Ag Aggregates Research Articles

One‐pot synthesis of silver particle aggregation as highly active SERS substrate

AbstractSilver particles with different degrees of aggregation were synthesized through a sodium dodecyl sulfate‐assisted one‐pot reaction in an aqueous medium. The products were characterized by transmission electron microscopy, scanning electron microscopy and UV‐visible spectroscopy. The results showed that the degree of aggregation of the Ag nanoparticles could be tuned by changing the reaction parameters, such as the reaction temperature and time. A possible formation process of the Ag aggregate is proposed on the basis of a series of experimental results. Moreover, the surface‐enhanced Raman scattering (SERS) effect of the Ag aggregates was evaluated by using rhodamine 6G as a Raman probe molecule. It was demonstrated that the SERS enhancement ability is related to the degree of aggregation of Ag particles, and a high SERS signal can be observed by selecting Ag nanoparticles with the proper degree of aggregation as substrates. Moreover, the aggregates showed good reproducibility and stability to SERS from organic molecules. Copyright © 2010 John Wiley & Sons, Ltd.

Condensation Behavior of Ag Aggregates on Liquid Surfaces

We report a condensation behavior of silver aggregates on silicone oil surfaces. The deposited Ag atoms diffuse and aggregate on the oil surface, and then form granular clusters and ramified islands. The apparent Ag coverage of the total area increases linearly with the nominal film thickness h for h < 0.9 nm. However, the coverage exhibits a fluctuation behavior for 1.0 nm < h < 2.5 nm. it is found that the anomalous behavior of the coverage is resulted from a characteristic material condensation process in the aggregates.

Effect of Surfactant Addition on the Structure of Silver Aggregates Produced by Galvanic Cell Reaction and Fabrication of Superhydrophobic Surfaces

The effect of cationic or anionic surfactant on the structure of the silver particles produced by galvanic cell reaction is studied. In the absence of any surfactant, both spherical and spindle-like Ag particles are produced, which exhibit binary structures with both micro- and nanoscale characteristics. Addition of cationic surfactant cetyltrimethylammonium bromide (CTAB) in the reaction solution results in the formation of spherical Ag particles with much smaller sizes. While anionic surfactant sodium dodecyl sulfate (SDS) results in the spindle-like Ag particles. Moreover, the rough Ag surfaces can be easily fabricated by direct deposition of the Ag aggregates onto the silicon surface from solution. After further chemisorption of a self-assembled monolayer of n-dodecanethiol, the Ag aggregates exhibit superhydrophobic properties.

CrN–Ag nanocomposite coatings: High-temperature tribological response

5 μm thick CrN–Ag composite layers, containing 22 at.% Ag solid lubricant, were deposited by reactive magnetron co-sputtering on 440C stainless-steel substrates at T s = 300, 400, 500, and 600 °C. Vacuum annealing experiments at T a = 425, 525, and 625 °C show that Ag diffuses to the coating surface to form lubricious surface aggregates and that the rate for Ag lubricant transport increases with T a > T s, as determined by quantitative electron microscopy surface analyses. However, the Ag remains in the CrN matrix for T a < T s. This is attributed to an increasing Ag aggregate size within the composite with increasing T s, leading to a decrease in the chemical potential and, in turn, negligible diffusion until T a exceeds T s. The tribological response during ball-on-disk sliding in ambient air against 6-mm-alumina balls was found to depend strongly on both T s and the testing temperature T t = 450, 550, and 650 °C. The key parameter that determines lubricant transport and, in turn, tribological properties is the temperature difference Δ T = T t − T s. For Δ T < 0, the lubricious Ag within the CrN matrix causes a moderate (25–35%) reduction in the friction coefficient from μ = 0.41–0.51 for pure CrN to μ = 0.31–0.34 for the composite. In contrast, at Δ T > 0, the Ag diffuses to the surface, partially transfers to the counterface, and forms a lubricious interface layer, reducing the friction by up to 60% to μ = 0.16–0.24. However, continuous wear at Δ T > 0 causes Ag depletion which results in an increasing friction and wear, and ultimately mechanical failure of the coating. The lubricant transport rate and, in turn, the lifetime before the Ag is depleted depend both strongly on Δ T, and can therefore be optimized for a given application.

Substituent-dependent structures and catalysis of benzimidazole-tethered N-heterocyclic carbene complexes of Ag(i), Ni(ii) and Pd(ii)

Homoleptic cationic benzimidazole-imidazolin-2-ylidene N-heterocyclic carbene (NHC = L) complexes of Ni(II) and Pd(II) have been prepared directly from the ligand precursor in salt form [H.L]Cl and from the transmetallation route via Ag(I). The N-tether of the imidazolinylidene ring imposes a significant influence on the nuclearity of the intermediate Ag(I)-NHC complexes and the geometric isomer outcome of the d(8) products. Use of a benzyl-substituted NHC gives [Ag(4)(L(Bn))(2)Cl(4)], 2a (from [HL(Bn)]Cl, 1a, and Ag(2)O) (Bn = benzyl), which shows an alignment of four silver atoms bridged by the difunctional C-N ligands and chlorides. Its transmetallation with NiCl(2)(PPh(3))(4) and PdCl(2)(MeCN)(2) results in double-metal salts 2[M(L(Bn))(2)](2+)[Ag(4)Cl(8)](4-) (M = Ni (3a) and Pd (4a)). The nuclearity of the Ag(4) aggregate is maintained in the transmetallation process. Their Ag-free forms [M(L(Bn))(2)]Cl(2) (M = Ni (5) and Pd (6)) were prepared by direct deprotonation of 1a with M(OAc)(2). The two carbenic carbon donor are cis- to each other in both 3a and 4a, thus imposing the weaker sigma-benzimidazole nitrogen donor to be trans to them. A sterically demanding mesityl pendant however gives the dinuclear dissymmetic [Ag(2)(L(Mes))(2)Cl(2)], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a 2-coordinated [Ag(I)(NHC)(2)] and 4-coordinated [Ag(I)(Imd)(2)Cl(2)] (Imd = imidazole). Transmetallation of the latter, or direct metallation from [HL(Mes)]Cl, 1b, gives [M(L(Mes))(2)]Cl(2) (M = Ni (3b) and Pd (4b)) with the two carbonic carbon trans to each other. The catalytic potential of 3b and 4b, which are more effective than 5 and 6, has been demonstrated by their high activities in Ni-catalyzed Kumada at room temperature and Pd-catalyzed Heck couplings of aryl and/or heteroaryl halides, respectively.

Rational reductive fusion of two heterometallic clusters: formation of a highly stable, intensely phosphorescent Au–Ag aggregate and application in two-photon imaging in human mesenchymal stem cells

An unprecedented Au-Ag alkynyl-diphosphine aggregate, obtained via CO-reduction of a mixture of simple reagents, exhibits intense room-temperature phosphorescence free from O(2) quenching, and serves as an excellent phosphorescence dye suited for both one- and two-photon imaging in human stem cells.

Photochemical synthesis of silver particles using water-in-ionic liquid microemulsions in high-pressure CO 2

Silver particles (Ag particles) were synthesized by the photoreduction of silver perchlorate (AgClO 4) in water-in-ionic liquid (IL) microemulsions consisting of nonionic surfactant Tween 20, water, and ionic liquids, [1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF 4]) or 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIm][BF 4])], mixed with a high-pressure (25 MPa) CO 2. The time evolution of the Ag particle formation by photoreduction was investigated by UV–Vis, cryo-TEM, extended X-ray absorption fine structure (EXAFS), and small-angle X-ray scattering (SAXS) measurements. In the particle formation process, aggregation and precipitation of Ag particles were suppressed under high-pressure CO 2. The average diameters of the metallic Ag particles prepared in water-in-[BMIm][BF 4] and water-in-[OMIm][BF 4] microemulsions were estimated from TEM to be 3.2 and 3.7 nm, respectively. SAXS analysis shows that the average diameters of the water droplets, which consisted of ionic precursors of AgClO 4 and Ag particles (or Ag aggregates), were estimated to be about 30–40 nm. In the process of Ag particle formation, the water droplet size under high-pressure CO 2 is more effectively regulated than that under ambient air, thereby preventing Ag particles from aggregating and precipitating.

CrN–Ag nanocomposite coatings: Tribology at room temperature and during a temperature ramp

5 µm thick CrN–Ag composite layers with 22 at.% Ag were deposited by reactive magnetron co-sputtering on 440C stainless steel substrates. Increasing the growth temperature from T s = 500 to 600 to 700 °C leads to Ag segregation within the CrN matrix and the formation of embedded lamellar Ag aggregates with increasing size, < 10 5, 9 × 10 6, and 7 × 10 7 nm 3, respectively. Ball-on-disk tests against 100Cr6 steel, followed by optical profilometry and energy dispersive spectroscopy, indicate that the Ag grains for T s = 500 °C are too small to facilitate an effective lubricious surface layer, resulting in a friction coefficient μ = 0.58 and a composite coating wear rate of 3.8 × 10 − 6 mm 3/Nm that are nearly identical to those measured for pure CrN with μ = 0.64 and 3.6 × 10 − 6 mm 3/Nm. The T s = 600 °C coating exhibits a Ag concentration which is 15% higher within than outside the wear track, and acts as a lubricious layer that reduces μ to 0.47 and yields a 16× and 2.4× lower wear rate for coating and counterface, respectively. T s = 700 °C leads to a dramatic increase in surface roughness and an associated increase in friction, μ = 0.85, and wear, 9.9 × 10 − 6 mm 3/Nm. Replacing the steel counterface with an alumina ball results in the lowest μ = 0.50 for T s = 500 °C, attributed to the presence of Ag and the relatively low hardness of 16.5 GPa for this particular coating. In contrast, friction and wear increase dramatically for T s = 600 °C, which is attributed to a breakdown of the lubricious Ag layer by the harder counterface. The transient friction coefficient μ t during experiments with continuously increasing testing temperature T t = 25–700 °C initially decreases for all samples, attributed to drying of the environment and an effective softening of both coating and counterface. For the T s = 500 °C coating, a temperature activated solid lubricant transport yields a lubricious Ag surface layer and a very low μ t = 0.05 at T t ~ 500 °C. All coatings exhibit an increasing μ t for T t > 500 °C, which is attributed to oxidative degradation.

Wavelength-Dependent Surface-Enhanced Resonance Raman Scattering by Excitation of a Transverse Localized Surface Plasmon

We measured the polarization dependences of surface-enhanced (resonance) Raman scattering (SER(R)S) and localized surface plasmon (LSP) resonance Rayleigh scattering for the same single Ag nanoaggregate to explore whether SER(R)S is dominated by the resonance of incident light with LSPs. We excited the LSPs polarized parallel to the short axis of the Ag nanoaggregate (transverse LSPs) using laser lines from an Ar ion laser (λ = 458, 488, and 514 nm). The polarization dependence of SERRS intensities is opposite to that of the transverse LSPs whose maximum wavelengths are below 500 nm. On the contrary, the polarization dependence of SERS intensities becomes similar to that of the transverse LSPs with the maximum wavelengths above 550 nm. The wavelength-dependent correlation between the transverse LSP and SER(R)S due to the absence and presence of just one junction on the short axis of the Ag aggregate will lead to an accurate understanding of the correlation between LSP and SER(R)S.

Ancillary Ligands and Spectator Cations as Controlling Factors in the Construction of Coordination and Hydrogen‐Bonded Networks with the tert‐Bu‐CC⊃Agn (n=4, 5) Supramolecular Synthon

Supramolecular networks constructed with the tBu--C[triple bond]C superset Ag(n) (n=4 or 5) metal-ligand synthon and trifluoroacetate have been transformed through the introduction of ancillary terminal nitrile ligands, from acetonitrile through propionitrile to tert-butyronitrile, giving rise to a 2D coordination network in AgC[triple chemical bond]CtBu3 AgCF(3)CO(2)H(2)O (1), a 2D hydrogen-bonded network in AgC[triple chemical bond]CtBu5 AgCF(3)CO(2)4 CH(3)CNH(2)O (2), a 2D hybrid coordination/hydrogen-bonded network in AgC[triple chemical bond]CtBu3 AgCF(3)CO(2)CH(3)CH(2)CN2 H(2)O (3), and another 2D coordination network in AgC[triple chemical bond]CtBu4 AgCF(3)CO(2) (CH(3))(3)CCN2 H(2)O (4). Concomitantly, the linkage modes between adjacent ethynide-bound Ag(n) aggregates in these compounds are also changed. A layer-type hydrogen-bonded host lattice in isostructural AgC[triple chemical bond]CtBu4 AgCF(3)CO(2)(R(4)N)(CF(3)CO(2)) 2 H(2)O (R(4)=BnMe(3), 5; R(4)=Et(4), 6; R(4)=nPr(4), 7) is obtained by introducing quaternary ammonium cations as guest templates, which occupy the interstices and thereby mediate the interlayer separation. Use of the bulky nBu(4)N(+) cation leads to disruption of the host network in AgC[triple bond]CtBu4 AgCF(3)CO(2)3[(nBu(4)N)(CF(3)CO(2))]H(2)O (8) with generation of a discrete dense nido-Ag(5) cluster.

Silver to erbium energy transfer in phosphate glasses

Silver and erbium co-doped sodium metaphosphate glasses were prepared by standard melt-quench technique. The samples underwent heat or 355 nm laser radiation exposure treatments in order to promote silver clustering. Absorption and photoluminescence spectra allowed distinguishing several active species related to silver, whose concentration in the samples depends on the treatment. A broad emission of silver related species has been detected in the visible region of the spectrum. The excitation spectra of the band and of the 1.5 μm luminescence of Er demonstrated energy transfer from Ag aggregates to Er. Differently from silicate glasses, in phosphates the silver to erbium energy transfer is very weak.

Enhancement of spontaneous and stimulated emission of a rhodamine 6G dye by an Ag aggregate

We have demonstrated that by adding the solution of aggregated silver nanoparticles to the solution of rhodamine $6\mathrm{G}\phantom{\rule{0.3em}{0ex}}\mathrm{dye}$, one can enhance the efficiency of spontaneous and stimulated emission. We attribute an increase of the spontaneous emission intensity of dye to the increase of the absorption efficiency caused by the field enhancements in metallic nanostructures associated with surface plasmons. The enhancement of the stimulated emission of dye, which has the same nature as the enhancement of absorption, was observed in the pump-probe and laser experiments.

Enhancement of surface plasmons in an Ag aggregate by optical gain in a dielectric medium

We have observed the compensation of loss in a metal by a gain in a dielectric medium in the mixture of an Ag aggregate and a Rhodamine 6G dye. The demonstrated sixfold enhancement of the Rayleigh scattering is the evidence of the enhancement of the surface-plasmon resonance. The reported experimental observation facilitates many applications of nanoplasmonics.

Effect of Ag aggregate on spectroscopic properties of Eu: Y2O3 nanoparticles

We have shown that excitation and emission spectra and emission kinetics of Eu: Y2O3 powder can be influenced by aggregated Ag nanoparticles. This demonstrates that basic physical and spectroscopic properties of novel advanced luminescent and gain media and, in particular, materials for random lasers can be engineered at the nanoscale level.

Optical Absorption of Ag Particles Dispersed in a SiO2 Amorphous Matrix

Composite materials of Ag species embedded in SiO2 matrix were prepared by the sol-gel method. Two kinds of sample preparation were used. In the first one, the Ag aggregates were synthesized using two different reduction solutions, obtaining fine particles with a quasi-spherical shape and particles with a dendrite-like form, that were later added to the SiO2 matrix. In the second one, the Ag aggregates were formed in the SiO2 matrix from silver nitrate solutions. The prepared samples were annealed in air at different temperatures. By using uv-visible spectroscopy, X-ray diffraction, EDS, DTA, TGA and SEM, the structures of all the samples were studied. It was found that the embedded species and the heat treatments modify strongly the optical properties of the samples.

Electronic structures of Ag/Ge(0 0 1) surfaces

The electronic states of Ag deposited Ge(0 0 1) surfaces at 100 K and their changes after the annealing at room temperature were investigated by using low-temperature angle-resolved photoelectron spectroscopy. Below 1 mono-layer of Ag, the surface did not show any metallic behaviors both at 130 K and after the annealing at room temperature. At 130 K, the Ag atoms adsorb as Ag ad-dimer at valley bridge site between the adjacent substrate dimer rows without breaking the Ge dimer structure. After the annealing at room temperature, the signal from the Ge dimer back-bond is significantly modified indicating changes in the dimer structure. All these findings are consistent with the previous observations by scanning tunneling microscopy: thermal diffusion and clustering of initially adsorbed Ag aggregates below 180 K; transformation of internal structure of Ag islands by further annealing above 240 K. The present study strongly supports the transformation as a restructuring process between Ag and Ge atoms at the interface.

Combining Layer-by-Layer Assembly with Electrodeposition of Silver Aggregates for Fabricating Superhydrophobic Surfaces

Taking advantage of the stability and penetrability of layer-by-layer (LbL) films, we develop a novel method to fabricate a branchlike structure of Ag aggregates on the matrix of a LbL polyelectrolyte multilayer by an electrodeposition technique. The morphology of Ag aggregates can be adjusted by electrodeposition time and potential. Moreover, after further chemisorption of a self-assembled monolayer of n-dodecanethiol, the as-prepared surface becomes superhydrophobic with a contact angle as high as 154 degrees and a tilt angle lower than 3 degrees.

Spectroscopic studies of liquid solutions of R6G laser dye and Ag nanoparticle aggregates

We have found that R6G laser dye in a concentration of 0.1 g l−1 mixed with a solution of aggregated silver nanoparticles exhibits a new emission band with a maximum at 612 nm. This band does not exist in pure dye of comparable concentration or in a mixture of dye with a solution of single silver nanoparticles. A qualitatively similar red-shifted emission band is observed in pure R6G dye at very high concentration (3.8 or 16.7 g l−1). In both cases, no changes occur to the shapes of the absorption spectra of the dye. We explain the observed spectral changes in terms of J-aggregates of R6G molecules whose formation is probable in the presence of Ag aggregates with a complicated surface structure and is much less likely in the case of adsorption of dye molecules on single Ag nanoparticles. Alternatively, many features observed in the experiment can be explained by an enhancement of the rates of spontaneous radiative transitions in the proximity of metallic particles, which is due to a modification of the local density of electromagnetic modes in the vicinity of metal surfaces at energies resonant with surface plasmon resonances.

Atomization Mechanism of Silver Deposited on Pyrolyzed Ascorbic Acid in Graphite Furnace Atomic Absorption Spectrometry

The atomization of 0.2 ng Ag deposited on ascorbic acid pyrolysed at various temperatures was observed and the activation parameters were estimated. The mass of Ag and the pyrolysis conditions were selected to form droplets without any dispersion on a PG wall and active carbon. The kinetic order obtained indicated the formation of Ag aggregates on the active carbon at temperatures below 1560 K and the formation of individually dispersed atoms of Ag on that at temperatures over 1700 K. The degree of interaction between the Ag atom and an active site on the high temperature treated active carbon was increased, resulting from their greater interaction.

Raman scattering from single Ag aggregates in presence of EDTA

We report on Surface Enhanced Raman Scattering (SERS) activity of silver colloids prepared by a radiolytic method followed by a chemical development by EthyleneDiAmineTetraAcetic (EDTA) acid. On these colloids, in presence of Rhodamine 6G and of EDTA, we have observed Single Molecule SERS spectra. We show that the spectra observed on single silver aggregates in presence of EDTA evidence a (photo)oxidative character of sites that exhibit a giant SERS enhancement.