Aqueous sol-gel synthesis route for the preparation of YAG: Evaluation of sol-gel process by mathematical regression model

Yttrium aluminium garnet (Y₃Al₅O₁₂, YAG) polycrystalline samples have been prepared by a simple aqueous sol-gel methodology. The influence of nineteen sol-gel processing variables on the formation of YAG has been investigated. Effects of different fabrication parameters on the phase purity and morphological properties of the compounds were studied by energy-dispersive spectrometry (EDS), X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). The parameters of the sol-gel processing such as pH of starting solution, concentration and nature of complexing ligand, temperature and duration of gelation, powder rehomogenization during annealing, duration and temperature of the final heat treatment were found to be the most significant. For the evaluation and verification of the experimental results the Brandon's model of a multiple regression was successfully used.     

Fabrication and optical properties of core-shell structured spherical SiO2@GdVO4:Eu phosphors via sol-gel process

Europium-doped nanocrystalline GdVO₄ phosphor layers were coated on the surface of preformed submicron silica spheres by sol-gel method. The resulted SiO₂@Gd_0.95Eu_0.05VO₄ core-shell particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra, low voltage cathodoluminescence (CL), time resolved PL spectra and kinetic decays. The XRD results demonstrate that the Gd_0.95Eu_0.05VO₄ layers begin to crystallize on the SiO₂ spheres after annealing at 600 ^°C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have spherical shape, narrow size distribution (average size ca. 600 nm), non-agglomeration. The thickness of the Gd_0.95Eu_0.05VO₄ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). PL and CL show that the emissions are dominated by ⁵D₀-⁷F₂ transition of Eu³⁺ (618 nm, red). The PL and CL intensities of Eu³⁺ increase with increasing the annealing temperature and the number of coating cycles. The optimum concentration for Eu³⁺ was determined to be 5 mol% of Gd³⁺ in GdVO₄ host.     

Thermoplastic and thermosetting properties of polyphenylsilsesquioxane particles prepared by two-step acid-base catalyzed sol-gel process

Polyphenylsilsesquioxane (PhSiO_3/2) particles were synthesized by a two-step acid-base catalyzed sol-gel process, in which phenyltriethoxysilane used as a starting alkoxide was hydrolyzed and condensed in the first step under two different acid-catalyzed conditions. The PhSiO_3/2 particles prepared under an acid-catalyzed process without using a solvent were thermally hardened after the first heating up to 200°C. In contrast, the PhSiO_3/2 particles prepared under an acid-catalyzed process with ethanol as a solvent were thermally softened with the repeated heating processes up to 200°C. From the gel permeation chromatography measurements, it was found that the average molecular weight of the particles prepared with the use of ethanol was decreased in comparison with that of the particles prepared without using ethanol. The addition of ethanol in the sols during hydrolysis and condensation reaction under the acid-catalyzed process caused the decrease in the average molecular weight, which should result in the changes of thermal property of PhSiO_3/2 particles from thermosetting to thermoplastic.     

Luminescent properties and energy transfer process of Sm3+-Eu3+ co-doped MY2(MoO4)4 (M=Ca,Sr and Ba) red-emitting phosphors

MY₂(MoO₄)₄:Sm³⁺ and MY₂(MoO₄)₄:xSm³⁺,yEu³⁺ (M=Ca, Sr and Ba) phosphors were successfully prepared using solid-state reaction route, and their luminescent properties and energy transfer process from Sm³⁺ to Eu³⁺ were systematically investigated. The results indicate that MY₂(MoO₄)₄:Sm³⁺ phosphors can be effectively excited by 407 nm near UV light originating from the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺, and exhibit a satisfactory red emission at 646 nm attributed to the ⁴G_5/2 → ⁶H_9/2 transition of Sm³⁺, in which the emission intensity of SrY₂(MoO₄)₄:Sm³⁺ is the strongest among the MY₂(MoO₄)₄:Sm³⁺ (M=Ca, Sr and Ba) phosphors. For Eu³⁺ co-doped MY₂(MoO₄)₄:Sm³⁺ samples, with increasing Eu³⁺ doping content, the main emission peaks of Sm³⁺ (approximately 646 nm) are decreased, but the emission peaks and intensity of Eu³⁺ are increased while the maximum intensity of luminescence at the Eu³⁺ concentration 0.9. The introduction of Eu³⁺ in the MY₂(MoO₄)₄:Sm³⁺ phosphors can remarkably generate a strong emission line at 616 nm, originating from the ⁵D₀→⁷F₂ transition of Eu³⁺ and Sm³⁺ (⁴G_5/2) → Eu³⁺ (⁵D₀) effective energy transfer process. The energy transfer mechanism from Sm³⁺ to Eu³⁺ was discussed in detail.     

Silica supported sulfated zirconia prepared by a sol-gel process: Effect of solvent evacuation procedure on the structural, textural and catalytic properties

Sol-gel process was used to prepare silica supported sulfated zirconia catalysts. The main parameter studied in this work was the gel drying method through four different ways of solvent evacuation. Textural, structural as well as the acidic properties of the four samples were studied using N₂ physisorption, X-ray diffraction, mass spectrometry, sulfur chemical analysis and adsorption-desorption of pyridine. The isomerization of n-hexane was used as a catalytic test. The surface areas and the pore distributions are highly affected by the drying mode. One of the four drying methods leads to a solid having improved textural properties and presenting both the crystalline ZrO₂ tetragonal phase and a particular type of sulfate mode bond. It seems that these three conditions are necessary for achieving high catalytic activity.     

Electrochromic and structural characteristics of mesoporous WO<Subscript>3</Subscript> films prepared by a sol-gel method

The preparation of electrochromic films of mesoporous tungsten trioxide from tungstic acid and tungstic hexaethoxide precursors with the addition of an organic stabiliser via a sol-gel method is reported. These films have been structurally characterised and both the film morphology and crystalline composition of the films were found to be significantly dependent on the temperature at which the films were annealed and upon the choice of precursor. Films annealed at lower temperatures consisted of amorphous and hexagonal tungsten trioxide, whereas films annealed above 500 °C comprised solely of monoclinic WO₃. The electrochromic activity of the films was found to be equally dependent on method of preparation, and both the composition and the structure of the WO₃ films were shown to clearly influence the colouration efficiency of the films.Dedicated to Zbigniew Galus on the occasion of his 70^th birthday.     

Electrochemical properties of Meldola's Blue immobilized on silica-titania phosphate prepared by the sol-gel method

The mixed oxide phosphate (≡SiO)₂Ti(O₃POH)₂ having a specific surface area of S_BET= 595 m² g⁻¹ and an average pore volume of 0.43 mL g⁻¹ was prepared by the sol-gel processing method. The material showed the following characteristics: Ti=11.6 wt% and P=10.5 wt%; ion exchange capacity of 0.60 mmol g⁻¹. Meldola's Blue (MLB) dye was adsorbed, by an ion exchange reaction, from an aqueous solution in a quantity of 0.62 mmol g⁻¹. The dye was strongly retained and was not easily leached from the matrix even in presence of 0.5 M electrolyte solution. Changing the solution pH between 2.5 and 7.0, the midpoint potential of the dye-adsorbed (≡SiO)₂ Ti(O₃POMLB)₂ matrix carbon paste electrode remained practically constant, i.e. about 20 mV vs. SCE. This is not the usual behaviour of MLB since its midpoint potential changes considerably in solution phase as the pH is changed. The modified electrode has proved to be stable and electrocatalytically active for hydrazine oxidation at pH 6. Received: 22 June 1999 / Accepted: 7 September 1999     

Boehmite sol properties and preparation of two-layer alumina membrane by a sol-gel process

A two layer ultrafiltration alumina membrane was prepared by a sol-gel process using boehmite sol as precursor. The sol was prepared by hydrolysation of aluminium tri-sec-butoxide. Sol properties, such as viscosity as a function of concentration and acidity, were investigated by using capillary viscometry, transmission electron microscopy and laser scattering photometry etc. The viscosity increased with an increase in concentration and a decrease in pH, while the particle shape and size of the sol were mainly determined by pH. The membrane prepared by a dipping procedure was characterized by both scanning electron microscopy and transmission electron microscopy. The results showed that the membrane thickness and surface morphology were affected by the dipping time, the viscosity and the temperature.     

Structural and spectroscopic properties of LaOF:Eu3+ nanocrystals prepared by the sol-gel Pechini method

A new method was used to obtain Eu(3+)-doped LaOF nanocrystals. The obtained nanocrystals were synthesized for the first time using a modified Pechini sol-gel method. The products were analyzed by X-ray powder diffraction and the Rietveld method. Optimal conditions for the synthesis were found. Luminescent properties of the tetragonal and rhombohedral LaOF:Eu(3+) nanocrystals were investigated by collecting excitation and luminescence spectra. The most effective dopant concentrations in both hosts were found. Luminescent lifetimes were also measured. The time-resolved luminescent traces showed both a growth and a decay, which pointed to energy transfer processes between Eu(3+) ions in the LaOF host. In order to explain these phenomena, an adequate mechanism has been proposed. Intensity parameters Ω(2), Ω(4) and quantum efficiencies were calculated using the Judd-Ofelt theory, allowing for an extensive study of the luminescent properties of Eu(3+) ion in the LaOF matrix.     

Preparation of nano-sized YAG:Eu3+ particles by a microwave-assisted polyol process and their luminescence properties

Phases of yttrium-aluminum binary oxide have been selectively prepared by selecting a polyol solvent in a microwave-assisted polyol process. Using 1,4-butanediol as a polyol solvent, nano-sized particles of a pure phase of YAG have been prepared. Furthermore, Eu³⁺-doped YAG has been synthesized by applying this method. We have demonstrated that 8 at% Eu³⁺-doped YAG (replacement of Y³⁺) gave the strongest emission when the content of Eu³⁺ was varied.     

Preparation and properties of amino-terminated anionic waterborne-polyurethane-silica hybrid materials through a sol-gel process in the absence of an external catalyst

A series of polymer-silica hybrid materials consisting of amino-terminated anionic waterborne-polyurethane (WPU) and inorganic silica particles have been prepared through a sol-gel process in the absence of an external catalyst. Typically, amino-terminated anionic WPU was first synthesized from polycaprolactone, dimethylol propionic acid, and 4,4′-methylenebis(cyclohexyl isocyanate) with specific molar ratios, followed by further reaction with triethylamine and triethylene tetramine to give as-prepared WPU. The WPU obtained was characterized by FTIR spectroscopy and gel permeation chromatography. Subsequently, a series of hybrid materials with different silica contents were prepared by performing sol-gel reactions with tetraethyl orthosilicate (TEOS) in an amino-terminated WPU matrix without the addition of an external catalyst. This was followed by examination by transmission electron microscopy and ²⁹Si solid-state NMR. The terminated primary amine groups attached to the as-prepared WPU chains functioned as an internal base catalyst for the sol-gel process of TEOS. The effect of composition on the thermal stability, mechanical strength, surface wettability, and optical clarity of the hybrid materials was evaluated by the thermogravimetric analysis, dynamic mechanical analysis, contact angle measurement, and UV-visible transmission spectroscopy, respectively.     

Pt/SiO2 catalysts obtained by the sol-gel method. Influence of the pH of gelation on the surface and catalytic properties

Pt/SiO₂ catalysts prepared by impregnation and sol-gel method from a platinum acetylacetonate precursor have been studied. The pH was varied in the range of 3–9. Important changes in the surface area and pore distribution were found. High metal dispersity was found in all the catalysts, being lower at pH 9.     

Synthesis and optical properties of novel red phosphors YNbTiO6:Eu3+ with highly enhanced brightness by Li+ doping

Highly luminescent euxenite phased YNbTiO₆:Eu³⁺ and Li⁺-doped YNbTiO₆:Eu³⁺ red phosphors have been prepared through a facile sol–gel combustion process and investigated for the first time. The introduction of Li⁺ ions into YNbTiO₆:Eu³⁺ is able to result in significant changes of the crystallinity and particle size, and bring a clear red-shift of absorption edge. A dominant red emission peak at 611 nm due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ was observed from photoluminescence spectra of the YNbTiO₆:Eu³⁺ and Li⁺-doped YNbTiO₆:Eu³⁺ phosphors. In particular, the emission intensity of the optimal Li⁺-doped YNbTiO₆:Eu³⁺ was examined to be close to 400% of commercial Y₂O₃:Eu³⁺ phosphor. The mechanism of the enhanced emission by Li⁺ doping was discussed.     

Preparation and optical properties of Bi2S3 microcrystallite doped glass and thin film by the sol-gel process

The Bi₂S₃ microcrystallite doped thin films and glass lumps have been successfully prepared by the sol-gel process from the hydrolysis of a complex solution of Si(OC₂H₅)₄ Bi(NO₃)₃ · 5H₂O and SC(NH₂)₂, and the size of the microcrystallites in glass heated for different times at 400°C was decided by the method of HRTEM. The optical transmission valley shifted towards longer wavelengths with longer heat-treatment time at 500°C in the Bi₂S₃ doped thin films, showing the experimental evidence of quantum size effects. The red-shift of emission peaks in luminenscence spectrum excited with longer wavelength is attributed to the broad distribution of particle size in Bi₂S₃ doped glass.     

Structural,optical and electric properties of nanocrystalline MgSe thin films deposited by chemical route using triethanolamine as a complexing agent

Semiconducting nanocrystalline thin films of magnesium selenide have been prepared using economic chemical bath deposition technique onto glass substrates at room temperature. The deposition bath consists of magnesium chloride, triethanolamine, hydrazine hydrate and selenium dioxide. The quantity of triethanolamine in the deposition bath was varied to study its effect on growth process as well as on physical properties of MgSe. The deposited films were characterized using X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. The effect of complexing agent (TEA) on optical and electrical properties is reported. It was found that as the triethanolamine in deposition bath increases, optical band-gap and electrical resistivity decreases. The thermo-emf measurement shows p-type nature of MgSe.     

Preparation of nano-sized YAG:Eu3+ particles by a microwave-assisted polyol process and their luminescence properties

Phases of yttrium-aluminum binary oxide have been selectively prepared by selecting a polyol solvent in a microwave-assisted polyol process. Using 1,4-butanediol as a polyol solvent, nano-sized particles of a pure phase of YAG have been prepared. Furthermore, Eu³⁺-doped YAG has been synthesized by applying this method. We have demonstrated that 8 at% Eu³⁺-doped YAG (replacement of Y³⁺) gave the strongest emission when the content of Eu³⁺ was varied.     

Enhancement of the analytical properties and catalytic activity of a nickel hexacyanoferrate modified carbon ceramic electrode prepared by two-step sol-gel technique: application to amperometric detection of hydrazine and hydroxyl amine

The electroless sol-gel technique was used for the construction of nickel hexacyanoferrat (NiHCF) modified carbon composite electrodes (CCEs).This involves two steps: formation of a carbon ceramic electrode fabricated by nickel powder and then immersing the electrode into a sodium- hexacyanoferate solution for the immobilization of NiHCF films. The cyclic voltammety of the resulting modified CCEs prepared under optimum conditions, shows a well defined surface redox couple due to the [Ni^IIFe^III/II(CN)₆]^−2/−1 system. The effect of different alkali metal cations in supporting electrolyte on the behavior of the modified electrode were studied. The charge transfer coefficient (α) and charge transfer rate constant (k_s) for modified films were calculated. Hydrazine and hydroxylamine have been chosen as a model to elucidate the electocatalytic ability and analytical parameters of NiHCF modified CCE prepared by one and two-step sol-gel techniques and these compounds determined amperometically at the surface of modified electrodes. The latter shows a good electocatalytic activity towards the oxidation of hydrazine and hydroxylamine in the pH range 3-8 in comparison with CCEs modified by homogeneous mixture of graphite powder, Ni(NO₃)₂ and Na₂[Fe(CN)₆], (one-step sol-gel technique). Furthermore, the catalytic rate constant, linear dynamic range, limit of detection, and sensitivity for hydrazine and hydroxylamine detections were evaluated and compared with CCEs prepared with one-step sol-gel method. The modified CCEs containing NiHCF shows good repeatability, short response time, t 90%<3 s, long term stability (3 months) and excellent catalytic activity. Furthermore, the method of preparation is rapid and simple and the modified electrodes are renewed by simple mechanical polishing and immersing in [Na₃Fe(CN]₆] solution.     

The morphological,optical and electrical properties of SnO2:F thin films prepared by spray pyrolysis

Combining the spray pyrolysis and the sol–gel techniques gives the possibility to produce Fluorine doped Tin oxide (SnO₂:F) thin films. Transparent conducting SnO₂:F thin films have been deposited on glass substrates by the spray pyrolysis technique. This technique for the fabrication of SnO₂:F filmsby combining sol–gel process and the spray pyrolysis technique ispresented in this paper. The Sol–gel precursors have been successfully prepared using SnCl₂·5H₂O and (Ac)F₃. The structural, electrical, and optical properties of these films were investigated. The high resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD) patterns of SnO₂:F films show that the gel films lead to a tetragonal structure. The X‐ray diffraction pattern of the films deposited at substrate temperature 530° , the orientation of the films was predominantly [110]. In addition, the surface chemical components were also examined by X‐ray photoelectron spectroscopy (XPS) showing the SnO₂:F deposited with the atomic concentration ratios Sn/F 1.82:1. The minimum sheet resistance was 50 Ω and average transmission in the visible wavelength range of 300 to 800 nm was 87.25%. Copyright © 2008 John Wiley & Sons, Ltd.     

The Analyses of Luminescent Properties and Structure of the Tetragonal Phase of Y2WO6:Eu3+ doped Phosphor

The structural, electronic and luminescent properties of the tetragonal phase of Y₂WO₆ phosphors doped with different amounts of Eu³⁺ have been studied. All samples were synthesized via solid-state methods, using NaCl as mineralizer. Through Rietveld refinement was found that the Eu³⁺ cations only occupy the Y1 (2c) sites of the host-matrix. The thermal stability of the undoped sample was investigated using simultaneous TG-DSC thermal analyses. The SEM analysis revealed that the doped and the undoped samples have tendency to form microcrystals with smooth surfaces and in both cases it is possible to observe some agglomerates of nanoparticles. Periodic Density Functional Theory calculations (DFT) were carried out to investigate the electronic properties of the undoped material; these results show good agreement with the experimental ones. The emission spectra of all doped-samples are dominated by the electric dipole transition (ED) ⁵D₀ → ⁷F₂ located at 613 nm, which indicates that the Eu³⁺ ions are located in non-centrosimetric sites in the host-structure.     

Small amount of water induced preparation of several morphologies for InBO3:Eu3+ phosphor via a facile boric acid flux method and their luminescent properties

Four kinds of morphologies for InBO₃:Eu³⁺ phosphor have been prepared via a facile boric acid flux method only by adjusting the small amount of added water. The prepared samples have been characterized by XRD, FT-IR, and SEM. It was found that the size and morphology of the samples could be effectively controlled by adjusting reaction temperature, reaction time, especially the small amount of added water, which plays an extremely critical role in the controlling morphology. The possible growth mechanisms for microsphere and flower-like morphologies were further discussed on the basis of time-dependent experiments. Furthermore, the luminescence properties of prepared InBO₃:Eu³⁺ samples have been investigated by photoluminescence (PL) spectra. The results show that the InBO₃:Eu³⁺ phosphors show strong orange emissions under ultraviolet excitation at 237 nm. The monodisperse microsphere sample possesses the highest PL intensity among above four morphologies, which can be used as a potential orange luminescent material.