8.1 The Dynamic Bubble Template Method
261
there is some change in the primary pore size (large pores), the secondary porosity
(i.e., the wall structure) seems to be invariant throughout the deposit thickness [2,
11]. As it can be assessed from the image of cross-sectionally polished deposits, the
even pore size is strongly related to the high dendrite length to pore wall thickness
ratio [11–13], albeit all porous structures appear to be dendritic at a high enough
magnification.
For finding the right balance between the gas evolution and the metal deposition,
the composition of the electrolyte solution must somewhat differ from that of the
typical plating baths. The concentration of the metal salts is smaller, but the acidity is
higher in solutions used for the dynamic bubble template method than for deposition
of smooth coatings. The acid concentration is often as high as 2 mol dm
–3 . For
transition metals whose ion can form a complex with ammonia, buffered solutions
containing an ammonium salt and ammonia also proved to be feasible to prevent the
chemical precipitation of metal hydroxides [11, 14–16]. The necessity of an intense
bubble formation rationalizes a relatively large current density which must definitely
be larger than the mass transport-limited current density of the metal deposition.
While the deposition of a compact metal layer seldom can be carried out with a
current density higher than −200 mA cm
–2 , porous structures are often produced
with current densities of about −3 A cm
–2 (typical value) or exceptionally up to
−5 A cm
–2 [17]. The duration of the sample preparation procedure with a total
thickness of at most a few tens of micrometers is between 5 s to 40 min, and very
thick deposits cannot be prepared due to the low mechanical strength of the resulting
structure.
When an electrode potential value is given for DHBT experiments, their elucidation of the experimental conditions is very difficult due to the necessity of compensating the ohmic drop between the working and reference electrode, and the reference electrode cannot be positioned close enough to the working electrode due to
the vigorous hydrogen evolution. Nevertheless, the application of the ohmic drop
correction during the bath optimization can yield valuable information on the kinetic
background of the DHBT process [16]. The problem of the correct electrode potential referencing is even more difficult when the cell voltage is given, which often
ranges to a few tens of volts. It is the pronounced opinion of the author of the present
work that not only is current control simpler in this case but also provides a higher
level of reproducibility. It is straightforward that stirring of the electrolyte solution
influences the DHBT process to a negligible extent only due to the impact of the
vigorous hydrogen evolution taking place right on the cathode surface.
Concerning the technical background of the DHBT experiments, the reactivity
of the solution components with the atomic hydrogen as an intermediate of the
hydrogen evolution has to be considered. The alkalization around the cathode in the
presence of ammonium salts may result in ammonia production, and organic acids
can also be reduced. The anode reaction is also of importance, and the penetration of
the undesired anode reaction product to the vicinity of the cathode must be strictly
controlled. In contrast to these requirements, many experimental works report that
the porous deposits were made in a single-compartment cell with a relatively small
anode–cathode distance, which was typically 2 cm.
261
there is some change in the primary pore size (large pores), the secondary porosity
(i.e., the wall structure) seems to be invariant throughout the deposit thickness [2,
11]. As it can be assessed from the image of cross-sectionally polished deposits, the
even pore size is strongly related to the high dendrite length to pore wall thickness
ratio [11–13], albeit all porous structures appear to be dendritic at a high enough
magnification.
For finding the right balance between the gas evolution and the metal deposition,
the composition of the electrolyte solution must somewhat differ from that of the
typical plating baths. The concentration of the metal salts is smaller, but the acidity is
higher in solutions used for the dynamic bubble template method than for deposition
of smooth coatings. The acid concentration is often as high as 2 mol dm
–3 . For
transition metals whose ion can form a complex with ammonia, buffered solutions
containing an ammonium salt and ammonia also proved to be feasible to prevent the
chemical precipitation of metal hydroxides [11, 14–16]. The necessity of an intense
bubble formation rationalizes a relatively large current density which must definitely
be larger than the mass transport-limited current density of the metal deposition.
While the deposition of a compact metal layer seldom can be carried out with a
current density higher than −200 mA cm
–2 , porous structures are often produced
with current densities of about −3 A cm
–2 (typical value) or exceptionally up to
−5 A cm
–2 [17]. The duration of the sample preparation procedure with a total
thickness of at most a few tens of micrometers is between 5 s to 40 min, and very
thick deposits cannot be prepared due to the low mechanical strength of the resulting
structure.
When an electrode potential value is given for DHBT experiments, their elucidation of the experimental conditions is very difficult due to the necessity of compensating the ohmic drop between the working and reference electrode, and the reference electrode cannot be positioned close enough to the working electrode due to
the vigorous hydrogen evolution. Nevertheless, the application of the ohmic drop
correction during the bath optimization can yield valuable information on the kinetic
background of the DHBT process [16]. The problem of the correct electrode potential referencing is even more difficult when the cell voltage is given, which often
ranges to a few tens of volts. It is the pronounced opinion of the author of the present
work that not only is current control simpler in this case but also provides a higher
level of reproducibility. It is straightforward that stirring of the electrolyte solution
influences the DHBT process to a negligible extent only due to the impact of the
vigorous hydrogen evolution taking place right on the cathode surface.
Concerning the technical background of the DHBT experiments, the reactivity
of the solution components with the atomic hydrogen as an intermediate of the
hydrogen evolution has to be considered. The alkalization around the cathode in the
presence of ammonium salts may result in ammonia production, and organic acids
can also be reduced. The anode reaction is also of importance, and the penetration of
the undesired anode reaction product to the vicinity of the cathode must be strictly
controlled. In contrast to these requirements, many experimental works report that
the porous deposits were made in a single-compartment cell with a relatively small
anode–cathode distance, which was typically 2 cm.