Spontaneous formation of silver particles in basic 2-propanol

Extracellular biosynthesis of silver nanoparticles: effects of shape-directing cetyltrimethylammonium bromide, pH, sunlight and additives

The work reported in this paper describes the preparation, morphology, stability and sensitivity of Ag-nanoparticles towards sunlight using Allium sativum, garlic extract for the first time. The synthesized silver particles show an intense surface plasmon resonance band in the visible region at 410 nm. The position of the wavelength maxima, blue and red shift, strongly depends on the sunlight and pH. TEM analysis revealed the presence of spherical, different size (from 5.0 to 30 nm) and garlic constituents bio-conjugated, stabilized and/or layered silver nanoparticles. The concentrations of garlic extract, cetyltrimethylammonium bromide, Ag(+) ions and reaction time play vital roles for nucleus formation and the growth processes. Sulfur-containing biomolecules of extract, especially cysteine, are responsible for the reduction of Ag(+) ions into metallic Ag(0). The agglomeration number of the silver nanoparticles (N Ag) and the average number of free electrons per particle (n fe) are calculated and discussed.

Fast process to decorate silver nanoparticles on carbon nanomaterials for preparing high-performance flexible transparent conductive films

This work demonstrates a fast process to decorate silver (Ag) nanoparticles onto the functionalized few-walled carbon nanotubes (f-FWCNTs) and graphene nanosheets (f-GNs). The Ag-coated carbon nanomaterials were used as fillers, which mixed with poly(3,4-ethylenedioxythiophene)-poly(4-stryensulfonate) (PEDOT:PSS) for preparing high optoelectronic performances of flexible transparent conductive films (TCFs). The Ag nanoparticles with a particle size of approximate 5 nm were uniformly distributed on the surfaces of the f-FWCNTs (Ag@f-FWCNTs) and the f-GNs (Ag@f-GNs). The Ag ions play the role of electron acceptors during the reduction process, which increases the hole concentrations in PEDOT:PSS, f-FWCNTs, and f-GNs, therefore enhancing the electrical conductivity of the TCFs. Additionally, the Schottky barrier was decreased because of the increase of work functions of the carbon fillers caused by Ag decoration. The X-ray diffraction spectrum of Ag@f-GNs depicts the formations of the face-centered cubic Ag nanoparticles, and the peak of the (002) graphene plane slightly shifted to the lower frequency, indicating that the f-GN interlayer was intercalated with Ag ions or Ag nanoparticles. When the mixture of 2.0 wt % Ag@f-FWCNTs and 8.0 wt % Ag@f-GNs containing PEDOT:PSS dispersant was coated onto a poly(ethylene terephthalate) (PET) substrate, outstanding optoelectronic properties with a sheet resistance of 50.3 Ω/sq and a transmittance of 79.73% at a wavelength of 550 nm were achieved.

Interactions of aqueous Ag+ with fulvic acids: mechanisms of silver nanoparticle formation and investigation of stability

This study investigated the possible natural formation of silver nanoparticles (AgNPs) in Ag(+)-fulvic acid (FA) solutions under various environmentally relevant conditions (temperature, pH, and UV light). Increase in temperature (24-90 °C) and pH (6.1-9.0) of Ag(+)-Suwannee River fulvic acid (SRFA) solutions accelerated the appearance of the characteristic surface plasmon resonance (SPR) of AgNPs. The rate of AgNP formation via reduction of Ag(+) in the presence of different FAs (SRFA, Pahokee Peat fulvic acid, PPFA, Nordic lake fulvic acid, NLFA) and Suwannee River humic acid (SRHA) followed the order NLFA > SRHA > PPFA > SRFA. This order was found to be related to the free radical content of the acids, which was consistent with the proposed mechanism. The same order of AgNP growth was seen upon UV light illumination of Ag(+)-FA and Ag(+)-HA mixtures in moderately hard reconstituted water (MHRW). Stability studies of AgNPs, formed from the interactions of Ag(+)-SRFA, over a period of several months showed that these AgNPs were highly stable with SPR peak reductions of only ~15%. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements revealed bimodal particle size distributions of aged AgNPs. The stable AgNPs formed through the reduction of Ag(+) by fulvic and humic acid fractions of natural organic matter in the environment may be transported over significant distances and might also influence the overall bioavailability and ecotoxicity of AgNPs.

Photocatalytic production of 1O2 and *OH mediated by silver oxidation during the photoinactivation of Escherichia coli with TiO2

Ag loaded TiO(2) was applied in the photocatalytic inactivation of Escherichia coli under ultraviolet (UV) and visible (Vis) light irradiations. Ag enhanced the TiO(2) photodisinfecting effect under Vis irradiation promoting the formation of singlet oxygen and hydroxyl radicals as identified by EPR analyses. Ag nanoparticles, determined on TEM analyses, undergo an oxidation process on the TiO(2)'s surface under UV or Vis irradiation as observed by XPS. In particular, UV pre-irradiation of the material totally diminished its photodisinfection activity under a subsequent Vis irradiation test. Under UV, photodegradation of dichloroacetic acid (DCA), attributed to photoproduced holes in TiO(2), was inhibited by the presence of Ag suggesting that oxidation of Ag(0) to Ag(+) and Ag(2+) is faster than the oxidative path of the TiO(2)'s holes on DCA molecules. Furthermore, photoassisted increased of Ag(+) concentration on TiO(2)'s surface enhances the bacteriostatic activity of the material in dark periods. Indeed, this latter dark contact of Ag(+)-TiO(2) and E. coli seems to induce recovering of the Vis light photoactivity promoted by the surface Ag photoactive species.

Silver nanoparticles to self-assembled films: green synthesis and characterization

In the present paper silver nanoparticle was synthesized by chemical reduction of silver nitrate by oxalic acid in aqueous solution. The nanoparticle film (self-assembled; mirror like illumination) on the wall of the clean glass surface was also observed after some days. The synthesized silver particles show an intense surface resonance plasmon band in the visible region at 425 nm. Transmission electron microscopy, selected areas electron diffraction, and UV-visible spectroscopy have been employed to characterize Ag-nanoparticles. The nanoparticle films were also observed using conventional visual and scanning electron microscope (spherical particles and size ranging from 23 to 245 nm). The transmission electron micrograph revealed that the average size of silver nanoparticle were ≤10 nm and 21-60 nm, respectively.

Synthesis and spectroscopic studies of stable aqueous dispersion of silver nanoparticles

A facile approach for the synthesis of stable aqueous dispersion of silver nanoparticles (AgNPs) using glucose as the reducing agent in water/micelles system, in which cetyltrimethylammonium bromide (CTAB) was used as capping agent (stabilizer) is described. The evolution of plasmon band of AgNPs was monitored under different conditions such as (a) concentration of sodium hydroxide, (b) concentration of glucose, (c) concentration of silver nitrate (d) concentration of CTAB, and (e) reaction time. AgNPs were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), fluorescence spectroscopy and FT-IR spectroscopy. The results revealed an easy and viable strategy for obtaining stable aqueous dispersion of AgNPs with well controlled shape and size below 30 nm in diameter.

Silver nanoparticles on titanate nanobelts via the self-assembly of weak polyelectrolytes: synthesis and photocatalytic properties

A weak-polyelectrolyte multilayer on a surface of titanate nanobelts (Ti-NBs) was utilized as a template for in situ Ag nanoparticle formation in the fabrication of Ag-loaded Ti-NBs nanocomposites. The polyelectrolyte multilayer (PEM) was fabricated using layer-by-layer self-assembly of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) on the surface of high-surface-area titanate nanobelts (Ti-NBs) synthesized using a hydrothermal procedure. The concentration of Ag nanoparticles in the PEM was controlled by repeating the ion-loading/reduction cycle. The subsequent annealing of the Ag/Ti-NBs-PEM nanocomposites yielded nanostructured crystalline Ag/Ti-NBs. Transmission electron microscopy (TEM) techniques (HRTEM, SAED) and x-ray powder diffraction (XRD) were employed to evaluate the morphological, structural and growth characteristics of the silver nanocrystallites in the Ag/Ti-NBs nanocomposites. The UV-vis photoactivity of the as-fabricated nanocomposites was monitored by the degradation of the cationic dye methylene blue (MB). An enhanced UV photo-efficiency was observed for the Ag/Ti-NBs nanocomposites compared with pure Ti-NBs. As-fabricated Ag(x)/Ti-NBs nanocomposites also exhibited visible photoactivity assisted by the near-field amplitudes of the localized surface plasmon resonance (LSPR) of the silver nanoparticles in the 1D nanocomposite.

Silver cluster formation, dynamics, and chemistry in metal-organic frameworks

Synthetic methods used to produce metal nanoparticles typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with the potential for both steric and chemical stabilization. We report a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions < or =1 nm, with a significant fraction existing as Ag(3) clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible.

In situ time-resolved XAFS studies of metal particle formation by photoreduction in polymer solutions

Formation mechanisms of metal particles (rhodium (Rh) and palladium (Pd) particles) in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by photoreduction were investigated by UV-vis, transmission electron microscopy (TEM), and in situ X-ray absorption fine structure (in situ XAFS). The average diameter of the Rh and Pd particles prepared by the photoreduction was estimated from TEM to be 2.2 and 2.0 nm, respectively. In situ energy-dispersive XAFS (in situ DXAFS) measurements were performed to investigate the reduction process of Rh(III) and Pd(II) aqua chlorocomplexes as well as their particle formation process. The reduction rate of these aqua chlorocomplexes could be quantitatively evaluated from the change of X-ray absorbance assigned to the contribution of metal-chloride bonds in these complexes. The reduction rate of Rh(III) aqua chlorocomplexes was found to be slower than that of Pd(II). It was also demonstrated that the reduction process of Rh(III) complexes possessed an induction period before the onset of Rh particle formation, although the Pd(II) complexes displayed no induction period, since the reduction of Pd(II) occurred immediately after mixing of an ethanol solution of Pd(II) complexes with aqueous PVP solutions.

Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics?

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Influence of an adsorption layer and its evolvement on the formation of Ag

The formation mechanism of Ag in an adsorption layer and the influence of temperature on the generation of Ag particles were studied. Two reaction systems were designed to explore the different reduction mechanisms of Ag+ in an adsorption layer and in bulk. A UV-vis adsorption spectrometer was employed to monitor the reaction process of Ag+ in two reaction systems. The results indicated that the formation mechanism of silver in an adsorption layer was largely different from that in alcohol bulk, which led to various morphologies of Ag particles in the two systems. Temperature experiments suggested that the induction time and morphology of Ag particles both changed greatly when temperature increased to 40 degrees C. The disappearance of physical-adsorption and the reaction occurring in alcohol bulk was the primary cause.

Mechanism of silver ion reduction in concentrated solutions of amphiphilic invertible polyesters in nonpolar solvent at room temperature

Fast formation and efficient stabilization of silver nanoparticles from [Ag(NH3)2]OH are achieved in concentrated nonpolar solutions of amphiphilic invertible polyesters based on poly(ethylene oxide) (PEO) and aliphatic dicarboxylic acids. Surface-modified silver nanoparticles able to be dispersed in both a polar and nonpolar medium are developed in the form of a ready-to-use colloidal solution with an enhanced silver concentration. The PEO fragments of polyesters form cavities (also called pseudo-crown ester structures) that can bind metal ions. The reduction of bound metal ions proceeds via oxidation of polyoxyethylene fragments. No chemical reducing agents are necessary in this approach. The polyesters act simultaneously as an efficient reducing agent and stabilizer. The main focus of the present research is to clarify the chemical mechanism of silver ion reduction in amphiphilic polyester solutions. A one-electron reduction mechanism is proposed to explain the formation of silver nanoparticles. The effect of the poly(ethylene oxide) fragment length and the polyester concentration are explored by examining several amphiphilic polyesters.

Surfactant assisted synthesis and spectroscopic characterization of selenium nanoparticles in ambient conditions

In this work, an attempt has been made to synthesize well-distributed stable selenium (Se) particles of nanosize dimensions via an aqueous micellar solution by the assistance of surfactants of two different polarities (anionic, sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and cationic, hexadecyltrimethylammonium bromide (CTAB)). The morphology of the particles was examined with transmission electron microscopy (TEM). X-ray analysis reveals that the particles have a monoclinic structure. The band gap of the particles was determined from UV-visible optical spectroscopic results. The size variation was estimated by employing a quantum confinement effect equation. The evolution of the selenium nanoparticles in AOT and CTAB micellar media was corroborated with the time-dependent absorption spectra. The influence of hydrazine hydrate concentrations on the formation kinetics of Se nanoparticles was also investigated. The capping ability of the surfactants has been quantitatively evaluated from Fourier transform infrared (FTIR) studies.

On the polyol synthesis of silver nanostructures: glycolaldehyde as a reducing agent

The polyol synthesis is a popular method of preparing metal nanostructures, yet the mechanism by which metal ions are reduced is poorly understood. Using a spectrophotometric method, we show, for the first time, that heating ethylene glycol (EG) in air results in its oxidation to glycolaldehyde (GA), a reductant capable of reducing most noble metal ions. The dependence of reducing power on temperature for EG can be explained by this temperature-dependent oxidation, and the factors influencing GA production can have a profound impact on the nucleation and growth kinetics. These new findings provide critical insight into how the polyol synthesis can be used to generate metal nanostructures with well-controlled shapes. For example, with the primary reductant identified, it becomes possible to evaluate and understand its explicit role in generating nanostructures of a specific shape to the exclusion of others.

Role of phenol derivatives in the formation of silver nanoparticles

We demonstrate that dihydroxy benzenes are excellent reducing agents and may be used to reduce silver ions to synthesize stable silver nanoparticles in air-saturated aqueous solutions. The formation of Ag nanoparticles in deaerated aqueous solution at high pH values suggests that the reduction of silver ions occurs due to oxidation of dihydroxy benzenes and probably on the surface of Ag2O. Pulse radiolysis studies show that the semi-quinone radical does not participate in the reduction of silver ions at short time scales. Nevertheless, results show that primary intermediates undergo slower transformation in the presence of dihydroxy benzenes than in their absence. This slow transformation eventually leads to the formation of silver nanoparticles. The Ag nanoparticles were characterized by UV-vis absorption spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). XRD and TEM techniques showed the presence of Ag nanoparticles with an average size of 30 nm.