Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 25

assisted method has advantages of shorter reaction time, reduced energy consumption,
better production yield, production of small particles, narrow particle size distribution,
and high purity (Liao et al., 2001; Kuhnert, 2002; Oliver, 2004; Hoz et al., 2005).
The microwave assisted method generally relies on the ability of the reaction
mixture to efficiently absorb microwave energy and convert it into heat, taking
advantage of the “microwave dielectric heating” phenomena (Gabriel et al., 1998;
Oliver, 2004). Dipole rotation and ionic conduction are the two main mechanisms
associated with microwave heating. When molecules are irradiated with microwaves,
they will attempt to align themselves with the electric field by rotation. Also, ionic
conduction contributes to the microwave heating effect, when the solution includes ions.
When ions move in the solution under the applied field, heat is generated by friction
losses ( Lidström et al., 2001; Dallinger and Kappe, 2007). Microwave heating occurs
through the interaction of electromagnetic radiation with the dipole moment of
molecules. Thus, polar solvents with a high dipole moment, such as water and ionic
liquids, are among the best solvents to use in microwave synthesis.
In the microwave process, temperature, solvent, reductant, and reaction time play
an important role in nanoparticles synthesis ( Lidström et al. 2001; Zhu et al., 2004;
Baldassari et al., 2005; Doolittle and Dutta, 2006). For example, the reaction
temperature, ramp time (°C/min), and reaction time are all critical factors in controlling
the size and shape of gold nanoparticles (Liu et al., 2003a). Using microwave radiation,
monodisperse gold nanoparticles around 15-20 nm in diameter could be produced from
a higher synthesis temperature (~125 °C), faster ramp rates (> 20 °C/min), and reaction
times of at least 15 min. In addition, Liu et al. (2005) demonstrated the synthesis of
silver nanomaterial using the microwave method in the presence of gold seeds; they then
explained the influence of reaction time and reaction temperature on the production of
silver nanomaterials (Liu et al., 2005).
2.2.8 Precipitation Method
One of the oldest and most conventional techniques for nanoparticle synthesis is
the precipitation method (Yonezawa et al., 2000; Fried et al., 2001; Chen and Hsieh,
2002). In precipitation reactions, metal precursors such as chloride, oxychloride, or
nitrate are dissolved in a common solvent such as water, and a base solution such as
sodium hydroxide or ammonium hydroxide solution is added to form nanoparticles (Han
et al., 1999; Gui et al., 2003; Willard et al., 2004). The precipitation of metals from
aqueous or nonaqueous solutions is generated by the chemical reduction of a metal
cation; precipitation reactions involve the simultaneous occurrence of nucleation,
growth, coarsening, and/or agglomeration processes. The nucleation process is a key
step of the precipitation process in that a large number of small particles will be formed.
Also, Ostwald ripening and aggregation affect the size, morphology, and properties of