Environmental Engineering Reference
In-Depth Information
FIGURE 14.2
(See color insert.)
Crystal structure of Fe
3
O
4
; green atoms are Fe
2+
, brown atoms are Fe
3+
, white atoms are oxy-
gen. (From Yang C., Wu J., Hou Y.,
Chemical Communications
, 47, 5130, 2011.)
14.2.1.1 Spherical Nanoparticles
Since Sugimoto's group fabricated monodispersed Fe
3
O
4
nanoparticles in 1980, various
methods have been developed for fabricating Fe
3
O
4
nanoparticles with narrow size dis-
tribution and good dispersion. In 2002, Sun's group developed a classic oleic acid (OA)-
oleyl amine (OAm) system that uses both OA and OAm as surfactants for synthesizing
monodispersed spherical Fe
3
O
4
nanoparticles [31]. A “seed-mediated” growth method was
developed to synthesize larger nanoparticles; the size of Fe
3
O
4
nanoparticles (3-20 nm)
can be tailored by altering the quantity of seeds. By modifying the strategy described
above, Xu et al. also prepared monodispersed Fe
3
O
4
nanoparticles without using polyol,
providing a low-cost route [36]. In such a system, OAm was used as both a reducing agent
and stabilizer, and the particle size was tunable by varying the volumetric ratio of benzyl
ether and OAm.
Solvothermal methods and high-temperature liquid phase methods have also been pro-
posed to synthesize spherical Fe
3
O
4
nanoparticles. Gao's group developed a solvothermal
method for synthesizing Fe
3
O
4
nanoparticles with a mean diameter of 25 nm [37]. In this
system, [Zn(CO
3
)
2
(OH)
6
] can accept Fe
2+
precipitates through −OH and then prevent the
agglomeration of Fe
2+
precipitates, and sequentially superparamagnetic Fe
3
O
4
nanopar-
ticles were obtained. Park et al. synthesized monodisperse iron oxide nanoparticles with
iron(III) oleates as the precursor by using a high-temperature liquid-phase method [38].
The particle size from 5 to 22 nm can be controlled by changing the decomposition tem-
perature and ageing time. γ-Fe
2
O
3
was the major phase for 5-nm iron oxide nanoparticles,
while the proportion of Fe
3
O
4
gradually increased with increasing particle size.
14.2.1.2 Octahedral, Dodecahedron, and Cubic Nanoparticles
It has been indicated that the shape of the particle is closely related to the crystallographic
surfaces that enclose the particle [39], and the key factor for synthesizing various Fe
3
O
4
nanoparticles is to tune the growth rate of speciic facets. As mentioned above, the relative
surface energies for Fe
3
O
4
are in the order of γ(111) < γ(100) < γ(110) < γ(220) owing to the
distances between these faces and the coordination number with neighboring atoms [40].
Therefore, the growth rate of the (111) plane is quicker than that of other planes, and the
octahedral shapes are the thermodynamically favored morphology.
