Uniform Silica Research Articles

Preparation of Phosphorus/Hollow Silica Microsphere Modified Polyacrylonitrile-Based Carbon Fiber Composites and Their Thermal Insulation and Flame Retardant Properties

Since thermal insulation materials with a single function cannot satisfy the increasing requirements for complex usage, the combination of thermal insulation with flame retardancy is desirable in multiple applications. Herein, phosphorous-containing flame retardant modifier (5.0 wt.%, 9.0 wt.%, and 13.0 wt.%) and hollow silica microsphere (130 nm of diameter) were composited with polyacrylonitrile-based carbon nanofibers via electrospinning technique. Electrospun phosphorous/silica/carbon nanofibers (P/HSM/CF) exhibited a uniform and clear fibrous structure (508, 170, and 1550 nm of average fiber diameter) with modified uniform and complete spherical silica. Reduced thermal conductivity (39.9–41.2 mW/m/K) and enhanced limiting oxygen index (29.5–33.5%) were achieved, enabling fire protection grade of fiber membrane from UL-94 V-1 grade to UL-94 V-0 grade efficiently. Moreover, favorable tensile strength (8.64–9.27 MPa) and elongation at break (43.28–48.54%) were obtained, presenting expected applications in structural components. The findings of this work provided a valuable reference for the fabrication of carbon nanofiber-based thermal insulation materials with excellent flame retardant and mechanical properties.

Electrochemically Induced Templated Sol-gel Deposition of Mesoporous Silica and Nanocomposites: The New Kid on the ECiD Block

A relatively new member of the group of electrochemically induced deposition is that of templated sol-gel deposition. Uniform and smooth mesoporous silica thin films were first fabricated by an ECiD process nicknamed electrochemically assisted self-assembly or EASA, introduced by Walcarius et al. The addition of cetyltrimethylammonium bromide or CTAB as organic template molecules to the tetraethyl orthosilicate (TEOS) sol-gel precursor solution resulted in silica thin films with highly ordered, hexagonal arrays of vertical mesopores of 2–3 nm in diameter. These films have been proposed for applications in electroanalysis and sensors where effects such as pre-concentration or perm selectivity have been exploited. Their synthesis combines the well-established field of templated sol-gel synthesis with an electrochemically induced deposition process. To prevent spontaneous gel formation, the silica sol-gel precursor solution has a mildly acidic pH which kinetically inhibits the gelation reaction. Only when a cathodic potential/current is applied to electrogenerate OH- ions from the pro-base (water mostly), the gelation reaction is catalyzed by the local increase in pH, resulting in the growth of silica gel films on the electrode. We will first review the principles of sol-gel reactions, before proceeding to details of ECiD of sol-gel films.

Preparation and Characterization of Uniform and Controlled Silica Encapsulating on Lithium Yttrium Fluoride-Based Upconversion Nanoparticles.

In this work, we present an advancement in the encapsulation of lithium yttrium fluoride-based (YLiF4:Yb,Er) upconversion nanocrystals (UCNPs) with silica (SiO2) shells through a reverse microemulsion technique, achieving UCNPs@SiO2 core/shell structures. Key parameters of this approach were optimized to eliminate the occurrence of core-free silica particles and ensure a controlled silica shell thickness growth on the UCNPs. The optimal conditions for this method were using 6 mg of UCNPs, 1.5 mL of Igepal CO-520, 0.25 mL of ammonia, and 50 μL of tetraethyl orthosilicate (TEOS), resulting in a uniform silica shell around UCNPs with a thickness of 8 nm. The optical characteristics of the silica-encased UCNPs were examined, confirming the retention of their intrinsic upconversion luminescence (UC). Furthermore, we developed a reliable strategy to avoid the coencapsulation of multiple UCNPs within a single silica shell. This approach led to a tenfold increase in the UC luminescence of the annealed particles compared to their nonannealed counterparts, under identical silica shell thickness and excitation conditions. This significant improvement addresses a critical challenge and amplifies the applicability of the resulting UCNPs@SiO2 core/shell structures in various fields.

Preparation of Superhydrophilic/Underwater Superoleophobic and Superhydrophobic Stainless Steel Meshes Used for Oil/Water Separation.

Robust membrane materials with high efficiency have attracted extensive attention in oil/water separation. In this work, carbon particles via candle combustion were firstly adsorbed on the surface of stainless steel meshes (SSMs), which formed a thin hydrophobic coating, and a rough structure was then constructed through chemical vapor deposition and high temperature calcination, with the resultant SSM surface wrapped with uniform silica coating possessing the characteristic of superoleophobicity underwater. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray powder diffraction (XRD) were used to characterize the modified SSMs. The prepared SSMs were superhydrophilic in air, and they had superoleophobicity underwater (157.4°). The separation efficiency of five oil/water mixtures was above 98.8%, and the separation flux was 46,300 L·m-2·h-1. After it was immersed in 1 mol/L NaOH, 1 mol/L HCl and 3.5 wt% NaCl for 24 h, respectively, the efficiency was still above 97.3%. Further immersion in the solution of dopamine and octadecylamine resulted in the transformation of superhydrophililc/superoleophobicity-underwater SSMs to superhydrophobic SSMs, and the resultant SSMs with reverse surface wettability was also used for the oil/water separation with good separation efficiency and separation flux.

Ultra-broadband electromagnetic wave absorbent: In-situ generated silica modified conductive carbon fiber aerogels

The research and development of dielectric microwave absorbing materials with broad electromagnetic (EM) response is a significant project in EM wave absorption field. To achieve high-performance absorption and strong interfacial bonding at the same time, thermal-assisted in-situ bonding technology was applied to fabricating the dielectric composite absorbing materials. Thanks to the combination of vacuum filtration and in-situ hydrothermal reaction, the as-prepared binary composite aerogel shows both strong interface contacting and good mechanical stability. In addition, the carbon nanofibers/silica composite aerogel (CSA) exhibits ultra-broad effective bandwidth covering from S to Ku band, originated from the uniform dispersed silica aerogel in conductive carbon fiber network. In details, for CSA1 sample, the maximum reflection loss (RL) values and effective absorption bandwidth reach −46.2 dB (1.8 mm) and 5.2 GHz (1.5 mm). Meanwhile, the optimum RCS reduction values reaches 16.2 dB m2 when the detection theta was set as 0°. For CSA2 sample, the effective absorption bandwidth reaches 8.64 GHz at 1.5 mm, and tends to possess lower frequency EM response covering the S-band. This work exhibits a kind of broad-bandwidth aerogel absorbers at low thickness, which shows huge potential in large-scale production of microwave absorbing devices.

Uniform pore structure silica aerogel powders prepared by pore repair method

Uniform pore structure silica aerogel powders prepared by pore repair method

Silica-Coated Micrometer-Sized Latex Particles.

A series of silica-coated micrometer-sized poly(methyl methacrylate) latex particles are prepared using a Stöber silica deposition protocol that employs tetraethyl orthosilicate (TEOS) as a soluble silica precursor. Given the relatively low specific surface area of the latex particles, silica deposition is best conducted at relatively high solids to ensure a sufficiently high surface area. Such conditions aid process intensification. Importantly, physical adsorption of chitosan onto the latex particles prior to silica deposition minimizes secondary nucleation and promotes the formation of silica shells: in the absence of chitosan, well-defined silica overlayers cannot be obtained. Thermogravimetry studies indicate that silica formation is complete within a few hours at 20 °C regardless of the presence or absence of chitosan. Kinetic data obtained using this technique suggest that the adsorbed chitosan chains promote surface deposition of silica onto the latex particles but do not catalyze its formation. Systematic variation of the TEOS/latex mass ratio enables the mean silica shell thickness to be tuned from 45 to 144 nm. Scanning electron microscopy (SEM) studies of silica-coated latex particles after calcination at 400 °C confirm the presence of hollow silica particles, which indicates the formation of relatively smooth (albeit brittle) silica shells under optimized conditions. Aqueous electrophoresis and X-ray photoelectron spectroscopy studies are also consistent with latex particles coated in a uniform silica overlayer. The silica deposition formulation reported herein is expected to be a useful generic strategy for the efficient coating of micrometer-sized particles at relatively high solids.

Geotechnical characteristics of siliceous North Sea sands

When deriving drained strength parameters for sandy soils, geotechnical engineers need to be cautious while applying the semi-empirical correlations that are mainly developed from the highly idealised uniform clean silica sands as often used in academia. Many studies have shown that changing fines content changes the mechanical behaviour of sand, and that grading curve and particle angularity affect the strength-dilatancy characteristics. However, when various parameters of sands encountered in nature differ from the commercially sourced silica sands, the counteracting effects are difficult to quantify. In this study, a database of classification tests and Consolidated Isotropic Drained axisymmetric strength tests as well as some accompanying cone penetration tests (CPTs) are collected for siliceous sands from the North Sea. From this extensive database of natural sands, simple expressions are developed to help predicting drained strength parameters with improved reliability. Furthermore, a correction factor, for a commonly applied CPT friction angle qc − φ′p , tx correlation accounting for the grading curve, is developed. These expressions can help in reducing the number of costly advanced strength tests and to gain more information from a soil investigation.

3D porous catalysts for Plasma-Catalytic dry reforming of Methane: How does the pore size affect the Plasma-Catalytic Performance?

The effect of pore size on plasma catalysis is crucial but still unclear. Studies have shown plasma cannot enter micropores and mesopores, so catalysts for traditional thermocatalysis may not fit plasma catalysis. Here, 3D porous Cu and CuO with different pore sizes were prepared using uniform silica particles (10–2000 nm) as templates, and compared in plasma-catalytic dry reforming. In most cases, the smaller the pore size, the higher the conversion of CH4 and CO2. Large pores reachable by more electrons did not improve the reaction efficiency. We attribute this to the small surface area and large crystallite size, as indicated by N2-sorption, mercury intrusion and XRD. While the smaller pores might not be reachable by electrons, due to the sheath formed in front of them, as predicted by modeling, they can still be reached by radicals formed in the plasma, and ions can even be attracted into these pores. An exception are the samples synthesized from 1 μm silica, which show better performance. We believe this is due to the electric field enhancement for pore sizes close to the Debye length. The performances of CuO and Cu with different pore sizes can provide references for future research on oxide supports and metal components of plasma catalysts.

Poly(ethylene imine) stabilized metal hydroxide nanoaggregates in reverse microemulsion system enabling synthesis of hollow silica catalytic nanoreactors as highly efficient semihydrogenation catalysts

In this work, we report a generic method of reverse microemulsion for syntheses of uniform and well-dispersed hollow porous silica nanospheres with metal (oxide) nanoparticles inside their cavities (MxOy@HPSNs or M@HPSNs), where the water phase consists of poly(ethylene imine) stabilized metal hydroxide nanoaggregates. During silica deposition, the mentioned nanoaggregates function as void templates as well as a platform to deliver catalytic functionality into voids. The subsequent calcination simultaneously gives hollow nanostructures and metal (oxide) nanoparticles inside central voids. The method applies to the metals that have hydroxide precipitates at basic conditions, and is suitable for most of catalytically active metals. As a demonstration, Pd@HPSNs illustrate high catalytic efficiency for selective hydrogenation of phenylacetylene at ambient H2 pressure, and the enhancement could be assigned to the effect of void confinement.

Modifying the Stöber Process: Is the Organic Solvent Indispensable?

The Stöber method is one of the most important and fundamental processes for the synthesis of inorganic (nano)materials but has the drawback of using a large amount of organic solvent. Herein, ethanol was used as an example to explore if the organic solvent in a typical Stöber method can be omitted. It was found that ethanol increases the particle size of the obtained silica spheres and aids the formation of uniform silica particles rather than forming a gel. Nevertheless, the results indicated that an organic solvent in the initial synthesis mixture is not indispensable. An initially immiscible synthesis method was discovered, which can replace the organic solvent-based Stöber method to successfully synthesize silica particles with the same size ranges as the original Stöber process without addition of organic solvents. Moreover, this process can be of further value for the extension to synthesis processes of other materials based on the Stöber process.

Structural, Optical, and Morphological Characterization of Silica Nanoparticles Prepared by Sol-Gel Process

In the current years, silica nanoparticles have become more favorable in various disciplines like medicine, nano-biotechnology, the food industry, and drug delivery due to their tunable physicochemical characteristics. In this paper, the silica nanoparticles were synthesized by hydrolysis and condensation of tetra-ethyl-ortho-silicate (TEOS) in an ethanolic medium using ammonia as a stimulator in the reaction. The chemical bond structures of silica nanoparticles were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR) which confirmed the existence of the Si-O bonds according to the different absorption peaks of the samples. The amorphous structure of these nanoparticles was certified by finding the board peaks in the X-Ray Diffraction (XRD) patterns. The elemental chemical composition of silica nanoparticles was investigated by Energy Dispersive X-Ray Spectroscopy (EDX) where 61.48wt % of silicon and 23.48wt% of oxygen were found. Almost round-shaped spherical and uniform silica nanoparticles with smooth surfaces were investigated by Scanning Electron Microscopy (SEM) measurement. The different particle sizes of silica nanoparticles within the range of 95±5.59 to 280±7.8 nm were found by controlling the concentration of TEOS. The optical absorption spectra and band gap calculations were also analyzed by Ultraviolet-Visible (UV-Vis) spectrophotometry for the different concentrations of TEOS. The results revealed that with increasing the concentration of TEOS, the absorption spectra of silica nanoparticles increased and their optical bandgap decreased from 3.92 eV to 3.79 eV.

Experimental measurement of particle crushing around model piles jacked in a calibration chamber

During installation of driven or jacked piles in sand, particle crushing occurs in a region below the base of these piles. Quantification of the degree and extent of particle crushing during driving or jacking in sand is necessary to improve current design methods. To study the effects of particle crushing on the capacity of piles jacked in uniform silica sand samples, a model pile was jacked and tested in a half-cylindrical calibration chamber with transparent observation windows in its front wall. Annular samples of crushed sand 3 mm in thickness were recovered after testing from several locations along the shaft and the base of the model piles and used to assess particle gradation changes and to calculate breakage parameters. Relationships between the load mobilized at the base of the model piles and three breakage parameters are proposed. Furthermore, the number of jacking strokes (ranging from 1 to 32 in this research) has a negligible effect on the degree of crushing produced during installation and on the measured stresses at the base of the model pile. Analyses of the digital images captured during pile jacking indicate that the silica sand particles in the path of the model pile crush mainly along irregularities and sharp edges. Different zones are identified below the model pile according to the degree of particle crushing. Digital images taken during static loading of the model pile allowed tracking of the displacement paths of individual sand particles during loading.

Facile ultrasonic-assisted synthesis of SiO2/ZnO core/shell nanostructures: A selective ethanol sensor at low temperatures with enhanced recovery

SiO2/ZnO core/shell and composite sensors with 22–97 wt% ZnO content were synthesized by a facile ultrasonic-assisted deposition-precipitation method. As for the core, uniform silica spherical particles were synthesized by a microemulsion technique. The prepared samples were characterized by XRD, EDS, SEM, DLS, and TEM analyses. The results confirm the formation of ZnO layers on silica core nanoparticles. Then, the sensors were applied for sensing 300 ppm of ethanol, carbon monoxide, toluene, trichloroethylene (TCE), or propane and 1% methane in air at 270–420 ˚C. Core/shell sensors containing more than 8 wt% silica showed a reduction in the maximum sensing temperature of at least 50 ˚C, compared to that of the pure ZnO sensor. At maximum sensing temperature, i.e., 270 °C, the selectivity (i.e., the ratio of sensor response to target to that of interfering gas) of core/shell sensors to ethanol, relative to the interfering gases, varied in the range of 1.8 × 103 to 3.24 × 105, compared to 61.1–5.4 × 103 for the pure ZnO sensor. This reveals a considerable enhancement in the selectivity of core/shell sensors towards ethanol. Compared to pure ZnO, the core/shell sensors containing more than 8 wt% silica show highly stable and reversible response-recovery properties with much lower drifts in the dynamic response to ethanol in 4 consecutive cycles. The best response-recovery property is for the SiO2/ZnO-74 wt% core/shell sensor with nearly no drift in response. The final steady response of the pure ZnO sensor, which happens after four consecutive cycles at 270 °C, is 50 times lower than the initial response. The effect of humidity on gas sensing responses is investigated for the pure ZnO and core/shell sensors. Under humid air, SiO2/ZnO sensors containing 74 wt% and 97 wt% ZnO have further shown increased durability of the responses, compared to the pure ZnO sensor. The finding here reveals the importance of silica addition as the core material to ZnO to increase the durability and reproducibility of the sensor under challenging operating conditions.

Modified Stöber synthesis of SiO2@Ag nanocomposites and their enhanced refractive index sensing applications

Core-shell type nanostructures (metal nanoparticles decorated dielectric spheres) are of particular interest based on their application potentials ranging from catalysis, bio-sensing to optical devices because of their inherent characteristic physico-chemical properties. Herein, we utilized silica (SiO2) as the dielectric cores and Ag as the metal shell nanostructures. In this study, we carefully modified the conventional Stöber process for the in-situ one step synthesis as well as functionalization of large and highly uniform silica particles with the universally soluble homogenous polymer PVP (polyvinyl pyrrolidone). This facilitates cohesive control of uniform mass transfer of Ag+ metal ions onto the silica surface, thereby enabling their sequential reduction to zerovalent Ag atoms (in the presence of slightly excess NaOH), as confirmed from the detailed spectroscopic and microscopic measurements. The precise role of PVP in the stabilization as well as sequential reduction of the Ag nanoparticles onto the dielectric silica surface is prudently demonstrated through FTIR data analysis. The nonlinear dependence of the plasmonic peak wavelength on the refractive index (RI) describes the intriguing light-matter interaction of core-shell type nanostructures, disentangling the surface plasmon contribution at the dielectric-metal interfaces in a rigorous manner, quantifying the plasmonic ultra-sensitivity and figure of merit of our as-prepared optical metamaterials.

Poly(acrylic acid) Enabling the Synthesis of Highly Uniform Silica Nanoparticles of sub‐100 nm

AbstractRobust synthesis of small colloidal silica (SiO2) nanoparticles with narrow size distributions (<5%) is challenging because the possible involvement of continuous nucleation and growth of SiO2 in the widely used Stöber process usually results in silica nanoparticles with large sizes and broad size distributions. Promoting heterogeneous nucleation and growth of silica is promising to tackle the challenges by in‐situ forming transient colloidal seeds that provide nucleation surfaces for silica condensate. In this work, poly(acrylic acid) (PAA) is introduced to the Stöber synthesis solutions, which are usually composed of ethanol (containing a trace amount of water), tetraethyl orthosilicate (TEOS), and ammonia, to mediate the nucleation of TEOS hydrolysates and growth of SiO2 nanoparticles. PAA reacts with ammonia to form soft PAA‐NH3 complex colloids that serve as transient seeds to facilitate heterogeneous nucleation and growth of SiO2 nanoparticles after adding TEOS to the synthesis solution. The number of PAA‐NH3 complex colloidal particles, which strongly depends on the molar ratio of PAA and ammonia, determines the number of the resulting SiO2 nanoparticles and thus the size of the SiO2 nanoparticles correspondingly according to the supply of TEOS. Highly uniform SiO2 nanoparticles with finely tuned diameters in the sub‐100 region have been successfully synthesized, and the synthesis protocol is feasible to be scaled up while maintaining the high quality of SiO2 nanoparticles.

Enhancing dehydration performance of isopropanol for flexible hybrid silica composite membranes with spray-coated active layer on polymers

Organically bridged silica has been proven to be promising precursors for fabricating silica-based organic–inorganic hybrid membranes with excellent permselectivity. However, the large-area deposition of defect-free hybrid silica membranes onto the polymer substrate has always been an enormous challenge. Herein, we reported a fast and facile approach for the large-area production of flexible silica-based hybrid membranes, where a thin and uniform ethylene-bridged silica active layer was coated on a microporous polymer substrate via a spray coating process with a single-pass method. First, the optimal deposition conditions were investigated and established, including sol concentrations and different solvent systems that are used to prepare hybrid silica sol, as well as various spray coating conditions. Subsequently, thus-produced flexible hybrid silica composite membranes exhibited superior isopropanol (IPA) dehydration performance by vapor permeation for a 90 wt% isopropanol aqueous solution, with water permeation flux reaching 4.0 kg m-2h−1 and with H2O/IPA separation factor exceeding 1000 (the selectivity of H2O/IPA is 770). This work has shown that the single-pass spray coating is a useful strategy for the large-scale fabrication of the flexible hybrid silica composite membrane.

Evaluation of the evaporation route of a liquid droplet on Au coated and non-coated glass surfaces

The contact angle was used to estimate the rate of evaporation of liquid droplets on bare glass or gold (Au) sputtered glass surfaces. The rate of evaporation of water (a pure liquid) was higher than non-pure liquid composed of a 3 wt% solution of silica nanoparticles (SNP) on these two solid supports. Despite using the same initial drop volume (1 µL) throughout the experiment, the base diameter of the liquid droplet after evaporation on the different surfaces interestingly showed variations. While the liquid–solid interface displayed slip-length and contact angle variations throughout the evaporation time, the slip-length variations were more pronounced with colloidal SNP on Au-sputtered glass surfaces than pure liquid on bare glass surface. Potential application of this study was also investigated in the surface control of uniform silica microwires from colloidal SNP on Au-sputtered glass surface under low temperature conditions.

Ultra-Fine Control of Silica Shell Thickness on Silver Nanoparticle-Assembled Structures.

To study the distance-dependent electromagnetic field effects related to the enhancement and quenching mechanism of surface-enhanced Raman scattering (SERS) or fluorescence, it is essential to precisely control the distance from the surface of the metal nanoparticle (NP) to the target molecule by using a dielectric layer (e.g., SiO2, TiO2, and Al2O3). However, precisely controlling the thickness of this dielectric layer is challenging. Herein, we present a facile approach to control the thickness of the silica shell on silver nanoparticle-assembled silica nanocomposites, SiO2@Ag NPs, by controlling the number of reacting SiO2@Ag NPs and the silica precursor. Uniform silica shells with thicknesses in the range 5–40 nm were successfully fabricated. The proposed method for creating a homogeneous, precise, and fine silica coating on nanocomposites can potentially contribute to a comprehensive understanding of the distance-dependent electromagnetic field effects and optical properties of metal NPs.

A promising radiolabeled drug delivery system for methotrexate: synthesis and in vitro evaluation of 99mTc labeled drug loaded uniform mesoporous silica nanoparticles

A promising radiolabeled drug delivery system for methotrexate: synthesis and in vitro evaluation of 99mTc labeled drug loaded uniform mesoporous silica nanoparticles