Shape-dependent acidity and photocatalytic activity of Nb2O5 nanocrystals with an active TT (001) surface

Nb(2)O(5) nanorods and nanospheres were synthesized, and their photocatalytic activity for methylene blue decomposition in water compared. Nb(2)O(5) nanorods clearly displayed higher activity, despite their comparable surface area. With a shape-dependent surface acidity, hydrothermal stability, and high photoactivity, these Nb(2)O(5) nanorods are a unique and exciting nanomaterial for non-classical photocatalytic mineralization of organic compounds in water.     

Flux line lattice symmetries in the borocarbide superconductor LuNi2B2C

We compare the results of small angle neutron scattering on the flux line lattice (FLL) obtained in the borocarbide superconductor LuNi₂B₂C with the applied field along the c- and a-axes. For H‖c the temperature dependence of the FLL structural phase transition from square to hexagonal symmetry was investigated. Above 10 K the transition onset field. H ₂(T), rises sharply, bending away from H _c2(T) in contradiction to theoretical predictions of the two merging. For H‖a a first order FLL reorientation transition is observed at H _tr=3–3.5 kOe. Below H _tr the FLL nearest neighbor direction is parallel to the b-axis, and above H _tr to the c-axis. This transition cannot be explained using nonlocal corrections to the London model.     

Gase

Ohne Zusammenfassung     

H-transfer rates in ion-neutral complexes versus those in intact aliphatic ions

The relative rates of H-transfer between partners in ion-neutral complexes were compared with those in intramolecular rearrangements using results of first differential photoionization mass spectrometry measurements. Complex-mediated H-transfers are inferred to have rates of the same order as those for intramolecular hydrogen rearrangements, suggesting a similar range of motion of the reactive sites in both types of reactions. It is also concluded that at their fastest H-transfers take place between the partners in ion-neutral complexes within at most the time of several rotations of the partners in the complexes. Copyright 1999 John Wiley & Sons, Ltd.     

Symmetry and Phase-Selected NMR Spectra of Liquid Crystalline Samples

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin- nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180°. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.     

Applications of MBMS and surface spectroscopic techniques in the study of reaction mechanisms in CBE; investigations of the reactivity of tritertiarybutylgallium and triisobutylgallium as alternative precursors for epilayer growth

Current approaches to the study of reaction mechanisms in CBE and investigations of the potential of triisobutylgallium and tritertiarybutylgallium as novel CBE precursors are reviewed. Surface spectroscopic techniques indicate that adsorbed iso-butyl radicals decompose to produce gaseous butene and hydrogen at significantly lower temperatures than in the corresponding process for ethyl radicals on GaAs, resulting in lowered growth temperatures and low temperature C incorporation levels in comparison to the results obtained with triethylgallium. A β-methyl migration occurs at higher temperatures causing C to deposit irreversibly on the surface in the presence of Al. Lowered temperatures for the β-hydride elimination reaction are also observed for adsorbed tertiarybutyl radicals and the absence of β-methyl groups avoids the facile C deposition process seen for iso-butyl. These potential advantages associated with tertiarybutyl ligands cannot be realized straightforwardly in CBE using tritertiarybutylgallium however, since steric crowding effects result in the inefficient total dissociation of the adsorbing precursor molecule.     

Chemisorption and corrosion at the metal-halogen interface: Overlayer growth and compound formation by bromine on Cr(100)

The interaction of bromine with Cr(100) has been studied in the regime 300–1300 K and over a wide range of coverage. The initial reaction results in the formation of an overlayer which exhibits a continuous series of compression structures whose end members are c(2 × 2) and c(2 × 4). The sticking probability of Br₂ remains constant at 0.9 ± 0.1 during this process. Beyond this stage, nucleation and growth of CrBr₂ commences at a rate which is ten times slower than that of the initial chemisorption reaction. The stoichiometry of the bromide phase is kinetically controlled and it grows by the Volmer-Weber mechanism to yield an epitaxial corrosion layer of a pseudohexagonal form of CrBr₂. Intensity changes and kinetic energy shifts in the Cr and Br Auger spectra indicate the occurrence of substantial valence charge transfer from metal to halogen. The structural, thermochemical and electronic properties of the system are discussed with reference to a specific model.