Chemistry Reference
In-Depth Information
the peak potential of peak IV
a
becomes independent of scan number.
However, the magnitude of this peak is too large to be explained by oxida-
tion of adsorbed Co(II)TSPc only. Therefore, it is believed that it corre-
sponds to the electrocatalytic oxidation of Co(II)TSPc, because when the
modified electrode is put into a blank pH 12 solution, this peak disappears.
The small increase of the reduction peak (I
c
,Fig. 7.2) is probably due to the
return peak corresponding with the newly formed oxidation peak, IV
a
.The
fact that the onset of peak I
c
is shifted to more positive potentials reveals
that, for scan numbers higher than 20, I
c
is the result of two overlapping
peaks.
Another observation in Fig. 7.2 is that the reduction peak (III
c
) increases
markedly, combined with a shift of peak potential towards less negative
potentials. It can be seen that, simultaneously with the increase of peak III
c
,
the fourth peak (IV
c
) decreases, which in fact is the opposite effect of that
observed in the first 20 scans. This confirms the interrelationship between
the increase/decrease of peaks III
c
and IV
c
.With continued scanning, even-
tually reduction peak IV
c
disappears completely. For all data obtained in
Fig. 7.1 and Fig. 7.2, it should be noted that the same potential region was
used. The reason for this is that, by using smaller potential regions, no elec-
trodeposition of Co(II)TSPc at gold was observed. By scanning only in the
positive or negative region, no marked immobilisation of Co(II)TSPc
species was observed, nor when potential regions smaller than those shown
in Fig. 7.1 and Fig. 7.2 were used. This indicates that both the negative and
positive regions are important in the electrodeposition reaction. In addi-
tion, it was observed that if the whole potential window was used and the
potential was first varied from 0 to -1.2 V vs. SCE, no changes were
observed in the negative region. When the potential was first varied from
0 to 0.6 V and then to -1.2 V vs. SCE, peaks II
c
and III
c
started to occur.
This means that the first adsorption of Co(II)TSPc occurs in the positive
region (probably because in this region, the electrode behaves as an anode
and Co(II)TSPc is negatively charged), while a stronger immobilisation is
obtained in the negative region.
7.2.2
Scan-rate study
In order to find out whether a peak is due to adsorption of species or to a
diffusion-controlled reaction of species, the influence of the peak current
on the scan rate was investigated. If a slope of 0.5 was found between log
I
p
and log
v
, the reaction corresponded to diffusion of Co(II)TSPc towards
the electrode surface, while for a slope equal to 1 the reaction corresponded
to oxidation or reduction of Co(II)TSPc adsorbed at the electrode surface.
It was found that peaks II
a
, III
a
,I
c
and II
c
correspond to adsorbed
Co(II)TSPc, while peaks III
c
and IV
c
are attributed to diffusion of
