Electron diffraction characterization of nanocrystalline materials using a Rietveld-based approach. Part I. Methodology.

Transmission electron microscopy is a powerful experimental tool, very effective for the complete characterization of nanocrystalline materials by employing a combination of imaging, spectroscopy and diffraction techniques. Electron powder diffraction (EPD) pattern fingerprinting in association with chemical information from spectroscopy can be used to deduce the identity of the crystalline phases. Furthermore, EPD has similar potential to X-ray powder diffraction (XRPD) for extracting additional information regarding material specimens, such as microstructural features and defect structures. The aim of this paper is to extend a full-pattern fitting procedure, broadly used for analysing XRPD patterns, to EPD. The interest of this approach is twofold: in the first place, the relatively short times involved with data acquisition allow one to speed up the characterization procedures. This is a particularly interesting aspect in the case of metastable structures or kinetics studies. Moreover, the reduced sampling volumes involved with electron diffraction analyses can better reveal surface alteration layers in the analysed specimen which might be completely overlooked by conventional bulk techniques. The first step forward to have an effective application of the proposed methodology concerns establishing a reliable calibration protocol to take into correct account the instrumental effects and thus separate them from those determined by the structure, microstructure and texture of the analysed samples. In this paper, the methodology for determining the instrumental broadening of the diffraction lines is demonstrated through a full quantitative analysis based on the Rietveld refinement of the EPD. In this regard, a CeO2 nanopowder reference specimen has been used. The results provide indications also on the specific features that a good calibration standard should have.

A new sample preparation protocol for SEM and TEM particulate matter analysis.

A new methodology has been developed to prepare electron microscopy, both SEM and TEM, specimens starting from particulate matter collected using environmental sampling systems. The approach is based on the extraction of the particles to be analyzed from the harvesting substrates. The extracted particles can be directly observed in an SEM, possibly in low-vacuum mode to prevent electrical charging. In order to prepare electron transparent samples, TEM observations require a further step, consisting in embedding the particles in an electron transparent carbon film deposited before dissolving the acetate extracting substrate. The protocol has been tested by analyzing particles collected during bench tests on brake pads and discs, carried out on a dynamometer equipped with a particulate matter sampling apparatus. The main advantages of the approach are: the complete extraction of the particulate matter specimens from the original substrates, that in this way do not interfere with the analyses; the extracted samples retain the topological information of the collection in the specimens prepared for SEM; possibility to be applied to any kind of particulate matter harvesting substrates.

Present knowledge and perspectives on the role of copper in brake materials and related environmental issues: A critical assessment.

This critical review presents several aspects related to the use of copper as a main component in brake pads in road vehicles. The compositions of these materials are attracting increasing interest and concern due to the relative contribution of wear products to particulate matter emissions in the environment as a result of braking action even though there has been a reduction in exhaust products from internal combustion engines. We review the data on the main wear mechanisms in brake systems and highlight the positive role of copper. However, similar to other heavy metal emissions, even the release of copper into the atmosphere may have important environmental and health effects. Thus, several replacement strategies are being pursued, and the positive and negative features will be critically reviewed. Additionally, the future perspectives in materials development will be discussed.

Inorganic chemistry in a nanoreactor: Au/TiO2 nanocomposites by photolysis of a single-source precursor in miniemulsion.

An original synthetic route, based on the combination of a single-source precursor, UV-photodegradation and inverse w/o miniemulsion, is used to prepare Au nanoparticles (NPs) dispersed on titania. The source of the nanocomposite materials is the photolabile single-source precursor AuCl4(NH4)7[Ti2(O2)2(cit)(Hcit)]2·12H2O, which is suspended in a w/o miniemulsion consisting of different surfactant/hydrocarbon/water formulations (surfactant: sodium dodecylsulfate (SDS) or Triton X-100) and subsequently irradiated with a UV lamp to promote its decomposition in the confined space of the droplets. Gold NPs that form at room temperature are found to be crystalline, while titanium dioxide occurs as an amorphous phase. Moreover, the average crystallite size of gold NPs ranges between 20 and 24 nm when using SDS and between 26 and 40 nm in the case of Triton X-100, after 4 and 8 hours of irradiation time, respectively. Scanning and transmission electron microscopies (SEM and TEM) are used to get information about the nanocomposite morphology and nanostructure, revealing that gold NPs are uniformly distributed on the titanium oxide surface. Furthermore, X-ray photoelectron spectroscopy (XPS) outcomes, besides confirming the formation of both metallic gold and titania, provide information about the high dispersion of Au NPs on the TiO2 surface. In fact, the Au : Ti atomic ratio is found to be 0.45-1.5 (1 : 2-1.5 : 1), which is higher than the value determined by starting from the precursor stoichiometry (0.25). Catalytic testing in the oxidation of 2-propanol shows that decomposition of the precursor in a miniemulsion provides a nanocomposite with enhanced activity compared to the decomposition in the aqueous phase.

XANES analysis of dried and calcined bones.

The structure of dried and calcined bones from chicken, bovine, deer, pig, sheep and chamois was examined using X-ray Absorption Near Edge Structure (XANES) spectroscopy. The oxygen K-edge absorption edge indicates that the surface of dried bone has a larger proportion of carbonate than the interior that is made up of phosphates. The phosphorus L and K edge clearly indicate that pyrophosphates, α-tricalcium phosphate (α-TCP) and hydrogen phosphates of Ca do not exist in either the dried bone or calcined bone and phosphorus exists as either ß-tricalcium phosphate (ß-TCP) or hydroxyapatite, both in the dried and calcined conditions. The Ca K-edge analysis indicates that ß-TCP is the likely form of phosphate in both the dried and calcined conditions.

Photodegradation of Pollutants in Air: Enhanced Properties of Nano-TiO(2) Prepared by Ultrasound.

Nanocrystalline TiO(2) samples were prepared by promoting the growth of a sol-gel precursor, in the presence of water, under continuous (CW), or pulsed (PW) ultrasound. All the samples turned out to be made of both anatase and brookite polymorphs. Pulsed US treatments determine an increase in the sample surface area and a decrease of the crystallite size, that is also accompanied by a more ordered crystalline structure and the samples appear to be more regular and can be considered to contain a relatively low concentration of lattice defects. These features result in a lower recombination rate between electrons and holes and, therefore, in a good photocatalytic performance toward the degradation of NO(x) in air. The continuous mode induces, instead, the formation of surface defects (two components are present in XPS Ti 2p(3/2) region) and consequently yields the best photocatalyst. The analysis of all the characterization data seems to suggest that the relevant parameter imposing the final features of the oxides is the ultrasound total energy per volume (E(tot)/V) and not the acoustic intensity or the pulsed/continuous mode.

Structure and optical properties of silica-supported Ag-Au nanoparticles.

Bimetallic Ag-Au nanoparticles are synthesized by sequential deposition of Au and Ag on amorphous silica by Radio Frequency (RF)-sputtering under mild conditions. Specimens are thoroughly characterized by a multi-technique approach, aimed at investigating the system properties as a function of the Ag/Au content, as well as the evolution induced by ex-situ annealing under inert (N2) or reducing (4% H2/N2) atmospheres. The obtained results demonstrate the possibility to obtain Ag-Au alloyed nanoparticles with controllable size, shape, structure, and dispersion under mild conditions, so that the optical properties can be finely tuned as a function of the synthesis and thermal treatment conditions.

Structural and magnetic properties of pure and Ca-doped LaCoO3 nanopowders obtained by a sol-gel route.

Pure and Ca-doped LaCoO3 nanopowders were prepared by a non-alkoxidic sol-gel route using cobalt(II) acetate, lanthanum(III) nitrate and calcium(II) acetate as oxide precursors. The structural evolution and magnetic properties of the samples were studied as a function of thermal treatments in air up to 1273 K. In particular, the microstructure and composition of the systems were analyzed by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS). Both pure and calcium-doped samples annealing at 973 K resulted in the formation of cubic LaCoO3 (average crystallite size <30 nm). This phase was fully retained in the calcium-doped materials even after annealing at higher temperatures, whereas a transition to the rhomboedral polymorph was detected in the pure samples at 1073 K. The magnetic behavior of the nanopowders was investigated as a function of temperature and applied field using both dynamic and static susceptibility measurements. Pure lanthanum cobaltite samples underwent a transition to an ordered state at 88 K, and their magnetic properties changed as a function of thermal treatments. As concerns calcium-doped samples, they ordered ferromagnetically at 171 and 185 K depending on the annealing temperature and displayed open hysteresis loops with coercive fields as large as 1.75 T at low temperatures.