Sol-gel Processing Route Research Articles

Solution-derived photocatalytic films for environmental cleaning applications

When photocatalytic water treatment is concerned, suspended catalyst in the aqueous phase is usually more efficient than immobilized on an inert support, but in the former case an undesirable separation/recycling step is needed. We have therefore concentrated on the preparation of immobilized catalysts in the form of films on glass and aluminium supports. The low-temperature sol-gel processing route to obtain transparent thin TiO2/SiO2 films for self-cleaning purposes and thicker TiO2/SiO2 coatings for efficient removal of pollutants in water and air are presented. The synthesis is based on a production of a nanocrystalline titania sol with a silica binder that after deposition does not require thermal treatment at high temperatures. Depending on the target application, some specific synthesis parameters and photocatalytic activity testing conditions are illustrated. For water-cleaning coatings fast kinetics is required, which was achieved by addition of a highly active titania powder into the sol. The same preparation procedure was used to prepare efficient air-cleaning coatings. On the other hand, self-cleaning films were thinner and transparent to keep the original appearance of the substrate and they solidified at ambient conditions. Advanced methodologies to evaluate photocatalytic activity of the films were applied.

Processing and properties of sol gel derived alumina–carbon nano tube composites

High purity alumina–carbon nano tube (CNT) composites were prepared by an aqueous sol–gel processing route. CNTs were dispersed in alumina sol containing appropriate amount of MgO precursor. Aqueous slurry of alumina was seeded into the sol followed by gelation, drying and calcination at 1000°C for 1h. The calcined powder consisting of alumina-coated CNTs and alumina was milled, sieved, dried, pressed and pressureless sintered at 1400–1600°C for 1h in nitrogen atmosphere. Sintered samples were further isostatically hot pressed at 1300°C and the properties were compared with the pressureless sintered samples. Phase formation was followed by XRD study, CNT retention was confirmed by Raman studies and the samples were further characterized for mechanical and microstructural properties.

Nanocomposite TiO2–SiO2 gel for UV absorption

Nano-sized particle TiO2-doped SiO2 gels have been synthesized for use as composites in which the UV absorption efficiency, the major factor for UV protection, can be enhanced. SiO2 gels having a mesoporous morphology have been synthesized via a sol–gel processing route using rice husk as the starting material and further treated using additions of TiO2 from two sources. The chemical purity of the SiO2 was measured by X-ray fluorescence analysis (XRF). Typical samples of pure TiO2, SiO2 and their composites were tested for surface characteristics using N2-sorption (BET surface area). All the different compositions processed have been characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), and imaged using Scanning and Transmission Electron Microscopy (SEM, TEM). The UV absorption values which determine the degree of UV-protection, were measured and the results discussed. The concentration of TiO2 loading, the particle size, and the surface characteristics are shown to relate to the degree of UV absorption and the measured energy band gap of the composites.

Glass-coated individual dispersed MWNTs in Alumina & its high temperature stability

An ultrasonication assisted sol-gel processing route for MWNTs/Alumina was proposed. Using aluminum ethoxide as an alumina precursor and ethanol/water as a solvent, the duration of the transformation from sol to gel under ultrasonication can be controlled by adjusting the water content. Purified MWNTs added under ultrasonication should be functionalized by alcohol at first and a well dispersion in alumina sol could be attained under the help of ultrasonic. With the evaporation of ethanol, sol transfers to gel and MWNTS dispersion can be kept in gel. Gel and calcine-powder show that individual MWNTs are enwrapped by amorphous alumina. As-received powders after a following heating-treatment under vacuum shows a good high stability for a glass coat formed on MWNTs. Raman results show the processing route has no obvious effect on the structure of MWNTs, even a high temperature (1273K) treatment is done under vacuum.

Nanostructured poly(vinyl alcohol)/bioactive glass and poly(vinyl alcohol)/chitosan/bioactive glass hybrid scaffolds for biomedical applications

Bone tissue engineering is the field of intense research for developing new 3D scaffolds with hierarchical and highly interconnected porous structure, which should match the properties of the tissue that is to be replaced. These materials need to be biocompatible, ideally osteoinductive, osteoconductive and mechanically well-matched. In the present study, we report the development and characterization of novel hybrid macroporous scaffolds of poly(vinyl) alcohol (PVA)/bioactive glass (BaG) through the sol–gel route. The organic–inorganic hybrids with three concentrations of PVA (80, 70 and 60 wt%) and bioactive glass (58SiO2–33CaO–9P2O5) were synthesized by foaming a mixture of polymer solution and bioactive glass via sol–gel precursor solution. PVA with two degree of hydrolysis, 98.5% (high degree) and 80% (low degree) was also investigated, in order to evaluate the influence of residual acetate group present in polymer chain on the final structure and properties of 3D porous nanocomposites produced (PVA/BaG). Bioartificial polymeric hybrids were also investigated in this work by blending PVA with chitosan (chi) and then reacting with bioactive glass reagents using sol–gel processing route (PVA/Chi/BaG). The microstructure, morphology and crystallinity of the hybrid porous scaffolds were characterized through synchrotron small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analysis. In addition, mechanical properties of hybrids were evaluated by uni-axial compression tests. Biocompatibility, cytotoxicity and cell viability assays were also performed by the MTT method with VERO cell culture. The results have given strong evidence that both hybrid systems of PVA/bioactive glass and PVA/chitosan/bioactive glass were successfully produced and extensively characterized with hierarchical macroporous 3D structure. FTIR spectroscopy associated with XRD and SAXS has proven to be important technique for investigating the formation of PVA/BaG and PVA/Chi/BaG hybrids as organic–inorganic network at micro-nanostructure order. Finally, the developed hybrids have presented suitable mechanical, morphological and cell viability properties for potential biological applications.

Excimer laser processing of ZnO thin films prepared by the sol–gel process

ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol–gel prepared with anhydrous zinc acetate [Zn(C 2H 3O 2) 2], monoethanolamine [H 2NC 2H 4OH] and isopropanol. The deposited films were dried at 50 and 300 °C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm −2 on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol–gel processing route differs significantly from ZnO sol–gel films prepared by conventional furnace annealing which requires temperatures above 450 °C for the formation of crystalline ZnO.

Surface Morphology of Anatase TiO<sub>2</sub> Thin Film by Sol-Gel Method

Sol-gel processing route has been used for the production of thin films from titanium isopropoxide (TIP). The surface properties and statistics results have been characterized by AFM; the coating process has been examined using SEM and optical microscope. The structure phase of TiO2 has been analyzed by XRD; the composition of TiO2 has been estimated through EDS; the thickness of film has been measured by polarized microscope in an interferometric technique. The coating films are always crystallined in the anatase structure with circular particles in nanometer size.

Immobilized enzyme reactor chromatography: Optimization of protein retention and enzyme activity in monolithic silica stationary phases

Our group recently reported on the application of protein-doped monolithic silica columns for immobilized enzyme reactor chromatography, which allowed screening of enzyme inhibitors present in mixtures using mass spectrometry for detection. The enzyme was immobilized by entrapment within a bimodal meso/macroporous silica material prepared by a biocompatible sol–gel processing route. While such columns proved to be useful for applications such as screening of protein–ligand interactions, significant amounts of entrapped proteins leached from the columns owing to the high proportion of macropores within the materials. Herein, we describe a detailed study of factors affecting the morphology of protein-doped bioaffinity columns and demonstrate that specific pH values and concentrations of poly(ethylene glycol) can be used to prepare essentially mesoporous columns that retain over 80% of initially loaded enzyme in an active and accessible form and yet still retain sufficient porosity to allow pressure-driven flow in the low μL/min range. Using the enzyme γ-glutamyl transpeptidase (γ-GT), we further evaluated the catalytic constants of the enzyme entrapped in capillary columns with different silica morphologies as a function of flowrate and backpressure using the enzyme reactor assay mode. It was found that the apparent activity of the enzyme was highest in mesoporous columns that retained high levels of enzyme. In such columns, enzyme activity increased by ∼2-fold with increases in both flowrate (from 250 to 1000 nL/min) and backpressure generated (from 500 to 2100 psi) during the chromatographic activity assay owing to increases in k cat and decreases in K M, switching from diffusion controlled to reaction controlled conditions at ca. 2000 psi. These results suggest that columns with minimal macropore volumes (<5%) are advantageous for the entrapment of soluble proteins for bioaffinity and bioreactor chromatography.

Sol-Gel Processing of Tellurium Oxide and Suboxide Thin Films with Potential for Optical Data Storage Application

A novel sol-gel processing route has been developed for the production of tellurium oxide (TeO2) and tellurium suboxide (TeOx) thin films. The process has been used to deposit thin films on glass substrates by the dip coating process. These films have been shown to undergo thermally induced, reversible changes in optical properties on heating at appropriate temperatures. Formation of TeOx was found to occur at low temperatures by direct breakdown of the sol-gel derived material even in the absence of reducing agents whilst the principal optical effects corresponded to the formation of TeOx and α-TeO2 at temperatures of <200°C and <450°C respectively. The relatively low temperatures required to induce significant optical effects are comparable with conventionally prepared TeOx and offer the potential application of these materials in optical data storage systems.

A sol-gel derived 0.9Pb(Mg1/2Nb2/3)O3–0.1PbTiO3 ceramic

Using inorganic chemicals, such as niobium pentachloride, titanium tetrachloride, lead nitrate, and magnesium nitrate, as the starting materials, 0.9PMN–0.1PT has been fabricated via a simple and low cost sol-gel processing route. A colloidal solution was first prepared by adding an aqueous lead nitrate solution into an ethanol solution of niobium and titanium chlorides. Magnesium nitrate was then mixed into the solution when chloride ions were removed by forming precipitates of PbCl2 with the excess lead nitrate. The gelation of the colloidal solution was facilitated in the presence of a small amount of polyethylene glycol (PEG 300) at 70 °C. A fine perovskite 0.9PMN–0.1PT powder was obtained when the resulting gel was dried at 300 °C for 4 h and subsequently calcined. It was observed that the sol-gel derived precursor underwent a pyrochlore phase at 500–600 °C, prior to the formation of a perovskite single phase at a calcination temperature of 850 °C. A sintered density of ˜98% theoretical density was obtained when the fine 0.9PMN–0.1PT powder was sintered at 1250 °C for 2 h and the sintered ceramic shows a maximum dielectric constant of 26,682, together with a room temperature dielectric constant of 19,206 at 1 kHz. The superb dielectric properties are correlated to the microstructural features of the sol-gel derived 0.9PMN–0.1PT, which has been characterized using techniques such as XRD, SEM, and TEM.

Sol–gel processing of hydroxyapatite

Hydroxyapatite, a calcium phosphate-based compound with numerous applications in the biological field was synthesized using the sol–gel processing route. The formation of hydroxyapatite and other compounds during the heat-treatment cycle were identified and characterized using thermal analyses and X-ray diffraction together with infrared and Raman spectroscopy. The influence of the addition of various organic alcohols (R–OH, R=CH3–, C2H5– and C3H8–) on the reaction results was studied. © 1998 Kluwer Academic Publishers

Sol-gel processing of NASICON thin-film precursors

Metal-alkoxides have been used to produce Nasicon solid electrolyte thin-film precursors via sol-gel processing route. The influence of various processing parameters e.g., solvent type, temperature, acid or base catalysts, pH, chelating agents, etc., on the hydrolysis-polycondensation (gelation) behaviour of the metal-alkoxides is discussed.

Combined use of XPS, IR and EDAX techniques for the characterization of ZrO 2-SiO 2 powders prepared by a sol-gel process

Powder precursor gels with composition xZrO 2-(100 × x)SiO 2, with selected values of x = 8, 14 and 75 mol%, were prepared by a sol-gel chemical processing route, using commercial silica gel and zirconyl chloride. A combination of X-ray photoelectron (XPS) and infrared (IR) spectroscopies has been applied for studying the chemical bondings in silicon, zirconium and oxygen as a function of the nominal zirconia compositions. Furthermore, ZrO 2/SiO 2 ratios based on semiquantitative XPS and energy-dispersive X-ray analyses (EDAX) showed changes of the surface chemical composition in the as-prepared samples as compared with the thermal-treated powders. These changes have been ascribed to some peculiarities in the processing chemical route when it is applied to this kind of composites.

Lead Oxide Coatings on Sol–Gel‐Derived Lead Lanthanum Zirconium Titanate Thin Layers for Enhanced Crystallization into the Perovskite Structure

An improved method is described for the sol–gel preparation of PLZT thin layers in the perovskite structure. The method uses a PbO cover coat. Details are reported for the sol‐gel processing route and heat‐treatment conditions. Through use of this method it is possible to prepare singlephase perovskite material with improved properties. The deleterious effect of additional phases—which are not present when a PbO cover coat is used—is attributed to Pb loss from the surface during thermal processing. Examples are given for PLZT thin layers integrated on Si with and without a PbO cover coat. The dielectric and ferroelectric properties were always found to be superior for coated structures.

Processing of a uniaxial ferroelectric Pb5Ge3O11 thin film at 450 °C with c-axis orientation

Uniaxial Pb5Ge3O11 thin films were successfully fabricated by the sol-gel processing route. Crack-free and c-axis oriented thin films (1600 Å) were observed on (111) Pt-coated Si substrates when heat treated at 450 °C for 15 min. The thin films exhibited well saturated P-E hysteresis loops with Pr=3.3 μC/cm2, Ps=3.7 μC/cm2, and Ec=135 kV/cm. Specifically, a 1600 Å film switched at 2.2 V. The possible applications such as nonvolatile ferroelectric memories and CCD IR image sensors without fatigue or retention problems, are discussed.