9.1 Synthesis of Nanoparticles on Electrodes
309
one single mediator can be used. The synthesis of Ag–Pd nanoparticles in a wide
composition range proved to be possible [35].
The mediators show a great variety, including both organic and inorganic
compounds. Among organic compounds, methyl viologen [23, 27, 29, 31, 32] and
its derivative, tetraviologen calix[4]resorcinol [22, 23, 26], are the most commonly
applied ones, but fullerenes (either C 60 or C 70 ) [21, 30] and anthracene [33] also
appear as mediator. Concerning inorganic mediators, O 2 [24, 28] was applied
first, and later Cr(III) and Co(III) triple-charged complexes such as [Cr(bipy) 3 ]
3+ ,
[Co(bipy) 3 ]
3+ and [Co(sep)]
3+ were tested [25] (bipy = 2,2
-bipyridyl, sep =
sepulchrate).
The solution has to be chosen in accord with the mediator to be used. Large
organic molecules requires less polar solvents (N,N-dimethylformamide being the
most common one), whereas processes mediated by metal complexes can take place
in aqueous solutions.
Nanoparticles obtained with the electrochemically mediated method are mostly
spherical. The diameter of the particles may vary even for the same metal depending
on the synthesis conditions. Taking Ag as an example, particles obtained in a
fullerene-mediated process are of 10–50 nm in diameter [21], the oxygen-mediated
Ag particles have a size distribution between 20 and 100 nm [24], and inorganic
Me(III) complexes as mediators result in the formation of Ag particles with mean
diameter ranging between 40 and 150 nm [25]. The application of stabilizers (like
polyvinyl pyrrolidone/PVP/) tentatively decreases the mean particle size [21]. The
particle size observation strongly depends on the method used (SEM, TEM, DLS
or AFM). An Ostwald ripening can be seen upon storage of particles for months in
spite of the stabilizer present. A 60-day-long storage of PVP-stabilized Ag particles
led to a doubling of the particle size as observed by DLS [24].
9.2 Metal Oxide Nanoparticles
9.2.1 General Aspects of the Electrosynthesis of Free Oxide
Particles
Methods discussed in Sect. 9.2 all belong to the large family of the coagulation
processes. Electrocoagulation is the method in which at least one of the reactants
that later form a precipitate is produced in a heterogeneous electrochemical reaction,
and the precipitate forms in a simple chemical reaction without a charge transfer
step (which is a clear distinction condition from mediated reactions where charge
transfer takes place between the dissolved reactants). The most common goal of
the electrocoagulation methods is to produce a colloidal adsorbent with very large
surface area that adsorbs pollutants that range from simple heavy metal ions to
dyes in industrial waste. For industrial purposes, electrocoagulation setups involve
a sacrificial anode that can be either Al or Fe for both economic and environmental
309
one single mediator can be used. The synthesis of Ag–Pd nanoparticles in a wide
composition range proved to be possible [35].
The mediators show a great variety, including both organic and inorganic
compounds. Among organic compounds, methyl viologen [23, 27, 29, 31, 32] and
its derivative, tetraviologen calix[4]resorcinol [22, 23, 26], are the most commonly
applied ones, but fullerenes (either C 60 or C 70 ) [21, 30] and anthracene [33] also
appear as mediator. Concerning inorganic mediators, O 2 [24, 28] was applied
first, and later Cr(III) and Co(III) triple-charged complexes such as [Cr(bipy) 3 ]
3+ ,
[Co(bipy) 3 ]
3+ and [Co(sep)]
3+ were tested [25] (bipy = 2,2
-bipyridyl, sep =
sepulchrate).
The solution has to be chosen in accord with the mediator to be used. Large
organic molecules requires less polar solvents (N,N-dimethylformamide being the
most common one), whereas processes mediated by metal complexes can take place
in aqueous solutions.
Nanoparticles obtained with the electrochemically mediated method are mostly
spherical. The diameter of the particles may vary even for the same metal depending
on the synthesis conditions. Taking Ag as an example, particles obtained in a
fullerene-mediated process are of 10–50 nm in diameter [21], the oxygen-mediated
Ag particles have a size distribution between 20 and 100 nm [24], and inorganic
Me(III) complexes as mediators result in the formation of Ag particles with mean
diameter ranging between 40 and 150 nm [25]. The application of stabilizers (like
polyvinyl pyrrolidone/PVP/) tentatively decreases the mean particle size [21]. The
particle size observation strongly depends on the method used (SEM, TEM, DLS
or AFM). An Ostwald ripening can be seen upon storage of particles for months in
spite of the stabilizer present. A 60-day-long storage of PVP-stabilized Ag particles
led to a doubling of the particle size as observed by DLS [24].
9.2 Metal Oxide Nanoparticles
9.2.1 General Aspects of the Electrosynthesis of Free Oxide
Particles
Methods discussed in Sect. 9.2 all belong to the large family of the coagulation
processes. Electrocoagulation is the method in which at least one of the reactants
that later form a precipitate is produced in a heterogeneous electrochemical reaction,
and the precipitate forms in a simple chemical reaction without a charge transfer
step (which is a clear distinction condition from mediated reactions where charge
transfer takes place between the dissolved reactants). The most common goal of
the electrocoagulation methods is to produce a colloidal adsorbent with very large
surface area that adsorbs pollutants that range from simple heavy metal ions to
dyes in industrial waste. For industrial purposes, electrocoagulation setups involve
a sacrificial anode that can be either Al or Fe for both economic and environmental