Showing papers on "Sol-gel published in 2007"

Graphene-silica composite thin films as transparent conductors.

Abstract: Transparent and electrically conductive composite silica films were fabricated on glass and hydrophilic SiOx/silicon substrates by incorporation of individual graphene oxide sheets into silica sols followed by spin-coating, chemical reduction, and thermal curing. The resulting films were characterized by SEM, AFM, TEM, low-angle X-ray reflectivity, XPS, UV−vis spectroscopy, and electrical conductivity measurements. The electrical conductivity of the films compared favorably to those of composite thin films of carbon nanotubes in silica.

Sol-gel with phase separation. Hierarchically porous materials optimized for high-performance liquid chromatography separations.

Abstract: Sol-gel processes for fabricating oxides or metalloxane polymers with controlled porous structures have been reviewed. Gel materials having controlled macropores are synthesized by polymerization-induced phase separation and concurrent sol-gel transition in a variety of chemical compositions. Several variations of tailoring mesopore structures within the macroporous materials are introduced, which enable one to design hierarchically porous metal oxide and metalloxane polymer materials. Applications of monolithic silica gels having hierarchical macro/mesoporous structure to the separation media of high-performance liquid chromatography, HPLC, are described.

Multiform oxide optical materials via the versatile pechini-type Sol-Gel process : Synthesis and characteristics

Abstract: This feature article highlights work from the authors' laboratories on the various kinds of oxide optical materials, mainly luminescence and pigment materials with different forms (powder, core−shell structures, thin film and patterning) prepared by the Pechini-type sol−gel (PSG) process. The PSG process, which uses the common metal salts (nitrates, acetates, chlorides, etc.) as precursors and citric acid (CA) as chelating ligands of metal ions and polyhydroxy alcohol (such as ethylene glycol or poly ethylene glycol) as a cross-linking agent to form a polymeric resin on molecular level, reduces segregation of particular metal ions and ensures compositional homogeneity. This process can overcome most of the difficulties and disadvantages that frequently occur in the alkoxides based sol−gel process. Using the PSG process, we are able to prepare luminescent powder materials that cannot be well synthesized by the solid-state reaction method, environmentally friendly and highly efficient phosphors that lack me...

Preparation and gas-sensing properties of Ce-doped ZnO thin-film sensors by dip-coating

Abstract: CeO 2 -doped ZnO thin-film gas sensors with different Ce/Zn ratios have been fabricated by dip-coating method, starting from zinc acetate dihydrate, cerium nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O) and anhydrous ethanol. Each layer was fired at 180 °C in a conventional oven for 30 min and the final coatings were sintered at 500 °C in a muffle furnace for 60 min. The microstructure and morphology of the films were characterized by XRD and FESEM, respectively. The resistance and sensitivities to volatile organic compounds were investigated on the static testing chamber. The X-ray diffraction (XRD) analysis of the films reveals the appearance of CeO 2 , tetravalent cerium dioxide whose valency is different from cerium nitrate hexahydrate. The results also show that as-prepared thin films with thickness of about 5 μm are polycrystalline with the structure of hexagonal wurtzite type. They consist of almost spherical particles with size ranging from 40 to 65 nm. Pure ZnO and Ce-doped ZnO thin-film sensors were prepared and tested for specific sensitivity to alcohol, acetone and benzene. It is observed that 1 at.% Ce–ZnO and 5 at.% Ce–ZnO are more sensitive to volatile organic compounds (VOCs), compared with other films with the different dopant concentration. The sensitivity of 5 at.% Ce–ZnO thin-film sensors to 100 ppm alcohol reaches 80 or so at 320 °C. 5 at.% Ce–ZnO thin-film sensors show good selectivity to alcohol, and thus can serve as alcohol-sensing sensors. A new physical model of the CeO 2 dopant influence on the gas-sensing properties of ZnO thin films is proposed. The addition of Ce to ZnO modified the particles size distribution, electrical conductivity, the catalytic activity and thus affected gas-sensing property to some extent.

Sol‐Gel Inks for Direct‐Write Assembly of Functional Oxides

Abstract: The ability to pattern oxide structures at the microscale in both planar and three-dimensional forms is important for a broad range of emerging applications, including sensors, micro-fuel cells and batteries, photocatalysts, solar arrays, and photonic bandgap (PBG) materials. Here, we report the fabrication of micro-periodic oxide structures by direct-write assembly of sol-gel inks. Specifically, we create both planar and three-dimensional (3D) architectures composed of submicron features, which are converted to the desired oxide phase upon calcination. Atomic force microscopy (AFM) and optical reflectivity measurements acquired on these micro-periodic structures reveal their high degree of structural uniformity. Several techniques have recently been introduced for patterning materials, including colloidal self-assembly, holographic lithography, and direct laser and ink writing approaches. Unfortunately, these approaches, apart from two notable exceptions, are confined to polymeric systems that lack the specific functionality required for a given application. As a consequence, the as-patterned structures require additional processing step(s) to produce the desired functional replicas. For example, 3D micro-fuel cells and photonic bandgap materials with inverse face centered cubic (fcc) structures have been templated from colloidal crystals, while silicon photonic crystals in both normal and inverse woodpile architectures have been templated from polymer structures produced by direct laser and ink writing, respectively. To circumvent the need for complicated templating schemes, we are developing a family of sol-gel inks that enable the direct ink writing (DIW) of functional oxides at the microscale. DIW is a layer-by-layer assembly technique, in which materials are fabricated in arbitrary planar and 3D forms with lateral dimensions that are two orders of magnitude lower than those achieved by ink-jet printing. Paramount to our approach is the creation of concentrated inks that can be extruded through fine deposition nozzles as filament(s), which then undergo rapid solidification to maintain their shape even as they span gaps in underlying layer(s). Unlike our prior efforts based on polyelectrolyte inks that require a reservoir-induced coagulation to enable 3D printing, these new inks can be directly printed in air providing exquisite control over the deposition process (e.g., the ink flow can now be started/stopped repeatedly during assembly). We first demonstrate this new ink design by creating a solgel precursor solution based on a chelated titanium alkoxide, titanium diisopropoxide bisacetylacetonate (TIA). TIA has an octahedral coordination of two isopropoxide and two acetylacetone (acac) groups about a central titanium (Ti) atom (Fig. 1a). This molecular structure is ideal, as it leads to the formation of soluble linear chains upon hydrolysis and condensation of the labile isopropoxide groups in the presence of a base catalyst. Subsequent slow hydrolysis of the acac groups leads to further gelation, which when coupled with solvent evaporation, results in the formation of a concentrated ink (Fig. 1b). An organic polymer, polyvinyl-pyrrolidone (PVP), is also incorporated to mitigate stresses that occur during drying and calcination of the as-patterned structures. We tailor the ink viscosity for DIW through micron-sized nozzles by regulating the solids content, defined as Ti + PVP concentration (wt %). The initial precursor solution contains 6.6 wt % solids and possesses a low viscosity ∼ 0.01 Pa·s, as shown in Figure 1c. However, upon concentrating the ink via solvent evaporation, a dramatic rise in viscosity is observed. For example, a nearly four-fold increase in ink concentration leads to a three orders of magnitude rise in its viscosity. Using an approximation of the Hagen–Poiseuille model, we estimate that the optimal ink viscosity ranges from 2.1– 4.2 Pa·s for the DIW conditions employed in this study (i.e., 1 lm deposition nozzle, 400 lm s deposition speed, and an applied pressure of 275–550 kPa). There is good agreement between the predicted values and those deemed optimal experimentally for microscale printing, as highlighted by the shaded region in Figure 1c. These inks not only flow readily, but are concentrated enough to rapidly solidify and maintain their cylindrical shape upon exiting the nozzle. The ink solidification mechanism can be understood by examining its elastic (G′) and viscous (G′′) moduli in the presence and absence of an ethanol solvent trap (Fig. 1d). The asprinted ink exhibits a liquid-like response (i.e., G′′> G′). HowC O M M U N IC A IO N

Photocatalytic Activity of Sol−Gel-Derived Nanocrystalline Titania

Abstract: Nanocrystalline titania (TiO2) powders have been synthesized via sol−gel, using an alkoxide precursor, under different processing conditions, and their photocatalytic activity has been investigated as a function of processing and material parameters through the decomposition of the methylene blue (MB) dye under exposure to the ultraviolet (UV) radiation (λ = 200−400 nm) in an aqueous solution. The nanocrystalline TiO2 powders with different morphology, crystallinity, average nanocrystallite size, surface area, and phase structure are obtained by controlling the ratio of molar concentrations of water and alkoxide (R) within the range of 5−60 and calcining the as-synthesized amorphous powders at higher temperatures (400−800 °C). The nanocrystalline TiO2 powders have been characterized using the scanning electron microscope (SEM), X-ray diffraction (XRD), and the Brunauer, Emmett, and Teller (BET) surface area measurement techniques while their photocatalytic activity was monitored using a UV−visible spectro...

Harnessing the sol-gel process for the assembly of non-silicate mesostructured oxide materials.

Abstract: Mesostructured non-silicate oxides, with well-defined organization on the 2–50 nm size scale, may play a pivotal role in advancing vital disciplines such as catalysis, energy conversion, and biotechnology. Herein, we present selected methodologies for utilizing the sol–gel process, in conjunction with organic-directed assembly, to synthesize a variety of mesostructured oxides. The nature of the inorganic precursor is critical for this process. We discuss the development of general routes for yielding stable, nanoscopic, hydrophilic, inorganic precursors compatible with organic co-assembly. In particular, we highlight the use and characterization of organic-acid-modified transition metal oxide sol–gel precursors that allow for the synthesis and processing of designer mesostructured oxides such as titania hybrids for optical applications and porous multicomponent metal oxides useful for catalysis.

Gold‐Nanoparticle‐Doped TiO2 Semiconductor Thin Films: Optical Characterization

Abstract: A simple procedure for creating titania sol-gel-based semiconductor thin films is described. Gold nanoparticles are doped homogeneously into the precursor mixture and the particles are homogeneously distributed in the resultant films when prepared using spin-coating. The effects of particle loading and annealing temperature on the optical properties of the resultant films are characterized. Ellipsometry, X-ray diffraction, atomic force microscopy, and surface plasmon spectroscopy are used to monitor the crystallization and porosity changes during film synthesis. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Fabrication of Hollow Hybrid Microspheres Coated with Silica/Titania via Sol−Gel Process and Enhanced Photocatalytic Activities

Abstract: In this paper, we reported a facile and effective approach for fabrication of polystyrene (PS)/silica/titania hybrid microspheres and their resultant hollow double shell hybrid spheres with a smooth and uniform shell of SiO2/TiO2. In this approach, the cationic polystyrene spheres were first synthesized by emulsifier-free emulsion polymerization using the cationic initiator. Subsequently, the PS/SiO2/TiO2 multilayer hybrid spheres were successfully formed by two-step sol−gel methods. The as-prepared hybrid spheres were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis, ζ potential measurement, and N2 adsorption/desorption isotherm analysis. The effects of the titanium tetrabutoxide content, surfactant content, and the midlayer of silica on the morphology, porosity and mean pore size, specific surface area, and thermal property of the obtained hybrid spheres were systematically evaluated...

Sol-gel modified Pechini method for obtaining nanocrystalline KRE(WO 4 ) 2 (RE = Gd and Yb)

Abstract: KRE(WO4)2 (RE = Gd and Yb) nanocrystalline powder was obtained by the modified sol-gel Pechini method. The precursor powder was calcined between 923 and 1023 K for a maximum of 6 h at air atmosphere. DTA-TG of the precursor powder shows that the temperature for total calcination is around 800–850 K. Molar ratio between the complexing agent and the metal ions in the first step of the method and molar ratio between the complexing agent and the ethylene glycol in the second step of the method were studied to optimize the preparation process. X-ray diffraction and IR spectroscopy were used to study the transformation from precursor powder into a crystalline monoclinic phase. Raman spectroscopy was used to study the vibrational structure of the nanoparticles. The Scherrer formula was used to confirm the grain sizes visualized by SEM and TEM techniques. Small nanoparticles in the range of 20–50 nm of monoclinic KREW have been successfully obtained by this methodology.

Preparation of Highly Monodispersed Hybrid Silica Spheres Using a One-Step Sol−Gel Reaction in Aqueous Solution

Abstract: The successful one-step preparation method of monodisperse hybrid silica particles was studied using organosilane chemicals in aqueous solution. In general, almost all of the hybrid silica materials were made by a complex method where organic materials were coated on the surface of silica substrate via chemical reaction. However, our novel method can be applied to prepare colloidal hybrid particles without using substrate material. This method has three advantages: (i) this simple method gives the opportunity to prepare hybrid particles with high monodispersity through the self-hydrolysis of various organosilane monomers in aqueous solution, (ii) this efficient method can be applied to load lots of organic functional groups on the surface of silica particles through a one-step preparation method using only organosilane, and (iii) this effective method can be used to control the particle size of the product by changing the experimental conditions such as the concentration of the precursor or the reaction temperature. Detailed characterization of the hybrid particles by scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis (TGA) was performed to elucidate the morphologies and properties of the hybrid silica particles.

Purification and stabilization of colloidal ZnO nanoparticles in methanol

Abstract: Purification and stabilization of colloidal ZnO nanoparticles synthesized from zinc acetate dihydrate and potassium hydroxide in methanol have been performed. Isopropanol and hexane were utilized to achieve the precipitation-redispersion washing procedure in methanol. The results from atomic absorption spectroscopy show that the concentration of K+ cation can be dramatically reduced by washing. X-ray diffraction and transmission electron microscopy results show that zinc layered double hydroxide formed in the ZnO precipitate can be effectively removed through concentrating the unpurified ZnO colloids by solvent evaporation. The purified ZnO nanoparticles can be fully redispersed in methanol, but become unstable with time due to the low concentration of acetate in the colloids. Fortunately, the unstable ZnO methanol colloids are found to become stable after addition of a small amount of hexane. The present study is of particular significance for the preparation of purified colloidal ZnO nanoparticles for device fabrication, functional ZnO coatings and polymer nanocomposite applications.

Role of Catalyst on Refractive Index Tunability of Porous Silica Antireflective Coatings by Sol-Gel Technique

Abstract: Porous silica coatings were prepared by acid- and base-catalyzed sol−gel method. Surface morphology studies by atomic force microscopy showed smooth surface for acid-catalyzed coatings while base catalysis resulted in coarse particle morphology. On increasing the tetraethyl orthosilicate to base molar ratio from 1:1 to 1:3, the mean particle size of the coating shifted from 30 to 100 nm while the pore size varied from 4.7 to 14 nm. Infrared spectral analysis exhibited a change in the ratio of integrated peak intensities of Si−O−Si to Si−OH in acid-and base-catalyzed silica coatings. Textural studies showed an increase in particle size and porosity with base concentration. Optical transmission and surface roughness of base-catalyzed samples were found to be higher than that of acid-catalyzed silica coatings.

A Sol–Gel Route To Synthesize Monolithic Zinc Oxide Aerogels

Abstract: The epoxide addition sol–gel process is a relatively new route to synthesize transition and main group metal oxide aerogels Zinc oxide monoliths were obtained by sol–gel processing of an alcoholic zinc nitrate solution with propylene oxide as the gelation initiator The alcogels were dried either by supercritical CO2 fluid extraction (aerogel) or by ambient temperature slow evaporation (xerogel) The resulting materials were characterized using powder X-ray diffraction (XRD), high-resolution scanning electron microscopy (HRSEM), nitrogen adsorption/desorption analysis, and photoluminescence (PL) Annealing of the aerogel at low temperatures (below 250 °C) yields a highly crystalline material which exhibits a significant increase in photoluminescence while retaining the inherent characteristics of the original aerogel, including high surface area (>100 m2/g) and porosity

Synthesis and structural characterization of P2O5-CaO-Na2O sol-gel materials

Abstract: Ternary phosphate-based glasses in the system P2O5–CaO–Na2O were synthesized using the sol–gel approach. Glasses in this system have the potential for use as bioactive materials. A mixture of mono- and dialkyl phosphate PO(OH)3−x(OC2H5)x (x = 1, 2) and alkoxides of sodium and calcium in an ethylene glycol solution were used as precursors. One of the compositions has also been synthesized by sonocatalysis (application of ultrasonic vibration to the sol). The systems synthesized, which remain fully amorphous even after calcination at 400 °C given the appropriate composition, have been characterized using X-ray diffraction (XRD). Thermal properties have been examined by means of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The structure of the phosphate network has been studied as a function of composition using Fourier transform infrared spectroscopy (FT-IR) and 31P MAS NMR.

Preparation of Mn-Zn ferrite nanoparticles from chemical sol-gel combustion method and the magnetic properties after sintering

Abstract: Nano-size Mn–Zn particles were synthesized by the sol–gel combustion method. These particles were prepared from metal nitrates and citric acid. The combustion process is an oxidation–reduction reaction in which the NO 3 – ion is oxidant and the carboxyl group is reductant. The thermal analysis of nitrate–citrate gels and the phase evolution of dried gel, as-burnt powder and sintered sample were investigated by DTA/TG and XRD techniques, respectively. SEM was also used to characterize the microstructure of the products. The effect of sintering temperature on the magnetic properties of the sample ferrite such as initial permeability ( μ i ) and hysteresis loop also investigated.

Physical and electrochemical characterization of nanocomposite membranes of Nafion and functionalized silicon oxide

Abstract: Nafion nanocomposite membranes were prepared from Nafion 117 and a systematic range of organically functionalized silicon alkoxide precursors using an in situ sol gel synthesis technique. The physical structure of the resulting nanocomposite membranes were characterized using small and wide-angle X-ray scattering, small angle neutron scattering, positron annihilation lifetime spectroscopy, and transmission electron microscopy. A structural model is proposed for three typical nanocomposite membranes (Nafion-TEOS, Nafion-MPTMS and Nafion-MPMDMS). The proton and methanol transport properties of the membranes included in the model were evaluated by impedance spectroscopy and pervaporation experiments, respectively, and correlated to their composite microstructure. In particular, this model explains the increased selectivity for transport over protons for nanocomposite membranes produced using (3-mercaptopropyl)methyldimethoxysilane as the silicon alkoxide precursor, which is more than six times higher than that of Nafion 117.