ORIGINAL RESEARCH
published: 14 August 2018
doi: 10.3389/fchem.2018.00302

Photoinduced Oxygen Evolution
Catalysis Promoted by
Polyoxometalate Salts of Cationic
Photosensitizers
Joaquín Soriano-López 1,2 , Fangyuan Song 3 , Greta R. Patzke 3* and
J. R. Galan-Mascaros 1,4*
1

Institute of Chemical Research of Catalonia, Barcelona Institute of Science and Technology, Tarragona, Spain,
Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Tarragona, Spain, 3 Department of Chemistry,
University of Zurich, Zurich, Switzerland, 4 ICREA, Passeig Lluis Companys, Barcelona, Spain

2

Edited by:
Soumyajit Roy,
Indian Institute of Science Education
and Research Kolkata, India
Reviewed by:
Graham Newton,
University of Nottingham,
United Kingdom
Samar Kumar Das,
University of Hyderabad, India
*Correspondence:
Greta R. Patzke
greta.patzke@chem.uzh.ch
J. R. Galan-Mascaros
jrgalan@iciq.es

The insoluble salt Cs15 K[Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ] (CsCo9 ) is tested as
heterogeneous oxygen evolution catalyst in light-induced experiments, when combined
with the homogeneous photosensitizer [Ru(bpy)3 ]2+ and the oxidant Na2 S2 O8 in neutral
pH. Oxygen evolution occurs in parallel to a solid transformation. Post-catalytic essays
indicate that the CsCo9 salt is transformed into the corresponding [Ru(bpy)3 ]2+ salt,
upon cesium loss. Remarkably, analogous photoactivated oxygen evolution experiments
starting with the [Ru(bpy)3 ](5+x) K(6−2x) [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]·(39+x)H2 O
(RuCo9 ) salt demonstrate much higher efﬁciency and kinetics. The origin of this improved
performance is at the cation-anion, photosensitizer-catalyst pairing in the solid state.
This is beneﬁcial for the electron transfer event, and for the long-term stability of the
photosensitizer. The latter was conﬁrmed as the limiting process during these oxygen
evolution reactions, with the polyoxometalate catalyst exhibiting robust performance in
multiple cycles, upon addition of photosensitizer, and/or oxidant to the reaction mixture.
Keywords: water splitting, oxygen evolution, polyoxometalates, photosensitizer, cobalt

Specialty section:
This article was submitted to
Inorganic Chemistry,
a section of the journal
Frontiers in Chemistry
Received: 30 April 2018
Accepted: 03 July 2018
Published: 14 August 2018
Citation:
Soriano-López J, Song F, Patzke GR
and Galan-Mascaros JR (2018)
Photoinduced Oxygen Evolution
Catalysis Promoted by
Polyoxometalate Salts of Cationic
Photosensitizers. Front. Chem. 6:302.
doi: 10.3389/fchem.2018.00302

Frontiers in Chemistry | www.frontiersin.org

INTRODUCTION
Sunlight is the preferred carbon-neutral energy source for competing with fossil fuels for energy
production, because solar radiation is readily accessible at almost any location on the surface of the
Earth (Cook et al., 2010). Artiﬁcial photosynthesis aims to mimic natural photosynthesis, where
sunlight is stored in the form of chemical bonds through reduction of CO2 into sugars, employing
H2 O as the ultimate source of electrons (Mcevoy and Brudvig, 2006). Therefore, an artiﬁcial
photosynthesis device would convert sunlight into spatially separated electron/hole pairs and store
its energy subsequently into chemical bonds by means of water splitting, obtaining hydrogen as
a clean fuel together with oxygen as the only side product (Lewis and Nocera, 2006; Balzani et al.,
2008; Barber, 2009). Unfortunately, the market introduction of commercial artiﬁcial photosynthesis
devices is still hampered by the lack of robust, inexpensive and eﬃcient water oxidation catalysts
(WOCs) (Dau et al., 2010; Seh et al., 2017).

1

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

Over the last decades, scientists have reported a wide variety
of new WOCs. Homogeneous organometallic compounds work
at fast oxygen evolution rates and oﬀer good processability
(Concepcion et al., 2008, 2009; Bozoglian et al., 2009; Blakemore
et al., 2010; Xu et al., 2010; Lloret-Fillol et al., 2011; Mccool
et al., 2011; Barnett et al., 2012; Duan et al., 2012; Liu
and Wang, 2012; Zhang et al., 2013; Goberna-Ferrón et al.,
2014). However, they often suﬀer from limited long-term
stability due to oxidative degradation of the organic ligands
in the harsh working conditions needed for water oxidation.
Precious-metal-based WOCs, for instance Ir-, and Ru-based
materials, have shown superior performance and stability for
water oxidation catalysis (Pillai et al., 2000; Youngblood et al.,
2009; Blakemore et al., 2010; Duan et al., 2012). Unfortunately,
the high production price due to metal scarcity questions
their viable implementation into commercial devices. Earth
abundant transition metal oxides and perovskites are a robust
alternative, but exclusively in alkaline media (Galán-Mascarós,
2015). Therefore, alternatives to the current state-of-the-art
catalysts are needed.
Polyoxometalates (POMs) have recently appeared as a
promising new catalyst class (Geletii et al., 2008; Sartorel
et al., 2008). When employed as WOCs, they combine the
most appealing features of homogeneous and heterogeneous
materials, and many of them can be obtained from inexpensive
raw materials. They are all-inorganic molecular clusters with
high stability under strongly oxidizing conditions. At the
same time, their molecular nature provides access to the
tunability and superior processing capabilities of homogeneous
catalysts for their easier implementation into devices (Pope,
1983; Pope and Müller, 2001). POMs have shown high
catalytic activity in water oxidation over a remarkable pH
range (0-10), and they retain their catalytic activity under
heterogeneous conditions as their corresponding insoluble salts,
or when anchored onto solid supports (Wu et al., 2012;
Guo et al., 2013; Quintana et al., 2013; Soriano-López et al.,
2013).
Among polyoxometalates, the cobalt-containing POMs (CoPOMs) have emerged as the most promising WOCs due to
their high eﬃciency and kinetics (Goberna-Ferrón et al., 2012;
Lv et al., 2012, 2014; Evangelisti et al., 2013). After Hill et al.
reported the OER activity of the [Co4 (H2 O)2 (PW9 O34 )2 ]10−
polyanion (Yin et al., 2010; Huang et al., 2011; Stracke
and Finke, 2011, 2013, 2014), we turned our attention to
the high nuclearity [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]16−
(Goberna-Ferrón et al., 2012, 2015; Soriano-López et al., 2013;
Co9 , Figure 1). Co9 shows good activity for photo-assisted water
oxidation in homogeneous conditions, exhibiting fast charge
transfer kinetics with the model [Ru(bpy)3 ]2+ photosensitizer
(bpy = 1,2-dipyridyl; Natali et al., 2017). It is also active in the
solid state when processed as an insoluble salt with alkaline metal
countercations (Soriano-López et al., 2013).
In this work we report the next required step on the road to
technological applications for Co9 in an artiﬁcial photosynthesis
platform, namely its combination with a photosensitizer in
a light-induced process in heterogeneous conditions. These
essays have been very successful with other POMs and

Frontiers in Chemistry | www.frontiersin.org

FIGURE 1 | Polyhedral representation of the polyanion
[Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]16− (Co9 ); WO6 , gray octahedra; PO4 ,
orange tetrahedra; CoO6 , pink octahedra.

water oxidation catalysts to assess photo-induced catalytic
performance, mechanistic considerations, and stability issues
(Puntoriero et al., 2010; Gao et al., 2013; Sartorel et al.,
2013; Al-Oweini et al., 2014; Xiang et al., 2014; Natali et al.,
2017). Our experiments conﬁrm the eﬃcient electron transfer
between catalyst and sensitizer, even when both species are
combined into an insoluble salt. The latter opens up interesting
possibilities for future combinations of cationic photosensitizers
with polyanionic WOCs for the construction of compact
functional photoelectrodes.

EXPERIMENTAL SECTION
Materials and Synthesis
Tris(2,2′ -bipyridyl)dichlororuthenium(II)
hexahydrate
and sodium persulfate were purchased from TCI and
Sigma-Aldrich (>99% purity) and used without further
puriﬁcation. The synthesis of Cs15 K[Co9 (H2 O)6 (OH)3
(HPO4 )2 (PW9 O34 )3 ]·39H2 O (CsCo9 ) was already reported
(Soriano-López et al., 2013). [Ru(bpy)3 ](5+x) K(6−2x) [Co9
(H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]·(39+x)H2 O
(RuCo9 )
was prepared by metathesis: A stoichiometric excess
of [Ru(bpy)3 ]Cl2 was added to a solution containing
Na8 K8 [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]·43H2 O (KCo9 ).
RuCo9 immediately precipitated as an orange powder. It was
ﬁltered, washed with water and acetone, and air-dried.

Material Characterizations
Elemental CHN analysis was performed with an Elemental
Microanalyzer Flash model 1112. Detection of Co, Ru, and
W was performed on an inductively coupled plasma atomic
emission spectrometer iCap 6500 (Thermo Fisher Scientiﬁc), and

2

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

K was detected on a 2,380 atomic absorption spectrophotometer
(Perkin-Elmer), both by Mikroanalytisches Labor Pascher
(Remagen/Germany). Thermogravimetric analyses were
performed with powder samples using a TGA/SDTA851 Mettler
Toledo with a MT1 microbalance. Dynamic light scattering
was used to measure the particle size distribution employing
a Malvern NanoZS analyzer. FT-IR spectra were collected in
the 3600–400 cm−1 range with a Bruker Optics FTIR Alpha
spectrometer equipped with a DTGS detector and a KBr
beamsplitter at 4 cm−1 resolution. Raman measurements
were acquired using a Renishaw inVia Reﬂex Raman confocal
microscope (Gloucestershire, UK) equipped with a diode
laser emitting at 785 nm at a nominal power of 300 mW, and
a Peltier-cooled CCD detector (−70◦ C) coupled to a Leica
DM-2500 microscope. X-ray photoelectron spectroscopy (XPS)
(K-ALPHA, Thermo Scientiﬁc SSTTI at University of Alicante)
was used to analyze the surface of the samples. All spectra were
collected using Al-Kα radiation (1486.6 eV), monochromatized
by a twin crystal monochromator, yielding a focused X-ray
spot with a diameter of 400 µm, at 3 mA x 12 kV. The alpha
hemispherical analyzer was operated in the whole energy band,
and 50 eV in a narrow scan to selectively measure the particular
elements.

SCHEME 1 | Schematic representation of the light-driven water oxidation
catalysis reaction, employing [Ru(bpy)3 ]2+ as photosensitizer, S2 O2− as
8
sacriﬁcial electron acceptor, and a polyoxometalate as water oxidation catalyst
(further side reactions giving rise to sulfate radical anions were omitted for
clarity).

Photoinduced Water Oxidation Catalysis

promotes oxygen evolution, which was monitored
using a ﬂuorescence O2 -sensor probe for increasing
amounts of CsCo9 (1–50 mg). The proposed net
reaction mechanism for light-driven water oxidation
catalyzed by POMs is depicted in Scheme 1. No oxygen
evolution was detected in the absence of any of the
components.
The reaction starts with fast kinetics immediately after
light irradiation, and slows down until oxygen evolution
stops reaching a plateau after 2 h. We analyzed the oxygen
production as a function of catalyst content (Figure 2 and
Table 1). The highest values of turnover number (TON) and
turnover frequency (TOF) obtained were 14.2 and 10.8 h−1 ,
for the minimum quantity used (1 mg, ≈ 0.1 µmol). In terms
of chemical yield (CY, see SI), a maximum 9.2% was reached
for intermediate CsCo9 contents (10 mg, ≈ 1 µmol) in the
investigated range. After oxygen evolution, CsCo9 was recovered
from the reaction vessel to perform structural characterization.
The FT-IR spectrum shows the typical Co9 bands within
the 1,100-400 cm−1 range, identical to those observed with
the freshly made CsCo9 . We also found additional bands
in the region between 1,200 and 1,600 cm−1 , which can be
attributed to the bipyridyl (bpy) ligand (Figure S1). The same
information is obtained from the Raman spectra (Figure S2).
Moreover, comparison of the XPS spectra (Figure S3) showed
the appearance of intense Ru peaks in the recovered CsCo9,
and disappearance of the expected Cs peak (Figure S4). The
data in their entirety suggest that cation exchange occurred
under turnover conditions, i.e., Cs+ cations are replaced by
[Ru(bpy)3 ]2+ cations. Indeed, the Raman spectrum of the
recovered CsCo9 is reminiscent of the corresponding Raman
spectrum of the salt obtained by addition of an excess of
[Ru(bpy)3 ]Cl2 to an aqueous K16 Co9 solution (Figure S9).

Oxygen evolution experiments were performed in a 6.7 mL
headspace Schlenk tube sealed with a rubber septum (PFTE).
The Schlenk tube was covered with aluminum foil, in order
to avoid an early light-induced reaction of the system, and
ﬁlled with 1 mM (9.4 mg) [Ru(bpy)3 ]Cl2 , 5 mM (14.9 mg)
Na2 S2 O8 , the desired amount of catalyst, and 12.5 mL of
40 mM KPi buﬀer solution at pH 7.0. Experiments employing
the RuCo9 salt as catalyst were performed with and without
addition of [Ru(bpy)3 ]Cl2 , the former for comparison in the
same conditions required for Co3 O4 experiments. Suspensions
were completely deaerated by purging with nitrogen. A
baseline of 20 min was recorded to ensure that no oxygen
leakage or side reactions took place. Next, the system was
exposed to the light of a blue LED lamp (wavelength at
peak emission = 465 nm; OSRAM Opto Semiconductors)
working at 0.20 A and 11.4 V. The concentration of oxygen
in the headspace was measured by employing a O2 -sensor
probe (Ocean Optics NeoFOX oxygen-sensing system
equipped with a FOXY probe). Turnover number (TON)
and turnover frequency (TOF) were estimated per Co9 content
as obtained from chemical analyses on fresh compounds
(see SI).

RESULTS AND DISCUSSION
Visible-Light-Driven Water Oxidation by
CsCo9 in Heterogeneous Conditions

Water oxidation experiments were carried out with
[Ru(bpy)3 ]2+ as a model photosensitizer and S2 O2−
8
as sacriﬁcial electron acceptor, in a suspension of the
insoluble salt Cs15 K[Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]
(CsCo9 ). Light irradiation (λ > 400 nm) of this mixture

Frontiers in Chemistry | www.frontiersin.org

3

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

FIGURE 2 | Oxygen evolution proﬁle of the CsCo9 salt in KPi (40 mM) buffer
at pH 7, with [Ru(bpy)3 ]2+ (1 mM) and S2 O2− (5 mM).
8
FIGURE 3 | Oxygen evolution proﬁle for solid RuCo9 in KPi (40 mM) buffer at
pH 7 with S2 O2− (5 mM).
8

TABLE 1 | Comparison of visible-light-driven oxygen evolution performance of
CsCo9 and RuCo9 catalysts(a) .
POM

Catalyst (mg)

CsCo9 (ﬁrst run)

10

20

50

TON

14.2

5.3

2.9

0.7

0.2

10.8

4.0

1.4

0.3

0.2

CY (%)

4.4

8.4

9.2

4.0

3.6

TON

27.3

20.3

16.7

7.2

2.7

TOF (h−1 )

19.1

11.9

17.0

9.0

4.3

CY (%)
POM

5

TOF (h−1 )

RuCo9

1

7.6

28.4

47.6

42.4

36.4

Catalyst (µm)

0.1

0.5

1

2

5

(Table S2, Figure S13). This Ru/POM stoichiometry is far
too low in comparison with our RuCo9 analyses (Table S1),
and thus it is not representative of the RuCo9 catalyst. The
obtention of this crystalline phase, though, precludes the
isolation of other salts with higher [Ru(bpy)3 ]2+ content, closer
to the present RuCo9 solid. The most plausible explanation
is that RuCo9 actually consists of a mixture of diﬀerent
[Ru(bpy)3 ]/Co9 salts, and their slightly diﬀerent solubility
and composition gives small deviations depending on the
given analytical technique. With all the analytical data taken
into account (Table S1), we assign an average stoichiometry
[Ru(bpy)3 ](5+x) K(6−2x) [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]·
(39+x)H2 O (RuCo9 ), where −1 < x < 1 (see Table S1
and Figure S5). This powder is insoluble in water at room
temperature with an average particle size of 374 nm (Figure S6).
When a suspension of RuCo9 in a solution of S2 O2−
8
is irradiated (λ > 400 nm), oxygen evolution starts. In this
case, the proposed reaction mechanism is analogous to that
depicted in Scheme 1, but with photosensitizer and catalyst
bound together in the solid state through electrostatic cationanion interactions. Remarkably, the measured oxygen evolution
in these conditions (Figure 3) is signiﬁcantly superior to the
ﬁrst run starting from photosensitizer in solution (Table 1
and Figure 4). The maximum TON (27.3) and TOF (19.1
h−1 ) values are doubled, and the CY showed a remarkable
increase up to 47.6%. Pulsed experiments conﬁrmed that
oxygen evolves exclusively when the light source is switched on
(Figure S7).

a TON, total turnover number after completion of the reaction; TOF, slope of the oxygen
evolution curve at the starting time; CY, total chemical yield after completion of the
reaction.

Visible-Light-Driven Water Oxidation by
RuCo9 in Heterogeneous Conditions

Addition of an excess of [Ru(bpy)3 ]Cl2 to an aqueous
KCo9 solution forms immediately an insoluble precipitate.
The presence of the [Ru(bpy)3 ]2+ cation and the
[Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]16−
anion
were
conﬁrmed by FT-IR spectroscopy (Figure S8) with the
signature bands for both molecular species. However, the
exact stoichiometry was diﬃcult to completely assess. We
carried out elemental CHN analyses, and metal ICP analyses,
along with thermogravimetry analyses, and they were not
fully consistent (Table S1). It is worthy to note at this point
that our attempts to crystallize this compound in order to
accurately characterize its composition and structure failed,
because slow diﬀusion between solutions of cation and
anion produce insoluble single crystals of the compound
[Ru(bpy)3 ]2 K12 [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]·xH2 O

Frontiers in Chemistry | www.frontiersin.org

Stability of the RuCo9 System

In order to determine the limiting agent in the photo-assisted
oxygen evolution reaction, we carried out diﬀerent tests.

4

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

FIGURE 5 | Top: Oxygen evolution with successive additions of oxidant
S2 O2− (5 mM) to a KPi (40 mM) buffered suspension of RuCo9 (10 mg,
8
0.9 µmol) at pH 7. Bottom: Oxygen evolution after addition of photosensitizer
[Ru(bpy)3 ]2+ (1 mM) and oxidant S2 O2− (5 mM) as solid reagents to a KPi
8
(40 mM) buffered suspension of recycled RuCo9 at pH 7 compared with the
same experiments starting from fresh CsCo9 .

Successive additions of S2 O2− to the as-used RuCo9 suspension
8
indicate that oxygen evolution activity is severely aﬀected after
each cycle (Figure 5 and Table 2), i.e., the system can barely
perform three cycles before reaching complete deactivation.
After deactivation, addition of an aliquot containing the
photosensitizer [Ru(bpy)3 ]2+ and S2 O2− to the reaction vessel
8
restarts oxygen evolution, with rates and yields comparable to
those obtained with CsCo9 (Figure 5). This behavior can only
be explained with deactivation of the photosensitizer in RuCo9
recycling experiments, probably due to oxidative degradation of

FIGURE 4 | Comparison of TON, TOF, and chemical yield (CY) for RuCo9
(blue) and CsCo9 (red). Experiments were carried out in a KPi (40 mM) buffer
at pH 7, with S2 O2− (5 mM). [Ru(bpy)3 ]2+ (1 mM) was added to the
8
suspension of CsCo9 , whereas no homogeneous photosensitizer was added
for the RuCo9 catalyst.

Frontiers in Chemistry | www.frontiersin.org

5

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

TABLE 2 | Comparison of the RuCo9 -catalyzed light-driven oxygen evolution
performance obtained for successive addition of S2 O2− (5 mM) to the reaction
8
vessela .
TON

TOF (h−1 )

CY (%)

1st cycle

16.7

17.0

47.6

2nd cycle

6.4

2.7

17.6

3rd cycle

2.1

0.7

5.6

a TON,

total turnover number at the ﬁnal reaction time; TOF, slope of the oxygen evolution
curve at the starting time; CY, total chemical yield at the ﬁnal reaction time.

the organic ligands, during the harsh working conditions. The
catalytic POM appears to be robust, since its performance is
maintained during successive cycles.

Analysis of Adventitious CoOx Formation

In water oxidation with cobalt-based catalysts, it is fundamental
to rule out the in situ formation of cobalt oxide CoOx , a
competent heterogeneous WOC. This could occur through Co2+
leaching from the RuCo9 salt, and the subsequent formation of
CoOx under oxidative conditions. Thus, we analyzed the as-used
RuCo9 with diﬀerent experimental techniques in the search for
traces of CoOx .
RuCo9 was recovered from the reaction vessel after the
visible-light-driven water oxidation experiments. The signature
FT-IR and Raman bands of the Co9 cluster remain identical
when compared with pristine RuCo9 , suggesting that the bulk
POM structure is maintained during the experiments. Raman
spectroscopy is particularly suited to detect even traces of CoOx
due to its high surface sensitivity, but no bands that could be
assigned to a CoOx species are present (Figures S8–S10).
As with fresh RuCo9 , the elemental and ICP analyses
showed small deviations, making diﬃcult to conﬁrm ﬁnal
stoichiometry. The numbers are not too diﬀerent from the
original stoichiometry (Table S1). However, we need to point
out that these analyses show a decrease for all elements, except
for W that increases. We assign this surprising result to the
deterioration of the compounds during working conditions
[triggered by the [Ru(bpy)3 ]2+ decomposition], making them
even more insoluble, and untractable.
Another powerful surface-sensitive technique is XPS. Pristine
and recovered RuCo9 salts display analogous XPS spectra
(Figure S11). The presence of CoOx should include the
appearance of a typical Co3+ peak below 780 eV (Chuang et al.,
1976; Tan et al., 1991; Hara et al., 2000). Close analysis of
the Co and O edges in search of such features that could be
assigned to the presence of an CoOx phase were negative. XPS
spectra of RuCo9 before and after oxygen evolution show intense
bands only in the 780-783 eV range, which diﬀer from those
expected for CoOx (Figure S12). This indicates that no cobalt
oxide amounts are formed during turnover conditions within the
detection limit of these techniques.
In order to gather additional indirect proof of the absence
of the signiﬁcant participation of cobalt oxide impurities, we
compared the photo-induced oxygen evolution reaction starting
from the RuCo9 salt to the Co3 O4 catalyst (Figure 6 and

Frontiers in Chemistry | www.frontiersin.org

FIGURE 6 | Comparison of the measured oxygen evolution employing
equimolar Co amounts for RuCo9 (blue) and Co3 O4 (green). The experiments
were performed in a KPi (40 mM) buffer at pH 7 with [Ru(bpy)3 ]2+ (1 mM) as
photosensitizer and S2 O2− (5 mM).
8

TABLE 3 | Comparison of the light-driven oxygen evolution data catalyzed by
RuCo9 and by Co3 O4 under the same reaction conditionsa,b .
Catalyst

TON

TOF (h−1 )

CY (%)

RuCo9

3.2

2.3

17.2

Co3 O4

0.9

0.3

12.8

a TON,

total turnover number at the ﬁnal reaction time; TOF, slope of the oxygen evolution
curve at the starting time; CY, total chemical yield at the ﬁnal reaction time.
b The experiments were performed in a KP (40 mM) buffer at pH 7 with [Ru(bpy) ]2+
i
3
(1 mM) as photosensitizer, S2 O2− (5 mM) as sacriﬁcial electron acceptor, and with
8
13.99 µmols of Co in the form of RuCo9 or Co3 O4 .

Table 3). For equimolar conditions, RuCo9 displays an overall
better performance, with faster onset kinetics and a higher
eﬃciency. This is incompatible with the attribution of the
catalytic activity observed for RuCo9 to very small traces of
CoOx , which may be below the detection limit of Raman or XPS
techniques.

CONCLUSIONS
We compared the heterogeneous catalytic activity of
two diﬀerent Co9 starting materials under visible-lightdriven water oxidation conditions at neutral pH. Direct
combination of CsCo9 with a homogeneous photosensitizer
yields a maximum turnover number (TON) of 14.2 and
a maximum turnover frequency (TOF) of 10.8 h−1 with
oxygen yields around 10%. Pre-catalytic incorporation of
the cationic photosensitizer into the polyoxometalate salt
through substitution of the alkali metal improves the oxygen

6

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

evolution notably, aﬀording chemical yields close to 50%.
We associate this improvement to two beneﬁcial eﬀects of
photosensitizer immobilization. On the one hand, the closer
cation-anion (photosensitizer/catalyst) interaction in the solid
state facilitates electron transfer, and therefore enhances the
oxygen evolution kinetics. Additionally, the incorporation of the
photosensitizer into the solid state partially improves its stability,
an additional beneﬁt to increase the eﬃciency of the overall
process.
Our experimental data indicate that oxygen evolution
eventually stops due to decomposition of the photosensitizer.
Successive additions of photosensitizer re-start the water
oxidation reaction at consistent rates, supporting the stable
catalytic performance of Co9 . We carried out careful surface
analyses on the as-used catalyst in the search of traces of
cobalt oxide. Neither Raman nor XPS spectroscopy showed any
feature that could be associated with CoOx species. Additionally,
RuCo9 exhibits superior catalytic performance than Co3 O4 .
Thus, the hypothetical presence of undetectable CoOx traces
cannot be responsible for the observed catalytic activity. This
supports the genuine catalytic activity of RuCo9 for photoinduced water oxidation as the ﬁrst example, to the best of
our knowledge, of an eﬀective photosensitizer/catalyst electron
transfer in an ionic salt. The superior performance of this
ionic composite opens up interesting perspectives for the use of
such materials in the development of compact photoanodes for
artiﬁcial photosynthesis.

AUTHOR CONTRIBUTIONS

REFERENCES

Dau, H., Limberg, C., Reier, T., Risch, M., Roggan, S., and Strasser, P. (2010).
The mechanism of water oxidation: from electrolysis via homogeneous
to biological catalysis. ChemCatChem 2, 724–761. doi: 10.1002/cctc.201
000126
Duan, L., Bozoglian, F., Mandal, S., Stewart, B., Privalov, T., Llobet, A., et al. (2012).
A molecular ruthenium catalyst with water-oxidation activity comparable to
that of photosystem II. Nat. Chem. 4, 418–423. doi: 10.1038/nchem.1301
Evangelisti, F., Car, P.-E., Blacque, O., and Patzke, G. R. (2013). Photocatalytic
water oxidation with cobalt-containing tungstobismutates: tuning the metal
core. Catal. Sci. Technol. 3, 3117–3129. doi: 10.1039/c3cy00475a
Galán-Mascarós, J. R. (2015). Water oxidation at electrodes modiﬁed with
earth-abundant transition-metal catalysts. ChemElectroChem 2, 37–50.
doi: 10.1002/celc.201402268
Gao, J., Cao, S., Tay, Q., Liu, Y., Yu, L., Ye, K., et al. (2013). Moleculebased water-oxidation catalysts (WOCs): Cluste-size-dependent dye-sensitized
polyoxometalates for visible-light-driven O2 evolution. Sci. Rep. 3:1853.
doi: 10.1038/srep01853
Geletii, Y. V., Botar, B., Köegerler, P., Hillesheim, D. A., Musaev, D. G., and
Hill, C. L. (2008). An all-inorganic, stable, and highly active tetraruthenium
homogeneous catalyst for water oxidation. Angew. Chem. Int. Ed. 47,
3896–3899. doi: 10.1002/anie.200705652
Goberna-Ferrón, S., Peña, B., Soriano-López, J., Carbó, J. J., Zhao, H., Poblet, J. M.,
et al. (2014). A fast metal-metal bonded water oxidation catalyst. J. Catal. 315,
25–32. doi: 10.1016/j.jcat.2014.04.010
Goberna-Ferrón, S., Soriano-López, J., Galán-Mascarós, J. R., and Nyman, M.
(2015). Solution speciation and stability of cobalt-polyoxometalate water
oxidation catalysts by X-ray scattering. Eur. J. Inorg. Chem. 2015, 2833–2840.
doi: 10.1002/ejic.201500404
Goberna-Ferrón, S., Vigara, L., Soriano-López, J., and Galán-Mascarós, J. R.
(2012). Identiﬁcation of a nonanuclear {CoII } polyoxometalate cluster as a
9
homogeneous catalyst for water oxidation. Inorg. Chem. 51, 11707–11715.
doi: 10.1021/ic301618h

GP and JRG-M proposed the concept. GP, JRG-M, and
JS-L designed the experiments. JS-L and FS carried out the
experiments. All authors analyzed the data and contributed to the
manuscript writing.

FUNDING
We would like to acknowledge the ﬁnancial support from the
Spanish Ministerio de Economía y Competitividad (MINECO)
through project CTQ2015-71287-R and the Severo Ochoa
Excellence Accreditation 2014-2018 SEV-2013-0319; and the
Generalitat de Catalunya (2017-SGR-1406 and the CERCA
Programme). GP and FS are grateful for ﬁnancial support by
the Swiss National Science Foundation (Sinergia Grant No.
CRSII2_160801/1) and by the University Research Priority
Program Solar Light to Chemical Energy Conversion (URPP
LightChEC) of the University of Zurich. This collaboration took
place in the context of the COST PoCheMoN action supported
by the European Research Area.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fchem.
2018.00302/full#supplementary-material

Al-Oweini, R., Sartorel, A., Bassil, B. S., Natali, M., Berardi, S., Scandola, F.,
et al. (2014). Photocatalytic water oxidation by a mixed-valent MnIII MnIV O3
3
manganese oxo core that mimics the natural oxygen-evolving center. Angew.
Chem. Int. Ed. 53, 11182–11185. doi: 10.1002/anie.201404664
Balzani, V., Credi, A., and Venturi, M. (2008). Photochemical conversion of solar
energy. ChemSusChem 1, 26–58. doi: 10.1002/cssc.200700087
Barber, J. (2009). Photosynthetic energy conversion: natural and artiﬁcial. Chem.
Soc. Rev. 38, 185–196. doi: 10.1039/B802262N
Barnett, S. M., Goldberg, K. I., and Mayer, J. M. (2012). A soluble
copper-bipyridine water-oxidatzion electrocatalyst Nat. Chem. 4, 498–502.
doi: 10.1038/nchem.1350
Blakemore, J. D., Schley, N. D., Balcells, D., Hull, J. F., Olack, G. W., Incarvito,
C. D., et al. (2010). Half-sandwich iridium complexes for homogeneous wateroxidation catalysis. J. Am. Chem. Soc. 132, 16017–16029. doi: 10.1021/ja104775j
Bozoglian, F., Romain, S., Ertem, M. Z., Todorova, T. K., Sens, C., Mola, J.,
et al. (2009). The Ru-Hbpp water oxidation catalyst. J. Am. Chem. Soc. 131,
15176–15187. doi: 10.1021/ja9036127
Chuang, T. J., Brundle, C. R., and Rice, D. W. (1976). Interpretation of the x-ray
photoemission spectra of cobalt oxides and cobalt oxide surfaces. Surf. Sci. 59,
413–429. doi: 10.1016/0039-6028(76)90026-1
Concepcion, J. J., Jurss, J. W., Brennaman, M. K., Hoertz, P. G., Patrocinio, A. O.,
Murakami Iha, N. Y., et al. (2009). Making oxygen with ruthenium complexes.
Acc. Chem. Res. 42, 1954–1965. doi: 10.1021/ar9001526
Concepcion, J. J., Jurss, J. W., Templeton, J. L., and Meyer, T. J. (2008).
One site is enough. catalytic water oxidation by [Ru(tpy)(bpm)(OH2 )]2+
and [Ru(tpy)(bpz)(OH2 )]2+ . J. Am. Chem. Soc. 130, 16462–16463.
doi: 10.1021/ja8059649
Cook, T. R., Dogutan, D. K., Reece, S. Y., Surendranath, Y., Teets, T. S., and Nocera,
D. G. (2010). Solar energy supply and storage for the legacy and nonlegacy
worlds. Chem. Rev. 110, 6474–6502. doi: 10.1021/cr100246c

Frontiers in Chemistry | www.frontiersin.org

7

August 2018 | Volume 6 | Article 302

Soriano-López et al.

Heterogeneous Photoinduced OER With Polyoxometalates

Seh, Z. W., Kibsgaard, J., Dickens, C. F., Chorkendorﬀ, I., Nørskov, J.
K., and Jaramillo, T. F. (2017). Combining theory and experiment
in electrocatalysis: insights into materials design. Science 355:eaad4998.
doi: 10.1126/science.aad4998
Soriano-López, J., Goberna-Ferrón, S., Vigara, L., Carbó, J. J., Poblet, J. M.,
and Galán-Mascarós, J. R. (2013). Cobalt polyoxometalates as heterogeneous
water oxidation catalysts. Inorg. Chem. 52, 4753–4755. doi: 10.1021/ic
4001945
Stracke, J. J., and Finke, R. G. (2011). Electrocatalytic water oxidation beginning
with the cobalt polyoxometalate [Co4 (H2 O)2 (PW9 O34 )2 ]10− : identiﬁcation
of heterogeneous CoOx as the dominant catalyst. J. Am. Chem. Soc. 133,
14872–14875. doi: 10.1021/ja205569j
Stracke, J. J., and Finke, R. G. (2013). Water oxidation catalysis beginning with
2.5 µM [Co4 (H2 O)2 (PW9 O34 )2 ]10− : investigation of the true electrochemically
driven catalyst at ≥600 mV overpotential at a glassy carbon electrode. ACS
Catal. 3, 1209–1219. doi: 10.1021/cs400141t
Stracke, J. J., and Finke, R. G. (2014). Distinguishing Homogeneous
from heterogeneous water oxidation catalysis when beginning with
polyoxometalates. ACS Catal. 4, 909–933. doi: 10.1021/cs4011716
Tan, B. J., Klabunde, K. J., and Sherwood, P. M. A. (1991). XPS studies of
solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobaltmanganese particles on alumina and silica. J. Am. Chem. Soc. 113, 855–861.
doi: 10.1021/ja00003a019
Wu, J., Liao, L., Yan, W., Xue, Y., Sun, Y., Yan, X., et al. (2012). Polyoxometalates
immobilized in ordered mesoporous carbon nitride as highly eﬃcient water
oxidation catalysts. ChemSusChem 5, 1207–1212. doi: 10.1002/cssc.201100809
Xiang, R., Ding, Y., and Zhao, J. (2014). Visible-light-induced water oxidation
mediated by a mononuclear-cobalt(II)-substituted silicotungstate. Chem. Asian
J. 9, 3228–3237. doi: 10.1002/asia.201402483
Xu, Y., Fischer, A., Duan, L., Tong, L., Gabrielsson, E., Åkermark, B. (2010).
Chemical and light-driven oxidation of water catalyzed by an eﬃcient
dinuclear ruthenium complex. Angew. Chem. Int. Ed. 49, 8934–8937.
doi: 10.1002/anie.201004278
Yin, Q., Tan, J. M., Besson, C., Geletii, Y. V., Musaev, D. G., Kuznetsov,
A. E., et al. (2010). A fast soluble carbon-free molecular water oxidation
catalyst based in abundant metals. Science 328, 342–345. doi: 10.1126/science.
1185372
Youngblood, W. J., Lee, S. H. A., Kobayashi, Y., Hernandez-Pagan, E. A., Hoertz,
P. G., Moore, T. A., et al. (2009). Photoassisted overall water splitting in a
visible light-absorbing dye-sensitized photoelectrochemical cell. J. Am. Chem.
Soc. 131, 926–927. doi: 10.1021/ja809108y
Zhang, M. T., Chen, Z., Kang, P., and Meyer, T. J. (2013). Electrocatalytic water
oxidation with a copper(II) polypeptide complex. J. Am. Chem. Soc. 135,
2048–2051. doi: 10.1021/ja3097515

Guo, S.-X., Liu, Y., Lee, C.-Y., Bond, A. M., Zhang, J., Geletii, Y. V., et al.
(2013). Graphene-supported [{Ru4 O4 (OH)2 (H2 O)4 }(γ-SiW10 O36 )2 ]10− for
highly eﬃcient electrocatalytic water oxidation. Energy Environ. Sci. 6,
2654–2663. doi: 10.1039/c3ee41892h
Hara, M., Waraksa, C. C., Lean, J. T., Lewis, B. A., and Mallouk, T. E. (2000).
Photocatalytic water oxidation in a buﬀered tris(2,2’-bipyridyl)ruthenium
complex-colloidal IrO2 system. J. Phys. Chem. A 104, 5275–5280.
doi: 10.1021/jp000321x
Huang, Z., Luo, Z., Geletii, Y. V., Vickers, J. W., Yin, Q., Wu, D., et al. (2011).
Eﬃcient light-driven carbon-free cobalt-based molecular catalyst for water
oxidation. J. Am. Chem. Soc. 133, 2068–2071. doi: 10.1021/ja109681d
Lewis, N. S., and Nocera, D. G. (2006). Powering the planet: chemical challenges
in solar energy utilization. Proc. Natl. Acad. Sci. U.S.A. 103, 15729–15735.
doi: 10.1073/pnas.0603395103
Liu, X., and Wang, F. Y. (2012). Transition metal complexes thast catalyze
oxygen formation from water: 1979–2010. Coord. Chem. Rev. 256, 1115–1136.
doi: 10.1016/j.ccr.2012.01.015
Lloret-Fillol, J., Codolà, Z., Garcia-Bosch, I., Gómez, L., Pla, J. J., and Costas,
M. (2011). Eﬃcient water oxidation catalysts based on readily available iron
coordination complexes. Nat. Chem. 3, 807–813. doi: 10.1038/nchem.1140
Lv, H., Geletii, Y. V., Zhao, C., Vickers, J. W., Zhu, G., Luo, Z., et al. (2012).
Polyoxometalate water oxidation catalysts and the production of green fuel.
Chem. Soc. Rev. 41, 7572–7589. doi: 10.1039/c2cs35292c
Lv, H., Song, J., Geletii, Y. V., Vickers, J. W., Sumliner, J. M., Musaev, D. G., et al.
(2014). An exceptionally fast homogeneous carbon-free cobalt-based water
oxidation catalyst. J. Am. Chem. Soc. 136, 9268–9271. doi: 10.1021/ja5045488
Mccool, N. S., Robinson, D. M., Sheats, J. E., and Dismukes, G. C. (2011). A Co4 O4
“cubane” water oxidation catalyst inspired by photosynthesis. J. Am. Chem. Soc.
133, 11446–11449. doi: 10.1021/ja203877y
Mcevoy, J. P., and Brudvig, G. W. (2006). Water-splitting chemistry of
photosystem II. Chem. Rev. 106, 4455–4483. doi: 10.1021/cr0204294
Natali, M., Bazzan, I., Goberna-Ferrón, S., Al-Oweini, R., Ibrahim, M., Bassil, B.
S., et al. (2017). Photo-assisted water oxidation by high-nuclearity cobalt-oxo
cores: tracing the catalyst fate during oxygen evolution turnover. Green Chem.
19, 2416–2426. doi: 10.1039/C7GC00052A
Pillai, K. C., Kumar, A. S., and Zen, J.-M. (2000). Naﬁon–RuO2 -Ru(bpy)2+
3
composite electrodes for eﬃcient electrocatalytic water oxidation. J. Mol. Catal.
A. 160, 277–285. doi: 10.1016/S1381-1169(00)00262-4
Pope, M. T. (1983). Heteropoly and Isopoly Oxometalates. Berlin: Springer.
Pope, M. T., and Müller, A. (2001). Polyoxometalate Chemistry From Topology via
Self-Assembly to Applications. Dordrecht: Kluwer Academic Publishers.
Puntoriero, F., La Ganga, G., Sartorel, A., Carraro, M., Scorrano, G., Bonchio,
M., et al. (2010). Photo-induced water oxidation with tetra-nuclear ruthenium
sensitizer and catalyst: a unique 4 × 4 ruthenium interplay triggering high
eﬃciency with low-energy visible light. Chem. Commun. 46, 4725–4727.
doi: 10.1039/c0cc00444h
Quintana, M., López, A. M., Rapino, S., Toma, F. M., Iurlo, M., Carraro, M., et al.
(2013). Knitting the catalytic pattern of artiﬁcial photosynthesis to a hybrid
graphene nanotexture. ACS Nano 7, 811–817. doi: 10.1021/nn305313q
Sartorel, A., Bonchio, M., Campagna, S., and Scandola, F. (2013). Tetrametallic
molecular catalysts for photochemical water oxidation. Chem. Soc. Rev. 42,
2262–2268. doi: 10.1039/C2CS35287G
Sartorel, A., Carraro, M., Scorrano, G., De Zorzi, R., Geremia, S., McDaniel, N.D.,
et al. (2008). Polyoxometalate embedding of a tetraruthenium(IV)-oxo-core by
template-directed metalation of [γ-SiW10 O36 ]8− : a totally inorganic oxygenevolving catalyst. J. Am. Chem. Soc. 130, 5006–5007. doi: 10.1021/ja077837f

Frontiers in Chemistry | www.frontiersin.org

Conﬂict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or ﬁnancial relationships that could
be construed as a potential conﬂict of interest.
Copyright © 2018 Soriano-López, Song, Patzke and Galan-Mascaros. This is an
open-access article distributed under the terms of the Creative Commons Attribution
License (CC BY). The use, distribution or reproduction in other forums is permitted,
provided the original author(s) and the copyright owner(s) are credited and that the
original publication in this journal is cited, in accordance with accepted academic
practice. No use, distribution or reproduction is permitted which does not comply
with these terms.

8

August 2018 | Volume 6 | Article 302