Project Parts

Researcher Title
Project Part 01: Coordination Project Günther Rupprechter (including USTEM Contribution),
Johannes Bernardi
The Coordination Project is concerned with administrative and organisational issues of the SFB (handled mostly by the Speaker/Board and the SFB Secretary), the organization and funding of SFB Symposia and Seminars, exchange of guest scientists and PhD students, and administrating a repair fund. The Coordination Project is also in charge of the collaboration with the USTEM (Johannes Bernardi, M. Stöger-Pollach, P. Schattschneider). The role of USTEM is two-fold. First, it provides nano-analytical support for all other SFB project parts. It provides chemical quantification and structural characterisation of catalytic materials on the nanometer scale by using TEM, high-resolution SEM, EELS, EDX and EFTEM within a temperature range from liquid nitrogen (77 K) up to 1280 K. Second, research within the SFB focuses on inelastic electron-matter interactions, in order to determine the electronic structure of reactive interfaces of zirconia, Yttrium stabilized zirconia (YSZ) and ceria, as well as metal-oxide interfaces and to determine optical properties of thin layered structures with high spatial resolution. The main technique is EELS, both in the low-loss regime (VEELS) for determining the dielectric properties, and in the core loss regime for determining the electronic structure via the energy loss near edge structure (ELNES). The dielectric behaviour is investigated by operating the TEM and the EELS spectrometer at very low beam energies down to 20 kV. A challenging topic is determination of interface states at metal-insulator interfaces (metal induced gap states – MIGS). The electronic structure within layered systems as well as at their internal and external interfaces are investigated by operating the TEM in scanning mode (STEM) and performing EELS experiments simultaneously with a resolution of

Project Part 02: In situ spectroscopy of chemical reactions on pure and doped ZrO2 thin films and zirconia-based metal-oxide systems, Günther Rupprechter, Institute of Materials Chemistry, Vienna University of Technology This Project Part examines the structure, adsorption and reaction properties of zirconia-based functional oxides using a two-tiered approach: i) UHV grown Surface Science model systems and ii) industrial-grade powder materials. Along these lines UHV-grown ultrathin ZrO2 layers have been examined by LEED and XPS, and the adsorption of probe molecules (CO, CO2, H2O) was examined by TPD, XPS, and PM-IRAS. In addition, Ni and Pd nanoparticles were grown and examined on the ZrO2 support. Industrial-grade ZrO2, ZrO2-CeO2, with and without metal (Ni, Cu, CuNi) nanoparticles were studied by HRTEM, (in situ) XRD, FTIR, TPD and flow reactor kinetics. Finally, thin perovskite-type PLD films were examined by XPS and TPD. The studies were performed jointly with Project Parts P1, P3, P4, P5, P9, P10, P11.
Key questions include the effect of surface structure/defects, oxide hydroxylation, and the promotional effect of oxide-metal interfaces. Comparison of spectroscopic and kinetics results under UHV and under ambient pressure allow to reveal “pressure gap” effects and comparison of model systems with technical materials elucidates the “complexity gap”.


Project 03: Catalytic synergisms at (bi)metallic and oxidic phase boundaries and interfaces, Bernhard Klötzer (S. Penner), Institute of Physical Chemistry, University Innsbruck Project Part 03 studies the reactivity of oxide-(bi)metal and oxide-oxide interfaces relevant for SOFC anode internal hydrocarbon reforming and the subsequent direct electro-oxidation processes. The active reforming/electrocatalyst metals Ni (Co, Pd) were modified by Cu (Ag, Au) in order to obtain bimetallic catalysts with good catalytic reforming activity and sintering stability while sufficiently blocking C diffusion and C whisker growth. By comparing purely (bi)metallic surfaces and those modified by submonolayers of ZrO2, YSZ and CeO2 ("inverse" model catalysts) with the respective epitaxially grown (bi)metallic nanoparticles embedded in ZrO2, YSZ and CeO2 supporting (mixed) oxide films ("real" model catalysts), we identified promotional electronic, ensemble, particle size and defect chemistry related effects, with a particular focus on catalytic synergisms located at the (bi)metal-oxide or oxide-oxide phase boundary. Molecular Beam (MB) adsorption- and reaction studies under UHV conditions as well as ambient-pressure high temperature catalytic reforming studies on the "inverse" model catalysts in UHV-compatible reaction cells are complemented by pre/post reaction characterization using a combination of ex-situ (near-)surface sensitive analytical techniques (XPS, LEIS, AES, TPD) with a special focus on the (near-)surface composition of the bimetallic model systems. For this purpose, an even larger "pool" of methods is available through close collaboration with Project Parts 02, 04 and 05 (PM-IRAS, PEEM, STM etc.). In order to understand redox- and segregation effects as well as thermodynamic phase stabilities under realistic “working” conditions, ambient-pressure operando analysis of the surface and surface-near regions by PM-IRAS and XPS/EXAFS are performed in close collaboration with Project Part 02 and the AC department at the FHI Berlin (Prof. R. Schlögl, Dr. Axel Knop-Gericke).


Project 04: In situ mesoscopic and microscopic visualisation of catalytic reactions on metal-oxide systems, Yuri Suchorski, Institute of Materials Chemistry, Vienna University of Technology

The group studies catalytic reactions on nanostructured and nanosized materials with a main focus on discontinuous oxide films grown on metal (single crystals and polycrystalline Ni, Pt) substrates (so-called “inverse” model catalysts) and nanosized tips (model for individual metal nanoparticles). Photoemission electron microscopy (PEEM), combined with a novel version of metastable He* atom impact electron spectroscopy (MAIES) with lateral resolution, allows in situ studies of catalytic reactions on a mesoscopic scale, and MAIES provides ultimate surface sensitivity (information strictly from the topmost surface layer). The field ion and field electron microscopes (FEM/FIM) allow such studies on a microscopic (down to sub-nm) scale. In order to reveal the role of the metal-oxide interface in the catalytic performance, spectroscopic measurements (XPS, IRAS) and microscopic structure studies (STM) of catalytic processes on UHV-grown inverse planar model catalysts are performed jointly with Project Parts P02 and P03, and with P05, respectively. While the PEEM technique delivers information about the spatio-temporal behaviour of a reaction on a planar metal-oxide system, FIM/FEM microscopy with nearly atomic resolution permits mimicking the catalytic behaviour of individual metal nanoparticles. Thus the “averaging problem” of most surface-sensitive methods is avoided. In these methods, data are collected simultaneously from a large number of catalytically active nanoparticles, thus “smoothing out” the characteristics of the individual nanoparticles (that may vary in size, shape, roughness, etc). Such “averaging” makes the detection of e.g. fluctuation-induced effects difficult. Such reaction-induced fluctuations, confined to a single particle (of a few nm in size), can lead to severe deviations from “mean-field” predictions causing, e.g., noise-induced kinetic transitions in the nanosized reaction systems. Within this project part, discontinuous ZrO2 films on Pt and Ni surfaces have been prepared and characterized together with P05 (STM), P02 (XPS, IRAS) and P03 (LEIS). Then, in situ PEEM and FEM/FIM are used to visualize in situ the catalytic CO oxidation reaction on the planar and “single particle” model systems. The discontinuous ZrO2 overlayers are then be modified by, e.g. CeO2, i.e. mixed oxide systems are prepared. This allows studying the role of the particular components of the mixed oxide overlayers for the CO oxidation reaction.


Project part 05: Zirconia films and zirconia-metal systems on the atomic scale, Michael Schmid, Institute of Applied Physics, Vienna University of Technology The aim of this project part is studying the properties of zirconia surfaces on the atomic scale, with STM as the main technique. As bulk ZrO2 and YSZ are electronic insulators, we have started by developing a suitable model system, ultrathin ZrO2 films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001), and we have thoroughly characterized these films, in close collaboration with the DFT group (project part P11). The preparation recipes and knowledge about the film properties were also supplied to the other project parts where ZrO2 films are studied by surface-science techniques (P02, P03, P04, P06). The ultrathin ZrO2 films are trilayer films, based on the trilayers in (111)-oriented cubic ZrO2, which is the lowest-energy facet, and therefore in many respects representative for surfaces of the bulk material. Taking advantage of different geometric distortions (buckling) of ZrO2/Pt3Zr(0001) and ZrO2/Pd3Zr(0001), we can determine the influence of these distortions on the properties of the oxide. Metal clusters on the ZrO2 films show higher density of nuclei and better wetting than on other oxygen-terminated oxides. This common motif, i.e., a stronger bonding of many species (including molecular H2O) to ZrO2 than to comparable oxide surfaces, is caused by the easy accessibility of the Zr cations in the ZrO2(111) structure.


Project Part 06: Atomic structure of functional oxide interfaces under operational conditions, Andreas Stierle, DESY Hamburg, Germany (formerly University Siegen, Germany) In this project part the structure of functional oxide surfaces and interfaces is investigated using advanced x-ray scattering techniques, allowing an in-situ characterization at the atomic scale. The primary focus lies in application-relevant conditions such as gas pressures in the bar regime and elevated temperatures, as well as externally applied voltages. The atomistic structure of oxide single-crystal surfaces and epitaxial oxide films is investigated as a function of the oxygen partial pressure at variable temperatures with focus on ZrO2 (Y stabilized), CeO2 and perovskites, which are key materials in solid oxide fuel cells (SOFC’s), in gas sensors or as supports in catalytic reactions. A novel setup for in-situ solid-state electrochemistry has been designed and commissioned. Proof-of-principle surface x-ray diffraction experiments have been performed on the interfacial structure of LSC/YSZ model electrodes under oxygen transport conditions at elevated temperatures in collaboration with project parts P09 and compared to results from impedance spectroscopy. For comparison, the structure of ultrathin ZrO2 films on Zr containing alloys has been investigated in collaboration with project part P05, who performed STM measurements on the same systems. The carbon chemistry on Ni surfaces has been investigated by surface sensitive x-ray diffraction in collaboration with project part P03.


Project Part 07: Growth and fundamental surface properties of perovskites, Ulrike Diebold, Institute of Applied Physics, Vienna University of Technology This project part strives to provide a detailed, fundamental understanding of the surfaces of selected perovskite oxides (i.e., ternary oxides with the principal formula ABO3). The intriguing richness of their physical and chemical properties makes these materials promising for many applications, in particular (in the context of this SFB) in SOFCs and catalysis. Yet the variability in stoichiometries and the complexity of (defect) structures has so far prevented the atomic-scale understanding that has been achieved on several, much simpler binary oxide systems. We strive to determine (and control) the geometric, electronic, and defect structure of selected model systems, and understand how these properties affect surface chemistry at the atomic level. We employ STM in combination with area-averaging spectroscopy and DFT modeling. As perovskites are often terminated with either a B (transition metal) or an A (alkaline earth metal) oxide layer, we have focused on two representative systems with such terminations: (i) SrTiO3 surfaces with a TiO2 termination in various reconstructions that can be prepared reproducibly and reversibly by varying the chemical potentials of Sr, Ti, and O, and (ii) cleaved single crystals of the Ruddlesden-popper series Srn+1RunO3n+1 that terminate with an essentially perfect SrO layer. In addition, we develop capabilities for the epitaxial growth and in-situ characterization of complex SrxLa1-xBO3 systems (B=Ti, Fe, Co) using Laser-MBE, non-contact AFM with atomic resolution, and cryogenic TPD of metal oxide single crystals.


Project Part 09: Active and resistive zones of electrochemical oxidation and reduction reactions on zirconia electrolytes, Jürgen Fleig, Institute for Chemical Technologies and Analytics, Vienna University of Technology The main goal of this project part is the clarification of electrochemical reaction paths and mechanisms of electrochemical reactions at electrodes on solid oxide ion conductors. Oxygen tracer experiments are employed with tracer diffusion in mixed conducting thin film electrodes and/or in the electrolyte. Impedance measurements give complementary information on the electrode kinetics. Both techniques are not only used in the standard way; rather, highly sophisticated measurement approaches and novel methods are developed. Many different electrode materials (LSC, LSF, STF, LSM, CGO, Pt, etc.) are investigated under various conditions (oxidizing and reducing atmosphere, low and high temperatures) and with or without applied voltage in order to reveal, among others, the role of stress, the importance of grain boundaries, the relevance of electron conduction, reaction orders, surface chemistry-property relations, etc. Numerous joint activities within the SFB such as joint beam times yield further complementary information on the electrochemical properties of fuel cell related functional oxide surfaces.


Project Part 10: Preparation and properties of metal oxides prepared from organically modified precursors by sol-gel processing, Ulrich Schubert, Institute of Materials Chemistry, Vienna University of Technology Metal oxides prepared by sol-gel processing from molecular precursors, such as hydrolyzable metal salts or metal alkoxides, are initially obtained in amorphous or nanocrystalline form, but can be converted to crystalline materials by high-temperature treatment. The employed metal alkoxides are often derivatized by organic ligands for various chemical or technological reasons.
In previous work of the group, a general method for the creation of monodisperse metal particles in silica has been developed by thermal decomposition of metal complexes tethered to the sol-gel network. This work was extended to metal nanoparticles in metal oxides relevant to other sub-projects of the SFB, especially Ni nanoparticles in a zirconia or ceria matrix.
The group has previously investigated the chemistry of metal alkoxide precursors substituted by a variety of bidentate ligands. The question addressed in the SFB is how the organic co-ligands of the precursors influence the morphology of the obtained oxides. The studies were exemplarily performed on zirconia in cooperation with other groups of the SFB.


Project part 11: Theory of oxide surfaces and metal/oxide interfaces, Josef Redinger, Institute of Applied Physics, Vienna, University of Technology The main goal of this project is to provide a detailed and fundamental theoretical insight into the functional oxides investigated in the context of the present SFB. The investigated oxide surfaces and interfaces pose a considerable challenge to theory, as standard DFT approaches, the present workhorse for computational materials science, usually lead to significant errors in the description of the electronic properties (e.g. band gaps) of correlated (insulating) materials and must be augmented or substituted by approaches like DFT+U or GW, or by employing hybrid functionals. In addition, the lack of non-local vdW-like contributions has to be compensated by finding appropriate approaches beyond DFT for weakly interacting systems such as physisorbed adsorbates or particular metal/oxide interfaces. This line of research focuses on an evaluation of structural and electronic properties, including an assessment of the thermodynamic stability and an evaluation of the chemical activity for the adsorption of small molecules in the case of (i) metal/oxide interface systems, like ZrO/Pt3Zr, ZrO/Pd3Zr, where an inclusion of vdW contributions is crucial to capture the correct properties, or CoO/Ir, where a close relationship between the magnetic properties and the structural deformations was revealed and (ii) surfaces of ABO3 perovskites like B-type terminated SrTiO3, which is extremely difficult to treat due to the complex reconstructions at the surface, or of the A-type terminated Ruddlesden-Popper series Sr3Ru2O7, as well as the bulk mixed ferrate LaxSr1-xFeO3-y. Most studies were performed jointly with Project parts P02, P05, P07, and P09.