For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to the stabilization of the tetragonal or cubic phase and increased oxygen ion conductivity. Most previous surface studies of yttria-doped zirconia were plagued by impurities, however. We have studied doping of pure, 5-monolayer ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). STM and LEED show that the tetragonal phase is stabilized by unexpectedly low dopant concentrations, 0.5 mol % Y2O3, even when the films are essentially fully oxidized (as evidenced by XPS core level shifts). XPS also shows Y segregation to the surface with an estimated segregation enthalpy of -23 ± 4 kJ/mol.

Starting from subsurface Zr0-doped “inverse” Pd and bulk-intermetallic Pd0Zr0 model catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS), in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch reactor cell. Both intermetallic precursors develop a Pd0–ZrO2 phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO2 from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO2. This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO2 reference catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive carbidic/dissolved C-species inside Pd0 toward CO at the Pd/ZrO2 phase boundary, which serves the role of providing efficient CO2 activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO2 islands at the surface of a quasi-infinite Pd0 metal bulk. Strongly delayed Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much less active interfacial ZrO2–Pd0 state.

X-ray photoelectron spectroscopy (XPS) of five-monolayer-thick ZrO2 films reveals a core level binding energy difference of up to 1.8 eV between the tetragonal and monoclinic phase. This difference is explained by positively charged oxygen vacancies in the tetragonal films, which are slightly reduced. Due to the large band gap of zirconia (≈5-6 eV), these charges shift the electron levels, leading to higher binding energies of reduced tetragonal films w.r.t. fully oxidized monoclinic films. These core level shifts have the opposite direction than what is usually encountered for reduced transition metal oxides. The vacancies can be filled via oxygen spillover from a catalyst that enables O2 dissociation. This can be either a metal deposited on the film, or, if the film has holes, the metallic (in our case, Rh) substrate. Our study also confirms that tetragonal ZrO2 is stabilized via oxygen vacancies and shows that the XPS binding energy difference between O 1s and Zr 3d solely depends on the crystallographic phase.

We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2-10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2-4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Above this thickness, films show a (2 x 1) or a distorted (2 x 2) surface structure with respect to cubic ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 degrees C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 degrees C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2, as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO2/Pt(111) system.

ZrO2/metal inverse model catalysts exhibit the strong metal–support interaction (SMSI) effect. Upon annealing under reducing conditions, an oxygen-deficient, ultrathin ZrO≈1.5 film covers the metal. Nevertheless, Zr retains its 4+ charge state.

A comprehensive study of water adsorption and desorption on an ultrathin trilayer zirconia film by experimental and computational methods shows good agreement with data for H2O/ZrO2 powder material.

Changes in chemical and physical properties resulting from water adsorption play an important role in the characterization and performance of device-relevant materials. Studies of model oxides with well-characterized surfaces can provide detailed information that is vital for a general understanding of water-oxide interactions. In this work, we study single crystals of indium oxide, the prototypical transparent contact material that is heavily used in a wide range of applications and most prominently in optoelectronic technologies. Water adsorbs dissociatively already at temperatures as low as 100 K, as confirmed by scanning tunneling microscopy (STM), photoelectron spectroscopy, and density functional theory. This dissociation takes place on lattice sites of the defect-free surface. While the In2O3(111)-(1 × 1) surface offers four types of surface oxygen atoms (12 atoms per unit cell in total), water dissociation happens exclusively at one of them together with a neighboring pair of 5-fold coordinated In atoms. These O-In groups are symmetrically arranged around the 6-fold coordinated In atoms at the surface. At room temperature, the In2O3(111) surface thus saturates at three dissociated water molecules per unit cell, leading to a well-ordered hydroxylated surface with (1 × 1) symmetry, where the three water OWH groups plus the surface OSH groups are imaged together as one bright triangle in STM. Manipulations with the STM tip by means of voltage pulses preferentially remove the H atom of one surface OSH group per triangle. The change in contrast due to strong local band bending provides insights into the internal structure of these bright triangles. The experimental results are further confirmed by quantitative simulations of the STM image corrugation.

Catalytic investigations on chemical-vapor-deposition (CVD)-prepared Pd/Zr-0/ZrOxHy inverse model catalysts and Pd/Zr intermetallic compound system in dry reforming of methane (DRM) are presented. DRM, which produces syngas, is an economically favourable way to operate an SOFC by reusing the already heated CO2 exhaust. The catalytic investigations of the Pd/Zr system yield important information for the design of novel electrode materials or external reforming catalysts. From a catalytic perspective, the initially bimetallic Pd-Zr pre-catalyst shows a distinct activity for dry reforming of methane. This activity can be ascribed to synergistic bifunctional cooperation of palladium and zirconium. Moreover, the investigations clearly demonstrate that metallic Zr is crucial to observe any activity. Therefore, different bulk and surface sensitive methods are used to follow the evolution of structural and redox changes of Zr. Studies of single-crystalline Pd(111) show that Zr-0 is located exclusively in subsurface layers after annealing in vacuum and prior to reaction.

A sputter deposition source for the use in ultrahigh vacuum (UHV) is described, and some properties of the source are analyzed. The operating principle is based on the design developed by Mayr et al. [Rev. Sci. Instrum. 84, 094103 (2013)], where electrons emitted from a filament ionize argon gas and the Ar+ ions are accelerated to the target. In contrast to the original design, two grids are used to direct a large fraction of the Ar+ ions to the target, and the source has a housing cooled by liquid nitrogen to reduce contaminations. The source has been used for the deposition of zirconium, a material that is difficult to evaporate in standard UHV evaporators. At an Ar pressure of 9×10-6 mbar in the UHV chamber and moderate emission current, a highly reproducible deposition rate of ≈1 ML in 250 s was achieved at the substrate (at a distance of ≈50 mm from the target). Higher deposition rates are easily possible. X-ray photoelectron spectroscopy shows a high purity of the deposited films. Depending on the grid voltages, the substrate gets mildly sputtered by Ar+ ions; in addition, the substrate is also reached by electrons from the negatively biased sputter target.

Due to the importance of zirconia (ZrO2) for many applications, such as solid oxide fuel cells or oxygen sensors, it is of big interest to study the material and its metal interfaces at atomic scale. This is only possible by preparing thin oxide films on a conductive substrate, since ZrO2 is an electronic insulator. Therefore, 5–6 ML thick ZrO2 films were prepared on a Rh(111) single crystal by sputter deposition in ultra-high vacuum (UHV). The growth and phase transformation of the films are studied in the first part of this work. Investigations by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) reveal the formation of a (2×1) row structure (w.r.t. cubic ZrO2) and that the Zr 3d5/2 is located at a binding energy of 183.2 eV (after post-annealing in O2 atmosphere at 650 ◦C), which is both confirming ZrO2 in tetragonal phase. ZrO2 is stable in the tetragonal phase up to a temperature of 700 ◦C. Above this temperature it transforms into monoclinic ZrO2, which can be seen in STM and LEED due to the formation of a (2×2) structure (w.r.t. cubic ZrO2) whose domains are rotated with respect to each other. Moreover, XPS reveals peak shifts of Zr 3d and O 1s to lower binding energies, with respect to the peaks in tetragonal phase. The Zr 3d5/2 peak is then positioned at 181.5 eV. The second part of this work deals with the investigation of metal nucleation on both ZrO2 phases. Fe, Ag, Au, and Rh were deposited on the 5–6 ML thick ZrO2 films by evaporation. The growth mode, the cluster density, and the cluster sizes were examined by STM and the following trend could be determined: The metals showed different strengths of metal-oxide bonding: Ag≈Au<Fe≈Rh. Additional XPS measurements revealed surprising results. On monoclinic ZrO2, the oxide peaks shift to higher binding energies according to the strength of metal-oxide bonding. The results do not match for tetragonal ZrO2, however. The oxide peaks are either not shifting at all or slightly shifting to lower binding energies. As a general trend, it was found, that the XPS peaks of tetragonal and monoclinic ZrO2 are closer after depositing metals on the oxid. This is probably induced by band bending of the oxide due to metal clusters.

The structure of the Fe3O4(110)-(1x3) surface was studied with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflection high energy electron diffraction (RHEED). The so-called one-dimensional reconstruction is characterised by bright rows that extend hundreds of nanometers in the [1-10] direction and have a periodicity of 2.52 nm in [001] in STM. It is concluded that this reconstruction is the result of a periodic faceting to expose 111-type planes with a lower surface energy.

Metal deposition on oxide surfaces usually results in adatoms, clusters, or islands of the deposited material, where defects in the surface often act as nucleation centers. Here an alternate configuration is reported. After the vapor deposition of Fe on the In_{2}O_{3}(111) surface at room temperature, ordered adatoms are observed with scanning tunneling microscopy. These are identical to the In adatoms that form when the sample is reduced by heating in ultrahigh vacuum. Density functional theory calculations confirm that Fe interchanges with In in the topmost layer, pushing the excess In atoms to the surface where they arrange as a well-ordered adatom array.

Two samples of amorphous nanoparticles FeNiB, one of them with SiO2 sheath around the core and one without, were investigated by transmission electron microscopy and magnetic measurements. The coating gives mean particle diameters of 4.3 nm compared to 7.2 nm for the uncoated particles. Magnetic measurements prove superparamagnetic behaviour above 160 K (350 K) for the coated (uncoated) sample. With use of effective anisotropy constant K-eff - determined from hysteresis loops - size distributions are determined both from ZFC curves, as well as from relaxation measurements. Both are in good agreement and are very similar for both samples. Comparison with the size distribution determined from TEM pictures shows that magnetic clusters consist of only few physical particles.