This document is the Accepted Manuscript version of a Published Work that appeared in final form in Coordination Chemistry
Reviews 304–305 (2015) 202–208 http://dx.doi.org/10.1016/j.ccr.2014.10.007

H2 generation and sulfide to sulfoxide oxidation with H2 O and
Sunlight with a model photoelectrosynthesis cell
Pau Farràs ; Carlo Di Giovanni ; John Noel Clifford ;
Emilio Palomares ; Antoni Llobet
a
b
c

Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16, E-43007 Tarragona, Spain
ICREA, Avda. Lluís Companys 28, Barcelona E-08030, Spain
Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193 Barcelona, Spain

Contents
1.
2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sacrificial molecular systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Light-driven water oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.
Light-driven oxidation of organic substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1. Homogeneous chemical, electrochemical and photochemical catalytic oxidation of MeSPh to MeS(O)Ph . . . . . . . . . . . . . . . . . . . . . .
Photoelectrochemical cells for the production of solar fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Light-driven water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Light-driven oxidation of organic substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1.
The interaction of the sensitized photoanode with the Ru catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2. Photocatalytic sulfide oxidation in heterogeneous phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3.
PESC cell assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

203
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a b s t r a c t
A summary of the recent advances in homogeneous light-driven catalytic oxidation reactions is provided
either using sacrificial agents or with complete functional cells. In this work, a photoelectrosynthesis model cell (PESC) for H2 generation together with methyl phenyl sulfide (MeSPh) oxidation to the
corresponding sulfoxide (MeS(O)Ph) with H2 O and using sunlight has been prepared and characterized. We have built a two compartment PESC connected through a Nafion proton exchange membrane.
The cathode consists of a platinum mesh in a pH 7.0 buffered solution. The photoanode consist on a
FTO/TiO 2 -P-bpy-RuII,2+ (where P-bpy-RuII,2+ is [RuII (P-bpy)(bpy) ]2+ and P-bpy is 2,2∗ -bipyridine-4,4∗ 2
bis(phosphonic acid)) electrode immersed in the same pH 7.0 solution. The anodic compartment also
contains the catalyst {[RuII (trpy)(H2 O)]2 (fL-pyr-dc)}(OAc) (trpy is 2,2∗ :6∗ ,2∗∗ -terpyridine and pyr-dc3−
is the pyrazolato-3,5-dicarboxylato trianion) abbreviated as [H2 O-RuII RuII -OH2 ]+ (2,2+ ) in homogenous
solution. The illumination under sun simulated 1.5 AMG (100 mW cm−2 ) together with an external input
of −0.43 V vs. SSCE led to the formation of hydrogen gas and the oxidation of the sulfide to sulfoxide. Last
but not least, all the significant reactions involved have been characterized both from a thermodynamic
and also from a kinetics viewpoint to analyze the system losses.

∗ Corresponding authors at: ICIQ, Chemistry, Avinguda Països Catalans, 43007 Tarragona, Tarragona, Spain. Tel.: +34 977920815.
E-mail addresses: epalomares@iciq.cat (E. Palomares), allobet@iciq.cat (A. Llobet).

1. Introduction
The area of light-driven oxidation reactions has experienced a
large development in recent years due to the renovated interest
in the generation of solar fuels. Current and predicted worldwide
energy demands are unsustainable with carbon-based fuels and a
transition from fossil to solar fuels is urgently needed [1–3]. Many
different strategies have been adopted by the scientific community
to generate solar fuels, from material-based to molecular systems,
and even the combination of both. Moreover, a solar fuel such as
hydrogen can be potentially produced either by the oxidation of
water (best case scenario), or by the oxidation of an organic substrate with water. Only until recently, most of the light-driven
oxidation systems used sacrificial reagents to avoid unwanted
recombination reactions. This review will only focus on molecular
systems and will provide the reader with an example of a functional
photoelectrosynthesis cell for the oxidation of an organic substrate
and the generation of H2 .
2. Sacrificial molecular systems
2.1. Light-driven water oxidation
The discovery in the mid-80s of molecular catalysts for water
oxidation led to the first examples of 3-component systems for
light-driven oxidation of water. They comprised of a molecular
ruthenium catalyst, a photosensitizer and a sacrificial electron
acceptor. However, their stability and performance was very
limited [4,5]. In both cases [Ru(bpy)3 ]2+ and Na2 S2 O8 were used
as photosensitizer and sacrificial electron acceptor, respectively.
After 20 years, the synthesis of more efficient water oxidation catalysts with lower overpotentials has fostered the generation of
more 3-component systems with better efficiencies. Molecular catalysts based on mononuclear and dinuclear Ru-complexes [6–10],
Ru polyoxometallates (Ru-POM) [11] and iron complexes [12] have
shown activity under sacrificial conditions. Some of them have
demonstrated good activities toward water oxidation when using
[Co(NH3 )5 Cl]Cl2 [7,8] as sacrificial acceptor.
More sophisticated molecular systems have been prepared
by combining catalytic and light-harvesting units on the same
molecules. These dyad systems are Ru-based photocatalysts that
are able to oxidize water in a 2-component system with a sacrificial reagent. Although their catalytic activity is low, it represents
a breakthrough in the area by opening new possibilities as photocatalysts for water oxidation. In all examples described, the activity
of the dyad is higher than the two separate components, a phenomenon ascribed to fast intramolecular electron transfer kinetics
that allows to reach the needed active higher oxidation states of
the catalyst [13,14]. However, all the examples above are based on
sacrificial reagents and the products generated, shown in Eq. (1),
are oxygen and protons. The latter need to be reduced elsewhere
to produce the desired solar fuel.
4hv

2H2 O−
7O2 + 4H+ + 4e−

E 0 = 1.23 V

As drawn, this reaction takes place in 100 % atom economy and it
generates a sulfoxide, which is a highly added value product, using
water and sunlight. Moreover, using sunlight and water avoids
the use of expensive and not very environmentally friendly oxidants such as CrO3 or MnO4 − . Furthermore, it generates molecular
hydrogen that is a green carbon-free fuel vector [16].
One approach of achieving this overall reaction is via a photoelectrochemical cell (PEC) as shown in Fig. 1. The overall reaction in
Eq. (2) can be divided in two half reactions occurring at the anode
and cathode as indicated below,
Anode : MeSPh + H2 O 7 MeS(O)Ph + 2H+ + 2e−
+

−

Cathode : 2H + 2e 7 H2

(3)
(4)

The overall reaction (2), is thermodynamically not favored and
thus the need of an external energy input such as light to drive it.
Since sunlight interacts with H2 O and MeSPh mainly at a vibrational
level a series of additional reactions involving light harvesting
molecules needs to be designed and coupled to suitable catalysts
in order to break and form the desired bonds at a reasonable rate.
The oxidation of substrates with visible light as indicated in
the anodic Eq. (3), has been described in the literature using
[Ru(bpy)3 ]2+ (bpy is 2,2∗ -bipyridine) as a light harvesting molecule
coupled to a sacrificial electron acceptor (SEA), such as sodium
persulfate or Co(III) salts like [Co(NH3 )5 Cl]2+ [17,18]. As before,
dyad systems with catalytic and photosensitizer units in the same
molecule have shown good turnover numbers (TN) for the oxidation of a large variety of substrates, such as alcohols, sulfides and
alkenes [19–26].
The oxidation of sulfide to sulfoxide involves formally the
addition of an oxygen atom to the sulfur atom. Thus, ideal catalysts for this transformation will involve typical transition metal
complexes in high oxidation states containing the M = O group.
Ru(II)-aqua polypyridyl complexes in general have been described
to carry out PCET type of reactions, reaching highly reactive
Ru(IV) = O species [27]. The latter is an excellent O-atom transfer
oxidant with regard to the oxidation of alkenes to epoxides and
sulfides to sulfoxides [28]. For this reason we have chosen the
dinuclear Ru-aqua complex, {[RuII (trpy)(H2 O)]2 (fL-pyr-dc)}(OAc)
(trpy is 2,2∗ :6∗ ,2”-terpyridine and pyr-dc3− is the pyrazolato3,5-dicarboxylato trianion) abbreviated as [H2 O RuII RuII OH2 ]+
(2,2+ ), where the trpy and bridging ligand are not written and
whose structure is shown in Chart 1. This complex undergoes a
series of PCET processes until it generates the reactive species that
is formally in oxidation state IV,IV, [O RuIV RuIV O]+ (4,4+ ). This
catalytic 4,4+ species is highly reactive and can transform alkenes
into epoxides (Eq. (5)) [29],

f

l+

O RuIV RuIV O

+ PhCH CHMe + H2 O 7

4,4+

f

HO − RuIII RuIII − OH

l+

+ PhCH(O)CHMe

(5)

3,3+

(1)

2.2. Light-driven oxidation of organic substrates
The oxidation of organic substrates with water and sunlight is a
challenge that has important implications both for basic research
science and for the green chemical industry. A model example of
this is the oxidation of sulfides to sulfoxides as indicated in the
equation below for the particular case of methyl phenyl sulfide
(MeSPh) [15],
2 hv

MeSPh + H2 O−7MeS(O)Ph + H2

G o ∼ 31.6 kcal/mol
=

(2)

Chart 1. Schematic drawings of the Ru-photosensitizer [RuII (P-bpy)(bpy)2 ]2+ (Pbpy-RuII,2+ ) and Ru-catalyst ({[RuII (trpy)(H2 O)]2 (fL-pyr-dc)} ,+ 2,2 + used in this
,
work.

and organic sulfides to sulfoxides as will be described here
(Eq. (6)).

3. Photoelectrochemical cells for the production of
solar fuels
3.1. Light-driven water splitting

2.2.1. Homogeneous chemical, electrochemical and
photochemical catalytic oxidation of MeSPh to MeS(O)Ph
We tested the capacity of 4,4+ to react with sulfides in homogeneous phase, chemically, electrochemically and photochemically.
Electrochemical experiments in aqueous solution at pH 7.0
show that the active catalytic species for sulfide oxidation is the 4,4+
as can be seen in Fig. 2, by the presence of an electrocatalytic wave
at the foot of the IV,IV oxidation wave (Eo IV,IV-IV,III = 0.74 V). Since
the oxidation of sulfide to sulfoxide involves a two electron process the reduced catalytic species, given the two-electron nature of
the sulfide oxidation, is [HO RuIII RuIII OH]+ , 3,3+ , as indicated in
the following equation:

f

O − RuIV RuIV − O

l+

+ MeSPh + H2 O 7

4,4+

f

l+

HO − RuIII RuIII − OH

+ MeS(O)Ph

(6)

3,3+

Chemically, under homogeneous conditions the active catalyst
can be generated with PhIO that acts as a sacrificial oxidant according to,

f

l+

HO − RuIII RuIII − OH

+ PhIO 7

3,3+

f

O − RuIV RuIV − O

l+

+ PhI + H2 O

(7)

4,4+

Thus a catalytic system can be built using 4,4+ 1.0 mM/MeSPh
2.0 M/PhIO 4.0 M/H2 O 4.0 M in DCM:EtOH (1:1). This gives 1.81 M
sulfoxide that represents an outstanding 1810 turnover number
(TN) with regard to the initial catalyst in 60 min. After this time the
conversion of the initial substrate reaches a value of 90.5% with a
remarkable TOFi of 3300 ks−1 .
The sulfide oxidation can also be photochemically induced
using [RuII (bpy)3 ]2+ , (abbreviated as bpy-RuII,2+ ; Eo (III/II) = 1.10 V)
and [CoIII (NH3 )5 Cl]2+ (abbreviated as CoIII ) as a sacrificial electron acceptor (SEA). A deoxygenated solution containing 2,2+
0.02 mM/MeSPh 20 mM/bpy-RuII,2+ 0.2 mM/CoIII 20 mM in a
0.05 M sodium phosphate buffer (pH 7.0) was irradiated by visible light for 30 min. Analysis of the product generation showed the
formation of 9 mM sulfoxide that represents 450 turnover numbers (TNs) with regard to the initial catalyst. After this time the
conversion of the initial substrate reaches a value of 90% with an
impressive initial TOFi of 250 ks−1 .
The generation of the active species was achieved sequentially
by the photochemically produced bpy-RuIII,3+ according to,

f

2 RuIII (bpy)3

l3+

f

f

bpy−RuII,2+

l2+

7

3,3+

bpy−RuIII,3+

2 RuII (bpy)3

l+

+ HO − RuIII RuIII − OH

f

l+

+ O RuIV RuIV O

+ 2H+

(8)

4,4+

The lower TOFi with regard to the chemical homogenous system is due to lower concentration of Ru(III) that is generated in
the photochemically induced experiment. In the previous case the
PhIO was used in a ratio of 1:4000 Cat:PhIO. Nevertheless the large
TN achieved enables the present system to be tried under more
restrictive conditions such as in heterogeneous phase.

An ideal functional device for the generation of hydrogen should
avoid the use of sacrificial reagents. Many examples can be found
in the literature for the production of hydrogen by using molecular
catalyst based on transition metal complexes and sacrificial electron donors. The sacrificial agents react fast and in an irreversible
manner so that they favor only the desired reactions. Furthermore, a
large excess of sacrificial molecules can be used to favor the desired
reaction. So far, only one example can be found in which oxidation
and reduction reactions are coupled together in a fully homogeneous molecular system for the oxidation of organic substrates and
hydrogen [30].
The best molecular catalysts for water oxidation, described in
the previous section with sacrificial reagents, have been incorporated in PEC cells, where oxygen and hydrogen are produced in the
anode and cathode, respectively. In all cases, oxygen is produced
in photoanodes based on dye-sensitized TiO2 with a Ru(bpy)3 -type
of complexes or porphyrins, along with a catalyst embedded in a
Nafion film or attached to the photosensitzer. On the other hand,
the cathode consists of a platinum electrode [31–40]. In general,
the PEC cells have shown small photocurrents, low efficiencies and
low stability. Meyer et al. have been able to increase the stability
of their chromophore-catalyst assembly on the electrode by the
deposition of a TiO2 layer by atomic layer deposition (ALD), producing a core-shell structure that also enhance the photocurrent of
their system [37]. More recently, photoanodes with chromophorecatalyst dyads have also been studied although only minor details
are given on the kinetics of the reaction. Recently several papers
on the kinetic studies of Ru-sensitized photoanodes in water have
been characterized [41,42].
3.2. Light-driven oxidation of organic substrates
A few examples exist of photocatalytic oxidation of organic substrates mediated by water using PESCs, although this topic is mainly
at its infancy and large developments need to be made before it can
be used in a generalized manner. One of the important issues that
should be overcome for this type of cells is the absence of sacrificial agents which significantly reduce the driving force toward the
productive reactions and for this reason most of the PESCs have
been described as a proof of concept level [43]. Some other examples can be found where the use of UV-light increases the efficiency
of the system and even avoids the use of an external bias to generate hydrogen [44,45]. For a functional device to work, UV-light
should be discarded as the share of this high-energy light within
the solar spectrum is very small and systems based on visible and
NIR light-harvesting molecules need to be used. It is thus crucial
for the advancement of this topic, to characterize all the reactions
involved in this type of cells both from a thermodynamic and kinetic
point of view, in order to be able to achieve technologically relevant
devices.
3.2.1. The interaction of the sensitized photoanode with the Ru
catalyst
In this work, we wanted to characterize all kinetic parameters affecting the performance of the cell. Given the excellent
behavior in homogenous phase we anchored [RuII (P-bpy)(bpy)2 ]2+
(P-bpy-RuII,2+ ; P-bpy is 2,2∗ -bipyridine-4,4∗ -bis(phosphonic acid),
see Chart 1) into TiO2 to generate a photoanode, labeled FTO/TiO2 P-bpy-RuII,2+ with a dye surface coverage of 4.1 × 10−8 mol cm−2
[46,47]. This photoanode was stable for days in a mixture of
0.1 M Na2 SO4 /0.83 mM NaH2 PO4 /1.17 mM Na2 HPO4 /0.69 M TFE

Fig. 1. Top, schematic drawing of the two compartment photoelectrochemical cell separated by a proton exchange membrane. Bottom, General Reaction Scheme involving
productive reactions. CB: conduction band of TiO2 ; D: photosensitizer; D* : excited state of the photosensitizer; D+ : oxidized photosensitizer; Ru-Cat: sulfoxidation catalyst
2,2+ ; PEM: proton exchange membrane, Nafion® .

(2,2,2-trifluoroethanol), and was tested with regard to its capacity
to interact in homogeneous phase with our sulfoxidation catalyst 3,3+ , generated under controlled potential electrolysis at
Eo app = 0.47 V.
Transient absorption spectroscopy (TAS) was used to measure
the electron transfer processes in FTO/TiO2 -P-bpy-RuII,2+ following
excitation at 450 nm in the presence and absence of different concentrations of 3,3+ (Fig. 3) and MeSPh substrate. The absorbance
decay after irradiation was measured at 650 nm which is where
bpy-RuIII,3+ has a broad absorption band. In the absence of catalyst the system decays with a half lifetime of about 3.3 ms, which is
attributed to the recombination reaction shown in eq. 9 (red arrow
6 in Fig. 5), once the electron injects into the TiO2 conduction band
which is known to happen within the picosecond time scale [48].
(−)TiO2 -P-bpy-RuIII,3+ 7 TiO2 -P-bpy-RuII,2+
catalyst 3,3+

t 1/2 ≈ 3.3 ms

(9)

bpy-RuIII,3+ signal decays

In the presence of the
the
faster as it is reduced by the catalyst according to Eqs. (10) and (11),
in a similar manner as it occurs when a dye is regenerated by a
red/ox electrolyte in a dye sensitized solar cell (DSC) [49].
(−)TiO2 -P-bpy-RuIII,3+ + [HO RuIII RuIII OH]+ 7
(−)TiO2 -P-bpy-RuII,2+ + [O RuIV RuIII OH]+ + H+

(10)

(−)TiO2 -P-bpy-RuIII,3+ + [O RuIV RuIII OH]+ 7
(−)TiO2 -P-bpy-RuII,2+ + [O RuIV RuIV O]+ + H+

(11)

As the catalyst concentration is increased from 30 to 220 fLM
the decay of the bpy-RuIII,3+ signal increases by almost an order of

Fig. 2. Square-wave voltammogram (SQWV) of 2,2+ in a pH = 7.0 sodium phosphate
buffer solution showing the active IV,IV oxidation state (solid line). CV curve of 2,2+
in a pH = 7.0 phosphate buffer in the presence (dashed line) and absence (dotted
line) of MeSPh showing the electrocatalytic wave. Starting scan at 0.0 V. A GC disk
was used as the working electrode, a platinum wire as the auxiliary electrode, and
SSCE as the reference electrode. SQWV parameters: potential increment of 0.004 V,
pulse amplitude of 0.025 V, and frequency of 1 Hz.

magnitude with a t1/2 ≈ 0.18 ms. A dependence of dye regeneration
on red/ox couple concentration has also been observed in DSCs [50].
The notable increase in signal intensity at slower timescales
(0.1–1 ms) is attributed to the formation of a new species, which
we ascribe to the 4,4+ state of the catalyst. The decay of the 4,4+
state is due to the oxidation of MeSPh with the catalyst returning
to its 3,3+ state (blue arrow 5 in Fig. 5). Due to the low signal to noise

Fig. 3. Transient absorption kinetics of FTO/TiO2 -P-bpy-RuII,2+ photoanodes in a
0.1 M Na2 SO4 /0.83 mM NaH2 PO4 /1.17 mM Na 2 HPO4 /0.69 M TFE aqueous solution
(blue) in the presence of 42.5 mM MeSPh. Black, in the absence of catalyst; red, with

Fig. 4. Hydrogen evolution from the cathodic compartment (Vgas phase = 6.5 mL) measured with a Clark electrode obtained with the PESC cell upon irradiation with a Xe
lamp (A > 400 nm, 0.1 W cm−2 ) at an applied bias of −0.43 V. The anodic half-cell
contains FTO/TiO2 -P-bpy-RuII,2+ (r = 6.4 × 10−8 mol cm−2 )/0.09 mM 2,2+ /MeSPh
35 mM while the cathodic half-cell consists of a Pt mesh. The supporting electrolyte
solution is made of a 0.1 M Na 2 SO4 /0.83 mM NaH2 PO4 /1.17 mM Na2 HPO4 /0.69 M
TFE up to a total volume of 5 mL in each compartment.

the addition of 30 fLM 3,3+ ; blue, with the addition of 220 fLM 3,3+ . TAS kinetics
were recorded under 0.1 Sun illumination (10 mW cm−2 ) and using laser excitation
pulses at 450 nm. TAS kinetics was recorded at 650 nm. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)

ratio of the kinetic traces we were not able to record the transient
spectra of the long-lived species, however, we note that the 4,4+
state of the catalyst (generated from 2,2+ at an applied potential
of 0.89 V vs. SSCE at pH 7.0; see SI) also has an absorption band
at 650 nm. Moreover, the t1/2 of the decay of the 4,4+ state (7 ms)
is in the order of magnitude found for the heterogeneous MeSPh
oxidation reaction described in next section.
3.2.2. Photocatalytic sulfide oxidation in heterogeneous phase
As in the previous section, the FTO/TiO2 -P-bpy-RuII,2+ photoanode in the presence of Co(III) as a sacrificial electron acceptor
were used to test its performance in heterogeneous phase. The
system: FTO/TiO2 -P-bpy-RuII,2+ (r = 6.4 × 10−8 mol cm−2 )/2,2+
0.02 mM/MeSPh 20 mM/Co(III) 20 mM/DMF 13 mM in 0.1 M
Na2 SO4 /0.83 mM NaH2 PO4 /1.17 mM Na2 HPO4 /0.69 M TFE buffer
solution gives 0.87 mM sulfoxide that represents 44 turnover
numbers (TNs) with regard to the initial catalyst in 135 min. After
this time the conversion of the initial substrate reaches a value
of 4.3% with a TOFi of 13 ks−1 (see SI for a product generation as
a function of time). Again as in the previous occasion the TOFi is
slightly reduced due to the lower amount of bpy-RuII,2+ generated
in the surface of TiO2 with regard to the bpy-RuII,2+ used in
homogeneous phase. All the above experiments show the capacity
of the system to act as a robust photoanode for the oxidation of
sulfide to sulfoxide in the presence of Co(III).
3.2.3. PESC cell assembly
Based on the good performance of the catalyst both in homogeneous and in hetergenous phase sacrificial reagents, we have
designed a new photoelectrosynthesis cell (PESC) (Fig. 1) where
the oxidation catalyst is present in homogeneous phase and thus
allowing a fine tuning of redox potentials. The oxidation catalyst has
to comply with two indispensable requirements: (a) the catalyst
has to act efficiently with regard to substrate oxidation and (b) the
active site of the catalyst needs to be generated very fast by the photoanode. For the latter we are using a [Ru(bpy)3 ]2+ type of molecule
attached at the surface of TiO2 [18]. This cell configuration benefits
from the fact that any catalyst that works in homogeneous phase
can be immediately incorporated into the cell. In contrast if the

catalyst needs to be anchored on solid devices then the corresponding homologue adequately functionalized will have to be
developed. On the other hand using catalysts in homogeneous
phase has the serious drawback that the ratio of anchoredphotosensitizing molecule vs. catalyst is generally very low.
In order to generate hydrogen, a small external bias is needed
when TiO2 is used as a photoanode [31,33,51]. As previously
reported, despite the fact that the conduction band edge of TiO2
is more negative than the thermodynamic potential of protons to
hydrogen, electrons in trap states below this band are not reducing
enough to generate hydrogen, and rapidly recombine according to
equation 9. Therefore, an applied potential bias is needed to remove
the electrons from the quasi-Fermi level of the semiconductor and
transfer them to the cathodic side of a potential PEC. The energy
level is dependent on the pH and the applied potential needs to
be measured for each system. In our cell using Pt-mesh as a cathode, the potential turned out to be −0.43 V as deduced from the I–V
curve shown in the SI.
Once the anodic and cathodic sites have been thoroughly studied independently, with the thermodynamics and kinetics known,
we connected them in the absence of sacrificial agents. A two
compartment PESC connected through a Nafion proton exchange
membrane, was built, as shown schematically in Fig. 1, with
the photoanode and cathode described in the previous sections.
Irradiation by visible-light (A > 400 nm, 0.1 W cm−2 ) and at an
applied bias of -0.43 V of the three-electrode PESC in 5 mL of 0.1 M
Na2 SO4 /0.83 mM NaH2 PO4 /1.17 mM Na2 HPO4 /0.69 M TFE solution,
containing FTO/TiO2 -P-bpy-RuII,2+ (r = 6.4 × 10−8 mol cm−2 )/2,2+
0.09 mM/MeSPh 35 mM/DMF 10 mM produces a photocurrent that
decays to a steady 14 fLA after 2000 s. In the cathodic compartment
310 nmol of H2 are formed and a total of 0.067 C are measured
(Fig. 4) that corresponds to a faradaic efficiency of 89%. A larger
amount of MeS(O)Ph (0.2 mM) could be detected in the anodic
compartment by 1 H NMR spectroscopy at the end of the reaction,
although its quantitative measurement is associated with a large
error. In the absence of the organic substrate or Ru-catalyst or Ruphotosensitizer, the currents obtained as well as product formation
were negligible in all cases.
The combination of reactions shown here is a proof of concept
of the viability of potential PESCs based on molecular catalysts in
homogeneous phase and proper dyes anchored in TiO2 as photoanodes. However there are a number of unproductive reactions that

Fig. 5. Summary of significant reactions that occur at the anode and their approximate timescales. See text for details. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)

need to be minimized in order to come up with more efficient cells.
These include recombination reactions such as,
(−)TiO2 -P-bpy-RuIII,3+ + [HO RuIII RuIV O]+ + H+ 7
TiO2 -P-bpy-RuIII,3+ + [HO RuIII RuIII OH]+

(12)

(−)TiO2 -P-bpy-RuIII,3+ + [O RuIV RuIV O]+ + H+ 7
TiO2 -P-bpy-RuIII,3+ + [O RuIV RuIII OH]+

(13)

that are represented as red arrows (7 and 8) in Fig. 5.
An additional problem to the cell reported here is the stability
of the anchored P-bpy-RuII,2+ under illumination as the reactions
proceed. It can be visually appreciated that the latter slowly deanchors from TiO2 .

is on the millisecond time scale (reed arrow 6 in Fig. 5). Moreover, upon addition of the Ru catalyst the decay becomes
bi-phasic with a fast phase corresponding to the efficient regeneration of the oxidized sensitizer (microsecond time scale,
arrows 3 and 4, Fig. 5) and a second phase that corresponds to
the oxidation of the substrate by the high oxidation states of the
catalyst (hundreds of miliseconds timescale; arrow 5, Fig. 5).
(c) The existence of the formation of higher oxidation states
derived from [HO RuIII RuIII OH]+ is indicated by the faster
decay of RuIII in FTO/TiO2 -P-bpy-RuIII,3+ , in the presence of the
catalyst. Furthermore the existence of the two electron oxidized
species [O RuIV RuIV O]+ is indirectly demonstrated by the
formation of the sulfoxide, since only the former is capable of
carrying out such oxidation.
(d) Recombination of the photo-injected electrons at the TiO2 and
the higher oxidation states of the catalyst (red arrows 7–8) are
lower than microseconds otherwise these reactions will preclude the formation of the final products.

4. Conclusions
This review is intended to give a short overview of the light
induced oxidative reactions and to show the different steps needed
for development of a PESC cell: from the identification of an active
oxidation catalyst to its inclusion in a full functional cell for the production of hydrogen. The following is a list of conclusions that can
be extracted from the work just presented, together with a graph
(Fig. 5) that helps to identify all the significant reactions involved
in the photoanode.
(a) The use of P-bpy-RuII,2+ sensitizer anchored in TiO2 facilitates
the formation of oxidative species of the catalyst Ru(IV) Ru(IV)
that are active toward the sulfoxidation of MeSPh. Importantly,
the III/II redox potential (1.10 V vs. SSCE) of the P-bpy-RuII,2+
sensitizer, is a pre-requisite to be able to generate the sufficient
thermodynamic driving force so that the catalyst can reach its
active site: the Ru(IV) Ru(IV) oxidation sate. A lower oxidation
potential of the sensitizer will compromise the electron transfer
kinetics from the dye to the catalyst and thus no hydrogen or
sulfoxide would be generated.
(b) The L-TAS shows that the recombination kinetics between the
photo-injected electrons at the TiO2 and the oxidized sensitizer

Finally, we have successfully built, step by step, a complete photoelectrosynthesis cell for simultaneous generation of hydrogen
and sulfoxide based on water and sunlight. We have thoroughly
studied and characterized all the potential reactions involved both
from a thermodynamic and also from a kinetics viewpoint. This
work represents a first step toward the use of sunlight and water
as sustainable and green oxidation agents using oxidation catalysts in homogeneous pahse, and sets up the basis for the rational
development of PESC for pure water splitting with sunlight.
Supporting information
Experimental details and additional spectroscopic, electrochemical, and catalytic data.
Acknowledgments
Support by MINECO CTQ-2013-49075-R, SEV-2013-0319 and
“La Caixa” are gratefully acknowledged. CDG is grateful for an FPI
grant from MINECO. EP and JC would like to thank ICIQ and ICREA
for their financial support.

References
[1] T. Faunce, S. Styring, M.R. Wasielewski, G.W. Brudvig, A.W. Rutherford, J.
Messinger, A.F. Lee, C.L. Hill, H. deGroot, M. Fontecave, D.R. MacFarlane, B. Hankamer, D.G. Nocera, D.M. Tiede, H. Dau, W. Hillier, L. Wang, R. Amal, Energy
Environ. Sci. 6 (2013) 1074.
[2] A. Harriman, Eur. J. Inorg. Chem. 4 (2014) 573.
˜
[3] S. Berardi, S. Drouet, L. Francàs, C. Gimbert-Surinach, M. Guttentag, C. Richmond, T. Stoll, A. Llobet, Chem. Soc. Rev. 43 (2014) 7501 (web).
[4] F.P. Rotzinger, S. Munavalli, P. Comte, J.K. Hurst, M. Graetzel, F.J. Pern, A.J. Frank,
J. Am. Chem. Soc. 109 (1987) 6619.
[5] P. Comte, M.K. Nazeeruddin, F.P. Rotzinger, A.J. Frank, M. Graetzel, J. Mol. Catal.
52 (1989) 63.
[6] J.L. Cape, J.K. Hurst, J. Am. Chem. Soc. 130 (2008) 827.
[7] L. Duan, Y. Xu, P. Zhang, M. Wang, L. Sun, Inorg. Chem. 49 (2009) 209.
[8] S. Roeser, P. Farràs, F. Bozoglian, M. Martínez-Belmonte, J. Benet-Buchholz, A.
Llobet, ChemSusChem 4 (2011) 197.
[9] K.J. Young, L.A. Martini, R.L. Milot, R.C. Snoeberger III, V.S. Batista, C.A. Schmuttenmaer, R.H. Crabtree, G.W. Brudvig, Coord. Chem. Rev. 256 (2012) 2503.
[10] Y. Xu, A. Fischer, L. Duan, L. Tong, E. Gabrielsson, B. Åkermark, L. Sun, Angew.
Chem. Int. Ed. 49 (2010) 8934.
[11] Y.V. Geletii, Z. Huang, Y. Hou, D.G. Musaev, T. Lian, C.L. Hill, J. Am. Chem. Soc.
131 (2009) 7522.
[12] G. Chen, L. Chen, S.M. Ng, W.L. Man, T.C. Lau, Angew. Chem. Int. Ed. 52 (2013)
1789.
[13] N. Kaveevivitchai, R. Chitta, R. Zong, M. El Ojaimi, R.P. Thummel, J. Am. Chem.
Soc. 134 (2012) 10721.
[14] D.L. Ashford, D.J. Stewart, C.R. Glasson, R.A. Binstead, D.P. Harrison, M.R. Norris,
J.J. Concepcion, Z. Fang, J.L. Templeton, T.J. Meyer, Inorg. Chem. 51 (2012) 6248.
[15] R.H. Holm, J.P. Donahue, Polyhedron 12 (1993) 571 (The data is extracted for
Me2 S).
[16] D. Kalita, B. Radaram, B. Brooks, P.P. Kannam, X. Zhao, ChemCatChem 3 (2011)
571.
[17] F. Li, M. Yu, Y. Jiang, F. Huang, Y. Li, B. Zhang, L. Sun, Chem. Commun. 47 (2011)
8949.
[18] S. Ohzu, T. Ishizuka, Y. Hirai, S. Fukuzumi, T. Kojima, Chem. Eur. J. 19 (2013)
1563.
[19] W. Chen, F. Rein, R. Rocha, Angew. Chem. Int. Ed. 121 (2009) 9852.
[20] S. Fukuzumi, T. Kishi, H. Kotani, Y. Lee, W. Nam, Nat. Chem. 3 (2010) 38.
[21] W. Chen, F. Rein, B. Scott, R. Rocha, Chem. Eur. J. 17 (2011) 5595.
[22] M. Hajimohammadi, N. Safari, H. Mofakham, F. Deyhimi, Green Chem. 13 (2011)
991.
[23] O. Hamelin, P. Guillo, F. Loiseau, M. Boissonnet, S. Menage, Inorg. Chem. 50
(2011) 7952.

[24] P. Guillo, O. Hamelin, G. Batat, G. Jonusauskas, N.D. McClenaghan, S. Menage,
Inorg. Chem. 51 (2012) 2222.
[25] P. Farras, S. Maji, J. Benet-Buchholz, A. Llobet, Chem. Eur. J. 19 (2013) 7162.
[26] D. Chao, W.F. Fu, Dalton Trans. 43 (2014) 306.
[27] S. Romain, L. Vigara, A. Llobet, Acc. Chem. Res. 42 (2009) 1944.
[28] M.H.V. Huynh, L.M. Witham, J.M. Lasker, M. Wetzler, B. Mort, D.L. Jameson, P.S.
White, K.J. Takeuchi, J. Am. Chem. Soc. 125 (2003) 308.
[29] C. Di Giovanni, A. Poater, J. Benet-Buchholz, L. Cavallo, M. Solà, A. Llobet, Chem.
Eur. J. 20 (2014) 3898.
[30] W. Singh, D. Pegram, H. Duan, D. Kalita, P. Simone, G. Emmert, X. Zhao, Angew.
Chem. Int. Ed. 51 (2011) 1653.
[31] W. Youngblood, S. Lee, Y. Kobayashi, E. Hernandez-Pagan, P. Hoertz, T. Moore,
A. Moore, D. Gust, T. Mallouk, J. Am. Chem. Soc. 131 (2009) 926.
[32] R. Brimblecombe, A. Koo, G. Dismukes, G. Swiegers, L. Spiccia, J. Am. Chem. Soc.
132 (2010) 2892.
[33] L. Li, L. Duan, Y. Xu, M. Gorlov, A. Hagfeldt, L. Sun, Chem. Commun. 46 (2010)
7307.
[34] W. Song, C.R.K. Glasson, H. Luo, K. Hanson, M.K. Brennaman, J.J. Concepcion, T.J.
Meyer, J. Phys. Chem. Lett. 2 (2011) 1808.
[35] G.F. Moore, J.D. Blakemore, R.L. Milot, J.F. Hull, H.E. Song, L. Cai, C.A.
Schmuttenmaer, R.H. Crabtree, G.W. Brudvig, Energy Environ. Sci. 4 (2011)
2389.
[36] Y. Gao, X. Ding, J. Liu, L. Wang, Z. Lu, L. Li, L. Sun, J. Am. Chem. Soc. 135 (2013)
4219.
[37] L. Alibabaei, M.K. Brennaman, M.R. Norris, B. Kalanyan, W. Song, M.D. Losego, J.J.
Concepcion, R.A. Binstead, G.N. Parsons, T.J. Meyer, Proc. Natl. Acad. Sci. U.S.A.
110 (2013) 20008.
[38] Y. Gao, L. Zhang, X. Ding, L. Sun, Phys. Chem. Chem. Phys. 16 (2014) 12008.
[39] X. Ding, Y. Gao, L. Zhang, Z. Yu, J. Liu, L. Sun, ACS Catal. 4 (2014) 2347.
[40] J.R. Swierk, T.E. Mallouk, Chem. Soc. Rev. 42 (2013) 2357.
[41] M.K. Brennaman, A. Patrocinio, W. Song, J. Jurss, J.J. Concepcion, P. Hoertz, M.
Traub, N. Murakami Iha, T.J. Meyer, ChemSusChem 4 (2011) 216.
[42] W. Song, M.K. Brennaman, J.J. Concepcion, J. Jurss, P. Hoertz, H. Luo, C. Chen, K.
Hanson, T.J. Meyer, J. Phys. Chem. C 115 (2011) 7081.
[43] W. Song, A.K. Vannucci, B.H. Farnum, A.M. Lapides, M.K. Brennaman, B.
Kalanyan, L. Alibabaei, J.J. Concepcion, M.D. Losego, G.N. Parsons, T.J. Meyer,
J. Am. Chem. Soc. 136 (2014) 9773.
[44] M. Antoniadou, P. Lianos, Catal. Today 144 (2009) 166.
[45] M. Antoniadou, D.I. Kondarides, D. Labou, S. Neophytides, P. Lianos, Sol. Energy
Mater. Sol. Cells 94 (2010) 592.
[46] P. Péchy, F.P. Rotzinger, M.K. Nazeeruddin, O. Kohle, S.M. Zakeeruddin,
R. Humphry-Baker, M. Grätzel, J. Chem. Soc., Chem. Commun. (1995)
65.
[47] K. Hanson, M.K. Brennaman, H. Luo, C.R.K. Glasson, J.J. Concepcion, W. Song, T.J.
Meyer, ACS Appl. Mater. Interfaces 4 (2012) 1462.
[48] F. Lakadamyali, A. Reynal, M. Kato, J.R. Durrant, E. Reisner, Chem. Eur. J. 18
(2012) 15464.
[49] J.N. Clifford, E. Palomares, M.K. Nazeeruddin, M. Grätzel, J.R. Durrant, J. Phys.
Chem. C 111 (2007) 6561.
[50] I. Montanari, J. Nelson, J.R. Durrant, J. Phys. Chem. B 106 (2002) 12203.
[51] A. Fujishima, K. Honda, Nature 238 (1972) 37.