Showing papers on "Sol-gel published in 1999"

Sol−Gel Synthesis and Hydrothermal Processing of Anatase and Rutile Titania Nanocrystals

Abstract: The alkoxide sol−gel synthesis of nanostructured TiO2 has been studied systematically to examine the processing parameters that control crystallite size and phase. Nonagglomerated, ultrafine anatas...

Silica Sol as a Nanoglue: Flexible Synthesis of Composite Aerogels

Abstract: Low-density nanoscale mesoporous composites may be readily synthesized by adding a colloidal or dispersed solid to an about-to-gel silica sol. The silica sol can "glue" a range of chemically and physically diverse particles into the three-dimensional silica network formed upon gelation. If the composite gel is supercritically dried so as to maintain the high porosity of the wet gel, a composite aerogel is formed in which the nanoscopic surface and bulk properties of each component are retained in the solid composite. The volume fraction of the second solid can be varied above or below a percolation threshold to tune the transport properties of the composite aerogel and thereby design nanoscale materials for chemical, electronic, and optical applications.

Inhibition of Crystallite Growth in the Sol-Gel Synthesis of Nanocrystalline Metal Oxides

Abstract: Crystal growth upon firing of hydrous transition metal oxide gels can be effectively inhibited by replacing the surface hydroxyl group before firing with another functional group that does not condense and that can produce small, secondary-phase particles that restrict advancing of grain boundaries at elevated temperatures. Accordingly, fully crystallized SnO 2 , TiO 2 , and ZrO 2 materials with mean crystallite sizes of ∼20, 50, and 15 angstroms, respectively, were synthesized by replacing the hydroxyl group with methyl siloxyl before firing at 500°C. An ultrasensitive SnO 2 -based chemical sensor resulting from the microstructural miniaturization was demonstrated.

Low temperature synthesis of nanocrystalline Y3Al5O12 (YAG) and Ce-doped Y3Al5O12via different sol–gel methods

Abstract: Nanocrystalline yttrium aluminium garnet (YAG, Y3Al5O12) and Ce-doped YAG ceramics were synthesized by two ‘soft chemistry' sol–gel processes using (i) mixtures of inorganic salts or oxides and (ii) mixtures of alkoxides of the respective elements. In the first approach the metal ions, generated by dissolving metal oxides or nitrates in acetic acid and/or water, were complexed by ethylene glycol to obtain the precursors for pure and doped YAG samples. In the alkoxide route monolithic gels were obtained by hydrolysis and condensation of a compositional mixture of Al, Y and Ce alkoxides in a PriOH solution. The molecular level mixing and the tendency of partially hydrolysed alkoxide species to form extended networks of cross-linked metal centers facilitates the structure evolution thereby lowering the crystallization temperature, in the latter case. The X-ray diffraction (XRD) patterns of the ceramic sintered at 700 °C were identical with the stoichiometric YAG composition which is the lowest temperature reported for the synthesis of crystalline and single phase Y3Al5O12 while well developed YAG phases in the non-alkoxide synthesis were obtained only at 1000 °C. Cerium doped YAG powders (CeO2 + Y3Al5O12) were synthesized using [NH4]2[Ce(NO3)6] (8 mol%) and [Ce3(OBut)9(ButOH)2] (5 mol%), as dopants. A homogeneous distribution of cerium in the YAG lattice was achieved in both cases. The thermal behaviour, phase transformations, composition and microstructural features in the gels and polycrystalline samples were studied by TG/DTA, XRD, FT-IR, solid-state 27Al MAS NMR spectroscopy, SEM, TEM, energy dispersive X-ray analysis and high resolution electron microscopy studies. The quality of the resulting products (homogeneity, crystallization temperature, grain size, grain size distribution, etc.) and economical aspects (synthetic skill, cost of precursors, etc.) of the two approaches are discussed.

Preparation of Ag/SiO2 nanosize composites by a reverse micelle and sol-gel technique

Abstract: Spherical nanosize Ag/SiO2 composite particles have been synthesized within reverse micelles via metal alkoxide hydrolysis and condensation. The size of the particles and the thickness of the coating can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) within the microemulsion. Composite particles in the size range 20−35 nm are produced. As the molar ratio of water to surfactant is increased above 10, the size distribution broadens. Absorption spectra have been used to dynamically monitor the reaction and growth. The effects of other synthesis parameters, such as the molar ratio of water to TEOS and the amount of base catalyst, are discussed. Possible mechanisms for the formation of the nanocomposite particles are also discussed.

Microemulsion mediated sol-gel synthesis of nano-scaled MAl2O4 (M=Co, Ni, Cu) spinels from single-source heterobimetallic alkoxide precursors

Abstract: Nanocrystalline aluminate spinels with cobalt, nickel or copper as bivalent cations have been prepared in different particle sizes using single-source heterometal alkoxides of the type [M{Al(OR) 4 } 2 ] (M II =Co, Ni, Cu; R=Pr i , Bu t ) in a microemulsion assisted sol-gel process. The compatibility of the metal stoichiometry in the precursor molecules with respect to the spinel requirement was established by elemental, spectroscopic and single crystal X-ray diffraction analyses. Synthesis of CoAl 2 O 4 using both single- and multi-component routes gave different results: whereas the single-source precursor approach yielded monophasic high purity spinels, phase contamination was evident in the diffractograms of ceramics obtained using a mixture of individual components. The alkoxide precursors were hydrolysed in a homogeneous dispersion of colloidal water droplets in a non-aqueous phase. The diameter of water droplets in the microemulsion can be tuned, inter alia, by varying the hydrophilic chain length of the surfactant molecules which influences the size distribution of nanoparticles in the MAl 2 O 4 ceramics. This parameter is used to obtain nano-spinels with particle sizes in the range 5-45 nm and reveals a qualitative correlation between the initial droplet size and the crystallite size of the resulting spinels. The TG-DTA measurements showed the thermal decomposition to be a three step process with crystallisation of the spinel phase occurring around 900 °C. 27 Al MAS NMR spectra reveal the inverse nature of the obtained spinels. XRD, TEM, SEM, EDX, FTIR and UV-VIS spectral studies were used for characterisation of the powders.

Iron-doped titania semiconductor powders prepared by a sol-gel method. Part I : Synthesis and characterization

Abstract: Samples of iron-doped titania containing different amounts of iron (0.5–5 wt%) were prepared from TiCl4 and Fe(III) acetylacetonate by the sol–gel method. The specimens were characterized by XRD, TPD, specific surface area (BET) measurements, SEM-EDX, XPS, atomic absorption, IR and diffuse reflectance spectroscopies. From the structural point of view, these samples are different from those obtained by impregnation of TiO2 (P-25) with Fe(NO3)3·9H2O or Fe(acac)3, seeming to present anatase as the unique phase and a continuous distribution of iron not only on the particle surface but also within particles, together with an external surface titania rich layer. The doped oxides show suitable properties for photocatalytic purposes.

Preparation and Physisorption Characterization of d-Glucose-Templated Mesoporous Silica Sol−Gel Materials

Abstract: The synthesis of mesoporous silica materials has been achieved with d-glucose as a nonsurfactant pore-forming agent in the sol−gel reactions of tetraethyl orthosilicate under basic or near neutral ...

Direct Synthesis and Characterization of Gold and Other Noble Metal Nanodispersions in Sol−Gel-Derived Organically Modified Silicates

Abstract: A general approach to prepare nanodispersions of noble metals in organically modified silicates is presented with particular emphasis on the synthesis of gold nanodispersion and its characterization. Organically modified sol−gel monomers containing amine functional groups are used to stabilize the metal salts before the reduction step and to cap and prevent coagulation of the metal sol after reduction and through gel formation, drying, and aging. Uniform spherical metal nanodispersions of Au, Pt, and Pd with average diameter of 4−6 nm were obtained. The particle size distribution and the average size of silver nanoparticles prepared by the same method were considerably larger. This was attributed to the lower stability of the silver−amine complex and to the lower affinity of amines to silver surfaces. Stable aqueous colloidal solutions and supported metal nanodispersions in porous films and monoliths can be prepared by this route.

Bioactive nanocrystalline sol-gel hydroxyapatite coatings.

Abstract: Sol-gel technology offers an alternative technique for producing bioactive surfaces for improved bone attachment. Previous work indicated that monophasic hydroxyapatite coatings were difficult to produce. In the present work hydroxyapatite was synthesized using the sol-gel technique with alkoxide precursors and the solution was allowed to age up to seven days prior to coating. It was found that, similar to the wet-chemical method of hydroxyapatite powder synthesis, an aging time is required to produce a pure hydroxyapatite phase. A methodology that has been successfully used to produce nanocrystalline hydroxyapatite thin film coatings via the sol-gel route on various substrates including alumina, Vycor glass, partially stabilized zirconia, Ti–6Al–4V alloy and single crystal MgO is described. Coatings produced on MgO substrates were characterized by X-ray diffraction and atomic force microscopy, while the analogous gels were examined with thermogravimetric and differential thermal analyses. The coatings were crack free and the surface was covered with small grains, of approximately 200 nm in size for samples fired to 1000 °C. Coating thickness varied between 70 and 1000 nm depending on the number of applied layers.

Preparation of nanostructured tin oxide using a sol-gel process based on tin tetrachloride and ethylene glycol

Abstract: A sol-gel process starting with tin tetrachloride and ethylene glycol as precursors, has been successfully used to prepare nanostructured tin oxide powders. The molecular structure evolution during the process has been identified using infrared spectroscopy and the underlying reaction mechanisms of the sol-gel process are proposed. Results suggest that the -OHCH2CH2OH- prevent Cl− ions from access to tin ions due to steric effect and hence increase the stability of the sol solution. Ethylene glycol functions not only as a complexion agent to form a polymer network but also as a spacer to modulate the distance between metal ions, preventing metal oxide particles from aggregation during earlier stages of organics removal. Further, conversion of xerogel to tin oxide are studied using thermogravimetric analysis, X-ray diffraction, and electron microscopy. It is found that cassiterite begins to form at a temperature as low as 250 °C when organics start to burn off. However, nanocrystalline tin oxide powders are formed only after the chemically bonded hydroxyl groups are completely removed at about 600 °C.

Polyimide-silica composite materials: How does silica influence their microstructure and gas permeation properties?

Abstract: Composite polyimide-silica materials have been synthesized via the sol-gel process and their gas transport properties studied. Structural characterizations have been performed showing that materials prepared with large concentration of silicon alkoxyde are composites made of silica particles embedded in the polyimide matrix while low-silicon alkoxyde concentration induces homogeneous materials. X-ray diffraction shows that the presence of silicon species induces modifications in the microstructure of the polyimide chains. These modifications have been confirmed by a shift of the glass transition temperature and density variations. Influence of the temperature and silicon species on the gas transport have been studied using various gases (nitrogen, oxygen, carbon dioxide, and methane) showing that gas permeation coefficients increase with the silicon species proportion. CO{sub 2} sorption measurements have been performed at various temperatures and the results have been analyzed in terms of the dual sorption theory. Activation energies have been calculated for the permeation and sorption mechanisms. The results show that silicon species contributes to the overall permeability.

Small-angle x-ray scattering study of sol-gel-derived siloxane-peg and siloxane-ppg hybrid materials

Abstract: Hybrid organic−inorganic two-phase nanocomposites of siloxane−poly(ethylene glycol) (SiO3/2−PEG) and siloxane−poly(propylene glycol) (SiO3/2−PPG) have been obtained by the sol−gel process. In these composites, nanometric siloxane heterogeneities are embedded in a polymeric matrix with covalent bonds in the interfaces. The structure of these materials was investigated in samples with different molecular weights of the polymer using the small-angle X-ray scattering (SAXS) technique. The SAXS spectra exhibit a well-defined peak that was attributed to the existence of a strong spatial correlation of siloxane clusters. LiClO4-doped siloxane−PEG and siloxane−PPG hybrids, which exhibit good ionic conduction properties, have also been studied as a function of the lithium concentration [O]/[Li], O being the oxygens of ether type. SAXS results allowed us to establish a structural model for these materials for different basic compositions and a varying [Li] content. The conclusion is consistent with that deduced fro...

A catalyst-free approach for sol–gel synthesis of highly mixed ZrO2–SiO2 oxides

Abstract: The synthesis of mixed ZrO2–SiO2 oxides has been carried out using a new non-acid–base alkoxide route. The chemical reactivity of zirconium n-propoxide (ZP) is first modified with the chelating agent acetylacetone (acac) before it is brought in contact with tetraethoxysilane (TEOS). It is found that the gelation rate of bi-component sols is much faster than those of the mono-components under similar preparation conditions. A self-catalytic effect of the resultant Zr–O–Si on Si–O–Si formation is thus identified. By employing FTIR/DTA/TGA/XRD techniques, the mixing level of the two oxides in terms of Zr–O–Si hetero-linkages has been investigated in detail. A linear relationship between the IR absorption band shift of Zr–O–Si linkages and the Zr content is established to indicate the amount of oxide mixing. Phase segregation and thermal decomposition of xerogels upon calcination are studied and factors responsible for good mixing are also discussed. Using this approach, a high level of mixing has been achieved for the ZrO2–SiO2 binary oxides (Zr up to 50 mol%) with an acac to ZP molar ratio in the range 0.2–0.4 in the absence of an acid or base catalyst.

Highly Efficient Organic/Inorganic Hybrid Nonlinear Optic Materials via Sol−Gel Process: Synthesis, Optical Properties, and Photobleaching for Channel Waveguides

Abstract: The thermally stable second-order nonlinear optic (NLO) hybrid materials were successfully prepared by a sol−gel process using dye-attached sol−gel monomers (ormosils) of difunctionalized or trifunctionalized second-order NLO chromophores attached to silicon atoms. The dye-attached sol−gel monomers were synthesized by the coupling reaction of 3-isocyanatopropyltriethoxysilane with the corresponding functionalized NLO chromophores in DMAc. The dye-attached sol−gel monomers were homopolymerized and copolymerized with TEOS by hydrolysis and polycondensation in the presence of a slight excess amount of acidified water (pH 3), yielding the hybrid precusors (or a homogeneous solution). The hybrid precursors were spin-coated on various substrates such as ITO glass and NaCl disk. Hybrid films were further polycondensed by curing at 170 °C for 3 h. The cured hybrid materials existed in monohydroxy (T2, Q3) and nonhydroxy (T3, Q4) states, indicating the formation of a highly cross-linked silicon−oxygen polymeric ne...

Preparation of Porous Anatase Coating from Sol-Gel-Derived Titanium Dioxide and Titanium Dioxide-Silica by Water-Vapor Exposure

Abstract: The effects of water vapor on the crystallization behavior of sol-gel-derived titanium dioxide (TiO2) thin films that contained 0-50 mol% silica (SiO2) were investigated. Anatase formed on exposure to water vapor at 60°-180°C, with a simultaneous decrease in the concentration of OH groups. An increase in the SiO2 content of the exposed films led to an increase in the average crystalline size. Because crystallization of the exposed films of the films was not accompanied by shrinkage, porous anatase coatings were obtained via exposure at a relatively low temperature. Phase separation of the immiscible TiO2-SiO2 system was induced with water vapor, which resulted in acceleration of the crystallization of the sol-gel films.