Optimization of accurate controlled formation of Au MNPs via non-thermal microplasma continuous glow discharge within aqueous electrochemical (GDE) method

In this work, micro plasma-induced non-equilibrium liquid chemistry was utilized to synthesize and controlled formation of gold metallic nanoparticles (Au MNPs) by governing the concentration of (HAuCl₄). These new approaches based on both plasma and liquid electrolytes contain charged species, and the interactions between the two phases represent a unique combination of physics, chemistry, and materials science. Continuous and stable DC glow discharge was done in home–made cavity to synthesize the definite sizes of (Au MNPs) by means of (3 kV) discharge voltage and (2 mA) discharge current for a period of (7 min) in aqueous solution of HAuCl₄ with four different concentrations of about 1 mM, 5 mM, 10 mM and 20 mM at room temperature. The atmospheric pressure plasma discharge between stainless steel capillary tube cathode electrode over the (HAuCl₄) solution and platinum plate as an anode dipped in solution for rapid formation of colloidal Au MNPs. Morphology aspects of the synthesized Au MNPs layer were studied by examining the (FE-SEM), HR-TEM images and X-ray difraction (XRD) pattern. Optical features of (Au MNPs) were considered via a UV–Vis beam spectrophotometer. These measurements showed that Au MNPs were organized by governing the concentration of HAuCl₄, and uniform Au MNPs with specific exclusive sizes were acquired. Grain size, specific surface area and optical stability of Au MNPs strongly be affected by the HAuCl₄ concentrations.     

High–Yield Production of Uniform Gold Nanoparticles with Sizes from 31 to 577 nm via One‐Pot Seeded Growth and Size‐Dependent SERS Property

In this work, uniform, quasi‐spherical gold nanoparticles (Au NPs) with sizes of 31–577 nm are prepared via one‐pot seeded growth with the aid of tris‐base (TB). Distinct from the seeded growth methods available in literature, the present method can be simply implemented by subsequently adding the aqueous dispersion of the 17 nm Au‐NP seeds and the aqueous solution of HAuCl₄ into the boiling aqueous TB solution. It is found that at the optimal pH range, the sizes of the final Au NPs and their concentrations are simply controlled by either the particle number of the Au seed dispersion or the concentration of the HAuCl₄ solution, while the latter enables us to produce large Au NPs at very high concentration. Moreover, as‐prepared Au NPs of various sizes are coated on glass substrates to test their surface‐enhanced Raman scattering (SERS) activities by using 4‐aminothiophenol (4‐ATP) molecules as probes, which exhibit “volcano type” dependence on the Au NP sizes at fixed excitation wavelength. Furthermore, the Au NPs with sizes of ≈97 and 408 nm exhibit the largest SERS enhancement at the excitation wavelength of 633 and 785 nm, respectively.     

Synthesis of Au nanorods via autocatalytic growth of Au seeds formed by sonochemical reduction of Au(I): Relation between formation rate and characteristic of Au nanorods

The sonochemical formation of Au seeds and their autocatalytic growth to Au nanorods were investigated in a one-pot as a function of concentration of HAuCl₄, AgNO₃, and ascorbic acid (AA). The effects of ultrasonic power and irradiation time were also investigated. In addition, the formation rate of Au nanorods was analyzed by monitoring the extinction at 400 nm by UV–Vis spectroscopy and compared with the growth behavior of Au seeds to nanorods. Most of the reaction conditions affected the yield, size, and shape of Au nanorods formed. It was confirmed that the concentration balance between HAuCl₄ and AA was important to proceed the formation of Au seeds and nanorods effectively. The formation rate became faster with increasing AA concentration and dog-bone shaped nanorods were formed at high AA concentration. It was also confirmed a unique phenomenon that the shape of Au nanorods changed even after the completion of the reduction of Au(I) in the case of short-time ultrasonic irradiation for Au seed formation.     

Catalytic properties of gold nanoparticles immobilized on the surfaces of nanocarriers

Gold nanoparticles are immobilized in the hydrophilic coronas of spherical micelle carriers for high catalytic activity. The micelle is formed by self-assembly of block copolymer, polystyrene-b-poly (acrylic acid), in basic aqueous solution (pH 10) and has a polystyrene core and a poly (acrylic acid) corona. The gold nanoparticles are anchored into the poly (acrylic acid) corona by in situ reduction of the mixture of HAuCl₄ and micelle with NaBH₄. The sizes of the gold nanoparticles can be adjusted by changing the content of the HAuCl₄. In the process of catalyzing p-nitrophenol to p-aminophenol, the reaction shows one-order kinetics, furthermore, the reaction rate increases with the concentration of composites as well as reaction temperature. Comparing the composites with polystyrene as core and poly (4-vinylpyridine)/Au as corona, the catalytic activity of the present composites is higher, which is ascribed to their hydrophilic corona structure.     

Synthesis of SERS active Au nanowires in different noncoordinating solvents

Au nanowires with length up to micrometers were synthesized through a simple and one-pot solution growth method. HAuCl₄ was reduced in a micellar structure formed by 1-octadecylamine and oleic acid in hexane, heptane, toluene and chloroform, respectively. As the non-polarity of noncoordinating solvents can affect the nucleation and growth rates of Au nanostructures, Au nanowires with different diameters could be obtained by changing the noncoordinating solvents in the synthetic process. The influences of the solvents on the morphology of Au nanowires were systematically studied. When using hexane as reaction solvent, the product turned to be high portion of Au nanowires with more uniform size than the others. Furthermore, surface-enhanced Raman scattering (SERS) spectrum of 2-thionaphthol was obtained on the Au nanowire-modified substrate, indicating that the as-synthesized Au nanowires have potential for highly sensitive optical detection application.     

Optimization of components in high-yield synthesis of block copolymer-mediated gold nanoparticles

The optimization to achieve stable and high-yield gold nanoparticles in block copolymer-mediated synthesis has been examined. Gold nanoparticles are synthesized using block copolymer P85 in gold salt HAuCl₄·3H₂O solution. This method usually has a very limited yield which does not simply increase with the increase in the gold salt concentration. We show that the yield can be enhanced by increasing the block copolymer concentration but is limited to the factor by which the concentration is increased. On the other hand, the presence of an additional reductant (trisodium citrate) in 1:1 molar ratio with gold salt enhances the yield by manyfold. In this case (with additional reductant), the stable and high-yield nanoparticles having size about 14 nm can be synthesized at very low block copolymer concentrations. These nanoparticles thus can be efficiently used for their application such as for adsorption of proteins.     

Formation and encapsulation of gold nanoparticles using a polymeric amine reducing agent

We report on the use of poly(allylamine) hydrochloride (PAH) as a reducing agent for the controlled formation of gold nanoparticles (AuNPs) in the size range of 5–50 nm. The formation of AuNPs using this polymer matrix allows for the AuNPs to be imbedded in the polymer matrix, once formed. The kinetics of AuNP formation are shown to be pseudo first-order in [HAuCl₄] at room temperature. The kinetics of AuNP formation are controlled by the ratio of reducing agent to HAuCl₄ as well as the overall concentration of the PAH and HAuCl₄. Additionally, at low PAH:HAuCl₄ mole ratios, the plasmon resonance wavelength can be controlled through the ratio of the reactants. This plamson resonance shift is shown to be related to AuNP size by means of TEM imaging data on the AuNPs.     

Model for UV induced formation of gold nanoparticles in solid polymeric matrices

UV irradiation of polymeric PMMA films containing HAuCl₄ followed by annealing at 60-80 °C forms gold nanoparticles directly within the bulk material. The kinetics of nanoparticle formation was traced by extinction spectra of nanocomposite film changes vs annealing time. We propose that UV irradiation causes HAuCl₄ dissociation and thus provides a polymeric matrix with atomic gold. The presence of an oversaturated solid solution of atomic gold in the polymeric matrix leads to Au nanoparticle formation during annealing. This process can be understood as a phase transition of the first order. In this paper we apply several common kinetic models of the phase transition for describing Au nanoparticle formation inside the solid polymer matrix. We compare predictions of these models with the experimental data and show that these models cannot describe the process. We propose that the stabilization effect of the matrix on the growing gold nanoparticles is important. The simplest model introducing some probability for the transition from growing nanoparticle to the non-growing, stabilized form is suggested. It is shown that this model satisfactorily describes the experimentally observed evolution of the extinction spectrum of Au nanoparticles forming in a polymer matrix.     

Diffusion-limited electrodeposition of ultrathin Au films on Pt(1 1 1)

The electrodeposition of Au on Pt(1 1 1) from electrolytes containing μM concentrations of was studied by in situ scanning tunneling microscopy. Under these conditions the Au flux is limited by diffusion in the electrolyte over a wide potential range, which allows to assess the effect of the electrochemical environment on the growth kinetics. Similar to gas phase metal deposition Au film growth proceeds via nucleation and lateral growth of Au monolayer islands, with the saturation island density strongly depending on the deposition potential and on the anion species in the electrolyte. For deposition in H₂SO₄ solution the saturation island density continuously increases with increasing potential between −0.2 and 0.5 V (SCE), whereas in Cl-containing H₂SO₄ it first decreases and then increases again. Following nucleation and growth theories this behavior can be attributed to potential-induced changes of the Au surface mobility, caused by changes in the density and structure of coadsorbed sulfate/bisulfate and chloride adlayers. Under conditions of high Au surface mobility multilayer growth proceeds via a typical Stranski-Krastanov growth mode, with layer-by-layer growth of a pseudomorphic Au film up to 2 ML and 3D growth of structurally relaxed islands at higher coverage, indicating thermodynamic control under these conditions.     

Novel fabrication and catalytic application of poly(ethylenimine)-stabilized gold–silver alloy nanoparticles

Novel synthesis of amine-stabilized Au–Ag alloy nanoparticles with controlled composition has been devised using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The composition of Au–Ag alloy nanoparticles was readily controlled by varying the initial relative amount of HAuCl₄ and AgNO₃. Due to the presence of abundant amine functional groups in PEI, which could act as the dissolving ligand for AgCl, the precipitation problem of Ag⁺ in the presence of Cl⁻ from the gold salt was avoided. On this basis, the relatively high concentrations of HAuCl₄ and AgNO₃ salts were used for the fabrication of Au–Ag alloy nanoparticles. The PEI thus plays triple roles in this study that include the co-reducing agents for HAuCl₄ and AgNO₃, the stabilizing agents for Au–Ag alloy nanoparticles, and even the dissolving agents for AgCl. As a novel material for use in catalysis, the Au–Ag alloy nanoparticles including pure Au and Ag samples were exploited as catalysts for the reduction of 4-nitrophenol in the presence of NaBH₄. As the Au content was increased in the Au–Ag alloy nanoparticles, the rate constant of the reduction was exponentially increased from pure Ag to pure Au.     

XAS and XPS examination of the Au–S nanostructures produced via the reduction of aqueous gold(III) by sulfide ions

The S L- and Au L₃-edge X-ray absorption fine structure and X-ray photoelectron spectra of nanoscale Au–S products formed via the reduction of aqueous HAuCl₄ by sulfide ions and immobilized onto graphite have been acquired. The TEY XANES and XPS spectra implied the formation of predominant polysulfide species and metallic gold, while the transmission spectra showed Au–S bonding, the share of which increased with increasing molar Na₂S/HAuCl₄ ratio in the reaction solution. The Au–S distances derived from EXAFS analysis changes from 2.31 Å to 2.325 Å with the concentration of sodium sulfide in solution, being longer than that for cuprite-type crystalline Au₂S (2.174 Å). It has been concluded from all the evidence that the surface of gold sulfide decomposes in air and in ultra-high vacuum even before X-ray irradiation.     

Rapid synthesis and characterization of ultra‐thin shell Au@SiO2 nanorods with tunable SPR for shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS)

The recently reported shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) is considered as the next generation of advanced spectroscopy for its surface and molecular generality. With the aim to utilize the virtues of shell‐isolated strategy and advance the SHINERS technique, we introduce a silane‐based rapid synthesis method of silica‐coating Au nanorods (Au@SiO₂ NRs) with manoeuvrable ultra‐thin shell and tunable SPR. The results demonstrate that the SPR of Au NRs could be optimized to obtain large Raman enhancement using either 633 nm or 785 nm laser. Differing from previously reported Au@SiO₂ NRs synthesis method, we can tune the silica shell thickness within several nanometers to maximize the Raman signal while effectively eliminating the exterior interference. And this advanced synthesis method has also significantly reduced the silica‐coating time from one day to ca. 1 h. This method as a new development of SHINERS technique has successfully got enhanced signal in solution Raman tests of malachite green, giving a great potential to be extended to in‐situ measurement for daily life detection. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and properties of Au-Fe3O4 and Ag-Fe3O4 heterodimeric nanoparticles

Monodisperse Au-Fe 3 O 4 heterodimeric nanoparticles (NPs) were prepared by injecting precursors into a hot reaction solution.The size of Au and Fe 3 O 4 particles can be controlled by changing the injection temperature.UV-Vis spectra show that the surface plasma resonance band of Au-Fe 3 O 4 heterodimeric NPs was evidently red-shifted compared with the resonance band of Au NPs of similar size.The as-prepared heterodimeric Au-Fe 3 O 4 NPs exhibited superparamagnetic properties at room temperature.The Ag-Fe 3 O 4 heterodimeric NPs were also prepared by this synthetic method simply using AgNO 3 as precursor instead of HAuCl 4.It is indicated that the reported method can be readily extended to the synthesis of other noble metal conjugated heterodimeric NPs.     

Phyllanthin-assisted biosynthesis of silver and gold nanoparticles: a novel biological approach

Abstract  The anisotropic gold and spherical–quasi-spherical silver nanoparticles (NPs) were synthesized by reducing aqueous chloroauric acid (HAuCl₄) and silver nitrate (AgNO₃) solution with the extract of phyllanthin at room temperature. The rate of reduction of HAuCl₄ is greater than the AgNO₃ at constant amount of phyllanthin extract. The size and shape of the NPs can be controlled by varying the concentration of phyllanthin extract and thereby to tune their optical properties in the near-infrared region of the electromagnetic spectrum. The case of low concentration of extract with HAuCl₄ offers slow reduction rate along with the aid of electron-donating group containing extract leads to formation of hexagonal- or triangular-shaped gold NPs. Transmission electron microscopy (TEM) analysis revealed that the shape changes on the gold NPs from hexagonal to spherical particles with increasing initial concentration of phyllanthin extract. The Fourier transform infrared spectroscopy and thermogravimetric analyses reveal that the interaction between NPs and phyllanthin extract. The cyclic voltammograms of silver and gold NPs confirms the conversion of higher oxidation state to zero oxidation state. Graphical abstract  Anisotropic gold and silver nanoparticles were synthesized by a simple procedure using phyllanthin extract as reducing agent. The rate of bioreduction of AgNO₃ is lower than the HAuCl₄ at constant concentration of phyllanthin extract. The required size of the nanoparticles can be prepared by varying the concentration of phyllanthin with AgNO₃ and HAuCl₄.      

Size-dependent transformation from Ag templates to Au–Ag nanoshells via galvanic replacement reaction in organic medium

The transformation from Ag templates to Au?CAg nanoshells via galvanic replacement reaction with HAuCl₄ was systematically studied in an organic medium in the presence of oleylamine. Decahedral (~43?nm in size) and triangular prism (~53?nm in edge length) Ag templates transformed into equiaxed and triangular prismatic Au?CAg nanoshells, respectively. The first step involved structural and morphological changes from Ag templates to Au?CAg nanoshells with an interior cavity. In the second step, the growth of the shells continued through the deposition of Au. The shell thickness increased from ~5 to ~10?nm for the equiaxed Au?CAg nanoshells (~39-nm interior cavity) and ~5 to ~8?nm for the triangular prismatic Au?CAg nanoshells (~52-nm interior edge length). Oleylamine not only served as a surfactant but also removed AgCl precipitates and reduced HAuCl₄. For the nanoshells derived from the ~20-nm Ag decahedrons, further reaction in excess HAuCl₄ collapsed the nanoshells into Au-rich solid fragments. However, the nanoshells derived from the ~43-nm Ag decahedrons, the nanoshell structure not only persisted in excess HAuCl₄, but its shell thickness also increased. The size-dependent transformation of these nanoshells is discussed.     

Selective synthesis of clinoatacamite Cu2(OH)3Cl and tenorite CuO nanoparticles by pH control

To investigate the shell deposited kinetics, CdSe quantum dots (QDs) and nanorods (NRs) with a maximum length of 17 nm were fabricated via organic synthesis routes. CdSe with a hexagonal crystal structure (wurtzite) favors epitaxial growth on the {002} surfaces when well-controlled conditions were used. The morphologies and sizes of CdSe samples depended strongly on chemicals and temperature. In the case of 320 °C, CdSe NRs with adjusted length of 7–17 nm were obtained from trioctylphosphine oxide (TOPO) and tetradecylphosphonic acid (TDPA). In contrast, short CdSe NRs (less than 10 nm) were created from octadecylphosphonic acid (ODPA) and trioctylamine (TOA). Spherical CdSe QDs were further fabricated using stearic acid (SA) and TOPO at 300 °C. CdSe cores were coated with Cd_0.5Zn_0.5S and CdTe shells. Anisotropic growth occurred during shell deposition because CdS shells grown preferentially on the {001} facet of the CdSe core. In the case of CdSe core prepared from TOPO and TDPA, CdSe/Cd_0.5Zn_0.5S core/shell samples prepared from long CdSe NRs (more than 10 nm) revealed a peanut morphology while the core/shell samples created from short ones (less than 10 nm) exhibited a spherical morphology. All of the CdSe/Cd_0.5Zn_0.5S core/shell samples revealed a similar length to that of the CdSe cores. This phenomenon was also observed for the core/shell samples fabricated using CdSe NRs prepared by ODPA and TOA. This is ascribed to the well-developed crystal structure of CdSe NRs fabricated using an organic synthesis at high temperature. In contrast, this anisotropic growth did not occur when spherical CdSe QDs prepared from SA and TOPO and the shell (Cd_0.5Zn_0.5S) coating carried out using SA and TOA. To indicate the shell depositing process, CdSe NRs fabricated using TDPA and TOPO were coated with a CdTe shell. CdTe monomers were deposited on the middle and tip parts of the CdSe NRs to form a tetrapod-like morphology at 220 °C. This is ascribed to the large difference of structure of CdSe (hexagonal) and CdTe (zinc blende).     

Silver,gold and the corresponding core shell nanoparticles: synthesis and characterization

Simple strategies for producing silver and gold nanoparticles (AgNP and AuNP) along with the corresponding core shell nanoparticles (Au–Ag and Ag–Au) by reduction of the metal salts AgBF₄ and HAuCl₄ by NaBH₄ in water will be presented. The morphologies of the obtained nanoparticles are determined by the order of addition of reactants. The obtained NPs, with sizes in the range 3–40 nm, are characterized by transmission electronic microscopy (TEM) and UV–Vis absorption spectroscopy, so as to evaluate their qualities. Moreover, a direct electrochemical detection protocol based on a cyclic voltammetry in water solution that involves the use of glassy carbon electrode is also applied to characterize the prepared NPs. The developed NPs and the related electroanalytical method seem to be with interest for future sensing and biosensing applications including DNA sensors and immunosensors.     

A novel method of nanocrystal fabrication based on laser ablation in liquid environment

Metal nanoparticles can be prepared by a novel technique that consists of the laser ablation of a solid target immersed in a water solution of a metal salt. Silicon was chosen as the most adequate target to synthesize silver and gold nanoparticles from a water solution of either AgNO₃ or HAuCl₄. The influence of both the silver nitrate concentrations and the irradiation time of the Si target on the optical properties of the Au and Ag nanoparticles have been investigated. The crystalline nature of the metal nanoparticles has been determined by X-ray diffraction (XRD). Average size and particle size distribution have been measured by means of TEM. The absorbance spectra show the characteristic band of the surface resonant plasmon of silver and gold nanoparticles.     

Preparation and characterization of amorphous hydrogenated carbon films containing Au nanoparticles from heat-treatment of polymer precursors

Amorphous hydrogenated carbon (aC:H) films containing Au nanoparticles have been successfully prepared by heat-treatment of the precursors including poly(phenylcarbyne) polymer and HAuCl₄ at 600 °C in Ar atmosphere. The microstructure and morphology of the obtained films were investigated by means of Raman, XPS, XRD, TEM, and AFM. The sheet resistivity of the films was measured by a four-point probe method. Moreover, a ball-on-disc test was employed to obtain information about the frictional properties and sliding wear resistance of the films. The results show that heat-treatment of the precursors at 600 °C causes the change of the polymer into amorphous hydrogenated carbon phase, and the reduction of AuCl₄^- anions into zero-valence Au. All of the AuaC:H films exhibit smooth morphologies, with the RMS roughness smaller than 0.80 nm. Au nanoparticles are well dispersed in the amorphous carbon matrix, with size ranging from several to tens of nanometers, and the particle size increases with increasing gold content. The incorporation of Au in the carbon matrix can drastically decrease the resistivity and the resistivity of composite films gradually decreases with increasing Au concentration. AuaC:H films with the Au concentration of 2% and 4% show much better friction-reduction and wear-resistance than aC:H film. PACS 81.15.Np; 81.07.-b; 81.40.Pq     

197Au Mössbauer study of the deactivation and reactivation of a carbon-supported AuCl 4 − hydrochlorination catalyst

Acetylene hydrochlorination catalysts consisting of activated carbon impregnated with a solution of HAuCl₄·xH₂O in aqua regia have been studied by¹⁹⁷Au Mössbauer spectroscopy. The relative amounts of AuCl ₄ ^? , of Au(0), and of an Au(J) species formed under certain process conditions were determined quantitatively. Deactivation of the catalyst at low and high temperatures was shown to be due to different mechanisms, and the reactivation of the catalyst by Cl₂ gas was studied.