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Novel Photocatalyst Based on Metastable ZrSnO Solid for Hydrogen and Oxygen Evolutions
4
Hiroaki Shirai, Naoya Akiyama, Naoyoshi Nunotani, and Nobuhito Imanaka*
Department of Applied Chemistry, Faculty of Engineering, Osaka University,
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-1 Yamadaoka, Suita, Osaka 565-0871
(Received <Month> <Date>, <Year>; CL-<No>; E-mail: <imanaka@chem.eng.osaka-u.ac.jp>)
Photocatalytic activity has been demonstrated for
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wavelength light below ca. 248 nm. For SnO , the bandgap
2
ZrSnO by controlling the bandgap and the bottom of the
(
3.1 eV [8]) is narrow compared to ZrO and applied as a
2
conduction band of ZrO and SnO . The single-phase with the
2
2
photocatalyst for organic compounds decomposition [10].
However, SnO cannot reduce water to hydrogen, because the
metastable ZrSnO solid was successfully obtained, and
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ZrSnO exhibited the photocatalytic activity for hydrogen and
2
4
oxygen production from water under the UV irradiation.
bottom of the conduction band is more positive than the redox
+
4+
potential of H /H . Therefore, the oxides composed of Zr
2
4
+
Keywords: Photocatalyst, Hydrogen production, Oxygen
production
and Sn is expected to possess both the photoreduction ability
and the narrow bandgap. In the previous studies, however, it
is difficult to obtain a single-phase of Zr-Sn oxide from the
phase diagram of ZrO -SnO binary system due to the high
2
2
In recent years, since emission of carbon dioxide and
depletion of fossil fuels are serious problems, clean and
renewable energy source is required. Hydrogen has been paid
great attention as a new energy carrier, because the emission
gas is only water. Especially, fuel cell converts from hydrogen
fuel into electric energy effectively, and this device has been
already commercialized as a hydrogen fuel cell vehicle.
Although the hydrogen gas is generally produced by steam
reforming method from fossil fuels involved with the
generation of carbon dioxide, this process does not solve the
fundamental problems. Electrolysis of water is another
method to produce hydrogen without emitting carbon dioxide;
however it requires electric power source. Photocatalytic
hydrogen production from water by using solar energy is an
ideal method, that is, clean, inexhaustible abundant,
sustainable, and environmentally harmonious method [1-4].
For the water splitting, both the reduction and oxidation
potentials of water have to be located within the bandgap of
the photocatalyst. In other words, the bottom of the
conduction band has to be more negative than the reduction
potential of water to produce hydrogen (0 V vs normal
hydrogen electrode [NHE]), and the top of the valence band
has to be more positive than the oxidation potential of water
to produce oxygen (+1.23 V vs NHE). Since photo-induced
stability of ZrO and SnO crystal structure [11-14]. While
2
2
only few studies of metastable ZrSnO have been reported
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[
15,16], its crystal structural properties still remain unclear.
Therefore, we synthesized the metastable ZrSnO solid by the
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co-precipitation method, to apply for its crystal structural
analysis and photocatalytic activity for water splitting was
investigated.
ZrSnO was prepared by the co-precipitation method. The
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ZrSnO precursor was precipitated from ZrOCl and SnCl
4
2
2
mixed solution using ammonia solution, and then calcined at
various temperatures. Figure 1 shows the thermogravimetric-
differential thermal analysis (TG-DTA) of the ZrSnO4
precursor. At ca. 100 °C, an endothermic peak was observed
with an abrupt weight loss due to the vaporization of the
adsorbed solvents, such as water and ammonia. A sharp
exothermic peak was explicitly observed at 568 °C. From the
X-ray powder diffraction (XRD) measurements (Figure S1), a
crystalline phase was observed at 600 °C, while the precursor
was amorphous phase, suggesting that the exothermic peak
was attributed to the crystallization. When calcining over
8
00 °C, the additional phase of SnO2 was observed. In
addition, for the sample after the TG-DTA up to 1400 °C, the
XRD pattern was assigned to be only ZrO and SnO (Figure
2
2
S2). Therefore, a broad exothermic peak at ca. 900 °C is
water splitting on rutile TiO electrodes was demonstrated in
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considered to be attributed to the decomposition of ZrSnO . A
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1
972 [5], photocatalytic activity of semiconductor has been
extensively studied. Almost efficient photocatalytic materials
5+
slight weight loss during the crystallization and the
decomposition might be affected by the decrease of residual
chloride ions, where a slight amount of chlorine species (0.65
at%) was detected for the sample calcined at 600 °C by X-ray
0
10
4+
4+
contain d or d metal ions [3-8], such as Ti , Zr , and Nb
0
3+
4+
5+
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for d ions and Ga , Sn , and Sb for d ions. Their metal
oxides hold electronic configurations suitable for water
splitting, because the top of valence band formed by O 2p
orbitals are generally located at ca. +3 V or higher vs. NHE,
which exceeds the oxidation potential of water (+1.23 V).
Therefore, the level of the conduction band and the bandgap
energy are the key factors for the metal oxides to drive the
water splitting reaction.
fluorescence analysis. From these results, the ZrSnO solid
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solution is the metastable phase, which decomposes at ca.
8
00 °C.
For the crystalline ZrSnO solid calcined at 600 °C, the
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XRD pattern was slightly different from the database [16]
Figure S3), while the prepared ZrO and SnO were single-
(
2
2
phases of the monoclinic ZrO and the tetragonal SnO phases,
2
2
In order to develop a novel photocatalyst for water
4
+
4+
0
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respectively (Figure S3). Although a few studies have
reported the single-phase of ZrSnO4 [15,16], the obvious
pattern has not been obtained and the structural analysis has
not been performed; e.g. the structure of ZrSnO4 has been
splitting, we focused on Zr and Sn as the d and d metal
ions, respectively. ZrO2 has large bandgap of 5.0 eV [9],
which leads to a high photoreduction ability. Nevertheless, the
excitation of electron requires high energy photon, i.e., short-
Shirai, Hiroaki
