Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 23

physical conditions (Adewuyi, 2001). The chemical effects from ultrasonic radiation
arise from acoustic cavitations in the solution, which leads to extreme conditions of high
temperature and high pressure. This condition promotes the formation, growth, and
collapse of bubbles in a liquid (Zhu et al., 2002).
There are several factors that affect the cavitation efficiency and
physicochemical properties of synthesized materials. These factors include the type of
solvent, solute, gas in the bubble, ultrasound power and frequency, and temperature of
the bulk solution, all of which can induce a dramatic effect on the yield and properties of
synthesized materials (Adewuyi, 2001; Okitsu et al., 2005). Cavities are more readily
formed when using solvents with a high vapor pressure (VP), low viscosity, and low
surface tension; however, the intensity of cavitation benefits from using solvents having
opposite characteristics (i.e., low VP, high viscosity, and surface tension, and density).

In the sonochemical method, a very high cooling rate (> 10 11 K/s) is obtained,
though this high cooling rate hinders the organization and crystallization of the
synthesized materials. For this reason, in all cases that precursors are volatile,
amorphous nanoparticles are obtained (Li et al., 2001). For example, Suslick et al.
(1991) reported that the sonication of Fe(CO) 3 in decane under argon yields 5-20
nanometer-sized amorphous iron particles (Suslick et al., 1991).

There are a few topics related to nanotechnology in which the sonochemical
method is superior to all other techniques. These areas are (Li et al., 2001; Gedanken,
2004):
1) Preparation of amorphous products (Sugimoto, 1994; Salkar et al., 1999; Liang

et al., 2003). Various metal and metal oxide nanoparticles can be synthesized
by using this method. Salkar et al. (1999) synthesized amorphous silver
nanoparticles of 20 nm size by sonochemical decomposition of an aqueous silver
nitrate under an atmosphere of argon-hydrogen, at a temperature of 10 °C. After
sonication, centrifusing and the washing process were conducted in the inert
atmosphere glove box (< 5 ppm O 2 ) to prevent the formation of any traces of
silver oxides. In addition, amorphous ZrO 2 nanoparticles were synthesized via
the sonochemical method. Ultrasonic irradiation was accomplished directly in
the mixture of Zr(NO 3 ) 4 ·5H 2 O and NH 3 ·H 2 O solution (Liang et al., 2003). Liang
et al. (2003) also synthesized crystalline ZrO 2 nanoparticles using amorphous
ZrO 2 when the temperature was increased between 300 and 1200 °C.

2) Insertion of nanomaterials into mesoporous materials (Chen et al., 2001; Landau

et al., 2001; Perkas et al., 2001). Chen et al. (2001) reported the synthesis of
palladium nanoparticles, incorporated into mesoporous silica. The preformed
mesoporous silica was put into the PdCl 2 solution (containing 0.2 mol/L
isopropanol) for several weeks to make the same concentrations of Pd 2+ and
isopropanol in the pores of mesoporous silica. After this process, the mixed