Topics in Current Chemistry (2020) 378:40
1 3
of evaporation, time spent in the reactor, and temperature. These studies concluded
that the heating time and temperature, along with the type of evaporation or reaction
taking place during the drying stage, will conform the particle structure as hollow,
dense, foam-like, etc. [148, 149]. Zheng et al. [150] recently reported that chloride
ions prevent phase transition from γ-Fe 2 O 3 to α-Fe 2 O 3 at higher temperatures, leading to higher magnetization, which highlights the importance of chloride ions in the
reaction. Das et al. proposed a new strategy to decrease size with high crystallinity
by adding ethanol to the ultrasonic pyrolysis [151]. It was explained that the faster
evaporation rate of ethanol compared to water, as well as a decrease in surface tension of the water–ethanol solution, led to the formation of smaller droplets and eventually smaller particles. Since the rate of evaporation of the solvent has been stressed
and linked to particle size, it might be interesting to see how methanol, or any other
solvent with a boiling point lower than that of ethanol, affects the size and crystallinity of particles.
The second category is Laser pyrolysis, a gas phase method that utilizes the heat
generated by a laser to heat the precursors and the flow of a gas or a mixture of gases
to produce nanoparticles. The sizes of the particles can be controlled by modulating
the power of the laser since a direct relationship exists between the two [152, 153].
Zhao et al. [154] were the first to improve on the TEA laser using a cw CO 2 laser,
which yielded particles with higher purity. There have also been reports on use of
this method to synthesize hybrid silica-iron oxide composites [155]. Laser pyrolysis has a new iteration, flame spray pyrolysis (FSP), which uses a flame to heat the
precursor [156]; the size of the nanoparticles can be controlled by varying the flame
length or the oxidant flow rate, and the precursor/fuel composition. Lower flow rate
of the oxidant leads to reduced flame length, with higher temperatures thus forming
smaller particles and vice versa [157].
The main advantage of this method is that it helps in achieving very high homogeneity and monodispersity irrespective of the complexity of particles, including
hybrid silica-iron oxide composites [155].
2.8 Sonochemical Method
This method utilizes acoustic cavitation, which means the formation, growth and
collapse of bubbles generated by ultrasound, to synthesize nanoparticles. Instead of
using high temperature or pressure directly, this method creates them indirectly by
using bubbles or cavities formed in the liquid by the acoustic waves. Further oscillation of such waves helps them gather and store ultrasonic energy, creating a hot spot
(~ 5000 K) and leading to the synthesis of particles of different shapes and sizes.
This method works for both volatile and non-volatile solvents [158–160]. The reaction medium was already considered the most important factor in controlling the
properties of nanoparticles by Suslick et al. [160] when they proposed the method,
since the bubbles formed will depend on the vapor pressure of the media. The nature
of the particles can also be altered by changing the ultrasonic frequencies based on
the inverse relationship between oxidation of Fe
2+
to Fe
3+
and ultrasonic frequencies
60
Reprinted from the journal
1 3
of evaporation, time spent in the reactor, and temperature. These studies concluded
that the heating time and temperature, along with the type of evaporation or reaction
taking place during the drying stage, will conform the particle structure as hollow,
dense, foam-like, etc. [148, 149]. Zheng et al. [150] recently reported that chloride
ions prevent phase transition from γ-Fe 2 O 3 to α-Fe 2 O 3 at higher temperatures, leading to higher magnetization, which highlights the importance of chloride ions in the
reaction. Das et al. proposed a new strategy to decrease size with high crystallinity
by adding ethanol to the ultrasonic pyrolysis [151]. It was explained that the faster
evaporation rate of ethanol compared to water, as well as a decrease in surface tension of the water–ethanol solution, led to the formation of smaller droplets and eventually smaller particles. Since the rate of evaporation of the solvent has been stressed
and linked to particle size, it might be interesting to see how methanol, or any other
solvent with a boiling point lower than that of ethanol, affects the size and crystallinity of particles.
The second category is Laser pyrolysis, a gas phase method that utilizes the heat
generated by a laser to heat the precursors and the flow of a gas or a mixture of gases
to produce nanoparticles. The sizes of the particles can be controlled by modulating
the power of the laser since a direct relationship exists between the two [152, 153].
Zhao et al. [154] were the first to improve on the TEA laser using a cw CO 2 laser,
which yielded particles with higher purity. There have also been reports on use of
this method to synthesize hybrid silica-iron oxide composites [155]. Laser pyrolysis has a new iteration, flame spray pyrolysis (FSP), which uses a flame to heat the
precursor [156]; the size of the nanoparticles can be controlled by varying the flame
length or the oxidant flow rate, and the precursor/fuel composition. Lower flow rate
of the oxidant leads to reduced flame length, with higher temperatures thus forming
smaller particles and vice versa [157].
The main advantage of this method is that it helps in achieving very high homogeneity and monodispersity irrespective of the complexity of particles, including
hybrid silica-iron oxide composites [155].
2.8 Sonochemical Method
This method utilizes acoustic cavitation, which means the formation, growth and
collapse of bubbles generated by ultrasound, to synthesize nanoparticles. Instead of
using high temperature or pressure directly, this method creates them indirectly by
using bubbles or cavities formed in the liquid by the acoustic waves. Further oscillation of such waves helps them gather and store ultrasonic energy, creating a hot spot
(~ 5000 K) and leading to the synthesis of particles of different shapes and sizes.
This method works for both volatile and non-volatile solvents [158–160]. The reaction medium was already considered the most important factor in controlling the
properties of nanoparticles by Suslick et al. [160] when they proposed the method,
since the bubbles formed will depend on the vapor pressure of the media. The nature
of the particles can also be altered by changing the ultrasonic frequencies based on
the inverse relationship between oxidation of Fe
2+
to Fe
3+
and ultrasonic frequencies
60
Reprinted from the journal