264
8 Porous Nanostructured Materials
the composition ratio of the constituents is similar to normal codeposition at high
relative Ni
2+ concentrations [15].
The application of bath additives proved to be a suitable strategy to tune the pore
size of the deposits. Surface-active components that stabilize the small bubbles and
prevent their coalescence lead to a reduction of the pore size. This is why acetic
acid is a popular component of the DHBT baths, being an acid source and a bubble
stabilizer at the same time [6]. Classical polymeric additives (e.g., polyethylene
glycol [18]), originally applied as brighteners for smooth coatings, play a similar
role in the DHBT method. A pronounced difference compared to the smooth coating
is that the blocking of the dendritic growth is undesired in the DHBT method; hence,
application of grain-refining additives (whose preferred adsorption site is the tip of
the growing crystals) is not necessary.
Interestingly, electrolyte components like sulphate salts were also found to be
effective in the hindrance of the bubble coalescence [4]. The role of the ammonium
salts is controversial since the liberation of NH 3 is expected due to the alkalization of
the solution around the growing foam, and ammonia can act as a complexing agent for
the metal ions, hence changing the deposition mechanism [10, 22, 24]. While ammonium ions were found to decrease the hydrogen evolution rate during the deposition
of porous Cu [22], an adverse effect was reported for Ag deposition [20, 35]. For
preparing a porous Ni layer, ammonium chloride was found to be indispensable [10,
29]. Ammonium chloride was applied in most of the baths used for various porous
deposits since the first study was published on a DHBT process [17]. Therefore,
the impact of ammonium ions as growth modifier and their buffering capability are
difficult to separate from the effect of halogenide ions. It was occasionally mentioned
that the maximum applicable current density from the viewpoint of the mechanical
stability of the deposit is strongly interrelated to the NH 4 Cl concentration in the
solution [17].
Halogenide ions are identified as promoters for the deposition, most likely due to
the bridging effect at the surface and the facilitation of the electron transfer reaction
between the metal and the adsorbed intermediate [6, 22]. The increase in the pore size
with halogenide ion concentration was also shown for bath not containing NH 4 Cl
as supporting electrolyte [4, 18]. Another additive, 3-mercapto-1-propanesulphonic
acid was found to lead to smooth Cu pore walls by decreasing the secondary porosity
[22] through a similar effect as brighteners and levelling agents work for planar
deposits. As it was stressed in Sect. 2.11, the impact of additives is a highly empirical
field where an a priori approach seldom works.
While the DHBT method was quite well characterized concerning the role of the
bath components, the role of the other deposition conditions received less attention.
The available information in the literature does not make it possible to establish
any trend concerning the current efficiency of the process since this parameter is
practically never given. It was demonstrated for porous Cu that both the deposit
grain size and the primary pore size decrease with increasing current density, but
the increase in temperature has an adverse effect [26]. The application of pulsed
potential was found to be suitable for tuning the morphological properties of porous
Cu deposits, even though the period of the potential modulation applied (3–20 ms)
8 Porous Nanostructured Materials
the composition ratio of the constituents is similar to normal codeposition at high
relative Ni
2+ concentrations [15].
The application of bath additives proved to be a suitable strategy to tune the pore
size of the deposits. Surface-active components that stabilize the small bubbles and
prevent their coalescence lead to a reduction of the pore size. This is why acetic
acid is a popular component of the DHBT baths, being an acid source and a bubble
stabilizer at the same time [6]. Classical polymeric additives (e.g., polyethylene
glycol [18]), originally applied as brighteners for smooth coatings, play a similar
role in the DHBT method. A pronounced difference compared to the smooth coating
is that the blocking of the dendritic growth is undesired in the DHBT method; hence,
application of grain-refining additives (whose preferred adsorption site is the tip of
the growing crystals) is not necessary.
Interestingly, electrolyte components like sulphate salts were also found to be
effective in the hindrance of the bubble coalescence [4]. The role of the ammonium
salts is controversial since the liberation of NH 3 is expected due to the alkalization of
the solution around the growing foam, and ammonia can act as a complexing agent for
the metal ions, hence changing the deposition mechanism [10, 22, 24]. While ammonium ions were found to decrease the hydrogen evolution rate during the deposition
of porous Cu [22], an adverse effect was reported for Ag deposition [20, 35]. For
preparing a porous Ni layer, ammonium chloride was found to be indispensable [10,
29]. Ammonium chloride was applied in most of the baths used for various porous
deposits since the first study was published on a DHBT process [17]. Therefore,
the impact of ammonium ions as growth modifier and their buffering capability are
difficult to separate from the effect of halogenide ions. It was occasionally mentioned
that the maximum applicable current density from the viewpoint of the mechanical
stability of the deposit is strongly interrelated to the NH 4 Cl concentration in the
solution [17].
Halogenide ions are identified as promoters for the deposition, most likely due to
the bridging effect at the surface and the facilitation of the electron transfer reaction
between the metal and the adsorbed intermediate [6, 22]. The increase in the pore size
with halogenide ion concentration was also shown for bath not containing NH 4 Cl
as supporting electrolyte [4, 18]. Another additive, 3-mercapto-1-propanesulphonic
acid was found to lead to smooth Cu pore walls by decreasing the secondary porosity
[22] through a similar effect as brighteners and levelling agents work for planar
deposits. As it was stressed in Sect. 2.11, the impact of additives is a highly empirical
field where an a priori approach seldom works.
While the DHBT method was quite well characterized concerning the role of the
bath components, the role of the other deposition conditions received less attention.
The available information in the literature does not make it possible to establish
any trend concerning the current efficiency of the process since this parameter is
practically never given. It was demonstrated for porous Cu that both the deposit
grain size and the primary pore size decrease with increasing current density, but
the increase in temperature has an adverse effect [26]. The application of pulsed
potential was found to be suitable for tuning the morphological properties of porous
Cu deposits, even though the period of the potential modulation applied (3–20 ms)