Titanium dioxide (TiO2) is a highly versatile material, owing to the optical and catalytic properties exhibited by its two common crystal forms: rutile and anatase. The rutile phase of TiO2 has a high refractive index and is useful for optical devices, such as waveguides. The photocatalytic activity of the anatase phase of TiO2 is widely applied in many fields, such as microorganism photolysis, medical treatment, environmental purification and photovoltaic cells. More recently, ordered nanostructures have been prepared using templating techniques. For example, ordered TiO2 nanotubes were synthesized using porous anodic alumina as templates via a sol-gel process. TiO2
nanowire arrays have also been synthesized using an electrochemical method.
Block copolymers (BCPs) are a versatile platform material since they can
self-assemble into various periodic structures for proper compositions and under adequate conditions, owing to the microphase separation between dissimilar blocks. A diblock copolymer, the simplest case, self-assembles into various equilibrium morphologies.
Block copolymers are good tools to be a template to let nanoparticle forming ordered structure.
In the thesis, we first study the surface modified TiO2 nanoparticles and the morphology of TiO2/ polystyrene-b-poly (methyl methacrylate) PS-b-PMMA
nanocomposite. Here, we first disperse surfactant-modified TiO2 nanoparticles into either block of a PS-b-PMMA diblock copolymer with an ordered lamellar and cylindrical phase. The pre-synthesized TiO2 nanoparticles were surface modified by different surfactant. The surfactant can be either hydrophilic or hydrophobic, with one of its ends tethered to a nanoparticle by an ionic bond or a covalent bond. This selectivity is important in designing the optical properties of nanoparticles-block copolymer hybrid systems. Moreover, we report the synthesis of an arrayed TiO2 nanostructure using ordered TiO2 seeds, which were synthesized and incorporated into one block of a thin PS-b-P4VP nanotemplate. This bottom-up growth process could be used to grow several kinds of metal or semiconducting materials via low temperature solution process, CVD, or furnace process. Furthermore, we study that single, aligned TiO2 nanoneedles having diameters in the tens of nanometers can be grown through a solution crystal growth process from patterned nanocavities under the influence of an electric field. The electric field, which we applied perpendicular to the substrate plane, drove the precursor solution into the cavities by overcoming the surface tension encountered and oriented the TiO2
nanoneedles during the growth process.
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Table 1-1. Relation between size and surface atoms.
Size (nm) Atoms Percentage of atoms at surface (%)
10 3*104 20
4 4*103 40
2 2.5*102 80
1 30 99
Table 1-2. Confinement by the infinite potential well.
Confinement length LA Energy E1 Temperature T
1 Ǻ 40 ev 4*105 K
1 nm 0.4 ev 4*103 K
1μm 0.4 μev 4m K
[Handbook of Nanostructured Materials and Nanotechnology. V4]
diblock
random mutiblock
triblock
graft copolymer
four arm startblock
Figure 1-1. Some block copolymer architectures.
[Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.2003.]
Figure 1-2. Randomly branched graft copolymers can be prepared by three general synthetic methods: 1) the ‘‘grafting to’’, 2) the ‘‘grafting from’’, and 3) the
‘‘grafting through’’.
[Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.2003.]
Figure 1-3. Schematic representation of the different types of block copolymers: a) coil-coil diblock copolymers, b) rod-coil diblock copolymers (total molecular weight >20,000 g/mol), and c) rod-coil diblock oligomers (total molecular weight <20,000 g/mol).
[Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.2003.]
Figure 1-4. The well-known structures of block copolymers in melt, solution or solid state.
Figure 1-5. Experimentally determined phase diagram for PS-PI diblock copolymers.
[Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.2003.]
Figure 1-6. Phase diagrams for ABA triblock copolymer melts with τ=0.25 (left) and τ
=0.5 (right). Solid lines give the disorder-to-order transition as (χNt) (f).
Dotted lines give the transitions between bcc and hex, and dashed lines the transition from hex to lam.
[Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.2003.]
Figure 1-7. ABC linear triblock copolymer morphologies. Microdomains are colored following the code of the triblock molecule in the top.
[Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.2003.]
Figure 1-8. A) Schematic illustration the density of state in metal and semiconductors. B) Variation of density of state of electrons with increase of the quantization dimension in quantum structure.
[Science 1995, 271, 933.]
59
Anatase [110](tetragonal) Rutile [101] (tetragonal)
79
Figure 1-9. The crystal structures of Anatase, Rutile and Brookite. Unit cells for each polymorph are shown by solid white lines.
(001) face
Ti In-plane O
[100]
[010]
Figure 1-10. Schematic illustration of the atomic arrangements on ideal TiO2 (110) and (001) single crystal faces.
Ti In-plane O
Bridging site O (110) face
[110]
[110]
Chapter 2