7.3 Combined MW/US and US/Flow Production of Nanoparticles
103
spray pyrolysis reactor (Fu et al. 2017). The same research group prepared luminescent carbon dots and used the chemical aerosol flow method to maximize productivity
(Fu et al. 2014).
Another example of high-quality nanoparticle production has been carried out
giving tuned particle properties and size in short production times using simultaneous
US and MW irradiation (Cravotto and Boffa 2014).
Nanoscaled ZnO is of very great interest because it is a common photocatalyst
that exhibits enhanced performance. The synergic effect that MW and US grant to
the preparation of well-defined flower-like nanostructures in surfactant-free conditions has been described by Li et al. The optimized procedure was performed via a
5 min sonication at 1000 W of a zinc acetate solution in hydroxide aqueous solution, followed by MW heating combined with pulsed US irradiation (1 s on and
2 s off) at a power of 500 W for 30 min (Li et al. 2011). More recently, the same
authors prepared hexagonal hourglass-like microstructures from the MW irradiation of ZnO with UV assistance and double sonication provided by two transducers
working at different frequencies (25 and 40 kHz) and fixed 220 W power (Li et al.
2013a, b). Ultrafine and highly crystalline particles of uniform size were obtained
by combining the MW oscillating field with the US acoustic streaming preventing
local overheating to afford more uniform microstructures. Li et al. (2013a, b) demonstrated another valuable example of the fine-tuning of the size and morphology of
ZnO nanostructures under combined MW and US irradiations. In that study, pH
value, zinc precursor and Zn/2-[4-(2-hydroxyethyll)-1-piperazinyl] ethanesulfonic
acid molar ratio were varied to afford a variety of ZnO micro- and nanostructures
(grenade-like, round-prism-like, spindle-like, rod-like, shuttle-like and flower-like).
A number of BiVO 4 nanostructures have been reported to promote the degradation
of numerous organic pollutants and microorganisms under visible light irradiation.
It has been also documented that both the shape and size of the nanostructures
have a significant effect on activity. The controlled fabrication of monocyclic BiVO 4
nanostructures generally requires harsh conditions, high temperature, long reaction
times and either the addition of organic additives or a specific template. Combined
MW/US irradiation is an attractive technique here as it enables BiVO 4 nanoparticles
(Zhang et al. 2013) with an average size of 150 nm to be prepared at pH 2. The
optimized procedure for BiVO 4 micro-/nanostructure preparation is performed in a
combined MW/US reactor at 110 °C (HC-300A, Beijing Xianghu Technology CO.,
Ltd, MW power 500 W, US 800 W, 2 s sonication and 1 interruption).
Porous manganese dioxide (MnO 2 ) has been synthesized by P. Wang et al. Their
detailed manuscript reported that the network structure varied according to its means
of preparation, either via US alone or in combination with MW irradiation. An
MW and US irradiation-intensified precipitation reaction has been used, in the presence of the structure-directing agent P123, to obtain manganese dioxide particles
with a looser network structure, more uniform dispersion, enlarged surface area and
enhanced electrochemical pseudocapacitance (Wang et al. 2014).
103
spray pyrolysis reactor (Fu et al. 2017). The same research group prepared luminescent carbon dots and used the chemical aerosol flow method to maximize productivity
(Fu et al. 2014).
Another example of high-quality nanoparticle production has been carried out
giving tuned particle properties and size in short production times using simultaneous
US and MW irradiation (Cravotto and Boffa 2014).
Nanoscaled ZnO is of very great interest because it is a common photocatalyst
that exhibits enhanced performance. The synergic effect that MW and US grant to
the preparation of well-defined flower-like nanostructures in surfactant-free conditions has been described by Li et al. The optimized procedure was performed via a
5 min sonication at 1000 W of a zinc acetate solution in hydroxide aqueous solution, followed by MW heating combined with pulsed US irradiation (1 s on and
2 s off) at a power of 500 W for 30 min (Li et al. 2011). More recently, the same
authors prepared hexagonal hourglass-like microstructures from the MW irradiation of ZnO with UV assistance and double sonication provided by two transducers
working at different frequencies (25 and 40 kHz) and fixed 220 W power (Li et al.
2013a, b). Ultrafine and highly crystalline particles of uniform size were obtained
by combining the MW oscillating field with the US acoustic streaming preventing
local overheating to afford more uniform microstructures. Li et al. (2013a, b) demonstrated another valuable example of the fine-tuning of the size and morphology of
ZnO nanostructures under combined MW and US irradiations. In that study, pH
value, zinc precursor and Zn/2-[4-(2-hydroxyethyll)-1-piperazinyl] ethanesulfonic
acid molar ratio were varied to afford a variety of ZnO micro- and nanostructures
(grenade-like, round-prism-like, spindle-like, rod-like, shuttle-like and flower-like).
A number of BiVO 4 nanostructures have been reported to promote the degradation
of numerous organic pollutants and microorganisms under visible light irradiation.
It has been also documented that both the shape and size of the nanostructures
have a significant effect on activity. The controlled fabrication of monocyclic BiVO 4
nanostructures generally requires harsh conditions, high temperature, long reaction
times and either the addition of organic additives or a specific template. Combined
MW/US irradiation is an attractive technique here as it enables BiVO 4 nanoparticles
(Zhang et al. 2013) with an average size of 150 nm to be prepared at pH 2. The
optimized procedure for BiVO 4 micro-/nanostructure preparation is performed in a
combined MW/US reactor at 110 °C (HC-300A, Beijing Xianghu Technology CO.,
Ltd, MW power 500 W, US 800 W, 2 s sonication and 1 interruption).
Porous manganese dioxide (MnO 2 ) has been synthesized by P. Wang et al. Their
detailed manuscript reported that the network structure varied according to its means
of preparation, either via US alone or in combination with MW irradiation. An
MW and US irradiation-intensified precipitation reaction has been used, in the presence of the structure-directing agent P123, to obtain manganese dioxide particles
with a looser network structure, more uniform dispersion, enlarged surface area and
enhanced electrochemical pseudocapacitance (Wang et al. 2014).