Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 55

of the fl ask. The bottom up approach would use similar chemistry but seek to halt
the growth of the particles of calcium sulfate before they became too large, thereby
producing a stable dispersion of calcium sulfate where the particle sizes are less
than 100 nm. The exact method used can vary enormously as will be seen here but
generally the dimensions of the particles are controlled by careful control of the
precipitation process.
The preparation of nanoparticles by the bottom up approach relies on the prin-
ciple of supersaturation (Zaizer and Lamer, 1948). In simple terms, at the start of
the reaction there is a homogeneous solution of the reagents (Figure 2.15). As the
reaction starts the product will begin to form; at very low concentrations it will
have some solubility in the medium. As the concentration of the product increases
the point of saturation will be reached. At this point the solution can no longer
support the ever-increasing amounts of product being formed. However, as the
product is being formed very rapidly and precipitation is limited by diffusion of the
molecules in the media, which is comparatively slow, the point of saturation may
be exceeded, producing a super saturated solution. At some point the supersatura-
tion of the solution is relieved by the formation of the precipitated particles. These
particles will be very small and represent the fi rst nuclei of the fi nal particles. It is
now more favourable for the formation of more product to occur at the surface of
the particle and therefore these fi rst nuclei essentially act as seed for the growth
of the particles. This is an idealised example of the processes involved and the
occurrence the formation of second batches of seed nuclei is not unusual.

Supersaturation occurs

Rapid relief of super saturation

Saturation
Concentration

Particle Growth

Rapid increase in product in solution

Time

Figure 2.15 The various stages in the formation of an idealised nanoparticle.