9.2 Metal Oxide Nanoparticles
311
makes an attempt to categorize the source–product permutations with the above
viewpoints:
– Fe 2 O 3 particles obtained from an Fe(III) solution [40]. The method described
made use of FeCl 3 as the iron source with a concentration range of 0.01–0.1 M. The
current density applied ranged from 0.15 to 2 A cm
–2 . The product composition
was Fe 2 O 3 , both the diffraction pattern and XPS line shape of which matching
closely to maghemite. Despite the vigorous hydrogen evolution on the cathode,
no Fe(II) species were found in the precipitate, which is probably due to fact that
the solubility of Fe(OH) 3 is much smaller than that of Fe(OH) 2 .
– Fe 3 O 4 particles from Fe(III) solution [45, 46]. This method involves the application of an ethanol solution with Fe(NO 3 ) 3 ·9H 2 O as solute and requires the very
accurate adjustment of the bath operation parameters to obtain magnetite nanoparticles. The salt concentration was found to be critical which has an optimum
value (~0.04 M) above which the increase in the concentration of both Fe(III) and
H 2 O leads to the formation of an amorphous non-magnetic precipitate instead
of magnetite nanoparticles. Also, a current density larger than 10 mA cm
–2 was
required to obtain nanoparticles with a current efficiency nearly 100%. In the case
when the current efficiency was close to 1, the process was formulated with the
following equation [46]:
3Fe
3+
+ 2NO
−
3 + 2H 2 O + 9e Fe 3 O 4 + 2NO
−
2 + 2H 2
(9.2)
which means that the reduction of Fe(III) to Fe(II) is a part of the particle formation process at the cathode (in other words, no Fe 2 O 3 particles have to form
that might later be reduced to Fe 3 O 4 ). It was the surprising experience of the
magnetite synthesis in ethanol that no additional stabilizer was necessary to obtain
monodispersed particles with diameter in the 4.5–8.9 nm range.
– Fe 3 O 4 particles from Fe(II) solution [41]. It was found that the particles formed
from aqueous Fe(ClO 4 ) 2 solution in the neighbourhood of the cathode have the
composition of Fe 3 O 4 . The key cathode process is the hydrogen evolution, and the
formation of the precipitate is accompanied with the oxidation of Fe(II) species
by dissolved oxygen. Since the anode used was Fe, it can be ruled out that the
occurrence of Fe(III) species is the anode reaction since it leads to Fe(II) formation only. The size range of the Fe 3 O 4 particles produced in the absence of any
stabilizing agent was 15 ± 6 nm.
– Fe 3 O 4 particles from solution or stoichiometric Fe(II)–Fe(III) mixtures [42–44,
47]. The methods applying Fe(II)–Fe(III) mixed solution corresponding the stoichiometry of the desired particles (1:2) makes it unnecessary to combine the
coagulation process with any redox transformation of the metal ions themselves.
Therefore, the key process is the hydroxide ion formation at the cathode and the
forthcoming precipitation in the solution with the release of water to achieve the
desired Fe 3 O 4 particle composition:
Fe
2+
+ 2Fe
3+
+ 8OH
−
Fe 3 O 4 + 4H 2 O
(9.3)
311
makes an attempt to categorize the source–product permutations with the above
viewpoints:
– Fe 2 O 3 particles obtained from an Fe(III) solution [40]. The method described
made use of FeCl 3 as the iron source with a concentration range of 0.01–0.1 M. The
current density applied ranged from 0.15 to 2 A cm
–2 . The product composition
was Fe 2 O 3 , both the diffraction pattern and XPS line shape of which matching
closely to maghemite. Despite the vigorous hydrogen evolution on the cathode,
no Fe(II) species were found in the precipitate, which is probably due to fact that
the solubility of Fe(OH) 3 is much smaller than that of Fe(OH) 2 .
– Fe 3 O 4 particles from Fe(III) solution [45, 46]. This method involves the application of an ethanol solution with Fe(NO 3 ) 3 ·9H 2 O as solute and requires the very
accurate adjustment of the bath operation parameters to obtain magnetite nanoparticles. The salt concentration was found to be critical which has an optimum
value (~0.04 M) above which the increase in the concentration of both Fe(III) and
H 2 O leads to the formation of an amorphous non-magnetic precipitate instead
of magnetite nanoparticles. Also, a current density larger than 10 mA cm
–2 was
required to obtain nanoparticles with a current efficiency nearly 100%. In the case
when the current efficiency was close to 1, the process was formulated with the
following equation [46]:
3Fe
3+
+ 2NO
−
3 + 2H 2 O + 9e Fe 3 O 4 + 2NO
−
2 + 2H 2
(9.2)
which means that the reduction of Fe(III) to Fe(II) is a part of the particle formation process at the cathode (in other words, no Fe 2 O 3 particles have to form
that might later be reduced to Fe 3 O 4 ). It was the surprising experience of the
magnetite synthesis in ethanol that no additional stabilizer was necessary to obtain
monodispersed particles with diameter in the 4.5–8.9 nm range.
– Fe 3 O 4 particles from Fe(II) solution [41]. It was found that the particles formed
from aqueous Fe(ClO 4 ) 2 solution in the neighbourhood of the cathode have the
composition of Fe 3 O 4 . The key cathode process is the hydrogen evolution, and the
formation of the precipitate is accompanied with the oxidation of Fe(II) species
by dissolved oxygen. Since the anode used was Fe, it can be ruled out that the
occurrence of Fe(III) species is the anode reaction since it leads to Fe(II) formation only. The size range of the Fe 3 O 4 particles produced in the absence of any
stabilizing agent was 15 ± 6 nm.
– Fe 3 O 4 particles from solution or stoichiometric Fe(II)–Fe(III) mixtures [42–44,
47]. The methods applying Fe(II)–Fe(III) mixed solution corresponding the stoichiometry of the desired particles (1:2) makes it unnecessary to combine the
coagulation process with any redox transformation of the metal ions themselves.
Therefore, the key process is the hydroxide ion formation at the cathode and the
forthcoming precipitation in the solution with the release of water to achieve the
desired Fe 3 O 4 particle composition:
Fe
2+
+ 2Fe
3+
+ 8OH
−
Fe 3 O 4 + 4H 2 O
(9.3)