Shaping Particles By Chemical Reaction And Diffusion

The shape of particles is crutial for their performance in applications. A versatile approach for obtaining shape-controlled particles relies on the understanding of the thermodynamics and kinetics involved in the crystallization of particles. Therefore, developing a versatile approach for controlling the shape of particles will greatly promote the industrial applications of particles, as well as benefit the development of crystallization science. To control the shape of particles, scientists invented sol-gel method, hydrothermal method, template method, etc. Most of these previous approaches are not based on mechanistic understanding, and difficult to be put into industrial application.Over decades, researchers discovered that, kinetic condition can alter the growth pathway of particles, leading to particles with shapes deviating from thermodynamic predictions. Based on this recognition, the kinetic approach was invented to control the shape of particles, and this approach is becoming more and more popular. Previous kinetic approaches mainly focus on adjusting the reaction rate of particle growth, which lacks more indepth understanding in the kinetic processes of particle growth. By viewing the growth of the particles, we find that, the mesoscale structures in the growing front the particles, including the contour curvature of the particles and the concentration distribution, are crutial for shape development of the particles. The transport rate of reactant towards the particle and their reaction rate influence the mesoscale structure in the growing front of the particle. Therefore, we propose a new kinetic approach, that is controlling the shape of particles via regulating the reaction and diffusion of chemicals. Our main findings in this thesis are shown in the follows:(1) Control on the shape development of CaCO3 by qualitatively manipulating the reaction-diffusion. By adjusting the reacion rate and diffusion rate of chemicals in the precipitation of CaCO3, we obtain sphere particles, dendritic structures and snow-shaped particles at different kinetic conditions. When the reaction rate is low and diffusion rate is high, the obtained particles show sphere shape, because it is a reaction-limited condition. When the reaction rate is high and diffusion rate is low, the particles show dendritic structures, because this is a diffusion-limited condition. When the interplay between reaction and diffusion comes to balance state, we obtain snow-shaped CaCO3 particles. Through Monte Carlo simulation, we confirm that the formation of snow-shaped particle is a result of compromise between reaction and diffusion. In this work, the importances of reaction and diffusion on the shape development of particles are verified.(2) Control on the shape development of silver particles by quantitatively manipulating the reaction-diffusion. Through adjusing the diffusion rate of diffusive units and reduction rate of Ag+, we obtain small crystals with smooth surface, rectangle plates and spindle rods. We find that the reaction-diffusion influences the particle growth in two aspects, which is the reason for the shape variation at different reaction-diffusion conditions. One of the two aspects is in terms of the growth pathway, which switches between classic layer-wise growth to coalescence growth; another one is in terms of the the growth direction which switches between isotropic mode and anisotropic mode. For the growth model, when the diffusion rate of Ag+ is low or reduction rate is low, the silver nuclei will preferentially aggregate through collision, favoring the coalesence growth pathway for silver growth, forming rectangle plates and spindle rod; when diffusion rate of Ag+ is high or reducion rate of Ag+ is high, the Ag+ deposits onto the surface of silver nuclei at fast pace, favoring the classic layer-wise growth for silver particle, leading to the formation of small crystals with smooth surfaces. For the growth direction, the switch happens when the particles grow through the coaleacence growth pathway. When the diffusion rate of building blocks is high or the reaction rate of building blocks formation is high, the growth direction of silver particles tends to follow the isotropic growth way, leading to the formation of rectangle plates; when the diffusion rate of building blocks is low or the reaction rate of building blocks formation is low, the growth direction of silver particles tends to follow the anisotropic growth, making the long axis of particles grow faster, forming spindle rods with higher aspect ratio.(3) Simulation studies on influences of reaction-diffusion on the shape development of particles. Growth models based on reaction-diffusion are built up to simulate the growth of snow-shape CaCO3 particles and silver particles. Through the simulation work, we confirm the effects of reaction-diffusion on the shape development of CaCO3 and silver particles. The snow shape of CaCO3 and spindle-rod shape of silver particles are both results of anisotropic growth. By disclosing the concentration distribution around the growing snow-shaped particles and silver aggregates, we find anisotropic concentration gradient around the growing particles, and the concentration gradient agrees well with the anisotropic direction of particle growth. Therefore, the concentration gradient created by diffusion-reaction is the key mechanism that determines the shape of particles. The shape development of CaCO3 and silver particles have been correlated by the mutual governing mechanism, that is the concentration gradient. Therefore, we verify that the reaction-diffusion approach promises a general protocol for shape-controlled synthesis of particles.