Synthesis of Monodisperse Spherical Nanocrystals
T.G. MyersVincent CreganRevathi BacsaMarc Calvo-SchwarzwälderMario Fernández-PendásB. J. FlorioAntonio MarquinaIain R. MoylesHelena Ribera PonsaSimone RusconiSusana Serna
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Nanoparticles, small units of matter with dimensions in the range1-100 nm, exhibit many advantageous size-dependent magnetic, electrical, chemical and optical properties, which are not observed at the micro scale or bulk. These properties are extremely sensitive to particle size, and thus the ability to produce monodisperse particles is critical. Due to its ease of use and flexibility, precipitation of nanoparticles from solution is one of the most widely used synthesis methods. The main disadvantage of this method is that the relationship between particle growth and system conditions is not fully understood. In practice, the optimal reaction conditions are usually ascertained empirically or intuitively. In this report we consider several different applied mathematical techniques to explain nanoparticle growth via the precipitation method. In particular, we describe the impact of size focussing and defocussing (or Ostwald ripening) on the evolution of the nanoparticle size distribution.Keywords:
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This work aims to evaluate the effect of Ostwald ripening on the crystallization of polymorphic phases by means of the kinetic equation model, which was adapted to describe the competition between the nucleation, the growth, and the Ostwald ripening of the different phases. The kinetic equation model is a very convenient way to simultaneously implement these three mechanisms and quantify their respective roles on the formation and the dissolution of the clusters. The polymorphic system studied is l-glutamic acid (LGlu), which exhibits two monotropic polymorphs: α LGlu and β LGlu. The model assumptions consider the main differences between both polymorphs (shape, solubility, interfacial energy, and growth rate) as well as the supersaturation decrease. The simulation results at various temperatures show that the crystallization process of small nuclei (<40 nm) can be greatly affected by the Ostwald ripening mechanism. In particular, Ostwald ripening can induce the total dissolution of the stable phase clusters, leading to a result in agreement with the Ostwald rule of stages. Thus, the numerical results suggest that the Ostwald ripening mechanism could explain the Ostwald rule of stages.
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Abstract The Ostwald ripening of crystalline precipitates in aqueous solutions is measured, the systems being H 2 O‐NaNO 3 and H 2 O‐Tetraethyl‐ammoniumiodide (TEAI). By comparison of the results at constant temperature with the theory, it is found that the growth kinetics of both substances seems to be determined by a second order reaction. For the application of Ostwald ripening in preparative chemistry and chemical engineering, it is suggested to age the precipitate at periodically changing temperature. This leads to an increase of the aging rate by more than one order of magnitude. The dependence of this increase on frequency and amplitude of the temperature changes is measured.
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The formation of stable transparent nanoemulsions poses two challenges: the ability to initially create an emulsion where the entire droplet size distribution is below 80 nm, and the subsequent stabilization of this emulsion against Ostwald ripening. The physical properties of the oil phase and the nature of the surfactant layer were found to have a considerable impact on nanoemulsion formation and stabilization. Nanoemulsions made with high viscosity oils, such as long chain triglycerides (LCT), were considerably larger ( D = 120 nm) than nanoemulsions prepared with low viscosity oils such as hexadecane ( D = 80 nm). The optimization of surfactant architecture, and differential viscosity eta D/eta C, has led to the formation of remarkably small nanoemulsions. With average sizes below 40 nm they are some of the smallest homogenized emulsions ever reported. What is more remarkable is that LCT nanoemulsions do not undergo Ostwald ripening and are physically stable for over 3 months. Ostwald ripening is prevented by the large molar volume of long chain triglyceride oils, which makes them insoluble in water thus providing a kinetic barrier to Ostwald ripening. Examination of the Ostwald ripening of mixed oil nanoemulsions found that the entropy gain associated with oil demixing provided a thermodynamic barrier to Ostwald ripening. Not only are the nanoemulsions created in this work some of the smallest reported, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insoluble triglyceride.
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We report a unique, cooperative mechanism that involves the interplay of Ge, Si, and Oxygen interstitials enabling an unusual Ostwald ripening and migration behavior of Ge nanocrystallites and quantum dots (QDs) embedded within a SiO2 matrix. In the presence of high Si interstitial fluxes with no supply of oxygen interstitials, the oxide surrounding the Ge QDs is decomposed by the Si interstitials, creating the volatile SiO reaction product and hence voids that enable the Ge QDs to grow by Ostwald ripening. When both Si and Oxygen interstitials are present in high concentrations, the Ostwald ripened Ge QD is further able to migrate towards the source of the Si interstitials. The QD movement occurs by virtue of the fact that the SiO created in front of the QD combines with O interstitials to regenerate SiO2 behind the Ge QD on its migration path. Thus, SiO influences the migration and Ostwald ripening behavior of the Ge QDs via a unique “Destruction-Construction” mechanism.
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Since the process of coarsening, the growth of the mean radius of the clusters of a new phase in solutions connected with a decrease of their number was described and discussed first by OSTWALD (1901), such a process is widely denoted as Ostwald ripening (LIESEGANG, 1911).
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This paper presents textural evolution of metamorphic rocks due to Ostwald ripening. First, Crystal size distributions (CSDs) of garnets in various metamorphic rocks are compared with the LSW (Lifshitz-Slyozov-Wagner) distribution for Ostwald ripening. There are two types of CSD. One is consistent with LSW distribution, the other is wider than LSW distribution. The former one has a homogeneous spatial disposition of garnets, and represents the progress of Ostwald ripen-ing. That CSD of the latter type is wider than LSW, is either; 1) because duration of Ostwald ripening is not sufficient, or 2) because spatial disposition of garnets is not homogeneous. LSW theory needs homogeneous spatial disposition of precipi-tated phase. Ostwald ripening in spatially heterogeneous system is researched by numerical simulation. The results suggest that heterogeneous structure, such as layered structure, are developed by Ostwald ripening. Shape of size distribution is important in two reasons; 1) material transport between layers by Ostwald ripening can be evaluated by comparisons of size distributions between layers, and 2) the width of initial size distribution strongly influences an evolution of layering structure.
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