Nanoparticles are engineered particles of between one and 100 nanometers in diameter, a size known as the nanoscale. To put this into context, a nanometer (nm) is one billionth of a meter, and the distance between individual atoms in a solid is between 0.1 nm and 0.4 nm. Nanoparticles therefore range in size from tens of atoms to around a million atoms. At their simplest, they may be somewhat irregular fragments of material, created by successive breaking up of larger particles. Nanoparticles may also be engineered molecules, with atoms arranged in precise geometric arrangements, such as carbon buckyballs. At their most complex, nanoparticles can form nanoscale machines and potentially even nano-bots.
In addition to engineered nanoparticles, they also occur naturally and are created by many conventional production processes, without intentional nanoengineering. Nanoparticles behave quite differently to microparticles and even fine particles. They will pass through almost all filters. At the nanoscale, particles are strongly effected by Brownian motion, meaning they generally will not settle as a sediment. They are also smaller than the wavelength of visible light, meaning that dispersions of nanoparticles do not reduce transparency.
Fullerenes are an important class of nanoparticles. They are named after Buckminster Fuller, the inventor of important three-dimensional truss structures, or ‘space frames’, including the octet truss and the geodesic dome. These are a form of pure carbon in which atoms are bonded to form a mesh made up of rings containing five to seven atoms. Graphene is a flat mesh of regular hexagonal rings, but the term fullerenes is generally used for molecules in which the mesh forms a sphere, ellipsoid or tube. Sphere’s and ellipsoids are sometimes referred to as buckyballs. The first buckyball to be discovered was buckminsterfullerene (C60) which contains sixty carbon atoms, arranged as twenty hexagons and twelve pentagons, C70 is another common buckyball. Cylindrical fullerenes are known as carbon nanotubes (CNTs), or sometimes buckytubes.
Fullerenes have many exciting properties such as extremely high mechanical strength, electrical conductivity and surface area. They may therefore have applications in lightweight structures, energy storage, solar cells and water purification. However, their use remains at an early stage of development. Although individual CNTs are 20 times as strong as any carbon fiber, to produce a composite material millions of nanotubes must be combined to form a fiber. CNTs slide easily past each other, resulting in fibers that are relatively weak. The same effect can be seen in the soft graphite used a pencils. Graphite is actually made up of layers of graphene which, although individually incredibly strong, slide past each other producing a soft bulk material. One day it may be possible to produce cross-linked bundles of CNTs that could greatly increase the strength of carbon fibers, but research is at an early stage. Current composites still rely on conventional carbon fibers for their strength and stiffness, but the addition of small quantities of graphene can improve inter-laminar shear strength, fracture toughness, and carbon-to-resin adhesion.