Exceptional poly(acrylic acid)-based artificial [FeFe]-hydrogenases for photocatalytic H2 production in water

Article abstract of DOI:10.1002/anie.201303110

Light, polymer, action: A set of water-soluble poly(acrylic acid) catalysts PAA-g-Fe₂S₂ containing {Fe₂S₂}, an [FeFe]-hydrogenase active-site mimic, is synthesized. This system, combined with CdSe quantum dots a

Full text of DOI:10.1002/anie.201303110

Angewandte
Chemie
Communications
H2 Production
Light, polymer, action: A set of water-
soluble poly(acrylic acid) catalysts PAA-g-
Fe2S2 containing {Fe2S2}, an [FeFe]-
hydrogenase active-site mimic, is syn-
thesized. This system, combined with
CdSe quantum dots and ascorbic acid,
has an exceptional turnover number and
initial turnover frequency (27135 and
3.6 sÀ1) for the photocatalytic production
of H2 in water, which is the highest
efficiency to date for [FeFe]-hydrogenase
mimics.
F. Wang, W.-J. Liang, J.-X. Jian, C.-B. Li,
B. Chen, C.-H. Tung,
L.-Z. Wu*
&&&& — &&&&
Exceptional Poly(acrylic acid)-Based
Artificial [FeFe]-Hydrogenases for
Photocatalytic H2 Production in Water
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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DOI: 10.1002/anie.201303110
H2 Production
Exceptional Poly(acrylic acid)-Based Artificial [FeFe]-Hydrogenases
for Photocatalytic H2 Production in Water**
Feng Wang, Wen-Jing Liang, Jing-Xin Jian, Cheng-Bo Li, Bin Chen, Chen-Ho Tung, and
Li-Zhu Wu*
Generation of hydrogen (H2) from water by solar-energy
conversion is considered a promising way to deal with the
energy crisis and climate change.[1,2] One of the key challenges
at this stage is to create catalysts for H2 production with high
efficiency and low cost. [FeFe]-hydrogenase, an enzyme in
algae, is the fastest proton reduction catalyst in nature known
to date.[3] The H2 production rate of the active site of [FeFe]-
hydrogenase, a Fe2S2 subunit coordinated by CO and CN
ligands, achieves turnovers as high as 6000–9000 molecular H2
per active site per second.[4] Such an efficient catalyst with its
noble-metal-free structure has aroused much interest in the
last decade.[5,6] With reference to the crystal structure of
natural [FeFe]-hydrogenase,[7,8] a large number of [FeFe]-
hydrogenase mimics have been synthesized on the basis of the
Fe2S2 cluster.[9–19] From a photochemical point of view,
molecular dyads and triads,[20–25] multi-component sys-
tems,[26–33] and assembled hybrid systems,[34–37] have been
developed. Although two systems with [FeFe]-hydrogenase
mimics as catalysts performed H2 photo-production with
a turnover number (TON) of over 200,[27,28] most of analogous
systems finished their photochemical H2 production with low
turnover numbers (TON < 5) in organic solutions or a mixture
of organic solvents and water.[12,38,39] In 2011, we designed
a robust water-soluble [FeFe]-hydrogenase mimic by linking
three hydrophilic ether chains to the Fe2S2 active site, and
achieved for the first time photocatalytic H2 production in
water.[29] With this water-soluble [FeFe]-hydrogenase mimic
as a catalyst, CdTe quantum dots (QDs) as a photosensitizer
(PS), and ascorbic acid (H2A) as a proton source and
sacrificial electron donor, the system exhibited a high effi-
ciency for photocatalytic H2 production (TON = 505). Since
then, incorporation of the Fe2S2 active site to a water-soluble
group,[29,30] protein,[31] and peptide,[32] even the hydrophobic
cavity of cyclodextrins,[33] has been developed to realize
photocatalytic H2 production in water because water is
believed to be an ideal solvent for proton mobility and has
non-toxic as well as economic advantages.[40,41] Indeed, the
TON of these water-soluble systems is enhanced in a range of
75 to 84.[30–32] Very recently, a systematic comparison sug-
gested that the efficiency and stability of photocatalytic
[FeFe]-hydrogenase systems in water is much better than in
organic solvents.[30] Nevertheless, the efficiency for H2 evolu-
tion is far less than the natural [FeFe]-hydrogenase (turnover
frequency (TOF) 6000–9000 molecule H2 per active site per
second).
Poly(acrylic acid) (PAA) is a common hydrophilic poly-
mer, widely used in drug-delivery, self-assembly, nanoparticle
modification, and bioimaging.[42–45] The carboxy group in the
PAA chain not only enhances the water-solubility of the
polymer, but also provides modification sites for functional-
ization. Herein we report that the Fe2S2 active site can be
anchored on the side chain of water-soluble PAA. With this
grafted polymer as a catalyst, CdSe QDs as a photosensitizer,
ascorbic acid as proton source and sacrificial electron donor,
we have successfully constructed the first set of polymer-
based [FeFe]-hydrogenase mimics for photocatalytic H2
production. The system shows exceptional TON (based on
the Fe2S2 subunit) and initial TOF of up to 27135 and 3.6 sÀ1,
respectively, for photocatalytic H2 production in water.
The water-soluble polymer catalyst PAA-g-Fe2S2 was
synthesized by stirring the amine-modified Fe2S2 precursor
Fe2S2-NH2, which is a deprotected product of Fe2S2-NHBoc,
in a mixed solution of CF3COOH and CH2Cl2 (v/v = 1/2), and
PAA (Mw = 1800) in the presence of PyBOP ((benzotriazol-1-
yloxy)tripyrrolidinophosphonium hexafluorophosphate) in
a
Et3N/DMF mixed solution at room temperature
(Scheme 1).[42,43] The crude polymer product was precipitated
in diethyl ether and dissolved in methanol three times and
finally obtained as red to yellow solids depending on the
grafting amount of Fe2S2 active site. The as-prepared product
was characterized by 1H NMR, IR, UV/Vis spectroscopy and
GPC (Supporting Information, Figure S1). The 1H NMR
spectra of PAA-g-Fe2S2 polymer and PAA in D2O are
shown in Figure 1. The signals at d = 7.56–7.36 ppm (Ha)
and d = 4.33 ppm (Hb) of the PAA-g-Fe2S2 polymer are
attributed to the aromatic protons and methylene protons
next to amide of the Fe2S2 moiety, respectively. The IR
spectrum of PAA-g-Fe2S2 exhibits a signal for cyanide (CN) at
2125 cmÀ1 and three characteristic CO signals at 2040, 1998,
and 1975 cmÀ1 of the Fe2S2 active site, which are the same as
those in the precursor Fe2S2-NHBoc (Figure 2b).[22] The UV/
Vis absorption spectra of PAA-g-Fe2S2 shows a strong char-
acteristic absorption of Fe2S2 moiety at 309 nm in water
(Figure 2a), which is a blue-shift of 5 nm compared to the
[*] F. Wang, W.-J. Liang, J.-X. Jian, C.-B. Li, Dr. B. Chen,
Prof. Dr. C.-H. Tung, Prof. Dr. L.-Z. Wu
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry & University
of Chinese Academy of Sciences, the Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: lzwu@mail.ipc.ac.cn
[**] We are grateful for financial support from the Ministry of Science
and Technology of China (2009CB220008, 2013CB834505 and
2013CB834804), the National Science Foundation of China
(21090343, 50973125 and 91027041), and the Knowledge Innova-
tion Program of the Chinese Academy of Sciences.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303110.
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pH 4.0, evolved H2 immediately with TON of 392
for 3 h of irradiation (blue LED, l = 450 nm). The
system showed high activity in the first 3 h; further
irradiation led to no obvious improvement in the
amount of H2 production. Control experiments
demonstrated that the presence of all of the
components in the system, catalyst, photosensitizer,
H2A, and light is necessary for photocatalytic H2
evolution.
The degradation of catalyst and consumption of
H2A influenced the rate of H2 production greatly.
Increasing or decreasing the pH value of the solu-
tion to either 5 or 2 resulted in the rate of H2
evolution dropping dramatically (Figure 3a), con-
sistent with the reported systems using ascorbic acid
as
a proton source and sacrificial electron
donor.[29–31] The H2 production rate also depends
on the concentration of photosensitizer. At fixed
concentration of catalyst PAA-g-Fe2S2-2 and H2A,
Scheme 1. The synthetic route to PAA-g-Fe2S2 for the photocatalytic H2 production.
Figure 1. The 1H NMR spectra of a) PAA and b) PAA-g-Fe2S2 in D2O.
absorption at 314 nm of precursor Fe2S2-NHBoc in CH3CN.
All of these results confirmed the Fe2S2 moiety was chemi-
cally grafted on the PAA chain.
Using different initial amount of precursor Fe2S2-NHBoc,
three PAA-g-Fe2S2 polymer catalysts (PAA-g-Fe2S2-1, PAA-
g-Fe2S2-2 and PAA-g-Fe2S2-3) with different amounts of
grafted Fe2S2 active site were prepared. All three polymer
catalysts are quite soluble in water. The grafting amount of
Fe2S2 active site for PAA-g-Fe2S2-1, PAA-g-Fe2S2-2,and PAA-
g-Fe2S2-3 is 3.86 ꢀ 10À4, 7.07 ꢀ 10À5, and 4.07 ꢀ 10À6 molgÀ1,
respectively, which was determined by inductively coupled
plasma-atomic emission spectrometry (ICP-AES) for the
relative amount of iron in samples.
The photocatalytic H2 production system was constructed
by using the PAA-g-Fe2S2 polymer as a catalyst, 3-mercapto-
propionic acid (MPA) stabilized CdSe QDs (MPA-CdSe
QDs) as the photosensitizer, and ascorbic acid (H2A) as the
proton source and sacrificial electron donor. The CdSe QDs
was selected as the photosensitizer not only for its broad
visible-light absorption, aqueous dispersion, and economical
advantage over precious metal photosensitizers, but also for
its simpler preparation than its counterpart CdTe QDs.
We investigated the H2 production activity of the system
using moderate Fe2S2 grafting amount polymer PAA-g-Fe2S2-
2. The system, containing PAA-g-Fe2S2-2 (0.25 mgmLÀ1),
MPA-CdSe QDs (0.08 mgmLÀ1), and H2A (0.01m) at initial
Figure 2. UV/Vis absorption (a) and IR (b) spectra of PAA (line a),
PAA-g-Fe2S2-1 (line b), and Fe2S2-NHBoc (Line c); the concentration of
samples for UV/Vis absorption is 0.067 mgmLÀ1 for PAA and for PAA-
g-Fe2S2-1 in aqueous solution, and 1.0ꢀ10À5 m for Fe2S2-NHBoc in
CH3CN.
the optimal concentration of photosensitizer was found to be
0.08 mgmLÀ1, the reaction was accelerated by increasing the
concentration of MPA-CdSe QDs from 0.008 mgmLÀ1 to
0.08 mgmLÀ1 in solution (Figure 3b). Further increasing the
concentration of CdSe QDs from 0.08 mgmLÀ1 to
0.8 mgmLÀ1, however, resulted in lower rate of H2 production
and aggregation of CdSe QDs. This change is probably due to
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PAA-g-Fe2S2-1 as catalyst under
optimized conditions (Supporting
Information). The TON (based on
Fe2S2 subunit) 27135 and initial TOF
3.6 sÀ1, to our knowledge, are the
highest values reported to date for
photocatalytic H2 production from
[FeFe]-hydrogenase mimics.
The photophysical properties of
the polymer-based systems were
studied to understand the mecha-
nism of the reaction. The average
diameter of MPA-CdSe QDs used
in our systems is 1.8 nm as deter-
mined by TEM. As shown in Fig-
ure 4a, the emission spectrum of
MPA-CdSe QDs shows a strong
excitonic emission at 470 nm and
a relative weak and broad surface
trap emission ranged from 520–
770 nm.[46] With the addition of
PAA-g-Fe2S2-1 or PAA-g-Fe2S2-2
(1.0 mg) into the aqueous solution
of MPA-CdSe QDs (0.08 mgmLÀ1,
3 mL), the excitonic and trap emis-
sions of CdSe QDs were both
quenched. The quenching efficiency
estimated at excitonic emission for
PAA-g-Fe2S2-1 and PAA-g-Fe2S2-2
is 81% and 34%, respectively.
Because of the small spectroscopic
overlap of absorption of PAA-g-
Fe2S2 and the emission of MPA-
CdSe QDs (Supporting Informa-
tion, Figure S5), the energy transfer
Figure 3. a) Photocatalytic H2 production in H2O at different pH values; sample concentration: PAA-
g-Fe2S2-2 (0.25 mgmLÀ1), MPA-CdSe QDs (0.08 mgmLÀ1), H2A (0.01m); b) Photocatalytic H2
production at initial pH 4.0 in H2O as a function of concentration of the photosensitizer (PS) MPA-
CdSe QDs; sample concentration: MPA-CdSe QDs (0.008 mgmLÀ1 for sample A, 0.08 mgmLÀ1 for
sample B, 0.8 mgmLÀ1 for sample C), PAA-g-Fe2S2-2 (0.25 mgmLÀ1), H2A (0.01m); c) Photocatalytic
H2 production at initial pH 4.0 in H2O as a function of catalysts with different loadings of Fe2S2
active site; PAA-g-Fe2S2-1 to PAA-g-Fe2S2-3 for sample A–C (0.25 mgmLÀ1), MPA-CdSe QDs
(0.08 mgmLÀ1), H2A (0.01m); d) Photocatalytic H2 production at initial pH 4.0 in H2O as a function
of ascorbic acid concentration; sample concentration: MPA-CdSe QDs (0.08 mgmLÀ1), PAA-g-Fe2S2-3
(0.25 mgmLÀ1), H2A (0.01m for line A, 0.05m for line B, 0.1m for line C). All samples were irradiated
by blue LED lamp (l=450 nm).
the light-filter effect at high concentration of photosensitizer.
Next, the H2 production activity of three PAA-g-Fe2S2
polymers, PAA-g-Fe2S2-1, PAA-g-Fe2S2-2, and PAA-g-Fe2S2-
3, was examined under the identical concentration of photo-
sensitizer and H2A at pH 4.0. As shown in Figure 3c, PAA-g-
Fe2S2-3, the one with the smallest Fe2S2 grafted amount, gives
rise to the highest H2 production efficiency (TON = 3536) of
the three PAA-g-Fe2S2 polymers for 3 h of irradiation. Finally,
the photocatalytic H2 production with the samples containing
between excited MPA-CdSe QDs to PAA-g-Fe2S2 would be
negligible if it occurs, so the emission quenching observed in
the cases of PAA-g-Fe2S2-1 and PAA-g-Fe2S2-2 is attributed to
the photoinduced electron transfer (PET) from the MPA-
CdSe QDs to Fe2S2 core of PAA-g-Fe2S2. The rate constant of
6.15 ꢀ 1012 mÀ1 sÀ1 for PETwas estimated at excitonic emission
by the Stern–Volmer equation (Supporting Information,
Figure S7). It was worth noting that when the same amount
of PAA-g-Fe2S2-3 or PAA was introduced in the system, the
emission intensity of MPA-CdSe QDs increased by 35% or
46%, respectively. The enhancement was also observed by
Yang and Gao et al. with the addition of PAA into the
aqueous solution of CdTe QDs. They explained and exper-
imentally demonstrated that the coordination between car-
boxy groups of PAA and cadmium ions on the surface of QDs
is responsible for this emission enhancement.[47,48] We believe
this coordination works the same way in our case. The PAA
chain wraps round the nanoparticles by coordination between
the carboxy groups and cadmium ions of CdSe QDs and thus
suppresses to some extent the non-radiative decay of MPA-
CdSe QDs. Actually, the PAA-g-Fe2S2 plays two roles for the
emission of MPA-CdSe QDs. On the one hand, the Fe2S2
active site in PAA-g-Fe2S2 quenches the emission of MPA-
CdSe QDs; on the other hand, the PAA chain suppresses the
PAA-g-Fe2S2-3
(0.25 mgmLÀ1),
MPA-CdSe
QDs
(0.08 mgmLÀ1) at different concentrations of H2A were
studied at initial pH 4.0. The rate of photocatalytic H2
production relies on the concentration of H2A (Figure 3d),
and the initial rate of H2 production obeys a first-order
dependence on the concentration of H2A (Supporting Infor-
mation, Figure S2). Surprisingly, the TON reached 27135 for
8 h of irradiation at 0.1m of H2A and the rate was as fast as
3.6 molecular H2 per Fe2S2 active site per second in the initial
30 min (Supporting Information). All of these results confirm
that the rate of photocatalytic H2 production depends on
pH value of solution, [photosensitizer] ([PS]), [HAÀ], and the
optical power of light source (Supporting Information, Fig-
ure S3). The quantum yield (QY) of the system for H2
production is 5.07%, which was obtained by the sample of
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energy level (Evb) of CdSe QDs is 0.57 eV,[53] and the excited-
state energy (E00) of MPA-CdSe QDs is 2.64 eV. Therefore,
the free-energy change (DG0) was determined to be À1.64 eV,
indicating that the electron transfer from excited MPA-CdSe
QDs to the Fe2S2 of PAA-g-Fe2S2 is exothermic.
From the results described above, we could speculate
about the general mechanism of the system. The photoexcited
electron in the conduction band of CdSe QDs transfers to the
Fe2S2 core of PAA-g-Fe2S2 and generates a one-electron
reduced FeIFe0 species. This active species further reacts with
a proton in catalytic cycle to produce H2.[39,50–52] As the
oxidative potential of H2A is negative enough to reduce the
CdSe QDs but too positive to reduce the Fe2S2 core of PAA-g-
Fe2S2 directly,[54] the oxidative CdSe QDs regenerates by
accepting an electron from the sacrificial electron donor
HAÀ. As reported in the literature and observed in our
experiments,[55] the photo-corrosion of CdSe QDs would lead
to aggregation of CdSe QDs in aqueous systems. But in this
case, the absorption and emission spectra of CdSe QDs
(0.08 mgmLÀ1) were almost unchanged for 4 h of irradiation,
and no obvious aggregation was observed in the presence of
PAA or PAA-g-Fe2S2 in aqueous solution (Figures S15
and S16). Evidently, the PAA moiety in PAA-g-Fe2S2 not
only functions as a water-soluble framework to bring the Fe2S2
core into water, but also plays a role in protecting MPA-CdSe
QDs from photo-corrosion and aggregation. Therefore, it is
reasonable to consider that the polymer chain of PAA wraps
around the MPA-CdSe QDs by coordination and thus
narrows distance between the photosensitizer and the Fe2S2
active site in aqueous solution. This unique structure prefers
to facilitate electron transfer from the excited MPA-CdSe
QDs to PAA-g-Fe2S2 and consequently improves the rate of
photocatalytic H2 production.
In summary, a new set of water-soluble polymer catalysts
PAA-g-Fe2S2 has been designed and successfully synthesized.
A system, containing PAA-g-Fe2S2 as the catalyst, CdSe QDs
as the photosensitizer, and ascorbic acid as proton source and
sacrificial electron donor, shows high efficiency for the
photocatalytic H2 production in water. The TON of 27135,
initial TOF of 3.6 sÀ1, are the highest known to date for
[FeFe]-hydrogenase mimics, competitive with those from
current state-of-the-art catalytic systems for H2 production.
The PAA chain of PAA-g-Fe2S2 plays three roles in the
system: 1) it is a framework to bring the Fe2S2 active site into
aqueous solution; 2) it is a good stabilizer protecting the
MPA-CdSe QDs from aggregation and enhancing the emis-
sion quantum yield of the CdSe QDs; 3) it narrows the
distance between the photosensitizer and the Fe2S2 core for
more efficient electron transfer (kET = 6.15 ꢀ 1012 mÀ1 sÀ1) from
the excited MPA-CdSe QDs to the Fe2S2 active site. The three
functions jointly contribute to a highly efficient photocatalytic
H2 production. Static-state and time-resolved spectroscopic
studies demonstrate that the electron transfer from the
excited MPA-CdSe QDs to the Fe2S2 catalyst center is the
key step to trigger the catalytic cycle. The unique perfor-
mance of the polymer-based [FeFe]-hydrogenase system
indicates that this approach is a promising strategy to improve
the photocatalytic efficiency of H2 production in water.
Extension of the present systems is ongoing in our laboratory.
Figure 4. a) The emission spectra of MPA-CdSe QDs (0.08 mgmLÀ1
)
in the absence and presence of PAA-g-Fe2S2-1 (0.33 mgmLÀ1) or PAA
(0.33 mgmLÀ1) in water; excitation wavelength: 400 nm. b) Transient
absorption spectrum of PAA-g-Fe2S2-1 (1.0 mgmLÀ1) and MPA-CdSe
QDs (0.08 mgmLÀ1) in H2O at pH 7.4 upon laser-pulsed by 355 nm
light.
mobility of the nanoparticles and enhances the emission of
MPA-CdSe QDs. In the cases of PAA-g-Fe2S2-1 and PAA-g-
Fe2S2-2, the quenching process dominates the overall emission
of MPA-CdSe QDs. In contrast, with PAA-g-Fe2S2-3 the
emission of MPA-CdSe QDs is enhanced because of the low
amount of Fe2S2 active site in PAA-g-Fe2S2-3.
The electron transfer from excited CdSe QDs to the Fe2S2
active site of PAA-g-Fe2S2 was evidenced by a flash photolysis
study at room temperature. Compared with signal silence of
a sample of MPA-CdSe QDs with PAA, the solution of CdSe
QDs and PAA-g-Fe2S2-1 at pH 7.4 gave a strong absorption at
430 nm, a broad absorption at the range of 550–650 nm, and
a relative strong absorption at 680 nm after excitation by laser
light at 355 nm. These absorptions are similar to those
reported for FeIFe0 species generated by reduction of
[FeFe]-hydrogenase mimics.[20,49] The decay monitored at
430 nm was mono-exponential with a lifetime of 449 ms
(Figure S11), which was quenched completely by adjusting
the pH to 5.2 by adding HCl to the solution. This quenching is
probably due to the formation of protonated FeIFeIIH species
in acidic solution.[39,50–52] The free energy change of electron
transfer from excited MPA-CdSe QDs to the Fe2S2 core was
estimated by the Rehm–Weller equation. The reductive
potential (E1/2) of À0.43 V (all potentials are versus the
normal hydrogen electrode (NHE)) of the Fe2S2 active site
was taken from cyclic voltammograms obtained using PAA-g-
Fe2S2-1 in aqueous solution (Figure S12). The valence band
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Received: April 13, 2013
Published online: && &&, &&&&
Keywords: [FeFe]-hydrogenases · hydrogen evolution ·
.
photocatalysis · polymers · quantum dots
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