The development of renewable green energy resources has received intensive
attention due to the limited fossil energy resources, significant increased demand for
energy and considerable environmental problems. Therefore various energy resources,
such as wind, tidal, chemical, nuclear, and solar energies have widely been considered
and explored to complement existing energy and partly substitute conventional fossil
sources. Among them, solar energy has been considered as one of the most attractive
solutions because of its permanent availability in abundant quantities. The solar energy
can be directly converted into electricity in solar cells using the photovoltaic effect.
However the rapid increase of the energy consumption can only be satisfied by cost
effective solar cell devices operating with high conversion efficiency of light to
electricity. Obviously the cost effectiveness and efficiency are interrelated. The
efficiency of solar cells strongly depends upon many device parameters that include
light absorption, effective induction of charge carriers, their transfer and collection. All
these parameters are related to optoelectronic properties of the materials used, their
architectures and contacts. Thus there are many parameters that have to be controlled
and optimized to achieve high conversion efficiency. In addition to large power plants,
small solar units can be used in high-value devices and in devices used for exploration
of our planetary system. In high value devices the cost of solar cells is not so important. In some consumer products, for example, in electric vehicles, their vast development is
based on advanced energy storage devices. Rechargeable lithium-ion batteries (LIBs)
have been regarded as one of most perspective devices for energy storage. However,
their specific capacity and power characteristics need to be further improved to meet
requirements for automobile and large-scale applications.
This thesis focuses on the design, synthesis and applications of novel titania (TiO2)
nanostructures. Specifically, arrays of rutile TiO2 nanorods, porous spherical and
wire-like TiO2 have been synthesized and applied as electrodes in solar cells and
lithium-ion batteries.
The first part is focused on synthesis of unique rutile TiO2 nanorods and their arrays
on a series of substrates, including silicon, silicon dioxide, sapphire and fluorine doped
tin oxide (FTO) substrates. The arrays were further used to construct dye-sensitized
solar cells (DSSCs) and organic/inorganic hybrid heterojunction solar cells. The
assembled DSSCs possess relatively low power conversion efficiency (around 1%) due
to the small surface areas of TiO2 and low dye molecule adsorption. However, the
hybrid solar cells based on TiO2/Sb2S3/P3HT heterojunction structures exhibit moderate
energy conversion efficiency of 1.8%, which is much higher than that made of
TiO2/P3HT structures (0.1%). In addition, the interfacial electronic structures of TiO2
nanorods and CuPc were characterized by x-ray photoelectron spectra and ultraviolet photoelectron spectra.
The second part reports on development of structures based on porous anatase TiO2
spheres with tunable sizes. The porous spheres were synthesized by a sol-gel method
followed by a rapid microwave-assisted hydrothermal treatment. The DSSCs employing
the porous TiO2 spheres exhibit a 5% power conversion efficiency, which is 40% higher
than that made of commercial Degussa P25 TiO2 nanoparticles, due to the higher
specific surface and enhanced light scattering within the photoanode films.
The third part presents a two-step method to optimize the nanoporous characteristics
of TiO2 samples as anodes in LIBs thus enabling a higher charge and discharge capacity
and a much better rate capability compared to dense TiO2 materials. A simple sol-gel
process is used to fabricate spherical titanium glycolates precursors followed by
subsequent hydrothermal or annealing treatments resulting, respectively, in highly
porous or dense TiO2 nanospheres. The fabricated TiO2 nanostructures have been
subsequently used to assemble lithium-half cells. Galvanostatic discharge-charge tests
indicate that the porous TiO2 nanospheres possess high and stable reversible capacities
of 229, 133, and 56 mAh g-1 at charge and discharge rates of 0.06, 0.6 and 6 C,
respectively; whereas the corresponding values for dense TiO2 nanospheres are 217, 45,
and ~1 mAh g-1. Such considerable improvement of the electrochemical activity is
attributed to the porous TiO2 nanostructures, and subsequent change in lithium ions diffusion length, and enables the possibility to optimize the high rate capability in
TiO2-based LIBs.
The final experimental parts deal with synthesis of highly porous wire-like TiO2
nanostructures as promising anodes in LIBs. These porous one-dimensional structures
are expected to enhance both the contact interface and charge transport characteristics.
The lithium half-cells made of the synthesized porous TiO2 wires present very high
lithium ion storage capacity, good rate capability, and stable cycle performance.
Galvanostatic charge/discharge tests indicate that the porous wire-like TiO2 samples
exhibit lithium ion storage capacities of 206.4, 167.1±0.7, 152.1±0.8, 139.7±0.3, and
116.1±1.1 mAh g-1 at 0.2, 0.5, 1, 2 and 5 C rates, respectively. Such improved
performance could be ascribed to their unique porous and 1-D nanostructures
facilitating better electrolyte penetration, higher diffusion rate of electrons and lithium
ions, and variation of accommodated volumes during the charge/discharge cycles.
In summary, this study demonstrates novel TiO2 nanostructures that have been
tailored for applications in solar cells and lithium ion batteries. The further performance
improvement of the solar cells and energy storage devices needs higher controlling the
particle size, and their distribution. Additionally, the surface/interface modifications and
controlled doping should be carried out and studied to construct the efficient solar cells
and energy storage devices.
| Date of Award | 16 Jul 2012 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Igor BELLO (Supervisor) |
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