Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

Cobalt–silicon mixed oxide materials (Co/Si=0.111, 0.250 and 0.428) were synthesised starting from Co(NO₃)₂·6H₂O and Si(OC₂H₅)₄ using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N₂ adsorption at −196 °C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co²⁺ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400 °C, the sample with lowest Co content appeared amorphous and contained only Co²⁺ tetrahedral complexes, while at higher cobalt loading Co₃O₄ was present as the only crystalline phase, besides Co²⁺ ions strongly interacting with siloxane matrix. At 850 °C, in all samples crystalline Co₂SiO₄ was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600 °C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.     

Pd-based sulfated zirconia prepared by a single step sol–gel procedure for lean NOx reduction

Pd-based sulfated zirconia catalysts have been prepared through a single step (one-pot) sol–gel preparation technique, in which both sulfate and Pd precursors were dissolved in an organic solution before the gelation step. Observation of the calcination procedure through TGA/DSC and mass spectrometry revealed that the addition of increasing amounts of Pd resulted in the evolution of organic precursor species at lower temperatures. In situ XRD experiments showed that tetragonal zirconia is formed at lower temperatures and larger zirconia crystallites are formed when Pd is added to the gel. Although tetragonal zirconia was the only phase observed through XRD, Raman spectra of samples calcined at 700 °C showed the presence of both the tetragonal and the monoclinic phase, indicating a surface phase transition. DRIFTS experiments showed NO species adsorbed on Pd²⁺ cations. Pd/SZ catalysts prepared through this single step method were active for the reduction of NO₂ with CH₄ under lean conditions. Calcination temperature had a significant effect on this activity, with samples calcined at 700 °C being much more active than those calcined at 600 °C, despite the observed transition to the monoclinic phase. This activity may be linked to observed changes in the surface sulfate species at higher calcination temperatures.     

Photoluminescence and photoactivity of titania particles prepared by the sol–gel technique: effect of calcination temperature

The photoluminescence (PL) characteristics of anatase titania particles prepared by the sol–gel method were investigated and correlated to their photocatalytic behavior with respect to the change of calcination temperature. It was found that the photoluminescence intensity measured at 77 K was gradually increased by increasing the calcination temperature due to the reduction of the internal defects which are responsible for the radiationless recombination of photoexcited electron/hole pairs. Also, the calcination temperature was found to influence the maximum peak position (λ) of the photoluminescence spectra of titania. That is, a blue shift of the photoluminescence spectrum occurs as a consequence of the enlargement of the energy-gap between the lowest excited state and the ground state of titania as increasing the calcination temperature. The quenching behavior of the photoluminescence at 77 K was monitored by in situ supplying oxygen at 77 K in order to investigate what happened to the surface of titania by the calcination. The quenching intensity was monotonically increased with increasing the calcinations temperature. Based on the above results, we concluded that the calcination of titania at higher temperature produces more surface-active sites easily reacting with oxygen molecules as well as improving the crystallinity of anatase phase. Consequently, higher temperature heat treatment of anatase titania particles makes it possible to get higher photoactivity as long as no significant rutile phase is formed.     

High surface area nickel aluminate spinels prepared by a sol–gel method

The oxide spinel NiAl₂O₄ and spinel-type solid solutions Al₂O₃–NiAl₂O₄ (at Ni/Al=1:4, and Ni/Al=1:8) were prepared by controlled hydrolysis of mixed metal alkoxides, followed by calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase cubic materials having the spinel-type structure. The cubic lattice parameter, a_o, was found to decrease gradually with increasing aluminium content of the mixed metal oxides. The specific surface area (determined by nitrogen adsorption at 77 K) was found to be in the range of 200–300 m² g⁻¹. The materials were found to be basically mesoporous, the most frequent pore radius being in the range 3.2–6.4 nm. IR spectroscopy of CO adsorbed at liquid nitrogen temperature gave a main band at 2186–2195 cm⁻¹, which was assigned to the C---O stretching vibration of surface Al³⁺CO adducts where coordinatively unsaturated Al³⁺ ions act as Lewis acid centres.     

Cr bound catalytically active sol–gel siloxane polymers

Several types of Cr bound siloxane polymers were prepared by various modes of polymerization. The co-polymerization of (EtO)₃SiPhCr(CO)₃ and Si(OMe)₄ by the sol–gel process, and its subsequent curing, led to a hydrogenation reactive polymer catalyst. Its catalytic reactivity was retained throughout several cycles, contrary to siloxane polymers prepared by different methods. The hydrogenation reaction was studied with methyl sorbate, 3-nonen-2-one, and 1-octyne. Regio- and stereoselectivities were studied. Cyclohexane as solvent was found to be superior to THF in retaining the catalytic activity upon recycling of the polymeric catalyst in the hydrogenation reactions.     

Synthesis of ceria–zirconia mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

Ceria–zirconia mixed oxide was successfully synthesized via the sol–gel process at ambient temperature, followed by calcination at 500, 700 and 900 °C. The synthesis parameters, such as alkoxide concentration, aging time and heating temperature, were studied to obtain the most uniform and remarkably high‐surface‐area cubic‐phase mixed oxides. The thermal stability of both oxides was enhanced by mutual substitution. Surface areas of the Ce_xZr_1?xO₂ powders were improved by increasing ceria content, and their thermal stability was increased by the incorporation of ZrO₂. The most stable cubic‐phase solid solutions were obtained in the Ce range above 50 mol%. The highest surface area was obtained from the mixed catalyst containing a ceria content of 90 mol% (200 m²/g). Temperature programmed reduction results show that increasing the amount of Zr in the mixed oxides results in a decrease in the reduction temperature, and that the splitting of the support reduction process into two peaks depends on CeO₂ content. The CO oxidation activity of samples was found to be related to its composition. The activity of catalysts for this reaction decreased with a decrease in Zr amount in cubic phase catalysts. Ce₆Zr₄O₂ exhibited the highest activity for CO oxidation. Copyright © 2006 John Wiley & Sons, Ltd.     

The role of thermal analysis in optimization of the electrochromic effect of nickel oxide thin films, prepared by the sol–gel method: Part II

Thin films and the corresponding xerogels were prepared from nickel acetate precursor using the sol–gel dip-coating technique. The differences in thermal stability of the two forms of samples were studied by dynamic and isothermal thermogravimetry. For thin films, the onset decomposition temperature of acetate groups was 230 °C and for the xerogel 250 °C. During thermal decomposition, the formation of nanosized nickel oxide took place. Carbonate ions, which were formed during thermal decompostion of acetate groups, remained either free or bidentately coordinated to nickel. In situ monochromatic optical transmittance changes showed that an optical stability up to the 100th cycle was already achieved for films heated for 15 min at the isothermal temperature (thermal decompositon 25%). Comparison of the results obtained for nickel sulfate (Part I) and nickel acetate precursors shows that at least two parameters, the precursor used and the degree of thermal treatment, have considerable influence on the thermal stability of the thin film and also on its electrochromic response during the cycling process.     

Density fractionated hollow silica microspheres with high-yield by non-polymeric sol–gel/emulsion route

Hollow silica microspheres were synthesized by non-polymeric sol–gel/emulsion technique using tetra ethyl orthosilicate (TEOS) as a source of silica. A sol mixture of TEOS, water, ethanol and acid was emulsified in a solution of light paraffin oil and surfactant (Span-80). Calcined spheres were density fractionated between density ranges: <1.0, 1.0–1.594, 1.594–1.74 and >1.74 g cm⁻³. The samples were characterized by optical and scanning electron microscopy with energy dispersive X-ray analysis, Fourier transform infrared spectroscopy and laser diffraction size analyzer. Spheres of densities lower than 1.74 g cm⁻³ were found to be hollow as observed from scanning electron microscopy (SEM) images and their yield was maximized to 100% by using a specific TEOS volume ratio with respect to volumes of surfactant and oil. Decreasing the calcination temperature from 700 to 500 °C enhances the yield of hollow spheres emphasizing importance of slower diffusion kinetics at lower calcination temperature. Outer diameters of spheres were between 5 and 60 μm with mean diameter expectedly increasing with increase in TEOS sol volume and with decrease in sphere density. It is proposed that silica shells form via hydrolysis and polycondensation at oil–water/ethanol interface in the water-in-oil emulsion, which subsequently form hollow spheres on removal of water–ethanol during calcination.     

Preparation and characterization of epoxy/SiO2 nanocomposites by cationic photopolymerization and sol–gel process

Epoxy/SiO₂ nanocomposite materials were prepared by cationic photopolymerization and sol–gel process using a novel epoxy oligomer (EP‐Si(OC₂H₅)₃) prepared by 3‐isocyanatopropyltriethoxysilane (IPTS)‐grafted bisphenol A epoxy resin and tetraethyl orthosilicate as inorganic precursor. The chemical structures of EP‐Si(OC₂H₅)₃ were characterized by Fourier transformed infrared spectroscopy. Transmission electron microscopy showed that the in situ generated nano‐SiO₂ dispersed uniformly in the EP matrix, and its average diameter is around 40 nm. The relationship between nanocomposite materials' thermal/mechanical properties and nano‐SiO₂ introduced were studied by thermogravimetric analysis, dynamic mechanical analysis, and impact strength test. The results showed that the nanocomposite materials' thermal and mechanical properties improved a lot with increase of the SiO₂ content. Copyright © 2013 John Wiley & Sons, Ltd.     

Polymer–filler interactions in sol–gel derived polymer/silica hybrid nanocomposites

The effect of polymer–filler interaction on solvent swelling and dynamic mechanical properties of the sol–gel derived acrylic rubber (ACM)/silica, epoxidized natural rubber (ENR)/silica, and poly (vinyl alcohol) (PVA)/silica hybrid nanocomposites has been described for the first time. Tetraethoxysilane (TEOS) at three different concentrations (10, 30, and 50 wt %) was used as the precursor for in situ silica generation. Equilibrium swelling of the hybrid nanocomposites in respective solvents at ambient condition showed highest volume fraction of the polymer in the swollen gel in PVA/silica system and least in ACM/silica, with ENR/silica recording an intermediate value. The Kraus constant (C) also followed a similar trend. In dynamic mechanical analysis, the storage modulus dropped at higher strain (>1%), which indicated disengagement of polymer segments from the filler surfaces. This drop was maximum in ACM/silica, intermediate in ENR/silica, and minimum in PVA/silica, both at 50 and 70 °C. The drop in modulus with theoretical volume fraction of silica (ϕ) was interpreted with the help of a Power law model ΔE′ = a₁ϕ, where a₁ was a constant and b₁ was primarily a filler attachment parameter. Strain dependence of loss modulus was observed in ACM/silica hybrid nanocomposites, while ENR/silica and PVA/silica nanocomposites showed almost strain‐independent behavior. The storage modulus showed sharp increase with increasing frequency in ACM/silica system, while that was lower in both ENR/silica (at higher frequency) and PVA/silica systems (in the entire frequency spectrum). The increase in modulus with ϕ also followed similar model ΔE′ = a₂ϕ proposed in the strain sweep mode. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2399–2412, 2005     

Preparation and characterization of a supported Si3N4 membrane via a non-aqueous sol–gel process

We report a novel fabrication process of a mesoporous Si₃N₄ membrane on an Al₂O₃ support via a non-aqueous sol–gel technique. The membrane was prepared by dipping an -Al₂O₃ support disk into a silicon diimide sol that was synthesized by catalytic ammonolysis of tris(dimethylamino)silylamine, H₂NSi(NMe₂)₃. The SEM image and nitrogen adsorption analysis indicate that amorphous Si₃N₄ layers with nano-sized pores have formed on the surface and also inside the pores of the Al₂O₃ support disk. The new membrane demonstrates high selective absorption of NO₂, suggesting a potential application as a selective filter for gas sensors.     

Coupled enzyme reaction microarrays based on pin-printing of sol–gel derived biomaterials

We report on the development of a new class of protein microarrays based on the co-immobilization of multiple components within a single pin-printed sol–gel array element. In the first case, the microarraying of a coupled two enzyme reaction involving glucose oxidase and horseradish peroxidase along with the fluorogenic reagent Amplex Red is demonstrated to allow “reagentless” fluorimetric detection of glucose. The second system involved the detection of urea using co-immobilized urease and fluorescein dextran, which works on the basis of a pH induced change in fluorescein emission intensity upon production of ammonium carbonate owing to hydrolysis of urea. In both the cases, it is demonstrated that the changes in intensity from the array are time-dependent, consistent with the enzyme catalyzed reaction, showing that such arrays can be used for kinetic studies. The rate of intensity change was also found to be dependent on the concentration of analyte added to the array, showing that such arrays could be useful for quantitative multianalyte biosensing. Inhibition of urease by the competitive inhibitor thiourea is also demonstrated on a microarray, demonstrating that sol–gel-based microarrays may find use in high-throughput drug-screening applications.     

Microbiosensor for acetylcholine and choline based on electropolymerization/sol–gel derived composite membrane

Amperometric enzyme biosensors for the determination of acetylcholine (ACh) and choline (Ch) have been described. For the fabrication of the biosensors, N-acetylaniline (nAN) was first electropolymerized on a Pt electrode surface to be served as a permselective layer to reject interferences. Bovine serum albumin (BSA) and choline oxidase (CHOD) were co-immobilized in a zinc oxide (ZnO) sol–gel membrane on the above modified Pt electrode for a Ch sensor, or CHOD, acetylcholinesterase (AChE) and BSA immobilized together for an ACh/Ch sensor. The poly (N-acetylaniline) (pnAN) film was the first time used for an ACh/Ch sensor and found to have excellent anti-interference ability, and the BSA in the sol–gel can improve the stability and activity of the enzymes. Amperometric detection of ACh and Ch were realized at an applied potential of +0.6 V versus SCE. The resulting sensors were characterized by fast response, expanded linear range and low interference from endogenous electroactive species. Temperature and pH dependence and stability of the sensor were investigated. The optimal ACh/Ch sensor gave a linear response range of 1.0 × 10⁻⁶ to 1.5 × 10⁻³ M to ACh with a detection limit (S/N = 3) of 6.0 × 10⁻⁷ M and a linear response range up to 1.6 × 10⁻³ M to Ch with a detection limit of 5.0 × 10⁻⁷ M. The biosensor demonstrated a 95% response within less than 10 s.     

Amperometric detection of cytochrome c by capillary electrophoresis at a sol–gel carbon composite electrode

Capillary electrophoresis (CE) was employed for the determination of cytochrome c using a wall-jet amperometric detector consisting a copper(I) oxide-modified sol–gel carbon composite electrode (CCE), which exhibits a sensitive electrocatalytic response for the oxidation of cytochrome c. The optimum conditions of separation and detection are 0.08 M NaOH for the separation solution, 12 kV for separation voltage and +0.60 V versus saturated calomel electrode (SCE) for the detection potential. Calibration was linear over the concentration range 1–600 μM with the limit of detection of 3.4 μM, based on a signal-to-noise ratio (S/N) of 3.     

Low-potential stable detection of β-NADH at sol–gel derived carbon composite electrodes

Sol–gel derived carbon composite electrodes, prepared from different non-silicate metal alkoxide precursors, offer a substantial decrease in the overvoltage of the NADH oxidation reaction (compared to ordinary carbon electrodes). Such promotion is attributed to acceleration of the proton-transfer step by the metal-oxide component of the composite. Passivation problems, accrued by accumulation of reaction products, are also greatly minimized. Both titania–, zirconia–sol–gel carbon composite electrodes thus offer a highly sensitive and stable anodic detection of NADH at +0.2 V. Greatly improved retention of the redox mediator Meldola Blue within the sol–gel network permits convenient measurements at NADH at −0.1 V. These improvements indicate great promise for the design of dehydrogenase-based amperometic biosensors. An intrinsic activation action by the metal-oxide component is also reported towards the oxidation of hydrazine, hydrogen peroxide, ascorbic acid and catechol. Low-potential detection of NADH is also illustrated at microfabricated titania/carbon screen-printed electrodes.     

Anionic surfactant‐aided preparation of high surface area and high thermal stability ceria/zirconia‐mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

This work is focused on the ceria zirconia mixed oxide prepared through a surfactant‐introduced synthesis method. High surface area nanoparticle mesoporous ceria/zirconia‐mixed oxide was successfully synthesized and characterized using various techniques. High surface area mesoporous fluorite‐structured CeO₂? ZrO₂ was obtained from the elimination of surfactants upon calcination. A surface area in excess of 205.6 m²/g was obtained after calcination at 500 °C, and dropped to 75.96 m²/g by heating at 900 °C. Temperature‐programming reduction (TPR) results showed that the lowest reduction temperature was obtained from the sample containing 40% zirconia content. Copyright © 2008 John Wiley & Sons, Ltd.     

Preparation and characterization of alumina supported silicalite membranes by sol–gel hydrothermal method

Two types of unsupported zeolites (silicalite-1 and silicalite-2) and porous alumina discs supports were prepared by the hydrothermal sol–gel synthesis method. The influence of the raw materials used as SiO₂ source, the temperature of the thermal treatment and the presence of the ceramic support on the crystallization of zeolites were studied. The reaction products were characterized by X-ray diffraction (XRD), IR spectroscopy (IR) and scanning electron microscopy (SEM) studies. The SiO₂ source had a significant effect on the final zeolite obtained: the use of colloidal silica sol (ZCS) as SiO₂ source in the synthesis led to ZSM-11 (silicalite-2) crystals, while the sodium silicate solution (ZSS) produced the ZSM-5 (silicalite-1) type. The presence of the alumina support influences the crystallization process of ZSM-5, as it improves nucleation and the ordering of the crystals.     

Properties of oligopyrrole doped dodecylbenzene–sulfonic acid prepared with different oxidants in reversed microemulsion

The calcium silicate systems were prepared by sol–gel methods—varying the synthesis conditions (precursor solutions, chemical compositions, catalysts, temperature and time of aging and heat treatment). The precursors of sol–gel procedures were calcium nitrate and tetraetoxysilane. Acetic acid, water, ammonia and nitric acid were applied as catalysts. The atomic and supramolecular structures of calcium silicate products have been varied from amorphous to crystalline phases depending strongly on the preparation conditions. The structures were investigated in atomic range with Fourier transform infrared (FTIR), X-ray diffraction (XRD) and in supramolecular ranges with small angle X-ray scattering (SAXS), ultra small angel X-ray scattering (USAXS) measurements. The mechanical properties could be most efficiently improved by heat treatments (min. 600 °C) and a post-treatment with tetraetoxysilane (TEOS). The mechanical strength of bulk biomaterials has been controlled by Brinell hardness test.     

TPR and XPS characterization of chromia–lanthana–zirconia catalyst prepared by impregnation and microwave plasma enhanced chemical vapour deposition methods

Chromia–lanthana–zirconia catalysts prepared by wet impregnation and microwave plasma enhanced chemical vapour deposition methods have been characterized by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The impregnation procedure requires large amounts of solvent and calcination at high temperatures producing Cr⁶⁺ species. Unlike this, it is found that the microwave plasma enhanced chemical vapour deposition (PECVD) method predominantly produces Cr³⁺ species on zirconia-based supports. Moreover, it has been shown that the dispersion of chromium species deposited on zirconia-based support by the PECVD method is higher than the dispersion of those prepared by wet impregnation. Thus, the advantages of PECVD over the impregnation method consist in this case in preventing the use of large amounts of solvent and avoiding the primary formation of poisonous Cr⁶⁺ species as well as in enabling the deposition of chromium species with high dispersion on zirconia-based supports.     

Synthesis of silica–silver wires by a sol–gel technique

SiO₂–Ag wires were synthesized by a sol–gel technique. A two step approach was followed, focusing mainly on the effect of acid concentration on the first stage and processing temperature on the second. This acid-catalyzed reaction on the first stage yielded SiO₂–AgCl wires with diameters as low as 800 nm average, and lengths ranging up to 100 μm, as determined by LV-SEM and TEM. A thermal treatment at different temperatures on the second step, under H₂ atmosphere, yields silica–silver unidirectional structures. The chemical composition of these structures was determined by EDS, indicating the presence of Si, O and Ag. The transformation of the wires as a function of temperature under reducing atmosphere was followed by electron microscopy analysis. At 400 °C and above the silica starts to cover the reduced silver while maintaining the unidirectional conformation, suggesting a tendency to form silver wires covered by a silica layer.