Nucleation Of Nanocrystals Research Articles

Autoclave mediated one-pot-one-minute synthesis of AgNPs and Au-Ag nanocomposite from Melia azedarach bark extract with antimicrobial activity against food pathogens.

BackgroundThe increasing use of nanoparticles and nanocomposite in pharmaceutical and processed food industry have increased the demand for nontoxic and inert metallic nanostructures. Chemical and physical method of synthesis of nanostructures is most popular in industrial production, despite the fact that these methods are labor intensive and/or generate toxic effluents. There has been an increasing demand for rapid, ecofriendly and relatively cheaper synthesis of nanostructures.MethodsHere, we propose a strategy, for one-minute green synthesis of AgNPs and a one-pot one-minute green synthesis of Au-Ag nanocomposite, using Melia azedarach bark aqueous extract as reducing agent. The hydrothermal mechanism of the autoclave technology has been successfully used in this study to accelerate the nucleation and growth of nano-crystals.ResultsThe study also presents high antimicrobial potential of the synthesized nano solutions against common food and water born pathogens. The multistep characterization and analysis of the synthesized nanomaterial samples, using UV-visible spectroscopy, ICP-MS, FT-IR, EDX, XRD, HR-TEM and FE-SEM, also reveal the reaction dynamics of AgNO3, AuCl3 and plant extract in synthesis of the nanoparticles and nanocomposite.ConclusionsThe antimicrobial effectiveness of the synthesized Au-Ag nanocomposite, with high gold to silver ratio, reduces the dependency on the AgNPs, which is considered to be environmentally more toxic than the gold counterpart. We hope that this new strategy will change the present course of green synthesis. The rapidity of synthesis will also help in industrial scale green production of nanostructures using Melia azedarach.

Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing

Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m.

Carboxymethyl chitosan functionalization of Bi2S3 quantum dots: Towards eco-friendly fluorescent core-shell nanoprobes

Designed bioengineered nanocomposites are emerging as a novel class of hybrid materials composed of natural aminopolysaccharides and inorganic semiconductors for biomedical and environmental applications. In this study, it is reported for the first time the synthesis and characterization of water-soluble Bi2S3 quantum dots (QDs) functionalized with O-carboxymethyl chitosan (O-CMC) as capping ligands. UV–vis spectroscopy, transmission electron microscopy, dynamic light scattering, zeta potential, and photoluminescence spectroscopy were used to characterize these nanohybrids. The results proved the hypothesis that O-CMC acted as a pH-dependent multi-functional ligand by altering the mechanisms of nucleation, growth and stabilization of water-soluble colloidal Bi2S3 nanocrystals under acidic, physiological and alkaline conditions, using an eco-friendly aqueous process at room temperature. Moreover, the O-CMC capping ligand and the relative molar ratios of the precursors in solution effectively controlled the diameters of the Bi2S3 QDs, which ranged from 2.8 to 12.8nm, and that exhibited luminescent properties in visible light.

Microwave synthesis of branched silver nanowires and their use as fillers for high thermal conductivity polymer composites

We report a rapid synthesis approach to obtain branched Ag nanowires by microwave-stimulated polyvinylpyrrolidone-directed polyol-reduction of silver nitrate. Microwave exposure results in micrometer-long nanowires passivated with polyvinylpyrrolidone. Cooling the reaction mixture by interrupting microwave exposure promotes nanocrystal nucleation at low-surfactant coverage sites. The nascent nuclei grow into nanowire branches upon further microwave exposure. Dispersions of low fractions of the branched nanowires in polydimethylsiloxane yield up to 60% higher thermal conductivity than that obtained using unbranched nanowire fillers. Our findings should be useful for realizing nanocomposites with tailored thermal transport properties for applications.

Temperature dependent magnetic coupling between ferromagnetic FeTaC layers in multilayer thin films

We report systematic investigations on temperature dependent magnetic coupling between ferromagnetic FeTaC layers and resulting magnetic properties of multilayer structured [FeTaC (~67nm)/Ta(x nm)]2/FeTaC(~67nm)] thin films, which are fabricated directly on thermally oxidized Si substrate. As-deposited amorphous films are post annealed at different annealing temperatures (TA=200, 300 and 400°C). Structural analyzes reveal that the films annealed at TA≤200°C exhibit amorphous nature, while the films annealed above 200°C show nucleation of nanocrystals at TA=300°C and well-defined α-Fe nanocrystals with size of about 9nm in amorphous matrix for 400°C annealed films. Room temperature and temperature dependent magnetic hysteresis (M–H) loops reveal that magnetization reversal behaviors and magnetic properties are strongly depending on spacer layer thickness (x), TA and temperature. A large reduction in coercivity (HC) was observed for the films annealed at 200°C and correlated to relaxation of stress quenched in during the film deposition. On the other hand, the films annealed at 300°C exhibit unusual variation of HC(T), i.e., a broad minimum in HC(T) vs T curve. This is caused by change in magnetic coupling between ferromagnetic layers having different microstructure. In addition, the broad minimum in the HC(T) curve shifts from 150K for x=1 film to 80K for x=4 film. High-temperature thermomagnetization data show a strong (significant) variation of Curie temperature (TC) with TA (x). The multilayer films annealed at 200°C exhibit low value of TC with a minimum of 350K for x=4 film. But, the films annealed at 400°C show largest TC with a maximum of 869K for x=1 film. The observed results are discussed on the basis of variations in magnetic couplings between FeTaC layers, which are majorly driven by temperature, spacer layer thickness, annealing temperature and nature of interfaces.

Composition-dependent electro-catalytic activities of covalent carbon-LaMnO3 hybrids as synergistic catalysts for oxygen reduction reaction

Through in-situ nucleation and growth of perovskite-type LaMnO3 nanocrystals on a modified carbon black, a set of LaMnO3/C hybrids with different LaMnO3 loadings (40wt%, 80wt%, 100wt% and 140wt%, relative to carbon) were facilely fabricated via hydrolysis and precipitation in an ethanol solution, followed by calcination at 700°C. The obtained LaMnO3 nanoparticles feature perovskite-type structure and are dispersed uniformly on the carbon surface. The electrochemical experiments in 1molL−1 NaOH solution illustrate that the hybrid catalyst (100wt% of LaMnO3) exhibits the highest ORR electrocatalytic activity and the lowest hydrogen peroxide yield. X-ray photoelectron spectroscopy (XPS) reveals that the catalyst with the best catalytic activity has the highest content of covalent COMn bonds formed at the interface between the LaMnO3 and carbon, which implies a close correlation between the loading and the covalent bond content within the hybrids. The synergy between the oxide and carbon, raised by the formed COMn bonds in their hybrid, is responsible for the remarkably improved ORR activity. This information is important to achieve synergy between non-noble metal oxides and carbon in their composites as efficient and economical ORR catalysts.

Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping.

Prior to this work, fully inorganic nanocrystals of cesium lead halide perovskite (CsPbX3, X = Br, I, Cl and Cl/Br and Br/I mixed halide systems), exhibiting bright and tunable photoluminescence, have been synthesized using conventional batch (flask-based) reactions. Unfortunately, our understanding of the parameters governing the formation of these nanocrystals is still very limited due to extremely fast reaction kinetics and multiple variables involved in ion-metathesis-based synthesis of such multinary halide systems. Herein, we report the use of a droplet-based microfluidic platform for the synthesis of CsPbX3 nanocrystals. The combination of online photoluminescence and absorption measurements and the fast mixing of reagents within such a platform allows the rigorous and rapid mapping of the reaction parameters, including molar ratios of Cs, Pb, and halide precursors, reaction temperatures, and reaction times. This translates into enormous savings in reagent usage and screening times when compared to analogous batch synthetic approaches. The early-stage insight into the mechanism of nucleation of metal halide nanocrystals suggests similarities with multinary metal chalcogenide systems, albeit with much faster reaction kinetics in the case of halides. Furthermore, we show that microfluidics-optimized synthesis parameters are also directly transferrable to the conventional flask-based reaction.

Interlayer coupling dependent magnetic properties in amorphous and nanocrystalline FeTaC based multilayer thin films

We report systematic studies on the effects of heat treatment, the number of multilayers and temperature on interlayer coupling dependent magnetic properties in amorphous and nanocrystalline ([FeTaC(y nm)/ Ta(1 nm)]n=1–4/ FeTaC(y nm)/substrate) multilayer structured thin films fabricated directly on thermally oxidized Si substrate at ambient temperature and post annealed at different elevated temperatures (TA). As-deposited films and the films annealed at 200 °C exhibit an amorphous structure. With an increase in TA ⩾ 300 °C, the nucleation of fine nanocrystals in a residual amorphous matrix appears and a fraction of such nanocrystals increases with increasing TA. The changes in the microstructure modify the interlayer coupling between FeTaC ferromagnetic layers due to the release of stress accumulated during film deposition and enhanced interface roughness with increasing TA. As a result, a change in the shape of the magnetic hysteresis (M-H) loop and multistep magnetization reversal process, where the number of steps in the M-H loop, their nature and positions strongly depend on the number of multilayers, TA and temperature, were observed. As-deposited films and the films annealed at 200 °C exhibit multistep magnetization reversal behavior only at temperatures below 80 K, but the films annealed above 200 °C show such multistep reversal behavior even at 300 K. This causes an unusual variation of temperature-dependent coercivity in these multilayer films having different microstructures. Furthermore, the coercivity due to individual or collective switching between FeTaC layers in these films varies unusually and is substantially influenced by the bottom FeTaC layer grown directly on the substrate. The observed results were discussed on the basis of variation in interlayer coupling with the multilayer structure, post annealing conditions and temperature. This provided evidence of controlling the soft magnetic properties and comprehensive study on the multistep magnetization reversal behavior in multilayer structured FeTaC based thin films.

Direct observation of atomic-level nucleation and growth processes from an ultrathin metallic glass films

Till date, there have been no direct atomic-level experimental observations of the earliest stages of the nucleation and growth processes of nanocrystals formed by thermally induced crystallization in ultrathin metallic glasses (MGs). Here, we present a study of the crystallization process in atomically thin and highly stable MG films using double spherical aberration-corrected scanning transmission electron microscopy (Cs-TEM). Taking advantage of the stability of MG films with a slow crystallization process and the atomic-level high resolution of Cs-TEM, we observe the formation of the nucleus precursor of nanocrystals formed by atom aggregation followed by concomitant coalescence and stepwise evolution of the shape of the nanocrystals with a monodispersed and separated bimodal size distribution. Molecular dynamics simulation of the atomic motion in the glass film on a rigid amorphous substrate confirms the stepwise evolution processes of atom aggregation, cluster formation, cluster movement on the substrate, and cluster coalescence into larger crystalline particles. Our results might provide a better fundamental understanding of the nucleation and growth processes of nanocrystals in thin MG films.

Hidden role of anion exchange reactions in nucleation of colloidal nanocrystals

We show the existence and importance of anion exchange reactions in colloidal chemistry.

Kinetics of nanocrystal synthesis in a microfluidic reactor: theory and experiment

A two-stage microreactor enables the quantitative evaluation of a kinetic model of nanocrystal nucleation and growth.

Seeded Growth Route to Noble Calcium Carbonate Nanocrystal.

A solution-phase route has been considered as the most promising route to synthesize noble nanostructures. A majority of their synthesis approaches of calcium carbonate (CaCO3) are based on either using fungi or the CO2 bubbling methods. Here, we approached the preparation of nano-precipitated calcium carbonate single crystal from salmacis sphaeroides in the presence of zwitterionic or cationic biosurfactants without external source of CO2. The calcium carbonate crystals were rhombohedron structure and regularly shaped with side dimension ranging from 33–41 nm. The high degree of morphological control of CaCO3 nanocrystals suggested that surfactants are capable of strongly interacting with the CaCO3 surface and control the nucleation and growth direction of calcium carbonate nanocrystals. Finally, the mechanism of formation of nanocrystals in light of proposed routes was also discussed.

In Situ TEM Nanoindentation Studies on Stress-Induced Phase Transformations in Metallic Materials

Although abundant phase transformations are in general thermally driven processes, there are many examples wherein stresses can induce phase transformations. Numerous in situ techniques, such as in situ x-ray diffraction and neutron diffraction, have been applied to reveal phase transformations. Recently, an in situ nanoindentation technique coupled with transmission electron microscopy demonstrated the capability to directly correlating stresses with phase transformations and microstructural evolutions at a submicron length scale. Here we briefly review in situ studies on stress-induced diffusional and diffusionless phase transformations in amorphous CuZrAl alloy and NiFeGa shape memory alloy. In the amorphous CuZrAl, in situ nanoindentation studies show that the nucleation of nanocrystals (a diffusional process) occurs at ultra-low stresses manifested by a prominent stress drop. In the NiFeGa shape memory alloy, two distinctive types of martensitic (diffusionless) phase transformations accompanied by stress plateaus are observed, including a reversible gradual phase transformation at low stress levels, and an irreversible abrupt phase transition at higher stress levels.

Hierarchical porous hydroxyapatite microspheres: synthesis and application in water treatment

Hydroxyapatite (HA) microspheres with hierarchical porous structure were synthesized using a facile gas diffusion method. The morphology and structure of the as-synthesized products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, and Brunauer–Emmett–Teller gas sorptometry analyses. Results showed that the change in pH resulting from ammonia diffusion provided the driving force for the formation of HA crystals. Under the cooperative influences of reaction time and reaction temperature, the nucleation and growth processes of HA nanocrystals were effectively controlled. Therefore, porous HA microspheres with large surface areas were produced at 60 °C for 24 h. On the basis of the time-dependent experiments, a possible formation mechanism of hierarchical porous HA microsphere was proposed. Moreover, hierarchical porous HA microspheres were investigated as adsorbent, and they exhibited a high adsorptive capacity for heavy metals Pb2+ and Cu2+ of approximately 213.58 ± 2.6375 and 59.68 ± 3.4436 mg/g, respectively. Therefore, hierarchical porous HA microspheres may be applied to heavy metal-contaminated water treatment.

Nonequilibrium-Plasma-Synthesized ZnO Nanocrystals with Plasmon Resonance Tunable via Al Doping and Quantum Confinement.

Metal oxide semiconductor nanocrystals (NCs) exhibit localized surface plasmon resonances (LSPRs) tunable within the infrared (IR) region of the electromagnetic spectrum by vacancy or impurity doping. Although a variety of these NCs have been produced using colloidal synthesis methods, incorporation and activation of dopants in the liquid phase has often been challenging. Herein, using Al-doped ZnO (AZO) NCs as an example, we demonstrate the potential of nonthermal plasma synthesis as an alternative strategy for the production of doped metal oxide NCs. Exploiting unique, thoroughly nonequilibrium synthesis conditions, we obtain NCs in which dopants are not segregated to the NC surfaces and local doping levels are high near the NC centers. Thus, we achieve overall doping levels as high as 2 × 10(20) cm(-3) in NCs with diameters ranging from 12.6 to 3.6 nm, and for the first time experimentally demonstrate a clear quantum confinement blue shift of the LSPR energy in vacancy- and impurity-doped semiconductor NCs. We propose that doping of central cores and heavy doping of small NCs are achievable via nonthermal plasma synthesis, because chemical potential differences between dopant and host atoms-which hinder dopant incorporation in colloidal synthesis-are irrelevant when NC nucleation and growth proceed via irreversible interactions among highly reactive gas-phase ions and radicals and ligand-free NC surfaces. We explore how the distinctive nucleation and growth kinetics occurring in the plasma influences dopant distribution and activation, defect structure, and impurity phase formation.

Sol–gel synthesis and crystallization kinetics of dysprosium-titanate Dy2Ti2O7 for photonic applications

We present a generic sol–gel approach for the preparation of nanocrystalline Dy2Ti2O7. This approach allows the preparation of powders and highly transparent thin films having nanocrystals of a tailored size. The thermal evolution of these nanocrystals was followed by conventional structural methods and furthermore, we determine both the kinetic parameters of the nanocrystal nucleation process from amorphous xerogel as well as the crystallization mechanism. The crystallization temperature was 798 ± 2 °C. The nanocrystal growth started by homogeneous nucleation with a constant nucleation rate followed by three dimensional growth. The activation energy of the crystallization was 780 kJ mol−1, and the energy of nanocrystal growth was 24.4 kJ mol−1. We demonstrate the existence of pure pyrochlore structure of Dy2Ti2O7 with the lattice parameter a equal to 10.2 Å. The structural evolution of nanocrystalline thin films was identical to that of the evolution of xerogels. A thin film of thickness 342 nm exhibited an optical transmission of 93.39% with a refractive index of 2.138 at 935 nm. These results provide fundamental information about the crystallization behavior of Dy2Ti2O7: they can be used to prepare pure nanocrystalline xerogels and highly transparent thin films with tailored structural properties which are suitable for photonic applications.

Formation of mono/bi-layer iron phosphate and nucleation of LiFePO4 nano-crystals from amorphous 2D sheets in charge/discharge process for cathode in high-performance Li-ion batteries

We prepared mono/bi-layer iron phosphate two-dimensional (2D) materials with 0.74nm/1.52nm thickness by means of a simple chemically induced precipitation method and post-processing. The mechanism of growth of the atomically thin 2D-sheet crystals was investigated by experimental measurements and theoretical calculations. The crystalline 2D sheets were easily oxidized to the amorphous phase in air, and LiFePO4 nano-crystals self-nucleated from amorphous 2D sheets in the charge/discharge process. The 2D sheets show excellent performance properties as cathode materials: high initial discharge capacity of 185mAhg−1 at 0.1C, stable cycling (98% capacity retention over 400 cycles), and high rate capability (107mAhg−1 at 20C) for Li-ion storage. A model for self-nucleation of the LiFePO4 nano-crystals involving double-center diffusion is discussed.

Induced synthesis of toroid-like lead sulfide nanocomposites in ethanol solution through a protein templating route

The toroid-like PbS nanocrystals have been prepared in zein ethanol solution based on self-assembly template of protein molecules. From transmission electron microscopy observation, the obtained samples were monodispersed with an average size of about 47 nm. The chemical composition and crystal structure of nanocomposites were determined by X-ray diffraction and energy-dispersive X-ray spectrum measurements. The interaction between PbS and zein was investigated through Fourier transform infrared, photoluminescence, circular dichroism (CD) spectra, and thermogravimetric analysis. The PbS nanocrystals could react with nitrogen and oxygen atoms of zein molecules through coordination and electrostatic force. The CD spectra results suggested that PbS nanocrystals induced the conformational transition of protein from α-helix to β-sheet and then self-assembled into ring or toroid nanostructure. The quenching of zein fluorescence induced by PbS nanocrystals also showed the change in the chemical microenvironments of the fluorescent amino acid residues in the protein structure. The key step of this facile, biomimetic route was the formation of self-assembly nanostructure of zein, which could regulate the nucleation and growth of toroid-like PbS nanocrystals.

Small Gold Nanoparticles as Crystallization “Catalysts”: Effect of Seed Size and Concentration on Au-ZnO Hetero Nanoparticles

This work reports the effect of seed nanoparticle size and concentration effects on heterogeneous crystal nucleation and growth in colloidal suspensions. We examined these effects in the Au nanoparticle-seeded growth of Au-ZnO heteronanocrystals under synthesis conditions that generate hexagonal, cone-shaped ZnO nanocrystals. It was observed that small (∼4 nm) Au seed nanoparticles form one-to-one Au-ZnO hetero dimers and that Au nanoparticle seeds of this size can also act as crystallization “catalysts” that readily promote the nucleation and growth of ZnO nanocrystals. Larger seed nanoparticles (∼9 and ∼11 nm) provided multiple, stable ZnO-nucleation sites, generating multicrystalline hetero trimers, tetramers, and oligomers.

Tuning Morphology of Nanostructured ZIF-8 on Silica Microspheres and Applications in Liquid Chromatography and Dye Degradation.

Zeolitic imidazolate framework-8 (ZIF-8) is one type of metal-organic framework (MOF) with excellent thermal and solvent stability and has been used extensively in separation, catalysis, and gas storage. Supported ZIF-8 structures can offer additional advantages beyond the MOF-only materials. Here, spheres-on-spheres (SOS) silica microspheres are used as support for the nucleation and growth of ZIF-8 nanocrystals. The surface functionalities (-SH, -COOH, and -NH2) of silica and reaction conditions are investigated for their effects on the ZIF-8 morphology. The use of SOS microspheres results in the formation of highly crystalline ZIF-8 nanostructured shell with varied sizes and shapes, ranging from spherical to cubic and to needle crystals. The SOS@ZIF-8 microspheres are packed into a column and utilized for separation of aromatic molecules on the basis of π-π interaction in high-performance liquid chromatography (HPLC). Furthermore, by thermal treatment in air, ZIF-8 nanocrystals can be transformed into ZnO coating on SOS silica microspheres. The SOS@ZnO microspheres show excellent photocatalytic activity, as measured by degradation of methyl orange in water, when compared to ZnO nanoparticles. This study has demonstrated the facile way of using SOS microspheres to prepare core-shell microspheres and their applications.