nm it becomes possible to correlate the chosen diffraction spots with their origins in the real image. The adjustment of the new objective aperture has to be done very carefully, it can easily outshine the observation screen or the CCD camera, so one can easily lose the designated position, but the selection of certain diffraction spots requires a very accurate positioning. Other than at larger apertures where slight drifts are not critical because of the visibility of the selected area and therefore easier readjustments, slightest drifts must be avoided. In conclusion, the new 2-µm objective aperture can be very helpful for the understanding and structural characterization of samples according their crystallinity, their growth behaviour or even defect studies. Keywords: objective aperture; focused ion beam; dark field

Optical properties of silver nanocube surfaces obtained by silane immobilization

Abstract Silver nanocubes were synthesized by the polyol method and immobilized on a surface in a simple approach using an aminopropyltriethoxysilane (APTES). The optical and structural properties of the polyvinylpyrrolidone (PVP) stabilized nanocubes were investigated in solution and on glass surfaces. The SERS enhancement factors at two excitation wavelengths for crystal violet were compared with electric fields arising in different nano¬particle configurations using finite-difference time-domain simulations. They are in agreement with the preferred face-to-face orientation in the nanoaggregates on the surfaces. The facile immobilization enables on-demand preparation and use of the nanocubes in real analytical applications.

Microstructural and Nanostructural FE-SEM, TEM, and STEM Investigations of High Strength Boron-Alloyed Steel Used in the Automotive Sector

Abstract More and more steel grades with martensitic microstructure are used for highly stressed structural components in the automobile branch. They are used in the field of light weight construction and for crash safety. Principally, press-hardened boron-alloyed steels with low carbon content are used here. In this study, the fine microstructure is investigated by means of FE-SEM and TEM microscopy. Possibilities but also limits of the FE-SEM investigation methodology are clearly pointed out and demonstrated presenting concrete examples. In addition to that, further microstructural investigations are performed using a modern scanning transmission electron microscope (STEM). The device allowed for switching from the TEM mode to the SEM mode and thus enabled an investigation of one and the same microstructural section in SEM and TEM conditions; first experiences are presented.