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structuration of porosities through mimicking natural systems, hierarchic-
ally structured porous materials can provide large surface areas for
reactions, interfacial transport, or dispersion of active sites at different
length scales of pores and they can shorten diffusion paths or reduce the
diffusion effect.
18
Zhou et al.
19
used natural leaves as a biotemplates to replicate all the
fine hierarchical structures of leaves as shown in Figure 4.6. They used a
pure inorganic structure of TiO
2
with the same hierarchy as leaves by a two-
step procedure composed of the infiltration of inorganic precursors and the
calcination of the biotemplates. All of the photosynthetic pigments were
replaced by artificial catalysts such as Pt nanoparticles. The artificial leaf
with catalyst components obtained was used for ecient light-harvesting
and photochemical hydrogen production. The average absorbance inten-
sities within the visible range increased 200-234% for the artificial leaves
compared with conventional TiO
2
nanoparticles prepared without biotem-
plates. This should certainly contribute to hierarchical architectures with all
the fine structures of leaves imprinted in artificial leaves. The photocatalytic
activity is much higher than that of TiO
2
nanoparticles prepared without
biotemplates and commercial nanoparticulate P25.
19
Similar structures were
also demonstrate by Zhu et al. by sonochemical methods.
20
The important
role of meso-macroporous structures in light harvesting photocatalysis has
been revealed. In the macro-mesoporous TiO
2
photocatalyst, the macro-
channels acted as a light-transfer path for introducing incident light onto
the inner surface of mesoporous TiO
2
. This allowed light waves to penetrate
deep inside the photocatalyst, making it a more ecient light harvester.
It is known that a wavelength of 320 nm is reduced to 10% of its original
intensity after penetrating a distance of only 8.5 mm on thin film TiO
2
.
The presence of macrochannels, however, makes it possible to illuminate
even the core TiO
2
particles with the emission from the four surrounding
UV sources. Considering the light absorption, reflection, and scattering
within such a hierarchical porous system, the effective light-activated surface
area can be significantly enhanced. Moreover, the interconnected TiO
2
nanoparticle arrays embedded in the mesoporous wall may allow highly
ecient photogenerated electron transport
d
n
3
r
4
n
g
|
9
.
through the macrochannel
network.
21
There are other approaches to prepare hierarchically structured nano-
particle films with submicron sized nanoparticle aggregates to realize high
eciency solar cells as shown in Figure 4.7. The synthesis of hierarchical
ZnO aggregates can be achieved by hydrolysis of zinc salt in a polyol medium
at 160 1C.
22
ZnO aggregates with either a monodisperse or polydisperse size
distribution can be prepared
23,24
by adjusting the heating rate during syn-
thesis and using a stock solution containing ZnO nanoparticles of 5 nm in
diameter. The hierarchically structured ZnO film is well packed by ZnO ag-
gregates with a highly disordered stacking, while the spherical aggregates
are formed by numerous interconnected nanocrystallites that have sizes
ranging from several
tens to several hundreds of nanometres.
22
The
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