262,185 research outputs found
Evolution of the Nonionic Inverse Microemulsion−Acid−TEOS System during the Synthesis of Nanosized Silica via the Sol−Gel Process
No full textThe cyclohexane−igepal inverse microemulsion, comprehensively established for the synthesis of silica nanoparticles in an ammonia-catalyzed sol−gel process, was alternatively studied with an acid-catalyzed sol−gel process. Tetraethyl orthosilicate (TEOS) was used as the silica precursor, while two different aqueous phases containing either HNO3 or HCl at two different concentrations, 0.1 and 0.05 M, were examined in the presence and in the absence of NaF, a catalyst of the condensation step. The evolution of the overall reacting system, specifically hydrolysis and polycondensation of reaction intermediates, was monitored in situ by time-resolved small-angle X-ray scattering. No size variation of the inverse micelles was detected throughout the sol−gel process. Conversely, the density of the micellar core increased after a certain time interval, indicating the presence of the polycondensation product. The IR spectra of the reacting mixture were in agreement with such a hypothesis. 1H and 13C NMR measurements provided information on the soluble species, the surfactant, and TEOS. The TEOS consumption was well fitted by means of an exponential decay, suggesting that a first-order kinetics for TEOS transpires in the various systems examined, with rate constants dependent not only on the acid concentration but also on its nature (anion specific effect), on the presence of NaF, and on the amount of water in the core of the inverse micelle. The self-diffusion coefficients, determined by means of PGSTE NMR, proved that a sizable amount of the byproduct ethanol was partitioned inside the inverse micelles. Characterization of the final product was carried out by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM), which concurrently confirmed that the silica isolated from the inverse nonionic microemulsion is not significantly different from the product of a bulk acid-catalyzed sol−gel synthesis. TEM micrographs illustrated particles with diameters smaller than the diameter of the inverse micelles as determined by SAXS, due to a shrinkage effect, in addition to nanostructured aggregates in the range 20−100 nm
Synthesis and optical properties of sub-micron sized rare earth-doped zirconia particles
No full textSub-micron sized crystalline particles of Eu3+ and Er3+-doped zirconia (ZrO2) were prepared via a wet chemical sol–gel route and post synthesis annealing. The doping was achieved by introduction of the respective rare earth salts into the zirconia precursor solution, with insitu generation of sodium chloride for stabilization of the particle surface during growth. A series of materials with differing europium content, nominally 0.3, 3 and 6 mol% within the ZrO2 lattice, were prepared for respective comparison and characterization of their optical properties following annealing at 700 °C. Average emission lifetimes of up to 2.3 ms were observed for the Eu3+-doped particles. Particle sizes, approximated from SEM micrographs, were observed in the range 250–400 nm. The synthesis of Er3+-doped ZrO2 particles (0.5 mol%) produced 300 nm sized particles which exhibited emission in the visible and infrared regions after annealing at 1000 °C. X-ray diffraction (XRD) with Rietveld analysis for phase quantification, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectra were used to characterize the samples
Encapsulation o Silica Particles by a Thin Shell of Poly(Methyl) Methacrylate
No full textThe encapsulation of inorganic particles in a thin polymer shell generates core-shell hybrid materials. This chapter summarizes the main contributions from the literature for the preparation of a specific example of such hybrid materials, core-shell particles composed of an inner silica core and a poly(methyl methacrylate) outer shell. The spherical silica particles used in the examples have been obtained by sol-gel methods, most commonly via the Stöber synthesis and the reverse phase microemulsion method. Depending on the choice of method for their encapsulation, the surfaces of the silica particles have been modified to promote formation of poly(methyl methacrylate) at their surface. Surface modification methods may include the covalent attachment of a polymerizable organosilane which may co-polymerize with the monomer in conventional radical polymerization or the covalent attachment of an initiator for controlled living radical polymerization. The methods presented in this chapter will include conventional in situ heterophase radical polymerization methods, namely emulsion, emulsifier-free emulsion, precipitation, dispersion, and vapour phase polymerization. We also provide several examples of surface initiated atom transfer radical polymerization, for the controlled growth of poly(methyl methacrylate) brushes on silica particle
Right-protected data publishing with provable distance-based mining
No full textProtection of one's intellectual property is a topic with important technological and legal facets. We provide mechanisms for establishing the ownership of a dataset consisting of multiple objects. The algorithms also preserve important properties of the dataset, which are important for mining operations, and so guarantee both right protection and utility preservation. We consider a right-protection scheme based on watermarking. Watermarking may distort the original distance graph. Our watermarking methodology preserves important distance relationships, such as: the Nearest Neighbors (NN) of each object and the Minimum Spanning Tree (MST) of the original dataset. This leads to preservation of any mining operation that depends on the ordering of distances between objects, such as NN-search and classification, as well as many visualization techniques. We prove fundamental lower and upper bounds on the distance between objects post-watermarking. In particular, we establish a restricted isometry property, i.e., tight bounds on the contraction/expansion of the original distances. We use this analysis to design fast algorithms for NN-preserving and MST-preserving watermarking that drastically prune the vast search space. We observe two orders of magnitude speedup over the exhaustive schemes, without any sacrifice in NN or MST preservation. © 1989-2012 IEEE
Structural and photoluminescence properties of ZrO2:Eu3+ @ SiO2 nanophosphors as a function of annealing temperature
No full textThe synthesis, morphology and luminescence properties of two systems comprising luminescent Eu3+-doped zirconium oxide nanocrystals embedded in an amorphous silica matrix are reported. The two systems, prepared with the same overall wt% composition of silica (75%) and EuxZr(1−x)O(2−x/2) solid solution (25%), have been annealed in the range 135–1000 °C and subsequently functionalized with (3-aminopropyl)triethoxysilane. Detailed X-ray diffraction analyses and transmission electron micrographs, combined with infrared spectroscopy and luminescence spectroscopy data, have been used to demonstrate the influence of annealing temperature on the: (i) nanostructure, (ii) luminescence properties and (iii) availability of superficial –OH groups for efficient surface functionalization. The optimum calcination temperature was found to be 700 °C for each series in terms of luminescence lifetime efficiency and post-functionalization efficiency with (3-aminopropyl)triethoxysilane
In situsynthesisofEu(Tp)3 complex inside the pores of mesoporous silica nanoparticles
No full textA routeforeuropium(III)tris-pyrazolylboratecomplex[Eu(Tp)3] formationinsidetheporesof
mesoporous silicananoparticles(MSNs)hasbeenestablishedtoyieldahighlyluminescentnanostruc-
tured hybrid.Twodifferentinsituchemicalprecipitationtechniqueshavebeenexploredforthe
nanoencapsulation oftheEu3+ complex[Eu(Tp)3] insidetheporechannelsofmesoporoussilica
nanoparticles, byvaryingthesequenceorderofprecursorimpregnation.Inthe first method,theEu
salt isintroducedintotheporesbywetimpregnationandremovalofthesolvent,followedbywet
impregnation oftheligand.Inthesecondapproach,theadditionsequenceisreversed.Theimportanceof
the additionsequencewasdemonstratedbythesuccessfulformationoftheeuropium(III)tris-pyrazolyl
boratecomplexintheporenetworkbyfollowingthe first approach.Theobservedpyrazol-1-ylborate
(Tp) toEu3+ intramolecular energytransfer,i.e.,theantennaeffect,verified theformationofthecomplex.
Photoluminescencespectroscopy,X-raydiffraction,N2 adsorption, FTIRspectroscopy,andTEMwereused
to characterizethematerial
Encapsulation of submicrometer-sized silica particles by a thin shell of poly(methyl methacrylate)
No full textPolymer encapsulation of submicrometer-sized silica particles by synthesis of the polymer shell,
poly(methyl methacrylate) under static conditions in a reaction medium free of surfactants and stabilizing
agents is described. The Stöber method, a base-catalyzed hydrolysis and condensation of tetraethyl
orthosilicate is used for the synthesis of the monodisperse colloidal dispersion of silica particles.
The silica particles are subsequently modified in situ with the surface grafting of the silane coupling
agent, 3-(trimethoxysilyl)propyl methacrylate. Encapsulation is achieved using tetraethyl orthosilicate
as a reaction medium, in which a thermally initiated radical polymerization of methyl methacrylate
is promoted in the presence of the modified particles by a seeding method which leads to a thin
coating of poly(methyl methacrylate), and hence silica core–shell particles. The complete encapsulation
of individual silica spheres by poly(methyl methacrylate) is visually evidenced by TEM microscopy which
reveals the presence of a polymer shell coating up to 10 nm. Evidence for the presence of a poly(methyl
methacrylate) shell is further corroborated by DSC/TGA, DRIFT-IR and NMR measurements
Structural and luminescence properties of europium(III)-doped zirconium carbonates and silica-supported Eu3+-doped zirconium carbonate nanoparticles
No full textThe synthesis, morphology and luminescence properties of europium(III)-doped zirconium carbonates prepared as bulk materials and as silica-supported nanoparticles with differing calcination treatments are reported. Transmission electron microscopy and X-ray diffraction analyses have, respectively, been used to study the morphology and to quantify the atomic amount of europium present in the optically active phases of the variously prepared nanomaterials. Rietveld analysis was used to quantify the constituting phases and to determinate the europium content. Silica particles with an approximate size of 30 nm were coated with 2 nm carbonate nanoparticles, prepared in situ on the surface of the silica core. Luminescence measurements revealed the role of different preparation methods and of europium-doping quantities on the optical properties observed
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A multinuclear solid-state magnetic resonance study on submicrometer-sized SiO2 particles encapsulated by a PMMA shell
No full textSubmicrometer-sized silica particles encapsulated by poly(methyl methacrylate) via a novel methodology have been characterized by means of solid-state high-resolution NMR techniques. The organic-inorganic composite prepared under static conditions, using our previously published method, comprised silicacore-shell particles of approximately 180 nm functionalized with 3-(trimethoxysilyl)propyl methacrylate and an outer coating of poly(methyl methacrylate) with an approximate 10 nm thickness, shown by transmission electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis and differential scanning calorimetry. A systematic solid-state nuclear magnetic resonance study based on Si-29 and C-13 high-resolution techniques was performed herein for the submicrometer particles at the various stages of synthesis, thus characterizing the organic and inorganic components as well as their interface. The presence of TMSPM and PMMA was confirmed and the structural features of the TMSPM species characterized. Moreover, from a comparative study of functionalized silica samples with different average particle size, it was possible to reveal the significant effect of this parameter on the condensation reactions involved in the synthesis of such samples
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