Showing papers by "Younan Xia published in 2002"

Shape-Controlled Synthesis of Gold and Silver Nanoparticles

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.

Crystalline Silver Nanowires by Soft Solution Processing

Abstract: This paper describes a soft, solution-phase approach to the large-scale synthesis of uniform nanowires of bicrystalline silver whose lateral dimensions could be controlled in the range of 30−40 nm, and lengths up to ∼50 μm. The first step of this procedure involved the formation of platinum nanoparticles by reducing PtCl2 with ethylene glycol heated to ∼160 °C. Due to their close match in crystal structure and lattice constants, these platinum nanoparticles could serve as seeds for the heterogeneous nucleation and growth of silver that was produced in the solution via the reduction of AgNO3 with ethylene glycol. When surfactants such as poly(vinyl pyrrolidone) (PVP) were present in this solution, the silver could be directed to grow into uniform nanowires with aspect ratios as high as ∼1000. Measurements of transport property at room temperature indicated that these nanowires were electrically continuous with a conductivity of approximately 0.8 × 105 S/cm.

Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone)

Abstract: A solution-phase approach has been demonstrated for the large-scale synthesis of silver nanowires with diameters in the range of 30−40 nm, and lengths up to ∼50 μm. The first step of this process involved the formation of Pt (or Ag) nanoparticles by reducing PtCl2 (or AgNO3) with ethylene glycol (EG) heated to ∼160 °C. These Pt (or Ag) nanoparticles could serve as seeds for the heterogeneous nucleation and growth of silver (formed by reducing AgNO3 with EG) because of their close match in crystal structure and lattice constants. In the presence of poly(vinyl pyrrolidone) (PVP), the growth of silver could be directed into a highly anisotropic mode to form uniform nanowires with aspect ratios as high as ∼1000. UV−visible spectroscopy, SEM, TEM, XRD, and electron diffraction were used to characterize these silver nanowires, indicating the formation of a highly pure phase, as well as a uniform diameter and bicrystalline structure. Both morphology and aspect ratios of these silver nanostructures could be varie...

Modifying the Surface Properties of Superparamagnetic Iron Oxide Nanoparticles through A Sol−Gel Approach

Abstract: This paper describes a sol−gel approach for the coating of superparamagnetic iron oxide nanoparticles with uniform shells of amorphous silica. The coating process has been successfully applied to particles contained in a commercial ferrofluid (e.g., the EMG 304 of Ferrofluidics) and those synthesized through a wet chemical process. The thickness of silica coating could be conveniently controlled in the range of 2−100 nm by changing the concentration of the sol−gel solution. Fluorescent dyes, for example, 7-(dimethylamino)-4-methylcoumarin-3-isothiocyanate (DACITC) and tetramethylrhodamine-5-isothiocyanate (5-TRITC), have also been incorporated into the silica shells by covalently coupling these organic compounds with the sol−gel precursor. These multifunctional nanoparticles are potentially useful in a number of areas because they can be simultaneously manipulated with an externally applied magnetic field and characterized in situ using conventional fluorescence microscopy.

CuO Nanowires Can Be Synthesized by Heating Copper Substrates in Air

Abstract: This paper describes a vapor-phase approach to the facial synthesis of cupric oxide (CuO) nanowires supported on the surfaces of various copper substrates that include grids, foils, and wires. A typical procedure simply involved the thermal oxidation of these substrates in air and within the temperature range from 400 to 700 °C. Electron microscopic studies indicated that these nanowires had a controllable diameter in the range of 30−100 nm with lengths of up to 15 μm by varying the temperature and growth time. Electron diffraction and high-resolution TEM studies implied that each CuO nanowire was a bicrystal divided by a (111) twin plane in its middle along the longitudinal axis. A possible mechanism was also proposed to account for the growth of these CuO nanowires.

Template-Engaged Replacement Reaction: A One-Step Approach to the Large-Scale Synthesis of Metal Nanostructures with Hollow Interiors

Abstract: This paper describes a general approach that generates nanoscale hollow structures of metals by reacting solutions of appropriate salt solutions with solid templates of a more reactive metal. Typical examples include Au3+, Pt2+, and Pd2+ salts and nanoparticles or nanowires of silver. The morphology, void space, and wall thickness of these hollow structures are all determined by the solid templates, which are completely converted into soluble species during the replacement reaction. Both electron microscopy and diffraction studies indicate that single crystalline hollow structures of metals can also be obtained when the templates are single crystals. These metallic hollow structures, having well-controlled sizes and shapes, are expected to find use in a number of applications that involve nanoscale encapsulation, drug delivery, plasmon photonics, and calorimetric sensing.

Increased sensitivity of surface plasmon resonance of gold nanoshells compared to that of gold solid colloids in response to environmental changes.

Abstract: Gold nanoshells have been synthesized by reacting aqueous HAuCl4 solutions with solid templates such as silver nanoparticles. The morphology, void space, and wall thickness of these hollow nanostructures were all determined by the templates, which were completely converted into soluble species during the replacement reaction. The surface plasmon peaks of these gold nanoshells were considerably red-shifted as compared to gold solid colloids having approximately the same dimensions. In addition, the surface plasmon resonance of gold nanoshells exhibited a much more sensitive response toward environmental changes even when compared with solid colloids with a mean size much smaller than that of gold nanoshells. For example, the sensitivity factor (i.e., the shift in peak position per unit change in the refractive index of the surrounding medium) was 408.8, 60.0, and 70.9 nm per refractive index unit for gold nanoshells with a mean diameter of 50 nm and wall thickness of 4.5 nm, gold solid colloids of 50 nm in...

Synthesis and Self-Assembly of Au@SiO2 Core−Shell Colloids

Abstract: Gold nanoparticles have been coated with amorphous silica to form spherical colloids with a core−shell structure The thickness of silica shells could be conveniently controlled in the range of tens to several hundred nanometers by changing the concentration of the sol−gel precursor or the coating time These core−shell colloids could serve as the building blocks to fabricate photonic devices In one demonstration, we showed that these core−shell particles could be assembled into long-range ordered lattices (or photonic crystals) over large areas that exhibited optical properties different from those crystallized from silica colloids Transmission spectra of these crystals displayed both features that correspond to the Bragg diffraction of a periodic lattice and the plasmon resonance absorption of gold nanoparticles Reflectance spectra taken from these crystals only showed peaks caused by Bragg diffraction In another demonstration, these core−shell colloids were assembled into chains of different config

Synthesis and characterization of stable aqueous dispersions of silver nanoparticles through the Tollens process

Abstract: This paper describes a simple and convenient procedure based on the Tollens process for the preparation of silver nanoparticles with a relatively narrow distribution in size. The starting reagents were similar to those commonly used in the electroless deposition of silver. Only under appropriate conditions, mixing of these reagents was able to generate stable aqueous dispersions of silver colloids rather than thin films of silver deposited on the surfaces of objects immersed in the plating solution (including the interior surface of the container). We have demonstrated the capability and feasibility of this approach by forming silver nanoparticles with dimensions in the range of 20–50 nm. These silver nanoparticles could exist as very stable dispersions in water, or as submonolayer coating on microscale colloids. We have also explored the use of light scattering simulation to study the oxidation (by air) of these nanoparticles.

Synthesis and Characterization of Uniform Nanowires of Trigonal Selenium

Abstract: This article describes a soft, solution-phase approach to the large-scale synthesis of uniform nanowires of trigonal selenium (t-Se) with lateral dimensions controllable in the range of ~10 to ~800 nm, and lengths up to hundreds of micrometers. These highly anisotropic, one-dimensional (1D) nanostructures were directly nucleated and grown from aqueous solutions without the help of any physical templates, such as channel-like structures etched in porous materials, or scaffolds assembled from surfactants or block-copolymers. The 1D morphology of the product was solely determined by the linear characteristics of the building blocks—i.e., the extended, helical chains of atoms contained in the crystalline lattice of t-Se. A blue shift was observed for the bandgap and interchain transition of these nanowires when their diameters were reduced from ~32 to ~10 nm. The photoconductivity of individual nanowires has also been measured using the four-probe method, and an increase by ~150 times was found when the sample was taken from the dark and exposed with ~3 lW lm ‐2 tungsten light. Since no exotic seeds were involved in this synthetic process, every nanowire (including both ends) should be made entirely of pure selenium, crystallized in the trigonal phase. We believe the protocol described here can be scaled up for the high-volume production of t-Se nanowires that can subsequently serve as the physical or chemical templates to generate 1D nanostructures of various kinds of functional materials. The synthetic strategy itself, may also be extendable to other systems containing chain-like building blocks. The single crystallinity and absence of kinks and other related defects in these nanowires should make them particularly useful in fabricating nanoscale electronic, optical, or mechanical nanodevices.

Silver Nanowires Can Be Directly Coated with Amorphous Silica To Generate Well-Controlled Coaxial Nanocables of Silver/Silica

Abstract: This paper describes a simple and convenient approach that allowed for the facial synthesis of silver/silica coaxial nanocables with well-controlled sheath thicknesses in the range of 2−100 nm. The lengths of these nanocables could be up to ∼50 μm. Such nanocables were prepared by directly coating bicrystalline silver nanowires with conformal sheaths of silica through a sol−gel process. The silver nanowires were, in turn, synthesized using a polyol method that involved the reduction of silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone). The thickness of silica coating could be controlled by changing the concentration of the precursor solution and/or the reaction time. Selective removal of the silver cores led to the formation of silica nanotubes having well-controlled dimensions and wall structures.

One-dimensional nanostructures of trigonal tellurium with various morphologies can be synthesized using a solution-phase approach

Abstract: This article describes a solution-phase, self-seeding approach to the large-scale synthesis of one-dimensional (1D) nanostructures of trigonal tellurium (t-Te) with diameters ranging from 50 to hundreds of nanometers, and lengths up to tens of micrometers. These highly anisotropic nanostructures were formed through the reduction of orthotelluric acid (or tellurium dioxide) by hydrazine at various refluxing temperatures. Nuclei formed in the reduction process had a strong tendency to grow along the c-axis due to the inherently anisotropic structure of t-Te. Depending on the solvent and refluxing temperature, the growth of t-Te nanostructures was found to follow two distinct paths. When the reaction was refluxed in water and at temperatures below 100 °C, the initial reduction products were a mixture of nanocrystallites of t-Te and spherical colloids of amorphous tellurium (a-Te). When this mixture was aged at room temperature, the a-Te colloids slowly dissolved into the solution and grew into nanowires on the nanocrystallites of t-Te. When the reaction was carried out in pure ethylene glycol (or mixtures with water) and refluxed at temperatures above 100 °C, the 1D nanostructures of t-Te were directly formed in the reduction process. The exact morphology of these anisotropic nanostructures was mainly controlled by the refluxing temperature (Tr); typical examples include spines (Tr < 100 °C), filaments (Tr = 100–160 °C), needles (Tr = 160–180 °C), and tubular structures (Tr > 180 °C). These uniform, relatively monodispersed 1D nanostructures could form stable dispersions in ethylene glycol or water, and be used as the building blocks or templates to generate more complex nanostructured materials.

Synthesis and Characterization of Crystalline Ag2Se Nanowires Through a Template‐Engaged Reaction at Room Temperature

Abstract: Single-crystalline Ag2Se nanowires have been successfully synthesized through a template-engaged topotactic reaction in which nanowires of trigonal selenium were transformed into Ag2Se by reacting with aqueous AgNO3 solutions at room temperature (RT). An interesting size-dependent transition between two crystal structures has also been observed for this newly synthesized one-dimensional system: The Ag2Se nanowires adopted a tetragonal structure when their diameters were less than ∼40 nm; an orthorhombic structure was found to be more favorable as the diameter of these nanowires was increased beyond 40 nm. Since this reaction can be carried out at ambient pressure and temperature, it should be straightforward to scale up the entire process for the high-volume production of Ag2Se nanowires with well-controlled sizes and crystal structures. These highly uniform nanowires of single-crystalline Ag2Se are potentially useful as photosensitizers, superionic conductors, magnetoresistive compounds, or thermoelectric materials. This work also represents the first demonstration of a template-engaged process capable of generating single-crystalline nanowires from the solution-phase and at RT.

Growth of Large Colloidal Crystals with Their (100) Planes Orientated Parallel to the Surfaces of Supporting Substrates

Abstract: Templating against two-dimensional (2D) regular arrays of square pyramidal pits etched in Si(100) wafers has been exploited to fabricate colloidal crystals with their (100) planes oriented parallel to the substrates (see Figure for an SEM image). The capability and feasibility of this method have been demonstrated by crystallizing 1.0, 0.48, and 0.25 μm polystyrene beads into 3D opaline lattices having such an orientation over areas as large as several square centimeters. Like their (111)-oriented cousins, these long-range ordered lattices of spherical colloids are useful in many areas such as photonics and porous materials. In particular, the ability to generate large colloidal crystals with adjustable spatial orientations will allow one to systematically investigate their photonic band structures in an effort to elucidate the structure–property relationship.

Colloidal Crystals Made of Polystyrene Spheroids: Fabrication and Structural/Optical Characterization

Abstract: This paper describes the fabrication and characterization of colloidal crystals constructed from spheroidal building blocks with well-controlled major-to-minor ratios. Such a crystalline lattice was fabricated by infiltrating an opaline lattice of monodispersed polystyrene spheres with an elastomer precursor such as poly(dimethylsiloxane) (or PDMS), followed by thermal curing and stretching of the composite film at a temperature higher than the glass transition temperature of polystyrene. In this process, the polystyrene spheres were transformed into spheroids through viscoelastic deformation, while the long-range order of this three-dimensional lattice was essentially preserved. Because of the low contrast in the refractive index, the colloidal crystal fabricated in the present work exhibited a stop band (rather than a complete band gap) in the optical regime. The position of this stop band was determined by the diameter of the polystyrene spheres and the elongation ratio of the elastomeric composite. Wh...

Toward a fully integrated positive-pressure driven microfabricated liquid analyzer

Abstract: A versatile integrated analyzer with a flow-programmed injection strategy and multiwavelength detection is described with applications toward sampling, flow injection analysis, and capillary separations. Continuous near-real-time sampling is a major benefit of the flow-programmed injection technique. Injection volumes ranging from 250 pL to several microliters were made without electrophoretic flow. Multiwavelength grating light reflection spectroscopy (GLRS) and transmission absorbance spectroscopy were performed simultaneously in a detection volume of 150 pL. The utility of these detection methods for refractive index (RI) and absorbance detection in capillary channels is demonstrated through analysis of salt, indicator, and dyes. GLRS is a unique, selective, and path-length-independent technique for probing RI, absorbance, and other optical properties. A limit of detection (LOD) of 170 μM was achieved for GLRS interferometric detection of FD&C Red #3, which corresponded to 2.6 fmol of analyte in the 15...

Synthesis and Optical Properties of Silver Bicrystalline Nanowires

Abstract: This paper describes a solution-phase route to the large-scale synthesis of silver nanowires with diameters in the range of 30-40 nm, and lengths up to ~50 m. The initial step of this synthesis involved the formation of Pt nanoparticles by reducing PtCl 2 with ethylene glycol (EG) refluxed at ~160 o C. These Pt nanoparticles could serve as seeds for the growth of silver (formed by reducing AgNO 3 with EG) through heterogeneous nucleation process because their crystal structures and lattice constants matched closely. In the presence of poly(vinyl pyrrolidone) (PVP), the growth of silver could be led to a highly anisotropic mode with formation of uniform nanowires. UV-visible spectroscopy was used to track the growth process of silver nanowires because different silver nanostructures exhibited distinctive surface plasmon resonance peaks at different frequencies. SEM, TEM, XRD, and electron diffraction were used to characterize these silver nanowires, indicating the formation of a highly pure face-centered cubic phase, as well as uniform diameter and bicrystalline structure. The morphology of these silver nanostructures could be varied from particles and rods to long wires by tuning the reaction conditions, including reaction temperature, and the ratio of PVP to silver nitrate. These silver nanowires could be used as sacrificial templates to synthesize gold nanotubes via a template-engaged replacement reaction. The dispersion of gold nanotubes exhibited a strong extinction peak in the red regime, which was around 760 nm. Keywords: Silver nanowires, seeded-growth, gold nanotubes, surface plasmon resonance

Photonic bandgap crystals by self-assembly

Abstract: This paper describes the use of confined self-assembly in organizing monodispersed spherical colloids into face-center-cubic crystalline lattices for photonic crystals applications. Using this method, we were able to conveniently control the thickness, the density and structure of defects, and the orientation of a crystal. Inverse opals of polymers and ceramic materials were also synthesized by templating corresponding precursors against three-dimensional colloidal crystals. As an extension to this method, we also demonstrated the hierarchical self-assembly that involved building blocks with sizes on two different scales, and its application in forming inverse opals.

Photonic crystals fabricated from gold-silica core-shell nanoparticles

Abstract: An approach to metallic photonic crystals is demonstrated by using gold-silica core-shell colloids as the building blocks. The formation of gold-silica core-shell nanoparticles involved a base-catalyzed hydrolysis of precursor TEOS and subsequent condensation of silica onto the surfaces of gold cores. The obtained core-shell colloids were monodispersed in size and their shell thickness could be controlled in the range of a few nanometers to about 500 nm. The core diameter could also be varied from ~5 nm to ~100 nm. The core-shell colloids were then employed as building blocks to self-assemble highly ordered three-dimensional photonic crystals using a non-lithographic method. The photonic band-gap properties were characterized by taking the transmittance and reflectance spectra.