Electrochemically Controlled Growth of AuPt Alloy Nanowires and Nanodendrites

The ability to control the morphology and phase structure of alloy nanowires is essential for the exploitation of their unique functional properties. This report describes the findings of an investigation of the growth mechanism in the electrochemically controlled growth of Au? Pt alloy nanostructures. By using a template‐free alternating‐current deposition method with different combinations of waveform, voltage, and frequency, controllability over the alloy morphology, composition, and phase structure has been clearly demonstrated for the growth of the nanostructures across the gap of two microelectrodes. The growth is proposed to involve an initial facet‐selective nucleation–growth process followed by two competing nucleation–growth pathways that are highly tunable by the applied frequency and voltage. The findings provided new insights into the mechanism that underlies the controlled fabrication of alloy nanowires and nanodendrites with structurally tailorable functional properties.     

Transformation of a close-packed Au nanoparticle/polymer monolayer into a large area array of oriented Au nanowires via E-beam promoted uniaxial deformation and room temperature sintering

Transformation of 2D Au nanoparticle (NP) arrays into large scale, ordered, and oriented nanorod/nanowire arrays supported on a transferrable polymer film has been accomplished. E-beam irradiation followed by room temperature aging of a suspended Au NP/polymethylmethacrylate (PMMA) polymer close packed monolayer results in one-dimensional nanoparticle aggregation, reorientation, and sintering into a high density array of oriented Au nanowires with coherent single-crystal-like interfaces. Molecular dynamics simulations of alkane-thiol capped Au NPs, interacting through the Vincent potential and undergoing 2D Poisson compression, account semiquantitatively for the qualitative features of the transformation. This fabrication approach should be extendable to directing 1D aggregation of highly anisotropic nanostructures in arbitrary NP systems.     

Controllable assembly of WO3 nanorods/nanowires into hierarchical nanostructures

The controlled synthesis of two novel h-WO3 hierarchical structures made of nanorods/nanowires has been successfully realized in a large scale via a simple hydrothermal method. It is demonstrated that the morphology of the final products is significantly influenced by adding different sulfates. The urchinlike and ribbonlike structures of WO3 can be selectively prepared by adding Rb2SO4 and K2SO4, respectively. The morphology evolvement and the growth mechanism were studied carefully. The sulfate-induced oriented attachment growth mechanism has been proposed for the possible formation mechanism of the ribbonlike sample. For urchinlike products, two growing stages are believed to be involved in the growth process. The current understanding of the growth mechanism of these nanostructures may be potentially applied for designing other oriented or hierarchical nanostructures based on 1D nanoscale building blocks through the direct solution-growth.     

Synthesis of sub-20-nm-sized bismuth 1-D structures using gallium-bismuth systems

Bismuth (Bi) nanowires are interesting one-dimensional systems due to the significant quantum confinement effects exhibited as a function of the wire diameters, and synthesizing Bi nanowires with sizes below 20 nm is of fundamental importance in understanding quantum effects. Here, we report a bulk synthesis method to synthesize ultrafine Bi nanowires and a new morphology of bismuth nanostructures, tapered whiskers. These tapered whiskers are about 10-20 mum in length and have diameters of 5-10 nm at the tip and 250-500 nm at the base. The synthesis method is based upon the multiple nucleation and basal growth of nanometer scale nuclei from molten gallium (Ga) melts that result from the low solubility of Bi in Ga and the low eutectic temperature of the Ga-Bi binary system. Adopting different methods of supplying bismuth and using variations in simple heating and cooling, we have synthesized a variety of bismuth nanostructures.     

Growth mechanism of penniform BaWO4 nanostructures in catanionic reverse micelles involving polymers

The formation of penniform BaWO4 nanostructures made of nanowires or nanobelts under the direction of a block copolymer in catanionic reverse micelles has been studied in detail. On the basis of the experimental results obtained from the BaWO4 crystallization in aqueous polymer solutions and careful transmission electron microscopy (TEM) observations of BaWO4 nanostructures formed in reverse micelles containing polymers, a detailed two-stage growth mechanism has been proposed for the formation of the penniform nanostructures in reverse micelles, which involves the polymer-controlled shaft formation (Stage 1) and the mixed surfactants-controlled barb growth (Stage 2). During Stage 1, poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) induced the formation of c-axis-oriented shuttle-like nanocrystals and the subsequent oriented attachment of these shuttle-like nanocrystals resulted in the formation of [100]-oriented shafts with many parallel [001]-oriented pricks. During Stage 2, [001]-oriented nanowires or nanobelts grew gradually from the pricks into barbs, leading to the formation of well-defined penniform BaWO4 nanostructures with the barb morphology essentially determined by the mixing ratio r of the anionic to cationic surfactants (i.e., nanowires were formed at r=1 while nanobelts were formed at r deviating from 1). The current understanding of the growth mechanism of penniform BaWO4 nanostructures in catanionic reverse micelles involving polymers may be potentially applied for designing a new synthesis system for the controlled synthesis of other hierarchical 1D nanostructures with desired architectures.     

Length‐Dependent Charge Generation from Vertical Arrays of High‐Aspect‐Ratio ZnO Nanowires

Aqueous chemical growth of zinc oxide nanowires is a flexible and effective approach to obtain dense arrays of vertically oriented nanostructures with high aspect ratio. Herein we present a systematic study of the different synthesis parameters that influence the ZnO seed layer and thus the resulting morphological features of the free‐standing vertically oriented ZnO nanowires. We obtained a homogeneous coverage of transparent conductive substrates with high‐aspect‐ratio nanowire arrays (length/diameter ratio of up to 52). Such nanostructured vertical arrays were examined to assess their electric and piezoelectric properties, and showed an electric charge generation upon mechanical compressive stress. The principle of energy harvesting with these nanostructured ZnO arrays was demonstrated by connecting them to an electronic charge amplifier and storing the generated charge in a series of capacitors. We found that the generated charge and the electrical behavior of the ZnO nanowires are strictly dependent on the nanowire length. We have shown the importance of controlling the morphological properties of such ZnO nanostructures for optimizing a nanogenerator device.     

Diketopyrrolopyrrole‐based polymer nanowires: Control of chain conformation and nucleation

Understanding the nucleation process is very important in growing polymer nanowires as it plays a decisive role in determining the crystal structure and size distribution. Polymer chain conformation determines whether the polymer chains could assemble to nuclei or not. Here, chain conformation and the nucleation process were controlled to grow 3,6‐bis‐(thiophen‐2‐yl)‐N,N′‐bis(2‐decyl‐1‐tetradecyl)‐1,4‐dioxo‐pyrrolo[3,4‐c]pyrrole and thieno[3,2‐b]thiophene (DT‐PDBT‐TT) nanowires. We changed the conformation of DT‐PDBT‐TT in solution and controlled the nucleation process by using a main solvent:cosolvent system. The main solvent was a low boiling point good solvent, and the cosolvent was consisted of two high boiling point solvents with different solubility. In fact, the chain conformation in the pristine solution was changed by choosing different main solvents (with H‐bond, π–π or none interaction with the main chain) and temperatures. The absorption spectrum and TEM images showed that trichloro ethylene (TCE) was the best main solvent because it has H‐bond interaction with the polymer main chain and DT‐PDBT‐TT conformation in it approaching unimer coil conformation, which is beneficial to grow nanowires. Mixed o‐dichlorobenzene (ODCB) and anisole (AS) with different ratios were used to changing the solubility step by step to control nucleation process. Only when marginal cosolvent (ODCB:AS = 1:1) was added, could it decrease the nuclei number and avoided the aggregates simultaneously. As the main solvent evaporated slowly, the nucleation and growth happened, leading to the nanowires formation. The resulting nanowires were about 63 nm in width and one to two microns in length. The width of the DT‐PDBT‐TT structures suggests that the polymer chains are oriented along the fibril axis. Our results indicated that there are two requirements for the nanowire formation, (1) the polymer chain conformation should approach unimer coil; (2) the nucleation density should be optimized, not too much and no aggregation happened. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 833–841     

Enhanced nucleation, growth rate, and dopant incorporation in ZnO nanowires

Pure and Co-doped ZnO nanowire arrays were grown on polished silicon substrates with high rates via an electrochemical technique. A negative potential applied to the substrate not only enhances the nucleation density on polished substrates more than 4 orders of magnitude but also increases the growth rate by 35 times over that obtained in the absence of the potential. Furthermore, incorporation of metallic dopants in ZnO nanowires was demonstrated in the low-temperature process. This fast growth technique provides a route to fabrication of low-cost highly oriented ZnO nanowires on polished substrate for industrial applications.     

氧化锌纳米线自组装定向生长动力学研究

研究了以极性高分子(如聚丙烯酰胺)长分子链作为自组装网络, 利用高分子软模板控制ZnO纳米点成核和ZnO纳米线定向生长, 从而使ZnO纳米线在半导体硅衬底上自组装生长的过程; 采用差示扫描量热法(DSC)测试了高分子络合-烧结法制备ZnO纳米线的结晶曲线, 对其结晶动力学进行了研究, 推导出结晶动力学方程为: 1－X_t＝exp(－7.475×10^－2t^1.9); 并利用热重(TG)测试结果, 通过热分解反应, 导出了反应动力学方程: dα/dT＝(3.76×10²³/Φ)e^－21340.8/T(1－α) ^2.8, 从而得到了化学反应速度随时间、浓度和温度变化的关系, 并用结果解释了实验现象.     

Conducting Polyaniline Nanowire Arrays Modified Electrode for High Performance Supercapacitor and Enhanced Catalysis of Nitrite Reduction

Vertically aligned conducting polymer nanowire arrays had great potential applications in supercapacitor electrode material and exhibited enhanced electrocatalytic behavior towards the reduction of nitrite. In this paper, a facial template‐free approach to synthesize large arrays of vertically aligned polyaniline (PANI) nanowires on electrochemically pretreated glassy carbon electrodes was reported by using a galvanostatic current method. The as‐prepared large arrays of PANI nanowires had very narrow diameters and were oriented perpendicular to the substrate, which was a benefit to the ion diffusion when being used as the supercapacitor electrode. The highest specific capacitance of PANI nanowire arrays was measured and kept high at a large charge‐discharge current density. Furthermore, it also can detect nitrite with ultrahigh sensitivity of 62.99 µA mM^?1 cm^?2 and a remarkable fast response time of less 1 s. The results indicated that the vertically aligned PANI nanowires could dramatically enhance the electrochemical performance.     

Selected-control synthesis of ZnO nanowires and nanorods via a PEG-assisted route

Long-chain polymer-assisted growth of one-dimensional (1D) nanostructures has been investigated in previous research. This kind mild method has lots of merits such as not requiring complex procedures, without template supporting etc. Can the short-chain polymer also be used to grow long nanowires? In the present work, a short-chain polymer (PEG400) was found to promote the formation of 1D ZnO nanostructures, which cannot be obtained by long-chain polymers (such as PEG10000). Moreover, nanowires and nanorods can be selectively synthesized by using short-chain polymers. The influence factors for the formation of 1D ZnO nanostructures were also investigated in detail. The XRD, Raman spectrum, XPS, SEM, TEM, ED, HRTEM, EDXA, and PL spectra have been provided for the characterization of the as-obtained nanowires and nanorods.     

MnO@Carbon Core–Shell Nanowires as Stable High‐Performance Anodes for Lithium‐Ion Batteries

A facile method is presented for the large‐scale preparation of rationally designed mesocrystalline MnO@carbon core–shell nanowires with a jointed appearance. The nanostructures have a unique arrangement of internally encapsulated highly oriented and interconnected MnO nanorods and graphitized carbon layers forming an external coating. Based on a comparison and analysis of the crystal structures of MnOOH, Mn₂O₃, and MnO@C, we propose a sequential topotactic transformation of the corresponding precursors to the products. Very interestingly, the individual mesoporous single‐crystalline MnO nanorods are strongly interconnected and maintain the same crystallographic orientation, which is a typical feature of mesocrystals. When tested for their applicability to Li‐ion batteries (LIB), the MnO@carbon core–shell nanowires showed excellent capacity retention, superior cycling performance, and high rate capability. Specifically, the MnO@carbon core–shell nanostructures could deliver reversible capacities as high as 801 mA h g^?1 at a high current density of 500 mA g^?1, with excellent electrochemical stability after testing over 200 cycles, indicating their potential application in LIBs. The remarkable electrochemical performance can mainly be attributed to the highly uniform carbon layer around the MnO nanowires, which is not only effective in buffering the structural strain and volume variations of anodes during repeated electrochemical reactions, but also greatly enhances the conductivity of the electrode material. Our results confirm the feasibility of using these rationally designed composite materials for practical applications. The present strategy is simple but very effective, and appears to be sufficiently versatile to be extended to other high‐capacity electrode materials with large volume variations and low electrical conductivities.     

Oriented attachment-based assembly of dendritic silver nanostructures at room temperature

How particles aggregate into an interesting dendritic structure has been the object of research for many years because of its importance in understanding physical processes involved and in designing novel materials. In this work, we for the first time describe an oriented attachment-based assembly mechanism for formation of different types of dendritic silver nanostructures at room temperature. It is found that the concentration of both AgNO(3) and p-aminoazobenzene (PA) molecules has a significant effect on the formation and growth of these novel nanostructures. Characterization by transmission electron microscopy (TEM) clearly shows that the dendritic silver nanostructures can be obtained through the preferential oriented growth along a crystallographically special direction. Interestingly, we observe that the oriented attachment at room temperature can also take place between relatively large single-crystalline silver particles with a diameter range from 20 to 60 nm, which may provide a new possibility for the design of novel metal nanostructures by using large metal nanoparticles as building blocks at room temperature. Moreover, a surface-enhanced Raman scattering (SERS) technique is used to investigate the role of PA molecules during the growth of the dendritic silver nanostructures.     

Defect-pit-assisted growth of GaN nanostructures: nanowires, nanorods and nanobelts

A new method using defect-pit-assisted growth technology to successfully synthesize the high-quality single crystalline GaN nanostructures by ammoniating Ga(2)O(3) films was proposed in this paper. During the ammoniating process, the amorphous middle buffer layer may unavoidably produce some defects and dislocations. Some defect pits come out, which have the lowest surface energy and can subsequently be used as a mask/template or act as potential nucleation sites to fabricate the GaN actinomorphic nanostructures. The as-prepared products are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results indicate that all the reflections of the samples can be indexed to the hexagonal GaN phase and the clear lattice fringes in HRTEM further confirm the growth of high-quality single-crystal GaN nanostructures. The SEM images show that the nanostructures have been realized under different experimental conditions exhibiting different shapes: nanowires, nanorods, and nanobelts. No particles or other nanostructures are found in the SEM study, demonstrating that the product possesses pure nanostructures. These nanostructures show a very good emission peak at 366 nm, which will have a good advantage for applications in laser devices using one-dimensional structures. Finally, the growth mechanism is also briefly discussed.     

Confinement Effects on the Crystallization of Poly(ethylene oxide) Nanotubes

In this work, we show the effects of nanoconfinement on the crystallization of poly(ethylene oxide) (PEO) nanotubes embedded in anodized aluminum oxide (AAO) templates. The morphological characteristics of the hollow 1D PEO nanostructures were evaluated by scanning electron microscopy (SEM). The crystallization of the PEO nanostructures and bulk was studied with differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The crystallization of PEO nanotubes studied by DSC is strongly influenced by the confinement showing a strong reduction in the crystallization temperature of the polymer. X-ray diffraction (XRD) experiments confirmed the isothermal crystallization results obtained by DSC, and studies carried out at low temperatures showed the absence of crystallites oriented with the extended chains perpendicular to the pore wall within the PEO nanotubes, which has been shown to be the typical crystal orientation for one-dimensional polymer nanostructures. In contrast, only planes oriented 33, 45, and 90° with respect to the plane (120) are arranged parallel to the pore's main axis, indicating preferential crystal growth in the direction of the radial component. Calculations based on classical nucleation theory suggest that heterogeneous nucleation prevails in the bulk PEO whereas for the PEO nanotubes a surface nucleation mechanism is more consistent with the obtained results.     

Carbon nanotubes with small and tunable diameters from poly(ferrocenylsilane)-block-polysiloxane diblock copolymers

Iron-containing nanostructures produced from various self-assembled poly(ferrocenylsilane)-block-polysiloxane thin films are catalytically active for the initiation and growth of high density, small diameter carbon nanotubes (CNTs). Moreover, the tube diameter and density can be tuned by adjusting the chain lengths of the block copolymer. Iron-containing nanostructures from poly(ferrocenylmethylethylsilane)-b-poly(methylvinylsiloxane) polymer with 25 repeat units of an iron-containing segment and 265 repeat units of a non-iron-containing segment are able to produce CNTs with diameters around or less than 1 nm. Lithographically selective growth of CNTs across a large surface area has been demonstrated using this polymer system. Under the same growth condition, it has been found that the yield of defect-free CNTs varies with the size of the catalytically active nanostructures, which are dictated by the chain lengths of the two blocks. This result indicates that, for a specific-sized catalyst nanocluster, a unique set of growth conditions is required for synthesizing high yield, defect-free CNTs. This finding further addresses the importance of using uniform-sized catalyst-containing nanostructures for consistently achieving high-yield and high-quality CNTs with a minimum number of defects and amount of amorphous carbon.     

Rationalization of nanowire synthesis using low-melting point metals

In this paper, we provide a theoretical basis using thermodynamic stability analysis for explaining the spontaneous nucleation and growth of a high density of 1-D structures of a variety of materials from low-melting metals such as Ga, In, or Sn. The thermodynamic stability analysis provides a theoretical estimate of the extent of supersaturation of solute species in molten metal solvent. Using the extent of maximum supersaturation, the size and density of critical nucleus were estimated and compared with experimental results using nucleation and growth of Ge nanowires using Ga droplets. The consistency of the proposed model is validated with the size and density of the resulting nanowires as a function of the synthesis temperature and droplet size. Both the experimental evidence and the theoretical model predictions point that the diameters of the resulting nanowires decrease with the lowering of synthesis temperatures and that the nucleation density decreases with the size of metal droplet diameter and increasing synthesis temperature.     

Direct synthesis of gallium oxide tubes,nanowires, and nanopaintbrushes

We demonstrate bulk synthesis of highly crystalline beta-gallium oxide tubes, nanowires, and nanopaintbrushes using molten gallium and microwave plasma containing a mixture of monatomic oxygen and hydrogen. Gallium oxide nanowires were 20-100 nm thick and tens to hundreds of micrometers long. Transmission electron microscopy (TEM) revealed the nanowires to be highly crystalline and devoid of any structural defects. Results showed that multiple nucleation and growth of gallium oxide nanostructures could easily occur directly out of molten gallium exposed to an appropriate composition of hydrogen and oxygen in the gas phase. These gallium oxide nanostructures should be of particular interest for optoelectronic devices and catalytic applications.     

Synthesis and growth mechanisms of one-dimensional strontium hydroxyapatite nanostructures

Strontium hydroxyapatite (SrHAp) nanowires with an aspect ratio of several hundreds were synthesized by controlling the growth conditions during a hydrothermal process. In the strontium phosphate system, it was found that the phase evolution changed with pH and that the aspect ratio of SrHAp was affected by the phases present before heating. Since the conditions for SrHAp nucleation prohibits one-dimensional growth, it was impossible to grow large-scale SrHAp nanowires using routine hydrothermal methods. Through thermodynamic considerations, the mechanisms of nanowire formation appear to involve the rapid release of the stored chemical potential in a metastable phase, which promotes the anisotropic growth of the most stable SrHAp nanostructures. Thereby, the conditions for both the nucleation of the SrHAp phase and the anisotropic growth were determined simultaneously, and considerable quantities of SrHAp nanowires were synthesized.     

Topochemical synthesis of cobalt oxide-based porous nanostructures for high-performance lithium-ion batteries

Two kinds of topochemical conversion routes from cobalt hydroxide precursors to cobalt oxide-based porous nanostructures are presented: pyrolysis in air and hydrothermal treatment by the Kirkendall diffusion effect. These cobalt hydroxide precursors were synthesized by a simple hydrothermal approach with sodium acetate as mineralizer at 200 °C. Detailed proof indicates that the process of cobalt hydroxide precursor growth is dominated by a nucleation, dissolution, renucleation, growth, and exfoliation mechanism. By the topochemical conversion processes several Co(3)O(4) nanostructures, such as cobalt oxide-coated cobalt hydroxide carbonate nanowires, cobalt oxide nanotubes, hollow cobalt oxide spheres, and porous cobalt oxide nanowires, have been synthesized. The obtained Co(3)O(4) nanostructures have also been evaluated as the anode materials in lithium-ion batteries. It was found that the as-prepared Co(3)O(4) nanostructures exhibited high reversible capacity and good cycle performance due to their porous structure and small size.