Novel ascorbic acid based ionic liquids for the in situ synthesis of quasi-spherical and anisotropic gold nanostructures in aqueous medium

A series of newly designed ascorbic acid based room temperature ionic liquids were successfully used to prepare quasi-spherical and anisotropic gold nanostructures in an aqueous medium at ambient temperature. The synthesis of these room temperature ionic liquids involves, first, the preparation of a 1-alkyl (such as methyl, ethyl, butyl, hexyl, octyl, and decyl) derivative of 3-methylimidazolium hydroxide followed by the neutralization of the derivatised product with ascorbic acid. These ionic liquids show significantly better thermal stability and their glass transition temperature (Tg) decreases with increasing alkyl chain length. The ascorbate counter anion of these ionic liquids acts as a reducing agent for HAuCl4 to produce metallic gold and the alkylated imidazolium counter cation acts as a capping/shape-directing agent. It has been found that the nature of the ionic liquids and the mole ratio of ionic liquid to HAuCl4 has a significant effect on the morphology of the formed gold nanostructures. If an equimolar mixture of ionic liquid and HAuCl4 is used, predominantly anisotropic gold nanostructures are formed and by varying the alkyl chain length attached to imidazolium cation of the ionic liquids, various particle morphologies can formed, such as quasispherical, raspberry-like, flakes or dendritic. A probable formation mechanism for such anisotropic gold nanostructures has been proposed, which is based on the results of some control experiments.     

One-step controlled synthesis of anisotropic gold nanostructures with aniline as the reductant in aqueous solution

We report a general and versatile method for controlled synthesis of anisotropic gold nanostructures through the reduction of HAuCl4 by aniline in aqueous solution, without the need for an additional stabilizer or capping agent. In this approach, the reduction kinetics of AuCl-4 can be altered by simply adjusting the initial pH and temperature, inducing the formation of a wide variety of anisotropic nanostructures such as dispersed or multilayered plates, wires with networked or paramecium-like structures, and ginger-shaped particles. AFM, TEM, XRD, EDX, FTIR, and UV-vis-NIR measurements were used to characterize the resulting gold nanostructures. Investigation reveals that in situ formed polyaniline serves effectively as a capping agent to direct the shape of gold nanostructures during the slow growth process. These as-synthesized gold nanostructures exhibit strongly shape-dependent optical properties. This facile approach may be extended to the synthesis of some other anisotropic metal nanostructures such as platinum or palladium.     

单分散纳米金尺寸的分步晶种生长控制

以聚乙烯吡咯烷酮(poly(vinylpyrrolidone), PVP)为保护剂, 硼氢化钠为还原剂, 合成了尺寸为(1.9±0.4) nm的单分散金胶体, 再以其作为一级晶种, 并分别用抗坏血酸和PVP为还原剂和保护剂, 通过改变各步晶种尺寸和氯金酸与晶种的摩尔比分步逐级合成了尺寸为3.2、4.7、6.3、8.0、10.3、14.0 nm的系列金纳米颗粒. 以LaMer模型为基础, 对分步晶种生长过程中影响金胶体产物尺寸分布(单分散性)的主要因素进行了讨论. 缓慢加入抗坏血酸并降低氯金酸对晶种的相对量对于单分散金纳米颗粒的控制合成有决定性作用. 快速加入抗坏血酸会因二次成核而导致金颗粒尺寸分布范围变宽.     

边长为微米级的银纳米片的简易合成与形成机理

用低温(60 ℃)溶剂热法, 以N,N-二甲基甲酰胺(DMF)为主还原剂和溶剂, 以聚乙烯吡咯烷酮(PVP)为辅助还原剂和晶面生长控制剂, 以硝酸银为前驱物, 大量制备了高纯度的、边长为微米级、宽厚比≥10的单晶银纳米片. 采用粉末X射线衍射(PXRD)、场发射扫描电镜(FE-SEM)、透射电子显微镜(TEM)等表征和分析了合成产物的成分、形貌和结构. 结果表明, 合成的银纳米片为面心立方单晶, 边长为1-4 μm, 厚度为50-100 nm. 考察了不同溶剂对银纳米结构的影响, 并提出了大尺寸的银纳米片的形成机理. 本文为调控单晶银纳米片的边长和宽厚比提供了一种新的可靠的动力学方法, 制备的长边长、大宽厚比的单晶银纳米片在聚合物基导电复合材料和电磁屏蔽材料方面有潜在的重要应用.     

Room Temperature Synthesis of Gold Nanokites in Polyvinyl Alcohol‐Sodium Dodecyl Sulfate Aggregations Aqueous Solution

Controlled synthesis of asymmetrical gold nanokites was realized by reducing HAuCl₄ with polyvinyl alcohol (PVA) in PVA‐sodium dodecyl sulfate (SDS) aggregations aqueous solution at room temperature without any additional reducing agents. The crystal structures, optical property and growth process of the gold nanokites were investigated by X‐ray diffraction (XRD), Vis‐NIR spectrum and transmission electron microscope (TEM). HAuCl₄ concentration dramatically influenced the morphologies of the products. When HAuCl₄ concentration was increased from 0.5 mmol·L⁻¹ to 2.2 mmol·L⁻¹, the products changed from network‐like nanostructures to nanokites, nanoleaves and microplates. SDS was indispensable for the formation of gold nanokites and without SDS super‐branched structures became the dominant products.     

紫外光辐照法制备金纳米盘及其反应机理

用紫外光辐照氯金酸、聚乙烯吡咯烷酮(PVP)和纳米金种子的混合溶液, 在室温下用30 min制备出尺度小于100 nm的截角三角形或六边形金纳米盘. X射线能谱和衍射分析表明粒子是以{111}面为盘状面的高纯面心立方金单晶, 红外透射光谱表明金粒子与PVP之间存在作用. 产物的可见吸收光谱表现出纳米盘的各向异性表面等离子体共振吸收峰. 不同实验条件下产物的吸收光谱分析表明: PVP起还原剂和包覆剂的作用; 高强度紫外光加速了反应进行; 种子对反应具有催化作用; 种子的加入量有最佳值, 在该值下纳米盘平均尺度最大(达80 nm), 吸收谱上的面内偶极共振峰位于950 nm处; 种子的加入量超过该值时, 纳米盘尺度变小, 面内偶极共振峰发生蓝移.     

Fabrication of gold nano- and microstructures in ionic liquids--a remarkable anion effect

Gold nano- and microstructures such as polyhedral crystals, large single-crystalline nanoplates, hollow trapeziform crystals, holey polyhedra, and dendrites were produced via microwave heating of HAuCl(4).4H(2)O in a variety of ionic liquids (ILs) in the absence of capping agents (polymers or surfactants) or additional reducing agents. The influence of the IL anions and cations on the topology (size, shape, etc.) of gold materials was studied in detail. The anions of the ILs control the topology of materials, whereas the cations used in the experiments exert less influence. It was also found that the HAuCl(4) concentration, reaction temperature, and heating method are key parameters that help to control the topological structures of the gold materials. For example, the thickness of the large single-crystalline nanoplates could be adjusted from 16 to 320 nm by varying the HAuCl(4) concentration and reaction temperature. This easy synthetic approach to gold nano- and microstructures is a seedless, one-step, fast, template-free route that shows good reproducibility and may be further developed to produce other types of metal nanostructures that satisfy specific applications.     

Formation of gold and silver nanostructures within polyvinylpyrollidone (PVP) gel

Study on reduction of Au(III) and Ag(I) and the formation of Au and Ag nanostructures was performed on the gels of metal precursor and PVP polymer mixture. Some comparing samples were prepared for better understanding the role of reactants on the reduction of metal ions and further growth of nanocrystals. The results suggest that, in addition to its function of generating stable colloids, PVP not only has a reducing effect on metal ions, but also acts as a crystal growth modifier. At low temperatures, the reducing effect of PVP is strong on Ag(I) ions in AgNO₃, while the reduction of complex Au(III) ions in HAuCl₄ is slow, involving two steps of Au(III)→Au(I)→Au. In the study of temperature disturbance on crystal growth, Au nanoplates of new and well-defined star shape were observed. The differences in the size and shape of nanoparticles are discussed from the colloid chemistry.     

Single-crystalline gold microplates: synthesis, characterization, and thermal stability

Single-crystalline gold microplates of several 10 microm in lateral size, characterized by hexagonal, truncated triangular, and triangular shapes with (111) planes as two basal surfaces, have been synthesized in large quantities through a solution phase process. Significantly, such anisotropic Au nanostructures exhibit remarkable optical properties, in which the dipole plasmon resonance shifting in the NIR region and the quadrupole plasmon resonance at approximately 820 nm were observed. Fragmentation of Au microplates is found when the temperature is higher than 450 degrees C, indicating they are not thermodynamically stable structure at high temperature. Investigations on the Au microplates upon heating suggest that the melting and collapsing start mainly at the edges that should be Au (110) facets. This work is valuable for Au nanostructures applied at elevated temperatures.     

In situ synthesis and characterization of multiwalled carbon nanotube/Au nanoparticle composite materials

An effective and facile in situ reduction approach for the fabrication of carbon nanotube-supported Au nanoparticle (CNT/Au NP) composite nanomaterials is demonstrated in this article. Linear polyethyleneimine (PEI) is ingeniously used as both a functionalizing agent for the multiwalled carbon nanotubes (MWNTs) and a reducing agent for the formation of Au NPs. This method involves a simple mixing process followed by a mild heating process. This approach does not need the exhaustive surface oxidation process of CNTs. The coverage of Au NPs on CNTs is tunable by varying the experimental parameters, such as the initial molar ratio of PEI to HAuCl4, the relative concentration of PEI and HAuCl4 to MWNTs, and the temperature and duration of the heat treatment. More importantly, even the heterogeneous CNT/Au composite nanowires are obtainable through this method. TEM, XPS, and XRD are all used to characterize the CNT/Au composite materials. In addition, the optical and electrocatalytic properties are investigated.     

Poly(vinyl pyrrolidone): a dual functional reductant and stabilizer for the facile synthesis of noble metal nanoplates in aqueous solutions

Poly(vinyl pyrrolidone) (PVP) has been extensively used in the solution-phase synthesis of many types of colloidal particles, where it is mainly considered as a steric stabilizer or capping agent with a major role to protect the product from agglomeration. In a recent study, we discovered that the hydroxyl end groups of PVP could also serve as a very mild reductant for kinetically controlled synthesis of Ag nanoplates with yields as high as 75%. Here we further demonstrate that hydroxyl-terminated PVP is also a well-suited reductant for the aqueous synthesis of circular, triangular, and hexagonal nanoplates made of other noble metals including Pd, Au, and Pt. The reduction kinetics of a metal salt by the hydroxyl end groups of PVP can be maneuvered in at least two different ways to facilitate the evolution of plate morphology: (i) by adjusting the molar ratio of PVP to the salt precursor and (ii) by altering the molecular weight of PVP. Unlike previously reported studies of Ag and Au thin plates, light was found to have a negligible role in the present synthesis.     

Crystal overgrowth on gold nanorods: tuning the shape, facet, aspect ratio, and composition of the nanorods

Electrochemically prepared Au nanorods were used as seeds for the overgrowth of thin shells of gold, silver, and palladium by using a mild reducing agent, ascorbic acid, in the presence of surfactants at ambient condition. The unique crystal facets of the starting nanorods results in anisotropic crystal overgrowth. The overgrowth rates along different crystallographical directions can be further regulated by adding foreign ions or by using different metal reduction methods. This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned. These surfactant-stabilized bimetallic Au(core)M(shell) (M=Au, Ag, Pd) nanorod colloids might serve as better substrates in surface-enhanced Raman spectroscopy as well as exhibiting enhanced catalytic properties.     

Fabrication of flower-like silver nanoparticles for surface-enhanced Raman scattering

The flower-like silver nanoparticles have been synthesized by reducing silver nitrate (AgNO₃) with ascorbic acid (AA) as the reductant and polyvinyl pyrrolidone (PVP) as the capping agent under vigorous stirring. Such flower-like nanoparticles are aggregates of small nanoplates and nanorods. They were tested as substrates for the surface-enhanced Raman scattering (SERS), showing high sensitivity for detecting Rhodamine 6G (R6G) at a concentration as low as 10^-7 mol/L. It has been found that replacing mechanical stirring with ultrasound sonication would drastically change the particle morphology, from flower-like nanoparticles to well-dispersed smaller nanoparticles. Furthermore, when trace amounts of NaCl were added into the reagents, well-dispersed Ag nanoparticles formed even in vigorous stirring. These phenomena can be explained with the diffusion and reactant supply during nucleation and growth of Ag nanoparticles.     

Green, effective chemical route for the synthesis of silver nanoplates in tannic acid aqueous solution

Silver nanoplates, with average size tunable from 50 to 500 nm, have been synthesized via a simple room-temperature tannic acid (TA) solution-phase chemical reduction method. The synthesis was a seedless process in which tannic acid was used as a reducing as well as a capping agent, and did not need any other surfactant or capping agent to direct the anisotropic growth of the silver nanoparticles. The morphology of silver nanoplates has been confirmed by transmission electron microscopy, the growth process of nanoplates has been studied by UV/vis spectroscopy. Control experiments have been explored for a more thorough understanding of the growth mechanism. It was found that both the concentrations of TA and the pH of solution were the key elements to control the morphology silver nanoplates. The optical in-plane dipole plasmon resonance bands of these silver plates could be tuned from 520 to 1100 nm.     

Organosulfur-functionalized Au, Pd, and Au-Pd nanoparticles on 1D silicon nanowire substrates: preparation and XAFS studies

A hybrid preparative method was developed to prepare organosulfur-functionalized Au nanoparticles (NPs) on silicon nanowires (SiNWs) by reacting HAuCl(4) with SiNW in the presence of thiol. A number of organosulfur molecules-dodecanethiol, hexanethiol, 1,6-hexanedithiol, and tiopronin-were used to functionalize the Au surface. Size-selected NPs ranging from 1.6 to 7.5 nm were obtained by varying the S/Au ratio and the concentration of HAuCl(4). This method was further extended to the preparation Pd and Pd-Au bimetallic NPs on SiNWs. The morphology of the metal nanostructures was examined by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The local structure and bonding of the SiNW-supported metal nanostructures were studied using X-ray absorption fine structures (XAFS) [including both X-ray near-edge structures (XANES) and extended X-ray absorption fine structures (EXAFS)] at the Au L(3)-, Pd K-, S K-, and Si K-edges. It was also found that the annealing of the thiol-capped Au NPs up to 500 degrees C transforms the surface of the thiol-capped NPs to gold sulfide, as identified using Au L(3)- and S K-edge XANES. We also illustrate that this preparative approach can be used to form size-controllable Au NPs on carbon nanotubes.     

Synthesis of silver nanoplates by two-dimensional oriented attachment

Synthesis of silver nanoplates was studied in the modified polyol method, where the nucleation and seed stage occurred in a poly(ethylene glycol) (PEG)-water mixture solution, and the growth stage happened in the PEG environment. The morphological evolution of nanoplates was characterized using UV, SEM, and TEM. Interestingly, plane nanostructures with unusual jagged edges were finally formed in our modified polyol method. Using TEM, we observed the medium state of fusion between two nanoplates, resulting in generating unusual jagged edges. Therefore, a novel two-dimensional oriented attachment occurred in our modified polyol method, which involves smaller nanoplates as the building blocks. Further control experiments showed that the presence of water could break this kinetic preferred reactivity, leading to the formation of nanoparticles.     

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.     

微波法制备3.5 G PAMAM树状大分子保护的金纳米粒子

以3.5 G PAMAM(3.5代聚酰胺-胺型)树状大分子为保护剂,利用微波法还原HAuCl₄溶液制备金纳米粒子.考察了当3.5 G PAMAM与HAuCl₄物质的量的比一定时,微波照射不同时间对金纳米粒子大小及形状的影响;以及同一照射条件下,3.5 G PAMAM与HAuCl₄不同的物质的量比值对金纳米粒子大小及形状的影响.利用紫外可见分光光度计、透射电子显微镜对其进行了表征.结果表明,当3.5 G PAMAM与HAuCl₄物质的量的比值一定时,金纳米粒子的形状和大小受微波照射时间长短的影响不大;适当延长照射时间,制得的金纳米粒子的分散性较好.在相同照射条件下,随着3.5 G PAMAM与HAu-Cl₄物质的量比值的减小,得到的金纳米粒子粒径逐渐变大,且分散性变差.     

Controlled Growth of Gold Nanoparticles in Aerosol-OT/Sorbitan Monooleate/Isooctane Mixed Reverse Micelles

Stable anisotropic gold nanoparticles were prepared by the reduction of tetrachloroauric acid with hydrazine in mixed reverse micelles formed with anionic surfactant Aerosol-OT and nonionic surfactant sorbitan monooleate (Span80) in isooctane. It was found that the Span80 serves not only as a structure modifier but also as a stabilizer for Au particles, to prevent their further growth and precipitation. The control of particle size, shape, and degree of dispersion was achieved by varying the process variables, such as molar ratio of reduction agent to metal salt, size of water droplets (omega(o)), concentration of metal salt, and sequence of addition of metal salt into the mixed reverse micelles. When the HAuCl(4) was injected directly into the mixed reversed micelles containing hydrazine, nonspherical gold nanoparticles, such as rods and cubes, were obtained at the molar ratio of hydrazine to HAuCl(4) of less than 1.0. The nonspherical Au particles were preferably formed at larger omega(o) value and lower gold salt loading. By the analyses of high-resolution electron microscope, electron diffraction pattern, and energy-dispersive X-ray analysis (EDX), the resultant particles have been found to be pure gold of face-centered cubic structure. Copyright 2000 Academic Press.     

A novel shape-selective fabrication of nanostructured silver

A novel protection-reduction technique is developed for the preparation of silver nanoparticles, nanorods and wheatear-like supramolecular nanostructures at room temperature using silver potassium cyanide [KAg(CN)2] as a silver source, vitamin C (Vc) as a reducing agent and polyvinylpyrrolidone (PVP) as a protecting agent. The concentration of KAg(CN)2, the mole ratios of PVP/Vc and KAg(CN)2/Vc have significant effects on the formation and growth of these novel nanostructures. This method may be extended to prepare novel nanostructures of other metals.