The Role And Mechanism Of Shaping Particles By Chemical Reaction And Mass Transfer

The diversity of morphologies and structures equips nanoparticles with a lot of unique physical and chemical properties,which enable nanomaterials to show promising applications in various fields,like optics,catalysis,electromagnetics,biology and so forth.Controllable synthesis and large-scale production of particles with specific shapes are the key to turn those potential applications into reality,so shape control is an absolutely essential issue in the nanoscience.Vapour deposition,precipitation,sol-gel synthesis and solvothermal methods have been developed for preparing nanoparticles in the past decades.Also,the particle shapes have already been effectively regulated by templates and surface capping.However,those conventional methods of shape control are mainly confined on the thermodynamic aspect,while an overwhelming majority of particle growth occurs far from the thermodynamic equilibrium and is dominated by kinetic factors,which result in the severe shortage of accesses to the shape evolution.Thus,a general theory and a universal methodology have not yet been built up for the manipulation of particle morphologies and structures.Based on the existed crystallization theories,material scientists were aware of the significance of kinetic processes for shape evolution.It was found that the interaction between chemical reaction and diffusion often leads to the emergence of intricate spatial and/or temporal structures.Reaction and diffusion were applied to explain the formation of some special patterns,and subsequently were developed two basic kinetic models,called diffusion-limited aggregation and reaction-limited aggregation.Therefore,by combining the former achievements and principles of chemical engineering,our research team proposed a new reaction-diffusion-based strategy to control the particle shapes,which preliminarily showed its power in shaping calcium carbonate and noble metal particles.In spite of nice regularities,our previous work was restrict to qualitative investigations for the parameters of kinetic models were not achieved,and thus general mechanisms of shape manipulation could not be set up.Accordingly,taking silver particles as the technically relevant model system,both qualitative and quantitative researches of shape regulations by the reaction-diffusion protocol are reported in this thesis.The effects of convection(mixing)are included as well,and a general reaction-mass transfer based mechanism of particle growth is ultimately founded.The main findings are shown in more detail as follows:(1)Regulation of silver particle shapes by qualitatively manipulating reaction and diffusion.A traditional solution-based reduction was applied to synthesize monodispersed silver dendrites.Reaction temperatures and chemical concentrations were initially adjusted,but no regularity was obtained for the shape variations,which indicated the limitation of shape manipulation by simply altering experimental parameters.Then,the acidity and solvent viscosity were adopted to regulate reaction and diffusion,respectively.It was found that dendritic structures were produced under fast reaction conditions.When reaction was reduced,the branches were suppressed,became compact and finally vanished.Diffusion depression brought about the decrease of particle sizes and also changed the orientations of branched structures.Sufficient monomer supply by quick diffusion favoured the simultaneous growth of both high-energy and low-energy facets,while reduced diffusion could only support the preferential growth of high-energy facets.Above discoveries implied the importance and necessity of the reaction-diffusion protocol for shaping particles.(2)Regulation of silver particle shapes by quantitatively manipulating reaction and diffusion.The electrochemical method was employed to deposit silver particles,in which reaction and diffusion were modulated by varying the applied potential and the solvent viscosity of the electrolyte,respectively.Reaction and diffusion were separately depicted by the standard rate constant of Ag+/Ag couple and the diffusion coefficient of Ag+,which were derived from cyclic voltammetry and chronocoulometry,respectively.These two parameters were coupled and both of them decreased with increasing solvent viscosity,merely a smaller declining amplitude for standard rate constant than diffusion coefficient.With negatively shifting the potential,the apparent rate constant calculated by Butler-Volmer formula increased exponentially and silver particles underwent a "polyhedron-dendrite-nanocrystal"shape variation in each solvent system.A transverse comparison of results from different solvent systems indicated that diffusion reduction induced by viscosity rise promoted better structural development at the starting potential of dendrites while made the ending potential of dendrites arise in advance.Then,the Damkohler number(Da)was introduced to quantify the diffusion and reaction processes during particle growth and to relate reaction-diffusion conditions and particle shapes,and it was found that almost all dendrites were produced in a fixed Da range.A Da-based concentration field mechanism of particle growth was put forward:diffusion control with small Da values generated local concentration gradients,which triggered Mullins-Sekerka(MS)instability on the growing interfaces and in turn favoured dendritic evolution;howbeit,large or extremely small Da and the accompanying reaction control or severe diffusion limitation could not provide the necessary concentration gradients for dendritic growth.Afterwards,the existence of local concentration fields and the rationality of the growth mechanism were verified by carefully designed pulsed electrodepostion.In addition,the same tendency of shape variation was achieved in more solvent systems and in other material systems(copper and gold),which implied the good generality of the reaction-diffusion method.(3)Regulation of silver particle shapes by manipulating convection(mixing).Batch experiments with solution-based reduction were initially employed to synthesize silver particles,and the dendritic structures were changed and even faded away via applying different mixing ratios and injection procedures of precursor and reducing agent solutions,indicating convention has significant effects on particle shapes.Then,continuous reactions were carried out in a T-mixer,and the mixing intensity quantified by previously determined mixing time using the Villermaux-Dushman protocol.It was found that slow mixing brought about plate-like particles,whose planar surfaces were(111)facets according to the crystallinity analysis,while fast mixing led to quite dendritic structures.Both the equivalent sizes and the widths of equivalent size distributions derived from analytical centrifugation decreased with improving the mixing quality.It was concluded:poor mixing led to low effective supersaturation,which gave rise to a preferential growth of high-energy facets of face-centered cubic crystals,and thus plate-like particles were generated;on the contrary,good mixing led to fast nucleation rates,and accordingly microscopic concentration gradients formed around the nuclei resulting in dendritic products;and better mixing quality led to smaller and more uniform particles.Therefore,a more general growth mechanism was achieved by incorporating mixing effects into the reaction-diffusion mechanism,and a kinetic embryo of the reaction-mass transfer methodology was established for continuous production of particles with controlled shapes.