Photoinduced Dynamics at the Water/TiO_{2}(101) Interface.

We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO_{2}(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO_{2} to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an O_{2c} site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO_{2} by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role.

Metastability of palladium carbide nanoparticles during hydrogen release from liquid organic hydrogen carriers.

Efficient hydrogen release from liquid organic hydrogen carriers (LOHCs) requires a high level of control over the catalytic properties of supported noble metal nanoparticles. Here, the formation of carbon-containing phases under operation conditions has a direct influence on the activity and selectivity of the catalyst. We studied the formation and stability of carbide phases using well-defined Pd/α-Al2O3(0001) model catalysts during dehydrogenation of a model LOHC, methylcyclohexane, in a flow reactor by in situ high-energy grazing incidence X-ray diffraction. The phase composition of supported Pd nanoparticles was investigated as a function of particle size and reaction conditions. Under operating conditions, we detected the formation of a PdxC phase followed by its conversion to Pd6C. The dynamic stability of the Pd6C phase results from the balance between uptake and release of carbon by the supported Pd nanoparticles in combination with the thermodynamically favorable growth of carbon deposits in the form of graphene. For small Pd nanoparticles (6 nm), the Pd6C phase is dynamically stable under low flow rate of reactants. At the high reactant flow, the Pd6C phase decomposes shortly after its formation due to the growth of graphene. Structural analysis of larger Pd nanoparticles (15 nm) reveals the formation and simultaneous presence of two types of carbides, PdxC and Pd6C. Formation and decomposition of Pd6C proceeds via a PdxC phase. After an incubation period, growth of graphene triggers the decomposition of carbides. The process is accompanied by segregation of carbon from the bulk of the nanoparticles to the graphene phase. Notably, nucleation of graphene is more favorable on bigger Pd nanoparticles. Our studies demonstrate that metastability of palladium carbides associated with dynamic formation and decomposition of the Pd6C and PdxC phases is an intrinsic phenomenon in LOHC dehydrogenation on Pd-based catalysts and strongly depends on particle size and reaction conditions.

In situ studies of the cathodic stability of single-crystalline IrO2(110) ultrathin films supported on RuO2(110)/Ru(0001) in an acidic environment.

We investigate with in situ surface X-ray diffraction (SXRD) and X-ray reflectivity (XRR) experiments the cathodic stability of an ultrathin single-crystalline IrO2(110) film with a regular array of mesoscopic rooflike structures that is supported on a RuO2(110)/Ru(0001) template. It turns out that the planarity of the single-crystalline IrO2(110) film is lost in that IrO2(110) oxide domains delaminate at a cathodic potential of -0.18 V. Obviously, the electrolyte solution is able to reach the RuO2(110) layer presumably through the surface grain boundaries of the IrO2(110) layer. Subsequently, the single-crystalline RuO2(110) structure-directing template is reduced to amorphous hydrous RuO2, with the consequence that the IrO2(110) film loses partly its adhesion to the template. From in situ XRR experiments we find that the IrO2(110) film does not swell upon cathodic polarization down to -0.18 V, while from in situ SXRD experiments, the lattice constants of IrO2(110) are shown to be not affected. The rooflike mesostructure of the IrO2(110) flakes remains intact after cathodic polarization to -0.18 V, evidencing that the crystallinity of IrO2(110) is retained.

Order-disorder phase transition of the subsurface cation vacancy reconstruction on Fe3O4(001).

We present surface X-ray diffraction and fast scanning tunneling microscopy results to elucidate the nature of the surface phase transition on magnetite (001) from a reconstructed to a non-reconstructed surface around 720 K. In situ surface X-ray diffraction at a temperature above the phase transition, at which long-range order is lost, gives evidence that the subsurface cation vacancy reconstruction still exists as a local structural motif, in line with the characteristics of a 2D second-order phase transition. Fast scanning tunneling microscopy results across the phase transition underpin the hypothesis that the reconstruction lifting is initiated by surplus Fe ions occupying subsurface octahedral vacancies. The reversible near-surface iron enrichment and reduction of the surface to stoichiometric composition is further confirmed by in situ low-energy ion scattering, as well as ultraviolet and X-ray photoemission results.

Atomic scale step structure and orientation of a curved surface ZnO single crystal.

We have investigated the surface structure of a curved ZnO-crystal, going from the (0001)-facet at 0° miscut to the (101¯4)-facet at a miscut of 24.8° using scanning tunneling microscopy and low energy electron diffraction. We find that the surface separates locally into (0001)-terraces and (101¯4)-facets, where the ratio between the facets depends on the miscut angle. In X-ray photoemission spectroscopy (XPS) the intensity of an O 1s component scaling with the step density of the surface is observed. No other facets were observed and the surface maintains a high degree of order over all angles. Such a curved ZnO crystal can be used for systematic studies relating the step density to the chemical reactivity using XPS to probe the curved surface at different positions.

A versatile nanoreactor for complementary in situ X-ray and electron microscopy studies in catalysis and materials science.

Two in situ `nanoreactors' for high-resolution imaging of catalysts have been designed and applied at the hard X-ray nanoprobe endstation at beamline P06 of the PETRA III synchrotron radiation source. The reactors house samples supported on commercial MEMS chips, and were applied for complementary hard X-ray ptychography (23 nm spatial resolution) and transmission electron microscopy, with additional X-ray fluorescence measurements. The reactors allow pressures of 100 kPa and temperatures of up to 1573 K, offering a wide range of conditions relevant for catalysis. Ptychographic tomography was demonstrated at limited tilting angles of at least ±35° within the reactors and ±65° on the naked sample holders. Two case studies were selected to demonstrate the functionality of the reactors: (i) annealing of hierarchical nanoporous gold up to 923 K under inert He environment and (ii) acquisition of a ptychographic projection series at ±35° of a hierarchically structured macroporous zeolite sample under ambient conditions. The reactors are shown to be a flexible and modular platform for in situ studies in catalysis and materials science which may be adapted for a range of sample and experiment types, opening new characterization pathways in correlative multimodal in situ analysis of functional materials at work. The cells will presently be made available for all interested users of beamline P06 at PETRA III.

Gas-Induced Segregation in Pt-Rh Alloy Nanoparticles Observed by In Situ Bragg Coherent Diffraction Imaging.

Bimetallic catalysts can undergo segregation or redistribution of the metals driven by oxidizing and reducing environments. Bragg coherent diffraction imaging (BCDI) was used to relate displacement fields to compositional distributions in crystalline Pt-Rh alloy nanoparticles. Three-dimensional images of internal composition showed that the radial distribution of compositions reverses partially between the surface shell and the core when gas flow changes between O_{2} and H_{2}. Our observation suggests that the elemental segregation of nanoparticle catalysts should be highly active during heterogeneous catalysis and can be a controlling factor in synthesis of electrocatalysts. In addition, our study exemplifies applications of BCDI for in situ 3D imaging of internal equilibrium compositions in other bimetallic alloy nanoparticles.

Interaction of Water with Graphene/Ir(111) Studied by Vibrational Spectroscopy.

Water in confinement exhibits altered properties in molecular arrangement, bonding, and interaction with its neighboring environment, as compared to its bulk counterpart. In this work, periodically arranged D2O nano droplets of ∼1 nm size on top of a graphene/iridium moiré superstructure were investigated by Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) under ultrahigh vacuum conditions at ∼120 K. The IR bands of D2O clusters differ significantly from those observed for bulk D2O amorphous solid water or crystalline ice phases. Blue-shifted symmetric and asymmetric stretching bands with narrower band widths and modified band intensity ratios were observed, pointing to an enhanced internal order and a reduced nearest neighbor distance. Furthermore, two IR bands of "dangling" deuterium atoms were detected originating from threefold coordinated water molecules at the surface of the clusters and at their interface to the graphene layer. The latter arose only with the transition from the water clusters to an amorphous solid water layer. We propose that upon coalescence, opposing local dipoles trigger a hydrogen bond rearrangement at the interface. Our results represent a first step toward an atomistic understanding of water in confinement.

Modulating the Mechanical Properties of Supercrystalline Nanocomposite Materials via Solvent-Ligand Interactions.

Supercrystalline nanocomposite materials with micromechanical properties approaching those of nacre or similar structural biomaterials can be produced by self-assembly of organically modified nanoparticles and further strengthened by cross-linking. The strengthening of these nanocomposites is controlled via thermal treatment, which promotes the formation of covalent bonds between interdigitated ligands on the nanoparticle surface. In this work, it is shown how the extent of the mechanical properties enhancement can be controlled by the solvent used during the self-assembly step. We find that the resulting mechanical properties correlate with the Hansen solubility parameters of the solvents and ligands used for the supercrystal assembly: the hardness and elastic modulus decrease as the Hansen solubility parameter of the solvent approaches the Hansen solubility parameter of the ligands that stabilize the nanoparticles. Moreover, it is shown that self-assembled supercrystals that are subsequently uniaxially pressed can deform up to 6 %. The extent of this deformation is also closely related to the solvent used during the self-assembly step. These results indicate that the conformation and arrangement of the organic ligands on the nanoparticle surface not only control the self-assembly itself but also influence the mechanical properties of the resulting supercrystalline material. The Hansen solubility parameters may therefore serve as a tool to predict what solvents and ligands should be used to obtain supercrystalline materials with good mechanical properties.

Growth of well-ordered iron sulfide thin films.

In this paper a growth recipe for well-ordered iron sulfide films and the results of their characterisation are presented. The film was studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). XRD data reveal that the film has a NiAs-like structure with Fe vacancies, similar to iron sulfides such as pyrrhotite and smythite, although no indication of any ordering of these vacancies was observed. LEED and STM results show that the film exhibits a 2 × 2 surface reconstruction. XPS data provide additional evidence for a large number of Fe vacancies, and the oxidation states of the Fe and S in the film are analysed.

Non-uniform nanosecond gate-delay of hybrid pixel detectors.

A simple experiment to characterize the gating properties of X-ray area detectors using pulsed X-ray sources is presented. For a number of time-resolved experiments the gating uniformity of area detectors is important. Relative gating delays between individual modules and readout chips of PILATUS2 series area X-ray detectors have been observed. For three modules of a PILATUS 300K-W unit the maximum gating offset between the modules is found to be as large as 30 ns. On average, the first photosensor module is found to be triggered 15 ns and 30 ns later than the second and the third modules, respectively.

Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties.

It is commonly accepted that the combination of the anisotropic shape and nanoscale dimensions of the mineral constituents of natural biological composites underlies their superior mechanical properties when compared to those of their rather weak mineral and organic constituents. Here, we show that the self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid molecules leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomechanical model, we determined that these exceptional mechanical properties are dominated by the covalent backbone of the linked organic molecules. Because oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.

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.

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.

In situ oxidation study of Pd-Rh nanoparticles on MgAl2O4(001).

Alloy nanoparticles on oxide supports are widely used as heterogeneous catalysts in reactions involving oxygen. Here we discuss the oxidation behavior of Pd-Rh alloy nanoparticles on MgAl2O4(001) supports with a particle diameter from 6-11 nm. As an In situ tool, we employed high energy grazing incidence X-ray diffraction at a photon energy of 85 keV. We find that physical vapor deposited Pd-Rh nanoparticles grow epitaxially on MgAl2O4(001) with a truncated octahedral shape over the whole concentration range. During our systematic oxidation experiments performed at 670 K in the pressure range from 10(-3) to 0.1 mbar, we observe for Rh containing nanoparticles the formation of two different Rh oxide phases, namely RhO2 and a spinel-like Rh3O4 phase. PdO formation is only observed for pure Pd nanoparticles. This oxidation induced segregation behavior is also reflected in the oxidation induced enlargement of the average nanoparticle lattice parameter towards to value for pure Pd. Our results have ramifications for the phase separation behavior of alloy nanocatalysts under varying reducing and oxidizing environments.

Transient heating of Pd nanoparticles studied by x-ray diffraction with time of arrival photon detection.

Pulsed laser heating of an ensemble of Pd nanoparticles, supported by a MgO substrate, is studied by x-ray diffraction. By time-resolved Bragg peak shift measurements due to thermal lattice expansion, the transient temperature of the Pd nanoparticles is determined, which quickly rises by at least 100 K upon laser excitation and then decays within 90 ns. The diffraction experiments were carried out using a Cu x-ray tube, giving continuous radiation, and the hybrid pixel detector Timepix3 operating with single photon counting in a time-of-arrival mode. This type of detection scheme does not require time-consuming scanning of the pump-probe delay. The experimental time resolution is estimated at 15 ± 5 ns, which is very close to the detector's limit and matches with the 7 ns laser pulse duration. Compared to bulk metal single crystals, it is discussed that the maximum temperature reached by the Pd nanoparticles is higher and their cooling rate is lower. These effects are explained by the oxide support having a lower heat conductivity.

Light-Induced Transformation of Virus-Like Particles on TiO2.

Titanium dioxide (TiO2) shows significant potential as a self-cleaning material to inactivate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and prevent virus transmission. This study provides insights into the impact of UV-A light on the photocatalytic inactivation of adsorbed SARS-CoV-2 virus-like particles (VLPs) on a TiO2 surface at the molecular and atomic levels. X-ray photoelectron spectroscopy, combined with density functional theory calculations, reveals that spike proteins can adsorb on TiO2 predominantly via their amine and amide functional groups in their amino acids blocks. We employ atomic force microscopy and grazing-incidence small-angle X-ray scattering (GISAXS) to investigate the molecular-scale morphological changes during the inactivation of VLPs on TiO2 under light irradiation. Notably, in situ measurements reveal photoinduced morphological changes of VLPs, resulting in increased particle diameters. These results suggest that the denaturation of structural proteins induced by UV irradiation and oxidation of the virus structure through photocatalytic reactions can take place on the TiO2 surface. The in situ GISAXS measurements under an N2 atmosphere reveal that the virus morphology remains intact under UV light. This provides evidence that the presence of both oxygen and UV light is necessary to initiate photocatalytic reactions on the surface and subsequently inactivate the adsorbed viruses. The chemical insights into the virus inactivation process obtained in this study contribute significantly to the development of solid materials for the inactivation of enveloped viruses.

Atomic structure and composition of the yttria-stabilized zirconia (111) surface.

Anomalous and nonanomalous surface X-ray diffraction is used to investigate the atomic structure and composition of the yttria-stabilized zirconia (YSZ)(111) surface. By simulation it is shown that the method is sensitive to Y surface segregation, but that the data must contain high enough Fourier components in order to distinguish between different models describing Y/Zr disorder. Data were collected at room temperature after two different annealing procedures. First by applying oxidative conditions at 10- 5 mbar O2 and 700 K to the as-received samples, where we find that about 30% of the surface is covered by oxide islands, which are depleted in Y as compared with the bulk. After annealing in ultrahigh vacuum at 1270 K the island morphology of the surface remains unchanged but the islands and the first near surface layer get significantly enriched in Y. Furthermore, the observation of Zr and oxygen vacancies implies the formation of a porous surface region. Our findings have important implications for the use of YSZ as solid oxide fuel cell electrode material where yttrium atoms and zirconium vacancies can act as reactive centers, as well as for the use of YSZ as substrate material for thin film and nanoparticle growth where defects control the nucleation process.

Reversible Ultrathin PtOx Formation at the Buried Pt/YSZ(111) Interface Studied In Situ under Electrochemical Polarization.

Three different platinum oxides are observed by in situ X-ray diffraction during electrochemical potential cycles of platinum thin film model electrodes on yttria-stabilized zirconia (YSZ) at a temperature of 702 K in air. Scanning electron microscopy and atomic force microscopy performed before and after the in situ electrochemical X-ray experiments indicate that approximately 20% of the platinum electrode has locally delaminated from the substrate by forming pyramidlike blisters. The oxides and their locations are identified as (1) an ultrathin PtOx at the buried Pt/YSZ interface, which forms reversibly upon anodic polarization; (2) polycrystalline ß-PtO2, which forms irreversibly upon anodic polarization on the inside of the blisters; and (3) an ultrathin α-PtO2 at the Pt/air interface, which forms by thermal oxidation and which does not depend on the electrochemical polarization. Thermodynamic and kinetic aspects are discussed to explain the coexistence of multiple phases at the same electrochemical conditions.

Chemical and Structural In-Situ Characterization of Model Electrocatalysts by Combined Infrared Spectroscopy and Surface X-ray Diffraction.

New diagnostic approaches are needed to drive progress in the field of electrocatalysis and address the challenges of developing electrocatalytic materials with superior activity, selectivity, and stability. To this end, we developed a versatile experimental setup that combines two complementary in-situ techniques for the simultaneous chemical and structural analysis of planar electrodes under electrochemical conditions: high-energy surface X-ray diffraction (HE-SXRD) and infrared reflection absorption spectroscopy (IRRAS). We tested the potential of the experimental setup by performing a model study in which we investigated the oxidation of preadsorbed CO on a Pt(111) surface as well as the oxidation of the Pt(111) electrode itself. In a single experiment, we were able to identify the adsorbates, their potential dependent adsorption geometries, the effect of the adsorbates on the surface morphology, and the structural evolution of Pt(111) during surface electro-oxidation. In a broader perspective, the combined setup has a high application potential in the field of energy conversion and storage.