Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 19

towards the center. The co-surfactants act as an electronegative “spacer” that minimizes
repulsions between the positively charged surfactant heads.
In the microemulsion process, selection of the surfactants plays a very important
role in nanoparticle synthesis. To this end, a variety of surfactants can be used in the
microemulsion process, such as cetyltrimethylammonium bromide (CTAB) (Zhang et
al., 2006), poly (oxyethylene) 5 nonyl phenol ether (NP5), poly (oxyethylene) 9 nonyl
phenol ether (NP9) (Fang et al., 1997), Sodium bis(2-ethylhexyl)sulphosuccinate (AOT),
and pentaethylene glycol dodecyl ether (PEDGE) (Eriksson et al., 2004). There are
various nanoparticles synthesized by using different precursors and surfactants. Some
examples are listed in Table 2.2.

Table 2.2 Examples of nanoparticles synthesized by microemulsion method.
Nanoparticle
Type

Particle
Size (nm)

Metal
Precursor

Surfactant Reference

Ag < 50 Ag(NO 3 ) AOT Maillard et al. (2002)

Ni 4.2 NiCl 2 CTAB Chen and Wu (2000)

NiO < 100 NiCl 2 TritonX-100 Han et al. (2004)

Pd/Pt 5 PdCl 2 , H 2 PtCl 6 AOT Wu et al. (2001)

Pt/TiO 2 - H 2 PtCl 2 PEDGE Kizling et al. (1996)

Fe/SiO 2 4.8-13 Fe(NO 3 ) 3 NP5 Hayashi et al. (2002)

It should be noted that the surfactant should be chemically inert with respect to
all other components of the microemulsion; CTAB that is stable against mild oxidizers is
one example of a surfactant in a microemulsion system. The counter ions of ionic
surfactants should affect the precursor in the reaction system. In a reaction involving
Ag + , for instance, the dissociated Br - ions of CTAB would cause the immediate

precipitation of AgBr.
There are other aspects that also need to be taken into consideration for particle
synthesis in microemulsions. These factors include the mass percentage of the aqueous
phase in the microemulsion, the average concentration of the reacting species in the
aqueous domain, the water-surfactant ratio and structure and properties of the
solubilizing water, and the dynamic behavior of the microemulsion (Cushing et al.,
2004). These factors affect synthesis properties, such as product size, particle size
distribution, agglomerate size, and phase of the final product.

2.2.4 Vapor Phase Reaction