Understanding and Exploration of the Biomineralization Mechanisms for the Controllable Synthesis of Nanomaterials

This thesis is mainly concerned with understanding the biomineralization mechanisms, and further extrapolating them for the controllable synthesis of transition metal compound nanomaterials on graphene sheets for energy storage applications in electrochemical capacitors and lithium ion batteries (LIB).;Firstly, we have studied the mimetic biomineralization process of CaCO 3 on a stearic acid or 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) Langmuir monolayer at the air-water interface by in-situ Brewster angle microscopy (BAM) and ex-situ electron microscopy. Amorphous calcium carbonate (ACC) precursors are directly nucleated from solvated ions prior to the crystal nuclei on a Langmuir monolayer. On a DPPC monolayer, numerous fresh ACC nanoparticles heterogeneously and continuously nucleated at the air-water interface are transformed into the metastable vaterite nanocrystals. Driven by the trend to decrease surface energy, the vaterite nanocrystals self-aggregate and grow into the loose-packed hollow ellipsoidal vaterite polycrystals. These nanocrystals in vaterite polycrystals are then gradually orientated in the same direction to evolve into tight-packed ellipsoidal mesocrystals. As the crystallization time is further increased, the metastable vaterite mesocrystals are eventually transformed into the most thermodynamically stable calcite crystals.;Secondly, organic and inorganic additives control over the shapes, sizes and phases of inorganic nanocrystals and arrange them into ordered structures from amorphous precursors in the organisms. This interesting phenomenon has galvanized many attempts to mimic the biomineralization process for synthesizing novel materials. We have studied the crystallization processes from small citrate molecules stabilized ACC precursors under cetyltrimethyl ammonium bromide (CTAB) micellar structures. Amorphous precursors, with a hydrated and disordered structure, are easily transformed and molded into CaCO 3 crystals with novel morphologies, such as, hollow radiating cluster-like particles, hollow sheaf-like crystals, and hollow rods, which are depended on CTAB micellar structures. Besides organic additives, inorganic dopants, such as, Mg2+ ion, are found to be another key factor to influence the polymorph and morphology. We combine two types of additives (Mg 2+ ion and a denatured collagen protein (gelatin)) to direct the mineralization of CaCO3. The polymorphs and morphologies critically depend on gelatin concentration at a given Mg2+ concentration. While, at a given gelatin concentration, the Mg molar percentages in the mother solution, although not a determining factor for the polymorphs, can affect the crystal micro- and nano-structures. The controlled crystallization can be rationalized by the interplay between Mg2+ and gelatin, which mutually enhances their uptake and regulate the concomitant mineralization. The biomineralization process can be divided into the nucleation of amorphous precursors and the subsequent amorphous to crystalline transformation.;Thirdly, on the basis of understanding the biomineralization mechanisms discussed above, we extrapolate it to synthesize transition metal compound nanomaterials on graphene sheets for energy storage application. We have applied a bio-inspired approach to prepare CoxNi1-xO (0≤x<1) nanorods on graphene sheets, breaking out the Co/Ni molar ratio limitation for the known stable mixed oxide spinel NiCo2O4. This success has allowed us to further screen the compositions for electrochemical capacitor. CoxNi1-xO/graphene composite electrodes achieve a peak specific capacitance as the Co/Ni molar ratio is closed to 1. This bio-inspired approach also is applied for anchoring Ni(OH)2 nanocrystals on graphene sheets. The size and morphology of the Ni(OH)2 nanocrystals can be controlled via altering the treated temperature during the Ostwald ripening process. The specific capacitance decreased with increasing Ni(OH) 2 nanocrystal size, whereas the cycling stability performance increased with increasing the stability of Ni(OH)2 in the nanocomposites. (Abstract shortened by UMI.).