Titanium (Ti) based compounds, such as titanium oxide, titanate and titanium carbide
are considered to be some of the most fascinating functional materials due to its
excellent electronic, chemical, physical and biocompatible properties. It has
demonstrated a wide rang of potential applications in photocatalyst, solar cell,
biological coating, sensing and supercapacitor, field emissions and so on. Currently, the
rapid development of nanoscience and nanotechnology has greatly motivated the
scientific community and industry to explore new features of both typical and novel
materials at the nanoscale level. In general, one-dimensional (l-D) structure are more
applicable to nanoelectronics and enhanced chemical reactions because of their special
shape which play important roles in outstanding physiochemical performances in terms
of fast electron transport and large surface area. The typical 1-D morphology, such as
nanowire (NW) and nanotube (NT), is considered as a superior candidate for achieving
excellent photonic, electrical and chemical properties. Much efforts have been
devoted to develop various methods (summed as "growing-up" and "etching down
methods) for synthesizing 1-D nanomaterials on metal surface directly, because the
formed nanostructure can strongly anchored on substrate to ensure small resistance and
strong bonding, making them suitable for a wide variety of applications.
In this thesis, we employed two different fabrication routes to construct 1-D
Ti-based nanotube arrays (NTAs) and nanofiber arrays (NFAs) on Ti or Ti alloy
substrate, respectively. Firstly, electrochemical anodic oxidation method has been the
foremost technique to fabricate TiO2 NTAs in fluorine-containing solution due to its
simple and high efficiency. The NTAs provides large surface area and unique
morphology for multifunctional applications. Secondly, thermochemical reaction is a
promising approach to fabricate NFAs on metal surface, which offers good electron
pathway to reduce the electron loss when chemical reaction happened on the surface.
After further adjusting the structure parameters and modifying the surface chemical
composition, it is capable to act as superhydrophoboic surface with controllable water
adhesion, excellent photocatalyst and promising biosensing platform as well as
supercapacitance electrode, etc. Here, we have discussed the fabrication of 1-D
nanostructure on Ti substrate and explored its potential applications.
The first chapter provides an overview of the advantages of 1D nanostructures
fabricated on conductive substrate, and described the fabrication methods and
applications of 1-D titanium based nanostructures.
The second chapter reports regulated water adhesion on superhydrophobic TiO2
NTAs. The special structure of TiO2 NTAs formed on the surface of Ti metal by
electrochemical anodization are interestingly applied for superhydrophobic surface with
controllable water adhesion due to the structural characteristics of TiO2 NTAs can be
controlled facilely by fine design of anodization process. By changing the structural
characteristics of nanotubes, such as by tubular inner diameter (D), length (L), density
(ρ) and surface roughness (Rq), the water adhesion on superhydrophobic TiO2 NTAs
can be regulated over a wide range from 4.4 μN to 89.6 μN. The cooperation effect
between the negative pressures induced by the volume change of sealed air-pockets and
the van der walls attraction at solid-liquid interface contributed to the water adhesion.
The superhydrophobic TiO2 NTAs with a high adhesive force was used as a
mechanical hand" to transfer water microdroplets without any loss, and the one with
extremely low adhesive force was utilized as a self-cleaning and anti-icing surface.
The third chapter reports heterostructured TiO2 nanoparticles/nanotube arrays via
in-situ hydrothermal formation from amorphous TiO2 nanotube arrays in water and its
enhanced photocatalytic activity. The anodized TiO2 NTAs possesses high light and
pollution absorption, thus allowing high photocatalytic efficiency. To enhance the
surface area, the heterostructured TiO2 NPs/NTAs composite structure composed of
anatase TiO2 NTAs and anatase NPs should exhibit improved photoelectrochemical
properties because they combine the merits of the large surface area of NPs and superior
electron transport properties of the NTs. In this chapter, it was found that the
amorphous TiO2 nanotube wall can be gradually etched by water molecules and
converted into anatase nanoparticles which are attached onto the remaining thinner
amorphous nanotube wall, resulting in the formation of the composite NPs/NTAs
structure. The formed TiO2 NPs/NTAs have an improved surface area that is about 1.4
times larger than that of the pristine TiO2 NTAs. The photodecomposition rate on the
anatase TiO2 NPs/NTAs photocatalyst is 2 times larger than that on the anatase TiO2
NTAs. The enhanced photocatalytic activity of the hererostructured TiO2 NPs/NTAs
arises from the large surface area of the TiO2 NPs and superior electron transport of
anatase TiO2 NT.
The fourth chapter reports hydrothermal synthesis of perovskite-type MTiO3 (M= Zn,
Co, Ni)/TiO2 NTAs from the amorphous TiO2 NTAs template. 1-D perovskite-type
titanate composed of titanium and alkaline earth or transition metals with a common
formula of MTiO3 has attracted much attention due to their excellent inherent chemical
and physical properties. In the previous chapter, we found that the amorphous TiO2
NTAs could gradually and spontaneously transform into anatase NPs via water-induced
dissolution and recrystallization. This self-transformation process provides a novel
method to fabricate perovskite-type titanate NTAs in neutral solutions by inserting
foreign metal ions into the octahedral TiO62- group during dehydration. By introducing
the acetate to control the recrystallization process, perovskite-type MTiO3 NPs are
precipitated on the original tubular wall and form the MTiO3/TiO2 NTAs. The result
provides a new method to fabricate perovskite-type MTiO3 without an alkaline
mineralizer.
The fifth chapter reports controlled fabrication of core-shell TiO2/C and TiC/C
nanofibers on Ti foils and Al ion-implanted Ti substrates and investigates their field
emission properties. Implantation of Al into Ti leads to in situ growth of TiC/C in lieu
of TiO2/C NFAs. This is because Al has a higher affinity to oxygen than Ti and Ti reacts
preferentially with C to form TiC. The Ti foil serves as both the Ti source and substrate
for the core-shell TiO2/C and TiC/C NFs to ensure strong bonding and small resistance
between the Ti substrate and the core-shell field emitters. The TiC/C NFs possess
better field emission properties with a turn on field (Eto) of 2.2 V μm-1 compared to an
Eto of 3.2 V μm-1 measured from the TiO2/C nanofiber arrays which can be ascribed to
the formation of the highly conductive TiC core thereby providing low resistance paths
for electron transport. The simple fabrication process and high conductivity render the
materials potentially useful in flat panel displays and other nanoelectronic devices.
The sixth chapter reports a novel non-enzymatic photoelectrochemical (PEC)
biosensing platform based on WO3 nanoparticles decorated core-shell TiC/C nanofiber
arrays via thermal evaporation of WO3 powders. Due to the drawbacks of
enzyme-based biosensors which suffer from complicated procedures during enzyme
immobilization and preservation, intrinsic insufficient stability of enzymes, and easy
inactivation by the external environment. PEC measurements are promising analytical
techniques for the determination of molecular in biological samples by combining the
advantages of both optical methods and electrochemical sensors and thus have a high
sensitivity and fast response. The principle of PEC biosensing is based on the
photocatalytic oxidation of molecules to produce photogenerated electron transfer
between the analyte and semiconductor electrode under light irradiation to amplify the
photocurrent response. In this chapter, sensitive PEC determination of bimolecular are
demonstrated without the assistance of enzyme and the detection limit of H2O2 and
glucose are 3.0x10-9 M and 3.3x10-8 M (S/N = 3) under low power irradiation,
respectively. Without irradiation, there is no respond towards glucose. The distinguish
ability and high sensitivity based on various PEC and EC response on WO3@TiC/C
NFs electrode towards different composition shows interesting application for
quantitative analysis of glucose.
The last chapter reports a robust electrode based on coaxial TiC/C-MnO2 core-shell
NFAs with excellent cycling stability for high-performance supercapacitors. The
composite structure is formed simply by combining the in situ chemical redox reaction
between the KMnO4 and the self-sacrificing carbon shell provided by TiC/C NFs. The
fabricated TiC/C-MnO2 core-double-shell NFs presents strong bonding between the
formed MnO2 shell and pristine NF and exhibits high specific discharge capacity of 645
F g-1 at current density of 1 A g-1 and 99% capacity retention after 5000
charge/discharge cycles. However, the thermal treatment conducted on the as-prepared
electrode decreases the initial capacitance, but the electrode undergoes capacitance
recovery through structural transformation from the crystalline cluster to layered
birnessite type MnO2 nanosheets as a result of dissolution and further electrodeposition
via charging/discharging cycling. 96.5% of the initial capacitance is retained after
1,000 cycles at high charging/discharging current density of 25 A g-1. After cycling, the
TiC/C NFs remain vertically rigid attributed to its highly robust mechanical properties.
The MnO2@TiC/C composite electrode exhibits remarkable specific capacitance,
excellent long-term cyclic stability and strong capacitance retention attribute to the
novel core-shell structure, indicating large potential to act as a high-performance SCs
electrode material.
| Date of Award | 16 Feb 2015 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Paul Kim Ho CHU (Supervisor) |
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