solutions, forming solid thorium compounds [141]. The first-order rate constants for Th(IV) beta-diketonate degradation were found to be (9.30–8) Â 10
À3 for
Th(HFAA) 4 and (3.80–4) Â 10
À3 min
À1 for Th(DBM) 4 , (T ¼ 92
C, I ¼ 3 W cm
À2 ).
The rate of the sonochemical reaction increased with increasing b-diketonate volatility and decreased with increasing hydrocarbon solvent vapor pressure. Solid
sonication products consisted of a mixture of thorium carbide ThC 2 and Th(IV) bdiketonate partial degradation products. The average ThC 2 particle size was estimated to be about 2 nm. ThC 2 formation was attributed to the high-temperature
reaction occurring within the cavitating bubble. The thorium b-diketonate partial
degradation products were formed in the liquid reaction zones surrounding the
cavitating bubbles.
It is more difficult to prepare III–V semiconductors than the II–VI. Two sonochemical investigations reported on the preparation of these materials. The first
paper details a safe method for the preparation of transition metal arsenides, FeAs,
NiAs, and CoAs [142]. At room temperature, well-crystallized and monodispersed
arsenide particles were successfully obtained under high-intensity ultrasonic irradiation for 4 h from the reaction of transition metal chlorides (FeCl 3 , NiCl 2 , and
CoCl 2 ), arsenic (which is the least toxic arsenic feedstock) and zinc in ethanol.
Different characterization techniques show that the product powders consist of
nanosize particles. The ultrasonic irradiation and the solvent are both important in
the formation of the product.
Another III–V semiconductor was prepared by Li and coworkers [143]. A room
temperature sonochemical method for the preparation of GaSb nanoparticles using less hazardous Ga and antimony chloride (SbCl 3 ) as the precursors has been
described. TEM and SAED results show that the as-prepared solid consists of
nanosized GaSb crystals with sizes in the 20–30 nm range. The photoacoustic
spectrum result reveals that the GaSb nanoparticles have a direct band gap of
about 1.21 eV. On the basis of the control experiments and the extreme conditions
produced by ultrasound, an ultrasound-assisted in situ reduction/combination
mechanism has been proposed to explain the reaction.
We will conclude this survey of the synthesis of nanomaterials by sonochemical
methods by mentioning that the most important material of the last decade, carbon nanotubes, were also synthesized by ultrasound radiation [144]. The carbon
nanotube is produced by applying ultrasound to liquid chlorobenzene with ZnCl 2
particles and to o-dichlorobenzene with ZnCl 2 and Zn particles. It is considered
that the polymer and the disordered carbon, which are formed by cavitational collapse in homogeneous liquid, are annealed by the inter-particle collision induced
by the turbulent flow and shockwaves.
6.2
Sonoelectrochemistry
The effect of ultrasound in electrochemistry, i.e. that the application of ultrasonic
energy can increase the rate of electrolytic water cleavage, was discovered as early
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
148
À3 for
Th(HFAA) 4 and (3.80–4) Â 10
À3 min
À1 for Th(DBM) 4 , (T ¼ 92
C, I ¼ 3 W cm
À2 ).
The rate of the sonochemical reaction increased with increasing b-diketonate volatility and decreased with increasing hydrocarbon solvent vapor pressure. Solid
sonication products consisted of a mixture of thorium carbide ThC 2 and Th(IV) bdiketonate partial degradation products. The average ThC 2 particle size was estimated to be about 2 nm. ThC 2 formation was attributed to the high-temperature
reaction occurring within the cavitating bubble. The thorium b-diketonate partial
degradation products were formed in the liquid reaction zones surrounding the
cavitating bubbles.
It is more difficult to prepare III–V semiconductors than the II–VI. Two sonochemical investigations reported on the preparation of these materials. The first
paper details a safe method for the preparation of transition metal arsenides, FeAs,
NiAs, and CoAs [142]. At room temperature, well-crystallized and monodispersed
arsenide particles were successfully obtained under high-intensity ultrasonic irradiation for 4 h from the reaction of transition metal chlorides (FeCl 3 , NiCl 2 , and
CoCl 2 ), arsenic (which is the least toxic arsenic feedstock) and zinc in ethanol.
Different characterization techniques show that the product powders consist of
nanosize particles. The ultrasonic irradiation and the solvent are both important in
the formation of the product.
Another III–V semiconductor was prepared by Li and coworkers [143]. A room
temperature sonochemical method for the preparation of GaSb nanoparticles using less hazardous Ga and antimony chloride (SbCl 3 ) as the precursors has been
described. TEM and SAED results show that the as-prepared solid consists of
nanosized GaSb crystals with sizes in the 20–30 nm range. The photoacoustic
spectrum result reveals that the GaSb nanoparticles have a direct band gap of
about 1.21 eV. On the basis of the control experiments and the extreme conditions
produced by ultrasound, an ultrasound-assisted in situ reduction/combination
mechanism has been proposed to explain the reaction.
We will conclude this survey of the synthesis of nanomaterials by sonochemical
methods by mentioning that the most important material of the last decade, carbon nanotubes, were also synthesized by ultrasound radiation [144]. The carbon
nanotube is produced by applying ultrasound to liquid chlorobenzene with ZnCl 2
particles and to o-dichlorobenzene with ZnCl 2 and Zn particles. It is considered
that the polymer and the disordered carbon, which are formed by cavitational collapse in homogeneous liquid, are annealed by the inter-particle collision induced
by the turbulent flow and shockwaves.
6.2
Sonoelectrochemistry
The effect of ultrasound in electrochemistry, i.e. that the application of ultrasonic
energy can increase the rate of electrolytic water cleavage, was discovered as early
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
148