Raman study of the variation in anatase structure of TiO<Subscript>2</Subscript> nanopowders due to the changes of sol–gel synthesis conditions

TiO₂ nanopowders were produced by sol–gel technique under different synthesis conditions. XRD results have shown that obtained nanopowders are in anatase phase, with the presence of a small amount of highly disordered brookite phase, whereas nanocrystallite size and amount of brookite slightly depend on sol–gel synthesis conditions. Raman measurements confirm these results. The analyses of the shift and width of the most intensive anatase E _g Raman mode by phonon confinement model suggest that anatase crystallite size should be in the range between 11 and 15 nm, what is in excellent correlation with XRD results. Obtained results have shown that Raman spectroscopy is a highly sensitive method for the estimation of anatase crystallite size as well as brookite content in TiO₂ nanopowders synthesized by variable sol–gel synthesis conditions.     

Sol–gel synthesis and structural characterization of niobium-silicon mixed-oxide nanocomposites

(Nb₂O₅) _x ·(SiO₂)_1−x gels of four different compositions with x = 0.025 (2.5Nb), 0.050 (5Nb), 0,10 (10Nb) and 0.20 (20Nb) were synthesized at room temperature from niobium penta-chloride and tetra-ethoxysilane and their structural evolution with the temperature was examined by X-ray diffraction, thermogravimetry/differential thermal analysis, Raman and IR spectroscopy (Fourier transform). The synthesis procedure tuned in this work allowed to obtain for each studied composition transparent chemical gels in which the niobium dispersion resulted to be strongly dependent on the Nb₂O₅ loading: it was on the atomic scale for the 2.5Nb and 5Nb gel samples whereas the gel structure of the 10Nb and 20Nb appears formed by phase separated niobia-silica nanodomains. All dried gels keep their amorphous nature up to 873 K, while at higher temperatures crystallization of T- and H-Nb₂O₅ polymorphs were observed according to the Nb₂O₅ loading: at low loading T-Nb₂O₅ was the main crystallising phase, whereas at higher one the H-Nb₂O₅ prevails. Particularly, T-Nb₂O₅ was the sole crystallising phase in the whole explored temperature range for the 2.5Nb, keeping its nanosize up to 1273 K for all samples except for the 20Nb.     

Synthesis of MgNb<Subscript>2</Subscript>O<Subscript>6</Subscript> nanocrystalline powders by an improved citrate sol–gel technique

MgNb₂O₆ nanocrystalline powders have been synthesized at a low temperature by improved citrate sol–gel method in this paper. The high quality solution of Nb⁵⁺ was prepared using Nb₂O₅ as the starting material. The crystal structure and microstructure of MgNb₂O₆ powders were characterized by XRD and SEM techniques, and the effects of preparation craft including pH value and the proportion of citric acid to the niobium ions on the crystal structure and microstructure of powders were also investigated. XRD and TG/DTA results show that the single phase of MgNb₂O₆ for synthesized powders can be obtained by calcining the precursor at 700 °C. SEM results indicate that the average particle size of MgNb₂O₆ exhibits a significantly dependence on the pH values and the proportion of citric acid to the niobium ions, where it was found that particle size of a 20 nm can be obtained for the MgNb₂O₆ powders by sol–gel process.     

Synthesis and characterization of ferroelectric SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> nanotubes arrays

Ferroelectric SrBi₂Ta₂O₉ nanotubes were fabricated by sol–gel dipping template technique and characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. They had a single orthorhombic perovskite structure, and most of SBT nanotubes showed highly preferential crystal growth along the [115] orientation. FE-SEM and TEM investigations showed that nanotubes have smooth wall morphologies and well-defined diameters corresponding to the diameter of the applied template. From HRTEM results, the clear lattice fringes indicated that the nanotubes are structurally uniform and well crystallized. The growth mechanisms of SBT nanotubes into the AAO templates were explored.     

Comparative study of the synthesis of KReO<Subscript>4</Subscript> using acrylamide sol–gel and solid-state reaction methods

KReO₄ was synthesized by solid-state reaction and for the first time by sol–gel method via acrylamide polymerization. X-ray diffraction analysis showed that a single phase of KReO₄ was obtained by both methods at 500–505 °C having a tetragonal unit cell. The solid-state reaction samples had a grain size of 5 μm and the sol–gel samples had fibrous aspect, agglomerated between 10 and 100 μm. By TEM it was observed nanocrystals of 100 nm, it is suggested that the xerogel fibers are formed by nanocrystals. From the results obtained, we concluded that the morphology is strongly influenced by the method of synthesis used.     

Synthesis of Si-MCM-41 from ternary SiO<Subscript>2</Subscript>–CTAOH–H<Subscript>2</Subscript>O system via dry gel conversion route

Steam assisted dry gel conversion method was employed for the synthesis of Si-MCM-41 from ternary SiO₂:CTAOH:H₂O systems wherein fumed silica was used as a source of silica. The influence of synthesis time, molar ratios of CTAOH/SiO₂ in dry gel and the water content at the bottom of autoclave on the quality and formation of mesophases has been investigated. Powder XRD, N₂ adsorption–desorption, TEM and hydrothermal stability test were the techniques used for sample characterization. Keeping molar ratio of CTAOH/SiO₂ constant, shorter synthesis time, lower unit cell parameter and d spacing were observed when steam assisted dry gel conversion method was employed in place of conventional hydrothermal method. There exists an optimum lower limit for water content at the bottom of autoclave for reducing the synthesis period. Keeping synthesis temperature and CTAOH/SiO₂ molar ratio fixed, Si-MCM-41 with improved hydrothermal stability was obtained by steam assisted dry gel conversion route.     

Synthesis and characterization of semiconductor tin oxide thin films on glass substrate by sol–gel technique

In this study, synthesis and characterization of semiconductor tin oxide (SnO₂) thin films on glass substrate were systematically investigated by using sol–gel technique for gas sensing applications. Turbidity, pH values, wettability and rheological properties of solution were measured by turbidimeter, pH meter, contact angle goniometer and rheometer machines before coating process. The thermal, structural, microstructural and optical properties of the coatings and powders made from the sols were extensively characterized by using DTA-TG, FT-IR, XRD, SEM-EDS, refractometer and spectrophotometer. Four different solutions, including 6, 8, 10 and 14 mL methanol content, were prepared by sol–gel technique to determine solvent influence on microstructure and semiconducting properties of the thin films. Refractive indiceses, band gaps, absorbance and transmittance values of SnO₂ thin films, containing different methanol quantity, were determined and their variations depending on solvent content were obtained. It is concluded that solvent content influences microstructural and semiconducting properties of Sn based oxide thin films notably.     

Aqueous colloidal sol–gel route to synthesize nanosized ceria-doped titania having high surface area and increased anatase phase stability

Nanocrystalline sol–gel derived titania doped with ceria (1, 2, 5 and 10-mole%) has been prepared from titanyl oxysulphate. The titania doped with 5-mole% CeO₂ after calcining to 500 °C, possesses specific surface area of 97 m² g⁻¹ and has anatase phase stability up to 900 °C. Moreover it retains a surface area of 37 m² g⁻¹ at 700 °C. In comparison, the undoped calcined material has anatase stability only up to 700 °C and specific surface area only 48 m² g^-1 and 6 m² g^-1 at 500 °C and 700 °C, respectively. The diffuse reflectance spectra show that, as the cerium content increases, the absorption undergoes a red shift and reaches the visible range. The exceptionally high phase stability, crystallinity and high surface area are due to the extremely fine particle size and effective doping achieved by the specific synthesis method. The results based on X-ray diffraction, specific surface area and diffuse reflectance spectra indicated that the maximum threshold limit of doping is up to a value of 5-mole%.     

Synthesis and characterization of sol–gel derived bioactive CaO–SiO<Subscript>2</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses containing magnetic nanoparticles

Magnetic bioglasses in the system CaO–SiO₂–P₂O₅ were prepared by interaction of acetic acid vapors with iron nitrate dispersed on the surface of sol–gel derived porous silicate network. Upon pyrolysis, the created iron acetate species transform into magnetic iron oxide nanoparticles. X-ray diffraction (XRD), FT-infrared (FT-IR) spectroscopy and surface area measurements (BET) were employed to monitor the evolution of glass structural features during the synthetic pathway as well as the structure and the texture of the resultant glasses. XRD, Raman spectroscopy and vibration magnetic measurements (VSM) revealed the features of magnetic phases, developed in the form of γ-Fe₂O₃ and magnetite. The obtained glasses exhibit in vitro bioactivity, expressed by spontaneous formation of hydroxyapatite on their surface after immersion in SBF at 37 °C, confirmed with μ-Raman and FT-IR spectroscopies.     

Carbothermal synthesis of ultra-fine zirconium carbide powders using inorganic precursors via sol–gel method

Ultra-fine zirconium carbide (ZrC) powders have been synthesized by carbothermal reduction reactions using inorganic precursors zirconium oxychloride (ZrOCl₂ · 8H₂O) as sources of zirconium and phenolic resin as the carbon source. The reactions were substantially completed at relatively lower temperatures (∼1400 °C/1 h) and the synthesized powders had a small average crystallite size (<200 nm) and a large specific area (54 m²/g). The oxygen content of the powder synthesized at 1400 °C/1 h was less than 1.0 wt%. The thermodynamic change process in the ZrO₂–C system and the synthesis mechanism were studied.     

Preparation, thermal, spectral and microscopic studies of calcium silicate hydrate–poly(acrylic acid) nanocomposite materials

A series of calcium silicate hydrate (C–S–H)-polymer nanocomposite (C–S–HPN) materials were prepared by incorporating poly(acrylic acid) (PAA) into the inorganic layers of C–S–H during precipitation of quasicrystalline C–S–H from aqueous solution. The as-synthesized C–S–HPN materials were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The XRD analysis of C–S–HPN materials suggest the intermediate organizations presenting intercalation of PAA within C–S–H and exfoliation of C–S–H. The SEM micrographs of C–S–H, PAA and C–S–HPN materials with different PAA contents exhibit the significant differences in their morphologies. The effect of the material’s composition on the thermal stability of a series of C–S–HPN materials along with PAA and C–S–H were studied by TG, DTA and DSC. Three significant decomposition temperature ranges were observed on the TG curves of all C–S–HPN materials.     

Synthesis of mesoporous NiO with crystalline walls by a simple sol–gel route

Mesoporous NiO particles with crystalline walls were prepared by a simple sol-gel technique. X-ray diffraction (XRD), N₂ adsorption–desorption, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) have been used to investigate the mesoporous NiO particles. The as-prepared mesoporous NiO possessed narrow pore in the range of mesopores and was stable up to 700 ^oC. Various characterization results showed that the mesostructure was formed through the aggregation of nanocrystals and stearic acid in the precursor played an important role in formation of the final mesoporous structures. Mesoporous Ni particles have also been successfully synthesized by reduction of the obtained mesoporous NiO at 700 ^oC for 30 min.     

Wet sol–gel silica matrices as delivery devices for phenytoin

Wet sol–gel silica matrices produced under different hydrolysis conditions were used as delivery devices to the active principle of an antiepileptic drug (phenytoin sodium), encapsulated during the condensation stage. Post-incorporation into dry silica powder was an alternative loading procedure. It was proven by infrared spectroscopy that neither the silica network nor the drug loose integrity by encapsulation. The kinetics of in vitro drug release was studied at 37 °C, to water and to artificial cerebrospinal fluid (ACSF). Emphasis has been given to the release to ACSF under dynamic conditions (with fluid renovation, emulating what occurs in the brain). Different delivery regimes were identified and correlated with the loading method and the matrix structure. Matrices with lower total porosity and smaller average pore size proved to be better for a long term release. Renovation of ACSF is relevant to assure a constant concentration of phenytoin in the vicinity of the device.     

Structural and Morphological Control in the Preparation of High Surface Area Zirconia

The methods for the formation of zirconia including precipitation from aqueous salts, sol–gel synthesis from zirconium alkoxides, and the templated synthesis using surfactants are described in this review. The surface areas obtained vary widely but invariably decrease upon prolonged calcination. Digestion of hydrous zirconia and incorporation of dopants such as lanthanum, yttrium, or sulfate ions can increase the surface area and thermal stability. However, these methods also affect the crystal phase of zirconia. The transformation from the metastable tetragonal to the monoclinic phase occurs during the cooling phase of calcination. Mechanisms for the stabilization of the tetragonal phase are discussed. Zirconia with well-ordered mesopores or in the form of hollow spheres can be prepared but lack thermal stability, unless doped with phosphates, silicates or sulfates.     

Sol–gel preparation and characterization of ceria stabilized zirconia minispheres

A novel processing technique based on sol–gel drop generation method has been developed to prepare fine zirconia minispheres for use as grinding media. Zirconium oxalate gel formation from the prepared sol was obtained in proper synthesis condition using Zirconium oxy-chloride octahydrate(ZrOCl₂·8H₂O) as starting material. The transparent oxalate gel was then added dropwise into the setting solution for the formation minispheres. To obtain the required fluidity and viscosity a suitable binder was mixed to the sol and stabilizing agent of required mol% was added to stabilize the phase formation. The addition of stabilizing agent transformation toughened the minispheres, with a complete retention of the tetragonal phase in the final product sintered at 1500 °C. Thermogravimetric analysis indicated the removal of most of the volatiles by 600 °C. Density and Crystallite size were found to be increasing linearly with sintering temperature. The phase identification, density variation, chemical decomposition, functional group specification and microstructural features for the dried and sintered final product were studied.     

Non-metal element detection by radio-frequency glow-discharge optical-emission spectrometry (rf-GD-OES) for determination of sol–gel-immobilized nucleotides

A new method is presented for qualitative and quantitative determination of non-metal elements present in organic analyte species immobilized in a sol–gel matrix. Nucleic acids were chosen as well-defined relevant biomolecules for which element ratios could be used for detection and identification. Solid, lyophilized powders of ribose-form nucleotides (AMP, ADP, and ATP) were immobilized/entrapped in a methlytrimethoxysilane (mTMOS) sol–gel matrix and monitored for P (I) 214.9 nm, C (I) 193.0 nm, O (I) 130.2, and N (I) 149.3 nm emission by radio-frequency glow-discharge optical-emission spectroscopy (rf-GD-OES). Using the sol–gel method, analytical blanks were obtained by use of un-doped sol–gels. Empirical formula calculations by use of P (I) and C (I), P (I) and N (I), and P (I) and O (I) emission response ratios demonstrate the versatility of the technique as an element and species-specific detector. Results show there is high correlation between phosphorus and oxygen emission responses and the extent of phosphorylation, demonstrating the capacity of the method to produce vital qualitative and quantitative information for the specific nucleotide. Absolute sub-nanogram detection limits were achieved for all the elements studied.     

Preparation of thick and hard coating films via sol–gel process with a low temperature treatment

SiO₂-polymer hybrid thick and hard coating films were successfully obtained via sol–gel process with a heat treatment at 200 °C. The films were achieved by control of hydrolysis and poly-condensation reaction of silica species. Key parameters were pH, water content and additional polymer in the coating solution. No-cracking and highly transparent films were formed with polymer content in a range from 28 to 42 mass% [vs. SiO₂]. Their thickness was varied from 2.6 to 3.5 μm with an increase in the polymer content. The haze values of the films after Taber abrasion were very low in a range from 1.3 to 2.7%, and the pencil hardness was over 9H. Moreover, no-damage was observed on the surface after steel wool abrasion with 4 kg loaded. These high mechanical strengths were come up to that of float glass, so that the coating films prepared by this process have a great potential for application to architectural and automotive window glass.     

Novel sol–gel derived cellular foam: reaction of an organotrialkoxysilane with sodium hypophosphite

(3-Triethoxysilylpropyl)succinic anhydride (TESP-SA) is an organo-functional silicon compound that can be converted into a polysilsesquioxane when it is hydrolyzed and subsequently subjected to a condensation reaction at elevated temperatures (>160 °C). If this process is performed without sodium hypophosphite (SHP), a hard solid material is obtained. In contrast, the condensation reaction of TESP-SA in conjunction with SHP results in the formation of a foamed, brittle material with closed macro cells. The foam was characterized by means of various analytical methods (FT-IR, ²⁹Si MAS NMR, XRD, TG-MS, SEM).

Single-phased white-light emitting CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>: Eu<Superscript>2+</Superscript>, Mn<Superscript>2+</Superscript> phosphors prepared by a sol–gel method

CaAl₂Si₂O₈: Eu²⁺, Mn²⁺ phosphors have been prepared by a sol–gel method. X-ray diffractometer, spectrofluorometer and UV–Vis spectrometer were used to characterize structural and optical properties of the samples. The results indicate that anorthite (CaAl₂Si₂O₈) directly crystallizes at 1000 °C in the sol–gel process. CaAl₂Si₂O₈: Eu²⁺, Mn²⁺ phosphors show two emission bands excited by ultraviolet light. Blue (around 415 nm) and yellow (around 575 nm) emissions originate from Eu²⁺ and Mn²⁺, respectively. With appropriate tuning of Mn²⁺ content, CaAl₂Si₂O₈: Eu²⁺, Mn²⁺ phosphors exhibit different hues and relative color temperatures.     

A method for producing gradient density aerogel

The Stardust Mission used gradient density aerogel as the hypervelocity particle capture medium in both the cometary and interstellar collection grids. The development and production of these materials was performed exclusively for this mission as a means of improving the efficiency of the collection process. The density of an aerogel can be dictated by controlling the ratio of the condensable silica to that of the solvent used in the aerogel precursor solution. A density gradient was established by gradually mixing the precursor solution for low density aerogel into the precursor solution for high density aerogel and continuously pumping the resultant mixture into a mold. The aerogel designed for the capture of cometary particles had a density gradient ranging from 10 mg/cm³ to 50 mg/cm³ across the 3 cm profile of each block. Whereas those designed for the capture of interstellar grains had a gradient ranging from 10 mg/cm³ to 20 mg/cm³ across a 1 cm profile. Since various physical properties, e.g., refractive index, thermal conductivity, acoustic impedance, dielectric constant, are correlated to the density, they also vary with the density. This method of establishing a density gradient in aerogel can be used to produce other material gradients, e.g., gradient oxide, gradient dopant, in any sol–gel based material.