Kinetics and Behavior of Hydrothermal Crystal Growth in Potassium Tantalate Niobate Particles

Ly, Tiffany

doi:https://doi.org/10.21985/n2-2dz6-8g53

Work

Kinetics and Behavior of Hydrothermal Crystal Growth in Potassium Tantalate Niobate Particles

Public

Download PDF

Download All Files (.zip)

Nanoparticle synthesis is capable of producing particles with any combination of structure, chemistry, size, shape, and surface. All of the different combinations of these physical properties can produce nanoparticles with almost countless materials properties suited for many applications. Given this interest in using nanoparticles in so many different fields, including electronics, catalysis, and biomedicine, there is also immense interest in understanding the correlation between the physical property of nanoparticles and their resulting functional properties. If any of these properties are phase-, composition-, size-, shape-, or surface-dependent, then methods to synthesis nanoparticles with these traits need to be developed. Therefore, understanding how thermodynamic and kinetic conditions influence nanoparticle growth behavior is essential for these studies. In this dissertation, this was done primarily through studying the growth behavior of oxide nanoparticles in hydrothermal syntheses. A kinetic regime model was proposed based upon the observation of two different growth morphologies on hydrothermally synthesized KTaO3 nanoparticles. Secondary electron imaging demonstrated that there were two dominant growth mechanisms: terrace nucleation, where surfaces are rough, and terrace growth, where surfaces are smooth. In the proposed model based upon standard step-flow growth, the rates of both mechanisms were established to be dependent on the chemical potential, or driving force, of the synthesis environment—terrace nucleation dominates under a higher driving force, and terrace growth dominates with a smaller driving force. This analysis illustrated the mechanism behind the formation of irregular rough particles as well as a method to achieve smooth well-faceted particles by enhancing the smoothing regime with terrace growth. The composition and chemical behavior of nanoparticle surfaces have significant effects on their growth behavior as well. Hydrothermally grown KNbO3, KTaO3, and KTa1-xNbxO3 particles were studied to examine the complex relationship between surface composition, phase, chemistry, and energetics. These may all be used as parameters to model the rates nanoparticle growth mechanisms and identify what conditions favor certain growth regimes. Two different composition-dependent growth modes were identified, where one type formed smooth surface facets, while the other resulted in roughened surfaces. Electron microscopy characterization, density functional theory calculations, and mathematical growth models were used to illuminate the role of surface properties and chemisorption on nanoparticle morphology. Surface energy reduction by chemisorption can increase the rate of terrace nucleation, driving the roughening of the lower surface energy nanoparticle surfaces. Properties of the synthesis environment can have considerable influence on the properties of the products grown. Potassium fluoride was added to the hydrothermal syntheses of KTa1-xNbxO3 and KTaO3 to investigate the effects of an additional mineralizer. One result demonstrated that potassium fluoride increased the solution stability of the tantalum precursor and therefore decreased its reaction rate, resulting in a change in the composition heterogeneity of tantalum, and niobium in the solid solution particles. When added in sufficient quantities to the solution, potassium fluoride also promoted the formation of particles with defect enhanced kinetic Wulff shapes in contrast to the typical nanocuboids produced. The increase in chemical potential of the solution because of fluoride enabled the formation of planar defects in the bulk, which accelerated growth in-plane to form particles with characteristic flat rectangular flake geometries. Thermodynamic modeling with density functional theory calculations suggested that potassium fluoride caused the formation of a defect phase Kn+1TanO3nF. A two-step heat sequence in a hydro-sauna environment was used to grow well-faceted LnScO3 particles. The dominant growth mechanisms and primary phases produced under different processing temperatures were identified to optimize the heating sequence temperatures. First, a high temperature was used to provide the appropriate thermodynamic conditions to nucleate the perovskite phase and increase size monodispersity. The second step lowers the temperature to enter the smoothing regime, where terrace growth encourages the formation of smooth, flat facets on the particle surfaces. The successful synthesis of LnScO3 particles with improved size and shape control was a demonstration of how crystal growth concepts can be used to design nanoparticle synthesis methods.

Creator