“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Sustainable Energy
Fuels, copyright © The Royal Society of Chemistry 2018 after peer review and technical editing by the publisher. To access the
final edited and published work see:

https://pubs.rsc.org/en/content/articlelanding/2018/se/c8se00146d#!divAbstract

A Hybrid Molecular Photoanode for Efficient Light Induced Water
Oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/

Sergi Grau,†a Serena Berardi,*†b Alicia Moya,c Roc Matheu,a Vito Cristino,b Juan José Vilatela,c Carlo
A. Bignozzi,b Stefano Caramori,b Carolina Gimbert-Suriñach,*a Antoni Llobet*a
A hybrid photoanode, made of a multilayered heterostructured WO3/BiVO4 semiconductor and a molecular water oxidation
catalyst Ru(tda)(py-pyr)2 (Ru-WOC, where tda is [2,2’:6’,2’’-terpyridine]-6,6’’-dicarboxylato and py-pyr is 4-(pyren-1-yl)-N(pyridin-4-ylmethyl)butanamide), is described. Both elements are linked by a highly conductive carbon nanotubes fibre film
(CNTf), which acts as charge transfer and anchoring platform, where the catalyst is attached through pi-pi stacking
interactions. Photoelectrochemical characterization of the resulting electrodes shows that the full photoanode
WO3/BiVO4/CNTf/Ru-WOC outperforms the bare WO 3/BiVO4 electrode at a potential range of 0.3 V to 0.8 V vs NHE at pH 7,
with current densities enhanced by 0.05 to 0.29 mA/cm2. Bulk electrolysis experiments and oxygen gas measurements prove
that the enhanced photocurrent is due to the catalytic water oxidation reaction. Detailed electrochemical impedance
spectroscopy (EIS) analysis is used to investigate the role of the multiple layers involved in the process. The CNTf/Ru-WOC
interface is responsible of increasing charge accumulation and also reducing recombination phenomena. On one hand, the
CNTf is able to hold the charge coming from the light absorbed by the WO3/BiVO4 semiconductor, as shown by the high
capacitive values observed for a WO 3/BiVO4/CNTf electrode in the whole range of studied potentials (0.15-0.85 V vs NHE).
On the other hand, the Ru-WOC transfers the charge to the solution by performing fast water oxidation catalysis. This is
supported by the low resistivity shown by the full WO 3/BiVO4/CNTf/Ru-WOC electrode at low potentials (E < 0.5 V vs NHE).
The robustness and high catalytic rate of the Ru-WOC ensures the proper performance of the hybrid photoelectrode device.
The latter is particularly important as it opens an avenue of opportunities to improve the performance of photoanodes for
the water oxidation reaction based on the easy modification of ligands of the molecular catalyst to tune its structural,
electronic and catalytic properties. This is a unique advantage with regard to the commonly used catalysts based on metal
oxide or oxy(hydroxides), with limited tunability.

Introduction
The conversion of solar energy is on the focus of worldwide
scientific research with the aim to turn into a more sustainable
energy management of our society. While photovoltaic (PV) and
solar thermal technologies are a reality with solar panels
generating part of global energy production, the conversion of
solar energy into clean fuels is still in a laboratory-scale stage.1
The latter strategy offers a solution to the mismatch between
sunlight intermittence and energy needs by converting the
sunlight energy into storable chemical bonds. One of the
reasons that hinders the implementation of such technology is
the difficulty to find an efficient light absorbing material
coupled to active electrocatalysts, either connected through
wires in a typical PV-electrochemical (PV-EC) configuration or
via semiconductor-electrolyte interfaces.1-3 The system needs
to effectively absorb light and produce long lived separated
charges to facilitate fast electron and hole transfer to catalysts

in order to drive the desired chemical reactions. In this regard,
the water oxidation reaction is one of the limiting process
within such a complex system due to high demands needed for
efficient catalysts, namely, a high thermodynamic potential and
large kinetic complexity. Indeed the latter complexity is a
consequence of the need to transfer four electrons and four
protons from two water molecules together with the
generation of an O-O bond (Figure 1).4-6 For this reason, great
efforts are devoted nowadays to the field of solar-to-fuel
conversion, specifically to find highly performant electroanodes
and photoanodes for the water oxidation reaction.2,7-9
Tungsten oxide (WO3) and bismuth vanadate (BiVO4) are two
promising semiconductor materials with suitable band gap and
valence band positon to carry out photoinduced water
oxidation.7,8 In particular, BiVO4 has shown excellent
photocurrent densities at potentials close to and even lower
than the thermodynamic value. However, the performances of
BiVO4 photoanodes are still far from its theoretical maximum
capacities.7,8 This problem is associated with the slow charge
carrier mobility as well as slow kinetics of the water oxidation

a. Institute

of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science
and Technology (BIST), Avda. Països Catalans 16, 43007 Tarragona, Spain.
b. Dept. of Chemistry and Pharmaceutical Sciences, University of Ferrara, Via di
Mortara 17, 44121, Ferrara, Italy.
c. IMDEA Materials Institute, C. Eric Kandel 2, 28906, Getafe, Madrid, Spain.
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 1

Please do not adjust margins

ARTICLE

Journal Name

reaction at the surface of the electrode. Four main strategies
have been used to overcome these unfavorable phenomena: i)
doping with metallic elements (particularly good results
obtained with W and Mo),10 ii) tuning morphology and
nanostructure of the material,10-12 iii) formation of
heterojunctions with other semiconductors (e.g. WO3) 13,14 or
conductive materials (e.g. graphene)7,15 and iv) modifying the
surface of the photoelectrode with suitable water oxidation
catalysts (metal oxides or oxy(hydroxides)).16,17 While the first
three strategies (i-iii) are designed to enhance charge carrier
mobility, charge separation and/or avoid recombination, the
latter strategy (iv) is mainly addressed to reduce the high kinetic
barrier associated with the water oxidation reaction.
Interestingly, recent works point to the poor effect of metal
oxide derivatives as water oxidation catalysts.17,18 Instead, they
attribute the enhancement of the photocurrent densities to
higher charge separation efficiency obtained with the presence
of the metallic oxide deposited on the surface of the
photoanode.17,18 These observations are in agreement with the
intrinsic limited catalytic activity of metal oxides such as cobalt
oxide that competes with the catalysis by the BiVO4 itself. In
contrast to metal oxide catalysts, molecular water oxidation
catalysts (WOC) have experienced a tremendous improvement
in catalytic rates, achieving maximum turnover frequencies that
are three to four orders of magnitude higher than the best
water oxidation catalysts based on oxides. Indeed, molecular
water oxidation catalysts outperform the naturally occurring
Oxygen Evolving Complex in Photosystem II, responsible for
oxygen release in the natural photosynthetic process in plants,
by two orders of magnitude.5,6,19 More recently, strategies to
anchor these molecular catalysts on electrode surfaces have
been developed, maintaining the high catalytic activity
observed in homogeneous conditions and in some cases even
increasing their performance.20-22
In this work, we couple a WO3/BiVO4 photoanode with a highly
active molecular WOC with the aim of enhancing the kinetics of
the chemical reaction, minimizing recombination and oxide
degradation, and thus improving the overall performance of the
photoelectrode. A carbon nanotube fibre interlayer film
between the photoactive material and the catalyst is used as
highly conductive platform that facilitates the anchoring of the
functionalized molecular WOC and allows fast charge transfer
between the two key components (semiconductor and catalyst)
of the photoelectrode (Figure 1).

Results
Fabrication of the hybrid photoanode

The multilayer architecture of the WO3/BiVO4 photoactive
material used in this study is shown in Figure 1. It is made of
three well defined layers, namely, a compact thin film of
tungsten oxide (WO3(C)), a mesoporous layer of the same
material (WO3(P))23 and a top layer of porous BiVO4.16 The inner
compact layer of WO3(C) is essential for isolating the back FTO
contact from the electrolyte solution, avoiding dark currents
associated with shunt phenomena. This effect is clearly seen in

Figure 1. Hybrid photoanode design studied in this work. WO3(C)
= compact tungsten oxide film, WO3= mesoporous tungsten
oxide layer, Ru-WOC = molecular ruthenium water oxidation
catalyst with pyrene groups in red and dangling carboxylate
group in blue, WNA = water nucleophilic attack, E(cat) =
oxidation potential of catalyst to give the Ru(V)-oxo active
species, E(H2O/O2) = thermodynamic potential of water
oxidation at pH 7.
Figure S5, which shows that in the absence of light the
electrode WO3(C)/WO3(P)/BiVO4 does not show any current in
the whole potential range while the electrode that lacks the
WO3 compact underlayer (WO3(P)/BiVO4) shows significant dark
currents from 0.75 V vs NHE. On the other hand, the
heterostructured WO3/BiVO4 porous material has shown
improved performance as compared to bare BiVO4
photoanodes.13,14 This improvement is attributed to the good
band alignment and synergistic effects between the two
semiconductors, leading to higher charge separation
efficiencies.14 Detailed structural, electrochemical and optical
characterization of the photoanodes used in this work is given
in Figures S1-S5 in the supporting information.
The water oxidation catalyst, Ru-WOC in Figure 1, was selected
for its high activity both in homogeneous and in heterogeneous
conditions.19,20,22 This catalyst is known to operate via an
electrophilic Ru(V)-oxo species that suffers a water nucleophilic
attack (WNA) by a water molecule forming an hydroperoxo key
derivative with a newly formed O-O bond. The high catalytic
rate that characterizes this catalyst is attributed to the role of
the dangling carboxylate group assisting the O-O bond
formation step (Figure 1, blue moiety of the Ru-WOC
complex).24 Several reports describe the direct attachment of
molecular catalysts on semiconducting materials, usually by
using phosphonates or carboxylate groups.25,26,27 In the present
case, when the Ru-WOC is attached to the WO3/BiVO4
photoanode, no improvement of the performance of the
electrode is observed. This result is due to deactivation of the
dangling carboxylate group, which interacts directly with the
metal oxide hindering its proper activation.27 This can be
avoided by using a conductive support that isolates the catalyst
from the photoelectrode surface. The support should guarantee
fast charge transfer from the semiconductor to the catalyst,
while maintaining the ruthenium catalytic centre unaltered. In
this regard, carbon nanostructures such as graphene, single or
multiwalled carbon nanotubes are good candidates for their
unique properties including high surface area, high thermal and
electrical conductivity as well as chemical inertness. In
particular, carbon nanotube fibres (CNTf) are excellent

2 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Please do not adjust margins
Journal Name

ARTICLE

conductive platforms. They are macroscopic arrays of CNTs
combining surface area around 250 m2/g, electrical conductivity
of 3.5 x 105 S/m, high electrochemical stability28 and the
possibility to produce them in a kilometer/day scale,29 for
example as free-form electrodes. In this direction, the hybrid
molecular photoanode device is prepared following a new
strategy; an approximately 1 × 1 cm2 of WO3/BiVO4 photoanode
is covered with an approximately 0.9 × 0.9 cm2 of a CNTf film
with a thickness of around 5µm, estimated based on fibre linear
density, winding rate and spinning time.29 Next, the resulting
WO3/BiVO4/CNTf is soaked in a 1mM methanolic solution of the
Ru-WOC precursor overnight to allow for the chemical
attachment of the ruthenium complex on the surface of the
electrode via pi-pi interactions of the pyrene groups of the
complex and the graphitic structure of the CNTf (Figure 1). After
this treatment, the electrodes are rinsed with clean methanol
to make sure that excess of the homogeneous catalyst is
removed and carefully dried with an air flow. In order to
estimate the loading of the catalyst on the surface of the
electrode, analogous experiments with non-photoactive
ITO/CNTf electrodes were done. Analysis of the current
underneath the Ru(III/II) wave in the cyclic voltammetry (CV) of
the resulting ITO/CNTf/Ru-WOC electrodes allow us to calculate
the amount of catalyst precursor on the CNTf film, giving values
up to 𝛤 = 15 nmol/cm2, comparable to those observed in
previous studies that used multiwalled carbon nanotubes
(Figure S9).20,22 This confirms that the free-standing format and
high surface area of CNTf makes them an attractive conducting
support in heterogeneous catalysis.
Photoelectrochemical performance

The photoelectrochemical performance of the photoanodes
was tested in a typical one-compartment three-electrodes
photoelectrochemical cell, as described in detail in the
supporting information. The CNTf are extremely black layers
that completely block incoming light, therefore all the
photoelectrochemical results presented in this study are based
on back illumination of the photoelectrodes. The
photoelectrochemical behaviour of the bare WO3/BiVO4
photoanodes improves over consecutive linear sweep
voltammetry (LSV) experiments as shown in Figure S8. Thus, all
electrodes were exposed to an electrochemical pre-treatment
until a constant photocurrent response was achieved previous
to any modification (usually 3-5 LSV cycles).
Figure 2 shows the chopped light LSV performance of a
WO3/BiVO4 photoanode and that of a WO3/BiVO4/CNTf
analogue in pH 7 buffered solution, showing a significant
decrease of the photocurrent when the CNTf is present
(compare red and black traces). This result is in agreement with
the CNTf film blocking the BiVO4 active centres responsible for
the water oxidation catalysis. Thus, while the CNTf film is highly
conductive, it is unable to perform the chemical reaction, in
other words it does not act as water oxidation catalysts under
these conditions. On the other hand, the presence of the RuWOC on the surface of the electrode significantly improves the
performance of the photocurrent, particularly in the range of

Figure 2. IR drop compensated chopped light LSV (5 mV/s) of
different photoanodes in pH 7 phosphate buffer. WO3/BiVO4
(red), WO3/BiVO4/CNTf (black) and WO3/BiVO4/CNTf/Ru-WOC
(blue).
0.3 V to 0.8 V vs NHE (compare blue trace with black and red
traces in Figure 2). The improvement goes from 0.05 mA/cm2 at
0.3 V up to 0.29 mA/cm2 at 0.7 V vs NHE. In order to make sure
that this enhancement is due to the catalyst and not to the
methanolic solution treatment during catalyst deposition, we
did analogous blank tests. Specifically, WO3/BiVO4/CNTf
electrodes were soaked in clean methanol overnight and
subsequently dried. The resulting treated electrodes did not
show a significant improvement of the performance, confirming
that the increased photocurrent observed in Figure 2 is due to
the presence of the catalyst (Figure S10).
Bulk electrolysis experiments show that the full electrode
WO3/BiVO4/CNTf/Ru-WOC outperforms the bare WO3/BiVO4
electrode (Figure 3, blue and red traces, respectively). We
attribute this photocurrent enhancement to the higher catalytic

Figure 3. Bulk electrolysis experiment at applied potential Eapp =
0.645 vs NHE in pH 7 phosphate buffer, IR drop was not
compensated. WO3/BiVO4 (red), WO3/BiVO4/CNTf (black) and
WO3/BiVO4/CNTf/Ru-WOC (blue).
activity of Ru-WOC compared to BiVO4. The integrity of the
molecular water oxidation catalyst after bulk electrolysis

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 3

Please do not adjust margins

ARTICLE

Journal Name

experiments was tested by carefully removing the CNTf/RuWOC film from the surface of the WO3/BiVO4 electrode and
placing it on a clean ITO electrode. As depicted in Figure S9, the
fingerprint of the ruthenium complex is still intact on the film.
Oxygen detection experiments were done to prove that the
enhanced
photocurrent
obtained
with
the
WO3/BiVO4/CNTf/Ru-WOC electrode is due to water oxidation
catalysis. An adapted Generator-Collector device was used,
which is inspired by previously reported procedures that have
been built for related photoelectrochemical systems (Figure 4
and Figures S11-S13 in the supporting information).30,31 In
particular, a slow scan rate LSV on the desired photoelectrode
(Generator) is performed while a controlled potential
electrolysis in a closely positioned FTO electrode (Collector) is
simultaneously run. In the Collector electrode, the generated
oxygen is reduced after diffusion through the solution from the
Generator electrode. Thus, the current observed in the
controlled potential electrolysis experiment is a measure of the
evolved gas due to the water oxidation reaction on the
photoelectrode. This method allows us to measure the oxygen
gas in situ and to control at what potential the chemical
reactions starts. As shown in Figure 4 and Figure S12, the oxygen
evolution reaction for the full hybrid electrode
WO3/BiVO4/CNTf/Ru-WOC starts at a potential 140 mV lower
than that of the bare electrode WO3/BiVO4 (ca. 0.41 V for the
former and 0.55 V for the latter). The faradaic efficiency
calculated
for
these
experiments
are
94%
for
WO3/BiVO4/CNTf/Ru-WOC and 97% for WO3/BiVO4 (Figure S13).
The delay in detecting the oxygen is attributed to the diffusion
process of the oxygen gas from the generator electrode to the
collector electrode (0.3 V from LSV in Figure 2 vs 0.41 V from GC method in Figure 4 and Figure S12).
Electrochemical impedance spectroscopy analysis

In order to get further insights into the role of the different
interfaces involved in the photocurrent generation,
electrochemical impedance spectroscopy (EIS) experiments
under illumination and different applied biases were carried
out. The results have been fitted using a circuital model adapted
from reported works (Figure 5A).14,32,33 Briefly, it consists of a
serial resistance of the electrochemical cell (R1), in series with a
R2-CPE2 mesh and an extended element (TL). The latter is
commonly used to represent the transmission line of
mesoporous films (WO3/BiVO4 in our case), through which the
photogenerated carriers travel overcoming the transport
resistance (Rtr) between the fused nanoparticles. The
approximation of a negligible capacitance contribution between
the particles is generally used. Furthermore, the model includes
a resistive contribution (Rct) due to the charge carrier
recombination (which eventually influences the charge transfer
to the electrolyte), as well as a capacitive contribution
accounting for the accumulation of photogenerated charges in
the film (CPEfilm). On the other hand, the R2-CPE2 mesh has been
introduced to take into account the FTO/WO3 interface or the
CNTf/electrolyte interface when the CNTf are present.

Figure 4. Oxygen gas detection experiments using an adapted
Generator-Collector method performed in pH 7. 30,31 Solid lines
correspond to LSV (5 mV/s) performed on the photoanode
electrode (Generator). The dotted lines correspond to the in situ
current observed on the Collector electrode due to O2 reduction
(Controlled Potential Electrolysis at Eapp = -0.355 V vs NHE, see
details in the supporting information). WO3/BiVO4 in the dark
(black), WO3/BiVO4 under 1 sun illumination (red),
WO3/BiVO4/CNTf/Ru-WOC
in
the
dark
(grey)
and
WO3/BiVO4/CNTf/Ru-WOC under 1 sun illumination (blue).
The experimental Nyquist plots of the different photoanodes,
together with the corresponding fittings, are reported in Figure
5B and Figure S14, evidencing the good agreement provided by
the selected circuital model. The major contribution of the total
resistance is given by Rct, which follows the same dependence
from the applied bias of the inverse of the differential resistance
(Figure S15). The exact values, obtained for the different
resistive and capacitive contributions, are reported in Table S1
and Table S2, respectively.
In the model, Rct represents the recombination through the
nanostructured film, and therefore its contribution is expected
to decrease for systems that can effectively transfer charge to
the electrolyte and perform fast water oxidation catalysis.
Figure 5C reports the bias dependence of the logarithm of Rct
and gives a quantification of this recombination phenomenon.
The presence of the Ru-WOC coupled to the CNTf reduces the
Rct value of the corresponding photoanode by ca. three times in
the whole range of potential (compare blue and black traces).
The charge transfer rate to the aqueous electrolyte is indeed
expected to be lower for the WO3/BiVO4/CNTf electrodes
without the molecular catalyst, due to the hydrophobicity of the
CNTf layer and the blockage of the active catalytic sites of BiVO4.
On the other hand, with regard to the bare WO3/BiVO4
electrode, the enhancement of the charge transfer by the RuWOC is only improved at low potentials, as evidenced by the
crossing point of the blue and red traces in Figure 5C.
Furthermore, it is worth noting that the WO3/BiVO4 electrode
shows a bell-shaped behaviour (red trace in Figure 5C) with a
minimum around 0.69 V vs NHE, i.e. at the inflection point of
the LSV curve of the photoelectrode (Figure S14). With regard

4 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Please do not adjust margins
Journal Name

ARTICLE

Figure 5. A, Equivalent circuit used to fit the EIS data of the
photoanodes. B, complex plane Nyquist plots for the different
electrodes measured under 1 sun illumination in pH 7
phosphate buffer at 0.440 V vs NHE. The corresponding fits are
also reported as thick solid lines. C, logRct vs potential. Legend
code: WO3/BiVO4 (red), WO3/BiVO4/CNTf (black) and
WO3/BiVO4/CNTf/Ru-WOC (blue).
to other resistive contributions, the resistance that we associate
with the inert FTO/WO3 interface for bare WO3/BiVO4 is < 15 
and it is substantially independent on the applied bias, as
expected when the interface is not directly involved in the
photocurrent generation mechanism.14 On the other hand, for
the WO3/BiVO4/CNTf electrodes, the R2 values are much higher
(in the range of 330-625 ), as well as bias-dependent. Thus, in
this case, the R2-CPE2 mesh has been assigned to the
CNTf/electrolyte interface, whose higher resistive contribution
covers the one due to the FTO/WO3 interface, that was

considered in the first place. This characteristic can be seen in
the Nyquist plots, resulting in an additional arc for the
WO3/BiVO4/CNTf electrode (Figure 5B, black trace). When the
latter is coupled with the Ru-WOC, the R2 values (assigned by
analogy to the functionalized CNTf/electrolyte interface
decrease by at least one order of magnitude (< 25 ) with
respect to the non-catalytic WO3/BiVO4/CNTf, as expected from
the improved surface kinetics in the presence of the catalyst,
which also induces wetting of the porous CNTf layer.
Furthermore, the R2 values are bias dependent for
WO3/BiVO4/CNTf/Ru-WOC, with a minimum at 0.64 V vs NHE,
close to the inflection point of the LSV curve (Figure S14 and
S15E). More information on the nanofibres/electrolyte
interface can be extracted from the capacitance values given by
CPE2, which follows a similar potential dependence for both
WO3/BiVO4/CNTf and WO3/BiVO4/CNTf/Ru-WOC (Figure S16).
These results support the hypothesis of reduced electron
recombination with surface trapped holes (that may translate
into WO3/BiVO4 defects, oxidized CNTf or oxidized catalyst) as
the potential increases.
In the case of the WO3/BiVO4 electrodes, the logarithm of the
capacitance associated to the charge accumulation in the film
(CPEfilm) displays a linear dependence on the applied bias, as
soon as the surface states have been emptied by the voltage, at
approximately 0.54 V vs NHE (Figure S17). This behaviour is
typical of a chemical capacitor, as expected for this kind of
electrodes.34 A higher CPEfilm value has been observed for
WO3/BiVO4/CNTf/Ru-WOC photoanodes, confirming that in the
presence of the catalyst a higher charge can be accumulated
and then transported to the electrolyte. It is worth noting that
the higher CPEfilm values for WO3/BiVO4/CNTf/Ru-WOC
correspond to lower Rct values in the 0.15-0.50 V vs NHE range,
i.e. where the photocurrent response is anticipated with
respect to the bare WO3/BiVO4 electrode (compare red and blue
traces in Figure 5C and Figure S17). Furthermore, we observe
that
CPEfilm
follows
same
bias
dependence
for
WO3/BiVO4/CNTf/Ru-WOC and WO3/BiVO4 electrodes at high
potentials (> 0.59 V vs NHE), while for lower values it results
essentially constant, as also observed in the case of
WO3/BiVO4/CNTf. The higher CPEfilm values obtained in the
latter case suggest a significant charge accumulation on the
CNTf layer (or on BiVO4 states covered by CNTf), which can be
followed by a good charge transfer to the electrolyte only when
the WOC is coupled to them.

Discussion
Figure 1 shows the potential values required to oxidize the
molecular catalyst attached on the surface of the
WO3/BiVO4/CNTf/Ru-WOC photoanode and generate the
Ru(V)=O active species (E(RuV/IV) = 1.3V vs NHE). This potential
should be easily overcome by the oxidative power of the low
BiVO4 valence band upon illumination (EVB, pH 7 = 2.2-2.4 V vs
NHE).35 Once the Ru(V)=O species is formed, the water
oxidation catalytic process is triggered and occurs at a
maximum rate of almost 8000 cycles per second. 19,20 Thus,
upon illumination, the electrode WO3/BiVO4/CNTf/Ru-WOC

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 5

Please do not adjust margins

ARTICLE

Journal Name

should be able to perform oxygen evolution catalysis. Figure 2
proves that indeed the WO3/BiVO4/CNTf/Ru-WOC shows
enhanced photocurrent at potentials lower than the
WO3/BiVO4 and WO3/BiVO4/CNTf electrodes (compare blue
with red and black traces). Bulk electrolysis and oxygen
measurement experiments confirmed that the enhanced
photocurrent is sustained over several minutes and that the
current is due to oxygen gas formation (Figure 3 and 4). More
importantly, the oxygen gas is detected at approximately 0.41 V
vs
NHE
using
the
catalyst
modified
electrode
WO3/BiVO4/CNTf/Ru-WOC, 140 mV before the bare
photoanode WO3/BiVO4 (Figure 4 and Figure S12) with faradaic
efficiencies close to 100%. These results not only prove that the
Ru-WOC performs the reaction but also, that the charge is
efficiently transferred all the way through the multilayered
architecture of the photoanode, i.e., from the inner
semiconductor, to the CNT fibres and finally to the catalyst. In
this regard, the CNT fibres form an effective and stable
interlayer with excellent charge transfer abilities. In addition,
their work function above the conduction band of BiVO4 might
contribute to charge separation by preventing recombination
phenomena.36 Also, the CNTf layer constitute an excellent
platform for the easy anchoring of molecular catalysts.
EIS analysis allowed us to differentiate two potential ranges in
which the CNTf/Ru-WOC interface have distinct influence on the
overall catalytic performance. At high potentials, although the
charge accumulation capacity of the complete photoanode
WO3/BiVO4/CNTf/Ru-WOC is higher than that of simple
WO3/BiVO4, the recombination is also higher, resulting in similar
performance (Figures 5C and Figure S17, E > 0.5 V vs NHE). At
this potential range, the water oxidation catalysis by the RuWOC starts competing with the catalysis by the BiVO4 active
centres of the bare WO3/BiVO4 electrode, that are available in
higher quantities in the absence of carbon nanotube fibres. On
the other hand, at low potentials the full electrode
WO3/BiVO4/CNTf/Ru-WOC outperforms the WO3/BiVO4
photoanode, as a result of higher charge accumulation capacity
and lower recombination phenomena (Figure 5C and Figure
S17, E < 0.5 V vs NHE). This is particularly remarkable at the 0.30.5 V vs NHE range where the WO3/BiVO4 photoanode has
practically no activity (Figure 1). This is a consequence of the
high kinetic barrier of the water oxidation catalysis at BiVO4
catalytic centres, which require higher potential to perform the
reaction and avoid recombination phenomena. In sharp
contrast, the specially designed Ru-WOC is able to perform the
catalytic reaction at a potential as low as 0.3 V vs NHE, that is,
ca. 300 mV higher than the semiconductor capacities (E = -0.05
V vs NHE, Figures S5). This value is close to those obtained for
the best BiVO4 based photoanodes described to date with onset
potentials for the water oxidation catalysis in the range of -0.2
V to 0.2 V vs NHE. Among them, the best performing examples
contain metal oxides (ruthenium, iron, cobalt or nickel) as water
oxidation
catalyst
on
the
surface
of
the
photoelectrode.11,12,17,18,37-39 Only limited examples of
photoanodes modified with molecular catalysts have been
reported with less efficiency than that of the photoanode
WO3/BiVO4/CNTf/Ru-WOC reported in this work.40,41,42

Importantly, the photoelectrode reported here can perform the
reaction 510 mV below the thermodynamic potential of the
water oxidation reaction at pH 7, allowing for the overlap with
state of the art photocathodes for the overall unassisted water
splitting reaction.37,39 This work highlights the promising role of
robust and fast true molecular water oxidation catalysts on
enhancing photoanode performance.

Summary and conclusions
The preparation of a multilayer heterostructured WO3/BiVO4
photoanode containing two layers of WO3 (compact and
mesoporous) and a porous BiVO4 film is described. The resulting
photoactive material is covered by a highly conductive carbon
nanotube fibre (CNTf) that is able to accumulate charge but
unable to perform the water oxidation reaction. Finally, the
surface of the CNTf is functionalized with a molecular catalyst
precursor (Ru-WOC) modified with pyrene groups forming pi-pi
stacking bonds. The Ru-WOC takes the charge accumulated on
the CNTf and rapidly transfers it to the solution by performing
the water oxidation reaction. The system works at low
potentials, where the bare WO3/BiVO4 is not active at all for the
oxygen evolution reaction. However, at high potentials, the
beneficial influence of the CNTf/Ru-WOC interfaces is not
sufficient to overcome the good performance of the bare
WO3/BiVO4. All these results have been rationalized by means
of EIS experiments that prove the advantage of using a highly
active molecular catalyst to trigger the water oxidation reaction
all the way down to 510 mV below the thermodynamic
potential of the reaction. Further improvements on the current
molecular based photoanode can be focused on increasing the
catalyst loading that should lead to enhanced current density at
low potentials.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
MINECO and FEDER (CTQ2016-80058-R, CTQ2015-73028-EXP,
SEV 2013-0319, ENE2016-82025-REDT, CTQ2016-81923-REDC),
and AGAUR (2017-SGR-1631) are gratefully acknowledged for
financial support. The project leading to this application has
received funding from the European Union’s Horizon 2020
research and innovation programmes under the ERC-STG
678565 and the Marie Skłodowska-Curie Grant Agreement No
705723. Dr. Marcel Risch is gratefully acknowledged for
providing and helping with experimental set-up.

Notes and references
1.
2.

N. S. Lewis, Science, 2016, 351.
K. Sivula and R. van de Krol, Nat. Rev. Mater., 2016, 1,
15010.

6 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Please do not adjust margins
Journal Name
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.

21.
22.
23.
24.
25.

26.
27.

ARTICLE

J. H. Montoya, L. C. Seitz, P. Chakthranont, A. Vojvodic, T.
F. Jaramillo and J. K. Nørskov, Nat. Mater., 2016, 16, 70.
X. Sala, I. Romero, M. Rodríguez, L. Escriche and A. Llobet,
Angew. Chem. Int. Ed., 2009, 48, 2842.
S. Berardi, S. Drouet, L. Francàs, C. Gimbert-Suriñach, M.
Guttentag, C. Richmond, T. Stoll and A. Llobet, Chem. Soc.
Rev., 2014, 43, 7501.
P. Garrido-Barros, C. Gimbert-Suriñach, R. Matheu, X. Sala
and A. Llobet, Chem. Soc. Rev., 2017, 46, 6088.
H. L. Tan, R. Amal and Y. H. Ng, J. Mater. Chem. A, 2017, 5,
16498.
S. S. M. Bhat and H. W. Jang, ChemSusChem, 2017, 10,
3001.
C. C. L. McCrory, S. Jung, I. M. Ferrer, S. M. Chatman, J. C.
Peters and T. F. Jaramillo, J. Am. Chem. Soc., 2015, 137,
4347.
Z.-F. Huang, L. Pan, J.-J. Zou, X. Zhang and L. Wang,
Nanoscale, 2014, 6, 14044.
T. W. Kim and K.-S. Choi, Science, 2014, 343, 990.
Y. Kuang, Q. Jia, H. Nishiyama, T. Yamada, A. Kudo and K.
Domen, Adv. En. Mat., 2016, 6, 1501645.
S. J. Hong, S. Lee, J. S. Jang and J. S. Lee, Energy Environ.
Sci., 2011, 4, 1781.
X. Shi, I. Herraiz-Cardona, L. Bertoluzzi, P. Lopez-Varo, J.
Bisquert, J. H. Park and S. Gimenez, Phys. Chem. Chem.
Phys., 2016, 18, 9255.
M.-Q. Yang, N. Zhang, M. Pagliaro and Y.-J. Xu, Chem. Soc.
Rev., 2014, 43, 8240.
J. A. Seabold and K.-S. Choi, J. Am. Chem. Soc., 2012, 134,
2186.
Y. Ma, A. Kafizas, S. R. Pendlebury, F. Le Formal and J. R.
Durrant, Adv. Funct. Mater., 2016, 26, 4951.
C. Zachaus, F. F. Abdi, L. M. Peter and R. van de Krol, Chem.
Sci., 2017, 8, 3712.
R. Matheu, M. Z. Ertem, J. Benet-Buchholz, E. Coronado, V.
S. Batista, X. Sala and A. Llobet, J. Am. Chem. Soc., 2015,
137, 10786.
J. Creus, R. Matheu, I. Peñafiel, D. Moonshiram, P.
Blondeau, J. Benet-Buchholz, J. García-Antón, X. Sala, C.
Godard and A. Llobet, Angew. Chem. Int. Ed., 2016, 55,
15382.
P. Garrido-Barros, C. Gimbert-Suriñach, D. Moonshiram, A.
Picón, P. Monge, V. S. Batista and A. Llobet, J. Am. Chem.
Soc., 2017, 139, 12907.
R. Matheu, I. A. Moreno-Hernandez, X. Sala, H. B. Gray, B.
S. Brunschwig, A. Llobet and N. S. Lewis, J. Am. Chem. Soc.,
2017, 139, 11345.
V. Cristino, S. Marinello, A. Molinari, S. Caramori, S. Carli,
R. Boaretto, R. Argazzi, L. Meda and C. A. Bignozzi, J. Mater.
Chem. A, 2016, 4, 2995.
R. Matheu, M. Z. Ertem, C. Gimbert-Suriñach, J. BenetBuchholz, X. Sala and A. Llobet, ACS Catal., 2017, 7, 6525.
M. K. Brennaman, R. J. Dillon, L. Alibabaei, M. K. Gish, C. J.
Dares, D. L. Ashford, R. L. House, G. J. Meyer, J. M.
Papanikolas and T. J. Meyer, J. Am. Chem. Soc., 2016, 138,
13085-13102.
M. Wang, Y. Yang, J. Shen, J. Jiang and L. Sun, Sustainable
Energy Fuels, 2017, 1, 1641.
L. Francàs, C. Richmond, P. Garrido-Barros, N. Planas, S.
Roeser, J. Benet-Buchholz, L. Escriche, X. Sala and A. Llobet,
Chem. Eur. J., 2016, 22, 5261.

28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.

E. Senokos, V. Reguero, J. Palma, J. J. Vilatela and R.
Marcilla, Nanoscale, 2016, 8, 3620.
V. Reguero, B. Alemán, B. Mas and J. J. Vilatela, Chem.
Mater., 2014, 26, 3550.
B. D. Sherman, M. V. Sheridan, C. J. Dares and T. J. Meyer,
Anal. Chem., 2016, 88, 7076-7082.
J. T. Kirner and R. G. Finke, ACS Appl. Mater. Interfaces,
2017, 9, 27625.
J. Bisquert, Phys. Chem. Chem. Phys., 2000, 2, 4185.
J. Bisquert, G. Garcia-Belmonte, F. Fabregat-Santiago and
A. Compte, Electrochem. Commun., 1999, 1, 429.
J. Bisquert, Phys. Chem. Chem. Phys., 2003, 5, 5360.
J. K. Cooper, S. Gul, F. M. Toma, L. Chen, P.-A. Glans, J. Guo,
J. W. Ager, J. Yano and I. D. Sharp, Chem. Mater., 2014, 26,
5365.
A. Moya, N. Kemnade, M. R. Osorio, A. Cherevan, D.
Granados, D. Eder and J. J. Vilatela, J. Mater. Chem. A,
2017, 5, 24695.
F. Jiang, Gunawan, T. Harada, Y. Kuang, T. Minegishi, K.
Domen and S. Ikeda, J. Am. Chem. Soc., 2015, 137, 13691.
F. Lin, D. Wang, Z. Jiang, Y. Ma, J. Li, R. Li, C. Li, Energy
Environ. Sci. 2015, 5, 6400.
P. Bornoz, F. F. Abdi, S. D. Tilley, B. Dam, R. van de Krol, M.
Graetzel, K. Sivula, J. Phys. Chem. C. 2014, 118, 16959.
K. S. Joya, N. Morlanés, E. Maloney, V. Rodionov, K.
Takanabe, Chem. Commun. 2015, 51, 13481.
W. Li, S. W. Sheehan, D. He, Y. He, X. Yao, R. L. Grimm, G.
W. Brudvig, D. Wang, Angew. Chem. Int. Ed. 2015, 54,
11428.
B. Liu, J. Li, H.-L. Wu, W.-Q. Liu, X. Jiang, Z.-J. Li, B. Chen, C.H. Tung, L.-Z. Wu, ACS Appl. Mater. Interfaces 2016, 8,
18577.

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 7

Please do not adjust margins

“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Sustainable Energy
Fuels, copyright © The Royal Society of Chemistry 2018 after peer review and technical editing by the publisher. To access the
final edited and published work see:

https://pubs.rsc.org/en/content/articlelanding/2018/se/c8se00146d#!divAbstract
Table of Contents

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 8

Please do not adjust margins