unchanged [134]. Narrow size distribution is obtained with an average particle size
of 2.8 nm. A communication [192] reporting only on the MWH part of these findings preceded this paper [134].
Nanosized zirconium oxide (ZrO 2 ) powders were prepared by adding NaOH to a
zirconyl chloride aqueous solution under microwave-hydrothermal conditions
[193]. The results showed that the tetragonal polymorph increased with increasing
NaOH concentration in the starting solution and reached a maximum value by
using 1 M ZrOCl 2 . The authors emphasize the simplicity of the method and that it
can lead to powders with desirable characteristics such as very fine size, narrow
size distribution, and good chemical homogeneity. The microwave-hydrothermal
treatments were conducted at 200 psi for 2 h. The time, pressure, and power were
computer controlled. TEM analysis confirmed the effect of concentration on particle size. In particular, the calculated average particle size ranged from 16 (G3) to 9
(G3) nm, with the ZrOCl 2 concentration varying from 0.5 M to 1 M. TEM observation of the particles revealed spherical-shaped particles with no agglomeration.
Forced hydrolysis preparation of zirconia sols and powders by microwave heating of zirconium tetrachloride solutions at a temperature of 180
C led in a few
minutes to monodispersed nanoscale zirconia particles [194]. Synthesis was performed using the above-mentioned RAMO system [189]. This process combines
the advantages of forced hydrolysis (homogeneous precipitation) and microwave
heating (very fast heating rates). Sols are colloidally stable, which means that after
6 months no sedimentation is observed and the size distribution given by photon
correlation spectroscopy (PCS) measurements does not change. For all synthesis
conditions (with or without HCl, zirconium salt concentration, and synthesis
time), zirconia polycrystalline particles were produced. According to the different
analyses, these zirconia polycrystalline particles were aggregates of small primary
clusters.
Zirconia, and polymer-stabilized tetragonal ZrO 2 nanopowders with an average
size of ca. 2.0 nm have been prepared by microwave heating in an aqueous solution containing Zr(NO3) 4 Á5H 2 O, PVA, and NaOH [195]. The photoluminescence
of the synthesized ZrO 2 fine particles has been investigated at two different excitations with an excitation wavelength of 254 nm; three fluorescence emissions at
402 nm, 420 nm, and 459 nm, respectively, could be observed. The PL spectrum
obtained at 412 nm excitation exhibited a maximum at 608 nm, with a weak satellite peak at 530 nm. The emissions that appear at short wavelength excitation are
ascribed to the near band-edge transitions.
Nanocrystalline SnO 2 powders of about 3 nm in size have been prepared by a
microwave irradiation heating technique from an aqueous solution in the presence of SnCl 4 and urea [196]. A bandgap estimated to be 4.5 eV is obtained from
the optical measurement of the nanoparticles. HRTEM pictures show that the asprepared SnO powders are crystalline with ca. 3 nm particle size, and the particles
are held together by an irregular network.
High-purity powders of SnO nanocrystallites with crystallite sizes less than
30 nm and surface areas up to 40 m
2 g
À1 have been synthesized by a solution
process in which an amorphous oxy–hydroxy precipitate of Sn
þ2 is crystallized
6.3 Microwave Heating 161
of 2.8 nm. A communication [192] reporting only on the MWH part of these findings preceded this paper [134].
Nanosized zirconium oxide (ZrO 2 ) powders were prepared by adding NaOH to a
zirconyl chloride aqueous solution under microwave-hydrothermal conditions
[193]. The results showed that the tetragonal polymorph increased with increasing
NaOH concentration in the starting solution and reached a maximum value by
using 1 M ZrOCl 2 . The authors emphasize the simplicity of the method and that it
can lead to powders with desirable characteristics such as very fine size, narrow
size distribution, and good chemical homogeneity. The microwave-hydrothermal
treatments were conducted at 200 psi for 2 h. The time, pressure, and power were
computer controlled. TEM analysis confirmed the effect of concentration on particle size. In particular, the calculated average particle size ranged from 16 (G3) to 9
(G3) nm, with the ZrOCl 2 concentration varying from 0.5 M to 1 M. TEM observation of the particles revealed spherical-shaped particles with no agglomeration.
Forced hydrolysis preparation of zirconia sols and powders by microwave heating of zirconium tetrachloride solutions at a temperature of 180
C led in a few
minutes to monodispersed nanoscale zirconia particles [194]. Synthesis was performed using the above-mentioned RAMO system [189]. This process combines
the advantages of forced hydrolysis (homogeneous precipitation) and microwave
heating (very fast heating rates). Sols are colloidally stable, which means that after
6 months no sedimentation is observed and the size distribution given by photon
correlation spectroscopy (PCS) measurements does not change. For all synthesis
conditions (with or without HCl, zirconium salt concentration, and synthesis
time), zirconia polycrystalline particles were produced. According to the different
analyses, these zirconia polycrystalline particles were aggregates of small primary
clusters.
Zirconia, and polymer-stabilized tetragonal ZrO 2 nanopowders with an average
size of ca. 2.0 nm have been prepared by microwave heating in an aqueous solution containing Zr(NO3) 4 Á5H 2 O, PVA, and NaOH [195]. The photoluminescence
of the synthesized ZrO 2 fine particles has been investigated at two different excitations with an excitation wavelength of 254 nm; three fluorescence emissions at
402 nm, 420 nm, and 459 nm, respectively, could be observed. The PL spectrum
obtained at 412 nm excitation exhibited a maximum at 608 nm, with a weak satellite peak at 530 nm. The emissions that appear at short wavelength excitation are
ascribed to the near band-edge transitions.
Nanocrystalline SnO 2 powders of about 3 nm in size have been prepared by a
microwave irradiation heating technique from an aqueous solution in the presence of SnCl 4 and urea [196]. A bandgap estimated to be 4.5 eV is obtained from
the optical measurement of the nanoparticles. HRTEM pictures show that the asprepared SnO powders are crystalline with ca. 3 nm particle size, and the particles
are held together by an irregular network.
High-purity powders of SnO nanocrystallites with crystallite sizes less than
30 nm and surface areas up to 40 m
2 g
À1 have been synthesized by a solution
process in which an amorphous oxy–hydroxy precipitate of Sn
þ2 is crystallized
6.3 Microwave Heating 161