# Photoelectrochemical Characterization of Sprayedα-Fe2O3 Thin Films: Influence of Si Doping and SnO2 Interfacial Layer

Authors: Yongqi Liang; Cristina S. Enache; Roel van de Krol
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

α-Fe2O3 thin film photoanodes for solar water splitting were prepared by spray pyrolysis of Fe(AcAc)3. The donor density in the Fe2O3 films could be tuned between 1017–1020 cm-3 by doping with silicon. By depositing a 5 nm SnO2 interfacial layer between the Fe2O3 films and the transparent conducting substrates, both the reproducibility and the photocurrent can be enhanced. The effects of Si doping and the presence of the SnO2 interfacial layer were systematically studied. The highest photoresponse is obtained for Fe2O3 doped with 0.2% Si, resulting in a photocurrent of 0.37 mA/cm2 at 1.23 VRHE in a 1.0 M KOH solution under 80 mW/cm2 AM1.5 illumination.


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## Body

## 1. Introduction

Not long after the
first report of  stable photoelectrochemical O2 evolution on single
crystalline TiO2 [1], significant research efforts were made to
search for new oxide materials with smaller bandgaps to enhance visible light
absorption [2]. Fe2O3 stands out with its nearly ideal
bandgap of 2.2 eV [3] and its high-photochemical stability in aqueous solutions
[4]. Though high-photoconversion efficiencies were already shown for
polycrystalline Fe2O3 pellets [5] and single crystalline
Fe2O3 samples [6], thin films are generally preferred in
order to avoid the high resistivities encountered in, for example, single
crystals, and to reduce the cost for fabrication. Very recently, an
exceptionally high photocurrent of 2.7 mA/cm2  at a bias of 1.23 V relative
to the reversible hydrogen electrode (RHE) under simulated AM1.5 sunlight was
reported for porous films synthesized using atmospheric pressure chemical vapor
deposition (APCVD) [7]. While this breakthrough represents a major step
forward, practical applications still require an additional ~4 times
improvement in photocurrent. In order to
achieve this, more detailed insights into the origin of the high photoresponse
are required.The key
ingredients for the high performance of the APCVD films seem to be (i) Si
doping, which is thought to induce a favorable morphology during film growth
[8], (ii) the presence of a thin SiO2 interfacial layer between the Fe2O3 and the transparent conducting substrate [7], and (iii) the addition of a cobalt
catalyst [7]. The aim of the present study is to reveal the effects of Si
doping and the addition of a thin interfacial layer (in this case SnO2)
between the transparent conducting substrate (TCO) and the Fe2O3.
Toward this end, thin dense undoped and Si-doped Fe2O3 films are
deposited onto conductive glass substrates by spray pyrolysis. Although spray
pyrolysis will not generally yield the high degree of texturing that is
required for optimal performance, the ease of doping and smooth film morphology
greatly simplifies the photoelectrochemical characterization of the material.
The photoresponse and the electrical characteristics of the deposited films are
studied for various Si concentrations and the effect of a thin SnO2 interfacial film is investigated.


## 2. Experimental

Thin films ofα-Fe2O3 (hematite) were prepared by spray pyrolysis of a 0.04⁢M Fe(AcAc)3 (Alfa Aesar, Karlsruhe, Germany)
solution in a mixed solvent (2 : 1) of ethyl acetate and ethanol. To avoid large
fluctuations of the temperature during spraying and to give the solvent enough
time to evaporate, the spray pyrolysis was carried out using 15 seconds on cycle, 45 seconds off cycle.
Compressed air was used as the carrier gas.Silicon doping was
achieved by addition of different amounts of TEOS (99.9%, Alfa Aesar, Karlsruhe, Germany) to the
precursor solution. All the Si concentrations mentioned hereafter refer to the
atomic Si : Fe ratio in the spray solution, though the Si concentration that
actually ends up in the deposited Fe2O3 films may be
somewhat different. The SnO2 interfacial layer was deposited by
spray pyrolysis of a 0.1⁢M SnCl4 (99%, Acros organics, NJ, USA) solution in ethyl acetate (99.5%, J. T. Baker, Deventer, Holland).FTO glass (SnO2:F,
TEC-15, Libbey-Owens-Ford,
Hartford, IN, USA, 2.5-mm thick) was used as a transparent conducting substrate for
photoelectrochemical experiments. Fused silica (ESCO, S1-UV grade, thickness of
1 mm) was used as a substrate for the film thickness determination by UV-vis absorption
measurements. All the substrates were cleaned by ultrasonic rinsing in pure
ethanol, and subsequently dried under N2 flow. During deposition,
the substrate was heated using a temperature-controlled hotplate.Structural
characterization of the films was carried out using a Bruker D8 Advance X-ray diffractometer in the grazing incidence
mode. A Renishaw 1000 spectrometer was used for microscopic Raman analysis.To determine the film
thickness, UV-vis absorption spectra of the SnO2 and Fe2O3 films on fused silica were measured with a Lambda 900 UV-vis spectrophotometer
(Perkin Elmer). For
the Fe2O3 films, the thickness was further verified by
inspection of the cross-section of the films using a JSM-6500F field emission scanning
electron microscope (SEM).A home-made
three-electrode electrochemical cell, fitted with a fused silica window, was
used for photoelectrochemical characterization. An aqueous solution of1.0⁢M
KOH (pH 14) was used as the electrolyte. The potential of the Fe2O3 working electrode (area: 0.283 cm2) was controlled by a potentiostat
(EG&G PAR 283), which was combined with a frequency response analyzer
(Solartron 1255, Schlumberger, Hampshire, England)
for impedance analysis. An Ag/AgCl electrode (REF321, Radiometer Analytical, Villeurbanne, France) was used as a
reference and a coiled Pt wire was adopted as the counter electrode. All the
potentials mentioned in this article are referred to Ag/AgCl (0.208 V versus
NHE at 25°C), unless stated otherwise.For spectral response
measurements, light from a200⁢W tungsten halogen lamp was coupled into a
grating monochromator (Acton SpectraPro 150i) via a 50-mm diameter fused silica
lens. Between the monochromator and the sample, an electronic shutter (Uniblitz
LS6) was placed, and high-pass colored glass filters (Schott, 2×3⁢mm) were used to remove second-order
diffracted light. The intensity of the monochromatic light was measured with a
calibrated photodiode (Ophir, Jerusalem, Israel, PD300-UV). For white-light measurements, a solar
simulator (EPS 1200S, KH
Steuernagel Lichttechnik GmbH, Mo¨rfelden-Walldorf, Germany) was used to provide simulated AM1.5
sunlight. The light from the solar simulator was directed to the sample by
mirror (UV-enhanced aluminum, Melles Griot, Cambridge, England) placed under a 45° angle. This resulted in an
illumination intensity of 80 mW/cm2  (i.e., 80% of AM1.5 light)
at the position of the sample, as measured with a calibrated Si photodiode. An
AM1.5 standard spectrum reported by NREL [9] was used for integrating the
monochromatic photocurrent and comparing it to the white light response.


## 3. Results and Discussion

Figure1(a) shows X-ray
diffraction results for the 0.1% Si-doped Fe2O3 films. At
350°C, the peak intensity distribution corresponds to that of regular
polycrystalline hematite. As the temperature is increased, the intensity of the
(110) diffraction peak becomes stronger, while the (104) diffraction becomes
less pronounced. For the samples without Si dopant, the same trend is observed.
Hence, (110) is preferred growth direction for sprayed Fe2O3 films at deposition temperatures of 400°C and higher.(a) XRD of Fe2O3 films doped with 0.1% Si
deposited at (a) 350°C, (b) 400°C, (c) 450°C. The diffraction pattern of the
TCO substrate fired at 400°C in the air is also shown for comparison. Undefined
peaks are marked with *. (b) Raman spectra of Fe2O3 samples doped with 0.2% Si deposited at (a) 350°C, (b) 450°C, (c) 500°C. (c) SEM
micrographs showing the typical morphology of the Fe2O3 films on TCO. (d) Cross-section of a 200 nm Fe2O3 film on
a fused silica substrate.
(a)(b)(c)(d)The preferred (110)
orientation implies that the c-axes of the crystal domains in the Fe2O3 film are parallel to the substrate surface. Since electron transport along the
c-axis is known to be about four orders of magnitude slower than in the other
directions [10, 11], the (110)
orientation observed here is much more favorable for electron transport than
the (001) oriented hematite films
deposited with, for example, ion-beam assisted CVD [12]. Moreover, the conductivity along the c-axis has been suggested to
be p-type in character [13, 14]. This implies that a (110) preferred
orientation would also be favorable for photoanodes made of vertically aligned
hematite nanowires, where efficient hole transport in the direction parallel to
the substrate is desired.In addition to the
hematite peaks, two other phases seem to be present in minor amounts as
indicated by small peaks at 29.8° and 32.1°. The small peak at 29.8°, which is
attributed to the presence of Fe3O4 [15], appears only in
the samples deposited at low temperatures (≤450°C). The origin of the peak at 32.1° is not
yet clear and can not be attributed to any of the known iron oxides. The
closest candidate would be β-Fe2O3 with a peak at 32.9°,
but this seems to be too far from the observed peak position. Moreover, β-Fe2O3 has been reported to disappear after annealing at high temperatures (550°C)
[16], whereas Figure 1(a) shows the peak at 32.9° to actually increase with
temperature.Figure1(b) shows
the Raman spectra of Fe2O3 doped with 0.2% Si. Compared
to the α-Fe2O3 peaks, the intensity of the Fe3O4 peak at 656 cm−1  [17] decreases with increasing deposition
temperature. This is consistent with the XRD results in Figure 1(a), and
supports our conclusion that Fe3O4 is only formed at
temperatures ≤450°C under the currently used conditions for
spray pyrolysis.As can be observed in
Figures1(c) and 1(d), spray pyrolysis of the Fe(AcAc)3 solution
yields compact films of uniform thickness. A thickness of 200 nm was reached
after 40 spray cycles (each cycle consists of 15 seconds spray on +45⁢seconds spray off), resulting in an average deposition rate of 20 nm/min
for spraying at 450°C. No significant differences in film morphology are
observed between undoped and Si-doped films.Figure2(a) shows the
measured photocurrent versus potential curves for Fe2O3 films doped with 0.2% Si. Comparison between curves a and b shows a
significant improvement of the photocurrent response, when a 5 nm SnO2 interfacial layer is present between the TCO substrate and the Fe2O3 film. This improvement is especially pronounced at lower potentials.
Specifically, at a potential of 0.23 V versus Ag/AgCl, which corresponds
to a reversible hydrogen potential of 1.23⁢VRHE, the
photocurrent increases from 0.04 mA/cm2  to 0.33 mA/cm2.
In fact, Figure 2(a) shows that the interfacial layer shifts the photocurrent
onset potential by ~0.2 V in the cathodic direction. Another effect of the
interfacial layer is a significant improvement of the reproducibility of the
photoresponse, especially for undoped samples. Without the interfacial layer,
photocurrent variations of up to a factor of five or more are observed between
samples made under the same conditions. With the interfacial layer, these
variations are within a factor of 2.(a)
Current-potential measurements under simulated 80 mW/cm2  AM1.5
sunlight for Fe2O3 doped with 0.2% Si with (a: front-side
illumination; a’: back-side illumination; d: dark) and without (b: front-side
illumination; b’: back-side illumination; c: dark) 5 nm SnO2 interfacial layer. The scan rate is 20mV/s. (b) Dependence of the photocurrent (front-side
illumination) at 0.23 V versus Ag/AgCl on the Si dopant concentration; a: with
SnO2 interfacial layer; b: without SnO2 interfacial
layer.
(a)(b)For the Si-doped
samples, front-side (electrolyte-side) illumination (curve a in Figure2(a))
always gives a higher photocurrent than back-side (substrate-side) illumination
(curve a’ in Figure 2(a)). This shows that the rate-limiting step in the
Si-doped samples is hole transport. In contrast, electron transport is found to
be rate limiting in undoped Fe2O3 samples. The
introduction of the SnO2 interfacial layer does not change the ratio
between the front-side photocurrent and the back-side photocurrent (curves b
and b’). Since front-side illumination shows higher photocurrents, all the data
discussed from this point forward are measured using front-side illumination.The thickness of the
SnO2 films was determined by UV-vis absorption measurements,
corrected for reflection, assuming an absorption coefficient of 1.3×105⁢cm−1 at 300 nm
[18]. For these measurements, the SnO2 films were deposited on fused silica substrates, and the growth rate was
assumed to be similar to that on the TCO substrates. Because of the small
thickness (~5 nm) and the large bandgap (>3.6 eV) of SnO2, it can
be safely assumed that the contribution of the interfacial layer to the photocurrent
is negligible. Furthermore, X-ray diffraction revealed that the SnO2 interfacial layer does not affect the preferred orientation of the Fe2O3 films.Figure2(b) shows the
photocurrent at 0.23 V versus Ag/AgCl for Fe2O3 films with various amounts of Si doping. For the films without SnO2 interfacial layer, the optimum Si concentration is ~1%, whereas an optimum
concentration range between 0.2 and –0.4% is found for films with an interfacial SnO2 layer. It should be noted that these concentrations are much lower than in
previously reported studies on Si-doped Fe2O3 [19, 20],
where a significant fraction of Si content was presumably present as a second
phase. As can be seen in Figure 2(b), the presence of an SnO2 interfacial layer is especially beneficial at low-Si concentrations.Previous reports have
shown that unintentional incorporation of donor-type species may also be
responsible for improved photoefficiencies. Examples of impurities that are
sometimes difficult to avoid are Si [8, 19], carbon in the case of organic
Fe-precursors such as Fe(AcAc)3, and chlorine when FeCl3 or FeCl2 are used as precursors [7]. The electrical conductivity can also be affected by
the presence of other iron oxide phases, such as FeO, Fe3O4,
or Fe2O3, which may be formed depending on the deposition
conditions and the presence of trace impurities [2, 16, 17].To rule out that the
beneficial effect of the SnO2 layer is caused by the diffusion of Sn
into the Fe2O3 film (i.e., unintentional doping), a
Mott-Schottky analysis was carried out. Figure 3(a) shows a Mott-Schottky plot
of an undoped Fe2O3 film recorded in the dark. The
presence of a 5 nm SnO2 interfacial layer has no significant effect
on the curve, which already indicates that no significant Sn diffusion into the
Fe2O3 layer occurs. According to the depletion layer
model, the capacitance of the semiconductor space charge layer Csc
changes with the applied
potential VA
according to
the Mott-Schottky equation,Mott-Schottky
measurement of Fe2O3 films (a) undoped, measured at 30 kHz
and (b) doped with 0.9% Si, measured at 1 kHz. The frequency was selected based
on the impedance spectra (not shown), in the range where the slope of log|−Z′′| versus log(ω) is close to −1, and the real part of impedance
is more or less constant [21].
(a)(b)(1)1Csc2=2eεrε0NdA2(VA−Vfb−kTe), where εr is the semiconductor dielectric
constant, A is the surface area of
the electrode, Nd is the
donor density, and Vfb is
the flatband potential. At potentials positive of −0.6⁢V, the Fe2O3 is fully depleted and the slope of the curve reflects the donor density in the
conducting FTO substrate. From the intercept of the dashed lines in Figure 3(a) and the dielectric constant of 80 for Fe2O3 [22], the
film thickness is calculated according to C=ε0εrA/d,
yielding a thickness value of 135 nm. The discrepancy between the calculated
value and the value determined from the SEM cross-section (200 nm, Figure 1(d))
is presumably caused by the surface roughness and/or the frequency dispersion
in the Mott-Schottky plots, which will be discussed in more detail below. A
donor density of 1.2×1017⁢cm−3 in
the Fe2O3 is calculated from the slope of the
Mott-Schottky plot between −0.8⁢V and −0.6⁢V. This value is quite small for a semiconducting
metal oxide indicating that (i) the sprayed Fe2O3 films
are of high quality with relatively few electronic defects and (ii) little, if
any, Sn diffuses from either the interfacial layer or the F:SnO2 conducting substrate into the Fe2O3 film. The beneficial effect
of the SnO2 interfacial layer is tentatively attributed either to
the passivation of surface states or to an improvement of the band alignment between
the Fe2O3 and the underlying TCO.After introducing 0.9%
Si dopant in the precursor solution, dramatic changes in the Mott-Schottky plot
are observed (Figure3(b)). From these data, a donor density of 1×1020⁢cm−3 is calculated.
However, considering the roughness of the surface on a microscopic scale and
the fact that the depletion layer extends only a few nanometers into the Fe2O3 film, the effective surface area will be larger than the geometrical area. As a
result, the true donor density will be somewhat smaller than the calculated
value. The fact that the curve concaves upwards in Figure 3(b) supports the
interpretation in terms of surface roughness effects [23]. If all Si atoms
present in the precursor solution are incorporated into the film and act as
ionized donors, a maximum donor density of 4×1020⁢cm−3 is expected.
This is within the same order of magnitude as the measured donor density,
indicating that a significant fraction of the Si atoms, is incorporated into
the film where they indeed act as ionized donors. This is a somewhat unexpected
result, since the pyrolysis of TEOS usually requires temperatures much higher
than 450°C [7].Another important
parameter of the semiconductor that can be derived from Mott-Schottky
measurements is the flat band potential. Although the flat band potential
depends on the exposed crystal plane and on the donor density inside the Fe2O3,
most reports agree that the flat band potential of Fe2O3 with a donor density of ~1017 cm−3 is −0.7⁢V in a 1.0⁢M
KOH solution [6, 24]. A somewhat more negative potential of −0.84⁢V versus
Ag/AgCl is found for our spray-deposited undoped Fe2O3 films (Figure 3(a)). However, it should be mentioned that an accurate
determination of the flat band potential is complicated by some degree of
frequency dispersion observed in the Mott-Schottky plots [25, 26].
Specifically, in the frequency range between 1 kHz and 30 kHz, the flat band
potential varies between −0.67⁢V and −0.96⁢V. The donor density shows
much less variation; values between 1.3×1017 cm−3 and 0.8×1017 cm−3 are observed
between 1 and 30 kHz. A more detailed analysis of the impedance spectra to
elucidate the origin of the frequency dispersion is still ongoing, but is
complicated by the fact that the equivalent circuits that are normally used to
model semiconducting photoanodes are not able to describe the measured data in
a satisfactory manner.To objectively compare
the photoelectrochemical performance of our films with the literature, the incidented
photon-to-current conversion efficiency (IPCE) has been measured as a function
of wavelength. The IPCE of the Fe2O3 films doped with
0.1% Si and deposited on top of a 5 nm SnO2 interfacial layer is
shown in Figure 4(a). A bias of 0.4 V versus Ag/AgCl was used, since no
accurate steady-state photocurrent values could be obtained at more negative
potentials due to a transient response when the light is switched on and off. A
maximum IPCE of 20% is found at 370 nm, which is comparable to the best
spray-deposited Fe2O3 samples reported so far [16],
though still a factor of about 2.5 below the samples made by APCVD [7]. At
wavelengths shorter than 370 nm, however, the IPCE shows a steeper decrease
than that reported in the literature for Fe2O3 [7, 16]. While the origin of this difference is not clear, it does explain
the relatively modest response of our samples to white-light
illumination.(a) IPCE curve
for 0.1% Si-doped Fe2O3 measured at 0.4 V versus Ag/AgCl.
(b) Momentary and integrated solar photocurrent of the sample obtained by
multiplying IPCE with the photon flux spectrum of AM1.5 sunlight. When
correcting for the actual experimental intensity of 80 mW/cm2,
an integrated photocurrent of 0.8×0.66=0.53mA/cm2 is obtained.
(a)(b)To compare the IPCE
spectra with the photoresponse under simulated AM1.5 sunlight, the IPCE data are
integrated over the standard AM1.5 solar spectrum and corrected for an actual intensity
of 80 mW/cm2 (Figure 4(b)). The photocurrent is assumed to depend
linearly on the light intensity, which is reasonable for the low-light
intensities used in this study (10–20μW/cm2).
The integrated photocurrent of 0.53 mA/cm2 is higher than the
measured AM1.5 photocurrent of 0.44 mA/cm2 (Figure 2(a)).
Although the reason for this discrepancy is not yet clear, this perhaps rather
conservative value for the photoresponse under simulated AM1.5 light is much
better suited for comparison with results from other labs than the often
reported photocurrents measured with xenon light sources. As pointed out
recently by Murphy et al., the high-UV content of xenon lamps often leads to
severe overestimation of the photoconversion efficiencies [3].


## 4. Conclusions

Thin-film hematite Fe2O3 photoanodes were deposited on transparent conducting glass substrates by spray
pyrolysis. A thin, 5 nm SnO2 interfacial layer between the Fe2O3 and TCO glass was found to be beneficial for both the reproducibility and the
photocurrent efficiency of the Fe2O3 films. While the
beneficial effect of an interfacial layer on the photoresponse of Fe2O3 has been previously reported for ultrathin (~1 nm) SiO2 films,
in this study we show that the beneficial effect of such an interfacial layer
is not limited to the material SiO2, and that electrons do not need
to be able to tunnel through the interfacial layer. Silicon concentrations
between 0.2 and 0.4% are found to give the highest photocurrents for
spray-deposited Fe2O3 films. The highest photoresponse
measured during this study was 0.37 mA/cm2 at 0.23 V versus
Ag/AgCl (1.23 V versus RHE) under 80 mW/cm2 AM1.5 illumination for
films doped with 0.2% Si and deposited on a 5 nm SnO2 interfacial
layer. Since hole transport is the rate limiting factor for these films,
further improvements of the efficiency requires nanostructured electrodes with
high aspect ratios in order to minimize the distance that the photogenerated
holes have to travel before reaching the semiconductor/electrolyte interface.
Development of suitable low-cost deposition techniques for such electrodes,
that can be readily scaled-up and that allow accurate control over dopant
concentrations, is one of the major challenges in the field of photocatalysis
and solar water splitting.



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*Source: 739864-2008-04-20.xml*