Dendritic Gold Research Articles

Dendritic (phosphine)gold(i) thiolate complexes: assessment of the molecular size through PGSENMR studies

The reactions of the tetraphosphine ligand DAB-G0-(PPh2)4 (DAB = 1,4-diaminobutane; G0 = Generation 0) or the octaphosphine ligand DAB-G1-(PPh2)8 (G1 = Generation 1) with the gold precursor [AuCl(tht)] (tht =tetrahydrothiophene) and the corresponding 4-substituted benzenethiolate lead to the (phosphine)gold(I) thiolate complexes [Au4(S-C6H4-X)4[(DAB-G0-(PPh2)4]] (X = F (11), MeO (12), Me (13) and NO2 (14)) and [Au8(S-C6H4-X)8[(DAB-G1-(PPh2)4]] (X = F (15), MeO (16), Me (17) and NO2 (18)). Complexes [Au4Cl4[(DAB-G0-(PPh2)4]] and [Au4(S-C6H4-OMe)4[(DAB-G0-(PPh2)4]] have been characterized by X-ray diffraction studies showing tetranuclear gold complexes in which the P-Au-X (X = Cl or S) structural units do not display aurophilic interactions. PGSE NMR studies of free ligands EN-G0-(PPh2)(4) (EN = 1,2-ethylenediamine), DAB-G0-(PPh2)(4), DAB-G1-(PPh2)8 and the (phosphine)gold(I) thiolate complexes permit the evaluation and comparison of the different molecular sizes depending on the ligand and the dendrimer generation.

Ionic Liquid-Assisted Growth of Single-Crystalline Dendritic Gold Nanostructures with a Three-Fold Symmetry

Hierarchical, three-fold symmetrical, single-crystalline gold dendrites were synthesized by the reaction between a zinc plate and a solution of HAuCl4 in the ionic liquid [BMIM][PF6]. The unique dendritic gold nanostructures show a three-order hierarchy (i.e., a three-fold symmetrical <111>-oriented trunk, three groups of trident-like <111>-oriented branches grown on the trunk, and many <111>-oriented nanorod leaves grown on the branches symmetrically), indicating an interesting fractal growth. According to the investigation on the growth process of the gold dendrites, it was proposed that gold nuclei nanocrystals first formed on the zinc substrate through a direct surface reaction and that the subsequent crystal growth preferentially took place on the preformed gold crystals through a primary cell reaction, leading to the formation of the final hyperbranched dendrites under nonequilibrium conditions. In contrast, AuZn alloy dendrites consisting of aggregated primary nanoparticles were produced when the ionic liquid solution was replaced by an aqueous solution. It was proposed that the significantly lowered ion diffusivity and reaction rate in the ionic liquid medium could largely contribute to the formation of the pure single-crystalline gold dendrites. Moreover, electrocatalytic measurements performed in alkaline solution suggested that the obtained dendritic gold nanostructures exhibited good electrocatalytic activity toward the oxidation of methanol, which might be related to the special three-order hierarchical architectures.

Fabrication of Dendritic Gold Nanoparticles by Use of an Ionic Polymer Template

A facile method for the fabrication of dendritic gold nanoparticles (NPs) by use of an ionic polymer template has been developed. In situ generation of an imidazolium-based (cationic) polymer, poly[1-methyl-3-(4-vinylbenzyl)imidazolium], with AuCl4- counteranions is achieved by addition of HAuCl4 into a solution containing poly[1-methyl-3-(4-vinylbenzyl)imidazolium chloride]. Subsequent reduction with NaBH4 in water or in a mixture of ethanol and water affords various NPs depending on the conditions, including large dendritic gold NPs that have been analyzed by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED). The structures of the dendritic gold NPs were found to depend on the ethanol concentration. Scanning electron microscopy (SEM) images of the ionic polymer reveal that the solvent used to deposit the polymer strongly influences its structure and may be correlated to the structure of the resulting NPs.

Poly(N-vinyl-2-pyrrolidone) (PVP)-Capped Dendritic Gold Nanoparticles by a One-Step Hydrothermal Route and Their High SERS Effect

Dendritic gold (Au) nanoparticles have been successfully synthesized by the one-step hydrothermal reduction of HAuCl4.4H2O using ammonium formate (AF) as a reducing agent in the presence of PVP. Effects of different reactant concentrations on the morphologies of obtained products have been systematically investigated. On the basis of the morphologies of the products observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), it has been found that an excessive number of AF molecules are the origin of the dendritic Au particles besides PVP as a stabilizer. AF molecules serve not only as a reductant but probably also as a capping reagent. The study implies that the use of two or more capping reagents with different adsorption abilities will be beneficial to the formation of hyperbranched Au nanoparticles. The new finding will have the potential to be extended to the construction of other highly branched noble metal nanoparticles only by one-step synthesis. In addition, as an example, application of the dendritic particles as an active material in surface-enhanced Raman scattering has been investigated by employing 4-aminothiophenol molecules as a probe.

Shape-Controlled Synthesis of Gold Nanoparticles Under UV Irradiation in the Presence of Poly(Ethylene Glycol)

Dendritic gold nanoparticles and triangular gold plates were synthesized via a simple photochemical reduction protocol. The shape of nanoparticles depended on the concentration of poly(ethylene glycol), which was used on the preparation in an aqueous solution. On the other hand, the size of nanoparticles was controlled by the duration of UV irradiation. The mechanism of morphology variation was discussed. Keywords: Gold nanoparticle, dendritic gold nanoparticle, triangular gold plate, photochemical reduction, poly(ethylene glycol), UV irradiation

Dendrite structure of electrolytic gold deposited from molten chlorides

The structure of gold deposits produced by electrolysis of molten eutectic NaCl-KCl-CsCl at 500–700°C in an inert atmosphere is studied. The initial process on the gold cathode is the epitaxial growth of a layer up to 3 μm thick with a smoothed surface and a considerably later growth of grain boundaries. Gradually, on the protruding parts of deposit, the growth of dendrites starts. The dendrites are the major form of gold deposits. Typical gold dendrites are two-dimensional 2D〈112〉 and 2D〈112〉–〈110〉 and three-dimensional 3D〈100〉. Upon supplying air into the atmosphere above the melt at high current densities at the initial period there appears a powder comprising particles of a rounded twisted shape. Probable mechanisms leading to the formation of the “rounded” powder are discussed.

Synthesis and Characterization of 3D Dendritic Gold Nanostructures and Their Use as Substrates for Surface-Enhanced Raman Scattering

Three-dimensional dendritic gold nanostructures (3D-DGNs) were prepared at room temperature through an aqueous/organic interfacial reaction of hydrogen tetrachloroaurate (HAuCl4, in aqueous phase) and 3,4-ethylenedioxythiophene (EDOT, in organic phase) without the use of any seeds, templates, surfactants, or stabilizers. SEM and TEM studies indicated that the 3D-DGNs compose of numerous nanowires with diameters around 30 nm and each wire brings a nanosheet head with thickness of 80 ± 5 nm. The effects of reaction conditions including the concentrations of HAuCl4 and EDOT, the reaction temperature, and the organic solvents on the morphology of 3D-DGNs were systematically investigated and a possible mechanism for the formation of 3D-DGNs was proposed. The 3D-DGNs can be used as substrates for surface-enhanced Raman scattering (SERS). They exhibited much higher surface enhancement than those of conventional electrochemically roughed gold surfaces and gold nanorods when 4-mercaptopyridine (4MP) was used as a molecule probe. The enhancement factor of 3D-DGN substrate was measured to be (1.9 ± 0.4) × 106, and the limit of detection (LOD) was determined to be lower than 1 × 10-10 M.

Seeding Method for Three-dimensional Dendritic Growth of Gold Nanoparticles Stabilized by Hexatrimethylammonium Bromide

Abstract Three-dimensional gold dendrites with fractal nanostructure were prepared with HAuCl4 and l-ascorbic acid via seeding growth method in the presence of hexatrimethylammomium bromide (HTAB) as a protecting agent and architecture soft template. Spherical gold, previously generated, plays as a seed for particle growth. The surfactant HTAB helps to induce various branched nanostructures. The growth of first branches on the seeds leads to the formation of second branches by seed amount.

Nanoparticle-aggregated 3D monocrystalline gold dendritic nanostructures

In this paper, through a simple and fast electroless metal deposition route, gold dendriticnanostructures are synthesized in aqueous conditions. The gold dendrites with a threefoldsymmetric characteristic were built up of numerous nanoparticles roughly 5–10 nm in size.The aggregated nanoparticles spontaneously experience a self-assembly process along crystallographic orientations and finally form a monocrystalline dendrite. An orientedattachment mechanism can be used to explain the nanoparticle-aggregated self-assemblyprocess.

Gold fractal structures spontaneously grown in sheared lamellar phase

In this paper, we report the effect of membrane composition and time on the morphology of gold nanoparticles grown in sheared lamellar phases from a 10−3 M KAuCl4 solution, using monoolein as the chemical reductant. Fractal, dendritic gold structures are synthesized when octadecylamine (ODA) is inserted in phosphatidylcholine (PC) or PC–monooleine bilayers, while without ODA, gold sphere-like particles are synthesized. Each morphology (dendrite or sphere-like particles) is macroscopically revealed by sample color. Time favors the growth of fractals at the expense of spherical particles as shown by a statistical analysis performed on 5 day-old and 21 day-old samples. Fractal structures disturb the lamellar arrangement as evidenced by small angle X-ray diffraction experiments, so that no multilamellar vesicles, usually resulting from dilution of the sheared lamellar phase, can be obtained as imaged by cryo-TEM analysis. Possible reasons for the formation of fractal structures are given.

One-step synthesis of 3D dendritic gold/polypyrrole nanocomposites via a self-assemblymethod

Novel spherical three-dimensional (3D) dendritic gold–polypyrrolenanocomposites were successfully prepared in the presence of an amphiphilicp-toluene sulfonic acid (TSA) as dopant and surfactant via a self-assemblyprocess which is based on the oxidation of pyrrole (Py) and the reduction ofthe chloroaurate ions, yielding PPy and Au(0) simultaneously. It was foundthat the probability of obtaining dendritic Au@PPy/TSA nanostructuresdepended on the concentration of TSA and the rate of addition of the oxidant(HAuCl4). It was also proposed that the supramolecular micelles formed by Py and TSA play therole of a ‘soft template’ to produce the dendritic Au@PPy/TSA nanocomposites.

Morphological selection of electroless metal deposits on silicon in aqueous fluoride solution

Well-defined silver and gold dendrites are prepared on silicon substrates in aqueous fluoride acid solution containing AgNO3 and KAuCl4, respectively. In cases of electroless platinum and copper deposition in fluoride acid solution, no corresponding metal dendrites but platinum microcrystallines and copper granular-crystallines are formed. Our experiments found that the morphological selection of electroless metal deposits is related to the microstructure of etched silicon. A possible template mechanism and affecting factors for the morphological selection of electroless metal deposits are discussed.

Polyelectrolyte multilayer as matrix for electrochemical deposition of gold clusters: toward super-hydrophobic surface.

This communication describes the use of polyelectrolyte multilayer as a preformed matrix in electrochemical deposition to adjust the morphology of gold clusters, and only the surface covered with dendritic gold clusters shows a super-hydrophobic property, with the contact angle above 150 degrees and the tilt angle less than 1.5 degrees , after the surface further chemisorbed with a self-assembled monolayer of n-dodecanethiol.

Simultaneous gold deposition and formation of silicon nanowire arrays

A method is presented for simultaneously producing oriented silicon nanowire arrays and depositing gold on a silicon substrate from aqueous HF solution containing KAuCl 4 near room temperature. Scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray were used to characterize the morphologies, structure and compositions of the samples. Experimental results show that these silicon nanowires are single crystals and the growth of gold dendrites plays an important role in the formation of silicon nanowire arrays. Possible mechanism was proposed to explain the formation of silicon nanowire arrays based on local passivation of the silicon dissolution induced by the growth of Au nuclei.

Comparative experimental study of ionic polymer–metal composites with different backbone ionomers and in various cation forms

An ionic polymer–metal composite (IPMC) consisting of a thin perfluorinated ionomer (usually, Nafion or Flemion) strip, platinum, and/or gold plated on both faces and neutralized by a certain amount of appropriate cations undergoes large bending motion when, in a hydrated state, a small electric field is applied across its thickness. When the same membrane is suddenly bent, a small voltage of the order of millivolts is produced across its surfaces. Hence IPMCs can serve as soft bending actuators and sensors. This coupled electrical–chemical–mechanical response of IPMCs depends on the structure of the backbone ionic polymer, the morphology and conductivity of the metal electrodes, the nature of the cations, and the level of hydration (or other solvent uptake). We have carried out extensive experimental studies on both Nafion- and Flemion-based IPMCs in various cation forms, seeking to understand the fundamental properties of these composites, to explore the mechanism of their actuation, and finally, to optimize their performance for various potential applications. The results of some of these tests on both Nafion- and Flemion-based IPMCs with alkali-metal or alkyl-ammonium cations are reported here. Compared with Nafion-based IPMCs, Flemion-based IPMCs with fine dendritic gold electrodes have higher ion-exchange capacity, better surface conductivity, higher hydration capacity, and higher longitudinal stiffness. They also display greater bending actuation under the same applied voltage. In addition, they do not display a reverse relaxation under a sustained dc voltage, which is typical of Nafion-based IPMCs in alkali-metal form. Flemion IPMCs thus are promising composites for application as bending actuators.

Occurrences of dendritic gold at the McLaughlin Mine hot-spring gold deposit

Two styles of gold dendrites are variably developed at the McLaughlin Mine. The most abundant occurrence is hosted by amber-coloured hydrocarbon-rich opal. Silica likely precipitated from a boiling hydrothermal fluid and complexed with immiscible hydrocarbons forming an amorphous hydrocarbon-silica phase. This phase likely scavenged particulate gold by electrostatic attraction to the hydrocarbon-silica phase. The dendritic nature of the gold is secondary and is the result of dewatering of the amorphous hydrocarbon-silica phase and crystallization of gold into syneresis fractures. The second style of dendritic gold is hosted within vein swarms that focused large volumes of fluid flow. The dendrites occur along with hydrocarbon-rich silica at the upper contact of the vein margins which isolated the dendrites allowing sufficient time for them to grow. In a manner similar to the amber-coloured opal, the dendrites may have formed by scavenging particulate gold by electrostatic attraction to the hydrocarbon-silica phase.

NATURAL GOLD DENDRITES FROM HYDROTHERMAL Au-Ag DEPOSITS: CHARACTERISTICS AND COMPUTER SIMULATIONS

Epithermal vein gold deposits in the western United States locally contain bonanza zones with extraordinary concentrations of gold in banded ores consisting of alternating gold-rich and barren silica bands. Gold-rich bands commonly consist of coalescing or isolated gold dendrites, which occur in a matrix of variably preserved colloidal silica. Most dendrites appear to have grown outward from near-vertical vein walls and have a rough radial symmetry. SEM images of dendrite surfaces suggest that they formed by the aggregation of spherical particles in the 10–100 nm range. Fractal dimension (Df) of true two-dimensional slices through the dendrites typically are in the 1.6–1.7 range, which compares favorably with the theoretical Df value of ~1.7 for 2-dimensional diffusion-limited colloid aggregation (DLA). Monte Carlo computer simulations best reproduce natural dendrite morphologies when particle randomness is large, sticking efficiency=1, and when particles are required to contact 3 adjacent particles on the dendrite as a prerequisite for aggregation. Thus, the physical transport and aggregation of gold colloids offers an attractive explanation for the origin of these enigmatic super high-grade gold ores.

Surface self-diffusion studied by microscopic measurements of crystallite profile evolutions

Dendritic gold crystallites on graphite are heated in ultra high vacuum up to less than 0.5 of the melting point ( T m ). Electron microscopy shows that the gold crystallites change their shapes by surface self-diffusion. The dendritic contours round off while the crystallite remains very flat (20 to 40 Å). The increase with time of the radii of dendrite tips is measured statistically. Such an evolution can be described by analogy to the blunting of either metal tips (Nichols and Mullins) or monoatomic cleavage tips (Höche and Bethge). Using this result, a new technique to measure surface self-diffusion coefficients ( D) is proposed. Test measurements have shown that this is an interesting, very sensitive method to measure D (down to 10 −13 cm −2 s −1) which enables measurements to be made in an unusual low temperature range (0.25 T m < T < 0.5 T m ). In special cases the dendrites are split by the surface self-diffusion which is qualitatively in agreement wih the theory.

Dendritic Gold

WILL one of my fellow-readers of NATURE be good enough to inform me, through its columns, with the name and publisher of such a work on mineralogy (short, if possible) as will give me the best information on the subject of the dendritic gold existing in sandstones in New Zealand, as reported in the Proceedings of the Wellington Society (NATURE, vol. xvi. p. 567).