SiO2 Xerogel Research Articles

Size effects on the magnetic properties of a system of ε-Fe2O3 nanoparticles embedded in a SiO2 xerogel matrix

A representative series of samples consisting of fine ε-Fe2O3 nanoparticles uniformly distributed over the SiO2 xerogel matrix with an iron oxide content of 5–33 wt % has been synthesized and studied by mutually complementary physical methods. It has been shown by the X-ray diffractometry and high-resolution electron microscopy examination that, with increasing iron oxide concentration, the average particle size <d> increases from 4 to 11 nm. According to the X-ray diffractometry and Mӧssbauer spectroscopy data, the samples with an iron oxide content of 5–20 wt % are single-phase, while at the highest Fe2O3 concentration (33 wt %), the β-Fe2O3 and α-Fe2O3 phases arise. As the average particle size <d> increases, a monotonic increase in coercivity HC and remanent magnetization MR of the synthesized systems at room temperature is observed, which is indicative of their magnetic hysteresis. The magnetic transition known to occur in the ε–Fe2O3 oxide manifests itself in all the investigated samples as a drastic change in the HC and MR values at 150–75 K. At the same time, a thorough analysis of the temperature dependence of the real part of the ac magnetic susceptibility χ′ has shown that the particles with a size smaller than a critical value of dC ∼6.5 nm do not undergo the magnetic transition and have a much lower magnetic anisotropy constant as compared with coarser particles (d > dC). It has been found that, in the low-temperature region, the magnetic moments of these fine particles experience superparamagnetic blocking. It has been established that the size effects that arise in ultra-fine ε–Fe2O3 particles has a serious impact on the macroscopic magnetic properties of the highly dispersed systems based on them.

Synthesis and Investigation of Novel Optical Active SiO2 Glasses with Entrapped YAG:Ce Synthesized via Sol-Gel Method.

We present a crack-free optically active SiO2 glass-composite material containing YAG:Ce synthesized via a modified sol-gel technique. A glass-composite material consisting of yttrium aluminum garnet doped with Ce3+ (YAG:Ce) was entrapped into a SiO2 xerogel. This composite material was prepared using a sol-gel technique with modified gelation and a drying process to obtain crack-free optically active SiO2 glass. The concentration of the YAG:Ce was from 0.5 to 2.0 wt%. All synthesized samples were characterized via X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, confirming their exceptional quality and structural integrity. The luminescence properties of the obtained materials were studied. Overall, the prepared samples' excellent structural and optical quality makes them great candidates for further investigation, or even potential practical application. Furthermore, boron-doped YAG:Ce glass was synthesized for the first time.

A usable concept for the indentation of thin porous films

Abstract The paper deals with the application of Pharr’s concept of the effectively shaped indenter as well as an extension of that concept – referred to as extrapolation method – on the determination of the yield strength of porous thin films. As an example, a 1066 nm thick porous SiO2 xerogel film having a porosity of about 50% and an average pore size in the range 3 – 4 nm was investigated. Three different spherical indenters with radii of 1.4, 3.11 and 110 lm were used in this investigation. Depending on the indenter radius and the applied load the physical nature of the sample deformation was different making the one or the other of the two methods more suitable for analysis. We found that the data received with the large indenter could be well analysed using Pharr’s concept, while for the two smaller indenters the extrapolation method had to be used. The yield strength values obtained with the two methods were in remarkable agreement (109 vs. 99 MPa).

High‐performance thermal insulator based on polymer foam and silica xerogel

AbstractIn this work, methyltrimethoxysilane (MTMS) was used as a precursor, n‐hexadecyltrimethylammonium bromide (CTAB) as a surfactant, and EtOH/H2O as a co‐solvent, while 10 mM HCl and urea were used as acid and base catalysts to prepare silica xerogels and xerogel inside the cells of a polyethylene (PE) foam via a sol–gel process combined with ambient pressure drying. The xerogels and PE‐silica xerogel foams were characterized by scanning electron microscope (SEM), Brunauer–Emmett–Teller method (BET), and thermal conductivity. The results indicate that the xerogels have a three‐dimensional nanoporous structure, and the specific surface areas of the SiO2 xerogel and PE‐xerogel foam are 536 and 377 m2/g, respectively. The PE‐xerogel foam exhibited very low thermal conductivity (22–24 mW/m.K) compared with conventional insulation materials on the market (30–50 mW/m.K). The thermal insulation properties of PE‐silica xerogels were increased by 45% and 49% compared with neat PE foam. Additionally, the compressive modulus and density of the PE‐xerogel foam was increased by the presence of the silica xerogel. These PE‐xerogel foams are believed to be excellent candidates for thermal insulation applications in terms of cost/performance.

Физико-химические свойства диоксидов кремния, полученных экстракцией из сернокислых растворов от кислотного разложения нефелина

The synthesis of silica was carried out by extraction from a silica-containing solution from the decomposition of nepheline using acetone and ethanol, followed by gelatinization of the organic phase. The structural and surface properties of the obtained sample of SiO2 were studied and the pH of the isoionic point of their surface was determined. It was found that the use of extraction does not have a destructive effect on the specific surface of the SiO2 xerogel and allows preserving its original microporous structure.

Synthesis of Mesoporous Silica Xerogel from Geothermal Sludge using Sulfuric Acid as Gelation Agent

A large amount of sludge is produced by the geothermal brine at the Dieng Geothermal power plant, exceeding 165 tons per month. This sludge is generally not utilized, except for use in landfills. The precipitate (sludge) is primarily composed of silica. The aim of this research is to synthesis mesoporous silica (SiO2) xerogel from geothermal sludge (GS) and to investigate the effects of pH as an effort to elevate the economic value of sludge through alkaline extraction followed by acidification. SiO2 xerogel was prepared by extracting the GS to become sodium silicate (Na2SiO3) assisted by a base NaOH and precipitated using H2SO4 as a gelation agent. The FTIR analysis of the SiO2 xerogel showed a group of silanol (Si-OH) and siloxane (Si-O-Si). The XRD analysis indicated that SiO2 xerogel was amorphous. Furthermore, it was observed from nitrogen absorption-desorption using BET (Breneur-Emmet-Teller) method test that decreased pH tends to the specific surface area increase, and the pore size becomes decrease. The largest specific surface area observed at SiO2 xerogel prepared at pH of 5.5 reached 400.10 m2/g with a pore size of 4.5 nm. The pore sized for all cases was in the range of 4 ~12 nm, indicating that the SiO2 xerogels were mesoporous. Pore size of the as-prepared silica affected the thermal stability property of the sample.

Nanocrystalline Zn2TiO4 films for distributed Bragg's reflectors operating in near infrared region

We present a generic sol-gel approach for the preparation of a distributed Bragg reflector (DBR) based on combining nanocrystalline Zn2TiO4 with amorphous SiO2 xerogel films. We determined the effects of the annealing temperatures on the structural and optical properties of nanocrystalline Zn2TiO4 films. The increasing size of nanocrystals and film's density caused a regular growth of the films' refractive indices at 632 nm from the value of 1.837–1.960. The acquired data were applied to design the DBR with tailored reflectance at 980 nm and the data predicted by the theoretical model were compared with experimental transmission and reflection spectra. Although the high optical losses caused by the Rayleigh scattering were observed at short wavelengths, the prepared films were fully transparent above 600 nm. The uniform dielectric structure reflecting over 97% of incident light at 980 nm was prepared from eight Bragg's pairs. The normalized spectral bandwidth was about 0.22. We proved the nanocrystalline films exhibiting scattering in UV-VIS spectral region can be used for the construction of the distributed Bragg's reflectors operating in near infrared region. The presented approach can be easily extended to other nanocrystalline films allowing the integration of nanocrystalline luminophores into advanced photonic structures.

Green Strategy to Develop Novel Drug-Containing Poly (ε-Caprolactone)-Chitosan-Silica Xerogel Hybrid Fibers for Biomedical Applications

A facile and green method was explored to prepare the tetracycline hydrochloride- (TCH-) loaded poly (ε-caprolactone)-chitosan-silica xerogel (PCL-CS-SiO2) hybrid fibers by using 90% acetic acid as a suitable solvent. The SEM results showed that those fibers exhibited a continuous, bead-free morphology, an average diameter of about 430 nm, and super-hydrophilicity ( θ water ≈ 0 ° ). The presence of SiO2 was found to enhance the thermal stability of the hybrid fibers, and the actual content of SiO2 was obtained by the TG measurement. Moreover, SiO2 xerogel as an important bioceramic endowed the hybrid fibers with good drug release behavior and in vitro bioactivity, suggesting their potential use as novel drug carriers for bone tissue engineering. The present work is expected to offer a green strategy to develop novel, multifunctional hybrid materials.

Sol–Gel Fabrication and Luminescence Properties of Multilayer Eu-Doped BaTiO3–SiO2 Xerogel Nanostructures

With the use of sol–gel method BaTiO3–SiO2 multilayer structures were fabricated on glass or fused silica substrates employing dipping or spinning procedure. The photonic stop band was observed in the reflection and transmission spectra of the multilayer structure annealed at [Formula: see text]C. It is redshifted with an increase of the thicknesses of BaTiO3 and SiO2 layers. For structures comprising Eu-doped BaTiO3 layers of different thicknesses photoluminescence with the main band at 614[Formula: see text]nm was detected. It is characterized with the double-exponential decay with the lifetimes of about 0.5[Formula: see text]ms and 1.1[Formula: see text]ms which do not depend significantly on the photonic stop band position.

Up-conversion white emission and other luminescence properties of a YAG:Yb2O3·Tm2O3·Ho2O3@SiO2 glass-nanocomposite.

We report on a glass-nanocomposite material consisting of yttrium aluminum garnet (Y3Al5O12, YAG) nanocrystals co-doped with Yb3+, Tm3+ and Ho3+ ions as well as entrapped into a SiO2 xerogel. This 94YAG·5Yb2O3·0.8Tm2O3·0.2Ho2O3@SiO2 (abbr. YAG:YbTmHo@SiO2) nanocomposite material has been prepared by sol–gel procedure. Its structure and morphology has been characterized by means of X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques as well as energy dispersive X-ray (EDX), X-ray photoelectron (XPS) and luminescence spectroscopies. The luminescent glass-nanocomposite exhibited an up-conversion effect under λexc = 980 nm and emission when excited under 355 nm in steady-state conditions. Then time-resolved luminescence emission was observed, when the sample was excited at 290 and 355 nm by a pulse laser. Average decay times for the SiO2 matrix and for some transitions of the Tm3+ and Ho3+ dopants present in the YAG:YbTmHo@SiO2 material have been evaluated. The luminescent nanocomposite when excited under 290 or 355 nm wavelengths in both conditions emits blue light. However, the nanocomposite is promising as a single-source white-light phosphor owing to its up-conversion luminescence under 980 nm excitation. Such optical features make the studied material an alternative phosphor.

Catalytic Properties of Lipase Entrapped as Lysates of Recombinant Strain-Producer rEscherichia coli/lip into Nanocarbon-in-Silica Composites in the Bioconversion of Triglycerides and Fatty Acids

Composite multi-component biocatalysts were prepared by entrapping lysates of a recombinant rE. coli/lip strain producing Thermomyces lanuginosus lipase into composite nanocarbon-containing matrices based on a SiO2 xerogel. The dependence of the lipase activity and operational stability on the type of the carbon component (nanotubes or nanospheres of different diameters) was studied in the bioconversion of triglycerides (hydrolysis and interesterification), as well as in the esterification of saturated fatty acids—namely, butyric (C4:0), capric (C10:0), and stearic (C18:0) acids—with isoamyl alcohol. It was shown that the biocatalytic properties were determined by both the texture parameters of the nanostructured carbon included and the type of enzymatic reaction performed. Biocatalysts without a nanocarbon component had the highest operational stability in the batch process of interesterification of sunflower oil with ethyl acetate; the half-life time was found to be 720 h at 40°C. Biocatalysts containing carbon nanotubes of ~21 nm in diameter were five to six times more active in the batch esterification process than biocatalysts without a nanocarbon component. Biocatalysts containing carbon nanotubes catalyzed the synthesis of esters in a binary organic solvent (hexane and diethyl ether) without a loss of activity for more than 500 h at 40°C.

Lasing of Some Red Laser Dyes in Annealed Silica Xerogel

The spectral and energy characteristics of generation in the red spectral region 650–720 nm were measured and analyzed for three laser dyes in preliminarily annealed SiO2 xerogel matrices under laser excitation λp = 588 nm in a nonselective cavity. The specific laser-energy output for two of them (LK678 and Ox170) in the matrices was 10–13% higher than in MeOH. NBA dye in the matrix generated two laser radiation bands in the 700–720 nm region with pumping Ep ≥ 80 mJ whereas its generation threshold in MeOH exceeded the maximum pumping energy of 140 mJ so that NBA generation was not observed. Laser emission spectra of the studied matrices in a nonselective cavity were red-shifted by ~1000 cm–1 from the fluorescence maximum. Such a shift could improve the characteristics of biosensors based on these matrices.

Synthesis of bioactive polyvinyl alcohol/silica hybrid fibers for bone regeneration

Polyvinyl alcohol/silica (PVA/SiO2) hybrid fibers were prepared via a sol–gel combined electrospinning technique, and their potential application in bone tissue engineering was evaluated in terms of in vitro bioactivity. When the PVA concentration was 8%, the obtained PVA/SiO2 hybrid fibers exhibited a uniform and continuous morphology, with an average fiber diameter of about 542.9 nm. The SiO2 xerogel phase was uniformly distributed within the PVA matrix, and its actual content was very close to the theoretical value. After 3 days of immersion in simulated body fluid (SBF), the PVA/SiO2 hybrid fibers displayed a vigorous precipitation of lamellar apatite crystals, reflecting their excellent in vitro bioactivity.

Physicochemical properties of TiO2–SiO2 xerogel modified by hydrogen peroxide

How the specific surface area and the amorphous-to-crystalline titania phase ratio in TiO2–SiO2 (14 mol % TiO2) xerogels change during the fivefold repeated cycles comprising the hydrogen peroxide treatment of the xerogel followed by drying and calcining of the binary material, was traced by the BET method, X-ray powder diffraction, and IR spectroscopy.

Facile synthesis silver nanoparticles on different xerogel supports as highly efficient catalysts for the reduction of p-nitrophenol

The paper addresses the effect of support on the structure and catalytic activity of Ag catalysts supported on SiO2, Fe2O3 and Al2O3 xerogel for catalytic reduction of p-nitrophenol. In order to study support effect of Ag catalysts, Ag-based xerogel catalysts (2wt.%, 5wt.%) were prepared by a facile two-step approach involving sol-gel method followed by calcination, and several techniques were employed to study the influence of different supports on the catalytic structure and reaction performances. The results showed the selected support is the main factor to determine the porous characteristic, porosities and the catalytic properties of the corresponding catalyst. Moreover, Ag-based xerogel catalysts on different supports were used for the conversion of p-nitrophenol (PNP) to p-aminophenol (PAP). The presence of different oxides as supports played a crucial role in the activity of Ag nanoparticles as reduction catalysts. The results revealed that 5wt.% Ag based catalysts exhibited better catalytic activity for the reduction of PNP to PAP with sodium borohydride at room temperature. The reduction reaction followed the pseudo first-order kinetics. The order of rate constants of the 5wt.% catalysts was Ag-Al2O3<Ag-SiO2<Ag-Fe2O3. It was found that these types of Ag-based xerogel catalysts can be used repetitively up to five times with no significant activity loss in the initial reduction rate.

On the origin of near-IR luminescence in SiO2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification

Near infrared photoluminescent bismuth(I) silanolate centers ((≡Si-O)3Si–O-Bi) were prepared on the surface of SiO2 xerogel, by the treatment in the vapors of bismuth(I) chloride. The optical properties of these groups are almost identical to that of photoluminescent centers in the bulk SiO2 glasses with bismuth as the single dopant.

Luminescent properties of composite scintillators based on PPO and o-POPOP doped SiO2 xerogel matrices

New composite scintillation detectors were obtained by incorporation of PPO and o-POPOP organic scintillators into porous sol–gel silica matrices. Composites possess high photoluminescence intensity and decay time in nanosecond range. The absolute light yield of composite scintillators at excitation by alpha-radiation is about 4000–5000 photons/MeV and the pulse–height resolution is about 30%. The investigations of time-resolved luminescence of composites performed under excitation by synchrotron radiation in the 3.7–25eV range have shown that the non-radiative energy transfer between host matrix and dopant molecules occurs via singlet states of SiO2 oxygen-deficient centers.

Structural, optical and magnetic properties of γ-irradiated SiO2 xerogel doped Fe2O3

The paper deals with the structure, morphology and magnetic properties of two different iron concentrations (20 and 33mol.%) of Fe2O3−SiO2 nanocomposites, prepared by sol–gel technique and exposed to different gamma-irradiation doses (0, 30 and 60kGy). The nanocomposites were investigated through XRD, TEM, SEM, FTIR and EPR measurements. Superparamagnetic iron (III) oxide nanoparticles with a narrow size distribution, dispersed over the amorphous silica matrix, are assumed to be present in all the samples before and after irradiation. Before irradiation, a lot of γ-Fe2O3 crystalline ferromagnetic nanoparticles are assumed to be formed particularly for sample containing 33mol.% Fe2O3, while exposing the samples to irradiation results in the transformation of γ- to α-Fe2O3. Iron concentrations and/or irradiation of the samples are assumed to cause changes in the bond angles and/or bond lengths of the structural silicate units within network, as well, the increase of more defect centers induced by irradiation as evident through the variations of the IR bands intensity. The EPR results show both intensity and line width increase with increasing Fe2O3 concentration. The EPR signals for the samples consist of a well defined symmetrical broad signal at g≈2.0 ascribed to antiferromagnetic interactions between the Fe2+ and Fe3+ clusters. Condensed clusters of Fe3+ ions are observed to give rise to a resonance line at g∼2 whose position and width do not depend on the Fe2O3 concentrations. The EPR signal intensity is observed to be significantly decreased in the sample 33mol.% or stabilized in the sample 20mol.% by γ-irradiation. This reflects simultaneous spin transformation from the high-spin state of Fe III to the low-spin state of Fe II. As a final point, an effort has been given to found the possibility to use one of these studied nanocomposite materials as candidate for radiation shielding purposes.

Synthesis of silica xerogel–poly(styrene sulphonic acid) nanocomposites as acid catalysts: effects of temperature and polymer concentration on their textural and chemical properties

A series of silica–poly(styrene sulphonic acid) (SiO2–PSSA) nanocomposites were prepared using the sol–gel route. Tetraethoxysilane and (3-aminopropyl)-triethoxysilane (APTES) were used as the SiO2 xerogel sources. The latter was used to provide anchoring sites for retaining the PSSA polymer via acid–base reactions between the amino groups and sulphonic acid sites (electrostatic interactions). During the synthesis, the Si/N atomic ratio was maintained constant (close to 11), whereas the temperature and the PSSA concentration (PSSA/APTES mol ratio, that is, the S/N at. ratio) were varied. The prepared organic–inorganic nanohybrids were characterised by chemical analysis, thermogravimetric analysis, N2 adsorption isotherms and DRIFT and 29Si and 13C MAS-NMR spectroscopic analyses. These materials were applied as catalysts in the dehydration of xylose to furfural and compared with a conventional sulphonic resin, with specific attention to the reusability properties of the catalysts. A synthesis temperature of 348 K and an initial S/N at. ratio between 4 and 8 provided the best balance between the textural, chemical and reutilisation properties of the nanocomposite. Regardless of the temperature and S/N at. ratio initially used for the synthesis, the catalysts deactivated during the first reuse cycle but reached a stable state in successive runs. In contrast, the sulphonic resin continuously and progressively deactivates.

Composite materials based on nanoporous SiO2 matrices and squarylium dye

Sol–gel SiO2 matrices with different contents of HF acid were synthesized. The matrices were annealed at 600°C and impregnated with squarylium dye SqD1 in methylmethacrylate (MMA) solvent. Xerogel nanoporous structure of SiO2 matrices was found to contain 5–12nm and 60–600nm pores. The size of large pores increases with the concentration of HF acid. For the synthesis of SiO2 matrices the ratio n(HF/TEOS)=0.25–0.38 is the most optimal. At other HF concentrations the formed matrices are prone to cracking at drying, annealing and impregnation with the dye solution in MMA. The location of the maxima of the absorption and luminescence bands of SqD1 dye in the composite SiO2:SqD1:ММА is found to be influenced by the surface state of SiO2 xerogel depending on the concentration of HF acid at the synthesis of the matrices.