Cu2O as a photocatalyst for overall water splitting under visible light irradiation

Article abstract of DOI:10.1039/a707440i

Photocatalytic decomposition of water into H₂ and O₂ on Cu₂O under visible light irradiation is investigated; the photocatalytic water splitting on Cu₂U powder proceeds without any noticeable decrease in the activity for more than 1900 h.

Full text of DOI:10.1039/a707440i

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Cu O as a photocatalyst for overall water splitting under visible light
2
irradiation
a
a
a
a
b
b
Michikazu Hara, Takeshi Kondo, Mutsuko Komoda, Sigeru Ikeda, Kiyoaki Shinohara, Akira Tanaka,
Junko N. Kondo and Kazunari Domen*a
a
a
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Nikon Corp., 1-10-1 Azamizodai, Sagamihara 228, Japan
b
Photocatalytic decomposition of water into H
Cu O under visible light irradiation is investigated; the
photocatalytic water splitting on Cu O powder proceeds
without any noticeable decrease in the activity for more than
900 h.
2
and O
2
on
was vigorously magnetically stirred. The cell was irradiated at
room temperature from one side with visible light (l > 460 nm)
from a 300 W Xe lamp with a cut-off filter. A closed gas
2
2
3
circulation and evacuation system (300 cm ) made of Pyrex
1
glass was connected to the reaction cell, and evolved gases were
directly transferred to a gas chromatograph to avoid any
contamination from air.
So far, many photocatalysts have been reported to decompose
–5
water into H
2
and O
2
under UV light irradiation.1 From the
Fig. 1 shows several typical time courses of H
evolution from Cu O under visible light irradiation ( > 460 nm).
The reaction system was evacuated after each run. As shown in
run 1, only O evolved for 10 h after the beginning of the
2 2
and O
view point of solar energy conversion, however, a photocatalyst
which works under visible light irradiation ( > 400 nm) is
indispensable, but such a photocatalyst has not yet been found.
2
2
In this report, we introduce Cu
semiconductor, which acts as a photocatalyst for overall water
2
O, a well-known p-type
reaction, and then the evolution of H
reaction proceeded. The rate of H
2
was observed as the
evolution increased
2
splitting under visible light irradiation (@600 nm).
gradually in the subsequent runs. The ratio of the amount of
evolved H to O (H /O ) was 0.8 in run 1 and increased to 1.8
The solid state physics of Cu
2
O, which abundantly exists as
2
2
2
2
cuprite in nature, has been extensively investigated for a long
time since Cu O is a simple metal oxide semiconductor with a
small band gap energy. As shown in an energy correlation
between the band gap model of Cu O and the redox potentials
of relevant electrode reactions in an aqueous solution at pH 7,
in run 4. After run 4, the ratio was between 2.0 and 2.5. The
reaction proceeds without any noticeable decrease in the
activity for more than 30 runs as shown in Fig. 1. The total
2
2
amounts of evolved H
mmol, respectively, and are comparable to the amount of Cu
2 2
and O for 1900 h reached 3.8 and 1.9
6
2
O
the conduction and valence band edges of Cu
2
O, which are
used (0.5 g, 3.5 mmol). Furthermore, there was no noticeable
difference in pH of the suspension before reaction (pH 7.3) and
after run 31 (pH 7.1). In order to elucidate the origin of the
separated by a band gap energy of 2.0–2.2 eV,7 seem to be
,8,9
available for reduction and oxidation of water, respectively.
1
8
Therefore, Cu
into H and O
such a photochemical reaction has not been accomplished on
any Cu O electrode since they undergo photodegradation in
aqueous solution. In fact, visible light irradiation (470 nm) of
a cathode-polarized Cu O single crystal electrode in aqueous
solution resulted in reduction of Cu
O to metallic Cu.10 For this
reason, overall water splitting on Cu O photocatalysts has not
2
O is, in principle, capable of decomposing water
evolved O
2
, an experiment using H
2
O was carried out. In
O (after reaction
3
2
2
under visible light excitation. However, as yet,
another small Pyrex cell (50 cm ), 0.1 g of Cu
2
1
6
3
18
for 400 h) suspended in a mixture of H
2
O (5 cm ) and H
2
O
3
2
(1 cm ) was irradiated with visible light ( > 460 nm). H and O
2
2
9
were stoichiometrically evolved after light irradiation and it was
confirmed by mass spectral analysis that the ratio,
2
16
O O:18O
16 18
2
O
2
:
2
, in the evolved O
254:94:13. The result indicates that the atomic ratio, O: O,
in the total amount of evolved O is 5.0 corresponding to that in
the mixed water. As a result, the evolved O is attributed to the
water cleavage on Cu O. The photoresponse on Cu O was
2
species for 24 h was
16
18
2
been investigated despite the band structure available for the
reaction. In this study, we confirmed the photocatalytic overall
2
2
water splitting on Cu
tion.
2
O powder under visible light irradia-
2
2
observed for visible light through a cut-off filter of 600 nm,
Cu
2
O powder prepared by the hydrolysis of CuCl was used in
PO
40 cm ) to a 5 m aqueous NaCl solution containing 0.04 mol of
while there was no photoresponse at l > 650 nm.
this study. CuCl was hydrolyzed by adding 1 m aqueous Na
3
4
3
(
run 1
run 2
run 3
run 4
run 9
run 14 run 31
3
CuCl (400 cm ) with vigorous stirring under an Ar flow. A
yellow precipitate was produced by the hydrolysis which was
washed with distilled water (200 cm ) 5–7 times followed by
3
150
2
decantation under vacuum and drying in vacuo. Cu O powder
was obtained by heating the yellow precipitate at 673 K for 24
h in vacuo, followed by boiling in water under an Ar atmosphere
100
to remove unreacted CuCl from Cu
surface area of Cu O were estimated to be 0.3–0.5 mm and 6 m
, respectively. Only the XRD pattern due to Cu O was seen
2
O. The particle size and
2
2
50
2
1
g
2
with no evidence for other diffraction patterns such as for CuO,
metallic Cu or other impurities. The XP spectra of Cu 2p and the
0
Cu LMM Auger spectra indicated that the surface of Cu
2
O was
O was
0
100
200
300
400 700
1000 1800
I 11,12
composed of Cu .
The band gap energy of Cu
2
t / h
(open circles) and O
O under visible light (l !460 nm) irradiation. Catalyst: 0.5 g, H
. The reaction system was evacuated with light irradiation after each
estimated at ca. 2.0 eV (l ca. 620 nm) by UV–VIS
spectroscopy.
Fig. 1 Time courses of H
Cu
cm
2
2
(filled circles) evolution in
O: 200
2
3
2
The photodecomposition of water was carried out in a Pyrex
3
cell with 0.5 g of Cu
2
O and 200 cm of distilled water, which
run. Time courses in runs 5–8, 10–13 and 15–30 are omitted.
Chem. Commun., 1998
357

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irradiation when O
2
in gas phase exceeds a certain pressure.15
Cu2+
Cu+
Cu+
Cu0
The photoadsorption largely depends on the O pressure as well
2
as on the wavelength and intensity of incident light, tem-
perature, etc. Although the dependence of photoadsorption on
O
2
pressure in a Cu
case has not yet been investigated, it is probable that preferential
photoadsorption inhibits the overall water splitting on the
O surface.
Although Cu
water decomposition under light irradiation from the results of
photoelectrochemistry, the present study has revealed Cu O to
be a photocatalyst able to decompose water into H and O
under visible light irradiation. The reaction mechanism on
Cu O is under investigation.
Recently, we have also found that CuFeO
under visible light irradiation, and detailed results will be
reported soon. CuFeO has a delafossite type layered structure
where the iron oxide layers are connected to each other through
2 2 2 2
O–H O–O /H system as in the present
O
2
Cu
2
(
a)
b)
2
O has been regarded as an unstable material for
(
2
Cu LMM Auger
XPS Cu 2p1/2 Cu 2p3/2
2
2
960
940
/ eV
920
910
920
930
E
Kinetic energy / eV
b
2
2
2 2
evolves H and O
Fig. 2 X-Ray photoelectron spectra of Cu 2p and Cu LMM Auger spectra
of Cu O before (a) and after (b) reaction for 400 h. The binding and kinetic
2
energies were referenced to the Au 4f7/2 level at 83.8 eV.
2
I
16–18
linear –O–Cu –O– bonds.
chains of linear bonds. This suggests that Cu containing
2
The Cu O lattice consists of
I
Fig. 2 shows the XP spectra of Cu 2p and Cu LMM Auger
I
peaks of Cu
noticeable difference in the XP spectra of Cu
Cu O powder was neither reduced nor oxidized after photo-
catalytic reaction. These results are in total contrast to the
observation on Cu O electrodes and strongly suggest that Cu
powder catalytically decomposes water into H and O under
visible light irradiation. To the best of our knowledge, such a
reaction on Cu O photocatalysts has not yet been reported. The
2
O before and after reaction for 400 h. There was no
materials with linear –O–Cu –O– bonds are available for the
overall water splitting under visible light irradiation. Such CuI
containing materials may become potential candidates for
2
O, indicating that
2
2
converting solar energy into H energy.
2
2
O
2
2
Notes and References
2
*
E-mail: kdomen@res.titech.ac.jp
reaction mechanism as well as the reason for the difference
between the electrode and the powder systems are still not clear.
Nevertheless it is inferred that the photocatalytic reaction on a
1 D. Duonghong, E. Borgarello and M. Graetzel, J. Am. Chem. Soc., 1981,
103, 4685.
2 K. Domen, S. Naito, T. Onishi, K. Tamaru and M. Soma, J. Phys.
Chem., 1982, 86, 3657.
Cu
2
O particle in distilled water is clearly different from the
O electrodes in
photoelectrochemical reaction on polarized Cu
2
an aqueous electrolyte. The quantum efficiency of the photo-
catalytic reaction was estimated at ca. 0.3% between 550 and
3
4
5
K. Domen, A. Kudo, A. Shinozaki, A. Tanaka, K. Maruya and T.
Onishi, J. Chem. Soc., Chem. Commun., 1986, 356.
Y. Inoue, T. Kubokawa and K. Sato, J. Chem. Soc., Chem. Commun.,
6
00 nm.
2
One of the characteristic features of the Cu O photocatalyst is
1
990, 1298.
K. Sayama and H. Arakawa, J. Chem. Soc., Chem. Commun., 1992,
50.
the excess evolution of O
stage of the reaction (runs 1 and 2). Cu
relatively large amount of oxygen in bulk as well as adsorbing
2
above the stoichiometry at the early
2
O is known to absorb a
1
6 H. Gerischer, J. Electroanal. Chem., 1977, 82, 133.
7 C. Kittel, in Introduction to Solid State Physics, 5th edn., Wiley, New
York, 1976, p. 341.
2
2
13,14
oxygen as O or O
the surface or in the bulk leads to p-type semiconducting
behaviour and unique oxidation catalysis of Cu O. The release
of these excess oxygen species from Cu O by visible light
irradiation may cause the excess evolution of O above the
stoichiometry at the early stage of the reaction. Another feature
to be noted is the O pressure dependence of the reaction.
As shown in runs 3 and 4 of Fig. 1, the evolution rates of H
and O became slow or stopped when the amount of evolved O
exceeded ca. 80 mmol which corresponded to 500 Pa of O
our system. In all runs after run 5, H and O evolved without
any significant decrease in the activity so long as the evolved
gas was evacuated before the pressure of O reached 500 Pa.
These results suggest that O at more than a certain pressure
500 Pa) in the reaction system inhibits the overall water
2
on the surface.
The excess oxygen on
8
9
P. W. Baumeister, Phys. Rev., 1961, 121, 359.
G. Nagasubramanian, A. S. Gioda and A. J. Bard, J. Electrochemcal
Soc., 1981, 128, 2158.
2
2
2
10 H. Gerischer, Ber. Bunsenges Phys. Chem., 1971, 75, 1237.
11 R. V. Siriwardane and J. A. Poston, Appl. Surf. Sci., 1993, 68, 65.
12 K. Domen, S. Naito, M. Soma, T. Onishi and K. Tamaru, J. Chem. Soc.,
Faraday Trans. 1, 1982, 78, 845.
2
2
2
2
13 H. D u¨ nwald and C. Wagner, Z. Phys. Chem. B, 1933, 40, 197.
14 B. J. Wood, H. Wise and R. S. Yolles, J. Catal., 1969, 15, 355.
15 Th. Wokenstein and IV. Karpenko, J. Appl. Phys., 1962, 33, 460.
2
in
2
2
1
1
6 A. Pabst, Am. Mineral., 1946, 31, 539.
7 R. D. Shannon, D. B. Rogers and C. T. Prewitt, Inorg. Chem., 1971 10,
2
7
13.
8 C. Prewitt, R. D. Shanonn and D. B. Rogers, Inorg. Chem., 1971, 10,
19.
2
1
(
7
splitting on Cu
photoadsorption of oxygen on the Cu
2
O. Such an inhibition might be attributed to the
O surface. p-Type
semiconductors are known to photoadsorb O under light
2
2
Received in Cambridge, UK, 15th October 1997; 7/07440I
358
Chem. Commun., 1998