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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 8 173 577 - 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 7 301 500 - 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: 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 , Electrocapillary Coupling at Metal Surfaces from First Principles: On the Impact of Excess Charge on Surface Stress and Relaxation(2018); ; We study the response of the surface stress to excess charge via ab initio simulation of metal surfaces in an external electric field. We focus on "simple" sp-bonded metals to gain insight into the mechanisms underlying electrocapillary coupling. Both the direct effect on the surface stress via charging of the bonds and the indirect effect resulting from the charge-induced relaxation are analyzed and discussed in relation to the trends of the coupling coefficients, which-owing to a Maxwell relation-are determined in terms of the response of the work function to strain. Al(111), Mg(0001), and Na(110) are investigated as prototypical sp-bonded metal surfaces with positive, vanishing, and negative coupling parameters, respectively. Mg(0001) and Al(111) exhibit an inward relaxation of the first atomic layer upon negative charging, whereas an outward relaxation occurs for Na(110). The indirect contribution of the relaxation to the coupling coefficient has the same sign as the total response and makes up about 30% of its magnitude for Al(111) and Na(110). Our study highlights that even the response behavior of the so-called simple metals is by no means readily captured within simple models.Publicationtype: Journal Article214 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A first principles study on the electronic origins of silver segregation at the Ag-Au (111) surface(2017-12-21); The special electronic structure of gold gives rise to many interesting phenomena, such as its color. The surface segregation of the silver-gold system has been the subject of numerous experimental and theoretical studies, yielding conflicting results ranging from strong Ag surface enrichment to Au surface segregation. Via a combined approach of density functional theory (DFT) and statistical physics, we have analyzed the segregation at the Ag-Au (111) surface with different Ag bulk concentrations. Interestingly, we observe a moderate Au surface segregation, which is due to a charge transfer from the less electronegative Ag to Au. Canonical Monte Carlo simulations suggest that the calculated concentration profile with a Au-enriched surface layer remains stable up to higher temperatures. However, the presence of adsorbed oxygen reverses the segregation behavior and leads to strong Ag enrichment of the surface layer.Publicationtype: Journal Article133 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Van der Waals Interaction Really Matters: Energetics of Benzoic Acid on TiO₂ Rutile Surfaces(2017-08-17); ; ; Density functional theory (DFT) has been applied to elucidate the adsorption structures and energetics of benzoic acid on TiO₂ (110), (100), and (011) rutile surfaces. We demonstrate that ab initio calculations of interacting carboxylic acids require an exchange-correlation functional with van der Waals (vdW) correction to yield reliable results, even for very small aliphatic species like acetic acid. On the (110) surface, benzoates dimerize due to intermolecular vdW interaction and form a 2 × 2 superstructure, which explains experimental findings already reported in the literature. The lateral vdW attraction between benzoates is even enhanced on (100) and (011) surfaces because of advantageous substrate periodicities, resulting essentially in only one geometrical adsorbate species.Publicationtype: Journal Article124 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Oxygen-vacancy driven electron localization and itinerancy in rutile-based TiO2(American Physical Society (APS), 2017-05-30); ; ; Oxygen-deficient TiO2 in the rutile structure as well as the Ti3O5 Magnéli phase is investigated within the charge self-consistent combination of density functional theory with dynamical mean-field theory. It is shown that an isolated oxygen vacancy (VO) in titanium dioxide is not sufficient to metallize the system at low temperatures. In a semiconducting phase, an in-gap state is identified at IG∼-0.75eV in excellent agreement with experimental data. Bandlike impurity levels, resulting from a threefold VO-Ti coordination as well as entangled (t2g,eg) states, become localized due to site-dependent electronic correlations. Charge localization and strong orbital polarization occur in the VO-near Ti ions, the details of which can be modified by a variation of the correlated subspace. At higher oxygen vacancy concentration, a correlated metal is stabilized in the Magnéli phase. A VO-defect rutile structure of identical stoichiometry shows key differences in the orbital-resolved character and the spectral properties. Charge disproportionation is vital in the oxygen-deficient compounds, but obvious metal-insulator transitions driven or sustained by charge order are not identified.Publicationtype: Journal Article139 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , How silver segregation stabilizes 1D surface gold oxide: A cluster expansion study combined with: Ab initio MD simulations(2017-05-09); ; ; Gold has many unique properties, some of which continue to be uncovered, such as the rich chemistry of gold at the nanoscale. In this study, gold surprises us again by the unusual stability of one-dimensional gold oxide structures on the surface of gold, which enhances in the presence of silver impurities. We employ first-principles calculations to investigate the surface segregation of silver in the presence of atomic-oxygen adsorbates arranged in chains on the Au(321) surface. Such 1D oxide chains have previously been suggested as the most stable form of adsorbed oxygen on gold. Although Ag-O bonds are expected to be generally stronger than Au-O bonds, we show that this does not hold for 1D oxide chains, where Au-O bonds seem to be at least as strong as Ag-O bonds. Remarkably, we find that up to very high surface concentrations of silver, the Ag atoms do not occupy positions within the oxide chain, but prefer locations next to it. Ab initio molecular dynamics simulations support this picture and reveal how oxide chains and silver atoms rearrange on the surface toward a lower-energy configuration.Publicationtype: Journal Article135 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Surface excess elasticity of gold: ab initio coefficients and impact on the effective elastic response of nanowiresPredicting the influence of the surface on the effective elastic properties of nanoscale structures and nanomaterials remains a challenge, which we here address on both levels, continuum and atomic. Density Functional Theory (DFT) computation at the atomic level yields the first reliable surface excess elastic parameters for the (111) and (001) surfaces of gold. At the continuum level, we derive closed-form expressions for the effective elastic behavior that can be combined with the DFT-derived excess elastic parameters to obtain the effective axial, torsion, and bending stiffness of circular nanowires with surface excess elasticity. The two approaches use different reference frames, and we emphasize the need for consistent stress definitions and for conversion between the separate stress measures when transferring results between the approaches. We present excess elastic parameters separately for Cauchy and 2nd Piola-Kirchhoff stresses, demonstrating that the conversion substantially modifies their numerical value and may even invert their sign. The results afford an assessment of the contribution of the surface excess elastic parameters to the effective elastic response of nanoscale beams or wires. This assessment sheds doubt on earlier suggestions relating experimental observations of an effective stiffening or softening at small size to the excess elasticity of clean surfaces.Publicationtype: Journal ArticleScopus© Citations 33 438 852