Silica Nanowires Research Articles

One‐Step Facile Synthesis and Luminescence Properties of Eu3+‐Doped Silica Nanowires

AbstractUniformly sized and shaped Eu3+‐doped silica nanowires were conventionally fabricated by the microemulsion method accompanied with a sol–gel process for the first time. The products were characterized by different techniques, and their luminescence properties were investigated and compared. The scanning electron microscopy and transmission electron microscopy images demonstrated that the as‐synthesized Eu3+‐doped silica nanowires were formed in high yield with a smooth surface and a uniform diameter of about 130–150 nm. The results of FTIR spectroscopy, energy‐dispersive spectroscopy, and X‐ray diffraction showed that the Eu3+ ions were doped into the silica network structure in the form of Si–O–Eu bonds. The effect of the Eu3+ concentration on the luminescent intensity was investigated, and the highest emission intensity was found at the composition of 20 mol‐% Eu3+. The obtained Eu3+‐doped silica nanowires exhibited characteristic luminescent emission of Eu3+ at λ = 613 nm and apparent concentration quenching above the concentration of 20 mol‐% Eu3+. The growth processes of the silica nanowires were explained in detail on the basis of a modified vapor–liquid–solid mechanism.

Effect of SiO 2 nano-particles and nano-wires on microstructure and pinning properties of YBa 2 Cu 3 O 7-d

A comparative study of the effects of nanosized silicon oxide nano-particle (NP) and nano-wire (NW) additions during the final processing stage on the microstructure and superconducting properties of polycrystalline YBa2Cu3Oy (YBCO, or Y-123) was carried out. Samples were synthesized in air using a standard solid state reaction technique by adding nanosized entities up to 1 wt. %. Phases, microstructure, superconductivity, have been systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrical measurements. TEM investigation shows the presence of inhomogeneities embedded in the superconducting matrix along with the presence of columnar defects in the case of SiO2 nano-particles added, however nano-wires tend to agglomerate by entangling with each other in the inter-grain regions. The critical current density of the samples was examined using current–voltage, ac-susceptibility and magnetization measurements. The results are discussed in the framework of Bean's critical state model. Flux pinning force density is calculated and the possible pinning mechanisms prevalent in sintered samples are determined. The flux creep activation energy is determined in the light of vortex dynamics exhibited by the frequency dependence of the AC susceptibility. We have clear indication that a relatively low concentration of nanosized silicon oxide improves the pinning properties of Y-123 and we also validated the impact of the shape of nanosized inclusions on the superconducting properties.

Catalyst-free approach for growth of graphene sheets on high-density silica nanowires by CVD

A novel “two-step annealing” method is proposed for the direct synthesis of graphene sheets on high-density dielectric silica nanowires without using metal catalysts. During the first annealing at 1000°C, the extremely thin SiO2 layer on Si substrate shrinks and forms dense nanoparticles. Using these silica nanoparticles as templates, graphene sheets and silica nanowires are synthesized simultaneously after the second annealing process at 800–850°C. The experiment results suggest that the graphene sheets grow along the nanowires and its crystalline quality and domain size are determined by the temperature and duration of the second annealing process.

Silica Nanowires Templated by Amyloid-like Fibrils.

Many peptides self‐assemble to form amyloid fibrils. We previously explored the sequence propensity to form amyloid using variants of a designed peptide with sequence KFFEAAAKKFFE. These variant peptides form highly stable amyloid fibrils with varied lateral assembly and are ideal to template further assembly of non‐proteinaceous material. Herein, we show that the fibrils formed by peptide variants can be coated with a layer of silica to produce silica nanowires using tetraethyl‐orthosilicate. The resulting nanowires were characterized using electron microscopy (TEM), X‐ray fiber diffraction, FTIR and cross‐section EM to reveal a nanostructure with peptidic core. Lysine residues play a role in templating the formation of silica on the fibril surface and, using this library of peptides, we have explored the contributions of lysine as well as arginine to silica templating, and find that sequence plays an important role in determining the physical nature and structure of the resulting nanowires.

Silica Nanowires Templated by Amyloid-like Fibrils.

Many peptides self-assemble to form amyloid fibrils. We previously explored the sequence propensity to form amyloid using variants of a designed peptide with sequence KFFEAAAKKFFE. These variant peptides form highly stable amyloid fibrils with varied lateral assembly and are ideal to template further assembly of non-proteinaceous material. Herein, we show that the fibrils formed by peptide variants can be coated with a layer of silica to produce silica nanowires using tetraethyl-orthosilicate. The resulting nanowires were characterized using electron microscopy (TEM), X-ray fiber diffraction, FTIR and cross-section EM to reveal a nanostructure with peptidic core. Lysine residues play a role in templating the formation of silica on the fibril surface and, using this library of peptides, we have explored the contributions of lysine as well as arginine to silica templating, and find that sequence plays an important role in determining the physical nature and structure of the resulting nanowires.

Detection of propane gas adsorbed in a nanometer layer on silica nanowire

In the present work, a miniature optical gas sensor for the leakage detection of propane is proposed. It is constructed by employing single mode silica nanowire Mach–Zehnder interferometer (MZI). Suitable chemical treatment of silica nanowire for propane gas adsorption onto it is suggested. In the proposed sensor, the idea is that, the propane gas gets adsorbed on treated surface of silica nanowire, which changes the refractive index of few nanometer thick layer on the nanowire surface. In theoretical modeling and simulation, full vectorial analysis of two and three layers tightly guiding waveguide structures are presented. Simulations include calculation of phase difference at the output end of MZI, phase sensitivity, detection limit and measurable refractive index over a practicable sensing length such as 1mm. Sensing characteristics are numerically investigated. The sensor shows merits over conventional gas sensors. The sensing characteristics show negligible dependence on temperature, vibration and humidity, which is discussed in details. The proposed gas sensor is featured by high sensitivity, low cost, low loss, miniaturize size, and shows possibility of early detection of leakage.

Silica nanowire conjugated with loop-shaped oligonucleotides: A new structure to silence cysteine proteinase gene in Leishmania tropica

The main aim of this study was to evaluate the capability of silica nanowire conjugated with loop-shaped oligonucleotides (SNWCLSOs) to silence cysteine proteinase b (Cpb) gene in Leishmania (L) tropica. On the other hand, its toxicity on amastigotes and mouse peritoneal macrophages was evaluated by 5-diphenyl-tetrazolium bromide (MTT) assay. For control, two loop-shaped oligonucleotides (LSO) were considered. LSO1 and LSO2 were 5ʹ-NH2-cccccaaaaaaaaaaaaaaaaaaaaaaaaaggggg-COOH-3ʹ and LSO2: 5ʹ-NH2-cccccttttttttttttttttttttttttttttttttttttttggggg-COOH-3ʹ, respectively. After 72h incubation at 37°C, AMSNW, LSO1, and LSO2 had no remarkable toxicity on L. tropica amastigote (2×105/mL) and mouse peritoneal macrophages (2×105/mL). In case of SNWCLSOs, they had high toxicity on L. tropica amastigote, but they had no effect on mouse peritoneal macrophages. At concentrations of 1, 10, and 25μg/mL, AMSNW, LSO1 and LSO2 had no effect on the gene expression. But, at concentration of 50 and 100μg/mL, decrease of gene expression was observed. In case of SNWCLSOs, they could dramatically decrease the gene expression. It could be concluded that since SNWCLSOs could silence Cpb gene with no remarkable toxicity, they are good choice for treat cutaneous leishmaniasis in future. As a new agent, it must be checked in vivo.

Natural Chrysotile-Based Nanowires Decorated with Monodispersed Ag Nanoparticles as a Highly Active and Reusable Hydrogenation Catalyst

Silver nanoparticles/silica nanowires (AgNPs/SiO2NWs) with the nearly uniform diameters of 35 ± 5 nm were successfully fabricated by two steps consisting of the preparation of the silica nanowires (SNWs) by chemical dispersing and acid leaching from the natural mineral chrysotile and an in situ reduction approach (for AgNPs). The highly dispersed AgNPs assembled on the surface of SiO2NWs through the in situ reduction of Ag+ by NaBH4 were confirmed by transmission electron microscopy (TEM) and UV–vis absorption spectra. The catalytic activities of the as-prepared Ag/SiO2NWs (silver nanoparticles/silica nanowires) nanocomposites with different concentrations were assessed via using a classical reaction based on the reduction process of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of NaBH4 as the reductant. The results demonstrated that all the nanocomposite catalysts exhibited high catalytic activities because the nearly monodispersed AgNPs were embedded on the surface of SiO2NWs, allowing...

Direct growth, characterization, and optical properties of silica nanowires on cordierite monolith

We report the direct synthesis of amorphous silica nanowires on cordierite monolith. Our approach is based on a simple catalyst-assisted growth using Sn particles that are formed via the mechanochemical reaction of SnO2 (s) with Al (s, l) or Si (s) during the initial phase of the growth process. The silica nanowires have a diameter of approximately 20nm and become densely packed upon repeatedly branching and merging with each other. The silica nanowire assemblies grow up to 50µm in length. The microscopic images reveal that the Sn catalysts govern the growth by controlling the formation of the nuclei on their surface. Remarkably, the Sn catalysts induce the growth of multiple silica nanowire assemblies and the formation of Sn-rich amorphous (Si,Al)Ox nanotube/Al-doped orthorhombic Mg2SiO4 nanofiber as a minor product. The Sn-catalyzed silica nanowires show strong blue emission.

Strain-induced structural modifications and size-effects in silica nanowires

This study investigates the structural transformations and properties of silica glass nanowires under tensile loading via molecular dynamics simulations using the BKS (Beest-Kramer-Santen) interatomic potential. Surface states of the elongated nanowires were quantified using radial density distributions, while structural transformations were evaluated via ring size distribution analysis. The radial density distributions indicate that the surface states of these silica nanowires are significantly different than those of their interior. Ring size analysis shows that the ring size distributions remain mainly unchanged within the elastic region during tensile deformation, however they vary drastically beyond the onset of plastic behavior and reach plateaus when the nanowires break. The silica nanowires undergo structural changes which correlate with strain energy and ring size distribution variations. It is also found that the ring size distribution (and strain energy) variations are dependent on the diameter of the silica nanowires. Interestingly, for ultrathin nanowires (diameters < 5 .0 nm), the variation of ring size distributions shows a distinct trend with respect to tensile strain, indicating that the surface states play a key role in both modifying the mechanical properties and structural characteristics. These results for ultrathin nanowires are consistent with prior theoretical and simulation predictions. The overall findings in this study provide key insights into the novel properties of nano-sized amorphous materials, and are aimed to inspire further experiments.

Facile fabrication of porous silicon nanowires using water-based polyvinyl alcohol/silicon tetraacetate solution

The fabrication of porous silicon nanowires has been studied for various applications such as lighting-emitting diodes, photovoltaics, and energy storage devices. A water-based spinning dope containing polyvinyl alcohol (PVA) and silicon tetraacetate precursor was electrospun to form nanowires, in which the silicon precursors were supported by PVA back-boned chains. The fabricated nanowires were calcined in an air furnace in order to remove the PVA polymer from the as-spun nanowires and form a silicon oxide nanowire. To reduce the silicon oxide nanowires, they were exposed to a magnesium vapor in a high-level vacuum (10−4torr) at high temperatures of 650–1100°C, followed by 0.5–3.0M HCl treatment to remove the formed MgO crystals in the reduced nanowires. The fabricated nanowires exhibited porous/linear structures with diameters ranging from 100nm to 300nm. This facile technique is suitable to mass-produce porous silicon nanowires for use in the various applications.

New pickering emulsions stabilized by silica nanowires

In this work, oil-in-water and water-in-oil emulsions stabilized by hydrophilic and amphiphilic silica nanowires separately were investigated by scanning electron microscopy (SEM), optical microscopy and visual observation. The effects of length, concentration and wettability of the nanowires on the stabilization of the emulsion were investigated. The hydrophilic silica nanowires of 10 μm length can be used to prepare stable oil-in-water emulsions, with a network forming on the surface of the emulsion droplet is believed to be beneficial for the emulsion stability; whereas, the amphiphilic silica nanowires of all three lengths (1.5, 2.5 and 5 μm) can produce water-in-oil emulsions stable for at least 4 months, and it was found that increasing either the length or concentration of silica nanowires resulted in the enhancement of the stabilization of emulsions by nanowires. Phase inversion from oil-in-water to water-in-oil can be easily achieved by increasing the oil volume fraction in the amphiphilic silica nanowire stabilized emulsions.

Silica nanowire–Au nanoparticle pea-podded composites: Synthesis and structural analyses

Nanoscale wires of silicon oxide, among which some with embedded gold nanoparticles, are synthesized by a standard procedure involving the heating of a gold-coated silicon wafer at temperature higher than 1273K. We report quantitative and systematic structural analyses of the silicon oxide nanowires with embedded gold nanoparticles performed by scanning electron microscopy, transmission electron microscopy, energy filtered transmission electron microscopy, X-ray diffraction, and optical transmittance measurements. For such pea-podded structures, the analyses allow us to correlate the diameter of the nanoparticles with the diameter of the nanowire in which they are embedded, and the diameter of the nanoparticles with their spacing. The results help in understanding the mechanism inducing the formation of such structures and indicate the coexistence of transport and instability processes during the growth stage.

Twinned Si nanowires grown by high temperature annealing of Au/Si system in vacuum

Periodically twinned Si nanowires were fabricated on Si surface by high-temperature annealing, in vacuum, of deposited colloidal Au nanoparticles. While performing the annealing process in a gas carrier with trace amounts of oxygen almost-cylindrical silica nanowires are obtained thanks to the stabilizing effect of the oxygen, faceted nanowires are obtained in vacuum condition. In this last case, nanowires with diameter in the 70–150nm range and length of some microns are obtained. They present an arrangement of periodically twinned segments with a rather uniform thickness along the entire growth length. A minimum surface energy and strain energy argument is used to explain the formation of periodic twins in the Si nanowires. The thickness of the periodic twinned segments is found to be linearly proportional to the nanowire diameter, and a constant volume model is used to explain the relation. By the fit of the experimental data, in particular, an estimation of the twin energy formation is obtained.

Growth of tapered silica nanowires with a shallow U-shaped vapor chamber: Growth mechanism and structural and optical properties

Traditional chemical vapor deposition method modified with a shallow U-shaped vapor chamber has been used to synthesize tapered bamboo shoot-like (BS-like) amorphous SiO2 nanowires (NWs) on Si (100) substrates without catalyst. The key innovation of this approach lies in a creation of swirling flow of the reactant vapors during the growth, which leads to a harvest of tapered silica NWs with lengths up to several microns. The unique structures and corresponding luminescence properties of the BS-like NWs were studied and their relationship with the evaporated active reactants was explored. A thermodynamic model that considers the critical role of the vapor flow during the growth is proposed to understand the structural and optical features. The shallow U-shaped vapor chamber-aided approach may provide a viable way to tailor novel structure of NWs for potential applications in nano-devices.

Origin of high elastic strain in amorphous silica nanowires

An understanding of the origin of elastic strain is extremely important for both crystalline materials and amorphous materials. Owing to the lack of a long range order in their structure, it is arduous to dynamically study the elastic mechanism of amorphous materials experimentally at atomic scale compared with their crystalline counterparts. Here, the elastic deformation mechanism of amorphous silica nanowires (NWs) has been studied for the first time via in situ elastic tensile tests in a transmission electron microscope. Radial distribution functions (RDFs) calculated from the corresponding selected area electron diffraction patterns (SAEDPs) at different strains were used to reconstruct a structural model based on the reverse Monte-Carlo (RMC) method. The result interestingly indicates that the elastic strain of silica glass NWs can be mainly attributed to the elastic elongation of the bond length accompanied by a change in the bond angle distribution. This work is useful for understanding the high strength of amorphous materials.

Self-assembly and growth mechanism of au nanoparticle chains in silica nanowires

We report the formation of a one-dimensional Au nanoparticle (NP) chain embedded in silica nanowires (NWs) and suggest a possible mechanism for the chain growth based on an alternative vapor-liquid-solid (VLS) process. The Au NP chain was self-assembled inside silica NWs, which were grown by high-temperature annealing of silicon substrates coated with a thick Au film. The Au NPs inside the NWs were observed to be formed by the sequent detachment of some Au from the top of a wick-like Au wire that extended from a Au lump, which was also the origin of the NWs. The results are attributed to the push-up growth of NWs by the continuous formation of a silicon-oxide phase at the top and/or on the side of a liquid Au lump via the VLS process.

Size effects on the fracture behavior of amorphous silica nanowires

Size effects on deformation behavior and fracture mechanism of amorphous silica nanowires (NWs) were studied by molecular dynamics simulation with reactive force field ReaxFF. We analyzed the evolution of voids during the tension deformation, and focused on the role of voids played in the fracture process of amorphous silica NWs. Our study shows that the smaller diameter NWs damage only from localized necking and the small growth of voids has no effect on its fracture behavior. While for the larger diameter NWs, there exists growth and coalescence of void inside the NWs and crack propagation on the surface of NWs. It is the combined action of these two mechanisms that leads to the final fracture. The evolution of the size of biggest void (BV) displays three distinct regions, which are corresponding to the different deformation stages of NWs. The voids begin to grow significantly after the yielding point, and the size of BV in the fracture stage increases much more quickly owing to the coalescence of voids. Moreover, the critical radius of BV at the start point of fracture stage increases from 2.6Å to 3.8Å with the increase of diameter of amorphous silica NWs.

Growth of silica nanowires in vacuum

Silica nanowires were grown on a Pt-coated Si substrate in flowing Ar, dynamic vacuum, and sealed vacuum tubes.

Substrate effect on nanoporous structure of silica wires by channel-confined self-assembly of block-copolymer and sol-gel precursors

Nanoporous silica wires of various wire diameters were developed by space-confined molecular self-assembly of triblock copolymer ethylene/propylene/ethylene (P123) and silica alkoxide precursor (tetraethylorthosilicate, TEOS). Two distinctive hard-templating substrates, anodized aluminum oxide (AAO) and track-etched polycarbonate (EPC), with channel diameters in the range between 10 nm and 200 nm were employed for space-confinement of soft molecular self-assembly driven by the block-copolymer microphase separation. It was observed in the scanning and transmission electron microscope (STEM) studies that the substrate geometry and material characteristics had pronounced effects on the structure and morphology of the silica nanowires. A “substrate wall effect” was proposed to explain the ordering and orientation of the intra-wire mesostructure. Circular and spiral nanostructures were found only in wires formed in AAO substrate, not in EPC. Pore-size differences and distinctive wall morphologies of the nanowires relating to the substrates were discussed. It was shown that the material and channel wall characteristics of different substrates play key roles in the ordering and morphology of the intra-wire nanostructures.