Free Ag Nanoparticles Research Articles

Effects of reduced graphene oxide on humic acid-mediated transformation and environmental risks of silver ions

The reduction of Ag+ mediated by natural organic matters has been demonstrated to be an important process of Ag+ transformation and would influence the risks of Ag+ and Ag-containing materials in aquatic environment. Considering the large production of carbon nanomaterials (CNMs) and their inevitable release into the environment, the effects of CNMs on Ag transformation are of considerable interest. This study demonstrated that the humic acid-mediated reduction of Ag+ to free Ag nanoparticles (AgNPs) in aqueous phase was suppressed by coexisting reduced graphene oxide (rGO). A large amount of Ag+ was reduced on rGO surface, resulting in the generation of AgNPs-rGO composites. rGO at concentrations of 1–2 orders of magnitude lower than those of Ag+ would exhibit significant effects. The X-ray absorption fine structure spectroscopy study showed that Ag+ was first adsorbed on rGO surface cooperatively with humic acid and then rapidly reduced to AgNPs. The hydroxylic-OH on rGO could participate in the AgNPs formation and was oxidized to carbonyl during the reduction of Ag+. Additionally, the formed AgNPs-rGO had a relatively lower environmental risk compared to AgNPs or rGO alone. Overall, these results improve our understanding of the interaction between CNMs and Ag+ in aquatic systems.

Interaction of multiply charged ions with large free silver nanoparticles: Multielectron capture, fragmentation, and sputtering phenomena

We report on the interaction of multiply charged ions at keV energies with free Ag nanoparticles with diameters of $\ensuremath{\sim}6$ nm (containing about 6600 Ag atoms). As the ionization energy increases only very slowly with the degree of ionization, multielectron capture processes are very likely to occur in peripheral collisions with large cluster sizes. However, due to the large particle size, the produced highly charged Ag nanoparticles are mostly stable. For projectile charge states below $q=8$ the geometrical cross section overcomes the cross section for peripheral electron transfer, and penetrating collisions become dominant. Therefore, these collisions can be described as ion collision with a nanosurface, where the distribution of small-size fragments with $nl20$ is due to sputtering events from the nanoparticle surface. This is in contrast to nanoparticles with a smaller diameter ($l2$ nm), where small fragments are produced by fission of multiply charged clusters.

Influence of the Silica Support on the Structure and the Morphology of Silver Nanoparticles: A Molecular Simulation Study

We investigate the effect of the support on the structure and morphology of deposited metallic nanoparticles via molecular simulations at zero temperature. We focus on silver clusters in contact with a substrate which exhibits weak interactions, as amorphous silica, frequently used in atomic deposition experiments. Three levels of approximation are used to describe the substrate: (i) the smooth wall approximation where the support is modeled by a square-well whose depth is related to the adhesion energy of the nanoparticle, (ii) an atomistic model of flat amorphous silica obtained from Monte Carlo simulations with effective pair potentials, and (iii) the same silica substrate partially melted in order to roughen the surface at the nanometric scale. The metal–metal interactions are modeled using tight-binding semiempirical potentials, and the metal–silica interactions are modeled with Lennard-Jones potentials fitted to ab initio or experimental data. The icosahedron/decahedron/truncated octahedron sequence found for free Ag nanoparticles is shown to be affected by the substrate (square-well), which favors truncated octahedrons deposited on their (111) facets. The aspect ratio is shown to follow the Wulff–Kaishew theorem for large clusters. The introduction of an atomistic description of the flat SiO2 surface does not change the results significantly, while the presence of a nanometric roughness induces drastic changes. Vertex orientations of nanoparticles are possible in the surface hollows, and decahedron structures are slightly favored for sizes smaller than 1200 atoms.

Ag–SiO2 nanocomposites with plum-pudding structure as catalyst for hydrogenation of 4-nitrophenol

Ag–SiO2 nanocomposites (NCs) with “plum-pudding” structure have been synthesized using a facile reverse micelle method. The as-synthesized Ag–SiO2 NCs were calcined at 623K to remove the surfactants and then characterized by XRD, FE-SEM, TEM, EDX, ICP and N2 adsorption–desorption. The obtained results demonstrated that many ultrafine Ag nanoparticles (NPs) with size of ~2nm are well dispersed in each SiO2 nanosphere (28nm). In comparison with free Ag NPs and SiO2 supported Ag NPs, the Ag–SiO2 NCs exhibited a better catalytic activity for the reduction of 4-nitrophenol with a turnover frequency of 489h−1 at room temperature. Ag–SiO2 NCs also displayed a superior catalytic activity for hydrogenation of 2-NP and 3-NP. The activation energy of Ag–SiO2 NCs was estimated to be about 34.4kJmol−1, which was lower than most of the reported values for the same reaction using Ag-based catalysts, indicating the superior catalytic performance of these plum-pudding structured catalysts.

Silver nanostructures from Ag(CN) 2 − reduction by citrate ions in the presence of dodecyl sulfate and Cu2+ ions. Synthesis and characterization

The synthesis of silver nanoparticles has been investigated using Ag(CN)2− species as precursor, citrate ions as reducing agent, and dodecyl sulfate ions as stabilizer, at pH 11 and 97 °C, in a batch stirred glass reactor. The role of Cu2+ ions in the synthesis was also studied. Bird-of-paradise flower-type nanostructures composed of AgCN nanowires having inside Ag and AgCN nanoparticles were produced in the absence of Cu2+ ions. The nanostructures slowly grew and transformed to AgCN nanowires with embedded Ag and AgCN nanoparticles, having a mean size of 9.7 ± 3.6 nm. The presence of Cu2+ ions in the synthesis significantly enhanced the production of the nanostructures. Nanowires having a thickness of 63 ± 33 nm and length of up to 20 μm were produced. Cu2+ ions also simultaneously lead to the synthesis of ordinary free Ag nanoparticles with a bimodal size distribution (mean sizes of 9.9 ± 3.9 and 65.5 ± 27 nm) and a low experimental formation kinetic rate constant of 1.22 × 10−4 s−1. Feasible mechanisms are presented for the origin of the AgCN nanowires, Ag and AgCN nanoparticles inside the nanowires, and for the free Ag nanoparticles. UV/Vis spectrometry was used to measure the surface plasmon resonance of the nanoparticles and the synthesis kinetic rate constant of the free Ag nanoparticles. ATR–FTIR spectroscopy, EDS–SEM, EDS–TEM, and HRTEM were used to characterize the size, crystal structure, texture, and chemical composition of the synthesis products.

One-pot hydrothermal synthesis of Ag-reduced graphene oxide composite with ionic liquid

A one-pot hydrothermal reaction was used to prepare a reduced graphene oxide sheets (RGO)-silver (Ag) nanoparticles composite using graphite oxide and silver nitrate as starting materials. It was found that graphene oxide could be well reduced under the hydrothermal conditions with ascorbic acid as the reductant, while the Ag nanoparticles were grown on the RGO surface simultaneously. The reduction of graphene oxide and synthesizing of Ag-RGO were confirmed by Fourier-transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, X-ray diffraction and X-ray photoelectron spectroscopy. Microscopy techniques (scanning electron microscopy, atomic force microscopy and transmission electron microscopy) have been employed to probe the morphological characteristics as well as to investigate the exfoliation of RGO sheets. The intensities of the Raman signals of RGO in the composite are greatly increased by the attached Ag nanoparticles, showing surface-enhanced Raman scattering activity. Besides, it was found that the antibacterial activity of free Ag nanoparticles is retained in the composite, suggesting that it can be used as RGO-based biomaterials.

Facile synthesis and application of Ag-chemically converted graphene nanocomposite

An in situ chemical synthesis approach has been employed to prepare an Ag-chemically converted graphene (CCG) nanocomposite. The reduction of graphene oxide sheets was accompanied by generation of Ag nanoparticles. The structure and composition of the nanocomposites were confirmed by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction. TEM and AFM results suggest a homogeneous distribution of Ag nanoparticles (5–10 nm in size) on CCG sheets. The intensities of the Raman signals of CCG in such nanocomposites are greatly increased by the attached silver nanoparticles, i.e., there is surface-enhanced Raman scattering activity. In addition, it was found that the antibacterial activity of free Ag nanoparticles is retained in the nanocomposites, which suggests they can be used as graphene-based biomaterials.

Theoretical study of the surface energy, stress, and lattice contraction of silver nanoparticles

We employ first-principles and empirical computational methods to study the surface energy and surface stress of silver nanoparticles. The structures, cohesive energies, and lattice contractions of spherical Ag nanoclusters in the size range $0.5--5.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are analyzed using two different theoretical approaches: an ab initio density functional pseudopotential technique combined with the generalized gradient approximation and the embedded atom method. The surface energies and stresses obtained via the embedded atom method are found to be in good agreement with those predicted by the gradient-corrected ab initio density functional formalism. We estimate the surface energy of Ag nanoclusters to be in the range of $1.0--2.2\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$. Our values are close to the bulk surface energy of silver, but are significantly lower than the recently reported value of $7.2\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ for free Ag nanoparticles derived from the Kelvin equation.

Higher Surface Energy of Free Nanoparticles

We present an accurate online method for the study of size-dependent evaporation of free nanoparticles allowing us to detect a size change of 0.1 nm. This method is applied to Ag nanoparticles. The linear relation between the onset temperature of evaporation and the inverse of the particle size verifies the Kelvin effect and predicts a surface energy of 7.2 J/m(2) for free Ag nanoparticles. The surface energy of nanoparticles is significantly higher as compared to that of the bulk and is essential for processes such as melting, coalescence, evaporation, growth, etc., of nanoparticles.