Full text of DOI:10.1557/JMR.2002.0324

Synthesis and photophysical properties of barium
indium oxides
J. Yin
Ecomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba,
Ibaraki 305-0047, Japan
Z. Zou
Photoreaction Control Research Center (PCRC), National Institute of Advanced Industrial Science
and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
J. Ye
Ecomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba,
Ibaraki 305-0047, Japan
(Received 27 February 2002; accepted 24 June 2002)
Barium indium oxides (BaIn2O4, Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8) were
synthesized by the citric process and characterized by powder x-ray diffraction. The
optical absorption properties of these compounds were investigated by UV–visible
diffuse reflectance spectroscopy. It was found that with the increase of the mole ratio
of In2O3 in the formula the optical absorption edges of these oxides shift to the longer
wavelength side monotonically. The photocatalytic H2 and O2 evolutions under visible
light irradiation (␭ > 420 nm) from aqueous CH3OH/H2O and AgNO3/H2O solutions
were performed. Among these oxides, BaIn2O4 was the most stable compound, and
other compounds were not stable chemically in the case of water and visible light
irradiation.
Much research has been performed on the BaO–In2O3
oxide system including synthesis, structure, defects,
and oxide ion conductivity.1–5 Among the system the
well-studied compounds include BaIn2O4, Ba4In6O13,
Ba2In2O5, Ba3In2O6, Ba5In2O8, etc. BaIn2O4 is crystal-
lized in monoclinic symmetry (space group C2h5 − P21/a,
a ס 1.4432 nm, b ס 0.5833 nm, c ס 2.0792 nm,
␤ ס 110.02°), and other compounds appear to be oxygen
deficient “multipled” perovskites.6,7 Kwestroo et al.3 re-
ported that the compounds BaIn2O4, Ba4In6O13, and
Ba2In2O5 are stable in moist air, Ba3In2O6 and Ba5In2O8
are sensitive to aqueous vapor, and Ba3In2O6 was con-
verted into hydrogarnet Ba3In2(OH)12 by reaction with
water. Recently Hashimoto et al. reported that Ba2In2O5
was subjected to structural transformation after incorpo-
ration of water vapor.8 Here we report the photophysical
properties and photocatalytic properties of the com-
pounds in the BaO–In2O3 system.
Polycrystalline barium indium oxide powders were
prepared by the citric process. The starting materials in-
cluding BaCO3, In(NO3)3 и 3H2O, and citric acid
[HOOCCH2C(OH)(COOH)CH2COOH] in chemical
stoichiometry were dissolved in a beaker with dilute ni-
tric acid solution. The mixed solution was heated on a hot
plate at a temperature about 100 °C and stirred by a
magnetic stirrer with a rotation rate 700 rpm. After the
water in the solution was evaporated completely, the ni-
tric group began decomposing, the solution became
denser, and then the magnetic stirrer was taken out. The
process in the beaker went on. When the mixed product
began self-igniting, the heating was stopped. The beaker
containing the residue was kept in a cabinet at 80 °C for
12 h. The dried residue was heated at 450 °C in a furnace
for 2 h; thus, the porous gray noncrystal powder was
obtained, in which some fine In2O3 particles with light
yellow color appeared. Finally the gray powders were
calcined at 1000 °C for 2 h. All of the compounds as
prepared show a light yellow color. The compound struc-
tures were checked by x-ray diffraction (XRD), (JEOL
JDX-3500, Tokyo, Japan). The UV–visible (VIS) diffuse
reflectance spectra were measured by using a UV–VIS
spectrometer (Shimadsu UV-2500PC, Tokyo, Japan).
Photocatalytic reactions were performed using a
closed gas circulation system and an outside-irradiation
type Pyrex cell. A 300-W Xe arc lamp was focused
through a shutter window and a 420-nm cut-off filter was
placed on the surface of the cell. The gases evolved were
determined with TCD gas chromatograph (Shimadsu
GC-8A, Tokyo, Japan), which was connected with a cir-
culating line to the system. H2 evolution ac-
tion was performed in aqueous CH3OH/H2O solution
with cocatalyst Pt (0.5 g, 0.2 wt% Pt-loaded powder
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photocatalyst; 50 ml of CH3OH; 220 ml of pure water).
a crystalline semiconductor it was shown that near the
band edge the optical absorption has the following
behavior:10
O2 evolution was performed in aqueous AgNO3 solu-
tion (0.5 g of photocatalyst powder; 5 mmol of AgNO3;
270 ml of pure water). In our experiments 1.0 wt% NiOx
was also loaded on the surfaces of the photocatalyst pow-
ders from aqueous Ni(NO3)2 solution. The Ni-loaded
photocatalysts were calcined at 500 °C in air for 1 h and
then calcined at 500 °C for 2 h in a mixed atmosphere
of Ar and H2 (Ar:H2 ס 9:1 in volume). Finally the
H2-treated photocatalysts were calcined at 200 °C for 1 h
in pure O2 atmosphere to get the NiOx /Ni bilayer struc-
ture on the surface of the photocatalysts.9
Figure 1 shows XRD patterns for the compounds in
the BaO–In2O3 system. All of the compounds were
well crystallized with pure phases. The crystal struc-
tures and the lattice parameters for these compounds
are listed in Table I. The UV–VIS diffuse reflectance
spectra for these compounds are shown in Fig. 2. For
n 2
ր
hv − E ͒
͑
g
␣ = A
,
(1)
h␯
where ␣, ␯, Eg, and A are absorption coefficient, light
frequency, band gap, and constant, respectively, and n
depends on the characteristics of the transition. With this
equation Butler determined the band gap of semiconduc-
tor WO3 by the following steps:11 firstly, plotting
ln(␣h␯) versus ln(h␯ − Eg) using an approximate value of
Eg and then determining the value of n with the slope
of the most straight line near the band edge; secondly,
plotting (␣h␯)2/n versus h␯ and then determining the in-
tercept of the most straight line near the band edge with
h␯ axis. The intercept will be the value of the band gap
Eg. With this method the values of n for these oxides
were determined as 4 from Fig. 2. This means that the
optical transitions for these indium oxides are all indi-
rectly allowed. The values of the band gaps for BaIn2O4,
Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8 are deter-
mined as 2.73, 2.76, 2.79. 2.80, and 2.85 eV, respec-
tively. It is obvious that with the increase of the mole
ratio of In2O3 in the formula the optical absorption edge
of the compounds shifts to the longer side of the wave-
length monotonically, indicating that the band gap be-
comes narrower.
Odaka et al. studied the electronic structure of indium
oxide (In2O3) by first-principles calculation,12 where the
calculated partial density of states (PDOS) supported
the conclusion that the valence band of indium oxide is
composed of O 2p states and the conduction band is com-
posed of the In 5s state. The same energy band structure
for indium oxide was reported by Tanaga et al. by using
cluster calculation.13 For the similar indium oxide system
In2O3(ZnO)m (m ס 3) the energy band calculation
FIG. 1. X-ray powder diffraction spectra for the compounds of the
BaO–In2O3 system as synthesized by the citric process.
TABLE I. Physical properties and chemical stabilities of compounds in the BaO–In2O3 system.
Compound formula
BaIn2O4
Crystal structure
Monoclinic
Lattice parameters
Energy gap (eV)
2.73
Chemical stabilitya
Yes
a ס 1.4432 nm
b ס 0.5833 nm
c ס 2.0792 nm
␤ ס 110.02°
Ba4In6O13
Ba2In2O5
Monoclinic
a ס 1.5187 nm
b ס 2.4527 nm
c ס 0.41867 nm
␤ ס 114.66°
a ס 1.67191 nm
b ס 0.60833 nm
c ס 0.59563 nm
a ס 0.4191 nm
c ס 2.1669 nm
a ס 0.4174 nm
c ס 2.943 nm
2.76
2.79
No
(under visible
light irradiation)
Orthorhombic
No
Ba3In2O6
Ba5In2O8
Tetragonal
Tetragonal
2.80
2.85
No
No
aIn water.
2202
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within the local density functional theory using the aug-
lowered. The optical absorptions as shown in Fig. 2 are
directly in correspondence with the electron excitations
from O 2p states to In 5s states.
mented spherical wave (ASW) method implied that
the electronic features of the materials are mainly con-
trolled by the oxygen 2p states at the top of the valence
band and by s-like states arising from the metal at the
bottom of the conduction band.14 So it is suggested that
the valence bands and the conduction bands in the energy
band structures for the compounds in the BaO–In2O3
system are composed of O 2p states and In 5s states,
respectively. On the basis of the band gaps as deter-
mined, the band structures of BaIn2O4, Ba4In6O13,
Ba2In2O5, Ba3In2O6, and Ba5In2O8 are schematically
plotted in Fig. 3, where we supposed that BaIn2O4,
Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8 have the
same valence band potential. With the increase of
the mole ratio of In2O3 in the formula. The levels of the
conduction bands for these compounds were slightly
To evaluate the photocatalytic activity of these indium
oxides sacrificial reagents CH3OH and AgNO3 were
employed in our experiments. The formation rates of
H2 evolution from CH3OH/H2O with cocatalyst Pt
(0.2 wt%) and O2 evolution from AgNO3 solution
(5 mmol) without any cocatalyst under visible light irra-
diation (␭ > 420 nm) for BaIn2O4 are shown in Fig. 4.
The rates of H2 and O2 formation for BaIn2O4 in the first
20 h were about 4.29 and 0.45 ␮mol/h, respectively. Af-
ter H2 evolution reaction the structure of the residue was
checked again by XRD; no new phase was observed,
while the sample after O2 evolution reaction showed a
small amount of metallic Ag phase as well as main-phase
BaIn2O4. The occurrence of the Ag phase can be ascribed
to the reduction reaction of Ag+ ions induced by the
photogenerated electrons. It means that the H2 and O2
evolution reactions were not due to the chemical reac-
tions between BaIn2O4 and water.
It is well known that the conduction band of the semi-
conductor is separated from the valence band by a band
gap Eg. Electrons will be excited to the conduction
band and holes will be left in the valence band when
irradiated by the light with energy larger or equal to the
band gap. Chemically if the bottom level of the conduc-
tion band is more negative than the redox potential of
H+/H2 (0 V versus SHE, pH ס 0) and the top level of the
valence band is more positive than the redox potential of
O2/H2O (1.23 V versus SHE, pH ס 0), the photogener-
ated electrons and holes will move to the surface of the
photocatalysts and cause redox reactions. So it means
FIG. 2. UV–VIS diffuse reflectance spectra of the compounds in the
BaO–In2O3 system.
FIG. 4. Formation rates of H2 evolution from CH3OH/H2O solution
with cocatalyst Pt (catalyst, 0.5 g; cocatalyst, 0.2 wt% Pt; CH3OH,
50 ml; H2O, 220 ml) and O2 evolution from AgNO3 solution (catalyst,
0.5 g; AgNO3, 5 mmol; H2O, 270 ml) under visible light irradiation
(300-W Xe lamp; ␭ > 420 nm) for BaIn2O4 photocatalyst.
FIG. 3. Suggested energy levels of the compounds in the BaO–In2O3
system.
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that the band structure of BaIn2O4 meets the electro-
later. The structural transformations for the compounds
Ba2In2O5, Ba3In2O6, and Ba5In2O8 when immersed in
pure water may be ascribed to the following similar
chemical reactions, just as reported by Kwestroo et al.2
and Hashimoto:8
chemical requirements for splitting water. Figure 5
shows the formation rate of H2 evolution from NiOx-
loaded BaIn2O4 particles suspending in pure water under
visible light irradiation (␭ > 420 nm). The formation rate
of H2 evolution is about 2.7 ␮mol/h. In the “dark” ex-
periments, in which the stirring process was kept on
while Xe lamp was turned off, no gas evolution was
observed, indicating that the reaction is induced by the ab-
sorption of visible light and excluding the so-called mecha-
nocatalysis. More than 467 ␮mol of H2 was obtained in a
prolonged 530 h, and the sample remain unchanged. The
turnover number of the reacted electrons to the amount of
Ni loaded on the surface of the sample reached 14. This
means that the reaction occurs catalytically.
H2 evolution reactions from CH3OH/H2O solutions
under the irradiation of visible light were also performed
on Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8, re-
spectively. The H2 evolution rates in the first 20 h for
Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8 are 11.12,
2.60, 5.17, and 1.68 ␮mol/h, respectively. XRD experi-
ments performed after H2 evolution reactions showed
that the structures of the compounds Ba2In2O5,
Ba3In2O6, and Ba5In2O8 were all changed, but only a
small amount of new phase was observed for Ba4In6O13.
The “dark” experiments were performed on these com-
pounds, where stirring was kept on while the Xe lamp
was turned off. It was found that in “dark” experiments
Ba4In6O13 was not subjected to the structural transfor-
mation, but Ba2In2O5, Ba3In2O6, and Ba5In2O8 reacted
directly with water to form new phases. Whether
Ba4In6O13 or the new phase produced after the reaction
with water should be responsible for H2 evolution is not
very clear now, and the further work will be reported
Ba In O + 6H O → Ba In OH͒
,
(2)
͑
2
3
2
6
2
3
12
or
Ba In O + nH O → Ba In O и nH O 0 Ͻ n Ͻ 1͒
.
͑
2
2
5
2
2
2
5
2
(3)
In summary, the compounds of the BaO–In2O3 system
including BaIn2O4, BaIn6O13, Ba2In2O5, Ba3In2O6, and
Ba5In2O8 with pure phases were synthesized by the citric
process. Their structures, optical properties, and photo-
catalytic activities under visible light irradiation were
investigated. The widths of their band gaps have direct
correlation with the mole ratio of In2O3 in the formula.
BaIn2O4 shows a high potential to act as photocatalyst
for evolving H2 from water under visible light irradia-
tion, and other compounds were not stable during the
photocatalytic reactions.
ACKNOWLEDGMENT
One of the authors (Dr. J. Yin) thanks the Japan So-
ciety for the Promotion of Science (JSPS) fellowship for
financial support.
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FIG. 5. Formation rate of H2 evolution from NiOx-loaded BaIn2O4
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