Summary - Hierarchical Nanostructures for Energy Devices - page 293

supercapacitor, hydrogen storage), and (3) energy ecient electronics
(display, sensors etc). Hierarchical nanostructuring includes highly porous
metal-organic frameworks, nanoparticle assembly with defined pore size,
and multiple-generation highly branched nanowire trees. These topics are
currently among the major issues in society. Energy-related issues have
increased since the recent energy crisis. However, widespread use of the next
generation of green energy devices is still limited by eciency and cost. Most
energy-related topics focus only on new material development. This topic
covers the fundamentals to state-of-the-art functional hierarchical nano-
structuring aspects.
Recent developments of hierarchical nanostructures have initiated con-
siderable research interest in energy devices. In this topic, applications of
hierarchical nanostructures in the emerging energy conversion and storage
areas were highlighted, ranging from solar cells, photocatalysis, photoelec-
trochemical water splitting to advanced energy storage devices such as Li ion
batteries and supercapacitors as well as displays. However, the potential
energy applications of hierarchical nanostructures are not only limited to
these but also include applications in other research areas such as fuel cells,
thermoelectric devices, piezoelectric devices, which are also being explored.
Furthermore, using hierarchical structures to harvest various types of
ambient power such as thermal, wind, vibration and electromagnetic energy
would also be very promising, providing a potentially endless source of
energy. 1
Hierarchical nanostructures are an extension of research into 1D nano-
structures to improve the limitation of low dimensional nanostructures. In
comparison to the fast progress of 1D nanostructures, the fabrication and
applications of 3D hierarchical structures have a relatively short history.
Further development in this research field requires improvements in
synthetic methods and novel fabrication processes to provide better control
of the structural complexity, composition uniformity, surface chemistry and
interface electronics, and last but not least, the yield, of branched nano-
structures. 1 While the chemical vapour method results in high-quality
nanobranches, it relies on expensive equipment and toxic source materials.
Therefore, developing simple and environmentally-friendly benign growth
methods such as multiple solution growth for the fabrication of 3D
semiconductor hierarchical structures are of great interest 1

d n 3 r 4 n g | 2

.

in a more

practical manner.
Hierarchical nanostructure research has a bright future in solving the
current limitations in energy devices. The ultimate goal is to push the energy
devices towards practical applications, which requires the development of
devices with high eciency, low cost and a long lifespan. Thus, the
optimization of the branched nanostructures and the improvement of the
energy conversion eciency will remain the future research focus. 1 On
the other hand, using 3D hierarchical nanostructures in energy applications
also has drawbacks in that it brings challenges in quantifying the charge
transport and recombination loss in terms of solar devices. Understanding