Kyushu University Global COE Program
Journal of Novel Carbon Resource Sciences, Vol. 3, pp. 1-5, Feb. 2011

Photocatalytic Properties of Ordered Double Perovskite Oxides
Hironori Iwakura*1, Hisahiro Einaga*2, Yasutake Teraoka*2*1Interdisciplinary Graduate School of Engineering Sciences, Kyushu University*2
Faculty of Engineering Sciences, Kyushu University(Received October 26, 2010; accepted November 25, 2010)

Ordered double perovskite oxides (A 2BB’O6, AA’BB’O6) which contained W, Ta, Nb and Ti were synthesized,
and the applicability of the oxides to photocatalytic reaction was discussed. The stability of the oxides strongly
depended on the constituent atoms, especially B-site cations. Ta-, Nb- and Ti-containing perovskites were stable
under two types of photocatalytic reaction conditions, namely, H 2 evolution from aqueous methanol solution andO2 evolution in Ag+ ion-containing aqueous solution. Among the perovskite oxides, La 2ZnTiO6 showed catalytic
activity for the latter reaction and the rate for O2 evolution normalized by catalyst surface area was one third of thatof TiO2.

1. Introduction

Photocatalytic reactions over semiconductors haveattracted much attention over several decades because
they can utilize solar energy for chemical reactions suchas overall water splitting1,2). Photocatalytic events insemiconductor are undertaken by absorption of near UV
or visible light to generate excited electron-hole pairs,
and these species migrate to catalyst surface and result
in redox reactions. The charge separation process, oneof the most important steps in photocatalytic reaction,strongly depends on the crystal structure and theconstituent elements in the catalyst.
So far, many photocatalysts have been developed withdifferent crystal structures, and most of them contain
W6+, Ta5+, Nb5+ and Ti4+ cations because these species are
effective in obtaining photocatalytic activities1,2).
Perovskite-type oxide has general formula of A n+Bm+O3(n + m = 6), and most of metal cations are known to bestably incorporated in the structure. Perovskite-typeoxides are also possible candidates for photocatalytic
materials, and several kinds of perovskite-type oxides
such as A+B5+O3 (A+ = Ag, Li, Na, K, B5+ = Ta, Nb)3,4) andA2+In0.5Nb0.5O3 (A2+ = Ba, Sr, Ca)5) have been reported tobe active for photocatalytic reactions.
Ordered double perovskite (A 2BB’O6, AA’BB’O6) iswell known as a subclass of perovskite-type oxide. In
the structure, two kinds of metal cations occupy B-sitein the same molar ratio, and the metal cations arrange
alternately at B-site6). The most favorable arrangement is
the rock-salt ordering as shown in Fig. 17), and in addition
to Ta5+, Nb5+ and Ti4+ cations, W6+ cation is also possible
A, A’-cation
BO6 octahedronB’O6 octahedron

Fig. 1

Crystal structure of ordered double perovskite
with rock-salt ordering of B-cations.

to be stably incorporated in ordered double perovskite6, 8).
In the present study, ordered double perovskites whichcontain W6+, Ta5+, Nb5+ and Ti4+ cations at B-sties havebeen synthesized and their photocatalytic properties
under UV-light irradiation have been investigated.
Photocatalytic H 2 production from aqueous methanol
solution and O2 evolution from Ag ion-containing
aqueous solution were used to investigate activities ofphotogenerated electron and hole, respectively. From theresults of photocatalytic activity, the applicability of theordered double perovskites to photocatalytic reactions
was discussed from the standpoint of catalyst stabilityand activity.

2. Experimental

2.1 Preparation and characterization of perovskite
oxidesPolycrystalline powders of perovskite-type oxides
were synthesized by a conventional solid-state reaction.
The starting materials are listed in the footnote ofTable 1. The materials were adequately mixed in thestoichiometoric ratio by a pestle and mortar and then bya universal ball mill. The mixture was calcined under theconditions which are also shown in Table 1. TiO2 (JRCTIO-4, identical to P25, SBET = 55 m2/g) was supplied
from Catalysis Society of Japan and used as a reference
photocatalyst.
The crystal structures of samples thus prepared wereexamined by powder XRD measurement using Cu-Kα
irradiation (Rigaku, RINT-2200) and were refined by
Rietveld method using RIETAN-2000 program9). The
surface area of a sample was determined by BET methodfrom N2 adsorption isotherm at 77 K (BEL JAPAN, Inc.,BELSORP-mini). Diffuse reflectance (DR) UV-VIS
spectra of samples were measured using Shimadzu UV3100 spectrophotometer equipped with an integrating
sphere attachment. α-Al2O3 powder was used as areference sample. The mixture of prepared sample andAl2O3 powder was used to obtain a reflectance spectrum,and the spectrum was transformed into the KubelkaMunk function.

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

Table 1Synthesis conditions and crystal structures of ordered double perovskites synthesized in this study

Compound a)

Calcination condition

Cell parameter (a, b, c / nm, β / o)Ba2CoWO6

1100 oC, 10 h

a = 0.810621(6)Ba2NiWO6o

1100 C, 10 h

a = 0.806676(6)Ba2CuWO61000 oC, 5 ha = 0.55668(3), c = 0.86289(5)Ba2ZnWO6o

1100 C, 10 h

a = 0.812161(6)Sr2CoWO61200 oC, 20 h

a = 0.558195(5), c = 0.798172(8)Sr2NiWO6o

1100 C, 5 h

a = 0.555965(4), c = 0.791745(7)Sr2CuWO61000 oC, 5 ha = 0.54283(1), c = 0.84131(2)Sr2ZnWO6o

1100 C, 20 h

a = 0.56321(4), b = 0.56089(3), c = 0.79235(4), β = 89.99(3)

LaBaCoTaO6 b)

1300 oC, 20 h

a = 0.8067(1)b)

o1300 C, 20 h

a = 0.8024(1)

LaBaCoNbO6 b)

1300 oC, 20 h

a = 0.8068(1)

LaBaNiTaO6b)o1300 C, 20 h

a = 0.8032(1)

LaBaZnNbO6 b)

1200 oC, 50 h

a = 0.80863(5)LaBaNiNbO6
La2ZnTiO6o1200 C, 20 h

a = 0.55746(1), b = 0.56167(1), c = 0.78922(2), β = 90.01(1)a) Starting material (purity, origin); La2O3 (>99.99%, KI), BaCO3 (>99%, KI), SrCO3 ( >99.9%, KA), Co3O4 ( >99.9%, KO), NiO( >98%, KI), CuO ( >99%, KI), ZnO ( >99.5%, KI), WO3 ( >99.9%, KI), Ta2O5 ( >99.9%, W), Nb2O5 ( >99.9%, KI), TiO2 ( >98%,KI), Origin: KI; Kishida Chem. Co., Ltd., KA; Kanto Chem. Co., Inc., KO; Kojundo Chem. Lab. Co., Ltd., W; Wako Pure Chem.
Industries.
b) Cell parameters were determined from the analysis of PXRD patterns. Those of the other samples were determined by Rietveld
refinement.

2.2 Catalyst stability test

Stability of catalyst samples under photoirradiation
was investigated in methanol-water solution (50 vol%)and aqueous solution which contained 0.02 M AgNO3.
Catalyst sample (50 mg) was dispersed into the solution(45 mL) and the suspension was transferred to a reaction
cell made of Pyrex glass and was stirred by a magnetic
stirrer. After deaeration of the suspension with Ar, UV
light was irradiated with a 500 W high-pressure mercury
lamp for 5 h. After the photoirradiation, the suspension
was filtered, and the obtained powder was dried. The
crystal structure of the powder was examined by PXRD
measurement.

2.3 Photocatalytic activity measurement

Photocatalytic activity of a catalyst was examined bya gas-closed circulation system equipped with a GCTCD (Shimadzu, GC-8A). Catalyst sample (0.3-0.5g) was dispersed into 500 mL of 0.01 M AgF aqueous
solution in an inner-irradiation-type reaction cell madeof Pyrex glass. The reaction cell was connected to thegas-closed circulation system, and then the suspension
was deaerated with stirring by a magnetic stirrer. Afterthe introduction of He gas, UV light was irradiated witha 100 W high-pressure mercury lamp, and the amountof O2 evolved was quantified by GC. After the reaction,
the amount of Ag deposition was quantified by ICP
measurement (Seiko Instruments Inc., SPS1700). The

catalyst surface after the photoreaction was examinedby means of SEM-EDX observation (Hitach ScienceSystems, Ltd., SEMEDX TypeN).

3. Results and discussion
3.1 Structural analysis

Table 1 summarizes the synthesis conditions and crystal
structures of double perovskites tested in this study. Thecalcination conditions were optimized to minimize
the amount of impurity phases for all the perovskite
oxides. For W- and Ti-containing double perovskites,
the lattice parameters and the cation arrangements of theordered double perovskites were consistent with thosein the previous reports10-13). Rietveld analyses confirmedthat these compounds had the rock-salt arrangement ofB-cations in the crystal structures.
In this study, we newly synthesized perovskite
oxides, LaBaB2+B’5+O6 (B2+ = Co, Ni, B’5+ = Ta, Nb) andLaBaZnNbO6. The characteristic diffraction lines fromcubic perovskite-type structure were observed in their
PXRD patterns (data are not shown). Concerning thecubic double perovskite, superlattice structure with aunit cell of 2ap × 2ap × 2ap, where ap is a lattice constantof primitive cubic ABO3 type perovskite, is formed dueto the alternate arrangement of B-cations. Therefore,
superlattice diffraction lines appear in the PXRD patterns
for these perovskites6). The superlattice diffraction lines
were clearly observed in the PXRD patterns of the Ta-

Photocatalytic Properties of Ordered Double Perovskite Oxides

and Nb-containing double perovskites, indicating that therock-salt arrangement of B-cations was also formed in
crystal structures of these samples.

3.2 Stability of ordered double perovskite underphotocatalytic conditions

It has been reported that ZnO suffers photocorrosion
when it is used in liquid-solid heterogeneousphotoreaction14), in marked contrast with TiO2 whichshowed high stability under the various photoreaction
conditions. Also, W-oxides easily decomposed to form
soluble tungstate species in alkali media. Therefore,
it is indispensable to investigate the catalyst stabilityof the perovskite oxides under the photoreaction
conditions. Table 2 shows the stabilities of the W-, Ta-,Nb- and Ti-containing ordered double perovskites inaqueous methanol solution and AgNO3-containing
aqueous solution under photoirradiation. After thephotoirradiation in the aqueous methanol solution,
A2+WO4 (A2+ = Ba, Sr,) and the other crystal phases weredetected by PXRD measurement for A2+2B2+WO6 (A2+ =Ba, Sr, B2+ = Cu, Zn), and only the diffraction lines fromperovskite phase were observed for A2+2B2+WO6 (A2+ =Ba, Sr, B2+ = Co, Ni), indicating that Co/W and Ni/Wsystems were stable and Cu/W and Zn/W systems wereunstable under the condition. For A2+2B2+WO6 (A2+ = Ba,Sr, B2+ = Co, Ni), however, diffraction lines from Ag2WO4(and Ag2O) in addition to perovskite phase were observed
after photoirradiation in the AgNO3-containing aqueoussolution. Thus, these perovskites were decomposed

Table 2

under the reaction conditions. These findings suggested
that W-containing perovskites were not suitable for thephotocatalytic reaction in aqueous media because of their
low stability.
As for the Ta-, Nb- and Ti-containing ordered double
perovskites, on the other hand, no differences in the crystal
structures were observed before and after photoirradiationin both of the aqueous solutions, indicating the highstability of the oxides under the conditions without thephotocorrosion of Ti and Zn oxides. The results suggested
the stability of ordered double perovskite depended
strongly on the nature of high valence B-cation.

3.3 Photocatalytic activity of La2ZnTiO6Catalytic test reaction was conducted with theperovskite oxides synthesized in this study, and ourstudy revealed that La2ZnTiO6 showed the activity for O2
evolution from Ag+ ion-containing aqueous solution. The
reaction mechanism on semiconductor photocatalysts,
such as TiO2 has been well studied and the reaction
pathway can be expressed in eqs.(1)-(3).

hvnull → e- .+ h+
Ag+ + e- → Ag0
2H2O + 4h+ → O2 + 2H+

…(1)
…(2)
…(3)

Here, e-, and h+ refer to excited electron and hole,respectively. The excited electron reduces Ag+ ion to Ag0
metal and hole oxidizes water to form O2.

Stability of ordered double perovskite under photoirradiation in methanol and AgNO3 aqueoussolutions a)
In a methanol aqueous solution

In an AgNO3 aqueous solution

CompoundStability a)

Stability a)

Ba2CoWO6

StableBa2CoWO6, Ag2WO4

Ba2NiWO6

StableBa2NiWO6, Ag2WO4, Ag2O

Ba2CuWO6

Ba2CuWO6, BaWO4-

Ba2ZnWO6

BaWO4, ZnO-

Sr2CoWO6

StableSr2CoWO6, Ag2WO4

Sr2NiWO6

StableSr2NiWO6, Ag2WO4, Ag2O

Sr2CuWO6

SrWO4, CuO-

Sr2ZnWO6

SrWO4, ZnO-

LaBaCoTaO6

StableStableLaBaNiTaO6

StableStable

LaBaCoNbO6-

StableLaBaNiNbO6-

Stable

LaBaZnNbO6-

StableLa2ZnTiO6

StableStablea) Stability was examined by PXRD patterns before and after photoirradiation in aqueous solutions of methanol and AgNO3.

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

Fig. 2 shows the time course for O2 evolution froma 0.01 M AgF aqueous solution over La2ZnTiO6. The
amount of O2 evolved increased with time up to 8 h,and then the O2 evolution stopped in dark, indicatingthat the O2 evolution photocatalytically proceeded overLa2ZnTiO6. The activity for O2 evolution over commonlyused TiO2 (P 25) was investigated and compared with thatof La2ZnTiO6 (Table 3). The total amount of O2 evolved
over La2ZnTiO6 was quite lower than that over TiO2 , but
the amount of O2 normalized by catalyst surface area was
one third of that of TiO2.
hνIntensity//a.a.u.
u.
Intensity

3737

3838

3939(b)
(b)
(a)
(a)

Dark
1010

83050
3050
2theta2theta // degree
degree7070Fig.
PXRD
patterns
La(a)beforeFig.
3 33PXRD
22ZnTiOFig.
PXRDpatterns
patternsofof
ofLa
La2ZnTiO
ZnTiO(a)before
beforeand6 66(a)and(b)
after
photoirradiation
in
a0.02
M
(b)
after
photoirradiation
in
a0.02
M
AgNO
and (b) after photoirradiation in a 0.02
M 3
aqueoussolution
AgNO
aqueoussolution.
3AgNO3 aqueous solution.

6100100 (b)
(b)

4(a)
(a)

2220012

3 4 5 6 7Reaction time / h8

119Fig.
course ofof OO22evolutionevolutionfrom
froma a0.01
0.01MM
Fig.2 2 Time course
AgF aqueousaqueous solution overAgF
overLaLa2ZnTiO
ZnTiO(0.5g).
6 6(0.52g).

1010 µmµm

Atomicconcentration
concentration/ /%%AtomicAmount of O2 evolved / μmol

10Ag
Ag

80
80
60
60
4040 La
La
202000PXRD studies confirmed that the reduction of Ag
to Ag0 proceeded on La2ZnTiO6 catalyst during thephotoirradiation in Ag+-containing aqueous solution.
Fig. 3 shows the PXRD patterns of the catalyst used forthe reaction. A characteristic XRD peak for Ag0 wasobserved at 2θ = 38.2 degree. The SEM-EDX observation
further provided the evidence for formation of Ag0
metal species on the catalyst surface. As shown in Fig.
4, deposition of rod-shape Ag metal over La2ZnTiO6 wasobserved, which further supported that the Ag reduction(eq.(2)) proceeded on the catalyst surface.
Table 3 also summarizes the results for thequantification of Ag metal deposited on La2ZnTiO6
after the reaction. According to the stoichiometry ineqs.(1)-(3), the theoretical molar ratio of Ag to O2 is4.0. For La2ZnTiO6, the ratio of Ag/O2 showed 5.7 andwas deviated from the theoretical value. As for the+La
La
AgAg
TiZnZnTi

Ti
Ti
Zn
Zn

AgAg

1122
Analysed
Analysed area
area

Fig.
(a) andresults oftheelemental
Fig.4 44 SEM
SEM image
image
thetheelemental
Fig.
SEM
image (a)(a)andandresults
resultsof ofelemental
analysis(b)ofLa
ZnTiO
after
266
analysis(b) of(b)La2ZnTiOof 6La
after
2ZnTiOafter
photoirradiation
analysis
photoirradiation
ina0.02MAgNOa 0.02
M AgNO33solution.
inphotoirradiation
a 0.02 M AgNO3inaqueousaqueousaqueous solution.
solution.
photocatalytic O2 evolution over TiO2 in an AgF aqueous
solution, Nishimoto et al. has reported that the ratioof Ag/O2 was larger than 4.0 due to the adsorption ofevolved O2 on the photocatalyst15). If one considers thatO2 adsorbs on the surface B-cations (Zn2+, Ti4+) in theratio of 1:1 for La2ZnTiO6, the amount of O2 adsorbedon the catalyst is estimated to be 2.4 μmol assuming 101̄
crystal plane. When this value is taken into account, theratio of Ag/O2 is 4.5 and became closer to the theoreticalvalue of 4.0.
Fig. 5 shows the DR UV-VIS spectra of La2ZnTiO6and TiO2 P25. The absorption edge for La2ZnTiO6 wasTable 3 Photocatalytic activity of La2ZnTiO6 and TiO2 for O2 evolution from a 0.01 M AgF aqueoussolution
SampleSpecific surface

O2 evolution a)Amount of Ag

Ratio ofarea / m2 g-1µmol b)µmol (m2-cat)-1

deposited / µmol c)

Ag/O2La2ZnTiO60.8

8.9

22.3

51

5.7

TiO255.31153.869.5--a) 0.5 g of La2ZnTiO6 and 0.3 g of TiO2 were used for the reaction, b) amount of gas evolved after 8 h photoirradiation,c) quantified by ICP measurement.

Photocatalytic Properties of Ordered Double Perovskite Oxides

observed at shorter wavelength than that for TiO2. Opticalband gap of La2ZnTiO6 was estimated from the spectraby extrapolating the linier portion to x axis, and the gap
was 3.9 eV. This value was much larger than that of TiO23.3 eV estimated from the spectrum. TiO2 (P 25) is wellknown as the mixture of anatase and rutile phases in theratio of 80% and 20%, respectively. Therefore, the opticalproperty of TiO2 (P 25) is governed by that of the major
phase, and the gap is close to that of anatase phase (3.2eV)2). DR UV-VIS spectrum of La2ZnTiO6 suggested theabsence of unreacted TiO2, because it did not absorb light
longer than about 350 nm. This excludes the possibility
that impurity TiO2 acted as the photocatalyst for thereaction and ensures the occurrence of photocatalytic O2evolution over La2ZnTiO6. La2ZnTiO6 showed almost noactivity for H2 generation in aqueous methanol solution.
This can be ascribed to the lack of H 2 generation sites,such as Pt, Rh, and RuOx on the surface of La2ZnTiO6
perovskite.

conditions, photoirradiation in aqueous methanol
solution or in Ag+ ion-containing aqueous solution.
On the other hand, Ta-, Nb- and Ti-containing ordered
double perovskites were stable under the same conditions,indicating that the stability of ordered double perovskite
depended mostly on the nature of high valence B-cation.
Among the perovskite oxides tested in this study,
La2ZnTiO6 showed the activity for O2 evolution and Ag
reduction. The reduction of Ag+ ion to Ag0 on the surface
was confirmed by PXRD and SEM-EDX studies. The
activity of La2ZnTiO6 for Ag reduction normalized by
the catalyst surface area was one third of that of TiO2.
Acknowledgements: This work was partly supported
by Grant-in-Aid for Scientific Research on Priority Areas(No. 19028050 and 20037053, “Chemistry of Concerto
Catalysis”) from Ministry of Education, Culture, Sports,
Science and Technology, Japan and a grant from theGlobal-Centre of Excellence in Novel Carbon ResourceSciences, Kyushu University.

La2ZnTiO6
La2ZnTiO6References

K-M function
250300350400450
Wave length / nm

500

Fig.
Absorption spectra
spectra ofof La
La22ZnTiO
ZnTiO66andand TiO
TiO22..
Fig.
5 5Absorption

4. Conclusions

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1)
2)
3)

TiO2
TiO2

To investigate the applicability of the W-, Ta-, Nb- andTi-containing ordered double perovskite to photocatalytic
reactions, 14 kinds of ordered double perovskites
were systematically synthesized and characterized
by PXRD measurements. The W-containing ordered
double perovskites decomposed under the photoreaction