Photosensitization of thin SnO2 nanocrystalline semiconductor film electrodes with metallodiporphyrin

Article abstract of DOI:10.1021/jp000140v

Sensitized photocurrent generation is observed with a porphyrin dyad (P_Zn-P) and its structural moieties: 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P) and Zn(II) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P_Zn). The dyes were adsorbed to saturation on a nanocrystalline SnO₂ thin film, employed as working electrode in a photoelectrochemical cell. The metallized and unmetallized moieties possess different singlet state energies and redox properties. In both, solution and adsorbed state, nearly complete singlet-singlet energy transfer from the P_Zn to P has been determined in the dyad. P_Zn is less efficient than P in the photocurrent generation, but is a suitable energy donor in the dyad molecule. The generation of photoelectrical effects by the dyad is less effective in comparison with P. Considering the oxidation potentials of the two moieties in P_Zn-P, a mechanism is proposed where the oxidized metallized porphyrin enhances the back electron-transfer process.

Full text of DOI:10.1021/jp000140v

7644
J. Phys. Chem. B 2000, 104, 7644-7651
Photosensitization of Thin SnO2 Nanocrystalline Semiconductor Film Electrodes with
Metallodiporphyrin
Fernando Fungo, Luis Otero, Edgardo N. Durantini, Juana. J. Silber, and Leonides E. Sereno*
Departamento de Qu´ımica y F´ısica, UniVersidad Nacional de R´ıo Cuarto, Agencia Postal 3,
5800 R´ıo Cuarto, Argentina
ReceiVed: January 11, 2000; In Final Form: March 23, 2000
Sensitized photocurrent generation is observed with a porphyrin dyad (PZn-P) and its structural moieties:
5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P) and Zn(II) 5-(4-carboxyphenyl)-10,15,20-
tris(4-methylphenyl) porphyrin (PZn). The dyes were adsorbed to saturation on a nanocrystalline SnO2 thin
film, employed as working electrode in a photoelectrochemical cell. The metallized and unmetallized moieties
possess different singlet state energies and redox properties. In both, solution and adsorbed state, nearly complete
singlet-singlet energy transfer from the PZn to P has been determined in the dyad. PZn is less efficient than
P in the photocurrent generation, but is a suitable energy donor in the dyad molecule. The generation of
photoelectrical effects by the dyad is less effective in comparison with P. Considering the oxidation potentials
of the two moieties in PZn-P, a mechanism is proposed where the oxidized metallized porphyrin enhances
the back electron-transfer process.
1. Introduction
driving force for excited-state charge injection.24,25 Unfortu-
nately, however, only the first monolayer of the adsorbed dye
produces a significant photoeffect, and the light-harvesting
efficiency of a single monolayer is very low. Nevertheless, in
a porous film of nanometer-sized semiconductor particles, the
effective surface area can be greatly enhanced, producing
substantial light absorption even with only a monolayer of dye
on each particle.26-35 As it has been pointed out,10,11,29 there
are several analogies between the natural photosynthesis and
the dye-sensitized nanocrystalline semiconductor electrodes.
Nature uses a similar method of absorption enhancement by
stacking the chlorophyll-containing thylakoid membranes of the
chloroplast to form the grana structure.10
Spectral sensitization of porous metal oxides by adsorbed
chlorophylls and related porphyrins has been studied using
ZnO,12 SnO2,13,14 and TiO2.10,11 Light harvesting and charge
separation efficiencies in a range of porphyrin dyes adsorbed
on nanostructured TiO2 were found comparable to those in
natural photosynthesis, with a very high (nearly 80%) incident-
photon-to-current efficiency (IPCE).10,11 Similarly, Kamat et al.
reported 15% efficiency for electron injection from adsorbed
chlorophyll a and b into SnO2.13,14 With such high efficiencies,
the photosensitization of oxide semiconductors has become a
practical means for solar energy conversion. Using Ru(II)
bipyridine complexes as photosensitizers, net power conversion
efficiencies up to 10% have been reported in diffuse daylight
on nanoporous TiO2 electrodes.26,36,37
Photoinitiated transmembrane electron transfer to yield long-
lived charge-separated states is the basic energy conversion
process of natural photosynthesis. In the protein environment
of photosynthetic reaction centers it is carried out by a series
of short-range, fast, and efficient electron-transfer steps, which
move electrons and holes to opposite sides of the membrane.
The actual photochemistry involves chlorophyll derivatives,
quinones, and carotenes. Photoinduced electron-transfer involv-
ing cyclic tetrapyrrole molecules (the special pair of bacterio-
chlorophylls) is the primary step in the mechanism of photo-
synthetic bacteria, where singlet-singlet energy transfer between
tetrapyrroles is also important.1 Given the crucial role of
photosynthesis in natural solar energy conversion, it is not
surprising that great effort has been made to mimic this solar
energy harvesting device.2,3
Synthetic multicomponent molecules that mimic both energy
and electron transfer have been reported4 and, in a recent work,
Gust et al. reported an artificial photosynthetic membrane.5 In
this membrane a vectorial photoinduced electron transfer is
produced in a carotene-porphyrin-naphthoquinone (C-P-Q)
molecular triad, which generates a proton pump. The photoge-
nerated C• +-P-Q• - state is coupled to proton translocation
by quinones that alternate between reduced and oxidized forms.5
Like the photosynthetic reaction center, many artificial
photosynthetic assemblies incorporate metalloporphyrins be-
cause they can be reversibly reduced and oxidized.6,7 Chloro-
phyll derivatives and several related porphyrins have been used
as light receptors in energy conversion devices,8-23 often in
achieving spectral sensitization of wide band gap semicon-
ductors.10-19
In this study we have focused on a porphyrin dyad and its
structural components (Figure 1), as photosensitizing adsorbates
on nanocrystalline SnO2 films, to extend the photoresponse of
this large band gap semiconductor. Porphyrin dyad containing
structural moieties with both different singlet state energies and
redox properties can undergo singlet-singlet energy transfer
or photoinduced electron transfer.38-43 Then, it is possible that
an “antenna” effect44 is present in the spectral sensitization of
a wide band-gap semiconductor. The use of a conventional
semiconductor electrode and a simple molecular sensitizer light
The energy difference between the conduction band edge of
an n-type semiconductor film and the oxidation potential of the
excited sensitizer (Eox*) present as an adsorbate provides a
* To whom correspondence should be addressed. E-mail: sereno@
arnet.com.ar
10.1021/jp000140v CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/20/2000

Photosensitization of Nanocrystalline SnO2 Films
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7645
Preparation of SnO2 Nanocrystalline Films. Optically
transparent electrodes were cut from indium tin oxide (ITO)
coated glass plates (1.3 mm thickness, 100 Ω/square) obtained
from Delta Technologies. The ITO slides were cleaned in hot
acetone in a Soxhlet apparatus and then rinsed several times,
first in 2-propanol and then in pure water (obtained from
Labonco equipment model 90901-01) in an ultrasound bath. An
SnO2 colloidal suspension (particle diameter 20-30 Å, Alfa
Chemicals) was used without further purification. A 1.5% SnO2
suspension was prepared by dilution of the commercial com-
pound with water containing a surfactant (0.01% Triton X-100,
Aldrich), and the pH was adjusted to 10 using NH4OH. ITO/
SnO2 electrodes were prepared by coating using Kamat’s
procedure.32 An aliquot of 0.1 mL of the diluted suspension
was spread onto a clean ITO surface with a 3.5 cm2 area,
followed by drying of the electrodes over a warm plate (5 min
at ∼90 °C). Finally, the SnO2 films were annealed at 450 °C
for 1 h. The resulting films, which are transparent in the visible
region, have strong absorption in the UV with an onset at around
355 nm. (This onset absorption corresponds to a bulk band gap
of 3.5 eV.)
The ITO/SnO2 electrodes were modified with P, PZn, and
dyad PZn-P porphyrins (Figure 1) by soaking the ITO/SnO2
films in a saturated petroleum ether solution of the dye for a
period of 2 h. The electrode was then washed with the same
solvent, dried in a nitrogen stream and stored in vials. The
violet-green coloration of the film confirms the adsorption of
the dyes. These electrodes will be referred to as ITO/SnO2/dye
or specifically ITO/SnO2/P; ITO/SnO2/PZn, and ITO/SnO2/
PZn-P for modification with P, PZn, or dyad PZn-P, respec-
tively. A copper wire was connected to the electrode surface
with an indium solder.
Figure 1. Structures of the sensitizing dyes.
absorption is often inefficient at monolayer coverage. One
possibility to improve the overall efficiency is the use of some
type of “antenna-sensitizer” molecular device. In principle, this
device would use intercomponent energy transfer from an
“antenna” chromophoric unit to a specific chromoforic unit that
behaves at the same time as an energy collector and as a charge
injection sensitizer.
Absorption and Emission Spectroscopy. Absorption spectra
were measured with Shimatzu 2020 spectrophotometer. The
fluorescence spectra of porphyrins in dichlomethane (DCM)
solution and on ITO/SnO2 electrodes were collected on a SPEX
Fluoromax instrument.
Electrochemistry. For cyclic voltammetry (CV), a poten-
tiostat-galvanostat EG & G Princeton Applied Research PAR-
273 and a conventional three-compartment Pyrex cell were used.
The working electrode was a Pt disk of 0.25 cm2. CV studies
were carried out in 1,2-dichloroethane (DCE, Merck) containing
0.1 M tetrabutylammonium hexaflourophosphate (TBAHFP,
Aldrich) as the supporting electrolyte. All of the potentials were
referred to a saturated calomel electrode (SCE) and were
corrected for IR drop by a positive feedback technique. Redox
potentials for the first anodic and cathodic charge transfers of
each molecule were calculated using the expression (Ep forward
- Ep backward)/2.
Photoelectrochemical Measurements. Photoelectrochemical
experiments were conducted in aqueous solutions (0.01 M) of
hydroquinone (H2Q, Aldrich, recrystallized from toluene), with
phosphate buffer (pH ) 5.2) prepared from 0.05 M NaH2PO4
(Anedra > 99%) and NaOH (Baker).17 These solutions were
degassed by bubbling with Ar and maintained in the top of the
cell by continuous stream.
The measurements were carried out in a 10 mm quartz
photoelectrochemical cell equipped with Ag/AgCl as the refer-
ence electrode and Pt foil as the auxiliary electrode. A battery-
operated, low-noise potentiostat, constructed in our laboratory,
was used in all photoelectrochemical measurements. The output
signal of the photocurrent was recorded on a Radiomiter-
Copenaghe X-t recorder and the same recorder was used for
measuring the generated photopotentials. Action spectra for ITO/
SnO2/dye electrodes were obtained by sending the output of a
2. Experimental Section
Synthesis of Porphyrine Dyes. The structure of the porphy-
rins used are shown in Figure 1. They were prepared using the
dipyrromethane method.45-47 The syntheses of 5-(4-carbomethox-
yphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P-ester),
5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin
(P), Zn(II) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl)
porphyrin (PZn), and 5-(4-acetamidophenyl)-10,15,20-tris(4-
methylphenyl) porphyrin (P-amide) have been previously
reported.46-50 P-ester was prepared by the reaction of 4-meth-
ylbenzaldehyde, 4-carboxymethylbenzaldehyde, and meso-(p-
tolyl) dipyrrometane in CHCl3. The mixture was catalyzed by
BF3‚O(Et)2 at room temperature. The chlorine compounds were
oxidized to the corresponding porphyrins with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ). P-ester was purified by flash
chromatography and basic hydrolysis of the P-ester afforded
the desired P compound.47
PZn was formed by the reaction between P and Zn(II) acetate
in methanol/dichloromethane. Flash chromatography yielded the
47
pure PZn
.
P-amide was prepared as described above for P, using
4-methylbenzaldehyde, 4-acetamidobenzaldehyde, and meso-
(p-tolyl)dipyrrometane. Flash column chromatography afforded
the amide porphyrin, which was hydrolyzed in basic medium
to yield 5-(4-aminophenyl)-10,15,20-tris(4-methylphenyl) por-
phyrin (P-amine).47
PZn-P dyad was synthesized from the acid chloride derivative
of PZn and P-amine in a mixture of toluene and pyridine at
room temperature. Flash column chromatography afforded the
pure dyad.
Spectroscopic data of the porphyrin derivatives coincide with
those previously reported.47

7646 J. Phys. Chem. B, Vol. 104, No. 32, 2000
Fungo et al.
150 W high-pressure Xe lamp (Photon Technology Instrument,
PTI) through a PTI high-intensity grating monochromator (∆λ
) 5 nm) and recording the resulting steady-state photocurrent.
The electrodes were located at the focus of monochromator
output (illuminated area: 1 cm2). All of the photoelectrochemi-
cal measurements were done in front face configuration. The
incident light intensities at different wavelengths were measured
with a Coherent Laser-Mate Q radiometer (sensitivity 1 µW).
3. Results and Discussion
Photochemistry and Electrochemistry in Solution. The
dyad P-PZn contains structural moieties with both different
singlet state energies and redox properties. Therefore, it is
possible that an intramolecular singlet-singlet energy transfer
or photoinduced electron transfer can occur after excitation.
These events can also be present when the dyad is in adsorbed
state, acting as spectral sensitizer of a wide band gap semicon-
ductor. To assess these effects, the photochemistry and the
electrochemistry of the three molecules were studied.
Investigation of the photochemistry of the monomers and
dyad molecules was done by steady-state absorption and
emission spectroscopy. The two porphyrin moieties in the dyad
molecule are closely related, except that the central hydrogen
atoms in the porphyrin bearing the acid group have been
replaced by zinc. The absorption spectra of these compounds
are shown in Figure 2. As can be observed, the spectrum of the
dyad (Figure 2c) features four Q-band maxima at 515, 551, 592,
and 648 nm, whereas the Soret band appears as a single
maximum at 418 nm. Consequently, the absorption spectrum
is identical to a linear combination of the spectra of the
component chromophores (Figure 2a, b), within experimental
error.38,46,47,51 Thus, linkage of the chromophores does not lead
to any significant changes in absorption band wavelengths,
widths, or relative extinction coefficients.
On the other hand, in the emission spectra the situation
changes. Figure 3 shows the corrected fluorescence spectra of
the monomers units and the dyad in DCM solution. The PZn
has maxima at 598 and 646 nm, whereas P has two maxima at
652 and 717 nm. The emission spectra of the dyad PZn-P
resemble that of the unmetallized porphyrin, with no significant
emission from the PZn moiety, even though both porphyrin
species absorb at the excitation wavelength (550 nm). These
results suggest efficient and rapid singlet-singlet energy transfer
from PZn to the free base.38,42
Analyzing the possible photophysic mechanisms, the energies
of the porphyrins’ first excited singlet states can be calculated
from the average of the frequencies of the longest wavelength
absorption maxima and the shortest wavelength emission
maxima. There is a significant energy gap between the PZn first
excited singlet state at 2.09 eV and that of P at 1.9 eV. This
means that the singlet energy transfer from the zinc porphyrin
to the free base is an energetically favorable process.
On the other hand, it has been reported51 that the reduction
and oxidation potentials of the dimeric porphyrins are similar
to those of the combined individual monomeric units, hence
the energy of the charge separated state in PZn+•-P-• has been
estimated from the first oxidation potential of PZn-ester and
the first reduction potential of P-amide (Table 1) as determined
from CV measurements. Typical results for P-amide are shown
in Figure 4.
As shown in Scheme 1, the PZn+•-P-• state lies at about
0.10 eV below PZn first excited singlet state (1PZn-P). However,
fluorescence excitation experiments reveal that singlet-singlet
energy transfer from PZn to the P moiety is essentially quantita-
Figure 2. Absorption spectra of porphyrins: in solution of DCM (s)
and in adsorbed state over ITO/SnO2/electrodes (- - -). (a) P; (b) PZn
,
and (c) PZn-P dyad. Absorbances have been normalized to one.
tive (Figure 5). When fluorescence is monitored at 717 nm
(where only P emits), it can be seen that absorption and

Photosensitization of Nanocrystalline SnO2 Films
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7647
Figure 5. Corrected fluorescence excitation (s) and absorption (• • •)
spectra of PZn-P in DCM solution. The emission was measured at
717 nm. Both spectra were normalized at 515 nm.
Figure 3. Corrected Fluorescence emission spectrum of: P (s), PZn
(- - -), and PZn-P dyad (• • •) in DCM, λex ) 550 nm. The spectra
have been normalized to one.
SCHEME 1
TABLE 1: Photoelectrochemical Properties of Dyes
a
a
dye
Eox Ered
SEb Eox* c ∆Ed IPCE %e LHE % Φinjηc
P
PZn
PZn-P
0.97 -1.18 1.90 -0.93 0.73 32 (24)
92
55
87
0.34
0.11
0.12
0.82
2.09 -1.27 1.07
6 (4)
1.90
11 (8)
PZn-esther 0.79 -1.27 2.09
P-amide 0.91 -1.19 1.90
a Ground-state oxidation and reduction potential (V vs SCE). b First
excited singlet state (eV). c Oxidation potential of excited dyes (V vs
SCE). d Driving force for photoinduced electron transfer ∆E) -e(Eox*
- EFB) (eV). e Maximum IPCE at soret bands under bias voltage )
0.1 V. In parentheses values without bias.
where quantitative singlet-singlet energy transfer was ob-
served.38,52
Properties of the ITO/SnO2/Dye Electrodes. Absorption
and Emission Spectra. Porphyrin adsorption onto the semi-
conductor produces strong violet-green coloration of the film.
The electrodes have absorbance values between 0.6 and 1 at
the absorption maximum (Soret band). Thus, the porosity of
the film allows high harvesting of the dye molecules onto the
semiconductor surface. The amount of the dyes adsorbed was
determined by soaking the dye-loaded electrodes in a known
volume of DCM for at least 2 h, after which the UV-visible
absorption spectrum of the resulting solution was obtained. For
ITO/SnO2/P, an average surface loading of ≈ 4 × 10-9 mol
cm-2 was obtained.
The electronic spectra of the studied compounds on the ITO/
SnO2/dye electrode and in DCM solution are shown in Figure
2. In the adsorbed films, the Soret and Q-bands show electronic
transitions similar to those observed for monomeric absorption
in DCM. However, on the electrodes the bands are broader and
shifted in comparison with those in solution. This may be caused
by the interaction of the porphyrins with the polar surface of
the electrode, as well as the possible formation of porphyrin
aggregates due to its high concentration onto the electrode.
Figure 4. Cyclic voltammogram at a platinum electrode (A ) 0.25
cm2) of P-amide 0.48 mM in DCE containing 0.1 M TBAHFP as
supporting electrolyte. Sweep rate ) 0.100 V/s.
fluorescence excitation spectra are essentially identical in the
500-700 nm range. This indicates that singlet-singlet energy
transfer is nearly complete.38 Thus, no matter which porphyrin
moiety is excited in the PZn-P dyad, always the excited singlet
state of P is formed. These results are in complete agreement
with those obtained for other structurally related porphyrins,
The emission spectrum of an ITO/SnO2/PZn-P electrode also
resembles that of the P porphyrin, with no significant emission

7648 J. Phys. Chem. B, Vol. 104, No. 32, 2000
Fungo et al.
Figure 8. Energy level diagram for the spectral sensitization of ITO/
SnO2/P electrode.
aqueous hydroquinone are thus consistent with the formation
of a light-filtering polymer that slowly coats the electrode
surface, producing a continuos drop in photocurrent.
Figure 6. Corrected fluorescence emission spectrum of ITO/SnO2/
PZn-P. λex ) 550 nm.
A photovoltage of ∼0.12 V was achieved following the
excitation of an ITO/SnO2/P electrode (λex) 420 nm, incident
intensity ) 1.55 mW cm-2), as is shown in Figure 7b. The
photovoltage observed in the absence of dye was negligible.
This confirmed that the photovoltage generation was entirely
initiated by the excitation of the dye.
Figure 8 shows a schematic illustration of a possible mech-
anism for the generation of anodic photocurrent in an ITO/
SnO2/P electrode in contact with aqueous H2Q. After production
of the excited state by photon absorption (hν), several pathways
are possible: (1) radiative relaxation, (2) thermal (nonradiative)
relaxation , or (3) interfacial charge injection. When the latter
occurs, the injected electron hops from particle to particle until
it ultimately reaches the ITO layer, producing observable
photocurrent. Although, the efficiency for charge injection is
adversely influenced by charge recombination (4) at the dye-
film interface, it can be partially suppressed in the presence of
the sacrificial reductant (in this case H2Q).
Effect of the H2Q Concentration. The variation of the
concentration of the sacrificial donor (H2Q) present in the
electrolyte affects the efficiency of photocurrent generation, as
shown in Figure 9 for an ITO/SnO2/P electrode. The photo-
current is maxima at H2Q ) 0.01 M. When H2Q is absent, the
anodic photocurrent is small and a cathodic current is observed
when the light is turned off, as the oxidized sensitizer retraps
photoinjected electrons. In the presence of H2Q, a higher
stationary anodic photocurrent is observed during illumination
and the cathodic current is suppressed. This observation is in
agreement with other dye-sensitized oxide electrodes.17 Similar
results were obtained with the PZn and PZn-P dyad.
Figure 7. On-off illumination cycles of (a) photocurrent and (b)
photovoltage at an ITO/SnO2/P electrode. Excitation wavelength λ )
425 nm, Iinc ) 1.55 mW cm-2 (photocurrent was obtained at 0.2 anodic
bias vs Ag/AgCl).
from the PZn moiety at the excitation wavelength of 550 nm
(Figure 6). This result suggests that singlet-singlet energy
transfer from the PZn to P occurs also when the dye is in
adsorbed state.
Dependence of the Photocurrent on the Applied Potential.
The effect of an applied potential on the observed photocurrent
generation is shown in Figure 10 for an ITO/SnO2/P electrode.
The observed photocurrent increases with anodic bias in every
case, which could be due to the increase in the collection
efficiency of the injected electrons. It has been reported that
the application of an external electrical bias to a dye-sensitized
nanocrystaline TiO2 film can result in a rapid acceleration of
the rate of charge recombination process, while the electron
injection kinetics are rather insensitive to it.54-56 When the
excited sensitizer injects electrons into the SnO2 particle, the
trapped electrons hop through the film to the ITO conducting
Photoelectrochemistry. The ITO/SnO2/dye electrodes are
photoelectrochemically active. Anodic photocurrents and pho-
topotentials are observed upon excitation of these electrodes
with visible light, indicating that the electrons flow from the
illuminated electrode to the ITO contact. A typical result for
ITO/SnO2/P electrode is shown in Figure 7. The photocurrent
is reproducible under repeated on-off illumination cycles and
decays by ∼15% over 2 h during continuous operation of the
cell. 1,4-Benzoquinone, formed by oxidation of H2Q in water,
has been reported to convert photochemically to 2-hydroxy-
1,4-benzoquinone, which in turn polymerizes to give a brownish-
black polymer.53 The decreased photocurrents observed with

Photosensitization of Nanocrystalline SnO2 Films
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7649
transfer from the dye to the semiconductor is the energy gap,
defined as ∆E) -e(Eox* - EFB),59 where Eox* is the oxidation
potential of the excited dye (estimate subtracting the excitation
energy from the redox potentials of the molecule in the ground
state24,25), EFB is the flat band potential of the semiconductor
(∼ -0.2 V vs SCE),32 and e is the electronic charge. The Eox
and Eox* potentials are shown in Table 1 for the monomers
that are present in the PZn-P dyad. The more negative Eox*
value for the metallized porphyrin should facilitate the electron
transfer from the excited state of the dye to the semiconductor.
As it has been observed, free-base tetracarboxyphenil porphyrin
exhibit 5- to 8-fold slower charge recombination kinetics than
the analogue Zn metallized porphyrin, when they are used in
the spectral sensitization of TiO2 nanostructured electrodes.55,56
On the other hand, the product Φinjηc, in the PZn-P molecule
is very similar to that of the PZn. Thus, a possible reason for
the low photocurrent efficiency in the dyad is the contribution
of PZn moiety to the back electron-transfer process.55,56 In the
case of other SnO2/chlorophyll13,14 and SnO2/Zn-porphyrin17
electrodes, the back electron-transfer process showed to be the
major limiting factor in achieving high IPCE values, producing
low efficiency of photon-to-photocurrent conversion.13,34,60
Consequently, it is proposed that in the dyad molecule the
oxidized state of the PZn moiety acts as fast electron acceptor
in the back electron transfer. The complete mechanism is
summarized in eqs 3-10
Figure 9. Photocurrent of an ITO/SnO2/P electrode as function of the
H2Q concentration. All the other conditions for photocurrents measure-
ments were the same as in Figure 7.
surface being the driving force the electric field produced by
the applied bias. With a cathodic bias, charge recombination
competes with charge injection.13,34 Upon illumination of the
electrodes under cathodic bias (i.e., < -200 mV), a decay of
anodic photocurrent and a growth of cathodic current upon
termination of illumination could be observed. The growth of
cathodic current arises from a back electron transfer from
reduced SnO2 nanoparticles to the photooxidized adsorbed dye.57
Photocurrent Action Spectra. The action spectrum (mea-
sured under 0.1 V positive bias) closely matches the absorption
spectrum of ITO/SnO2/dye in all cases, see Figure 11. The
incident-photon-to-photocurrent efficiencies (IPCE) were evalu-
ated from eq 1.27,32,58
PZn-P
PZn-P
9
hν8 PZn-P1
hν8 P1Zn-P
(3)
(4)
9
ki
9
P1Zn-P
8 PZn-P1
(5)
(6)
(7)
P1Zn-P + SnO2 f PZn+•-P + SnO2 (e)
PZn-P1 + SnO2 f PZn-P+• + SnO2 (e)
IPCE (%) ) 100 (isc1240)/(Iinc λ)
(1)
where isc is the short circuit photocurrent (A cm-2), Iinc is the
incident light intensity (W cm-2), and λ is the excitation
wavelength (nm). The close correspondence between the pho-
tocurrent action spectrum and the absorption spectrum of the
electrodes confirms that photosensitization has successfully
extended the photocurrent response of the electrodes into the
visible region.
ket
PZn-P+• 98 PZn+•-P
(8)
kb1
P+•Zn-P + SnO2 (e)
PZn-P+• + SnO2 (e)
98 PZn-P + SnO2
(9)
kb2
98 PZn-P + SnO2
(10)
Maximum IPCE of ∼6%, 32%, and 11% were obtained at
the Soret band of ITO/SnO2/PZn, ITO/SnO2/P, and ITO/SnO2/
PZn-P, respectively, by using [H2Q] ) 0.01 M and bias voltage
) 0.1 V. There are several reasons for this difference, and they
may be linked to the factors that determine IPCE, namely, the
light harvesting efficiency (LHE) of the dye, the charge injection
yield (Φinj) from the excited dye to the semiconductor, and the
charge collection efficiency, ηc, of the system. The IPCE in
terms of these parameters can be written27,35 as:
Both porphyrin moieties in dyad can be excited by light
absorption (eqs 3 and 4), but the lack of fluorescence from the
metallized unit (both in solution and in adsorbed state) indicates
that energy transfer (eq 5) can compete with the electron transfer
process to the semiconductor (eq 6). Thus, it is probable that
most of the photocurrent is generated from the excited state of
the free base moiety (eq 7). The heterogeneous electron transfers
(eqs 6 and 7) indicate the formation of two different radical
cations, (P+•Zn-P and PZn-P+•), but the electron-transfer process
from the zinc porphyrin to the oxidized free base (eq 8) is an
exergonic process, as it is inferred from the 0.12 V difference
in the oxidation potentials (estimated from the first oxidation
potential of PZn-ester and P-amide, Table 1). Hence, it is
proposed that the presence of the oxidized metallized porphyrin
enhances the back electron-transfer process55,56 (eq 9), producing
lower photocurrent yields in the dyad molecule than in P.
Moreover, the photocurrent generation efficiency for the
IPCE % ) LHE (%) Φinj ηc
(2)
Table 1 summarizes the results of IPCE, LHE, and the product
Φinjηc for the three dyes at the Soret band. As it can be observed,
PZn gives lower photocurrent efficiency than the unmetallized
porphyrin, despite that the driving force for charge injection is
higher in the metallized one. For a dye adsorbed on a
semiconductor the driving force for photoinduced electron

7650 J. Phys. Chem. B, Vol. 104, No. 32, 2000
Fungo et al.
Figure 10. Dependence of incident photon-to-current efficiency IPCE
(%) on applied bias for ITO/SnO2/P. Electrode was excited at absorption
maximum of the dye, 425 nm.
ITO/SnO2/PZn-P electrode, taken at the Q-band where only
the free base moiety absorbs (i.e., 650 nm), is always around
three times lower than in the ITO/SnO2/P electrode. If there
were no effects of the presence of PZn moiety in the dyad, both
photocurrents should be near the same. On the other hand, an
effect resulting from the orientation of the dyad over the SnO2
surface (i.e., whether it is the P moiety or the PZn moiety that
gets attached to SnO2 surface) might also play a role in
determining the IPCE of the dyad, and cannot be rejected.
4. Conclusions
The phophyrin dyad molecule and its moieties have been used
in the spectral sensitization of wide band gap semiconductor
colloidal particles. The SnO2 film modified with porphyrin dyes
exhibits photoresponse in the visible region. The close match
between the photocurrent action spectrum and the absorption
spectrum shows that the photosensitization mechanism is
operative in extending the photocurrent response of ITO/SnO2/
dye electrode to the visible.
Nearly complete singlet-singlet energy transfer from the PZn
to P has been observed in the PZn-P dyad, even when the dye
is in the adsorbed state. Moreover, the photocurrent efficiency
in the dyad and PZn are lower than the P moiety, although the
Eox* value is more negative for the metallized porphyrin, which
should facilitate the electron transfer from the excited state of
the dye to the semiconductor. Consequently PZn is less efficient
than P in the heterogeneous charge-transfer process, but is a
suitable energy donor, acting as “antenna” in the dyad molecule.
The fact that in the generation of photoelectrical effects the dyad
is less effective in comparison with P is explained considering
that the metallized porphyrin enhances the back electron-transfer
process, producing low photocurrent yield.
Note Added in Proof. When this manuscript was in the
publication process, a work was published by Koehorst, R. B.
M.; Boschloo, G. K.; Savenije, T. J.; Goosens, A.; Schaafsma,
T. J. J. Phys. Chem. B 2000, 104, 2371. This paper deals with
the spectral sensitization of TiO2 substrates by porphyrin
heterodimers. Monolayers of these dimers enhance the spectral
response of the semiconductor with respect to the monomeric
sensitizer by intramolecular energy transfer.
Figure 11. Absorption spectra (s) and photocurrent action spectra,
IPCE (O) of ITO/SnO2/dye electrodes modified with (a) P, (b) PZn
,
and (c) PZn-P dyad. All other conditions for photocurrent measure-
ments were the same as in Figure 7.

Photosensitization of Nanocrystalline SnO2 Films
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7651
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Acknowledgment. We are grateful to Consejo Nacional de
Investigaciones Cient´ıficas y Te´cnicas (CONICET-Argentina);
Agencia Nacional de Promocio´n Cient´ıfica y Tecnolo´gica
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