Rare earth (Eu, Tb) centered composite gels Si-O-M (M = B, Ti) through hexafluoroacetyl-acetone building block: Sol-gel preparation, characterization and photoluminescence

This report focuses on the syntheses of a series of novel photoactive composite xerogels materials in which the functionalized hexafluoroacetylacetone (HFAASi) organic components are grafted into the different inorganic networks (SiO₂-B₂O₃ or SiO₂-TiO₂) via covalent bonds through a sol-gel process. Subsequently, the physical characterization and especially photoluminescent properties of the resulting xerogel materials are studied in detail. Except for composite xerogels linked to SiO₂-TiO₂ networks, all of these composite xerogels exhibit homogeneous microstructures and morphologies, suggesting that molecular-based materials are obtained with strong covalent bonds between the organic β-diketone ligand and inorganic matrices. In addition, the ternary rare earth composite gels present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary ones, indicating that the introduction of the second ligands (phen) can sensitize the luminescent emission of the rare earth ions in the ternary hybrid systems. It should be especially noted that these composite xerogels based on Si-O-B networks possess not only higher thermal stability but also stronger luminescent intensities than the other systems linked to different inorganic networks.     

Ethyl group as matrix modifier and inducer of ordered domains in hybrid xerogels synthesised in acidic media using ethyltriethoxysilane (ETEOS) and tetraethoxysilane (TEOS) as precursors

Hybrid silica xerogels favourably combine the properties of organic and inorganic components in one material; consequently these materials are useful for multiple applications. The versatility and mild synthetic conditions provided by the sol-gel process are ideal for the synthesis of hybrid materials. The specific aims of this study were to synthesise hybrid xerogels in acidic media using tetraethoxysilane (TEOS) and ethyltriethoxysilane (ETEOS) as silica precursors, and to assess the role of the ethyl group as a matrix modifier and inducer of ordered domains in xerogels. All xerogels were synthesised at pH 4.5, at 60 °C, with 1:4.75:5.5 TEOS:EtOH:H₂O molar ratio. Gelation time exponentially increased with the ETEOS molar ratio. Incorporation of the ethyl groups into the structure of xerogels reduced cross-linking, increased the average siloxane bond length, and promoted the formation of ordered domains. As a result, a transition from Qⁿ to Tⁿ signals detected in the ²⁹Si NMR spectra, the Si–O structural band in the FTIR spectra shifted to lower wavelength, and a new peak in the XRD pattern at 2θ < 10° appeared in the XRD patterns. Mass spectroscopy detected fragments with high numbers of silicon atoms and a polymeric distribution.     

Synthesis of sustainable silica xerogels/aerogels using inexpensive steel slag and bean pod ash: A comparison study

Low cost silica xerogels/aerogels were synthesized from steel slag and bean pod ash by sol–gel method. Comparison study showed differences between structural, morphological, textural, thermal and physical properties of the silica xerogels and aerogels. Formation of amorphous structure and silica network was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analyses, respectively. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses revealed that silica xerogels had smaller interlinked network in contrast to silica aerogels. Typical type IV isotherm was observed for all samples in N₂ adsorption-desorption isotherms. The highest surface area was determined as 371 m² g⁻¹ for silica aerogel synthesized from steel slag. Particle size of silica aerogels was lower than that of the silica xerogels. The more porous structure made silica aerogels desirable materials with lower bulk density and thermal conductivity when compared to silica xerogels. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) exhibited high thermal stability of the silica xerogels/aerogels. Although silica xerogels had highly hydrophilic structure, contact angle of silica aerogels synthesized from steel slag and bean pod ash was 60° and 74°, respectively. The comparison study will give a new point of view about differences between silica xerogels and aerogels synthesized from by-products or inorganic/organic waste instead of silicon alkoxides.     

Atomic ordering in xZrO2·(1−x) SiO2 xerogels (x=0.3, 0.5) by X-ray diffraction and reverse Monte Carlo simulations

Atomic distribution functions for xZrO₂·(1 − x) SiO₂ xerogels (x = 0.3, 0.5) have been obtained by X-ray diffraction experiments. Three dimensional structure models that closely reproduce the experimental structural data have been constructed by reverse Monte Carlo simulations. The model results suggest that the investigated xerogels do not exhibit any characteristics of a phase segregation and have a homogeneous structure at the atomic scale. In particular, the local atomic ordering in 0.5ZrO₂·0.5 SiO₂ xerogels heat treated to 770 K has been found to be similar to that in crystalline ZrSiO₄. The question as to why a phase segregation occurs on the crystallization of xZrO₂·(1 − x) SiO₂ xerogels (x − 0.3, 0.5) has been addressed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Growth and Optical Properties of Quantized CdS Crystallites in TEOS Silica Xerogels

Quantized CdS crystallite-doped tetraethylorthosilicate (TEOS) silica xerogels are prepared by the sol-gel method. In this method, cadmium acetate [Cd(CH₃COO)₂2H₂O]-doped TEOS alcogel is formed by the hydrolysis and polycondensation of ethanolic TEOS in the presence of hydrochloric acid (HCl) and ammonium hydroxide (NH₄OH) catalysts and Cd(CH₃COO)₂.2H₂O. The CdS crystallites are formed in the alcogel by the reaction of Cd(CH₃COO)₂.2H₂O present in the gel and methanolic sodium sulfide (Na₂S), which is added over the alcogel. The effect of CdS/TEOS, EtOH/TEOS, S/Cd molar ratios, and temperature on the optical properties and CdS crystallite sizes in the xerogels are studied. A blue shift is observed in optical absorption spectra by decreasing the CdS/TEOS molar ratio from 2 × 10^–2 to 1 × 10^–4. It is observed that the crystallite size is increased from 1.6 to 3.4 nm by increasing the EtOH/TEOS molar ratio from 2 to 20, respectively, for a constant CdS/TEOS molar ratio of 5 × 10^–4. Emission spectra of xerogels are measured and found that the emission peak maxima shifted toward lower energies (higher wavelengths) by increasing the CdS/TEOS molar ratio in the xerogels. It is known from the X-ray diffraction (XRD) measurements of CdS-doped xerogels that the CdS crystallite structure in the xerogels is hexagonal wurtzite. The crystallite sizes were calculated from the XRD patterns and tight bonding calculations. There is a significant change in the color and size of CdS crystallite in the xerogels with a variation in temperature from 200 to 400°C.     

Preparation of organic and carbon xerogels using high-temperature–pressure sol–gel polymerization

To prepare organic gels at temperatures higher than normal boiling point of solvent, a method was developed using sol–gel polymerization in atmosphere saturated by vapor of solvent. To illustrate the advantages of proposed method, two series of gels were prepared using the conventional (T_curing = 70 °C) and the high temperature (T_curing = 140–170 °C) sol–gel polymerization. While no drying shrinkage was observed in our proposed method, 5–18% linear shrinkage occurred in conventional method depending on resin concentration in sol. Moreover, rising of curing temperature reduced the required time for preparation of organic gels from 5 days to lower than 5 h. The effects of processing parameters were investigated on physical and mechanical properties of organic xerogels. The results revealed that resin concentration significantly affects both density and compressive strength of final xerogels. While the curing temperature had no obvious effect on density, the raising of curing temperature significantly enhance the strength of organic xerogels. Carbon xerogels prepared by pyrolysis of novolac aerogels in inert atmosphere. The textures of the carbon xerogels were denser than corresponding organic xerogels, as evidenced by scanning electron microscopy (SEM) images. N₂ adsorption tests indicated that carbon aerogels were mainly meso or macroporous depending on resin concentration in initial sol.     

Luminiscent dye Rhodamine 6G doped monolithic and transparent TEOS silica xerogels and spectral properties

Sol–gel process was used for the preparation of Rhodamine 6G (R6G) doped silica xerogels, using tetraethylorthosilicate [TEOS, Si(OC₂H₅)₄] as the precursor for the silica network. Silica alcosol was prepared by hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of citric acid (CTA) catalyst. The ethanolic R6G was added to the alcosol to trap R6G molecules inside the SiO₂ gel network during the gelation of the TEOS alcosol. The effect of CTA/TEOS molar ratio on the gelation time of the R6G doped TEOS alcosol, transparency and monolithicity of the R6G doped silica xerogel was studied by varying the CTA/TEOS molar ratio from 1.2×10⁻⁴ to 180×10⁻⁴ by keeping the molar ratios of TEOS:EtOH:H₂O:R6G constant at 1:5:7:9.2×10⁻⁶, respectively. It was found that the minimum (<70 h) gelation time was observed at higher and lower CTA/TEOS molar ratios of 72×10⁻⁴ where as maximum (>180 h) gelation time was observed for CTA/TEOS molar ratio of 72×10⁻⁴. While opaque and monolithic R6G doped SiO₂ xerogels were obtained for <4.8×10⁻⁴ CTA/TEOS molar ratios, whereas cracked and transparent xerogels were obtained for >120×10⁻⁴ molar ratios of CTA/TEOS. Transparent, homogeneous and monolithic samples were obtained between 4.8×10⁻⁴ and 120×10⁻⁴ of CTA/TEOS molar ratios. Leaching out property was studied by using water, methanol and ethanol solvents for the R6G doped SiO₂ xerogels of 9.2×10⁻⁶ and 12×10⁻⁴ of R6G/TEOS and CTA/TEOS molar ratios, respectively, and found that R6G molecules were trapped in the pores of the SiO₂ network.Bleaching out phenomena of the R6G doped SiO₂ xerogels was studied by focusing the high intensity light on some part of the samples for a period of 1 h and found that the pores were continuous in SiO₂ network. Visible spectra of R6G in water, ethanol, SiO₂ alcosol and xerogel were taken for 1.6×10⁻⁴ M R6G and observed that there were two absorption peaks at 499 and 525 nm in the spectrum of R6G in water due to dimerization of R6G molecules and only one absorption peak at 530 nm in the spectra of ethanol, SiO₂ alcosol and xerogel because of monomerization of R6G molecules. Visible spectra of the R6G doped silica xerogels for varying R6G/TEOS molar ratios from 9.2×10⁻⁸ to 9.2×10⁻⁵ were taken and found the red shift (5–10 nm) with increasing the R6G/TEOS molar ratio from 9.2×10⁻⁸ to 9.2×10⁻⁵. The effect of temperature on these sample was studied by varying the temperature from 50 to 300 °C and found that the R6G doped silica samples were stable up to 200 °C. IR spectra were taken for pure R6G powder and R6G doped silica xerogels of 9.2×10⁻⁸ and 9.2×10⁻⁵ R6G/TEOS molar ratios and found that most of the peaks present in pure R6G powder spectrum were absent in the spectra of trapped R6G SiO₂ xerogels. This shows that, the SiO₂ network hinders the rotational and vibrational transitions of R6G when it is caged in the SiO₂ network. The peaks related to bending motion in R6G molecules were not disturbed by the SiO₂ network     

Preparation and luminescence of some [K2PtCl6] materials

The preparation of Cs₂TaOCl₅ and Cs₂TaSCl₅, new materials with the [K₂PtCl₆] structure, is described. Luminescence has been measured for these materials and other pure [K₂PtCl₆] materials.     

Mechanochemical Route for the Preparation of Nanosized Aluminum and Gallium Sulfide and Selenide

Aluminum and gallium chalcogenides are promising, yet relatively underexplored photoelectric and semiconducting materials. Many studies confirmed the applicability of gallium sulfides and selenides in photovoltaics and as electrode materials. In present work, a simple and convenient mechanochemical method has been developed for preparation of aluminum and gallium sulfide and selenide from elemental powders. The products have been characterized by X-ray powder diffraction (XRPD) and transmission electron microscopy (TEM), including energy-dispersive X-ray spectroscopy (EDX). The following products have been obtained: Al₂S₃, Al₂Se₃, Ga₂S₃, and Ga₂Se₃, with crystallite sizes in the range 8–12 nm.     

Synthesis and characterization of fluoride xerogels

Using coprecipitation from aqueous solutions, we have synthesized transparent fluoride xerogels with both hexagonal (NdF₃, PrF₃, and CeF₃; tysonite structure) and cubic (Sr_0.6Y_0.4F_2.4, Ba₄Y₃F₁₇:Bi, and Ba₄Y₃F₁₇:Yb; fluorite structure) symmetries. As shown by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy, the transparent xerogels have a hierarchical structural organization: primary nanoparticles 20–30 nm in size form agglomerates about 100 nm in size, which in turn form a “skeleton” with many voids and channels up to hundreds of nanometers in size.     

In vitro release of cisplatin from sol–gel processed organically modified silica xerogels

SiO₂, SiO₂/PEG and SiO₂/PDMS xerogels were examined as polymeric carriers for the controlled release of cisplatin—an antineoplasmic medicine. Drug/carrier systems were prepared by the sol–gel method. The effect of organic substitution of the silica xerogel matrix and drying conditions on the release of cisplatin was evaluated. Based on the presented results of the study it may be stated that sol–gel method is useful for entrapping a cisplatin in the pores of organically modified silica gels and for releasing cisplatin mainly in the way of diffusion from the pores of the lattice under the in vitro conditions. The use of organic impurities in silica gel increased the release of cisplatin from xerogel (from 62% to 97% within 7 days), and thermal treatment of all xerogels with cisplatin at the temperature of 80 °C resulted in the acceleration of the drug release (2 days) and increase of the released drug (89–98%).     

Rapid and Low‐Temperature Salt‐Templated Production of 2D Metal Oxide/Oxychloride/Hydroxide

2D materials have played an important role in electronics, sensors, optics, electrocatalysis, and energy storage. Many methods for the preparation of 2D materials have been explored. It is crucial to develop a high‐yield, rapid, and low‐temperature method to synthesize 2D materials. A general, fast (5 min), and low‐temperature (≈100 °C) salt (CoCl₂·6H₂O)‐templated method is proposed to prepare series of 2D metal oxides/oxychlorides/hydroxides in large scale, such as MoO₃, SnO₂, SiO₂, BiOCl, Sb₄O₅Cl₂, Zn₂Co₃(OH)₁₀ 2H₂O, and ZnCo₂O₄. The as‐synthesized 2D materials possess an ultrathin feature (2–7 nm) and large aspect ratios. Additionally, these 2D metal oxides/oxychlorides/hydroxides exhibit good electrochemical properties in energy storage (lithium/sodium‐ion batteries) and electrocatalysis (hydrogen/oxygen evolution reaction).     

One-pot synthesis of supported-nanoparticle materials in ionic liquid solvents

Highly porous supported-nanoparticle materials were synthesized by a rational method involving the encapsulation of poly(vinylpyrrolidone) (PVP)-stabilized Au nanoparticles into titania xerogels employing room temperature ionic liquids (1-butyl-3-methylimidazolium hexaflurophosphate, [BMIM]PF₆) as a medium followed by solvent extraction of the ionic liquid and calcination of the materials. The materials were thoroughly characterized by TEM, nitrogen adsorption-desorption isotherms, and XRD. After calcinations at 350 °C, the Au-titania system resulted in the formation of a highly mesoporous materials with BET surface areas of 200 m²/g and average pore sizes of 3-5 nm. These materials can find potential applications in catalysis and photocatalysis.     

Luminiscent dye Rhodamine 6G doped monolithic and transparent TEOS silica xerogels and spectral properties

Sol–gel process was used for the preparation of Rhodamine 6G (R6G) doped silica xerogels, using tetraethylorthosilicate [TEOS, Si(OC₂H₅)₄] as the precursor for the silica network. Silica alcosol was prepared by hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of citric acid (CTA) catalyst. The ethanolic R6G was added to the alcosol to trap R6G molecules inside the SiO₂ gel network during the gelation of the TEOS alcosol. The effect of CTA/TEOS molar ratio on the gelation time of the R6G doped TEOS alcosol, transparency and monolithicity of the R6G doped silica xerogel was studied by varying the CTA/TEOS molar ratio from 1.2 × 10^-4 to 180 × 10^-4 by keeping the molar ratios of TEOS:EtOH:H₂O:R6G constant at 1:5:7:9.2× 10^-6, respectively. It was found that the minimum (<70 h) gelation time was observed at higher and lower CTA/TEOS molar ratios of 72 × 10^-4 where as maximum (>180 h) gelation time was observed for CTA/TEOS molar ratio of 72 × 10²⁴. While opaque and monolithic R6G doped SiO₂ xerogels were obtained for <4.8 × 10^-4 CTA/TEOS molar ratios, whereas cracked and transparent xerogels were obtained for >120 × 10^-4 molar ratios of CTA/TEOS. Transparent, homogeneous and monolithic samples were obtained between 4.8 × 10^-4 and 120 × 10^-4 of CTA/TEOS molar ratios. Leaching out property was studied by using water, methanol and ethanol solvents for the R6G doped SiO₂ xerogels of 9.2 × 10^-6 and 12 × 10^-4 of R6G/TEOS and CTA/TEOS molar ratios, respectively, and found that R6G molecules were trapped in the pores of the SiO₂ network.

Bleaching out phenomena of the R6G doped SiO₂ xerogels was studied by focusing the high intensity light on some part of the samples for a period of 1 h and found that the pores were continuous in SiO₂ network. Visible spectra of R6G in water, ethanol, SiO₂ alcosol and xerogel were taken for 1.6 × 10^-4 M R6G andobserved that there were two absorption peaks at 499 and 525 nm in the spectrum of R6G in water due to dimerization of R6G molecules and only one absorption peak at 530 nm in the spectra of ethanol, SiO₂ alcosol and xerogel because of monomerization of R6G molecules. Visible spectra of the R6G doped silica xerogels for varying R6G/TEOS molar ratios from 9.2 × 10^-8 to 9.2 × 10^-5 were taken and found the red shift (5–10 nm) with increasing the R6G/TEOS molar ratio from 9.2 × 10²⁸ to 9.2 × 10^-5. The effect of temperature on these sample was studied by varying the temperature from 50 to 300 8C and found that the R6G doped silica samples were stable up to 200 8C. IR spectra were taken for pure R6G powder and R6G doped silica xerogels of 9.2 × 10^-8 and 9.2 × 10^-5 R6G/TEOS molar ratios and found that most of the peaks present in pure R6G powder spectrum were absent in the spectra of trapped R6G SiO₂ xerogels. This shows that, the SiO₂ network hinders the rotational and vibrational transitions of R6G when it is caged in the SiO₂ network. The peaks related to bending motion in R6G molecules were not disturbed by the SiO₂ network     

Porous graphitic materials obtained from carbonization of organic xerogels doped with transition metal salts

Porous carbons with a well developed graphitic phase were obtained via the pyrolysis of FeCl₃-, NiCl₂-, and CoCl₂-doped organic xerogels. Doping was realized through salt solubilization in a water/methanol solution of resorcinol and furfural. Carbon xerogels with tailored particles, porous morphology and various degrees of graphitization were obtained depending of the water/methanol ratio and the salt content and type in the starting solution of substrates. When obtained via pyrolysis, carbon xerogels retain the overall open-celled structure exhibiting depleted microporosity and a well-developed mesoporic region that expands into macropores. The removal of metal leads to carbon xerogels with specific surface areas between 170 and 585 m²/g and pore volume up to 0·76 cm³/g. The possibility of enhancing the porosity of xerogels via templating with colloidal silica was also investigated. It was assumed that from the three investigated salts, FeCl₃ makes the best choice for graphitization catalyst precursor to obtain uniformly graphitized mesoporous carbon xerogels. The obtained carbon samples were characterized by means of SEM, TEM, X-ray diffraction, Raman spectroscopy, N₂ physisorption and thermogravimetric analysis.     

Composition control in Cu2ZnSnS4 thin films by a sol–gel technique without sulfurization

Kesterite Cu₂ZnSnS₄ (CZTS) thin films with a smooth, compact and crack-free morphology are obtained via a sol–gel method without sulfurization process. Non-toxic ethylene glycol is selected as solvent, while Cu(CH₃COO)₂, Zn(CH₃COO)₂, SnCl₂·2H₂O and thiourea are used as raw materials. Chemical composition dependence of CZTS films on pre-annealing and post-annealing process is comprehensively investigated. The analysis of energy dispersive X-ray indicates that composition control of CZTS films can be easily realized by the preparation of precursor solution and varying the annealing conditions.     

Nanocrystalline Oxide Ceramics Prepared by High-Energy Ball Milling

Studies of grain size effects in nanocrystalline materials require a preparation technique which allows adjustment of the grain size. We prepared various nanocrystalline ceramics by high-energy ball milling. The investigated systems are the oxide ceramics Li₂O, LiNbO₃, LiBO₂, B₂O₃, TiO₂ as monophase materials and the composite material Li₂O : B₂O₃. The average grain size was adjusted by variation of the milling time. It was determined via line broadening of X-ray diffraction patterns (XRD) and directly with transmission electron microscopy (TEM). Thermal stability and thermally induced grain growth of the samples can be observed with differential thermal analysis and X-ray analysis. Further information concerning the structure of these heterogeneously disordered materials was extracted from nuclear magnetic resonance (NMR) and infrared spectroscopy. Li diffusion in the lithium-containing compounds is studied with ac conductivity measurements, as well as [⁷Li] NMR relaxation spectroscopy. The TiO₂ is interesting for research on catalytic activity. Ball milling not only causes particle size reduction, but may also lead to phase transitions and chemical reactions. This was verified with XRD.     

Fabrication and properties of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> composites

Composites in the form of precipitated powders, hybrid xerogels, and SiO₂ core/TiO₂ shell particles have been produced via hydrolysis of precursors (alkoxides and inorganic derivatives of titanium and silicon) and have been characterized by differential thermal analysis, X-ray diffraction, adsorption measurements, and macroelectrophoresis. The results demonstrate that heat treatment of the composites leads to crystallization of the titanium-containing component and, accordingly, reduces their specific surface area. Hydrothermal treatment enables the fabrication of materials in which TiO₂ nanocrystals are evenly distributed over an amorphous SiO₂ matrix.     

Photodeposition of Pt nanoparticles on TiO2-carbon xerogel composites

Carbon xerogels synthesized from polycondensation of resorcinol with formaldehyde, having specific surface areas in the range 650 to 990 m² g⁻¹ and variable degrees of surface oxidation, are used to prepare TiO₂-carbon xerogel composites by sol-gel methods. These composite materials are used to support Pt nanoparticles (5 wt.%) by the photodeposition technique. After a high temperature reduction treatment at 773 K, the obtained materials were characterized in order to assess the interactions between the phases Pt, TiO₂ and carbon xerogel. It is observed that the carbon xerogel acts as an adhesive agent of the TiO₂ and Pt particles, enhancing the interaction between the metal and the composite support.