Research description

The group focuses on visualization of surface reactions in real time on a mesoscopic scale using photoemission electron microscopy (PEEM) and on a nanoscale using field emission (FEM) and field ion (FIM) microscopy. Catalytic reactions on nanostructured and nanosized materials are studied with a main focus on discontinuous oxide films grown on single crystal metal (Pt) substrates (so called "inverse" model catalysts) and on nanosized platinum metal (Pt, Rh) tips. Spectroscopic measurements (XPS) and microscopic structure studies (STM) of catalytic processes on UHV-grown inverse planar model catalysts (CeOx/Pt) are performed in order to reveal the role of the metal-oxide interface in the catalytic performance. The group also carries out studies on model systems mimicking the catalytic behaviour of individual metal nanoparticles. This allows avoiding the "averaging problem" of most surface-sensitive methods, i.e. averaging the data over a huge 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. The fluctuations are confined to a single particle (a few nm in size) and can lead to severe deviations from "mean-field" reaction causing e.g. noise-induced kinetic transitions in the nanosized reaction systems.

 Surface-specific "Surface Science" methods (Oberflächenanalytik) applied in our lab include

• X-ray photoelectron spectroscopy (XPS)
• Photoemission Electron Microscopy (PEEM)
• Low energy electron diffraction (LEED)
• Auger electron spectroscopy (AES)
• Field emission microscopy (FEM)
• Field ion microscopy (FIM)
• Field ion appearance energy spectroscopy (FIAES), "spectromicroscopy"

PEEM

Photoemission Electron microscopy (PEEM) is a tool for surface imaging on a mesocopic scale (100 nm - 10.000 nm) and is based on utilizing local variations in electron emission to generate image contrast. The excitation is usually produced by UV light, synchrotron radiation or X-ray sources. PEEM is a parallel imaging instrument, i.e. it creates a complete picture of the photoelectron distribution emitted from the imaged surface region at any given time. In combination with high-speed video techniques this allows real time studies of complex spatio-temporal surface phenomena such as surface diffusion and catalytic reactions. Typically, this technique was applied to processes on single crystal surfaces and allowed e.g. to reveal the oscillating changes in surface structure and coverage during catalytic CO oxidation on Pt (Prof. G. Ertl, Nobel Prize in Chemistry 2007). In our group, this technique is used to visualize reactions on discontinuous oxide films grown on single crystal metal (Pt) substrates (so called "inverse" model catalysts) and on nanostructured materials.

FEM, FIM

Field emission (FEM) and field ion microscopy (FIM) are analytical techniques based on field electron (field ion) emission in a projection-type microscope. These microscopes can be used to image the arrangement of surface atoms or adsorbed species at the surface of a nanosized metal tip. FIM was the first technique able to spatially resolve individual atoms (E.W. Müller, 1956). Both FEM and FIM are parallel imaging techniques and (contrary to STM) create a complete picture of the electron/ion emission distribution from the entire imaged surface area. This permits studying of spatially correlated phenomena. FEM/FEM based techniques allow investigations of local surface reaction kinetics on an atomic-scale level. The studied sample, the apex of a nanosized Pt or Rh tip, exhibits a heterogeneous surface formed by differently oriented nanofacets and can thus serve as a suitable model for a catalytic particle of comparable dimensions. However, in contrast to a catalyst particle, the tip surface can be prepared reproducibly by field evaporation and subsequently characterized with atomic resolution by imaging in the FIM. This technique, as well as the FEM can then be used to visualize in situ catalytic reactions such as CO oxidation on the platinum metal nanofacets, laterally resolved on the nanoscale.

 Studies on model systems mimicking the catalytic behaviour of individual metal nanoparticles using the (FIM/FEM) microscopies allow avoiding the "averaging problem" of most surface-sensitive methods, i.e. averaging the data over a huge number of catalytically active nanoparticles, thus "smoothing out" the characteristics of the individual nanoparticles (that may vary in size, shape, roughness, etc). This makes possible e.g. the detection of fluctuation-induced effects. The fluctuations are confined to a single particle (a few nm in size) and can lead to severe deviations from "mean-field" reaction causing e.g. noise-induced kinetic transitions in the nanosized reaction systems.

FIAES

By using probe-hole techniques (a small hole in the FIM screen corresponding in size to one or few surface atomic sites projected on the screen) the ions emitted from these sites can be collected and the energy of the field ions of reacting entities emitted from selected surface sites can be analyzed in a mass-to-charge resolved retarding potential experiment (field ion appearance energy spectroscopy, FIAES). Using the thermionic cycle, the binding energy of the (neutral) atoms or molecules adsorbed on the selected surface sites can be obtained from the retarding potential experiments in a straightforward way.