Prof Richard Haverkamp, of Massey University, explains why the small size of some nanoparticles helps them become good catalysts.
DR RICHARD HAVERKAMP
The structure of the nanoparticle or the size and how sharp the little corners are, these things all effect the chemical properties. If you've got a large chunk of something, say a big cube, or even a big sphere of a material, the surface is essentially flat on a small scale, so as you zoom in and zoom in, even if its on a curved sphere, when you zoom right in it looks essentially flat. And so what that means is that the atoms on that surface, they’re all bonded with neighbours around them, and atoms underneath, so they've got a certain connection with other atoms of the same type. And as you increase the curvature you can image an atom starts to sit up on its own a bit more, so it no longer has neighbours on either side because they've curved away. An atom sitting on the surface is a bit more exposed, its out there on its own. And chemical reactions are all about atoms forming new bonds, and if you've got an atom that‘s already bonded with all its mates it can't do very much sometimes. But if the atom is out on its own, its got some bonds available to react. So this is why for some substances such as gold, when its a large piece its not reactive because its all joined up with its other gold atoms, but when its on a small one the radius of curvature gets very small, so the atoms stuck out on their own and so then they want to react and bond with things. So for a lot of catalysts that situation is quite important where the particles have to be small because it exposes all these other bonds that wouldn't be available otherwise. So that effects the reactivity of the material enormously.
Dr Aaron Marshall
Stem images of nanoparticles with kind permission of Dr Richard Haverkamp and Springer Science and Business Media (published in Journal of Nanoparticle Research (2007), 9:697-700, Pick your carats: nanoparticles of gold-silver-copper alloy produced in vivo by R.G Haverkamp, A.T Marshall and D. van Agterveld.)