Site-specific nucleation and growth kinetics in hierarchical nanosyntheses of branched ZnO crystallites

Here we report a site-specific sequential nucleation and growth route to the systematic building of hierarchical, complex, and oriented ZnO micro/nanostructures in solution nanosynthesis. Structures and morphologies of the products were confirmed by results from X-ray diffraction and scanning electron microscopy studies. The organic structure-directing agents (SDAs), diaminopropane and citrate, are found to play different roles in controlling the evolution of these new morphologies. Through the selective adsorptions of SDAs on different crystal facets of the primary ZnO rods, we have alternated the hierarchical growth of secondary and tertiary new complex nanostructures. Roles of the SDA concentration, nucleation time, and growth kinetics in the solution hierarchical ZnO nanosyntheses have all been systematically investigated.     

Enhanced nucleation, growth rate, and dopant incorporation in ZnO nanowires

Pure and Co-doped ZnO nanowire arrays were grown on polished silicon substrates with high rates via an electrochemical technique. A negative potential applied to the substrate not only enhances the nucleation density on polished substrates more than 4 orders of magnitude but also increases the growth rate by 35 times over that obtained in the absence of the potential. Furthermore, incorporation of metallic dopants in ZnO nanowires was demonstrated in the low-temperature process. This fast growth technique provides a route to fabrication of low-cost highly oriented ZnO nanowires on polished substrate for industrial applications.     

Particle nucleation and growth models

This review concentrates on the progress of modeling the nucleation process of particles by the balanced nucleation-growth (BNG) process. The BNG model will be compared with other models that try to predict material nucleation. Compared to other models, the BNG model allows quantifying the nucleation rate, maximum growth rate, and supersaturation during the nucleation period as a function of nucleation efficiency and maximum growth rate of the crystals. From this model, equations are derived that correlate the number of stable crystals formed with molar addition rate of reactants, solubility of the crystals, and temperature. The BNG model predicts the experimental result that many crystallization processes result in a limited number of crystals followed by growth. The model also predicts that factors like diffusion and kinetically controlled growth process, Ostwald ripening agents and growth restrainers control the crystal number. Equations are given for each of the variables that agree with experiments. The BNG model predicts the conditions for renucleation (formation of new crystals during precipitation). It leads to new equations for the prediction of crystal number and crystal size during controlled continuous precipitation in the continuous stirred tank reactor (CSTR) as a function of precipitation conditions.     

Kinetics of electrochemical nucleation and growth

A united scheme for the kinetics of electrochemical nucleation and the growth of a new phase is presented. The peculiarities of ion-transfer kinetics during electrochemical phase formation are analysed. The influence of the exchange current density at the electrolyte/cluster of the new phase interface on the nucleation rate, the nucleation induction time and the growth rate is reported.     

Crystal growth and classical nucleation theory

Calculations were performed of the crystal growth rates in lithium disilicate glass in the low-temperature regime where homogeneous nucleation is observed. The computations were executed using the gain-loss (Becker–Doring) equations that form the framework of Classical Nucleation Theory (CNT). The growth rates were obtained in several different ways, using various choices for the kinetic model, the generalized diffusion coefficient, and the physical input data. The results of these calculations are compared with recently obtained experimental values of the growth rates.     

Metadynamics simulations of ice nucleation and growth

The metadynamics method for accelerating rate events in molecular simulations is applied to the problem of ice freezing. We demonstrate homogeneous nucleation and growth of ice at 180 K in the isothermal-isobaric ensemble without the presence of external fields or surfaces. This result represents the first report of continuous and dynamic ice nucleation in a system of freely evolving density. Simulations are conducted using a variety of periodic simulation domains. In all cases the cubic polymorph ice I(c) is grown. The influence of boundary effects on estimates of the nucleation free energy barrier are discussed in relation to differences between this and earlier work.     

聚苯胺的成核及生长机理

本文通过恒电位阶跃法研究了聚苯胺在不同介质中的成核与膜的生长过程动力学。结果表明, 在硫酸介质中, 成核过程为扩散控制下的三维连续成核, 得到疏松、多孔的膜; 而在高氯酸介质中, 成核则是电化学动力学控制下的二维成核过程。在高电位时(E>1.02V, vs, SCE)为二维连续成核过程, 而在较低的电位时, 主要表现为二维瞬时成核, 膜层呈网状且致密。     

Control of the ZnO nanowires nucleation site using microfluidic channels

We report on the growth of uniquely shaped ZnO nanowires with high surface area and patterned over large areas by using a poly(dimethylsiloxane) (PDMS) microfluidic channel technique. The synthesis uses first a patterned seed template fabricated by zinc acetate solution flowing though a microfluidic channel and then growth of ZnO nanowire at the seed using thermal chemical vapor deposition on a silicon substrate. Variations the ZnO nanowire by seed pattern formed within the microfluidic channel were also observed for different substrates and concentrations of the zinc acetate solution. The photocurrent properties of the patterned ZnO nanowires with high surface area, due to their unique shape, were also investigated. These specialized shapes and patterning technique increase the possibility of realizing one-dimensional nanostructure devices such as sensors and optoelectric devices.     

Secondary nucleation in the formation of methane crystal hydrate

The kinetic data on crystallization and a morphological analysis of a layer of CH₄ · 6H₂O hydrate crystals formed on the surface of water as a result of methane absorption showed that secondary nucleation occurred during hydrate crystallization. The mutual arrangement of crystals in the layer revealed photographically in situ was evidence that part of nuclei produced on the surface of previously formed crystals went away from the surface into solution and grew there independently of “mother” crystals, although the probability of such transfer into an immobile solution remained low. In view of this, a model of crystal growth generating secondary crystals was developed.     

Secondary nucleation and accessible surface in insulin amyloid fibril formation

At low pH insulin is highly prone to self-assembly into amyloid fibrils. The process has been proposed to be affected by the existence of secondary nucleation pathways, in which already formed fibrils are able to catalyze the formation of new fibrils. In this work, we studied the fibrillation process of human insulin in a wide range of protein concentrations. Thioflavin T fluorescence was used for its ability to selectively detect amyloid fibrils, by mechanisms that involve the interaction between the dye and the accessible surface of the fibrils. Our results show that the rate of fibrillation and the Thioflavin T fluorescence intensity saturate at high protein concentration and that, surprisingly, the two parameters are proportional to each other. Because Thioflavin T fluorescence is likely to depend on the accessible surface of the fibrils, we suggest that the overall fibrillation kinetics is mainly governed by the accessible surface, through secondary nucleation mechanisms. Moreover, a statistical study of the fibrillation kinetics suggests that the early stages of the process are affected by stochastic nucleation events.     

Understanding nucleation and growth using computer simulation

Atomistic simulation has been used to examine two distinct problems: (1) the growth of calcium carbonate and how growth inhibitors may operate at the atomic scale and (2) the external surface properties of a zeolite and how these influence the transport of reactants or products at the surface. In the first study we have examined how the class of monophosphonate growth inhibitors disturbs the assembly of calcite at a growing surface. We show that a retarding growth inhibition mechanism is that of kink-blocking, where the poison irreversibly binds to a kink site preventing kink–kink annihilation or step assembly in the vicinity of the impurity. Secondly, we show that two models proposed by Terasaki (O. Terasaki, J. Elec. Micro. 43 (1994) 337), are stable and provide an external surface with characteristic steric and chemical properties. To assess the question of transport, we used a probe molecule of benzene to navigate the energy barrier that separates the external surface and internal surface. We find that the barrier is smaller than that in bulk which suggests that for molecules with relatively small kinetic radii, the rate determining step in the transport process will be in the host medium or crystal bulk.     

Three-dimensional nucleation and growth under galvanostatic conditions

The nucleation and growth of new phase clusters have been studied under galvanostatic conditions. Time dependences of the overpotential, number, and size of clusters formed on the electrode have been calculated from a model of the electrocrystallization process.     

Galvanostatic nucleation and growth under diffusion control

A theory is proposed for galvanostatic three-dimensional nucleation and growth of new phase clusters controlled by diffusion of depositing ions in the bulk of the electrolyte to the clusters surface. The mathematical model is used to calculate the time dependences of the overpotential, number, and size of clusters formed on the electrode.     

Homogeneous nucleation and growth of melt in copper

Molecular dynamics simulations are conducted to investigate homogeneous nucleation and growth of melt in copper described by an embedded-atom method (EAM) potential. The accuracy of this EAM potential for melting is validated by the equilibrium melting point obtained with the solid-liquid coexistence method and the superheating-supercooling hysteresis method. We characterize the atomistic melting process by following the temperature and time evolution of liquid atoms. The nucleation behavior at the extreme superheating is analyzed with the mean-first-passage-time (MFPT) method, which yields the critical size, steady-state nucleation rate, and the Zeldovich factor. The value of the steady-state nucleation rate obtained from the MFPT method is consistent with the result from direct simulations. The size distribution of subcritical nuclei appears to follow a power law similar to three-dimensional percolation. The diffuse solid-liquid interface has a sigmoidal profile with a 10%-90% width of about 12 A near the critical nucleation. The critical size obtained from our simulations is in reasonable agreement with the prediction of classical nucleation theory if the finite interface width is considered. The growth of melt is coupled with nucleation and can be described qualitatively with the Johnson-Meh-Avrami law. System sizes of 10(3)-10(6) atoms are explored, and negligible size dependence is found for bulk properties and for the critical nucleation.     

Theory of cyclic voltammetry for electrochemical nucleation and growth

The theory of cyclic voltammetry for processes of 3D nucleation, diffusion-controlled growth, and dissolution is proposed. The cases of single metal cluster formation and multiple nucleation are studied. The main features of cyclic voltammograms are analyzed: current increase on forward scan during nucleation, nucleation loop, maximum cathode current on reverse scan, and anodic peak of metal stripping.     

Experimental analysis of non-isothermal transformations involving nucleation and growth

A critical examination is made of a recently suggested experimental technique which utilizes the Johnson-Mehl-Avrami transformation rate equation to describe nonisothermal transformations. It is shown that for transformations involving nucleation and growth the technique has limited applicability.     

Melting dynamics of superheated argon: nucleation and growth

We investigate the microscopic melting process of a superheated argon solid using molecular dynamics simulations. We characterize the melting dynamics by following the temperature and time evolutions of liquid atoms and demonstrate the formation of a critical liquid nucleus via fluctuations and subsequent growth. The critical liquid nucleus size (about 120 atoms) obtained from our direct simulations is in accord with the prediction of the classical nucleation theory. The dynamic nucleation and growth of liquid also agree with the Johnson-Mehl-Avrami law, and the growth exponent n approximately 3 at the early stage followed by a substantial increase in n thereafter.     

Evolution of surface width in electrochemical nucleation and growth

The evolution of surface width during growth of copper islands is studied in the kinetic regime, allowing control of island shape as well as island density. We show that film roughness at island coalescence is determined by island density, island shape, and the island growth kinetics. The connection between the kinetics of island growth and film growth provides the scientific basis for control of surface roughness.     

Taxonomy of nucleation and growth processes in isotropic systems

We construct a taxonomy of transient behavior for one component coupled nucleation and growth based on linear and nonlinear stability analysis of a two species model. The two species are the condensable species and the condensed droplets or particles, respectively. Our model reproduces previously reported asymptotically stable and oscillatory transient nucleation behavior. Additionally, we show previously unreported possibilities of multiple stationary states and combinations of stationary states with limit cycles. We predict that multiple stationary states may be observed under near critical conditions. Estimates of suitable conditions for experimental verification are given for several systems of interest.