306
9 Electrosynthesis of Nanostructures Without a Coating …
• The classical thermodynamic explanation that considers the energy of the bulk
and the surface (which is the indirect consideration of the surface curvature) treats
the material as a continuum. However, the particle size is often close to the size
of the atoms, which means that the validity of the continuum treatment is by far
not guaranteed. It was shown for Ag nanoparticle synthesis with optical methods
that Ag 4
2+ clusters play an important role in the particle formation [2], and Cu
clusters formed from about 14 Cu atoms also proved to be stable [3]. It is clear
that quantization effects, concerning both the incorporation of new atoms and the
transfer of one electron, also play role in the particle formation.
• The classical treatment can be applied for nearly spherical particles. However, the
growth of nanoparticles with a large aspect ratio [5, 7, 10, 11] would certainly
require an explanation including kinetic terms with the involvement of the surface
energy of various crystal faces, which is beyond the scope of the classical
thermodynamic approach.
It is noteworthy that the micelle encapsulation approach used for Ni nanoparticle
synthesis in an anode process was also reported [18]. The electrochemical setup
applied in the latter study was very similar to those described for cathodic particle
generation processes and made it possible to obtain Ni particles with about 5 nm mean
diameter. Since a two-chamber cell was not used in either of the studies mentioned
hitherto, it remains a question what is the role of the anodic and cathodic processes
in the nanoparticle formation.
At last, an example for a completely adverse process is to be brought up in which
the primary product of the electrode reaction is an anion which is oxidized in the
solution to form nanoparticles. In this method, selenium particles were dispersed in
a carbon paste electrode, and the reduction of selenium resulted in Se
2– ions [19].
The dissolved oxygen in the solution was the oxidation agent. The oxidation of the
Se
2– ions led to either the production of elemental selenium directly or that of Se
4+
species, depending on the concentration ratio of the reactant, and the Se
4+ could also
react with Se
2– in a synproportionation reaction. Nearly spherical Se nanoparticles
with 43–85 nm diameter could be obtained this way in the presence of a suitable
stabilizing agent.
9.1.2 Mediated Formation of Metal Nanoparticles
with Electrochemical Generation of a Reducing Agent
The mediated nanoparticle formation is based on the electrochemical formation of at
least one of the reactants of the redox reaction that leads to the nanoparticle formation.
The indispensable electrochemical step of the mediated nanoparticle formation is the
generation of a reducing agent on the cathode that later react with the metal ions in
the solution. Although it is principally possible that the solution used contain a
metal salt as solute and this configuration is common in lab-scale research, practical
aspects dictate that the metal ions are generated in the anode reaction in large-scale
9 Electrosynthesis of Nanostructures Without a Coating …
• The classical thermodynamic explanation that considers the energy of the bulk
and the surface (which is the indirect consideration of the surface curvature) treats
the material as a continuum. However, the particle size is often close to the size
of the atoms, which means that the validity of the continuum treatment is by far
not guaranteed. It was shown for Ag nanoparticle synthesis with optical methods
that Ag 4
2+ clusters play an important role in the particle formation [2], and Cu
clusters formed from about 14 Cu atoms also proved to be stable [3]. It is clear
that quantization effects, concerning both the incorporation of new atoms and the
transfer of one electron, also play role in the particle formation.
• The classical treatment can be applied for nearly spherical particles. However, the
growth of nanoparticles with a large aspect ratio [5, 7, 10, 11] would certainly
require an explanation including kinetic terms with the involvement of the surface
energy of various crystal faces, which is beyond the scope of the classical
thermodynamic approach.
It is noteworthy that the micelle encapsulation approach used for Ni nanoparticle
synthesis in an anode process was also reported [18]. The electrochemical setup
applied in the latter study was very similar to those described for cathodic particle
generation processes and made it possible to obtain Ni particles with about 5 nm mean
diameter. Since a two-chamber cell was not used in either of the studies mentioned
hitherto, it remains a question what is the role of the anodic and cathodic processes
in the nanoparticle formation.
At last, an example for a completely adverse process is to be brought up in which
the primary product of the electrode reaction is an anion which is oxidized in the
solution to form nanoparticles. In this method, selenium particles were dispersed in
a carbon paste electrode, and the reduction of selenium resulted in Se
2– ions [19].
The dissolved oxygen in the solution was the oxidation agent. The oxidation of the
Se
2– ions led to either the production of elemental selenium directly or that of Se
4+
species, depending on the concentration ratio of the reactant, and the Se
4+ could also
react with Se
2– in a synproportionation reaction. Nearly spherical Se nanoparticles
with 43–85 nm diameter could be obtained this way in the presence of a suitable
stabilizing agent.
9.1.2 Mediated Formation of Metal Nanoparticles
with Electrochemical Generation of a Reducing Agent
The mediated nanoparticle formation is based on the electrochemical formation of at
least one of the reactants of the redox reaction that leads to the nanoparticle formation.
The indispensable electrochemical step of the mediated nanoparticle formation is the
generation of a reducing agent on the cathode that later react with the metal ions in
the solution. Although it is principally possible that the solution used contain a
metal salt as solute and this configuration is common in lab-scale research, practical
aspects dictate that the metal ions are generated in the anode reaction in large-scale