8.2 Superhydrophobic Porous Surfaces by Electrodeposition
271
again after air exposure [72]. The accumulation of airborne organic adsorbates was
concluded from the temporal change of the infrared spectra of porous surfaces [85].
Therefore, one can speculate that the hydrophilic-to-hydrophobic transition with
time was due to the condensation of apolar impurities on the porous surface that
changes the surface energy. The oxide formation on the surface was also named as
a possible reason for the hydrophobic properties observed, which was verified by
either the electrochemical corrosion behaviour of the deposits [78] or the change
in oxide-related XPS peaks of the metal atoms [70, 82]. The oxidation of Co was
more significant with the increase in storage time than that of Ni. Electrodeposited
porous Zn [80] and Sn [71] surfaces are oxide-covered already in the as-received
state without ageing, and time dependence of the hydrophobicity was not mentioned
for these materials.
8.2.3 Post-deposition Impregnation of Deposits with High
Surface Roughness
The time dependence of the contact angle of as-received metal deposits with large
surface roughness verifies the necessity of a reproducible production of hydrophobic
surfaces so that the spontaneous adsorption of impurities should not be relied on. This
is why several methods were developed for the surface treatment of porous deposits
that result in a reproducible surface state, many of them exhibiting a better layer
adherence than the spontaneous adsorption of airborne hydrocarbons. Electroplating
of porous surface layers with a high surface roughness is not a strong necessity,
although much desired. The general experience is that the post-deposition treatment
yields the better results, the larger porosity the electroplated surface exhibits. Therefore, the principles concerning the electrodeposition of the surface layers are the
same as for layers without a post-deposition treatment, and the deposition conditions are often the same. When a metal of technical importance is treated with a
hydrophobic cover layer, the process often involves several steps [87–90], in accord
with the forthcoming sequence: (i) Deposition of the first compact protecting layer,
either by electrochemical or electroless manner. The key factor for the deposition of
this layer is the compatibility with the technical metal substrate (like an Mg or Al
alloy) and the good adherence; (ii) Deposition of a second layer with large porosity;
(iii) Post-deposition treatment of the surface.
The post-deposition treatment can happen by simply immersing the workpiece
coated with a layer of high surface area into a solution of polymer (like polypropylene
[91] or vulcanized polymer [92]). Then, the solvent of polymer solution is evaporated
and the protecting hydrophobic organic coating is dried onto the porous surface. In
this case, the protecting layer is attached to the metal surface with weak adhesion
force only, but the insolubility of the polymer in aqueous solution provides the
desired hydrophobicity. If the post-deposition treatment is carried out with plasmapolymerization of fluorinated hydrocarbons [76, 93, 94], coatings with a thickness
271
again after air exposure [72]. The accumulation of airborne organic adsorbates was
concluded from the temporal change of the infrared spectra of porous surfaces [85].
Therefore, one can speculate that the hydrophilic-to-hydrophobic transition with
time was due to the condensation of apolar impurities on the porous surface that
changes the surface energy. The oxide formation on the surface was also named as
a possible reason for the hydrophobic properties observed, which was verified by
either the electrochemical corrosion behaviour of the deposits [78] or the change
in oxide-related XPS peaks of the metal atoms [70, 82]. The oxidation of Co was
more significant with the increase in storage time than that of Ni. Electrodeposited
porous Zn [80] and Sn [71] surfaces are oxide-covered already in the as-received
state without ageing, and time dependence of the hydrophobicity was not mentioned
for these materials.
8.2.3 Post-deposition Impregnation of Deposits with High
Surface Roughness
The time dependence of the contact angle of as-received metal deposits with large
surface roughness verifies the necessity of a reproducible production of hydrophobic
surfaces so that the spontaneous adsorption of impurities should not be relied on. This
is why several methods were developed for the surface treatment of porous deposits
that result in a reproducible surface state, many of them exhibiting a better layer
adherence than the spontaneous adsorption of airborne hydrocarbons. Electroplating
of porous surface layers with a high surface roughness is not a strong necessity,
although much desired. The general experience is that the post-deposition treatment
yields the better results, the larger porosity the electroplated surface exhibits. Therefore, the principles concerning the electrodeposition of the surface layers are the
same as for layers without a post-deposition treatment, and the deposition conditions are often the same. When a metal of technical importance is treated with a
hydrophobic cover layer, the process often involves several steps [87–90], in accord
with the forthcoming sequence: (i) Deposition of the first compact protecting layer,
either by electrochemical or electroless manner. The key factor for the deposition of
this layer is the compatibility with the technical metal substrate (like an Mg or Al
alloy) and the good adherence; (ii) Deposition of a second layer with large porosity;
(iii) Post-deposition treatment of the surface.
The post-deposition treatment can happen by simply immersing the workpiece
coated with a layer of high surface area into a solution of polymer (like polypropylene
[91] or vulcanized polymer [92]). Then, the solvent of polymer solution is evaporated
and the protecting hydrophobic organic coating is dried onto the porous surface. In
this case, the protecting layer is attached to the metal surface with weak adhesion
force only, but the insolubility of the polymer in aqueous solution provides the
desired hydrophobicity. If the post-deposition treatment is carried out with plasmapolymerization of fluorinated hydrocarbons [76, 93, 94], coatings with a thickness