Environmental Engineering Reference
In-Depth Information
TiO
2
nanotube film with tunable tube lengths was developed by a two-step
anodization process and then transferred onto FTO glass via a layer of TiO
2
nanoparticle paste [
190
-
195
]. This process, however, results in closed-bottom
tube-ends. This is problematic because the interface between the TiO
2
nanotube
arrays and the TiO
2
nanoparticle layer might cause near-UV light absorption and
front surface light reflection and block the diffusion of the redox reagents into the
underlying TiO
2
nanoparticles coating on the collecting FTO substrate. Thus, a
number of methods have been developed to fabricate free-standing TiO
2
nanotube
arrays, including critical point drying, dissolution of the Ti substrate in water-free
CH
3
OH/Br
2
solution, solvent evaporation, ultrasonic agitation, chemically assisted
delamination and potential shock [
196
-
201
]. For example, Li et al. removed the
caps of the closed-bottom TiO
2
nanotubes by immersing the as-prepared free-
standing TiO
2
nanotube film in an oxalic acid solution. As compared to the closed-
end TiO
2
nanotube-based DSSC, the opened-end TiO
2
nanotube-based device
exhibited an increase in one-sun efficiency from 5.3 to 9.1 %, yielding a 70 %
enhancement [
202
].
A novel version of DSSCs was introduced to overcome the light illumination
problem of TiO
2
nanotube-based electrodes, namely, three-dimensional dye-sen-
sitized solar cells (3D DSSCs). In this system titanium wires or meshes are utilized
instead of titanium foils or sheets to fabricate TiO
2
anodized nanotubes [
203
-
208
].
Misra et al. used a TiO
2
nanotube-based wire as a working electrode (Fig.
10
a-c)
and a platinum wire as a counter electrode in a DSSC. This DSSC achieved a
conversion efficiency of 2.78 % under AM 1.5 simulated solar light (Fig.
10
d).
The prototype device is capable of achieving long distance transport of photo-
generated electrons and multi-directional light harvesting from surrounding to
generate electricity [
205
]. Wang et al. developed a new type of 3D DSSCs with
double deck cylindrical Ti meshes as the substrates. Here, one of the Ti meshes
was anodized to in situ synthesize the self-organized TiO
2
nanotube layer to serve
as the photoanode. Another Ti mesh was platinized through electrodeposition as
the counter electrode. This all-Ti 3D DSSC exhibited the highest conversion
efficiency of 5.5 % under standard AM 1.5 sunlight [
204
].
In addition, vertically ordered TiO
2
nanotube arrays also face the serious
problem of insufficient surface area due to the large diameter of nanotubes and
considerable free space between nanotubes. This leads to poor dye adsorption
capacity when applied in DSSCs [
209
]. In light of this limitation, many strategies
have been explored [
46
,
103
,
210
-
212
]. For example, a common combination of
TiO
2
nanotubes and nanoparticles was realized by treating the as-anodized nano-
tubes with a TiCl
4
solution which hydrolyzed to yield nanoparticles. This can
increase the nanotube surface area and bridge any cracks resulting from annealing,
and thus improve the conversion efficiency of nanotube-based DSSCs [
213
,
214
].
Recently, novel hierarchical-structured TiO
2
nanotube arrays have been prepared
by combining the two-step electrochemical anodization with a hydrothermal pro-
cess. The resulting DSSCs exhibited good performance and applicability [
212
].
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