Free-electron lasers (FELs) enable the study of the ultrafast dynamics of photocatalytic reactions by time-resolved X-ray photoelectron spectroscopy (tr-XPS) with femtosecond time resolution. In an optical pump - soft X-ray probe photoemission experiment conducted at the free-electron laser in Hamburg (FLASH), we observed the ultrafast oxidation of CO to CO2 on rutile TiO2(110) by monitoring the O 1s core level region. Within 800± 250 fs after laser excitation, CO2 as a product of the photooxidation of CO is detected. Based on density functional theory calculations, we propose that the oxygen activation pathway for the CO oxidation is initiated via an O2-TiO2 charge transfer complex directly excited by the 770 nm pump laser. Our results give insight into the fundemental understanding of photocatalytic processes of TiO2 polymorphs relevant for the design of more efficient photoctalaysts.

The electrocatalytic production of hydrogen is pivotal for the sustainable generation of hydrogen fuel and hydrogen as reducing agent. Metal cations in alkaline electrolytes can facilitate or significantly impede this reaction, depending on electrolyte pH, catalyst surface, cation identity, and concentration. Still, the underlying mechanisms governing these effects remain elusive, in part due to the lack of direct characterization of how cations interact with the electrocatalyst surface and interfacial water. Here, using Surface X-ray Diffraction combined with ab initio Molecular Dynamics, we elucidate the formation of a cation layer at the electrochemical interface during hydrogen evolution on a hexagonally reconstructed Au(100) model catalyst under varying pH, potential, cation concentration, and electrolyte composition. Specular Crystal Truncation Rod measurements show that as the potential becomes more negative, Cs+ cations increase in surface coverage and move progressively closer to the interface, revealing that the positions of the inner- and outer-Helmholtz planes continuously shift with potential. Slightly smaller cation-to-surface distances and higher coverages are observed in alkaline media compared to acidic environments. Ab initio Molecular Dynamics simulations reveal that the Cs+ ions move into the first water layer as they approach the surface at more negative potentials. No water molecules can then reside between the ions and the Au surface, leading to a broken solvation symmetry and an opening angle. This opening is initially established by an orientation and slight distortion of the solvation shell, while at shorter ion-surface distances the ion sheds ∼10% of its solvation, enabling a closer approach to the surface. Finally, in Li+/Cs+ electrolyte mixtures at pH 3, we observe the preferential accumulation of Cs+ at the electrochemical interface, albeit in a more disordered fashion compared to the Li+-free situation. This is expected to enhance interfacial mobility and influence electrocatalytic activity beyond traditional "structure making/breaking" descriptions.

In situ nuclear forward scattering shows a thermally induced cation exchange between a ^{57}Fe_{3}O_{4} thin-film and a Fe_{3}O_{4} (001) substrate predominantly in the octahedral sublattice for a temperature range between 470 and 710 K. The overall activation barrier in this temperature range is found to be 19±32  kJ/mol, which is significantly lower than expected from extrapolating a bulk diffusion model. This observation can be attributed to the large out-of-equilibrium cation deficit as determined by surface x-ray diffraction. Despite the relatively low hopping barrier, the diffusion constant is about 5 orders of magnitude lower than expected for magnetite having an equilibrium cation stoichiometry. The results are relevant for applications relying on the near-surface structure and stoichiometry of magnetite, and we argue that the correlation between cation diffusion and stoichiometry may play a role for a wider range of oxide materials.

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.

A novel contact–separation triboelectric generator concept is proposed in this paper, which consists of a limestone-based mounting putty and a metallized polyester (PET/Al) sheet. This is an attempt to explore tacky materials for power generation and extend the operational frequency bandwidth compared to existing TriboElectric NanoGenerators (TENGs). Moreover, the proposed design is very cost-effective and easy to build. Unlike traditional TENGs, which generate power solely due to a charge developing on the surface, the putty also replies on charge developed inside the material. Parametric study was conducted to determine the optimal putty thickness in a shaker test at 40 Hz. It was found that a putty layer at 0.6 mm thick yielded maximum power generation. During the separation phase, the electrical breakdown between triboelectric layers allows most existing electrons to flow back from the ground due to rapid charge removal at the interface. We are able to achieve a peak power of 16 mW in a shaker test at 40 Hz with an electrical load of 8 MΩ, which corresponds to a power density of 25.6 W/m2. A peak power of 120 mW in a manual prototype generator is achieved, which operates at approximately 2 Hz. Since putty material has less tackiness than double-sided tape, we are able to expand the frequency bandwidth up to 80 Hz, which is significantly higher than a TENG (typically <10 Hz). The mounting putty material contains limestone with approximate 31 nm of mean grain size mixed with synthetic rubber materials. Elasticity from rubber and the nanohardness of calcite crystallites allow us to operate a putty generator repeatedly without the concern of grain fracture. Also, a durability test was conducted with up to 250,000 contact–separation cycles. In summary, comparable performance is achieved in the proposed putty generator to benefit energy harvesting and sensor applications.

We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain an atomistic picture of the catalyst's structure under in situ oxidizing and reducing conditions. The Cu/ZnO model catalysts were investigated at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. As the nanoparticles swell during oxidation, they maintain their epitaxy on basal ZnO (000 +/- 1) surfaces, whereas on the vicinal ZnO (10 (1) over bar4) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H-2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas the orientation of Cu NPs on vicinal ZnO was only partially restored. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No Cu-Zn bulk alloy formation is observed. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates.

A novel contact-separation triboelectric generator concept is proposed in this study, which is composed of a double-sided tape with acrylic adhesive material and a metalized polyester (PET/Al) film (an aluminum layer coating on one side). The proposed concept is very cost-effective and easy to fabricate compared to existing triboelectric nanogenerators (TENGs), which require special equipment and sophisticated procedure to build. The strong bonding nature of acrylic adhesive on the tape induces a significant charge when contacting. The peak power generation depends on the induced pressure at the impact. During the separation phase, the air breakdown between triboelectric layers allows most existing electrons to flow back from the ground due to rapid charge removal at the interface. A higher voltage can be generated when the PET is interfaced with the double-sided tape compared to the Al-acrylic configuration because of the effect of triboelectric series and a Schottky barrier formation for electrons at the tape-Al interface during contact. A double-electrode configuration with an assembly of Al/PET-tape-PET/Al significantly improved the performance, in which a 21.2 mW peak power is achieved compared to 7.6 mW in the single-electrode design with tape-PET/Al assembly when excited at 20 Hz in a shaker test. This double-electrode triboelectric generator can power 476 LEDs with an active area of 38 mm × 25 mm. Moreover, a direct power of a 650 nm laser diode was demonstrated. In summary, the proposed triboelectric generator concept using tacky materials shows the potential for higher-energy harvesting via triboelectrification and advances the state of the art by offering low cost and easy fabrication options. It is expected that such newly proposed triboelectric generators are able to meet power requirements in many engineering applications.

A proof of concept is shown for the design of a high pressure heterogeneous catalysis reaction cell suitable for surface sensitive x-ray diffraction and x-ray reflectometry over planar samples using high energy synchrotron radiation in combination with mass spectrometry. This design enables measurements in a pressure range from several tens to hundreds of bars for surface investigations under realistic industrial conditions in heterogeneous catalysis or gaseous corrosion studies.

We investigated Cu/ZnO model catalysts for methanol synthesis to obtain an atomistic picture of activation and deactivation processes under in situ oxidizing and reducing conditions. We have investigated Cu nanoparticles with different shapes and aspect ratios grown epitaxially on basal and vicinal ZnO surfaces at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. During oxidation, they maintain their epitaxy on basal ZnO (000-1) surfaces, whereas on the vicinal ZnO (10-14) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas a random orientation is preferred for the (10-14) surface. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No proof for Cu-Zn alloy formation is found. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates, which is detrimental to the catalyst’s performance.

The microscopic understanding of the atomic structure and interaction at carboxylic acid/oxide interfaces is an important step towards tailoring the mechanical properties of nanocomposite materials assembled from metal oxide nanoparticles functionalized by organic molecules. We have studied the adsorption of oleic acid (C17H33COOH) on the most prominent magnetite (001) and (111) crystal facets at room temperature using low energy electron diffraction, surface X-ray diffraction and infrared vibrational spectroscopy complemented with molecular dynamics simulations used to infer specific hydrogen bonding motifs between oleic acid and oleate. Our experimental and theoretical results give evidence that oleic acid adsorbs dissociatively on both facets at lower coverages. At higher coverages, the more pronounced molecular adsorption causes hydrogen bond formation between the carboxylic groups, leading to a more upright orientation of the molecules on the (111) facet in conjunction with the formation of a denser layer, as compared to the (001) facet. This is evidenced by the C=O double bond infrared line shape, in depth molecular dynamics bond angle orientation and hydrogen bond analysis, as well as X-ray reflectivity layer electron density profile determination. Such a higher density can explain the higher mechanical strength of nanocomposite materials based on magnetite nanoparticles with larger (111) facets.

The role of metal-support epitaxy on shape and size heterogeneity of nanoparticles and their response to gas atmospheres is not very well explored. Here we show that an ensemble of Pd nanoparticles, grown on MgO(001) by deposition under ultrahigh vacuum, mostly consists of two distinctly epitaxially oriented particles, each having a different structural response to redox cycles. X-ray reciprocal space patterns were acquired in situ under oxidizing and reducing environments. Each type of nanoparticle has a truncated octahedral shape, whereby the majority grows with a cube-on-cube epitaxy on the substrate. Less frequently occurring and larger particles have their principal crystal axes rotated ±3.7° with respect to the substrate's. Upon oxidation, the top (001) facets of both types of particles shrink. The relative change of the rotated particles' top facets is much more pronounced. This finding indicates that a larger mass transfer is involved for the rotated particles and that a larger portion of high-index facets forms. On the main facets of the cube-on-cube particles, the oxidation process results in a considerable strain, as concluded from the evolution to largely asymmetric facet scattering signals. The shape and strain responses are reversible upon reduction, either by annealing to 973 K in vacuum or by reducing with hydrogen. The presented results are important for unraveling different elements of heterogeneity and their effect on the performance of real polycrystalline catalysts. It is shown that a correlation can exist between the particle-support epitaxy and redox-cycling-induced shape changes.

A new concept for temporal gating of synchrotron X-ray pulses based on laser-induced thermal transient gratings is presented. First experimental tests of the concept yield a diffraction efficiency of 0.18%; however, the calculations indicate a theoretical efficiency and contrast of >30% and 10(-5), respectively. The full efficiency of the pulse picker has not been reached yet due to a longrange thermal deformation of the sample after absorption of the excitation laser. This method can be implemented in a broad spectral range (100 eV to 20 keV) and is only minimally invasive to an existing setup.

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.

We investigated the orientation and morphology of Cu nanoparticles grown under ultrahigh-vacuum conditions on ZnO(0001), ZnO(000 (1) over bar), and ZnO(10 (1) over bar4) single crystal surfaces by scanning tunneling microscopy, high-energy grazing incidence X-ray diffraction, low-energy electron diffraction, and scanning electron microscopy. The (111) oriented Cu NPs on basal ZnO showed only small area fractions of high indexed Cu(225) and Cu(331) facets. Cu NPs grown on ZnO(10 (1) over bar4) show alignment of Cu [111] with the ZnO [0001] direction, which is at an angle of 24.8 degrees to the ZnO(10 (1) over bar4) surface normal. Because of this tilt, the NPs exhibit a shape with a larger fraction of high indexed facets such as (335), (221), (113), and (55 (1) over bar). In addition, the direct interaction of subsequent Cu(111) planes to the underlying substrate results in unequal amounts of ABCA and ACBA stacked NPs. Small NPs are found to interact strongly with the vicinal surface, giving rise to a surface corrugation with a multiple of the surface step distance. The high density of low-coordinated Cu surface atoms potentially increases the overall catalytic activity for methanol synthesis and CO2 hydrogenation reactions.

With the goal to delevop the fabrication of a new type of Pt-nanoparticle carbon-support electrocatalyst, we investigate the carbon embedding of Pt cluster superlattices grown on the moire of a monolayer of hexagonal boron nitride (h-BN) on Ir(111). Our combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) study establishes conformal C embedding of the Pt clusters on h-BN/Ir(111) without deterioration of superlattice order, preferential and strong binding of the embedding carbon to the Pt clusters, and upon annealing the formation of a homogeneous amorphous carbon (a-C) matrix. There are indications that while the a-C matrix and the Pt clusters bind strongly to each other, upon annealing both weaken their binding to h-BN.

Femtosecond X-ray laser pulses synchronized with an optical laser were employed to investigate the reaction dynamics of the photooxidation of CO on the anatase TiO2(101) surface in real time. Our t...

Inelastic X-ray scattering (IXS) on an In0.17Ga0.83As/GaAs0.8P0.2 multiple quantum well (MQW) superlattice has been conducted to investigate the potential for phonon bottlenecks in low dimensional materials. This work shows that the measured spectra are in good agreement with an adiabatic bond charge model prediction and back-folded phonon modes make large contributions to the broadening of peaks observed in the spectra. The high-lying mode at 45 meV in the MQW is attributed to vibrations of Ga and P and confirmed by both experiment and theory. The acoustic phonons have a dominant contribution from the Ga and As components, and the contribution from InAs is small and only visible at around 29.7 meV. Low energy optical modes resulting from back-folding might be a key to increased electron-phonon coupling in the material system. The suitability of utilizing IXS as a means to investigate phonon modes in low dimensional materials is also discussed.