Ostwald Ripening Process Research Articles

Towards the Understanding of Sintering Phenomena at the Nanoscale: Geometric and Environmental Effects

One of the technologically most important requirements for the application of supported metal nanoparticles (NPs) to the field of heterogeneous catalysis is the achievement of thermally and chemically stable systems under reaction conditions. For this purpose, a thorough understanding of the different pathways underlying coarsening phenomena is needed. In particular, in depth knowledge must be achieved on the role of the NP synthesis method, geometrical features of the NPs (size and shape), initial NP dispersion on the support (interparticle distance), support pre-treatment (affecting its morphology and chemical state), and reaction environment (gaseous or liquid medium, pressure, temperature). This study provides examples of the stability and sintering behavior of nanoscale systems monitored ex situ, in situ, and under operando conditions via transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray absorption fine-structure spectroscopy. Experimental data corresponding to physical-vapor-deposited and micelle-synthesized metal (Pt, Au) NPs supported on TiO2, SiO2 and Al2O3 will be used to illustrate Ostwald-ripening and diffusion coalescence processes. In addition, the role of the annealing environment (H2, O2, water vapor) on the stability of NPs will be discussed.

Facile hydrothermal synthesis of ultra‐long manganese oxyhydroxide nanorods and their transformation into MnO2and Mn2O3nanorods

Single-crystal manganese oxyhydroxide (MnOOH) nanorods with high aspect ratios have been successfully prepared employing a simple hydrothermal route based on the redox between potassium permanganate (KMnO4) and polyvinyl pyrrolidone (PVP). By adjusting the reaction temperature, reaction time and the mass ratio of PVP/KMnO4, MnOOH nanorods with lengths up to 20 μm and diameters in the range of 100–400 nm could be facilely prepared in high yield. The as-prepared MnOOH nanorods were characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Fourier transformed infrared spectroscopy. The formation mechanism of MnOOH nanorods was preliminarily discussed based on the Ostwald's ripening process. Furthermore, MnO2 and Mn2O3 nanorods with high aspect ratios were synthesised by calcination of MnOOH nanorods.

Chemical and Kinetically Controllable Nucleation, Aggregation-Coalescence, and Ostwald Ripening Processes in the Synthesis of Magnetic Co–B Nanoparticles

This article reports a mechanistic study of the formation of magnetic nanoparticles via aggregation-coalescence and Ostwald ripening processes considering the kinetically controllable nucleation of the Co–B alloy. Citric acid or sodium citrate determines the pH of the medium and modifies the kinetics of the precursor conversion by means of the NaBH4 hydrolysis processes involved. The subsequent steps of aggregation-coalescence and Ostwald ripening become consequently altered and even hindered by the final silica coating, and on the basis of these results, we report the important chemical role of the citrate ion precursor in the whole process.

Does Morphology of a Metal Nanoparticle Play a Role in Ostwald Ripening Processes?

The Ostwald ripening theory is commonly adopted to rationalize the growth of large metal nanoparticles that are formed at the expense of small-sized nanoparticles of higher chemical potential energies. This classical theory did not describe whether the morphology of a metal nanoparticle plays a role in controlling the shape evolution in the ripening processes. Here we show the direct observation of shape evolution among Ag nanoparticles of different morphologies in solutions, and experimental measurements of the relative chemical potential energies (or the electrochemical oxidation potentials) of Ag NPs of different morphologies, including, nanocubes, nanospheres, triangular plates, and decahedral nanoparticles. Theoretical calculation shows that when the diameter of a metal NP is beyond 35 nm, the influence of particle sizes on the oxidation potentials becomes very small. Chemical etching of Ag NPs by Fe(NO3)3 results in preferential removal of atoms at sharp edges/corners, and negative shift in the oxid...

Facile solution routes for the syntheses of GeTe nanocrystals

Solution-based synthetic routes are attractive strategies for synthesizing GeTe materials, because they have the potential to impart morphology control on the crystallites and permit liquid-based processing of films and patterned structures. Two liquid phase reaction systems for GeTe nanoparticles (NPs) have been studied using GeCl2·dioxane and trioctylphosphine-tellurium (TOP-Te) in oleylamine (OLA) as solvent and reducing agent and using GeCl2·dioxane and (Et3Si)2Te in trioctylphosphine oxide (TOPO) without the use of any reducing agent. The morphology of the GeTe powders had a strong dependence on the Te source and reaction medium. The SEM image of NPs obtained by the reaction of GeCl2·dioxane and (Et3Si)2Te reveals that with an increase in reaction time (2, 10, 15 and 30 min), the size of the NPs increases and their shape becomes uniform. However, it is interesting to observe that after 30 min, the morphology of the nanoparticles was maintained even after longer reaction times i.e., the duration of heating had no pronounced influence on the size and morphology of the nanocrystals after a particular period of time. Reaction with TOP-Te leads to the formation of irregular GeTe nanocrystals through the so called Ostwald-ripening process. However, with (Et3Si)2Te as Te source, a ligand exchange reaction mechanism has been proposed leading to the formation of well-dispersed GeTe nanoparticles of uniform shape.

Fluorescence Spectrum of CdTe Quantum Dots Solution in Ostwald Ripening Process at Room Temperature

We have developed a new, simple, green and very reproducible aqueous synthesis method for the preparation of different sizes CdTe QDs without the use of any pyrophoric organometallic precursors. Transmission electron microscope image demonstrates the shape, monodispersity, average size and size distribution. Two different sizes CdTe QDs are mixed and standing for three months in aqueous solution. Through UV-Vis absorption spectroscopy and fluorescence spectrum detection, we confirm Ostwald ripening process can also occur in CdTe QDs mixed solution for a long time at room temperature.

Cobalt–iron cyanide hollow cubes: Three-dimensional self-assembly and magnetic properties

Cobalt–iron cyanide hollow cubes have been synthesized via a poly(vinylpyrrolidone) assisted solvothermal route. A unique formation process: self-assembly followed by Ostwald ripening process, has been put forward to take account for the construction of hollow cubes. The rod-like nanocrystals first assemble as porous cubes via an oriented attachment process. Then, the porous cubes undergo an Ostwald-ripening process, which create interior spaces and result in the formation of hollow cubes. The magnetic property investigation reveals that K 0.22Co 0.58Fe 2.2(CN) 6 hollow cubes exhibit a ferromagnetic behavior.

Facile Synthesis of Hierarchical Conducting Polymer Nanotubes Derived from Nanofibers and Their Application for Controlled Drug Release

Hollowing of inorganic nano/micromaterials can be realized by well-known Ostwald ripening and Kirkendall processes. However, hollowing of organic polymer without sacrificial templates is seldom seen. Herein, we demonstrate for the first time that conducting polymer nanofibers can be transformed into nanotubes by a simple, effective, and controllable swelling-evaporation strategy. By controlling swelling degree, surface-patterned poly(o-toluidine) (POT) nanofibers, and nanotubes with controllable inner diameter can be fabricated. Novel characteristics of such nanotubes, such as patterned surfaces and sealed tips, will endow them with enhanced properties when applied in fields as controlled drug delivery, noble metal nanocatalysts supporter, and adsorbent for heavy-metal ions.

Thermodynamic analysis of nucleation in confined space: Generalized Gibbs approach

A general thermodynamic analysis of nucleation-growth processes in confined space in initially metastable states of the ambient phase is performed based on the generalized Gibbs approach to the description of heterogeneous systems. In particular, it is shown analytically how the parameters of critical clusters and clusters in stable equilibrium with the ambient phase depend on the volume of the system for initially fixed intensive state parameters of the ambient phase. Qualitatively, the results are shown to be similar independent on the boundary conditions employed. It is demonstrated further that the behavior of systems in confined space is directly related to the kinetics of phase transformation processes in spatially extended systems, when ensembles of clusters are formed. The results of the thermodynamic analysis of cluster formation and growth in a confined space are employed then, in particular, to the derivation of kinetic equations for the description of the process of coarsening or Ostwald ripening. In the analysis of both the nucleation in confined space and the description of Ostwald ripening, no specific assumptions concerning the equations of state of the system under consideration and the number of components both in the ambient and newly evolving phases are made. Consequently, the results are of very general nature and hold always as far as the necessary condition for the possibility of a phase transformation is fulfilled.

Sintering Kinetics of Soft-Doped PZT (54/46) Systems

The influence of dopant concentration on PZT (54/46) systems doped with lanthanum and/or niobium is studied. The sintering kinetics is presented for 1 wt% of the dopant used to find the main mechanism which drives this process. The results were compared with a phenomenological model for viscous sintering and solid state sintering. The exponent obtained for viscous sintering in PZTN, PLZT and PLZTN were 0.05, 0.01, and 0.23 respectively, which indicate that the process is reactive liquid in all cases. In the other hand, the exponent obtained for solid state sintering were 6.61, 5.68, and 1.23 respectively, and prevalence Ost-wald ripening and coalescence process together. Both dopants inhibit the grain growth and accelerate the sintering process, which increases with dopant concentration and the combination of both dopants. Shoro-hod-Olevsky model was applied for explain grain growth evolution, but does not coincide strictly with the applied model, which suggest that the process is very complex.

BiOCOOH hierarchical nanostructures: Shape-controlled solvothermal synthesis and photocatalytic degradation performances

Different shaped bismuth oxide formate (BiOCOOH), including spherical-like, sponge-like, tremella-like, flower-like hierarchical nanostructures have been successfully synthesized through a facile and template-free solvothermal process in different solvents. The products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflection spectroscopy (DRS) and nitrogen adsorption. Based on the electron microscopy observations of the products obtained under different experimental conditions, a possible growth mechanism which involves Ostwald ripening and self-assembly process was proposed. It was found the solvent plays a crucial role in the formation of the BiOCOOH hierarchical nanostructures. The optical properties, Brunauer–Emmett–Teller (BET) surface areas and photocatalytic activities on the degradation of Rhodamine B (RhB) of the different BiOCOOH hierarchitectures were also evaluated. The results indicated that the synthesized sponge-like BiOCOOH hierarchical nanostructures possessed favorable recycling characteristics, and exhibited the highest photocatalytic activity compared with other BiOCOOH hierarchitectures, because of its largest surface area (30.28 m2 g−1) and large band gap (3.46 eV). This work not only provides new strategies for the controllable synthesis of other novel hierarchical materials, but also develops new promising photocatalysts for degrading organic pollutants and explores significant potential applications in environment pollution.

Synthesis of Zn 3(OH) 2V 2O 7· nH 2O hierarchical nanostructures and their photoluminescence properties

The controlled synthesis of Zn 3(OH) 2V 2O 7· nH 2O hierarchical structures has been successfully realized in a large scale via a simple hydrothermal method. It was demonstrated that the morphologies of the final products are significantly affected by the quantity of hexamethylenetetramine, reaction temperature and reaction time. Optimum amount of sodium sulfate plays a crucial role in the development of crystallinity of the products. The morphology evolvement and the growth mechanism were discussed, and sulfate induced oriented attachment and temperature facilitated Ostwald ripening process were proposed for the possible formation mechanism. The structure and morphology of those products were characterized by X-ray diffraction (XRD), Raman spectrum, field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersion spectrum (EDS). Furthermore, the photoluminescence properties of those products were researched. Excellent visible light emission ranging from 400 to 700 nm was exhibited via room temperature photoluminescence (PL) measurement.

Stability of Th(IV) polymers measured by electrospray mass spectrometry and laser-induced breakdown detection

The formation of polynuclear thorium hydroxide complexes in acidic aqueous solutions is observed by electrospray mass spectrometry. Size, number of hydroxide ligands and charge of all complexes in solution are identified independently. By repeated measurements over a time span of up to 418 days the stability of the polymers is observed. At concentrations above [Th(IV)]tot = 10 mM pentamers and dimers are the dominant stable species. In contrast, the fractions of pentamers initially present at [Th(IV)]tot = 0.2 mM decrease within less than 4 weeks, correlated with the formation of nm-sized colloids observed by LIBD indicating an Ostwald-ripening like process.

Aqueous Solution Synthesis of CaF2 Hollow Microspheres via the Ostwald Ripening Process at Room Temperature

Nearly monodispersive CaF2 hollow microspheres were synthesized by a facile aqueous solution route from the mixed aqueous solutions of CaCl2, Na2WO4, and NaF at room temperature. The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and N2 adsorption-desorption techniques. The CaF2 hollow microspheres have an average diameter of about 1.5 microm and a hollow interior of 0.5 microm. The shell is composed of numerous single-crystalline nanoparticles with diameter of about 20 nm. The morphologies and diameters of the CaF2 products are strongly dependent on the experimental parameters, such as the concentration of the aqueous NaF solution and the reaction temperature. The synthetic experiments indicate that the growth process of CaF2 hollow microspheres involves first the formation of CaWO4 solid microspheres and then the formation of CaF2 solid microspheres through the reaction between CaWO4 and F(-) ions controlled by the difference of the solubility product for CaWO4 and CaF2. Phenomenological elucidation based on TEM observations and XRD patterns of intermediate products at different precipitation stages indicates that the formation mechanism for the CaF2 hollow microspheres is related to the Ostwald ripening mechanism. N2 adsorption-desorption measurement shows that the CaF2 hollow microspheres possess a high Brunauer-Emmett-Teller surface area and porosity properties. The synthetic procedure is straightforward and represents a new example of the Ostwald ripening mechanism for the formation of inorganic hollow structures in an aqueous solution at room temperature.

An algorithm for emulsion stability simulations: account of flocculation, coalescence, surfactant adsorption and the process of Ostwald ripening.

The first algorithm for Emulsion Stability Simulations (ESS) was presented at the V Conferencia Iberoamericana sobre Equilibrio de Fases y Diseño de Procesos [Luis, J.; García-Sucre, M.; Urbina-Villalba, G. Brownian Dynamics Simulation of Emulsion Stability In: Equifase 99. Libro de Actas, 1st Ed., Tojo J., Arce, A., Eds.; Solucion’s: Vigo, Spain, 1999; Volume 2, pp. 364–369]. The former version of the program consisted on a minor modification of the Brownian Dynamics algorithm to account for the coalescence of drops. The present version of the program contains elaborate routines for time-dependent surfactant adsorption, average diffusion constants, and Ostwald ripening.

Formation of ordered arrays of gold particles on silicon and silicon-dioxide by nanoindentation patterning

AbstractOrdered arrays of gold particles have been fabricated on gold-coated Si(100) surfaces by pre-patterning the surface with a nanoindenter. During thermal annealing the Au is observed to accumulate within the residual indents. Once nucleated, the Au particles grow at the expense of smaller surface particles via an Ostwald-ripening process. The size of the Au particles is controlled by the initial thickness of the deposited Au layer, the size of the indentation (which is controlled with a high degree of precision), and the annealing conditions. Particles of ˜200 nm dimensions are formed in indents of ˜1 μm dimensions whilst nanoparticles of ˜20 nm are observed in the smallest indents made (˜50 nm). We have also demonstrated patterning of Au by indentation of a Au layer sandwiched between two SiO2 films deposited on Si by plasma-enhanced chemical vapour deposition. Here, cracking of the SiO2 layer occurs allowing Au to diffuse to the surface at the indented locations during post-indentation annealing.

ChemInform Abstract: Efficient and Recyclable Catalyst of Palladium Nanoparticles Stabilized by Polymer Micelles Soluble in Water for Suzuki—Miyaura Reaction, Ostwald Ripening Process with Palladium Nanoparticles.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Direct Observation of Single Ostwald Ripening Processes by Molecular Dynamics Simulation

Ostwald ripening is an important growth process in many scientific disciplines ranging from material science, geology, biophysics, and product formulation. Here ripening of argon clusters in a vapor phase is observed directly in constant energy molecular dynamics simulations serving as a model system for large-time scale ripening processes. Starting from an initial metastable equilibrium between the vapor phase and two clusters Ostwald ripening is initiated by the addition of kinetic energy. This mimics local thermal fluctuations in a larger system. It appears that there is not necessarily a close encounter of two clusters before ripening sets in. Also no static density bridge between two ripening clusters is observed. The onset of ripening is rather related to the different evaporation dynamics of clusters of different size. It can start at the moment energy is added or with some delay, depending on the difference in cluster size and dynamics.

Hydrothermal synthesis of ordered nanolamella-composed Y 2O 3:Eu 3+ architectures and their luminescent properties

A series of Y 2O 3:Eu 3+ samples with interesting three-dimensional (3D) microarchitectures were synthesized through a facile hydrothermal method in the presence of hexamethylenetetramine (HMT) followed by a subsequent heat treatment. The as-formed precursor via the hydrothermal process could transform to cubic Y 2O 3 with the same morphology after annealing at 500 °C for 2 h. The Y 2O 3 microstructures, with an average diameter about 10–20 μm, were composed of nanolamellas with thickness of about 50 nm. Besides a probable self-assembly mechanism, the Ostwald Ripening process was proposed to explain the formation of the architectures on the basis of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The as-synthesized lamella-composed Y 2O 3:Eu 3+ samples showed a strong red emission corresponding to 5D 0→ 7F 2 transition (612 nm), suggesting promising applications in optoelectronics.

Efficient and Recyclable Catalyst of Palladium Nanoparticles Stabilized by Polymer Micelles Soluble in Water for Suzuki-Miyaura Reaction, Ostwald Ripening Process with Palladium Nanoparticles

The Suzuki-Miyaura cross-coupling reaction of ArX (I, Br) with Ar′B(OH) 2 , catalyzed by Pd-containing water-soluble micelle formed by PS-PEO copolymer and N-cetylpyridinium chloride as a surfactant, was studied in water and methanol. The reaction was performed under mild conditions (temperature up to 50 °C) and the catalyst can be recycled (5 runs) by ultrafiltration without any loss of its catalytic activity. The phenomenon was observed, consisting in growing Pd-nanoparticle size because the small nanoparticles dissolve to form larger nanoparticles in the course of reaction as well as in the presence of the only reagent, ArI. In the latter case, additionally, the formation of very small, not completely dissolved Pd nanoparticles, obviously having a high catalyst activity, is observed. This observation, together with the catalyst activity data might have an implication for a leaching mechanism for the studied catalytic system.