Size-controlled synthesis of mesoporous silica nanoparticles using rice husk by microwave-assisted sol–gel method

Mesoporous silica nanoparticles with distinct characteristics like particle size, tunable pores, and high surface area have received much interest for environmental remediation, energy conversion, and biological applications. In this work, we synthesized spherical silica nanoparticles with tunable particle size and mesoporous properties using a low-cost silica source (rice husk) and polyethylene glycol (PEG) via microwave-assisted sol–gel synthesis. The formation of an amorphous silica structure was found using XRD and FTIR analysis. FESEM analysis showed that altering the PEG concentration from .01 to .005 M produced spherical silica nanoparticles with 100–500 nm in size. Nitrogen adsorption–desorption demonstrated that silica nanoparticles obtained with .005, .007, and .01 M of PEG had unique pore sizes and distributions, with specific surface areas of 51.475, 62.367, and 84.251 m²/g, respectively. These results might be due to PEG molecules’ capping effect, which acts as a soft template to regulate particle size, pore size, and dispersion by interacting with sodium silicate precursor. Hence, this approach can be a facile and cost-effective method to prepare mesoporous silica nanoparticles with controllable nanoscale characteristics for suitable applications.     

Modification of the glass surface by titanium dioxide films synthesized through the sol–gel method

The glass surface has been modified by titanium dioxide films synthesized using the sol-gel method. The sol-gel synthesis has been optimized by the experimental design with the use of the Latin square method. The film composition has been evaluated using X-ray powder diffraction analysis and optical measurements. The photoinduced hydrophilicity of the modified glass has been investigated.     

Microreactor-assisted synthesis of α-alumina nanoparticles

In this paper, nanosized α-Al₂O₃ powders were synthesized with the aid of a high throughput impinging stream microreactor. It was demonstrated that the as-prepared powders exhibited better dispersity and more homogeneous distribution of particle size than that prepared by conventional parallel flow precipitation method due to the drastic collisions and homogeneous explosive nucleation in microchannel during the precipitation process. The effects of calcinating temperature and time on the morphology, phase composition and particle size of α-Al₂O₃ were systematically studied. Comparatively well dispersed and crystallized α-Al₂O₃ powders with higher sintering activity were obtained by calcinating the as-prepared precursors at 1200 °C for 2 h in air. The specific surface area of α-Al₂O₃ powders was above 20 m² g⁻¹ and average particle size was about 110 nm with higher sintering behavior. From room temperature to 1520 °C, the green compact exhibited shrinkage of 19.34%. This work opens a door for developing ultrafine α-Al₂O₃ powders with uniform size distribution, high crystallinity, and excellent thermal expansion property.     

Ultrafast sol–gel synthesis of graphene aerogel materials

Graphene aerogels derived from graphene-oxide (GO) starting materials recently have been shown to exhibit a combination of high electrical conductivity, chemical stability, and low cost that has enabled a range of electrochemical applications. Standard synthesis protocols for manufacturing graphene aerogels require the use of sol–gel chemical reactions that are maintained at high temperatures for long periods of time ranging from 12 h to several days. Here we report an ultrafast, acid-catalyzed sol–gel formation process in acetonitrile in which wet GO-loaded gels are realized within 2 h at temperatures below 45 °C. Spectroscopic and electrochemical analysis following supercritical drying and pyrolysis confirms the reduction of the GO in the aerogels to sp² carbon crystallites with no residual carbon–nitrogen bonds from the acetonitrile or its derivatives. This rapid synthesis enhances the prospects for large-scale manufacturing of graphene aerogels for use in numerous applications including sorbents for environmental toxins, support materials for electrocatalysis, and high-performance electrodes for electrochemical capacitors and solar cells.     

Crystallization kinetics of Nd-substituted yttrium iron garnet prepared through sol–gel auto-combustion method

Nd substituted Y₃Fe₅O₁₂ (YIG) was synthesized using sol–gel auto-combustion method. The phases present in the samples were identified using XRD. The asprepared dried gel was found to transform to the rare earth iron garnet phase at lower temperature when the Nd concentration is lower. Higher concentration of Nd resulted in the formation of orthoferrite as the major phase. The Curie temperature was recorded using thermomagnetic analysis and it was found to increase with the concentration of Nd. The crystallization kinetics of the Nd-substituted garnet phase was studied using TG–DTA. Two stage crystallization was observed from the DTA analysis for Nd₁Y₂Fe₅O₁₂. The activation energy for the formation of Nd₁Y₂Fe₅O₁₂ was 610 kJ/mol, found by using Johnson–Mehl–Avrami (JMA) equation. Three dimensional growth mechanism dominate at lower heating rates for the Nd-substituted YIG.     

Synthesis of nanocrystalline gadolinium doped ceria via sol–gel combustion and sol–gel synthesis routes

Gadolinium doped ceria (GDC) has been investigated as a promising material for application as an electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFC). In this work, 10GDC powders (Gd_0.1Ce_0.9O_1.95) were synthesized by sol–gel combustion and sol–gel synthesis routes using the same complexing agents in both procedures. The thermal behavior of Gd–Ce–O precursor gels was investigated by TG–DSC measurements. X-ray diffraction (XRD) analysis was used for the characterization of phase purity and crystallinity of synthesized samples. Scanning electron microscopy (SEM) was employed for the estimation of surface morphological features. Nitrogen adsorption–desorption (BET model) was used for evaluation of specific surface area. The surface composition was determined by X-ray photoelectron spectroscopy (XPS). Electrical properties of synthesized ceramic samples were studied by means of impedance spectroscopy.     

Controllable synthesis of metastable tetragonal zirconia nanocrystals using citric acid assisted sol–gel method

Metastable tetragonal ZrO₂ nanoparticles and nanosheets were synthesized with citric acid assisted sol–gel method. In this approach, zirconium acetylacetonato, Zr(acac)₂, citric acid (CA) and ethylene glycol (EG) were used as the source of Zr⁴⁺, the chelating, and solvent agent, respectively. The effects of heat treatment on zirconia phase evolution were investigated. We demonstrate that pure tetragonal nanocrystalline zirconia can be obtained with CA: EG mole ratio=5:1 and calcination temperature 490 °C. The microstructure of the products was characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Raman scattering. Finally, Photoluminescence (PL) of nanosheets and nanoparticles were also investigated.     

Nontemplate sol–gel synthesis of catalytically active mesoporous titanosilicates

A method for the nontemplate sol–gel synthesis of micro-mesoporous and mesoporous titanosilicates is developed using commercially available raw materials (a mixture of oligomer esters of orthosilicic acid (Ethylsilicate-40) and a water–alcohol solution of titanium(IV) ethoxide). The effect of the pH values of the reaction mixture on sol–gel synthesis is studied using the characteristics of the porous structure of titanosilicates. Samples of amorphous structure with different textural characteristics are obtained by varying the pH in the range of 3–10: specific surface area, 320–740 m²/g; micropore volume, 0.04–0.15 cm³/g; and mesopore volume, 0–0.92 cm³/g. It is shown that a sample of mesoporous titanosilicate obtained with alternating pH values has maximum catalytic activity in the oxidation reaction of 4-tert-butylphenol (TBP) in aqueous solutions of hydrogen peroxide. (Conversion of 4-tert-butylphenol, 45 wt %; selectivity of the formation of 4-tert-butylcatechol, 66 wt %. Conditions: 10 wt % of titanosilicate; TBP/H₂O₂ = 1/2; 75°C; 1 h).     

Morphology controlled synthesis of ZnO particles through the oxidation of Al–Zn mixture

ZnO particles with different morphologies were synthesized through a simple oxidation process of Al–Zn mixtures in air. The morphologies significantly depended on the Zn content in Al–Zn mixture and the oxidation time. Rod-based brushes, typical tetrapods, and novel tetrapods with triangular wedges were synthesized with the increase of Zn content in Al–Zn mixture. The morphology was also changed from rod to tetrapod shape with oxidation time. The results indicate that the concentration of Zn and the oxidation time might be responsible for the different morphologies of ZnO particles. XRD spectra showed that the ZnO particles were a hexagonal wurtzite structure.     

A novel route to obtain B4C nano powder via sol–gel method

Boron carbide nanopowders and nanowhiskers were synthesized using phenolic resin and boron alkoxide as precursors through sol–gel process in water–solvent–catalyst–dispersant system. The effects of soaking time on free carbon content and synthesized B₄C particles morphology were evaluated at 1270 °C. The synthesis process of B₄C nanopowders was completed at 1270 °C after 1 h while B₄C nanowhiskers were heterogeneously nucleated and grown from obtained nanopowders after 3 h.     

Low-temperature combustion synthesis of nanocrystalline HoFeO3 powders via a sol–gel method using glycin

Single phase nanocrystalline HoFeO₃ powders were successfully synthesized by the sol–gel self-propagation combustion method using glycin (C₂H₅NO₂) as the chelating reagent at a low combustion temperature. Four different mole ratios of glycin to metal nitrate (G/M) were used to prepare HoFeO₃ powders in the experiment. The XRD patterns indicate monophasic HoFeO₃ powders can be formed by further calcination, the SEM micrographs show that the nano-sized grains with distinguishable boundaries had been obtained. The M–H curves show HoFeO₃ powders had the characteristic of antiferromagnetism at 50 K, while the powders had the characteristic of paramagnetism as the ambient temperature reached 100 K or 300 K. The FC/ZFC magnetic measurement results demonstrate that there was a transition from antiferromagnetism to paramagnetism in HoFeO₃ nanopowders as the temperature was increased.     

Microwave-assisted sol–gel synthesis of alpha alumina nanopowder and study of the rheological behavior

In the present work, alpha alumina nanopowder was synthesized via a sol–gel route. After preparation of bohemite (AlOOH) sol, carbon black was added and the resultant sol was dried and calcined in microwave furnace for 10 min. XRD results showed that alpha alumina was the only crystalline phase with specific surface area, mean diameter and crystallite size of 51 m² g^?1, 100 and 25 nm, respectively. Rheological measurements revealed that the optimal content of Tiron at pH=10 is 1 and 0.1 g per 100 g nano- and micron-alumina (1.5 m² g^?1), respectively. Furthermore, the optimum solid content of the slips was determined as 35–45 and 70 wt.% for nano- and micron-alumina, respectively.     

A sol–gel approach for the room temperature synthesis of Al-containing micelle-templated silica

A two-step sol–gel method has been developed for the synthesis of Al-containing micelle-templated silica (Al-MTS) with Si/Al atomic ratios in the range 8–45. The synthesis proceeds at ambient temperature, atmospheric pressure, acid medium and for only 3 h. For the calcined materials, a decrease of both the d₁₀₀ spacing and the pore size, as well as an increase in the wall thickness, are observed with ncreasing aluminum content. Thus, the average pore size changed from 2.4 nm (Si/Al=45) to 1.4 nm (Si/Al=8), i.e. the pore size reaches the micropore range without varying the chain length of the surfactant used as micellar templating agent. The ²⁷Al NMR-MAS spectra show that in the as-synthesized samples the aluminum is present as tetrahedrally coordinated species, which are partially transformed into octahedral aluminum upon calcination. Since the synthesis takes place in the absence of alkali cations, the Al-MTS materials are directly obtained in the acid form, making a subsequent ion exchange and calcination treatments superfluous.     

Green gelatine-assisted sol–gel synthesis of ultrasmall nickel oxide nanoparticles

Nickel oxide nanoparticles (NiO NPs) were synthesised using a sol–gel method in a gelatinous medium. Gelatine was used as a size-limiting polymerisation agent for the growth of NiO NPs. X-ray diffraction (XRD) analysis revealed that increasing the calcination temperature increased the crystallite size and decreased the size of the lattice constant. The size-strain plot method (SSP) was used to measure the individual contribution of grain sizes and micro strain on the peak broadening of NiO NPs. Transmission electron microscopy (TEM) showed the ultrasmall size of the NiO NPs with a narrow size distribution(10±0.2 nm). The band gap value of NiO NPs was calculated using ultraviolet–visible (UV–Vis) spectroscopy and decreased with increased calcination temperature.     

Processing and properties of sol gel derived alumina–carbon nano tube composites

High purity alumina–carbon nano tube (CNT) composites were prepared by an aqueous sol–gel processing route. CNTs were dispersed in alumina sol containing appropriate amount of MgO precursor. Aqueous slurry of alumina was seeded into the sol followed by gelation, drying and calcination at 1000 °C for 1 h. The calcined powder consisting of alumina-coated CNTs and alumina was milled, sieved, dried, pressed and pressureless sintered at 1400–1600 °C for 1 h in nitrogen atmosphere. Sintered samples were further isostatically hot pressed at 1300 °C and the properties were compared with the pressureless sintered samples. Phase formation was followed by XRD study, CNT retention was confirmed by Raman studies and the samples were further characterized for mechanical and microstructural properties.     

Study of cation distribution for Cu–Co nanoferrites synthesized by the sol–gel method

Nano sized polycrystalline soft ferrite particles with composition Cu_1−xCo_xFe₂O₄ (x =0.1, 0.3, 0.5, 0.7, 0.9) were synthesized by the sol–gel technique. The existence of well-defined single cubic spinel structure was confirmed in all the samples by X-ray diffraction. The crystallite size found by XRD varied from 14.8 to 34.0 nm. The microstructure was also characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Slight expansion of the unit cell was detected with the increase of Cobalt concentration, which may be attributed due to larger ionic radius of Co²⁺. Lattice parameter ranged from 8.34 Å to 8.37 Å for Co²⁺ from 0.1–0.9. The distribution of cations amongst A- and B-sites of the lattice was estimated by X-ray diffraction by using the R-factor technique. The results showed that both Cu²⁺ and Co²⁺ ions occupy mainly the B-site while Fe³⁺ ions were equally distributed among A- and B-sites. The data obtained from cation distribution analysis was used to determine the magnetic moment for each sample and VSM studies were also carried out to validate these calculations. Magnetic measurements showed that the saturation magnetization (Ms) and coercivity (Hc) increased with increasing cobalt content.     

Preparation of alumina ultrafine powders through acrylamide,starch and glutaric dialdehyde mediated sol–gel method

Ultrafine alumina powders were synthesized through acrylamide (AM), starch and glutaric dialdehyde mediated aqueous sol–gel process, respectively. Sol and gel formed gradually during drying of the solution due to polymerization reaction between functional groups. The used AM, starch and glutaric dialdehyde could be applied to form perfect matrix for the entrapment of metal ions, giving rise to ultrafine crystalline alumina particles during heating treatment. Al₂O₃ nanoparticles with γ crystalline phase were obtained via heating treatment of the dried precursor in air. Then the γ phase transforms to α phase and pure α-Al₂O₃ powders could be obtained when the heating temperature was 1473 K. Our results provide a new way of aqueous sol–gel process.     

Microwave assisted synthesis of Dy2Ti2O7 ultrafine powders by sol–gel method

Dy₂Ti₂O₇ ultrafine powders ranging from 100 to 300 nm were successfully synthesized by sol–gel method. Particularly, the dried gel precursor was treated at different temperatures (700–1000 °C) via microwave-heating, which contributed to decreasing the grain size and reaction time. The phase composition and structural evolution of the final products were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, the resultant powders were selected to fabricate ceramics and rubber based absobers. Their sinterability, mechanical properties and neutron absorption ability were also studied. Results showed that the highest flexural strength of 99.0 MPa were obtained for Dy₂Ti₂O₇ samples when sintered at 1600 °C for 2 h in air atmosphere. Meanwhile, the neutron absorption rate of Dy₂Ti₂O₇ ceramics and rubber based absorbers could reach 97.39% and 80.00% respectively when the thickness of samples was set as 5.0 mm.     

One-step synthesis of barium hexaferrite nano-powders via microwave-assisted sol–gel auto-combustion

Single phase barium hexaferrite nano-powders have been innovatively one-step synthesized via a microwave-assisted sol–gel auto-combustion in a specially designed quartz vessel using citric acid and ethylene diamine tetraacetic acid as composite chelating agents and freeze-drying technique to remove sols’ moisture. The auto-combustion product powder is characterized by fluffy particle aggregates with the crystallites ranging from 50 to 100 nm in diameter and containing single magnetic domains for each with a low apparent coercive field of 260 Oe and a high saturation magnetization of 64.1 emu/g. The direct formation of barium hexaferrite is believed to result from the effective improvement in the spatial distribution homogeneity of metal ions and oxidant in the gels. Moreover, the quartz vessel with barium hexaferrite ceramic pad on sample's support and film-strips on its interior wall can effectively build up a favorable temperature environment to promote the direct formation of barium hexaferrite with microwave assistance during the gel's uniform auto-combustion.     

Effect of p-aminobenzoic acid on synthesizing ordered mesoporous alumina via the sol–gel method

Ordered mesoporous alumina (OMA) samples were prepared via the sol–gel method. The triblock copolymer P123 and aluminum isopropoxide were chose as the template and aluminum source, respectively. The p-aminobenzoic acid (PABA) worked as both the interfacial protector and the assisted template. Different techniques, including X-ray diffractions, N₂ physisorption measurement and transmission electron microscope, were employed to study the structural properties of the prepared samples. The results showed that the mesostructure ordering and specific surface area of the samples highly depend on the quantity of the added PABA. The samples after calcinated at 800 °C with the n(PABA)/n(Al³⁺) molar ratio lying in 0.30 exhibited high specific surface areas (301 m² g^?1) and retained an ordered mesostructures. It was indicated that the carboxyl groups of PABA have multiple functions in the formation of OMA with a long-range ordering and high specific surface area. The carboxyl groups of PABA can slow down the hydrolysis-condensation reaction of aluminum isopropoxide, reduce the influence of chloride ions on self-assembly process. The amino groups of PABA can also react with the hydrogen ions to form quaternary ammonium salt, which can neutralize the charges of the chloride ions, protect the balance of inorganic–organic interface, further to improve the mesostructure ordering and specific surface area of the prepared OMA.