Phase and structure control of metal compound and carbon hybrid nanofibers using thermodynamic redox reaction

Abstract

Precise phase control in nanoscale synthesis and spontaneous assembly in macroscopic dimension are important but difficult for practical implementation of nanomaterials. In frequently-encountered situations where multiple components are involved in synthesis, the most of difficulties about mass production and reproducibility are related to the lack of thermodynamic and kinetic predictability. Inspired by chemical metallurgy theories, an insight to solve the limitations in nanomaterial synthesis has been suggested in this thesis. Metal compound/carbon nanofibers are receiving great attention for the energy and environmental applications. Metal compound acts an active material which shows high electrochemical activity. Electrically conductive carbon (C) nanofiber is a supporter which maintains high-aspect ratio one-dimensional (1D) structures. In this thesis, a new functionality of C nanofiber has been pioneered as a determinant to control the spontaneous of atomic species and final structures of metal compound. It is based on oxidation based C decomposition, which produces gaseous decomposition products such as carbon monoxide (CO) and carbon dioxide (CO2). Based of the reaction between O2 and C, the degree of C decomposition can be precisely controlled by the oxygen partial pressure (pO2). In multi-atomic component system, the reaction between the elements can generate many intriguing compounds. For inducing the reaction between selected elements, Ellingham diagram and phase diagram can contribute to predict processing parameters and material composition. Based on this theoretical background, an innovative fabrication methodology for controlling the phase and structures of active materials and C nanofibers has been developed via predictive synthesis. Escaping from the control in solution process, it is based on gas-solid reactions which can realize the mass production of complex multi-atomic nanomaterials with high uniformity. The first focus is the demonstration of selective C oxidation induced reductive metal structure control. Based on the oxidation Gibbs free energy (ΔG) of C, metals are categorized into reductive and oxidative metals. In the case of reductive metals such as Co, Ni, Cu, Pt, etc., selective redox reaction induces C oxidation and metal reduction. According to the pO2 in this condition, full-filled C nanofibers with embedded metal nanoparticles, metal/C core/shell nanofibers, hollow and porous C nanofibers are formed. The mechanism of this structure evolution according to the pO2 has been studied by the catalyzed C oxidation at the surface of metallic species. The second focus is an in-depth study about the effect of C porosity on the structures of oxidative metallic species. The size and distribution of Sn, which shows high capacity active material in Li-ion battery anode, was optimized by managing the outward Sn diffusion during calcination. For minimizing the volumetric expansion based material degradation, the strategy for inducing Sn nanoparticles fully-embedded has been established. By utilizing the pressure equilibrium to derive reverse direction of Boudouard reaction, C porosity could be controlled by ambient conditions. Finally, selective redox reaction scheme has been verified in multi-atomic component system to control molybdenum disulfide (MoS2) inside C nanofibers. The redox reactions of Mo-S-C-O were categorized according to the pO2. Especially, at the pO2 between ΔG of C and Mo oxidation, C is decomposed by combustion with MoS2 formation. In ternary phase diagrams, Mo-S-C-O mole fraction was determined to form MoS2 and gaseous C oxide such as CO and CO2. The calcination in this region was induced to control MoS2 structures by modulating the vertical stacking and lateral growth of MoS2 stacking unit. Through this methodology various MoS2 structures such as length, stacking number, distribution, and alignment were induced. In this thesis, it has been revealed that the structure and phase of metal compound/C nanofibers can be precisely controlled by the predicted parameters from the same metal precursor and polymer matrix nanofibers. This customized fabrication has a potential to tuning the properties of metal compound/C nanofibers according to the various applications such as electronic, chemical, energy, and environment fields.