A copper(i) dye-sensitised TiO2-based system for efficient light harvesting and photoconversion of CO2 into hydrocarbon fuel

Article abstract of DOI:10.1039/c2dt30865g

A new copper(i) complex with the ability to bind to TiO₂ was synthesised and successfully employed as a solar cell sensitizer. Furthermore, we demonstrated that the copper(i) dye-sensitised TiO₂-based photocatalyst exhibits impressiv

Full text of DOI:10.1039/c2dt30865g

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A copper(I) dye-sensitised TiO2-based system for efficient light harvesting and
photoconversion of CO2 into hydrocarbon fuel†
Yong-Jun Yuan,a Zhen-Tao Yu,*a,b Ji-Yuan Zhanga and Zhi-Gang Zou*a
Received 20th April 2012, Accepted 22nd June 2012
DOI: 10.1039/c2dt30865g
commonly used rare heavy-metal (poly-)pyridine complexes,6
A new copper(I) complex with the ability to bind to TiO2 was
efforts have been focused on exploration of hybrid systems
based exclusively on earth-abundant elements for the conversion
of solar energy into electricity (dye-sensitised solar cells,
DSSCs);7 however, such photo collectors that are capable of
efficient charge separation are still relatively scarce, particularly
those capable of CO2 photoconversion.
synthesised and successfully employed as a solar cell sensi-
tizer. Furthermore, we demonstrated that the copper(I) dye-
sensitised TiO2-based photocatalyst exhibits impressive effec-
tiveness for the selective photoreduction of CO2 to CH4
under visible light.
The alarming climate-change issue triggered by global warming
has generated great interest in the exploration of effective and
clean approaches to holding and reducing greenhouse-gas emis-
sions, especially carbon dioxide induced by the extensive use of
fossil fuels.1 Among the advances developed thus far, the light-
driven reduction of carbon dioxide directly into a high-energy-
content fuel is attractive because it is emerging as a promising
form of artificial photosynthesis that converts solar energy into
stored chemical energy.2 The catalysts typically used to achieve
such a demanding task are semiconductor metal oxide particles,
such as TiO2 and some non-titanates with the frequent use of
metallic clusters that contain Cu, Pt or Rh as active sites. These
materials are popular because of their ability to assist in the
photoreduction of carbon dioxide into various hydrocarbons.3,4
However, most of the previously developed catalysts rely on
ultraviolet light for excitation, which mainly concerns the con-
version efficiency and selective chemistry of the reactions and
therefore limits their potential number of applications. The
creation of new low-cost photocatalysts that can function with
visible light, facilitate the complicated multielectron reaction and
achieve high efficiency and selectivity has proven challenging.
Toward this end, spectral sensitisation of wide-band-gap semi-
conductor oxides, such as TiO2, allows the use of the main
visible-light range of the solar spectrum via photoinduced inter-
facial electron transfer.5 In this manner, hybrid materials based
on the preferred TiO2 nanoparticles combine the dissimilar phys-
ical and chemical potentials of organic–inorganic species while
eliminating or reducing their particular individual limitations.
With respect to the cost and environmental demand of the
An alternative pathway toward the conversion of CO2 involves
the use of transition-metal complexes as molecular photocata-
lysts,8 although most of these important systems suffer from a
tendency to promote only partial reduction to CO. One of the
main reasons behind this shortcoming is that the selective for-
mation of CH4 from CO2 is significantly more difficult than the
CO2-to-CO conversion process, which involves the participation
of only two electrons.9 The tailoring of a metal complex that
contains a low-cost, environmentally benign and abundant metal
as an alternative functional species to the more precious and
less-abundant systems for CO2 photoreduction is highly desir-
able. Copper(I) complexes represent a good choice of catalyst in
this respect. As an electrocatalyst, copper complexes have been
shown to be capable of the reductive coupling of CO2 to
oxalate.10 Because of their d10 configuration, copper(I) com-
plexes, particularly the copper(I) bis(phenanthroline)-type com-
plexes, exhibit a long-lived photoexcited state compared to those
of other redox-active first-row-transition-metal analogues. This
long-lived excited state represents a set of key electrochemical
and photophysical properties that render such materials suitable
for application in a solar-energy-conversion scheme.11 However,
a copper(I)–phenanthroline derivative was found to exhibit low
efficiency when used as a sensitiser.11d Recent work on
copper(I)–bipyridine complexes has shown that the photo-
sensitising behaviour of these complexes makes them strong
candidates for DSSC applications.12 These complexes provide
an exciting prospect for the use of Cu(I) sensitisers to drive elec-
tron-transfer processes in CO2 photoreduction.
In the course of our search for efficient components for solar
energy conversion,9a we designed and synthesised an air-stable
copper(I) complex 1 to serve as a photosensitiser for TiO2. The
1/TiO2 hybrid system provides a good platform to promote the
photoreduction of CO2 with high efficiency and selectivity under
visible-light irradiation. The structure of 1 is depicted in Fig. 1.
The methyl groups are appended at the 6- and 6′-position of
2,2′-bipyridine (bpy) to provide protection around the labile
metal copper(I), and the presence of these groups is believed to
increase the excited-state lifetime of the cuprous complex.12
aEco-Materials and Renewable Energy Research Center, National
Laboratory of Solid State Microstructures, Department of Materials
Science and Engineering, Nanjing University, Nanjing, Jiangsu 210093,
P.R. China. E-mail: yuzt@nju.edu.cn, zgzou@nju.edu.cn
bState Key Laboratory of Coordination Chemistry, Nanjing University,
Nanjing, 210093, P.R. China
†Electronic supplementary information (ESI) available: All the exper-
imental full details and DFT computational procedures. See DOI:
10.1039/c2dt30865g
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Fig. 1 Chemical structure of cationic copper(I) complex 1.
The cyanoacetic acid moieties at the end position act as anchor-
ing groups to ensure their grafting onto the semiconductor
surface, which has been extensively adopted as an efficient
acceptor unit for the construction of metal-free dye sensitisers
with high molar extinction coefficients for DSSCs; such semi-
conductor surfaces thereby achieve directional electron transport
for the implementation of efficient light-harvesting devices.
This desired complex [Cu(bpy)2]+ was easily synthesised in
good yield through the metalation of the functionalised bpy
ligand with one-half equivalent of [Cu(CH3CN)4]PF6 in
CH3OH–CH2Cl2 (1 : 1, v/v) and was characterised using conven-
tional spectroscopic and analytical methods (ESI†). Complex 1
is stable in solution and in the solid state in air, which indicated
that modification of the bpy ligand with methyl groups effec-
tively stabilises the Cu(I) species. In terms of the electronic
absorption properties, similar to those of bipyridyl complexes of
copper(I), complex 1 in MeOH produces an intense characteristic
band in the UV region with an absorption maximum centred at
310 nm (extinction coefficient ε = 5.54 × 104 M−1 cm−1) that
was assigned to ligand-based π → π* transitions. The expected
metal-to-ligand charge transfer (MLCT) absorption for 1 typi-
cally appears in the visible region centred at 496 nm with ε
8033 M−1 cm−1 (Fig. S2†), which accounts for the dark red
appearance of the complex.12 When the complex is bound to the
surface of titania (Degussa P-25), as verified using IR spec-
troscopy, the particles are rendered dark red, and the original
MLCT bands appear at approximately the same wavelength as
the band of the complex in solution. The solid-state absorption
bands are, however, broader and red-shifted compared to those
in the solution spectra, which is related to π–π stacking or aggre-
gation in the solid state. Two strong bands appear in the IR spec-
trum of 1 at 1389 and 1628 cm−1, in accordance with the
characteristic symmetric and asymmetric stretching frequencies
of the carboxylate group (Fig. S3†). After being adsorbed onto
TiO2, the complex exhibits two related bands at 1418 and
1631 cm−1. Complex 1 is non-emissive in air-saturated MeOH
solution (the only common solvent in which it is slightly
soluble) at room temperature, which may be due to the presence
of some quenchers (most likely MeOH) or due to the excited-
state structural dynamics of 1.13 The complex is also non-emis-
sive in the solid state, most likely because of the aggregation
effect. Although a few copper(I)–bipyridine complexes have
been previously employed as sensitisers in DSSC applications,
the photophysical characterisation for the excited state has been
less extensively investigated.12 This is unfortunate for sensitisers
because the performance of solar conversion devices relies on
both the MLCT absorption profile and the excited-state proper-
ties. We have tried to determine the excited-state lifetime with
the help of a pulsed Nd:YAG laser; however, no information
Fig. 2 Photocurrent density–voltage (J–V) characteristic curves for
DSSC device with 1 recorded under illumination of simulated solar light
(AM 1.5G, 100 mW cm−2). Inset: photograph of a 1-coated TiO2 film
on FTO.
could be extracted, which indicated that the electrons do not stay
in the excited state for an expected prolonged time in 1. The
decrease in rigidity of the bpy fragment compared to that of the
phenanthroline should promote deterioration of the excited-state
properties of the bipyridine-type complex.14
To determine the identity of the lower-energy MLCT band in
1, density functional theory (DFT) calculations were performed
on the cation of 1 using the B3LYP method with the LANL2DZ
basis set (Figs. S4–S6†). The HOMO is populated over the
Cu dπ orbitals and partially on the pyridine rings of the ligand
whereas the lowest unoccupied molecular orbital (LUMO)
resides primarily on the π system of the bpy moieties and on the
anchoring units, which serve as an acceptor through which the
excited electrons of the complex are directionally injected into
TiO2. The presence of the LUMO on the ligand, especially on
the aromatic rings of the ligand, may facilitate electron injection
because it provides electronic mixing between the bpy π* orbital
into which the electron is excited and the TiO2 acceptor orbitals.
The good separation of the frontier molecular orbitals is
beneficial for the charge migration on the electron excitation
from the HOMO level to the LUMO level.
The light-harvesting property of complex 1 on a nanocrystal-
line TiO2 electrode was initially confirmed through the demon-
stration of a typical DSSC device. Complex 1 achieved efficient
sensitisation of nanocrystalline TiO2, as illustrated in Fig. 2. The
photovoltaic performance of the solar cell was measured under
standard AM 1.5G conditions at a light intensity of 100 mW cm−2.
The cell yielded a short-circuit current (Jsc) of 4.69 mA cm−2
,
an open-circuit voltage (Voc) of 570 mV and a fill factor (FF)
of 78.8%, and achieved an overall energy conversion efficiency
(η) of 2.2%, which is slightly lower than the previous value
reported for the copper(I)-based DSSCs (2.3% with higher Jsc of
5.9 mA cm−2).12 For purposes of comparison, a cell with the art
ruthenium dye of N719 was also fabricated under the same con-
ditions, and it gave a η value of 7.8%. The results clearly imply
that complex 1 works properly for harvesting light in the cell
and that the injected electrons are effectively transported, which
results in the generation of electricity. The photon-to-current
conversion mechanism, which is similar to the photosynthesis
mechanism of plants, is well understood.12a In DSSCs, the sensi-
tiser captures sunlight, which results in the injection of an
electron into the conduction band of the semiconductor TiO2.
The injected electrons diffuse through the nanoparticle porous
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strong IR bands appeared at lower frequencies of 1392 and
1604 cm−1 for the 1/TiO2 sample after it was used as a photo-
catalyst, which is attributed to the adsorption of CO2 or the inter-
mediate products on the surface of the 1/TiO2.
These results demonstrate that electron transfer from the
copper complex to CO2 is effective. The separation of electrons
and holes by visible light is also evidenced by the surface photo-
voltage spectrum of 1-derivatised TiO2 (Fig. S9†), which exhi-
bits an obvious signal in response to visible light (λ > 420 nm).
The occurrence of this signal indicated that the photogenerated
electron–hole pairs were effectively separated.17 This result also
suggested that the introduction of complex 1 is beneficial to the
separation of the charges in TiO2 under irradiation from visible
light.18,19 The formation mechanism of hydrocarbons from CO2
photoreduction remains to be elucidated. The possible process is
shown in the ESI.†20 The inactivity of the parent TiO2 in this
reaction revealed that the Cu(I) moiety was responsible for the
visible-light activity towards catalytic CO2 reduction.2e,3c The
copper(I) species apparently serves the function of harvesting
visible photons to drive rapid electron injection into the conduc-
tion band of the TiO2, as confirmed by its effective performance
in a DSSC device.
In conclusion, we have achieved selective CO2 visible-light
conversion based on the functionalisation of TiO2 with an artifi-
cial light-harvesting copper(I) complex. The results may upgrade
copper to the metal of choice in CO2 reduction catalysts. The
superior performance of the copper complexes may be due to a
modification of the key structural and electronic features that
sustain efficient CO2 reduction. Further detailed investigations
are also needed to obtain an exact mechanistic understanding of
the reaction process. We believe that these investigations would
contribute significantly to the exploitation of new efficient light-
harvesting complexes for the creation of sustainable artificial
photosynthesis.
Fig. 3 CH4 generation catalyzed by solid 1/P-25 under visible-light
irradiation (λ > 420 nm).
network to the photoanode electrode, where they are collected
for powering
a load. However, the energy-to-electricity
capability is lower than that of N719, which is primarily attribu-
ted to the relatively narrow optical absorbance of the copper
complex in the visible region and to its relatively short excited-
state lifetime. Like other transition-metal complex sensitisers
fixed on the surface of TiO2 for solar energy conversion,7,15
the copper complex exhibits a short-lived excited state may
negatively impact the electron-transfer dynamics and, therefore,
the performance of the device. Nevertheless, our results
strongly indicate that the potential of this copper(I) material is
promising and competitive for light-harvesting applications in
next-generation devices.
In view of these encouraging energy-conversion results,
further investigations were conducted to assess the activity of the
system for the photocatalytic conversion of CO2. Upon
irradiation with visible light (radiation λ > 420 nm), CO2 photo-
reduction was established using water vapour as an electron
donor over 0.1 g of a solid sample that contained 1/P-25 in a
special gas–solid reaction system.9a The reduction product
during the experiment was monitored and confirmed by gas
chromatography analysis. The results showed that 1/P-25 was
active in the selective reduction of CO2 to only CH4 as the
product over H2O under these conditions. The amount of CH4
production increased stepwise with increasing time, until a
plateau value of ca. 7 μmol g−1 was reached after 24 h of
irradiation (Fig. 3). In the absence of 1, 1/TiO2 or CO2, no CH4
generation was observed under identical gaseous conditions. In
addition, the reaction in which 100 mg of pure 1 was used alone
showed also no detectable CH4 production over 10 h. Therefore,
the photoreduction of carbon dioxide is involved in the genera-
tion of methane. Although the photoreduction of CO2 to
methane has been achieved by several Ru(II) complexes,8,9,16
such impressive activity for the clean formation of CH4 rep-
resents a significant advance given that the metal on which the
complex is based is inexpensive and abundant. The activity
observed in the present reduction of CO2 is lower than that
reported for previous noble-metal-loaded inorganic semiconduc-
tor catalysts that require UV irradiation;2 however, it is compar-
able with the iridium-complex sensitiser that was aided by the
presence of active Pt sites on a TiO2 support.9a After the photo-
reduction process, the absorption spectrum of 1/TiO2 was almost
unchanged from that of the parent compound (Fig. S7†), i.e., the
compound appears to be stable towards photocatalysis. However,
This research was financially supported by the National
Science Foundation of China (Grant No. 20901038) and the
Fundamental Research Funds for the Central Universities (Grant
Nos. 1106021342 and 1117021302). We are also grateful to the
Scientific Research Foundation for the Returned Overseas
Chinese Scholars, State Education Ministry.
Notes and references
1 (a) N. Armaroli and V. Balzani, Angew. Chem., Int. Ed., 2007, 46, 52–66;
(b) D. G. Nocera, Inorg. Chem., 2009, 48, 10001–10017.
2 (a) J. Barber, Chem. Soc. Rev., 2009, 38, 185–196; (b) N. D. McDaniel
and S. Bernhard, Dalton Trans., 2010, 39, 10021–10030;
(c) M. R. Hoffmann, J. A. Moss and M. M. Baum, Dalton Trans., 2011,
40, 5151–5158; (d) S. C. Roy, O. K. Varghese, M. Paulose and
C. A. Grimes, ACS Nano, 2010, 4, 1259–1278; (e) A. Kubacka,
M. Fernández-García and G. Colón, Chem. Rev., 2012, 112, 1555–1614.
3 (a) G. Centi, S. Perathoner, G. Winè and M. Gangeri, Green Chem.,
2007, 9, 671–678; (b) V. P. Indrakanti, J. D. Kubicki and H. H. Schobert,
Energy Environ. Sci., 2009, 2, 745–758; (c) C. C. Yang, Y. H. Yu,
B. V. D. Linden, J. C. S. Wu and G. Mul, J. Am. Chem. Soc., 2010, 132,
8398–8406.
4 (a) O. K. Varghese, M. Paulose, T. J. LaTempa and C. A. Grimes, Nano
Lett., 2009, 9, 731–737; (b) N. M. Dimitrijevic, B. K. Vijayan,
O. G. Poluektov, T. Rajh, K. A. Gray, H. He and P. Zapol, J. Am. Chem.
Soc., 2011, 133, 3964–3971.
5 (a) A. Fihri, V. Artero, M. Razavet, C. Baffert, W. Leibl and
M. Fontecave, Angew. Chem., Int. Ed., 2008, 47, 564–567; (b) J. Zhang,
P. Du, J. Schneider, P. Jarosz and R. Eisenberg, J. Am. Chem. Soc., 2007,
9596 | Dalton Trans., 2012, 41, 9594–9597
This journal is © The Royal Society of Chemistry 2012

View Online
129, 7726–7727; (c) W. Kim, T. Tachikawa, T. Majima, C. Li, H. J. Kim
and W. Choi, Energy Environ. Sci., 2010, 3, 1789–1795; (d) E. Reisner,
D. J. Powell, C. Cavazza, J. C. Fontecilla-Camps and F. A. Armstrong,
J. Am. Chem. Soc., 2009, 131, 18457–18466; (e) W. J. Youngblood,
S. H. A. Lee, Y. Kobayashi, E. A. Hernandez-Pagan, P. G. Hoertz,
T. A. Moore, D. Gust and T. E. Mallouk, J. Am. Chem. Soc., 2009, 131,
926–927.
6 (a) T. W. Woolerton, S. Sheard, E. Reisner, E. Pierce, S. W. Ragsdale and
F. A. Armstrong, J. Am. Chem. Soc., 2010, 132, 2132–2133; (b) S. Sato,
T. Morikawa, S. Saeki, T. Kajino and T. Motohiro, Angew. Chem., Int.
Ed., 2010, 49, 5101–5105; (c) S. Sato, T. Arai, T. Morikawa, K. Uemura,
T. M. Suzuki, H. Tanaka and T. Kajino, J. Am. Chem. Soc., 2011, 133,
15240–15243.
7 (a) S. Ferrere and B. A. Gregg, J. Am. Chem. Soc., 1998, 120, 843–844;
(b) C. L. Linfoot, P. Richardson, K. L. McCall, J. R. Durrant,
A. Morandeiara and N. Robertson, Sol. Energy, 2011, 85, 1195–1203.
8 (a) C. Finn, S. Schnittger, L. J. Yellowlees and J. B. Love, Chem.
Commun., 2012, 48, 1392–1399; (b) A. J. Morris, G. J. Meyer and
E. Fujita, Acc. Chem. Res., 2009, 42, 1983–1994; (c) D. Walther,
M. Ruben and S. Rau, Coord. Chem. Rev., 1999, 182, 67–100.
9 (a) Y. J. Yuan, Z. T. Yu, X. Y. Chen, J. Y. Zhang and Z. G. Zou,
Chem.–Eur. J., 2011, 17, 12891–12895; (b) R. Maidan and I. Willner,
J. Am. Chem. Soc., 1986, 108, 8100–8101; (c) I. Willner, R. Maidan,
D. Mandler, H. Dürr, G. Dörr and K. Zengerle, J. Am. Chem. Soc., 1987,
109, 6080–6086.
J.-F. Nierengarten and J.-P. Sauvage, J. Chem. Soc., Dalton Trans., 1994,
1649–1654.
12 (a) N. Robertson, ChemSusChem, 2008, 1, 977–979; (b) T. Bessho,
E. C. Constable, M. Graetzel, A. H. Redondo, C. E. Housecroft,
W. Kylberg, Md. K. Nazeeruddin, M. Neuburger and S. Schaffner, Chem.
Commun., 2008, 3717–3719; (c) B. Bozic-Weber, E. C. Constable,
C. E. Housecroft, P. Kopecky, M. Neuburger and J. A. Zampese, Dalton
Trans., 2011, 40, 12584–12594; (d) S. Sakaki, T. Kuroki and T. Hamada,
J. Chem. Soc., Dalton Trans., 2002, 840–842.
13 (a) D. G. Cuttell, S. M. Kuang, P. E. Fanwick, D. R. McMillin and
R. A. Walton, J. Am. Chem. Soc., 2002, 124, 6–7; (b) L. X. Chen,
G. B. Shaw, I. Novozhilova, T. Liu, G. Jennings, K. Attenkofer,
G. J. Meyer and P. Coppens, J. Am. Chem. Soc., 2003, 125, 7022–7034.
14 J. A. Treadway, B. Loeb, R. Lopez, P. A. Anderson, F. R. Keene and
T. J. Meyer, Inorg. Chem., 1996, 35, 2242–2246.
15 (a) Y. Tachibana, S. A. Haque, I. P. Mercer, J. R. Durrant and D. R. Klug,
J. Phys. Chem. B, 2000, 104, 1198–1205; (b) P. Qu, D. W. Thompson
and G. J. Meyer, Langmuir, 2000, 16, 4662–4671; (c) A. Vlček Jr.,
Coord. Chem. Rev., 2000, 200–202, 933–977.
16 (a) O. Ozcan, F. Yukruk, E. U. Akkaya and D. Uner, Appl. Catal., B,
2007, 71, 291–297; (b) T. V. Nguyen, J. C. S. Wu and C. H. Chiou,
Catal. Commun., 2008, 9, 2073–2076.
17 Z. T. Yu, J. J. Xu, Y. S. Jiang, Z. Shi, Y. Guo, D. J. Wang and J. S. Chen,
J. Mater. Chem., 2003, 13, 2227–2233.
18 (a) W. J. Youngblood, S. H. A. Lee, K. Maeda and T. E. Mallouk, Acc.
Chem. Res., 2009, 42, 1966–1973; (b) G. J. Meyer, Inorg. Chem., 2005,
44, 6852–6864; (c) J. C. S. Wu, Catal. Surv. Asia, 2009, 13, 30–40.
19 (a) M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii and M. Honda, J. Phys.
Chem. B, 1997, 101, 2632–2636; (b) C. Wang, R. L. Thompson,
J. Baltrus and C. Matranga, J. Phys. Chem. Lett., 2010, 1, 48–53.
20 C. A. Kelly and G. J. Meyer, Coord. Chem. Rev., 2001, 211, 295–315.
10 R. Angamuthu, P. Byers, M. Lutz, A. L. Spek and E. Bouwman, Science,
2010, 327, 313–315.
11 (a) N. Armaroli, Chem. Soc. Rev., 2001, 30, 113–124; (b) R. M. Everly
and D. R. McMillin, J. Phys. Chem., 1991, 95, 9071–9075;
(c) M. Ruthkosky, C. A. Kelly, M. C. Zaros and G. J. Meyer, J. Am.
Chem. Soc., 1997, 119, 12004–12005; (d) N. Alonso-Vante,
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