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Figure 8.1 Example of the hierarchical structure in nature that is composed of a
trunk with 1
st
generation branches (a) and multiple generation branches
(b) attached to it.
wire-type structures, tree-like hierarchical structures
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are formed while
spherical particles comprise urchin-like or chestnut-like hierarchical struc-
tures
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when combined with wire-type branches. In this chapter, the words
''hierarchical structure'' rather signify structures with a distinct backbone
(also called a trunk or core) to which a plethora of branches are attached as
seen in Figure 8.1. Therefore, tetrapod,
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cauliflower
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and chrysanthemum
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structures are not examined in this chapter, though they are also often re-
ferred to as hierarchical structures in a broader sense.
Among various materials and applications, this chapter focuses on hier-
archical nanostructures of materials for photo-electro-chemical (PEC) cell
applications, which are largely classified as photovoltaics and water splitting
for fuel (hydrogen) generation.
.
8.2 Importance of the ''Hierarchical'' Structure in
PEC Cell Applications
Tweaking and improving the properties of existing materials could enhance
cell eciency by a relatively modest factor. In contrast, structural recon-
figuration such as introducing nanowires instead of nanoparticles, for ex-
ample, has shown remarkable results in eciency improvement.
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It has
been verified through numerous research studies that 1-D crystalline
nanostructures such as nanowires and nanotubes significantly improve the
electron diffusion length by providing a direct conduction pathway to the
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