A novel process to prepare a hollow silica sphere via chitosan-polyacrylic acid (CS-PAA) template

Hollow silica spheres were synthesized by using a chitosan-polyacrylic acid (CS-PAA) template. Polyacrylic acid (PAA) was titrated into the dissolved chitosan solution to form CS-PAA particles. For forming the core-shell particle, colloidal silica which was prepared from homogeneous nucleation was mixed with a solution of CS-PAA particles. Colloidal silica was adsorbed on to the surface of the CS-PAA particles to form a shell structure. CS-PAA could be removed from the core of the core-shell particles by adjusting with HCl to pH < 1 for forming the hollow silica structure. The outside particle size diameter, shell thickness, specific surface, pore diameter and pore volume of the final hollow silica sphere are about 200 nm, 20 nm, 350.95 m²/g, 5.4078 nm and 0.516768 cm³/g, respectively.     

Preparation of spherical silica particles in reverse micro emulsions using silicon tetrachloride as precursor

Silica powders with spherical shape have been prepared by precipitation reaction of silicon tetrachloride with de-ionized water in the micro emulsion system. Two kinds of micro emulsion systems were used. One was cyclohexane/polyoxyethylene nonyl phenyl ether (NP-5)/de-ionized water system, and the other was heptane e/NP-5/de-ionized water. Transmission electron microscope (TEM), thermo gravimetric analysis (TG), X-ray diffraction (XRD) patterns, infrared spectrum (IR) were employed to characterize the micro structure, shape, thermal properties, phase, and molecular structure of the obtained silica. The results show that silica featuring spherical shapes and uniform characteristics was yielded by the precipitation method using silicon tetrachloride with de-ionized water in the micro emulsion system. A decreased tendency for silica particles agglomeration could be achieved by using lower mole ratio of water and SiCl₄, and the particle size was 5.74 nm when mole ratio of water and SiCl₄ was 4 in NP-5/cyclohexane/deionized water micro emulsion system. The silica prepared by these two systems was amorphous silica and thermally stable after 600 °C treatment. The structures of silica prepared from the different micro emulsion systems were similar; organic molecules can be removed completely from the silica through 600 °C treatment.     

An alumina mat with a nano microstructure prepared by centrifugal spinning method

Ceramic fiber products specially alumina mat because of low thermal conductivity and high melting point are used as high temperature insulating materials. Alumina has so high melting point (T_m > 2040 °C) that its mat can be produced through sol–gel method. In this research alumina mat has been manufactured by sol–gel spinning method using our laboratory-designed centrifugal spinneret. The desired viscosity of sol for spinning is 150 P. Phase transformation of the product begins at 600 °C and there is not any amorphous phase at 800 °C and theta alumina (θ-Al₂O₃) is the main phase. In this work, transformation of transitional phase to alpha alumina (α-Al₂O₃) takes place from 1000 °C to 1200 °C. The optimum percent of silica in alumina mat is 4 wt%. Fibers constitute network structure that their average diameter is about 10 μm and contains very fine grains (100 nm). The silica percent concerning the limits of this study (<10 wt%) does not effect on fiber diameter, but grain size decreases from about 200 nm to less than 100 nm while increasing silica percent.     

Deformation of colloidal silica particles using MeV Si ion irradiation

Spherical submicrometer-sized silica particles were prepared by the Stöber process, from a reaction mixture containing tetraethoxysilane, ammonia and ethanol, and deposited onto silicon wafers. While the properties of these SiO₂ particles depend on their size, size distribution and shape, monodisperse spherical particles were obtained with a narrow size distribution. The samples were then irradiated at room temperature with Si ions at two energies (4 MeV and 6 MeV) and fluences up to 5 × 10¹⁵ Si/cm², at an angle of 45° with respect to the sample surface. The size, size distribution and shape of the silica particles were determined using scanning electron and atomic force microscopes. By means of the Si ion irradiation, the as-prepared spherical silica particles can be turned into ellipsoidal particles, as a result of an increase of the particle dimensions perpendicular to the ion beam and a decrease in the direction parallel to the ion beam. This effect increases with the ion energy and fluence, and depends on the electronic energy loss processes to which the impinging ions are subjected. Our results also suggest that the particle deformation is more important in the case of larger as-prepared silica colloids.     

Silica xerogels and aerogels synthesized with ionic liquids

The synthesis of silica gels with ionic liquids (IL) as either additives or co-solvents is described. The wet gels were either dried by supercritical CO₂ or by evaporation to obtain aerogels or xerogels, respectively. The effects of two ionic liquids: 1-butyl-3-methyl imidazolium tetrafluororate (IL1) and 1-butyl-4-methyl pyridinium tetrafluoroborate (IL2), on the structural and textural characteristics of gels were investigated. IL2 showed a more important influence than IL1, on the gelation time and gel structure, according to solid state NMR investigations. With both types of ionic liquids, the average pore radius of xerogels increased from ≈2 nm at an IL to Si molar ratio n_IL/n_Si = 0.07, to ≈4 nm at n_IL/n_Si ≈ 1.5 and the size distributions were rather well centered about each mean radius. Hence, ionic liquids appeared as interesting additives to target gels with a given pore size.     

Sintering process of amorphous SiO2 nanoparticles investigated by AFM,IR and Raman techniques

We report an experimental investigation on the effects of thermal treatments at different temperatures (room—1270 K) and for different duration (0–75 h) on amorphous silica nanoparticles (fumed silica) in powder tablet form. Three types of fumed silica are considered, comprising nearly spherical particles of 40 nm, 14 nm and 7 nm mean diameter. The experimental techniques used here are Raman and infrared absorption (IR) spectroscopy together with atomic force microscopy (AFM). Raman and IR spectra indicate that the structure of nanometer silica particles is significantly different with respect to that of a bulk silica glass. In particular, the main differences regard the positions of the IR band peaked at about 2260 cm^?1, the Raman R-band peaked at about 440 cm^?1 and the intensity of the D1 and the D2 Raman lines, related to the populations of 4- and 3-membered rings, respectively. Our data also indicate that, under thermal treatments, the structure of fumed silica samples is significantly changed, gradually relaxing towards that pertaining to ordinary bulk silica. These changes are interpreted here on the basis of the morphological information provided by the AFM measurements and assuming a two-shell structure for the fumed silica primary particles.     

Structural investigation of alumina silica mixed oxide gels prepared from organically modified precursors

The development of the structure of silica/alumina gels was investigated by small angle X-ray scattering (SAXS) through all stages of the preparation process, that is gelation, aging, drying and calcination. Two series of gels were compared; the first was prepared from a single-source precursor, obtained by reaction of 3-oxoethyl-6-trimethoxysilyl-hexan-2-one (OTH-H) with Al(O^sBu)₃. The gels of the second series were prepared from the same precursors, but under conditions where hydrodeacylation of OTH-H occurs. It turned out that both series showed a similar structural development of the gels. Immediately after the start of the reaction small primary particles are formed, the size of which (r = 0.7 ± 0.1 nm) remains constant through the gelation and aging process. In some samples, Si(OEt)₄ was added as an additional source for Si/O which was incorporated between the primary particles. Condensation proceeds by a slower aggregation of the primary particles, which initially form a rather open network. This network densifies during aging and drying.     

Structural and luminescence properties of amorphous SiO2 nanoparticles

We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO₂. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO₂ nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the defects responsible for the photoluminescence are localized on a surface shell of the particles and not simply on their surface. In addition it is found that the generation efficiency of these defects depends on the structural properties of the SiO₂ matrix in the surface shell.     

Synthesis and X-ray structural characterization of NiO nanoparticles obtained through gelatin

Nanoparticles of fcc-NiO phase were obtained by heating the dried resin resultant of a mixture of gelatin and NiCl₂ · 6H₂O in aqueous solution. The average particle size and microstrain were calculated from the line broadening of X-ray powder diffraction peaks, and these values were between 15 nm and 78 nm, and 0.056% and 0.172%, respectively. The Rietveld refinement method was applied to all diffraction patterns. The particle size, obtained from this procedure, changes as a function of temperature, heating time and the remarkable reduction due to the addition of NaOH to the solution, which can be attributed to the presence of NaCl crystals and carbon encapsulating NiO nanoparticles during the heating. The heating temperature was in the range of 350–700 °C. Thermo-gravimetric analysis showed that the majority of organic fraction starts to disappear after 300 °C.     

Study on the gel casting of fused silica glass

Slip casting process is usually applied for the forming of fused silica products. Segregation always occurs and it will results in density deviation. By using gel casting process, green is fabricated by means of in situ polymerization with a three-dimensional network, holding the particles together and eliminating the tendency of migration. To prepare gel casting slurries, premix solutions were composed of acrylamide and N,N′-methylenebisacrylamide. Solid loading was kept 60% and the average particle size of silica powder 8–8.5 μm. Lactic acid was introduced as a dispersant to regulate the pH value 3–4. Mechanism of the dispersant was investigated by studying ζ-potential at different pH. Ammonium persulfate (NH₄)₂S₂O₄ was added as an initiator. Gelation took place with the help of initiator at 50–60 °C. Nanometer silica was introduced to boost sinterability so that the density and bending strength of fused silica ceramics have been increased to 2.1 g/cm³ and 40 Mpa, respectively.     

Correlation between the luminescence properties and the surface structures of submicron silica particles

We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Stöber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar⁺ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing.     

A facile route for the fabrication of CeO2 nanosheets via controlling the morphology of CeOHCO3 precursors

The morphology and size of the functional oxide rare earth material CeO₂ are crucial for its efficient applications. CeO₂ nanosheets, which are 30–50 nm in thickness and about 300 nm in the lateral direction, have been synthesized via controlling the morphology of CeOHCO₃ precursors by a facile hydrothermal technique. The dependences of morphologies and phase transitions of CeOHCO₃ precursors on reaction time, chemical compositions are investigated in detail. Results show that the precursors exhibit changes both in morphology and phase structure as reaction time elongated. One-dimensional nanowire morphology with an orthorhombic phase structure is obtained when the reaction time is less than 3 h, while two-dimensional nanosheet with a hexagonal phase structure is formed when the reaction time is more than 9 h. The detailed transition process is recorded by snapshots of products between 3 and 9 h. Experimental results reveal that the amount of acetic acid is a key parameter for the nucleation and crystal growth of CeOHCO₃ nanosheets. A defect driven dissolving and recrystallization mechanism is proposed for the phase and morphology transition behavior of the precursors. The CeO₂ nanosheets are obtained after thermal decomposition of the precursors. Compared to the nanodisk, the CeO₂ nanosheets show a blue shift in the UV–Vis absorption spectra due to the quantum confinement effect.     

Epoxide assisted sol–gel synthesis of perovskite-type LaMxFe1−xO3 (M = Ni, Co) nanoparticles

Perovskite-type LaM_xFe_1−xO₃ (M = Ni, Co) nanoparticles were synthesized by a sol–gel method using propylene oxide as a gelation agent. The resulting nanoparticles show a narrow size distribution with particles in the 30–50 nm range. A highly homogeneous wet gel was formed during the hydrolysis and condensation of the precursor salts. This high homogeneity allows a substantial reduction of the calcination temperature and time required for the formation of the perovskite phase as compared with the solid-state and other wet solution routes, reducing drastically the aggregation of the particles during calcination.     

The effect of calcination temperature on the crystallinity of TiO2 nanopowders

TiO₂ nanopowders have been prepared using 0.1 M titanium tetraisopropoxide (TTIP) in varied pH aqueous solution containing TMC and NP-204 surfactants. Only the powder acquired from a solution of pH=2 has a regular particle size distribution. Anatase phase powders are obtained by calcination in nitrogen in the 250–500°C temperature range. When calcined at 400°C, the diameter of the nanoparticles is approximately 10 nm with a specific surface area of 106.9 m²/g. As the calcination temperature is increased, the particle size increases. Rutile phase powders are formed at calcination temperatures above 600°C.     

Ultrafine Co1−xZnxFe2O4 particles synthesized by hydrolysis: Effect of thermal treatment and its relationship with magnetic properties

Co_1−xZn_xFe₂O₄ (x = 0, 0.2 and 0.4) fine powders with particles size of 3 nm were prepared by hydrolysis method. The powders were annealed at 500 °C for 3 h. With heat treatment, the average particles size increased to 12 nm with corresponding increase in blocking temperature, saturation magnetization and reduced remanence. A significant increase in coercive field was found only for the pure CoFe₂O₄.     

The nanostructure of fire opal

Fire opal, a transparent orange variety of opal, which does not diffract visible light, is built from the random accumulation of granular particles of hydrated silica, about 20 nm in size. This opal variety does not present the structure most commonly associated with precious opal, that is, a regular three-dimensional network of amorphous silica spheres about 200 nm in diameter. About 60 samples (from Mexico, Brazil, Kazakhstan, Ethiopia, Tanzania, Slovakia and USA) were documented using scanning electron and atomic force microscopy. This work demonstrates that nanograins are the elementary building blocks of this variety of opal.     

Thermal decomposition and new luminescence bands in wet,dry, and additional oxygen implanted silica layers

Wet and dry silica oxide layers have been treated thermally up to T_a = 1300 °C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 10¹⁷ ions/cm² and high thermal treatment T > 1100 °C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.     

Preparation and characterization of amorphous silica nanowires from natural chrysotile

By chemical dispersing and acid leaching, silica nanowires have been prepared from the natural mineral, chrysotile. X-ray fluorescence analysis (XRF), thermogravimetric analysis and differential thermogravimetric analysis (TGA–DTA), powder X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to characterize the silica nanowires. The results indicate that the chemical composition of the silica nanowires is SiO_1.8 · 0.6H₂O, and although the silica is amorphous, its structure is regular to some extent. The structural unit of the silica nanowires is the [SiO₄] tetrahedron and six-member silicon–oxygen ring with the tetrahedral positioned alternately up and down in the six-member ring. The silica nanowires are well-dispersed and have cylindrical morphology and smooth surface, with lengths over 10 μm and diameters of 30–60 nm.     

Preparation of core–shell nanospheres of silica–silver: SiO2@Ag

We prepared SiO₂@Ag core–shell nanospheres: silver nanoparticles (～4 ± 2 nm in diameter) coated silica nanospheres (～50 ± 10 nm in diameter). The preparation route is a modification of the Stöber method, and involves the preparation of homogeneous silica spheres at room temperature, combined with the deposition of silver nanoparticles from Ag⁺ in solution, by using water/ethanol mixtures, tetraethyl-orthosilicate as Si source and silver nitrate as Ag source in a single-pot wet chemical route without an added coupling agent or surface modification, which leads to the formation of core@shell homogeneous nanospheres. We present the preparation and characterization of the SiO₂@Ag core–shell nanospheres and also of bare silica spheres in the absence of silver, and propose a reaction mechanism for the formation of the core–shell structure.     

Influence of UV-exposure on the crystallization and optical properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass. Photoinduced crystalline phase precipitation results in refractive index variations in UV exposed areas of PTR glass. The precipitation of silver containing particles which occurs during photo-thermo-refractive process increases the optical absorption of the samples in the range 350 nm to NIR wavelengths and the growth of sodium fluoride crystals and their aggregation increases light scattering in visible and NIR regions. We show that one effect of the UV-exposure is a decrease in the crystallization temperature by ～50 °C compared to the unexposed areas as measured by differential scanning calorimetry, which we attribute to an increase in nucleation rate. Using spectro-photometric measurements, a linear function is fitted to the changes in the amplitude of the absorption band of the silver containing particles versus the UV-dosage. The root mean square scatter of the data from the linear function is better than 0.99 and the slope of the function is 0.32 ± 0.01 cm/J. The IR absorption of PTR sample, measured by laser calorimetry shows that the increase of the absorption in infrared region at 1.1 μm, is due to the tail of the absorption band of silver containing particles having maximum at 465 nm. We finally show that after hyper-development, one effect of UV-exposure at 325 nm on the crystallization kinetics of PTR glasses is a decrease in particle sizes from micron size to nanometers size. But additional investigations demonstrate that smaller dosage UV-exposures (a few tens of milliwatts) increase the optical scattering by one order of magnitude. Optical micrographs taken after UV-exposure and hyper-development reveal the use of smaller dosages enhances nucleation rate without preventing the growth of large crystals and therefore induces higher scattering.