Environmental Engineering Reference
In-Depth Information
conversion and storage devices. For example, the increasing surface-to-volume
ratio and special surface area facilitate sufficient reaction or interaction between
the nanostructured material and surrounding electrolyte [
37
]. Energy conversion
and transport in nanostructured materials are also different from those in bulk
materials due to the quantum size effects on energy carriers such as photons,
electrons, and molecules [
38
]. For example, efficient light harvesting to create
charge carriers in materials happens at the scale of several hundreds of nanome-
ters, near the wavelength of light.
However, the mean free path of the excited charge carriers is shorter than the
wavelength of light, requiring the small length scales afforded by nanostructures.
Considering the need for both efficient photon absorption and effective collection of
excited charge carriers in devices, it is necessary to design structures on a scale
commensurate with both the wavelength of light and charge migration lengths
simultaneously. One such option is one-dimensional (1D) nanostructures (e.g.,
nanotubes or nanowires), which have one dimension larger than the wavelength of
light, and another dimension shorter than the mean free path of charge carriers [
39
].
Accordingly, current efforts in nanoscience and nanotechnology for energy appli-
cations are concentrating on utilizing these nanoscale effects to produce efficient
energy technologies such as solar cells, fuel cells, and batteries. Researchers are
eager to exploit cost-effective process to prepare high-performance nanostructures
for a more sustainable energy economy [
6
,
40
-
44
].
Many properties of nanostructured TiO
2
films, such as surface area, shape, grain
size, and grain boundary density, will significantly impact the performances of
energy conversion devices [
45
-
47
]. The methodology used to fabricate the
nanostructured films is an essential factor to tailor the properties of TiO
2
nano-
structures [
48
]. To date, significant progress has been achieved in the preparation
of TiO
2
nanomaterials. A variety of film preparation techniques have been
developed and employed for the formation of diversiform nanostructured TiO
2
,
including nanoparticles [
49
], nanorods [
50
], nanowires [
51
], nanotubes [
52
],
nanosheets [
53
], and mesoporous structures [
54
]. In most cases, nanostructured
TiO
2
materials can be prepared either by dry or wet processes. In the past decades,
a number of methods, such as sol-gel [
55
], hydrothermal/solvothermal processes
[
56
], electrochemical anodization [
57
], electrospinning [
58
], electrospray [
59
],
electrodeposition [
60
], directional chemical oxidation [
61
], ultrasound and
microwave irradiation [
62
], laser pyrolysis [
63
], and chemical/physical vapor
deposition [
64
], have been developed to control the size, morphology, and uni-
formity of TiO
2
nanostructures simultaneously. The following sections further
elaborate on some of the above-mentioned preparation methods for preparing cost-
effective, high-performance TiO
2
nanostructures for energy applications, such as
DSSCs, hydrogen generation, and photocatalysis.
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