An oxynitride, TaON, as an efficient water oxidation photocatalyst under visible light irradiation (λ ≤ 500 nm)

Article abstract of DOI:10.1039/b202393h

Under visible light irradiation (λ = 420-500 nm), a tantalum oxynitride, TaON, functions as a stable and very efficient photocatalyst for oxidation of water into O₂ with a sacrificial electron acceptor (Ag⁺).

Full text of DOI:10.1039/b202393h

An oxynitride, TaON, as an efficient water oxidation photocatalyst under
visible light irradiation (l 5 500 nm)
Go Hitoki,a Tsuyoshi Takata,a Junko N. Kondo,a Michikazu Hara,a Hisayoshi Kobayashib and
Kazunari Domen*ac
a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama,
226-8503, Japan. E-mail: kdomen@res.titech.ac.jp
b Department of Chemical Technology, College of Science and Industrial Technology, Kurashiki University
of Science and the Arts, 2640 Nishinoura, Tsurajima-cho, Kurashiki, 712-8505, Japan
c
Core Research for Evolutional Science and Technology, Japan Science and Technology Co. (CREST, JST),
2-1-13 Higashi-ueno, Daito-ku, 110-0015, Japan
Received (in Cambridge, UK) 7th March 2002, Accepted 7th June 2002
First published as an Advance Article on the web 10th July 2002
Under visible light irradiation (l = 420–500 nm), a tantalum
oxynitride, TaON, functions as a stable and very efficient
photocatalyst for oxidation of water into O2 with a sacrificial
electron acceptor (Ag+).
Many oxide photocatalysts have been developed for energy
problem and the increasingly stringent standards of environ-
mental regulations.1,2 Among these, Ta2O5 and some tantalates
such as NaTaO3 work as highly active photocatalysts and can
even decompose water efficiently, with proper modification.3
Fig. 1 UV–visible diffuse reflectance spectra of TaON and Ta2O5.
Most of these tantalum oxides have band gap energies that are
too large to efficiently use solar radiation, however. In these
Photooxidation of H2O into O2 and photoreduction of H+ into
H2 were used as test reactions to evaluate the photocatalytic
capabilities of TaON. The reactions were carried out in a Pyrex
reaction vessel connected to a closed gas circulation system
constructed of glass. For the photooxidation of water into O2,
the reaction was performed in an aqueous AgNO3 solution (0.01
M; 200 ml) containing 0.40 g of the TaON catalyst and 0.2 g of
La2O3 powder. The La2O3, a basic oxide, was added to maintain
the pH of the solution at around 8 during the reaction. The H2
evolution was examined in an aqueous solution (200 ml)
containing 0.40 g of TaON loaded with 3 wt% Pt, and 20 ml of
methanol as a sacrificial donor. The reaction solution was
evacuated several times to remove air, followed by irradiation
with a 300 W Xe lamp equipped with a cut-off filter (l = 420
nm). The evolved gas was analyzed by gas chromatography.
Fig. 2 shows the time course of O2 evolution under visible
light (l > 420 nm) irradiation. No reaction took place in the
dark. Upon visible light irradiation, O2 evolution started and the
initial rate was 660 mmol h21, which corresponds to a quantum
yield of 34%.† The rate of O2 evolution decreased with reaction
time due to the decrease in Ag+ concentration and because the
surface of TaON became covered with metallic Ag particles that
obstructed photon absorption into the TaON. The total amount
of evolved O2 (500 mmol) corresponds closely to the amount
Ta5+-based oxides, the valence bands predominantly consist of
O 2p orbitals whose potential energy levels are located at a deep
position of about 3 V vs. NHE (or about 27.44 V vs. vacuum
level)4 Due to this fact, it is generally not feasible to find a
transition metal oxide photocatalyst that functions under visible
light irradiation. Although NiO-modified InTaO4 was recently
reported as an effective photocatalyst for water splitting under
visible light irradiation,5 the absorption in the visible light
region was not significant and the photocatalytic efficiency was
very low.
Recently, we have been studying early transition metal
(oxy)nitrides as photocatalysts. (Oxy)nitrides containing Ti4+,
Nb5+ and Ta5+ have reasonable absorption in the visible light
region, i.e. the absorption edges are at 500–650 nm, correspond-
ing to band gap energies of 1.9–2.5 eV. Interestingly, some of
these materials show activity for water oxidation as well as
reduction under visible light irradiation (l 4 420 nm), and they
are essentially stable during the photoreactions. In this commu-
nication, we report the photocatalytic behavior of one such
oxynitride, TaON, because it shows very high activity for water
oxidation under visible light irradiation.
b-TaON is a yellow–yellowish-green oxynitride that has the
same structure as baddeleyite, monoclinic ZrO2.6 From the band
structure of b-TaON, as calculated by density functional theory
(DFT),7 the bottom of the conduction band is based on empty
Ta 5d orbitals similar to Ta2O5, whereas the top of the valence
band consists of a hybridization of N 2p and O 2p orbitals in
which the N 2p contribution is larger than that of O 2p. The
potential energy of the hybridized orbital is higher than that of
an O 2p orbital in an oxide, resulting in a smaller band gap
energy sufficient to absorb visible light.
TaON was prepared by heating Ta2O5 powder in an
atmosphere of flowing NH3 (flow rate: 20 ml min21) at 1123 K
for 15 h. The X-ray diffraction (XRD) pattern (data not shown)
coincided with that of b-TaON in the literature.6 Fig. 1 shows
UV–visible diffuse reflectance spectra of TaON and Ta2O5. The
absorption band-edge of TaON is at ca. 500 nm, being shifted
by about 170 nm from that of Ta2O5, and the band gap energy
was estimated to be 2.5 eV. Elemental analysis revealed that the
composition of the sample was TaO1.24N0.84, indicating that the
prepared ‘TaON’ was nonstoichiometric and defective.
Fig. 2 Time course of O2 (5) evolution from TaON under visible light
irradiation (l > 420 nm). TaON: 0.4 g, La2O3: 0.2 g, 200 mL 0.01 M
AgNO3 solution.
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that can be evolved by the stoichiometric reduction of Ag+
(Ag+: 2000 mmol, O2: 500 mmol). It was confirmed by XRD, X-
ray photoelectron spectroscopy (XPS) and inductive coupling
plasma (ICP) analysis that after the reaction for 5.5 h, all Ag+ in
the solution was deposited on TaON as metallic Ag0. There was
no difference in the XRD patterns of the catalyst before and
after the reaction except for the presence of Ag metal. In the
early stage of the reaction (first 1–2 h), a small amount of N2
evolution was detected. This was attributed to the oxidation of
N32 in the TaON into N2. However, the amount of N2 from
assumed photogenerated holes corresponded to less than 1% of
the catalyst, and further degradation of TaON did not occur.
Actually, in the second run after another AgNO3 (2000 mmol)
addition, no N2 evolution was observed. The slower rate of O2
evolution was due to Ag metal deposition on TaON. O2
evolution under visible light irradiation was also observed in a
Na2S2O8–Na2SO4 (Na2S2O8: 1.0 3 1022 M, Na2SO4: 5.0 3
1022 M, 200 ml) aqueous solution containing TaON although
the rates of O2 evolution was ca. 1/4 that of AgNO3. This
indicates that TaON in visible light functions as a photocatalyst
Fig. 4 Dependence of the initial rates of H2 (1) and O2 (5) evolution on the
cut-off wavelength of incident light, and UV–visible diffuse reflectance
spectrum of TaON. H2 and O2 evolution were examined with a Pt-deposited
TaON (0.4 g), 10 vol% aqueous methanol solution and a TaON (0.4 g),
La2O3 (0.2 g), 0.01 M AgNO3 solution system, respectively.
22
for oxidation of water even in the presence of S2O8 as an
electron acceptor.
In summary, TaON was found to be a novel visible light-
driven photocatalyst. Especially, photooxidation of water on
TaON proceeded very efficiently. On the other hand, the
photocatalytic activity for H2 evolution was not high. By both of
electrochemical analysis and UV photoelectron spectroscopy
for the TaON film, the bottom of the conduction band and the
top of the valence band were estimated to be ca. 20.3 and 2.2
V vs. NHE at pH 0, respectively, suggesting that TaON has a
sufficient potential for reduction of H+ in spite of the small H2
evolution. This is currently under investigation. The details will
be published soon.
The time course of H2 evolution on TaON in visible light (l
> 420 nm) is shown in Fig. 3. H2 evolution occurred as a result
of the reduction of H+ to H2 by excited electrons in TaON.
Simultaneous evolution of CO2 due to the oxidation of methanol
was also confirmed. After evacuation of the solution, H2
evolution proceeded without any decrease in activity, indicating
that TaON also functions as a photocatalyst for photoreduction
of H+ into H2 under visible light irradiation. The estimated
quantum efficiency was ca. 0.2%, which was much lower than
that of the water oxidation. Although a very small level of N2
evolution was observed in the early stage of the reaction, N2 did
not evolve during subsequent reactions. This indicates that
TaON is essentially stable during the H2 evolution reaction.
Fig. 4 shows the dependence of the H2 or O2 evolution rates
on the cut-off wavelength of the incident light. The rates were
determined in the early stages of the reactions (first 1–2 h). The
rates of both H2 and O2 evolution decreased with increasing cut-
off wavelength, and the longest wavelength available for both
photoreactions was estimated to be ca. 500 nm. This corre-
sponds to the absorption edge of TaON. As a result, these
photoreactions are considered to proceed by the bandgap
transition.
This work was supported by the Core Research for Evolu-
tional Science and Technology (CREST) program of the Japan
Science and Technology Co. (JST).
Notes and references
† Quantum efficiencies (F) were calculated using the following equation:
F(%) = (AR/I) 3 100, where A represents the coefficient based on the
reaction (for H2 evolution: 1; for O2 evolution: 4), R represents the H2 or O2
evolution rate (molecules h21), and I represents the rate of absorption of
incident photons (9.6 3 1021 photons h21 at l = 420–500 nm). We
assumed that visible light at l
< 500 nm was available for the
photoreactions because TaON did not work at l > 500 nm, as shown in Fig.
4.
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Fig. 3 Time course of H2 evolution (1) from TaON under visible light
irradiation (l > 420 nm). Pt-deposited TaON: 0.4 g, 200 mL methanol
solution (distilled water 180 mL, methanol 20 mL).
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