Abstract: Multiwall carbon nanotubes grown by the catalytic decomposition of acetylene over supported Co catalyst were subjected to wet and dry oxidation in order to remove the unwanted products and the catalyst traces. The effects of the purification treatment on the Co content was monitored by physical methods: Rutherford Backscattering Spectrometry (RBS). Particle Induced X-Ray Emission (PIXE) and X-Ray Fluorescence (XRF). The purified products were investigated by microscopic methods: TEM. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and STM. The KMnO4/H2SO4 aqueous oxidation procedure was found to be effective in reducing the Co content while damaging only moderately the outer wall of the nanotubes. Treatment in HNO3/H2SO4 yields a bucky-paper like product and produces the increase of the Si and S content of the sample. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: Hematite-titania nanocomposites, eventually functionalized with gold nanoparticles (NPs), are designed and developed by a plasma-assisted strategy, consisting in: (i) the plasma enhanced-chemical vapor deposition of -Fe2O3 on fluorine-doped tin oxide substrates; the radio frequency-sputtering of (ii) TiO2, and (iii) Au in controlled amounts. A detailed chemicophysical characterization, carried out through a multitechnique approach, reveals that the target materials are composed by interwoven -Fe2O3 dendritic structures, possessing a high porosity and active area. TiO2 introduction results in the formation of an ultrathin titania layer uniformly covering Fe2O3, whereas Au sputtering yields a homogeneous dispersion of low-sized gold NPs. Due to the intimate and tailored interaction between the single constituents and their optical properties, the resulting composite materials are successfully exploited for solar-driven applications. In particular, promising photocatalytic performances in H-2 production by reforming of water-ethanol solutions under simulated solar illumination are obtained. The related insights, presented and discussed in this work, can yield useful guidelines to boost the performances of nanostructured photocatalysts for energy-related applications.

Abstract: Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb1.5Ba2.5Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb1.5Ba2.5Bi2Fe6O16 belong to the perovskite-based AnBnO3n2 homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell ap as ap√2 × ap × nap√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(1̅01)p crystallographic shear (CS) planes. The CS operation results in (1̅01)p-shaped perovskite blocks with a thickness of (n 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb1.5Ba2.5Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb1.5Ba2.5Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned FeFe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 AnFenO3n2 (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623632 K.