310
9 Electrosynthesis of Nanostructures Without a Coating …
affordability. On the cathode, hydrogen is evolved, which itself is not used but the
alkalination of the medium provides the hydroxide ions in the system that makes
colloidal precipitates with the metal cations. The precipitate is filtered out from the
solution together with the pollutant that is adsorbed on the colloid surface, which
means that the balance of the process is the input that includes the electrical current,
the waste solution and some inexpensive metal, but the output is the purified solution
and the concentrated waste.
In industrial electrocoagulation processes, both the particle size and its homogeneity are of secondary importance, while the adsorption capability of the colloidal
particles is the key parameter to optimize. However, when electrocoagulation is used
for synthetic processes to obtain nanoparticles, the optimization aims at the particle
size and the size distribution. Since the resulting particles are often superparamagnetic, they are also named as superparamagnetic iron oxide nanoparticles (SPIONs).
The forthcoming chapters will detail such processes. Since SPIONs are by far the
most frequently produced materials with electrocoagulation methods, the various
versions of electrocoagulation will be presented with Fe x O y particles as examples.
The processes are partly analogous to various other coagulation methods that are
summarized in [36] and those that lead to well-adherent coatings on electrode (see,
e.g., [37–39]). Thereafter, processes will be just shortly mentioned for all other
materials.
9.2.2 Cathodic Processes for Obtaining Iron Oxide Particles
The common feature of the “cathodic” processes of iron oxide nanoparticle preparation is that the solution used contains an iron salt already at the time of the start of
the experiment. Therefore, the mean iron concentration of the bath is constant during
the experiment also in the case when an iron (or steel) electrode is used as anode
whose dissolution provides the replenishment of the solution with the ions that form
the precipitate. Besides, the oxidation state of the dissolved Fe source can be taken
as known.
Since the key reaction partner of the precipitate formation is the hydroxide ions,
it is straightforward that aqueous solutions can be used [40–44], and the alkalization
takes place in the neighbourhood of the cathode due to the hydrogen evolution. Even
in the cases when ethanol is used as solvent, the solution must contain water in order
to provide a source for hydroxide ions, originating either solely from the crystalline
form of the iron salt applied [45, 46] or from the salt and the water content of the
solvent together [47]. It is generally assumed that the precipitation leads to some iron
hydroxide in the first step and the oxide particles are obtained after the decomposition
of these particles by releasing water.
The cathodic electrosynthesis of iron oxide nanoparticles is very divergent from
the viewpoint whether the formation of the final product takes place by a simple
precipitation reaction (possibly combined with the release of some water) or some
further oxidation/reduction reaction has to be taken into account. The list below
9 Electrosynthesis of Nanostructures Without a Coating …
affordability. On the cathode, hydrogen is evolved, which itself is not used but the
alkalination of the medium provides the hydroxide ions in the system that makes
colloidal precipitates with the metal cations. The precipitate is filtered out from the
solution together with the pollutant that is adsorbed on the colloid surface, which
means that the balance of the process is the input that includes the electrical current,
the waste solution and some inexpensive metal, but the output is the purified solution
and the concentrated waste.
In industrial electrocoagulation processes, both the particle size and its homogeneity are of secondary importance, while the adsorption capability of the colloidal
particles is the key parameter to optimize. However, when electrocoagulation is used
for synthetic processes to obtain nanoparticles, the optimization aims at the particle
size and the size distribution. Since the resulting particles are often superparamagnetic, they are also named as superparamagnetic iron oxide nanoparticles (SPIONs).
The forthcoming chapters will detail such processes. Since SPIONs are by far the
most frequently produced materials with electrocoagulation methods, the various
versions of electrocoagulation will be presented with Fe x O y particles as examples.
The processes are partly analogous to various other coagulation methods that are
summarized in [36] and those that lead to well-adherent coatings on electrode (see,
e.g., [37–39]). Thereafter, processes will be just shortly mentioned for all other
materials.
9.2.2 Cathodic Processes for Obtaining Iron Oxide Particles
The common feature of the “cathodic” processes of iron oxide nanoparticle preparation is that the solution used contains an iron salt already at the time of the start of
the experiment. Therefore, the mean iron concentration of the bath is constant during
the experiment also in the case when an iron (or steel) electrode is used as anode
whose dissolution provides the replenishment of the solution with the ions that form
the precipitate. Besides, the oxidation state of the dissolved Fe source can be taken
as known.
Since the key reaction partner of the precipitate formation is the hydroxide ions,
it is straightforward that aqueous solutions can be used [40–44], and the alkalization
takes place in the neighbourhood of the cathode due to the hydrogen evolution. Even
in the cases when ethanol is used as solvent, the solution must contain water in order
to provide a source for hydroxide ions, originating either solely from the crystalline
form of the iron salt applied [45, 46] or from the salt and the water content of the
solvent together [47]. It is generally assumed that the precipitation leads to some iron
hydroxide in the first step and the oxide particles are obtained after the decomposition
of these particles by releasing water.
The cathodic electrosynthesis of iron oxide nanoparticles is very divergent from
the viewpoint whether the formation of the final product takes place by a simple
precipitation reaction (possibly combined with the release of some water) or some
further oxidation/reduction reaction has to be taken into account. The list below