Hydrothermal Synthesis of Native Elements and Simple Oxides - Hydrothermal Technology - page 594

Figure 9.21 Dendrite magnetite crystals
grown in 5 M NH 4 Cl solution 5.3 M mol
hydrogen [100] .

NH 4 Cl solution at 500 C under 1000 kg/cm 2 . Figure 9.21 shows the dendritic mag-
netite crystals grown in 5 M NH 4 Cl solution in 5.3 M mol hydrogen.
Chen and Xu [101] have prepared Fe 3 O 4 particles of nanometer size hydrother-
mally from iron (II) 2-methoxyethoxides (Fe(OMOE) 2 ), using 2-methoxyethanol
(MOE) water mixed solvent as the medium. As in the TiO 2 preparation, here also

the reaction conditions such as solvent, temperature, and time usually have impor-
tant effects on the resultant products. These authors have shown that the particle
size of Fe 3 O 4 powders increase with prolonged reaction time.
In recent years, hydrothermal synthesis of magnetite in the presence or absence
of organic chelating agents, oxidation of ferrous solutions using KNO 3 , and
solvothermal methods have become popular [102,103] . The ability to direct the
morphology of magnetite particles may be a key feature in the development of
new applications, particularly if other physical, chemical, and electronic properties
are retained. The formation of iron oxide phases is often influenced by the addi-
tion of secondary additives to aqueous systems containing either soluble or insolu-
ble precursors of the iron oxides. There have been quite a few reports dealing
with the influence of transition metals on the dissolution

precipitation reactions
in which iron oxides can participate under hydrothermal conditions. McGarvey
and Owen [100] have obtained magnetite under hydrothermal conditions at 150 C
and 175 C in the presence of hydrazine and copper (II) oxide. In the absence of
CuO, octahedral crystals of magnetite corresponding to the literature data are
found. In the presence of CuO, magnetite crystals with distorted morphology were
prepared [102] in which the crystal faces were absent, leaving a framework
structure of corners and edges. (See Figure 9.22 .) Substitution of NiO, ZnO, or
Ag 2 O produced somewhat different results. Thus copper, as a result of having
three accessible oxidation states (Cu(O), Cu 1 1 , and Cu 1 2 ) and rapid electron
transfer between the species, is able to participate in a redox catalytic mechanism
involving hydrazine to facilitate the reduction of hematite to magnetite as well as
directing its morphology. Uchida et al. [103] have prepared micaceous iron oxide
by the oxidation of iron with pressurized oxygen in concentrated sodium hydrox-
ide solution at elevated temperature. Thus, the preparation of iron oxide under
hydrothermal conditions is an attractive field.