Geoscience Reference
In-Depth Information
from 180
C to 250
C in most of the cases. The typical size of the products varies
from 20 nm to 200 nm depending upon the starting materials and the experimental
temperature. Iron oxides of spinel and magnetic structures are very important for
their unique magnetic properties, which can be varied systematically through
dopants like Co, Ni, Zn, and Mn.
Wu et al.
[142]
have prepared nanowire arrays of Co-doped magnetite under
hydrothermal conditions at 200
C using ferrous chloride, cobalt chloride, and
sodium hydroxide. These nanowires are believed to possess a single magnetic
domain which can be regarded as small wire-like magnets.
Wan et al.
[136]
have proposed a soft-template-assisted hydrothermal route to
prepare single crystal Fe
3
O
4
nanorods with an average diameter of 25 nm and
length of 200 nm at 120
C in 20 h. The formation of these Fe
3
O
4
nanorods has
been ascribed to ethylenediamine, which plays a crucial role not only as a base
source but also as a soft template to form single crystal Fe
3
O
4
nanorods.
Figure 10.32
shows the Fe
3
O
4
nanorods obtained through a soft-template-assisted
hydrothermal route. Teja and Koh
[134]
have extensively reviewed the synthesis of
all the magnetic phases of magnetite under hydrothermal conditions. Hematite nano-
particles
10 nm in size were produced from ferric nitrate and ferric ammonium
sulfate solutions using this method
[143]
. Magnetite nanoparticles were also pro-
duced using ferrous sulfate, with the addition of urea
[144]
. It was noted that the
particle size and morphology were strongly dependent on operating temperature
and processing time. The particle size increased as the operating temperature
increased and different forms of iron oxide were produced from the same precursor
by changing the temperature. For instance, mixtures of 6-line ferrihydrite (a weakly
crystalline form of iron oxide) and hematite were formed at 250
,
350
C, while
350
C.
Figure 10.33
shows very interesting results on the iron oxide nanoparticles mod-
ification with different organic ligand molecules. By adding different amounts of
the same functional groups as modifier reagents, affects on the particle size,
arrangement and morphology of the particles obtained under supercritical hydro-
thermal conditions (400
C and 30 MPa) can be altered. By adding decanoic and
oleic acids to the starting precursors, cubic and spherical magnetite particles were
obtained with a mean size of about 25 nm and 5 nm, respectively. Without the
modifier it yielded hematite particles. However, under subcritical conditions, the
addition of oleic acid to the precursor resulted in the formation of nanowires up to
85 nm length.
Kominami et al.
[145]
have prepared Ta
2
O
5
nanoparticles through solvothermal
routes and have studied their photocatalytic properties. They used tantalum penta-
butoxide in toluene at 200
pure hematite was obtained at temperatures
.
300
C in the presence of water. Ta
2
O
5
powder of
200 m
2
/g was obtained.
20
100 nm size showing high surface area of
.
150]
have worked out in detail a continuous syn-
thesis (flow technique) of fine metal oxide particles using supercritical water as the
reacting medium. They have shown that fine metal oxide particles are formed
when a variety of metal nitrates are contacted with supercritical water in a flow
system. They postulated that the fine particles were produced because supercritical
Adschiri and coworkers
[146
