Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 38

importance of surface chemistry when describing and considering nanoparticulate
materials.
However, the chemistry of the surface of a nanoparticle can also vary signifi cantly
from that of the core of the nanoparticle. In some cases this variation in properties
allows some nanoparticles to form stable dispersions having a signifi cant effect on
the types of interactions the nanoparticle will have with other materials. As such it
is erroneous to consider the surface chemistry of any nanoparticle to approximate
well to that of the core material. An excellent example of this can be found by
examining silica nanoparticles. The particle itself has the approximate molecular
formula (SiO 2 ) n where n is large ( 20). This implies an infi nite lattice, which is not

in fact possible at the surface due to the bonding constraints surrounding the geom-
etry of the silicon atoms. These need to form a tetrahedral structure and therefore
need to form bonds in three dimensions. This results in a number of terminal Si

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O
or Si-OH groups on the surface of the particle. The Si-OH groups can ionise to
from SiO − and H + pairs if the particles are suspended in a suitable solvent, and this
in turn gives rise to the stability of the particle dispersion. Clearly the formula
would be better represented as a core of SiO 2 with a shell of SiO(OH) 2 .
This variation between the chemistry of the core and the surface is particularly
relevant when considering inorganic (including carbon particles such as Carbon
Black (CB), C 60 or Carbon nanotubes (CNT)) or metallic nanoparticles. Although
the surface chemistry of polymer nanoparticles will also be variable between the
core and the surface the reasons for this are different. This means that the surface
of nanoparticles prepared from what might initially appear to be an inert material
such as silver can rapidly oxidise to give a very thin layer of silver oxide at the
particles surface (Schnippering et al. , 2007 ).
The atoms at the surface of any inorganic material are not inert because they
may lower their energy by binding to other molecules. This energy associated with
the surface is called the surface energy. In fact, even bulk inorganic materials have
this property and it is therefore routine, in experiments where interactions with the
surface are to be studied, to clean the surface with ion bombardment or heating
whilst the material is held in a high vacuum. This results in removing any adsorbed
molecules for the surface. In the case of nanomaterials their surface is so large they
have a massive surface energy. If the surface is not capped with a strongly bound
molecule that prevents interactions between particles, the surface energy will be
minimised by interactions between the particles themselves. In fact, were it not for
the high surface energy of nanoparticles many of them would remain suspended
indefi nitely in aqueous media at room temperature and pressure.
Three forces are acting on any particle suspended in a medium (Figure 2.7).
Gravity pulls the particles down causing sedimentation; friction (with a component
of buoyancy) works against gravity; molecules of the medium collide with the par-
ticles resulting in Brownian motion. The laws and dynamics of these processes have
been well understood for over a century and have proven to be well behaved theo-
ries which have been applied in many instruments since. The competing effects of
gravity and friction are described well by the Stokes law equation, which relates
the rate of sedimentation to the square of the particle radius, meaning a larger
particle will sediment much more rapidly than a small one. However, the particles

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