Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 26

the nanoparticles. In addition, the supersaturation conditions necessary to induce
precipitation are usually the result of a chemical reaction.
Qu et al. (1999) demonstrated the synthsis of Fe 3 O 4 nanoparticles with FeCl 3 ,
Na 2 SO 3 and ammonia. Na 2 SO 3 solution was put into the FeCl 3 solution under the stirring
condition. After mixing Fe 3+ with SO 3 2- , an intermeadiate solution containing complex
ions was formed. This solution was poured quickly into the diluted ammonia solution
under the vigorous stirring condition. After the reaction, black powders of Fe 3 O 4 were
formed.

In addition, there are some other examples of using the precipitation method to
synthesize nanoparticles (see Table 2.4 below). In these synthesizing processes, reaction
conditions such as the rate of reactant addition and stirring must be controlled to ensure
the appropriate product size, morphology, and particle size distribution. The size and
morphology of the precipitated nanoparticles can be manipulated through the control of
synthesis parameters such as pH, metal cation concentration, and the type of
precipitating agent. However, the concentration of the metal species in the initial
reaction solution has been found to have the largest effect on the overall nanoparticle
size. For example, low concentrations limited particle growth, although the resultant
particles are generally more uniform in size; at a high concentration, particle growth is
increased with a subsequent loss of size uniformity. A precipitating agent also affects the
overall particle size, as well as the phase purity of the particle formed (Willard et al.,
2004).

Table 2.4 Nanoparticles synthesized by precipitation methods.
NP Type Size (nm) Reference NP Type Size (nm) Reference

Chen et al.
(1997)

Yonezawa et
al. (2000)

Fe (Fe 3 O 4 ) 20-200 Ag < 5

Cabral-Prieto
et al. (1998)

Tang et al.
(2008)

-FeOOH 7-48 ZnO 10-40

Qu et al.
(1999)

Song et al.
(2005)

Fe 3 O 4 < 10 Al 2 O 3 7-11

2.2.9 Hydrothermal/Solvothermal Method

A hydrothermal/solvothermal process can be used to heat a solution to
temperatures well above its boiling point by increasing the autogenous pressures
resulting from heating. Performing a chemical reaction under such conditions is referred
to as solvothermal processing or hydrothermal processing, when water is used as the
solvent.