Photosensitization of different ruthenium(II) complex dyes on TiO2 for photocatalytic H2 evolution under visible-light

Article abstract of DOI:10.1016/j.cplett.2008.06.001

Hydrogen production over dye-sensitized Pt/P25 under visible-light irradiation was investigated by using methanol or TEOA as an electron donor. Ru₂(bpy)₄L₁-PF₆ shows the best photosensitization due to its larges

Full text of DOI:10.1016/j.cplett.2008.06.001

Chemical Physics Letters 460 (2008) 216–219
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Chemical Physics Letters
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Photosensitization of different ruthenium(II) complex dyes on TiO2 for
photocatalytic H2 evolution under visible-light
a
a
a
a
a
Tianyou Peng a,b, , Ke Dai , Huabing Yi , Dingning Ke , Ping Cai , Ling Zan
*
a Department of Chemistry, Wuhan University, Wuhan 430072, PR China
b State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogen production over dye-sensitized Pt/P25 under visible-light irradiation was investigated by using
methanol or TEOA as an electron donor. Ru2(bpy)4L1–PF6 shows the best photosensitization due to its
largest conjugation system, widest range of visible-light and ‘antenna effect’ among the used three
Ru(II)-bipyridyl dyes. Ru2(bpy)4L1–PF6 loosely linked with TiO2 also exhibit more steady and higher
increases in H2 evolution upon prolonging the irradiation time than the tightly linked N719. The dynamic
equilibrium between the linkage of ground dye and divorce of oxidized dye from TiO2 can enhance the
electron-injection and hinder the backward transfer, and then improve the H2 evolution efficiency.
Ó 2008 Elsevier B.V. All rights reserved.
Received 25 January 2008
In final form 1 June 2008
Available online 5 June 2008
1. Introduction
The firmly bound dye like N3 can be stabilized near TiO2 sur-
faces, and leads to fast electron-injection of the excited dye into
Photocatalytic water-splitting over semiconductors has re-
ceived much attention due to its potential application in H2 pro-
duction [1,2]. As a widely applied photocatalyst, TiO2 can only
absorb UV light due to its wide energy gap. The development of
photocatalyst that can effectively harvest the visible-light of sun-
light is indispensable for H2 production. An important way to im-
prove the H2 production over TiO2 is coupling with narrow
energy-band semiconductor and doping metal and/or nonmetal
element. However, a reliable and reproducible photocatalyst of this
type has seldom been established [2].
The dye photosensitization on TiO2 has been reported in the
dye-sensitized nanocrystalline solar cell (DSSCs) [3], in which
dye was considered as antenna to collect energy and then trans-
ferred it to TiO2. Similarly, dye-sensitized TiO2 have also been ap-
plied for photocatalytic H2 production from suspension
containing sacrificial [4–7] or nonsacrificial reagents [8–10]. Up till
now, xanthene [6,7], merocyanine [11,12], and Ru(II) complex
[5,8–10], have been used as sensitizer of TiO2 for H2 evolution.
Most dyes can not oxidize water by themselves to O2 and conse-
quently are decomposed under irradiation without sacrificial re-
agent, because O2 evolution from water is much more difficult
than the kinetically simpler process of H2 evolution [4–7,13]. This
inability to produce O2 over sensitizers has prevented the con-
struction of dye-sensitized photocatalysts. Therefore, it is still
meaningful to seek more effective sensitizer for the photocatalytic
H2 production.
substrate because the probability of this process should depend
on the overlap of the wave functions of the donor and acceptor
[5,14], but injected electrons also possibly transfer backward the
oxidized dye due to its tight linkage with TiO2 [14]. Kajiwara et
al. have also thought that it was probable that the thermal re-ori-
entational motion of the loosely attached molecules enhanced the
electron transfer [5]. Therefore, the linkage mode of dye with TiO2
definitely influences the electron transfer. Herein, three kinds of
dye possessing different terminal groups, which can attach to
TiO2 through tight or loose linkage, were selected to compare their
photosensitizing behaviors.
2. Experimental
P25 (TiO2, Degussa) and H2PtCl6ꢀ6H2O were obtained from com-
mercial sources. N719 [(n-Bu4N)2-cis-Ru(dcbpy)2(SCN)2], one of
the best sensitizer for DSSCs with the same structure as N3 dye
[8], was purchased from Solaronix and used as reference substance
because it can tightly attach to TiO2 through its carboxyl groups
[8]. Ru(bpy)2(him)2–NO3 and Ru2(bpy)4L1–PF6 were prepared
according to previous literatures [15,16]. These complexes have
no terminal group like N719 and can just loosely link with TiO2.
Fig. 1 gives structures of those Ru(II)-bipyridyl dyes.
H2PtCl6ꢀ6H2O(0.01 g) and P25(0.38 g) were added into 175 ml
of 2.0 M Na2CO3 solution in a Pyrex glass cell, and then exposed
to 500 W Hg-lamp for 5 h under stirring. After centrifugation, the
sample was washed and dried at 120 °C to obtain 1.0 wt% Pt/P25.
This Pt/P25 was immersed in 1 ꢁ 10ꢂ4 M dye aqueous solution
for 72 h, then washed and dried at 80 °C to obtain dye-sensitized
Pt/P25.
* Corresponding author. Address: Department of Chemistry, Wuhan University,
Wuhan 430072, PR China. Fax: +86 27 6875 4067.
E-mail address: typeng@whu.edu.cn (T. Peng).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.06.001

T. Peng et al. / Chemical Physics Letters 460 (2008) 216–219
217
Fig. 2. UV–Vis spectra of the three Ru(II)-bipyridyl dyes.
the reaction of surface hydroxyl of TiO2 and oxygen atoms from
the azo-benzene carboxylic spacer (L). Similarly, Ru(bpy)2(him)2–
NO3 can also loosely attach to TiO2 through hydrogen-bonds be-
tween un-coordinated N atom in imidazoles and surface hydroxyl.
Therefore, it is reasonable to conclude that there exists different
electron transfer mechanism between the tightly and loosely at-
tached dyes in Pt/P25 suspension, which will be further discussed
below.
Fig. 1. Molecular structures of three Ru(II)-bipyridyl dyes.
3.2. Reaction of H2 evolution over dye-sensitized Pt/P25
H2 production reactions were carried out in an outer irradia-
tion-type photoreactor (Pyrex glass). A 500 W Xe-lamp was colli-
mated and focalized into 5 cm2 parallel faculae. A cut-off filter
was employed to obtain the visible-light irradiation (k > 420 nm).
100 ml aqueous suspension containing photocatalyst (40 mg) and
sacrificial reagent solution (20 ml triethanolamine (TEOA) or meth-
anol) was irradiated from top after thoroughly removing air. H2
evolution rate was analyzed with gas chromatograph (TCD, 5 Å
molecular sieve).
Primary results showed that there was no obvious H2 produc-
tion over Pt/P25 without dye or sacrificial reagent under visible-
light irradiation, indicating that the band-gap excitation was not
occurred. H2 evolution was detected in water–methanol solution
containing dye-sensitized Pt/P25, and partially produced by the
conversion of methanol in this system [20]. Therefore, to check
whether this reaction was photocatalytic or not, the reaction was
repeated but using TEOA as sacrificial reagent, which could not
contribute H2 production during the photocatalytic reaction [7].
Table 1 shows the H2 evolution rates over dye-sensitized Pt/P25
under visible-light for 1 h, and Fig. 3 presents the H2 production
amount as a function of time from different sacrificial reagents.
As can be seen, all dyes show obvious photosensitization, indicat-
ing dyes can be efficiently excited and inject electrons into TiO2,
and then transport it to the loaded Pt to produce H2 [8,12]. Further-
more, a much higher H2 yield is obtained by using methanol in
comparison with TEOA, which is similar to the previous report
[20]. The following are the reasons for rationalizing this
phenomenon.
3. Results and discussion
3.1. UV–Vis absorption spectra of dyes
The kmax in visible range for Ru2(bpy)4L1–PF6, Ru(bpy)2(him)2–
NO3 and N719 is 456, 436 and 502 nm (Fig. 2), respectively. Ru2
(bpy)4L1–PF6 and Ru(bpy)2(him)2–NO3 show broader absorption
bands due to their larger molecular areas and delocalized conjuga-
tion system, although their kmax are slightly shorter than N719 due
to the presence of carboxyl groups. These properties of Ru(bpy)2
(him)2–NO3 and Ru2(bpy)4L1–PF6 are beneficial for the visible-light
absorption and electron-injection of the excited dye [3,5]. Ru(II)-
bipyridyl dyes have been used as sensitizers in DSSCs, and elec-
tron-injection from the excited dye to TiO2 occurs efficiently
[17,18]. For photocatalytic H2 production, however, the solution
must contain sacrificial reagent due to lack of Iꢂ3 =Iꢂ redox mediator,
otherwise Ru(II) dye might be decomposed during the light irradi-
ation [19–21]. Therefore, methanol or TEOA was used as sacrificial
reagent at the present system.
First, methanol can be oxidized to CH2O, HCOOH and trans-
formed to CO2 and H2 in Pt/TiO2 suspension [19–22], some metha-
nol was consumed during the reaction and the formed CH4 and H2
were identified in the gas mixtures [19,20]. These findings indicate
that H2 is partially contributed from methanol, and the following
reactions might take place in the water–methanol solution
[12,19–22].
Dye þ h
m
! Dyeꢃ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
Dyeꢃ ! Dyeþ þ eꢂðCBÞ
eꢂðCBÞ ! eꢂðPtÞ
In the present work, H2 evolution over Ru2(bpy)4L1–PF6 or Ru(b-
py)2(him)2–NO3 sensitized Pt/P25 indicates that the electron-injec-
tion did occur, although the two dyes have no any carboxyl group
like N719 to tightly link with TiO2. Still, Ru2(bpy)4L1–PF6 can
loosely attach to TiO2 through oxygen-bridge bonds derived from
eꢂðPtÞ þ Hþ ! 1=2H2
Dyeþ þ CH3OH ! Dye þ CH3OHꢄþ
CH3OHꢄþ ! CH2OHꢄ þ Hþ

218
T. Peng et al. / Chemical Physics Letters 460 (2008) 216–219
Table 1
Effect of different dyes on the H2 production over Pt/P25 from different sacrificial reagent solutions
Dye sensitized Pt/P25
H2 evolution rate (TONa) from CH3OH solution
mol/g h)
H2 evolution rate (TONa) from TEOA solution
mol/g h)
Dye adsorption amount on 40 mg catalyst
mol)
(l
(
l
(l
Ru2(bpy)4L1–PF6
Ru(bpy)2(him)2–NO3
N719
874.3 (7055.1)
561.3 (2178.0)
271.5 (2636.7)
53.3 (554.3)
48.8 (209.5)
43.7 (445.5)
0.062
0.027
0.024
a
Moles of H2 produced per mole of ruthenium complex.
These phenomena can be rationalized by considering the following
reasons.
The injected electrons can also transfer backward the oxidized
dye due to the tight linkage between N719 and TiO2 [14]. Whereas
this electron backward transfer can be retarded in Ru2(bpy)4L1–PF6
and Ru(bpy)2(him)2–NO3 sensitized Pt/P25, because the coordina-
tion circumstances of Ru need change to meet with the increase
of valence state after the electron-injection. And this change likely
means the divorce of oxidized dyes from TiO2 due to their loose
linkage as well as the adsorbed ground state molecules usually
locating the position of minimum energy on the surface [5]. Any-
way, those loose linkages such as oxygen-bridges and hydrogen-
bonds might promote the electron-injection, and the oxidized
dye can divorce timely from TiO2 to avoid the backward transfer,
which results in higher H2 production efficiency.
In addition to the widest visible-light harvesting (Fig. 2) due to
its larger molecular area and conjugation system [4–7], the highest
H2 evolution efficiency over Ru2(bpy)4L1–PF6 sensitized Pt/P25 can
be ascribed to the more efficient electron transfer. The excited elec-
trons of this dye can be transferred through two ways, MLCT
(metal-to-ligand) and MMCT (metal-to-metal), while Ru(bpy)2
(him)2–NO3 and N719 conduct a single electron transfer (MLCT).
Amadelli et al. have reported two Ru(bpy)2(CN)2-units in a trinu-
clear dye [(Ru(bpy)2(CN)2)2Ru(dcbpy)2] acted like antenna to ab-
sorb light and transported it to the other Ru(dcbpy)2-unit
connected with TiO2 through the tight chemical bonds [3]. Accord-
ing to this view, ‘antenna effect’ must exist in Ru2(bpy)4L1–PF6 sen-
sitized Pt/P25, which can enhance the visible-light harvesting and
electron transfer efficiency. Whereas the TON of Ru(bpy)2(him)2–
NO3 sensitized Pt/P25 in both sacrificial reagent solution are lower
than those over Ru2(bpy)4L1–PF6 or N719 sensitized Pt/P25. It is
possibly ascribed to the multilayer adsorption of Ru(bpy)2(him)2–
NO3 on the surface of catalyst due to the much higher adsorption
amount (Table 1), which lead to the retardation of electron-injec-
tion and then to a lower TON value. The further researches are
now under investigation.
Fig. 3. Time courses of the photocatalytic H2 production over the dye-sensitized Pt/
P25.
CH2OHꢄ ! HCHO þ Hþ þ eꢂ
ð7Þ
ð8Þ
HCHO þ H2O ! HCO2H þ H2 or
HCHO þ OHꢄ ! ꢄCHO þ H2O ! HCOOH þ Hþ þ eꢂ
HCOOH ! CO2 þ H2 or
ð9Þ
ð10Þ
ð11Þ
ð12Þ
HCOOH þ OHꢄ ! ꢄCOOH þ H2O
ꢄCOOH þ OHꢄ ! CO2 þ H2O
Second, TEOA can be oxidized by means of losing one of the un-
paired electrons of N atom [23], so the protonated TEOA are diffi-
cult to oxidize. The pH value might have effects on the
adsorption of TEOA, and then on the reactions of H2 evolution
and TEOA oxidation. Namely, the photocatalytic H2 evolution
should occur at an optimal pH [23] but the present system is not
possibly at the optimal value.
Third, the turn over numbers (TON) of photocatalytic H2 evolu-
tion in TEOA solution are much lower than those in methanol solu-
tion (Table 1), also indicating that H2 is at least partially produced
by the conversion of methanol [20].
Theoretically, N719 should show a better photosensitization
than the other dyes due to its much tight fixation, which is benefi-
cial for the dye adsorption and electron-injection [5,14]. However,
N719 exhibits the lowest photosensitization among the three dyes
from both sacrificial reagent solutions (Table 1). For example, the
TON of N719 sensitized Pt/P25 in both sacrificial reagent solutions
are also much lower than those over Ru2(bpy)4L1–PF6 sensitized
Pt/P25. H2 evolution rates over Ru2(bpy)4L1–PF6 and Ru(bpy)2-
(him)2–NO3 sensitized Pt/P25 in methanol solution are about 3.22
and 2.04 times as that over N719 sensitized Pt/P25, respectively.
3.3. Comparative investigation on the steady H2 evolution over dye-
sensitized Pt/P25
To observe the reproducibility of H2 production, H2 evolution
rates were checked every 1 h, in which the next portion of metha-
nol or TEOA (10 mL) was added into the same dye-sensitized Pt/
P25 suspension. Ru2(bpy)4L1–PF6 and Ru(bpy)2(him)2–NO3 sensi-
tized Pt/P25 exhibit more steady and much higher increases in
H2 evolution than N719 sensitized one upon prolonging the irradi-
ation time (Fig. 3), while H2 evolution for N719 almost remain
unchangeable after 3 h irradiation. Namely, N719 possesses several
advantages such as stability and less influence of pH, but it shows
the worst photosensitization in comparison with the loosely linked
dyes.
Ru2(bpy)4L1–PF6 sensitized Pt/P25 have the pronounced advan-
tages of high H2 evolution efficiency and preferable durability. It
may be attributed to the stable chemical structures, the lower risk
of backward electron transfer and/or even the photodegradation

T. Peng et al. / Chemical Physics Letters 460 (2008) 216–219
219
due to the oxidized dye molecules can be timely disengaged from
TiO2 as described above. The best photosensitization of Ru2-
(bpy)4L1–PF6 is a demonstration of the operation of ‘antenna effect’
in TiO2 sensitization process. Apparently, the photosensitizing
behavior on Pt/P25 was related not only to the linkage mode be-
tween the dye and TiO2, but also to the coordination circumstances
and physicochemical properties of Ru(II)-bipyridyl dyes. The dy-
namic equilibrium between the linkage of ground state dye with
TiO2 and the divorce of oxidized dye from the surfaces seems to
be beneficial for electron injecting and hindering the electron
transfer backward, which can improve the electron transfer effi-
ciency and then the H2 evolution efficiency [4–7].
transfer from highly absorbing chromophoric groups by tuning
the molecular components.
Acknowledgments
This work was supported by National ‘863’ Foundation of China
(2006AA03Z344), Natural Science Foundation of China (20573078),
and Program for New Century Excellent Talents in University of
China (NCET-07-0637).
References
[1] A. Fujishima, K. Honda, Nature (London) 238 (1972) 37.
[2] Z.G. Zou, J.H. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625.
[3] R. Amadelli, R. Argazzi, C.A. Bignozzi, F. Scandola, J. Am. Chem. Soc. 112 (1999)
7099.
4. Conclusions
[4] H. Misawa, H. Sakuragi, Y. Usui, K. Tokumaru, Chem. Lett. (1983) 1021.
[5] T. Kajiwara, K. Hashimoto, T. Kawai, T. Sakata, J. Phys. Chem. 86 (1982) 4516.
[6] T. Shimizu, T. Iyoda, Y. Koide, J. Am. Chem. Soc. 107 (1985) 35.
[7] R. Abe, K. Hara, K. Sayama, K. Domen, H. Arakawa, J. Photochem. Photobiol. 37a
(2000) 63.
[8] R. Abe, K. Sayama, H. Sugihara, J. Solar Energy Eng. 127 (2005) 413.
[9] Y. Kim, S. Salim, M.J. Huq, T.E. Mallouk, J. Am. Chem. Soc. 113 (1991) 9561.
[10] Y. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, J. Phys. Chem. 97 (1993)
11802.
[11] R. Abe, K. Sayama, K. Domen, H. Arakawa, Chem. Phys. Lett. 379 (2003) 230.
[12] R. Abe, K. Sayama, H. Arakawa, Chem. Phys. Lett. 362 (2002) 441.
[13] K. Sayama, K. Mukasa, R. Abe, Y. Abe, H. Arakawa, Chem. Commun. (2001)
2416.
An efficient H2 production over dye-sensitized Pt/P25 under vis-
ible-light irradiation was demonstrated in the presence of sacrifi-
cial reagent. Three Ru(II)-bipyridyl complex show different
photosensitization due to their differences in coordination circum-
stances, physicochemical properties and interaction with TiO2. The
more steady increase in H2 evolution for Ru2(bpy)4L1–PF6 indicates
that the use of ‘antenna’ sensitizer may constitute a viable strategy
to overcome problems of light-harvesting in the photosensitization
process. The loosely attached Ru2(bpy)4L1–PF6 have the pro-
nounced advantages of high H2 evolution and preferable durability
in comparison with N719. The dynamic equilibrium between the
linkage of ground dye and divorce of oxidized dye seems to play
a crucial role in the photochemical behavior in dye-sensitized Pt/
P25, which can enhance the electron-injection and hinder the
backward transfer, and then improve the H2 evolution efficiency.
These results suggested interesting possibilities for the preparation
of more efficient dyes that can couple the functions of a sensitizer,
which is bound to the surface of TiO2 and inject the photoelectron,
and an antenna, which can realize the intra-molecular energy
[14] A. Hagfeldtt, M. Gratzel, Chem. Rev. 95 (1995) 49.
[15] S. Faham, M.W. Day, W.B. Connick, et al., Acta Crystallogr. 55D (1999)
379.
[16] P. Cai, M.X. Li, C.Y. Duan, F. Lu, D. Guo, Q.J. Meng, New J. Chem. 29 (2005) 1011.
[17] B. O’Regan, M. Graetzel, Nature (London) 353 (1991) 737.
[18] K. Hara, Y. Tachibana, Y. Ohga, et al., Sol. Energy Mater. Sol. Cells 77 (2002) 89.
[19] L. Millard, M. Bowker, J. Photochem. Photobiol. 148A (2002) 91.
[20] A. Galinska, J. Walendziewski, Energ. Fuel 19 (2005) 1143.
[21] T. Tachikawa et al., J. Phys. Chem. 108B (2004) 19299.
[22] K. Battacharyya, S. Varma, D. Kumar, A.K. Tripathi, M.M. Gupta, J. Nanosci.
Nanotechnol. 5 (2005) 797.
[23] K. Kalyanasundaram, J. Kiwi, M. Gratzel, Helv. Chim. Acta 61 (1978) 2720.