Buchbesprechungen

Ohne Zusammenfassung     

Buchbesprechungen

Ohne Zusammenfassung     

Linear and nonlinear transport across carbon nanotube quantum dots

We present a low energy-theory for non-linear transport in finite-size interacting single-wall carbon nanotubes. It is based on a microscopic model for the interacting p_z electrons and successive bosonization. We consider weak coupling to the leads and derive equations of motion for the reduced density matrix. We focus on the case of large-diameter nanotubes where exchange effects can be neglected. In this situation the energy spectrum is highly degenerate. Due to the multiple degeneracy, diagonal as well as off-diagonal (coherences) elements of the density matrix contribute to the nonlinear transport. At low bias, a four-electron periodicity with a characteristic ratio between adjacent peaks is predicted. Our results are in quantitative agreement with recent experiments.     

The particle size effect on the oxygen reduction reaction activity of Pt catalysts: influence of electrolyte and relation to single crystal models

The influence of particle size on the oxygen reduction reaction (ORR) activity of Pt was examined in three different electrolytes: two acidic solutions, with varying anionic adsorption strength (HClO(4) < H(2)SO(4)); and an alkaline solution (KOH). The experiments show that the absolute ORR rate is dependent on the supporting electrolyte; however, the relationship between activity and particle size is rather independent of the supporting electrolyte. The specific activity (SA) toward the ORR rapidly decreases in the order of polycrystalline Pt > unsupported Pt black particles (~30 nm) > high surface area (HSA) carbon supported Pt nanoparticle catalysts (of various size between 1 and 5 nm). In contrast to previous work, it is highlighted that the difference in SA between the individual HSA carbon supported catalysts (1 to 5 nm) is rather trivial and that the main challenge is to understand the significant differences in SA between the polycrystalline Pt, unsupported Pt particles, and HSA carbon supported Pt catalysts. Finally, a comparison between measured and modeled activities (based on the distribution of surface planes and their SAs) for different particle sizes indicates that such simple models do not capture all aspects of the behavior of HSA carbon supported catalysts.     

Tunable mixed amorphous–crystalline cellulose substrates (MACS) for dynamic degradation studies by atomic force microscopy in liquid environments

Atomic force microscopy in liquid environments (L-AFM) became a state of the art technique in the field of enzymatic cellulose degradation due to its capability of in situ investigations on enzymatic relevant scales. Current investigations are however limited to few substrates like valonia cellulose, cotton linters and processed amorphous cellulose as only these show required flatness and purity. Structurally monophasic, these substrates confine conclusions regarding enzymatic degradation of mixed amorphous–crystalline substrates as commonly found in nature. To exploit the full potential of the technique, cellulose substrates with multiphase properties, flat topology and purity are therefore absolutely required. In this study we introduce a special preparation route based on highly crystalline Avicel PH101^® cellulose and the ionic liquid 1-butyl-3-methylimmidazolium chloride as dissolution reagent. As comprehensively shown by atomic force microscopy, wide angle X-ray scattering, Raman spectroscopy and electron microscopy, the developed material allows precise control of its polymorphic composition by means of cellulose types I and II embedded in an amorphous matrix. Together with the tunable composition and flat topology over large areas (>10 × 10 µm²) the material is highly suited for L-AFM studies.     

Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pt-transition metal alloys: Pt-skin versus Pt-skeleton surfaces

The surface properties of PtM (M = Co, Ni, Fe) polycrystalline alloys are studied by utilizing Auger electron spectroscopy, low energy ion scattering spectroscopy, and ultraviolet photoemission spectroscopy. For each alloy initial surface characterization was done in an ultrahigh vacuum (UHV) system, and depending on preparation procedure it was possible to form surfaces with two different compositions. Due to surface segregation thermodynamics, annealed alloy surfaces form the outermost Pt-skin surface layer, which consists only platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components. The measured valence band density of state spectra clearly shows the differences in electronic structures between Pt-skin and sputtered surfaces. Well-defined surfaces were hereafter transferred out from UHV and exposed to the acidic (electro)chemical environment. The electrochemical and post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable during and after immersion to an electrolyte. In contrast all sputtered surfaces formed Pt-skeleton outermost layers due to dissolution of transition metal atoms. Therefore, these three different near-surface compositions (Pt-skin, Pt-skeleton, and pure polycrystalline Pt) all having pure-Pt outermost layers are found to have different electronic structures, which originates from different arrangements of subsurface atoms of the alloying component. Modification in Pt electronic properties alters adsorption/catalytic properties of the corresponding bimetallic alloy. The most active systems for the electrochemical oxygen reduction reaction are established to be the Pt-skin near-surface composition, which also have the most shifted metallic d-band center position versus Fermi level.