Size-dependent melting of silica-encapsulated gold nanoparticles

We report on the size dependence of the melting temperature of silica-encapsulated gold nanoparticles. The melting point was determined using differential thermal analysis (DTA) coupled to thermal gravimetric analysis (TGA) techniques. The small gold particles, with sizes ranging from 1.5 to 20 nm, were synthesized using radiolytic and chemical reduction procedures and then coated with porous silica shells to isolate the particles from one another. The resulting silica-encapsulated gold particles show clear melting endotherms in the DTA scan with no accompanying weight loss of the material in the TGA examination. The silica shell acts as a nanocrucible for the melting gold with little effect on the melting temperature itself, even though the analytical procedure destroys the particles once they melt. Phenomenological thermodynamic predictions of the size dependence of the melting point of gold agree with the experimental observation. Implications of these observations to the self-diffusion coefficient of gold in the nanoparticles are discussed, especially as they relate to the spontaneous alloying of core-shell bimetallic particles.     

Size-dependent endocytosis of single gold nanoparticles

Herein we investigate the size-dependent force of endocytosing single gold nanoparticles by HeLa cells. The results reveal that both the uptake and unbinding force values are dependent upon the size of gold nanoparticles.     

Size-dependent spectroscopic properties of conjugated polymer nanoparticles

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.     

Laser-assisted synthesis of Au-Ag alloy nanoparticles in solution

By using laser-induced heating, we prepared Au-Ag nanoalloys via three different procedures: (i) mixture of Au nanoparticles and Ag(+) ions irradiated by a 532 nm laser, (ii) mixture of Au and Ag nanoparticles irradiated by a 532 nm laser, and (iii) mixture of Au and Ag nanoparticles irradiated by a 355 nm laser. Procedure i is advantageous for the production of spherical alloy nanoparticles; in procedures ii and iii, nanoalloys with a sintered structure have been obtained. The morphology of the obtained nanoalloys depends not only on the laser wavelength but also on the concentration of nanoparticles in the initial mixture. When the total concentration of Ag and Au nanoparticles in the mixture is increased, large-scale interlinked networks have been observed upon laser irradiation. It is expected that this selective heating strategy can be extended to prepare other bi- or multi-metallic nanoalloys.     

Size-dependent interaction of Au nanoparticles and graphene sheet

We report enhancement of the mechanical stability of graphene through a one-step method to disperse gold nanoparticles on the pristine graphene without any added agent.     

From spontaneous dispersion to mechanical alloying

Phenomena of spontaneous dispersion, mechanical alloying, and plastic deformation of metals and ionic crystals in the air and in the presence of surfactants within a wide range of thermodynamic and mechanical forces are compared. Various manifestations of Rehbinder’s effect and the role of the main mechanisms of mass transfer (diffusion, dislocation, and interstitial) in the aforementioned processes at stresses both lower and much higher than the yield stress are demonstrated.     

Synthesis of block copolymer-stabilized Au-Ag alloy nanoparticles and fabrication of poly(methyl methacrylate)/Au-Ag nanocomposite film

[Poly(2-(N,N-dimethylamino)ethyl methacrylate)]-b-poly(methyl methacrylate)-b-[poly(2-(N,N-dimethylamino)ethyl methacrylate)] (M(n)=45,000; 20K-5K-20K; PDI = 1.2) block copolymer surfactant stabilized amphiphilic gold-silver alloy nanoparticles (Au-Ag(PDMA-b-PMMA-b-PDMA)) has been synthesized in both water and in organic medium. The block copolymer stabilized pre-made alloy nanoparticles were successfully dispersed in hydrophobic poly(methyl methacrylate) homopolymer matrix (PMMA) of molecular weight 30,000. The successful synthesis of alloy nanoparticles was accessed by Transmission Electron Microscope (TEM), Energy Dispersed X-ray (EDX), and UV-visible spectrophotometric analysis. The surface functionality of the nanoparticles was confirmed by quantitative determining the grafting density of polymer chain around the nanoparticle surface using combination of thermo gravimetric (TGA) and TEM analysis. The hydrodynamic diameter of the alloy particles including the polymer chains was obtained from dynamic light scattering measurement (DLS). The mechanism of synthesis of high concentration of Au-Ag alloy particles from HAuCl(4) and AgNO(3) (in presence of Cl(-) from reduction of gold salt) metal particles precursors and the successful preparation of poly(methyl methacrylate)/gold-silver nanocomposite films have been discussed.     

Size-dependent shape evolution of silica nanoparticles into hollow structures

Hollow silica nanoparticles can be spontaneously generated without a template on the basis of the porous nature of silica and the high surface energy on the nanometer scale. We show that solid silica particles synthesized by the Stober and microemulsion methods initially develop small pores inside the nanoparticles under slightly basic conditions as a result of base-catalyzed etching. With further reaction, those small seed pores merge into a single void to reduce the surface energy of small pores, generating well-defined hollow nanoparticles. This behavior is unique to nanometer-sized porous materials, and the shape evolution is size-dependent, reinforcing the importance of evaluating the reactivity and structural changes of nanomaterials as well as their physical properties in different size ranges. The mechanism described here provides a simple way to generate uniform hollow nanoparticles of porous materials.     

Production of Au-Ag alloy nanoparticles by laser ablation of bulk alloys

Gold-silver alloy nanoparticles can be produced by pulsed laser irradiation of bulk alloy metals in water, preserving the stoichiometry of the target metals.     

Size-dependent peroxidase-like catalytic activity of Fe3O4 nanoparticles

Peroxidase-like catalytic properties of Fe3O4 nanoparficles （NPs） with three different sizes, synthesized by chemical coprecipitation and sol-gel methods, were investigated by UV-vis spectrum analysis. By comparing Fe3O4 NPs with average diameters of 11, 20, and 150 nm, we found that the catalytic activity increases with the reduced nanoparticle size. The electrochemical method to characterize the catalytic activity of Fe3O4 NPs using the response currents of the reaction product and substrate was also developed.     

Size-dependent studies in ferromagnetic nanoparticles dispersed ferroelectric liquid crystal mixtures

In the present study, ferromagnetic nickel nanoparticles (NiNPs) of size (~20 nm, 40 nm) into ferroelectric liquid crystal (FLC) mixture has been dispersed and investigated. Effect of size of NiNPs on the electro-optic, dielectric and optical properties of FLC mixture have been studied and discussed. A minor improvement in spontaneous polarisation, rotational viscosity and faster response time in NiNPs-FLC samples than pure FLC is noticed. Goldstone mode of relaxation frequency ~100 Hz is detected in all samples and follow a Debye type relaxation behaviour. In addition, it is observed that size of NiNPs does not have any remarkable effect on relaxation frequency and dielectric strength. A single absorption peak at 363, 362 Hz is also noticed in pure FLC and NiNPs-FLC samples.     

Structure of nm-sized InSb clusters formed by spontaneous alloying

The negative ion photoelectron spectrum of⁷Li ₂ ^– is reported at 488 nm (2.540 eV). Three electronic bands are observed in this spectrum and are assigned to the following photodetachment transitions:⁷Li₂,X ¹ _g ⁺ +e ^{– 7}Li ₂ ^– ,X ² _u ⁺ ;⁷Li₂,a ³ _u ⁺ +e ^{– 7}Li ₂ ^– ,X ² _u ⁺ ; and⁷Li₂,A ¹ _u ⁺ +e ^{– 7}Li ₂ ^– ,X ² _u ⁺ . The electron affinity of⁷Li₂ is determined to be 0.437±0.009 eV, leading to an anion dissociation energy,D ₀, of 0.865±0.022 eV for the ground state of⁷Li ₂ ^– . A Franck-Condon analysis of the⁷Li₂,X ¹ _g ⁺ +e ^{– 7}Li ₂ ^– ,X ² _u ⁺ band yields the following spectroscopic constants for the ground state of⁷Li ₂ ^– :B _e =0.502±0.005 cm^–1,r _e =3.094±0.015 Å, and _e =232±35 cm^–1.     

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.     

Size-dependent ultrafast electronic energy relaxation and enhanced fluorescence of copper nanoparticles

The energy relaxation of the electrons in the conduction band of 12 and 30 nm diameter copper nanoparticles in colloidal solution was investigated using femtosecond time-resolved transient spectroscopy. Experimental results show that the hot electron energy relaxation is faster in 12 nm copper nanoparticles (0.37 ps) than that in 30 nm copper nanoparticles (0.51 ps), which is explained by the size-dependent electron-surface phonon coupling. Additional mechanisms involving trapping or energy transfer processes to the denser surface states (imperfection) in the smaller nanoparticles are needed to explain the relaxation rate in the 12 nm nanoparticles. The observed fluorescence quantum yield from these nanoparticles is found to be enhanced by roughly 5 orders of magnitude for the 30 nm nanoparticles and 4 orders of magnitude for the 12 nm nanoparticles (relative to bulk copper metal). The increase in the fluorescence quantum yield is attributed to the electromagnetic enhancement of the radiative recombination of the electrons in the s-p conduction band below the Fermi level with the holes in the d bands due to the strong surface plasmon oscillation in these nanoparticles.     

Size-dependent interaction of silica nanoparticles with different surfactants in aqueous solution

The size-dependent interaction of anionic silica nanoparticles with ionic (anionic and cationic) and nonionic surfactants has been studied using small-angle neutron scattering (SANS). The surfactants used are anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethyl ammonium bromide (DTAB), and nonionic decaoxyethylene n-dodecylether (C(12)E(10)). The measurements have been carried out for three different sizes of silica nanoparticles (8, 16, and 26 nm) at fixed concentrations (1 wt % each) of nanoparticles and surfactants. It is found that irrespective of the size of the nanoparticles there is no significant interaction evolved between like-charged nanoparticles and the SDS micelles leading to any structural changes. However, the strong attraction of oppositely charged DTAB micelles with silica nanoparticles results in the aggregation of nanoparticles. The number of micelles mediating the nanoparticle aggregation increases with the size of the nanoparticle. The aggregates are characterized by fractal structure where the fractal dimension is found to be constant (D ≈ 2.3) independent of the size of the nanoparticles and consistent with diffusion-limited-aggregation-type fractal morphology in these systems. In the case of nonionic surfactant C(12)E(10), micelles interact with the individual silica nanoparticles. The number of adsorbed micelles per nanoparticle increases drastically whereas the percentage of adsorbed micelles on nanoparticles decreases with the increase in the size of the nanoparticles.     

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.     

Low-temperature metallic alloying of copper and silver nanoparticles with gold nanoparticles through digestive ripening

We describe a remarkable and simple alloying procedure in which noble metal intermetallic nanoparticles are produced in gram quantities via digestive ripening. This process involves mixing of separately prepared colloids of pure Au and pure Ag or Cu particles and then heating in the presence of an alkanethiol under reflux. The result after 1 h is alloy nanoparticles. Particles synthesized according to this procedure were characterized by UV-vis spectroscopy, EDX analysis, and high-resolution electron microscopy, the results of which confirm the formation of alloy particles. The particles of 5.6+/-0.5 nm diameter for Au/Ag and 4.8+/-1.0 nm diameter for Cu/Au undergo facile self-assembly to form 3-D superlattice ordering. It appears that during this digestive ripening process, the organic ligands display an extraordinary chemistry in which atom transfer between atomically pure copper, silver, and gold metal nanoparticles yields monodisperse alloy nanoparticles.     

Size-dependent carrier dynamics in CdS nanoparticles by femtosecond visible-pump/IR-probe measurements

Ultrafast photoexcited carrier dynamics in CdS nanoparticles prepared by an AOT/n-heptane reversed micelle system were investigated by a femtosecond visible-pump/mid-IR probe technique. A mid-IR probe beam was found to mainly probe the ultrafast dynamics of photoexcited electrons in the conduction band. Dispersions of CdS nanoparticles with 8 different mean diameters from 2.9 to 4.1 nm were prepared by tuning the mole ratio between water and AOT (W = [H(2)O]/[AOT]) in the reversed micelle systems. The excited state lifetime strongly depended on the mean size of CdS nanoparticles with a maximum around a mean diameter of 3.5 nm. This result was explained by considering the balance between the carrier recombination rates via surface states and those via interior states. The relationship between the excited state lifetime and the size of CdS nanoparticles was drastically changed when the surface was terminated by thiol molecules.     

Evolution of catalytic activity of Au-Ag bimetallic nanoparticles on mesoporous support for CO oxidation

We report a novel Au-Ag alloy catalyst supported on mesoporous aluminosilicate Au-Ag@MCM prepared by a one-pot synthesis procedure, which is very active for low-temperature CO oxidation. The activity was highly dependent on the hydrogen pretreatment conditions. Reduction at 550-650 degrees C led to high activity at room temperature, whereas as-synthesized or calcined samples did not show any activity at the same temperature. Using various characterization techniques, such as XRD, UV-vis, XPS, and EXAFS, we elucidated the structure and surface composition change during calcination and the reduction process. The XRD patterns show that particle size increased only during the calcination process on those Ag-containing samples. XPS and EXAFS data demonstrate that calcination led to complete phase segregation of the Au-Ag alloy and the catalyst surface is greatly enriched with AgBr after the calcination process. However, subsequent reduction treatment removed Br- completely and the Au-Ag alloy was formed again. The surface composition of the reduced Au-Ag@MCM (nominal Au/Ag = 3/1) was more enriched with Ag, with the surface Au/Ag ratio being 0.75. ESR spectra show that superoxides are formed on the surface of the catalyst and its intensity change correlates well with the trend of catalytic activity. A DFT calculation shows that CO and O2 coadsorption on neighboring sites on the Au-Ag alloy was stronger than that on either Au or Ag. The strong synergism in the coadsorption of CO and O2 on the Au-Ag nanoparticle can thus explain the observed synergetic effect in catalysis.