Photocurrent generation from semiconducting manganese oxide nanosheets in response to visible light

Article abstract of DOI:10.1021/jp0500485

Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light irradiation (?? < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and UV-visible absorption spectra. The band gap energy was estimated to be 2.23 eV on the basis of the photocurrent action spectrum. The molecular thickness of a??0.5 nm may facilitate the charge separation of excited electrons and holes, which is generally very difficult for strongly localized d-d transitions. The monolayer film of MnO₂ nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16% in response to the monochromatic light irradiation (?? = 400 nm), which is comparable to those for sensitization of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer number of MnO₂ nanosheets, although the optical absorption was enhanced. The recombination of the excited electron-hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer through multilayered nanosheets. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0500485

J. Phys. Chem. B 2005, 109, 9651-9655
9651
Photocurrent Generation from Semiconducting Manganese Oxide Nanosheets in Response
to Visible Light
Nobuyuki Sakai, Yasuo Ebina, Kazunori Takada, and Takayoshi Sasaki*
AdVanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba,
Ibaraki 305-0044, Japan
ReceiVed: January 4, 2005; In Final Form: March 6, 2005
Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light
irradiation (λ < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and
UV-visible absorption spectra. The band gap energy was estimated to be 2.23 eV on the basis of the
photocurrent action spectrum. The molecular thickness of ∼0.5 nm may facilitate the charge separation of
excited electrons and holes, which is generally very difficult for strongly localized d-d transitions. The
monolayer film of MnO2 nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16%
in response to the monochromatic light irradiation (λ ) 400 nm), which is comparable to those for sensitization
of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer
number of MnO2 nanosheets, although the optical absorption was enhanced. The recombination of the excited
electron-hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer
through multilayered nanosheets.
Introduction
1 nm and a lateral size ranging from submicrometers to several
tens of micrometers. These unusual structural features with
extremely high two-dimensional anisotropy are expected to
evolve novel chemical and physical properties that differ from
those for granular bulk oxides. In practice, titania nanosheets
exhibit a larger band gap energy than that for bulk TiO2 due to
quantum size effects.20 MnO2 nanosheets have an absorption
peak centered at around 380 nm, which is tailed to the visible
regime,17,21 while the parent layered manganese oxide has
featureless absorption over an entire wavelength range of
200-800 nm. In the present work, we fabricated the MnO2
nanosheet electrodes and examined the photoelectrochemical
properties of these electrodes.
The transition metal oxides such as TiO2 and ZnO are well-
known semiconductors that can generate photocurrent under UV
light irradiation.1-6 These oxides are characterized by their
electronic structure with empty d-shell (d ) 0) or with fully
occupied orbitals (d ) 10). In these oxides, charge separation
takes place via photon absorption associated with a gap energy
between valence and conduction bands, which are mainly gen-
erated from oxygen 2p and metal 3d or 4s orbitals, respectively.
This electronic transition involving two different atoms may
stabilize the charge separation and allow the migration of carriers
having a relatively longer lifetime (10-100 ns),7-10 which is
the origin of photocatalytic activities and photovoltaic conver-
sion capabilities of these oxides. However, their large band gap
(>3 eV), which requires UV light to excite, leads to a
disadvantage in solar energy utilization. Extensive efforts have
been made to overcome this problem by dye-sensitization11,12
and introduction of impurity level by anion doping.13
Transition metal oxides with partially filled d-levels may be
attractive because they absorb visible light involving d-d
transitions. However, because the d electrons are confined in
central transition metal ions, migration of the excited carriers
into surfaces and/or interfaces is difficult, and the excited
electrons are rapidly recombined with the holes. This may be
the main reason there have been no reports on photocurrent
generation from oxides with d ) 1-9 configurations except
for a few exceptional cases.14 These difficulties can be overcome
with a new modification of nanosized oxides as described below.
Recently, several groups including our laboratories have
reported the synthesis of functional oxide nanosheets, for
example, Ti0.91O2,15,16 MnO2,17,18 and Ca2Nb3O10,19 by delami-
nating precursor crystals of a layered oxide into its elementary
layers. The crystallites have an ultrathin thickness of less than
Experimental Section
All chemicals were of >99.9% purity or of analytical grade.
Polyethylenimine (PEI, MW ) ∼7.5 × 105) and polydiallyldi-
methylammonium (PDDA, MW ) ∼1 × 105 to 2 × 105)
chloride were obtained from Aldrich Co. and used without
further purification. Ultrapure water, filtered by a Milli-Q
reagent water system to a resistivity of >17 MΩ cm, was used
throughout the experiments. Propylene carbonate (Aldrich, H2O
content <0.002%) and LiClO4 (Wako, anhydrous) were used
as purchased without further purification.
The MnO2 nanosheets were synthesized by delaminating a
layered protonic manganese oxide (H0.13MnO2‚0.7H2O) into
colloidal single layers according to well-established procedures
described previously:17 Briefly, a precursor layered manganese
oxide of K0.45MnO2, which was prepared by calcining a
stoichiometric mixture of K2CO3 and Mn2O3 at 750 °C, was
converted into a protonic form by acid-digesting in a 1 mol
dm-3 HCl solution. The resulting protonic manganese oxide of
H
0.13MnO2‚0.7H2O (1 g) was shaken vigorously with 250 cm3
of 5.2 mmol dm-3 tetrabutylammonium hydroxide solution. This
suspension after shaking for 10 days was centrifuged at 10 000
rpm for 30 min to remove an unexfoliated material. This
* Corresponding author. Phone: +81-29-860-4313. Fax: +81-29-854-
9061. E-mail: sasaki.takayoshi@nims.go.jp.
10.1021/jp0500485 CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/09/2005

9652 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Sakai et al.
procedure produced a deep dark-brown colored suspension
containing well-dispersed exfoliated nanosheets of manganese
oxide (∼0.5 nm in thickness and 0.1-1 µm in lateral size). The
MnO2 nanosheets are negatively charged in their solution due
to their mean valency of Mn (3.87).
The MnO2 nanosheets were deposited on an ITO-coated
quartz glass plate (sheet resistance ) 10 Ω/square) with the
flat surface (Ra ) 0.2 nm) via the electrostatic self-assembly
method reported previously.21 ITO-coated quartz glass substrates
were cleaned by sonication in acetone, ethanol, and ultrapure
water for 30 min each and stored in water until use. The cleaned
substrate was pretreated with a PEI solution (2.5 g dm-3, pH
9) for 20 min to turn the surface positively charged. The
MnO2 nanosheets and PDDA were sequentially adsorbed from
Figure 1. UV-vis absorption spectra (ITO substrate was employed
as a baseline) before (solid line) and after (dotted line) the photocurrent
generation (λ > 375 nm, Q ) 7.4 mC cm-2). The data for the film on
a quartz glass chip are also shown as a broken trace.
the colloidal suspension of MnO2 nanosheets (0.08 g dm-3
,
pH 9) and a PDDA solution (20 g dm-3, pH 9), respectively.
The substrate was immersed in each solution for 20 min and
then rinsed meticulously with ultrapure water. Deposition
of the PDDA/nanosheet bilayer was repeated n times to syn-
thesize a multilayer assembly composed of n layer pairs,
(MnO2/PDDA)n-1/MnO2/PEI/ITO.
Photoelectrochemical measurements were carried out in a
conventional three-electrode, single-compartment glass cell,
fitted with a synthesized quartz window, using a potentiostat
(SI1287, Solartron). The MnO2 nanosheet electrodes served as
the working electrode. The counter and the reference electrodes
were platinum black wire and Ag/Ag+/acetonitrile () +0.49
V vs NHE), respectively. Propylene carbonate containing 0.1
mol dm-3 LiClO4 was employed as the supporting electrolyte
without further purification. A 500-W Xe lamp (XEF-501S, San-
ei Electric) was used as the excitation light source through a
monochromator (H-10UV, Jobin Yvon) to obtain monochro-
matic light (fwhm ) 15 nm). The incident light was irradiated
onto MnO2 nanosheet electrodes from the front face through
the quartz window and the electrolyte unless noted otherwise.
The irradiated light intensity was monitored by a spectroradi-
ometer (USR-30, Ushio) and was estimated by taking into
account the optical absorption of quartz window and electrolyte.
All electrode potential values are given with respect to Ag/Ag+
() +0.49 V vs NHE) in the present paper.
Figure 2. In-plane XRD patterns for (a) ITO and monolayer film of
MnO2 nanosheets (b) before and (c) after the photocurrent generation
(λ > 375 nm, Q ) 7.4 mC cm-2). The circles in the figure indicate the
diffraction from the two-dimensional hexagonal architecture of MnO2
nanosheets. The wavelength of the incident X-ray is 0.13988 nm.
AFM image (not shown) revealed a dense monolayer coverage
of the substrate surface with the nanosheets.
The in-plane X-ray diffraction (XRD) technique could detect
diffraction peaks from the monolayer film of MnO2 nanosheets
on the ITO coated quartz glass substrate. Upon self-assembly
of MnO2 nanosheets, two new peaks appeared at 2θ ) 32.9°
and 58.6° (indicated by circles in Figure 2b) in addition to many
reflections from ITO substrate (Figure 2a). These two peaks
with d-values of 2.47 and 1.43 Å are attributable to 10 and 11
reflections from the two-dimensional hexagonal architecture of
MnO2 nanosheet.
A spectrophotometer (U-4000, Hitachi) was employed to
record UV-vis absorption spectra for films on ITO/quartz glass
substrates. In-plane X-ray diffraction (XRD) measurements were
carried out using synchrotron radiation X-rays at Photon Factory,
BL-3A, KEK. All measurements were conducted under ambient
conditions.
Results and Discussion
Photocurrent Generation from Monolayer Films of MnO2
Nanosheet Electrodes. We detected the anodic photocurrent
from MnO2 nanosheets, which were deposited as a monolayer
on ITO coated quartz glass as shown in the inset of Figure 3.
The generation of anodic photocurrent indicates the n-type
semiconducting nature of this material.10 A full understanding
of the redox reactions of excited electrons and holes associated
with photocurrent generation is not available at the present stage.
We speculate that the electrons may reduce water and/or
molecular oxygen dissolved in the electrolyte, while the holes
may oxidize water and/or propylene carbonate used as electro-
lyte. The photocurrent action spectrum (Figure 3), in which the
incident photon-to-electron conversion efficiency (IPCE) is
plotted as a function of excitation wavelength, shows the onset
wavelength for the photocurrent generation of around 500 nm,
indicating that the photoelectric conversion occurred in response
to visible light. The IPCE for the monolayer film of MnO2
Film Fabrication. The formation of the monolayer film was
monitored by UV-vis absorption spectra, which showed an
absorption peak characteristic of MnO2 nanosheets (Figure 1).
As the back face of the substrate was covered with a masking
tape during the layer-by-layer assembly of multilayer films, the
MnO2 nanosheets were only adsorbed on the front face of the
substrate, which is coated with ITO thin film. The data on a
bare quartz glass substrate shown for comparison represent the
intrinsic spectral feature of MnO2 nanosheets.17,21 Some differ-
ence in profile between the film on the ITO coated quartz glass
and that on the bare quartz glass substrate is attributed to the
interference effect from the ITO layer, which is difficult to
correct after the deposition of MnO2 nanosheets. The absorbance
of ∼0.04 at 380 nm is almost comparable to that for the
monolayer loading of the nanosheets, which was found in our
previous study on the multilayer film assembly.21 In fact, an

Photocurrent Generation from MnO2 Nanosheets
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9653
Figure 4. Variation of the square root of IPCE (η) times hν with photon
energy for the monolayer film of MnO2 nanosheet electrode at 0.4 V.
Inset shows the band gap energy for the MnO2 nanosheet electrode
with various numbers of layer pairs.
Figure 3. Photocurrent action spectra for monolayer film of MnO2
nanosheet/ITO electrode at 0.4 V vs Ag/Ag+ in a propylene carbonate
solution containing 0.1 mol dm-3 LiClO4. The solid line shows the
optical absorption spectra of MnO2 nanosheet normalized to the
photocurrent profile. Inset shows periodic anodic photocurrent genera-
tion from the monolayer film of MnO2 nanosheet/ITO electrode under
monochromatic light centered at 450 nm.
nanosheet electrode was approximately 0.16% under a mono-
chromatic light (λ ) 400 nm) at 0.4 V, which is comparable to
that from dyes, for example, ruthenium complexes, adsorbed
in monolayer on the flat single-crystal surface.12 The spectral
profile in a wavelength range of 350-500 nm closely matches
the optical absorption feature of the MnO2 nanosheets, confirm-
ing that the observed photocurrent arises from MnO2 nanosheets.
To the best of our knowledge, this is the first report on
photocurrent generation from manganese oxides, although a
wide variety of compositions, crystal structures, and morphol-
ogies have been reported so far in this system.22
It is worth considering why the photocurrent involving d-d
transitions is generated in the present system. The two-
dimensional structure with an ultrathin thickness of less than 1
nm may play an essential role in the separation of the excited
electron-hole pairs. The MnO2 nanosheet is composed only of
one plane of Mn ions sandwiched by two hexagonal planes of
oxygen atoms. The two oxygen planes are stacked in a relation
to provide octahedral sites for Mn ions, producing a coplanar
oxide nanosheet. The atomic architecture normal to the sheet
is comparable to molecules or metal complexes, while that
parallel to the sheet corresponds to bulk crystals. Such an
ultrathin MnO2 nanosheet (∼0.5 nm) may facilitate the excited
electrons and holes to migrate into ITO and interface with
electrolyte, respectively, before their loss via recombination.
To estimate the band gap energy for the MnO2 nanosheet
electrode, the square root of the IPCE (η) times the photon
energy, (ηhν)0.5, is plotted against photon energy hν (Figure
4).23 This plot gives a straight line in a photon energy range
close to the absorption threshold, suggesting that the MnO2
nanosheets have an indirect electronic transition near the band
gap. The band gap energy for the monolayer film of the MnO2
nanosheet electrode is derived as 2.23 eV from the intercept of
the linear portion with the abscissa.
We also examined the dependence of the photocurrent
generation on the applied potential of the MnO2 nanosheet
electrodes (Figure 5). Under each applied potential, the mono-
layer film of MnO2 nanosheet electrode was irradiated with
monochromatic light centered at 400 nm. The behavior of
photocurrent generation was dependent on the direction of
potential change. When the potential was swept from -0.5 to
1.0 V by a 0.1-V difference, the photocurrent appeared to flow
at -0.3 V and increased with increasing the potential until 0.4
V and then decreased in more positive potentials. When the
potential was changed from 1.0 to -0.5 V, the photocurrent
Figure 5. Dependence of the IPCE values on applied potential for the
monolayer film of MnO2 nanosheet electrode. The incident light source
was monochromatic light centered at 400 nm.
was almost constant from 0.8 to 0.4 V and gradually decreased
as the potential becomes more negative, and became undetect-
able at -0.4 V. In both cases, the onset potential is at around
-0.35 V, and the anodic photocurrent was observed under more
positive potential than the onset potential. This value is
consistent with the redox potential of Mn3+/Mn4+ of the MnO2
nanosheet observed in the cyclic voltammogram, which will
be published elsewhere.
The hysteresis observed in the potential range between 0.0
and 0.9 V may suggest that the Mn3+ state is rather superior
for the photocurrent generation to Mn4+ state. The Mn3+ state,
which is produced by partial reduction of MnO2 nanosheets due
to polarization at -0.5 V, is gradually oxidized to the Mn4+
state during the potential sweep in a positive direction. The
decrease of the Mn3+/Mn4+ ratio brings about the attenuation
of the photocurrent in a potential range of 0.4-0.9 V. In contrast,
the potential change in a negative direction from 1.0 V does
not change the oxidation state of Mn4+ ions, showing that the
photocurrent magnitude enhances with larger polarization from
the onset potential and becomes saturated in a potential range
of 0.4-0.8 V. This behavior is similar to that for well-known
n-type semiconductors such as TiO2.10
On the basis of the results shown above, the energy level
diagram of the MnO2 nanosheets can be described as shown in
Figure 6. Mn 3d energy level is split into higher (eg) and lower
(t2g) energy levels due to the ligand field of MnO6 octahedra.
The optical absorption of the MnO2 nanosheets in the visible
light range principally originates from d-d transitions of Mn
ions. The energy level of the Mn 3d eg state should be equal to
the redox potential of Mn3+/Mn4+ of MnO2 nanosheets because

9654 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Sakai et al.
Figure 7. Dependence of the IPCE value (λ ) 400 nm) on the layer
number of MnO2 nanosheets.
Photoelectrochemical Behaviors on Multilayer Films of
MnO2 Nanosheets. We have also examined the photocurrent
generation from multilayer films of MnO2 nanosheet electrodes.
The success in multilayer formation was confirmed by UV-
vis absorption spectra. The band gap energies for MnO2
nanosheet electrode with various layer numbers estimated from
the photocurrent action spectra are plotted in the inset of Figure
4. They were nearly independent of the layer number of MnO2
nanosheets. Dependence of the photocurrent generation on the
applied potential was also similar to that for the monolayer film
of MnO2 nanosheet electrodes, showing the onset potential of
-0.35 V for photocurrent generation. These results indicate that
the nanosheet is electronically isolated in the multilayer as-
semblies without affecting the electronic state of neighboring
nanosheets.
The quantum efficiency declined with increasing the layer
number. Even though the optical absorption was enhanced, the
IPCE value reduced contrarily (Figure 7). This tendency may
reflect the essential nature of the phenomenon. The recombina-
tion of the excited electron-hole pairs rather than the charge
transfer via some redox reactions at the interface predominantly
proceeds when the carriers need to migrate a longer distance
through the stacked multiple numbers of nanosheets.
This phenomenon raises a presumption that only a topmost
layer of multilayer films of MnO2 nanosheets plays a role in
generating the photocurrent. To confirm this hypothesis, we
examined the dependence of photocurrent generation on the
direction of incident light. When the light was irradiated from
the back face of the electrode, the incident intensity of irradiated
light to the topmost layer in contact to the electrolyte decreases
to ca. 40% at λ ) 400 nm, because the 9 layers of MnO2
nanosheets behind the topmost layer absorb ca. 60% of incident
photon. The IPCE value at λ ) 400 nm was actually damped
down to approximately 40% in comparison with the case of
the front-face illumination (Figure 8). This strongly supports
the photosensitivity of only the topmost layer. The photoexcited
carriers within the other layers are extinct via recombination.
Moreover, the increase of the distance between the topmost layer
and ITO electrode decreased the IPCE value, as shown in Figure
7. These behaviors may be compatible with the process of the
photocurrent generation discussed above for the monolayer film.
This may also provide a reasonable explanation why no
photocurrent has been observed from the bulk materials of
manganese oxide.28
Figure 6. Schematic illustration of energy level diagram of MnO2
nanosheet in comparison with titania nanosheet.
the Mn3+ state in the MnO6 octahedra takes a high spin state,
whose electrons occupy the eg state, due to its low ligand-field
strength. The agreement between the redox potential of
Mn3+/Mn4+ of MnO2 nanosheets and the onset potential of
photocurrent generation strongly suggests that the excited energy
level of MnO2 nanosheets corresponds to the energy level of
the Mn 3d eg state. The band gap energy corresponds to the
energy difference between Mn 3d t2g and eg states. Therefore,
the photocurrent can be generated by an excitation of electrons
from the Mn 3d t2g level to the Mn 3d eg level. Mn 3d t2g states
and eg states serve as the “valence band” and the “conduction
band” as semiconductor, respectively.
It is interesting to compare the energy diagram with that for
the titania nanosheet (Figure 6), which shows a large band gap
due to size quantization effects.20 The energy levels of Mn 3d
t2g and eg are higher and lower than the valence and the
conduction bands of titania nanosheet, respectively. Alternate
layer-by-layer assembly of titania and MnO2 nanosheets may
create two-dimensional multiple-quantum-well structures.10,24
Stability of MnO2 Nanosheet Electrodes toward Photo-
current Generation. A serious problem of some semiconduc-
tors in practical applications is the instability against photo-
chemical processes including photocurrent generation, which
is typically found for CdS and CdSe despite their sensitivity to
visible light.25-27
The MnO2 nanosheets were found to be stable against the
photocurrent generation. Because the hexagonal two-dimen-
sional unit cell of the nanosheet crystallite (a ≈ 0.285 nm)
contains one formula unit, an ideal monolayer film without gaps
and overlaps of the nanosheets has 1.4 × 1015 Mn atom cm-2
.
If the MnO2 nanosheet is assumed to undergo photodecompo-
sition as eq 1 upon light irradiation, 0.92 mC cm-2 of the
photocurrent should be consumed to decompose the monolayer
of MnO2 nanosheet.
MnO2 + 4h+ f Mn4+ + O2v
(1)
Even after the photocurrent of 7.4 mC cm-2, which is 8 times
that required to decompose the entire monolayer film of MnO2
nanosheet, was generated, the in-plane XRD peaks correspond-
ing to MnO2 nanosheets did not appear to change as shown in
Figure 2c. UV-vis absorption spectra (Figure 1) also showed
a negligible difference before and after the photocurrent
generation. These data indicate that the MnO2 nanosheet was
stable without dissolution itself against photocurrent generation.
Conclusions
The MnO2 nanosheet electrode generates anodic photocurrent
under visible light irradiation without dissolution itself. The two-
dimensional ultrathin oxide system may account for the charge

Photocurrent Generation from MnO2 Nanosheets
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9655
(3) Bard, A. J. J. Phys. Chem. 1982, 86, 172.
(4) Gerischer, H. Electrochim. Acta 1990, 35, 1677.
(5) Tryk, D. A.; Fujishima, A.; Honda, K. Electrochim. Acta 2000,
45, 2363.
(6) Memming, R. Semiconductor Electrochemistry; Wiley-VCH:
Weinheim, 2001.
(7) Martin, S. T.; Herrmann, H.; Choi, W.; Hoffmann, M. R. J. Chem.
Soc., Faraday Trans. 1994, 90, 3315.
(8) Martin, S. T.; Herrmann, H.; Hoffmann, M. R. J. Chem. Soc.,
Faraday Trans. 1994, 90, 3323.
(9) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. J. Phys. Chem.
B 1997, 101, 4265.
(10) Ohko, Y.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. A 1997,
101, 8057.
(11) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.
(12) Gra¨tzel, M. Nature 2001, 414, 338.
Figure 8. Photocurrent action spectra for 10-layer film of MnO2
nanosheet electrodes. The direction of the incident light is from (a) the
front and (b) the back face of the electrode.
(13) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science
2001, 293, 269.
(14) Dare-Edwards, M. P.; Goodenough, J. B.; Hamnett, A.; Nicholson,
N. D. J. Chem. Soc., Faraday Trans. 2 1981, 77, 643.
separation of excited electrons and holes involving strongly
localized d-d transitions. Combination with electron donors or
acceptors sandwiched by the layers of MnO2 nanosheets may
enhance the quantum efficiency of photocurrent generation in
multilayer film of MnO2 nanosheet electrodes. The findings here
should be of significant interest from a fundamental viewpoint
as a new effect of nanoscale materials, which may lead to
utilization of localized d electrons for photoinduced electron-
transfer reactions, and be of great importance from technology
as a new possibility in photoelectric conversion involving visible
light harvesting, which may lead to achievement of extremely
thin film solar cells with two-dimensional ultrathin nanosheets.
Furthermore, the electronic diagram obtained here may shed
light on the mechanism of photocatalytic reactions with
manganese oxides.29,30
(15) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa,
H. J. Am. Chem. Soc. 1996, 118, 8329.
(16) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682.
(17) Omomo, Y.; Sasaki, T.; Wang, L. Z.; Watanabe, M. J. Am. Chem.
Soc. 2003, 125, 3568.
(18) Yang, X.; Makita, Y.; Liu, Z.-h.; Sakane, K.; Ooi, K. Chem. Mater.
2004, 16, 5581.
(19) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2002, 14, 1455.
(20) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc.
2004, 126, 5851.
(21) Wang, L. Z.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina,
Y.; Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873.
(22) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319.
(23) Kavan, L.; Gra¨tzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J.
J. Am. Chem. Soc. 1996, 118, 6716.
(24) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061.
(25) Miller, B.; Heller, A. Nature 1976, 262, 680.
Acknowledgment. This work was supported by CREST of
Japan Science and Technology Agency (JST). We thank Dr.
K. Fukuda and Prof. I. Nakai of Science University of Tokyo
for the in-plane XRD analysis.
(26) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J. Am.
Chem. Soc. 1977, 99, 2839.
(27) Ellis, A. B.; Bolts, J. M.; Kaiser, S. W.; Wrighton, M. S. J. Am.
Chem. Soc. 1977, 99, 2848.
(28) Hardee, K. L.; Bard, A. J. J. Electrochem. Soc. 1977, 124, 215.
(29) Cao, H.; Suib, S. L. J. Am. Chem. Soc. 1994, 116, 5334.
References and Notes
(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37.
(2) Bard, A. J. Science 1980, 207, 139.
(30) Segal, S. R.; Suib, S. L.; Tang, X.; Satyapal, S. Chem. Mater. 1999,
11, 1687.