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.     

A study of the acidic and catalytic properties of pure and sulfated zirconia–titania and zirconia–silica mixed oxides

Protonated pyridine (PyH⁺) was not found on ZrO₂ (Z) or ZrO₂–TiO₂ (ZT), but was detected on sulfated oxides (ZS, ZTS) by IR spectroscopy. In contrast, ZrO₂–SiO₂ samples containing about 30–80 mol% ZrO₂ showed Brønsted acidity both in nonsulfated (ZS) and sulfated (ZSS) forms. The total acidity was determined by NH₃TPD. Introduction of sulfate ions increased the sitespecific catalytic activity (TOF) in the conversion of cyclopropane or nhexane. The effect of sulfate ions was more significant on samples rich in zirconia. Results suggest that Zr is homogeneously distributed in ZS samples rich in silica. Zirconiabound dimeric sulfate, generating strong acidity, could not be formed in these preparations due to the absence of fairly large ZrO₂ domains.     

In-situ sol–gel synthesis of zirconia networks in flexible and conductive composite films

The conductive and stretchable films with improved hardness are suitable for the fabrication of flexible electronic devices. This study describes the sol–gel technique to prepare a novel conductive and flexible film consisting of epoxidized natural rubber (ENR), doped polyaniline (PD) and zirconia. The zirconia networks were directly synthesized in-situ in ENR/PD solution and flexible conductive composite film with improved hardness was obtained. The morphology study revealed the size of PD decreased significantly from 77.89 ± 43.95 nm to 4.32 ± 1.13 nm and highest electrical conductivity of 1.9 × 10⁻³ S/cm was achieved with 10 wt.% percolation threshold of zirconia precursor. The binding energy of Zr3d_5/2 and Zr3d_3/2 decreased, suggesting that zirconia was converted to the lower oxidation state. Furthermore, the shape of PD changed from spherical to rod-like structure with root mean square value of 2 nm, while the hardness and reduced modulus improved to 1.72 MPa and 36.7 MPa, respectively.     

Sol–gel synthesis and catalytic properties of vanadium phosphates

Vanadium phosphates with variable specific surface areas were synthesized from vanadium alkoxides and phosphoric acid by precipitation in various solvents. The catalytic properties were studied in the reaction of methanol oxidation. Although the specific activities increase with the specific surface areas (BET), the intrinsic activities seem to depend on the particle morphology. The selectivities do not vary within a large range. The same method of precipitation was applied to prepare vanadium phosphates on a silica support. It is shown that the supported phase is highly dispersed, and that these supported catalysts are more stable than silica‐supported vanadium oxide for methanol oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

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.     

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.     

Effect of synthesis methods and aging on synthesis of uvarovite garnet by ceramic and sol–gel processes

Abstract

The uvarovite garnet (CaO)₃ (Cr₂O₃ )(SiO₂ )₃ has been synthesised (mineralised with borax to facilitate diffusion of precursors) by several sol–gel methods. Two routes for uvarovite formation have been observed: where CaCrO₄ forms as an intermediate phase; and where metastable pseudowollastonite (α-CaSiO₃ ) forms as an intermediate phase. Synthesis via CaCrO₄ appears to be more reactive in unaged samples. The reactivity of samples can be directly related to the chemical homogeneity of raw powders, and two methods of synthesis, Pechini and alkoxide, were found to stand out for their reactivity. Aging of raw powders for 6 months inhibits nucleation. Consequently, samples without nucleating agents (i.e. chlorides), such as gels from alkoxides, lead to the stabilisation of amorphous material or metastable phases (α-CaSiO₃ ) and, as a result, hinder uvarovite formation. In contrast, aged samples involving heterogeneous nucleation agents, as in the Pechini method with CaCl₂ as precursor, enhance reactivity.     

Use of a design-of-experiments approach for preparing ceria–zirconia–alumina samples by sol–gel process

In this work we successfully obtained ceria–zirconia–alumina samples by the sol–gel technique. These materials were prepared under acidic or basic conditions, using either nitric acid or ammonium hydroxide as the catalyst. A design-of-experiments approach was used in order to optimize the specific surface area, pore structure, and thermal stability of the prepared samples. It was observed that the addition of ceria and zirconia did not affect the formation of γ-Al₂O₃. The highest surface areas and smallest pore sizes were observed for specimens obtained under acidic conditions and with low to intermediate concentrations of cerium and water. The increase of the heat treatment temperature from 600 °C to 1000 °C led to both a decrease of the surface area and an increase of the mean pore size. This behavior is due to the coalescence of pores upon calcination. Samples with a high concentration of ceria showed an expressive thermal instability at high temperatures. On the other hand, the addition of zirconia increased the thermal stability of these materials. In general, samples with improved thermal stabilities were obtained under basic conditions.     

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).     

Size-controlled synthesis of nano α-alumina particles through the sol–gel method

Nano α-alumina particles were synthesized by a sol–gel method using aqueous solutions of aluminum isopropoxide and 0.5 M aluminum nitrate. 1/3-benzened disoulfonic acid disodium salt (SDBS) and sodium bis-2-ethylhexyl sulfosuccinate (Na(AOT)) were used as surfactant stabilizing agents. Solution was stirred for different periods (24, 36, 48 and 60 h) at 60 °C. The samples were then analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Introduction of surfactant stabilizing agents and different stirring times will affect the size and shape of particle formed and also the degree of aggregation. SDBS, however, produced better dispersion, finer particles and spherical shape nanoparticles, compared to Na(AOT). The finest particle size (20–30 nm) was obtained at 48 h stirring time with SDBS surfactant.     

Sol–gel synthesis of Nd-doped BiFeO3 multiferroic and its characterization

Neodymium doped bismuth ferrite (BiFeO₃, BFO) nanoparticles were successfully synthesized by a facile sol–gel route. The influence of annealing temperature, time, Bi content and solvent on the crystal structure of BFO was studied. Results indicated that the optimum processing condition of BFO products was 550–600 °C/1.5 h with excess 3–6% Bi and ethylene glycol as solvent. On the other hand, Nd³⁺ ion was introduced into the BFO system and the effect of Nd³⁺ concentration on the structure, magnetic and dielectric properties of BFO were investigated. It was found that the magnetization of BFO was enhanced significantly with Nd³⁺ substitution, being attributed to the suppression of the spiral cycloidal magnetic structure led by the crystal structure transition. Furthermore, with increasing Nd³⁺ content, the dielectric constant was found to decrease while the dielectric loss was enhanced, which was mainly due to the hoping conduction mechanism with the reduction of oxygen vacancies.     

Preparation of nano-AlN powder by sol–gel foaming and its sintering properties

Uniformly dispersed nano-sized aluminum nitride powders were prepared by the sol–gel foaming method using aluminum nitrate as the aluminum source, sucrose as the carbon source, and ammonium chloride as the foaming agent. The effects of ammonium chloride content on the particle size and the sintering properties of aluminum nitride were investigated. The results showed that when the molar ratio of ammonium chloride to aluminum nitrate was .5, the colloidal foams were uniform, large, and fluffy, and amorphous alumina precursors with uniform particles could be prepared. Aluminum nitride powder with a particle size of 22–27 nm can be obtained by calcining these precursors in nitrogen atmosphere at 1400°C for 2 h. At the same time, aluminum nitride bulk material with a relative density of 95% can be obtained by sintering the compact samples in nitrogen atmosphere at 1700°C for 2 h.     

Synthesis of TiO2–Ag nanocomposite with sol–gel method and investigation of its antibacterial activity against E. coli

TiO₂–Ag nanocomposite was prepared by the sol–gel method and an azeotropic distillation with benzene was used for dehydration of the gel. Because of gel dehydration by distillation method a nanopowder with a surface area of 230 m²/g was produced which decreased to 80 m²/g after calcination. TEM micrographs and XRD patterns showed that spherical nanosized Ag particles (≈ 10 nm) were deposited among TiO₂ particles. The antibacterial activity of calcined powder at 300 and 500 °C was studied in the presence and in the absence of UV irradiation against Escherichia coli as a model for Gram-negative bacteria. The antibacterial tests confirmed the powder calcined at 300 °C possessed more antibacterial activity than the pure TiO₂, amorphous powder and the powder calcined at 500 °C under UV irradiation. In the absence of UV, the reduction in viable cells was observed only with calcinated powder at 300 °C.     

Effect of addition of surface modified nanosilica into silica–zirconia hybrid sol–gel matrix

An organic–inorganic hybrid sol (MZ) comprising a methacrylate functionalized silane matrix (M) and zirconium-n-propoxide (Z) was prepared using sol–gel technique. Two methodologies were adopted to modify the hybrid sol for generating nanocomposite coatings viz., (a) addition of acrylic surface modified silica nanoparticles (N) of diameter ～20 nm to the sol to enhance their compatibility with the hybrid sol–gel matrix and (b) in-situ formation of a three dimensional silica network by addition of tetraethoxy silane (T) to the sol MZ. In the first methodology, the sols were prepared with six different weight ratios of the nanoparticles to the sol, i.e. 0, 0.01, 0.05, 0.1, 0.25 and 1 which were labelled as MZ+Nx where x=0, 1, 2, 3, 4 and 5 respectively. The prepared sols were dip coated on 100 mm×100 mm polycarbonate substrates followed by thermal curing at 130 °C. The coatings were characterized for their mechanical properties like pencil scratch hardness, scratch resistance using scratch tester, nanoindentation hardness, and abrasion resistance as well as visible light transmittance. FT-IR studies were also carried out on heat-treated gels derived from the sols. A maximum pencil scratch hardness of 3H was obtained for the MZ+T coatings and these coatings withstood a critical load of 4.3±0.7 N before failure during scratch test. The maximum nanoindentation hardness of 3.8±0.01 GPa was obtained for the MZ+N5 coatings. The abrasion resistance of MZ+T coatings was higher when compared to MZ+N0 and MZ+N5 coatings. The scratch and nanoindentation hardness were seen to be better for an in-situ formed –Si–O–Si– network in the hybrid sol when compared to those obtained from coatings generated by external addition of acrylic surface modified silica nanoparticles. The difference in properties was attributed to the level of interaction between the nanoparticles and hybrid sol–gel matrix.     

Synthesis of nanosized zirconium carbide powders by a combinational method of sol–gel and pulse current heating

Zirconium carbide nanopowders were synthesized by a novel method combining the advantages of sol–gel method and rapid synthesis using pulse current heating. The core-shelled structure of ZrO₂/C mixture was obtained during the sol–gel process, and further heat treatment in SPS led to the fast formation of ZrC. The particle size of ZrO₂ played an important role in the synthesis of nanosized ZrC powders. In addition, the coalescence and grain growth of ZrC particles could be also limited due to the fast heating rate. As a result, the reactions were thoroughly completed at a relatively low temperature and ZrC nanopowders of 60–100 nm were obtained. The corresponding powders also had low oxygen content (∼0.64 wt%) and residual carbon content (∼0.27 wt%). Additive-free ZrC powders could be sintered to ∼99% relative density with an average grain size of 0.8 μm at low temperature of 1750 °C.     

Novel and green approach for the nanocrystalline magnesium oxide synthesis and its catalytic performance in Claisen–Schmidt condensation

Magnesium oxide is one of the most promising solid base material having catalytic applications. A novel, simple, efficient, inexpensive, additives free and green synthesis protocol for magnesium oxide nano-particles using concentrated solar energy has been reported. Using magnesium acetate and 1, 4-butanediol magnesium oxide nanoparticles in the range of 5–20 nm was prepared. The driving force for the nanoparticles synthesis was obtained by using concentrated solar energy as a dual energy source in the form of radiational and thermal effect. The resulting magnesium oxide nanoparticles are found to be excellent and reusable catalyst for the Claisen–Schmidt condensation under solvent free condition.     

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 characterization of sol–gel titania membranes

In this work we present a structural characterization of sol–gel titania membranes obtained in both supported and unsupported forms. We used two commercial grade alumina supports obtained from Whatman and Rojan Advanced Ceramics. The unsupported membranes were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), differential scanning calorimetry (DSC), nitrogen sorption, and X-ray powder diffraction (XRD). Morphological studies were performed in both supported and unsupported membranes using a field emission scanning electron microscope (FE-SEM). In order to evaluate the performance of the supported membranes, single-gas permeation experiments were carried out at room temperature with nitrogen, helium, and carbon dioxide. We concluded from nitrogen sorption experiments that increasing the membrane heat treatment temperature leads to samples with lower specific surface areas and greater pore sizes. Close packed titania particles of uniform size were observed in SEM micrographs of unsupported membranes. The SEM analyses also revealed the presence of titania coatings on supported membranes. Some of the obtained membranes showed a separation capacity for He/CO₂ and He/N₂ larger than that expected for the Knudsen mechanism in the investigated pressure range. However, a good part of the analyzed samples showed an improvement of their separation capacity with increasing the feed pressure.