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Moreover, branched nanowires from the backbone structure usually have a
much smaller size and higher density,
2,10
resulting in a larger surface area as
well as more ecient charge collection and separation. Therefore, backbone-
branch NW type hierarchical heterostructures combine these advantages.
Several studies on hierarchical heterostructures have been reported, in-
cluding ZnO/In
2
O
3
(backbone/branch) by an evaporation method,
68
BaCrO
4
by a catanionic reserver micelle system,
69
and Si/SiO
2
by VLS method.
70
Among various materials, we will focus on heterostructures of the PEC cells
for water splitting in this section.
Xi and coworkers synthesized Fe
3
O
4
/WO
3
(core-branch) heterostructures
showing enhanced photoconversion capability with respect to pure WO
3
or
Fe
3
O
4
nanostructures as depicted in Figure 8.12(a).
4
The branches, WO
3
nanoplates, have a relatively narrower bandgap (2.6-2.8 eV) enabling pho-
tocatalytic activity in the visible range, while the Fe
3
O
4
microsphere cores
provide a fast transport route for the photogenerated charges due to the high
d
n
3
r
4
n
g
|
4
.
Figure 8.12
(a) Schematic illustration showing formation of the Fe
3
O
4
/WO
3
core-
shell structures. Adapted with permission from ref. 4. (Copyright 2011,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.) (b) Schematic
illustration of the Si/TiO
2
nanotree heterostructure and working mech-
anism of water splitting. The electron-hole pairs separated at the
semiconductor-electrolyte interface conduct the individual half reac-
tions of water splitting with the help of co-catalysts, without any applied
bias. Adapted with permission from ref. 28. (Copyright 2013, ACS.)
(c) SEM image of the Si/TiO
2
nanotree heterostructures. Adapted with
permission from ref. 28. (Copyright 2013, ACS.) (d) SEM image
of Si/ZnO heterostructures.
1
Adapted with permission from ref. 71.
(Copyright 2013, ACS.)
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