spherical-shaped gold nanoparticles, with a mean diameter of 5.2 nm, located in
the pores, most of which are less than 6 nm in diameter. The ultrasonic irradiation
time dependence of optical absorption for the soaked porous solid sample, as
measured by the variation in absorbance at 310 and 544 nm, indicated the reduction of Au(III) ions, and the nucleation and aggregation of gold nanoparticles
within the pores of MSP silica. Additionally, the reaction rates estimated phenomenologically by the absorbance decay at 310 nm for both the porous sample and the
corresponding soaking solution, showed the enhancement of the sonochemical
reduction rate of Au(III) ions within the pores of mesoporous silica. It is assumed
that the extensive liquid–solid interfacial zones in the pores, due to the high specific surface area and great porosity of the mesoporous solid, are the major regions
where the efficient sonochemical reduction induced by the cavitation takes place.
In the latest of their publications related to the insertion of gold into MSP silica
[110], the same authors discuss the structural changes that the silica skeleton undergoes upon sonication. The structure of mesoporous silica after sonochemical
preparation of gold nanoparticles within its pores was studied by a nitrogen adsorption technique. It was shown that the structural parameters, such as specific
surface area (SSA), porosity (P), and the mean pore diameter (l(p)) were increased
significantly after ultrasonic irradiation. It is suggested that the collision of Au
nanoparticles with the pore walls and localized erosion induced by the asymmetric
implosive collapse of cavities on the extensive liquid–solid interface are responsible
for the structural change in the mesoporous solid.
One paper by the same group reports on the sonochemical insertion of palladium nanoparticles loaded within mesoporous silica [111]. The formation of Pd
nanoparticles (5–6 nm in diameter) was restricted by the coalescence of the sonochemically reduced Pd atoms inside the confined volumes of the porous solid.
One of the most exciting sonochemical preparation procedures of MSP titania
did not use any surfactant [112]. Yu and his coworkers used monodispersed TiO 2
sol particles, which were formed initially by ultrasound-assisted hydrolysis of acetic
acid-modified titanium isopropoxide. Then, the mesoporous spherical or globular
particles, which have a narrow pore size distribution, were produced by controlled
condensation and agglomeration of these sol nanoparticles under high intensity
ultrasound irradiation. The mesoporous TiO 2 has a wormhole-like structure and a
lack of long-range order. Nitrogen adsorption results indicate that the mesoporous
TiO 2 retains mesoporosity with a narrow pore size distribution and high surface
area to at least 673 K. The thermal stability of mesoporous TiO 2 is attributed to
its thick inorganic walls, consisting of TiO 2 nanoparticles. A TGA study shows
that this synthetic method is environmentally friendly. The photocatalytic activity
of mesoporous TiO 2 for the oxidation of acetone in air was measured. As-prepared
mesoporous TiO 2 has a negligible activity due to its amorphous structure. Calcined
mesoporous TiO 2 shows a better activity than thecommercial photocatalyst P25.
The reasons for the high activity of mesoporous TiO 2 are discussed.
In a continuing paper Yu and coworkers reported [113] on the preparation of
mesoporous TiO 2 with a bicrystalline (anatase and brookite) framework, which was
synthesized directly under high-intensity ultrasound irradiation. The synthesis was
6.1 Sonochemistry 141
the pores, most of which are less than 6 nm in diameter. The ultrasonic irradiation
time dependence of optical absorption for the soaked porous solid sample, as
measured by the variation in absorbance at 310 and 544 nm, indicated the reduction of Au(III) ions, and the nucleation and aggregation of gold nanoparticles
within the pores of MSP silica. Additionally, the reaction rates estimated phenomenologically by the absorbance decay at 310 nm for both the porous sample and the
corresponding soaking solution, showed the enhancement of the sonochemical
reduction rate of Au(III) ions within the pores of mesoporous silica. It is assumed
that the extensive liquid–solid interfacial zones in the pores, due to the high specific surface area and great porosity of the mesoporous solid, are the major regions
where the efficient sonochemical reduction induced by the cavitation takes place.
In the latest of their publications related to the insertion of gold into MSP silica
[110], the same authors discuss the structural changes that the silica skeleton undergoes upon sonication. The structure of mesoporous silica after sonochemical
preparation of gold nanoparticles within its pores was studied by a nitrogen adsorption technique. It was shown that the structural parameters, such as specific
surface area (SSA), porosity (P), and the mean pore diameter (l(p)) were increased
significantly after ultrasonic irradiation. It is suggested that the collision of Au
nanoparticles with the pore walls and localized erosion induced by the asymmetric
implosive collapse of cavities on the extensive liquid–solid interface are responsible
for the structural change in the mesoporous solid.
One paper by the same group reports on the sonochemical insertion of palladium nanoparticles loaded within mesoporous silica [111]. The formation of Pd
nanoparticles (5–6 nm in diameter) was restricted by the coalescence of the sonochemically reduced Pd atoms inside the confined volumes of the porous solid.
One of the most exciting sonochemical preparation procedures of MSP titania
did not use any surfactant [112]. Yu and his coworkers used monodispersed TiO 2
sol particles, which were formed initially by ultrasound-assisted hydrolysis of acetic
acid-modified titanium isopropoxide. Then, the mesoporous spherical or globular
particles, which have a narrow pore size distribution, were produced by controlled
condensation and agglomeration of these sol nanoparticles under high intensity
ultrasound irradiation. The mesoporous TiO 2 has a wormhole-like structure and a
lack of long-range order. Nitrogen adsorption results indicate that the mesoporous
TiO 2 retains mesoporosity with a narrow pore size distribution and high surface
area to at least 673 K. The thermal stability of mesoporous TiO 2 is attributed to
its thick inorganic walls, consisting of TiO 2 nanoparticles. A TGA study shows
that this synthetic method is environmentally friendly. The photocatalytic activity
of mesoporous TiO 2 for the oxidation of acetone in air was measured. As-prepared
mesoporous TiO 2 has a negligible activity due to its amorphous structure. Calcined
mesoporous TiO 2 shows a better activity than thecommercial photocatalyst P25.
The reasons for the high activity of mesoporous TiO 2 are discussed.
In a continuing paper Yu and coworkers reported [113] on the preparation of
mesoporous TiO 2 with a bicrystalline (anatase and brookite) framework, which was
synthesized directly under high-intensity ultrasound irradiation. The synthesis was
6.1 Sonochemistry 141