Functionalized silica synthesized by sol-gel process

This work considers some aspects of the chemistry involved in the preparation and description of silicon oxide functionalized by sol-gel process. In this work we studied the synthesis and measured the properties of silicon oxide functionalized with 3-chloropropyl, through a sol-gel process. Thermogravimetic analysis, infrared spectra, and elemental analyses were measured. The samples were prepared in the following proportions of tetraethylorthosilicate (TEOS): 3-chloropropyl trimethoxisilane molar ratio: 1:0, 1:1, 2:1, 3:1 and 4:1. The thermogravimetric data for the resulting materials established the 'minimum formulae' 2:0, 3:1, 4.1, 7:1 and 11:1, respectively. As expected, the relative amount of water is inversely proportional to the presence of propyl groups. Infrared data show Si-C and -CH2-vibration modes at 1250 to 1280 and 2920 to 2940 cm(-1), respectively. Thermogravimetric data and infrared spectra showed that inorganic polymers contained organic polymers. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Response of L-alanine and 2-methylalanine minidosimeters for K-Band (24 GHz) EPR dosimetry

Minidosimeters of L-alanine and 2-methylalanine (2MA) were prepared and tested as potential candidates for small radiation field dosimetry. To quantify the free radicals created by radiation a K-Band (24 GHz) EPR spectrometer was used. X-rays provided by a 6 MV clinical linear accelerator were used to irradiate the minidosimeters in the dose range of 0.5-30 Gy. The dose-response curves for both radiation sensitive materials displayed a good linear behavior in the dose range indicated with 2MA being more radiation sensitive than L-alanine. Moreover, 2MA showed a smaller LLD (lower limit detection) value. The proposed system minidosimeter/K-Band spectrometer was able to detect 10 Gy EPR spectra with good signal-to-noise ratio (S/N). The overall uncertainty indicates that this system shows a good performance for the detection of dose values of 20 Gy and above, which are dose values typically used in radiosurgery treatments. (c) 2007 Elsevier B.V. All rights reserved.

Synthesis of hybrid silicates containing porphyrins incorporated by a sol-gel process and their properties

Porphyrin was incorporated in a silicate network, via a covalent bond, by grafting a functional group with 3-aminopropyltriethoxysilane, using a sol-gel process. We have carried out the synthesis and measured the absorption spectra, nuclear magnetic resonance spectra, infrared (IR) spectra, luminescence spectra and lifetime of these hybrid silicates, porphyrinosilicas. These samples contained the following free-base porphyrins: meso-tetrakis-p-chlorobenzoylporphyrin, meso-tetrakis-2,6-dichloro-3-chlorosulfonylphenylporphyrin. The obtained porphyrinosilicas have similar absorption and luminescence spectra to the free base porphyrins in solution. IR spectra confirm the formation of monomeric species. Lifetime measurement for porphyrinosilica reveals that 32% +/- 2% of porphyrin is covalently bonded to the silica network. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Electron scattering and effects of sources of light on photoconductivity of SnO2 coatings prepared by sol-gel

Electro-optical properties of sol-gel derived 2 mol% antimony or niobium doped tin dioxide films have been measured. The electron density has been calculated considering all the relevant scattering mechanisms and experimental conductivity data measured in the range -197 to 25 degrees C. The results support the hypothesis that both ionised impurity scattering and grain boundary scattering have comparable effects in the resistivity of coatings, for free electron density congruent to 5 x 10(18) cm(-3). We have measured variation of photoconductivity excitation with wavelength using xenon and deuterium lamp as light sources. Results show that the main band in the photoconductivity spectrum is dependent on the spectral light source emission, the excitation peak reaching 5 eV (deuterium lamp). This band is due to the recombination process involving oxygen species and photogenerated electron-hole pairs. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Preparation and characterization of spherical silica-porphyrin catalysts obtained by the sol-gel methodology

This work describes the synthesis of a first-generation iron porphyrin catalyst entrapped in a silica matrix by the sol-gel route, leading to spherical particles. The catalyst was synthesized by the method of Stober, through hydrolysis and condensation of the alkoxysilane TEOS in a mixture of alcohol, water and ammonia, in the presence of the iron porphyrin Fe(TPP)Cl. The relation between particle morphology and catalytic activity of the different Fe(TPP)-SiO2, obtained using different H2O/silane molar ratios and ammonia concentrations in the xerogel syntheses, was studied.The obtained catalysts were characterized by UV-vis spectroscopy, NMR Si-29. thermogravimetric analysis and transmission electron microscopy. Their ability to catalyze (Z)-cyclooctene epoxidation and cyclohexane oxidation was tested using iodosylbenzene as oxygen donor; the oxidation products were analyzed by gas chromatography and the catalysts obtained in a form of particles spherical and monodispersed showed to be a promising catalytic system for selective oxidation. (c) 2005 Elsevier B.V. All rights reserved.

Small angle X-ray scattering study of surface modified tin oxide nanoparticles prepared by sol-gel route

The surface properties of SnO2 nanoparticles were modified by grafting ionic (Tiron (R). (OH)(2)C6H2(SO3Na)(2)(H2O)-H-.) or non-ionic (Catechol (R). C6H4-1,2-(OH)(2)) capping Molecules during aqueous sol-gel processing to improve the redispersibility of powdered xerogel. The effect of the amount of grafted organic molecules on the redispersibility of powders in aqueous solution at several basic pH values was Studied. The nanostructural features of the colloidal suspensions were analyzed by small angle X-ray scattering (SAXS) measurements. Irrespective of the nature and amount of grafted molecules, complete redispersion was obtained in aqueous solution at pH = 13. The redispersion at pH = 11 results in a mixture of dispersed primary particles and aggregates. The proportion of well dispersed nanoparticles and aggregates (and their average size) can be tuned by the quantity of grafted ionic molecules.

Dynamical study of ZnO nanocrystal and Zn-HDS layered basic zinc acetate formation from sol-gel route

We have pointed out that zinc based particles obtained from ethanolic solution of a zinc acetate derivative (zinc oxy-acetate, Zn4O(Ac)(6)) are a mixture of nanometer sized ZnO, zinc oxy-acetate, and zinc hydroxide double salt (Zn-HDS). The knowledge of the mechanisms involved in the formation of ZnO and Zn-HDS phases, and the evolution of Zn species in reaction medium was monitored in situ during 14 h by simultaneous measurements of UV-vis absorption and extended X-ray absorption fine structures (EXAFS) spectra. This spectroscopic monitoring was initialized just after the addition of an ethanolic lithium hydroxide solution ([LiOH]/[Zn] = 0. 1) to the reaction medium kept under controlled temperature (40 degrees C). This study points out the first direct evidence of the reaction between ZnO nanoparticles and unreacted zinc oxy-acetate to form a Zn-HDS phase. The dissolution of ZnO and the reprecipitation of Zn-HDS are induced by the gradual release of water mainly produced by ethanol esterification well evidenced by gas chromatography coupled to mass spectroscopy and FT-IR measurements.

Sol-gel prepared Pt-modified oxide layers: Synthesis, characterization, and electrocatalytic activity

Thin films of pure RuO2 and IrO2 and mixed Ru0.5Ir0.5O2 oxide modified with Pt particles were prepared by a sol-gel method in the form of thin films of similar to 2 mu m thickness on Ti substrates. Surface morphology of these Pt- modified oxides was examined by scanning electron microscopy and was found to exhibit a significant influence of the chemical composition of the oxide matrix. Element mapping showed homogeneous distribution of the metals. X- ray diffraction and X- ray photoelectron spectroscopy analyses showed that these films consist of metallic Pt particles dispersed in an oxide matrix. Cyclic voltammetry in acid solutions showed that the sol- gel prepared layers have relatively high Pt surface areas. The electrocatalytic activity of these materials toward the anodic oxidation of formaldehyde and methanol was compared in terms of onset potential and current density and was found to follow the sequence: Pt- Ru0.5Ir0.5O2/ Ti > Pt- RuO2/ Ti > Pt- IrO2/ Ti.

Microwave synthesis of YAG : Eu by sol-gel methodology

Yttrium-aluminum oxides are interesting compounds and they have been extensively used as host for lasers and phosphors, due to their stable physical and chemical properties. The fabrication of yttrium-aluminum garnet (YAG) has been investigated thoroughly. Single-crystal YAG is expensive and to produce it a new way has been investigated. This process consists of modifying the methodology of reagents mixture and the process of heating them. The microwave irradiation is used to heat-treat the oxide mixture. The traditional synthesis of YAG powders occurs through the reaction of aluminum and yttrium powders at high temperatures. With this work we investigated the preparation of YAG by non-hydrolytic sol-gel route as an alternative methodology to obtain yttrium-aluminum matrix from inorganic precursors (yttrium and aluminum chloride). The preparation of the gel was carried out in an oven-dried glassware. The AlCl3, YCl3 and ethanol were reacted in reflux under argon atmosphere. Europium III chloride was added as a structural probe. The powder was dried and heat-treated in modified microwaves. The samples were pre-treated at 50 and 800 C during I h and then heated in microwaves for 30 s, 2 and 4 min. The formation process and structure of the powders were studied by means of X-ray diffraction (XRD), photoluminescence (PL) and transmission electronic microscopy (TEM). XRD presents only picks corresponding to the YAG phase and confirmed by TEM. PL date showed that the YAG phase was formed in 2 min with the samples pre-treated at 50 C. For the samples pretreated at 800 degrees C, the YAG phase appears in 30s. The excitation spectra present a maximum of 394 nm corresponding to the L-5(6) level and emission spectra of Eu III ion present bands characteristic transitions arising from the D-5(0) -> F-7(J) (J= 1, 2, 3, 4) monifolds excited at their maximum. The magnetic dipole D-5(0) -> F-7(1) transition presents more intensity than the electric dipole D-5(0) -> F-7(2) transition. This methodology showed efficiency in obtaining YAG phase. (c) 2006 Elsevier B.V. All rights reserved.

Improvement of the Mo/TiO2-Al2O3 catalyst by the control of the sol-gel synthesis

Traditional hydrotreating catalysts are constituted by molybdenum deposited on Al2O3 promoted by nickel and phosphorous. Several studies have shown that TiO2-Al2O3 mixed oxides are excellent supports for the active phases. Results concerning the preparation, characterization and testing of molybdenum catalyst supported on titania-alumina are presented. The support was prepared by sol-gel route using titanium and aluminum isopropoxides, the titanium one chelated with acetylacetone (acac) to promote similar hydrolysis ratio for both the alcoxides. The effect of nominal molar ratio [Ti]/[Ti+Al] on the microstructural features of nanometric particles was analyzed by X-Ray Diffraction, N-2 Adsorption Isotherms and Transmission Electron Microscopy. The catalytic activity of Mo impregnated supports was evaluated using the thiophene hydrodesulfurization at different temperatures and atmospheric pressure. The pores size distribution curve moves from the micropores to the mesopores by increasing the Ti contents, allowing the fine tuning of average size from 2.5 to 6 nm. Maximal (367 m(2).g(-1)) and minimal (127 m(2).g(-1)) surface area were found for support containing [Ti]/[Ti+Al] ratio equal to 0.1 and 1, respectively. The good mesopore texture of alumina-titania support with [Ti]/[Ti+Al] molar ratio between 0.3 and 0.5 was found particularly valuable for the preparation of well dispersed MoS2 active phase, leading to HDS catalyst with somewhat higher activity than that prepared using a commercial alumina support.

Effect of rinsing with water immediately after APF gel application on enamel demineralization in situ

This double-blind crossover in situ study evaluated the effect of rinsing with water immediately after the application of acidulated phosphate fluoride (APF) on enamel demineralization. APF application was followed by: no rinsing or consuming of liquids or solids for the next 30 min; immediately washing with water jet or drinking of a glass of water. All treatments showed a statistically significant decrease in enamel mineral loss compared to the control (p < 0.05), but the differences among treatments were not significant (p > 0.05). The data suggest that the recommendation of asking patients to refrain from drinking water following professional APF application does not seem to have an influence on the anticaries effect of fluoride. Copyright (C) 2005 S. Karger AG, Basel.

Er-doped silica-based waveguides prepared by different techniques: RF-sputtering, sol-gel and ion-exchange

Erbium-activated silica-based planar waveguides were prepared by three different technological routes: RF-sputtering, sol-gel and ion exchange. Various parameters of preparation were varied in order to optimize the waveguides for operation in the NIR region. Particular attention was devoted to the minimization of the losses and the increase of the luminescence efficiency of the metastable I-4(13/2) state of the Er3+ ion. Waveguide properties were determined by m-line spectroscopy and loss measurements. Waveguide Raman and luminescence spectroscopy were used to obtain information about the structure of the prepared films and about the dynamical processes related to the luminescence of the Er3+ ions.

Electroluminescence of zinc oxide thin-films prepared via polymeric precursor and via sol-gel methods

Zinc oxide (ZnO) is an electroluminescent (EL) material that can emit light in different regions of electromagnetic spectrum when electrically excited. Since ZnO is chemically stable, inexpensive and environmentally friendly material, its EL property can be useful to construct solid-state lamps for illumination or as UV emitter. We present here two wet chemical methods to prepare ZnO thin-films: the Pechini method and the sol-gel method, with both methods resulting in crystalline and transparent films with transmittance > 85% at 550 nm. These films were used to make thin-film electroluminescent devices (TFELD) using two different insulator layers: lithium fluoride (LiF) or silica (SiO2). All the devices exhibit at least two wide emission bands in the visible range centered at 420 nm and at 380 nm attributed to the electronic defects in the ZnO optical band gap. Besides these two bands, the device using SiO2 and ZnO film obtained via sol-gel exhibits an additional band in the UV range centered at 350 nm which can be attributed to excitonic emission. These emission bands of ZnO can transfer their energy when a proper dopant is present. For the devices produced the voltage-current characteristics were measured in a specific range of applied voltage. (C) 2007 Elsevier B.V. All rights reserved.