A simple large-scale synthesis of very long aligned silica nanowires

A simple method based on the thermal oxidation of Si wafers has been discovered to provide a large-scale synthesis of very long, aligned silica nanowires. The as-grown product was characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence. The obtained SiO₂ nanowires had no metal contaminations, ultralong lengths of millimeters, and most diameters of ∼50 nm. The PL spectra of the SiO₂ nanowires showed a strong and stable green emission at 540 nm. The nucleation and growth of the SiO₂ nanowires were investigated.     

Shear-induced alignment and nanowire silica synthesis in a rigid crystalline surfactant mesophase

Highly aligned stringlike silica nanostructures are obtained through templated synthesis in the columnar hexagonal structure of a rigid crystalline surfactant mesophase. A two-step procedure is used to first shear-align the surfactant mesophase and then conduct synthesis under quiescent conditions in the mesophase. The mesophase retains its alignment for extended periods, allowing materials synthesis to be decoupled from the application of shear. The observations have significant implications in the control of ceramic microstructure morphology and transitions from nonaligned to aligned nanowire type structures.     

Shear-induced orientation of a rigid surfactant mesophase

An optically clear, crystalline, gel-like mesophase is formed by the addition of water to a micellar solution consisting of a mixture of 0.85 M anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and a 0.42 M zwitterionic surfactant phosphatidylcholine (lecithin) in isooctane. At 25 degrees C and water to AOT molar ratio of 70, the system has a columnar hexagonal microstructure with randomly oriented domains. The shear-induced orientation and subsequent relaxation of this structure were investigated by rheological characterization and small-angle neutron scattering (SANS). The rheological response implies that the domains align under shear, and remain aligned for several hours after cessation of shear. Shear-SANS confirms this picture. The sheared gel mesophase retains its alignment as the temperature is increased to 57 degrees C, indicating the potential to conduct templated polymer and polymer-ceramic composite materials synthesis in aligned systems.     

Cryo-field emission scanning electron microscopy imaging of a rigid surfactant mesophase

The aerosol OT/ L-alpha-phosphatidylcholine/isooctane/water system forms a rigid mesophase that transitions from reverse hexagonal to multilamellar in structure at specific water contents. This study shows that characteristics of ordered liquid-crystalline mesophases can be distinguished and imaged in high clarity using cryo-field emission scanning electron microscopy (cryo-FESEM). The reverse hexagonal phase consists of bundles of long cylinders, some with length scales of over 2 microm, that are randomly oriented as part of a larger domain. Cryo-imaging allows the visualization of the intercylinder spacings and the details of transitions from one domain to another. The multilamellar structured mesophase consists of spherical vesicles of 100 nm to 10 microm in diameter, with intervening noncrystalline isotropic regions. Coexistence regions containing both the reverse hexagonal and lamellar structures are also observed in the transition from the reverse hexagonal to the lamellar phase. These results complement and qualitatively verify our earlier studies with small-angle neutron scattering, high-field nuclear magnetic resonance spectroscopy, and freeze-fracture direct imaging transmission electron microscopy. The information is useful in understanding materials templating in these rigid systems.     

A water-soluble hexa-peri-hexabenzocoronene: synthesis, self-assembly and role as template for porous silica with aligned nanochannels

A water-soluble hexa-peri-hexabenzocoronene was prepared and shown to undergo ordered columnar self-assembly either in water solution or bulk and therefore served as template for the fabrication of porous silica with aligned nanochannels.     

Molten gallium as a catalyst for the large-scale growth of highly aligned silica nanowires

The vapor-liquid-solid (VLS) process is a fundamental mechanism for the growth of nanowires, in which a small size (5-100 nm in diameter), high melting point metal (such as gold and iron) catalyst particle directs the nanowire's growth direction and defines the diameter of the crystalline nanowire. In this article, we show that the large size (5-50 microm in diameter), low melting point gallium droplets can be used as an effective catalyst for the large-scale growth of highly aligned, closely packed silica nanowire bunches. Unlike any previously observed results using gold or iron as catalyst, the gallium-catalyzed VLS growth exhibits many amazing growth phenomena. The silica nanowires tend to grow batch by batch. For each batch, numerous nanowires simultaneously nucleate, grow at nearly the same rate and direction, and simultaneously stop growing. The force between the batches periodically lifts the gallium catalyst upward, forming two different kinds of products on a silicon wafer and alumina substrate. On the silicon wafer, carrot-shaped tubes whose walls are composed of highly aligned silica nanowires with diameters of 15-30 nm and length of 10-40 microm were obtained. On the alumina substrate, cometlike structures composed of highly oriented silica nanowires with diameters of 50-100 nm and length of 10-50 microm were formed. A growth model was proposed. The experimental results expand the VLS mechanism to a broader range.     

A novel green template for the synthesis of mesoporous silica

Mesoporous pure silicas and functionalized silica with a narrow pore size distribution centered at 3.8 nm were prepared by a novel template, amphiphilic dendritic polyglycerol. The resulting silica materials were characterized by electron microscopy; nitrogen adsorption; (1)H, (13)C, and (29)Si solid-state cross-polarization magic-angle spinning NMR spectroscopy. It was shown that the template could be completely removed from the pure and functionalized silica in an environmentally friendly way by means of a simple water extraction procedure. Furthermore, it was shown that these materials could be easily functionalized, for example, by employing aminopropyl groups. Thus, a new environmentally friendly pathway to this fascinating class of silica material has been opened.     

A facile route for shape-selective synthesis of silica nanostructures using poly-L-lysine as template

A facile method for the shape-selective synthesis of silica nanostructures using a reversemicroemulsion -mediated template(RMMT) technique is reported.In this method,positive poly-Llysine (PLL) is selected as template due to its configuration diversity.By adjusting pH and concentration, PLL demonstrates various secondary structures containing random coil,α-helix andβ-sheet,which result in the formation of silica nanorods,silica nanospheres and silica nanotubes in the reversemicroemulsion system,respectively.Thus,the shape-selective synthesis of silica nanostructures might be achieved by using PLL as structural template in the reverse-microemulsion system.     

A simple hydrothermal method for the large-scale synthesis of single-crystal potassium tungsten bronze nanowires

The large-scale synthesis of single-crystal K(x)WO(3) tungsten bronze nanowires has been successfully realized by a hydrothermal method under mild conditions. Uniform K(0.33)WO(3) nanowires with diameters of 5-25 nm and lengths of up to several micrometers are obtained. It is found that the morphology and crystallographic forms of the final products are strongly dependent on the sulfate and citric acid, which may act as structure-directing and soft-reducing agent, respectively. Some other influential factors on the growth of tungsten bronze nanowires, such as temperature and reaction time, are also discussed. It is worth noting that other alkali metal tungsten bronzes such as (NH(4))(x)WO(3), Rb(x)WO(3), and Cs(x)WO(3) could also be selectively synthesized by a similar route. Thus, this novel and efficient method could provide a potential mild route to selectively synthesize various tungsten bronze on-dimensional nanomaterials.     

A simple approach to the synthesis of silver nanowires by hydrothermal process in the presence of gemini surfactant

Silver nanowires were synthesized by the hydrothermal route in aqueous solution of gemini surfactant 1,3-bis(cetyldimethylammonium) propane dibromide (16-3-16) at a relatively low temperature. The as-prepared silver nanowires were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), transmission electron microscope (TEM), electron diffraction (ED), and UV-vis absorption spectrum. The obtained silver nanowires are of high aspect ratios with an average diameter of approximately 30 nm and length ranging from several to tens of micrometers.     

Synthesis of silica particles with lamellar and wormhole-like bi-modal mesopores using anionic surfactant as the template

Silica particles with lamellar and wormhole-like bi-modal mesopores have been synthesized using anionic surfactant (N-lauroylsarcosine sodium) as the template. The particles with diameters of 300―500 nm possess bi-modal mesopores with pore sizes of 3 nm and 12 nm, which were ascribed to the disordered wormhole-like mesophase and lamellar mesophase, respectively. The BET surface area of the particles was 536 m2/g and the pore volume was 0.83 cm3/g. The lamellar mesophase and cylindrical mesophase were formed...     

Direct synthesis of aligned silicon carbide nanowires from the silicon substrates

Aligned silicon carbide nanowires were synthesized directly from the silicon substrates via a novel catalytic reaction with a methane-hydrogen mixture at 1,100 degrees C, with a mean diameter of 40 nm and length of 500 microm; they consist of a single-crystalline zinc blende structure crystal in the [111] growth direction; X-ray diffraction, Raman, and infrared spectroscopy confirm the synthesis of high-purity silicon carbide nanowires.     

Hemicyanine dye as a surfactant for the synthesis of bicontinuous cubic mesostructured silica

In this paper, we developed a facile way to synthesize highly ordered optically active MCM-48 at room temperature, by using mixtures of hemicyanine dye N-alkyl-2-[p-(N,N-diethylamino)-o-(alkyloxy)]pyridinium bromide (denoted as o-CnPOCm, Scheme 1) and cetyltrimethylammonium bromide (CTAB) as the structure-directing agents. The mesoporous materials were systematically characterized by powder X-ray diffraction, transmission electron microscopy, nitrogen sorption, and thermogravimetry. The resultant MCM-48 exhibits unusually high thermal stability. For example, in the case of o-C(2)POC(14), it can retain its cubic structure even under calcinations at 900 degrees C for 5 h, although the pore size is shifted to the micropore region because of shrinkage of the framework. The typical surface area and pore volume are 980 m(2)/g and 0.44 cm(3)/g, respectively, for the powder calcined under such a high temperature. This is the first report of room-temperature synthesis of MCM-48 with such good thermal stability using cationic-cationic mixed surfactant as the structure-directing agent. The fluorescence lifetimes of the as-synthesized mesostructured MCM-48 were also measured, and the result showed that the incorporated dye molecules have a 1 order of magnitude longer lifetime than that of free species in solution, showing that the hemicyanine dye molecules are well dispersed within the CTAB surfactant matrix. Furthermore, we compared eight other dye congeners (Scheme 1) to fully investigate the mesophase resulting from the dye-CTAB system. The results show that, upon addition of the dye surfactant to the starting mixtures, the mesostructured silica undergoes an intrinsic phase-transition process; however, specific dye geometry is required to obtain MCM-48 at room temperature. Those functionalities as well as the designed synthesis of this novel mesostructured MCM-48 material promise a bright future in multifunctional optical and electric nano- and microdevices (e.g., waveguides, laser, light-emitting diodes, etc.) and also shed light on the self-assembly behavior in complex colloidal system.     

A novel method for synthesis of silica nanoparticles

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

31P and 1h NMR as probes of domain alignment in a rigid crystalline surfactant mesophase

A viscous reverse hexagonal surfactant mesophase containing bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and alpha-phosphatidylcholine (lecithin), with comparable volume fractions of isooctane and water, was characterized by Fourier transform (31)P and (1)H NMR spectroscopy. Shear alignment was reflected through both (31)P NMR and (1)H NMR spectra. A complicated (31)P spectrum was observed as a result of superposition of chemical shifts according to the distribution of crystalline domains prior to shear. The initially disordered samples with polydomain structures become macroscopically aligned after Couette shear. (31)P NMR chemical shift anisotropy characteristics are used to elucidate orientation of the hexagonal phase. Interestingly, (1)H NMR spectra exhibit spectral changes upon shear alignment closely corresponding with that of (31)P NMR spectra. These observations complement the findings of mesophase alignment obtained using SANS and imply that (31)P and (1)H NMR spectroscopy can be used as probes to define microstructure and monitor orientation changes in this binary surfactant system. This is especially beneficial if these mesophases are used as templates for materials synthesis.     

In situ generation of the silica shell layer--key factor to the simple high yield synthesis of silver nanowires

a-Silica encapsulated silver nanowires (diameter of 25 +/- 5 nm, average length of 10 mum) have been synthesized by reacting (Me3Si)4Si with AgNO3 in nearly quantitative yield. Formation of the a-silica shell layer (1-3 nm) in situ appears to be one of the most important factors in this simple process.     

A simple and highly efficient method for the synthesis of chalcones by using borontrifluoride-etherate

Chalcones are secondary metabolites of terrestrial plants, precursors for the biosynthesis of flavonoids and exhibit various biological activities. Condensation of substituted acetophenones (2a-12a) with various aromatic aldehydes (1b-7b) in the presence of BF₃-Et₂O at room temperature gave chalcones in 75-96% yield.     

Starch-assisted synthesis of polypyrrole nanowires by a simple electrochemical approach

Starch, one of the most commonly used polysaccharides, has been adopted for the first time as morphology-directing agent to the electrochemical synthesis of polypyrrole (PPy) nanowires on various electrodes.     

Transparent electrodes fabricated via the self-assembly of silver nanowires using a bubble template

To shore up the demand of transparent electrodes for wide applications such as organic light emitting diodes and solar cells, transparent electrodes are required as an alternative for indium tin oxide electrodes. Herein the self-assembly method with a bubble template paves the way for cost-effective fabrication of transparent electrodes with high conductivity and transparency using self-assembly of silver nanowires (AgNWs) in a bubble template. AgNWs were first dispersed in water that was bubbled with a surfactant and a thickening agent. Furthermore, these AgNWs were assembled by lining along the bubble ridges. When the bubbles containing the AgNWs were sandwiched between two glass substrates, the bubble ridges including the AgNWs formed continuous polygonal structures. Mesh structures were formed on both glass substrates after air-drying. The mesh structures evolved into mesh transparent electrodes following heat-treatment. The AgNW mesh structure exhibited a low sheet resistance of 6.2 Ω/square with a transparency of 84% after heat treatment at 200 °C for 20 min. The performance is higher than that of transparent electrodes with random networks of AgNWs. Furthermore, the conductivity and transparency of the mesh transparent electrodes can be adjusted by changing the amount of the AgNW suspension and the space between the two glass substrates.