Evaporation-Induced Self-Assembly of Amino-Functionalized Mesoporous Silica Thin Films by Sol–Gel Process

Microporous and mesoporous materials have found great utility as catalysts and adsorption media because of their large internal surface area. Since the discovery of surfactant-templated mesoporous silica materials in 1992, the new developing periodic mesoporous organosilica material (PMO) has received much attention. In this present work, a PMO material, amino-functionalized mesoporous silica thin films with 3D hexagonal structure, were synthesized by self-assembly co-condensation of tetraethoxy silane (TEOS) and 3-aminopropyltriethoxysiliane (APTES) precursors in various ratios, in the presence of cetyltrimethyl ammonium bromide acting as the structure-directing agent. The thin films were characterized by small-angle X-ray diffraction, grazing incidence small angle X-ray scattering, Fourier transform infrared, transmission electron microscope, thermogravimetric analysis, and N₂ adsorption. Results show that when the APTES/(TEOS+APTES) ratio was 0.10, the mesoporous organosilica thin film has high degree of order, an average pore size of 2.16 nm, a large BET-specific surface area of 1560 m²/g and a large pore volume of 0.76 cm³/g.     

Characterization of Nanocrystalline Forsterite Fiber Synthesized via the Sol–Gel Process

An alkoxide sol–gel route was developed to prepare forsterite fibers. Transparent precursor gel fibers were converted to the crystalline phase of forsterite by heating above 550°C. Fibers heated at 1100°C for 2 h showed a porous microstructure with nanocrystalline sizes of about 50–100 nm. The structural evolution of fibrous gel was characterized. Effects of water and ethanol content on microstructures and strengths of fired fibers are also discussed. Some properties of the fired fibers were investigated.     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.     

Preparation of C70-Doped Solid Silica Gel via Sol–Gel Process

We reported the first preparation of C₇₀-doped solid gel. Its UV-visible spectrum was recorded.     

Synthesis of Porous Silica Foams via a Novel Vacuum-Induced Sol–Gel Method

Mesoporous silica foams having a macroporous skeleton and well-ordered internal mesoporous structure were synthesized by a novel vacuum-induced sol–gel process. This contrasts with the usual route involving expensive sacrificial microspheres. The resulting materials had a tri-modal pore size distribution and were characterized by nitrogen physisorption, X-ray diffraction, transmission electron microscopy.     

Positronium Annihilation Study of Porosity in Silica Xerogels Prepared by a Sol–Gel Process

Nano- and mesopores of bulk silica xerogels were studied by means of positron annihilation lifetime measurement as well as nitrogen physisorption. Effects of an alkaline catalyst concentration in the sol–gel process on the pore sizes and specific surface area were investigated. The pore size was estimated from the ortho -positronium lifetime. Remarkable structural changes were found around a relative alkaline concentration of 10⁻⁴ (a molar ratio of sodium hydroxide to tetramethyl ortho -silicate). Above this particular catalyst concentration, the nanopore radius decreased from 0.45 to 0.35 nm, whereas the mesopore radius increased from 1 to 8 nm. The positron annihilation lifetime technique was effective in the evaluation of the nano- and mesoscopic structural changes of silica xerogel.     

Sol–Gel Synthesis of Cadmium Telluride-Microcrystal-Doped Silica Glasses

The sol–gel process has been applied to the preparation of small-sized CdTe-doped silica glasses. Gels synthesized by the hydrolysis of a complex solution of Si(OC₂H₅)₄, Cd(CH₃COO)₂· 2H₂O, and Te were heated from 350° to 600°C in a H₂─N₂ atmosphere to form fine cubic CdTe crystals. The size of CdTe crystals, determined from the line broadening of X-ray diffraction pattern, increases from 4 to 9 nm in diameter with increasing heat-treatment temperature. The optical absorption edge shifts to the higher-energy side as the size of the CdTe crystals decreases. This phenomenon is interpreted in terms of a quantum confinement effect of electron and hole in the CdTe microcrystals.     

Preparation of Small-Particle-Size, Semiconductor CdS-Doped Silica Glasses by the Sol–Gel Process

The sol–gel process has been applied successfully to the preparation of small-particle-size CdS-doped silica glasses with a significant quantum size effect. Gels prepared through the hydrolysis of a complex solution of Si(OC₂H₅)₄ and Cd(CH₃COO)₂·2H₂O were heated at 500°C, then reacted with H₂S gas to form fine, hexagonal, CdS-microcrystal-doped glasses. The optical absorption edge is blue shifted by ∼0.4 eV compared with the bulk absorption value of CdS crystal. This result is interpreted in terms of a quantum-confinement effect of small crystal size.     

Structural Characterization of Mesoporous Silica Nanofibers Synthesized Within Porous Alumina Membranes

Mesoporous silica nanofibers were synthesized within the pores of the anodic aluminum oxide template using a simple sol–gel method. Transmission electron microscopy investigation indicated that the concentration of the structure-directing agent (EO₂₀PO₇₀EO₂₀) had a significant impact on the mesostructure of mesoporous silica nanofibers. Samples with alignment of nanochannels along the axis of mesoporous silica nanofibers could be formed under the P123 concentration of 0.15 mg/mL. When the P123 concentration increased to 0.3 mg/mL, samples with a circular lamellar mesostructure could be obtained. The mechanism for the effect of the P123 concentration on the mesostructure of mesoporous silica nanofibres was proposed and discussed.     

Textural and Structural Properties of Mesoporous Silica Synthesized Under Refluxing Conditions

Mesoporous silica with pore sizes of 3–6 nm has been synthesized under refluxing and autogenous pressure conditions of hydrothermal synthesis from precursor gels having different alkaline pH. The mesoporous silica prepared is characterized by powder X-ray diffraction, nitrogen adsorption-desorption measurement and scanning electron microscopy. Thermal stability has been tested by XRD analysis of mesoporous silica after thermal treatment at 823 K, 6 h; 1023 K, 1 h and 1223 K, 1 h. The results indicate that the mesoporous silica prepared under refluxing condition from precursor gel of pH 11 has large surface area (ca.1103 m² g^{− 1}) and pore volume (ca. 0.868 cm³ g^{− 1}) and is thermally stable at 1223 K. The surface area, pore volumes and pore wall thickness increase as the pH of the precursor gel is increased for refluxing condition of synthesis. The comparison of textural properties revealed that the refluxing condition is advantageous over autogenous pressure condition for obtaining mesoporous silica with higher surface area (852 m² g^{− 1}), pore volume (0.894 cm³ g^{− 1}) and pore diameter > 4 nm with wall thickness of 1.59 nm, when synthesized from precursor gel of pH 9.2. The ²⁹Si NMR spectra showed that a great part of the Si atoms exists as silanol groups. The mesoporous silica made at the lower pH (9.2) under refluxing conditions have more condensed framework. In calcined mesoporous silica, the proportion of partly condensed silica (Q³) is higher than fully condensed silica (Q⁴).     

Preparation and Nonlinear Optical Properties of Quantum-Sized CuCl-Doped Silica Glass by the Sol–Gel Process

The sol–gel process has been applied successfully to the preparation of small-sized CuCl-doped silica glass with a significant quantum-sized effect. Gel synthesized by the hydrolysis of a complex solution of Si(OC₂H₅)₄ and CuCl with an HCl catalyst is heated to 900°C to form fine, cubic CuCl crystals. Above 700°C, absorption peaks observed at ∼370 and ∼380 nm are attributed to the excitation of confined Z_1,2 and Z₃ excitons, respectively, in the CuCl microcrystals. As the size of the CuCl crystals decreases, exciton energy shifts to the higher-energy side. The resonant third-order nonlinear susceptibility at 77 K is 1.1 × 10⁻⁸ esu, which originates from the enhancement by the quantum-sized effect of the exciton.     

Synthesis of Hydroxyapatite Nanopowders via Sucrose-Templated Sol–Gel Method

The present research describes synthesis of hydroxyapatite (HAp) nanopowders using a sol–gel route with calcium nitrate and ammonium hydrogen phosphate as calcium and phosphorous precursors, respectively. Sucrose is used as template material, and alumina is added as a dopant to study its effects on particle size and surface area. Synthesized powders are characterized using X-ray diffractometry, BET surface-area analysis, and transmission electron microscopy. Results show that alumina stabilizes the HAp crystalline phase. Average particle size of mesoporous HAp samples is between 30 and 50 nm with surface area of 51–60 m²/g.     

SBA-15分子筛形成过程的介观动力学模拟及实验研究

为了研究三嵌段共聚物的自组装过程和SBA-15的形成机理,采用介观动力学(MesoDyn)方法模拟介孔分子筛SBA-15形成过程中模板剂P123与硅源试剂TEOS的协同自组装过程,并通过向体系引入稳恒剪切力来代替实际外力模拟了SBA-15六角介观相的形成过程,直观地展现了P123/硅酸物种超分子聚集体在SBA-15结构形成过程中的作用。模拟结果表明,P123与TEOS在水溶液中通过协同作用能够自组装形成尺寸均一的球状超分子聚集体,这是硅基有序介孔分子筛SBA-15形成过程中的前驱体:疏水的PPO嵌段团缩在该超分子聚集体的内部,在SBA-15分子筛形成中起决定性的致孔作用;亲水的PEO嵌段与包裹在胶束外部的TEOS相互交织在一起,其使得SBA-15在制备后期脱除模板剂后形成一些与主介孔相互连接的无规则微孔。在引入剪切力作用后,该P123/TEOS超分子聚集体不再自组装形成球状胶束,而是被拉扯成圆柱状的胶束,并且最终排列成为具有规则的六角形状的聚集结构,这一六角介观相则是介孔分子筛SBA-15的结构基础。今采用模拟条件,用水热法合成了高质量的SBA-15分子筛,通过XRD和高分辨透射电镜分析表明,模拟SBA-15形成过程的结果与实验合成的SBA-15具有相互吻合的完美二维六方结构。     

Synthesis of Nanoparticulate Silica Composite Membranes by the Pressurized Sol–Gel Technique

A crack-free silica composite membrane has been synthesized from a nanoparticulate silica sol (particle diameter <10 nm) by a pressurized sol–gel coating technique developed in this study. The microporous silica layers with an estimated pore radius of 0.78 nm were deposited inside the pores (average pore size of 0.1 μm) of slip cast a-alumina support tubes. The microstructure of the coated layer was controlled by adjusting sol properties and pressurizing conditions. The room-temperature intrinsic permeability of N₂ through the silica membrane layer after heat treatment at 200°C is about 4.9 × 10⁻¹² mol·m/m²·s· Pa, and the mechanism of gas transport is Knudsen flow. The thermal stability of the silica composite membrane is excellent up to 500°C.     

Synthesis of Nanocrystalline Enstatite Fiber Via Alkoxide Sol–Gel Process

An alkoxide sol–gel route to precursors to transparent enstatite fibers is described. The fibrous gels were drawn from acetic acid-modified viscous solutions. The amount of acetic acid was found to be the crucial factor in the spinnability. Infrared studies indicated that the acetate anion behaved as a ligand and modified the molecular structure of the enstatite gel. The fibrous gel was amorphous after drying by X-ray, and began to convert to enstatite at temperature as low as ∼600°C. The enstatite fibers were nanocrystalline up to 1100°C. At 1300°C, transparent and dense enstatite fibers were produced, with the room temperature dielectric constants of around 5.2–5.8 (at 1 MHz).     

Thio Sol–Gel Process for the Synthesis of Titanium Disulfide

A thio sol–gel process for the synthesis of titanium disulfide using titanium alkoxide as the metal source is demonstrated. The alkoxide reacts at room temperature with H₂S to form a precipitate which is a precursor to the sulfide. Through infrared spectroscopy and chemical analysis of the powder and gas chromatographic analysis of the liquid byproducts of the reaction, it is shown that a partially sulfidized alkoxide precursor forms through the displacement of alkoxy groups from the alkoxide by sulfur-bearing thiol groups. This thiolysis reaction is very similar to that which occurs in the case of sol–gel reactions to form oxides. The alkoxy–thiol species then undergo condensation–polymerization by the liberation of H₂S. When it is heat-treated in H₂S, this precursor transforms to TiS₂ at ∼700°C. Systematic heat treatments have been performed and the formation of TiS₂ has been observed using X-ray diffractometry. The change in the morphology has also been studied using scanning electron microscopy.     

Lowering of Preparation Temperatures of Anatase Nanocrystals-Dispersed Coatings via Sol–Gel Process with Hot Water Treatment

A longer period of time was necessary to form anatase nanocrystals on silica–titania gel coatings at lower hot water treatment temperature, whereas considerable amounts of anatase nanocrystals were observed even at 38°C after a treatment of 65 h. The coatings pre-heated at 60°C for 1 h and treated with hot water at 55°C for 15 h showed higher photocatalytic activity for methylene blue bleaching than the coatings prepared under the conditions with higher processing temperature close to 100°C. Such a high photocatalytic activity of the coatings obtained through low processing temperatures can be ascribed to a large specific surface area of the coatings and a smaller crystal size of the precipitated anatase.     

Sol–Gel Synthesis of PZT–Glass Nanocomposites Using a Simple System and Characterization

A simple inorganic system has been developed for the sol–gel synthesis of piezoelectric PZT–glass nanocomposites. Nanocrystalline PZT are nucleated from the amorphous xerogel through controlled heat treatment at 600°C. The formation of the crystalline phase, particle size, morphology, kinetics, and mechanism of crystallization of PZT in the glass matrix have been studied using X-ray diffraction, scanning electron microscopy, and differential thermal analysis. The piezoelectric characteristics of the PZT–glass compositions have been determined, and are correlated with the microstructure and the crystalline phase of PZT.     

Mesoporous AlPO4 Glass from a Simple Aqueous Sol–Gel Route

Transparent and colorless AlPO₄ gel and glass are prepared via a simple aqueous sol–gel route using aluminum lactate and phosphoric acid as precursors. The stoichiometric AlPO₄ glass derived from this sol–gel route has a mesoporous structure with a surface area of 504 m²/g after calcination to 600°C. With increasing gel-to-glass processing temperature, the average degree of P/Al connectivity increases. After sample calcination at 600°C for 4 h, ²⁷Al MAS NMR spectra indicate that aluminum is almost completely converted into AlO₄ units. ²⁷Al{³¹P} rotational echo double resonance and ³¹P{²⁷Al} rotational echo adiabatic passage double resonance NMR experiments as well as ²⁷Al and ³¹P MAS NMR results further confirm that the prepared AlPO₄ glass has a three-dimensional network based on alternating Al(OP)₄ and P(OAl)₄ tetrahedral units in analogy to the local structure of crystalline AlPO₄.     

Synthesis of Platinum/Silica Nanocomposite Particles by Reverse Micelle and Sol–Gel Processing

Pt/SiO₂ nanosized particles have been synthesized using a reverse micelle technique combined with metal alkoxide hydrolysis and condensation. The size of the particles and the thickness of the coating can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) within the microemulsion. The average size of synthesized particles was in the size range of 21–42 nm. The effects of synthesis parameters, such as the molar ratio of water to TEOS and the molar ratio of water to surfactant, are discussed.