Andreas Menzel

Shape-Dependent Activity of Platinum Array Catalyst

We produced millions of morphologically identical platinum catalyst nanoparticles in the form of ordered arrays epitaxially grown on (111), (100), and (110) strontium titanate substrates using electron beam lithography. The ability to design, produce, and characterize the catalyst nanoparticles allowed us to relate microscopic morphologies with macroscopic catalytic reactivities. We evaluated the activity of three different arrays containing different ratios of (111) and (100) facets for an oxygen-reduction reaction, the most important reaction for fuel cells. Increased catalytic activity of the arrays points to a possible cooperative interplay between facets with different affinities to oxygen. We suggest that the surface area of (100) facets is one of the key factors governing catalyst performance in the electrochemical reduction of oxygen molecules.

In Situ Surface X-Ray Scattering Observation of Long-Range Ordered ( 19 × 19 ) R23.4 ° ­ 13 CO Structure on Pt(111) in Aqueous Electrolytes

Presented herein is the experimental observation of the long-range ordered (√19 × √19)R23.4°-13CO structure on Pt(111) in aqueous electrolytes by in situ surface X-ray scattering. The results confirmed the presence of two mirrored domains suggested earlier on the basis of scanning tunneling microscopy and infrared measurements. Based on the weak intensity of the second order adlattice reflections and earlier results obtained by other techniques, a further refinement of the (√19 × √19) structure with tilted CO molecules is proposed. The hystereses observed in transitions between (2 × 2 )- 3 CO and (√19 × √19)R23.4°-13CO phases, as well as in CO adsorption and stripping, with change in electrode potential are discussed.

Temperature-Induced Ordering of Metal/Adsorbate Structures at Electrochemical Interfaces

The influence of temperature changes in water-based electrolytes on the atomic structure at the electrochemical interface has been studied using in situ surface X-ray scattering (SXS) in combination with cyclic voltammetry. Results are presented for the potential-dependent surface reconstruction of Au(100), the adsorption and ordering of bromide anions on the Au(100) surface, and the adsorption and oxidation of CO on Pt(111) in pure HClO(4) and in the presence of anions. These systems represent a range of structural phenomena, namely metal surface restructuring and ordering transitions in both nonreactive spectator species and reactive adsorbate layers. The key effect of temperature appears to be in controlling the kinetics of the surface reactions that involve oxygenated species, such as hydroxyl adsorption and oxide formation. The results indicate that temperature effects should be considered in the determination of structure-function relationships in many important electrochemical systems.

Polarization-dependent resonant anomalous surface X-ray scattering of CO/Pt(111)

Polarization dependence of resonant anomalous surface X-ray scattering (RASXS) was studied for interfaces buried in electrolytes and in high-pressure gas. We find that RASXS exhibits strong polarization dependence when the surface is only slightly modified by adsorption of light elements such as carbon monoxide on platinum surfaces. σ- and π-polarization RASXS data were simulated with the latest version of ab initio multiple scattering calculations (FEFF8.2). Based on the simulation, we find that Pt layer to C distances in the (2 × 2)-3CO structure are ~ 2.0 A and ~ 1.6 A for atop CO and 3-fold CO, respectively. Some considerations are presented for the origin of the polarization dependence in RASXS.

Characterization of off-axis MgB2 epitaxial thin films for planar junctions

Using scanning tunneling spectroscopy, we perform a full mapping of the quasiparticle density of states of magnesium diboride (MgB2) epitaxial thin films grown on (110) yttrium stabilized zirconia (YSZ) by hybrid physical-chemical vapor deposition. The films have critical temperatures of 40 K. X-ray measurements show an epitaxial MgB2 growth having the c-axis tilted by 32° with respect to the normal to the substrate, consistent with the atomic force microscopy images of the sample. Scanning tunneling spectroscopy clearly finds that the spectroscopic peak associated to the π gap is reduced on most of the film surface and the feature representative of the σ gap is present, with different intensity, on the majority of the sample’s surface, which is consistent with x-ray measurements.

Fabrication of platinum nano-array model catalysts

We fabricated and characterized one- and two- dimensional nanoscale arrays of platinum for study of model catalysts. One-dimensional arrays of nanoscale facets were fabricated by annealing a high-index plan of platinum single crystals. The high-index plane forms rows of alternating two low-index facets, (111) and (100), widths of which are ~10 nanometers. Two-dimensional arrays were fabricated lithographically from the epitaxial films of platinum grown on SrTiO3 substrates. Electron beam lithography was used to create precisely registered square arrays of millions of identical platinum nanocrystals with ~30 nm in diameter.

Chapter 12 – Resonance Elastic and Inelastic X-ray Scattering Processes for In-situ Investigation of Electrochemical Interfaces

Publisher Summary This chapter discusses the resonance elastic and inelastic x-ray scattering processes for the in-situ investigation of electrochemical interfaces. X-ray-in and x-ray-out methods with high-energy and high-brilliance x-rays are some important techniques that can be used for the study of electrochemical interfaces. The most widely used technique is x-ray absorption spectroscopy (XAS). An advantage of the x-ray techniques is that they are highly versatile. They have been powerful in case of both single-crystal electrode interfaces and nano-structured electrochemical interfaces. In addition, they serve as promising tools for the surfaces of nanoparticles. This chapter elaborates on the potential new x-ray applications beyond ordinary XAS. Most techniques are possible only because of third-generation synchrotron x-ray sources. The scattering probability of x-rays at an atom can be enhanced when the photon energy is close to an absorption edge of the atom. This is similar to classical resonance phenomena. The discreteness of electronic energy levels in atoms or molecules is inherently quantum mechanical in nature. A quantum mechanical description of the resonance phenomena has also been reviewed in the chapter.