1 3
Topics in Current Chemistry (2020) 378:29
able to reduce the metal to form nanoparticles. The advantages of using ultrasound
is its simplicity, application in ambient conditions, energy efficiency, rapid reactions
and fast synthesis, uniformity in formation and distribution [22–24]. An analysis
of the important reports that have been published on the sonochemical synthesis of
semiconductor photocatalysts are presented in the following paragraphs. The first
photcatalyst described is TiO 2 , which is often the most recommended photocatalyst.
Pinjari et al. [25] studied the synthesis of TiO 2 based on the sol–gel process,
focusing on understanding the effect of calcination and sonication time. These
authors noted that the use of ultrasound and the sonication time did affect the phase
transformation from anatase to rutile, a very important observation considering the
possible application of this phase transformation in the photocatalysis. Typically,
rutile content and crystallinity increased with calcination time and both were higher
for the catalyst obtained using ultrasound-assisted approach (US in Fig. 2) compared
with the silent conditions without use of ultrasound (NUS in Fig. 2). They also demonstrated that optimum sonication time was required to obtain the best rutile content
in the obtained catalyst and that the use of ultrasound resulted in higher yields, with
an actual yield of 95% compared to the 83.22% yield with the conventional approach
without ultrasound [25]. In another study [26], the same group reported their elucidation of the effect of ultrasonic amplitude on the phase transformation, reporting
that an optimum amplitude does exist for obtaining the best results for the yield,
rutile content and the crystallinity. The reported effect of amplitude on rutile content
and crystallinity has been reproduced in Fig. 3 to demonstrate a quantitative understanding of the results. The existence of the optimum amplitude was attributed to
the fact that the introduced heat energy due to collapse of cavities is not utilized for
the phase transformation. Also, the process of acoustic shielding based on too much
cavitation around the surface of transducer reduces the effective energy transfer and
hence lower cavitational intensity is observed that in turn drives lower phase transformation. The vaporization of 2-propanol due to excess heat energy dissipation at
higher power dissipations locally shifts the equilibrium, resulting in lower yields and
also lower cavitational intensity, again contributing to lower rutile content.
A mesoporous TiO 2 having a wormhole structure was effectively synthesized
using very high-intensity ultrasound [20]. The first step included the formation of
bare-TiO 2 using ultrasound-assisted hydrolysis and subsequent controlled condensation, and the second step included tailoring the particle size using high-intensity
ultrasound. The reported advantages of this process were rapid synthesis, high activity and thermal stability. Similarly, the synthesis of mesoporous bicrystalline TiO 2
containing anatase and brookite phases showing high photocatalyst activity was
reported using a tri-block copolymer [27]. The application of ultrasound resulted in
the enhanced content of the brookite phase, which was favourable for higher activity.
The other reported benefit of the use of ultrasound was obtaining mesoporous TiO 2
with narrow pore size distribution. In addition, a reduction in the synthesis time was
observed based on enhancements in the hydrolysis of the precursor, crystallization
and extraction of the surfactants used during the synthesis.
Apart from the application of ultrasound for the synthesis of native TiO 2 , there
have also been reports of ultrasound being used for the synthesis of composites
based on TiO 2 . Sonochemical synthesis was used to develop a novel-shaped TiO 2 /
75
Reprinted from the journal
Topics in Current Chemistry (2020) 378:29
able to reduce the metal to form nanoparticles. The advantages of using ultrasound
is its simplicity, application in ambient conditions, energy efficiency, rapid reactions
and fast synthesis, uniformity in formation and distribution [22–24]. An analysis
of the important reports that have been published on the sonochemical synthesis of
semiconductor photocatalysts are presented in the following paragraphs. The first
photcatalyst described is TiO 2 , which is often the most recommended photocatalyst.
Pinjari et al. [25] studied the synthesis of TiO 2 based on the sol–gel process,
focusing on understanding the effect of calcination and sonication time. These
authors noted that the use of ultrasound and the sonication time did affect the phase
transformation from anatase to rutile, a very important observation considering the
possible application of this phase transformation in the photocatalysis. Typically,
rutile content and crystallinity increased with calcination time and both were higher
for the catalyst obtained using ultrasound-assisted approach (US in Fig. 2) compared
with the silent conditions without use of ultrasound (NUS in Fig. 2). They also demonstrated that optimum sonication time was required to obtain the best rutile content
in the obtained catalyst and that the use of ultrasound resulted in higher yields, with
an actual yield of 95% compared to the 83.22% yield with the conventional approach
without ultrasound [25]. In another study [26], the same group reported their elucidation of the effect of ultrasonic amplitude on the phase transformation, reporting
that an optimum amplitude does exist for obtaining the best results for the yield,
rutile content and the crystallinity. The reported effect of amplitude on rutile content
and crystallinity has been reproduced in Fig. 3 to demonstrate a quantitative understanding of the results. The existence of the optimum amplitude was attributed to
the fact that the introduced heat energy due to collapse of cavities is not utilized for
the phase transformation. Also, the process of acoustic shielding based on too much
cavitation around the surface of transducer reduces the effective energy transfer and
hence lower cavitational intensity is observed that in turn drives lower phase transformation. The vaporization of 2-propanol due to excess heat energy dissipation at
higher power dissipations locally shifts the equilibrium, resulting in lower yields and
also lower cavitational intensity, again contributing to lower rutile content.
A mesoporous TiO 2 having a wormhole structure was effectively synthesized
using very high-intensity ultrasound [20]. The first step included the formation of
bare-TiO 2 using ultrasound-assisted hydrolysis and subsequent controlled condensation, and the second step included tailoring the particle size using high-intensity
ultrasound. The reported advantages of this process were rapid synthesis, high activity and thermal stability. Similarly, the synthesis of mesoporous bicrystalline TiO 2
containing anatase and brookite phases showing high photocatalyst activity was
reported using a tri-block copolymer [27]. The application of ultrasound resulted in
the enhanced content of the brookite phase, which was favourable for higher activity.
The other reported benefit of the use of ultrasound was obtaining mesoporous TiO 2
with narrow pore size distribution. In addition, a reduction in the synthesis time was
observed based on enhancements in the hydrolysis of the precursor, crystallization
and extraction of the surfactants used during the synthesis.
Apart from the application of ultrasound for the synthesis of native TiO 2 , there
have also been reports of ultrasound being used for the synthesis of composites
based on TiO 2 . Sonochemical synthesis was used to develop a novel-shaped TiO 2 /
75
Reprinted from the journal