tors in the solution, leading to the enhancement of cathodic
photocurrents.12
Second, to investigate the electron transfer between the
excited aggregates and the electron acceptors or donor in the
solution, the e†ect of a typical electron acceptor, methyl-
viologen (MV2`), on the photocurrent generation from the
Ðlms was investigated. The cathodic photocurrents for all the
Ðlms increased with the addition of MV2` to the solution
(shown in Table 1), because the strong electron acceptor
MV2`12 could accept electrons from the Ðlm and accelerate
the rate of electron transfer from the Ðlm to the solution,
leading to the enhancement of the cathodic photocurrents. As
an example, Fig. 7 shows the dependence of the photocurrent
for the Id Ðlm on the concentration of MV2` in solution.
The e†ect of oxygen on the electron transfer process was
also investigated. When O was removed from the solution by
Fig. 7 Relationship between MV2` concentration and photo-
currents under ambient conditions (113 mW cm~2 white light, 0.5 M
KCl solution) for Ðlm Id.
2
bubbling N , the cathodic photocurrents decreased gradually
2
and reached stable values until saturation of N in the solu-
2
tion, because it could act as an electron acceptor to form a
superoxide anion radical,12 and accelerate the electron trans-
mated 498 nm light with an intensity of 6.14 ] 1015 photon
cm~2 s~1, the efficiencies of photoelectric conversion are 0.03,
0.05, 0.06 and 0.1% for Ðlms Ia, Ib, Ic and Id, respectively.
Compared with Ðlm Ia, the quantum efficiencies can be
enhanced about 1.6, 2.0 and 3.3 times for Ib, Ic and Id, respec-
tively, by just changing the component of the subphase from
H O to CuCl , ZnCl and HgCl respectively.
fer from the Ðlm to the solution. When H Q was added to the
2
solution, the cathodic photocurrents for the four Ðlms
decreased so quickly that they became anodic photocurrents
(shown in Table 1). The experimental results indicated that the
presence of a strong electron donor is unfavorable to cathodic
photocurrent generation.12
2
2
2
2
Mechanism of photoinduced electron transfer in the Ðlm
modiÐed ITO electrodes
Some factors in photoinduced electron transfer
The observed cathodic photocurrents indicate that the photo-
generated electrons Ñow from the Ðlms to solution. In order to
explore this process in detail, the di†erent factors a†ecting
these processes were investigated as follows.
It is known that photoinduced electron transfer involves elec-
tron transfer within the di†erent energy levels.12h14 In order
to elucidate the detailed mechanism of electron transfer under
the di†erent conditions, cyclic voltammetric studies (sweep
rate \ 100 mV s~1) were carried out in neutral 0.1 M KCl
solution to estimate the redox potentials of the Ðlms on the
ITO electrode. For example, it can be seen from Fig. 8 that
there is no peak in the range 0È1.2 V for the blank ITO elec-
trode, but an irreversible oxidation potential peak was
observed when Ðlm Ib modiÐed the ITO electrode. Under the
same conditions, the oxidation potentials of the dyeÈdye`
couples for the Ðlms are 0.798 V (IaÈIa`), 0.828 V (IbÈIb`),
0.858 (IcÈIc`) and 0.903 V (IdÈId`) vs. SCE (shown in Table
2). Hence the HOMO energy levels available for donating
electron are estimated to be [5.54, [5.57, [5.60 and [5.64
eV for Ðlms Ia, Ib, Ic and Id, respectively. The band gaps are
518 nm (2.39 eV) for all Ðlms and the LUMO energy levels
available for accepting electrons can be obtained as [3.15,
[3.18, [3.21 and [3.25 eV for Ia, Ib, Ic and Id, respectively,
First, to study electron transfer between the ITO electrode
and LB Ðlms, the e†ect of bias voltage was investigated. All
the cathodic photocurrents increased with the potential of the
working electrode more negative than the SCE electrode for
all the Ðlms (shown in Table 1), and vice versa, indicating that
photocurrents Ñow in the same direction as the applied nega-
tive voltage. Such a negative voltage on the ITO electrode can
form a strong electric Ðeld within the LB Ðlms (ca. 3 nm),
which can accelerate electron transfer from the ITO electrodes
to the holes in the aggregates, and then from Ðlms to accep-
on the absolute scale. The conduction band E and valence
c
band E of the ITO electrode are known to be ca. [4.5 and
v
[8.3 eV on an absolute scale, respectively.12 The reduction
potential of MV2` is about [4.51 eV, and the oxidation
potential of H Q is [4.61 eV on the absolute scale.12 Thus a
2
mechanism could be proposed as shown in the Scheme 2.
When the dye aggregates are excited, the direction of photo-
currents is dependent not only on the excited dye aggregates,
but also on the nature of the redox couples around the elec-
trode in the solution. For example, a cathodic photocurrent is
generated through the following process: the dye aggregates
Fig. 8 Cyclic voltammetric graphs of the blank ITO and Ib Ðlm-
modiÐed ITO electrode (0.1 M KCl, sweep speed \ 100 mV s~1).
Table 2 HOMO and LUMO energy levels (eV) for Ðlms Ia, Ib, Ic and Id
Energy level
Iaa
Ibb
Icc
Idd
ITOe
HOMO
LUMO
Band gap
[5.54
[3.15
[5.57
[3.18
[5.60
[3.21
[5.64
[3.25
[8.3
[4.5
2.39(518 nm)
2.39(518 nm)
2.39(518 nm)
2.39(518 nm)
3.82(325 nm)
a Ia: Ðlm fabricated from pure water. b Ib: Ðlm fabricated from 10~5 M CuCl subphase. c Ic: Ðlm fabricated from 10~5 M ZnCl subphase. d Id:
2
2
Ðlm fabricated from 10~5 M HgCl subphase. The energy level of the dyes were obtained from the oxidation potentials from the cyclic voltam-
2
metry of the dye monolayers in 0.1 M KCl solution (sweep rate \ 100 mV s~1). e Values cited from ref. 12.
3052
Phys. Chem. Chem. Phys., 2000, 2, 3049È3053