Environmental Engineering Reference
In-Depth Information
these 1D nanostructures gives them excellent electron percolation pathways for
directional charge transfer between interfaces. For example, the mobility of
electrons in 1D nanostructures is typically several orders of magnitude higher than
in semiconductor nanoparticle films [
153
]. In comparison with other 1D mor-
phologies, nanotubes provide a larger interfacial area due to their external and
internal surfaces, which is beneficial for surface area-dependent applications [
154
].
Studies on TiO
2
have shown that vertically oriented nanotube arrays are
remarkably efficient when applied in sensors, water splitting, DSSCs, and photo-
catalysis [
155
-
160
].
In the past decade, self-organized oxide nanotube arrays have attracted exten-
sive scientific and technological interest. Thus, TiO
2
nanotube arrays have been
synthesized by a variety of methods, including template deposition (e.g., AAO
templates, ZnO nanorod templates, or organic templates) [
161
], electrochemical
anodization [
162
], and hydrothermal techniques [
163
]. Among these, an inex-
pensive and straightforward approach that leads to well-behaved nanotubes is the
anodization method, which enables precise control over the resulting tube diam-
eter, tube length, and overall morphology by adjusting various parameters such as
the pH, concentration and composition of electrolyte, applied potential, growth
time, and temperature of the anodization process [
164
-
167
]. Interestingly, the
addition of fluoride ions tends to control the overall development of nanotube
architecture (Fig.
8
b). There are several excellent reviews detailing the growth
mechanism in anodic oxidation [
168
-
171
].
Normally, the anodization process can be divided roughly into three stages: (1)
electrochemical oxidization of the titanium surface which results in the formation
of an initial TiO
2
barrier layer, corresponding to the first current drop; (2) chemical
etching of TiO
2
by F
-
to form TiF
6
2-
, resulting in nanotube formation that leads to
a current increase; and (3) the growth of nanotubes, which results in a slow current
decrease [
172
]. Briefly, the nanotube growth is determined by the equilibrium
between anodic oxidation and chemical dissolution. The anodic oxidation rate is
mainly controlled by the anodic potential, while the chemical dissolution rate is
controlled by the electrolyte acidity and F
-
concentration [
173
]. As previously
mentioned, since the discovery by Gong and co-workers in 2001, TiO
2
nanotube
fabrication has been intensively investigated over the past decade. Consequently, a
variety of nanotubular architectures have also been explored. In particular, by
varying the voltage during the growth, new self-organized TiO
2
morphologies
could be obtained: bamboo-type nanotubes [
174
], branched nanotubes [
175
],
periodic nanotubes [
176
], ridged nanotubes [
177
], double-walled nanotubes [
178
],
and multilayer nanotubes [
179
]. In the following sections, some representative
fabrication
processes
developed
recently
for
high-performance
photovoltaic
applications are discussed in greater detail.
TiO
2
nanotubes prepared via anodization of Ti foil are attached to the Ti
substrate with closed bottom. In most cases, the use of Ti foil leads to TiO
2
nanotube arrays supported on Ti substrate. However, in many applications,
detached TiO
2
nanotube layers are required. TiO
2
nanotube arrays grown in situ on
opaque titanium foil are difficult to apply to high-efficiency DSSCs because the
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