G. Parteder

Structural and vibrational properties of two-dimensional MnxOy layers on Pd(100) : Experiments and density functional theory calculations

Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd(100) at (sub)monolayer coverage. Amongst the multitude of phases experimentally observed we focus our attention on four structures which can be classified into two distinct regimes, characterized by different building blocks. Two oxygen-rich phases are described in terms of MnO(111)-like O-MnO trilayers, whereas the other two have a lower oxygen content and are based on a MnO(100)-like monolayer structure. The excellent agreement between calculated and experimental scanning tunneling microscopy images and vibrational electron energy loss spectra allows for a detailed atomic description of the explored models.

The two-dimensional cobalt oxide (9 × 2) phase on Pd(100).

The two-dimensional (2D) Co oxide monolayer phase with (9 × 2) structure on Pd(100) has been investigated experimentally by scanning tunneling microscopy (STM) and theoretically by density functional theory (DFT). The high-resolution STM images reveal a complex pattern which on the basis of DFT calculations is interpreted in terms of a coincidence lattice, consisting of a CoO(111)-type bilayer with significant symmetry relaxation and height modulations to reduce the polarity in the overlayer. The most stable structure displays an unusual zig-zag type of antiferromagnetic ordering. The (9 × 2) Co oxide monolayer is energetically almost degenerate with the c(4 × 2) monolayer phase, which is derived from a single CoO(100)-type layer with a Co(3)O(4) vacancy structure. Under specific preparation conditions, the (9 × 2) and c(4 × 2) structures can be observed in coexistence on the Pd(100) surface and the two phases are separated by a smooth interfacial boundary line, which has been analyzed at the atomic level by STM and DFT. The here described 2D Co oxide nanolayer systems are characterized by a delicate interplay of chemical, electronic, and interfacial strain interactions and the associated complexities in the theoretical description are emphasized and discussed.

One-Dimensional Oxide-Metal Hybrid Structures: Site-Specific Enhanced Reactivity for CO Oxidation

Oxide nanostructures in low dimensions on well-defined metal surfaces form novel hybrid systems with tremendous potential and impact in fundamental research and for emerging nanotechnologies. The coupling of a quasi-one-dimensional oxidetype line structure to the atoms of a metal substrate, such as achieved by the decoration of the steps of a vicinal metal surface with oxidic stripes, creates a hybrid system with novel geometric and electronic structures and with well-defined phase boundary effects. Such systems contain regular arrays of low-coordinated atom sites, which are recognised as the most active reaction centers in supported metal-oxide nanocatalysts. The quasi-one-dimensional (1D) oxide–metal hybrid structures employed herein consist of nickel oxide nanowires attached to the step atoms of a vicinal Rh(553) surface. The decoration of the Rh steps with monoatomic rows of Ni adatoms followed by selective oxidation generates pseudomorphically strained 1D stripes of NiO2 stoichiometry, [2,3] which are geometrically and electronically coupled to the Rh step atoms. Herein we report that these NiO2/Rh nanowires exhibit a high reactivity for a prototypical model reaction of both fundamental and practical interest, namely the oxidation of CO, as compared to the bare stepped Rh(553) surface. The elementary Langmuir–Hinshelwood (LH)-type reaction step COad+Oad! CO2› (the latter desorbs from the surface immediately under reaction conditions) has been probed by adsorbing CO onto the O-covered surfaces at 90 K followed by stepwise annealing to elevated temperatures to initiate the oxidation reaction. The reactant species at the surface have been followed experimentally by high-resolution core-level X-ray photoelectron spectroscopy (HR-XPS) with the use of synchrotron radiation, and theoretically by ab initio density functional theory (DFT) calculations. The XPS spectra allow us to follow the changes in the oxygen content of the surface during the reaction, and to identify the adsorption site of the reactants. Figure 1 shows the relevant structures and the spectroscopic core-level fingerprints in form of O1s HR–XPS spectra of the two surfaces (Figure 1a): the oxygen-precovered Rh(553) and the NiO2 decorated Rh(553) surface. Figures 1b,c display the corresponding schematic geometry models. The oxygen exposure of the bare Rh(553) surface under the employed conditions yields a (p2mg+2 1)-O structure of chemisorbed oxygen, which is characterised by a zig-zag arrangement of Oad at the Rh step edges and a (2 1)-O structure at the terraces (Figure 1b). The chemisorbed oxygen gives rise to a single component in the O1s spectrum (Figure 1a, upper curve), because all O atoms are coordinated to three Rh surface atoms and step and terrace sites cannot be separated. The O1s spectrum of the NiO2decorated Rh(553) surface (Figure 1a, lower curve) is significantly different from the one of the bare O Rh(553) surface in that it displays a major spectral structure at a binding energy (BE) of 528.9 eV (labelled NiO2) and a minor component at 529.4 eV (Oad), which are assigned, on the basis of DFT calculations, to oxygen atoms bound to the Ni atoms at the step edges and to chemisorbed oxygen atoms at the terraces. Each Ni step atom is coordinated to four oxygen atoms and the stoichiometry of the nickel oxide nanostripes is thus formally NiO2; [3] the DFT model in Figure 1c also contains a single line of chemisorbed oxygen atoms in hollow sites. We note that the Oad emission intensity is weaker than expected on the basis of the model shown in Figure 1c, which is ascribed to photoelectron diffraction effects. It is important to mention at Figure 1. a) O1s XPS core-level spectra of the O-covered (p2mg+2 1)-O(upper curve) and NiO2-decorated Rh(553) surfaces (lower curve). b) and c) Models of the (p2mg+2 1)-Oand NiO2-decorated Rh(553) surfaces, respectively (light grey: Rh; red: oxygen; dark yellow: Ni). d) Reduced NiO zigzag structure.

The (100)→(111) Transition in Epitaxial Manganese Oxide Nanolayers

The growth and structure of epitaxial MnO(100) and MnO(111) nanolayers on Pd(100) surfaces have been investigated. We found that despite the large lattice mismatch to the Pd(100) substrate MnO(100) layers can be kinetically stabilised at low temperatures (≤350°C) and at oxygen pressures between 2x10−7 mbar and 5x10−7 mbar. Annealing in ultra-high vacuum to 650°C or, alternatively, deposition of manganese metal in oxygen pressure < 1x10−7 mbar causes the transformation of the MnO(100) to a polar MnO(111) surface. It is suggested that the growth of MnO(111) layers is energetically preferred over MnO(100) due to the epitaxial stabilisation at the metal-oxide interface.