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FULL PAPER

Photoelectrochemical Water Splitting

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Boosting Photoelectrochemical Water Oxidation of Hematite
in Acidic Electrolytes by Surface State Modification
Peng-Yi Tang, Li-Juan Han, Franziska Simone Hegner, Paul Paciok, Martí Biset-Peiró,
Hong-Chu Du, Xian-Kui Wei, Lei Jin, Hai-Bing Xie, Qin Shi, Teresa Andreu,
Mónica Lira-Cantú, Marc Heggen, Rafal E. Dunin-Borkowski, Núria López,
José Ramón Galán-Mascarós, Joan Ramon Morante,* and Jordi Arbiol*
For the design of a beneficial device structure, in which both electrodes are exposed
to the same medium, and considering
that the hydrogen evolution is most efficiently carried out in acidic electrolyte and
the advantages of the proton exchange
membrane, a robust photoanode would
be highly desirable.[10–15] Nonetheless the
development of an efficient and affordable photoanode, which is stable in acidic
electrolyte, imposes a great challenge and
limits the large-scale implementation of
economically viable PEC water-splitting.
In light of this challenge, much attention
has been drawn to the development of efficient and affordable photoanode systems
adapted to acidic electrolytes.
Hematite is arguably the most desirable
photoanode material. On one hand, its
relatively small bandgap of 1.9-2.1 eV and
its suitably aligned valence band level perfectly match the thermodynamic energy
requirements needed to drive water oxidation.[4,10] On the other hand, it is made from the most abundant transition metal on Earth crust, iron. Unfortunately, the
bare hematite surface is catalytically very poor, and therefore
requires modification with water-oxidation catalysts (WOCs) in
order to extract the thermodynamic power stored when light is
absorbed.

State-of-the-art water-oxidation catalysts (WOCs) in acidic electrolytes
usually contain expensive noble metals such as ruthenium and iridium.
However, they too expensive to be implemented broadly in semiconductor
photoanodes for photoelectrochemical (PEC) water splitting devices. Here,
an Earth-abundant CoFe Prussian blue analogue (CoFe-PBA) is incorporated
with core–shell Fe2O3/Fe2TiO5 type II heterojunction nanowires as composite
photoanodes for PEC water splitting. Those deliver a high photocurrent
of 1.25 mA cm−2 at 1.23 V versus reversible reference electrode in acidic
electrolytes (pH = 1). The enhancement arises from the synergic behavior
between the successive decoration of the hematite surface with nanolayers
of Fe2TiO5 and then, CoFe-PBA. The underlying physical mechanism of
performance enhancement through formation of the Fe2O3/Fe2TiO5/
CoFe-PBA heterostructure reveals that the surface states’ electronic levels
of hematite are modified such that an interfacial charge transfer becomes
kinetically favorable. These findings open new pathways for the future design
of cheap and efficient hematite-based photoanodes in acidic electrolytes.

1. Introduction
Photoelectrochemical (PEC) water splitting devices, using
Earth-abundant semiconductor materials, have long been considered to be the ‘‘Holy Grail’’ of solar energy conversion.[1–9]
Dr. P.-Y. Tang, Dr. H.-B. Xie, Dr. M. Lira-Cantú, Prof. J. Arbiol
Catalan Institute of Nanoscience and Nanotechnology (ICN2)
CSIC and BIST
Campus UAB
Bellaterra 08193, Barcelona, Catalonia, Spain
E-mail: arbiol@icrea.cat
Dr. P.-Y. Tang, M. Biset-Peiró, Dr. Q. Shi, Dr. T. Andreu
Prof. J. R. Morante
Catalonia Institute for Energy Research (IREC)
Jardins de les Dones de Negre 1, Sant Adrià del Besòs,
Barcelona 08930, Catalonia, Spain
E-mail: jrmorante@irec.cat
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aenm.201901836.

Dr. L.-J. Han, Dr. F. S. Hegner, Prof. N. López, Prof. J. R. Galán-Mascarós
Institute of Chemical Research of Catalonia (ICIQ)
BIST
Avinguda Països Catalans 16, Tarragona 43007, Catalonia, Spain
Dr. P.-Y. Tang, Dr. P. Paciok, Dr. H.-C. Du, Dr. X.-K. Wei, Dr. L. Jin,
Dr. M. Heggen, Prof. R. E. Dunin-Borkowski
Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons
and Peter Grünberg Institute
Forschungszentrum Jülich GmbH
52425 Jülich, Germany
Prof. J. R. Galán-Mascarós, Prof. J. Arbiol
ICREA
Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain

DOI: 10.1002/aenm.201901836

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Figure 1. A) 3D Atomic supercell models with solvent accessible surface illustrating the synthetic procedure for Fe2O3/Fe2TiO5/CoFe PBA photoanodes.
SEM images of B) Fe2O3, C) Fe2O3/Fe2TiO5, and D) Fe2O3/Fe2TiO5/CoFe PBA electrodes.

Regarding efficient WOCs in acidic electrolyte, many
researchers have hitherto devoted their efforts to explore cheap,
effective alternatives to the state-of-the-art ruthenium (Ru)- and
iridium (Ir)-based WOCs.[14–18] For example, cobalt-containing
polyoxometalates,[16] Ti-stabilized MnO2,[19] W1−xIrxO3–δ,[20]
NixMn1−xSb1.6–1.8Oy,[21] Fe-TiOx,[22] iron (III) oxide,[23] cobaltdoped hematite,[24] and CoFe-PBA[25,26] WOCs have been substantially explored. For a successful WOC-functionalized photoanode, it is necessary to consider the utilization of light capture
of semiconductors and the catalytic effect of WOCs simultaneously, that is to say, boosting the performance of the WOCs
without compromising the light absorption features.[11,12] Up to
date, few reports have appeared on smart integration of hematite with WOCs, and most of them related to noble Ir-based
catalysts,[11,27–29] with which a maximum photocurrent response
of 0.66 mA cm−2 at 1.23 V versus reversible reference electrode (RHE) in acidic electrolyte (pH = 1.01) was obtained.[11]
Thus, even by coupling with noble Ir-based WOCs, the photocurrent response of hematite based photoanodes in acidic
electrolyte remains much lower than its theoretical value
(12.5 mA cm−2).[30]
Meanwhile, it is well established that the surface states present in the bandgap of hematite, mediate hole transfer and plays
a vital role in determining its PEC performance.[31,32] There are
two types of surface states, intrinsic surface states derived from
the loss of translational bulk crystal symmetry, and extrinsic
surface states due to chemical bond formation/surface interaction with a secondary species.[33,34] While it is difficult to
completely remove intrinsic surface states, they can be modified by depositing a secondary species,[35] which has recently
been demonstrated.[28,36–38] For instance, our previous investigation about ITO/Fe2O3/Fe2TiO5/FeNiOOH photoanodes in
alkaline electrolytes reveals that the surface states of hematite

Adv. Energy Mater. 2019, 1901836

can be modified by atomic layer–deposited Fe2TiO5 and photoelectrodeposited FeNiOOH.[38] Moreover, hematite photoanodes
were combined with a CoFe-PBA resulting in enhanced photocurrent response in neutral electrolyte.[37] Despite these observations in neutral and alkaline electrolytes, rare reports on the
performance of hematite-based photoanodes in acidic media
have been published, despite the extraordinary technological
interest, as described above.
With the aim of designing cheap and efficient hematitebased photoanodes in acidic electrolyte, we decided to merge
these two previous strategies. First, we fabricated core–shell
Fe2O3/Fe2TiO5 type II heterostructured nanowires, as a surface-modification approach to enhance photocatalytic activity.
Second, we decorated these nanowires with a nanolayer of an
acid-stable WOC, the CoFe-PBA (Scheme S1, Supporting Information). These photoanodes were prepared on fluoride-doped
tin oxide (FTO) glass electrodes in three steps: hydrothermal
deposition of Fe2O3; atomic layer deposition (ALD) of Fe2TiO5;
and finally, chemical bath deposition of CoFe-PBA, as displayed
in Figure 1A. These heterostructures are able to produce the
highest photocurrent response in acid media ever observed
for a hematite-based photoanode, when made by scalable processes, and earth-abundant materials, opening new strategies
for hematite-based PEC water splitting in acidic electrolyte.

2. Results and Discussion
2.1. Processing and Structural Characterization
Vertically aligned Fe2O3 nanowires with diameters ranging
from 100 to 200 nm (Figure 1B) were first grown on a FTO
substrate via a hydrothermal method.[38] Then, a thin TiO2

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layer was coated onto the Fe2O3 nanowires by 30 ALD cycles.
The surface coated TiO2 was subsequently transformed into
Fe2TiO5 through a post-sintering process in ambient atmosphere at 750 °C for 30 min. As displayed in Figure 1C, the
Fe2O3/Fe2TiO5 heterostructured nanowires are homogeneous
without changing the nanowire-like architecture. Subsequently,
the obtained Fe2O3/Fe2TiO5 composite nanowires were subjected to a chemical bath for 2 h in the presence of the CoFePBA precursor at 60 °C to produce Fe2O3/Fe2TiO5/CoFe-PBA
heterostructured nanowires. Its scanning electron microscope
(SEM) image in Figure 1D reveals that the diameter of these
nanowires did not change compared to the Fe2O3/Fe2TiO5 ones,
indicating the ultrathin CoFe-PBA coating. The sample crystallinity and chemical composition were further analyzed via
energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), Raman
spectrum, and Fourier transform infrared (FT-IR) spectrum in
Figures S1-S5 in the Supporting Information, evidencing the
existence of hematite, Fe2TiO5, and CoFe-PBA species in the
corresponding electrodes.
The structure, crystallography, and spatial distribution of
hematite, Fe2TiO5, and CoFe-PBA species were further investigated by aberration-corrected scanning transmission electron microscopy (AC-STEM) in high angle annular dark-field
(HAADF) mode. On one hand, the HAADF STEM images of
Fe2O3 and Fe2O3/Fe2TiO5 electrodes on the top and middle
rows of Figure 2 show the atomic ordering of the hematite
matrix. On the other hand, the Fe2TiO5 species in the Fe2O3/
Fe2TiO5 electrode are shown as a blurred ultrathin shell
on the surface of the hematite nanowires (middle rows of
Figure 2 and Figure S10, Supporting Information), in good
agreement with the maps obtained by STEM combined with
electron energy loss spectroscopy (EELS) in Figure S12 in the
Supporting Information conducted on the same region. The
additional atomic resolution HAADF STEM imaging in combination with the STEM-EELS compositional maps of the Fe2O3
and Fe2O3/Fe2TiO5 electrodes are included in Figures S6–S12
in the Supporting Information, confirming the core–shell
nanowires structure of the Fe2O3/Fe2TiO5 electrode. Notably,
coordination polymers are especially susceptible to the electron
beam damage, hindering stable atomic-level HAADF STEM
observation of the CoFe-PBA.[25,38–41] Thus, we employed bright
field HRTEM to monitor the surface structure evolution of the
Fe2O3/Fe2TiO5/CoFe-PBA electrodes.
Figure 2A displays a representative TEM image of a Fe2O3/
Fe2TiO5/CoFe-PBA nanowire. According to Figure 2B-D, the
nanoparticles attached to the composite nanowire can be
assigned to CoFe-PBA species. The hematite and Fe2TiO5
phases dominate the nanowires matrix, as identified by the
HRTEM and its corresponding power spectrum in Figure 2E-F.
Moreover, the corresponding frequency filtered image
(Figure 2G) clearly illustrates the presence of a localized hematite nanowire core and an ultrathin pseudo-brookite shell.
Figure 2E and Figure S13 in the Supporting Information show
that the fine CoFe-PBA shell on the nanowires surface tends
to possess an amorphous structure, whereas bigger CoFe-PBA
nanoparticles present lattice fringes denoting its good crystallinity, as displayed in Figure 2C and Figure S14 in the Supporting Information.

Adv. Energy Mater. 2019, 1901836

The spatial elemental distribution of Fe2O3/Fe2TiO5/CoFePBA electrodes was further characterized via HAADF STEM
combined with EELS. In addition to the elemental signals from
the CoFe-PBA nanoparticles, we also found the presence of
C, N, O, Co, and Fe surrounding the nanowire matrix in the
STEM-EELS maps shown in Figure 3 and Figures S15-S16 in
the Supporting Information.[25] These results evidence that the
surface amorphous region observed in Figure 2E is indeed an
ultrathin CoFe-PBA shell at the surface of the Fe2O3/Fe2TiO5
nanowires. Additionally, the statistical diameter size distributions of Fe2O3, Fe2O3/Fe2TiO5, and Fe2O3/Fe2TiO5/CoFe
PBA nanowires in Figure S17 in the Supporting Information
reveal that Fe2O3, Fe2O3/Fe2TiO5, Fe2O3/Fe2TiO5/CoFe PBA
nanowires have average diameter size of 168 ± 43, 174 ± 63,
and 185 ± 70 nm, respectively. The average diameter size of
these nanowires did not significantly change with the coating
of Fe2TiO5 and CoFe PBA, which is consistent with the SEM
results.

2.2. Photoelectrochemical Performance
The detailed PEC performance measured for these photoanodes is displayed in Figure 4. Cyclic voltammtry (CV) measurements in the dark (Figure 4A) show a positive shift of the
onset potential of hematite upon coating with the ultrathin
Fe2TiO5 shell, consistent with our previous work,[38] whereas
modification with CoFe-PBA reduces the onset potential of
the Fe2O3/Fe2TiO5 electrode, which demonstrates the positive
catalytic effect of CoFe-PBA. Under light irradiation, the
CV in Figure 4B and the statistical data in Figure S18 in the
Supporting Information reveal that pristine Fe2O3 electrodes
exhibit a very low photocurrent response of 0.12 mA cm−2 at
1.23 V versus RHE, the thermodynamic potential for the oxygen
evolution reaction.[42] Upon Fe2TiO5 deposition, the photocurrent density increases significantly above 1.0 V versus RHE,
reaching 0.90 mA cm−2 at 1.23 V versus RHE (Figure 4B). The
onset potential is further improved for the Fe2O3/Fe2TiO5/CoFePBA electrode. This parameter was used to optimize the Fe2O3/
Fe2TiO5/CoFe-PBA processing (Figures S19-S20, Supporting
Information). According to the statistical data in Figure S19
in the Supporting Information, we reached a maximum PEC
performance with electrodes coated with CoFe-PBA by a
chemical bath reaction at 60 °C for 2 h, giving 1.25 mA cm−2
at 1.23 V versus RHE. To the best of our knowledge, this is the
highest photocurrent value observed for hematite-based photoanodes in acidic electrolyte (see Table S1, Supporting Information). Moreover, it is better than the photocurrent response for
Fe2O3/Fe2TiO5 (0.90 mA cm−2, Figure 4B) and Fe2O3/CoFe-PBA
(0.62 mA cm−2, based on the statistical data in Figures S21-S22,
Supporting Information) electrodes indicating a synergic effect
in combining core–shell Fe2O3/Fe2TiO5 type II heterojunction
with the CoFe-PBA WOC.
The chopped light photocurrent–potential curves in Figure 4C
show smaller photocurrent transients for the Fe2O3/Fe2TiO5/
CoFe-PBA electrodes, in particular in the potential range of 1.2–
1.7 V versus RHE. This reduction of the photocurrent transient
indicates that the electron–hole recombination is suppressed by
the Fe2TiO5 and CoFe-PBA modification, further demonstrating

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Figure 2. Top row: (Left) HAADF STEM image showing the atomic ordering at the edge region of the Fe2O3 electrode. (Middle) the corresponding
colored fast Fourier transform power spectrum (FFT) indicates that the nanowires crystallize in the hematite phase as visualized along the [2–21] direction. (Right) Atomic resolution HAADF STEM image of the green squared region showing the ordering of Fe, while O atoms are almost not visible in
HAADF STEM mode due to the their weak Z-contrast. Middle row: (Left) HAADF STEM image showing the atomic ordering at the edge region of the
Fe2O3/Fe2TiO5 electrode. (Middle) The corresponding colored FFT spectrum indicates that the nanowires matrix is hematite as visualized along the
[2–21] direction. (Right) Atomic resolution HAADF STEM image of the blue squared region showing the typical ordering of Fe atoms in hematite. The
Fe2TiO5 shell is observed as a blurred ultrathin shell (≈1 nm) on the surface of the hematite matrix since the heights of hematite-core and Fe2TiO5
shell are different. (the inset shows the atomic model of Fe and O atoms visualized from the [2–21] direction, with Fe atoms marked as red and O
atoms marked as green). Bottom row: A) low magnification bright field TEM images showing the general morphology of the Fe2O3/Fe2TiO5/CoFe-PBA
nanowires. B) HRTEM detail showing the yellow squared interface area in (A). C) Magnified HRTEM detail of the selected surface nanoparticle and
D) corresponding power spectrum indicating that the nanoparticle attached to the nanowire matrix crystallized in the cubic CoFe-PBA phase, as
visualized along the [1–11] direction. E) HRTEM image of the nanowire surface region squared in purple in (B). The white dotted line is marking
an amorphous CoFe-PBA region. F) Corresponding FFT spectrum indicating that the nanowire heterostructure is mainly composed of hematite
and pseudobrookite as visualized along the [−441] and [001] directions, respectively. G) Frequency filtered structural map of the hematite (red) and
pseudobrookite (green), showing their atomic stack sequence.

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Figure 3. High magnification EELS chemical composition maps obtained from the blue rectangle area in the ADF-STEM micrograph of a nanowire
extracted from the Fe2O3/Fe2TiO5/CoFe-PBA electrode. Individual Fe (red), C (green), Sn (blue), N (purple), Ti (indigo), and Co (yellow) maps and
their composites.

its advantage. Moreover, the UV–vis absorptance, Tauc plots,
IPCE, and APCE spectra of the Fe2O3, Fe2O3/Fe2TiO5, and
Fe2O3/Fe2TiO5/CoFe PBA electrodes in Figure S23 in the
Supporting Information further evidence that the enhanced
PEC performance of Fe2O3/Fe2TiO5/CoFe-PBA electrodes is
attributed to the synergetic effect from Fe2TiO5 and CoFe-PBA.
The PEC stability of these three electrodes was investigated
by chronoamperometry at a constant applied working potential of 1.23 V versus RHE (Figure 4D) at pH = 1 for 24 h.
The photocurrent response of Fe2O3 electrodes shows a slow
but continuous decrease, maintaining about 40% of the initial photocurrent response after 24 h test. In contrast, Fe2O3/
Fe2TiO5 and Fe2O3/Fe2TiO5/CoFe-PBA electrodes follow a similar trend, showing an initial drop in photocurrent during the
first 2 h and show no further sign of fatigue during the rest of
the stability measurement, retaining around 80% of the initial
photocurrent response after 24 h test. Additionally, we monitored the evolved oxygen in the case of Fe2O3/Fe2TiO5/CoFePBA electrodes by a calibrated Fibox O2 detector in a gastight
cell during the first 2 h water oxidation at 1.23 V versus RHE
(Figure S24A, Supporting Information). The theoretical oxygen
yield was calculated from the total charge passed during PEC
water oxidation. Faradaic Efficiencies above 94% were demonstrated (Figure S24B, Supporting Information), indicating
that the photocurrent response is mainly originating from
the water oxidation process. The enhanced stability of Fe2O3/
Fe2TiO5 and Fe2O3/Fe2TiO5/CoFe-PBA electrodes compared
to Fe2O3 electrodes is further confirmed by the CV curves of
these electrodes after 24 h stability test in Figure S25 in the
Supporting Information. Moreover, the SEM images of Fe2O3
electrodes after 24 h stability measurement in Figure S26A-C
in the Supporting Information reveal that the attenuation of
the photocurrent response in Fe2O3 electrodes is derived from
its nanowires structure degradation in acidic electrolyte. In
contrast, the degradation of Fe2O3/Fe2TiO5, and Fe2O3/Fe2TiO5/
CoFe-PBA electrodes’ nanowires are substantially suppressed,
as displayed in Figure S26D-I in the Supporting Information. Therefore, we assign the drastically enhanced stability of
Fe2O3/Fe2TiO5/CoFe-PBA electrodes to the dual protective
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effect provided by the Fe2TiO5 and the CoFe-PBA (Figure 4E),
both of which are stable in acidic electrolytes. [19,22,25,43]

2.3. Mechanistic Investigation via Photoelectrochemical
Impedance Spectroscopy (PEIS)
It is well established that the catalytic activity of photoanodes is
strongly dependent on the characteristics of the surface states at
the semiconductor-electrolyte interface (SEI).[44–46] While those
surface states can limit water oxidation kinetics by acting as
electron–hole recombination centers, they can also have a beneficial influence on water oxidation, promoting electron transfer
across the interface, dependent on their respective kinetics.[47]
In particular, electrical active surface states presented in the
hematite bandgap are supposed to play a vital role in PEC water
oxidation; thus, a deeper investigation is required to probe their
effect on charge transfer at the SEI (Figure 4E).[38,48–51]
We employed CV and PEIS techniques to monitor the
evolution of such surface states in hematite,[44,45] which was
suggested to be an iron-oxo intermediate by operando IR
spectroscopy[48] and density functional theory calculations[52,53]
and how it is influenced by successive Fe2TiO5 and CoFe-PBA
deposition. As displayed in Figure 5A, the precatalytic feature
in the CV, which is related to adding and removing electrons
to/from the surface states, changes with the addition of Fe2TiO5
and CoFe-PBA.[48] Their significant impact on the surface
states was further suggested by PEIS. The equivalent circuits
in Figure S29 in the Supporting Information were used to fit
the obtained data in Figures S27-S28 in the Supporting Information; the obtained resistances and capacitances are shown in
Figures S30-S31 in the Supporting Information.
From the fitted surface states or trap capacitance Ctrap,
we estimated the density of surface states (DOSS) with
Equation (1)[32,36,54,55]
N SS (E ) =

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C trap (E )
q

(1)

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Figure 4. A) Cyclic voltammetry under dark, B) cyclic voltammetry under illumination, C) chopped light photocurrent–potential curves, and D) photoelectrochemical stability test operated at 1.23 V versus RHE of the Fe2O3, Fe2O3/Fe2TiO5, and Fe2O3/Fe2TiO5/CoFe-PBA electrodes for 24 h. All polarization potentials reported here are relative to the RHE, and current densities are based on the geometric area. J (mA cm−2) represents the current density
response under light illumination. E) Zoom in view of the atomic supercell model with solvent accessible surface of Fe2O3, Fe2O3/Fe2TiO5, and Fe2O3/
Fe2TiO5/CoFe-PBA nanowires show the modified surface interfaces.

where Nss (E) is the DOSS (cm−2 eV−1) as a function of the
applied potential and q is the electron charge (1.602 × 10−19 C).
As shown in Figure 5B, the energy and density distribution

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of the surface states Nss[32,54] follows the order Fe2O3 < Fe2O3/
Fe2TiO5 < Fe2O3/Fe2TiO5/CoFe-PBA across the entire surface states dominated region (0.86 to 1.46 V). The extended

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Figure 5. A) CV curves scanned immediately in dark at 20 mV s−1 after holding the electrode potential at 1.85 V versus RHE for 1 min under illumination (the inset shows its magnified plot). B) DOSS as a function of the applied potential. Error bars stem from the goodness of the EIS data fittings.
C) Kinetic scheme of the charge generation and transfer processes at the SEI at 1.23 V versus RHE under illumination of these electrodes. Green and
white areas represent electron filled and empty states, respectively. The dotted lines marked region in the CB filled states refer to photogenerated electrons with the same relative area as the empty states at the VB; the exceeding green regions highlight the doping levels in these electrodes. The green
arrows denote the charge generation process upon photons absorption; the yellow arrows denote the hole trapping process at SS (surface states);
the red arrows denote the hole transfer process from the SS to the electrolyte; the purple arrows denote electron transfer from CB states to the FTO
substrates. The thickness and shape of the arrows reveal the relative rates of the charge transfer processes, where the dotted lines mean the slowest rate
(Fe2O3 electrode) and the thickest lines means the fastest rate (Fe2O3/Fe2TiO5/CoFe-PBA electrode). The light indigo shaded areas refer to the relative
overlapping of the DOSS and water density of states. E: electrode potential; Ec,s: surface CB edge potential; EF: Fermi level of the semiconductors that
matches the electrode potential E) and the O2/H2O couple’s thermodynamic potential (1.23 V vs RHE); ESS: center potential of the SS distribution; Ev,s:
surface VB edge potential; λ : redox couple reorganization energy. It is worth noting that the relative size of the DOSS distribution for these electrodes
has been intentionally enlarged to highlight the SS. D): Total surface state density (Nss), donor density (Nd), and their ratio (Nss/Nd) plot. Nd was
estimated from the slopes of the Mott–Schottky plots (Figure S30, Supporting Information), whereas Nss was obtained from integration of the DOSS
profiles. Color bar with a unit of µm is plotted at the right Y axis for Nss/Nd. E) Ratio of the charge transfer rate constant (kct) and the sum of kct and
trapping rate constant (ktrapping) at different potentials.

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surface states distribution from 0.86 to 1.46 V in unmodified
Fe2O3 electrodes probably spans inside the CB, where recombination with CB electrons may occur. Moreover, it triggers a
deleterious Fermi level pinning, which also contributes to its
low photocurrent response.[54] Upon surface modification, the
DOSS maximum shifts to more positive potentials, i.e., further
into the bandgap, which minimizes overlap with the conduction band (Figure 5C). Further, its shape coincides well with the
cathodic CV curves obtained after holding the electrodes at a
potential of 1.85 V versus RHE for 1 min (Figure 5A), which
also indicates the correct utilization of the equivalent circuit
model for PEIS fitting.[44,48] Consequently, the ultrathin Fe2TiO5
and CoFe-PBA coatings indeed work together modifying
the density and energy level of the surface states in hematite
photoanodes.
Assuming surface states mediated charge-transfer (CT), the
CT rate constant (kct) at a certain electrode polarization potential (E), is proportional to Equation (2)[28,32,54–56]
kct ∝

∫

E

Ev,s

N ss f (E ) DH2O (E ) dE

(2)

in which f(E) is the Fermi–Dirac distribution indicating the
fraction of occupied surface states and DH2O(E) is the water
density of states (cm−2 eV−1). Given that the inelastic hole trapping process mediated by surface states is fast enough,[53] the
photocurrent response is proportional to kct,[57] depending on
the overlap between the filled surface states and the filled water
density of states. There is, thus, a direct correlation between
the percentage of available filled surface states (larger DOSS)
near the thermodynamic potential for water oxidation and
the observed photocurrent response at 1.23 V versus RHE
because of the required isoenergetic hole transfer process at
the SEI.[38,54,55] As illustrated in Figure 5C, the Fe2O3/Fe2TiO5/
CoFe-PBA electrode possesses the highest photocurrent
response at 1.23 V versus RHE due to the maximum energy
level matching between the surface states of the photoanodes
and the water density of states.
Furthermore, a combined comparison of the Nss, Nd, and
Nss/Nd ratio is presented in Figure 5D. The pristine Fe2O3
electrodes present a relatively high Nss/Nd ratio but poor PEC
performance, indicating that a large Nss/Nd ratio does not
guarantee a good photocurrent response due to the lack of
donors and low electrical conductivity. For the Fe2O3/Fe2TiO5
electrodes, the Nd is promoted via Ti doping, and this enables
a higher photocurrent. In the case of the Fe2O3/Fe2TiO5/CoFePBA electrode, Nss and Nd are both numerous enough to further increase the photocurrent response.
The charge transfer efficiency at the SEI is first estimated by
Equation (3)[33,54,57,58]
Transfer efficiency ( % ) =

R trapping
kct
=
kct + ktrapping Rct,trap + R trapping

(3)

where kct and ktrapping are the charge transfer and trapping
rate constants, respectively, and Rct and Rtrapping are the corresponding resistances. The calculated charge transfer efficiency from PEIS is shown in Figure 5E. In the case of the
Fe2O3/Fe2TiO5/CoFe-PBA electrode, over 60% of the holes are
Adv. Energy Mater. 2019, 1901836

transferred into the electrolyte at 1.23 V versus RHE, which
is almost 10 times as high as for pristine Fe2O3. Additionally,
the calculated charge transfer efficiency of these electrodes
is in good agreement with the steady-state current-voltage
relationship (Figure 4B) and the charge separation efficiencies
(Figure S32, Supporting Information) of Fe2O3, Fe2O3/Fe2TiO5
and Fe2O3/Fe2TiO5/CoFe-PBA electrodes obtained via comparing the cyclic voltammetry measurements in electrolyte with
and without a hole scavenger, further confirming the highest
charge transfer efficiency of the Fe2O3/Fe2TiO5/CoFe-PBA
electrodes.[32]

3. Conclusion
In summary, we have successfully integrated Fe2O3 nanowires
with an ultrathin Fe2TiO5 heterojunction and CoFe-PBA decoration for enhanced PEC water splitting in acidic electrolyte
(pH = 1). Thanks to the combination of core–shell Fe2O3/
Fe2TiO5 type II heterojunction nanowires and the catalytic function of CoFe-PBA, Fe2O3/Fe2TiO5/CoFe-PBA composite photoanodes are able to deliver 1.25 mA cm−2 photocurrent at 1.23 V
versus RHE, almost one order of magnitude photocurrent
increment in comparison to the pristine Fe2O3 nanowires. By
a systematic electrochemical investigation, the enhanced PEC
performance of the Fe2O3/Fe2TiO5/CoFe-PBA composite electrode can be attributed to the modified surface states density
after successive coatings, as well as the enhanced donor density
derived from inevitable Ti doping during the high temperature
sintering.[38] This work suggests that simultaneously employing
the synergy of core–shell Fe2O3/Fe2TiO5 type II heterojunction
and CoFe-PBA WOCs could be an effective approach to improve
the PEC performance of photoanodes in acidic electrolytes,
bringing new promise toward effective solar-fuel generation.

4. Experimental Section
Chemicals and Materials: All chemical reagents were purchased from
Sigma-Aldrich and used without further purification. All solutions were
prepared with Milli-Q water (≈18.2 MΩ cm resistivity). FTO substrate
(735167-1EA, 7Ω sq−1) was purchased from Sigma-Aldrich and precleaned before using as substrates.
FTO Pre-Clean Process: FTO substrates were cut into small pieces
(area: 1 cm × 3 cm) and washed by sonicating in a (1:1:1) mixture
of acetone (99.9%), isopropanol (99.9%), and water. After rinsing
thoroughly with distilled water, the FTO substrates were washed in
ethanol (Fluka, 99.8%) and then dried in air at 300 °C for 1 h (heating
rate: 8.5 °C min−1). Then, part of the FTO substrates (≈1 cm × 2 cm) was
covered using a polymer tape (Kaptons Foil, VWR International). The
uncoated part of the FTO was later employed as electric contact for the
working electrodes in the photoelectrochemical cell.
Fe2O3 Electrodes: Hematite nanowires were prepared according to
our previously published procedure.[38] Typically, a 200 mL Teflon-lined
stainless-steel autoclave was filled with 60 mL aqueous mixture solution
of 0.15 M ferric chloride (FeCl3, 97%), 1 M sodium nitrate (NaNO3, 99%)
and 316 µL hydrochloric (HCl, wt 37%). 6 pieces of FTO substrates were
put into the autoclave, which was sealed and heated at 95 °C for 4 h. A
homogenous layer of iron oxyhydroxide (FeOOH) nanowires was grown
onto the FTO substrate. After that, the FeOOH-coated FTO substrates
were washed with deionized water to remove any residual salts, and
subsequently pre-sintered in air at 550 °C (heating rate: 8.5 °C min−1)

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for 2 h to convert FeOOH nanowires into hematite nanowires. To
further reduce the surface defective sites and improve the crystallinity,
the obtained hematite nanowires were post-sintered at 750 °C in air for
additional 30 min and cooled down to the room temperature in 1 min.
Fe2O3/Fe2TiO5 Electrodes: The obtained hematite samples, after a
pre-sintering process (550 °C for 2h), were further subjected to an ALD
TiO2 process. The ALD TiO2 was performed in a R200 picosun atomic
layer deposition system at 150 °C with TiCl4 (99.9%) and water as the
precursors in an 8 mbar N2 flow atmosphere with a growth rate of
0.27 Å per cycle. The pulse time for the TiCl4 and water were 0.1 s and
the purge time was 10 s. The thickness of TiO2 coating onto the Fe2O3
nanowires can be controlled by changing the ALD deposition cycle. In
this case, the optimized TiO2 layer corresponds to 30 cycles according
to the previous report.[38] After that, a post-sintering process at 750 °C
for 30 min was performed to transform the surface ALD TiO2 into
Fe2TiO5.[38,54]
Fe2O3/Fe2TiO5/CoFe-PBA Electrodes: The obtained Fe2O3/Fe2TiO5
electrodes were further coated with CoFe-PBA in a chemical bath.[25]
Chemical bath deposition of CoFe-PBA on the Fe2O3/Fe2TiO5
electrodes were carried out according to the following procedure:
First, Co(NO3)26H2O (700 mg) and K3Fe(CN)6 (350 mg) powder were
dissolved in 40 mL of Milli-Q water under vigorous stirring. After that,
one piece of Fe2O3/Fe2TiO5 electrodes was immersed in a 5 mL glass vial
with 4 mL freshly prepared mixture solution containing Co(NO3)26H2O
+ K3Fe(CN)6. The glass vial was sealed and then heated at 60 °C for
different reaction times in the oven. Finally, the obtained samples were
rinsed with Milli-Q water to remove any impurities and were dried in the
oven at 60 °C overnight.
Structural and Morphological Characterization: The grazing incidence
X-ray diffraction analyses were conducted on a Bruker D4 X-ray
powder diffractometer via using the Cu Ka radiation (1.54184 Å) and
a 1D Lynkeye detector, which was equipped with a Gobel mirror in
the incident beam and equatorial Soller slits in the diffracted beam
(51 incidence angle, 2° per step). The surface morphology of the
electrodes was characterized via using a field emission gun scanning
electron microscope (FE-SEM, Zeiss Series Auriga microscopy)
equipped with an electron dispersive X-ray spectroscopy detector. X-ray
photoelectron spectroscopy was performed with a Phoibos 150 analyser
(SPECS GmbH, Berlin, Germany)) in an ultrahigh vacuum condition
(base pressure 4 × 10−10 mbar) with a monochromatic aluminium Kα
X-ray source (1486.74 eV). The energy resolution was 0.8 eV based
on the FWHM measurement of the Ag 3d5/2 peak for a sputtered
silver foil. Infrared absorption spectroscopy was performed with a
ThermoScientific NICOLET iS50 FT-IR spectrometer. Raman Spectrum
was conducted at InVia-RENISHAW with incident wavelength: 514 nm.
Optical properties of all electrodes were characterized by using a UV–
vis spectrophotometer (Lambda 950, Perkin Elmer) equipped with an
integrating sphere (150 mm diameter sphere covered with Spectralon
as the reflecting material, Perkin Elmer). Absorbance (A) measurements
were obtained from measured reflectance (R, %) and transmission
(T, %), using a wavelength range from 350 to 800 nm and a step of 5 nm,
respectively. All the samples for HRTEM and ADF-STEM were produced
via using a mechanical process.[38] HRTEM and ADF-STEM images were
obtained by using a FEI Tecnai F20 field emission gun microscope with a
0.19 nm point-to-point resolution at 200 kV equipped with an embedded
Quantum Gatan image filter for EELS analyses. Atomic resolution AC
HAADF STEM and further EELS-STEM analyses were conducted at a
FEI TITAN 80–300 STEM operated at 300 kV and a TITAN G3 50–300
PICO operated at 80 kV.[59,60] Images were analyzed via using Gatan
Digital Micrograph software. The Eje-Z, Rhodius, and JMOL software
packages were employed for the atomic supercell modeling with the
corresponding crystal phase parameters of each species obtained from
the inorganic crystal structure database (ICSD).[61–63] Specifically, to
further identify the crystal phases via HRTEM, HAADF STEM, and to
probe the spatial distribution of these components in the composite
hematite electrodes, we created crystal models based on the single
crystal data found in the ICSD. With these crystal models, the diffraction
patterns visualized from different zone axes of each species could be

Adv. Energy Mater. 2019, 1901836

simulated. Then, the simulated diffraction pattern was compared with
the FFT spectrum obtained on the atomic resolution HRTEM and
HAADF STEM experimental images for the identification of the crystal
phases in the composite hematite electrodes.
Photo-Electrochemical Measurements: Photocurrent density (j, mA
cm−2) versus applied potential (E, V) curves were conducted using a
three-electrode cell. The working, counter, and reference electrodes
were the composite hematite photoanodes (1 cm2 geometric area),
a Pt wire, and an Ag/AgCl (3 M KCl) reference electrode (Metrohm,
E = 0.203 versus NHE), respectively. The utilized electrolyte was a
0.1 M NaNO3 + 0.1 M HNO3 solution (pH = 1), which was purged
with N2 during the experiments. CV was taken using a computercontrolled potentiostat (VMP3, BioLogic Science Instruments). CV
scan was from 0.30 V versus Ag/AgCl to 1.60 V versus Ag/AgCl,
with a scan rate of 20 mV s−1. The photocurrent density is calculated
based on the geometric area. All potentials were corrected at 80%
for the ohmic drop, which was determined using the automatic
current interrupt (CI) method implemented by the potentiostat,[25]
and were converted with respect to the RHE: E (V versus RHE) = E
(V versus Ag/AgCl) + 0.0592 × pH + 0.203. Light illumination calibration
was performed using a 150 W AM 1.5G solar simulator (Solar Light
Co., 16S-300-002 v 4.0) with an incident light intensity set at 1 Sun
illumination (100 mW cm−2), as measured via using a thermopile
(Gentec-EO, XLPF12-3S-H2-DO) coupled with an optical power meter
(Gentec-EO UNO). In the PEC characterization, the light came from
the front side (hematite-electrolyte interface, front side illumination).
All the electrodes were repeated at least three times, and the statistical
photocurrent response data at 1.23 V versus RHE are included in the
Supporting Information.
Faradaic Efficiency Measurement: The O2 generated under
chronoamperometric conditions (1.23 V versus RHE) during 2 h and
under 1 Sun illumination was measured with the calibrated Fibox
detector immersed in the electrolyte in a gastight cell. The oxygen
evolution efficiencies were determined from the total amount of charge
Q (C) passed through the cell. Assuming that four holes are needed
to produce one O2 molecule, the theoretical yield can be calculated as
follows:
nO 2 =

Q
4F

(4)

where F is the Faraday constant. The total mole of oxygen produced was
quantitatively determined by using a calibrated Fibox detector with a
temperature sensor.
Incident Photon to Current Efficiency (IPCE): IPCE was characterized
using a xenon light source (Abet 150 W Xenon Lamp) coupled with a
monochromator (Oriel Cornerstone 260 1/4 m monochromator). The
wavelength was scanned from 350 to 800 nm (step: 10 nm step−1)
keeping the voltage fixed at 1.23 V versus RHE. The IPCE was calculated
based on the following equation[38]

(

)

IPCE ( % ) = (1240/λ ) × I/Jlight × 100

(5)

where I is the photocurrent density (mA cm−2) obtained using a
potentiostat recording the I-T curve at 1.23 V versus RHE, λ is the
incident light wavelength (nm) from monochromatic, and Jlight
(mW cm−2) is the power density of monochromatic light at a specific
wavelength. A source meter (Keithley Instruments Inc., model no. 2400)
coupled with the standard Silicon Photodiode (Thorlabs, S120VC) was
used to measure the power density of monochromatic light.
PEIS Data: PEIS data were obtained with an alternate current
perturbation of 5 mV in amplitude and a 100 mHz to 105 Hz frequency
range, both in the dark and under illumination, and under selected direct
current potentiostatic conditions (0.30 to 1.60 V vs Ag/AgCl). Nyquist
plots (imaginary vs real components of impedance, ZIm vs ZRe) were
fitted to the corresponding equivalent circuits via using Z-fit (BioLogic
Associates). Fitted capacitances and resistances are calculated based on
the electrode geometric area (1 cm2). Error bars were derived from the
goodness of the EIS data fittings.

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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.

Acknowledgements
This work was supported by the Spanish Ministerio de Economia y
Competitividad (MINECO, Grants CTQ2015-71287-R, CTQ2015-71287-R,
and CTQ2015-68770-R) and the coordinated Project ValPEC (ENE201785087-C3), the BIST Ignite Project inWOC2 and the Generalitat de
Catalunya (2017 SGR 90, 2017 SGR 327, 2017 SGR 329, 2017 SGR 1246,
and 2017 SGR 1406). ICN2 acknowledges the support from the Severo
Ochoa Program (MINECO, Grant SEV-2017-0706). ICN2, ICIQ, and IREC
are funded by the CERCA Programme/Generalitat de Catalunya. P.Y.T.
acknowledges the scholarship support of DAAD short term grant. H.C.D.
acknowledges support from the Deutsche Forschungsgemeinschaft
(SFB 917). F.S.H. thanks the “LaCaixa”-Severo Ochoa International
Programme (Programa internacional de Becas “LaCaixa”- Severo Ochoa)
for a Ph.D. fellowship. P.P. and M.H. thank for the support by the Federal
Ministry for Economic Affairs and Energy (BMWi) (Fundingregistration
number: 03ET6080E). M.L. thanks the COST Action StableNextSol
project MP1307, supported by COST (European Cooperation in Science
and Technology). H.X. acknowledges the Spanish MINECO through the
Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295
for the postdoctoral contract. The authors acknowledge Dr. Guillaume
Sauthier from the ICN2 for the XPS measurements and Dr. Nina
M. Carretero from IREC for their help in Faradaic efficiency measurement.
Part of the the was performed in the framework of Universitat Autònoma
de Barcelona Materials Science Ph.D. program.

Conflict of Interest
The authors declare no conflict of interest.

Keywords
acidic electrolyte, hematite, photoelectrochemical water splitting, surface
states
Received: June 7, 2019
Revised: July 1, 2019
Published online:

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