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UNTERRICHT
It is interesting to see, that the current effi ciency increases with
increasing current density. Furthermore, as to be expected, the
effi ciencies are independent from the substrate because after
a short deposition time they are completely covered with
nickel.
b) electroless deposition
Electroless metal deposition means the reduction of metal
ions by a reducing agent that is present in the solution.
Metal ion + Red ---> Metal + Ox
This technique is especially advantageous when a layer of uniform
thickness is desired on a material of complicated shape
or when many similar objects are to be treated,
The most prominent examples for such reactions are the copper
deposition with formaldehyde in alkaline solution and the reduction
of nickel with sodium hyphosphate. In the latter case not only
nickel ions are reduced, also some hypophosphate is reduced to
phosphorus. The product is a mixture of glassy nickel-phosphorus
and hexagonal nickel, supersaturated with phosphorus. We
will return to this reaction in the next paragraph. In order to stabilize
copper ions in alkaline medium a complex former, in this
case ethylendiamonotetraacetate, EDTA, must be present.
In order to obtain a metal deposit only on a certain surface,
it is necessary that the reaction occurs with negligible rate in
the homogeneous phase and on the remaining other surfaces.
This is achieved if the reduction reaction is heterogeneously
catalyzed by the metal‘s surface. Hence, electroless deposition
must be an autocatalytic process. If one wants to metallize an
isolating surface like a polymer surface very small particles of
a metallic catalyst like palladium must be deposited fi rst. The
electroless deposition can easily be monitored with an EQCM.
If, for instance, during the deposition a copper surface is the
working electrode in an electrochemical cell, polarization leads
to an external current. This current is the algebraic sum of the
cathodic copper deposition current and the anodic formaldehyde
oxidation current. The EQCM permits to monitor the cathodic
current as a function of the potential because it is the
only reaction that leads to a mass change of the electrode. The
anodic current then is the difference between the total current
and the cathodic current. Schumacher et al 22 detected by use
of the EQCM that an intermediate of the formaldehyde oxidation
catalyzes the copper reduction from the Cu-EDTA-complexes. It
can be demonstrated by cathodic polarization that the copper
reduction current is zero when no formaldehyde is present and
increases with increasing formaldehyde concentration in spite
of the fact that the necessary electrons are supplied by the
outer circuit 23 , see Fig. 13. Also when by anodic
168
Ag 2.4 3.0 0.17
Ni 2.4 3.1 0.18
Au 2.4 3.0 0.17
Ag 4.5 7.6 0.43
Ni 4.5 7.8 0.44
Au 4.5 8.4 0.47
BUNSEN-MAGAZIN · 9. JAHRGANG · 5/2007
Fig. 13
Dependence of the copper reduction rate on the concentration of formaldehyde.
IEQCM is the current, calculated from the rate of frequency change.
polarization the copper reduction current vanishes the formaldehyde
oxidation rate becomes zero. Copper reduction forms
the active sites for the formaldehyde oxidation 24 .
c) anodic stripping
In this paragraph we want to demonstrate that a combination of
current vs. potential curves with mass loss vs. potential curves
can be used to determine the composition of two-component
mixtures. As an example we will discuss results obtained with
galvanic deposits from nickel and H3PO3 containing solutions.
Similarly as with electroless deposition from Nickel and H3PO2
containing solutions the deposit contains nickel and phosphorus.
With fi lms of a thickness in the range of 500 to 700 nm
the fi lm can be totally oxidized during a single anodic sweep.
Unexpectedly the voltammogram shows two well separated
peaks 25 , shown in Fig. 14 for a sample of the overall composition
Ni78P22. During the scan
Fig. 14
Voltammogramm of a NixPy film during anodic stripping
one can continually record the mass loss. There are two potential
regions where the mass loss occurs and they coincide
with the regions where the oxidation occurs. It can be shown
that the oxidation leads to nickel in the oxidation state +2 and