28 Copper Nucleation on Nickel from Pyrophosphate-Based Polyligand Electrolyte
445
was used as counter electrode. Saturated Ag/AgCl reference electrode was used in
the measurements, and all the potentials are given in the reference to this electrode.
28.3 Experimental Results and Discussion
28.3.1 Cyclic Voltammetry Study
Cyclic voltammetry study was performed in the potential range from −0.2 to
−1.5 V on Ni electrode from week alkaline electrolyte, containing ammonia and
pyrophosphate ions as ligands. The potential scan was initiated in the negative
direction from the open circuit potential (E OCP ) to different switching potentials
E λ . In forward scanning copper deposition on nickel begins at a potential of −0.55 V
(Fig. 28.1a). At a potential of −0.7 V, the cathode process speed begins to be
substantially limited by the lack of discharging copper ions in the near-electrode
layer. The current rise at the cathode branch at a potential of −1 V is due to
beginning of simultaneous hydrogen evolution and copper deposition. The copper
ion reduction in the process of backward scanning occurs at a slower rate. However,
on the layer of deposited copper, they continue reduction at potentials greater than
−0.55 V too. Two crossovers (E CO and E N ) are observed on CVA, a feature that
is characteristic of a new phase formation. The crossover E N at the potential of
−0,65 V is just a crossover potential at which nucleation and growth take place with
a measurable rate [43]. The crossover E CO at the potential of −0,3 V is associated
with potential of reversible metal/ion reaction, if it does not change position with
shifting the scan border [50]. As it is shown in Fig. 28.1a, the value of E CO and E N
is independent of the E λ value. It is possible to estimate the nucleation overvoltage
out of the difference between the E N and E CO potentials [18, 19], which is 0.35 V
in this case. At the anodic branch of the CVA, the deposited copper layer dissolves;
therefore, in the second cycle, the cathode branch is reproduced. Shifting the cathode
border of the potential scan to the negative side up to −1.2 V leads to the change in
the ratio of the anode peak height. This may be due to a change in the structure of
the copper layer deposited at different potential values. When the scan direction was
reversed in the switching potential E λ = −1.5 V (Fig. 28.1b), the copper deposit
accumulates on nickel electrode during the first cycle. It does not have time to
dissolve during the anode part of the cycle. Therefore the copper deposition on the
second cycle starts on the surface of oxidized deposit at a larger potential. In the
case of obtaining CVA only in the cathode region (Fig. 28.1c), the deposition in the
second cycle also starts not on nickel but on the surface of copper deposit from the
E CO potential.
445
was used as counter electrode. Saturated Ag/AgCl reference electrode was used in
the measurements, and all the potentials are given in the reference to this electrode.
28.3 Experimental Results and Discussion
28.3.1 Cyclic Voltammetry Study
Cyclic voltammetry study was performed in the potential range from −0.2 to
−1.5 V on Ni electrode from week alkaline electrolyte, containing ammonia and
pyrophosphate ions as ligands. The potential scan was initiated in the negative
direction from the open circuit potential (E OCP ) to different switching potentials
E λ . In forward scanning copper deposition on nickel begins at a potential of −0.55 V
(Fig. 28.1a). At a potential of −0.7 V, the cathode process speed begins to be
substantially limited by the lack of discharging copper ions in the near-electrode
layer. The current rise at the cathode branch at a potential of −1 V is due to
beginning of simultaneous hydrogen evolution and copper deposition. The copper
ion reduction in the process of backward scanning occurs at a slower rate. However,
on the layer of deposited copper, they continue reduction at potentials greater than
−0.55 V too. Two crossovers (E CO and E N ) are observed on CVA, a feature that
is characteristic of a new phase formation. The crossover E N at the potential of
−0,65 V is just a crossover potential at which nucleation and growth take place with
a measurable rate [43]. The crossover E CO at the potential of −0,3 V is associated
with potential of reversible metal/ion reaction, if it does not change position with
shifting the scan border [50]. As it is shown in Fig. 28.1a, the value of E CO and E N
is independent of the E λ value. It is possible to estimate the nucleation overvoltage
out of the difference between the E N and E CO potentials [18, 19], which is 0.35 V
in this case. At the anodic branch of the CVA, the deposited copper layer dissolves;
therefore, in the second cycle, the cathode branch is reproduced. Shifting the cathode
border of the potential scan to the negative side up to −1.2 V leads to the change in
the ratio of the anode peak height. This may be due to a change in the structure of
the copper layer deposited at different potential values. When the scan direction was
reversed in the switching potential E λ = −1.5 V (Fig. 28.1b), the copper deposit
accumulates on nickel electrode during the first cycle. It does not have time to
dissolve during the anode part of the cycle. Therefore the copper deposition on the
second cycle starts on the surface of oxidized deposit at a larger potential. In the
case of obtaining CVA only in the cathode region (Fig. 28.1c), the deposition in the
second cycle also starts not on nickel but on the surface of copper deposit from the
E CO potential.