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Item type:Publication, Access status: with files , Mechanical properties of TiO2/carboxylic-acid interfaces from first-principles calculationsNature forms structurally complex materials with a large variation of mechanical and physical properties from only very few organic compounds and minerals. Nanocomposites made from TiO2 and carboxylic-acids, two substances that are available to nature as well as materials engineers, can be seen as representative of a huge class of natural and bio-inspired materials. The hybrid interfaces between the two components are thought to determine the overall properties of the composite. Yet, little is known about the atomistic processes at those interfaces under load and their failure mechanisms. The present work models the stress-strain curves of TiO2/carboxylic-acid interfaces in the slow deformation limit for different facets and binding modes, employing density functional theory calculations. Contrary to former hypotheses, the interface rarely fails through a de-bonding of the molecule, but rather through a surface failure mechanism. Furthermore, a stress-release mechanism is discovered for the bi-dentate binding mode on the {101} facet. Deriving mechanical properties, such as the interface strength, strain at interface failure, and the elastic modulus, allows a comparison with experimental results. The calculated strengths and elastic moduli already agree qualitatively with properties of nanocomposites, despite the simplifications in the model consisting of periodic sandwich structures. The results presented here will help to improve these materials and can be directly integrated in multi-scale simulations, in order to reach a more accurate quantitative description.Publicationtype: Journal Article13 40 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Surface structure of magnetite (111) under oxidizing and reducing conditions(IOP Publishing, 2022-02-22); ; ; ; We report on differences in the magnetite (111) surface structure when prepared under oxidizing and reducing conditions. Both preparations were done under UHV conditions at elevated temperatures, but in one case the sample was cooled down while keeping it in an oxygen atmosphere. Scanning tunneling microscopy after each of the preparations showed a different apparent morphology, which is discussed to be an electronic effect and which is reflected in the necessity of using opposite bias tunneling voltages in order to obtain good images. Surface x-ray diffraction revealed that both preparations lead to Fe vacancies, leading to local O-terminations, the relative fraction of which depending on the preparation. The preparation under reducing conditions lead to a larger fraction of Fe-termination. The geometric structure of the two different terminations was found to be identical for both treatments, even though the surface and near-surface regions exhibit small compositional differences; after the oxidizing treatment they are iron deficient. Further evidence for the dependence of iron vs oxygen fractional surface terminations on preparation conditions comes from Fourier transform infrared reflection-absorption spectroscopy, which is used to study the adsorption of formic acid. These molecules dissociate and adsorb in chelating and bidentate bridging geometries on the Fe-terminated areas and the signal of typical infrared absorption bands is stronger after the preparation under reducing conditions, which results in a higher fraction of Fe-termination. The adsorption of formic acid induced an atomic roughening of the magnetite (111) surface which we conclude from the quantitative analysis of the crystal truncation rod data. The roughening process is initiated by atomic hydrogen, which results from the dissociation of formic acid after its adsorption on the surface. Atomic hydrogen adsorbs at surface oxygen and after recombination with another H this surface hydroxyl can form H2O, which may desorb from the surface, while iron ions diffuse into interstitial sites in the bulk.Publicationtype: Journal ArticleScopus© Citations 10 72 334 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Heterogeneous adsorption and local ordering of formate on a magnetite surface(2021-04-22); ; ; ; We report a novel heterogeneous adsorption mechanism of formic acid on the magnetite (111) surface. Our experimental results and density functional theory (DFT) calculations give evidence for dissociative adsorption of formic acid in quasibidentate and chelating geometries. The latter is induced by the presence of iron vacancies at the surface, making oxygen atoms accessible for hydrogen atoms from dissociated formic acid. DFT calculations predict that both adsorption geometries are energetically favorable under our experimental conditions. The calculations prove that the locally observed (√3 × √3)R 30° superstructure consists of three formate molecules in a triangular arrangement, adsorbed predominantly in a chelating geometry. The results show how defects can stabilize alternative adsorption geometries, which is a crucial ingredient for a detailed atomistic understanding of reaction barriers on magnetite and other oxide surfaces, as well as for the stability of carboxylic acid based nanocomposite materials.Publicationtype: Journal ArticleScopus© Citations 8 173 574 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Modeling charge redistribution at magnetite interfaces in empirical force fieldsMagnetite shows enormous potential from biocompatible hybrid materials to heterogeneous catalysis. However, a detailed atomistic understanding of magnetite in complex nanostructures and at interfaces is required to unfold these potentials. Methods capable of treating (several) thousands of atoms and achieving an optimal balance between accuracy and efficiency are therefore in great demand. Here, a new empirical force field for the (001) and (111) magnetite surfaces is developed using partial point charges derived from ab initio Bader charge analyses. An accurate description of electrostatic interactions enables the modeling of magnetite-organic and magnetite-water interfaces. Consequently, surface charge redistribution is proposed as the most relevant mechanism for the surface reconstruction of magnetite and the bidentate binding of ligands. The produced force field results are in excellent agreement with the latest findings on magnetite. The approach can be further applied to magnetite nanoparticles and easily extended to oxide and other ionic crystal surfaces.Publicationtype: Journal ArticleScopus© Citations 13 197 710 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Understanding interfaces in metal-oxide/organic-acid hybrid materials from first-principles calculationsMetal-oxide/organic-acid interfaces play an important role in various applications ranging from hybrid materials to catalysis. The present work studies the structure and energetics of these interfaces on the atomic scale. Employing first-principles calculations and a novel structure search tool, the most stable interface structures are determined, finding a restructuring of magnetite (Fe3O4) surfaces, and identifying shape-influencing factors for titania (TiO2) nanoparticles. Finally, the facet dependence of mechanical interface properties is examined, yielding deeper insights in the origin of failure in hybrid materials.Publicationtype: Doctoral Thesis315 296 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Shape-controlling effects of hydrohalic and carboxylic acids in TiO2 nanoparticle synthesisThe ability to synthesize nanoparticles (NPs), here TiO2, of different shapes in a controlled and reproducible way is of high significance for a wide range of fields including catalysis and materials design. Different NP shapes exhibit variations of emerging facets, and processes such as adsorption, diffusion, and catalytic activity are, in general, facet sensitive. Therefore, NP properties, e.g., the reactivity of NPs or the stability of assembled NPs, depend on their shape. We combine computational modeling based on density functional theory with experimental techniques such as transmission electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray powder diffraction to investigate the ability of various adsorbates, including hydrohalic and carboxylic acids, to influence NP shape. This approach allows us to identify mechanisms stabilizing specific surface facets and thus to predict NP shapes using computational model systems and to experimentally characterize the synthesized NPs in detail. Shape-controlled anatase TiO2 NPs are synthesized here in agreement with the calculations in platelet and bi-pyramidal shapes by employing different precursors. The importance of the physical conditions and chemical environment during synthesis, e.g., via competitive adsorption or changes in the chemical potentials, is studied via ab initio thermodynamics, which allows us to set previous and new results in a broader context and to highlight potentials for additional synthesis routes and NP shapes.Publicationtype: Journal ArticleScopus© Citations 7 301 494 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Carboxylic acid induced near-surface restructuring of a magnetite surface(Macmillan Publishers Limited, part of Springer Nature, 2019-08-09); ; ; ; A fundamental knowledge of the interaction of carboxylic acids, such as formic acid, with magnetite surfaces is of prime importance for heterogeneous catalysis and the synthesis of novel materials. Despite this, little is known about the atomic scale adsorption mechanisms. Here we show by in-situ surface X-ray diffraction that the oxygen rich subsurface cation vacancy reconstruction of the clean magnetite (001) surface is lifted by dissociative formic acid adsorption, reestablishing a surface with bulk stoichiometry. Using density functional theory, the bulk terminated, fully formic acid covered surface is calculated to be more stable than the corresponding clean, reconstructed surface. A comparison of calculated and experimental infrared bands supports the bidentate adsorption geometry and a specific adsorption site. Our results pave the way for a fundamental understanding of the bonding mechanism at carboxylic acid/oxide interfaces.Publicationtype: Journal ArticleScopus© Citations 16 401 347 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Energy harvesting near room temperature using a thermomagnetic generator with a pretzel-like magnetic flux topology(2019-01-01); ; ; ; To date, there are very few technologies available for the conversion of low-temperature waste heat into electricity. Thermomagnetic generators are one approach proposed more than a century ago. Such devices are based on a cyclic change of magnetization with temperature. This switches a magnetic flux and, according to Faraday’s law, induces a voltage. Here we demonstrate that guiding the magnetic flux with an appropriate topology of the magnetic circuit improves the performance of thermomagnetic generators by orders of magnitude. Through a combination of experiments and simulations, we show that a pretzel-like topology results in a sign reversal of the magnetic flux. This avoids the drawbacks of previous designs, namely, magnetic stray fields, hysteresis and complex geometries of the thermomagnetic material. Our demonstrator, which is based on magnetocaloric plates, illustrates that this solid-state energy conversion technology presents a key step towards becoming competitive with thermoelectrics for energy harvesting near room temperature.Publicationtype: Journal Article252 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adsorption of acetone on rutile TiO2: a DFT and FTIRS studyAcetone adsorbed on rutile TiO 2 nanoparticles was investigated with respect to its energetic, vibrational, and chemical properties. Temperature-dependent ultrahigh-vacuum Fourier transform infrared spectroscopy measurements for different acetone dosages (4.5-900 L) give insights into the acetone adsorption behavior. Those experiments indicate thermal-induced reactions of acetone on rutile TiO 2 surfaces yielding new species. Density functional theory calculations were performed to investigate acetone adsorption on rutile TiO 2 (110). Particularly, the importance of sampling the adsorption configuration space is shown. Adsorption geometries that are energetically significantly more favorable than the commonly used high-symmetry configurations are presented. To facilitate the comparability to the experiment, theoretical infrared spectra were computed using density functional perturbation theory for various acetone adsorption geometries using different exchange-correlation functionals. Additionally, computational spectra were obtained for several species which are potential products from reactions of acetone on TiO 2 surfaces. The investigated species are η 2 -acetate, η 2 -diolate, η 1 -enolate, and mesityl oxide. For η 1 -acetone, experimental and calculated spectra fit well for low temperatures, whereas for elevated temperatures, emerging bands indicate the formation of diolate.Publicationtype: Journal ArticleScopus© Citations 23 382 512 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Interfacial Thermal Resistance in Magnetocaloric Epoxy-Bonded La-Fe-Co-Si Composites(2018-08); ; ; ; Magnetocaloric composites made from La−Fe−Co−Si particles and an epoxy binder matrix exhibit mechanical stability and good magnetocaloric properties, but also a large characteristic time for thermal transport. Here, the origin of this large time constant is examined by comparing two measurement techniques, direct and contactless, to finite-element simulations based on a tomographic dataset of the sample. The combination of the low thermal conductivity of the epoxy matrix and a thermal resistance at the interface between epoxy and La−Fe−Co−Si is shown to be in good agreement in simulations and experiments. The findings help to disentangle the role of the thermal conductivity and the interfacial thermal resistance for the heat flow in magnetocaloric composites. It is shown that the low thermal conductivity of the epoxy alone cannot explain the large time constant and possibilities for using the interfacial thermal resistance to tailor anisotropic thermal conductivity for directional heat transfer in magnetocaloric composites are presented.Publicationtype: Journal Article193