Showing papers on "Sol-gel published in 2001"

Use of Epoxides in the Sol−Gel Synthesis of Porous Iron(III) Oxide Monoliths from Fe(III) Salts

Abstract: Iron oxide-based porous solids were prepared by a sol−gel process using Fe(III) salts in various solvents. It was observed that the addition of propylene oxide to Fe(III) solutions resulted in the formation of transparent red-brown monolithic gels. The resulting gels were converted to either xerogels by atmospheric drying or aerogels by supercritical extraction with CO2(l). Some of the dried materials were characterized by nitrogen adsorption and desorption analysis and transmission electron microscopy (TEM). The results of those analyses indicate that the materials have high surface areas (∼300−400 m2/g), pore sizes with mesoporic dimensions (2−23 nm), and a microstructure made up of 5−10 nm diameter clusters of iron(III) oxide. The dependence of both gel formation and its rate was studied by varying the epoxide/Fe(III) ratio, the Fe(III) precursor salt, amount of water (H2O/Fe(III)) present, and the solvent employed. All of these variables were shown to affect the rate of gel formation and provide a con...

New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

Abstract: We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+, Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Nanostructured energetic materials using sol-gel methodologies

Abstract: We have utilized a sol–gel synthetic approach in preparing nano-sized transition metal oxide components for new energetic nanocomposites. Nanocomposites of Fe 2 O 3 /Al(s), are readily produced from a solution of Fe(III) salt by adding an organic epoxide and a powder of the fuel metal. These materials can be processed to aerogel or xerogel monolithic composite solids. High resolution transmission electron microscopy (HRTEM) of the dried energetic nanocomposites reveal that the metal oxide component consists of small (3–10 nm) clusters of Fe 2 O 3 that are in intimate contact with ultra fine grain (UFG) ∼25 nm diameter Al metal particles. HRTEM results also indicate that the Al particles have an oxide coating ∼5 nm thick. This value agrees well with analysis of pristine UFG Al powder and indicates that the sol–gel synthetic method and processing does not significantly perturb the fuel metal. Both qualitative and quantitative characterization has shown that these materials are indeed energetic. The materials described here are relatively insensitive to standard impact, spark, and friction tests, results of which will be presented. Qualitatively, it does appear that these energetic nanocomposites burn faster and are more sensitive to thermal ignition than their conventional counterparts and that aerogel materials are more sensitive to ignition than xerogels. We believe that the sol–gel method will at the very least provide processing advantages over conventional methods in the areas of cost, purity, homogeneity, and safety and potentially yield energetic materials with interesting and special properties.

The defect structure of sol–gel-derived silica/polytetrahydrofuran hybrid films by FTIR

Abstract: The structural perturbations induced by polytetrahydrofuran (PTHF) on sol–gel hybrid films were identified by Fourier-transform infrared (FTIR) spectroscopy. The films were prepared by spin-coating, from aged solutions containing tetraethylorthosilicate (TEOS) as the inorganic precursor and different concentrations of PTHF. All the spectra reveal a set of bands that may be associated with structural defects of the silica network. The hybrid films show an additional `defect' band at 560 cm −1 , assigned to a skeletal vibration of 4-fold siloxane rings, whose intensity grows as the polymer content or molecular weight increases. The relative intensity of two components of the νasSi–O–Si mode (resolved by deconvolution of the band at ∼ 1080 cm −1 ) grow accordingly. They were thus assigned to the longitudinal (LO) and transverse optical (TO) modes of that vibration in 4-fold siloxane rings. Simultaneously, the bands assigned to the νSi–O− mode and to the νSi–O(H) mode of unreacted silanol groups (obtained by deconvolution of the band at ∼ 950 cm −1 ) increase. These conjugated observations lead to the conclusion that the polymer hinders the condensation reactions, being responsible for a more porous structure, with retention of a larger proportion of 4-fold siloxane rings. For high concentrations of high molecular weight PTHF, the defect structure of the films indicates that a partial segregation of the polymer occurs.

Synthesis and electrochromic properties ofmesoporous tungsten oxide

Abstract: The electrochromic properties of mesoporous tungsten oxide are described and compared to standard sol–gel films of WO3. The block copolymer template used in the synthesis was removed by solvent extraction and calcination methods, leading to mesoporous tungsten oxides which range from amorphous to crystalline depending on the removal treatment. The pores are not ordered but do exhibit a fairly regular diameter with an average size of 4 to 5 nm. The mesoporous tungsten oxides exhibit a well behaved electrochromic response and their electrochemical and optical reversibilities are better than those of the analogous sol–gel film. In addition, the mesoporous films exhibit higher rates for coloration and bleaching. The high surface area of these materials is responsible for this behavior and also leads to multiple peaks in the voltammetric scans.

Preparation and properties of transparent conductive aluminum-doped zinc oxide thin films by sol–gel process

Abstract: Highly conductive and transparent aluminum-doped zinc oxide thin films have been prepared from the solution of zinc acetate and aluminum nitrate in ethanol by the sol–gel process. The effect of changing the aluminum-to-zinc ratio from 0 to 5 at. % and annealing temperature from 0 °C to 700 °C in air has been investigated. The resistivities of thin films were measured as a function of annealing temperature and also as a function of aluminum dopant concentration in the solution. As-deposited films have high resistivity and high optical transmission. Annealing of the as-deposited films in air leads to a substantial reduction in resistivity. The films have a minimum value of resistivity of 1.5×10−4 Ω cm for 0.8 at. % aluminum-doped zinc oxide and a maximum transmission of about 91% when deposited on glass substrates. X-ray diffraction measurements employing Cu Kα radiation were performed to determine the crystallinity of the ZnO:Al films which showed that the films were polycrystalline with a hexagonal structure when annealed at 500 °C.

Preparation of microstructure compatible porous supports by sol–gel synthesis for catalyst coatings

Abstract: To enhance the inner surface of microchannel reactors, porous metal oxide coatings based on alumina, silica, and titanium oxide have been developed as support for catalytically active components. The oxides were made by the sol–gel process. Thin film coating was accomplished by dip coating using an Fe-based alloy as support. The influence of the sol composition, sol viscosity, and thermal treatment procedure on the surface enhancement factor and the porosity of the coating was investigated. The resulting surface enhancement factor reached 800 m 2 /m 2 .

Imprinted Polysilsesquioxanes for the Enhanced Recognition of Metal Ions

Abstract: Imprinting techniques have been used to synthesize organic−inorganic hybrid polymers with ionic recognition characteristics. 1,2-bis(triethoxysilyl)ethane (BTSE) monomers were copolymerized with metal ion complexes of N-[3-(trimethoxysilyl)propyl]ethylenediamine ((TMS)en) in the presence of supramolecular assemblies of cetyltrimethylammonium chloride (CTAC), via room temperature, hydrolytic, sol−gel procedures. Subsequent extraction of the surfactant and metal ions results in a porous material with a high affinity for the metal ion template. Selective sorbents have been synthesized for Cu(II), Ni(II), and Zn(II). Adsorption studies indicate that these materials preferentially sequester the metal ion template from mixed solutions and exhibit fast uptake kinetics. These materials have been characterized by nitrogen gas adsorption, small-angle X-ray scattering, solid state 13C NMR, and UV−visible spectroscopy.

Effects of Li and Mg doping on microstructure and properties of sol-gel ZnO thin films

Abstract: Zinc oxide thin films doped with Li and Mg were prepared by the sol-gel method, and effects of doping on microstructure and electrical properties were examined The doped films exhibited c-axis-orientation after final heating at 500°C for 30 min in flowing oxygen The ZnO crystallite size increased by doping and the surface of the films became rougher The current density of the films was reduced by doping probably due to the formation of acceptor levels (Li-doping) and the reduction of oxygen defects (Mg-doping) The film with a nominal composition of Zn085Li010Mg005O showed the lowest current density of 17×10−6 A cm−2 in the present study

Structural, Optical, and Photoelectrochemical Properties of Nanocrystalline TiO2−In2O3 Composite Solids and Films Prepared by Sol−Gel Method

Abstract: Nanocrystalline titanium and indium oxides and TiO2−In2O3 composites have been prepared by sol−gel technique using concentrated hydrous titanium dioxide and indium hydroxide sols. Structure, optical, and photoelectrochemical properties of the composites with different molar ratios of TiO2 to In2O3 have been studied. The binary oxide films and xerogels annealed at temperatures from 200 to 700 °C have a nanocrystalline structure without the chemical interaction between the oxides. However, there is a strong mutual influence of the components of the binary oxide systems, during their annealing which manifests itself in thermogravimetry and differential scanning calorimetry measurements and in a marked effect on the oxide crystallite size. Highly transparent TiO2−In2O3 films fabricated by spin-coating technique exhibit extreme dependencies of the various optical and photoelectrochemical properties on the film composition. The band gap edge of the composite films is blue-shifted, as compared with that of singl...

Sol-Gel Processed TiO2-Based Nano-Sized Powders for Use in Thick-Film Gas Sensors for Atmospheric Pollutant Monitoring

Abstract: Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dope TiO2 nano-sized powders. The thermal decomposition behaviour of the precursors was studied using simultaneous thermogravimetric and differential thermal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the powders heated to 400°C were crystalline in the anatase TiO2 structure. The pure TiO2 powder heated to 850°C showed the rutile structure. The addition of Ta and Nb inhibited the anatase-to-rutile phase transformation up to 950–1050°C. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired for 1 h at different temperatures in the 650–1050°C range. Scanning electron microscopy (SEM) observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at a nanometric level even at 950°C, though grain growth was observed with increasing temperature. The gas-sensitive electrical response of the thick films were tested in laboratory, in environments with CO in dry and wet air. Conductance measurements showed a good gas response only for the nanostructured titania-based films. For field tests, the prototype sensors were placed beside a conventional station for atmospheric pollutant monitoring. The electrical response of the thick films was compared with the results of the analytical instruments. The same trend was observed for both systems, demonstrating the use of gas sensors for this aim.

Alkoxy sol-gel derived Y3−xAl5O12:Tbx thin films as efficient cathodoluminescent phosphors

Abstract: Thin-film Y3−xAl5O12:Tbx3+ (YAG:Tb) phosphor derived from a sol-gel chemistry is analyzed by x-ray diffraction, scanning electron microscopy, photoluminescence (PL) and cathodoluminescence (CL). The metal alkoxides organic precursors were chosen as the starting materials to form the sol-gel. This liquid sol-gel was spin coated on sapphire and silicon substrates to form the uniform thin films, then crystallized by annealing. The PL intensity of the crystallized film at 545 nm green emission was 15 times higher than that of the as-coated noncrystalline film. CL measurements show that luminance and efficiency are comparable to the films deposited by other techniques.

Preparation of titania mesoporous materialsusing a surfactant-mediated sol–gel method

Abstract: Small titania particles were generated within the template of a non-ionic surfactant (TX-100) reverse micelle assembly by the sol–gel method from titanium(IV) isopropoxide. Cyclohexane was used as the reaction solvent to form a reverse micelle system with a low water to surfactant ratio. Gelling, i.e., hydrolysis and polycondensation, were observed by means of viscosity measurements. The transparent raw gels obtained were dried under vacuum at 372 K. The dry gels were then either thermally decomposed directly to TiO2 or the organic content (surfactant) was substantially reduced first by supercritical fluid extraction with CO2 and then thermally treated. Special attention was paid to the effect of temperature on the extent of crystallisation of individual crystallographic phases of TiO2. Gels, partly decomposed gels and gels after the supercritical fluid extraction were investigated by XRD, XPS, IR, TPR, thermogravimetry, mass spectrometry, elemental analysis, UV–vis, SEM and extensive gas adsorption measurements. Their properties were related to the structure and other features of the final nanosized TiO2 particles. The anatase form of TiO2 with a very high specific surface area and well-developed mesoporous and microporous structure was prepared.

Sol-gel PZT and Mn-doped PZT thin films for pyroelectric applications

Abstract: Thin films of ferroelectric lead zirconate titanate (PbZr0.3Ti0.7O3 PZT30/70) and manganese-doped lead zirconate titanate ((Pb(Zr0.3Ti0.7)1-xMnx)O3, where x = 0.01, PM01ZT30/70; and x = 0.03, PM03ZT30/70) have been prepared using sol-gel processing techniques. These materials can be used as the pyroelectric thin films in uncooled infrared detectors. The thin films were prepared via a sol-gel route based on a hybrid solvent of methanol and ethanol with acetic acid, ethanolamine and ethylene glycol as additives. The final solution is non-moisture sensitive and stable. Films deposited on Pt/Ti/SiO2/Si substrates and annealed on a hot plate at 500-530 °C for a few minutes were seen to fully crystallize into the required perovskite phase and showed excellent ferroelectric behaviour, demonstrated by reproducible hysteresis loops (Pr = 33-37 µC cm-2, Ec( + ) = 70-100 kV cm-1, Ec(-) = -170 to -140 kV cm-1). The pyroelectric coefficient (p) was measured using the Byer-Roundy method. At 20 °C, p was 2.11×10-4 C m-2 K-1 for PZT30/70, 3.00×10-4 C m-2 K-1 for PM01ZT30/70 and 2.40×10-4 C m-2 K-1 for PM03ZT30/70 thin films. The detectivity figure-of-merit (FD) was 1.07×10-5 Pa-0.5 for PZT30/70, 3.07×10-5 Pa-0.5 for PM01ZT30/70 and 1.07×10-5 Pa-0.5 for PM03ZT30/70. These figures compare well with values reported previously.