Synthesis of Submicron-Sized Titania-Coated Silica Particles with a Sol-Gel Method and Their Application to Colloidal Photonic Crystals

A previously proposed method for preparing monodispersed titania particles was extended to preparation of titania-coated silica spherical particles. The core silica particles with an average size of 264 nm were prepared with Stöber method. The titania-coating was performed in ethanol/acetonitrile solvent in the presence of silica particles by a sol-gel method with the use of titanium tetraisopropoxide (TTIP) and methylamine (MA) catalyst. Average size of the silica-titania particles decreased from 457 to 292 nm with an increase in concentration of silica particles. Coefficient of variation for the particle size was less than 5%. Colloidal crystals could be fabricated with a dip-coating technique and a sedimentation process, respectively. Measurements of reflectance revealed peaks based on the Bragg diffraction. Those peaks red-shifted with an increase in titania shell thickness because of a high refractive index of titania. Annealing at high temperature transformed crystal structure of titania shell from amorphous to anatase (500°C) and rutile (1000°C), which led to red-shift of reflection peak because of an increase in refractive index of titania due to the crystallization.     

Preparation of titania-coated polystyrene particles in mixed solvents by ammonia catalysis

A mixed-solvent method was developed to coat polystyrene (PS) spheres with smooth, homogeneous shells of amorphous titania by ammonia catalysis. The TEM images showed that, in the presence of ammonia, the thickness of titania shells could be controlled in the range of 8-65 nm by varying the concentration of titanium tetrabutoxide (TBOT) in the ethanol/acetonitrile mixed solvents with an appropriate volume ratio. The diffusion-controlled mechanism of the mixed solvents and the catalysis mechanism of ammonia were investigated. After the calcination of core-shell particles for 2 h at 500 degrees C, spherical hollow titania shells could be obtained, and the surfaces of the particles remained quite smooth and homogeneous. The XRD analysis indicated that calcination promoted the transformation of amorphous titania into an anatase phase.     

Fabrication of hydroxyl group modified monodispersed hybrid silica particles and the h-SiO(2)/TiO(2) core/shell microspheres as high performance photocatalyst for dye degradation

The monodisperse hybrid silica particles (h-SiO(2)) were firstly prepared by a modified sol-gel process and the surface was modified in situ with double bonds, then abundant carboxyl moieties were introduced onto the surface of the silica core via thiol-ene click reaction. Afterward, the h-SiO(2)/TiO(2) core/shell microspheres were prepared by hydrolysis of titanium tetrabutoxide (TBOT) via sol-gel process in mixed ethanol/acetonitrile solvent, in which the activity of TBOT could be easily controlled. The carboxyl groups on the surface of silica particles promote the formation of a dense and smooth titania layer under well control, and the layer thickness of titania could be tuned from 12 to 100nm. The well-defined h-SiO(2)/TiO(2) core/shell structures have been confirmed by electron microscopy and X-ray photoelectron spectroscopy studies. After calcination at 500°C for 2h, the amorphous TiO(2) layer turned into anatase titania. These anatase titania-coated silica particles showed good photocatalytic performance in degradation of methyl orange aqueous solution under UV light.     

Simple route to monodispersed silica-titania core-shell photocatalysts

A monodispersed silica-titania core-shell photocatalyst was synthesized via a sol-gel route without the need of pH adjustment, cationic polyelectrolytes, or surfactants in a process where silica spheres were impregnated with hydrolyzed titanium tetrabutoxide, incubated at room temperature, and then condensed using an ethanol/water (1:1) solvent. Four coating cycles in a 10% v/v titania sol produced homogeneous titania shells. The quality of catalysts was assessed quantitatively using Rietveld analysis of powder X-ray diffraction patterns combined with X-ray fluorescence spectrometry. During calcination, the anatase-to-rutile transformation was delayed to 1000 degrees C, which is approximately 300 degrees C higher than usually observed. The thermal stability and surface area of titania were enhanced through the slow crystal growth of anatase. The photocatalytic activity of the core-shell photocatalysts calcined at 400-600 degrees C was found to be proportional to the thickness of titania but did not directly correlate with the surface area.     

Facile preparation of transparent monolithic titania gels utilizing a chelating ligand and mineral salts

Highly homogeneous transparent titania gels have been successfully prepared from titanium alkoxide by a sol–gel method utilizing chelating agent, ethyl acetylacetate (EtAcAc), in the presence of strong acid anions. Only catalytic amount of a strong acid anion suppress the rapid hydrolysis of titanium alkoxide by blocking the nucleophilic attack of HO⁻ and H₂O, and the resultant moderate sol–gel reactions thus afford homogeneous gelation, leading to transparent monolithic titania gels. Gelation time can be widely controlled by changing amounts of water, chelating agent and salt. The ability of salts to suppress the too abrupt sol–gel reactions is strongly dependent on the electronegativity of anions and valence of cations. With employing NH₄NO₃ as a suppressing electrolyte, the obtained titania gels can be converted to pure TiO₂ by simple washing and heat-treatment, and transformations to anatase and rutile structures were found to start at 400 and 600 °C, respectively.     

Synthesis of spherical polymer and titania photonic crystallites

The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 microm in diameter and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.     

Mechanisms and kinetics of noncatalytic ether reaction in supercritical water. 1. Proton-transferred fragmentation of diethyl ether to acetaldehyde in competition with hydrolysis

Noncatalytic reaction pathways and rates of diethyl ether in supercritical water are determined in a quartz capillary by observing the liquid- and gas-phase 1H and 13C NMR spectra. The reaction is investigated at two concentrations (0.1 and 0.5 M) in supercritical water at 400 degrees C and over a water-density range of 0.2-0.6 g/cm3, and in subcritical water at 300 and 350 degrees C. The neat reaction (in the absence of solvent) is also studied for comparison at 0.1 M and 400 degrees C. The ether is found to decompose through (i) the proton-transferred fragmentation to ethane and acetaldehyde and (ii) the hydrolysis to ethanol. Acetaldehyde from reaction (i) is consecutively subjected to the unimolecular and bimolecular redox reactions: (iii) the unimolecular proton-transferred decarbonylation forming methane and carbon monoxide, (iv) the bimolecular self-disproportionation producing ethanol and acetic acid, and (v) the bimolecular cross-disproportionation yielding ethanol and carbonic acid. Reactions (ii), (iv), and (v) proceed only in the presence of hot water. Ethanol is produced through the two types of disproportionations and the hydrolysis. The proton-transferred fragmentation is the characteristic reaction at high temperatures and is much more important than the hydrolysis at densities below 0.5 g/cm3. The proton-transferred fragmentation of ether and the decarbonylation of aldehyde are slightly suppressed by the presence of water. The hydrolysis is markedly accelerated by increasing the water density: the rate constant at 400 degrees C is 2.5 x 10(-7) s(-1) at 0.2 g/cm3 and 1.7 x 10(-5) s(-1) at 0.6 g/cm3. The hydrolysis becomes more important in the ether reaction than the proton-transferred fragmentation at 0.6 g/cm3. In subcritical water, the hydrolysis path is dominant at 300 degrees C (0.71 g/cm3), whereas it becomes less important at 350 degrees C (0.57 g/cm3). Acetic acid generated by the self-disproportionation autocatalyzes the hydrolysis at a higher concentration. Thus, the pathway preference can be controlled by the water density, reaction temperature, and initial concentration of diethyl ether.     

Preparation and properties of silicon- and titanium-containing hybrid nanocomposite films based on ethyl cellulose

Nanocomposite hybrid films containing silicon and titanium compounds in the polymer matrix are prepared through the sol-gel method via the hydrolytic polycondensation of Si and Ti alkoxides (tetraethoxysilane and titanium tetrabutoxide) in the THF solution of a hydrophobic polymer, ethyl cellulose. Their structure and properties are studied with the use of a complex of physicochemical methods. During the hydrolysis of tetraethoxysilane and the subsequent polycondensation of the reaction products, silicon atoms are incorporated into the polymer and form -O-Si-O-bonds involving hydroxyl groups of ethyl cellulose. In the sol-gel method, titanium alkoxide yields nanosized particles of titanium dioxide that play the role of fillers in the polymer matrix. Titanium-containing films show solubility in THF and, after prolonged contact with the solvent, precipitate titanium dioxide from the solution. Hybrid films containing silicon are insoluble owing to the formation of a chemical network between polymer molecules and Si-OH groups of the products of hydrolysis of silicon alkoxide, as confirmed by the IR data. It is shown that the amounts and types of alkoxides and the diameters of the structures formed in the polymer matrix via the sol-gel procedure affect the hydrophilicity levels of ethyl cellulose hybrid films and their abilities to swell in water and aqueous solutions of organic dyes (brilliant blue and methylene blue). Ethyl cellulose hybrid films are hydrophilic, and they facilitate the removal of dye molecules from aqueous solutions. The best properties are featured by the films containing nanosized particles of titanium dioxide in the polymer matrix.     

Preparation of sub-micrometer silica shells using poly(1-methylpyrrol-2-ylsquaraine)

Poly(1-methylpyrrol-2-ylsquaraine) precipitates from reaction solution as uniformly spherical particles with a diameter of 1.3 microm. Upon heating, the particles reduce in diameter until extinction at approximately 630 degrees C. Treatment of the particles with 9:1 tetraethoxysilane:ethanol solution, and subsequent hydrolysis in dilute acid, results in a polymer core-silica shell structure. Removal of the core, upon heating to 660 degrees C, results in an amorphous silica shell with a diameter half that of the initial template sphere. It has been found that the silica shells produced by this method are able to encapsulate organic dyes upon soaking of the shells in chloroform solutions of the dyes, and further washings with fresh chloroform did not remove the dyes. The production of crystalline titanium dioxide shells was also achieved through the use of the polysquaraine particles as a spherical template.     

Formation and characterization of high surface area thermally stabilized titania/silica composite materials via hydrolysis of titanium(IV) tetra-isopropoxide in sols of spherical silica particles

A direct synthetic route leading to titania particles dispersed on nonporous spherical silica particles has been investigated; 5, 10, and 20% (w/w) titania/silica sols mixtures were achieved via hydrolyzation of titanium tetra-isopropxide solution in the mother liquor of a freshly prepared sol of spherical silica particles (St?ber particles). Titania/silica materials were produced by subsequent drying and calcination of the xerogels so obtained for 3 h at 400 and 600 degrees C. The materials were investigated by means of thermal analyses (TGA and DSC), FT-IR, N(2) gas adsorption-desorption, powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). In spite of the low surface area (13.1 m(2)/g) of the pure spherical silica particles calcined at 400 degrees C, high surface area and mesoporous texture titania/silica materials were obtained (e.g., S(BET) ca. 293 m(2)/g for the 10% titania/silica calcined at 400 degrees C). Moreover, the materials were shown to be amorphous toward XRD up to 600 degrees C, while reasonable surface areas were preserved. It has been concluded that dispersion of titania particles onto the surface of the nonporous spherical silica particles increase their roughness, therefore leading to composite materials of less firm packing and mesoporosity.     

Monodisperse hollow titania nanospheres prepared using a cationic colloidal template

Titania-coated polymeric nanoparticles were synthesized based on the cationic colloidal particles which were prepared by surfactant-free emulsion copolymerization of styrene and butylacrylate in the presence of a cationic monomer, methacryloxyethyltrimethyl ammonium chloride (MOTAC) using azobis(isobutylamidine)hydrochloride (AIBA) as an initiator. These cationic particles were stabilized by poly(vinylpyrrolidone) (PVP). Then, these particles were dispersed in ethanol and mixed with titanium(IV) butoxide. Negatively charged titania precursors were rapidly hydrolyzed onto the cationic surfaces of colloidal particles. Subsequently, the samples were heated to 450 degrees C to form anatase TiO2 and to remove the colloidal template, which resulted in hollow nanospheres. The hollow titania particles were characterized with zeta analyzer, transmission electron microscopy, scanning electron microscopy, light scattering, X-ray diffraction, thermogravimetric analysis and FT-IR.     

Surface modification of polyaniline using tetraethyl orthosilicate

A well-dispersible conducting polyaniline/silica hybrid is prepared by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) on the surface of polyaniline in water/ethanol solution. It provides a simple and environmentally sound route for preparing the processable conducting polyaniline/silica hybrid at the nanometre level. The conductivity of polyaniline/silica hybrid is 2.43 S cm(-1) at 25 degrees C, and its powder is easily dispersed in the anhydrous ethanol or aqueous solution without any stabilizer. In addition, the structure, morphology and cyclic voltammorgram of this hybrid are also reported.     

The Effect of Alcohol Solvents on the Porosity and Phase Composition of Titania

Bimodally porous titania powders were made by hydrolysis of titanium tetraisopropoxide (TTIP) dissolved in various alcohols (methanol, ethanol, isopropanol, and sec-butanol). The specific surface area (SSA) of the powders dried at 150 degrees C ranged from 332 to 624 m(2)/g as determined by nitrogen adsorption. At excess alcohol concentration, the SSA of the dried powders decreased in the order of sec-butanol, iso-propanol, ethanol, and methanol at a constant alcohol/TTIP molar ratio. The pore size distribution was bimodal with fine intraparticle pore diameters at 1-6 nm and larger interparticle pore diameters at 30-120 nm as determined by nitrogen adsorption isotherms. The average intraparticle pore diameter decreased with increasing alcohol concentration for methanol and ethanol, while it was rather constant at 3.3 nm, irrespective of alcohol concentration for iso-propanol and sec-butanol. The evolution of particle phase composition was determined by X-ray diffraction ranging from amorphous to crystalline anatase and rutile largely proportional to the calcination temperature and to a lesser extent on the type and concentration of alcohols. Copyright 2000 Academic Press.     

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.     

Preparation of highly photocatalytic active nano-sized TiO2 particles via ultrasonic irradiation

A novel method for preparing highly photoactive nano-sized TiO2 photocatalysts with anatase and brookite phases has been developed by hydrolysis of titanium tetraisoproproxide in pure water or a 1:1 EtOH-H2O solution under ultrasonic irradiation; the photocatalytic activity of TiO2 particles prepared by this method exceeded that of Degussa P25.     

低温下溶胶凝胶法制备TiO2纳米晶

从含过量水的溶胶出发,在室温下得到了TiO₂纳米晶。通过红外光谱,透射电子显微镜法和X射线粉末衍射法对含有过量水的溶胶体系中TiO₂纳米晶的室温形成机理进行了研究。与传统的溶胶凝胶法相比,在改良的溶胶凝胶体系中,在缩聚反应之前由于水过量使得钛的先驱体快速且充分的水解,从而生成[TiO₆]基团,随之形成TiO₂纳米晶。晶粒的尺寸为约3.5 nm,该法得到的TiO₂纳米晶比传统溶胶凝胶法得到的TiO₂纳米晶和商用光催化剂德固赛P25具有更好的光催化活性。     

TiO<Subscript>2</Subscript>-based nanopowders obtained from different Ti-alkoxides

Pure TiO₂ and S-doped TiO₂ sol–gel nanopowders were prepared by controlled hydrolysis-condensation of titanium alkoxides. The influence of different Ti-alkoxides (tetraethyl-, tetraisopropyl- and tetrabutyl-orthotitanate) used in obtaining TiO₂ porous materials in similar conditions (water/alkoxide ratio, solvent/alkoxide ratio, pH and temperature of reaction) has been investigated. The relationship between the synthesis conditions and the properties of titania nanosized powders, such as thermal stability, phase composition, crystallinity, morphology and size of particles, BET surface area and the influence of dopant was investigated. The nature of the alkyl group strongly influences the main characteristics of the obtained oxide powders, fact which is pointed out by thermal analysis, X-ray diffraction, TEM and BET surface area measurements.     

Two step, hydrolytic-solvothermal synthesis of redispersible titania nanocrystals and their gas-sensing properties

A titanium chloromethoxide solution was prepared by reacting TiCl₄ with methanol, followed by water addition. The starting solutions were characterized by Fourier Transform Infrared (FTIR) spectroscopy, evidencing that the in situ generated water results in early hydrolysis of the chloroalkoxide. The solution was reacted with molten dodecylamine at room temperature, obtaining a white slurry of amorphous titania nanoparticles. Stable, redispersible TiO₂ nanocrystals could be prepared by subsequent solvothermal treatment in oleic acid at 250???C. The use of oleic acid was essential for obtaining crystalline structures, while other surfactants prevented crystallization. The nanocrystals were characterized by X-ray Diffraction and Transmission Electron Microscopy, confirming the formation of anatase TiO₂ nanocrystals with a mean size of 3.3?nm. The TiO₂ nanocrystals were used for fabricating gas-sensing devices, which were tested towards ethanol vapors. The initial small size of the nanocrystals, and the limited size growth during the high-temperature sensor operation, result in remarkable sensing performances if compared with bulk titania sensors.     

Synthesis and photoluminescence of titania nanoparticle arrays templated by block-copolymer thin films.

High-density arrays of titania nanoparticles were prepared using a polystyrene-b-poly(ethylene oxide) block copolymer (PS-b-PEO) as a template and a titanium tetraisopropoxide based sol-gel precursor as titania source via a spin-coating method. The hydrophilic titania sol-gel precursor was selectively incorporated into hydrophilic PEO domains of PS-b-PEO and form titania nanoparticle arrays, due to a microphase separation between the PS block and the sol-gel/PEO phase. Field emission scanning electron microscopy (FESEM) and scanning probe microscopy (SPM) images showed that the uniformity and long-range order of the titania/PEO domains improved with increasing sol-gel precursor amount. Grazing incidence small-angle X-ray scattering (GISAXS) results indicate that the ordered structures exist over large length scales. Titania nanocrystal arrays of anatase modification were obtained by calcination at 600 degrees C for 4 h. After calcination, separated particles were observed for low and medium amounts of sol-gel precursors. Films with higher precursor amounts showed wormlike structures due to the aggregation between neighboring particles. Removal of the polymer matrix via UV treatment leads to highly ordered arrays of amorphous titania while retaining the domain size and interparticle distance initially present in the hybrid films. Photoluminescence (PL) properties were investigated for samples before and after calcination. The PL intensity increases with the increasing amount of sol-gel precursor. Bands at 412 nm were ascribed to self-trapped exitons and bands at 461 and 502 nm to oxygen vacancies, respectively. Uncalcined or UV-treated samples also showed PL properties similar to calcined samples, indicating that the local environment of the titanium atoms is similar to the environment of the crystalline anatase modification.     

Crystallization behaviors of nanosized MgO particles from magnesium alkoxides

The effects of the size of the alkoxy group on the thermal decomposition behavior of magnesium alkoxides (magnesium methoxide and ethoxide) and the crystallization behavior of MgO was investigated using thermogravimetry, Fourier-transformed infrared spectroscopy, X-ray powder diffraction, and transmission electron microscopy. As the size of the alkyl group increased, the decomposition temperature decreased and resultant MgO crystallization of the alkoxide precursor was enhanced. In an inert N(2) atmosphere, the decomposition temperature of magnesium ethoxide was about 260 degrees C, which was lower than that of magnesium methoxide by approximately 70 degrees C. The degree of the crystallization of MgO particles from the ethoxide was also significantly higher than that of the methoxide. This result is explained in terms of the O-R bonding strength of the alkoxide. With use of the Kissinger method, the activation energy for the thermal decomposition of magnesium alkoxide was found to be dependent on the size of the alkyl group. The activation energies were 161+/-23 and 130+/-24 kJ/mol for the magnesium methoxide and the magnesium ethoxide, respectively.