Environmental Engineering Reference
In-Depth Information
number of seeds. The second method relies on deposition of silica from silicic acid solution
[88]. It has been proven that the silicic acid method is more eficient for covering a higher
proportion of the magnetite surface than the TEOS method [89]. The particle size can be
controlled from tens to several hundreds of nanometers by changing the ratio of SiO
2
/
Fe
3
O
4
or repeating the coating procedure [90]. The third method is an emulsion method,
in which the silica coating is conined and controlled by the introduction of micelles or
inverse micelles [91,92]. In addition, the pyrolysis method has also been proposed for fab-
ricating submicronic silica-coated magnetic-sphere aerosols [93,94].
Owing to the presence of surface silanol groups, various coupling agents can be attached
to these magnetic particles by covalently bonding between silanol groups and speciic
ligands of coupling agents [95,96]. For example, amine groups have often been introduced
on the surface of silica-coated magnetite nanoparticles by hydrolysis and condensation of
an organosilane [97-99].
Gold is another important inorganic coating highly adequate to implement functionality
to MNPs. To date, some protocols have been developed for fabricating MNPs coated with
gold [100-103]. For example, core-shell-structured Fe/Au nanoparticles have been prepared
by a reverse-micelle approach [100]. Water-soluble Au-coated magnetite nanoparticles can
be prepared by the reduction of Au(III) onto the surface through iterative hydroxylamine
seeding [101]. The Au shell is favorable for protecting the Fe core and providing further
organic functionalization. Magnetic gold nanoshells have been developed: Fe
3
O
4
nanopar-
ticles was stabilized by oleic acid and 2-bromo-2-propionic acid, and gold seed nanopar-
ticles can be attached to amino-modiied silica particles; then, the growth of a complete
gold shell ultimately leads to the formation of superparamagnetic gold nanoshells [102].
Recently, Desai et al. have also reported the fabrication of superparamagnetic FeAs@C
core-shell nanoparticles with a fairly high blocking temperature (
T
B
) via a hot injection
precipitation technique—the proposed growth mechanism is shown in Figure 14.7 [103].
The synthesis involved the use of triphenylarsine (TPA) and Fe(CO)
5
as the Fe and As pre-
cursor, respectively, and hexadecylamine (HDA) as a surfactant. TPA reacts with Fe(CO)
5
by ligand displacement at moderate temperatures (300°C). Addition of HDA assists in the
HDA capped nuclei
Burst of nucleation of Fe at
t
= 0
At the end of the reaction at
t
= 3 hrs
FeAs
FeAs
TPA starts reacting from outside
Carbon
coating
As-rich
FeAs
Fe-rich
core
FeAs
At
t
= 45 min
FIGURE 14.7
Proposed growth mechanism of FeAs@C nanoparticles. (From Desai P, Song K, Koza JA et al.,
Chemistry of
Materials
, DOI: 10.1021/cm303632c, 2013.)
