Hierarchical Nanostructures for Energy Devices
Chapter 2: Fundamentals of Hierarchical Nanostructures
Published:29 Oct 2014
Jinhwan Lee, Seung Hwan Ko, 2014. "Fundamentals of Hierarchical Nanostructures", Hierarchical Nanostructures for Energy Devices, Seung H Ko, Costas P Grigoropoulos, Royal Society of Chemistry
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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 cm3 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 become nanometre sized, the percentage of surface atoms cannot be ignored anymore and the nanomaterial system starts to show characteristics of both surface atoms and bulk atoms. Depending on the size, the ratio between the surface atoms and bulk atoms and the resultant characteristics of the nanomaterials also show size-dependent properties. As the nanomaterial becomes smaller, the system shows more surface-atom-dominated characteristics with huge surface energy.