Although a broad definition, we categorise
nanomaterials as those which have structured
components with at least one dimension less than
100nm. Materials that have one dimension in the
nanoscale (and are extended in the other two dimensions)
are layers, such as a thin films or surface coatings. Some
of the features on computer chips come in this category.
Materials that are nanoscale in two dimensions (and
extended in one dimension) include nanowires and
nanotubes. Materials that are nanoscale in three
dimensions are particles, for example precipitates, colloids
and quantum dots (tiny particles of semiconductor
materials). Nanocrystalline materials, made up of
nanometre-sized grains, also fall into this category. Some
of these materials have been available for some time;
others are genuinely new. The aim of this chapter is to
give an overview of the properties, and the significant
foreseeable applications of some key nanomaterials.
Two principal factors cause the properties of
nanomaterials to differ significantly from other
materials: increased relative surface area, and quantum
effects. These factors can change or enhance properties
such as reactivity, strength and electrical characteristics.
As a particle decreases in size, a greater proportion of
atoms are found at the surface compared to those
inside. For example, a particle of size 30 nm has 5% of
its atoms on its surface, at 10 nm 20% of its atoms, and
at 3 nm 50% of its atoms. Thus nanoparticles have a
much greater surface area per unit mass compared with
larger particles. As growth and catalytic chemical
reactions occur at surfaces, this means that a given mass
of material in nanoparticulate form will be much more
reactive than the same mass of material made up of
larger particles.
To understand the effect of particle size on surface area, consider an American Silver Eagle coin. This silver dollar contains 31 grams of coin silver and has a total surface area of approximately 3000 square millimeters. If the same amount of coin silver were divided into tiny particles – say 10 nanometer in diameter – the total surface area of those particles would be 7000 square meters (which is equal to the size of a soccer field – or larger than the floor space of the White House, which is 5100 square meters). In other words: when the amount of coin silver contained in a silver dollar is rendered into 10 nm particles, the surface area of those particles is over 2 million times greater than the surface area of the silver dollar! (We borrowed this excellent example from the Particular website)
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