with microwave heating [197]. Microwave heating was found to have selectively
accelerated SnO crystallization, but not the concurrent Sn
þ2 to Sn
þ4 oxidation,
which otherwise prevails in the conventional thermal heating process. Control
studies give a strong indication of a non-temperature effect of the microwave irradiation in the present process.
Microwave-hydrothermal synthesis of titanium dioxide under various reaction
conditions was reported by Komarneni [198]. Crystallization of rutile from TiOCl 2
solutions was found to be extremely rapid. Titanium dioxide, particle size, morphology and polymorph can be controlled by changing various parameters, such
as: concentration, pH, pressure (or temperature), time, and anionic species. The
main advantages of microwave-hydrothermal processing of TiO 2 are: (1) rapid
heating to required temperature and (2) extremely rapid kinetics of crystallization.
Rutile was the only crystalline phase when various concentrations (3 M, and 2 M)
of TiOCl 2 solutions were treated at a variety of pressures (190, 100, 50 and 25 psi)
for 2 h. The yield of rutile was 95% at all pressures, which showed that the crystallization of this solution was practically complete. When the TiOCl 2 was further
reduced, a mixture of anatase and rutile phases was obtained.
Another microwave study yielded only the anatase phase [199]. In this work TiO 2
was synthesized from the alkoxide by the polyol method using various polyols (1,4butanediol, 1,5-pentanediol, or 1,6-hexanediol) under MW radiation. The authors
demonstrated that the crystallite size, which was always less than 10 nm, could be
controlled by the quantity of added water and by the nature of the polyol (see Table
1 in [199]).
A group of binary oxide nanophase (titanates and zirconates) materials were prepared using a microwave-assisted soft-chemical route [200]. BaTiO 3 , Ba 6 Ti 17 O 40 ,
BaZrO 3 and PbTiO 3 were prepared from BaCl 2 hydrate, Pb(Ac) 2 , Ti(OPri) 4 and
ZrOCl 2 . All reactions were performed in ethylene glycol, which acted both as a
solvent and as a growth regulating agent, under atmospheric pressure in a microwave reactor.
The Ni/NiO composite was prepared by using the fast method of microwaveassisted oxidation [201]. Amorphous Ni nanoparticles were used as a precursor,
and the oxidizing agent was oxygen. By using vapors of H 2 O 2 , almost complete
oxidation of nickel was achieved.
CuO nanoparticles with an average size of ca. 4 nm have been successfully prepared by microwave irradiation, using copper(II) acetate and sodium hydroxide as
the starting materials and ethanol as the solvent [202]. The as-prepared CuO
nanoparticles have regular shape, narrow size distribution and high purity. The
band gap is estimated to be 2.43 eV according to the results of the optical measurements of the CuO nanoparticles.
Before summarizing this section and emphasizing the advantages of using microwave radiation for the synthesis of nanoparticles, we would like to remind the
reader that perhaps the largest body of work in this field was conducted in the
preparation of nanosized chalcogenides. However, following our own guidelines
mentioned above, we have left it to another review [3]. Almost all the authors
quoted in this review have mentioned the short reaction time in the microwave
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
162
accelerated SnO crystallization, but not the concurrent Sn
þ2 to Sn
þ4 oxidation,
which otherwise prevails in the conventional thermal heating process. Control
studies give a strong indication of a non-temperature effect of the microwave irradiation in the present process.
Microwave-hydrothermal synthesis of titanium dioxide under various reaction
conditions was reported by Komarneni [198]. Crystallization of rutile from TiOCl 2
solutions was found to be extremely rapid. Titanium dioxide, particle size, morphology and polymorph can be controlled by changing various parameters, such
as: concentration, pH, pressure (or temperature), time, and anionic species. The
main advantages of microwave-hydrothermal processing of TiO 2 are: (1) rapid
heating to required temperature and (2) extremely rapid kinetics of crystallization.
Rutile was the only crystalline phase when various concentrations (3 M, and 2 M)
of TiOCl 2 solutions were treated at a variety of pressures (190, 100, 50 and 25 psi)
for 2 h. The yield of rutile was 95% at all pressures, which showed that the crystallization of this solution was practically complete. When the TiOCl 2 was further
reduced, a mixture of anatase and rutile phases was obtained.
Another microwave study yielded only the anatase phase [199]. In this work TiO 2
was synthesized from the alkoxide by the polyol method using various polyols (1,4butanediol, 1,5-pentanediol, or 1,6-hexanediol) under MW radiation. The authors
demonstrated that the crystallite size, which was always less than 10 nm, could be
controlled by the quantity of added water and by the nature of the polyol (see Table
1 in [199]).
A group of binary oxide nanophase (titanates and zirconates) materials were prepared using a microwave-assisted soft-chemical route [200]. BaTiO 3 , Ba 6 Ti 17 O 40 ,
BaZrO 3 and PbTiO 3 were prepared from BaCl 2 hydrate, Pb(Ac) 2 , Ti(OPri) 4 and
ZrOCl 2 . All reactions were performed in ethylene glycol, which acted both as a
solvent and as a growth regulating agent, under atmospheric pressure in a microwave reactor.
The Ni/NiO composite was prepared by using the fast method of microwaveassisted oxidation [201]. Amorphous Ni nanoparticles were used as a precursor,
and the oxidizing agent was oxygen. By using vapors of H 2 O 2 , almost complete
oxidation of nickel was achieved.
CuO nanoparticles with an average size of ca. 4 nm have been successfully prepared by microwave irradiation, using copper(II) acetate and sodium hydroxide as
the starting materials and ethanol as the solvent [202]. The as-prepared CuO
nanoparticles have regular shape, narrow size distribution and high purity. The
band gap is estimated to be 2.43 eV according to the results of the optical measurements of the CuO nanoparticles.
Before summarizing this section and emphasizing the advantages of using microwave radiation for the synthesis of nanoparticles, we would like to remind the
reader that perhaps the largest body of work in this field was conducted in the
preparation of nanosized chalcogenides. However, following our own guidelines
mentioned above, we have left it to another review [3]. Almost all the authors
quoted in this review have mentioned the short reaction time in the microwave
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
162