Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Chemistry

Supervisor

Dr. Yang Song

Abstract

Nanomaterials have been extensively studied due to their distinctive properties such as surface effect, small-size effect and quantum size effect. In recent year, investigations of the structural and phase transformations of nanomaterials under high pressure are receiving increasing attentions. In addition to composition and synthetic routes, pressure provides a clean way to adjust interatomic distance and hence affect the crystal structure and thus properties of the nanostructured materials.Two nanomaterials (i.e. TiO2 and Li4Ti5O12) with different morphologies are studied in this thesis.

In part I, the high-pressure behaviours of four hydrothermal synthesized 1D rutile TiO2 nanomaterials (i.e. nanowires, nanorods, flower-like nanorods and nanotubes) were studied using Raman spectroscopy and synchrotron X-ray diffraction for the first time. A new morphology of flower-like nanorods was the first time obtained by this synthesis method. The phase transition sequence of these four 1D rutile TiO2 nanomaterials is consistent with previous study. However, an interesting high-pressure behaviour was observed on nanowires that the reflection of rutile phase was observed in all patterns upon compression and decompression, suggesting a unique phase transition phenomenon unprecedented in TiO2 nanomaterials. Comparative studies of these 1D rutile TiO2 nanomaterials show strongly contrasting morphology-dependent compression behaviours: (1) Upon compression, nanotubes show the highest phase transition pressure of 20.8 GPa. (2) A higher compressibility along a-axis was observed compared to c-axis. (3) A relatively low bulk modulus of TiO2 nanotubes (bulk modulus of 193 GPa) was found indicating that among these 1D rutile TiO2 nanomaterials, nanotubes has the highest compressibility.

In part II, another titania based energy material: nanostructured Li4Ti5O12 has been studied under high pressure. Nanostructured Li4Ti5O12 (LTO) as promising anode materials in lithium ion battery (LIB) has shown excellent yet morphology-dependent performance in LIB operations. However, the structural origin that influences the material performance at microscopic level remains unclear. Here using combined in situ Raman spectroscopy and synchrotron X-ray diffraction, we comparatively investigated the structural stability of two nanostructured LTO materials with different morphologies by application of external pressure up to 27 GPa. In particular, nanoflower-like Li4Ti5O12 spheres (LTO-1) and Li4Ti5O12 nanoparticles (LTO-2) were rendered to high pressures using diamond anvil cells and their structural evolutions were characterized upon compression and decompression. Raman measurements show that both LTO materials undergo pressure-induced structural disorder but with different reversibility upon decompression. X-ray results further confirmed Raman measurements, but also allow the quantitative analysis of pressure dependence of crystal structures. Structural refinements of diffraction patterns yield morphology dependent bulk modulus of the two LTO materials, which reveals critical information about the intrinsic lattice strain and vacancies. These different structural characteristics, when compared with another spinel structure of lithium titanium dioxide, allows the interpretation of different performance between LTO-1 and LTO-2 for LIB operations. This study thus contributes to the understanding of the important factors that may influence the electrochemical performance and help with the design of new LTO based anode materials for LIB.

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