Showing papers by "Younan Xia published in 2004"

Electrospinning of Nanofibers: Reinventing the Wheel?†

Abstract: Electrospinning provides a simple and versatile method for generating ultrathin fibers from a rich variety of materials that include polymers, composites, and ceramics. This article presents an overview of this technique, with focus on progress achieved in the last three years. After a brief description of the setups for electrospinning, we choose to concentrate on the mechanisms and theoretical models that have been developed for electrospinning, as well as the ability to control the diameter, morphology, composition, secondary structure, and spatial alignment of electrospun nanofibers. In addition, we highlight some potential applications associated with the remarkable features of electrospun nanofibers. Our discussion is concluded with some personal perspectives on the future directions in which this wonderful technique could be pursued.

Direct Fabrication of Composite and Ceramic Hollow Nanofibers by Electrospinning

Abstract: Hollow nanofibers with walls made of inorganic/polymer composites or ceramics have been prepared by electrospinning two immiscible liquids through a coaxial, two-capillary spinneret, followed by selective removal of the cores. The capability and feasibility of this technique have been demonstrated by the fabrication of titania/polymer or anatase nanotubes whose size and wall thickness could be independently varied by controlling a set of experimental parameters. The presence of a sol−gel precursor in the sheath liquid was necessary for the formation of stable, coaxial jets and hollow fibers with robust walls. The circular cross-section, uniform size, and well-controlled orientation of these long hollow nanofibers are particularly attractive for use in fabricating fluidic devices and optical waveguides.

Mechanistic study on the replacement reaction between silver nanostructures and chloroauric acid in aqueous medium.

Abstract: The replacement reaction between silver nanostructures and an aqueous HAuCl4 solution has recently been demonstrated as a versatile method for generating metal nanostructures with hollow interiors Here we describe the results of a systematic study detailing the morphological, structural, compositional, and spectral changes involved in such a heterogeneous reaction on the nanoscale Two distinctive steps have been resolved through a combination of microscopic and spectroscopic methods In the first step, silver nanostructure (ie, the template) is dissolved to generate gold atoms that are deposited epitaxially on the surface of each template Silver atoms also diffuse into the gold shell (or sheath) to form a seamless, hollow nanostructure with its wall made of Au−Ag alloys The second step involves dealloying, a process that selectively removes silver atoms from the alloyed wall, induces morphological reconstruction, and finally leads to the formation of pinholes in the walls Reaction temperature was f

Polyol Synthesis of Silver Nanoparticles: Use of Chloride and Oxygen to Promote the Formation of Single-Crystal, Truncated Cubes and Tetrahedrons

Abstract: Single-crystal cubes and tetrahedrons of silver with truncated corners/edges have been prepared for the first time in high yields by reducing silver nitrate with ethylene glycol heated to 148 °C in the presence of poly(vinyl pyrrolidone) and a trace amount of sodium chloride. These nanoparticles were relatively monodisperse in size and shape, and their dimensions could be readily controlled in the range of 20 to 80 nm by varying the reaction time and other experimental parameters. We propose that the defects inherent in twinned nuclei of silver led to their selective etching and dissolution by chloride and oxygen (from air), leaving only the single crystalline ones to grow into nanoscale cubes and tetrahedrons.

Electrospinning Nanofibers as Uniaxially Aligned Arrays and Layer‐by‐Layer Stacked Films

Abstract: The conventional procedure for electrospinning has been modified to generate nanofibers as uniaxially aligned arrays over large areas. The key to the success of this method was the use of a collector composed of two conductive strips separated by an insulating gap of variable width. Directed by electrostatic interactions, the charged nanofibers were stretched to span across the gap and became uniaxially aligned arrays. Two types of gaps have been demonstrated: void gaps and gaps made of a highly insulating material. When a void gap was used, the nanofibers could readily be transferred onto the surfaces of other substrates for various applications. When an insulating substrate was involved, the electrodes could be patterned in various designs on the solid insulator. In both cases, the nanofibers could be conveniently stacked into multi-layered architectures with controllable hierarchical structures. This new version of electrospinning has already been successfully applied to a range of different materials that include organic polymers, carbon, ceramics, and composites.

Ethylene glycol-mediated synthesis of metal oxide nanowires

Abstract: A simple and convenient method has been demonstrated for large-scale synthesis of metal oxide (including TiO2, SnO2, In2O3, and PbO) nanowires with diameters around 50 nm and lengths up to 30 µm. In a typical procedure, tetraalkoxyltitanium, Ti(OR)4 (with R = –C2H5, –iso-C3H7, or –n-C4H9), was added to ethylene glycol and heated to 170 °C for 2 h under vigorous stirring. The alkoxide was transformed into a chain-like, glycolate complex that subsequently crystallized into uniform nanowires. Similarly, nanowires made of tin glycolate were synthesized by refluxing SnC2O4·2H2O in ethylene glycol at 195 °C for 2 h, and nanowires consisting of indium and lead glycolates were prepared by adding In(OOCC7H15)(OiPr)2 and Pb(CH3COO)2 to ethylene glycol, followed by heating at 170 °C for 2 h. The nanowires could be readily collected as precipitates after the reaction solutions had been cooled down to room temperature. By calcining at elevated temperatures, each glycolate precursor could be transformed into the corresponding metal oxide without changing the wire-like morphology. Electron microscopic and XRD powder diffraction studies were used to characterize the morphology, crystallinity, and structure of these nanowires before and after calcination at various temperatures. A plausible mechanism was also proposed to account for the one-dimensional growth of such nanostructures in a highly isotropic medium. This mechanism was supported by XRD, FT-IR, solid state 13C-NMR, and TGA measurements. As a demonstration of potential applications, the polycrystalline nanowires made of SnO2 were used as functional components to fabricate sensors that could detect combustible gases (CO and H2) with greatly enhanced sensitivity under ambient conditions.

Single-Crystal Nanowires of Platinum Can Be Synthesized by Controlling the Reaction Rate of a Polyol Process

Abstract: Platinum nanowires of approximately 100 nm in length and approximately 5 nm in diameter have been synthesized by reducing H(2)PtCl(6) with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) and a trace amount of Fe(3+) or Fe(2+). The wires were generated at the final stage of the synthesis, which involved the formation of several intermediate species. The Fe(3+) or Fe(2+) ions had dual functions in the synthesis: they induced aggregation of Pt nanoparticles into larger structures that served as the nucleation sites, and they greatly reduced the reaction rate and supersaturation level to induce anisotropic growth. The reaction mechanism was studied by X-ray photoelectron spectroscopy (XPS) and UV-vis spectral analysis. The Pt nanowires could be readily separated from the surfaces of the agglomerates by sonication and obtained as pure samples by centrifugation.

Bottom-Up and Top-Down Approaches to the Synthesis of Monodispersed Spherical Colloids of Low Melting-Point Metals

Abstract: We report two different, solution-based approaches that have allowed us to process metals with melting points below 400 °C as monodispersed spherical colloids, in copious quantities, and with diameters controllable in the range of 100 to 600 nm. Bismuth was selected as a typical example to demonstrate the concepts. The production of monodispersed bismuth particles was realized by either thermally decomposing bismuth acetate in boiling ethylene glycol (the bottom-up approach) or by emulsifying molten drops of bismuth in boiling di(ethylene glycol) (the top-down approach), followed by quenching with cold ethanol. Depending on the concentration of Bi precursor and the stirring rate, the diameters of these uniform spherical colloids could be readily varied from 100 to 600 nm. The synthetic protocols have also been extended to prepare uniform spherical colloids from other metals with relatively low melting points, and typical examples include Pb, In, Sn, Cd, and their alloys.

Polyol Synthesis of Platinum Nanoparticles: Control of Morphology with Sodium Nitrate

Abstract: Morphological control over platinum nanoparticles was realized by varying the amount of NaNO3 added to a polyol process, where H2PtCl6 was reduced by ethylene glycol to form PtCl42- and Pt0 at 160 °C. As the molar ratio between NaNO3 and H2PtCl6 was increased from 0 to 11, the morphology of Pt nanoparticles evolved from irregular spheroids with rounded profiles to tetrahedra and octahedra with well-defined facets. Absorption spectroscopy studies suggest that nitrate was reduced to nitrite by PtCl42- in the early stage of the synthesis, and the nitrite could then form stable complexes with both Pt(II) and Pt(IV) species. As a result, the reduction of Pt precursors by ethylene glycol was greatly slowed. This change in reaction kinetics altered the growth rates associated with different crystallographic directions of the Pt nanocrystals and ultimately led to the formation of different morphologies.

Synthesis and optical properties of nanorattles and multiple-walled nanoshells/nanotubes made of metal alloys.

Abstract: The galvanic replacement reaction between silver and chloroauric acid has been exploited as a powerful means for preparing metal nanostructures with hollow interiors. Here, the utility of this approach is further extended to produce complex core/shell nanostructures made of metals by combining the replacement reaction with electroless deposition of silver. We have fabricated nanorattles consisting of Au/Ag alloy cores and Au/Ag alloy shells by starting with Au/Ag alloy colloids as the initial template. We have also prepared multiple-walled nanoshells/nanotubes (or nanoscale Matrioshka) with a variety of shapes, compositions, and structures by controlling the morphology of the template and the precursor salt used in each step of the replacement reaction. There are a number of interesting optical features associated with these new core/shell metal nanostructures. For example, nanorattles made of Au/Ag alloys displayed two well-separated extinction peaks, a feature similar to that of gold or silver nanorods. The peak at approximately 510 nm could be attributed to the Au/Ag alloy cores, while the other peak was associated with the Au/Ag alloy shells and could be continuously tuned in the spectral range from red to near-infrared.

Nanofibers of Conjugated Polymers Prepared by Electrospinning with a Two‐Capillary Spinneret

Abstract: Nanofibers of conjugated polymers and their blends were described. It was demonstrated that the nanofibers can be fabricated by co-electrospinning the solutions of polymers with a spinnable matrix polymer solution. It was also found that the morphology and diameters of the resulting fibers can be controlled by adjusting the processing parameters. Compared with spin-cast films, the polymer chains in electrospun fibers were found to have a more extended conformation and better spatial orientation.

Use of electrospinning to directly fabricate hollow nanofibers with functionalized inner and outer surfaces.

Abstract: A simple and versatile technique, electrospinning, which could be modified to fabricate long, hollow nanofibers with independently functionalized inner and outer surfaces was investigated. It has been demonstrated that a variety of materials such as polymers, ceramics, and carbon could be prepared as uniform fibers by electrospinning with well-controlled sizes, compositions, and morphologies. A tight control over the surfaces of hollow nanofibers greatly enhance their performance in applications related to nanofluidics, the fabrication of nanoscale lasing structures, and the prediction of advanced catalysts. The co-electrospinning process allows for the fabrication of nanofibers as assemblies with controllable, hierarchical structures, in addition to the control of surface chemistries. The results show that co-electrospinning with coaxial, dual-capillary spinneret is capable of generating hollow nanofibers with controllable hierarchical structures.

Synthesis and crystallization of hybrid spherical colloids composed of polystyrene cores and silica shells.

Abstract: The Stober method has been adopted to prepare hybrid core-shell particles by coating the surfaces of monodisperse polystyrene beads with uniform silica shells. Polystyrene beads with diameters in the range of 0.1-1.0 microm have been successfully demonstrated for use with this process, and the thickness of the silica coating could be controlled in the range of 50-150 nm by adjusting the concentration of tetraethoxysilane, the deposition time, or both. The morphology and surface smoothness of the deposited silica were found to strongly depend on a number of parameters such as the surface functional groups on the polymer beads, the pH value of the medium, and the deposition time. Hollow spheres made of silica could be obtained by selectively removing the polymer cores via calcination in air at an elevated temperature or by wet etching with toluene. These core-shell colloids were also explored as building blocks to fabricate long-range ordered lattices (or colloidal crystals) that exhibited stop bands different from those assembled from spherical colloids purely made of either polystyrene or silica.

Edge spreading lithography and its application to the fabrication of mesoscopic gold and silver rings.

Abstract: A new form of edge lithography, edge spreading lithography (ESL), has been demonstrated and applied to the formation of coinage metal rings. In this process, alkanethiols are delivered from a flat PDMS stamp to the surface of a metal film through a two-dimensional array of spherical silica colloids. The thiols further spread on the metal surface, forming highly ordered SAMs in the form of a ring pattern. Following lift-off of beads, the pattern in the SAMs can be transferred into the metal film through wet chemical etching, with SAMs serving as the resist. The dimensions of the rings can be readily controlled by several parameters such as the beads diameter, the concentration of the thiol solution, and the contact time between the stamp and the silica beads.

Ag Nanowires Coated with Ag/Pd Alloy Sheaths and Their Use as Substrates for Reversible Absorption and Desorption of Hydrogen

Abstract: When the surfaces of Ag nanowires were coated with thin sheaths of Pd/Ag alloys, they exhibited hydrogen absorption/desorption behaviors and capacities similar to those of pure Pd powders or nanotubes. The stronger mechanical strengths of these cable-like nanostructures also allowed them to undergo more absorption/desorption cycles with no morphological changes. These nanostructures can be used as a good model system to study the interaction between hydrogen and metal alloys with relatively low concentrations of Pd (<10%) with respect to structural, thermodynamic, and kinetic features.

A Solution-Phase, Precursor Route to Polycrystalline SnO2 Nanowires That Can Be Used for Gas Sensing under Ambient Conditions.

Abstract: This paper describes a solution-based, precursor method for the facile synthesis of uniform nanowires containing rutile SnO2 nanocrystallites. In a typical procedure, nanowires of ∼50 nm in diamete...

Single crystalline nanowires of lead: Large-scale synthesis, mechanistic studies, and transport measurements

Abstract: A solution-phase, precursor method has been demonstrated for the large-scale synthesis of single crystalline nanowires of lead with uniform diameters in the range 50−90 nm and lengths up to several...

Welding and patterning in a flash

Abstract: Light-absorbing materials can exhibit exceptional photothermal effects as their characteristic dimensions approach the nanoscale. A flash welding technique that exploits the peculiar photothermal response of polyaniline nanofibres opens new avenues for processing and patterning polymer-based materials and devices.

Comparative study of monolayers self-assembled from alkylisocyanides and alkanethiols on polycrystalline Pt substrates.

Abstract: This paper compares the properties of self-assembled monolayers (SAMs) derived from octadecylisocyanide (ODI) and octadecanethiol (ODT) on polycrystalline Pt substrates. Both monolayers formed at a...

Geometry and surface state effects on the mechanical response of Au nanostructures

Abstract: A study of ultra-thin gold films and thin-walled nanoboxes has confirmed that length scales in terms of dislocation spacing can predict flow stress. Initial stages of deformation conform to linear hardening with average dislocation spacing controlled by the number of geometrically necessary dislocations in a pile-up. Later stages of deformation exhibit parabolic behavior with Taylor hardening interpreted in terms of a dislocation density described by the total line length of prismatic loops per unit volume. Comparisons of 20 and 40 nm thick planar films could be made to 205 nm high hollow gold nanoboxes with a wall thickness of 24 nm. These highly constrained, ultra-thin planar films demonstrated increased hardness from about 2 to 10 GPa with strains of 20 percent while less constrained nanoboxes increased from 0.8 to 4 GPa for the same strain magnitude.

One-dimensional nanostructures of chalcogens and chalcogenides

Abstract: This paper presents an overview of solution-phase methods for generating one-dimensional (1D) nanostructures of chalcogens and chalcogenides. The first part describes self-seeding techniques for the formation of pure Se, pure Te, and alloy nanostructures. Se and Te are interesting for their inherent properties such as chirality, photoconductivity, piezoelectricity, and high reactivity. The specific morphology (i.e., nanowires, nanorods, and nano-tubes) and dimensions of these nanostructures could be tightly controlled. The second part describes a versatile template-engaged technique for topotactic transformation of chalcogen nanostructures into chalcogenides. This transfor-mation enables the formation of 1D nanostructures of materials that do not have intrinsically anisotropic crystal structure. These 1D nanostructures will play an important role in the fabrication of nanoscale devices and in the study of electronic size-confinement effects.