314
9 Electrosynthesis of Nanostructures Without a Coating …
the particles does not take place on the electrode surface in an electrochemically
assisted nucleation process, Eq. 9.1 is clearly not applicable here. The explanation
given by the authors for the current density dependence of the particle size refers
to the diffusion length of the ions and its relation to the thickness of the Helmholtz
layer. This explanation was not elaborated quantitatively and neglects the fact that
anions are assumed to accumulate in the vicinity of the anode.
– Synthesis of Fe 3 O 4 particles [49–54]. Although the goal of the synthesis modes
discussed in this section was to obtain Fe 3 O 4 , the formation of either Fe 2 O 3
or FeOOH as a competing reaction pathway was also evidenced [49, 51, 52,
54]. The formation of a product mixture itself indicates the complexity of the
processes. Since the synthesis conditions show a great variety, no unambiguous
reaction mechanism can be given. The spectrum of solutions used includes pure
water [52], aqueous solutions with various supporting electrolytes and complexing
agents [49, 50, 52, 54] and water–ethanol mixtures [51, 53]. The current density
applied scales somewhat with the conductivity of the solution: while it was
smaller than 1 mA cm
–2 for pure water, values between 2 and 200 mA cm
–2
were chosen for solutions containing a supporting electrolyte (the larger current
density values typically belong to larger interelectrode distances). The particle
size varied between 5 and 40 nm. In contrast to several experiments discussed
above, the increase in the current density led to an increase in the particle size [52,
54], which is yet to be explained. Mössbauer spectra indicated that the particles
obtained cannot be identified as a pure phase of either magnetite of maghemite.
Since the primary goal of the papers discussed so far was the synthesis and the characterization of nanoparticles, the systematic change of the experimental parameters
did not make it possible to conclude a fully feasible particle formation mechanism.
The anodic polarization curve of iron in various water–ethanol mixtures indicated
[51] that the working regime of the sacrificial anode falls to the potential region
where Fe(III) species are stable. This was called the “transpassive region”, even
though the experiment was performed in the presence of LiCl as supporting electrolyte which promotes iron dissolution instead of allowing the formation of a passive
oxide layer. The possibility of the Fe(III) species in the dissolution of the sacrificial
anode indicated that an additional oxidation process is not necessary for the formation mechanism of either Fe 3 O 4 or Fe 2 O 3 particles. Rather, the formation of oxide
particle mixtures is a natural consequence of the experimental conditions.
Concerning the reaction mechanism of formation of various iron oxide particles,
those works can be recommended that aimed at the scrutiny of the particle formation
mechanism by systematically changing a large number of experimental conditions
and performing in situ observations [55, 56]. The authors of these works concluded
that the Fe(OH) 2 formation is the initial step of the oxide particle formation, and
an oxidation steps follows with the participation of dissolved oxygen. This is partly
contradictory to the result of the works in which oxide particle formation in deaerated
solutions were observed. In spite of the large number of unresolved controversies,
the occurrence of two processes could be convincingly excluded: (i) reduction of
Fe(III)-containing particles by hydrogen evolved on the cathode in the formation of
9 Electrosynthesis of Nanostructures Without a Coating …
the particles does not take place on the electrode surface in an electrochemically
assisted nucleation process, Eq. 9.1 is clearly not applicable here. The explanation
given by the authors for the current density dependence of the particle size refers
to the diffusion length of the ions and its relation to the thickness of the Helmholtz
layer. This explanation was not elaborated quantitatively and neglects the fact that
anions are assumed to accumulate in the vicinity of the anode.
– Synthesis of Fe 3 O 4 particles [49–54]. Although the goal of the synthesis modes
discussed in this section was to obtain Fe 3 O 4 , the formation of either Fe 2 O 3
or FeOOH as a competing reaction pathway was also evidenced [49, 51, 52,
54]. The formation of a product mixture itself indicates the complexity of the
processes. Since the synthesis conditions show a great variety, no unambiguous
reaction mechanism can be given. The spectrum of solutions used includes pure
water [52], aqueous solutions with various supporting electrolytes and complexing
agents [49, 50, 52, 54] and water–ethanol mixtures [51, 53]. The current density
applied scales somewhat with the conductivity of the solution: while it was
smaller than 1 mA cm
–2 for pure water, values between 2 and 200 mA cm
–2
were chosen for solutions containing a supporting electrolyte (the larger current
density values typically belong to larger interelectrode distances). The particle
size varied between 5 and 40 nm. In contrast to several experiments discussed
above, the increase in the current density led to an increase in the particle size [52,
54], which is yet to be explained. Mössbauer spectra indicated that the particles
obtained cannot be identified as a pure phase of either magnetite of maghemite.
Since the primary goal of the papers discussed so far was the synthesis and the characterization of nanoparticles, the systematic change of the experimental parameters
did not make it possible to conclude a fully feasible particle formation mechanism.
The anodic polarization curve of iron in various water–ethanol mixtures indicated
[51] that the working regime of the sacrificial anode falls to the potential region
where Fe(III) species are stable. This was called the “transpassive region”, even
though the experiment was performed in the presence of LiCl as supporting electrolyte which promotes iron dissolution instead of allowing the formation of a passive
oxide layer. The possibility of the Fe(III) species in the dissolution of the sacrificial
anode indicated that an additional oxidation process is not necessary for the formation mechanism of either Fe 3 O 4 or Fe 2 O 3 particles. Rather, the formation of oxide
particle mixtures is a natural consequence of the experimental conditions.
Concerning the reaction mechanism of formation of various iron oxide particles,
those works can be recommended that aimed at the scrutiny of the particle formation
mechanism by systematically changing a large number of experimental conditions
and performing in situ observations [55, 56]. The authors of these works concluded
that the Fe(OH) 2 formation is the initial step of the oxide particle formation, and
an oxidation steps follows with the participation of dissolved oxygen. This is partly
contradictory to the result of the works in which oxide particle formation in deaerated
solutions were observed. In spite of the large number of unresolved controversies,
the occurrence of two processes could be convincingly excluded: (i) reduction of
Fe(III)-containing particles by hydrogen evolved on the cathode in the formation of