Photochemistry of azobenzene in sol-gel systems

The photophysical and photochemical behavior of azobenzene incorporated into sol-gel systems was studied. Sols doped with azobenzene were prepared by the hydrolysis of tetramethoxysilane. The absorption spectra of azobenzene in sols with and without HCl as a catalyst showed photochromism; UV irradiation changes trans-azobenzene to cis-azobenzene, which returns to trans-azobenzene by successive visible light irradiation. The results indicate that azobenzene is not protonated in the sol prepared by the present conditions. The absorption spectra showed that the azobenzene-doped xerogel prepared without HCl is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption did not affect photochromic behavior and photo-reversible changes were observed in the xerogels. UV photolysis of the azobenzene-doped xerogel prepared with HCl so produced protonated benzo[c]cinnoline that photochromic behavior was deteriorated. Surface modified xerogels were prepared from the mixture of tetraethoxysilane and methyltriethoxysilane in order to make clear the effect of the surface silanol groups. It was shown that the formation of the protonated benzo[c]cinnoline is suppressed by the introduction of Si-CH₃. These present results confirm that the acidic sites of Si-OH₂⁺ of xerogels play an important role in the photochemical reaction of azobenezene in silica xerogels.     

Characterization of low-density silica xerogel formed into microspheres

Low-density silica xerogels were formed into spherical pellets using a process in which a xerogel suspension containing sodium alginate was solidified through free fall in a solution of divalent ions that induce the droplet solidification. Pellet preparation was also carried out at laboratory scale by using a syringe instead of the material-consuming unit used for scaling up. When beads are made from a suspension of dried-and-calcined xerogel material, the textural properties such as porosity and surface area are significantly altered. Both properties are reduced by 30-50% compared to the starting reference material. Properties were improved when a suspension of dried xerogel that is less sensitive to contact with water was used: specific surface area was maintained at its initial level and porosity decreased by 25%.     

The structural evolution of alkoxy-derived gel during drying

The effects of drying temperature on the structural evolution of alkoxy-derived silica gel prepared using various catalysts have been investigated. The dependence of specific surface area, S_g, reflecting the structure, on the temperature of drying was remarkable for a non-catalyzed xerogel. The effect of drying temperature on the S_g of an ammonia-catalyzed xerogel was also found but was not very large. The S_g of xerogels obtained by drying at 60°C was always higher by 50% than the gels dried at 30°C without regard to the aging temperature. The S_g of xerogels from HCl-catalyzed solution was of the order of several m²/g, however, the S_g of the aerogel obtained by hypercritical drying of the wet gel from a similar solution was about 800 m²/g. These phenomena were understood on the basis of SAXS measurements on both wet gels and aerogels.     

Structural evolution up to 1100 °C of xerogels prepared from TEOS sonohydrolysis and liquid phase exchanged by acetone

Silica xerogels were prepared from sonohydrolysis of tetraethoxysilane and exchange of the liquid phase of the wet gel by acetone. Monolithic xerogels were obtained by slow evaporation of acetone. The structural characteristics of the xerogels were studied as a function of temperature up to 1100 °C by means of bulk and skeletal density measurements, linear shrinkage measurements and thermal analyses (DTA, TG and DL). The results were correlated with the evolution in the UV-Vis absorption. Particularly, the initial pore structure of the dried acetone-exchanged xerogel was studied by small-angle X-ray scattering and nitrogen adsorption. The acetone-exchanged xerogels exhibit greater porosity in the mesopore region presenting greater mean pore size (∼4 nm) when compared to non-exchanged xerogels. The porosity of the xerogels is practically stable in the temperature range between 200 °C and 800 °C. Evolution in the structure of the solid particles (silica network) is the predominant process upon heating up to about 400 °C and pore elimination is the predominant process above 900 °C. At 1000 °C the xerogels are still monolithic and retain about 5 vol.% pores. The xerogels exhibited foaming phenomenon after hold for 10 h at 1100 °C. This temperature is even higher than that found for foaming of non-exchanged xerogels.     

Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Undoped and Mn²⁺-doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and fluorescence and Raman spectroscopies. Raman measurements over the range 4–1200 cm⁻¹ showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn²⁺ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn²⁺ hampers the destruction of three- and four-membered rings. In the low-wave number region (4–100 cm⁻¹), doping with manganese ions was found to affect the position of the boson peak. The boson peak profiles were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm).     

Solid-state NMR and XANES studies of lithium and silver silicate gels synthesized by the sol-gel route

The objective of this study is to understand the effect of low temperature sol-gel synthesis on the microstructural properties of lithium [xLi₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] and silver [xAg₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] silicate xerogels via solid state nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) techniques. The Li silicate xerogels were analyzed with solid-state ⁷Li and ²⁹Si NMR and the Ag silicate xerogels were studied with Ag XANES. At high Li loading, ⁷Li NMR shows quadrupolar satellite transitions attributed to LiNO₃, a phase also found with X-ray diffraction (XRD). At low Li loading, both NMR and XRD results show an amorphous xerogel. The silicate network is monitored with ²⁹Si NMR and shows evidence of Li incorporation. For the Ag silicate xerogels, Ag-L-III XANES spectral studies show a local environment similar to AgNO₃ for low Ag loading levels, and an increased Ag oxidation for higher Ag loading levels. Si K edge spectra show only an amorphous phase, with no evidence of a crystalline quartz phase. The electrical conductivity of the lithium silicates was estimated from impedance data and the highest conductivity is exhibited by the 0.3Li₂O-0.7SiO₂ composition xerogel. The conductivity dependence on loading level strongly suggests that the observed conductivity is due to Li⁺ mobility. However, further experimental studies are needed to rule out the possibility that the conductivity is, at least in part, due to H⁺ mobility. Variation in conductivity is explained qualitatively using existing theoretical models.     

Emission enhancement of Eu(III) and/or Tb(III) ions entrapped in silica xerogels with ZnO nanoparticles by energy transfer

Luminescent materials consisting of Eu(III) and/or Tb(III) ions and quantum dots of the ZnO semiconductor were immobilized in oxide xerogels by the impregnation method. The material’s excitation spectra showed the characteristic absorption bands of lanthanide(III) ions and sharp bands at low wavelengths attributed to excitons in the semiconductor nanoparticles. The luminescence emission of Ln(III) ions (Ln = Tb and/or Eu) showed very low intensity when typical excitation wavelengths for both ions were used (λ_exc = 394 for Eu(III), 350 nm for Tb(III)). In three-component materials (ZnO quantum dots and Ln(III) ions in oxide xerogels) only the Tb(III) emission could be improved by using an excitation wavelength corresponding to the position of one of the exciton absorption bands. The energy transfer enhancing the Eu(III) emission was possible in the four-component system (ZnO quantum dots plus Tb(III) and Eu(III) incorporated into the oxide matrix). The best results of the Eu(III) emission intensity were obtained when the silica xerogel served as the matrix of the four-component material thermally treated at 80 °C.     

Effect of the counter-ion of the basification agent on the pore texture of organic and carbon xerogels

Organic and carbon xerogels were prepared by polycondensation of resorcinol with formaldehyde in water, followed by evaporative drying and, eventually, pyrolysis. The pH of the precursor’s solution was fixed at 6.0 in all cases by adding various hydroxides as basification agent. Three alkali metal hydroxides (LiOH, NaOH and KOH) and three alkaline earth metals hydroxides (Ca(OH)₂, Ba(OH)₂, Sr(OH)₂) were used. It was found that the pore texture of the organic and carbon xerogels is totally independent on the cation size, but depends on the charge and concentration of the counter-cation. Indeed, the pore size of the alkaline earth metal loaded samples is larger than that of the alkali metal-doped xerogels. As a matter of fact, to reach the same initial pH, the concentration in alkali metal hydroxide must be twice that of the alkaline earth metal base. The effect of ions on the pore texture was thus attributed to electrostatic effects on the microphase separation process that occurs prior to gelation.     

Preparation and characterization of monolithic methylsilicone xerogels doped with liquid-phase synthesized TiO2

The xerogels of methylsilicone doped with TiO₂ were prepared by incorporation of liquid-phase prepared TiO₂ in methylsilicone matrix. The microstructure and properties of the composite xerogels were investigated by Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), N₂ sorption, and wide-angle X-ray powder diffraction (XRD). The results indicated that the as-prepared TiO₂ particles could be well dispersed in methylsilicone matrix. The different amounts of TiO₂ doped in methylsilicone oligomer solution resulted in different reaction process and microstructure. The thermal stability of the composite monolith was enhanced after doping TiO₂ particles.     

Preparation of silica microspheres coated with V2O5 xerogel and V2O5/WO3 composite xerogel via adapted Stöber process

In the present study, we have synthesized spherical silica particles covered with either layered V₂O₅ xerogel or V₂O₅/WO₃ mixed xerogel via an adapted Stöber method using an ammonia solution of methyltriethoxysilane (MTES). The SEM images revealed the presence of a lamellar V₂O₅ xerogel layer coating the silica particles, as evidenced by the (0 0 1) diffraction lines. Both materials have voltammetric behavior that is quite similar to that of the V₂O₅ xerogel. The catalytic activity of the silica particles coated with the V₂O₅/WO₃ mixed xerogel was evaluated in the epoxidation of styrene using iodosylbenzene as an oxidant.     

Changes in the porous structure of silica gels during the gel-to-glass conversion

The changes in porous structure during the conversion from the gel to the SiO₂ glass have been studied through the xerogel and aerogel routes. For both gels, selective collapse of relatively large pores was observed during the first contraction stage (ΔL/L₀ = 0−38%). In the xerogel route, the size distribution of smaller pores less than 100 Å in diameter also changed slightly, but there was no difference in the pores in the aerogel route. In the second stage, both the large pores which remained and the smaller pores collapsed at the same time. However, it was characteristic that once the aerogel was converted to the porous structure it contained uniformly sized pores (150 Å in diameter) during the densification. This phenomenon requires spherical particles (500–1000 Å in diameter) which we observed using a SEM arranged in a near close-packed structure. The results indicate that the contraction of the aerogel occurs by the rearrangement of the particles which move individually, while the particles in the xerogel route behave like a cluster.     

Erbium-activated silica xerogels: spectroscopic and optical properties

Silica-based sol–gel glasses activated by Er³⁺ ions are attractive materials for integrated optics (IO) devices such as frequency upconverters and optical amplifiers. Monolithic erbium-activated silica xerogels with erbium content ranging from 0 up to 40 000 ppm were prepared by the sol–gel technique. Samples were densified by thermal treatment in air at 950°C for 120 h. The densification degree and the relative content of hydroxyl groups were studied by Raman spectroscopy. Refractive indices were measured at 632.8 and 543.5 nm by a prism coupling technique. Green to blue and violet upconversion luminescence upon continuous-wave excitation at 514.5 nm was observed for all samples. Emission at 1.5 μm, characteristic of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions, was observed at room temperature for all samples upon continuous-wave excitation at 980 nm. For the 5000 Er/Si ppm-doped xerogel, a photoluminescence was observed and a lifetime of 8 ms for the metastable ⁴I_13/2 level was measured.     

Effects of poly(vinyl alcohol) additions on the structure of silica xerogels

Poly(vinyl alcohol)/silica hybrid xerogels were prepared from sonohydrolysis of tetraethoxysilane (TEOS) and additions of water-solution of poly(vinyl alcohol) (PVA). The samples were studied by small-angle X-ray scattering (SAXS), nitrogen adsorption, and differential scanning calorimetry (DSC). On drying at room temperature the resulting xerogels exhibit a fairly bimodal porous structure composed by small mesopores and micropores. The pore size distribution of the mesopores was found to follow approximately a power-law with the pore size. The micropore structure was associated to an evolution at a high resolution level of the mass fractal structure of the original wet gels. The role of the PVA addition on the pore structure of the xerogels is to diminish the specific surface area and the pore volume without to change substantially the pore mean size.     

Spherical and lamellar octadecylsilane hybrid silicas

Silicas bearing different contents of octadecylsilane groups were synthesized by the sol–gel method and characterized by solid-state magic angle spin ¹³C nuclear magnetic resonance spectroscopy, Raman spectroscopy, attenuated total reflectance infrared spectroscopy, small angle X-ray scattering, laser light scattering and atomic force microscopy. A structural model for such hybrid materials is proposed in which spherical or lamellar morphology, fern-like or Porod’s structure, are proposed depending on the tetraethyl orthosilicate (TEOS)/octadecylsilane (ODS) molar ratio. The effect of the ODS addition time on the chain conformation and on the particle morphology and texture was also investigated. The degree of organization lowered as the amount of ODS increased. The nanostructured xerogel obtained in the case of pure TEOS evolutes from spherical to lamellar patterns, as the amount of octadecylsilane is increased.     

Methods for the preparation of bimetallic xerogel catalysts designed for chlorinated wastes processing

The aim of this work is to simplify and generalize the synthesis procedure of bimetallic supported catalysts by sol–gel process. For Pd–Ag/SiO₂ co-gelled xerogels catalysts a number of synthesis procedures were compared: use of one or two specific alkoxides able to form a chelate with palladium and/or silver cations, reagent mixing in one or two steps, use of industrial grade chemicals instead of laboratory grade chemicals. The catalysts obtained are quite similar: same metal dispersion, same tailored morphology, same localization and accessibility of Pd–Ag alloy nanoparticles inside microporous silica, same activity and selectivity for hydrodechlorination of 1,2-dichloroethane into ethylene. For catalyst production at large scale the synthesis can be achieved in one step with 3-(2-aminoethyl)aminopropyltrimethoxysilane of industrial grade as chelating alkoxide, tetraethylorthosilicate (TEOS) of industrial grade and ethanol denatured with diethyl phthalate.     

溶剂热合成球形Cu2ZnSnS4 （CZTS）微粉的研究

本文采用高压溶剂热法合成了铜锌锡硫(CZTS)粉体,采用XRD、SEM测试方法对合成产物的物相、形貌进行了表征。探讨了温度,时间以及表面活性剂等因素对溶剂热合成CZTS粉体形貌、物相和性能的影响。结果表明:高压溶剂热法制备的CZTS目标粉体纯度高,制备周期短,产物形貌为球形的片状集合体。反应进程随着反应温度的升高而加快,随着保温时间的延长而趋于完全。加入部分表面活性剂对团聚现象改善并不明显。     

Low-temperature specific heats of porous silica xerogels of low densities

Low-temperature specific heat measurements have been performed in porous silica xerogels with densities varying from 670 to 1730 kg m⁻³ to study the low-energy vibrational dynamics. The specific heat, C_p, shows a bump in the temperature range above 4 K, when reported in a plot of C_p/T³ against the temperature, T. The bump is almost independent of the sample density and is close to the boson peak observed in melt-quenched amorphous silica (a-SiO₂). At temperatures <4 K, an additional contribution to that predicted by the Debye theory is observed. It follows an approximately linear temperature dependence (C_exc=aT^1+v, v being equal to about 0.25). In the xerogel with the largest density, specific heat of about a factor 5 larger than that of a-SiO₂ is measured, which increases with decreasing sample density. By comparison with the corresponding properties of a-SiO₂, we conclude that the disorder introduced by the presence of pores does not measurably affect the excess density of vibrational states in a frequency range of the boson peak (BP), but increases the density of the two-level systems (TLS).     

Effects of organic content and H2O/TEOS molar ratio on the porosity and pore size distribution of hybrid naphthaleneaminepropylsilica xerogel

The hybrid naphthaleneaminepropylsilica material was obtained by a sol-gel route, varying the organic loading and the water/TEOS molar ratio. Infrared spectroscopy was used to identify the organic and inorganic phases. The morphology of the hybrid material was studied by using scanning electron microscopy and N₂ adsorption-desorption isotherms. It was observed that the increase in the organic content produces a decrease in the size and volume of the pores as well as in the surface area of the xerogel. The best porosity was obtained for water/TEOS molar ratio between 4 and 6.     

Luminescence of CaS and MnS nanocrystallites co-activated sol-gel derived silica xerogel

CaS and MnS nanocrystallites co-activated sol-gel derived silica xerogel has been prepared by sol-gel processing. Their photoluminescence characteristics have been evaluated and compared with those of the undoped silica xerogel. Two emission bands have been observed from the doped sample, one at 440 nm while the other at 580 nm. CaS and MnS nanocrystallites embedded in sol-gel derived silica xerogel show sharp emission band. The novel luminescence phenomenon is attributed to the luminescent centers of CaS and MnS in the silica xerogel.     

Ordered structure and preferred orientation of boehmite films prepared by the sol-gel method

Several boehmite films were produced using the sol-gel route. The drying process, the film structure evolution and the final texture of the samples were investigated by means of combined X-ray diffraction and weight variation. A remarkable (0 1 0) preferred orientation (about 97% of the crystallites) was found for xerogel films with thickness ranging from 1.5 to 24 μm. The ordered structure is already present in the sol state and is appeared and enhanced during drying, before gelling. This phenomenon may be attributed to the particular boehmite structure that induces special crystallite organization in the aquagel.