Preparation and Photoelectrochemical Characterization
of Porphyrin-Sensitized -Fe
2
O
3
Thin Films
Francisco V. Herrera,
a
Paula Grez,
a
Ricardo Schrebler,
a,z
Luis A. Ballesteros,
a
Eduardo Muñoz,
a
Ricardo Córdova,
a,
*
Hernán Altamirano,
a
and
Enrique A. Dalchiele
b,
*
a
Instituto de Química, Facultad de Ciencias, Pontificia Universidad Católica de Valparaíso, Casilla 4059,
Valparaíso, Chile
b
Instituto de Física, Facultad de Ingeniería, 11000 Montevideo, Uruguay
The photoelectrochemical response of electrodeposited hematite -Fe
2
O
3
thin films, whose surface has been sensitized by the
adsorption of 5,10,15,20-tetrakis4-carboxyphenylporphyrin TCPP, was studied. The -Fe
2
O
3
thin films were obtained by the
annealing of an electrodeposited -FeOOH precursor layer. The structural and morphological characteristics of the resulting
hematite films were studied by X-ray diffraction, scanning electron microscopy, and atomic force microscopy techniques. The
semiconducting characteristics of unmodified and sensitized TCPP hematite films were determined by electrochemical impedance
measurements and by Mott–Schottky analysis. The hematite films exhibited an n-type behavior and a donor carrier concentration
N
D
=3 10
17
cm
-3
for both unmodified and TCPP-sensitized ones. However, it was observed that the presence of TCPP shifts
the energy of the surface states to higher values. The photoelectrochemical response of the unmodified and sensitized -Fe
2
O
3
electrodes was followed by means of photovoltammetry and photocurrent–time transient techniques in a 0.1 M NaOH
+ 0.1 M KI solution under 25 mW/cm
2
white light illumination. The results showed that the photocurrent response exhibited by
the TCPP-sensitized hematite films was 70% higher than the unmodified ones.
© 2010 The Electrochemical Society. DOI: 10.1149/1.3357257 All rights reserved.
Manuscript submitted December 11, 2009; revised manuscript received February 10, 2010. Published April 9, 2010.
In the past years, several studies have been carried out showing
that hematite -Fe
2
O
3
is a promising semiconducting material for
photoelectrochemical and photocatalysis applications due to its sta-
bility, abundance, and environmental compatibility, as well as con-
venient bandgap and position of the valence band.
1-10
Iron oxide
satisfies most of the requirements to be a very good material for
solar energy conversion because its bandgap value allows harvesting
up to 40% of the incident solar radiation. Despite the good proper-
ties presented by -Fe
2
O
3
, several problems must be solved to con-
vert this material into an optimal one for certain applications. For
instance, one of these problems corresponds to the short length of
diffusion that presents the photogenerated holes when this semicon-
ductor is illuminated 20 nm
11
and 2–4 nm
12
. In fact, the poor
efficiency exhibited by -Fe
2
O
3
photoanodes that have been em-
ployed for water oxidation has been attributed to this situation.
11
This is because the light can reach a length of penetration of ca. 100
nm = 1.6 10
7
m
-1
at 500 nm
13
within iron oxide, implying
that most of the holes generated in the bulk of the material would
have to be recombined with electrons before they reach the surface.
Only those generated near the semiconductor/electrolyte interface
are able to oxidize water molecules. In this sense, it is assumed that
hematite thin films or nanostructured -Fe
2
O
3
could improve the
performance of this semiconductor.
14-18
In previously published
works, we described an electrochemical synthetic route for nano-
structured hematite -Fe
2
O
3
thin-film preparation.
1,2
This oxide
was obtained from the precursor iron oxyhydroxide -FeOOH,
akaganeite
3
electrochemically prepared as a thin film onto a con-
ductor glass fluorine-doped tin oxide FTO SnO
2
:F substrate. In
the synthesis of the precursor film, the potential cycling and pulsed
potential techniques using the reduction reaction of H
2
O
2
as OH
-
ion generator was employed in an electrolytic solution that con-
tained FeIII–F complex species. Studies of the electrochemical
formation mechanism of the -FeOOH phase done by electrochemi-
cal quartz crystal microbalance and voltammetric techniques have
been presented and discussed in detail in our previously published
work.
2
Then, the -Fe
2
O
3
phase was obtained upon heat-treatment
of this -FeOOH precursor at 520°C.
3
In fact, thermal dehydration
of -FeOOH films at high temperatures above 400°C leads to the
thermodynamically stable phase of FeIII oxide, the -Fe
2
O
3
phase,
19,20
and, according to the following chemical equation
2-FeOOH → -Fe
2
O
3
+H
2
O 1
The ironIII oxide hematite films prepared in these conditions
presented a small bandgap energy of 2.0 eV, which agrees well with
previously reported values of 1.9–2.2 eV for -Fe
2
O
3
thin films
4-6
depending on the crystalline status and methods of preparation.
5
Recent studies have focused on the sensitization of the semicon-
ductor photoelectrode surface in photoelectrochemical solar cells to
enhance the overall efficiency of light energy conversion.
21-23
More-
over, the performance of the photoelectrochemical solar cells is not
only determined by the solar light absorption charge carrier genera-
tion, but also by the transfer and collection of electrical charge
carriers.
21-24
These scopes can be achieved through the use of a
sensitizer in several ways: i harvesting incident photons with a
greater efficiency charge carrier generation, ii improving the cap-
ture and transport of charge carriers within the nanostructured semi-
conductor to achieve efficient charge separation at the electrode sur-
face, iii diminishing the charge recombination, and iv charge
injection from an excited sensitizer into large bandgap semiconduc-
tors. Furthermore, these sensitizer species can increase the sensitiv-
ity and selectivity of these interfaces, improving the catalytic prop-
erties of the electrode, because these species are able to improve the
charge transfer in the electrodic interface. Charge transfer occurs
through the molecular structure of these sensitizer species. Several
sensitizers have been explored to be coupled with different semicon-
ductor materials; examples of these molecular structures include
dye-sensitized semiconductor solar cells DSSCs, assemblies of in-
organic semiconductors with conjugated polymers, as well as de-
vices using quantum dots, fullerenes, and carbon nanotubes.
21-23
For
instance, the Grätzel photoelectrochemical solar cells are a typical
example of the use of a dye sensitizer.
25
These are based on the dye
of molecules of organic type or metals of transition that are adsorbed
on a highly porous nanocrystalline TiO
2
. In this case, the visible
light excites the dye-sensitizer molecules from the ground state, lo-
cated energetically in the semiconductor bandgap, to an excited state
resonant with the conduction band.
26
Another type of molecules that can be considered as candidates
to be used as sensitizer for semiconductor photoelectrodes is consti-
tuted by porphyrins. Porphyrin, which is similar in structure to chlo-
rophyll, is well-known for its strong light absorption that allows it to
participate in energy and electron transfer processes.
27-33
This mac-
* Electrochemical Society Active Member.
z
E-mail: rschrebl@ucv.cl
Journal of The Electrochemical Society, 157 5 D302-D308 2010
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