Chemistry Reference
In-Depth Information
reduction peaks are observed: the first one at -0.41 V vs. SCE (II
c
) and a
third peak at -1.15 V vs. SCE (IV
c
) grow with increasing scan number; the
second reduction peak around -0.8 V vs. SCE (III
c
) decreases, and its peak
maximum shifts towards more negative potentials.
The increase of the oxidation peaks, II
a
and III
a
, and the reduction peaks,
II
c
and IV
c
, and the decrease of the gold oxide reduction peak, I
c
, continue
until scan 20. Continuation of the scanning beyond scan 20 results in dif-
ferent behaviour. It is remarkable for scan 20 that the charge related to the
two oxidation waves (II
a
and III
a
) and the charge related to reduction wave
II
c
is almost the same and equal to 4.1 ± 0.3 mC. For peak I
c
,a charge of
5.0 ± 0.2 mC is obtained. Assuming this is due to gold oxide reduction, it
would correspond to a Co(II)TSPc coverage of 70%, which is somewhat
low and rather unexpected. Therefore, it is presumed that peak I
c
at scan
20 is not only due to a fraction of the gold surface that is still uncovered
and able to be oxidised and reduced, but also that this peak is the return
peak of the second oxidation peak (III
a
). An additional indication that peak
I
c
in scan 20 is not exclusively associated with gold oxide reduction can be
found in the shift of the peak maximum towards less positive potentials
with increasing scan number, despite the fact that the peak intensity is
decreasing. An opposite effect would be expected if peak I
c
in scan 20 was
due to gold oxide reduction only.
Taking account of the charge associated with peak III
a
, this implies that
about 1 mC of the 5 mC of peak I
c
measured in scan 20 is still due to gold
oxide reduction, corresponding to 95% CoTSPc coverage. Therefore, it can
be concluded that peaks I
c
and III
a
in scan 20 are related to the same
reversibly behaved redox system. Reversibility can be proven by the fact
that the half-wave potentials for both waves are the same, and that the peak
potential for the anodic wave (III
a
) does not shift with scan number.
It is also important to note that for peak III
c
, the peak potential shifts
towards more negative potentials without a decrease in peak current in the
first scans. Only after a few scans does the peak height start to decrease
and, simultaneously, peak IV
c
starts to grow. This indicates that the decrease
of peak III
c
and increase of peak IV
c
are related to each other. The increase
with scan number of the previously mentioned peaks is due to the deposi-
tion of Co(II)TSPc at the gold surface. With each scan, the fraction of
deposited Co(II)TSPc increases, which results in higher peaks because
more adsorbed Co(II)TSPc is present at the surface in the next scan. This
feature also explains the gradual decay of peak I
c
. Covering the gold surface
with Co(II)TSPc prevents its oxidation and reduction, and therefore the
gold oxide formation and reduction peaks (I
a
and I
c
) decrease.
The second reduction peak due to CoTSPc (III
c
) is reasonably large and
shifts towards more negative potentials with scan number. This shift also
ends with scan 20. Therefore, it is presumed that this peak is to be attrib-
uted to the reduction of Co(II)TSPc in solution (not adsorbed) and that the
