Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 18

of the linear polymeric chain that occurs in acidic conditions. An alcoholic solvent was
added to control the reaction rate and particle size. In Stöber's experiment, the reaction
rate was found to be faster with methanol than n -butanol. In addition, particle sizes
obtained under comparable conditions were smallest in methanol and largest in n -
butanol; however, the addition of more alcohol led to a wide variance in size
distribution. Similarly, different ligand sizes in the precursors affected the reaction rate
and particle size, as smaller ligands brought about a faster reaction rate and smaller
particle size. Also, temperature was found to be an important factor in terms of reaction
rate and generation of small nuclei. A high temperature favors a fast hydrolysis reaction
rate and results in high supersaturation, which in turn leads to the formation of larger
number of small nuclei.
In addition, through this method various kinds of metal oxide nanoparticles such
as -Fe 2 O 3 , ZrO 2 and TiO 2 can be synthesized (Hu et al., 1998; Wei et al., 1999; Wang
et al., 2008). This method uses inexpensive starting chemicals, that is, inorganic metal
salts (i.e., FeCl 3 , ZrOCl 2 · x H 2 O, and Ti(SO 4 ) 2 ). For example, Wang et al. (2008) used
FeCl 3 and HCl solution to synthesize -Fe 2 O 3 nanoparticles. This solution was heated to
100 °C by 8 °C/min in a water bath. In this process, aging time affected the size and
morphology of -Fe 2 O 3 . With an increase in the aging time, irregularly shaped particles
transformed into bar-shaped nanoparticles. A long aging time would permit the
occurrence of Oswalt ripening to further narrow the size distribution.
2.2.3 Microemulsion Method
The microemulsion method has been used to synthesize nanoparticles by
precipitation (Schwuger et al., 1995). Microemulsion acts as an interesting alternative
reaction medium for the production of nanoparticles. For example, by adding a reducing
agent into the microemulsion system or by mixing with nanodroplets, these
microemulsions can be filled with different reactants thereby enabling the synthesis of
metallic or metal oxide nanoparticles. Boutonnet et al. (1982) first reported the synthesis
of monodisperse metal nanoparticles by microemulsion method in the early 1980s
(Boutonnet et al., 1982).
Controlled nucleation and growth of metal clusters occurs in the interior of
surfactant aggregates. In this process, an ionic salt (i.e., Fe(BF 4 ) 2 or anhydrous FeC1 3 ) is
dissolved in the hydrophilic interior of the micelles, while the surrounding continuous
hydrophobic oil limits nucleation and growth to the micelle interior volume (Wilcoxon
and Provencio, 1999). The addition of a co-surfactant (i.e., an alcohol) in the reaction
system plays a role in controlling the interfacial tension. From this, microemulsions are
spontaneously synthesized without the need for significant mechanical agitation.
Through the ion-dipole interactions with the polar co-surfactant, the surfactant forms
spherical aggregates in which the polar (ionic) ends of the surfactant molecules orient