Preparation and Characterization of HgxCd1-xS and PbxCd1-xS Quantum Dots and Doped Thin Films

Thin silica and silica-titania films doped with sulfide nanocrystals of controlled size were fabricated by a method based on the preparation of colloidal particles and their introduction into a glassy matrix through the sol-gel method. Colloidal sols of composition Hg_xCd_1-xS and Pb_xCd_1-xS (with _x ranging from 0 to 1) were prepared and used to dope alkoxide solutions for the deposition of thin silica and silica-titania films. Optical absorption spectra were taken on both precursor colloidal sols and derived doped films. X-ray diffraction characterization gave structural information on the nature and size of particles in powders obtained by precipitation from colloidal sols and in doped films. The advantages and limits of the investigated systems are discussed in light of possible applications. The nonlinear properties of the most interesting PbS-doped planar waveguides have been investigated in the near-infrared, at 1.064 m. A reversible nonlinear effect was measured, with n ₂ values ranging from -5 to -20×10^-9 cm²/kW.     

Microporous Silica Xerogel Membrane with High Selectivity and High Permeance for Carbon Dioxide Separation

The asymmetric microporous silica xerogel membrane was prepared by sol-gel method. A multi-layer (asymmetric) structure was obtained by silica xerogel layer on top of mesoporous silica layer (mean pore diameter of 4 nm), supported by porous alumina tube (mean pore diameter of 0.1 m). The ratio of the permeances, CO₂/N₂ for this membrane attained more than 60 at 298 K. This value is considerably higher than the theoretical Knudsen value (CO₂/N₂ = 0.8). The gas permeances of this membrane were about 5–10 times higher than the previous membrane. Application of the membrane to an enrichment of CO₂ from air was also investigated. CO₂ in the air (about 300 ppm) was concentrated to more than 1000 ppm by using the membrane at 298 K.     

Ultra-low expansion glass from gels

Pyrolysis of mixed titanium and silicon metal halides produces a commercial glass (7.4% TiO₂) with ultra-low thermal expansion that is essentially zero over the temperature range of 0 to 300°C. A colloidal particulate gel process involving potassium silicate, titania sol and formamide gel reagent was found to produce glass compositions with similar low expansion behavior. Due to the strongly basic nature of the precursor solutions, special titania sols had to be prepared that were stable in these alkali silicate solutions. The preferred TiO₂ sols were those containing quaternary ammonium stabilizing counter-ions. These sols served not only as the source of homogeneously distributed titania, but they may also serve as nucleating species that contribute to particle growth and pore size control of the gel network. The large pore (0.3 µm) TiO₂/SiO₂ gel structures were easily dealkalized, dried and sintered to uncracked glass shapes. Plates up to 9.5 cm×6.6 cm× 0.5 cm thick and some intricate cast shapes were produced and their glass properties evaluated.     

Catalytic and Physicochemical Properties of Oxidative Condensation Products in the Oxidative Dehydrogenation of Propane by Sulfur Dioxide on SiO2

The effect of the deposition of oxidative condensation products in the reaction of oxidative propane dehydrogenation in the presence of SO₂ on the catalytic, acid–base, and texture characteristics of silica was studied. It was found that the oxidative condensation products exhibited high catalytic activity in this reaction. The carbonization of silica from 0 to 40 wt % was accompanied by an increase in the yield of propylene from 3.4 to 46 mol % (640°C; a C₃H₈/SO₂/He + N₂ mixture, 10 : 10 : 80 vol %). Further accumulation of condensation products resulted in a considerable decrease in the pore volume and radius; this imposed diffusion limitations on both propane conversion and selectivity to propane conversion products. The nature of active and deactivated condensation products was studied by DRIFT spectroscopy, diffuse-reflectance UV–VIS spectroscopy, EPR spectroscopy, XPS, thermal analysis, and electron microscopy.     

Preparation and Structure of Microporous Silica Membranes

Silica sols have been prepared in an alcoholic solution by hydrolysis and condensation of TEOS (tetra-ethyl-ortho-silicate) molecules as a function of water and nitric acid concentration. The polymers are intended as precursors for ceramic, gas separation membranes. These molecules show fractal behavior as determined by SAXS (Small Angle X-ray Scattering). Microporosity of dried and calcined silica polymers is determined by N₂-adsorption at 77 K. Fractal dimension and porosity increase with increasing acid concentration. Both the sol structure and the drying kinetics determine the porosity values. N₂-adsorption isotherms are not very suitable for the determination of pore size distributions of microporous silica.     

Effect of Method Used for Synthesis of Silica Sols on Pore Characteristics of Silica Gels on Their Base

Problems associated with synthesis of silica gels with prescribed pore characteristics from stabilized silica sols with controlled colloid particle size, obtained in the metastable solubility region of the system SiO₂-H₂O (8.5-9.5), are considered.     

Influence of polyvinylpyrrolidone (PVP) on alumina sols prepared by a modified Yoldas procedure

Al₁₃ polycations containing alumina sols, prepared by a modified Yoldas procedure were mixed with polyvinylpyrrolidone (PVP). Although Al speciation in freshly prepared sols was not affected by PVP addition the decay rate of Al₁₃ polycations was slightly decreased in PVP-containing sols. PVP does not show any influence on particle size and particle growth. The influence of PVP addition on viscosity and flow behavior of modified Yoldas sols depends on their solids content and molar ratio.     

Bis(triethoxysilyl)ethane (BTESE)-derived silica membranes: pore formation mechanism and gas permeation properties

Based on their high performance in gas and liquid-phase separations, 1,2-bis(triethoxysilyl)ethane (BTESE)-derived organosilica membranes have attracted much attention. To improve performance, we focused on the acid molar ratio (AR) in sol preparation and its effect on the pore formation mechanism during sol-gel processing. BTESE-derived sols with AR?=?10^?4–10⁰ were prepared, and the effect of the AR on the gel structure was evaluated in detail via FT-IR, nuclear magnetic resonance (NMR), N₂ adsorption, and positron annihilation lifetime (PAL) measurements. The chemical structure of the gels was confirmed by FT-IR and NMR and showed that sols with the largest number of silanol groups (AR?=?10^?2) experienced a significant increase in condensation during the firing process. The porous structures of fired gels characterized by N₂ adsorption and PAL measurement showed that the AR?=?10^?2 fired gel consisted of a larger number of small pores that had formed during the firing process. Single-gas permeation experiments showed high H₂ permeance (5–9?×?10^?7?mol/(m²?Pa?s)) and H₂/CF₄ selectivity (700–20,000). The gas permselectivity (He/H₂, H₂/N₂, and H₂/CF₄) was highest for the intermediate AR (=10^?2), which corresponded to the greatest amount of silanol groups in unfired gels and confirmed that small pores had formed from the condensation of silanol groups during firing.

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Deposition of Titania Nanoparticles on Spherical Silica

Titania coated silica nanoparticles were prepared through a sol-gel process using peptized TiO₂ nano-sols. The TiO₂ sols were obtained by peptization, the process of redispersing a coagulated colloid, and were coated on SiO₂ particles by the control of the weight ratio of TiO₂/SiO₂ and the pH of the mixture in aqueous solution. At pH 4.5 the difference of zeta-potential between SiO₂ and TiO₂ maximized and then the TiO₂-coated SiO₂ particles with highest TiO₂ contents (20%) were obtained without the self-aggregation of TiO₂ sols. The morphologies of particles were characterized with field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) and the isoelectric points (IEP) of particles were measured by zeta potential. The nature of exposed titanium ions on the surface of titania-coated silica nanoparticles was elucidated by X-ray photoelectron spectroscopy (XPS).     

Membrane Emulsification Using Sol-Gel Derived Macroporous Silica Glass

A macroporous silica glass membrane with continuous cylindrical pores was prepared by a sol-gel process using phase separation. The applicability of the sol-gel derived macroporous silica to the membrane emulsification process was evaluated in comparison with a conventional SPG (Silasu Porous Glass) membrane.Aqueous colloidal silica in one side of the membrane was emulsified through the pores under an applied pressure to a toluene bath containing surfactant. With the sol-gel and SPG membranes with respective median pore diameters of 0.6 and 1.0 m, emulsions with almost the same droplet size centered around 3 m were obtained. The permeation rate of the sol-gel derived membrane was about 1.6 times faster than SPG, which reflected higher pore volume of the former one.     

Sol-Gel Processing and Luminescent Properties of Rare-Earth Oxyfluoride Materials

Luminescent rare-earth oxyfluoride materials were prepared by a sol-gel method using trifluoroacetic acid as a fluorine source. Crystalline (La, Eu)OF powders and thin films were obtained by heating gels at typically 600–800°C. Transparent SiO₂-LaOF glass-ceramic thin films were also prepared by mixing the trifluoroacetate sols and silica sols, spin-coating on silica glass substrates and heating at temperatures 600–900°C. Eu³⁺ ions doped in the oxyfluoride materials exhibited a strong red ⁵D₀ ⁷F₂ emission (611 nm) by a charge-transfer (O^2–-Eu³⁺) excitation with ultra-violet radiation (265 nm). It was strongly suggested that the Eu³⁺ activators were preferentially incorporated into the crystalline LaOF phase in the SiO₂-LaOF glass-ceramics.     

Formation of TiO2 Sols,Gels and Nanopowders from Hydrolysis of Ti(OiPr)4 in AOT Reverse Micelles

Titania sols, gels and nanopowders have been produced by the controlled hydrolysis of tetraisopropyltitanate (TPT) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Particle formation and aggregation have been investigated by photon correlation spectroscopy, the crystal phases by FT-Raman spectroscopy, and the crystallite dimensions of the precipitates by transmission electron microscopy. Nanoparticles could be produced at relatively high Ti(IV) concentrations (up to 0.05 mol dm^–3). These nanoparticles aggregated into sols, with colloid sizes of 20–300 nm, eventually forming gelatinous precipitates. The kinetics of particle formation and aggregation were controlled by varying the primary process parameters [TPT], [H₂O]/[AOT] (w₀), and [H₂O]/[Ti(IV)] (R), yielding a range of products including stable, transparent sols, precipitates and monolithic gels. The aggregation kinetics and physical properties of the sols depended strongly on w₀. Different titania phases were produced, depending on w₀; w₀ 6 yielded amorphous particles, while w₀ 10 produced anatase. The dimensions of the crystallites were comparable to those of the parent reverse micelles. A model was developed to interpret the effect of the primary process parameters on colloidal stability: (1) nucleation to form primary crystallites occurs by rapid hydrolysis and condensation reactions within the reverse micelle and (2) subsequent colloidal growth by aggregation occurs by reverse micellar exchange, where the rate of growth is governed by electrostatic and steric stability factors which increase as [AOT]/[TPT] (S) and residual [H₂O]/[AOT] (w_r) increase.     

The Effect of Nanoscaled Metal Oxide Sols on the Structure and Properties of Glycidoxypropyltrimethoxysilane Derived Sols and Gels

Glycidoxypropyltrimethoxysilane (GPTS) is frequently used as precursor for the preparation of sol-gel derived nanoscaled hybrid polymers. The influence of nanoscaled metal oxide sols of silica, boehmite, zirconia and ceria on reactions of GPTS in ethanolic hydrolysates and in corresponding gels (epoxide ring-opening, condensation degree) was examined by liquid- and solid-state ¹³C and ²⁹Si NMR with regard to a better correlation between structure and material properties. Generally, a higher condensation degree of RSi(O_0.5)₃ units of GPTS is found after addition of metal oxide sols compared to GPTS without additives. The metal oxide sols (10 mole% series) cause an epoxide ring-opening up to 20% in GPTS hydrolysates after 24 h. A nearly complete ring opening was found in the boehmite and silica containing hybrid gels whereas gels containing ceria and other types of silica only show a low degree of ring-opening. The results show an accelerated ring-opening with increasing content of AlO/OH-species in silica sols. ¹³C NMR studies reveal that the epoxide ring-opening does not completely lead to polyether structures but to considerable amounts (up to 40%) of ethylether groups which can influence the material properties (hardness).     

Preparation and microstructure of sol-gel derived silver-doped silica

Silver-doped silica was prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄) in the presence of a silver nitrate (AgNO₃) solution by two different synthesis methods. In the first synthesis route, sol-gel mixtures were prepared using an acid catalyst. In the second synthesis route, silver-doped silica gels were formed by two-step acid/base catalysis. For the same concentration of silver dopant [AgNO₃]/[TEOS] = 0.015 acid-catalyzed sol-gel formed a microporous silica with an average pore size of <25 Å whereas the two-step catalyzed silica had an average pore size of 250 Å and exhibited a mesoporous structure when fully dried. The differences in the pore size affected the silver particle formation mechanism and post-calcination silver particle size. After calcination at 800 °C for 2 h the acid-catalyzed silica contained metallic silver particles size with an average particle size of 24 ± 2 nm whereas two-step catalyzed silica with the same concentration of [AgNO₃]/[TEOS] = 0.015 contained silver nanoparticles with an average size of approximately 32 ± 2 nm. Mechanisms for silver particle formation and for silica matrix crystallization with respect to the processing route and calcination temperature are discussed.     

CVD-Titania on Mesoporous Silica Gels

Different amounts of CVD-titania ( from 2.3 to 19.2 wt%) with amorphous and crystalline (anatase) phases were synthesized on silica gels (Kieselgel 40, 60, and 100) and characterized by means of XRD, IR, DTG, and adsorption methods. The amounts of titania depend strongly on the pore size distribution of the support, as the narrower the pores, the lower the deposit concentration due to diminution of the accessibility of narrower pores and deceleration of titania grafting in them. A portion of CVD-titania filling matrix pores is rather amorphous than that forming on the outer (external) surfaces of silica gel grains, as anatase crystallites have the average size of 70 nm for KG 40/TiO₂ ( = 6.5 wt% including 26% of anatase and 74% of amorphous titania), 21 nm (KG 60/TiO₂ at = 11 wt%, 16% anatase) and 16 nm (KG 100/TiO₂ at = 19.2 wt%, 29% anatase), which are larger than the average pore size of the silica gels. The crystallite size decreases with increasing average pore diameter.     

Preparation of Silica Microspheres Containing Ag Nanoparticles

Two sol-gel fabrication processes were investigated to make silica spheres containing Ag nanoparticles: (1) a modified Stöber method for silica spheres below 1 m size, and (2) a SiO₂-film formation method on spheres of 3–;7 m size. The spheres were designed to incorporate silver nanoparticles of high ⁽³⁾ in a spherical optical cavity structure for the resonance effect. For the incorporation, interaction between [Ag(NH₃)₂]⁺ ion and Si-OH was important. In the Stöber method, the size of the silica spheres was determined by a charge balance of plus and minus ions on the silica surface. In the film formation method, the capture of Ag complex ion on the silica surface depended on whether the surface was covered with OH groups or not. After doping [Ag(NH₃)₂]⁺ into silica particles or SiO₂ films on the spheres, these ions w ere reduced by NaBH₄ to form silver nanoparticles. From plasma absorption at around 420 nm wavelength and TEM photographs of nanometer-sized silver particles, their formation inside the spherical cavity structures was confirmed.     

Preparation of octadecyl modified column gels using heat-treated silicas and their retention behaviour in high-performance liquid chromatography

Summary The effect of silanol groups on three types of octadecylmodified column gels using heat-treated silicas by calcination has been studied by high-performance liquid chromatography. After heat-treating at 180°C, 500°C and 950°C, the silicas treated with octadecyldimethylchlorosilane were used for the measurement of physical and chemical analysis. From elemental carbon analysis data, the reactive silanol group concentrations, _OH(s), were determined to be 2.0 in the 180°C treated silica, 2.1 in the 500°C treated silica and 1.6 in the 950°C treated silica, respectively (original silica: mean pore diameter 116 Å, specific surface area 298 m²/g, pore volume 1.22 ml/g, particle size 5.0 m). The separation factors, , of pyridine versus phenol were measured to be 0.79 on 180°C treated silica, 0.91 on 500°C treated silica and 1.98 on 950°C treated silica, using acetonitrile-water mixtures as the eluent. And then, on the basis of the physico-chemical and chromatographic data, the three types of octadecyl modified column gels using heat-treated silicas by calcination have been compared.     

Evaluation of hybrid silica sols for stable microporous membranes using high-throughput screening

Microporous membranes are a promising option for energy-efficient molecular separations. Long-term hydrothermal stability of the membrane material is of prime importance for several industrial processes. Here, a short overview of silica-based membrane materials and their hydrothermal stability is presented. Following this, the development of a series of organic–inorganic hybrid silica sols is described, based on α,ω-bis(triethoxysilyl)-precursors with bridging methane, ethane, propane, and benzene groups. High-throughput screening was used to scan a range of sol parameters, followed by membrane preparation from the most promising sols. These organic–inorganic hybrid silica (HybSi^®) membranes were used in dewatering of lower alcohols by pervaporation. Separation factors up to 200 were found for ethanol/water mixtures, and up to 23 for methanol/water mixtures. Modest permselectivity values for hydrogen over nitrogen were found, ranging up to 20.7 for the shortest bridging group. It was concluded that the length of the organic bridge has a clear effect on the pore size distribution and the selectivity of the membrane.     

Surface silylation and pore structure development of silica aerogel composites from colloid and TEOS-based precursor

Flexible aerogel-fiber composites were prepared by silylation and ambient drying of colloidal silica and tetraethylorthosilicate (TEOS)-based sol. After immersing glass fiber matrices into silica sol with colloid-based, colloid/TEOS-based, and TEOS-based silica sol, it was surface-modified in a trimethylchlorosilane/n-hexane solution and heat-treated at 230 °C in ambient atmosphere. Surface silylation of silica aerogel synthesized from colloid and TEOS-based silica sols showed different behaviors. For colloid silica gel, it was comprised of small sized mesopores because colloid-based silica gel has dense networks through great degrees of hydrolysis and condensation. On the contrary, TEOS-based aerogel was consisted of relatively large-sized pores because of comparatively lesser degree of hydrolysis and condensation. Through this study, we can know that the pore structures of silica aerogel could be controlled by choosing colloid or TEOS-based precursor and surface silylation reaction.     

Effect of non-ionic surface active agents on TEOS-derived sols,gels and materials

The sol-gel process, starting from tetraethylorthosilicate precursor, is a suitable technique for the preparation of silica thin films. The use of specific organic additives, like non ionic surface-active agents, drastically modifies the gelation process and allows the preparation of microporous materials with a high microporous volume. The effects of additives on the sol, gel and material characteristics have been investigated by several methods such as ²⁹Si NMR, QELS, SAXS (for sols and gels), and N₂ adsorption, FESEM (for fired materials). It appears that the interactions of surface active agents with TEOS derived species limit condensation reactions and particle growing. A brittle gel structure is generated which leads to highly porous microporous silica after the elimination of organic chains by thermal treatment at 450°C. The material porous texture (specific surface area, pore size distribution and porous volume) can be varied especially by varying the surface active agent chain length and quantity. This kind of sol-gel system is suitable to prepare microporous silica membranes candidate for gas separation or catalytic reactor applications.