Erbium enhanced formation and growth of photoluminescent Er/Si nanocrystals

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Photoluminescent Er/Si-nanocrystal composites were obtained after annealing Er doped silicon suboxide (SiOx) thin films. The filmswere prepared by reactive sputtering (Ar/O-2 atmosphere) with a pure silicon target partially covered with metallic Er. The presence of Er in the resulting films strongly influences Si nanocrystal nucleation and growth during thermal treatment at temperatures between 300 and 1300 degrees C. A correlation between Er photoluminescence (PL) spectra, Er speciation and Si nanocrystal properties indicated that PL bands and their intensity are directly influenced by the nanocrystal size and density, and their vicinity to the Er3+ centers. This correlation is explained by considering Er centers as promotor for SiOx disproportionation, locally increasing SiOx concentration which leads to formation of Si-0 nanocrystals in the vicinity of Er. (C) 2013 Elsevier B.V. All rights reserved.536196201Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)LNLS - National Synchrotron Light Laboratory, BrazilFWO-VlaanderenFlemish GovernmentFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Co-assessment of cell cycle and micronucleus frequencies demonstrates the influence of serum on the in vitro genotoxic response to amorphous monodisperse silica nanoparticles of varying sizes

Serum proteins have been shown to modulate the cytotoxic and genotoxic responses to nanomaterials. The aim was to investigate the influence of serum on the induction of micronuclei (MN) by nanoparticles (NPs) of different sizes. Therefore, A549 human lung carcinoma cells and amorphous monodisperse silica nanoparticles (SNPs) were used as models. Assessment of the cell viability, cell cycle changes and induction of MN by SNPs ranging from 12 to 174 nm was performed in presence or absence of serum, applying the in vitro flow cytometry-based MN assay. Here, it has been demonstrated that serum has an influence on these end points, with a lower cell viability in absence of serum compared with the presence of serum. Further, cell cycle changes, specifically, G(1) and S-phase arrest, were observed in absence of serum for four out of six SNPs tested. A size-dependent MN induction was observed: larger SNPs being more active in absence of serum. In addition, the serum influence was characterised by a size-dependency for cytotoxic and genotoxic effects, with a higher influence of serum for smaller particles. The data indicate that the in vitro micronucleus assay in presence and absence of serum could be advised for hazard assessment because it demonstrates a higher sensitivity in serum-free conditions than in conditions with serum. However, this recommendation applies only if the cell line used is able to proliferate under serum-free conditions because cell division is a prerequisite for MN expressio

Evolution of the crystal growth mechanism of zeolite W (MER) with temperature

The morphology of zeolite W (MER topology) synthesized from Hydrated Silicate Ionic Liquids (HSILs) shows a distinct temperature dependence, reflected in a fundamental difference in the underlying crystal growth mechanism as revealed by Atomic Force Microscopy (AFM). Zeolite W crystals obtained at 90 °C develop in a highly supersaturated solution through birth and spread growth, whereas synthesis at 175 °C results in elongated, spiral grown zeolite W particles. Supersaturation was measured through the concentration of dissolved aluminate, being the limiting species. The evolution of the aluminum concentration during crystallization at different temperatures was monitored with 27Al Nuclear Magnetic Resonance (NMR) spectroscopy. Supersaturation conditions determine the nucleation rate, the prevailing crystal growth mechanism, and resulting crystal morphology.The morphology of zeolite W (MER topology) synthesized from Hydrated Silicate Ionic Liquids (HSILs) shows a distinct temperature dependence, reflected in a fundamental difference in the underlying crystal growth mechanism as revealed by Atomic Force Microscopy (AFM). Zeolite W crystals obtained at 90 °C develop in a highly supersaturated solution through birth and spread growth, whereas synthesis at 175 °C results in elongated, spiral grown zeolite W particles. Supersaturation was measured through the concentration of dissolved aluminate, being the limiting species. The evolution of the aluminum concentration during crystallization at different temperatures was monitored with 27Al Nuclear Magnetic Resonance (NMR) spectroscopy. Supersaturation conditions determine the nucleation rate, the prevailing crystal growth mechanism, and resulting crystal morphology.status: publishe

Framework flexibility-driven CO₂ adsorption on a zeolite

Adsorption of the greenhouse gas CO₂ by ordered porous materials, although cost-efficient and eco-friendly, has not yet achieved large-scale practical application. Here, we report that zeolite framework flexibility can be exploited by tuning a subtle interplay between extra framework cations with framework oxygen and adsorbed guest molecule (CO₂). This phenomenon has been demonstrated on the Na^{+}, K^{+} and Rb^{+} orms of the small-pore zeolite gismondine with a Si/Al ratio of 3.0, which show marked cation-dependent hystereses in their CO₂ isotherms. A detailed analysis of the structures by solid state NMR and XRD revealed that the framework dynamics and the adsorption behavior is governed by the extra framework cations, which strive for coordination either with framework oxygen, guest molecules, or both. The K+ and Rb+ forms display very high CO₂ working capacities (2.8 and 3.0 mmol g^{-1}) for a 50 : 50 CO₂/CH_{4} mixture under mild temperature swing conditions (25–100 °C at 1.0 bar), as well as high CO₂/CH_{4} selectivities (36 and 28) at 25 °C. Given the excellent thermal and chemical stability of zeolites compared to other classes of adsorbents, and the here reported tuneable flexing action effected by the cations, zeolites displaying framework flexibility harbour great potential for fuel gas applications

Porous solids get organized

A powerful combination of analytical techniques is used to shed light on the complex crystallizations of porous solids. Molecular recognition creates the seeds of order from which complex lattices grow

Template-aluminosilicate structures at the early stages of zeolite ZSM-5 formation. A combined preparative, solid-state NMR and computational study

Species at three stages in the self-assembly of zeolite ZSM-5 have been studied with one- and two-dimensional magic-angle-spinning 13C, 27Al, 29Si, and 1H NMR spectroscopy and compared with the earlier proposed structures: (1) precursor species containing 33-36 T sites around a tetrapropylammonium (TPA) cation, (2) nanoslabs consisting of a flat 4 × 3 array of such precursors, and (3) the final TPA-ZSM-5 zeolite. Synthesis was carried out in D2O to suppress the water and silanol protons. Under such conditions, the effective Si-H and Al-H distances measured with 29Si-{1H} and 27Al-{1H} rotational echo double resonance (REDOR) reflect the interactions between TPA cations and the surrounding aluminosilica. The 29Si-{1H} REDOR curves for Q4-type silicon atoms at the three mentioned stages are closely similar, as well as the observed 27Al-1H REDOR curve for the precursor species compared to that for the TPA-ZSM-5. This indicates that in addition to externally attached TPA, there is also internal TPA already incorporated at an early stage into the aluminosilicate in a similar way as in the final zeolite, in accordance with the earlier proposed MFI self-assembly pathway (Kirschhock et al. Angew. Chem. Int. Ed. 2001, 40, 2637). However, the effective distances extracted from the initial REDOR curvatures are significantly (10-15%) larger than those computed for the model. Since there is no temperature effect, we tentatively assign this difference to a reduction of the 29Si-1H and 27Al-1H interactions by multispin decoherence effects or self-decoupling caused by proton spin diffusion. By assuming the computed model distances and fitting Anderson-Weiss curves to the observed REDOR data, we obtain similar decoherence times in the order of 0.1 ms. The observed 29Si-{1H} REDOR dephasing for the Q3 sites in the precursors is significantly faster than that for the Q4 sites. This is tentatively ascribed to a partial deuteron-proton back exchange at the silanol positions

Combined in situ 29Si NMR and small-angle X-ray scattering study of precursors in MFI zeolite formation from silicic acid in TPAOH solutions

Silicic acid powder was dissolved and polymd. in a concd. aq. tetrapropylammonium (TPA) hydroxide soln. at room temp. Two complementary techniques were employed to follow this process leading to silicalite-1 zeolite upon heating. The formation of small silicates and specific oligomers involved in the assembly of silicalite-1 nanoprecursors was investigated using 29Si NMR. Small-angle X-ray scattering (SAXS) was used to follow processes at a colloidal level. Dissoln. and polymn. of silicic acid could then be related to events occurring at both mol. and colloidal scales. The appearance of very well-defined colloidal particles was linked to a specific intermediate already obsd. in systems using an org. and monomeric silica source. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) using synchrotron radiation showed a linear growth of the av. crystal diam., which was slower than of that encountered in Na+ contg. synthesis mixts. Using the results presented here, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. This model is in agreement with rising evidence of a common mechanism involving nanoblock aggregation for org. mediated crystn. of high-silica zeolites