Attachment-driven morphology evolvement of rectangular ZnO nanowires

The rectangular cross-sectional ZnO nanowires were synthesized in a solution method. An attachment-driven growth mechanism was proposed for the morphology evolvement of ZnO nanocrystals from nanoparticles to nanoplates and eventually to nanowires. Due to the pileup attachment of the nanoplates to recrystallize into nanowires, unique one-dimensional (1D) ZnO nanowires with the rectangular cross section were obtained, which is different from those nanowires in the previous reports. It is the first time the evidence that "oriented attachment" can occur not only for nanoparticles but also for nanoplates was obtained, suggesting that "oriented attachment" is an intrinsic behavior for nanosized materials. According to the growth model proposed based on the direct TEM observations, ZnO nanocrystals can be easily controlled as nanoparticles, nanoplates, or nanowires by tuning the synthetic parameters.     

Wires, plates, flowers, needles, and core-shells: diverse nanostructures of gold using polyaniline templates

A simple and versatile method for the synthesis of a wide range of polyaniline (PANI)-based 1D and 2D gold nanostructures of uniform size distribution with high colloidal stability is demonstrated. All the nanostructures were synthesized from oligoaniline-coated gold nanoparticle precursors. The nanostructures include nanowires of various sizes, nanoplates, and flower-like nanoparticles. These nanowires showed a pH-dependent shape transformation. Needle-like aggregates of Au/PANI were formed as the pH of the nanowire solution changed to 2.5. At higher pH (10.2), nanowires converted into spherical nanoparticles. Core-shell particles of Au/PANI composites have been achieved by the reversal of the pH of the nanowire from 10.2 to 2.9. The morphology of the nanostructures was studied by TEM and SEM. FTIR, UV-vis, XRD, and LDI MS were utilized for the characterization of the chemical composition of the nanostructures. A mechanism for the nanowire growth is proposed.     

Thermal aqueous solution approach for the synthesis of triangular and hexagonal gold nanoplates with three different size ranges

The synthesis of gold nanoplates was carried out in an aqueous solution by thermal reduction of HAuCl(4) with trisodium citrate in the presence of cetyltrimethylammonium bromide (CTAB) surfactant in just 5-40 min. The sizes of the gold nanoplates can be varied from as small as tens of nanometers in width, to several hundreds of nanometers, and even a few microns in width by changing the reagent concentrations, solution temperature, and the reaction time. A [CTAB]/[HAuCl(4)] ratio of 6 in the reaction solution was found to be favorable for the formation of gold nanoplates. The nanoplates possess well-defined shapes with sharp edges. The small nanoplates exhibit mainly a triangular shape, while larger nanoplates show a mixture of triangular, hexagonal, truncated triangular, and other symmetrical structures. The nanoplates are composed of essentially (111) lattice planes, as revealed by both XRD and TEM results. Nanoplates with widths from several hundreds of nanometers to a few microns absorb light strongly in the near-infrared region. The growth mechanism of these nanoplates was investigated. The ability to synthesize gold nanoplates with these different size ranges in large scale in aqueous solution using simple CTAB capping surfactant should allow more diverse applications of gold nanoplates.     

有机分子CTAB对银纳米颗粒形貌的影响

报道了一种有效调节银纳米颗粒形貌的特殊方法.在不同浓度的CTAB有机分子作用下,片状三角形银纳米颗粒形貌发生改变,形成圆形和纺锤形等特殊形貌的银纳米片,研究了CTAB浓度对银纳米颗粒形貌的影响,从实验结果分析了银纳米颗粒形貌发生改变的主要因素.     

Growth mechanisms of phthalocyanine nanowires induced by Au nanoparticle templates

We combine X-ray reflectivity and scanning electron microscopy measurements to investigate the mechanisms involved in the growth of vertical arrays of phthalocyanine nanowires directed by templates of Au nanoparticles. The study has been carried out for H(16)CuPc at different substrate temperatures. It is shown that three organic morphologies evolve during the growth: 1D nanostructures on top of the Au nanoparticles, a multilayer film on the substrate and a layer wetting the gold nanoparticles. For substrate temperatures below 100 °C there is a coexisting and competing growth of the three structures, whereas beyond this temperature the 1D growth on the nanoparticles is predominantly favored. The observance of two regimes with the temperature is characterized by two different activation energies. Both the length of the 1D structures and the thickness of the multilayer film can be precisely controlled by the substrate temperature which is of importance for application of vertical organic nanowires as donor/acceptor architecture in organic solar cells.     

Shape tunable synthesis of anisotropic gold nanostructures through binary surfactant mixtures

In recent years, much effort has been made to produce gold (Au) nanorods of different sizes through the use of binary surfactant mixtures via a seed-mediated growth approach. However, how the ratio of two different surfactants influences the shape of the resulting Au nanoparticles remains to be elucidated. Here, we report the shape-controlled synthesis of Au nanoparticles using a binary surfactant mixture of CTAB (cetyltrimethylammonium bromide) and DDAB (didodecyldimethylammonium bromide) via a silver-assisted seed-mediated growth approach. Decreasing the CTAB/DDAB ratio results in a shape transition from Au nanorods to elongated tetrahexahedra and finally to Au bipyramids. The results showed significant improvement in the yield of Au bipyramidal type nanoparticles in different sizes (nm to μm) by using binary surfactant mixtures without any need for shape selection procedure. By varying the pH and concentration of ascorbic acid, we can control the shape and size of Au nanoparticles (i.e., truncated bipyramids, dogbones, and nanodumbbells) at fixed CTAB/DDAB ratios. A preliminary growth mechanism has been proposed based on the change in the mixed micelle soft-template induced by the increasing concentration of DDAB and reaction parameters (i.e., pH, concentration of ascorbic acid). These results constitute the advances in the understanding for synthesizing anisotropic Au nanoparticles of tunable optical properties via engineering the design of a soft-template. These anisotropic Au nanoparticles, especially, bipyramids of different morphologies and sizes are potential candidates for the enhancement of the optical response and developing label-free biosensing devices.     

Temperature-controlled growth of ZnO nanowires and nanoplates in the temperature range 250-300 degrees C

Starting from a mixture of Zn and BiI3, we grew nanowires and nanoplates on an oxidized Si substrate at relatively low temperatures of 250 and 300 degrees C, respectively. The ZnO nanowires had diameters of approximately 40 nm and grew along the [110] direction rather than the conventional [0001] direction. The nanoplates had thicknesses of approximately 40 nm and lateral dimensions of 3-4 microm. The growth of both the nanowires and nanoplates is dominated by the synergy of vapor-liquid-solid (VLS) and direction conducting. Analysis of photoluminescence spectra suggested that the nanoplates contain more oxygen vacancies and have higher surface-to-volume ratios than the nanowires. The present results clearly demonstrate that the shapes of ZnO nanostructures formed by using BiI3 can be controlled by varying the temperature in the range 250-300 degrees C.     

Effects of alkylated polyethylenimines on the formation of gold nanoplates

Mono- and dialkylated polyethylenimines (PEI-1R and PEI-2R) were used for the facile synthesis of gold nanoplates with a preferential growth direction along the Au (111) plane. It was found that polymer hydrophobicity greatly influenced the nanoparticle morphology. PEI-1R in the acidic aqueous solution with a smaller degree of alkylation effectively adsorbed on the surface of gold nanoplates with the protonated ethylenimine groups rather than being aggregated in the bulk aqueous phase to form polymer aggregates as compared to the situation for PEI-2R. Loose alkylated PEI aggregates in acidic solution promote the formation of gold nanoplates by means of the anion-induced cation adsorption on certain crystallographic facets during the growth of gold particles. Without incorporating alkyl groups, however, the TEM image of the gold colloid solution with PEI showed only the formation of spherical gold nanoparticles by the same process. The morphology of gold nanoparticles was tuned not only by varying the degree of alkylation of PEI samples but also by the solvent type and pH value of the solution. By utilizing differently alkylated PEIs as reducing agents, this facile synthetic procedure can selectively result in the formation of gold nanoplates at room temperature without an extra inducing process.     

Formation process of two-dimensional networked gold nanowires by citrate reduction of AuCl4- and the shape stabilization

Gold nanowires with a two-dimensional (2-D) network structure were formed by citrate reduction of AuCl4- with a low concentration of citrate. The structure change during the growth processes was observed by transmission electron microscopy (TEM) and the variation in concentrations of gold species in the aqueous solution was monitored by UV-vis spectra and Inductively Coupled Argon Plasma Emission Spectrophotometer (ICAP). The formation of 2-D gold nanowires was induced by the small amount of reducing agent because the preliminary gold nanoparticles formed by reduction of AuCl4- were thermodynamically unstable in the aqueous solution due to the insufficient capping of citrate. One of the key points of nanowire formation is the preferential adsorption of AuCl4- instead of citrate ions on the surface of the preliminary gold particles, which results in an attracting force between gold nanoparticles. We propose a hit-to-stick-to-fusion model, in which gold nanoparticles adhere by the attraction force and stick together, causing selective deposition of reduced gold metallic species on the concave surface of the two sticking particles, followed by fusion into nanowires. Nanowires then connect with each other, forming a network structure. The evidence obtained from TEM observation of transformation from gold nanowires on a TEM grid to large nanoparticles by hydrogen gas reduction and time-resolved measurements of gold ions suggest that gold ions not only are crucial for the growth of gold nanowires but also play an important role in stabilizing the shape of gold nanowires during the formation process. This method for synthesizing 2-D gold nanowires is simple and relatively easy application to the synthesis of other metallic nanowires such as silver or platinum is expected.     

Controlling the length and shape of gold nanorods

In this paper, we report a new approach to fabricate gold nanowires by controlling the volume of the growth solution. The shape evolutions ranging from fusiform nanoparticles to 1-D rods were observed. Increasing the addition of the growth solution can control the length of nanorods. The length of the rods can be extended to 2 microm, and nanorods with aspect ratios of up to approximately 70 could be obtained.     

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.     

Synthesis of flexible, ultrathin gold nanowires in organic media

Gold nanoparticles are very interesting because of their potential applications in microelectronics, optical devices, analytical detection schemes, and biomedicine. Though shape control has been achieved in several polar solvents, the capability to prepare organosols containing elongated gold nanoparticles has been very limited. In this work we report a novel, simplified method to produce long, thin gold nanowires in an organic solvent (oleylamine), which can be readily redispersed into nonpolar organic solvents. These wires have a characteristic flexible, hairy morphology arising from a small thickness (<2 nm) and an enormous length (up to several micrometers), with the possibility of adjusting the dimensions through modification of the growth conditions, in particular, the gold salt concentration. Despite their extreme aspect ratio, the wires are stable in solution for long periods of time but easily break when irradiated with high-energy electron beams during transmission electron microscopy.     

Fine-tuning of catalytic tin nanoparticles by the reverse micelle method for direct deposition of silicon nanowires by a plasma-enhanced chemical vapour technique

The reverse micelle method was used for the reduction of a tin (Sn) salt solution to produce metallic Sn nanoparticles ranging from 85 nm to 140 nm in diameter. The reverse micellar system used in this process was hexane-butanol-cetyl trimethylammonium bromide (CTAB). The diameters of the Sn nanoparticles were proportional to the concentration of the aqueous Sn salt solution. Thus, the size of the Sn nanoparticles can easily be controlled, enabling a simple, reproducible mechanism for the growth of silicon nanowires (SiNWs) using plasma-enhanced chemical vapour deposition (PECVD). Both the Sn nanoparticles and silicon nanowires were characterised using field-emission scanning electron microscopy (FE-SEM). Further characterisations of the SiNW's were made using transmission electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy. In addition, dynamic light scattering (DLS) was used to investigate particle size distributions. This procedure demonstrates an economical route for manufacturing reproducible silicon nanowires using fine-tuned Sn nanoparticles for possible solar cell applications.     

Synthesis and self-assembly of highly monodispersed quasispherical gold nanoparticles

We report the synthesis of cetyltrimethylammonium bromide (CTAB) assisted seed mediated growth of highly pure and monodispersed quasispherical gold nanoparticles (QAuNPs) and their self-assembly on the silica/glass substrates. The seed-mediated growth approach was modified to prepare size-tunable monodispersed QAuNPs with sizes ranging from 20 to 150 nm. The larger, more uniform seeds and lower CTAB concentration resulted in the formation of relatively large QAuNPs with improved monodispersity (relative standard deviation (RSD) of ～5-8%) and high purity in their shapes. In addition, CATB-capped QAuNPs can be spontaneously assembled into closely packed and highly aligned superstructures with well-defined mutillayers (two to six layers) on silica substrates. Furthermore, CATB-capped QAuNPs can easily construct density-controllable QAuNP chips by electrostatic self-assembly, showing their promising applications for single-nanoparticle plasmonic sensors.     

Density-controlled growth of aligned ZnO nanowires sharing a common contact: a simple, low-cost, and mask-free technique for large-scale applications

An effective, low cost, simple, and mask-free pathway is demonstrated for achieving density control of the aligned ZnO nanowires grown for large-scale applications. By a slight variation of the thickness of the thermally evaporated gold catalyst film, a significant change in the density of aligned ZnO nanowires has been controlled. The growth processes of the nanowires on an Al(0.5)Ga(0.5)N substrate has been studied based on the wetting behavior of gold catalyst with or without source vapor, and the results classify the growth processes into three categories: separated dots initiated growth, continuous layer initiated growth, and scattered particle initiated growth. This study presents an approach for growing aligned nanowire arrays on a ceramic substrate with the simultaneous formation of a continuous conducting electrode at the roots, which is important for device applications, such as field emission.     

Fabrication and Formation Mechanism of Ag Nanoplate‐Decorated Nanofiber Mats and Their Application in SERS

We report a new simple method to fabricate a highly active SERS substrate consisting of poly‐m‐phenylenediamine/polyacrylonitrile (PmPD/PAN) decorated with Ag nanoplates. The formation mechanism of Ag nanoplates is investigated. The synthetic process of the Ag nanoplate‐decorated PmPD/PAN (Ag nanoplates@PmPD/PAN) nanofiber mats consists of the assembly of Ag nanoparticles on the surface of PmPD/PAN nanofibers as crystal nuclei followed by in situ growth of Ag nanoparticles exclusively into nanoplates. Both the reducibility of the polymer and the concentration of AgNO₃ are found to play important roles in the formation and the density of Ag nanoplates. The optimized Ag nanoplates@PmPD/PAN nanofiber mats exhibit excellent activity and reproducibility in surface‐enhanced Raman scattering (SERS) detection of 4‐mercaptobenzoic acid (4‐MBA) with a detection limit of 10^?10 m , making the Ag nanoplates@PmPD/PAN nanofiber mats a promising substrate for SERS detection of chemical molecules. In addition, this work also provides a design and fabrication process for a 3D SERS substrate made of a reducible polymer with noble metals.     

Block copolymer-mediated synthesis of size-tunable gold nanospheres and nanoplates

We have successfully controlled the size and shape of isotropic and anisotropic gold nanocrystals through a one-step reaction by using amphiphilic polyethylene oxide-polystyrene oxide block copolymers as both reductant and stabilizing agents in water solution. Spherical or quasispherical nanoparticles were obtained at room temperature with tunable mean sizes and polydispersities depending on reaction conditions, that is, on copolymer block length, and copolymer and gold salt concentrations. By moderate increases of reaction temperature up to 65 degrees C, progressive formation of single-crystalline gold nanoplates in good yields takes place (up to 70%) without the necessity of additional reactants or growing solutions. These nanoplates are characterized by lateral mean sizes between 0.1-1.2 microm depending on copolymer concentration and reaction temperature, with mainly truncated or rounded triangular shapes with {111} planes as two basal surfaces. This allows us to tune the surface plasmon band of the nanoplates from ca. 850 nm to more than 1100 nm, well inside the near-infrared region (NIR), which enables the use of these type of nanostructures as a very promsing materials in applications such as optical coatings, SERS, and cancer cell hyperthermia. We proposed that the growth of these nanostructures can stem from a decrease in the reaction rate as temperature increases due to an enhanced copolymer hydrophobicity, which gives rise to a structure of interacting micelles formed from the fluid via a percolation transition (known as "soft gel") at elevated temperatures. In this way, reduction becomes slow enough to allow kinetic control of the reaction, and preferential adsorption of the copolymer molecules/micelles on certain crystallographic planes can favor the growth of certain nanocrystal facets to give the final structure. This alternative water-based system provides a more convenient and environmentally benign route to the synthesis of shape-controlled noble-metal nanocrystals in high yield because it does not involve toxic organic solvents or reagents and serves as a bridge between two frontline discipline: the block copolymeric science and anisotropic nanoparticles.     

硫化锌与硫化锌/氧化锌异质结纳米线的化学气相沉积法制备与表征

介绍一种利用石墨还原快速制备大量硫化锌纳米线的方法,并分别合成了超晶格型、双轴型、核/壳型的硫化锌/氧化锌异质结纳米线。所合成的硫化锌纳米线存在六方纤锌矿和立方闪锌矿两种晶型,纳米线长度达几十微米,直径在20-50 nm,直径均匀且产量很高。在具有双轴型的硫化锌/氧化锌异质结中,首次发现具有超结构特征的氧化锌。HRTEM分析表明,硫化锌/氧化锌超晶格异质结界面为ZB-ZnS(111)∥ZnO(0001),而核/壳型异质结界面为W-ZnS(0001)∥ZnO(0001),这三个晶面分别为各自晶体的极性面,即所合成的硫化锌/氧化锌异质结中极性面相互平行。对ZnS 和ZnS/ZnO 异质结的生长机制进行了探讨,并对硫化锌纳米线与硫化锌/氧化锌异质结的光学性质进行了分析。     

Synthesis and optical properties of triangular gold nanoplates with controllable edge length

Gold triangular nanoplates with a uniform size were synthesized by a simple seeds-growth method in a lamellar lyotropic liquid crystal (LLC) medium consisting of F127, n-C₄H₉OH and H₂O. The edge length of gold nanoplates can be adjusted from tens to several hundreds nanometers (and even a few micrometers) by varying the concentration of Au³⁺ and the seeds solution volume. The optical properties of the synthesized gold nanoplates were studied. The vis-NIR spectra of the synthesized gold nanoplates exhibited a good linear correlation between the in-plane plasmon resonance λ_max and the average edge length of the corresponding nanoplates.

     

Influence of biomass and gold salt concentration on nanoparticle synthesis by the tropical marine yeast Yarrowia lipolytica NCIM 3589

Cell-associated gold nanoparticles and nanoplates were produced when varying number of Yarrowia lipolytica cells were incubated with different concentrations of chloroauric acid (HAuCl₄) at pH 4.5. With 10⁹ cells ml⁻¹ and 0.5 or 1.0 mM of the gold salt, the reaction mixtures developed a purple or golden red colour, respectively, and gold nanoparticles were synthesized. Nanoparticles of varying sizes were produced when 10¹⁰ cells ml⁻¹ were incubated with 0.5, 1.0 or 2.0 mM chloroauric acid salt. With 3.0, 4.0 or 5.0 mM HAuCl₄, nanoplates were also observed. With 10¹¹ cells ml⁻¹ nanoparticles were synthesized with almost all the gold salt concentrations. The cell-associated particles were released outside when nanoparticle-loaded cells were incubated at low temperature (20 °C) for 48 h. With increasing salt concentrations and a fixed number of cells, the size of the nanoparticles progressively increased. On the other hand, with increasing cell numbers and a constant gold salt concentration, the size of nanoparticles decreased. These results indicate that by varying the number of cells and the gold salt concentration, a variety of nanoparticles and nanoplates can be synthesized. Fourier transform infrared (FTIR) spectroscopy revealed the possible involvement of carboxyl, hydroxyl and amide groups on the cell surfaces in nanoparticle synthesis.