Study Of Self-assembly And Growth Mechanism Of Nano/Micro Boehmite

Self-assembly is becoming an efficient and promising method to obtain nanomaterials. In present years, nano/micro materials with zero dimensional, one dimensional, two dimensional and three dimensional structures have been synthesized by self-assembly and their formation mechanisms have been brought out. In the dissertation, nano/micro boehmite and alumina with three dimensional urchin-like and flowerlike structures were synthesized hydrothermally by self-assembly. The influences of preparation parameters on the morphologies were investigated. The formation mechanisms and growing models of three dimensional boehmite structures are proposed and established on the basis of experimental results. The main points are summarized as follows:Three dimensional urchin-like boehmite structures were synthesized hydrothermally with AlCl3·6H2O serving as the precursor in an ethanol-water mixed solution system. Gamma-alumina with the same morphology was obtained when the samples were calcined at 600℃. The samples were characterized by a series of methods and the influence of precursor type, alcohol type, volume ratio of EtOH/H2O, reaction temperature and reaction time on the final morphologies of boehmite were investigated. The formation mechanism was proposed on the basis of experimental results. The cooperative mechanism between the competition between the surface free energy (F) and the self-assembly free energy (△G) and acid erosion plays a key role. In the beginning, the boehmite nanoplates were obtained resulted with the crystal layer structure of boehmite. Then the nanoplates aggregated into blocks via hydrogen bonds to minimize the surface free energy. The surface free energy was dominant in the stage. The irregular nanoplates evolved uniform nanorods on the effect of hydrochloric acid with increasing time. Finally, the urchin-like AlOOH structures consisting of nanorods were prepared. The nanorods contacted each other via hydrogen bonds. The self-assembly free energy was dominant in the stage. The reason·of the effect of the reaction factors on the final morphologies was explained by the mechanism.Three dimensional flowerlike AlOOH structures were obtained with cetyl trimethyl ammonium bromide (CTAB) serving as the template via a hydrothermal method in a water solution system. After calcination, the boehmite structures could be transformed into gamma-alumina nanostructures while keeping their morphology. The samples were characterized by a series of methods and the influence of mixing sequence, molar ratio of aluminum salts to urea, molar amount of CTAB, reaction temperature and reaction time on the final morphologies of boehmite were investigated. The formation mechanism was proposed on the basis of experimental results and explained the reason of the influence of the reaction factors on the morphologies. The template reagent CTAB plays a crucial role on the formation of the flowerlike boehmite.The novel three dimensional boehmite structures were prepared with sodium aluminate and urea serving as precursors via a seed separation and hydrothermal reaction coupling method in a water solution system. Gamma-alumina with the same morphology was obtained when the samples were calcined at 600℃. The samples were characterized by a series of methods. The influences of molar ratio of sodium aluminate to urea, reaction temperature and reaction time on the final morphologies of boehmite were investigated. The cooperative mechanism between the oriented attachment and base erosion is responsible for the morphology of boehmite and explain the reason of the effect of reaction factors on the morphology of boehmite. In the beginning, the formed nanosheets self-attached through stacking by lattice fusion to generate irregular nanorods. Then the boehmite nanorods became uniform and self-assembled the flowerlike structures via hydrogen bonds by an oriented attachment mechanism with increasing time.