Fundamentals of Hierarchical Nanostructures - Hierarchical Nanostructures for Energy Devices - page 7

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CHAPTER 2
Fundamentals of Hierarchical
Nanostructures

JINHWAN LEE* a AND SEUNG HWAN KO b

a Department of Mechanical Engineering, Korea Advanced Institute of
Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon
305-701, Korea; b Applied Nano and Thermal Science (ANTS) Lab
Department of Mechanical Engineering, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
*Email: [e-mail]

2.1 Introduction
Due to a large surface-to-volume ratio and quantum confinement effects,
nanoscale materials show distinctive optical, mechanical, chemical, thermal
and electronic properties compared with their bulk counterparts. A large
fraction of their atoms are located at the surface. For example, a material
that is 5 cm 3 possesses almost 0% (

.

10 5 %) surface atoms, but when the
cube is divided 24 times into 1 nm-sized cubes, the percentage of surface
atoms increases to 80% so that the same mass of nanomaterial will have
enough surface area to cover an entire football field. The surface atom
percentage explains why the nanomaterials' properties are size dependent.
The atoms at the surface have weak bonding (because atoms or molecules on
a surface possess fewer nearest neighbors) compared with bulk atoms,
which means that the atoms at the surface have a tendency to react easily to
external perturbation or energy. Because bulk materials have a very small
fraction of surface atoms (almost 0%), they show bulk-atom-dominated
material properties, which we know well. However, when the materials

B