Synthesis and Mechanism of Composition and Size Dependent Morphology Selection in Nanoparticles of Ag-Cu Alloys Processed by Laser Ablation Under Liquid Medium

We have synthesized Ag-Cu alloy nanoparticles of four different compositions by using the laser ablation technique with the target under aqueous medium. Following this, we report a morphological transition in the nanoparticles from a normal two-phase microstructure to a structure with random segregation and finally a core shell structure at small sizes as a function of Cu concentration. To illustrate the composition dependence of morphology, we report observations carried out on nanoparticles of two different sizes: similar to 5 and similar to 20 nm. The results could be rationalized through the thermodynamic modeling of free energy of phase mixing and wettability of the alloying phases.

High quality oxide-free metallic nanoparticles: a strategy for synthesis through laser ablation in aqueous medium

The paper explores the synthesis of oxide-free nanoparticles of Ag and Cu through laser ablation of pure targets under aqueous medium and tuning the quality and size through addition of Polyvinylpyrrolidone (PVP) in the medium. The size distribution of nanoparticles reduces from 37 +/- 30 nm and 13 +/- 5 nm to 32 +/- 12 nm and 4 +/- 1 nm for Ag and Cu with changes in PVP concentration from 0.00 to 0.02 M, respectively. Irregular shaped particles of Ag with Ag2O phase and a Cu-Cu2O core-shell particles form without the addition of PVP, while oxide layer is absent with 0.02 M of PVP. The recent understanding of the mechanism of particle formation during laser ablation under liquid medium allows us to rationalize our observation.

Synthesis and Characterization of Au@Pt Nanoparticles with Ultrathin Platinum Overlayers

Gold-core platinum-shell (Au@Pt) nanoparticles with ultrathin platinum overlayers, ranging from submonolayer to two monolayers of platinum atoms, were prepared at room-temperature using a scalable, wet-chemical synthesis route. The synthesis involved the reduction of chloroauric acid with tannic acid to form 5 nm (nominal dia.) gold nanoparticles followed by addition of desired amount of chloroplatinic acid and hydrazine to form platinum overlayers with bulk Pt/Au atomic ratios (Pt surface coverages) corresponding to 0.19 (half monolayer), 0.39 (monolayer), 0.58 (1.5 monolayer) and 0.88 (2 monolayers). The colloidal particles were coated with octadecanethiol and phase-transferred into chlroform-hexane mixture to facilitate sample preparation for structural characterization. The structure of the resultant nanoparticles were determined to be Au@Pt using HRTEM, SAED, XPS, UV-vis and confirmed by cyclic voltammetry (CV) studies. Monolayers of octadecanethiol coated Au@Pt nanoparticles were self-assembled at an air-water interface and transfer printed twice onto a gold substrate to form bilayer films for electrochemical characterization. Electrochemical activity on such films was observed only after the removal of the octadecanethiol ligand coating the nanoparticles, using a RF plasma etching process. The electrochemical activity (HOR, MOR studies) of Au@Pt nanoparticles was found to be highest for particles having a two atom thick platinum overlayer. These nanoparticles can significantly enhance platinum utilization in electrocatalytic applications as their platinum content based activity was three times higher than pure platinum nanoparticles.

Phase formation and stability of alloy phases in free nanoparticles: some insights

This paper explores phase formation and phase stability in free nanoparticles of binary alloys. A procedure for estimating the size and composition dependent free energies incorporating the contributions from the interfaces has been presented. Both single phase solid solution and two phase morphology containing interphase interfaces have been considered. A free energy scenario has been evaluated for two binary alloy systems Ag-Ni and Ag-Cu to predict the microstructure of the alloy nanoparticles at different size ranges as a function of composition. Both Ag-Cu and Ag-Ni systems exhibit wide bulk immiscibility. Ag-Ni nanoparticles were synthesized using the wet chemical synthesis technique whereas Ag-Cu nanoparticles were synthesized using laser ablation of a Ag-Cu target immersed in distilled water. Microstructural and compositional characterization of Ag-Ni and Ag-Cu nanoparticles on a single nanoparticle level was conducted using transmission electron microscopy. Nanoparticle microstructures observed from the microscopic investigation have been correlated with thermodynamic calculation results. It is shown that the observed two phase microstructure consisting of Ag-Ni solid solution in partial decomposed state coexisting with pure Ag phases in the case of Ag-Ni nanoparticles can be only be rationalized by invoking the tendency for phase separation of an initial solid solution with increase in nanoparticle size. Smaller sized Ag-Ni nanoparticles prefer a single phase solid solution microstructure. Due to an increase in particle size during the synthesis process the initial solid solution decomposes into an ultrafine scale phase separated microstructure. We have shown that it is necessary to invoke critical point phenomenon and wetting transition in systems showing a critical point that leads to phase separated Ag-Ni nanoparticles providing a catalytic substrate for the nucleation of equilibrium Ag over it. In the case of the Ag-Cu system, we report the experimental observation of a core shell structure at small sizes. This can be rationalized in terms of a metastable solid solution. It is argued that the nucleation barrier can prevent the formation of biphasic morphology with an internal interface. In such a situation, demixing of the solid solution can bring the system to a lower energy configuration. This has lead to the observed core-shell morphology in the Ag-Cu system during room temperature synthesis.

Comparative study of magnetic ordering in bulk and nanoparticles of Sm0.65Ca0.35MnO3: Magnetization and electron magnetic resonance measurements

To explore the effect of size reduction to nanoscale on the hole doped Sm0.65Ca0.35MnO3 compound, dc magnetic measurements and electron magnetic resonance (EMR) were done on bulk and nanoparticle samples in the temperature range 10 <= T <= 300 K. Magnetization measurement showed that the bulk sample undergoes a charge ordering transition at 240K and shows a mixed magnetic phase at low temperature. However, the nanosample underwent a ferromagnetic transition at 75 K, and the charge ordered state was destabilized on size reduction down to nanoscale. The low-temperature ferromagnetic component is found to be enhanced in nanoparticles as compared to their bulk counterpart. Interestingly around room temperature, bulk particles show higher magnetization where as at low temperature nanoparticles show higher magnetization. Ferromagnetism in the bulk is due to super exchange where as ferromagnetism in nanoparticles is due to uncompensated spins of the surface layer. Temperature variation of EMR parameters correlates well with the results of magnetic measurements. The magnetic behaviour of the nanoparticles is understood in terms of the core shell scenario. (C) 2015 AIP Publishing LLC.

Cytotoxicity of Ultrasmall Gold Nanoparticles on Planktonic and Biofilm Encapsulated Gram-Positive Staphylococci

The emergence of multidrug resistant bacteria, especially biofilm-associated Staphylococci, urgently requires novel antimicrobial agents. The antibacterial activity of ultrasmall gold nanoparticles (AuNPs) is tested against two gram positive: S. aureus and S. epidermidis and two gram negative: Escherichia coli and Pseudomonas aeruginosa strains. Ultrasmall AuNPs with core diameters of 0.8 and 1.4 nm and a triphenylphosphine-monosulfonate shell (Au0.8MS and Au1.4MS) both have minimum inhibitory concentration (MIC) and minimum bactericidal concentration of 25 x 10(-6)m Au]. Disc agar diffusion test demonstrates greater bactericidal activity of the Au0.8MS nanoparticles over Au1.4MS. In contrast, thiol-stabilized AuNPs with a diameter of 1.9 nm (AuroVist) cause no significant toxicity in any of the bacterial strains. Ultrasmall AuNPs cause a near 5 log bacterial growth reduction in the first 5 h of exposure, and incomplete recovery after 21 h. Bacteria show marked membrane blebbing and lysis in biofilm-associated bacteria treated with ultrasmall AuNP. Importantly, a twofold MIC dosage of Au0.8MS and Au1.4MS each cause around 80%-90% reduction in the viability of Staphylococci enveloped in biofilms. Altogether, this study demonstrates potential therapeutic activity of ultrasmall AuNPs as an effective treatment option against staphylococcal infections.

Sizable photocurrent and emission from solid state devices based on CdS nanoparticles

CdS nanoparticles exhibit size dependent optical and electrical properties. We report here the photocurrent and I-V characteristic studies of CdS nanoparticle devices. A sizable short circuit photocurrent was observed in the detection range governed by the size of the clusters. We speculate on the mechanisms leading to the photocurrent and emission in these nanometer scale systems.

Static analysis of infinite laminated cylindrical shell panel

This paper presents an approximate three-dimensional elasticity solution for an infinitely long, cross-ply laminated circular cylindrical shell panel with simply supported boundary conditions, subjected to an arbitrary discontinuous transverse loading. The solution is based on the principal assumption that the ratio of the thickness of the lamina to its middle surface radius is negligible compared to unity. The validity of this assumption and the range of application of this approximate solution have been established through a comparison with an exact solution. Results of classical and first-order shear deformation shell theories have been compared with the results of the present solution to bring out the accuracy of these theories. It is also shown that for very shallow shell panels the definition of a thin shell should be based on the ratio of thickness to chord width rather than the ratio of thickness to mean radius.

Nanoporous Pt with High Surface Area by Reaction-Limited Aggregation of Nanoparticles

Nanoporous structures with high active surface areas are critical for a variety of applications. Here, we present a general templateless strategy to produce such porous structures by controlled aggregation of nanostructured subunits and apply the principles for synthesizing nanoporous Pt for electrocatalytic oxidation of methanol. The nature of the aggregate produced is controlled by tuning the electrostatic interaction between surfactant-free nanoparticles in the solution phase. When the repulsive force between the particles is very large, the particles are stabilized in the solution while instantaneous aggregation leading to fractal-like structures results when the repulsive force is very low. Controlling the repulsive interaction to an optimum, intermediate value results in the formation of compact structures with very large surface areas. In the case of Pt, nanoporous clusters with an extremely high specific surface area (39 m(2)/g) and high activity for methanol oxidation have been produced. Preliminary investigations indicate that the method is general and can be easily extended to produce nanoporous structures of many inorganic materials.

Antibacterial Applications of Silver Nanoparticles Synthesized by Aqueous Extract of Azadirachta Indica (Neem) Leaves

Silver nanoparticles are known to have bactericidal effects. A new generation of dressings incorporating antimicrobial agents like silver nanoparticles is being formulated to reduce or prevent infections. The particles can be incorporated in materials and cloth rendering them sterile. Recently, it was found that aqueous silver ions can be reduced by aqueous extract of plant parts to generate extremely stable silver nanoparticles in water. Apart from being environmentally friendly process, use of Neem leaves extract might add synergistic antibacterial effect of Neem leaves to the biosynthesized nanoparticles. With this hypothesis the biosynthetic production of silver nanoparticles by aqueous extract of Neem leaves and its bactericidal effect in cotton cloth against E. Coli were studied in this work. Silver nanoparticles were synthesized by short term (1 day) and long term (21 days) interaction of Neem extract (20% w/v) and 0.01 M AgNO3 solution in 1:4 mixing ratio. The synthesized particles were characterized by UV visible spectroscopy, transmission electron microscopy, and incorporated into cotton disks by (i) centrifuging the disks with liquid broth containing nanoparticles, (ii) in-situ coating process during synthesis, and (iii) coating with dried and purified nanoparticles. The antibacterial property of the nanoparticles coated cotton disks was studied by disk diffusion method. The effect of consecutive washing of the coated disks with distilled water on antibacterial property was also investigated. This work demonstrates the possible use of biologically synthesized silver nanoparticles by its incorporation in cloths leading them to sterilization.

Structural and optical investigation of semiconductor CdSe/CdS core-shell quantum dot thin films

Highly luminescent CdSe/CdS core-shell nanocrystals have been assembled on indium tin oxide (ITO) coated glass substrates using a wet synthesis route. The physical properties of the quantum dots (QD) have been investigated using X-ray diffraction, transmission electron microscopy and optical absorption spectroscopy techniques. These quantum dots showed a strong enhancement in the near band edge absorption. The in situ luminescence behavior has been interpreted in the light of the quantum confinement effect and induced strain in the core-shell structure.

Magnetization, magnetotransport and electron magnetic resonance studies of nanoparticles and nanowires of Pr0.5Sr0.5MnO3

In this paper, we present the preparation and characterization of nanoparticles and nanowires of Pr0.5Sr0.5MnO3 (PSMO). The main results of this investigation are as follows: (a) a comparison with the properties of the bulk material shows that the ferromagnetic (FM) transition at 270 K remains unaffected but the anti-ferromagnetic (AFM) transition at TN = 150 K disappears in the nanoparticles, (b) the size induced ground state magnetic phase (below 150 K) is predominantly FM, coexisting with a residual AFM phase, and (c) the temperature dependence of magnetic anisotropy shows complex behaviour, being higher in the nanoparticles at high temperatures and lower at moderately lower temperatures in comparison with the bulk. The results obtained from the extensive magnetization, magnetotransport and electron magnetic resonance studies made on various samples are presented and discussed in detail.

X-ray diffraction of permalloy nanoparticles fabricated by laser ablation in water

Permalloy (NiFeMo) nanoparticles were fabricated by laser ablation of bulk material in water with a UV pulsed laser. Transmission electron microscope images showed that approximately spherical particles about 50 nm in diameter were formed in the ablation process. All diffraction peaks corresponding to the bulk material were present in the nanoparticles. In addition to these peaks several new peaks were observed in the nanoparticles, which were attributed to nickel oxide.

Synthesis of nanoparticles of the giant dielectric material, CaCu3Ti4O12 from a precursor route

A complex oxalate precursor, CaCu3(TiO)(4)(C2O4)(8)center dot 9H(2)O, (CCT-OX), was synthesized and the precipitate that obtained was confirmed to be monophasic by the wet chemical analyses, X-ray diffraction, FTIR absorption and TG/DTA analyses. The thermal decomposition of this oxalate precursor led to the formation of phase-pure calcium copper titanate, CaCu3Ti4O12, (CCTO) at a parts per thousand yen680A degrees C. The bright-field TEM micrographs revealed that the size of the as synthesized crystallites to be in the 30-80 nm range. The powders so obtained had excellent sinterability resulting in high density ceramics which exhibited giant dielectric constants upto 40000 (1 kHz) at 25A degrees C, accompanied by low dielectric losses.

Simplified dispersion curves for circular cylindrical shells using shallow shell theory.

An alternative derivation of the dispersion relation for the transverse vibration of a circular cylindrical shell is presented. The use of the shallow shell theory model leads to a simpler derivation of the same result. Further, the applicability of the dispersion relation is extended to the axisymmetric mode and the high frequency beam mode.