Nanometer-sized Silica Particles Research Articles

Preparation of fluoroalkyl end-capped vinyltrimethoxysilane oligomer/micro-sized silica composites possessing superoleophilic/superhydrophobic characteristic: application to selective removal of aromatic compounds from aqueous methanol solution by using these composites

Fluoroalkyl end-capped vinyltrimethoxysilane oligomer [RF-(CH2–CHSi(OMe)3)n-RF; n = 2, 3; RF = fluoroalkyl group: RF-(VM)n-RF] was found to undergo the sol–gel reaction in the presence of micro-sized silica particles under alkaline conditions to afford the corresponding fluorinated oligomer/micro-sized silica composites [RF-(VM-SiO3/2)n-RF/μ-SiO2]. The obtained composites were applied to the surface modification of glass to reveal a superoleophilic/superhydrophobic characteristic on the modified surface. In contrast, fluoroalkyl end-capped vinyltrimethoxysilane oligomer/silica nanocomposites [RF-(VM-SiO3/2)n-RF/n-SiO2], which were prepared in the presence of nanometer-sized silica particles under similar sol–gel reactions, provided an oleophobic/superhydrophobic property on the surface. RF-(VM-SiO3/2)n-RF/μ-SiO2 composites were applied to the solid-phase extraction sorbent to isolate the transparent colorless oil from water-in-oil (W/O) emulsion. These composite powders were also applicable to the effective removal of traditional organic dyes, such as rhodamine B, acid red 289, and fluorescein from aqueous methanol solution. Of particular interest, it was demonstrated that RF-(VM-SiO3/2)n-RF/μ-SiO2 composites, of whose micro-sized silica gel is surface-functionalized with amino units, can exhibit a similar superoleophilic/superhydrophobic characteristic on the composite surface, giving the selective removal of fluorinated phenol-type aromatic compounds from aqueous methanol solutions, although the removal ability for the corresponding non-fluorinated ones was not observed at all under similar conditions.

Formation and Preservation of Microbial Palisade Fabric in Silica Deposits from El Tatio, Chile.

Palisade fabric is a ubiquitous texture of silica sinter found in low temperature (<40°C) regimes of hot spring environments, and it is formed when populations of filamentous microorganisms act as templates for silica polymerization. Although it is known that postdepositional processes such as biological degradation and dewatering can strongly affect preservation of these fabrics, the impact of extreme aridity has so far not been studied in detail. Here, we report a detailed analysis of recently silicified palisade fabrics from a geyser in El Tatio, Chile, tracing the progressive degradation of microorganisms within the silica matrix. This is complemented by heating experiments of natural sinter samples to assess the role of diagenesis. Sheathed cyanobacteria, identified as Leptolyngbya sp., were found to be incorporated into silica sinter by irregular cycles of wetting, evaporation, and mineral precipitation. Transmission electron microscopy analyses revealed that nanometer-sized silica particles are filling the pore space within individual cyanobacterial sheaths, giving rise to their structural rigidity to sustain a palisade fabric framework. Diagenesis experiments further show that the sheaths of the filaments are preferentially preserved relative to the trichomes, and that the amount of water present within the sinter is an important factor for overall preservation during burial. This study confirms that palisade fabrics are efficiently generated in a highly evaporative geothermal field, and that these biosignatures can be most effectively preserved under dry diagenetic conditions.

Effect of steric hindrance on surface wettability of fine silica powder modified by n- or t-butyl alcohol

Fumed silica is an important industrial material, which is widely used in medical, cosmetic, and electronic products. One important industrial application of the fumed silica powder is using it as fillers, which are key materials for reinforcing the high-performance/high-functional industrial products. However, the use of fumed silica as filler is limited to micrometer or millimeter sized particles, because nanometer sized silica particles tend to aggregate. In this study, the surface of fine silica powder is modified with n- and t-butyl alcohols, which exhibit different steric hindrance effects. The surface wettability of each modified silica powders is determined in two ways: macroscopic and microscopic wettability. Macroscopic wettability refers to preference dispersion test and microscopic wettability refers to evaluation of the molecular level by Fourier transform infrared spectroscopy (FT-IR). The modification ratio of each sample is confirmed by thermogravimetry-differential thermal analysis (TG-DTA), and the hydrophobicity of these modified silica powders is evaluated by the preference dispersion test. Molecular level evaluation of surface wettability by FT-IR confirms an obvious structural difference due to the steric hindrance of the n- and t-butoxy groups on the surface of silica. In addition, a correlation between macroscopic and microscopic evaluation results for the surface wettability of modified silica powders is confirmed.

Research Regarding a Correlation Core–Shell Morphology–Thermal Stability of Silica–Silver Nanoparticles

Silica nanoparticles repeatedly inhaled lead to acute and chronic lung inflammation and finally to pulmonary fibrosis and lung cancer, people with chronic respiratory diseases like asthma or allergic rhinitis being even more susceptible to their toxic effects. In order to reduce these above-mentioned toxic effects, the aim of this study was to engineer the environmental silica nanoparticles with silver and subsequent thermal treatment. Nanometer-sized and spherical silica particles were synthesized in a homogeneous state, using a simple one-pot chemical method. An applicable approach resulted in silver particles forming over the surface of the silica. The outcome was materialized in extremely small silver particles attached to silica core particles. Playing their well-known decisive role, precursors and catalysts effectively controlled the size of silver and silica particles. The synchronized structure of the synthesized particles was revealed by the electrostatic repulsion among the silica spheres and the electrostatic attraction between silica spheres and silver particles. The morphological images are revealed by means of a scanning electron microscope. The formation of silver–silica composite particles was confirmed by using infrared spectroscopy and X-ray photoelectron spectroscopy analysis. Following thermal analysis, the results concerning the thermal stability of the prepared particles provided higher temperature applications.

Facile formation of superhydrophobic silica-based surface on aluminum substrate with tetraethylorthosilicate and vinyltriethoxysilane as co-precursor and its corrosion resistant performance in corrosive NaCl aqueous solution

A facile sol–gel method has been developed to create a superhydrophobic surface on aluminum substrate with tetraethylorthosilicate (TEOS) and vinyltriethoxysilane (VTES) as co-precursor at room temperature. Firstly, nanometer sized silica particles were self assembled on the substrate through the hydrolysis of TEOS. Then, the silica particles were modified with vinylsiloxane through the hydrolysis and condensation polymerization of VTES. The emphasis was focused on investigating the influence of the molar ratio between NH3·H2O and ethanol on the surface morphology and wetting property. The surface morphology was observed by scanning electron microscopy (SEM), the chemical composition and bonding state of the surface were explored by energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectra (XPS), and the wetting property of the surface was investigated by water contact angle measurement (WCA). The modified silica-based surface possessed the greatest static contact angle of 154.9°, exhibiting excellent superhydrophobic property. A hierarchical microstructure with spherical microparticles of around 2μm decorated with nanoparticles of around 450nm was observed on the film surface. The surface was covered by hydrophobic vinyl groups via the decoration of silica microparticles with vinyl-terminated siloxane nanoparticles as evidenced by EDS, FTIR and XPS. The corrosion resistant performance and durability of the superhydrophobic silica-based surface formed on aluminum substrate in corrosive NaCl solution were estimated by electrochemical impedance spectroscopy (EIS) measurements. The appropriate equivalent circuit model was put forward to fit and analyze the EIS data. The electrochemical result revealed that the corrosion resistant performance of aluminum was improved greatly by the superhydrophobic treatment.

Preparation and Characterization of SiO2/SiO2 Core–Shell Microspheres as RP-HPLC Stationary Phase

Novel SiO2/SiO2 core–shell monodisperse silica spheres for high-performance liquid chromatography packing materials are prepared by the layer-by-layer self-assembly technique. The core–shell silica spheres consist of micrometer-sized porous silica spheres as the core and a thin mesoporous silica shell formed from multilayer nanometer-sized silica particles. In addition, a reversed-phase packing by bonding octadecyltrichlorosilane on SiO2/SiO2 particles through alkyl-modified method is prepared and characterized. The results show that the carbon content of the new reversed-phase stationary phase increases by approximately 45% compared with the uncoated octadecyl-bonded SiO2 stationary phase. Eight kinds of tested aromatic compounds are well-separated on the packing and the peaks are symmetrical, which demonstrates that the packing acts as an excellent reversed-phase chromatographic stationary phase.

Further Consideration of Viscoelastic Two Glass Transition Behavior of Nanoparticle-Filled Polymers

Tsagaropoulos and Eisenberg [Macromolecules 1995, 28, 396; Macromolecules 1995, 28, 6067] reported a second loss tangent (tan δ) peak in temperature-dependent viscoelastic data for various un-cross-linked polymers filled with nanometer-sized silica particles. This peak, occurring at temperatures as much as 100 °C above the primary tan δ peak (glass-to-rubber softening transition), was ascribed to the glass transition of immobilized chains near the particles. This research is often cited as support for the existence of severely retarded segmental motion of polymer near the surfaces of small particles (glassy polymer shell). We offer a different interpretation of the results from Tsagaropoulos and Eisenberg, reinforced by our recent measurements on particle-filled polybutadiene and consideration of other literature data. Particles restrict the flow of some polymer chains, thus resulting in incomplete terminal relaxation, and partial cross-linking of unfilled polymers produces the same higher temperature tan δ peak. Polymer chains which are adsorbed onto filler surfaces are immobilized from a flow relaxation (chain diffusion/reptation) standpoint, but segmental relaxation (glass transition) is not substantially altered by small particles as a general rule.

Wide range excitation of visible luminescence in nanosilica

The visible luminescence of nanometer-sized silica particles (7 nm mean diameter) was investigated using time resolved spectroscopy. This luminescence is characterized by a wide excitation in the visible and ultraviolet range. The emission spectrum is centred at 2.72 eV with a full width at half maximum of 0.70 eV when excited above 3.5 eV, whereas it progressively empties on the high energy side when excited below 3.5 eV. Moreover, the lifetime falls in the ns timescale and decreases on increasing the emission energy. These features are due to the exceptionally broad inhomogeneous distribution of the emitting centres peculiar to the silica nanoparticles.

Nanoparticles of Mesoporous SO3H‐Functionalized Si‐MCM‐41 with Superior Proton Conductivity

Nanometer-sized mesoporous silica particles of around 100-nm diameter functionalized with a large amount of sulfonic acid groups are prepared using a simple and fast in situ co-condensation procedure. A highly ordered hexagonal pore structure is established by applying a pre-hydrolysis step in a high-dilution synthesis approach, followed by adding the functionalization agent to the reaction mixture. The high-dilution approach is advantageous for the in situ functionalization since no secondary reagents for an effective particle and framework formation are needed. Structural data are determined via electron microscopy, nitrogen adsorption, and X-ray diffraction, proton conductivity values of the functionalized samples are measured via impedance spectroscopy. The obtained mesoporous SO(3)H-MCM-41 nanoparticles demonstrate superior proton conductivity than their equally loaded micrometer-sized counterparts, up to 5 x 10(-2) S cm(-1). The mesoporosity of the particles turns out to be very important for effective proton transport since non-porous silica nanoparticles exhibit worse efficient proton transport, and the obtained particle size dependence might open up a new route in rational design of highly proton conductive materials.

Study of the Sol−Gel Reaction Mechanism in Supercritical CO2 for the Formation of SiO2 Nanocomposites

Direct sol-gel reactions in supercritical CO2 (scCO2) have attracted significant interest for synthesizing nanomaterials by reacting alkoxides with a carboxylic acid. In this study, the hydrolysis of silicon alkoxides (TEOS or TMOS) was carried out using scCO2 as the solvent to generate silica nanoparticles within the matrix of polyethylene for the synthesis of polymeric nanocomposites. This methodology provides advantages of combining the sol-gel reactions and drying into a one-step process for producing polymer nanocomposites. The synthesized polymer silica composites were characterized by SEM, FTIR, and XPS. When the TEOS loading was below 10 wt % Si content, nanometer-sized silica particles were formed that were well dispersed within the polyethylene matrix. The mechanism of the silicon alkoxides reacting with acetic acid in scCO2 was further studied using online GC-MS and offline 13C NMR. Oligomer structures with a bridging methoxyl group between the two silicon atoms and the acetate monodentate were observed. This study suggests a new sol-gel pathway in scCO2 that is different from the hydrolysis-condensation reactions using the conventional sol-gel process.

Probing polymer nanocomposite morphology by small angle neutron scattering

Polyamide nanocomposite films were prepared from nanometer-sized silica particles having particle radius of gyration (Rg) of about 66 A and trimesoyl chloride-m-phenylene diamine-based polyamides having macromolecular units of about 100–140 A. The nanoscale morphology of the samples was characterized using small angle neutron scattering (SANS). SANS reveals that silica nanoparticles interact well with the polyamide units only at limited silica loading.

Characterization of physical structure of silica nanoparticles encapsulated in polymeric structure of polyamide films

Polyamide nanocomposite films were prepared from nanometer sized silica particles and trimesoyl chloride– m-phenylene diamine based polyamides. The type of silica nanoparticles used is commercial LUDOX ® HS-40 and the particle size characterized by the radius of gyration ( R g ) is about 66 Å. The immediately prepared films were easily broken into particles to form colloidal-like dilute suspension of the silica–polyamide composite particles in D 2O–H 2O solutions for SANS measurements, that in this dilute system SANS data the complication of scattering data from the interacting particles is minimized. At about 60% D 2O of the sample solution, the silica is contrasted out, therefore the SANS profiles are predominantly from the organic polyamide scattering. The SANS profile of the sample solutions measured at 90% D 2O clearly indicates scattering from both silica and polymer. The scattering heterogeneities for two-phase system were evident from the validity of the Debye–Bueche expression in case of the nanocomposite with high silica loading. At limited silica loading of the nanocomposite, interaction between the silica and polymer chains forming core–shell morphology was observed. The transport properties of the membranes made from the nanocomposite films were measured on a batch type test kit with an aqueous solution of 500 ppm dioxane concentration at pressures ranging from 50 to 200 psig, and correlated to their composite structure.

Preparation of high performance nanocomposite elastomer: effect of reaction conditions on in situ silica generation of high content in natural rubber

Effect of amines on an in situ silica generation in natural rubber was investigated, and n-hexylamine, n-heptylamine and n-octylamine were found to increase the in situ silica content. The nanometer sized silica particles up to ca. 80 parts per hundred rubber by weight were generated in situ in the rubber matrix via a sol–gel reaction of tetraethoxysilane. Additionally, dispersion of the silica in the rubbery matrix was more homogeneous than that of commercial silica dispersed by a conventional mechanical mixing. In this in situ silica generation, the polarity and solubility in water of amine were influential factors for controlling the in situ silica content in the rubbery matrix. The obtained high in situ silica filled natural rubber was useful to prepare high performance nanocomposite elastomers.

The [formula omitted] center as a probe of structural properties of nanometer-sized silica particles

A Q-band electron spin resonance (ESR) study is reported of E′ type point defects observed in ∼7nm-sized fumed silica nanoparticles following 10-eV irradiation to photodissociate H from passivated defects. In a comparative study with bulk silica (suprasil), the E′ center is used as an atomic probe to get more in depth information on the network structure of the nm-sized particles. The nanoparticles were brought into contact with ‘bulk’ Si/SiO2 entities at an elevated temperature in vacuum (Tan=1105°C), i.e., the presence of an Si/SiO2 interface. As a result of this post manufacture treatment, the E′ density increased drastically (>order of magnitude), enabling us to resolve hyperfine (hf) structure of the E′ centers located in the core region of the nanoparticles. Two doublet structures are observed, one each assigned to O2Si–H entities and the primary 29Si hf structure of the E′ centers. Analysis of these hf spectra reveals interesting information on the network structure of the core region of the nanoparticles: (1) Fumed silica is found to contain relatively less hydrogen than suprasil. (2) An increased 29Si hf splitting (439±2G) is observed compared to bulk silica (418±2G), indicating that the E′ centers located in the core of the nanoparticles exhibit on average a slightly more pyramidal defect structure, and moreover, providing evidence that the fumed silica particles are densified compared to standard bulk silica, possibly originating from the presence of more low-membered rings (n<5) in the nm-sized silica network.

Formation and Photoluminescence Characterization of Transparent Silica Glass Prepared by Solid-Phase Reaction of Nanometer-Sized Silica Particles

We investigate the structural transformations of nanometer-sized silica particles during the solid-phase sintering process using infrared and Raman spectroscopy and field emission scanning electron...

Influence of Thermal Treatments on the Photoluminescence Characteristics of Nanometer-Sized Amorphous Silica Particles

We have studied the photoluminescence (PL) characteristics of nanometer-sized amorphous silica particles after appropriate annealing in air and in vacuum. A broad visible PL band peaking at ∼450 nm, which is characterized by a nonexponential decay in the order of nanoseconds, has been found to develop when the samples are heat-treated in air. On the other hand, the vacuum-heated samples do not show such an increase in PL in the visible region but instead show a development of PL in the near-ultraviolet region. The different PL characteristics observed between the air- and vacuum-heated samples are discussed in terms of the different dehydroxylation reactions in air and in vacuum. Possible models of the respective emission centers are presented on the basis of the density functional theory calculations.

Mesostructured Silica with Bush-like Morphology and Its Transformation into Nanometer-sized Mesoporous Silica Particles

Abstract A novel bush-like morphology of mesostructured silica having stems, branches, and leaves has been grown under mild basic conditions via auxiliary solvent evaporation induced self-assembly process.

Enhanced blue photoluminescence from SiCl 4-treated nanometer-sized silica particles

We propose a structural model of the blue-light-emitting center in silicon- and silica-based nanostructured materials on the basis of the dehydroxylation reaction of a pair of geminal silanol groups. This model predicts that the blue-light emission can be enhanced by introducing a large amount of surface silanol groups and the appropriate subsequent annealing. This enhancement of the PL emission is indeed observed in nanometer-sized amorphous silica particles with SiCl 4.

Structure and properties of silica glass prepared by solid-state sintering of nanometer-sized silica particles

Structure and properties of silica glass prepared by solid-state sintering of nanometer-sized silica particles

Unique commercial applications of the sol-gel process in Japan

Two unique commercial applications of the sol-gel process in Japan are presented. One application involves alkoxide-derived nanometer-sized silica particles used in the final polishing of silicon wafers for the fabrication of integrated circuits. The particles are cocoon-like in shape and have almost replaced conventional abrasives because of the advantages over spherical particles of similar size in terms of obtaining high polishing efficiency for good surface finish. The other application concerns the treatment of paper with an alkoxide solution for water repellent and oil resistance properties, which leads to new products for disposable tableware or cooking ware for microwave oven use.