Growth and Melting of Metallic Nanoclusters in Glass: A Review of Recent Investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-an-le X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi- or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Waoner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T(m) on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T(m) is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established.

A portable Compton spectrometer for clinical X-ray beams in the energy range 20-150 keV

Primary beam spectra were obtained for an X-ray industrial equipment (40-150 kV), and for a clinical mammography apparatus (25-35 kV) from beams scattered at angles close to 90 degrees, measured with a CdTe Compton spectrometer. Actual scattering angles were determined from the Compton energy shift of characteristic X-rays or spectra end-point energy. Evaluated contribution of coherent scattering amounts to more than 15% of fluence in mammographic beams. This technique can be used in clinical environments. (C) 2010 Elsevier Ltd. All rights reserved.

A single-step procedure for the preparation of palladium nanoparticles and a phosphine-functionalized support as catalyst for Suzuki cross-coupling reactions

We here report the preparation of supported palladium nanoparticles (NPs) stabilized by pendant phosphine groups by reacting a palladium complex containing the ligand 2-(diphenylphosphino)benzaldehyde with an amino-functionalized silica surface The Pd nanocatalyst is active for Suzuki cross-coupling reaction avoiding any addition of other sources of phosphine ligands The Pd intermediates and Pd NPs were characterized by solid-state nuclear magnetic resonance and transmission electron microscopy techniques The synthetic method was also applied to prepare magnetically recoverable Pd NPs leading to a catalyst that could be reused for up to 10 recycles In summary we gathered the advantages of heterogeneous catalysis magnetic separation and enhanced catalytic activity of palladium promoted by phosphine ligands to synthesize a new catalyst for Suzuki cross-coupling reactions The Pd NP catalyst prepared on the phosphine-functionalized support was more active and selective than a similar Pd NP catalyst prepared on an amino-functionalized support (C) 2010 Elsevier Inc All rights reserved

A model for the stabilization of atomic hydrogen centers in borate glasses

A previously proposed model describing the trapping site of the interstitial atomic hydrogen in borate glasses is analyzed. In this model the atomic hydrogen is stabilized at the centers of oxygen polygons belonging to B-O ring structures in the glass network by van der Waals forces. The previously reported atomic hydrogen isothermal decay experimental data are discussed in the light of this microscopic model. A coupled differential equation system of the observed decay kinetics was solved numerically using the Runge Kutta method. The experimental untrapping activation energy of 0.7 x 10(-19) J is in good agreement with the calculated results of dispersion interaction between the stabilized atomic hydrogen and the neighboring oxygen atoms at the vertices of hexagonal ring structures. (C) 2009 Elsevier B.V. All rights reserved.

Self-assembling of phenothiazine compounds investigated by small-angle X-ray scattering and electron paramagnetic resonance spectroscopy

Small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) have been carried out to investigate the structure of the self-aggregates of two phenothiazine drugs, chlorpromazine (CPZ) and trifluoperazine (TFP), in aqueous solution. In the SAXS studies, drug solutions of 20 and 60 mM, at pH 4.0 and 7.0, were investigated and the best data fittings were achieved assuming several different particle form factors with a homogeneous electron density distribution in respect to the water environment. Because of the limitation of scattering intensity in the q range above 0.15 angstrom(-1), precise determination of the aggregate shape was not possible and all of the tested models for ellipsoids, cylinders, or parallelepipeds fitted the experimental data equally well. The SAXS data allows inferring, however, that CPZ molecules might self-assemble in a basis set of an orthorhombic cell, remaining as nanocrystallites in solution. Such nanocrystals are composed of a small number of unit cells (up to 10, in c-direction), with CPZ aggregation numbers of 60-80. EPR spectra of 5- and 16-doxyl stearic acids bound to the aggregates were analyzed through simulation, and the dynamic and magnetic parameters were obtained. The phenothiazine concentration in EPR experiments was in the range of 5-60 mM. Critical aggregation concentration of TFP is lower than that for CPZ, consistent with a higher hydrophobicity of TFP. At acidic pH 4.0 a significant residual motion of the nitroxide relative to the aggregate is observed, and the EPR spectra and corresponding parameters are similar to those reported for aqueous surfactant micelles. However, at pH 6.5 a significant motional restriction is observed, and the nitroxide rotational correlation times correlate very well with those estimated for the whole aggregated particle from SAXS data. This implies that the aggregate is densely packed at this pH and that the nitroxide is tightly bound to it producing a strongly immobilized EPR spectrum. Besides that, at pH 6.5 the differences in motional restriction observed between 5- and 16-DSA are small, which is different from that observed for aqueous surfactant micelles.

On the Stabilization of Gold Nanoparticles over Silica-Based Magnetic Supports Modified with Organosilanes

The immobilization of gold nanoparticles (Au NPs) on silica is made possible by the functionalization of the silica surfaces with organosilanes. Au NPs could only be stabilized and firmly attached to silica-support surfaces that were previously modified with amino groups. Au NPs could not be stabilized on bare silica surfaces and most of the NPs were then found in the solution. The metal-support interactions before and after the Au NP formation, observed by X-ray absorption fine structure spectroscopy (XAFS), indicate a stronger interaction of gold-(III) ions with amino-modified silica surfaces than with the silanol groups in bare silica. An amino-modified, silica-based, magnetic support was used to prepare an active Au NP catalyst for the chemoselective oxidation of alcohols, a reaction of great interest for the fine chemical industry.

Semiconductor nanoparticle modeling via density functional theory

In this work, a 2.0 nm nanoparticle (low limit synthesized system) is compared to possible simplified models: passivated clusters, small (1.3 nm) nanoparticles and sets of plane surfaces. Our density functional theory results suggest that even when geometric aspects are properly described by the simplifications considered, electronic properties might be very different, especially when edge atoms are not properly taken into account in the nanoparticle`s modeling. In addition, we propose a protocol that might help future theoretical descriptions of nanoparticles.

Enzyme immobilization on Ag nanoparticles/polyaniline nanocomposites

We show a simple strategy to obtain all efficient enzymatic broelectrochemical device, in which urease was immobilized oil electroactive nanostructured membranes (ENMs) made with polyaniline and silver nanoparticles (AgNP) stabilized in polyvinyl alcohol (PAni/PVA-AgNP). Fabrication of the modified electrodes comprised the chemical deposition of polyaniline followed by drop-coating of PVA-AgNP and urease, resulting in a final ITO/PAni/PVA-AgNP/urease electrode Configuration. For comparison. the electrochemical performance of ITO/PAni/urease electrodes (without Ag nanoparticles) was also studied. The performance of the modified electrodes toward Urea hydrolysis was investigated via amperometric measurements, revealing a fast increase in cathodic current with a well-defined peak upon addition of urea to the electrolytic solution. The cathodic currents for the ITO/PAni/PVA-AgNP urease electrodes were significantly higher than for the ITO/PAni/urease electrodes. The friendly environment provided by the ITO/PAni/PVA-AgNP electrode to the immobilized enzyme promoted efficient catalytic conversion of urea into ammonium and bicarbonate tons. Using the Michaelis-Menten kinetics equation, a K(M)(aPP) of 2.7 mmol L(-1) was obtained. indicating that the electrode architecture employed may be advantageous for fabrication of enzymatic devices with improved biocatalytic properties. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

Influence of different solvents on the structural, optical and morphological properties of CdS nanoparticles

CdS is one of the most important II-VI semiconductors, with applications in solar cells, optoelectronics and electronic devices. CdS nanoparticles were synthesized via microwave-assisted solvothermal technique. Structural and morphological characterization revealed the presence of crystalline structures presenting single phase with different morphologies such as ""nanoflowers"" and nanoplates depending on the solvent used. Optical characterization was made by diffuse reflectance and photoluminescence spectroscopy, revealing the influence of the different solvents on the optical properties due to structural defects generated during synthesis. It is proposed that these defects are related to sulfur vacancies, with higher concentration of defects for the sample synthesized in ethylene glycol in comparison with the one synthesized in ethylene diamine. (C) 2011 Elsevier B.V. All rights reserved.

Joint Experimental and Theoretical Analysis of Order Disorder Effects in Cubic BaZrO(3) Assembled Nanoparticles under Decaoctahedral Shape

Periodic first-principles calculations based on density functional theory at the B3LYP level has been carried out to investigate the photoluminescence (PL) emission of BaZrO(3) assembled nanoparticles at room temperature. The defect created in the nanocrystals and their resultant electronic features lead to a diversification of electronic recombination within the BaZrO(3) band gap. Its optical phenomena are discussed in the light of photoluminescence emission at the green-yellow region around 570 nm. The theoretical model for displaced atoms and/or angular changes leads to the breaking of the local symmetry, which is based on the refined structure provided by Rietveld methodology. For each situation a band structure, charge mapping, and density of states were built and analyzed. X-ray diffraction (XRD) patterns, UV-vis measurements, and field emission scanning electron microscopy (FE-SEM) images are essential for a full evaluation of the crystal structure and morphology.

Hierarchical Assembly of CaMoO(4) Nano-Octahedrons and Their Photoluminescence Properties

Hierarchical assemblies of CaMoO4 (CM) nano-octahedrons were obtained by microwave-assisted hydrothemial synthesis at 120 degrees C for different times. These structures were structurally, morphologically and optically characterized by X-ray diffraction, micro-Raman spectroscopy, field-emission gun scanning electron microscopy, ultraviolet-visible absorption spectroscopy and photoluminescence measurements. First-principle calculations have been carried out to understand the structural and electronic order-disorder effects as a function of the particle/region size. Supercells of different dimensions were constructed to simulate the geometric distortions along both they and z planes of the scheelite structure. Based on these experimental results and with the help of detailed structural simulations, we were able to model the nature of the order-disorder in this important class of materials and discuss the consequent implications on its physical properties, in particular, the photoluminescence properties of CM nanocrystals.

Effect of Different Solvent Ratios (Water/Ethylene Glycol) on the Growth Process of CaMoO(4) Crystals and Their Optical Properties

In this paper, calcium molybdate (CaMoO(4)) crystals (meso- and nanoscale) were synthesized by the coprecipitation method using different solvent volume ratios (water/ethylene glycol). Subsequently, the obtained suspensions were processed in microwave-assisted hydrothermal/solvothermal systems at 140 degrees C for 1 h. These meso- and nanocrystals processed were characterized by X-ray diffraction (X R I)), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR). ultraviolet visible (UV-vis) absorption spectroscopies, held-emission gun scanning electron microscopy (FEG-SEM). transmission electron microscopy (TEM). and photoluminescence (PL) measurements. X RI) patterns and FT-Raman spectra showed that these meso- and nanocrystals have a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a large absorption band situated at around 827 cm(-1), which is associated with the Mo-O anti-symmetric stretching vibrations into the [MoO(4)] clusters. FEG-SEM micrographs indicated that the ethylene glycol concentration in the aqueous solution plays an important role in the morphological evolution of CaMoO(4) crystals. High-resolution TEM micrographs demonstrated that the mesocrystals consist of several aggregated nanoparticles with electron diffraction patterns of monocrystal. In addition, the differences observed in the selected area electron diffraction patterns of CaMoO(4) crystals proved the coexistence of both nano- and mesostructures, First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level were employed in order to understand the band structure find density of states For the CaMoO(4). UV-vis absorption measurements evidenced a variation in optical band gap values (from 3.42 to 3.72 cV) for the distinct morphologies. The blue and green PI. emissions observed in these crystals were ascribed to the intermediary energy levels arising from the distortions on the [MoO(4)] clusters clue to intrinsic defects in the lattice of anisotropic/isotropic crystals.

From Rational Design of Drug Crystals to Understanding of Nucleic Acid Structures: Lamivudine Duplex

A DNA-like duplex of nucleosides is probable to exist even without the 5`-phosphate groups needed to assemble the chain backbone. However, double-stranded helical structures of nucleosides are unknown. Here, we report a duplex of nucleoside analogs that is spontaneously assembled due to stacking of the neutral and protonated molecules of lamivudine, a nucleoside reverse transcriptase inhibitor (NTRI) widely used in anti-HIV drug combinatory medication. The left-handed lamivudine duplex has features similar to those of i-motif DNA, as the face-to-face base stacking and the helix rise per base pair. Furthermore, the protonation pattern on alternate bases expected for it DNA-like duplex stabilized by pairing of neutral and protonated cytosine fragments was observed for the first time in the lamivudine double-stranded helix. This structure demonstrates that hydrogen bonds can substitute for covalent phosphodiester linkage in the stabilization of the duplex backbone. This interesting example of spontaneous molecular self-organization indicates that the 5`-phosphate group could not be a requirement for duplex assembly.

Driving forces in substitution reactions of octahedral complexes: The influence of the competitive effect

The reaction of cis-[RuCl(2)(P-P)(N-N)] type complexes (P-P = 1,4-bis(diphenylphosphino)butane or (1,1`-diphenylphosphino)ferrocene; N-N = 2,2`-bipyridine or 1,10-phenantroline) with monodentate ligands (L), such as 4-methylpyridine, 4-phenylpyridine and benzonitrile forms [RuCl(L)(P-P)(N-N)](+) species Upon characterization of the isolated compounds by elemental analysis, (31)P{(1)H} NMR and X-ray crystallography it was found out that the type of the L ligand determines its position in relation to the phosphorus atom. While pyridine derivatives like 4-methylpyridine and 4-phenylpyridine coordinate trans to the phosphorus atom, the benzonitrile ligand (bzCN), a good pi acceptor, coordinates trans to the nitrogen atom. A (31)P{(1)H} NMR experiment following the reaction of the precursor cis-[RuCl(2)(dppb)(phen)] with the benzonitrile ligand shows that the final position of the entering ligand in the complex is better defined as a consequence of the competitive effect between the phosphorus atom and the cyano-group from the benzonitrile moiety and not by the trans effect. In this case, the benzonitrile group is stabilized trans to one of the nitrogen atoms of the N-N ligand. A differential pulse voltammetry experiment confirms this statement. In both experiments the [RuCl(bzCN)(dppb)(phen)]PF(6) species with the bzCN ligand positioned trans to a phosphorus atom of the dppb ligand was detected as an intermediate complex. (c) 2009 Elsevier Ltd. All rights reserved.

Molecular conformation of the racemic indan derivative (+/-)-1-trans-3-(3,4-dichlorophenyl)-2,3-dihydro-1H-indene-1-carboxamide

This paper presents the structural characterization of the indan derivative (+/-)-1-trans-3-(3,4-dichlorophenyl)-2,3-dihydro-1H-indene-1-carboxamide, which was unambiguously determined by X-ray diffraction (XRD) to be a racemate (R/S: 50/50) crystallizing in an achiral crystal structure (P2(1)/c, a = 9.3180(1) , b = 7.9070(2) , c = 19.7550(4) , beta = 103.250(1)A degrees, V = 1416.75(5) (3) and Z = 4). The diastereomers are related by the inversion symmetry and linked by H bond forming a dimer. The crystal packing is stabilized by hydrogen bonds, including the classical one responsible for the formation of centrosymmetric dimers, and non-classical ones involving C-H center dot center dot center dot O and C-H center dot center dot center dot pi-aryl interactions. The intra and intermolecular geometry of the title compound is compared to the (+/-)-1-trans-3-(3,4-dichlorophenyl)-2,3-dihydro-1H-indene-1-carboxylic acid one, which also present an achiral crystal structure from racemates (R/S: 50/50). The two indan derivatives crystallize in a very similar unit cell.