Two-step synthesis and visible-light-driven photocatalytic water oxidation activity of AW(O,N)3 (A = Sr, La, Pr, Nd and Eu) perovskites

Article abstract of DOI:10.1016/j.jcat.2016.09.001

To expand the family of transition metal oxynitride perovskites, the two-step synthesis of a series of tungsten-based metal oxynitride perovskites (EuW(O,N)₃, NdW(O,N)₃, PrW(O,N)₃, LaW(O,N)₃, and SrW(O,N)₃) and their visible-light-driven photocatalytic water oxidation activity with the assistance of CoO_x (2 wt% Co) cocatalyst were studied in this work. The XRD results revealed that the cubic perovskite LnW(O,N)₃ (Ln = Pr, Nd, and Eu) and SrW(O,N)₃ phases and tetragonal perovskite LaW(O,N)₃ phase were successfully synthesized by nitriding their corresponding oxide precursors at 900 °C for 10–25 h under an NH₃ flow, with minor secondary phases in only PrW(O,N)₃ and NdW(O,N)₃. The highly porous structures of EuW(O,N)₃, LaW(O,N)₃, and SrW(O,N)₃ were formed from the segregation of nanocrystals with average sizes of 140, 92, and 160 nm, respectively. The surfaces of the NdW(O,N)₃ and PrW(O,N)₃ crystal structures were covered with plate-like crystals which can be identified as W₅N₄. No clear absorption edges were observed in the UV–Vis diffuse reflectance spectra of the tungsten-based metal oxynitrides owing to the extensive amount of reduced tungsten species (W⁵⁺ and W⁴⁺) or metallic tungsten and anion deficiency. Within 5 h of the photocatalytic water oxidation half-reaction, the CoO_x-loaded SrW(O,N)₃ crystal structures exhibited the highest photostability and O₂ evolution rate of 3.3 μmol h^?1 compared with CoO_x-loaded LnW(O,N)₃ (Ln = La, Pr, Nd, and Eu) crystal structures due possibly to the highest O/N ratio and more positively positioned top of valence band of SrW(O,N)₃. The present work is expected to stimulate research into the development of more stable and efficient tungsten-based metal oxynitride perovskites in the future.

Full text of DOI:10.1016/j.jcat.2016.09.001

Journal of Catalysis 344 (2016) 29–37
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Two-step synthesis and visible-light-driven photocatalytic water
oxidation activity of AW(O,N) (A = Sr, La, Pr, Nd and Eu) perovskites
3
a
a
b
c
d
Kenta Kawashima , Mirabbos Hojamberdiev , Hajime Wagata , Ehsan Zahedi , Kunio Yubuta ,
e
a,f,
⇑
Kazunari Domen , Katsuya Teshima
a
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Department of Applied Chemistry, Meiji University, 1-1-1-D615 Higashimita, Tama, 214-8571 Kawasaki, Japan
Department of Physical Chemistry, Shahrood Branch, Islamic Azad University, Shahrood, Iran
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
b
c
d
e
f
Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
To expand the family of transition metal oxynitride perovskites, the two-step synthesis of a series of
Received 30 July 2016
Revised 28 August 2016
Accepted 1 September 2016
tungsten-based metal oxynitride perovskites (EuW(O,N)
O,N) ) and their visible-light-driven photocatalytic water oxidation activity with the assistance of
CoO (2 wt% Co) cocatalyst were studied in this work. The XRD results revealed that the cubic perovskite
LnW(O,N) (Ln = Pr, Nd, and Eu) and SrW(O,N) phases and tetragonal perovskite LaW(O,N) phase were
successfully synthesized by nitriding their corresponding oxide precursors at 900 °C for 10–25 h under an
NH flow, with minor secondary phases in only PrW(O,N) and NdW(O,N) . The highly porous structures
of EuW(O,N) , LaW(O,N) , and SrW(O,N) were formed from the segregation of nanocrystals with average
sizes of 140, 92, and 160 nm, respectively. The surfaces of the NdW(O,N) and PrW(O,N) crystal struc-
tures were covered with plate-like crystals which can be identified as W . No clear absorption edges
were observed in the UV–Vis diffuse reflectance spectra of the tungsten-based metal oxynitrides owing
3 3 3 3
, NdW(O,N) , PrW(O,N) , LaW(O,N) , and SrW
(
3
x
3
3
3
Keywords:
3
3
3
Oxynitride
Perovskite
Water splitting
Tungsten
3
3
3
3
3
5 4
N
Visible light
5
+
4+
to the extensive amount of reduced tungsten species (W and W ) or metallic tungsten and anion defi-
ciency. Within 5 h of the photocatalytic water oxidation half-reaction, the CoO -loaded SrW(O,N) crystal
structures exhibited the highest photostability and O evolution rate of 3.3 mol h compared with
CoO -loaded LnW(O,N) (Ln = La, Pr, Nd, and Eu) crystal structures due possibly to the highest O/N ratio
and more positively positioned top of valence band of SrW(O,N) . The present work is expected to stim-
x
3
ꢀ1
2
l
x
3
3
ulate research into the development of more stable and efficient tungsten-based metal oxynitride per-
ovskites in the future.
Ó 2016 Elsevier Inc. All rights reserved.
1
. Introduction
Hydrogen is clean and renewable energy that can eventually
gas [1]. In regard to reducing the loads on the environment, the
production of hydrogen by solar-driven photocatalytic water split-
ting has been considered as one of the most effective and simplest
approaches [2]. As known, the semiconducting materials for photo-
catalytic water splitting should at least have the following: (a) the
chemical stability during the photocatalytic reaction and (b) the
appropriate band structure for water oxidation and reduction [3].
Since the Honda-Fujishima effect was first reported [2], various
substitute the depleting fossil fuels. Hydrogen is mainly produced
by steam reforming, electrolysis, methane cracking, etc. However,
the current production of hydrogen by those processes has brought
large loads on the environment. For instance, electrolysis mainly
relies on electric power generated by burning the fossil fuels,
steam reforming needs liquefied petroleum gas (LPG) or naphtha
0
10
semiconducting materials, such as d - and d -type transition
and discharges CO
2
gas, and methanol cracking also emits CO
2
metal oxides (e.g. TiO
2
[4], SrTiO
3
[5], NaTaO
[8], Zn GeO
3
[6], AMWO
[9], AgInW
6
(
[
A = Rb, Cs; M = Nb, Ta) [7], Ga
2
O
3
2
4
2
O
8
10], etc.) have been demonstrated for achieving an effective over-
loaded with NiO
cocatalyst has shown a high apparent quantum efficiency of
approximately 56% at 270 nm [11]. Al-doped SrTiO loaded with
⇑
Corresponding author at: Department of Environmental Science and Technol-
ogy, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-
all water splitting. For instance, La-doped NaTaO
3
8
553, Japan. Fax: +81 26 269 5550 (K. Teshima).
E-mail address: teshima@shinshu-u.ac.jp (K. Teshima).
3
http://dx.doi.org/10.1016/j.jcat.2016.09.001
021-9517/Ó 2016 Elsevier Inc. All rights reserved.
0

3
0
K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
Rh2ꢀyCr
ciency of 30% at 360 nm [12]. However, those oxides have a wide
band gap energy (E > 3 eV), and an available region of photons is
limited to the ultraviolet region (k < 400 nm). In the near-UV
region (k = 315–400 nm), sun light possesses only 2.9% of the total
solar photons [13]. On the other hand, 44.4% of the total solar pho-
tons is present in the visible light region (k = 400–700 nm) [13].
Therefore, it is indispensable to search for semiconducting materi-
y
O
3
cocatalyst has also achieved an apparent quantum effi-
observed by field-emission-type scanning electron microscopy
(FE-SEM, JSM-7600F, JEOL), at an acceleration voltage of 2 kV,
equipped with an energy dispersive X-ray spectrometer (EDS).
The specific surface area (SBET) was obtained by using the Brunauer,
g
Emmett, and Teller (BET) method from
2
N gas adsorption-
desorption isotherm at 77 K (BELSORP-mini, BEL Japan, Inc.) on
the crystal structures degassed at 100 °C for 5 h in vacuum. The
ultraviolet–visible (UV–vis) diffuse reflectance spectra were
recorded on a JASCO V-670 spectrophotometer equipped with an
g
als with narrow band gap energy (E > 1.23 eV) and appropriate
band structure in order to achieve efficient solar energy conversion
by photocatalytic water splitting.
4
integrating sphere, and BaSO was used as a reference. The surface
chemical compositions of oxynitride samples were analyzed by X-
ray photoelectron spectroscopy (XPS, JPS-9010MC, JEOL) using a
non-monochromatic Mg Ka X-ray source. The XPS profiles were fit-
ted using a Gaussian–Lorentzian function, and the peak positions
Oxynitrides with narrower band gaps and suitable band struc-
tures are regarded as an emerging class of inorganic materials that
can effectively utilize solar energy for photocatalytic reactions. The
0
10
d - and d -type transition metal oxynitrides, such as LaTiO
Zr ON [15], ANbO N (A = Ba, Sr, Ca) [16], LaNbON [16], TaON [17],
ATaO N (A = Ba, Sr, Ca) [18], LaTaON [19], (Zn1+xGe)(N ) [20],
and (Ga1ꢀxZn ) [21] have been exhibited to be effective
2
N [14],
were normalized by positioning the C 1s peak at 284.5 eV.
2
2
2
2
2
2
2 x
O
2.3. Photocatalytic water oxidation activity test
x
)(N1ꢀxO
x
photocatalysts for water splitting under visible light.
Although tungsten doping has previously shown an enhance-
ment in the photocatalytic water splitting activities of BaTaO
The photocatalytic water oxidation half-reactions were per-
formed in a glass cell connected to a closed gas circulation system.
First, 50 mg of CoO -loaded AW(O,N) (A = Sr, La, Pr, Nd or Eu) and
(as a pH buffer) were dispersed in 300 mL of
(>99%, Wako Pure Chemical Industries, Ltd.) aque-
2
N
x
3
[
22], LaTiO N [23], and Ta [24], tungsten-based metal oxyni-
2
3 5
N
2
1
2 3
00 mg of La O
tride perovskites have not been studied for photocatalytic water
splitting under visible light. In this work, we have synthesized five
tungsten-based metal oxynitride perovskites (AW(O,N) , A = Sr, La,
3
Pr, Nd, or Eu) and studied their photocatalytic water oxidation
activities. The change of the A-site cation in the tantalum-based
oxynitrides was previously reported to alter the band structure
0 mM AgNO
3
ous solution (as a sacrificial electron scavenger) in the glass cell
under magnetic stirring. The visible light source was 300 W Xe
lamp with a cutoff filter (k > 420 nm) and light intensity of
ꢀ2
1
1 mW cm
2 2
at 360 nm. The evolved gases (O and N ) were
detected by a gas chromatograph (GC-8A, TCD, Ar gas carrier, Shi-
madzu) attached to the closed gas circulation system. As a cocata-
lyst for O evolution, CoO (2 wt% Co) nanoparticles were loaded by
2 x
[
25]. Here we also elucidate the effect of the A-site cation change
on photocatalytic water oxidation activities of five tungsten-
based metal oxynitride perovskites.
?
unknown
2
. Experimental
(
(
e)
d)
2
.1. Synthesis of oxide precursors and AW(O,N)
3
(A = Sr, La, Pr, Nd or
Eu) perovskites
ICDD PDF 08-0490
SrWO4
SrWO
thesized by a solid state reaction. SrCO
Pr (99.9%), Nd (99.9%), WO (>95%) purchased from Wako
Pure Chemicals Industries, Ltd., and Eu (>99.95%) purchased
4
, La
2 2
W O
9
, Pr
2
2
W O
, Nd W
9 2 2
9
O , and Eu
2
2
W O
9
were syn-
3
(>95%), La
O (99.99%),
2 3
2
O
3
2
O
3
3
2 3
O
from Kanto Chemical Co., Inc., were manually mixed in stoichio-
metric ratios by dry mixing using an agate mortar and pestle for
ICDD PDF 70-7873
La W O
2
2
9
3
0 min, and the homogenized mixtures were placed into alumina
crucibles and covered loosely with alumina plates. The mixture-
containing alumina crucibles were heated in an electric furnace
(c)
(b)
?
?
?
???
?
?
?
?
at 1100 °C for 3 h for SrWO
, and 24 h for La
to 500 °C at a cooling rate of 150 °C h , and then cooled naturally
to room temperature. To synthesize AW(O,N) (A = Sr, La, Pr, Nd or
Eu) perovskites, the as-synthesized SrWO , La and Eu
precursors were nitrided at 900 °C for 10 h, while the as-
synthesized Pr and Nd precursors were nitrided at
00 °C for 25 h under an NH flow (200 mL min ) [26,27]. Further,
and NdW(O,N) samples (250 mg) were acid treated
aqueous solution for 40 min to remove
impurities, rinsed with deionized water, and dried at 100 °C for
4
, 5 h for Pr
2 2 9 2 2 9 2
W O , Nd W O , and Eu -
ICDD PDF 21-0739
Pr W O
ꢀ1
2
W O
9
2 2 9
W O at a heating rate of 100 °C h , cooled
2
2
9
ꢀ1
3
4
2
2
W O
9
2 2 9
W O
?
?
?
?
?
?
2
2
W O
9
W
2 2
O
9
ICDD PDF 77-3808
Nd W O
ꢀ1
9
3
2
2
9
the PrW(O,N)
3
3
with 300 mL of 5 M HNO
3
(
a)
?
?
?
?
?
1
2 h.
ICDD PDF 23-1073
Eu W O
2
2
9
2.2. Characterization
The X-ray diffraction (XRD) patterns were collected using a
2
θ / degree
Miniflex II (Rigaku) diffractometer with Cu K
a (k = 0.15418 nm)
radiation in the 2h scan range of 10–70°. The morphology and size
of the synthesized oxide precursors and oxynitride samples were
Fig. 1. XRD patterns of (a) Eu
(e) SrWO precursor crystals.
2 2
W O
9
, (b) Nd
2 2
W O
9
, (c) Pr
2 2
W O
9
, (d) La
2 2 9
W O , and
4

K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
31
2 2 9 2 2 9 2 2 9 2 2 9 4
Fig. 2. SEM images of (a) Eu W O , (b) Nd W O , (c) Pr W O , (d) La W O , and (e) SrWO precursor crystals.
immersing the oxynitride samples in aqueous solution of Co
NO O (>99.5%, Wako Pure Chemical Industries, Ltd.), fol-
ꢁ6H
lowed by heating at 700 °C for 1 h under an NH flow
(
3
)
2
2
SrWO4
3
ꢀ1
(
200 mL min ) and reoxidizing at 200 °C for 1 h in air.
La W O
9
3. Results and discussion
2
2
3.1. Characterization of the synthesized oxide precursors and
oxynitride perovskites
Pr W O
9
2
2
The XRD patterns of tungsten-based metal oxide precursors
synthesized by a solid state reaction are shown in Fig. 1. As shown
in Fig. 1a–c, the monoclinic phases of Eu
2
W
2
O
9
(ICDD PDF# 23-
(ICDD PDF#
1-0739) were formed along with some minor unidentified phases.
Nd2W O
2
9
1
2 2 9 2 2 9
073), Nd W O (ICDD PDF# 77-3808), and Pr W O
2
Particularly, the diffraction peaks in Fig. 1c belonging to the Pr
phase were slightly shifted to a lower angle due to the forma-
tion of oxygen deficiency enlarging the ionic radii of
2 2
W -
O
9
Eu W O
9
2
2
3
+
2+
praseodymium ions (1.18 Å for Pr , 1.37 Å for Pr ) and tungsten
6
+
5+
4+
ions (0.60 Å for W , 0.62 Å for W , 0.66 Å for W ) having the
Wavelength / nm
coordination numbers of 9 and 6, respectively [24,28]. In contrast,
the triclinic La
2 2 9 4
W O (ICDD PDF# 70-7873) and tetragonal SrWO
Fig. 3. UV–vis diffuse reflectance spectra of Eu
2 2 9 2 2 9 2
W O (pink), Nd W O (blue), Pr -
(
ICDD PDF# 08-0490) were synthesized in a completely pure phase
2
W O
9
(green), La (orange), and SrWO (red) precursor crystals. (For interpre-
2
W
2
O
9
4
by a solid state reaction (Fig. 1d and e), indicating that no diffrac-
tion peaks assignable to impurity phases were detected.
tation of the references to color in this figure legend, the reader is referred to the
web version of this article.)

3
2
K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
W0.62ON
W
W N
WN ?unknown
were also observed due to the transitions between the electronic
energy levels of Eu, Nd, and Pr atoms. Seven absorption bands
5
4
are noted for Eu
2
W
2
O
9
at 376.0, 382.0, 394.5, 415.0, 465.0, 533.5,
(
(
(
e)
d)
c)
7
5
7
5
and 579.5 nm corresponding to the
F
0
? G
3
,
F
0
? G
transi-
2
,
7
? 5L
7
5
7
5
, 7
5
7
5
F
0
6
,
F
0
? D
3
,
F
0
? D
2
F
0
? D
1
, and
F
0
? D
0
tions, respectively [32]. There are fourteen absorption bands for
Nd at 354.0, 359.0, 384.5, 423.0, 435.5, 464.0, 474.0, 478.0,
2 2 9
W O
514.5, 528.5, 586.5, 629.5, 684.0, and 748.5 nm associated with
4
4
4
4
4
2
4
2
the
I
9/2 ? D5/2
,
I
9/2 ? D3/2
,
I
9/2 ? P3/2
,
I
9/2 ? D3/2
,
,
,
4
2
4I9/2 ? G3/2
4
4I9/2 ? G11/2
4
4I9/2 ? K15/2
2
I
9/2 ? D1/2
,
,
,
4
4
4G7/2
4
4I9/2 ? K13/2, 4
2
9/2 ? G7/2, 4
4
5/2, 4
2
I
9/2 ? G9/2
,
,
,
I
G
I9/2 ? H11/2
4I9/2 ? F9/2
4
I
9/2 ? S3/2, and 4F7/2 transitions, respectively [32].
4
Four absorption bands were detected for Pr
2 2 9
W O at 450.5,
?
?
3
3
1
?
?
?
4
76.0, 488.0, and 600.0 nm assignable to the
H
4
? P
transitions, respectively [32]. As
2 2 9
shown in Fig. 3, the colors of the Eu , Nd , and Pr W O
2
,
6
I ,
3
3
3
3
3
1
4 1 4 0 4 2
H ? P , H ? P , H ? D
2
W O
2 9
2
2
W O
9
(
(
b)
a)
crystals are pale pink, dark violet, and light green, respectively,
because of electronic transitions between the ionic ground state
and energy levels stemming from 4f electron configurations of
?
3
+
6
3+
3
3+
2
Eu (4f ), Nd (4f ), and Pr (4f ) [33].
2 2 9 4
Eu , Nd , Pr , La W O , and SrWO have been
2
2
W O
9
2
W
2
O
9
W
2 2
O
9
demonstrated to have potential applications in lasers, medical
devices, rare earth ions hosts, electrochemical (membranes for
gas separation and fuel cells) and luminescent devices, lossless
capacitors, and photocatalytic materials for hydrogen generation.
However, the main aim of this work was to study the thermal con-
2
θ / degree
2 2 9 2 2 9 2 2 9 2 2 9 4
version of the Eu W O , Nd W O , Pr W O , La W O , and SrWO
Fig. 4. XRD patterns of (a) EuW(O,N)
3 3 3
, (b) NdW(O,N) , (c) PrW(O,N) , (d) LaW(O,
N) , and (e) SrW(O,N) crystal structures.
3
3
crystals into their oxynitride perovskites and to evaluate their pho-
tocatalytic water oxidation activity under visible light.
3
Fig. 4 shows the XRD patterns of AW(O,N) (A = Sr, La, Pr, Nd or
Eu) crystal structures synthesized by nitridation of their corre-
sponding oxide precursors. As shown in Fig. 4a, the cubic per-
The SEM images of tungsten-based metal oxide precursor crys-
tals synthesized by a solid state reaction are displayed in Fig. 2. As
shown in Fig. 2b and c, the Nd and Pr crystals with
average sizes of about 5.5 and 6.2 m have idiomorphic shapes
with clear edges because the Nd + WO and Pr + WO binary
2
W
2
O
l
9
2
W
2
O
9
ovskite EuWO1.58
successfully formed from the monoclinic Eu
through nitridation, in which Eu was changed to Eu in order
to give an access to the cubic perovskite [34]. No diffraction peaks
assignable to the impurity phases were detected. When the mono-
N
1.42 (ICDD PDF# 76-3153) phase was
crystals
2
2 9
W O
2
O
3
3
2
O
3
3
3+
2+
systems behaved as a self-flux, allowing the crystals to grow freely
and to gain their idiomorphic shapes in the supersaturated high-
temperature non-aqueous solution. Compared to the Nd
and Pr crystals, the Eu and La crystals were
formed in quasi-spherical forms with smooth surfaces and average
sizes of about 6.5 and 8.6 m, respectively (Fig. 2a and d). As disso-
2 2 9
W O
clinic Nd
NdW(O,N)
2
W
2
O
9
and Pr
2
W
2
O
9
crystals were nitrided, the perovskite
phases (crystal structures need to be
W
2 2
O
9
2
W O
2 9
2 2 9
W O
3
and PrW(O,N)
3
refined) appeared along with minor secondary phases belonging
to W (ICDD PDF# 65-4761), WN (ICDD PDF# 75-1012), W
ICDD PDF# 04-0806), 0.62ON (ICDD PDF# 25-1254), and
unknown phases (Fig. 4b and c). Fig. 4d and e reveals that the
diffraction peaks of the nitrided triclinic La and tetragonal
SrWO crystals fully correspond to the tetragonal phase of
LaWO0.6 2.4 (ICDD PDF# 84-1682) and the cubic phase of
SrWO2.05 0.95 (ICDD PDF# 88-0828), respectively. Previously, Li
l
5 4
N
lution is a mass transfer-controlled process, a slight deviation from
equilibrium may lead to the growth of quasi-spherical crystals of
Eu W O and La W O that initially had clear facets with rough
2 2 9 2 2 9
surfaces having nearly the same growth rate [29]. Similarly, the
effect of solubility on the growth rate was also noticed previously
in the ferrite system. That is, the prolonged heating of acicular
2 4 2 4
NiFe O and ZnFe O particles in the NaCl-KCl flux at 900 °C caused
the particles to adopt a somewhat rounded shape through the par-
(
W
2 2 9
W O
4
N
N
et al. [34] theoretically predicted the formability of these
tungsten-based metal oxynitride crystal phases by applying the
tolerance and octahedral factors. Note that our experimental
results have also shown a good agreement with these reported
results from theoretical calculations about the tolerance and octa-
hedral factors for predicting the formability of perovskite struc-
tures in oxynitrides [34].
ticle deformation in the particle-growth stage due to high solubil-
ity [30]. On the contrary, the SrWO
shape because of low solubility of the reactants, and the formed
chunk-like particles have an average size of 12.8 m (Fig. 2e).
Fig. 3 shows the UV–vis diffuse reflectance spectra of the Eu
(pink line), Nd (blue line), Pr (green line), La
(orange line), and SrWO
synthesized by a solid state reaction. The Eu
, La , and SrWO precursor crystals exhibited absorp-
tion edges at about 370, 356, 358, 343, and 270 nm, respectively,
and their band gap energies (E ) were estimated to be about
, 3.48 eV for Nd , 3.47 eV for Pr
, and 4.59 eV for SrWO according to the
= hc/k, where h is the Planck’s constant, c is
4
particles do not have a clear
l
2
2
-
-
2
W O
9
2 2
W O
9
2
2
W O
9
2
W
Fig. 5 shows the SEM images of the as-synthesized Eu
2 2 9
W O ,
O
9
4
(red line) precursor crystals
, Nd , Pr
Nd
2 2
W O
9
, Pr , La , and SrWO crystals before (two col-
2
W
2
O
9
2
W
2
O
9
4
2
W
2
O
9
2
W
2
O
9
2
-
umns to the left) and after (two columns to the right) nitridation at
2
W O
9
W
2 2
O
9
4
ꢀ1
9
00 °C for 10 and 25 h under an NH
3
flow (200 mL min ). As
shown, all tungsten-based metal oxide precursors had smooth sur-
faces that were eventually changed after nitridation despite the
oxynitride crystal structures maintained the outlines of oxide pre-
cursor crystals. The EuW(O,N) , LaW(O,N) , and SrW(O,N) pos-
3 3 3
sessed highly porous structures formed from the strong
segregation of nanocrystals with average sizes of 140, 92, and
g
3
3
.35 eV for Eu
.62 eV for La
2
W
2
O
9
2
2
W O
9
2 2 9
W O ,
2
W
2
O
9
4
Planck’s equation, E
g
ꢀ1
the light speed (m s ), and k is the wavelength of light (nm)
31]. In addition, in the UV–Vis diffuse reflectance spectra of the
Eu , Nd , and Pr crystals, the absorption bands
[
1
60 nm, respectively. These porous structures were formed possi-
bly because of the phase transitions from oxides to oxynitrides
W
2 2
O
9
W
2 2
O
9
2 2 9
W O

K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
33
(
(
a)
b)
before nitridation
after nitridation
3
μm
μm
500 nm
before nitridation
3 μm
500 nm
after nitridation
3
500 nm
3 μm
500 nm
(
c)
before nitridation
after nitridation
3
μm
μm
μm
500 nm
before nitridation
3 μm
3 μm
3 μm
500 nm
after nitridation
(
(
d)
e)
3
500 nm
before nitridation
500 nm
after nitridation
3
500 nm
500 nm
2 2 9 2 2 9 2 2 9 2 2 9 4
Fig. 5. SEM images of (a) Eu W O , (b) Nd W O , (c) Pr W O , (d) La W O , and (e) SrWO before (two columns to the left) and after (two columns to the right) nitridation.
and the lattice condensation process as well as lattice shrinkage
light absorption and a color change to dark gray may be attributed
to the negative shift of the top of valence band stemmed from the
due to the partial replacement of O2 with N in the oxide precur-
ꢀ
3ꢀ
2ꢀ
3ꢀ
sor crystals during nitridation process [35,36]. The measured speci-
partial replacement of O with N in the oxide precursor crystals
5+
4+
fic surface areas of EuW(O,N)
1
3
, LaW(O,N)
2.3, 13.1, and 11.9 m g , respectively. In contrast, on the sur-
faces of the NdW(O,N) and PrW(O,N) crystal structures, the
plate-like crystals with lateral sizes of about 550 (thickness of
0 nm) and 580 (thickness of 68 nm) nm were formed, respec-
tively. As previously reported [37], the tungsten nitrides such as
N, W , and W have a hexagonal system, and
the hexagonal W N phase tends to form plate-like crystals [37].
Hence, these plate-like crystals possibly belong to the hexagonal
tungsten nitride (W ) phase, which is in good agreement with
the XRD results. The measured specific surface areas of NdW(O,
3
, and SrW(O,N)
3
were
and the accumulation of reduced tungsten species (W and W )
2
ꢀ1
and metallic tungsten, creating massive defect states within the
bandgap of tungsten-based metal oxynitrides [22,23,35,38,39].
3
3
Compared to the spectra of the Eu
crystals, the absorption peaks of f-f transitions were not observed
in the UV–Vis spectra of the EuW(O,N) , NdW(O,N) , and PrW(O,
N) crystal structures probably because the absorbance of the
2 2 9 2 2 9 2 2 9
W O , Nd W O , and Pr W O
1
3
3
W
2
5
N
4
, W2.54
N
4
5
N
8
3
2
metallic tungsten impurities masks that of the f-f transitions.
To study the distribution of elements and chemical composi-
tions, the AW(O,N) (A = Sr, La, Pr, Nd or Eu) crystal structures were
3
5
N
4
analyzed by energy-dispersive X-ray spectroscopy (EDS), and the
results are shown in Figs. 7 and 8. The EDS elemental mapping
images, shown in Fig. 7, reveal that the Eu, Nd, Pr, La, Sr, W, O,
and N elements are evenly distributed throughout the tungsten-
based metal oxynitride crystal structures, implying that the partial
2
ꢀ1
N)
3
and PrW(O,N)
Fig. 6 shows the UV–vis diffuse reflectance spectra of the EuW
O,N) (pink), NdW(O,N) (blue), PrW(O,N) (green), LaW(O,N)
orange), and SrW(O,N) (red) crystal structures. In comparison
3
were 10.6 and 9.8 m g , respectively.
(
(
3
3
3
3
3
2
ꢀ
3ꢀ
with oxide precursors, the nitrided samples exhibited an upward
shift in absorption in the visible-light region (k = 300–800 nm)
without indicating clear absorption edges. This upward shift in
replacement of O with N was uniform. In addition, the EDS
spectra shown in Fig. 8 confirmed that the nitrided crystals do
not contain any impurity elements.

3
4
K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
SrW(O,N)3
LaW(O,N)3
PrW(O,N)3
NdW(O,N)3
EuW(O,N)3
The Eu 4d, Nd 4d, Pr 4d, La 4d, Sr 3d, W 4f, O 1s, and N 1s XPS
core-level spectra of the EuW(O,N) , NdW(O,N) , PrW(O,N) , LaW
O,N) , and SrW(O,N) crystal structures are shown in Fig. 9. The
Eu 4d XPS core-level spectrum of EuW(O,N) exhibited four peaks
at 127.5, 132.4, 135,6, and 141.1 eV, which correspond to the Eu
3
3
3
(
3
3
3
2+
2
+
3+
3+
4
d5/2, Eu 4d3/2, Eu 4d5/2, and Eu 4d3/2 spin-orbital splitting,
respectively, indicating the presence of mixed-valence europium
3
+
2+
(
Eu and Eu ) [40]. According to the previous report [41], the oxi-
dation states of W and Eu can be tuned in EuW(O,N) by varying
the O/N ratio. Hence, the as-synthesized EuW(O,N)
3
3
+
crystal struc-
2
6+
5+
tures can be formed with formal compositions Eu
1ꢀx x 1+x
W W O -
3
+
5+
4+
N
2ꢀx and Eu
W
1ꢀx
W
x
O
1+x
N
2ꢀx. The peaks at 122 and 125 eV in the
are originated from the
Nd 4d5/2 and Nd 4d3/2 spin-orbital splitting [42]. For PrW(O,N) , the
Nd 4d XPS core-level spectrum of NdW(O,N)
3
3
Pr 4d peaks are found to be at 116 eV, which are possibly associated
with Pr 4d3/2 [43]. The peaks in the La 4d XPS core-level spectrum of
LaW(O,N)
and La 4d3/2 at 100.9 and 103.8 eV, respectively [44]. In the Sr 3d XPS
core-level spectrum of SrW(O,N) , there are two spin-orbit compo-
3
can be assigned to two spin-orbit components of La 4d5/2
Wavelength / nm
3
nents at 133.4 (Sr 3d5/2) and 135.1 eV (Sr 3d3/2) [45]. The O 1s XPS
core-level spectra of the tungsten-based metal oxynitrides have
two components at 531.2 and 533 eV, which are ascribed to the
W–O bonds and crystallized water, respectively [46,47]. The N 1s
Fig. 6. UV–vis diffuse reflectance spectra of EuW(O,N)
3 3
(pink), NdW(O,N) (blue),
PrW(O,N) (green), LaW(O,N) (orange), and SrW(O,N) (red) crystal structures. (For
3
3
3
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Fig. 7. EDS elemental mapping images of (a) EuW(O,N)
corresponding oxide precursors at 900 °C for 10 and 25 h under an NH
3
, (b) NdW(O,N)
3
, (c) PrW(O,N)
flow.
3 3 3
, (d) LaW(O,N) , and (e) SrW(O,N) crystal structures synthesized by nitridation of their
3

K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
35
Fig. 8. EDS spectra of (a) EuW(O,N)
oxide precursors at 900 °C for 10 and 25 h under an NH
3
, (b) NdW(O,N)
3
, (c) PrW(O,N)
flow.
3 3 3
, (d) LaW(O,N) , and (e) SrW(O,N) crystal structures synthesized by nitridation of their corresponding
3
3 3 3 3 3
Fig. 9. Eu 4d, Nd 4d, Pr 4d, La 4d, Sr 3d, W 4f, O 1s, and N 1s XPS core-level spectra of (a) EuW(O,N) , (b) NdW(O,N) , (c) PrW(O,N) , (d) LaW(O,N) , and (e) SrW(O,N) crystal
structures.
XPS core-level spectra of the tungsten-based metal oxynitrides have
also two components at 397.6 and 400.7 eV, which are attributed to
the nitrogen species introduced in the substitutional sites and
adsorbed on the crystal surface, respectively [48,49]. As can be seen,
as in its interstitial sites compared to other tungsten-based metal
oxynitride perovskites synthesized in this work. The W 4f XPS
core-level spectra of the tungsten-based metal oxynitrides were
deconvoluted into six peaks located at around 32.5, 34, 34.6, 35.6,
4
+
5+
7/2
7/2, W f ,
the SrW(O,N)
3
contains more nitrogen species on its surface as well
36.2, and 37.8 eV, which can be assigned to the W
f

3
6
K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
W4+
f
5/2, W6+
f
7/2, W5+
f
5/2 and W6+
f
5/2, respectively [50]. The XPS
of photogenerated holes for water oxidation can be expressed in
the following reaction formulas [54]:
4
+
results confirmed that tungsten exists in oxidation states of W
,
5
+
6+
W
, and W , implying that the substantial amount of oxygen and
3
ꢀ
þ 6hþ ! N
2
2
2
N
2
nitrogen vacancies is present in the synthesized tungsten-based
metal oxynitride perovskites.
2
ꢀ
þ 4hþ ! O
O
2
ðfrom photocorrosionÞ
3.2. Photocatalytic water oxidation activity of AW(O,N)
3
(A = Sr, La, Pr,
O þ 4hþ ! O
ðfrom water oxidationÞ þ 4Hþ
H
2
2
Nd or Eu) crystal structures
Apparently, the ratio of the evolved O
ratio in the LnW(O,N) (Ln = Eu, Nd, Pr or La) crystal structures (e.g.,
2.2, and LaWO0.6 2.4). Com-
2 2
/N gases is similar to the O/N
The photocatalytic water oxidation activity of the tungsten-
based metal oxynitride perovskites was evaluated under visible
3
EuWO1.58
N
1.42, NdWO0.8
N
2.2, PrWO0.8
N
N
light, and the reaction time courses of O
in the presence of AgNO over the CoO -loaded AW(O,N)
La, Pr, Nd or Eu) crystal structures are plotted in Fig. 10. For the
CoO -loaded LnW(O,N) (Ln = La, Pr, Nd or Eu) crystal structures,
the amounts of O and N gases evolved within 5 h of photocat-
alytic water oxidation half-reactions are 24.9, 38.5, 20.5, and
0.8 mol for O and 32.5, 57.8, 28.5, and 8.1 mol for N , respec-
tively. It is possible that the evolution of N gas was caused by the
photocorrosion of the CoO -loaded LnW(O,N) (Ln = La, Pr, Nd or
Eu) crystal structures [51]. Presumably, the lack of active sites for
evolution on the surfaces of the LnW(O,N) (Ln = La, Pr, Nd or
Eu) crystal structures, which inhibits the supply of photogenerated
holes for O evolution, can be responsible for their photocorrosion
52,53]. The photocorrosion of the CoO -loaded LnW(O,N) (Ln = La,
Pr, Nd or Eu) crystal structures accompanied by the consumption
2
evolution from water
paring the photocatalytic water oxidation activity of the
lanthanide-containing tungsten-based metal oxynitride per-
3
x
3
(A = Sr,
ovskites, the CoO
higher ratio of the evolved O
loaded EuW(O,N) , NdW(O,N)
This difference in the ratio of the evolved O
x
-loaded LaW(O,N)
/N gases compared to the CoO
, and PrW(O,N) crystal structures.
/N gases indicates that
3
with vacant 4f orbitals (4f ) may have higher photo-
3
crystal structures showed
x
3
2
2
x
-
2
2
3
3
3
2
2
0
1
l
2
l
2
the LaW(O,N)
catalytic activity than EuW(O,N)
2
3
, NdW(O,N)
3
3+
, and PrW(O,N)
3
with
7
2+
6
3
3+
2
x
3
partly filled 4f orbitals (4f (Eu ), 4f (Eu ), 4f (Nd ), and 4f
3
+
(Pr ), respectively). As shown in Scheme 1b, this can be rational-
ized by considering that the partly filled 4f orbitals may become
the trap level acting as a recombination center for the photogener-
O
2
3
2
ated electrons and holes [55]. From XRD results, the PrW(O,N)
3
and
[
x
3
NdW(O,N) crystal structures contained some impurity phases
3
which mainly are assigned to tungsten oxynitride and nitride.
SrW(O,N)
LaW(O,N)
PrW(O,N)
NdW(O,N)
3
3
3
3
EuW(O,N)
3
Time / h
Time / h
2 2 x 3
Fig. 10. Reaction time courses for O and N evolution over CoO -loaded AW(O,N) (A = Sr, La, Pr, Nd or Eu) perovskites.
%
:ꢀ2ꢆ1ꢂ ꢀ% 6UDQG/Dꢂ
&:ꢀ2ꢆ1ꢂ ꢀ& 3Uꢆ1GDQG(Xꢂ
ꢇ
ꢇ
HOHFWURQ
&
%ꢀ:ꢁGꢂ
&%ꢀ:ꢁGꢂ
+ꢃꢈ+ꢄ
ꢉ9YVꢊ1+(ꢂ
/
QꢅIꢀ/Q 3Uꢆ1GDQG(Xꢂ
ꢀ
2ꢄꢈ+ 2
ꢄ
ꢀ
ꢋꢊꢄꢇ9YVꢊ1+(ꢂ
9
%ꢀ2ꢃ1ꢄSꢂ
9%ꢀ2ꢃ1ꢄSꢂ
KROH
:
DWHUꢌR[LGDWLRQꢌUHDFLWRQ
3KRWRFRUURVLRQ
+
2N3- + 6h → N
+
2
H O + 4h → O2
(from water
2
2
+
2-
+
oxidation)+ 4H
2O + 4h → O (from photocorrosion
)
2
Scheme 1. Schematic representation of photocatalytic water oxidation over (a) BW(O,N)
3
3
(B = Sr and La) and (b) CW(O,N) (C = Pr, Nd and Eu) crystal structures.

K. Kawashima et al. / Journal of Catalysis 344 (2016) 29–37
37
According to the previous report [54], the tungsten oxide and
nitride showed the photocatalytic activities for H evolution under
visible light irradiation. Presumably, the impurities can act as the
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74–78.
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3
, NdW(O,N)
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, and PrW(O,N)
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, indicating that the
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. On the other hand, the
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The synthesized tungsten-based metal oxynitride perovskites have
highly porous structures. Within 5 h of the photocatalytic water
3
(A = Sr, La, Pr, Nd or Eu) perovskites
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oxidation half-reactions, the CoO
high stability and O evolution rate of 3.3
CoO -loaded EuW(O,N) , NdW(O,N) , PrW(O,N)
x
-loaded SrW(O,N)
3
exhibited
mol h compared with
, and LaW(O,N)
ꢀ1
2
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x
3
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3
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(
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[
[
[
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The authors declare that there are no conflict of interests.
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Acknowledgment
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