9.2 Metal Oxide Nanoparticles
313
The mean diameter of the non-stabilized particles varied between 16 and 19 nm, while
the size increment upon the application of additives exceeded much the effect that
could be explained merely by the molecular size of the stabilizing agent that may
coat the particles. The mean diameters of the coated particles were 42 nm (additive: polyethylene glycol [47]), 82 nm (EDTA [42]), 52 nm (dextran [43]), 64 nm
(glucose [44]) and 70 nm (dextran [44]). At the same time, the X-ray diffraction
patterns revealed a mean crystallite size below 10 nm for both bare and coated
particles, indicating that the particles seen with TEM or DLS may be composed of
smaller grains. Nevertheless, the large particle size increment with the application
of additives raises questions on the generally accepted mechanism of the particle
stabilization by hindering the growth process.
9.2.3 Iron Oxide Synthesis with Sacrificial Anodes
In Sect. 9.2.2, systems were discussed in which the cathode process plays a key role
in the nanoparticle synthesis since the metallic precursor species were present in the
solution as dissolved compounds, even though in some cases a sacrificial Fe anode
was applied. This chapter deals with the processes in which the only source of iron
in the system is the anode reaction.
If the both the cations and the anions needed for the coagulation are provided in
the anode and cathode reactions, respectively, the optimization of the setup requires
much attention. The zone of the particle formation is the interelectrode space where
the oversaturation of the solution is the highest. No wonder, studies dealing with
the anodic synthesis of iron oxide particles all report that the fine tuning of the
interelectrode distance (typically 1–2 cm) and the current density are crucial for the
successful nanoparticle synthesis [48–54]. Following a similar classification to that
applied for the cathodic synthesis, various pursuits can be distinguished as presented
below:
– Synthesis of Fe 2 O 3 particles [48]. The method based on a DMF–water mixture as
this solvent is the only one in which Fe(III) species are formed directly in the
anode process. The water content was limited to at most 5 vol.%. While the high
water content may decrease the stability of the Fe(III) species formed on the anode,
water itself is indispensable for the OH
– formation on the cathode. The supporting
electrolyte was 0.05–0.15 M tetraoctylammonium bromide. Although it was not
mentioned in the original work, the bromide ions may play a role in the solvation
of Fe(III) species in the solution. Oxidation of bromide ions to bromine was not
discussed. The absence of Fe(II) species in the solution was checked with a redox
titration with chromate ions. Since the polarization behaviour was not reported, it
cannot be assessed whether the oxidation of Fe takes place in a step-wise process
with any soluble Fe(II) species that may be stable in a moderately positive potential
interval. The diameter of the particles decreased as a result of the increase in
current density, varying between about 3.5 and 7.5 nm. Since the formation of
313
The mean diameter of the non-stabilized particles varied between 16 and 19 nm, while
the size increment upon the application of additives exceeded much the effect that
could be explained merely by the molecular size of the stabilizing agent that may
coat the particles. The mean diameters of the coated particles were 42 nm (additive: polyethylene glycol [47]), 82 nm (EDTA [42]), 52 nm (dextran [43]), 64 nm
(glucose [44]) and 70 nm (dextran [44]). At the same time, the X-ray diffraction
patterns revealed a mean crystallite size below 10 nm for both bare and coated
particles, indicating that the particles seen with TEM or DLS may be composed of
smaller grains. Nevertheless, the large particle size increment with the application
of additives raises questions on the generally accepted mechanism of the particle
stabilization by hindering the growth process.
9.2.3 Iron Oxide Synthesis with Sacrificial Anodes
In Sect. 9.2.2, systems were discussed in which the cathode process plays a key role
in the nanoparticle synthesis since the metallic precursor species were present in the
solution as dissolved compounds, even though in some cases a sacrificial Fe anode
was applied. This chapter deals with the processes in which the only source of iron
in the system is the anode reaction.
If the both the cations and the anions needed for the coagulation are provided in
the anode and cathode reactions, respectively, the optimization of the setup requires
much attention. The zone of the particle formation is the interelectrode space where
the oversaturation of the solution is the highest. No wonder, studies dealing with
the anodic synthesis of iron oxide particles all report that the fine tuning of the
interelectrode distance (typically 1–2 cm) and the current density are crucial for the
successful nanoparticle synthesis [48–54]. Following a similar classification to that
applied for the cathodic synthesis, various pursuits can be distinguished as presented
below:
– Synthesis of Fe 2 O 3 particles [48]. The method based on a DMF–water mixture as
this solvent is the only one in which Fe(III) species are formed directly in the
anode process. The water content was limited to at most 5 vol.%. While the high
water content may decrease the stability of the Fe(III) species formed on the anode,
water itself is indispensable for the OH
– formation on the cathode. The supporting
electrolyte was 0.05–0.15 M tetraoctylammonium bromide. Although it was not
mentioned in the original work, the bromide ions may play a role in the solvation
of Fe(III) species in the solution. Oxidation of bromide ions to bromine was not
discussed. The absence of Fe(II) species in the solution was checked with a redox
titration with chromate ions. Since the polarization behaviour was not reported, it
cannot be assessed whether the oxidation of Fe takes place in a step-wise process
with any soluble Fe(II) species that may be stable in a moderately positive potential
interval. The diameter of the particles decreased as a result of the increase in
current density, varying between about 3.5 and 7.5 nm. Since the formation of