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Topics in Current Chemistry (2020) 378:7
chemical effects of ultrasound derive from acoustic cavitation. The collapse of
the bubbles in the liquid generates a huge amount of energy from the conversion
of the kinetic energy of the liquid motion into heating. The high local temperature and pressure, combined with extraordinarily rapid cooling, provide a unique
means of driving chemical reactions under extreme conditions [69]. The implosion
of the bubbles creates extreme conditions that enable synthesis to be conducted on
the benchtop in liquid at room temperature that in other cases would have required
high temperature and pressure or long reaction times [70]. During the preparation of
nanomaterials with ultrasonic irradiation, the phenomena responsible for sonochemistry can be characterized under “primary sonochemistry”, “secondary sonochemistry”, and “physical modifications” [71]. Xu et al. reported “primary sonochemistry”
as the reaction occurring inside the collapsing bubbles and “secondary sonochemistry” as the reaction in solution phase occurring outside the bubbles. These phenomena are responsible for the chemical effect of ultrasound and occur only if the reaction is sonication-sensitive or when the energy released during cavitation collapse
participates as reaction intermediate [70]. In particular, sonolysis of water produces
highly reactive H
·
and OH
·
radicals, which can be utilized for various sonochemical
reactions. These generated free radicals can further react with each other to form
new molecules and radicals or diffuse into the bulk liquid to serve as oxidants. The
reaction that produces free radicals can occur within the collapsing bubble (thermolytic center), at the interface between the bubble and bulk liquid, or in the adjacent
liquid. Several studies have reported that the predominant effect in heterogeneous
sonochemical reactions used for the production or modification of semiconductors
is the physical one [72]. In particular, it is the impact of jets of liquid at high speed
on the particle surface that can cause erosion and corrosion phenomena, by modifying the particle surfaces. In this way, surface nanostructures or disaggregated particles can be generated [71].
The mechanisms governing ultrasound applications can be summarized as follows [73].
• Homogeneous reactions that proceed via radical mechanisms are affected by
sonication, while ionic reactions are not affected by ultrasound.
• Heterogeneous reactions involving ionic species are influenced mainly by the
physical effects of cavitation (e.g., by the reduction in particle size). In this type
of reaction, the chemical effects are not dominant, so it is important to select the
appropriate operating parameters.
• Heterogeneous reactions involving radicals or combined mechanisms (ionic and
radical) are significantly influenced by the ultrasound effect. The radical reactions are intensified by the presence of ultrasound, but the physical effects also
greatly affect the mass transfer rate, improving its efficiency.
As regards the ultrasonic application for photocatalyst synthesis, the preparation
of anatase and rutile TiO 2 was reported by Huang et al. [74]. The authors compared
the sol–gel method with the sonochemistry procedure, highlighting that the ultrasound process resulted in a perfectly crystalline TiO 2 structure. The procedure used
to obtain the titania photocatalyst typically involves the treatment of the precursor
233
Reprinted from the journal
Topics in Current Chemistry (2020) 378:7
chemical effects of ultrasound derive from acoustic cavitation. The collapse of
the bubbles in the liquid generates a huge amount of energy from the conversion
of the kinetic energy of the liquid motion into heating. The high local temperature and pressure, combined with extraordinarily rapid cooling, provide a unique
means of driving chemical reactions under extreme conditions [69]. The implosion
of the bubbles creates extreme conditions that enable synthesis to be conducted on
the benchtop in liquid at room temperature that in other cases would have required
high temperature and pressure or long reaction times [70]. During the preparation of
nanomaterials with ultrasonic irradiation, the phenomena responsible for sonochemistry can be characterized under “primary sonochemistry”, “secondary sonochemistry”, and “physical modifications” [71]. Xu et al. reported “primary sonochemistry”
as the reaction occurring inside the collapsing bubbles and “secondary sonochemistry” as the reaction in solution phase occurring outside the bubbles. These phenomena are responsible for the chemical effect of ultrasound and occur only if the reaction is sonication-sensitive or when the energy released during cavitation collapse
participates as reaction intermediate [70]. In particular, sonolysis of water produces
highly reactive H
·
and OH
·
radicals, which can be utilized for various sonochemical
reactions. These generated free radicals can further react with each other to form
new molecules and radicals or diffuse into the bulk liquid to serve as oxidants. The
reaction that produces free radicals can occur within the collapsing bubble (thermolytic center), at the interface between the bubble and bulk liquid, or in the adjacent
liquid. Several studies have reported that the predominant effect in heterogeneous
sonochemical reactions used for the production or modification of semiconductors
is the physical one [72]. In particular, it is the impact of jets of liquid at high speed
on the particle surface that can cause erosion and corrosion phenomena, by modifying the particle surfaces. In this way, surface nanostructures or disaggregated particles can be generated [71].
The mechanisms governing ultrasound applications can be summarized as follows [73].
• Homogeneous reactions that proceed via radical mechanisms are affected by
sonication, while ionic reactions are not affected by ultrasound.
• Heterogeneous reactions involving ionic species are influenced mainly by the
physical effects of cavitation (e.g., by the reduction in particle size). In this type
of reaction, the chemical effects are not dominant, so it is important to select the
appropriate operating parameters.
• Heterogeneous reactions involving radicals or combined mechanisms (ionic and
radical) are significantly influenced by the ultrasound effect. The radical reactions are intensified by the presence of ultrasound, but the physical effects also
greatly affect the mass transfer rate, improving its efficiency.
As regards the ultrasonic application for photocatalyst synthesis, the preparation
of anatase and rutile TiO 2 was reported by Huang et al. [74]. The authors compared
the sol–gel method with the sonochemistry procedure, highlighting that the ultrasound process resulted in a perfectly crystalline TiO 2 structure. The procedure used
to obtain the titania photocatalyst typically involves the treatment of the precursor
233
Reprinted from the journal