Colloidal SiO2 Research Articles

Photocatalytic air purification: Comparative efficacy and pressure drop of a TiO 2-coated thin mesh and a honeycomb monolith at high air velocities using a 0.4 m 3 close-loop reactor

Photocatalytic purifiers, which are in development for treating gaseous effluents at flow rates generally higher than several tens of m 3 h −1, usually employ TiO 2-coated materials either as planar or folded fibrous filters, or as honeycomb monoliths. Our primary objective was to compare the photocatalytic efficacy of two types of these materials using a ca. 0.4 m 3 close-loop, air tight, photocatalytic reactor we built at EDF. This loop – through which air can be flowed at rates from 16 to 1800 m 3 h −1, i.e. 0.11–12.25 m s −1 – includes a paralleliped (560 mm × 295 mm × 200 mm) into which alternate banks of lamps and TiO 2 coated-materials (entrance area = 408 cm 2) can be accommodated. The materials tested were (i) an Ahlstrom-supplied, thin, non-woven tissue dried at room temperature after impregnation with both Degussa TiO 2 P-25 and colloidal SiO 2 used as a binder and (ii) an aluminum honeycomb-shaped material we coated with Degussa TiO 2 P-25 and dried at room temperature. U-shaped lamps emitting at 254 nm and having an electrical power of 18 or 35 W were employed. The geometries of the materials and the spatial arrangements of the lamps and materials provided an irradiance of the materials as high and homogeneous as possible according to our modeling [Catal. Today 122 (2007) 66–77]. Methanol was chosen as the test pollutant because it is easily mineralized, which minimized the inhibition of the photocatalytic activity by intermediate products. In the recirculation regime, the initial rates of methanol removal observed for the folded tissue were multiplied by at least four – depending on the numbers of materials and banks of three 35 W lamps – when the honeycomb material was utilized. Several factors can add to cause this high increase in efficacy: distinct shapes and TiO 2 supports, differences in photons scattering, and the reduced accessibility of reactants to TiO 2 because of SiO 2. Regarding energy consumption, the use of two photocatalytic materials sandwiching one bank of three 35 W lamps instead of one material in-between two banks of three lamps led to about the same efficacy. The honeycomb material was also tested with toluene at an initial concentration corresponding to the same amount of carbon as when methanol was the pollutant. The removal rate and CO 2 formation rate were lower for toluene than for methanol, which can be easily explained by a lower reactivity with respect to oxidation, a smaller adsorbed amount and a higher competition with the more numerous degradation intermediate products. That comparison illustrates the need of trials for every effluent to be treated. Considerable differences in pressure drop between the two materials clearly demonstrated another interest of using honeycomb shapes, at least for the materials and configurations investigated. Furthermore, a deactivation, together with a yellowing, was noticed at high irradiance for the Ahlstrom tissue, which included cellulose fibers. Folding the material enables one to employ lower irradiances for minimizing this effect, while maintaining the efficacy because of the increase in the material area in the reactor, as well as slightly lowering the pressure drop.

Self-assembled SiO_2 photonic crystal infiltrated by Ormosil:Eu(DBM)_3phen phosphor and its enhanced photoluminescence

Luminescence films were prepared by infiltration of the tris(dibenzoylmethane) mono(1, 10-phenanthroline) europium incorporated ormosil into colloidal SiO(2) photonic crystal templates. Because a stopband of the template was not overlapped with the PL excitation and emission bands, the stopband did not suppress the PL intensity. The PL intensity of the infiltrated film into the template was about 13.1 times higher than that of the plane film prepared without the template. Three major terms, which are the mass term, the scattering term, and the crystallinity term, were considered as factors that improve the PL intensity. The relative ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2.

Synthesizing high α-phase Si 3N 4 powders containing sintering additives

Fine grain α-phase silicon nitride (Si 3N 4) ceramic powders were produced via carbothermic reduction of colloidal SiO 2, which contained pre-mixed additives of sintering aids primarily consisting of oxides such as MgO and Y 2O 3. The powders that were pre-mixed in the starting reactants were chosen based on the final powder composition and on type and amount of the secondary phases desired for sintering. After synthesizing, powder properties were examined using standard characterization techniques (XRD, SEM, BET, etc). This technique of ceramic synthesis has advantages in providing nitride-based ceramic powders, which contain secondary in situ phases that are distributed as sintering additives. Silicon nitride ceramic powders synthesized using this method might therefore be readily sintered because the homogeneously distributed sintering additives were present in the starting materials. In this work, the processing parameters are described in terms of the synthesis conditions.

Fabrication of Hydroxyapatite with Three-dimensional Pore Frame by Colloidal Template Method

SiO2 colloidal template was prepared through Stber method,followed by gravitational sedimentation.Hydroxyapatite(HA) with highly ordered three-dimensional pores was fabricated through infiltrating the template with HA precursor,sintering and then removing the template by immersing in NaOH solution.The samples were characterized by X-ray diffraction(XRD),transmission electron microscope(TEM),field emission scanning electron microscope(FESEM).The effects of the concentration and amount of HA precursor,and the time of immersion in NaOH solution on the preparation technique,the porous structure and the porosity were investigated.The results show that the Zeta potential of SiO2 spheres increase after surface modification by H2O2,thus improving the order degree of the silica colloidal template.The sizes of SiO2 colloid spheres and the pores formed by dissolved spheres are about 250nm and 200nm,respectively.There are a great number of micro-pores with dimension of 20nm on the wall of the HA pore formed by removing SiO2 spheres.The well-ordered HA with three-dimension pores are fabricated on the condition that the HA precursor concentration is 0.8mol/L,the volume ratio of HA precursor to the whole template is 30%,immersion in 4mol/L NaOH solution for 3d.

Double Direct Templated Hollow ZnS Microspheres Formed on Chemically Modified Silica Colloids

The effect of surface chemistry of SiO2 colloids dispersed in a nonionic surfactant based lyotropic liquid crystal (LLC) on the double direct templating of hollow mesostructured ZnS capsules was investigated. In double direct templating, a LLC templates the mineralization of ZnS on the surface of a colloidal particle. Removal of the colloid results in a periodically mesostructured hollow capsule. After ZnS mineralization and a subsequent etching of the sacrificial SiO2 core, the ZnS mesostructure was imaged using transmission electron microscopy (TEM). The highest quality mesostructure was observed for poly(acrylic acid) (PAA) functionalized colloids, while other surface chemistries, including bare silica, poly(ethylene glycol), amine, and thiol surface chemistries yielded much less favorable results. In the PAA system, the order of the hexagonally arranged 3 nm mesopores in the ZnS shell closely matched that of the lyotropic liquid crystal template. Detailed TEM revealed the presence of regions where the...

Rapid fabrication of crystalline films with double layers and dual members by spin-coating method

Ordered colloidal particle films with double layers and dual members could be prepared by stepbystep spincoating method. Monolayer film of larger sized PS or SiO2 colloids was utilized as template to selfassemble second layer of SiO2 colloidal particles with smaller diameter. The size ratio γ of smaller and larger colloidal particles was 0.20—0.56 in the composite films. Larger （L） and smaller （S） dual member colloidal particles formed composite crystalline doublelayer films with multifarious arraymodes, whose structures could be expressed as LSx （here, L represents larger sized particles, S represents smaller colloidal particles, and x represents the stoichiometry of larger and smaller colloidal particles）, and x=1, 2, …, 13. The performance of composite colloidal films were influenced mainly by several factors, including spincoating velocity, spincoating time, viscosity of suspension medium, number density of colloidal suspension, and the wetting properties of substrates, etc.. On the prerequisite of complete wetting between colloidal suspension and substrate, appropriate number density of colloidal suspension, velocity and time of spincoating were the necessary conditions for ordered selfassembling to crystalline film of colloidal particles by spincoating method.

Stability of hydrogen-bonded hydroxypropylcellulose/poly(acrylic acid) microcapsules in aqueous solutions

Hydroxypropylcellulose (HPC), a cellulose derivative, and poly(acrylic acid) (PAA) were assembled onto colloidal SiO2 particles in acidic solutions in a layer-by-layer manner, using hydrogen bonding as the driving force. Monodisperse hollow microcapsules were prepared by the removal of the sacrificial template. Long-term stability of the hydrogen-bonded capsules under various conditions was studied with dynamic light scattering. In contrast to an expected quick response to pH change upon mixing with phosphate buffered saline (PBS), a slow swelling of the capsule wall was observed, which is attributed to the hydrophobicity and semi-rigidity of the HPC chains involved in the hydrogen-bonded capsule wall. Because of the reversible nature of hydrogen bonding, all HPC/PAA capsules eventually disintegrate completely regardless of pH of the media. High ionic strength slows down the swelling and disintegration of the capsule wall by effective screening of the repulsive interaction among the ionized groups, however, it cannot totally prevent the capsule wall from disassembly. By covalent crosslinking, the capsule wall can be locked firmly. The resultant capsules can withstand high pH of the media and present a good long-term stability because they do not disintegrate with time. The gradual disintegration of hydrogen-bonded capsules may find applications in controlled drug delivery.

Si3N4세라믹스의 균열 치유와 굽힘 강도 특성

Crack-healing behavior of Si₃N₄ composite ceramics has been studied as functions of heat-treatment temperature and amount of additive SiO₂ colloidal. Results showed that optimum amount of additive SiO₂ colloidal and coating of SiO₂ colloidal on crack could significantly increase the bending strength. The heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature depends on the amount of additive SiO₂ colloidal. Crack healing strength was far the better cracked specimen with SiO₂ colloidal coating on crack surface. After heat treatment at the temperature 1,273 K in air, the crack morphology almost entirely disappeared by scanning prob microscope. At optimum healing temperature 1,273 K, the bending strength with additive SiO₂ colloidal 0.0 wt.% without SiO₂ colloidal coating recovered to the value of the smooth specimens at room temperature for the investigated crack sizes 100 ㎛. But that with SiO₂ colloidal coating increase up to 140 %. The amount of optimum additive SiO₂ colloidal was 1.3 wt.% and crack healed bending strength with SiO₂ colloidal coating increase up to 160 % to smooth specimen of additive SiO₂ colloidal 0.0 wt.%. Crack closure and rebonding of the crack due to oxidation of cracked surfaces were suggested as a dominant healing mechanism operating in Si₃N₄ composite ceramics.

A study of intensity of tribochemical contact interaction between a polishing compound and sapphire in machining

The paper presents the findings of a study of the intensity of tribochemical contact interaction between sapphire (α-Al2O3) and a polishing compound based on colloidal silica SiO2, which occurs in polishing precision electronic and optical components. It has been established that the main factor that intensifies this interaction is the acceleration of a chemical reaction between Al2O3 and SiO2 with increasing contact temperature, which experimentally validates the chemical nature of the tribochemical polishing process and allows a significant reduction of its labor intensity.

Influence of the substrate, metal overlayer and lattice neighbors on the focusing properties of colloidal microspheres

Calculations of the field distribution in colloidal SiO2 microspheres are presented. Two cases are considered: small particles on a Si substrate irradiated by the 266 nm light, and larger ones, covered with a gold film and irradiated at 800 nm. Substrate, neighboring spheres and sputtered metal overlayer all significantly modify the field pattern and magnitude. Reflected light is focused inside the spheres, which may lead to their damage. The results can be useful in the analysis of microspheres-assisted nano-patterning.

Layers of xCuS-SiO2·nH2O nanocomposite, synthesized by the layer-by-layer technique

For the first time conditions were determined for the synthesis of CuxSiO2+x·nH2O nanostructured layers by consecutive adsorption of copper ammine cations and adagulation of colloidal SiO2 particles and also for the synthesis of xCuS-SiO2·nH2O nanocomposite layers by consecutive surface adsorption of copper cations and HS− anions. These layers were studied by means of UV and visible transmission spectroscopy, X-ray spectral microanalysis, and scanning electron microscopy. Schemes of the surface reactions were constructed on the basis of this experimental material.

Chemical Mechanical Polishing of Transparent Nd:YAG Ceramics

Transparent Nd:YAG ceramics which are very hard and brittle materials, are very difficult to be polished. There are many micro scratches or damages on the surface after mechanical polishing with Al2O3. In order to remove micro scratches or damages, chemical mechanical polishing (CMP) was adopted to manufacture Nd:YAG ceramics. In the polishing experiment, Pellon and Chemcloth pads were utilized for chemical mechanical polishing of Nd:YAG ceramics. Colloidal SiO2 was selected as the polishing slurry in two different polishing environments, acidity and alkalinity. The surface roughness was determined by using atomic force microscope. In this study, four polishing experimental combinations that each combination contains one of the two pads and one of the two polishing environments were carried out in the optimum polishing condition. Then the high quality surface of transparent Nd:YAG ceramics with the best surface roughness of < 0.2 nm RMS and few micro scratches or damages is obtained by adopting CMP process with Chemcloth pad and colloidal SiO2 in acidic condition.

Mechanical properties of metakaolin-based geopolymers with molar ratios of Si/Al ≈ 2 and Na/Al ≈ 1

The mechanical properties of four different types of geopolymers, but of the same composition (Na/Al ≈ 1, Si/Al ≈ 2 molar ratio), made using a combination of precursors, were determined. The four types were: (i) sodium aluminate (NaAlO2/NaOH solution), Ludox (colloidal SiO2 solution) and metakaolin (MK), (SAGP), (ii) NaOH, fumed silica and MK (FSGP), (iii) Ludox, NaOH and MK (LGP) and (iv) commercial sodium silicate and MK (SGP). The highest crushing strength (CCS) value obtained was for SGP (70 MPa) and the lowest value was for SAGP (16 MPa). The highest modulus of rupture (MOR) value obtained was for LGP (9 MPa) and the lowest value was for SAGP (3 MPa). The fracture toughness (K1c) and Young’s modulus (E) showed the same trend. The effect of adding sand (40 wt%) on their mechanical properties was also determined. The K1c values increased up to 65% and E values increased up to 80% compared to samples free of sand. However, CCS and MOR values did not change much and gave mixed results. Overall, porosity is found to be the chief microstructural variable limiting the mechanical properties of the geopolymers. The properties of the geopolymers are compared with those of ordinary Portland cement.

Inverse conjugated polymer opals and their luminescent spectra

Colloidal silica spheres are usually used for making opal photonic crystals. With the aim to create tunable or otherwise functional opals we have coated colloidal SiO2 spheres with conjugated polymers, including: polyaniline sulfonic acid, polypyrrole and MEH-PPV (poly [2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]). Spheres coated with MEH-PPV were dispersed in H2O and coated with polyelectrolytes that charged the colloidal surfaces allowing the polymer coated spheres to self-assemble by sedimentation under gravity into close packed structures with well ordered crystalline domains. Inverse MEH-PPV/polypropylene photonic crystals were fabricated and their photoluminescence was blue shifted compared to a uniform MEH-PPV film.

Monodisperse SiO<sub>2</sub> Nanospheres Prepared by Batch/Semibatch Process and Its Opals

Nearly monodispersed SiO2 nanospheres, with different size and size standard deviation smaller than 5%, have been prepared by using batch/semibatch process. The SiO2 colloids were used as building blocks to self-assemble into the colloidal crystals, which photonic band gaps are close to the theoretical values, and can be modified by changing the size SiO2 nanospheres. With the increase of the size of SiO2 spheres, the photonic band gap red shift.

Identifying when microbes biosilicify: The interconnected requirements of acidic pH, colloidal SiO 2 and exposed microbial surface

This study demonstrates discernible biosilicification of natural microbial mats through batch laboratory experiments. Identification of the geochemical requirements for this process to occur includes thermodynamically favorable, but sluggish silica reaction kinetics associated with acidic conditions, and the necessity for colloidal silica rather than dissolved silicic acid species. This study provides the first results to bridge the apparent literature discrepancy between widespread, in-situ observations of microbial silicification, and the inability to demonstrate a detectable microbial impact in this process under well-constrained laboratory conditions. We compared the silica scavenging abilities of three natural microbial mats collected from Yellowstone National Park (YNP) hotsprings, relative to those of both abiotic particle (TiO 2) and solution controls at constant, near-saturated aqueous silica concentrations, while experimental pH and temperature conditions were varied, using both dissolved and colloidal SiO 2 forms. We specifically evaluated three microbial mats sampled from YNP sites all exhibiting saturation with respect to amorphous SiO 2, but possessing variable pH and temperature conditions that should reflect differential kinetics (and therefore biological opportunity) relative to silica polymerization: (1) most biologically favorable, acidic-mesophile (AM: pH 3, T = 35 °C); (2) biologically possible, but less opportune, alkaline, mesophile (ALK-M: pH 8, T = 35 °C) and (3) unlikely to be biologically favorable, alkaline-thermophile (ALK-T: pH 8, T = 80 °C). Comparison of field and laboratory results substantiates the requirements for thermodynamically favorable, but kinetically slower SiO 2 polymerization conditions. Results show that acidic moderate temperature conditions were required for an observable biosilicification impact. Moreover, they also identified for the first time, the necessity specifically for colloidal silica forms which are surface bound under acidic pH conditions, to distinguish discernible biosilicification compared to mineral particle controls. Results also highlight the important influence of mat surface characteristics in this process, specifically the extent of live, non-mineralized, exposed biological mat surface. Greater colloidal SiO 2 scavenging abilities are associated with non-mineralized microbial mat surfaces than with mineral particle surfaces or microbial mat surfaces encrusted with authigenic silica. These results are the first to demonstrate that biosilicification can be a microbially mediated, discernible geobiological process, shedding new light on the longstanding argument in the literature, and opening the door for more sensitive evaluation of this phenomenon in natural systems.

A new study on the kinetics of Stöber synthesis by in-situ liquid 29Si NMR

Liquid-state 29Si NMR was used to investigate the hydrolysis and condensation kinetics of ammonia-catalyzed tetraethoxysilane (TEOS) in methanol system. The reactive rate constants were calculated by applying first-order reaction approximation and the steady state approximation theory. The reaction orders with respect to TEOS, ammonia and water were derived, as well as the activation energies and the Arrhenius constants. It was found that the formation of intermediate species Si(OH)(OEt)3 was the rate-limiting step and its reaction rate equation was rTEOS=7.41×10−3[TEOS][NH3]0.333[H2O]0.227. Higher reactive temperature benefited the hydrolysis of TEOS. The results presented here indicated quantificationally that the formation of colloidal SiO2 particles was controlled by the initial hydrolysis of TEOS.

Glass formation and gelation in the system K2O-SiO2-H2O

Systems produced in interaction of colloid SiO2 particles with potassium hydroxide solutions form, as a result of aging, solid transparent gels and jellies. Depending on the pH value, the systems formed retain the colloid structure or take features of a homogeneous glassy body, with the glass formation possible within a rather large area in the K2O-SiO2-H2O triangle, adjacent to the K2O-SiO2 line.

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Abstract Simple soft-solution method has been developed to synthesize films and powders of TiO2 and mixed TiO2–SiO2 at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO2, we used aqueous acidic medium containing TiOSO4 and H2O2, which results in a peroxo-titanium precursor while colloidal SiO2 has been added to the precursor for the formation of TiO2–SiO2. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO2 (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO2–SiO2 mixed films and powders, TiO2 (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO2 and mixed TiO2–SiO2 films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO2 films. These compact films are highly photoactive when TiO2 is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO2 colloids are added into the solution.

Surface charge density on silica in alkali and alkaline earth chloride electrolyte solutions

The surface charge density of colloidal SiO 2 (Aerosil 380) was measured in alkali chloride (0.067 and 0.20 M LiCl, NaCl, and KCl) and alkaline earth chloride (0.067 M MgCl 2, CaCl 2, SrCl 2, BaCl 2) solutions. Measurements were conducted at 25°C by potentiometric titrations using the constant ionic medium method in a CO 2-free system. The experimental design measured surface charge for solutions with constant ionic strength as well as constant cation concentration. Alkali chloride solutions promote negative surface charge density in the order LiCl < NaCl < KCl to give the “regular” lyotropic behavior previously reported. In contrast, the alkaline earth chloride solutions exhibit a reversed lyotropic trend with increasing crystallographic radius where increasing negative charge is promoted in the order BaCl 2 < SrCl 2 < CaCl 2 < MgCl 2. The origin of the opposing affinity trends is probed by testing the hypothesis that this reversal is rooted in the differing solvent structuring characteristics of the IA and IIA cations at the silica–water interface. This idea arises from earlier postulations that solvent structuring effects increase entropy through solvent disordering and these gains must be much greater than the small, positive enthalpy associated with electrostatic interactions. By correlating measured charge density with a proxy for the solvent-structuring ability of cations, this study shows that silica surface charge density is maximized by those electrolytes that have the strongest effects on solvent structuring. We suggest that for a given solid material, solvation entropy has a role in determining the ionic specificity of electrostatic interactions and reiterate the idea that the concept of lyotropy is rooted in the solvent-structuring ability of cations at the interface.