A diketopyrrolopyrrole dye-based dyad on a porous TiO2photoanode for solar-driven water oxidation

Article abstract of DOI:10.1039/d0sc04509h

Dye-sensitised photoanodes modified with a water oxidation catalyst allow for solar-driven O2 evolution in photoelectrochemical cells. However, organic chromophores are generally considered unsuitable to drive the thermodynamically demanding water oxidation reaction, mainly due to their lack of stability upon photoexcitation. Here, the synthesis of a dyad photocatalyst (DPP-Ru) consisting of a diketopyrrolopyrrole chromophore (DPPdye) and ruthenium-based water oxidation catalyst (RuWOC) is described. The DPP-Ru dyad features a cyanoacrylic acid anchoring group for immobilisation on metal oxides, strong absorption in the visible region of the electromagnetic spectrum, and photoinduced hole transfer from the dye to the catalyst unit. Immobilisation of the dyad on a mesoporous TiO2 scaffold was optimised, including the use of a TiCl4 pretreatment method as well as employing chenodeoxycholic acid as a co-adsorbent, and the assembled dyad-sensitised photoanode achieved O2 evolution using visible light (100 mW cm-2, AM 1.5G, λ > 420 nm). An initial photocurrent of 140 μA cm-2 was generated in aqueous electrolyte solution (pH 5.6) under an applied potential of +0.2 V vs. NHE. The production of O2 has been confirmed by controlled potential electrolysis with a faradaic efficiency of 44%. This study demonstrates that metal-free dyes are suitable light absorbers in dyadic systems for the assembly of water oxidising photoanodes.

Full text of DOI:10.1039/d0sc04509h

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A diketopyrrolopyrrole dye-based dyad on a porous
TiO2 photoanode for solar-driven water oxidation†
Cite this: DOI: 10.1039/d0sc04509h
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Daniel Anton-Garcıa,
Julien Warnan and Erwin Reisner
´
´
Dye-sensitised photoanodes modified with a water oxidation catalyst allow for solar-driven O2 evolution in
photoelectrochemical cells. However, organic chromophores are generally considered unsuitable to drive
the thermodynamically demanding water oxidation reaction, mainly due to their lack of stability upon
photoexcitation. Here, the synthesis of
a
dyad photocatalyst (DPP-Ru) consisting of
a diketopyrrolopyrrole chromophore (DPPdye) and ruthenium-based water oxidation catalyst (RuWOC) is
described. The DPP-Ru dyad features a cyanoacrylic acid anchoring group for immobilisation on metal
oxides, strong absorption in the visible region of the electromagnetic spectrum, and photoinduced hole
transfer from the dye to the catalyst unit. Immobilisation of the dyad on a mesoporous TiO2 scaffold was
optimised, including the use of a TiCl4 pretreatment method as well as employing chenodeoxycholic
acid as a co-adsorbent, and the assembled dyad-sensitised photoanode achieved O2 evolution using
visible light (100 mW cmꢀ2, AM 1.5G, l > 420 nm). An initial photocurrent of 140 mA cmꢀ2 was generated
in aqueous electrolyte solution (pH 5.6) under an applied potential of +0.2 V vs. NHE. The production of
O2 has been confirmed by controlled potential electrolysis with a faradaic efficiency of 44%. This study
demonstrates that metal-free dyes are suitable light absorbers in dyadic systems for the assembly of
water oxidising photoanodes.
Received 17th August 2020
Accepted 10th September 2020
DOI: 10.1039/d0sc04509h
rsc.li/chemical-science
be overcome by covalently linking the chromophore to the
catalyst, forming a dyadic system in which the catalyst is placed
Introduction
farther away from the electrode surface (Fig. 1). Dyads thereby
enable fast interfacial electron transfer from the dye to semi-
conductor electrode, and intramolecular quenching of S+ by the
WOC combined with slow recombination of semiconductor
electrons with holes accumulated in the oxidised WOC.3,10
Dyads have previously been constructed using Ru-based dyes
and [RuII(bda)] WOCs, and have been integrated in TiO2 pho-
toanodes either by immobilisation of the synthesised
assembly,11 or by in situ polymerisation.12–14 However, dyad
photoanodes typically rely on precious-metal chromophores,
and the need for reduced cost has led to the exploration of
earth-abundant chromophores with more intense visible light
absorption, embodied by metal porphyrins15 and organic dyes.16
An example of a zinc porphyrin-based dyad has been reported,
but the chromophoric unit lacks sufficient oxidation potential
for light-activation of the [Ru(bda)]-based WOC.15 Organic dyes,
frequently designed with push–pull architectures, have been co-
immobilised with [RuII(bda)]-type catalysts on dye-sensitised
photoanodes, albeit with low efficiencies.17–23
The integration of a molecular dye and a water oxidation cata-
lyst (WOC) onto an n-type metal oxide (e.g., titanium dioxide,
TiO2) semiconductor (SC) lm on a conductive substrate (e.g.,
uorine-doped tin oxide, FTO), produces a dye-sensitised pho-
toanode for visible light-driven O2 evolution in photo-
electrochemical (PEC) cells.1,2 Dye-sensitised photoanodes
operate by photoexcitation of the dye (S), which results in ultra-
fast (typically sub-ns) electron injection from the excited state
(S*) to the conduction band (CB) of the semiconductor.3 This
step is followed by hole transfer to the WOC, which regenerates
the oxidised dye (S+). Repeated cycles allow the catalyst to
accumulate four holes to oxidise water to O2.4–6 Ruthenium
complexes are the most commonly employed molecular WOCs
due to their fast O2 evolution rates at low overpotentials, with
[RuII(bda)(pic)2] (bda ¼ 2,20-bipyridine-6,60-dicarboxylic acid,
pic ¼ 4-picoline) displaying benchmark performance.7,8
Co-immobilisation of the dye and WOC on an electrode
results in fast electron–hole recombination at the molecule–
electrode interface, thereby resulting in limited efficiency.6,9
These unfavourable recombination dynamics can in principle
Diketopyrrolopyrroles (DPPs) are a class of chromophores
known for their high photostability and intense light absorp-
tion, which can be tuned to absorb even red and infra-red
photons.24,25 Immobilisation onto metal oxides in both n-type
and p-type dye-sensitised solar cells (DSSCs) has been ach-
ieved using a surface anchoring group.26–31 Recently, DPPs were
Department of Chemistry, University of Cambridge, Lenseld Road, Cambridge CB2
1EW, UK. E-mail: reisner@ch.cam.ac.uk
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0sc04509h. Raw data related to this publication are available at the
University of Cambridge data repository: DOI: 10.17863/CAM.57382.
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Fig. 1 (a) Schematic representation of the water oxidising dyad-sensitised photoanode. (b) Structure of DPP-Ru with design features and
electron transfer events.
also co-immobilised with molecular catalysts on colloidal TiO2 DPP-Ru in 31% yield. The crude product contained a mixture of
nanoparticles, and CuCrO2 delafossite and NiO photocathodes DPPdye, DPP-Ru, [RuII(bda)(pic)2] and [RuII(bda)(DPPdye)2], and
for visible light-driven H2 evolution.32,33
accounts for the lower yield. The compounds were puried by
In this study, we report the synthesis, optical properties, column chromatography, and the composition and purity were
1
electrochemistry and PEC performance of a bespoke molec- conrmed by H, 13C and 11B NMR spectroscopy (Fig. S1–S5†),
ular dyad, DPP-Ru (Fig. 1), where a tailor-made DPP chromo- high-resolution mass spectrometry, infrared spectroscopy and
phore is covalently linked to a [RuII(bda)]-type WOC. The elemental analysis (see ESI for details†).
chromophore unit contains a pyridine moiety to coordinate to
the ruthenium catalyst, and an alkyl spacer chain to provide
Photophysical properties
exibility for the dimerisation of the Ru unit for improved
catalysis.11,34–36 A thiophene unit increases planarity and shis
the absorption maximum to longer wavelengths, allowing
more incident solar photons to be harvested.37,38 The incor-
poration of bulky alkyl chains on the DPP core allow for
increased hydrophobicity and limit p–p aggregation of the
molecule.25 DPP-Ru is then immobilised on a porous TiO2
electrode via a cyanoacrylic acid anchoring group, which
allows for localisation of the LUMO near the electrode surface,
facilitating electron injection into the bulk of the TiO2 semi-
conductor.30 Optimisation of immobilisation conditions and
surface coverage results in a dyad-sensitised photoanode for
light-driven water oxidation to O2.
The UV-vis absorption spectra of DPPdye
, DPP-Ru and
[RuII(bda)(pic)2] were recorded in N,N-dimethylformamide
(DMF) solution (Fig. 2a). The spectrum of DPPdye features an
intense band at 499 nm (3 ¼ 27.9 mMꢀ1 cmꢀ1), matching the
highest intensity of the solar spectrum, and a tailing absorption
up to 560 nm. This characteristic DPP-absorption is attributed
to a p–p* HOMO–LUMO transition. Density functional theory
(DFT) calculations on similar DPP chromophores have indi-
cated that this transition originates from the DPP core and
extends to the cyanoacrylic acid group.39 The second band at
390 nm can be attributed to a HOMOꢀ1 to LUMO and HOMO to
LUMO+1 transition.39
Due to the break in conjugation between the dye and the
catalyst, introduced by the ethylene-bridge, only a marginal
change in the DPP absorption spectrum is observed upon
complexation with the Ru centre in DPP-Ru. The dyad also
Results and discussion
Synthesis and characterisation
The DPP-Ru dyad (Fig. 1b) was prepared by a convergent features a strong absorption band at 302 nm, also observed in
synthesis protocol, rstly forming the bespoke chromophore, the [RuII(bda)(pic)2] spectrum, which is characteristic of
DPPdye, followed by complexation to the ruthenium moiety, [RuII(bda)] complexes.41
RuWOC [RuII(bda)(dmso)(pic)] (dmso ¼ dimethyl sulfoxide)
Upon photoexcitation at 499 nm, the emission spectrum of
(Scheme 1). The synthesis of DPPdye started by preparation of DPPdye shows a broad band centred at 587 nm, with a vibronic
the bridging moiety, 1, through a two-step synthesis from 4- shoulder at 627 nm, in aerated DMF solution (Fig. S6†). The
picoline (Scheme S1†). A Suzuki cross-coupling with DPPI large Stokes shi is typical of phenyl anked DPP dyes, due to
(synthesised by a previously reported procedure)39 afforded their torsion angle.42 The emission trace is similar for DPP-Ru
DPPII with a yield of 95%. A Knoevenagel condensation with (Fig. S7†). Absolute quantum yield measurements for DPPdye
cyanoacetic acid in the presence of piperidine afforded DPPdye reached 65%, whereas DPP-Ru achieved 4%. This efficient
in 84% yield. Finally, reux of RuWOC (synthesised following luminescence quenching is indicative of an intramolecular
a previously reported procedure)40 with DPPdye in methanol electron transfer from the ruthenium centre to the excited
(MeOH) and triethylamine afforded the dye-catalyst assembly chromophoric unit.43
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Scheme 1 Synthesis of the water oxidising DPP-Ru dyad. Conditions: (a) [Pd(PPh3)4], Na2CO3, tetrahydrofuran/H2O 2 : 1, 87 ꢂC, N2, overnight; (b)
piperidine, tetrahydrofuran, reflux, N2, overnight; (c) MeOH, drops of triethylamine, reflux, N2, overnight.
immobilisation, conrming that the molecules retain their
absorption properties on the electrode (Fig. 2b).
Electrochemical properties
Using cyclic voltammetry (CV), the electrochemical properties of
DPPdye and DPP-Ru were examined in solution (DMF) and when
chemisorbed on a mesoporous indium tin oxide (mITO; particle
size < 50 nm, lm thickness ꢁ3 mm)45–47 electrode in acetonitrile
(MeCN), containing tetrabutylammonium tetrauoroborate
(TBABF4, 0.1 M) as the supporting electrolyte. MeCN was used
for the electrochemical experiments with mITO due to the lower
solubility of the molecules in MeCN than DMF, which increases
their anchoring stability on the electrode. For irreversible
oxidations, the half-peak potentials (E(p/2)) were used to esti-
mate the thermodynamic oxidation potential (E(S+/S)).48 For
DPPdye, an irreversible oxidation is observed with a potential of
+1.22 V vs. normal hydrogen electrode (NHE) in DMF solution
and of +1.29 V vs. NHE for the mITO|DPPdye electrode in MeCN
(Fig. S8†).
When dissolved in DMF (Fig. S9a†), DPP-Ru features
a reversible oxidation at +0.60 V vs. NHE assigned to the RuIII/
RuII couple.34 A second oxidation, attributed to the DPP unit,
was observed at E(S+/S) ¼ +1.29 V vs. NHE. Upon immobilisation
(Fig. S9b†), the RuIII/RuII couple could not be observed, possibly
due to the high capacitance of the ITO electrodes or spatial
separation from the electrode. The oxidation of the DPP unit
was observed at +1.34 V vs. NHE, similar to the value recorded in
DMF solution.
The electrochemical properties were also conrmed in
aqueous sodium acetate (NaOAc, 0.1 M, pH 5.6) solution, in
Fig. 2 (a) UV-vis absorption spectra of DPPdye, [RuII(bda)(pic)2] and
DPP-Ru in DMF solution. (b) Normalised UV-vis spectra of DPPdye and
DPP-Ru on a mTiO2 film coated on a glass slide, and in DMF solution.
which the oxidation of the chromophore on a mITO|DPPdye
electrode was observed at +1.29 V vs. NHE (Fig. S10a†). A
signicantly higher current, attributed to catalysis, was ob-
tained for a mITO|DPP-Ru electrode (Fig. S10b†).
Thus, the oxidation potential of the DPP unit in both organic
and aqueous conditions is more positive than the reported
onset of catalysis of [RuII(bda)(pic)2] (Ecat ¼ +1.1 V vs. NHE), and
should therefore provide sufficient driving force for water
oxidation.34
Given the energy of the 0–0 transition for DPPdye and DPP-Ru,
(E0–0 ¼ 2.24 eV, Fig. S6 and S7†), the oxidation potential of the
excited chromophore (E(S+/S*)) in DMF solution can be
Immersion of a mesoporous TiO2 lm (anatase nano-
particles, ca. 20 nm diameter, ca. 6 mm thick) coated on a glass
slide in a solution of DPPdye and DPP-Ru in dichloromethane
(DCM) leads to a strong colouration of the electrodes, demon-
strating the affinity of the molecules to the metal oxide
surface.44 The spectra also remain largely unchanged upon
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estimated to be ꢀ1.02 and ꢀ0.95 V vs. NHE, respectively. This
allows for sufficient thermodynamic driving force for electron
injection into the conduction band of TiO2 at a wide range of pH
values (ECB(TiO2) ¼ ꢀ0.57 V vs. NHE at pH 7),49 and conrms
that the dye meets all of the thermodynamic requirements to be
incorporated in
oxidation.
a dyad-sensitised photoanode for water
Photoelectrochemistry under sacricial conditions
PEC experiments were carried out at room temperature in a N2-
purged one-compartment three electrode electrochemical cell
using a platinum counter electrode, a Ag/AgCl/KClsat reference
electrode and a sensitised TiO2 lm (mTiO2, procedure in ESI†)
as the working photoelectrode. Linear sweep voltammetry (LSV)
experiments were performed under chopped light irradiation
and a potential of +0.2 V vs. NHE was applied for chro-
noamperometry experiments. UV-ltered simulated solar light
was used for all PEC measurements (100 mW cmꢀ2, AM 1.5G, l
> 420 nm), avoiding direct excitation of the TiO2 semiconductor.
To evaluate the maximum photocurrent that can be extrac-
ted from the dye, without the kinetic limitations of water
oxidation catalysis, PEC measurements were performed on
a mTiO2|DPPdye electrode in the presence of triethanolamine
(TEOA) as a sacricial electron donor in aqueous electrolyte
solution (0.1 M, pH 7). The photoanodes were prepared by
soaking mTiO2 electrodes in a solution of DPPdye (0.2 mM in
DCM) overnight, followed by rinsing and drying in air (see ESI
for details†). Photocurrents of up to 1.3 mA cmꢀ2 were observed
for the mTiO2|DPPdye electrode (Fig. 3a), which conrms the
feasibility of electron injection into the CB of TiO2. These
currents are much higher than those typically obtained for
organic dyes on TiO2 electrodes in aqueous conditions with an
electron donor, and slightly lower than the ones obtained in
aqueous DSSCs, albeit without any electrode or electrolyte
optimisation.17,23,50–54 During a four hour chronoamperometry
experiment (Fig. S11†), a steady decrease of the photocurrent
and electrode decolouration was observed, which can be
attributed to dye decomposition, and to hydrolysis and
desorption of the carboxylate anchoring group at neutral
pH.55,56
Fig. 3 (a) Linear sweep voltammogram of a mTiO2|DPPdye electrode
with chopped illumination (v ¼ 5 mV sꢀ1). Conditions: aqueous TEOA
solution (0.1 M, pH 7), 100 mW cmꢀ2, AM 1.5G, l > 420 nm, N2 purged,
room temperature. (b) Chronoamperometry results at +0.2 V vs. NHE
under chopped light illumination of TiCl4–mTiO2|DPP-Ru and TiCl4–
mTiO2|DPP-Ru/CDCA electrodes incubated in a solution of 0.1 mM
DPP-Ru in MeOH with varying CDCA concentrations (0 or 20 mM).
Conditions: NaOAc buffer (0.1 M, pH 5.6), 100 mW cmꢀ2, AM 1.5G, l >
420 nm, N2 purged, room temperature.
To improve the photocurrent response, the TiO2 electrodes
were treated with a titanium tetrachloride solution (TiCl4,
TiCl4–mTiO2, details in ESI†), a straightforward method used to
improve the efficiency of DSSCs by increasing the electron
diffusion coefficient.57,58 PEC experiments were performed as
described above, with immobilisation of DPP-Ru on TiCl4–
mTiO2 electrodes carried out in different solvents (MeOH, DMF
and DCM) to identify the optimised immobilisation conditions
(Fig. S13†). In agreement with previous studies using Ru and
porphyrin photoabsorbers, in which the immobilisation solvent
plays a role in the ordering of the molecules on the surface, and
thus on the electron transfer dynamics, higher photocurrents
were obtained when using a polar protic solvent.9,59–61 The
currents observed with TiCl4–mTiO2|DPP-Ru electrodes were
signicantly higher than for the untreated mTiO2|DPP-Ru
electrodes. Interestingly, the initial photocurrent of the TiCl4–
mTiO2|DPPdye electrode was similar to that of the TiCl4–
mTiO2|DPP-Ru electrode under identical conditions
(Fig. S14a†). However, the UV-visible absorption spectrum of
TiCl4–mTiO2|DPPdye aer the PEC experiment showed
Photoelectrochemical water oxidation
For water oxidation catalysis, the sacricial electron donor
solution was replaced by an aqueous NaOAc solution (0.1 M, pH
5.6). The mTiO2 electrodes were immersed in a solution of DPP-
Ru (0.1 mM) in MeOH overnight, followed by rinsing and
drying in air. During the LSV experiments (Fig. S12a†),
photocurrents were observed with an onset of ꢀ0.43 V vs. NHE,
approximately 60 mV more positive than the conduction band
of TiO2 (ECB(TiO2) ¼ ꢀ0.49 V vs. NHE at pH 5.6).49 At more
positive potentials, the photocurrents spike at 200 mA cmꢀ2, but
quickly decay aerwards. This response can be attributed to an
initial fast electron injection from the dye into TiO2, followed by
charge accumulation and recombination between the electrode
and the oxidised dyad.5,9 At ꢀ0.1 V vs. NHE, a net photocurrent
of 18 mA cmꢀ2 was observed.
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decomposition of the chromophore, whereas the spectrum of desorption of the molecule or decomposition of the chromo-
TiCl4–mTiO2|DPP-Ru remained largely unchanged (Fig. S14b†). phore, but rather to the detachment or decomposition of the
Therefore, the high photocurrents of the TiCl4–mTiO2|DPPdye WOC. Different deactivation mechanisms have been proposed
electrode in the absence of hole scavenger can be attributed to for [RuII(bda)]-catalysts, which usually occur during the rate-
light-driven dye degradation.
limiting steps when the Ru centre is in the higher oxidation
The origin of the modest performance of TiCl4–mTiO2|DPP- states.66 The effect of CDCA addition, which increases the
Ru may be ascribed to aggregation of the dyad on the electrode. distance between dyad molecules on the electrode surface, is
The presence of aggregates has been shown to slow down unknown both on the decomposition pathways and the
electron injection, and shorten the lifetime of the radical dye dimerisation pathway, to which the early catalytic onset is
cation, leading to a much lower power conversion in DSSCs.62 attributed.34 Further work utilising pump–probe spectroscopic
Aggregate formation can be limited by addition of a co- techniques could be used in future studies to gain insight on
adsorbent, oen chenodeoxycholic acid (CDCA), which the role of CDCA in the system.
improves the efficiency of DSSCs despite decreasing the loading
of the dye on the electrode.26,27,31,39 Furthermore, the use of co-
adsorbents can impact the PEC performance by altering the
wettability of the electrode or the CB level of TiO2.53,63,64 While
this approach has been successfully implemented to stabilise
dyad-sensitised photocathodes for H2 evolution,65 it has not yet
been explored in water oxidising photoanodes.
Oxygen quantication
To evaluate the faradaic efficiency (FE) of our photoanode for
oxygen evolution, collector–generator (CG) cells were fabricated
as described previously.67 Illumination of a bare TiCl4–mTiO2
electrode for 10 min (Fig. S21†) leads only to a negligible
photocurrent background, due to the 420 nm cut-off lter pre-
The loading of the dyad on the surface was optimised in two
venting band gap excitation of the TiO2 semiconductor. While
stages. Firstly, the concentration of CDCA in the immobilisation
a high photocurrent was produced by the TiCl4–mTiO2|DPPdye
bath was varied. Despite this leading to a lower DPP-Ru loading
electrode (Fig. S22†), this only leads to a marginal current
(Fig. S15a†), higher photocurrents were reached when CDCA
increase by the collector, demonstrating that no O2 originates
was added to the immobilisation bath (Fig. S16†). Similar
from the dye-sensitised electrodes in the absence of the WOC
loading and photocurrents were observed for all concentrations
unit.
of CDCA studied. The loading of the dyad on the surface and
The fully assembled TiCl4–mTiO2|DPP-Ru/CDCA electrode
photoanode performance could be further controlled by varying
displays an initial photocurrent of 140 mA cmꢀ2 aer 10 s
the concentration of DPP-Ru in the immobilisation bath while
illumination, which decays to 17 mA cmꢀ2 over the course of
keeping a constant concentration of CDCA (Fig. S17 and S18†).
10 min of PEC operation. In contrast to control experiments,
Decreasing the dyad concentration resulted in lower absor-
an increase in the O2 reduction current by the collector elec-
bance of the lms and lower photocurrents. Furthermore,
trode was recorded for the dyad photoanode, corresponding
increasing the concentration resulted in a higher absorbance,
to a FE for O2 of 44 ꢃ 3.2% (Fig. 4). Trapped O2 in the porous
but without an accompanied increase in photocurrent
response.
electrode cannot be accounted for and hence lowers collector
efficiency (details in the ESI†). In addition, the moderate FE
Further optimisation of the PEC conditions was attempted
by varying the electrolyte solution (Fig. S19†). A similar peak
current was obtained in sodium sulfate (0.1 M, pH 7) solution
with a faster decrease in photocurrent, which is consistent with
hydrolysis of the molecule at neutral pH.
The optimised conditions for PEC experiments were an
aqueous NaOAc electrolyte solution (0.1 M, pH 5.6) with a TiCl4–
mTiO2|DPP-Ru/CDCA electrode prepared by immobilisation in
MeOH solution (0.1 mM DPP-Ru and 20 mM CDCA), capable of
affording a light harvesting efficiency close to unity up to
530 nm (Fig. S20†). In the chronoamperometry experiment
(Fig. 3b), photocurrents of 140 mA cmꢀ2 aer 10 s illumination
are observed in the presence of the co-adsorbent, which repre-
sent a 3.5 fold increase compared to PEC experiments in the
absence of CDCA.
The decrease in photocurrent could arise from decomposi-
Fig. 4 Collector–generator experiment for a TiCl4–mTiO2|DPP-Ru/
CDCA electrode prepared by immobilisation in MeOH solution
(0.1 mM DPP-Ru and 20 mM CDCA). Conditions: NaOAc buffer (0.1 M,
pH 5.6) with NaClO4 (0.4 M), 100 mW cmꢀ2, AM 1.5G, l > 420 nm, N2
purged, room temperature. (a) Chronoamperometry photocurrent
trace of the generator with an applied potential of +0.2 vs. NHE. (b)
Chronoamperometry current trace of the collector with an applied
potential of ꢀ0.6 vs. NHE.
tion of the chromophore, implied by the irreversibility of its
oxidation, as observed in cyclic voltammetry measurements.
Nonetheless, the UV-vis spectra of TiCl4–mTiO2|DPP-Ru/CDCA
electrodes aer the experiment (Fig. S15b†) show only a slight
decrease in the absorption band at 499 nm, suggesting
continued integrity of the chromophore on the electrode. The
decrease in photocurrent is therefore not attributed to
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can also be partially attributed to decomposition of the
photocatalyst.
Conclusions
We present an organic dye-ruthenium catalyst dyad consisting
of a DPP dye and a [RuII(bda)]-type complex. The chromophore
displayed strong light absorption in the visible part of the
electromagnetic spectrum, and suitable thermodynamics for
electron injection into the conduction band of TiO2 and hole
transfer to the Ru WOC. The dyad was then integrated in a TiO2-
based photoanode for light-driven water oxidation. Incorpora-
tion of CDCA as a co-adsorbent was shown to signicantly
increase the photocurrents from 40 to 140 mA cmꢀ2 in aqueous
sodium acetate solution (0.1 M, pH 5.6), despite a lower dyad
loading. The FE for O2 evolution was found to be 44% and
corresponds to a TONcat of 2.3 and a TONdye of 9.2. UV-vis
absorption measurements indicated that the decrease in
current during photolysis was mainly associated to catalyst
detachment or decomposition rather than dyad desorption or
chromophore decomposition. This study shows that a metal-
free dye with sufficient oxidising power can be covalently
linked to a molecular catalyst for catalytic O2 evolution on
a dyad-sensitised photoanode. Key techniques for accommo-
dating chromophores with strong intermolecular p–p stacking
interactions have been highlighted, and the signicant benets
of CDCA co-adsorption on molecular dye-sensitised photo-
anodes for water oxidation has been demonstrated. Future
experiments with time-resolved spectroscopy can be used to
gain insight on the role of CDCA in enhancing the PEC
performance, and serve as a blueprint for subsequent molecular
design.
Performance and comparison with state-of-the-art
Inductively coupled plasma optical emission spectrometry
(ICP-OES) based on Ru determination, aer digestion of fresh
TiCl4–mTiO2|DPP-Ru/CDCA electrodes in nitric acid, revealed
an initial loading (G0) of 11.7 ꢃ 1.04 nmol cmꢀ2 of the dyad.
This loading is lower than for other molecules on mesoporous
TiO2 electrodes, but in line with the large steric footprint of the
dyad and the presence of CDCA.15,45,68,69 This translates to
a turnover number (TON) of 2.3 ꢃ 0.6 for O2 evolution for the
catalyst (TONcat) and 9.2 ꢃ 2 for the dye (TONdye). ICP-OES
revealed a loading of 7.1 ꢃ 0.7 nmol cmꢀ2 of the dyad aer
the experiment, suggesting catalyst detachment or desorption
of the dyad assembly as partly responsible for the decreasing
photocurrent.
A molecular dyad made of a zinc porphyrin chromophore
and a [RuII(bda)] WOC was previously reported and immobi-
lised on a TiO2 electrode.15 CV measurements showed that in
aqueous conditions the chromophore did not possess sufficient
driving force to activate the catalyst. Despite this, during
photolysis, O2 was measured via gas chromatography corre-
sponding to a FE of 33% and a TONcat of 1.3, but the role of
direct excitation of TiO2 was not ruled out under the employed
experimental conditions. A Ru dye-[RuII(bda)] catalyst dyad was
able to evolve O2 with a FE of 30% on TiO2 and 74% on SnO2/
TiO2 electrodes.11 The FE values for O2 evolution reported here
compare favourably to the precious-metal chromophore dyad,
and show for the rst time catalytic turnover of both catalyst
and dye using a dyad with a metal-free chromophore. The
infancy of organic chromophores compared to Ru dyes for PEC
in aqueous conditions is reected in the superior stability of the
Ru-dye based water oxidation dyad, but future organic chro-
mophore development and optimisation opens the door to the
Conflicts of interest
There are no conicts to declare.
Acknowledgements
replacement of precious metal with earth-abundant This work was supported by the Christian Doppler Research
chromophores.
Association (Austrian Federal Ministry for Digital and Economic
Affairs and the National Foundation for Research, Technology
and Development), the OMV Group (E. R. and J. W.), an EPSRC
PhD DTA studentship (EP/M508007/1; D. A.-G.). We wish to
thank Dr Charles Creissen, Prof. Nikolay Kornienko, and Dr
Andreas Wagner for helpful discussions. We also thank Dr Nina
Despite higher dye loadings, photocurrents obtained for co-
immobilised systems with organic push–pull dyes on SnO2/TiO2
electrodes are typically low and FEs for O2 evolution are in the
range of 10%.17,23 Embedding the chromophore in a thin metal
oxide layer by atomic layer deposition (ꢁ1 nm, TiO2 or Al2O3)
and further immobilisation of the catalyst has been studied as Heidary and Dr Khoa Ly for their help in preparing the artwork.
an alternative way of limiting oxidative decomposition of the We appreciate suggestions and comments on the manuscript
chromophore.18,19,22 Improved efficiencies, between 11% and
from Dr Carla Casadevall and Esther Edwardes Moore.
49%, were thus reached. However, comparison with these
systems is limited since TON values have not been reported. A
TONcat of 3.0 and a TONdye of 2.4 were reported for a bor-
Notes and references
ondipyrromethene chromophore co-immobilised with a func-
tionalised [RuII(bda)] catalyst on a TiO2 electrode, with a FE for
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analogous catalyst, a TONcat of 27 and a TONdye of 14 were ob-
tained with a FE of 64%.20 Thus, the results obtained highlight
the benet of dyadic systems compared to a co-immobilised
approach in making an efficient use of dye molecules relative
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