8.2 Superhydrophobic Porous Surfaces by Electrodeposition
275
present [118, 125]. For these reasons, the salt film produced at the cathode may
contain higher-valency metal ions. Although some studies draw the attention to the
possible formation of a mixed-valency product [127], the XPS data published do not
indicate that higher-valency cations might be also present in the salt films produced
[119, 122, 127, 128]. It was shown only for the electrolysis performed in aqueous
Ce(NO 3 ) 3 solution that the cathodic process leads to the formation of a CeO 2 coating
[134] (Although an oxidation process taking place during the cathodic deposition is
unusual, it occurs also during the cathodic deposition of ruthenium oxide, and the
oxidation process is due to the instability of the Me(OH) 3 compounds in both cases.).
Anodic processes for obtaining porous hydrophobic salt film coatings are much
less common than cathodic ones. If the solution contains the salt of the metal that
forms the salt film in the anodic process, the salt has to contain the metal in an
intermediate oxidation state so that oxidation could be possible. This happens when
a Ce(NO 3 ) 3 solution is anodized to form CeO 2 [135]. The CeO 2 surface showed
a time-dependent hydrophobicity and a hydrophobic–hydrophilic transition upon
plasma cleaning, indicating that the hydrophobicity of the surface was due to impurity
adsorption.
Another means of the anodic salt film formation is when the substrate itself
is used as a conversion anode. This film formation mode was demonstrated with
Zn as anode in the presence of tetradecanoic acid [136]. Many deposition conditions (ethanol used as solvent, 30 V cell voltage applied) were similar to those
used in the cathodic processes. The anodic fatty acid salt coating showed intrinsic
hydrophobicity, similarly to the cathodically obtained ones.
The structure of the salt film deposits was analyzed with XRD method in a few
works. For the high-angle range of the diffractogram, mostly the spacing of the
–CH 2 – units can be determined [131, 133]. The periodicity of the unit cell of the
compound produced (i.e., the metal ion—apolar chain pair—metal ion sequence) can
be seen in the low-angle range (between 2° < 2 < 20°). Since usually no standards
are available for comparison, the exact deposit structure is not known, and only the
layered nature of the deposit can be concluded from the presence of the higher-order
reflections [120, 129]. The most common method of the check of the presence of
the hydrophobic layer is the record of the infrared spectra from the surface and its
comparison with that of the fatty acid used. In most cases, the agreement is nearly
perfect, and the presence of the metal ions in the deposit just negligibly modifies the
spectra.
8.2.5 Electroplated Hydrophobic Polymers with High
Surface Roughness
Electropolymerization of organic monomers is a common method to modify metal
surfaces. When redox-active polymer films are produced electrochemically, the films
are mostly hydrophilic and can be oxidized and reduced reversibly at least in a
275
present [118, 125]. For these reasons, the salt film produced at the cathode may
contain higher-valency metal ions. Although some studies draw the attention to the
possible formation of a mixed-valency product [127], the XPS data published do not
indicate that higher-valency cations might be also present in the salt films produced
[119, 122, 127, 128]. It was shown only for the electrolysis performed in aqueous
Ce(NO 3 ) 3 solution that the cathodic process leads to the formation of a CeO 2 coating
[134] (Although an oxidation process taking place during the cathodic deposition is
unusual, it occurs also during the cathodic deposition of ruthenium oxide, and the
oxidation process is due to the instability of the Me(OH) 3 compounds in both cases.).
Anodic processes for obtaining porous hydrophobic salt film coatings are much
less common than cathodic ones. If the solution contains the salt of the metal that
forms the salt film in the anodic process, the salt has to contain the metal in an
intermediate oxidation state so that oxidation could be possible. This happens when
a Ce(NO 3 ) 3 solution is anodized to form CeO 2 [135]. The CeO 2 surface showed
a time-dependent hydrophobicity and a hydrophobic–hydrophilic transition upon
plasma cleaning, indicating that the hydrophobicity of the surface was due to impurity
adsorption.
Another means of the anodic salt film formation is when the substrate itself
is used as a conversion anode. This film formation mode was demonstrated with
Zn as anode in the presence of tetradecanoic acid [136]. Many deposition conditions (ethanol used as solvent, 30 V cell voltage applied) were similar to those
used in the cathodic processes. The anodic fatty acid salt coating showed intrinsic
hydrophobicity, similarly to the cathodically obtained ones.
The structure of the salt film deposits was analyzed with XRD method in a few
works. For the high-angle range of the diffractogram, mostly the spacing of the
–CH 2 – units can be determined [131, 133]. The periodicity of the unit cell of the
compound produced (i.e., the metal ion—apolar chain pair—metal ion sequence) can
be seen in the low-angle range (between 2° < 2 < 20°). Since usually no standards
are available for comparison, the exact deposit structure is not known, and only the
layered nature of the deposit can be concluded from the presence of the higher-order
reflections [120, 129]. The most common method of the check of the presence of
the hydrophobic layer is the record of the infrared spectra from the surface and its
comparison with that of the fatty acid used. In most cases, the agreement is nearly
perfect, and the presence of the metal ions in the deposit just negligibly modifies the
spectra.
8.2.5 Electroplated Hydrophobic Polymers with High
Surface Roughness
Electropolymerization of organic monomers is a common method to modify metal
surfaces. When redox-active polymer films are produced electrochemically, the films
are mostly hydrophilic and can be oxidized and reduced reversibly at least in a