304
9 Electrosynthesis of Nanostructures Without a Coating …
the relevant studies, the chemical name of the additive mentioned, its abbreviation
and the formula given show a large discrepancy [6–9, 12], and sometimes nanocubes
are obtained instead of spherical nanoparticles under conditions that are formally
analogous to other studies [12]. It was mentioned that Ag
+ ions are necessary to
grow rod-like Au nanoparticles [10], although the desired concentration was not
defined. The change of the supporting electrolyte to a non-stabilizing conventional
salt (like NaClO 4 ) leads to a layer formation instead of particle production [1].
The cathode was a platinum sheet in all studies mentioned above. The role of the
cathode material has not been studied yet; therefore, it cannot be established whether
the good electrocatalytic capability of platinum plays any role in the formation of
nanoparticles. All metals from which nanoparticles were electrosynthesized can be
easily plated onto platinum in the absence of the particle-stabilizing surfactant; therefore, it is likely that the same surfactant can make a film on the Pt cathode, too. It is
mentioned in the works of one research group only [1, 2] that the metal deposition
and nanoparticle formation are competing processes.
It has to be mentioned here that the cathode potential has to be limited for metal
particle formation without a deposition process. First, this is because a too high
cathodic overvoltage can lead to both solvent decomposition at a high rate and a
reduction of the solute that destroys the deposition-preventing film. Secondly, the
noble metal electrode applied for particle production may also undergo a transformation in the presence of tetraalkylammonium salt, as it is described in a number of
studies [13–17]. This transformation involves an in-depth insertion of the tetraalkylammonium cations into the metal, similarly to intercalation processes. The electrochemical cation insertion into the noble metals is reversible but is accompanied with
the ramification of the cathode.
All methods described in the literature combine the dissolution of the metal to
be precipitated in the form of nanoparticles and the reduction of its ion. Hence, the
sacrificial anode is always made of the material of the particles to be produced, even if
it is a noble metal. Due to stability reason, metallic particles that can be obtained with
this technique are composed of relatively stable metals such as Pd [4], Au [5–9] Ag
[1, 2] and Cu [3]. Interestingly, the nature of the anion of the supporting electrolyte
(i.e., complexing or non-complexing) does not seem to be important. Beside the
most common bromide salts, tetraalkylammonium acetate [2] and nitrate [3] salts
have also been reported as appropriate solution components that allow the dissolution
reaction at the anode. What is the most unexpected experience concerning the metal
cation–anion compatibility is that silver can readily dissolve in the presence of a
bromide salt [1], despite AgBr is notorious of its weak solubility.
The solvent seems to be related to the material dissolved. If the nanoparticles are
made of gold (which is not sensitive to either water or dissolved oxygen), aqueous
media are sufficient [5–7], although isopropanol is also commonly used as solvent [8,
9]. Occasionally, acetone was used as co-solvent, in particular when shape-modifying
additive with large hydrophobic groups has to be applied. Acetonitrile proved to be
suitable for Ag nanoparticles [1, 2], while an acetonitrile/tetrahydrofuran solvent
mixture was applied to synthesize Pd nanoparticles [4]. Surprisingly, no organic
9 Electrosynthesis of Nanostructures Without a Coating …
the relevant studies, the chemical name of the additive mentioned, its abbreviation
and the formula given show a large discrepancy [6–9, 12], and sometimes nanocubes
are obtained instead of spherical nanoparticles under conditions that are formally
analogous to other studies [12]. It was mentioned that Ag
+ ions are necessary to
grow rod-like Au nanoparticles [10], although the desired concentration was not
defined. The change of the supporting electrolyte to a non-stabilizing conventional
salt (like NaClO 4 ) leads to a layer formation instead of particle production [1].
The cathode was a platinum sheet in all studies mentioned above. The role of the
cathode material has not been studied yet; therefore, it cannot be established whether
the good electrocatalytic capability of platinum plays any role in the formation of
nanoparticles. All metals from which nanoparticles were electrosynthesized can be
easily plated onto platinum in the absence of the particle-stabilizing surfactant; therefore, it is likely that the same surfactant can make a film on the Pt cathode, too. It is
mentioned in the works of one research group only [1, 2] that the metal deposition
and nanoparticle formation are competing processes.
It has to be mentioned here that the cathode potential has to be limited for metal
particle formation without a deposition process. First, this is because a too high
cathodic overvoltage can lead to both solvent decomposition at a high rate and a
reduction of the solute that destroys the deposition-preventing film. Secondly, the
noble metal electrode applied for particle production may also undergo a transformation in the presence of tetraalkylammonium salt, as it is described in a number of
studies [13–17]. This transformation involves an in-depth insertion of the tetraalkylammonium cations into the metal, similarly to intercalation processes. The electrochemical cation insertion into the noble metals is reversible but is accompanied with
the ramification of the cathode.
All methods described in the literature combine the dissolution of the metal to
be precipitated in the form of nanoparticles and the reduction of its ion. Hence, the
sacrificial anode is always made of the material of the particles to be produced, even if
it is a noble metal. Due to stability reason, metallic particles that can be obtained with
this technique are composed of relatively stable metals such as Pd [4], Au [5–9] Ag
[1, 2] and Cu [3]. Interestingly, the nature of the anion of the supporting electrolyte
(i.e., complexing or non-complexing) does not seem to be important. Beside the
most common bromide salts, tetraalkylammonium acetate [2] and nitrate [3] salts
have also been reported as appropriate solution components that allow the dissolution
reaction at the anode. What is the most unexpected experience concerning the metal
cation–anion compatibility is that silver can readily dissolve in the presence of a
bromide salt [1], despite AgBr is notorious of its weak solubility.
The solvent seems to be related to the material dissolved. If the nanoparticles are
made of gold (which is not sensitive to either water or dissolved oxygen), aqueous
media are sufficient [5–7], although isopropanol is also commonly used as solvent [8,
9]. Occasionally, acetone was used as co-solvent, in particular when shape-modifying
additive with large hydrophobic groups has to be applied. Acetonitrile proved to be
suitable for Ag nanoparticles [1, 2], while an acetonitrile/tetrahydrofuran solvent
mixture was applied to synthesize Pd nanoparticles [4]. Surprisingly, no organic