Synthesis of nanowires and amorphous/crystalline $CaCo_3$ structures inspired by biomineralization

Abstract

Biological materials often provide inspirations for the design and synthesis of advanced materials. For the formation of structures, biological materials prefer growth by biologically controlled self-assembly in ambient conditions as opposed to the highly energy-consuming process by which engineering materials are fabricated. In addition, the elements for biological materials are limited to some light elements which are abundant in nature such as Ca, P, Si, C, and so on. In spite of the limitation on selecting elements, biological organisms can produce structural, protective or functional materials which fairly meet the requirements for their survivals. All the processes concerning the formation of inorganic materials in biological organisms could be defined as biomineralization. When the strategies on the formation of biomineral can be used for material processing, there will be a lot of advantages in terms of energy efficiency and abundance of the base elements, considering energy and environmental problems with resource depletion. In this study, various advanced materials were synthesized by understanding and utilizing the biomineralization. Mollusc shells, the most representative biomineral, are composed of $CaCO_3$ (aragonite and calcite), organic matrix and other elements such as sodium and potassium. Mollusc shell release $CO_2$, organics and elements including sodium by thermal heating, which can be considered as thermodynamically reversed process of biomineralization. Single crystalline sodium titanate nanowire having photoactivity for water splitting was grown from titania (P25) and oyster shell by thermal heating at $700\degC$ in air. $CO_2$ and sodium have been demonstrated as essential elements for one-dimensional crystal growth of sodium titanate from titania. Ostwald ripening is considered to be the mechanism for the growth of nanowires from titania nanoparticles. In addition, recrystallization should be followed for precursor material to be...