Environmental Engineering Reference
In-Depth Information
process [63]. Flower-like Fe
3
O
4
superstructures can also been prepared by using a hydro-
thermal method without any surfactant or organic solvent (Figure 14.6c) [68] or using an
ultrasound-assisted hydrothermal method.
In addition, many other Fe
3
O
4
hierarchical superstructures with special morphologies,
such as Fe
3
O
4
microspheres assembled by tetrahedral nanocrystals [71], porous hollow
Fe
3
O
4
beads constructed with rodlike nanoparticles [72], and nanoparticles-assembled
Fe
3
O
4
dendritic patterns [73] have also been fabricated.
14.2.3 Chemical-Functionalized MNMs
14.2.3.1 Monomeric Coating
Functional groups, including carboxylates, phosphates, and sulfates, are known to bind to
the surface of magnetites [74]. Their introduction is favorable for both dispersibility into
aqueous media and the adsorption of toxic compounds because of increasing the active
polar groups.
Citric acid can be adsorbed on the surface of the magnetite nanoparticles by coordinat-
ing via one or two of the carboxylate functionalities, depending on steric necessity and
the curvature of the surface [75]. At least one carboxylic acid group can be exposed to the
solvent, keeping the surface negatively charged and hydrophilic. It has been indicated that
carboxylates have important effects on the growth of iron oxide nanoparticles and their
magnetic properties [76,77]. Increasing concentrations of citric acid can lead to signiicant
decreases in the crystallinity of the iron oxides formed and the presence of citrate can
result in changes in the surface geometry. Other coating molecules, such as gluconic acid,
dimercaptosuccinic acid, and phosphorylcholine, have also been used as the coatings of
iron oxide.
Alkane sulfonic and alkane phosphonic acid surfactants have been applied as eficient
binding ligands on the surface of Fe
2
O
3
nanoparticles for dispersion in organic solvents
[78]. Two possible bonding schemes for the phosphonate ions on Fe
3+
have been proposed
by Yee et al. [79]; that is, one or two O atoms of the phosphonate group binding onto the
surface. Furthermore, the phosphate ions form bidentate complexes with adjacent sites on
the iron oxide surface [80].
14.2.3.2 Inorganic Materials
Many inorganic materials such as silica [81], gold [82], or gadolinium(III) [83] have been
exploited to coat iron oxide nanoparticles not only to prevent the oxidation of iron oxide
but also to help bind various ligands to the nanoparticle surface. Such iron-based MNMs
have an inner iron oxide core and an outer metallic shell of inorganic materials.
Silica has often been exploited as a coating material for magnetic nanoparticles (MNPs)
[84]. The inert silica coating on the surface of MNMs prevents their aggregation in liquid,
improves their chemical stability, and helps bind various ligands. The nanoparticles are
negatively charged and can be used for the removal of heavy metal cations. Three differ-
ent approaches have been developed to synthesize magnetic silica nanospheres. The irst
method refers to the well-known Stöber process, in which silica was formed
in situ
by the
hydrolysis and condensation of a sol-gel precursor, such as tetraethyl orthosilicate (TEOS)
[85,86]. For example, silica colloids loaded with SPIONs can be fabricated by using this
process [87]. The inal size of silica colloids depends on the concentration of iron oxide
nanoparticles and the type of solvent because the size of silica is closely related to the
