Environmental Engineering Reference
In-Depth Information
100
90
80
70
60
50
40
30
20
10
0
Physical
Chemical
Biological
68%
12%
37%
14%
24%
3%
15%
11%
21%
19%
15%
3%
Fungi mediated
Bacteria mediated
Protein mediated
Gas-phase deposition
Electron beam lithography
Pulsed laser ablation
Laser-induced pyrolysis
Powder ball milling
Aerosol
20%
10%
27%
Coprecipitation
Microemulsion
Hydrothermal
Electrochemical deposition
Sonochemical
ƒermal decomposition
FIGURE 14.1
(See color insert.)
Comparison of published work (up to date) on the synthesis of SPIONs by three different
routes. (From Mahmoudi M., Sant S., Wang B. et al.,
Advanced Drug Delivery Reviews
, 63, 24, 2011.)
14.2.1 Simple Nanocrystals
Generally, the magnetic component of MNMs used for water treatment is magnetite Fe
3
O
4
(or its products of oxidation, γ-Fe
2
O
3
). Magnetite is much easier to synthesize and much
cheaper than other magnetic materials composed of other metals (Co) or other oxides (of
the type MFe
2
O
4
). Therefore, many expanding synthetic works on MNMs have evolved
from around the preparation of Fe
3
O
4
crystals. Fe
3
O
4
has a cubic inverse spinel structure
with a space grouping of
Fd-3m
[29]. The lattice constant is
a
= 0.839. In the unit cell (Figure
14.2), the oxygen ions form an
fcc
close packing, and the iron ions occupy interstitial tet-
rahedral sites that are occupied with Fe(II) and octahedral sites that are occupied by both
Fe(II) and Fe(III), symbolized as [Fe]
A
[Fe
2+
Fe
3+
]
B
O
4
[30]. The high-index planes usually have
higher surface energy [31] and the sequence is γ(111) < γ(100) < γ(110) < γ(220) for the face-
centered-cubic phase [32]. Fe
3
O
4
crystals are usually overlaid with octahedral and mixed
octahedral/tetrahedral layers along the <111> direction. Faster growth along the <100>
direction can result in octahedral particles, while faster growth along the <111> direc-
tion can lead to cubic particles. Various morphologies of Fe
3
O
4
crystals can be prepared
by controlling the growth rate of different facets of the nuclei through the introduction of
surfactants [33] and templates [34], or regulating the speciic synthetic conditions [35].
