Hierarchical Nanostructures for Photo-Electro-Chemical Cells - Hierarchical Nanostructures for Energy Devices - page 182

representative fabrication strategies for hierarchical structures are

introduced.

8.4.1 Combination of Hydrothermal Growth and Seed
Engineering (Solution-phase Chemical Route)
The hydrothermal growth method is a low-cost, scalable and facile way to
produce metal oxide nanostructures such as ZnO, 35 TiO 2 , 36 In 2 O 3 37 and
WO 3 . 38,39 Sometimes, the word ''hydrothermal'' is used only when the precursor
solution is aqueous while the expression ''solvothermal'' is adopted when a
non-aqueous precursor solution is utilized. However, in many instances,
''hydrothermal method'' has a broader meaning encompassing both cases.
For hierarchical structures synthesized via the hydrothermal method,
nano-size branches are grown from either secondary seed nanoparticles 2 or
nuclei generated on the surface of the backbone structure during an inter-
mediate treatment. 10 ZnO is the most actively researched material as a
hydrothermally grown metal oxide due to the low synthesis temperature and
water-based non-toxic precursors. Therefore, many types of hierarchical
structures have been reported for PEC applications, especially for DSSCs.
Ko et al. 2 and Herman et al. 3 reported ZnO ''nanoforest'' structures of high
density, long-branched ''tree-like'' multi generation hierarchical photo-
anodes that can significantly increase power conversion eciency due to
substantially enhanced surface area, enabling higher dye loading and light
harvesting as well as due to reduced charge recombination by direct con-
duction along the crystalline branches as shown in Figure 8.6. They have
shown that complex and hierarchical ZnO nanowire photoanodes could be
fabricated via a low-cost, all-solution processed hydrothermal method
incorporating seed particle deposition steps and utilization of capping
polymers. Lee et al. 10 reported high eciency DSSCs with densely-packed,
omni-directionally branched TiO 2 nanostructures grown by the hydro-
thermal method. Through the so-called ''2 þ 1 method'', which comprises
two hydrothermal growth steps with an intermediate high-concentration
TiCl 4 treatment on upright backbone NWs (Figure 8.7(a)), the branches grew
in all directions from densely distributed needle-like seeds on jagged
cylindrical surfaces of the backbone NWs, resulting in a dramatically in-
creased surface area as shown in Figure 8.7(b). There are also many vari-
ations of hierarchical TiO 2 nanowire structures through solution-phase
routes. Details of fabrication methods have been reported. 32,38,40,41
Zhuge et al. 42 demonstrated TiO 2 nanotubes coated with an outer nano-
crystalline and mesoporous TiO 2 shell. They used hydrothermally-grown
ZnO nanowires as a template for the layer-by-layer coated TiO 2 nanotube.
The ZnO core is dissolved by mild etching in TiCl 4 solution to form TiO 2
nanotubes which transform to the hierarchical structure through additional
layer-by-layer coating followed by hydrothermal crystallization (Figures 8.7(c)
and (d)). The DSSC eciency with the hierarchical TiO 2 nanotube structure
is 30% higher than with the original nanotube arrays.

d n 3 r 4 n g | 4

.