Showing papers by "Younan Xia published in 2003"
TL;DR: A comprehensive review of 1D nanostructures can be found in this article, where the authors provide a comprehensive overview of current research activities that concentrate on one-dimensional (1D) nanostructure (wires, rods, belts and tubes).
Abstract: This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures—wires, rods, belts, and tubes—whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostructures that have been synthesized in relatively copious quantities using chemical methods. We begin this article with an overview of synthetic strategies that have been exploited to achieve 1D growth. We then elaborate on these approaches in the following four sections: i) anisotropic growth dictated by the crystallographic structure of a solid material; ii) anisotropic growth confined and directed by various templates; iii) anisotropic growth kinetically controlled by supersaturation or through the use of an appropriate capping reagent; and iv) new concepts not yet fully demonstrated, but with long-term potential in generating 1D nanostructures. Following is a discussion of techniques for generating various types of important heterostructured nanowires. By the end of this article, we highlight a range of unique properties (e.g., thermal, mechanical, electronic, optoelectronic, optical, nonlinear optical, and field emission) associated with different types of 1D nanostructures. We also briefly discuss a number of methods potentially useful for assembling 1D nanostructures into functional devices based on crossbar junctions, and complex architectures such as 2D and 3D periodic lattices. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials might be directed.
8,259 citations
TL;DR: Sun et al. as mentioned in this paper demonstrated an approach based on the polyol process for the large-scale synthesis of silver nanowires with uniform diameters, which can be used to synthesize 30−60 nm in diameter and 1−50 μm in length.
Abstract: We have recently demonstrated an approach based on the polyol process for the large-scale synthesis of silver nanowires with uniform diameters (see Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833. Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736). Although the capability and feasibility of this method have been successfully illustrated with the production of silver nanowires 30−60 nm in diameter and 1−50 μm in length, the growth mechanism of this process is yet to be elucidated. Here we report some progress on this matter: First, electron microscopy studies on microtomed samples indicated that the cross sections of such silver nanowires had a pentagonal shape, together with a 5-fold twinned crystal structure. Second, the side surfaces (bounded by {100} facets) and the ends (bounded by {111} facets) of each nanowire were shown to have significant difference in reactivity toward dithoil molecules, with the side su...
1,524 citations
TL;DR: In this article, a collector consisting of two pieces of electrically conductive substrates separated by a gap whose width could be varied from hundreds of micrometers to several centimeters.
Abstract: Electrospinning has been applied to prepare uniaxially aligned nanofibers made of organic polymers, ceramics, and polymer/ceramic composites The key to the success of this method was the use of a collector consisting of two pieces of electrically conductive substrates separated by a gap whose width could be varied from hundreds of micrometers to several centimeters As driven by electrostatic interactions, the charged nanofibers were stretched to span across the gap and thus to become uniaxially aligned arrays over large areas Because the nanofibers were suspended over the gap, they could be conveniently transferred onto the surfaces of other substrates for subsequent treatments and various applications Materials that have been successfully incorporated into this procedure include conventional organic polymers, graphite carbon, and metal oxides By controlling the parameters for electrospinning, we have also fabricated a number of simple device structures, for example, an individual nanofiber spanning
1,460 citations
TL;DR: In this article, the Langmuir−Blodgett technique was used to assemble monolayers (with areas over 20 cm2) of aligned silver nanowires that are ∼50 nm in diameter and 2−3 μm in length.
Abstract: Langmuir−Blodgett technique was used to assemble monolayers (with areas over 20 cm2) of aligned silver nanowires that are ∼50 nm in diameter and 2−3 μm in length. These nanowires possess pentagonal cross-sections and pyramidal tips. They are close-packed and are aligned parallel to each other. The resulting nanowire monolayers serve as excellent substrates for surface-enhanced Raman spectroscopy (SERS) with large electromagnetic field enhancement factors (2 × 105 for thiol and 2,4-dinitrotoluene, and 2 × 109 for Rhodamine 6G) and can readily be used in ultrasensitive, molecule-specific sensing utilizing vibrational signatures.
1,322 citations
TL;DR: In this paper, the authors describe a procedure based on electrospinning for generating nanofibers of anatase with controllable diameters and porous structures, which can be converted into anatase without changing their morphology via calcination in air at 500 °C.
Abstract: This paper describes a procedure based on electrospinning for generating nanofibers of anatase with controllable diameters and porous structures. When an ethanol solution containing both poly(vinyl pyrrolidone) (PVP, Mw ≈ 1 300 000) and titanium tetraisopropoxide was injected through a needle under a strong electrical field, composite nanofibers made of PVP and amorphous TiO2 were formed (with lengths up to several centimeters) as a result of electrostatic jetting. These nanofibers could be subsequently converted into anatase without changing their morphology via calcination in air at 500 °C. The average diameter of these ceramic nanofibers could be controlled in the range from 20 to 200 nm by varying a number of parameters such as the ratio between PVP and titanium tetraisopropoxide, their concentrations in the alcohol solution, the strength of the electric field, and the feeding rate of the precursor solution. Both supported and free-standing mats consisting of anatase nanofibers have been successfully ...
1,213 citations
TL;DR: It is demonstrated that this new approach could be extended to generate polycrystalline nanowires of other metal oxides such as In2O3 and anatase TiO2 and could be used for gas sensing with improved sensitivity and reversibility 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...
948 citations
TL;DR: In this article, a replacement reaction between the surface of a nanoscale template and the solution of an appropriate salt precursor is used to synthesize metal nanostructures with well-defined hollow interiors.
Abstract: We have recently developed a simple and versatile route to the large-scale synthesis of metal nanostructures with well-defined hollow interiors. The key step of this process involves a replacement reaction between the surface of a nanoscale template and the solution of an appropriate salt precursor. The capability and feasibility of this method have been demonstrated by preparing hollow nanostructures of gold with a range of different morphologies (e.g., triangular rings, prism-shaped boxes, cubic boxes, spherical capsules, and tubes). In addition to gold, this method also worked well for other metals such as platinum and palladium (see Figure for an SEM image of Pd nanotubes). These metal nanostructures with hollow interiors are useful as fillers for generation of ultralight composites; they are also interesting for new types of applications that include use as components for highly sensitive colorimetric sensors and as effective, recoverable catalysts.
947 citations
TL;DR: In this article, the authors show that the light present in an ordinary chemical laboratory was sufficiently strong to transform some spherical nanoparticles into small triangular nanoplates, and that refluxing provided the driving force to facilitate the Ostwald ripening process.
Abstract: Silver has been prepared as triangular nanoplates with sharp corners and thin nanobelts by refluxing an aqueous dispersion of spherical colloids of silver with an average diameter of 3.5 nm. The spherical colloids of silver were, in turn, generated by reducing silver nitrate with sodium borohydride in the presence of poly(vinyl pyrrolidone) (PVP) and sodium citrate. Our studies indicate that the light present in an ordinary chemical laboratory was sufficiently strong to transform some spherical nanoparticles into small triangular nanoplates. Refluxing provided the driving force to facilitate the Ostwald ripening process−growth of these plate-like seeds at the expense of spherical nanoparticles. In addition, refluxing might also selectively disrupt the organic layers on the surfaces of the small triangular nanoplates, driving these plates to assemble into thin nanobelts due to the strong dipole−dipole interaction between adjacent plates. These nanostructures of silver with unique planar shapes might find u...
724 citations
TL;DR: In this paper, the authors synthesize single-crystalline nanoboxes of gold with poly(vinyl pyrrolidone) (PVP) in large quantities by reducing silver nitrate with ethylene glycol in the presence of PVP.
Abstract: Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-crystalline nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technologically important because of the strong correlation between these parameters and optical, electrical, and catalytic properties.
590 citations
TL;DR: In one demonstration, it is shown that the thickness of a closed polymer shell could be obtained by mapping the electrons backscattered from the core and shell, and the plasmon resonance band of the gold cores was used as an optical probe to follow the diffusion kinetics of chemical reagents across the polymer shells.
Abstract: Gold nanoparticles have been conformally coated with amorphous silica (using a sol−gel method) and then an organic polymer (via surface-grafted, atom transfer radical polymerization) to form spherical colloids with a core−double-shell structure. The thickness of silica and polymer shells could be conveniently controlled in the range of tens to several hundred nanometers by changing the concentration of the reagent and/or the reaction time. Selective removal of the silica layer (through etching in aqueous HF) led to the formation of hollow polymer beads containing movable gold cores. This new form of core−shell particles provides a unique system for measuring the feature size and transport property associated with hollow particles. In one demonstration, we showed that the thickness of a closed polymer shell could be obtained by mapping the electrons backscattered from the core and shell. In another demonstration, the plasmon resonance band of the gold cores was used as an optical probe to follow the diffus...
544 citations
TL;DR: Spectral measurements indicated that the positions of surface plasmon resonance (SPR) bands for these nanoparticles could be tuned by varying all these parameters, and gold nanoshells with hollow interiors were shown to exhibit a much higher sensitivity to environmental changes than gold solid colloids with roughly the same size.
Abstract: This paper presents several solution-phase methods for the large-scale synthesis of metal nanoparticles with controllable compositions (e.g., spherical nanoparticles of gold/silver alloys), morphologies (e.g., nanospheres, triangular nanoplates, circular nanodisks, and nanocubes of silver), and structures (e.g., solid vs. hollow colloids). Spectral measurements indicated that the positions of surface plasmon resonance (SPR) bands for these nanoparticles could be tuned by varying all these parameters. The number of SPR peaks was found to increase as the symmetry of the nanoparticles decreased. In addition to their use as chromophores with strong extinction coefficients, these nanoparticles could serve as a platform to probe binding events of chemical and biochemical species on their surfaces. Gold nanoshells with hollow interiors were, in particular, shown to exhibit a much higher sensitivity to environmental changes than gold solid colloids with roughly the same size.
TL;DR: The capability and feasibility of this approach have been demonstrated with the organization of polymer latex or silica beads into homo-aggregates, including circular rings, polygonal and polyhedral clusters; and linear, zigzag, and spiral chains.
Abstract: Colloidal aggregates with well-controlled sizes, shapes, and structures have been fabricated by dewetting aqueous dispersions of monodispersed spherical colloids across surfaces patterned with two-dimensional arrays of relief structures (or templates). The capability and feasibility of this approach have been demonstrated with the organization of polymer latex or silica beads into homo-aggregates, including circular rings; polygonal and polyhedral clusters; and linear, zigzag, and spiral chains. It was also possible to generate hetero-aggregates in the configuration of HF and H2O molecules that contained spherical colloids of different sizes, compositions, densities, functions, or a combination of these features. These uniform, well-defined aggregates of spherical colloids are ideal model systems to investigate the aerodynamic, hydrodynamic, and optical properties of colloidal particles characterized by non-spherical shapes and/or complex topologies. They can also serve as a new class of building blocks to generate hierarchically self-assembled structures that are expected to exhibit interesting features valuable to areas ranging from condensed matter physics to photonics.
TL;DR: Sun et al. as discussed by the authors showed that the galvanic replacement reaction between Ag nanoparticles and an aqueous HAuCl4 solution has been demonstrated as a simple and convenient route to metal nanostructures with hollow interiors and highly crystalline walls.
Abstract: The galvanic replacement reaction between Ag nanoparticles and an aqueous HAuCl4 solution has recently been demonstrated as a simple and convenient route to metal nanostructures with hollow interiors and highly crystalline walls (see, for example, Sun, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 481. Sun, Y.; Xia, Y. Science, 2002, 298, 2176). However, the details of morphological, compositional, structural, and spectral changes involved in the entire process of this template-engaged reaction is yet to be elucidated. The experimental results described in this letter indicate that the templating process proceeded through two distinctive steps: (i) formation of pinhole-free nanoshells with homogeneous, uniform walls of Au/Ag alloys via a combination of the replacement reaction and alloying; and (ii) formation of porous nanoshells (nanocages) through a dealloying process, in which Ag was selectively dissolved from the walls made of Au/Ag alloys. As alloying and dealloying proceeded, the surface plasmon r...
TL;DR: In this article, the authors describe the fabrication and characterization of colloidal crystals whose stop bands could be varied through the application of a liquid, and demonstrate a photonic paper/ink system where color patterns could be conveniently generated on the surface of a thin film of polystyrene beads by writing with a Pilot pen.
Abstract: This article describes the fabrication and characterization of colloidal crystals whose stop bands could be varied through the application of a liquid. Such a colloidal crystal was generated by infiltrating the voids within an opaline lattice of polystyrene beads with a liquid prepolymer to poly(dimethylsiloxane), followed by thermal curing. When a liquid (e.g., a silicone fluid, hexane, or octane) capable of swelling the elastomer matrix was applied to the surface of this crystal, the lattice constant and thus the wavelength of Bragg-diffracted light was increased. For instance, the color of light diffracted from a colloidal crystal made of 175 nm polystyrene beads could be varied from violet to green, orange, and red simply by swelling it with different solvents. On the basis of this mechanism, we further demonstrated a photonic paper/ink system where color patterns could be conveniently generated on the surface of a thin film of colloidal crystal by writing with a Pilot pen, by screen printing, or by m...
TL;DR: Hollow nanostructures of platinum have been synthesized by reducing PtCl2 with alcohol in the presence of selenium nanowires or colloids by varying the template, reaction time, and the concentration of PtCl 2.
Abstract: Hollow nanostructures of platinum have been synthesized by reducing PtCl2 with alcohol in the presence of selenium nanowires or colloids. The Se template could be removed by soaking the resultant Se@Pt nanostructures in hydrazine or by heating them to 200−250 °C. The size and wall thickness of the polycrystalline hollow nanostructures could be controlled by varying the template, reaction time, and the concentration of PtCl2.
TL;DR: In this paper, a simple procedure for fabricating randomly and uniaxially aligned nanofibers of nickel ferrite by electrospinning was described, and magnetic hysteresis scans were performed.
Abstract: This letter describes a simple procedure for fabricating randomly and uniaxially aligned nanofibers of nickel ferrite by electrospinning. Polycrystalline nanofibers of NiFe2O4 with an average diameter of 46 nm have been prepared by electrospinning a solution that contained poly(vinyl pyrrolidone) and alkoxide precursors to nickel and iron oxides, followed by hydrolysis and calcination at 550 °C in air. Magnetic hysteresis scans were performed on these nanofibers, in comparison with a powder sample prepared using the conventional sol–gel process. Significant differences in magnetic properties were noted between these two samples, and these differences seemed to be associated with the size and morphological differences between the nanofibers and the powders.
Patent•
09 Dec 2003TL;DR: In this paper, methods for forming nanostructures of various shapes are disclosed, such as nanocubes, nanowires, nanopyramids and multiply twinned particles of silver.
Abstract: Methods for forming nanostructures of various shapes are disclosed. Nanocubes, nanowires, nanopyramids and multiply twinned particles of silver may by formed by combining a solution of silver nitrate in ethylene glycol with a solution of poly(vinyl pyrrolidone) in ethylene glycol. Hollow nanostructures may be formed by reacting a solution of solid nanostructures comprising one of a first metal and a first metal alloy with a metal salt that can be reduced by the first metal or first metal alloy. Nanostructures comprising a core with at least one nanoshell may be formed by plating a nanostructure and reacting the plating with a metal salt.
TL;DR: This method provides another route to asymmetric dimers made of colloidal particles that could be different in size, chemical composition, surface functionality, density or sign of surface charge, bulk property, or a combination of these properties.
Abstract: Asymmetric dimers consisting of gold microcrystals and spherical silica colloids have been fabricated by depositing thin films of gold onto the spherical colloids to form half-shells, followed by annealing at elevated temperatures. The capability and feasibility of this procedure have been demonstrated with silica and titania beads of 0.2−2 μm in diameter and γ-Fe2O3/polystyrene@SiO2 core−shell particles 0.5 μm in size. The dimensions of gold microcrystals could be conveniently varied in the range of 100−650 nm by controlling the thickness of gold films and/or the diameter of the spherical colloids. This method provides another route to asymmetric dimers made of colloidal particles that could be different in size, chemical composition, surface functionality, density or sign of surface charge, bulk property, or a combination of these properties.
TL;DR: In this paper, a solution-based method for the large-scale synthesis of selenium nanowires was described, where trigonal Se seeds were formed during sonication and grew over the course of hours at the expense of amorphous Se colloids in a process similar to Ostwald ripening.
Abstract: This paper describes a solution-based method for the large-scale synthesis of selenium nanowires. Sonication was used as the driving force for both nucleation and dispersion. The trigonal Se seeds were formed during sonication and grew over the course of hours at the expense of the amorphous Se colloids in a process similar to Ostwald ripening. The resultant nanowires were typically single crystals of trigonal Se whose morphologies could be tuned by adjusting the reaction conditions. A variety of solvents are demonstrated for use with this process. We also demonstrate patterned growth of these nanowires on substrates to form extensive 2D networks and isolated clusters. The morphologies of the products were examined with a combination of microscopic and spectroscopic techniques.
TL;DR: The processing of an achiral material into helical mesostructures having well-controlled handedness should allow us to explore new properties that this material otherwise does not exhibit and may provide valuable insights to improve understanding on the mechanism(s) by which pure enantiomers with a particular sense of rotation evolved in nature.
Abstract: Physical constraint and capillary force have been combined to provide a generic approach to assemble achiral building blocks such as monodisperse spherical colloids into helical mesostructures. The key component of this process is an array of V-shaped grooves anisotropically etched in the surface of a Si(100) wafer. The structural arrangement among the spherical colloids is determined by the ratio between the width of the V-grooves and the diameter of the colloids. Double-layered structures with a helical morphology will be formed when this ratio falls between 2.70 and 2.85. The exact handedness of these helical structures could be controlled by varying the relative orientation of capillary force with respect to the longitudinal axis of the spirals. The processing of an achiral material into helical mesostructures having well-controlled handedness should allow us to explore new properties that this material otherwise does not exhibit. The self-assembly process may also provide valuable insights to improve...
TL;DR: In this paper, a simple and convenient procedure that allowed for the fabrication of large colloidal crystals with their (100) planes oriented parallel to the surfaces of supporting substrates was described, and the capability and feasibility of this approach have been demonstrated by crystallizing spherical colloids (>250 nm in diameter) into (100)-oriented crystals over areas as large as several square centimeters.
Abstract: This paper describes a simple and convenient procedure that allowed for the fabrication of large colloidal crystals with their (100) planes oriented parallel to the surfaces of supporting substrates. Such crystals were grown by templating against two-dimensional (2D) regular arrays of square pyramidal pits or V-shaped grooves etched in the surfaces of Si(100) substrates. The capability and feasibility of this approach have been demonstrated by crystallizing spherical colloids (>250 nm in diameter) into (100)-oriented crystals over areas as large as several square centimeters. We further demonstrated that inverse opals with (100) crystallographic orientation could be fabricated by templating liquid precursors (such as a UV-curable prepolymer) against these colloidal crystals. In addition to silicon templates whose fabrication involved extensive use of cleanroom facilities, multiple copies of polymer templates having similar functions were also generated by replica molding, a process that could be performed...
TL;DR: In this paper, a solution-phase route to the facile synthesis of lead nanowires with lateral dimensions as thin as 35 nm and lengths up to 250 μm was described.
Abstract: This paper describes a solution-phase route to the facile synthesis of lead nanowires with lateral dimensions as thin as 35 nm and lengths up to 250 μm. These lead nanowires were single crystalline in structure, and electron transport measurements on individual nanowires (using the four-probe method) indicate a phase transition to the superconducting state around 7.13 K, a temperature similar to the value (7.20 K) reported for bulk lead. Different from polycrystalline samples, no significant variation in the transition temperature was observed for nanowires with diameters down to 50 nm.
TL;DR: A comprehensive review of 1D nanostructures can be found in this paper, where the authors provide a comprehensive overview of current research activities that concentrate on one-dimensional (1D) nanostructure (wires, rods, belts and tubes).
Abstract: This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures—wires, rods, belts, and tubes—whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostructures that have been synthesized in relatively copious quantities using chemical methods. We begin this article with an overview of synthetic strategies that have been exploited to achieve 1D growth. We then elaborate on these approaches in the following four sections: i) anisotropic growth dictated by the crystallographic structure of a solid material; ii) anisotropic growth confined and directed by various templates; iii) anisotropic growth kinetically controlled by supersaturation or through the use of an appropriate capping reagent; and iv) new concepts not yet fully demonstrated, but with long-term potential in generating 1D nanostructures. Following is a discussion of techniques for generating various types of important heterostructured nanowires. By the end of this article, we highlight a range of unique properties (e.g., thermal, mechanical, electronic, optoelectronic, optical, nonlinear optical, and field emission) associated with different types of 1D nanostructures. We also briefly discuss a number of methods potentially useful for assembling 1D nanostructures into functional devices based on crossbar junctions, and complex architectures such as 2D and 3D periodic lattices. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials might be directed.
TL;DR: In order to enhance the available signal, Kirkpatrick-Baez focusing optics have been introduced in the 34-ID-C beamline at APS.
Abstract: Coherent X-ray Diffraction is applied to investigate the structure of individual nanocrystalline silver particles in the 100nm size range. In order to enhance the available signal, Kirkpatrick-Baez focusing optics have been introduced in the 34-ID-C beamline at APS. Concerns about the preservation of coherence under these circumstances are addressed through experiment and by calculations.
01 Dec 2003
28 Oct 2003
TL;DR: In this article, a template-engaged displacement reaction has been demonstrated to prepare metal nanostructures with hollow interiors by reacting solutions of appropriate salts with solid metal nano-structures.
Abstract: A general approach involved template-engaged displacement reaction has been demonstrated to prepare metal nanostructures with hollow interiors by reacting solutions of appropriate salts with solid metal nanostructures. For example, silver nanostructures with various morphologies including triangular plates, cubes, spheres, rods and wires have been used as templates to react with an aqueous chloroauric acid solution. The reaction led to the formation of hollow nanostructures with shapes similar to that of silver templates. The void space, wall thickness, and crystalline structure of these hollow structures were determined with the silver templates, which were converted into soluble species during the displacement reaction. Elemental analysis and electron microscopic studies indicated that these hollow structures were made of gold/silver alloys. The capability and feasibility of this method have also been demonstrated by preparing nanotubes made of different metals (e.g., gold/silver, palladium/silver, and platinum/silver alloys). The hollow nanostructures of gold/silver alloys exhibited significantly different surface plasmonic properties from their solid counterparts. For instance, the extinction peaks of the nanoshells of gold/silver alloy with roughly spherical shape were considerably red-shifted as compared to solid colloids of silver or gold having approximately the same dimensions. The high extinction coefficient in the red and near infrared regimes should make these nanoshells particularly useful as components in fabricating plasmonic devices and labels in probing the desired biomolecules.