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). ₃...[ Read more ]

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 CaCO₃ 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. The degree of transformation is found to be closely related to the surface pressure of DPPC monolayer, revealing that the surface energy plays a vital role in determining the transformation process. However, the biomineralization process is exactly carried out in bulk solution rather than at the air-water interface of Langmuir monolayer. To mimic the biomineralization process more realistically, we further study it in an insoluble chitosan scaffold. During the mimetic biomineralization process, small citrate molecules is used for controlling the nucleation and stabilization of ACC precursors, just like the acidic biomacromolecules in the biominerals, and insoluble chitosan scaffold provides a scaffolding framework for crystallization. The crystallization process is almost consistent with that on a Langmuir monolayer, following an amorphous precursor pathway: the supersaturation solution → amorphous precursors → nanocrystals → polycrystals → mesocrystals → single 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 CaCO₃ 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 (Mg²⁺ 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. An intriguing question is that the issue in which process organic molecules control the polymorphism of magnesium-containing calcium carbonate remains elusive. Hence, we have studied the roles of organic molecules with different functional groups added in the different processes in controlling the polymorph of magnesium-containing calcium carbonate. It’s found that polymorph selection is controlled by the functional groups of organic molecules added into the nucleation process of amorphous precursor rather than added in the transformation process. Specifically, when added in the nucleation process of amorphous precursor, hydroxyl and amine groups induce a preferential transformation from amorphous to the thermodynamically metastable aragonite and carboxyl groups to the thermodynamically most stable calcite, whereas little difference is effected when these functional groups were introduced into the amorphous to crystalline transformation process.

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 Co_xNi_1-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. Co_xNi_1-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. Similarity, MFe₂O₄ (M=Co, Ni) nanoparticles with ~ 30 nm in diameter are uniformly and densely dispersed on N-doped graphene sheets (NGS) with around 4 atomic % of N/C atom ratio via the bio-inspired method. Introducing NGS into the composites have enhanced lithium insertion/desertion performance, in comparison with MFe₂O₄ nanoparticles. More importantly, the cycleability performance has been significantly improved. The MFe₂O₄/NGS composite electrode had the specific capacity of 957 mAh/g (M=Co) and 1229 mAh/g (M=Ni), while MFe₂O₄ electrodes only retained the specific capacities of 348 mAh/g for CoFe₂O₄ and 439 mAh/g for NiFe₂O₄ after 80 cycles. Even after 245 cycles, the CoFe₂O₄/NGS composite electrode can retain the specific capacity up to 873 mAh/g, and the NiFe₂O₄/NGS composite electrode achieved the specific capacity high to 1013 mAh/g after 120 cycles. NGS not only acts as spaces and a conductive additive to prevent the aggregation of MFe₂O₄ nanoparticles and increase the electrical conductivity to provide more electrochemically active insertion/extraction lithium sites, but also offers sites itself for lithium insertion/desertion during charge and discharge process, and thus enhance Lithium insertion/extraction performance.