Catalysis of nickel ferrite for photocatalytic water oxidation using [Ru(bpy)3]2+ and S2O82-

Article abstract of DOI:10.1021/ja309771h

Single or mixed oxides of iron and nickel have been examined as catalysts in photocatalytic water oxidation using [Ru(bpy)₃]²⁺ as a photosensitizer and S₂O₈^2- as a sacrificial oxidant. The catalytic activity of nickel ferrite (NiFe₂O ₄) is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O₂ yield and O₂ evolution rate under ambient reaction conditions. NiFe₂O₄ also possesses robustness and ferromagnetic properties, which are beneficial for easy recovery from the solution after reaction. Water oxidation catalysis achieved by a composite of earth-abundant elements will contribute to a new approach to the design of catalysts for artificial photosynthesis.

Full text of DOI:10.1021/ja309771h

Communication
pubs.acs.org/JACS
Catalysis of Nickel Ferrite for Photocatalytic Water Oxidation Using
[Ru(bpy)3]2+ and S2O8
2−
Dachao Hong,† Yusuke Yamada,† Takaharu Nagatomi,† Yoshizo Takai,† and Shunichi Fukuzumi*,†,‡
†Department of Material and Life Science, Graduate School of Engineering, Osaka University, and ALCA, Japan Science Technology
Agency (JST), Suita, Osaka 565-0871, Japan
‡Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
cobalt oxides for photocatalytic water oxidation using [Ru-
(bpy)3]2+ (bpy = 2,2′-bipyridine) as a photosensitizer and S2O8
2−
ABSTRACT: Single or mixed oxides of iron and nickel
as a sacrificial oxidant.18 However, foreign-metal-ion doping of
iron oxides, which are much more earth-abundant than cobalt
have been examined as catalysts in photocatalytic water
oxidation using [Ru(bpy)3]2+ as a photosensitizer and
2−
S2O8 as a sacrificial oxidant. The catalytic activity of
nickel ferrite (NiFe2O4) is comparable to that of a catalyst
containing Ir, Ru, or Co in terms of O2 yield and O2
evolution rate under ambient reaction conditions. NiFe2O4
also possesses robustness and ferromagnetic properties,
which are beneficial for easy recovery from the solution
after reaction. Water oxidation catalysis achieved by a
composite of earth-abundant elements will contribute to a
new approach to the design of catalysts for artificial
photosynthesis.
oxides, has yet to be reported for photocatalytic water oxidation.
We report herein a highly active and robust photocatalytic
water oxidation catalyst composed of iron oxide doped with
foreign elements. Before the iron oxide was doped with foreign
metal ions, catalysis by Fe3O4, which contains Fe2+ and Fe3+ ions,
was compared with that of Fe2O3, which is known as the most
stable form under ambient conditions.15 Fe3O4 was chosen
because it is isostructural to Co3O4, which shows high activity in
photocatalytic water oxidation.16 Next, the Fe2+ ion of Fe3O4 was
replaced by another divalent metal ion (Ni2+, Mg2+, or Mn2+) to
improve the activity and robustness of the catalyst under water
oxidation conditions. In comparisons of MFe2O4 (M = Mg, Mn,
Fe, Ni) catalysts in terms of O2 evolution rate and O2 yield, the
rtificial photosynthesis that directly converts solar energy
Ainto chemical energy is one of the most promising systems highest activity for the photocatalytic water oxidation was
for realizing a sustainable energy cycle.1 Artificial photosynthesis
exhibited by nickel ferrite (NiFe2O4), which has often been used
for organic oxidation reactions.21 The O2 yields obtained usinig
systems are composed of at least three functional units; a catalyst
for water oxidation (2H2O → O2 + 4H+ + 4e−) to extract
NiFe2O4 are comparable to those using Co3O4. Additionally, the
ferromagnetic properties of NiFe2O4 are quite beneficial,
allowing the catalyst to be recovered from the solution after
the reaction using a magnet without any loss. The superior
catalysis of NiFe2O4 shown in the photocatalytic water oxidation
was also found in electrochemical water oxidation. The condition
of the surface of NiFe2O4 after the photocatalytic water oxidation
was investigated by X-ray photoelectron spectroscopy (XPS).
The highly active and robust WOC composed of only the earth-
abundant elements of Fe and Ni is disclosed in the photocatalytic
system for the first time.
electrons and protons, a catalyst for reduction of protons or other
chemicals to produce fuels, and light-harvesting and charge-
separation molecules for solar energy harvesting and utiliza-
tion.1g For the construction of truly sustainable systems, the use
of noble or minor metals in each unit should be avoided.
Photocatalytic hydrogen evolution was recently achieved with Ni
nanoparticles instead of Pt nanoparticles,2 and a long-lived
charge-separated state can be established with a donor−
acceptor-linked dyad that does not include metal ions.3 On the
other hand, developing efficient water oxidation catalysts
(WOCs) with earth-abundant elements still remains the most
challenging task for artificial photosynthesis.4
Most of the previously reported WOCs contain the precious
metals iridium and ruthenium as active species.5−20 Previous
screening of various metal oxides suggested that cobalt oxides
show relatively high activity among nonprecious metals.15,16
Thus, much effort has been devoted to improving the catalytic
activity of cobalt oxide by various methods. A distinguished
example is the use of cobalt phosphate, which exhibits high
catalytic activity in the electrocatalytic water oxidation.1c
Homogeneous cobalt complexes have also been used as
precursors for WOCs, which include residues derived from
organic ligands during the photocatalytic water oxidation
reaction.17 Additionally, doping with trivalent metal ions such
as La3+ has been reported to improve the catalytic activity of
The photocatalytic water oxidation was performed in a
phosphate buffer (50 mM, pH 8.0, 2.0 mL) containing a metal
oxide catalyst, Na2S2O8 (5.0 mM) as a two-electron oxidant, and
[Ru(bpy)3]SO4 (0.25 mM) as a photosensitizer. The solution
was purged with Ar gas for 10 min in a vial (∼1 cm i.d.) and then
sealed with a rubber septum. The reaction was started by
irradiating the solution with a Xe lamp (500 W) through a
transmitting glass filter (λ > 420 nm) with vigorous magnetic
stirring at room temperature. The overall photocatalytic cycle of
water oxidation with Na2S2O8, [Ru(bpy)3]SO4, and a WOC is
depicted in Scheme 1. Photoinduced electron transfer from
[Ru(bpy)3]2+* (where the * denotes the excited state) to S2O8
2−
Received: October 3, 2012
Published: November 19, 2012
© 2012 American Chemical Society
19572
dx.doi.org/10.1021/ja309771h | J. Am. Chem. Soc. 2012, 134, 19572−19575

Journal of the American Chemical Society
Communication
Scheme 1. Cycle of Photocatalytic Water Oxidation with
Na2S2O8 and [Ru(bpy)3]2+ Using a Water Oxidation Catalyst
Table 1. O2 Evolution Rates (RO ) and O2 Yields of WOCs in
the Photocatalytic Water Oxidation
2
a
b
catalyst
RO (μmol s−1 g−1
)
O2 yield (%)
2
NiFe2O4
NiO
5.3 0.2
3.0 0.1
1.3 0.1
1.9 0.2
4.8 0.1
74
38
21
29
64
4
2
2
3
3
Fe2O3
Fe3O4
Co3O4
a
O2 evolution rates (normalized by the catalyst mass) after 10 min of
photoirradiation (λ > 420 nm) of an aqueous buffer solution (pH 8.0,
2 mL) containing the catalyst (0.50 g L−1), Na2S2O8 (5.0 mM), and
[Ru(bpy)3]2+ (0.25 mM). The reported values are averages of three
repetitive examinations. O2 yields, defined as twice the amount of O2
per mole of Na2S2O8. The reported values are averages of three
repetitive examinations.
•−
affords [Ru(bpy)3]3+, SO42−, and SO4•−. The produced SO4
,
which is known to be a very strong oxidant [E0(SO4•−/SO42−) =
2.6 V vs NHE], can oxidize another [Ru(bpy)3]2+ to produce 2
equiv of [Ru(bpy)3]3+ in the overall photoinduced process.22
Finally, [Ru(bpy)3]3+ can oxidize water in the presence of the
WOC to evolve O2. However, decomposition of the photo-
sensitizer by nucleophilic attack of OH− or water on [Ru-
(bpy)3]3+ under neutral or basic conditions competes with
electron transfer from the WOC to [Ru(bpy)3]3+, leading to low
O2 evolution yields.20 Thus, highly active WOCs must improve
the lifetime of the photosensitizer and the O2 yield.
To confirm the effect of the crystal phase of the iron oxides on
the catalytic activity for the photocatalytic water oxidation, Fe2O3
and Fe3O4 were synthesized by reported methods23a,b and
characterized by powder X-ray diffraction (PXRD) measure-
ments (Figure 1a). Co3O4, which is known as an active WOC,
was also synthesized and characterized by PXRD as a reference.18
All of the PXRD peaks were clearly indexed as the spinel
structure for Fe3O4 (magnetite) and Co3O4 or the corundum
structure for Fe2O3 (hematite).
b
The time courses of O2 evolution with these metal oxides are
shown in Figure 1b. Table 1 tabulates the O2 evolution rates
(RO ) and O2 yields obtained for all of the metal oxides studied.
2
No O2 evolution was confirmed from a reaction solution without
a catalyst. The stoichiometric amount of O2 evolution is 5.0 μmol
in the present reaction systems, because Na2S2O8 is a two-
electron acceptor. A comparison between the reaction systems
with iron oxides indicates that the amount of O2 obtained after 30
min of photoirradiation with Fe3O4 (1.5 μmol) was larger than
that with Fe2O3 (1.0 μmol). Even when the concentration of
Fe3O4 was reduced to half in the reaction solution, the same
amount of O2 evolution (1.5 μmol) was achieved, with a similar
O2 evolution rate [Figure S1 in the Supporting Information
(SI)]. After the first run of the photocatalytic reaction, Fe2O3 was
recovered from the reaction solution by centrifugation for further
experiments, and Fe3O4 was collected by a magnet. A fresh buffer
solution containing Na2S2O8 (5.0 mM) and [Ru(bpy)3]SO4
(0.25 mM) was added to the collected particles for the repetitive
examination under photoirradiation. No significant change in the
total amount of O2 evolution was observed in the second and
third runs from the reaction solutions with these three catalysts.
These results indicate that Fe3O4 is a better WOC than Fe2O3 in
the photocatalytic system. However, the catalysis by Fe3O4 is
inferior to that by Co3O4 in terms of RO and O2 yield (i.e., the O2
2
evolution of 3.2 μmol with Co3O4 is more than double of that
with Fe3O4).
To improve the catalytic activity and robustness of iron-based
oxides for photocatalytic water oxidation, the Fe2+ ions in Fe3O4,
which are easily oxidized to Fe3+ under highly oxidizing
conditions, were substituted with Ni2+, Mg2+, and Mn2+ while
maintaining the spinel structure. A series of spinel MFe2O4 (M =
Ni, Mg and Mn) catalysts were synthesized by reported
methods23c−e and characterized by PXRD (Figure 1c), which
confirmed the spinel structure. Figure 1d shows the time courses
of O2 evolution with NiFe2O4, MgFe2O4, and MnFe2O4 in the
photocatalytic water oxidation. While the amounts of O2 evolved
from reaction solutions with MgFe2O4 (0.95 μmol) and
MnFe2O4 (0.42 μmol) were smaller than that with Fe3O4 (1.5
μmol), the amount of O2 evolved from the reaction solution with
NiFe2O4 (3.7 μmol) was higher than that with Co3O4 (3.2
μmol). NiFe2O4 could be easily collected from the solution after
the reaction because of its ferromagnetic properties (Figure S2).
The high O2 yield with NiFe2O4 was maintained even after the
10th run in 5 h (Figure S3). The NiFe2O4 catalyst was examined
before and after the reaction by PXRD and transmission electron
Figure 1. (a) PXRD patterns of Fe2O3, Fe3O4, and Co3O4. Each peak is
labeled with its hkl index. (b) Time courses of O2 evolution under
photoirradiation (Xe lamp, λ > 420 nm) of a phosphate buffer solution
(pH 8.0, 2.0 mL) containing Na2S2O8 (5.0 mM), [Ru(bpy)3]SO4 (0.25
mM), and either Fe2O3, Fe3O4, or Co3O4 (0.50 g L−1) at room
temperature in three repetitive examinations. (c, d) Same as (a, b) but
using NiFe2O4, MgFe2O4, and MnFe2O4.
19573
dx.doi.org/10.1021/ja309771h | J. Am. Chem. Soc. 2012, 134, 19572−19575

Journal of the American Chemical Society
Communication
microscopy (TEM). No significant change in either the PXRD
pattern or the morphology of the NiFe2O4 catalyst was observed
(Figures S4 and S5). Although the NiFe2O4 was contaminated
with a small amount of Fe2O3, the high catalytic activity of
NiFe2O4 can be ascribed to the pure NiFe2O4 because the
addition of Fe2O3 to NiO or Fe3O4 showed no significant
improvement in the O2 yield (Figure S6). The catalytic activity of
the MFe2O4 series decreased with M in the order of Ni > Fe > Mg
> Mn. The catalytic activity of NiO for the photocatalytic water
oxidation was also examined (Figure S7) and found to be ∼1.9
μmol, which is only half that with NiFe2O4. These results clearly
indicate that NiFe2O4 is a highly active and robust catalyst for the
photocatalytic water oxidation.
overpotential of NiFe2O4 for the electrochemical water oxidation
(η = 0.43 V) is comparable to the reported overpotentials of
catalysts such as cobalt phosphate,1c CoOx,26 and nickel
borate27(Table S1 in the SI). At ∼0.8 V, a small redox couple
assignable to the redox of Ni2+ species appears. A similar redox
couple has been assigned to Ni2+/Ni3+ for a nickel oxide
electrode formed on Ni in alkaline solution.28 Recently, the
oxidized nickel species at anodic potentials has been assigned as
Ni4+ in nickel borate by X-ray absorption near-edge structure
spectra.27 Further investigation is necessary to clarify the valence
of the active nickel species of NiFe2O4, but the growth of this
oxidation peak ensures that a high-valent nickel species is an
active species for the water oxidation. The onset potentials for
water oxidation with both Fe2O3 and Fe3O4 were observed at
∼1.1 V. The anodic currents with Fe2O3 and Fe3O4 at 1.5 V were
as small as 190 and 290 μA, respectively. These results suggest
that incorporation of Ni2+ ions enhances the water oxidation
ability of iron oxides, which can exhibit high activity for
photocatalytic water oxidation.
A critical issue for a series of spinel compounds under highly
oxidizing conditions is oxidation of divalent metal species. The
oxidation may lead to microphase separation and deactivation of
the catalyst. It has been reported that magnetite Fe3O4 is oxidized
to form Fe2O3 under highly oxidizing conditions.29 NiO is rather
stable, but its transformation to nonstoichiometric nickel oxide
under highly oxidizing conditions has been reported.30 Thus,
confirmation of the surface conditions of each component after
water oxidation was necessary. The change in the surface
conditions of NiFe2O4 before and after the photocatalytic
reaction was observed by XPS, which was performed in the Fe
2p3/2, Ni 2p3/2, O 1s, Ru 3d, and C 1s energy regions. No peak
was observed in the Ru 3d region. The binding energy of each
element was corrected using the C 1s peak from residual carbon
(284.8 eV). Figure 3a displays the XPS spectra for the Ni 2p3/2
As the catalytic activities of heterogeneous catalysts are usually
compared after normalization by the specific surface area, the RO
2
values calculated from the initial slopes (10 min) of the time
courses were normalized by the Brunauer−Emmett−Teller
surface areas determined by N2 adsorption measurements at 77 K
(48 m2 g−1 for NiFe2O4, 45 m2 g−1 for Fe3O4, 34 m2 g−1 for
Co3O4, and 150 m2 g−1 for NiO) to obtain apparent turnover
frequencies (TOFs). The apparent TOF of 0.11 μmol s−1 m−2
obtained with NiFe2O4 was higher than those with Fe3O4 (0.042
μmol s−1 m−2) and NiO (0.020 μmol s−1 m−2). These results
indicate that the high activity of NiFe2O4 results from the
composite effect of nickel and iron oxides. The apparent TOF
with NiFe2O4 is slightly smaller than that with Co3O4 (0.14 μmol
s−1 m−2). However, the O2 yield obtained with NiFe2O4 (74%)
was higher than that with Co3O4 (64%) by 10%. The O2 yield
with NiFe2O4 is also higher than those reported for catalysts
containing precious metals such as IrO2 particles (69%, pH
5.0)15 and RuO2 particles (22%, pH 5.0)15 and abundant
elements such as MnxOy particles (55%, pH 5.8)19 and
comparable to that with LaCoO3 particles (74%, pH 7.0).18
Thus, NiFe2O4 composed of earth-abundant elements is one of
most active catalysts for the photocatalytic water oxidation.
The superior catalysis of NiFe2O4 for the photocatalytic water
oxidation was scrutinized under electrocatalytic conditions. The
electrochemical deposition of nickel or cobalt on the surface of
hematite Fe2O3 has previously been reported to improve the
catalysis of electrochemical water oxidation by Fe2O3.24,25 Figure
2 shows cyclic voltammograms (CVs) of water using a working
electrode modified with a metal oxide catalyst in a pH 8.0 buffer
solution. The anodic currents with NiFe2O4 started growing at
∼0.8 V vs SCE and reached more than 650 μA at 1.5 V vs SCE,
which is larger than those with iron-based oxides. The
Figure 3. XPS spectra of NiFe2O4 before and after the reaction in the (a)
Ni 2p3/2, (b) Fe 2p, and (c) O 1s energy regions.
peak at 854.8 eV with a weak satellite peak at 861 eV for NiFe2O4
samples before and after the reaction. The binding energies of
these peaks indicate that the Ni species in the samples are Ni2+ by
comparison with the Ni 2p3/2 peak positions of Ni metal and NiO
(Figure S8a). The similar intensity ratios for the main and
satellite peaks for the two samples indicate that the surface
conditions were the same even after the photocatalytic water
Figure 2. CVs in a buffer solution (pH 8.0) with a carbon paste working
electrode (A = 0.071 cm2) containing no metal oxide (gray), 5%
NiFe2O4 (orange), Fe3O4 (black), or Fe2O3 (red) (standard calomel
electrode; Pt wire counter electrode; scan rate 100 mV s−1). The inset
shows the initial range of the electrocatalytic current.
19574
dx.doi.org/10.1021/ja309771h | J. Am. Chem. Soc. 2012, 134, 19572−19575

Journal of the American Chemical Society
Communication
(2) Yamada, Y.; Miyahigashi, T.; Kotani, H.; Ohkubo, K.; Fukuzumi, S.
Energy Environ. Sci. 2012, 5, 6111.
oxidation was performed under highly oxidizing conditions.
Figure 3b displays the XPS spectrum for the Fe 2p3/2 peak at
710.2 eV with a weak satellite peak at 723.7 eV for NiFe2O4
before the reaction and that at 710.6 eV with a weak satellite peak
at 724.1 eV after the reaction. These peaks are assigned to Fe3+ by
comparison with the Fe 2p3/2 peaks of Fe2O3 and Fe metal
(Figure S8b). Although the main Fe 2p3/2 peak from the sample
after the reaction was slightly shifted in the direction of higher
binding energy, the same separation between the main and
satellite peaks in the two samples and the similarity of the peak
shapes, including the satellite peaks, over the whole energy
region between 700 and 730 eV strongly indicate that there was
no change in the valence state of Fe3+. The absence of changes in
the surface conditions of NiFe2O4 before and after the reaction
was also supported by the absence of a shift in the O 1s peak
(Figure 3c). Thus, NiFe2O4 is highly robust even during the
photocatalytic water oxidation.
(3) (a) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N.
V.; Lemmetyinen, H. J. Am. Chem. Soc. 2004, 126, 1600. (b) Hoshino,
M.; Uekusa, H.; Tomita, A.; Koshihara, S.; Sato, T.; Nozawa, S.; Adachi,
S.; Ohkubo, K.; Kotani, H.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134,
4569. (c) Kotani, H.; Ohkubo, K.; Fukuzumi, S. Faraday Discuss. 2012,
155, 89.
(4) Du, P.; Eisenberg, R. Energy Environ. Sci. 2012, 5, 6012.
(5) Najafpour, M. M.; Ehrenberg, T.; Wiechen, M.; Kurz, P. Angew.
Chem., Int. Ed. 2010, 49, 2233.
(6) Fillol, J. L.; Codola,
Costas, M. Nat. Chem. 2011, 3, 807.
̀ ́
Z.; Garcia-Bosch, I.; Gomez, L.; Pla, J. J.;
(7) Yin, Q. S.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.;
Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. Science 2010, 328,
342.
(8) Murakami, M.; Hong, D.; Suenobu, T.; Yamaguchi, S.; Ogura, T.;
Fukuzumi, S. J. Am. Chem. Soc. 2011, 133, 11605.
(9) Poulsen, A. K.; Rompel, A.; McKenzie, C. J. Angew. Chem., Int. Ed.
In summary, we have demonstrated for the first time superior
catalysis of photocatalytic water oxidation by a material
composed of only earth-abundant elements, NiFe2O4. This
catalyst possesses high catalytic activity as well as durability in
photocatalytic water oxidation with Na2S2O8 and [Ru(bpy)3]2+,
as evidenced by the maintenance of a high O2 yield after 10
repeated uses. Cyclic voltammetry studies of electrocatalytic
water oxidation with NiFe2O4 suggested that a high-valent nickel
species is the active species for the photocatalytic water
oxidation. This has important implications for the exploitation
of efficient WOCs to expand the use of iron-based oxides for
water oxidation.
2005, 44, 6916.
(10) Polyansky, D. E.; Muckerman, J. T.; Rochford, J.; Zong, R.;
Thummel, R. P.; Fujita, E. J. Am. Chem. Soc. 2011, 133, 14649.
(11) Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.;
Llobet, A.; Sun, L. Nat. Chem. 2012, 4, 418.
(12) Fukuzumi, S.; Kato, S.; Suenobu, T. Phys. Chem. Chem. Phys. 2011,
13, 17960.
(13) Kaveevivitchai, N.; Chitta, R.; Zong, R.; El Ojaimi, M.; Thummel,
R. P. J. Am. Chem. Soc. 2012, 134, 10721.
(14) Hong, D.; Murakami, M.; Yamada, Y.; Fukuzumi, S. Energy
Environ. Sci. 2012, 5, 5708.
(15) Harriman, A.; Pickering, I. J.; Thomas, J. M.; Christensen, P. A. J.
Chem. Soc., Faraday Trans. 1 1988, 84, 2795.
(16) Jiao, F.; Frei, H. Energy Environ. Sci. 2010, 3, 1018.
(17) Hong, D.; Jung, J.; Park, J.; Yamada, Y.; Suenobu, T.; Lee, Y.-M.;
Nam, W.; Fukuzumi, S. Energy Environ. Sci. 2012, 5, 7606.
(18) Yamada, Y.; Yano, K.; Hong, D.; Fukuzumi, S. Phys. Chem. Chem.
Phys. 2012, 14, 5753.
(19) Jiao, F.; Frei, H. Chem. Commun. 2010, 46, 2920.
(20) Morris, N. D.; Mallouk, T. E. J. Am. Chem. Soc. 2002, 124, 11114.
(21) (a) Fouad, O. A.; Abdel, H. K. S.; Rashad, M. M. Top. Catal. 2008,
47, 61. (b) Miki, J.; Asanuma, M.; Tachibana, Y.; Shikada, T. J. Catal.
1995, 151, 323. (c) Turek, W.; Strzezik, J.; Krowiak, A. React. Kinet.
Mech. Catal. 2012, 107, 115.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section, time courses of O2 evolution under
different conditions, photograph of NiFe2O4 attracted by a
magnet, comparison of overpotentials, PXRD patterns, TEM
images, and XPS spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp
■
(22) Kaledin, A. L.; Huang, Z.; Geletii, Y. V.; Lian, T.; Hill, C. L.;
Musaev, D. G. J. Phys. Chem. A 2010, 114, 73.
(23) (a) Elizarova, G. L.; Matvienko, L. G.; Parmon, V. N. J. Mol. Catal.
1987, 43, 171. (b) Wang, L.; Li, J.; Jiang, Q.; Zhao, L. Dalton Trans.
2012, 41, 4544. (c) Cheng, Y.; Zheng, Y.; Wang, Y.; Bao, F.; Qin, Y. J.
Solid State Chem. 2005, 178, 2394. (d) Deng, H.; Chen, H.; Li, H. Mater.
Chem. Phys. 2007, 101, 509. (e) Wang, L.; Li, J.; Wang, Y.; Zhao, L.;
Jiang, Q. Chem. Eng. J. 2012, 181−182, 72.
(24) (a) Liao, P.; Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012, 134,
13296. (b) Landon, J.; Demeter, E.; Inoglu, N.; Keturakis, C.; Wachs, I.
E.; Vasic, R.; Frenkel, A. I.; Kitchin, J. R. ACS Catal. 2012, 2, 1793.
(25) Liu, Y.; Yu, Y.-X.; Zhang, W.-D. Electrochim. Acta 2012, 59, 121.
(26) Stracke, J. J.; Finke, R. G. J. Am. Chem. Soc. 2011, 133, 14872.
(27) Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.;
Yachandra, V. K.; Nocera, D. G. J. Am. Chem. Soc. 2012, 134, 6801.
(28) Lyons, M. E. G.; Brandon, M. P. Int. J. Electrochem. Sci. 2007, 3,
1386.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by Grants-in-Aid (20108010
and 24350069) from MEXT (Japan) and by NRF/MEST
(Korea) through the WCU (R31-2008-000-10010-0) and GRL
(2010-00353) Programs (to S.F.). We acknowledge Research
Centre for Ultra-Precision Science & Technology for TEM
measurements and Prof. Norimitsu Tohnai at OU for XRD
measurements.
REFERENCES
■
(29) Tang, J.; Myers, M.; Bosnick, K. A.; Brus, L. E. J. Phys. Chem. B
2003, 107, 7501.
(30) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd
ed.; Butterworth−Heinemann: Oxford, U.K., 1997.
(1) (a) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006,
103, 15729. (b) Eisenberg, R.; Gray, H. B. Inorg. Chem. 2008, 47, 1697.
(c) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072.
(d) Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E. Acc.
Chem. Res. 2009, 42, 1966. (e) Concepcion, J. J.; Jurss, J. W.;
Brennaman, M. K.; Hoertz, P. G.; Patrocinio, A. O. T.; Murakami Iha, N.
Y.; Templeton, J. L.; Meyer, T. J. Acc. Chem. Res. 2009, 42, 1954.
(f) Romain, S.; Vigara, L.; Llobet, A. Acc. Chem. Res. 2009, 42, 1944.
(g) Fukuzumi, S. Eur. J. Inorg. Chem. 2008, 1351.
19575
dx.doi.org/10.1021/ja309771h | J. Am. Chem. Soc. 2012, 134, 19572−19575