Environmental Engineering Reference
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(c)
(d)
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fiGure 18.4
x-ray diffraction patterns of (a) coo, (b) Zno, (c) Fe
3
o
4
, and (d) BaTio
3
. Reprinted with permission from Ref. [47].
© 2008, Royal society of chemistry.
18.4
sonocheMical synthesis of nanoparticles
sonochemical study is concerned with understanding the effect of sonic waves and wave properties on chemical systems. The
chemical effects of ultrasound do not come from a direct interaction with molecular species. Instead, sonochemistry arises from
acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid, which creates high pressure and tem-
perature followed by a high rate of cooling [49]. These acoustic cavitations are the driving force in a variety of reactions and in
materials synthesis. The effectiveness of cavitations depends on the contents of the collapsing bubble and hence on the nature
of the solvent. high-boiling hydrocarbons are highly effective in ultrasound methodology because of their low vapor pressure
and extremely high temperature within bubbles during cavitations. Nowadays, ultrasound-assisted synthesis is gaining attention
for the synthesis of nanocrystalline materials. The methodology works due to acoustic cavitations, that is, formation, growth,
and immediate collapse of bubbles in the solvent. Normally an ultrasonic horn is used for the synthesis of various metal and
metal oxide nanoparticles (Fig. 18.6).
18.4.1
Metal nanoparticle synthesis by sonochemical route
dhas and Gedanken reported palladium nanoclusters at room temperature by sonochemical reduction of palladium acetate,
Pd(ch
3
co
2
)
2
, using myristyltrimethylammonium bromide, [ch
3
(ch
2
)13N(ch
3
)
3
Br] (NR
4
x), in ThF or methanol. Apart from
having a stabilizing effect, NR
4
x acts as a reducing agent, probably due to the decomposition that occurs at the liquid-phase
region immediately surrounding the collapsing cavity and provides reducing radicals. The obtained Pd nanoclusters are catalyt-
ically active toward carbon-carbon coupling, in the absence of phosphine ligands, to a moderate extent of 30 conversions [50].
Fujimoto et al. reported novel metal Pd and Pt nanoparticles by sonochemical reduction of h
2
Ptcl
6
or K
2
Pdcl
4
solutions,
wherein atmospheric gas effect on the particle size distribution was investigated. The particle size of Pd and N
2
(Pd/N
2
) were
found to be 3.6 ± 0.7 nm under Ar (Pd/Ar) and 2.0 ± 0.3 nm in (Pd/N
2
). In the case of Pt, a smaller and sharper distribution of the
particle size was observed under a xe atmosphere. These can be explained in terms of a hot-spot temperature created by
acoustic cavitation [51].
Bimetallic nanoparticles composed of gold and palladium were synthesized by Mizukoshi et al. using the sonochemical
method. Under ultrasound irradiation in an aqueous medium, Au (III) and Pd (II) ions of sodium tetrachloroaurate (III) dihydrate
and sodium tetrachloropalladate (II) were reduced by sodium dodecyl sulfate (sds). In addition to having a stabilizing effect,
