Crystallization Of Micro-Structured Zn And Nb Transitional Metal Oxides

A key limitation for various applications of transitional metal oxides (TMOs) is the lack of remarkably economic, simple and versatile methods for effectively generating micro structures with well-defined morphologies. Precise manipulation of the crystallization process would enable one to fabricate TMO microstructures with unique optical, electronic, or magnetic properties so as to construct functional devices. Crystallization is a complex issue involved in the growth process of functional materials. The understanding of crystallization processes offers valuable insights into the scope and nature of materials chemistry, which represents an inspiration toward future innovations in seeking highly efficient and unique materials synthesis strategies. In this dissertation, we studied the microscopic crystallization process of Zn and Nb TMOs on the basis of solution chemistry.1. Slovothermal processes were developed to controllably crystallize hexagonal ZnO nanorings, hierarchical ZnO nanosheet assemblies, and multishelled ZnO hollow superstructures in ethanol, ethylene glycol-water, and isopropanol-acetone-water solution system, respectively. The etching growth mechanism of ZnO nanoring was discussed in detail. The Zn(NO3)2 reagent concentration and glycol/water volume ratio were varied to determine its effect on the formation of hierarchical ZnO nanosheet assemblies, and the erosion effect of glycol on twinned ZnO microdisks was investigated. It was also found that hierarchical Ostwald ripening was responsible for the crystallization of multilevel homogeneous ZnO core-shell structure, which depended on the spatial distribution and solution process of primary crystallites.2. A precursor thermal conversion route was exploited to chemically synthesize sheet-like dense ZnO architectures, Mg0.2Zn0.8O alloys, and NaNbO3 nanobelt arrays. The growth kinetics of Zn5(OH)g(CO3)2 and Nay(H3O)Nb6O19·14H2O based on the surface oxidation and corrosion reactions of metal were studied, and the influences of heat treatment on morphology and optical properties of ZnO and NaNbO3 products were analyzed. The morphological retainment and structural changes in solid-phase transformation process were highlighted. The influences of annealing temperature on the Mg concentration in ZnO matrix and on the band gap of Mgo.2Zno.8O alloy were investigated, which indicated that the Mg concentration in the as-obtained alloys was a function of annealing temperature, thereby facilely tuning the band gap.3. A sacrificial template strategy was developed to crystallize NaNb3O8 microtube arrays, NaNbO3 microrod arrays, and new-phase K2Mb2O6 microtube arrays. Mb2O5 rod-like arrays on the Nb foils were controllably crystallized by using (NH4)2SO4 and CO(NH2)2 as different pH mediators in hydrothermal conditions. These well-aligned Mb2O5 rod-like arrays were used as a sacrificial precursor to demonstrate how a wide range of niobate arrays could be obtained by dynamically controlling interfacial reactions between alkali solution and Nb2O5 solid template in localized crystallization processes. The chemical synthesis of highly ordered Nb-based binary and ternary oxide arrays provids a much deeper understanding of crystallization behaviors during a sacrificial-templating process.4. A solution-phase ion exchange approach was developed to controllably crystallize NaNbO3 crystals with K2Nb2O6 precursor. K2Nb2O6 hollow spheres were grown first via a hydrothermal route. The metastable compound was chemically active in NaOH solution, readily crystallizing NaNbO3 microcrystals by a spontaneous ion exchange and recrystallization process. The crucial influences of ion exchange duration, NaOH concentration, surfactant, and reaction medium (water, ethylene glycol/water, ethylenediamine/water) on the morphology and the crystal phase have also been established. The current chemical route provides a mild recipe to crystallize NaNbO3, which is highly expected to be extended to grow more niobate crystals with controlled compositions, structures, and particle morphologies.