Topics in Current Chemistry (2020) 378:40
1 3
particles were shown to possess higher saturation magnetization. They also observed
differences in hematite particle size and morphology when using different precursors, with iron acetate giving rise to smaller spherical particles, while iron nitrate
led to larger, quasi cubic particles. These differences were due to the water content as well as the presence of nitrate and carboxylate in the precursors [108]. More
recently, Hu et al. [109] reported a new explosion-assisted sol–gel method in which
they used ferric nitrate as precursor and citric acid as chelating agent to form a gel.
The gel was then homogenized and heated with picric acid to attain highly pure,
well dispersed and crystallized magnetite nanoparticles ranging from 3 to 20 nm.
The synthesis was proposed to be resulting from the combined action of the complexing of citric acid with metal ions, and the explosion, thus explaining the important role of citric acid, not only as a carbon source, but also to allow the combustion
and reduction of the dried gel simultaneously. The chemistry of the sol–gel method
is vast, with the involvement of different precursors, gelators as well as chelators,
but it is beyond the scope of this review. It is, however, nicely explained by Danks
et al. [110].
This method is more recommended for synthesizing thin films [111] and nanocomposites [112, 113] since it can form thin films in just 2 min if the heating source
is changed to microwaves, and pure phases can be formed by using high microwave
power (600–800 W).
2.6 Microemulsion Method
The microemulsion method is a form of coprecipitation performed in a confined
space such as micelles. It generally involves two immiscible liquids with surfactants
forming the interfacial layer [114], and is classified as either the water-in-oil method
or oil-in-water method [115]. Inouye et al. [116] were the first to report the synthesis
of magnetic particles using this method, exploiting the faster oxidation of ferrous
ions in micelles.
In water-in-oil microemulsion, a hydrophobic phase is used with aqueous droplets separated by a surfactant [117]. The most common surfactants used are PVP
and cetyltrimethylammonium bromide (CTAB). In this method, particles generally collide and coalesce, and break again, leading to the growth of particles, the
particle size being determined by the size of the droplets. In a final step, particles
are centrifuged and lyophilized to get pure nanoparticles [118–120]. Many articles have been published on the use of this method to synthesize iron oxide nanoparticles [121–124]. Although surfactant concentration is not shown to affect the
size, precursor concentration and temperature are important influencers, together
with pH [125] and the choice of surfactant [126, 127]. Recently, Singh et al. [128]
showed the importance of ionic concentration and temperature on the morphology, size and crystallinity by claiming that, in order to obtain monophasic particles, [Fe
2+
] and [Fe
3+
] should be ≤ 0.09 M and ≤ 0.184 M, respectively, with a
temperature range of 65–72 ℃. They also observed changes in the morphology of
the particles, from cubes to pentagons to spheres, when increasing the concentration of the surfactant (CTAB) between 0.01 and 0.1 M, but they did not describe
58
Reprinted from the journal
1 3
particles were shown to possess higher saturation magnetization. They also observed
differences in hematite particle size and morphology when using different precursors, with iron acetate giving rise to smaller spherical particles, while iron nitrate
led to larger, quasi cubic particles. These differences were due to the water content as well as the presence of nitrate and carboxylate in the precursors [108]. More
recently, Hu et al. [109] reported a new explosion-assisted sol–gel method in which
they used ferric nitrate as precursor and citric acid as chelating agent to form a gel.
The gel was then homogenized and heated with picric acid to attain highly pure,
well dispersed and crystallized magnetite nanoparticles ranging from 3 to 20 nm.
The synthesis was proposed to be resulting from the combined action of the complexing of citric acid with metal ions, and the explosion, thus explaining the important role of citric acid, not only as a carbon source, but also to allow the combustion
and reduction of the dried gel simultaneously. The chemistry of the sol–gel method
is vast, with the involvement of different precursors, gelators as well as chelators,
but it is beyond the scope of this review. It is, however, nicely explained by Danks
et al. [110].
This method is more recommended for synthesizing thin films [111] and nanocomposites [112, 113] since it can form thin films in just 2 min if the heating source
is changed to microwaves, and pure phases can be formed by using high microwave
power (600–800 W).
2.6 Microemulsion Method
The microemulsion method is a form of coprecipitation performed in a confined
space such as micelles. It generally involves two immiscible liquids with surfactants
forming the interfacial layer [114], and is classified as either the water-in-oil method
or oil-in-water method [115]. Inouye et al. [116] were the first to report the synthesis
of magnetic particles using this method, exploiting the faster oxidation of ferrous
ions in micelles.
In water-in-oil microemulsion, a hydrophobic phase is used with aqueous droplets separated by a surfactant [117]. The most common surfactants used are PVP
and cetyltrimethylammonium bromide (CTAB). In this method, particles generally collide and coalesce, and break again, leading to the growth of particles, the
particle size being determined by the size of the droplets. In a final step, particles
are centrifuged and lyophilized to get pure nanoparticles [118–120]. Many articles have been published on the use of this method to synthesize iron oxide nanoparticles [121–124]. Although surfactant concentration is not shown to affect the
size, precursor concentration and temperature are important influencers, together
with pH [125] and the choice of surfactant [126, 127]. Recently, Singh et al. [128]
showed the importance of ionic concentration and temperature on the morphology, size and crystallinity by claiming that, in order to obtain monophasic particles, [Fe
2+
] and [Fe
3+
] should be ≤ 0.09 M and ≤ 0.184 M, respectively, with a
temperature range of 65–72 ℃. They also observed changes in the morphology of
the particles, from cubes to pentagons to spheres, when increasing the concentration of the surfactant (CTAB) between 0.01 and 0.1 M, but they did not describe
58
Reprinted from the journal