9.2 Metal Oxide Nanoparticles
315
Fe 3 O 4 ; (ii) direct oxidation or reduction of the precipitated particles on either of the
electrodes. The variety of the experimental conditions used makes it natural that no
uniform picture can be outlined on the anodic iron oxide particle formation process.
9.2.4 Co-precipitation of Mixed Magnetic Oxide Particles
Containing Iron
The co-precipitation method is based on similar processes to the iron oxide nanoparticle formation, except for that the solution contains the ions of other metallic
elements. Since the solubility of Fe(OH) 2 and Fe(OH) 3 is smaller than that of the
hydroxide of the other metals present, these additives do not precipitate in their simple
oxide/hydroxide phase but co-precipitate with iron oxide only. The metal beside iron
(that all are present in the bath as Me
2+ cations) can be Sr [57, 58], Mn [59], Zn
[59] and Co [58], either as a single additive or a part of an additive mixture to form
ternary oxides [58, 59]. The element combination of choice is rationalized to obtain
magnetic nanoparticles that are similar in composition to hard magnetic ferrites.
Two solution types have been elaborated for the synthesis of ferrate-like nanoparticles. The suitable pH range of the bath based on Sr(NO 3 ) 2 and Fe(NO 3 ) 3 was 1–3,
while the solution containing chloride salts could be operated between pH 1 and 12.
Ternary oxides were obtained from mixed chloride-nitrate baths. While a relatively
large Sr
2+ concentration could be used in these baths (c = 0.1–1.5 M), the concentration of other metal cations was always ≤60 mM. Although the formation process of
the binary and ternary oxides is described in all relevant papers as if the cations other
than iron entered the iron-containing precipitate after the formation of the particles,
this view has not been strictly evidenced. It is unlikely that a solid-state diffusion
might lead to the mixed multielement oxide particles (if so, the process should also
work ex situ, but no such results are available). Therefore, it is the opinion of the
author of this monograph that the formation of the mixed oxide nanoparticles is a
simultaneous co-precipitation of the constituents. For stabilizing the particles, an
organic stabilizer (aniline) was applied in a concentration of a few hundred ppm.
Concerning other synthesis conditions, the temperature range was 20–80 °C and the
upper limit of the current density applied was 250 mA cm
–2 .
The metal ratio in the nanoparticles varied in a wide range. In SrFe w O z particles,
iron was always the predominant metallic component, and Sr proved to be nearly an
impurity with 12 < w < 400 [57]. In contrast, for MnZn i Fe j O k in occurred that Mn
was the majority component with i = 0.6 and j = 0.4 [59]. The comparison with
the solution composition from which the MnZn i Fe j O k particles were obtained tells
that Zn has a stronger doping ability than Mn. The data available do not make it
possible to establish clear trends, especially since the materials collected from the
anode, from the cathode and from the solution were of dissimilar composition even
in the same experiment. For all compositions reported, the particle size was in the
range from 17 to 32 nm.
315
Fe 3 O 4 ; (ii) direct oxidation or reduction of the precipitated particles on either of the
electrodes. The variety of the experimental conditions used makes it natural that no
uniform picture can be outlined on the anodic iron oxide particle formation process.
9.2.4 Co-precipitation of Mixed Magnetic Oxide Particles
Containing Iron
The co-precipitation method is based on similar processes to the iron oxide nanoparticle formation, except for that the solution contains the ions of other metallic
elements. Since the solubility of Fe(OH) 2 and Fe(OH) 3 is smaller than that of the
hydroxide of the other metals present, these additives do not precipitate in their simple
oxide/hydroxide phase but co-precipitate with iron oxide only. The metal beside iron
(that all are present in the bath as Me
2+ cations) can be Sr [57, 58], Mn [59], Zn
[59] and Co [58], either as a single additive or a part of an additive mixture to form
ternary oxides [58, 59]. The element combination of choice is rationalized to obtain
magnetic nanoparticles that are similar in composition to hard magnetic ferrites.
Two solution types have been elaborated for the synthesis of ferrate-like nanoparticles. The suitable pH range of the bath based on Sr(NO 3 ) 2 and Fe(NO 3 ) 3 was 1–3,
while the solution containing chloride salts could be operated between pH 1 and 12.
Ternary oxides were obtained from mixed chloride-nitrate baths. While a relatively
large Sr
2+ concentration could be used in these baths (c = 0.1–1.5 M), the concentration of other metal cations was always ≤60 mM. Although the formation process of
the binary and ternary oxides is described in all relevant papers as if the cations other
than iron entered the iron-containing precipitate after the formation of the particles,
this view has not been strictly evidenced. It is unlikely that a solid-state diffusion
might lead to the mixed multielement oxide particles (if so, the process should also
work ex situ, but no such results are available). Therefore, it is the opinion of the
author of this monograph that the formation of the mixed oxide nanoparticles is a
simultaneous co-precipitation of the constituents. For stabilizing the particles, an
organic stabilizer (aniline) was applied in a concentration of a few hundred ppm.
Concerning other synthesis conditions, the temperature range was 20–80 °C and the
upper limit of the current density applied was 250 mA cm
–2 .
The metal ratio in the nanoparticles varied in a wide range. In SrFe w O z particles,
iron was always the predominant metallic component, and Sr proved to be nearly an
impurity with 12 < w < 400 [57]. In contrast, for MnZn i Fe j O k in occurred that Mn
was the majority component with i = 0.6 and j = 0.4 [59]. The comparison with
the solution composition from which the MnZn i Fe j O k particles were obtained tells
that Zn has a stronger doping ability than Mn. The data available do not make it
possible to establish clear trends, especially since the materials collected from the
anode, from the cathode and from the solution were of dissimilar composition even
in the same experiment. For all compositions reported, the particle size was in the
range from 17 to 32 nm.