Alcoholic Solvent Effect on Silica Synthesis—NMR and DLS Investigation

The solvent effect on the synthesis of silica particles has been elucidated through the examination of hydrolysis rate and particle growth by 29Si NMR and Dynamic Light Scattering studies, respectively, in various alcoholic solvents. Kinetic data obtained from the NMR experiments indicate that the rate of hydrolysis is fastest in 1-butanol and the rates follow an unusual order of 1-butanol > methanol > ethanol > 2-propanol > 1-propanol. This order of the rates of hydrolysis is rationalized with two opposing factors, steric hindrance reducing the rate and decrease of hydrogen bonding enhancing the rate. The final particle size increases as the alcohol molecular weight increases. This might be due to decrease in polarity of the alcohols.     

Solvent Effects on Particle Formation in Hydrolysis of Tetraethyl Orthosilicate

Particle formation in ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) was studied in aqueous solvents that contained ethanol, 1-propanol or 1-butanol. The reactions were performed at a TEOS concentration of 0.2 kmol/m³, a H₂O concentration of 5 kmol/m³ and a NH₃ concentration of 1 kmol/m³. Average particle size was the largest for 1-butanol and the smallest for ethanol. No specific relation of the average diameter to hydrolysis rate was observed in the reactions. Difference in dielectric constant of the solvent and particle surface potential in the reactions indicated that the magnitudes of interparticle repulsion in the solvents corresponded with the order of the average particle sizes. This suggested that the particle sizes were dominated by the interparticle repulsion.     

Formation of Silica Nanoparticles by Hydrolysis of TEOS Using a Mixed Semi-Batch/Batch Method

A new method for preparing silica nanoparticles, which consists of a two-stage semi-batch/batch hydrolysis reaction of tetraethylorthosilicate (TEOS), is presented. A relatively slow rate of hydrolysis of the TEOS occurred during the semi-batch process, which resulted in larger silica particles with a narrower size distribution. This was in direct contrast to the batch process. An example of reduction in particle size for an initial semi-batch and subsequent batch reaction is shown. On completion of the initial semi-batch step, the silica particles had a diameter of 106 nm. As the subsequent batch reaction proceeded, the mean size of the particles decreased to 23 nm. In this work, it was found that the optimal conditions for the silica nanoparticles using this mixed method were as follows; (TEOS: 0.5 M, H₂O: 6.0 M, NH₄OH: 0.2 M, feed rate: 5.0 ml/min, temperature: 42.5°C). In conclusion, a mixedsemi-batch/batch system suggested a new probability for the synthesis of nanoparticles.     

Size, volume fraction, and nucleation of Stober silica nanoparticles

29Si NMR, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) are used to monitor the synthesis of silica nanoparticles from the base-catalyzed hydrolysis of TEOS in methanol and ethanol. The reactions are conducted at a [TEOS] =0.5 M, low concentrations of ammonia ([NH(3)] =0.01-0.1 M), and [H(2)O] =1.1-4.4 M to resolve the initial size of the first nuclei and to follow their structural evolution. It is found that after an induction period where there is a buildup of singly hydrolyzed monomer, the first nuclei are fractal and open in structure. Interestingly, the nuclei are twice as large in ethanol (R(g) approximately 8 nm) as those in methanol (R(g) approximately 4 nm). The data suggest that the difference in primary particle size is possibly caused by a higher supersaturation ratio of the singly hydrolyzed monomer in methanol than in ethanol if it is assumed that the surface energy of the first nuclei is the same in methanol and ethanol. The particle number concentration and the volume fraction of the silica particles are calculated independently from the SAXS, DLS, and 29Si NMR results. Finally, the rate of nucleation is obtained from the particle number concentrations.     

Facile synthesis of silver core - silica shell composite nanoparticles

Combining metal nanoparticles and dielectrics (e.g. silica) to produce composite materials with high dielectric constant is motivated by application in energy storage. Control over dielectric properties and their uniformity throughout the composite material is best accomplished if the composite is comprised of metal core - dielectric shell structured nanoparticles with tunable dimensions. We have synthesized silver nanoparticles in the range of 40-100nm average size using low concentration of saccharide simultaneously as the reducing agent and electrostatic stabilizer. Coating these silver particles with silica from tetraalkoxysilanes has different outcomes depending on the alcoholic solvent and the silver particle concentration. A common issue in solution-based synthesis of core-shell particles is heterogeneous nucleation whereupon two populations are formed: the desired core-shell particles and undesired coreless particles of the shell material. We report the formation of Ag@SiO(2) core-shell particles without coreless silica particles as the byproduct in 2-propanol. In ethanol, it depends on the silver surface area available whether homogeneous nucleation of silica on silver is achieved. In methanol and 1-butanol, core-shell particles did not form. This demonstrates the significance of controlling the tetraalkoxysilane hydrolysis rate when growing silica shells on silver nanoparticles.     

Particle formation in the hydrolysis of tetraethyl orthosilicate in pH buffer solution

Particle formation in the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) was studied by varying pH (9.5-11) with the basic catalysts NH3, methylamine (MA), and dimethylamine (DMA) in the presence of 5 mol/m3 CH3COOH, which was chosen to suppress time variations of pH and ionic strength during the reaction. Spherical particles were formed for MA and DMA at catalyst concentrations of 0.02-0.2 kmol/m3 and for NH3 at catalyst concentrations of 0.1-1.5 kmol/m3. In a common range of catalyst concentrations for spherical particle formation, average particle size was largest for DMA and smallest for NH3. Hydrolysis rate of TEOS could be quantified by the use of buffer systems as a function of TEOS and OH- concentrations. A specific relation was not found between the hydrolysis and the particle size. The zeta potential of silica particles measured in the reaction solvent was in the order DMA < MA < NH3, and ionic strength, estimated from pH in the reactions, was in the order DMA approximately equal to MA > NH3. This suggested that the particle sizes were controlled by electrostatic particle interactions.     

Synthesis of monodisperse fluorescent core-shell silica particles using a modified Stober method for imaging individual particles in dense colloidal suspensions

Core-shell silica particles, with a diameter of 1.5 mum, containing a dye fluorescein isothiocyanate (FITC), are synthesized by the hydrolysis and condensation of tetraethylorthosilicate (TEOS). Sodium dodecyl sulfate (SDS) is added to synthesize fluorescent core particles with the diameter of approximately 1 mum. In the addition of SDS, the surface charge reduced by counterions (Na+) of the surfactant leads to a higher degree of aggregation of the primary particles and the formation of larger secondary particles. The particle growth kinetics confirms the aggregation growth model for the synthesis of monodisperse silica particles, and also shows the dependence of final particle size on colloidal stability resulting from the addition of SDS. Light and X-ray scattering data reveal that the final particles have compactly packed structures with smooth surfaces. The seeded growth technique is then used to form a silica shell layer on the fluorescent core. The added amount of water and NH4OH has significant effects on shell formation. Finally, the final core-shell silica particles are modified by chemisorption of octadecanol at the surface to be dispersed in organic solvents. Octadecyl-coated silica particles are sterically stabilized in silica index-matching solvents such as chloroform and hexadecane to directly image separate particles using confocal microscopy. In chloroform, the organophilic silica particles disperse well, whereas in hexadecane they form a volume-filling gel structure at room temperature.     

Preparation and microstructure of sol-gel derived silver-doped silica

Silver-doped silica was prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄) in the presence of a silver nitrate (AgNO₃) solution by two different synthesis methods. In the first synthesis route, sol-gel mixtures were prepared using an acid catalyst. In the second synthesis route, silver-doped silica gels were formed by two-step acid/base catalysis. For the same concentration of silver dopant [AgNO₃]/[TEOS] = 0.015 acid-catalyzed sol-gel formed a microporous silica with an average pore size of <25 Å whereas the two-step catalyzed silica had an average pore size of 250 Å and exhibited a mesoporous structure when fully dried. The differences in the pore size affected the silver particle formation mechanism and post-calcination silver particle size. After calcination at 800 °C for 2 h the acid-catalyzed silica contained metallic silver particles size with an average particle size of 24 ± 2 nm whereas two-step catalyzed silica with the same concentration of [AgNO₃]/[TEOS] = 0.015 contained silver nanoparticles with an average size of approximately 32 ± 2 nm. Mechanisms for silver particle formation and for silica matrix crystallization with respect to the processing route and calcination temperature are discussed.     

Nanocomposites Fe2O3/SiO2-Preparation by Sol-Gel Method and Physical Properties

Magnetic nanocomposites γ-Fe₂O₃/silica were prepared by a one-step sol-gel method. The sol was prepared by TEOS (tetraethyl orthosilicate) acid hydrolysis in the presence of an iron salt soluble in methanol. After gelation and drying, the transparent samples were characterized after treatment at different temperatures. The particle size, observed by HR TEM, was in the range of 2–10 nm and depended on Fe-concentration and heating temperature. Magnetic measurements showed either a ferromagnetic or a superparamagnetic behaviour and could be explained by the particle size. The dependence of the magnetic behaviour on the particle size was also studied by Mössbauer spectroscopy. The samples in which the Fe₂O₃ particle size was approximately 10 nm showed magnetic splitting (sextet) at room temperature, while smaller particles (2–3 nm) showed this splitting only at the temperature of liquid helium. The optomagnetic properties of the samples were also measured (Kerr effect).     

Combination of Silica Sol and Potassium Silicate via Isothermal Heat Conduction Microcalorimetry

Isothermal heat conduction microcalorimetry was utilized as a novel characterization method to investigate the polymerization processes of silica with both thermodynamic and kinetic parameters when the combination of silica sol and potassium silicate was stirred at temperatures of 25.0, 35.0, and 45.0°C. The silica polymerization was characterized by the greater enthalpy change at each higher temperature and by the reaction orders of the silica sol and potassium silicate, which varied rapidly, instantaneously, and constantly from low to high all the time, up and down in an alternate manner. When the reaction order of the silica sol and potassium silicate was 3.0, the maximum rate constant occurred at 25.0°C (k=1.22×10^?4mol^?2·dm⁶·s^?1). The two temperature regions (25.0–35.0°C region with a faster rate and 35.0–45.0°C region with a lower rate) reflected a two‐stage oligomerization of silica monomers with different oligomers formed in a two‐step anionic mechanism. The measurements of particle size and pH value showed that the colloidal particles in the mixed silica sol and potassium silicate first dissolved, then "active" silica in the potassium silicate redeposited to make a distinct particle size distribution (Z‐average size, 33.0–14.9 nm at 25.0°C) influenced both by pH value (9.82–11.97 at 25.0°C) and the mass fraction (53, 65, 75, and 85 mass/%) of the silica sol in the mixture. The processes of combination of the silica sol and potassium silicate did not result from acid‐base neutralization reactions but from a complex polymerization of the "active" silica components which relate to silica monomers oligomerization with heat evolved (the total enthalpy changes, 1.6234–3.3882 J).     

Ammonia Catalyzed Hydrolysis-Condensation Kinetics of Tetraethoxysilane/Dimethyldiethoxysilane Mixtures Studied by <Superscript><Emphasis Type="Bold">29</Emphasis></Superscript>Si NMR and SAXS

In-situ ²⁹Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the ammonia catalyzed hydrolysis and condensation of the mixed systems of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) dissolved in methanol. With ammonia catalysis, the hydrolysis reaction orders for TEOS and DDS in the mixed systems remained first order, which is similar to that observed for their corresponding single silane component precursor systems. The hydrolysis rate constant for TEOS in the mixed systems was larger than that of TEOS in the single silane component precursor systems. Meanwhile, the hydrolysis rate constants of DDS in the mixed precursor systems were smaller than those of DDS in the single silane component precursor systems. The hydrolysis and condensation kinetics showed more compatible hydrolysis-condensation relative rates between TEOS and DDS, which remarkably affected the final microstructure of the resulting silica particles. Small angle X-ray scattering (SAXS) experiments showed a typical double fractal structure in the particulate networks.     

Synthesis of mesoporous Stöber silica nanoparticles: the effect of secondary and tertiary alkanolamines

The effect of secondary (diethanolamine) and tertiary (triethanolamine) alkanolamines as catalysts on the formation of mesoporous Stöber silica nanoparticles by sol–gel method was studied. The particles were characterized by thermogravimetry and differential thermal analysis, Fourier transform infrared spectroscopy, N₂ physisorption measurements, and field emission scanning electron microscopy. By using ammonia and different alkanolamines as catalysts, the Brunauer–Emmet–Teller (BET) surface area and pore volume increased in the order of ammonia < diethanolamine < triethanolamine. A maximum BET surface area of 140.1 m² g^?1 and pore volume of 0.66 cm³ g^?1 were obtained from triethanolamine catalyzed silica particles. The average particle size of silica prepared by ammonia and different alkanolamines as catalysts decreased in the order of ammonia > diethanolamine > triethanolamine. The role of different alkanolamines on the textural properties and particle size of silica is explained in terms of their relative steric hindrance and basicity.     

An NMR Study of MeTMS Based Coatings Filled with Colloidal Silica

Coatings were prepared by mixing MeTMS and an aqueous colloidal silica. Mixing of an MeTMS hydrolysis mixture with the aqueous colloidal silica is only possible, without flocculation of the colloidal silica particles, within a certain time window. ²⁹Si NMR was used to follow the hydrolysis/condensation reactions of MeTMS, whereas ¹H NMR was used to monitor the reaction of the MeTMS monomers and oligomers with the silica surface in the coating liquid. The reaction of MeTMS with the surface of the SiO₂ particles is determined by the oligomer size. Typical SiO₂ surface coverage is less than 3 molecules/nm² (approximately one monolayer). Large MeTMS oligomers and/or cyclic species do not react with the SiO₂ surface. These species are probably too apolar to react or absorb at the SiO₂ surface. Flocculation of the aqueous colloidal silica occurs due to the low polarity of the hydrolysis mixture. The water content and the degree of condensation of the MeTMS determine the width of the time window.     

Hematite template route to hollow-type silica spheres

Hollow-type silica spheres with controlled cavity size were prepared from Fe₂O₃-SiO₂ core-shell composite particles by selective leaching of the iron oxide core materials using acidic solution. The spherical Fe₂O₃ core particles with a diameter range of 20-400 nm were first prepared by the hydrolysis reaction of iron salts. Next, the Fe₂O₃-SiO₂ core-shell particles were prepared by the deposition of a SiO₂ layer onto the surface of Fe₂O₃ particles using a two-step coating process, consisting of a primary coating with sodium silicate solution and a subsequent coating by controlled hydrolysis of tetraethoxysilicate (TEOS). The Fe₂O₃ core was then removed by dissolving with acidic solution, giving rise to hollow-type silica particles. Scanning electron microscopy clearly revealed that the cavity size was closely related to the initial size of the core Fe₂O₃ particle. According to the cross-sectional view obtained by transmission electron microscopy, the silica shell thickness was about 10 nm. The porous texture of the hollow-type silica particles was further characterized by nitrogen adsorption-desorption isotherm measurements.     

Effect of the polymeric coating over Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> particles used for magnetic separation

In this work, the synthesis of magnetite nanoparticles by two variant chemical coprecipitation methods that involve reflux and aging conditions was investigated. The influence of the synthesis conditions on particle size, morphology, magnetic properties and protein adsorption were studied. The synthesized magnetite nanoparticles showed a spherical shape with an average particle size directly influenced by the synthesis technique. Particles of average size 27 nm and 200 nm were obtained. When the coprecipitation method was used without reflux and aging, the smallest particles were obtained. Magnetite nanoparticles obtained from both methods exhibited a superparamagnetic behavior and their saturation magnetization was particle size dependent. Values of 67 and 78 emu g⁻¹ were obtained for the 27 nm and 200 nm magnetite particles, respectively. The nanoparticles were coated with silica, aminosilane, and silica-aminosilane shell. The influence of the coating on protein absorption was studied using Bovine Serum Albumin (BSA) protein.      

Microstructure evolution in Stöber-type silica nanoparticles and their in vitro apatite deposition

Prepared via Stöber-type sol–gel routes were three types of silica particles of <1 μm in size: pure silica, Ca-involving silica, and chitosan/alginate-coated silica with a polymershell-silica core structure. Calcium ions were impregnated in the organic layers of the polymer-coated silica particle. The sol–gel procedure was applied to tetraethoxysilane dissolved in an ethanol/water mixture, while Ca–silica was derived from CaCl₂-containing ethanol/water solutions. Scanning and transmission electron micrograph analyses indicated that those silica particles consisted of ~10 nm primary particles, the Ca–silica particles (~500 nm) were larger than the Ca-free ones (~200 nm) and that their size increased with the Ca concentration in the precursor solutions. From ¹H- and ²⁹Si- magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra and ²⁹Si cross-polarization NMR spectra, the mechanism of primary particle agglomeration and degradation of the secondary particles in saline were discussed in terms of the content of H₂O molecules and >Si–OH as well as hydrogen bonding interactions among them. In addition, the Ca–silica and core-shell silica deposited apatite in Kokubo’s simulated body fluid. Thus, the present Ca–silica and polymer-coated silica particles were suggested to be applicable to injectable bone fillers for bone generation.     

Studies of mechanism of silica polymerization reactions in the combination of silica sol and potassium sodium waterglass via isothermal heat conduction microcalorimetry

Isothermal heat conduction microcalorimetry was adopted as a novel characterization method to investigate the polymerization processes of silica when the combination of silica sol and potassium sodium silicate was stirred at 25.0, 35.0, and 45.0 °C. Thermodynamic and kinetic parameters were simultaneously obtained. The enthalpy change was greater at each higher temperature. The reaction orders (m, n) instantaneously varied, up and down in an alternate manner. At 25.0, 35.0, and 45.0 °C, the rate constants were different; the maximum rate constant occurred at 25.0 °C. These phenomena reflect a two-stage oligomeric mechanism of silica monomers. The measurements of particle size showed the complex chemical composition of aqueous silicates, which can be qualitatively designated by the particle size distribution in two parts. The results further indicate that the colloidal particles in the mixed silica sol and silicates first dissolved. Then the “active” silica in the silicates redeposited to make a distinct particle size distribution influenced by K⁺ and Na⁺ ions as well as by temperature.     

反相微乳液法制备纳米SiO2的研究

在壬基酚聚氧乙烯5醚(NP-5)/环己烷/氨水的反相微乳液体系中，进行正硅酸乙酯(TEOS)的水解、缩合反应，得到粒径在30～50 nm的单分散纳米SiO₂胶体。红外光谱法(FTIR)及透射电子显微镜(TEM)观察证明了纳米SiO₂粒子的生成。反相微乳液体系相图的研究表明，水相为氨水比纯水有较窄的W/O型微乳区。氨水微乳液是碱催化TEOS水解、缩合制备纳米SiO₂粒子的适宜体系。当体系中TEOS的浓度增大时，粒子的粒径随之增大。降低NP-5     

Preparation and characterization of polypyrrole-silica colloidal nanocomposites in water-methanol mixtures

The effect of methanol cosolvent on the synthesis of polypyrrole-silica colloidal nanocomposites using ultrafine silica sols in combination with both FeCl3 and APS oxidants has been investigated. Two protocols were evaluated: the addition of methanol to an aqueous silica sol and the addition of water to a methanolic silica sol. The latter protocol proved to be more robust, since it allowed colloidally stable dispersions to be prepared at higher methanol content (up to 50 vol% using the APS oxidant). This allowed greater control over the particle size of the nanocomposite particles. In general, the spectroscopic data, the particle size range, silica contents and electrical conductivities of these nanocomposites were similar to those reported earlier for purely aqueous formulations. Polypyrrole contents ranged from 49 to 71% by mass and particle diameters varied from around 160 to 360 nm. In terms of colloid stability, the APS oxidant was preferred for nanocomposite syntheses in the presence of methanol. However, the FeCl3 oxidant generally gave higher conductivities and narrower size distributions under comparable conditions. HF etching experiments combined with transmission electron microscopy studies indicated that, to a first approximation, these nanocomposite particles had core-shell morphologies, with a hydrophobic polypyrrole core and a hydrophilic silica shell that compose approximately one monolayer of silica sol particles. Finally, aqueous electrophoresis measurements suggested that the polypyrrole-silica nanocomposites were silica-rich and that the methanolic silica sol was more hydrophobic (lower surface charge density) than the aqueous silica sol.     

A novel method for synthesis of silica nanoparticles

A sequential method has been used, for the first time, to prepare monodisperse and uniform-size silica nanoparticles using ultrasonication by sol-gel process. The silica particles were obtained by hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol medium and a detailed study was carried out on the effect of different reagents on particle sizes. Various-sized particles in the range 20-460 nm were synthesized. The reagents ammonia (2.8-28 mol L(-1)), ethanol (1-8 mol L(-1)), water (3-14 mol L(-1)), and TEOS (0.012-0.12 mol L(-1)) were used and particle size was examined under scanning electron microscopy and transmission electron microscopy. In addition to the above observations, the effect of temperature on particle size was studied. The results obtained in the present study are in agreement with the results observed for the electronic absorption behavior of silica particles, which was measured by UV-vis spectrophotometry.