Direct formation of SiO<Subscript>2</Subscript>/SnO<Subscript>2</Subscript> composite nanoparticles with high surface area and high thermal stability by sol–gel-hydrothermal process

SnO₂/SiO₂ composite nanoparticles were prepared by sol–gel-hydrothermal process and their physico-chemical structure and photocatalytic property were investigated. The results of XRD, TEM and FT-IR indicated that SnO₂ crystallites with the tetragonal rutile structure were well-developed directly during hydrothermal process. The SnO₂/SiO₂ composite nanoparticles owned narrow size distribution, large specific surface area, and good thermal stability. As the presence of 25.0 wt% SiO₂, the SnO₂ nanoparticles were about 4.0 nm in diameter and the specific surface area was 259.0 m²/g. After calcination at 800 °C, the crystalline grain size maintained 16.2 nm and the surface area still remained 132.6 m²/g. The SnO₂/SiO₂ composite nanoparticles showed better photocatalytic activity than pure SnO₂ nanoparticles.     

Formation and characterization of phosphate-modified silicate materials derived from sol–gel process

Phosphate-containing silicate materials prepared using sol–gel method from Si(OC₂H₅) were investigated at the variation of the amount of phosphate modifier from 5 to 50 wt% in term of P₂O₅. Chemical composition, textural and structural properties of these materials were characterized by FTIR-spectroscopy, TEM, X-ray diffraction and nitrogen adsorption. It was shown that the materials posse monomodal pore size distribution of 5–20 nm for the samples dried at 100 °C and 40–60 nm for the specimens calcined at 600 °C. The mean pore size and surface area depended on the amount of phosphoric acid. Before the stage of high temperature treatment phosphoric acid, introduced into the structure of the materials as a modifying agent, was uniformly distributed inside a porous space of the material and was not chemically bonded with silicate. After high temperature treatment both chemical interaction of silicate with phosphate, providing the formation of silicate-phosphate structures, as well as redistribution of free modifier from the bulk of granules to their surface took place. The polyphosphate layer is formed on the material surface closing the internal porous space. However, in this case a part of the phosphate modifier remains chemically unbound to SiO₂ structure.     

Thermal degradation studies on PMMA–HET acid based oligoesters blends

Imparting thermal stability to polymethyl methacrylate (PMMA) without affecting its optical clarity is attempted by incorporating HET acid based oligoesters. Pure PMMA and PMMA containing five and 20 wt% of four different oligoesters are separately prepared using bulk polymerization. The thermal properties of the materials studied using DSC, TG, TG–FTIR and Pyr–GC–MS are presented. The main volatile degradation products identified are CO, HCl, CO₂, H₂O, hexachlorocyclopentadiene, hexachloroendomethylene tetrahydrophthalic acid/anhydride and methyl methacrylate. A detailed mechanism for the influence of the degradation products of HET acid based oligoesters on the thermal degradation of PMMA is also presented.     

Preparation of superhydrophobic PET fabric from Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> hybrid: geometrical approach to create high contact angle surface from low contact angle materials

In this work we prepare high contact Poly Ethylene Terephthalate (PET) fabric surface from low contact angle materials. Superhydrophobic PET fabric is prepared by coating the fabric with hybrid Al₂O₃–SiO₂ sol. In this case, the high contact angle Al₂O₃–SiO₂ hybrid is created from low contact angle Al₂O₃ and SiO₂ precursors. PET treated with hybrid Al₂O₃–SiO₂ exhibit Water Contact Angle (WCA) as 150°, while PET treated with individual Al₂O₃ sol or SiO₂ sol exhibits lower WCA, (Al₂O₃ WCA = 137°; SiO₂ WCA = 141°). FESEM and AFM investigations show that the hybrid Al₂O₃–SiO₂ sol and individual Al₂O₃ or SiO₂ sol imparted different roughness geometry on the PET fabric surface. We observe surface structure of fish fin-like, particle-like and hybrid fin-particle for treated PET fabric with; Al₂O₃, SiO₂ and hybrid Al₂O₃–SiO₂ sol, under FESEM and AFM observations.AFM observations show the evolution of roughness (Ra) dimension of different surface structures with the order of: SiO₂ < Al₂O₃ < Al₂O₃–SiO₂ (Ra = 31, 63 and 273 nm). We believe that the disparity of the surface geometries lead into different surface WCA. FTIR spectra of Hybrid Al₂O₃–SiO₂ shows additional peak at 902, 850, 557, and 408 cm⁻¹ which can be ascribed to the hybridization structure.     

Thermal stability and UV-shielding properties of polymethyl methacrylate and polystyrene modified with calcium carbonate nanoparticles

The influences of nanosized CaCO₃ on the thermal and optical properties embedded in poly(methyl methacrylate) (PMMA) and polystyrene (PS) were investigated. Calcium carbonate nanoparticles were synthesized by in situ deposition technique, and its nano size (32–35 nm) was confirmed by scanning electron microscope (SEM) and X-ray studies. Nanocomposites samples of PMMA/CaCO₃ and PS/CaCO₃ were prepared with different filler loading (0–4 wt%) of CaCO₃ nanoparticles by solution mixing technique. The Fourier transform infrared analysis confirmed that CaCO₃ nanoparticles were present in the polymers matrices. The morphology and elemental composition of nanocomposites were evaluated by SEM and energy dispersive X-ray spectroscopy. The thermal properties of nanocomposites were characterized by differential scanning calorimetric, thermogravimetric, and differential thermogravimetry analysis, and the results indicate that the incorporation of CaCO₃ nanoparticles could significantly improve the thermal properties of PMMA/CaCO₃ and PS/CaCO₃ nanocomposites. The glass transition temperature (T _g ) and decomposition temperature (T _d ) of nanocomposites with 4 wt% of CaCO₃ nanoparticles were increased by 30 and 24 K in case of PMMA/CaCO₃ and 32  and 15 K in the case of PS/CaCO₃ nanocomposites, respectively. The obtained transparent nanocomposites films were characterized using UV–Vis spectrophotometer which shows the transparencies of nanocomposites are almost maintained in visible region while the intensity of absorption band in ultraviolet (UV) region is increased with CaCO₃ nanoparticles contents and these composites particles could enhance the UV-shielding properties of polymers.     

PMMA-SiO<Subscript>2</Subscript> organic–inorganic hybrid films: determination of dielectric characteristics

Organic–inorganic hybrid thin films have been prepared by a modified sol–gel route using tetraethyl orthosilicate as the inorganic (silica) source, methyl methacrylate (MMA) as the organic source, and 3-trimetoxysilylpropyl methacrylate as the coupling agent. The films were prepared by spin coating on Si (100) p-type substrates and subsequently heat-treated at 90 °C. Fourier transform infrared results reveal a set of absorption bands associated with the formation of both PMMA and SiO₂ phases in the hybrid films. Capacitance–voltage (C–V) characterization was carried out on metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures, with the hybrid films as the insulator layer to evaluate the electrical properties. We present a detailed comparative analysis of the dielectric constant obtained from C–V characterization in the frequency range of 1 kHz–1 MHz. For the PMMA-SiO₂ hybrid material the dielectric constant values obtained were around 9.5 at 1 MHz which is superior to the values reported for thermally grown SiO₂ and pure PMMA materials. The interface state density for PMMA-SiO₂ on Si was approximately 10¹⁰ cm⁻², which is comparable to the standard SiO₂/Si structures. Due to the electrical behavior and low processing temperatures this hybrid dielectric is a very promising candidate for flexible electronic devices and its subsequent implementation does not require complex equipment.     

Structure and multiferroic properties of Bi<Subscript>5</Subscript>FeTi<Subscript>3</Subscript>O<Subscript>15</Subscript> thin films prepared by the sol–gel method

The Bi₅FeTi₃O₁₅ (BFTO) films of layered structure have been fabricated on Pt/Ti/SiO₂/Si substrates by the sol–gel method. The thermal decomposition behaviors of precursor powder were examined using thermo-gravimetric and differential scanning calorimeters analysis. The optimal heat treatment process for BFTO films were determined to be low-temperature drying at 200 °C for 4 min and high-temperature drying at 350 °C for 5 min followed by annealing at 740 °C for 60 min, which led to the formation of compact films with uniform grains of ~300 nm. The structural, surface topography, ferroelectric and magnetic properties of the films were investigated. The remnant polarization (2P _r) of BFTO thin films under an applied electric field of ~550 kV/cm are determined to be 67.5 μC/cm² . Meanwhile, the weak ferromagnetic properties of the BFTO films were observed at room temperature.     

Preparation and characterization of transparent TiO2 thin films coated on fused-silica substrates

The transparent TiO₂ thin films coated on fused-SiO₂ substrates were prepared by the sol–gel method and spin-coating technique. Effects of calcination temperature on crystal structure, grain size, surface texture, and light transmittance of the films were investigated. After calcining at 600–1,200 °C, the thicknesses of the TiO₂ films were all around 80 nm and the molecular structures of the films were anatase, even at 1,200 °C. The calcined TiO₂ films had the ultraviolet light (wavelength 200–400 nm) transmittances of ≤29% and the visible light (wavelength 400–800 nm) transmittance of ≥72%. By photocatalytically decomposing the methylene blue (MB) in water, the photocatalytic activities of the TiO₂ thin films were measured and represented using the characteristic time constant (τ) for the MB degradation. While the films prepared at 1,000 and 1,200 °C photodecomposed about 54 mol% of the MB in water (the corresponding τ ≈ 14.8 h) after exposing to 365-nm UV light for 12 h, the films prepared at 600 and 800 °C had smaller τ (≈9.0 h) and photodecomposed about 74 mol% of the MB in water at the same testing conditions.     

Hepta isobutyl polyhedral oligomeric silsesquioxanes (hib-POSS)

Six polyhedral oligomeric silsesquioxanes (POSSs) with general formula R₇ R′₁ (SiO_1.5)₈, where R- was an isobutyl group and R′- a variously substituted phenyl group, namely hepta isobutyl polyhedral oligomeric silsesquioxane (hib-POSS), were prepared and their composition was checked by elemental analysis and ¹ H NMR spectroscopy. The degradation of compounds obtained was studied by simultaneous differential thermal analysis/thermogravimetry (DTA/TG) technique, in both inert (flowing nitrogen) and oxidative (static air atmosphere) environments, in order to draw useful information about their thermal stability. Experiments, performed in the 35–700 °C temperature range, showed different behaviour between the two used atmospheres. The formation of volatile compounds only, with an about complete mass loss, was observed under nitrogen, while a solid residue (≈40–50% in every case), due to the formation of SiO₂, as indicated by the FTIR spectra, was obtained in static air atmosphere. The results obtained were discussed and compared, and the classifications of resistance to thermal degradation in the studied environments were made. A comparison between the thermal stabilities of hib-POSSs and analogous cyclopentyl POSSs previously studied was also performed.     

High surface area sol–gel alumina–titania nanocatalyst

Alumina–titania mixed oxide nanocatalysts with molar ratios = 1:0.5, 1:1, 1:2, 1:5 have been synthesized by adopting a hybrid sol–gel route using boehmite sol as the precursor for alumina and titanium isopropoxide as the precursor for titania. The thermal properties, XRD phase analysis, specific surface area, adsorption isotherms and pore size details along with temperature programmed desorption of ammonia are presented. A specific surface area as high as 291 m²/g is observed for 1:5 Al₂O₃/TiO₂ composition calcined at 400 °C, but the same composition when calcined at 1,000 °C, resulted in a surface area of 4 m²/g, while 1:0.5 composition shows a specific surface area of 41 m²/g at 1,000 °C. Temperature programmed desorption (of ammonia) results show more acidic nature for the titania rich mixed oxide compositions. Transmission electron microscopy of low and high titania content samples calcined at 400 °C, shows homogeneous distribution of phases in the nano range. In the mixed oxide, the particle size ranges between 10–20 nm depending on titania content. The detailed porosity data analysis contributes very much in designing alumina–titania mixed oxide nanocatalysts.     

Preparation,characterization and photocatalytic properties of TiO<Subscript>2</Subscript> nanostructured spheres synthesized by the Sol–Gel method modified with ethylene glycol

Monodispersed nanostructured TiO₂ spheres were obtained by the Sol–Gel method modified with ethylene glycol. The sample morphology and surface textural properties were characterized by X-ray diffraction (XRD), N₂-physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and diffuse reflectance spectroscopy (DRS). The SEM image showed spheres with sizes ranging from 600 to 700 nm. In addition, HRTEM micrographs reveal hexagonal grains slightly elongated (20 nm). The powders present a BET surface area of 116 m² g⁻¹. Samples without thermal treatment and those treated at 400 °C both showed characteristic reflections of the anatase phase. The photocatalytic activity of the prepared TiO₂ spheres was determined by degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution. Kinetics parameters have displayed than the nanostructured material present a reaction half-life time of 30 min and it was two times faster than commercial TiO₂ (P25).     

Fabrication and characterization of TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> composite nanoparticles and polyurethane/(TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>) nanocomposite films

TiO₂–SiO₂ composite nanoparticles were prepared by a sol–gel process. To obtain the assembly of TiO₂–SiO₂ composite nanoparticles, different molar ratios of Ti/Si were investigated. Polyurethane (PU)/(TiO₂–SiO₂) hybrid films were synthesized using the “grafting from” technique by incorporation of modified TiO₂–SiO₂ composite nanoparticles building blocks into PU matrix. Firstly, 3-aminopropyltriethysilane was employed to encapsulate TiO₂–SiO₂ composite nanoparticles’ surface. Secondly, the PU shell was tethered to the TiO₂–SiO₂ core surface via surface functionalized reaction. The particle size of TiO₂–SiO₂ composite sol was performed on dynamic light scattering, and the microstructure was characterized by X-ray diffraction and Fourier transform infrared. Thermogravimetric analysis and transmission electron microscopy (TEM) employed to study the hybrid films. The average particle size of the TiO₂–SiO₂ composite particles is about 38 nm when the molar ratio of Ti/Si reaches to1:1. The TEM image indicates that TiO₂–SiO₂ composite nanoparticles are well dispersed in the PU matrix.     

Thermal degradation of lignin–phenol–formaldehyde and phenol–formaldehyde resol resins

Resol resins are used in many industrial applications as adhesives and coatings, but few studies have examined their thermal degradation. In this work, the thermal stability and thermal degradation kinetics of phenol–formaldehyde (PF) and lignin–phenol–formaldehyde (LPF) resol resins were studied using thermogravimetric analysis (TG) in air and nitrogen atmospheres in order to understand the steps of degradation and to improve their stabilities in industrial applications. The thermal stability of samples was estimated by measuring the degradation temperature (T _d), which was calculated according to the maximum reaction rate criterion. In addition, the ash content was determined at 800 °C in order to compare the thermal stability of the resol resin samples. The results indicate that 30 wt% ammonium lignin sulfonate (lignin derivative) as filler in the formulation of LPF resin improves the thermal stability in comparison with PF commercial resin. The activation energies of degradation of two resol resins show a difference in dependence on mass loss, which allows these resins to be distinguished. In addition, the structural changes of both resins during thermal degradation were studied by Fourier transform infrared spectroscopy (FTIR), with the results indicating that PF resin collapses at 300 °C whereas the LPF resin collapses at 500 °C.     

Antibacterial glass films prepared on metal surfaces by sol–gel method

This paper describes the preparation and characterization of glass films consisting of SiO₂, Li₂O, Na₂O, K₂O or MgO in varying compositions on stainless steel and aluminum substrates by sol–gel method. Silver phosphate or silver incorporated zeolite was also introduced into the sols for obtaining antibacterial effect. The SiO₂/Li₂O/Na₂O system having the composition of 85:5:10 wt% was found as the optimum for obtaining a stable sol and film formation. The films were investigated by scanning electron microscopy (SEM) and electron dispersive analysis by X-ray (EDX), Fourier transformed infrared (FTIR) spectroscopy, thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA). Homogenous films having 300 ± 20 nm thicknesses were formed by spin coating and then by curing at 500 °C for 1 h. Obtained films had high adherence to the metal substrates and they were also durable in acidic, basic or NaCl environments. They also presented a powerful antibacterial effect against E. coli.     

Selective extraction of sulfonamides from food by use of silica-coated molecularly imprinted polymer nanospheres

We report the use of nanospheres prepared by coating silica with molecularly imprinted polymer (MIP) for sulfamethoxazole (SMO). The resulting SiO₂–SMO–MIP nanoparticles have highly improved imprinting and adsorption capacity, and can be used for separation and determination of sulfonamides in eggs and milk. In the synthesis, monodispersed SiO₂ nanoparticles (Si–NP) of diameter 80 nm were amino-modified by reaction with 3-aminopropyltriethoxylsilane. The acryloyl monolayer was then grafted onto the amine-modified Si–NP. Finally, the MIP films were coated on to the surface of Si–NP by the copolymerization of the vinyl end groups with functional monomer (acrylamide), cross-linking agent (ethylene glycol dimethacrylate), initiator (azo-bis-isobutyronitrile), and template molecule (sulfamethoxazole). The resulting SiO₂–SMO–MIP–NP were characterized by transmission electron microscopy, scanning electron microscopy, and Fourier transform infrared spectrometry. The adsorption properties were demonstrated by equilibrium rebinding experiments and Scatchard analysis. The results showed that the binding sites of the SiO₂–SMO–MIP were highly accessible, and the maximum adsorption capacity of the SiO₂–SMO–MIP for SMO was 20.21 mg g⁻¹. The selectivity of the SiO₂–SMO–MIP–NP obtained was elucidated by using SMO and structurally related sulfonamides. The results indicated that the SiO₂–SMO–MIP had significant selectivity for SMO. The feasibility of removing SMO and sulfadiazine (SDZ) from food samples was proved by use of spiked milk and eggs. A method for the separation and determination of trace SMO and SDZ in milk and egg samples was developed, with recoveries ranging from 69.8% to 89.1%.     

Some studies on the phase formation and kinetics in TiO<Subscript>2</Subscript> containing lithium aluminum silicate glasses nucleated by P<Subscript>2</Subscript>O<Subscript>5</Subscript>

Lithium aluminum silicate (LAS) glasses of compositions (wt%) 10.6Li₂O–71.7SiO₂–7.1Al₂O₃–4.9K₂O–3.2B₂O₃–1.25P₂O₅–1.25TiO₂ were prepared by the melt quench technique. Crystallization kinetics was investigated by the method of Kissinger and Augis–Bennett using differential thermal analysis (DTA). Based on the DTA data, glass ceramics were prepared by single-, two-, and three-step heat treatment schedules. The interdependence of different phases formed, microstructure, thermal expansion coefficient (TEC) and microhardness (MH) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-mechanical analysis (TMA), and microhardness (MH) measurements. Crystallization kinetics revealed that Li₂SiO₃ is the kinetically favored phase with activation energy of 91.10 kJ/mol. An Avrami exponent of n = 3.33 indicated the dominance of bulk crystallization. Based upon the formation of phases, it was observed that the two-stage heat treatment results in highest TEC glass ceramics. The single-step heat treatment yielded glass ceramics with the highest MH.     

Kinetics analysis of colloidal crystallization of silica spheres modified with polymers on their surfaces in acetonitrile

 The nucleation and growth rates in the colloidal crystallization of silica spheres (136 nm in diameter) modified with polymers on their surface were measured by time-resolved reflection spectroscopy. The polymers were poly(maleic anhydride-co-styrene) [P(MA-ST)] and poly(methyl methacrylate) (PMMA). The induction period for nucleation decreased sharply when the sphere concentration increased. The crystal growth process consisted of a fast growing step leading to metastable crystals (rate v ₁) and a slow growth rate accompanied by the formation of stable crystals. The crystal size of the P(MA-ST)/SiO₂ particles decreased from 0.4 to 0.06 mm, whereas v ₁ increased from 13 to 37 μm/s, when the particle concentration increased. The slow step was also observed for almost all the samples but was not analyzed since the rate was too small. For PMMA/SiO₂ dispersions, the crystal size (0.17–0.3 mm) and v ₁ (43–166 μm/s) did not show any relation to the particle concentration but showed a linear relationship with the molecular weight of PMMA. These results suggest the important role of the excluded-volume effects of the polymer layers around the silica surface. The contribution of the repulsion due to the electrical double layers is still effective in the colloidal crystallization in acetonitrile. Received: 6 June 2001 Accepted: 20 September 2001     

Photo catalytic oxidation of TNT using TiO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> nano-composite aerogel catalyst prepared using sol–gel process

The degradation of nitro aromatics like trinitrotoluene (TNT) released in the waste water from explosive process plants is the serious problem due to toxic and explosive nature of TNT. The poor response of TNT to biodegradation enhanced the gravity of the problem. We have demonstrated that high specific surface area TiO₂–SiO₂ nano-composite aerogel is promising photo catalyst in successful treating of TNT contaminated aqueous solution. The TiO₂–SiO₂ composite aerogel with nominal content of 20 and 50% TiO₂, used as catalyst, were prepared by co-precursor sol–gel method using titanium isopropaxide and tetramethylorthosilicate as source of titania and silica, respectively. The XRD studies confirmed formation of anatase phase of crystalline TiO₂ with nano sized crystallites. The TiO₂–SiO₂ aerogel showed specific surface area of 1,107 and 485 m²/g for the aerogels containing 20 and 50% TiO₂, respectively. The 100 ppm TNT solution was treated, in 700 ml capacity reaction vessel, using H₂O₂ oxidizer and TiO₂–SiO₂ aerogel catalyst in presence of UV light (8 W UV lamp). Using TiO₂–SiO₂ (50/50) aerogel with surface area of 485 m²/g, we succeeded to reduce the TOC to 1 ppm within 3.5 h where as using TiO₂/SiO₂ (20/80) aerogel with surface area of 1,107 m²/g, the TOC was reduced to about only 7 ppm in the same time. It revealed that the combination of high TiO₂ content and high specific surface area is an important factor to achieve effective and faster degradation of TNT for complete mineralization.     

Synthesis and characterization of sol–gel derived Ag(Nb,Ta)O<Subscript>3</Subscript> nanopowder

Perovskite-type Ag(Nb_0.6Ta_0.4)O₃ nanopowder was prepared by the sol–gel process from the AgNO₃, Ta₂O₅ and Nb₂O₅, with help of K₂CO₃, avoiding use of strong corrosive acid or expensive niobium ethoxide and tantalum ethoxide. The results suggested that thermal decomposition of the xerogel took place when the xerogel was heated at 450 °C. Well-crystallized single-phased powder was obtained at low temperature about 680 °C. With the heat-treatment temperature increasing (680–1,100 °C), the intensity of the diffraction peaks increased. The crystallite size determined by Scherer formula and the result suggested that higher temperature lead to larger crystallite size. Moreover, the average grain size 30–50 nm was estimated by a field emission scanning electron microscope. The influence of holding time on microstructures indicated that the homogeneous and small grains were obtained at 800 °C for 2–4 h while larger ones for 8–16 h.     

Highly Uniform Nano and Mesostructures of Silica Obtained by Reverse Micellar and Hydrothermal Methods

Silica nanoparticles with controlled size and morphology and a high degree of monodispersity have been synthesized using single and double microemulsion systems employing a cationic surfactant (CTAB) and a non-ionic surfactant (Tergitol). Depending on the type of surfactant aggregate acting as templates, very different morphologies were formed. Nanospindles of silica of ~200–300 nm in length and nanofibres could be obtained by suitably controlling the reverse micellar methodology. The hydrothermal method resulted in highly porous and uniform spheres of diameter ~300 nm which appears to be formed from aggregates of small silica nanoparticles of size ~10 nm. The surface area of the nanoparticles (119 m²/g) was found to be much higher than the corresponding bulk SiO₂ (500–600 nm) which had a surface area of 22 m²/g. The hydrothermally obtained spheres of size ~300 nm show a surface area of 35 m²/g. In honor of Prof. C.N.R. Rao, FRS, on his 75th birth anniversary.