Chemistry Reference
In-Depth Information
IVa
10
I
(
m
A)
1-11
5
1-11
0
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0
0.2
,2
0.4
0.6
1-11
-5
E
(V) vs. SCE
1-11
-10
-15
1-11
-20
7.2
Current-potential curves recorded at a gold-disc electrode in a pH
=
12 buffer solution in the absence (curve 1) and presence (curves
2-7) of 8.07
¥
10
-
3
mol l
-
1
Co(II)TSPc at a scan rate of 50 mV s
-
1
and a
temperature of 298.0 K as a function of scan number. Scan
numbers are (1) 20, (2) 30, (3) 40, (4) 50, (5) 60, (6) 70, (7) 80, (8)
90, (9) 100, (10) 110 and (11) 120. (Reprinted from
Journal of
Electroanalytical Chemistry
, Vol 567 No 2, De Wael et al, 'Study of
the deposition of . . .', pp 167-73 (2004), with permission from
Elsevier.)
charge-transfer kinetics are faster at gold than at a Co(II)TSPc-modified
gold electrode. As the coverage of the gold electrode increases, the kinet-
ics of Co(II)TSPc reduction slow down, which results in the observed shift
of the reduction wave. In addition, note that the peak current decreases
with scan number - an additional indication that changes in charge-
transfer kinetics are occurring.
Figure 7.2 shows curves obtained during the following 100 scans, from scan
20 (curve 1) to scan 120 (curve 11). It can be seen that both oxidation peaks
(II
a
and III
a
) and reduction peak (II
c
), and the associated charges, decrease
with increasing scan number, with a maximum decline of 27%.This indicates
that Co(II)TSPc initially adsorbed at the electrode surface is again released.
A possible explanation is a reorganisation of the deposited layer(s) to obtain
a thermodynamically more stable condition. At the same time, a third oxi-
dation peak (IV
a
) is observed which increases with scan number. Its peak
potential is initially dependent on scan number, but eventually the peak
potential becomes independent of scan number. This can be explained by
the fact that, initially, at lower scan numbers, there is an overlap between
peaks III
a
and IV
a
. As soon as there is no more interference from peak III
a
,
