Stabilization
All nanostructured materials possess a huge surface
energy due to the large surface area; thus, they are
thermodynamically unstable or metastable. Overcoming
the large surface energy to prevent the nanostructures
from growing is one of the great challenges in the synthesis
of nanomaterials [32]. Nanoparticles, exclusively
colloidal particles, in a short distance, are attracted to
each other by the van der Waals force. If there is no
counteracting force, the particles will aggregate and the
colloidal system will be destabilized. The stability is
achieved when the repulsion forces balance the attraction
forces by electrostatic stabilization and/or steric
stabilization.
There are several types of colloidal metal stabilizers
which depend on the type of metal, method of preparation,
and the application of the resultant metallic nanoparticles.
For example, polymers having functional
groups such as -NH2, -COOH, and -OH have high affinity
for metal atoms; however, the use of stabilizers is not
desirable for some applications such as catalysis. For
example, activities of supported metal nanoparticle
catalysts by coordination capture method are higher
than those of polyvinyl-pyrrolidone (PVP)-stabilized
metal colloidal catalysts [33,34]. Due to functional
groups namely C = O and N, and long polymer chains,
PVP can associate with the metal nanoparticles [35,36].
The functional groups containing lone pairs of electrons
help in the stabilization of metal nanoparticles at their
surfaces by covalent interaction, whereas the polymer
chain restricts aggregation of metal nanoparticles by
steric hindrance. For example, the long chains of PVP
stretch out around nickel atom on the surface of the
crystal, causing a steric hindrance effect and thus
prevent particle growth effectively [37]. Apart from
this, PVP is a biocompatible polymer. Hence, nanoparticles
synthesized in PVP can be used in biological
applications.