232
A. Gupta and C. Srivastava
and the CV run completed at 0.0 V. This part of CV is henceforth referred as reverse
sweep. In Fig. 1, during the cathodic sweep for the Sn CV (circles), the peak at −
0.660 V is observed. This peak corresponds to reduction of Sn over the GO substrate
according to the reaction
Sn
+2
(aq.) + 2e
−
→ Sn (s)
(1)
After the peak in the cathodic sweep, the observed current density decreases and
then increases at more negative potentials. This continuous increase in current density
corresponds to the hydrogen evolution reaction. In the reverse sweep (anodic sweep),
a peak is observed at −0.309 V which corresponds to oxidation (dissolution) of the
Sn metal that got deposited on the GO substrate.
The Co CV curve (squares) is also presented in Fig. 1 in which the applied potential
was varied between +0.5 and −1.5 V. In the forward sweep, the reduction peak
occurred at −1.292 V. The electrolyte for Co electrodeposition contained 10 mM
CoSO 4 .7H 2 O and 1 M Na 2 SO 4 (anhydrous) at pH 2.5. Under these conditions, the
Co exists in the electrolyte as [Co(H2O) 6 ]
2+ ionic complex. Thus, the peak in the
forward sweep can be associated to the reduction of Co complex [23] into Co metal
over the GO substrate as:
Co(H 2 O) 6
2+ + 2e
−
→ Co + 6H 2 O
( 2 )
As the potential decreases further in the forward sweep, the continuous increase
in current density is observed which corresponds to hydrogen evolution reaction. In
the reverse scan, a current crossover is observed. Two current density peaks were
observed during the reverse sweep: a primary oxidation peak at −0.102 V and a
shoulder peak at +0.018 V. Both the peaks correspond to the oxidation of Co. These
two oxidation peaks correspond to the different allotropic forms of Co (hexagonal
and simple cubic) [24].
To comment on the free energies of nucleation and critical nucleus size, the knowledge of equilibrium potential for metal deposition from the electrolyte is necessary.
If the CV curve exhibits a crossover during the reverse sweep, then the crossover at
algebraically more positive potential values is taken as the equilibrium potential. In
the case of Co CV experiment, the crossovers are observed at −1.036 and −0.650 V.
Therefore, the equilibrium potential is taken as −0.650 V. In the CV cases where
the crossovers are not observed, the potential values in the reverse sweep where the
current density decreases to zero are taken as the equilibrium potential. Therefore,
the equilibrium potential for Sn CV experiment is taken as −0.550 V. The inset in
Fig. 1 represents the similar CV curves but the x-axis is represented as overpotential
axis which is produced by putting the applied voltage values with respect to the
respective equilibrium values for Sn and Co.
A. Gupta and C. Srivastava
and the CV run completed at 0.0 V. This part of CV is henceforth referred as reverse
sweep. In Fig. 1, during the cathodic sweep for the Sn CV (circles), the peak at −
0.660 V is observed. This peak corresponds to reduction of Sn over the GO substrate
according to the reaction
Sn
+2
(aq.) + 2e
−
→ Sn (s)
(1)
After the peak in the cathodic sweep, the observed current density decreases and
then increases at more negative potentials. This continuous increase in current density
corresponds to the hydrogen evolution reaction. In the reverse sweep (anodic sweep),
a peak is observed at −0.309 V which corresponds to oxidation (dissolution) of the
Sn metal that got deposited on the GO substrate.
The Co CV curve (squares) is also presented in Fig. 1 in which the applied potential
was varied between +0.5 and −1.5 V. In the forward sweep, the reduction peak
occurred at −1.292 V. The electrolyte for Co electrodeposition contained 10 mM
CoSO 4 .7H 2 O and 1 M Na 2 SO 4 (anhydrous) at pH 2.5. Under these conditions, the
Co exists in the electrolyte as [Co(H2O) 6 ]
2+ ionic complex. Thus, the peak in the
forward sweep can be associated to the reduction of Co complex [23] into Co metal
over the GO substrate as:
Co(H 2 O) 6
2+ + 2e
−
→ Co + 6H 2 O
( 2 )
As the potential decreases further in the forward sweep, the continuous increase
in current density is observed which corresponds to hydrogen evolution reaction. In
the reverse scan, a current crossover is observed. Two current density peaks were
observed during the reverse sweep: a primary oxidation peak at −0.102 V and a
shoulder peak at +0.018 V. Both the peaks correspond to the oxidation of Co. These
two oxidation peaks correspond to the different allotropic forms of Co (hexagonal
and simple cubic) [24].
To comment on the free energies of nucleation and critical nucleus size, the knowledge of equilibrium potential for metal deposition from the electrolyte is necessary.
If the CV curve exhibits a crossover during the reverse sweep, then the crossover at
algebraically more positive potential values is taken as the equilibrium potential. In
the case of Co CV experiment, the crossovers are observed at −1.036 and −0.650 V.
Therefore, the equilibrium potential is taken as −0.650 V. In the CV cases where
the crossovers are not observed, the potential values in the reverse sweep where the
current density decreases to zero are taken as the equilibrium potential. Therefore,
the equilibrium potential for Sn CV experiment is taken as −0.550 V. The inset in
Fig. 1 represents the similar CV curves but the x-axis is represented as overpotential
axis which is produced by putting the applied voltage values with respect to the
respective equilibrium values for Sn and Co.