Showing papers on "Sol-gel published in 2000"

TiO2 thin films by a novel sol–gel processing for gas sensor applications

Abstract: Novel thin films of titanium dioxide dispersed in a polymeric matrix have been prepared by a chemically modified sol–gel technique. Nanostructured films of pure TiO2 in the anatase form are obtained after annealing at 500°C. SEM, TEM and TG/DTA are used for the structure characterisation of TiO2 films. The role of the polymer in controlling the microstructure is confirmed. The first application of this technique in gas sensor field is presented in this work. Ethanol and methanol sensing properties are tested and reported. TiO2 sensors can detect very well concentration required for breath analysers.

Aqueous Sol−Gel Process for Protein Encapsulation

Abstract: Porous silica materials made by low-temperature sol−gel process are promising host matrixes for encapsulation of biomolecules. To date, researchers have focused on sol−gel routes using alkoxides such as tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS) for encapsulation of biomolecules. These routes lead to formation of alcohol as a byproduct that can have a detrimental effect on the activity of entrapped biomolecules. We have developed a novel aqueous sol−gel process to encapsulate biological molecules (such as enzymes, antibodies, and cells) that uses neutral pH and room temperature and does not generate alcohol as a byproduct. The process uses sodium silicate as precursor and is carried out in two steps: preparation of a low-pH silicate sol followed by gelation at neutral pH with a suitable buffer containing biomolecules. Two enzymes widely used in biosensing applications, horseradish peroxidase (HRP) and glucose-6-phosphate dehydrogenase (G6PDH), were used to prepare enzyme-doped s...

Analysis of the noble metal catalytic additives introduced by impregnation of as obtained SnO2 sol–gel nanocrystals for gas sensors

Abstract: In order to clarify the role of the noble metal additives in the gas sensing mechanisms, three of the most common catalytic additives, such as Pd, Pt and Au, have been introduced in a sol–gel obtained tin oxide base material. The additives nominal weight concentrations used were 0.2% and 2%, and they were introduced in the precipitated tin oxide. A posterior calcination treatment was carried out, during 8 h, at the temperatures of 250°C, 400°C, 450°C, 600°C, 800°C and 1000°C. Structural and surface analysis of these nanopowders have been performed. Identification and localisation of metallic, 2+ and 4+ oxidised states of the used noble metals are discussed, and experimental evidences about their effects on the sensor performance are presented. Likewise, effects of their presence on the nanoparticle characteristics, and also on the material sensitivity to CO and CH4, are analysed and discussed.

Blue CoAl2O4 particles prepared by the sol-gel and citrate-gel methods

Abstract: Bright blue CoAl2O4 particles were prepared by the sol−gel and citrate−gel methods using aluminum sec-butoxide, cobalt salts, and citric acid as oxides precursors. Both methods start from sols of the precursor alkoxides and salts, and involve formation of homogeneous solid intermediates, reducing atomic diffusion processes during thermal treatment. This important feature results in a substantial lowering of the time and temperature needed for the formation of the desired compounds. The stages of the formation of CoAl2O4, as well as the characterization of the resulting compounds were done using XRD, FTIR, UV−VIS, SEM, and TGA/DTA techniques. The structure, coloration, particle size, and temperature of formation of the resulting CoAl2O4 phases were found to depend on the precursors and methods used for preparation and the calcination temperature. The lowest temperature for preparation of the blue cobalt aluminate of about 700 °C was obtained using the citrate−gel method. This temperature is much lower than...

The Sol-Gel Process as a Basic Technology for Nanoparticle-Dispersed Inorganic-Organic Composites

Abstract: Nanoparticles containing hybrid materials became of interest for many areas in the last decade. The reason for this is the fact that, in addition to the molecular inorganic-organic hybrid network, the physical, electronical, optical or catalytical properties of nanoparticles resulting from the inorganic crystalline, glassy or metallic properties also can be used for the material tailoring. For this reason, a survey is given over some interesting developments. Furthermore, in case studies, examples are given for the effect of nanoparticles on the two component Ormosil type of hybrids composed of ethyl ortho-silicate (TEOS) and methylethoxy(methoxy)triethoxy silane (MTEOS, MTMOS). It was shown that the 6 nm SiO2-containing nanocomposite hybrid sols can be dried in form of thick films up to 14 μm after a one step dip-coating process and densified crack-free. This is attributed to the increase of relaxation ability and flexibility. This nanocomposite based on TEOS, MTEOS and particulate SiO2 has been used to develop an industrial process for a new type of environmentally friendly glass fiber mat with a temperature resistance up to 600°C.

Sol-Gel Synthesis of Nano-Scaled BaTiO3, BaZrO3 and BaTi0.5Zr0.5O3 Oxides via Single-Source Alkoxide Precursors and Semi-Alkoxide Routes

Abstract: Sol-gel synthesis of nano-sized BaTiO3, BaZrO3 and BaTi05Zr05O3 ceramics using alkoxide and semi-alkoxide routes has been investigated and the pervoskites obtained have been compared with respect to crystallisation temperature, crystallite size and compositional purity Heterometal alkoxides containing two (for BaTiO3 and BaZrO3) and three (for BaTi05Zr05O3) different metals were used as single-source precursors in the alkoxide route while semi-alkoxide synthesis was performed by reacting barium hydroxide or acetate with Ti and/or Zr alkoxides Semi-alkoxide synthesis also produces stoichiometric and phase-pure oxides, however, at temperatures higher than 1000°C At temperatures below 1000°C, BaCO3 and small amounts of other undesired phases (eg, BaTi2O4) were present in the oxides derived from semi-alkoxide synthesis Thermal behaviour, studied by TGA/DTA measurements, shows that thermal decomposition occurs in three major steps and depends on the educt composition and the synthesis route Among alkoxide derived powders, crystalline BaTi05Zr05O3 phase is formed at 400°C while complete crystallisation of BaMO3 ceramics occurs around 600°C The cubic to tetragonal phase transition for BaTiO3 is clearly observed at relatively low-temperature of 800°C The stoichiometry and phase homogeneity of the obtained powders were demonstrated by energy dispersive X-ray analysis and powder diffractometry The averaged crystallite size of the obtained nano-ceramics was evaluated using the FormFit programme SEM and TEM observations revealed a high microstructural uniformity

Nanostructured hybrid organic-inorganic lanthanide complex films produced in situ via a sol-gel approach

Abstract: Communication: Nanostructural hybrid organic-inorganic lanthanide complex films were prepared in situ by use of a novel sol-gel precursor containing pendant triethoxy-silyl and carboxyl groups (see Figure). The resulting transparent and crack-free films gave rise to strong red or green emission, even at low lanthanide ion concentration. Phase separation and lanthanide ion aggregation were controlled at the nanoscale.

Transparent Anatase Nanocomposite Films by the Sol–Gel Process at Low Temperatures

Abstract: We have successfully prepared transparent anatase nanocomposite films on various types of substrates, including organic polymers, using the sol–gel method at temperatures <100°C under ambient pressure. This novel process is based on the findings that (i) anatase nanocrystals are uniquely formed in sol–gel-derived SiO2–TiO2 films that have been subjected to a hot-water treatment, and (ii) the addition of an organic polymer such as poly(ethylene glycol) in the films accelerates the formation of anatase nanocrystals. The film coating on the substrates is a promising candidate for use as a photocatalyst to decompose environmental pollutants and harmful microorganisms.

The synthesis of nanosized TiO2 powder using a sol-gel method with TiCl4 as a precursor

Abstract: Nanosized TiO2 powder with anatase structure was synthesized by a sol-gel method using TiCl4 ethanol solution as a precursor. The grain size of TiO2 powder was homogenous and was about 10 nm after the precursor was calcined at 500 °C for 1 hour. Anatase TiO2 powder formed after the precursor was calcined at a temperature ranging from 300 °C to 550 °C. The gelatinizing mechanism of TiCl4 in ethanol solution can be described as followings. When mixed with ethanol, TiCl4 reacted with ethanol to form TiCl x (OCH2CH3)4 − x species and HCl gas. During gelatinizing process, TiCl x (OCH2CH3)4 − x species absorbed water from atmosphere to form Ti(OH)4 precursor, which was polymerized to be an inorganic polymer. The formation of inorganic polymer of Ti(OH)4 was intensified with gelatinizing time. In contrast, the organic component was removed from the precursor. The formation of anatase TiO2 can also be promoted by increasing gelatinizing time. The influence of alcohol on the reacting progress and dispersivity was also studied. The size and activity of alcohol molecule were found to have influence on the polymerization and mineralization degree of the precursor and the dispersivity of TiO2 powders.

Sol-gel derived hydroxyapatite coatings on titanium substrate

Abstract: Biomaterials, in particular those used for orthopaedic prostheses, consist of a metallic substrate, exhibiting excellent mechanical properties, coated with a ceramic layer, which guarantees resistance to the corrosion and an elevated bioactivity. In this paper the preparation of sol-gel films of hydroxyapatite, HA (Ca10(PO4)6(OH)2), on titanium substrate is described. The samples were obtained through the dip-coating method, starting from a colloidal suspension of hydroxyapatite. In order to increase the adhesion between the HA film and the metallic substrate, the same substrate has been preliminarily coated either with titanium oxide, TiO2 (in the anatase or rutile phase), or calcium titanate, CaTiO3 (perovskite). Also these latter films have been deposited from a sol-gel solution. The characterization of the films through XRD, SEM, and AFM gave good results for the crystallinity of the deposited HA; for what concerns the sample morphology, the films turned out to be homogeneous and crack-free.

FT-IR spectroscopic investigations on sol–gel-derived coatings from acid-modified titanium alkoxides

Abstract: Sol–gel-derived coatings with potentially bioadhesive properties for application on implant metals were prepared from the reaction products of titanium alkoxides and endogenous organic molecules, ie, trans-propene-1,2,3-tricarboxylic acid, l-cysteine, l-leucine and taurine Spin coated titanium substrates and amorphous solids isolated from the modified alkoxide sols were characterized by means of FT-IR spectroscopy Free functional groups, like thiol, carboxylate and sulphonate could be detected On the basis of spectroscopic data, coordination modes were deduced and molecular structure proposals were given for the new species

Highly oriented 3D-hexagonal silica thinfilms produced with cetyltrimethylammonium bromide

Abstract: Mesoporous silica thin films have been produced by sol–gel chemistry in the presence of cetyltrimethylammonium bromide (CTAB) template. The films were deposited on silicon or glass substrates by dip-coating and underwent different treatments to eliminate the CTAB and create porosity. As-prepared and treated coatings exhibit good optical quality. Their structures were fully characterised by transmission electron microscopy (TEM) performed on film cross-sections and by X-ray diffraction (XRD) in θ–2θ scan mode, as well as in transmission mode using two different scattering geometries. The films exhibit large and homogeneous domains organised in a 3D-hexagonal (P63/mmc) structure with the c axis normal to the surface throughout their whole thickness. Numerical analysis of the TEM pictures confirms the space group deduced from the XRD measurements. To our knowledge, these are the first reported thin films obtained by dip-coating in the presence of CTAB which show such extended and highly mono-oriented 3D-hexagonal (P63/mmc) domains. The film thickness, porosity and refractive index were evaluated by ellipsometry for the various treated films.

Synthesis conditions for hexagonal mesoporous silica layers

Abstract: Hexagonal mesoporous silica layers were prepared by the sol–gel route using silicon alkoxides as silica precursors and alkyltrimethylammonium bromides to form the templating liquid crystal mesophase. The synthesis conditions required to obtain well-ordered crack-free layers were investigated. Thin layers exhibiting these properties were deposited from diluted sols. X-Ray diffraction enabled characterisation of the thermal evolution of their ordered structure and crystalline texture. Their porosity was experimentally measured from nitrogen adsorption–desorption analyses carried out directly on the thin layers. Two main synthesis parameters were identified. The first is the ageing time of the sol before deposition: 29Si NMR showed that the disappearance of the ordered structure in the layers is related to the appearance of the Q3 species in the sol. The second important synthesis parameter is the surfactant volume fraction in the medium after the removal of the volatile components. Using hexadecyltrimethylammonium bromide, well-ordered hexagonal layers were obtained for surfactant volume fractions ranging from 0.5 to 0.65, as expected from the corresponding water–surfactant binary diagram. For surfactants with shorter alkyl chains, the domain of existence of the hexagonal layers shifts to higher surfactant volume fractions in agreement with the evolution previously observed on the water–surfactant binary diagrams.

Synthesis and characterization of nanocomposite of polyimide–silica hybrid from nonaqueous sol–gel process

Abstract: Polyimide–silica (PI–SiO2) hybrids with a nanostructure was obtained using the nonaqueous sol–gel process by polycondensation of phenyltriethoxysilane in a polyamic acid solution. Self-catalyzed hydrolysis of phenyl-substituted akoxysilane and modification on the polyimide structure are applied and result in highly compatible PI–SiO2 hybrids. Transparent PI–SiO2 with a high silica content of about 45% was thus obtained. The prepared PI–SiO2 films were characterized by infrared spectrometry, 29Si-NMR, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. These characterizations showed the silica influence on the properties of the hybrid. The thermal expansion coefficient of the PI–SiO2 and the temperature correlation were also established for probing the potential for application. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1609–1618, 2000

Preparation, structure and properties of TiO2–PVP hybrid films

Abstract: The organic–inorganic hybrid films of TiO 2 –PVP have been prepared by the sol-gel process. The TiO 2 was obtained by the hydrolysis of titanium n -butoxide (TNBT) under the catalysis of HCl. The hybrid film has a smooth surface with average roughness less than 3 nm. The thermal stability and structure of the gels have been studied by TGA and FTIR spectra, respectively. The TiO 2 –PVP films are transparent and their transmittance is high up to 93% in the visible light range. The microhardness and dielectric properties for the hybrid material films are dependent on the annealing temperature. The microhardness of the film reaches 2.65 GPa after heat treatment at 270°C for 0.5 h.

Structural study of 3D-hexagonal mesoporous spin-coated sol–gel films

Abstract: Mesostructured films deposited on glass substrates by spin coating are studied by X-ray diffraction, small angle X-ray scattering using grazing incidence (GISAXS) and high resolution electron microscopy. The films generally present a gradient of ordering from the air/sol interface to the interior of the film, with a 3D-hexagonal mesophase near the surface and a disordered micellar structure at the interior. However, the 3D-hexagonal ordering and the texturing can be extended over all the film thickness by adjusting the composition of the deposited solution. In particular, critical values are clearly observed both for the size of silica units forming the walls in the mesophase and for the film thickness. All these results can be understood from the competition between structuration and gelation of the films during alcohol removal.

Catalysts and the structure of SiO2 sol-gel films

Abstract: In the fabrication of silica films by sol-gel synthesis, the role of various catalysts is systematically examined. The precursor solutions were made by mixing tetraethylorthosilicate (TEOS), ethanol and water in the molar ratio of 1 : 2 : 2. The spin coated films were thermally treated at various temperatures, and characterised using ellipsometry, molecular probe ellipsometry (MPE), infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and optical scattering. By employing different catalysts with the same concentration (0.1 M in water), it was found that dramatic effects on the porosity, optical quality, shrinkage, thickness, structural evolution during thermal treatment, and film stress can be observed.

Sol-Gel and Ultrafine Particle Formation via Dielectric Tuning of Inorganic Salt-Alcohol-Water Solutions.

Abstract: Under some conditions, inorganic salts can be as good precursors for sol–gel-type processing as those obtained from expensive metalloorganic precursors such as alkoxides. In this work, the formation of monodispersed hydrous zirconia microsphere particles (particularly nanosized) and gels was achieved in solutions of zirconyl chloride dissolved in alcohol–water mixed solvents. The dielectric property of the mixed alcohol–water solvent directly affects the nucleation and growth of zirconia clusters/particles in homogeneous solutions. A lower dielectric constant of mixed solvent corresponds to a lower solubility of inorganic solute and, thus, a shorter induction period for nucleation as well as higher solid particle growth kinetics. Dynamic light scattering (DLS) was used to monitor the homogeneous nucleation and growth processes, while final particles and gels were studied by scanning electron microscopy (SEM) and high-temperature X-ray diffraction (HTXRD). The sol–gel processes in the mixed solvent system can be adjusted using the processing parameters, including the initial inorganic salt concentration ( C ), alcohol/aqueous medium volume ratio of the mixed solution (RH), incubation temperature ( T ), incubation time ( t ), concentration of hydroxypropyl cellulose (HPC), and ammonia neutralization. Monodispersed submicron and nanoscale ( −3 g/cm 3 ) and ammonia neutralization. Initial salt concentration affects the particle size significantly. Gel materials were obtained under conditions of low RH (1.0). Microstructure and transparency of gels changed significantly from low (0.05 M) to high (0.2 M) concentration of the metal salt. We have also demonstrated that monodispersed particle production can be achieved not only at low temperatures (