J Mater Sci: Mater Electron(2023)34:281(0123456789().,-volV)(0123456789().,-volV)

Enhanced photocatalytic activities under visible lightof double-perovskite oxide semiconductor Ba2Tb(Bi,Sb)O6 with mixed-valence
Dayal Chandra Roy1, Michiaki Matsukawa1,* , Takanori Yonai1, Minami Arakida1,
Haruka Taniguchi1, Kazume Nishidate1, Sumio Aisawa1, Akiyuki Matsushita2, and
Lin Shiqi2
12Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
National Institute for Materials Science, Tsukuba 305-0047, Japan

Received: 24 August 2022

ABSTRACT

Accepted: 16 November 2022

In this study, we showed the effect of Sb substitution on lattice structures,
physical properties and photocatalytic activities for the double-perovskite oxides with mixed valence. High quality powders of Ba2 Tb(Bi1x Sbx )O6(0:1  x  1:0) were successfully synthesized using a citrate pyrolysis approach.
Through partial substitution of Sb at Bi site of this compound, it was possible todesign oxidative photocatalysts at lower x, and reduction-type ones at higher x.
These findings are discussed on the basis of the conduction and valence bandedge potentials of a series of Ba2 (Tb, Pr)(Bi, Sb)O6 photocatalysts. The Sb substituted sample with the x = 0.6 composition predicted from the machinelearning procedure exhibited the highest performance of methylene blue
degradation, which is comparable to typical degradation rates for good perovskite catalysts.

Ó The

Author(s),underexclusive licence to Springer
Science+Business Media, LLC,
part of Springer Nature 2023

1 Introduction
Pseudo-cubic perovskite oxides have been extensively studied in the field of solar light active photocatalysis for water splitting into H2 and O2
evolution as alternative materials for TiO2 [1–4]. In
this crystal structure of ABO3 , the larger and smaller
cations occupy the A and B sites, respectively. The
perovskite structures have large potential to designnovel functional materials based on the selective
occupancy of cations at the A and B sites. Although

Address correspondence to E-mail: matsukawa@iwate-u.ac.jp

https://doi.org/10.1007/s10854-022-09542-6

there are so many reports on the photocatalytic
properties for these candidate oxides, detailed studies on these compounds have been desired for higher
performances under visible light illumination.
Double-perovskite-based oxides with the generalformula AA0 BB0 O6 provide a wider possibility todesign various kinds of functional materials by tuning suitable cations at AA0 and/or BB0 sites. In particular, the double-perovskite-based compounds with
B-site substitution belonging to A2 BB0 O6 have been
investigated because of a variety of the physical281

Page 2 of 10

properties and potential applications [5]. Recently,
rare-earth double-perovskites Ba2 LnBiO6 (Ln: rareearth element) have been examined on the view pointof photocatalysis materials [6]. In particular, it isshown that the Ba2 PrBiO6 semiconductor exhibits
good performance, which is related to the mixed
valence state of rare-earth ion [6, 7]. Furthermore, themagnetic susceptibility data for the Ba2 PrBiO6 compound revealed that the average valences of Pr ion
reach the intermediate values between trivalent andtetravalent states [8].
For fabrication of visible light driven photocatalysts, it is one of the key factors to tailor the energy
band gap separating between the valence band (VB)
and conduction band (CB) of their semiconductors[9, 10]. Moreover, it is of importance to adjust a relative relation between the upper VB and lower CB
positions, and surface redox reactions in photocatalysis [11]. It is needed that the absolute positions of the
VB and CB edges are optimized to effectively facilitate photocatalytic oxidation and reduction processes. Our goal of higher photocatalytic activity isachieved by reducing the rapid recombination ofelectrons and holes and promoting the photo-excitedcharge separation, before the redox reaction at thesurface [9, 12]. In our previous research [13], it has
been proposed that the coexistence between Pr3þ andPr4þ ions contributes to the separation of photogenerated charge carries. Further, we extended photocatalytic researches to the Ba2 TbBiO6 system [14].
In this paper, we demonstrate crystal structures,
physical,andphoto-active
properties
for
Ba2 Tb(Bi1x Sbx )O6 , for our further understanding ofphotocatalytic activities of double-perovskite compounds with mixed valence states. First, the experimental setups are briefly illustrated. In the section of
results and discussion, we describe the structuralphase diagram, the mixed valence states, andthe energy band gaps for the Ba2 Tb(Bi1x Sbx )O6 .
system, on the basis of tolerance factors, and bandcalculations. For both the Pr- and Tb-based compounds, we evaluate the photocatalytic activities ofthe MB reductive degradation and the 2-propanoloxidative decomposition as a function of band gap
energy. Furthermore, these findings are well
explained in terms of CB and VB edge potentials of
the Ba2 (Tb, Pr)(Bi, Sb)O6 photocatalysts [15, 16]. For
comparison, the structural, physical and photocatalytic data for BaBiO3 prepared by the same approach

J Mater Sci: Mater Electron (2023)34:281

are given since its chemical formula Ba2 Bi3þ Bi5þ O6 istaken as a pristine compound of Ba2 Ln3þ Bi5þ O6[17, 18]. Finally, the concluding section is given.

2 ExperimentHigh quality powders of Ba2 Tb(Bi1x Sbx )O6 with x ¼
0:1; 0:1; 0:5; 0:6; and 1.0, and BaBiO3 were synthesizedusing the citrate pyrolysis approach [13, 14]. In thefirst step, at 70–80  C, stoichiometric mixtures of highpurity Ba(NO3 )2 , Tb4 O7 , Bi2 O3 , and Sb were dissolved in nitric acid solution. After adding citric acidand neutralizing it with aqueous ammonia, we
obtained the porous products through the self-ignition process using halogen-lamp stirrer. (Molar ratioof metal cations, citric acid, and nitric acid, Ba2 Tb(Bi,Sb)O6 :C6 H8 O7 :HNO3 = 1:6:10.) In the next step, theprecursors were ground and the resultant fine powders were annealed in air (or nitrogen gas) at 900–
1000  C for 48–96 h, in order to synthesize the double-perovskite phase. Powder X-ray diffraction data
for all the samples were collected with a diffractometer (Ultima IV, Rigaku). The lattice parameters
were calculated from the X-ray diffraction data using
the least-squares fits [19], as listed in Table 1. The
surface area of powder samples was obtained by BET
measurement on N2 absorption at 77 K (BELSORPmini II , Microtrac). The polycrystalline films for thescanning electron microscope (SEM) measurement
were fabricated from the Ba2 Tb(Bi1x Sbx )O6 powders
using an electrophoretic deposition technique [20].
Typical SEM images for Ba2 Tb(Bi1x Sbx )O6(x = 0.5) powders will be given in Fig. 2.
For our estimation of energy band gaps, opticalmeasurements were conducted using a diffuse
reflectance spectroscopy (V550, JASCO Co.). We
applied the Kubelka–Munk (KM) conversion to the
diffuse reflection spectrum and obtained the opticalband gap energies by the extrapolation [6, 13, 17].
Moreover, electronic band structures for the doubleperovskite structure compound Ba2 Tb(Bi1x Sbx )O6
were calculated using the density functional theory
as implemented in the ‘Quantum ESPRESSO’ [21, 22].
We utilized the Perdew–Burke–Ernzerhof (PBE)
functional [23] for the exchange-correlation effect.
Pseudopotentials of ‘pslibrary’ [24] were adopted torepresent the core electronic states. The cut off energyof 40 Ry and the 4  4  4 Monkhorst–Pack grid for
the K-point sampling were applied.

J Mater Sci: Mater Electron (2023)34:281

Page 3 of 10

281

Table 1 Structural and physical properties, and photocatalytic performance for Ba2 Tb(Bi1x , Sbx ) O6 .
Sb x

Crystal symmetrya (Å)b (Å)c(Å)

leff (lB )Eg (eV)

CO2 (ppm/g)

MB (%)

0.0
0.1
0.5
0.6
1.0
BaBiO3

Monoclinic
Monoclinic
Cubic
Cubic
Cubic
Monoclinic

6.1223a6.10698.4814a8.46118.4506
6.19106.09036.0809–––6.14078.61108.5916–––8.6780

8.91a
8.89
8.86a
9.13
–
–

0.92a
0.94
2.45a
2.59
–
0.83b
2.10c

221
204
32
–
–
–

9.7a
–
80.7a
85.8
81.3
10.7

Crystal symmetry, lattice parameters, effective magnetic moment, energy gap, gas evolution of CO2 , and MB degradation are summarized
for x ¼ 0:0; 0:1; 0:5; 0:6 and 1.0. The CO2 and MB data for photocatalytic activities were collected at an irradiation time of 90 min and
60 min, respectively. For comparison, the data for BaBiO3 prepared by the citrate method are given, which will be also shown in Figs. S1and S2aReference [14]bIndirectc

DirectFor our understanding of the valence state of Tbion, we estimated the values of effective magnetic
moment (leff ) through applying the Curie–Weiss law
to the magnetic susceptibility data. Here, the temperature variation of dc magnetization was measured
under the magnetic field cooling process of 0.1 T
using a superconducting quantum interference
device magnetometer (MPMS, Quantum Design).
Finally, we demonstrated the methylene blue (MB)degradation as a function of the Sb content, to evaluate photocatalytic reduction performance of theBa2 (Tb, Pr)(Bi1x Sbx )O6 fine powders. A 300 W Xe
lamp system including UV and IR filters was utilized
as the light source (Cermax LX300F, Excelitas Technologies). In reducing environment, it is a common
understanding that aqueous MB is decolorized tocolorless leuco-MB, LMB [25, 26], and it has been
widely used for the model pollution. The MB aqueous solution of 50 mL (10 ppm) and Ba2 (Tb, Pr)(Bi,Sb)O6 powders of 0.2 g were prepared . Before visible
light irradiation, the suspension of the photocatalyst
powders was stirred in the dark for 30 min. We
measured the temporal bleaching of MB with the
UV–Vis spectrometer (V730, JASCO Co.). The MB
solution of 3 mL was transferred into the test cell forchemical analysis at the regular time interval. For
comparison, we examined the photo-induced
decomposition of 2-propanol (IPA) using a gas
chromatography system (GC-2014, Shimadzu Co.).
After photocatalytic oxidation of the IPA gas, theamount of resultant CO2 was recorded as a functionof time, as described in [13, 27].

3 Results and discussion
Figure 1 shows the X-ray diffraction data for the Sb
substituted Ba2 Tb(Bi1x Sbx )O6 compounds. Typical
SEM image of the cubic Ba2 Tb(Bi0:5 Sb0:5 )O6 is shownin Fig. 2. The polycrystalline grains with sub-micron
order were uniformly distributed on the surface ofelectrophoretical deposition film. A SEM photographfor the Sb-free sample has been presented in [14].
Furthermore, the morphologies for the Ba2 PrBiO6(x ¼ 0) and the Sb substituted Ba2 Pr(Bi1x Sbx )O6(x ¼ 0:1 and 1.0) have been examined in Fig. 2a in[13] and Fig. S2a and b in [13], respectively. Theseexperimental data revealed that the Sb substitution at
Bi sites promotes the refinement of polycrystalline
grains. This fact seems to be close to good performance of photocatalysis as well as other factors, as
will be discussed later.
The lower Sb substituted sample was well fitted on
the basis of monoclinic structure model as the parent
sample [14]. The samples with higher Sb contents(0:5  x  1:0) crystallized in a cubic phase. The lattice
parameters estimated for the end-member x ¼ 1:0
sample were in good agreement with previous data[8, 28]. In Fig. 3, the structural parameters for all thesamples are plotted as a function of Sb content, which
are also listed in Table 1.

281

Page 4 of 10

J Mater Sci: Mater Electron (2023)34:281

For our understanding of the valence state of Tbion, we estimated the values of effective magnetic
moment (leff ) through applying the Curie–Weiss law
to the magnetic susceptibility data (not shown). For
example, we evaluated leff = 8.89 lB at the x ¼ 0:1
and 9.13 lB at x ¼ 0:6, as listed in Table 1. Using thesimilar equation as described in [13, 29], the ratio of
Tb3þ to Tb4þ ions is ranging from 0.51:0.49 at x ¼ 0 to
0.64:0.36 at x ¼ 0:6. The magnetization measurements
on both the present Tb and previous Pr-based samples revealed that their mixed valence states are acommon property over the whole range of the Sb
content [13]. In addition, it is another evidence that
X-ray photo-emission spectroscope study on the Prbased sample has revealed a dominant peak of thetrivalent ion accompanied by a shoulder structure oftetravalent ion [30].
Next, we discuss the stability of the crystallographic phase diagram in Fig. 3 using the tolerance
factor considering the mixed valence state of the Tb
ions. The tolerance factor for Ba2 Tb(Bi1x Sbx )O6 isgiven in the form of the following equation,rBa þ rO
t ¼ pﬃﬃﬃr þr;
2 Tb 2 M þ rO

Fig. 1 Powder X-ray diffraction data (black dots) for the
monoclinic Ba2 Tb(Bi1x Sbx )O6 (x ¼ 0:0 and 0.1) and cubic
Ba2 Tb(Bi1x Sbx )O6 (0:5  x  1:0). X-ray diffraction curves (redlines) were computed using the corresponding lattice parameters,
which will be given in Table 1 (Color ﬁgure online)

Fig. 2 SEM image of the cubic Ba2 Tb(Bi0:5 Sb0:5 )O6

where rBa , rO , rTb , and rM¼ðBi;SbÞ are the ionic radii ofeach element (in details, refer to [31]). In general, it iswell-known that the tolerance factor represents a

Fig. 3 Lattice parameters (left hand side) and tolerance factor(right-hand side) as a function of Sb content for
pﬃﬃﬃ
pﬃﬃﬃ
Ba2 Tb(Bi1x Sbx )O6 . (c ¼ c= 2, a ¼ a= 2). The data points
for the x ¼ 0; 0:1; 0:5; and 0.6 samples show the tolerance factor
estimated from the ratio of Tb3þ to Tb4þ (see the text). For
comparison, a solid line denotes the tolerance factor for the mixed
valence state of Tb ion (Tb3þ :Tb4þ = 1:1)

J Mater Sci: Mater Electron (2023)34:281

lattice distortion of the ABO3 perovskite structure. As
for the trivalent and tetravalent ions of Tb, we evaluated the tolerance factors using the chemical for3þ 5þ
5þ
mula
such
as
Ba2þ
and
2 Tb Bi1x Sbx O6
2þ
4þ
4þ
4þ
Ba2 Tb M11x M2x O6 , respectively [14]. Here, we
5þ5þ4þ
set M14þ = Bi3þ= Sb3þ
0:5 Bi0:5 and M2
0:5 Sb0:5 . In Fig. 3,
the tolerance factor was shown for a mixed valence
3þ
4þ
5þ
5þ
4þ
4þ
state of Ba2þ
2 Tb0:5 Tb0:5 Bið1xÞ=2 Sbx=2 M1ð1xÞ=2 M2x=2 O6(0:0  x  1:0). The data points for the x ¼ 0; 0:1; 0:5;
and 0.6 samples are displayed as their tolerance factors considering the ratio of Tb3þ to Tb4þ from themagnetic moments as mentioned before. The tolerance factor for Ba2 Ln(Bi, Sb)O6 as a function of theionic radius of Ln (Ln: rare-earth element) has beenreported in [28]. The crystallographic phase diagram
for the double-perovskite rare-earth compounds isseparated into the monoclinic, rhombohedral, andcubic regions. The crystal structures obtained for the
lower and higher Sb substituted samples are in reasonable agreement with the phase diagram reported.
For the x ¼ 0:5 sample, the ratio of Tb3þ to Tb4þ listed
in Table 1 gives rise to the value of t = 0.973, which islocated just above the phase boundary between cubic
and rhombohedral structures [32]. This finding isrelated to the stability of the cubic phase for higher Sb
substituted samples [33].
Here, we give some comments on the factors
affecting B-site cation ordering. The charge difference
between the B and B0 cation oxidation states isdefined as DZB , whereas we represent the ionicradius difference between the two states as DrB . For
the compounds with the trivalent, intermediate, andtetravalent ions of Tb, we obtain DZB ¼ 2; 1; and 0,
respectively. In the case of DZB ¼ 2; the B-site cations
3þ 05þ
of A2þ
O6 perovskites tend to order with
2 B B
4þ 04þ
increasing DrB [5]. On the other hand, A2þO6
2 B B
perovskite compounds with DZB ¼ 0 are disorderedwhen DrB \0:17 Å. Accordingly, it is expected thatthe present mixed valence compounds exhibit apartially disordered phase.
Applying the KM conversion to the diffuse reflection spectrum, we obtained the typical optical spectraof the KM conversion for the x ¼ 0:1 and 0.6 samples(Fig. 4). Additionally, we evaluated the optical band
gap energies by the extrapolation [6, 13, 17], as shownin the inset of Fig. 4. For the direct and indirect
optical transitions, we utilized the absorption coefficient ðaKM ep Þ2 vs. ep plot and ðaKM ep Þ1=2 vs. ep plot,

Page 5 of 10281

respectively (in details, see [6, 13]) . Here, aKM , and ep
represent the absorption coefficient, and the photon
energy. For the Sb substituted samples, we estimated
Eg to be 0.94 eV at x ¼ 0:1 and 2.59 eV at x ¼ 0:6; aslisted in Table 1. The Sb substitution for Bi enlarges
the band gap energy as well as that in the case of the
Pr-based compound. Following our previous workon Ba2 Pr(Bi1x Sbx )O6 [34, 35], we examined theinfluence of the Sb substitution on band gap energies
on the basis of first-principle electronic structure
calculation. In Fig. 5, we exhibit our calculated results
for both the monoclinic Ba2 TbBiO6 and cubic
Ba2 TbSbO6 compounds. We adopted the experimentally determined lattice parameters as listed inTable 1. The atomic position parameters listed inTable 2 were used [8, 28]. Here, we utilized thedensity functional theory as implemented in the
Quantum ESPRESSO [21, 22], to study the structuraland electronic properties of the corresponding double-perovskite. The direct band gaps were evaluatedto be 1.90 eV and 3.77 eV for the parent and endmember samples. Our previous findings predict thatthe 6s orbitals of the Bi5þ ion occupy the bottom of
the CBs, while the top of the VBs is composed of the
6s orbitals of the Bi3þ ion. Accordingly, it is reasonable that Bi5þ is partially replaced by Sb5þ withincreasing Sb content, resulting in the band gap

Fig. 4 KM conversion to diffuse reﬂection spectrum for
Ba2 Tb(Bi1x , Sbx )O6 with x ¼ 0:1 and 0.6. Inset shows opticalband gap estimation. For x ¼ 0:6; ðaKM ep Þ2 vs. ep plot for the
direct band gap was used. For the indirect band gap, we applied
plots of ðaKM ep Þ1=2 vs. ep to the inset data for the x ¼ 0:1 sample.
The values of Eg were given by the extrapolation method

281

J Mater Sci: Mater Electron (2023)34:281

Page 6 of 10

Fig. 5 Energy band structures and total density of states near the
Fermi levels for a monoclinic Ba2 TbBiO6 and b cubic
Ba2 TbSbO6 . Band gap values are 1.90 eV and 3.77 eV for theformer and latter compounds, respectively
Table 2 Atomic position parameters
Ba2 TbBiO6 atom

(I2/m) site

x

y

z

Ba
Tb
Bi
O1
O2

4i
2c
2b
8j
4i

0.50360.5
00.242100.436100.5
00.5
0.25400
00.25
000:03340
0.25890

Ba2 TbSbO6 atom

 site(Fm3m)

x

y

z

Ba
Tb
Sb
O

8c
4b
4a
24e

0.25
0.5
0
0.237

0.25
0.5
0
0

0.25
0.5
0
0

opening. Furthermore, we found out that the partial
B-site disordering of the double-perovskite compound Ba2 Tb(Bi1x Sbx )O6 causes narrowing the bandgaps. For the present calculation, the B-site disordering effect has not been considered, which isprobably related to the overestimation of the band
gap for the parent sample. For the pristine perovskite
BaBiO3 , the optical measurement as shown in Fig. S2
revealed an indirect energy gap of 0.83 eV followed
by a direct one of 2.1 eV. This result is in good
agreement with the several theoretical predictions
based on the density functional approach [35, 36].
Furthermore, we demonstrated the reductive
degradation of methylene blue under visible lightirradiation for the Ba2 (Tb, Pr)(Bi, Sb)O6 citratepyrolysis samples. Figure 6a shows the typicalabsorbance spectra for the MB degradation of
Ba2 TbBi0:4 Sb0:6 O6 . When the peak intensities aroundk ¼ 665 nm are responsible for the MB concentrations, the MB degradation rate (%) is expressed asðCð0Þ  CðtÞÞ=Cð0Þ  100. Here, C(0) and C(t) represent at the initial and final concentrations at different
time intervals, respectively. In Fig. 6b, c, the MB
degradation data for Ba2 Tb(Bi1x , Sbx ) O6 and
Ba2 Pr(Bi1x , Sbx ) O6 are plotted as a function of time
under visible light irradiation. The reductive degradation rates of MB to colorless LMB rapidly increased
with substituting Sb at the Bi site. For the Tb-based
samples with heavy Sb content, their higher performances were observed. For the Ba2 Pr(Bi1x , Sbx ) O6
system, the end-member sample Ba2 PrSbO6 exhibited
highest degradation among all the Pr-based samples.
Here, we comment on the reason for selecting thex ¼ 0:6 composition in the Tb-based compounds.
Recently, with the aid of materials informatics suchas machine learning, it is possible to design newmaterials with the desired properties under minimum experiments or simulations [37]. We applied
the machine learning program (Common Bayesian
Optimization library, COMBO.exe [38]) to the three
data sets (Sb content, MB degradation) obtained forthe both end-member and intermediate samples(x ¼ 0:0; 0:5 and 1.0). As a result of this procedure, thex ¼ 0:6 composition was recommended as next candidate material. In fact, the experimental data in
Fig. 6b strongly support the present prediction
though there are small differences in the MB degradation rate. The band gap for the x ¼ 0:6 sample
showed 2.59 eV as listed in Table 1, which is slightly
larger than the values for the x ¼ 0:5 and 1.0 samples.
Additionally, the Sb substitution at Bi sites promoted
the refinement of powder samples as shown in ourprevious study [13]. Around x ¼ 0:6; it is expectedJ Mater Sci: Mater Electron (2023)34:281

(a)

(c)

Page 7 of 10

281

(b)

(d)

Fig. 6 a Typical absorbance spectra of MB degradation undervisible light illumination for Ba2 TbBi0:4 Sb0:6 O6 . MB degradation
vs. irradiation time for b Ba2 Tb(Bi1x Sbx )O6 and c Ba2 Pr(Bi1x
Sbx )O6 . The solid line denotes the MB only decomposition data. d

Visible light driven CO2 evolution vs. irradiation time for the
Ba2 Tb(Bi1x Sbx )O6 powder samples. The data for the x ¼ 0 and
0.5 samples prepared from the different batches were collectedthat the photocatalytic active sites on the powder
surfaces are optimized against the number of MB
molecules. These factors contribute to the highest
performance for MB degradation. Furthermore, inTable 3, the MB degradation rates for typical perovskite compounds such as manganite, titanate, ferrite and niobium perovskites were cited. Our present
data seem to be comparable to other similar studies.
For the pristine perovskite BaBiO3 with Eg ¼ 0:83 eV,
we obtained low MB degradation rate similar to thatof the Ba2 TbBiO6 . Compared with the previous
studies on BaBiO3 [17, 18], we note that the present
sample is isostructural with the previous one provided by Tang. However, the latter sample with arelatively larger gap of 2.05 eV exhibited high photocatalytic performance of MB degradation as listed

in Table 3. The discrepancies in the energy band gap
and its associated photo-induced activity are probably related to partial B-site disordering. Additionally,
the photo-induced reaction on the surface of powders
strongly depends on sample synthesis approaches[18].
In addition to the MB degradation, we examined
the photocatalytic oxidation activity of the powder
samples using gaseous 2-propanol (IPA). Figure 6dshows the visible light driven CO2 evolution vs.
irradiation time for the Ba2 Tb(Bi1x , Sbx ) O6 powder
samples. The CO2 concentration for the samples withlight Sb content showed a rapid enhancement at theinitial time and then remained a steady rise at further
irradiation time. This stable behavior is responsible
for the CO2 absorption process on the surface of the

281

J Mater Sci: Mater Electron (2023)34:281

Page 8 of 10

Table 3 Comparison withdifferent perovskite
photocatalysts for MB
degradation rate

Catalyst

Initial MB (mg/L)

Mass (mg)Time (min)Degradation rate (%)

Ba2 Tb(Bi, Sb)O6
BaBiO3 [17]Nd0:7 Ca0:3 MnO3 [39]Fe-doped CaTiO3 [40]Y-doped BiFeO3 [41]NaNbO3 [42]

10
15.3
10
10
50
10

200
300
40
100
100
160

90
60
180
180
120
180

100
100
96
100
97.6
99.3

powders, leading to a reduction of the photo-induced
CO2 gas concentration. For the x ¼ 0:5 sample, we
detected no remarkable quantity of CO2 .
Our photocatalytic data for both the MB degradation and IPA decomposition are scaled with the band
gap energy in Fig. 7a. The data for CO2 evolution and
MB decomposition rate were collected at a lightirradiation time of 90 min. The unit for CO2 concentration was transformed by the BET surface area ofthe samples. For both the Pr- and Tb-based compounds, we conclude that the photocatalytic reduction process of MB bleaching is in contrast to theoxidative one of IPA decomposition with respect tothe magnitudes of band gap energy.
In Fig. 7b, we illustrate schematic plot of the bandedge potential with respect to normal hydrogenelectrode (NHE) for the Ba2 (Tb, Pr)(Bi, Sb)O6 compounds. For the Sb-free compounds, their VB edge
potentials are located to facilitate oxidation reaction.
However, the lower CB level is not suitable for promoting reduction reaction. On the other hand, thehigherCB
edge
potentials
forboth
Ba2 Tb(Bi0:4 Sb0:6 )O6 and Ba2 PrSb O6 are suitable forpromoting effective reduction process. Here, the CBedge potential (ECB ) of the Ba2 (Tb, Pr)(Bi, Sb)O6
semiconductor is estimated using the following
empirical equation [15, 16],
ECB ¼ v  Ee  0:5Eg ;where v and Ee are the electronegativity of thesemiconductor and the energy of free electrons on thehydrogen scale (  4:5 eV), respectively. For example,
the electronegativity of the present Ba2 Tb(Bi1x , Sbx )O6 semiconductor is calculated by the geometricaverage of the electronegativities of its constituent
atoms in the following form [16, 17],v ¼ ½v2 ðBaÞvðTbÞv1x ðBiÞvx ðSbÞv6 ðOÞ1=10 ;where the atomic electronegativity on the right-handside is given by the arithmetic average of the atomic
electron affinity and the first ionization energy [15].

(a)

(b)

Fig. 7 a Photocatalytic activity vs. band gap energy. The
published data for Ba2 Pr(Bi1x Sbx )O6 are referred from ourprevious work [13, 14]. b Schematic illustration of the band edge
potential with respect to normal hydrogen electrode (NHE) forBa2 (Tb, Pr)(Bi, Sb)O6 and BaBiO3 compounds

According to this expression, we evaluated the CBedge potential ECB to be 0.34 eV and 0:45 eV for theparent and x ¼ 0:6 substituted samples, respectively.
In a similar way, for the Pr-based photocatalytic endmember compounds, the values of ECB were estimated to be 0.23 eV at x ¼ 0:0 and 0:51 eV at
x ¼ 1:0. For the parent compounds with Eg  1 eV, theoxidation reactions concerning the IPA decomposition process are in progress through the photo-excited holes near the VB edge. The heavily Sb

J Mater Sci: Mater Electron (2023)34:281

substituted compounds with Eg  2:5 eV exhibit thestrong degradation of MB and their CB edges are
optimized for the photocatalytic reductive reactions.
Finally, we give some comments on the mixed
valence states. As mentioned in Introduction, themixed valence states between trivalent and tetravalent rare-earth ions are related to the improvement inphotogenerated charge separation. In addition, theionic radius differences between trivalent andtetravalent ones give rise to the B-site partial disordering and then suppress the band gap energies ofthe ordered double-perovskite compounds. Moreover, for the highly Sb substituted samples, theirmixed valence states contribute to the stability of the
cubic phase through the higher tolerance factors as
discussed above.

4 Conclusion
We investigated the influence of Sb substitution at B
sites on the lattice structures, the magnetic valence
states, the band gap energies, and photocatalyst
performance for the double-perovskite-based semiconductors.
High
qualitypowdersof
Ba2 Tb(Bi1x Sbx )O6 (0:1  x  1:0) were successfully
synthesized using the citrate pyrolysis approach. The
single-phase powder samples with the light andheavy Sb contents crystallized in the monoclinic andcubic structures, respectively. These findings are in
reasonable agreement with the structural phase diagram previously reported, through the tolerance
factor considering the mixed valence states. We
examined the band gap widening associated with thepartial substitution of Sb through performing theoptical reflectance measurement and first-principle
electronic structure calculation.
Through the partial substitution of Sb at Bi site ofthis compound, it was possible to design oxidative
photocatalysts at lower x, and reduction-type ones athigher x. The photocatalytic properties were well
explained in terms of CB and VB edges of the Ba2 (Tb,
Pr)(Bi, Sb)O6 photocatalysts. For the x ¼ 0:6 sample,
we experimentally observed the highest performanceof MB degradation, which was derived from themachine learning prediction. These data seem to becomparable to typical degradation rates for good
perovskite catalysts. For comparison, the structural,
physical and photocatalytic data for BaBiO3 synthesized by the same approach were shown since its

Page 9 of 10281

chemical formula Ba2 Bi3þ Bi5þ O6 was considered as apristine compound of double-perovskite oxide
semiconductor.

Acknowledgements
The SEM measurement was performed with the helpof Mr. K. Sasaki and Mr. M. Ito. The authors thankthe Super-Computing and Information Sciences
Center of Iwate University, the Cyberscience Centerof Tohoku University, and the Supercomputer Centerof the Institute for Solid State Physics of University ofTokyo, for providing the computational resources.

Author contributions
DCR: Material preparation, Data collection andanalysis, Writing—Original draft preparation. MM:
Writing—Reviewing and Editing, Supervision, Project administration, Funding acquisition. TY: Material
preparation, Data collection. MA: Material preparation, Data collection. HT: Data analysis. KN: Data
analysis, Conceptualization, Funding acquisition. SA:
Methodology. AM: Conceptualization, Methodology.
LS: Data collection.

Funding
This work was partially supported by MEXT Grandsin-Aid for Scientific Research (JPSJ KAKENHI Grant
Nos. JP19K04995 and JP21K05140).

Data availability
Data will be made available on request.

Declarations
Conflict of interest On behalf of all authors, thecorresponding author states that there is no conflict ofinterest.
Supplementary Information: The online version ofthis article (https://doi.org/10.1007/s10854-022-095
42-6) contains supplementary material, which isavailable to authorized users.

281

Page 10 of 10

References
J. Shi, L. Guo, Prog. Nat. Sci. Mater. Int. 22, 592–615 (2012)
P. Kanhere, Z. Chen, Molecules 19, 19995–20022 (2014)
E. Grabowska, Appl. Catal. B 186, 97–126 (2016)
A. Fujishima, K. Honda, Nature 238, 37–38 (1972)
S. Vasala, M. Karppinen, Prog. Solid State Chem. 43, 1–31
(2015)
6. T. Hatakeyama, S. Takeda, F. Ishikawa, A. Ohmura, A.
Nakayama, Y. Yamada, A. Matsushita, J. Yea, J. Ceram. Soc.
Jpn 118, 91–95 (2010)
7. A. Matsushita, T. Nakane, T. Naka, H. Isago, Y. Yamada, Jpn.
J. Appl. Phys. 51, 121802-1-5 (2012)
8. W.T.A. Harrison, K.P. Reis, A.J. Jacobson, L.F. Schneemeyer,
J.V. Waszczak, Chem. Mater. 7, 2161–2167 (1995)
9. X. Chen, S. Shen, L. Guo, S. Mao, Chem. Rev. 110,
6503–6570 (2010)
10. D. Sudha, P. Sivakumar, Chem. Eng. Process. 97, 112–133(2015)
11. H.W. Eng, P.W. Barnes, B.M. Auer, P.M. Woodward, J. Solid
State Chem. 175, 94–109 (2003)
12. B. Ohtani, Catalysts 3, 942–953 (2013)
13. A. Sato, M. Matsukawa, H. Taniguchi, S. Tsuji, K. Nishidate,
S. Aisawa, A. Matsushita, K. Zhang, Solid State Sci. 107,10635-1-7 (2020)
14. T. Senzaki, M. Matsukawa,T. Yonai, H. Taniguchi, A. Matsushita,T. Sasaki, M. Hagiwara, Chap. 9. In: Functional
Materials Synthesis and Physical Properties, ed by M. Bartoli (IntechOpen, 2022). https://doi.org/10.5772/intechopen.
100241.
15. A.H. Nethercot, Phys. Rev. Lett. 33, 1088–1091 (1974)
16. M.A. Butter, D.S. Ginley, J. Electrochem. Soc. 125, 228–232(1978)
17. J.W. Tang, Z.G. Zou, J.H. Ye, J. Phys. Chem. C 111,
12779–12785 (2007)
18. B. Men, J. Zhang, C. Diao, X. Liu, H. Zheng, J. Mater. Sci.
Mater. Electron. 111, 12779–12785 (2018)19. F. Izumi, K. Momma, Solid State Phenom. 130, 15–20 (2007)
20. M. Kawachi, N. Sato, E. Suzuki, S. Ogawa, K. Noto, M.
Yoshizawa, Physica C 357–360, 1023–1026 (2001)
21. P. Giannozzi et al., J. Phys. Condens. Matter 21, 395502(2009)
22. P. Giannozzi et al., J. Phys. Condens. Matter 29, 465901(2017)
23. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77,
3865–3868 (1996)
24. A. Dal Corso, Comput. Mater. Sci. 95, 337–350 (2014)
25. W.S. Wang, N. Xie, L. He, Y.D. Yin, Nat. Commun. 5,5459-1-7 (2014)
1.
2.
3.
4.
5.

J Mater Sci: Mater Electron (2023)34:281

26. Y.N. Liu, X. Zhou, X. Wang, K. Liang, X.K. Yang, C.C.
Shen, M. Imran, S. Sahar, A.W. Xu, RSC Adv. 7,30080–30085 (2017)
27. T. Murase, H. Irie, K. Hashimoto, J. Phys. Chem. B 108,
15803–15807 (2004)
28. S. Otsuka, Y. Hinatsu, J. Solid State Chem. 227, 132–141 (2015)
29. We estimated the ratio (y) by using l2eff ¼ yl2eff ðTb3þ Þ þ ð1 yÞl2eff ðTb4þ Þ with leff (Tb3þ )=9.72 lB and leff (Tb4þ )=7.94 lB
30. K. Onodera, T. Kogawa, M. Matsukawa, H. Taniguchi, K.
Nishidate, A. Matsushita, M. Shimoda, J. Phys. Conf. Ser.
969, 012122-1-6 (2018)
31. R. Shannon, Acta Crystallogr. A 32, 751 (1976)
32. W.T. Fu, D.J.W. Ijdo, J. Solid State Chem. 178, 2363–2367(2005)
33. P.J. Saines, B.J. Kennedy, M.M. Elcombe, H.H. Harris, L.Y.
Jang, Z. Zhang, J. Solid State Chem. 181, 2941–2952 (2008)
34. H. Taniguchi, M. Matsukawa, K. Onodera, K. Nishidate, A.
Matsushita, IEEE Trans. Magn. 55, 2400404-1-4 (2019)35. K. Nishidate, A. Adiko, M. Matsukawa, H. Taniguchi, A.
Sato, A. Matsushita, S. Tanibayashi, M. Hasegawa, Mater.
Res. Express 7, 065505-1-8 (2020)
36. C. Franchini, A. Sanna, M. Marsman, G. Kresse, Phys. Rev.
B 81, 085213-1-7 (2010)
37. R. Tamura, T. Osadab, K. Minagawa, T. Kohata, M. Hirosawa, K. Tsuda, K. Kawagishi, Mater. Des. 198, 109290-1-11(2021)
38. K. Terayama, K. Tsuda, R. Tamura, Jpn. J. Appl. Phys. 58,
098001-1-4 (2019)39. P. Amalthi, J. Judith Vijaya, L. John Kennedy, A. Mustafa, M.
Bououdina, J. Phys. Chem. Solids 169, 110860-1-17 (2022)
40. H. Yang, C. Han, X. Xue, J. Environ. Sci. 26, 1489–1495(2014)
41. N.S. Abdul Satar, R. Adnan, H.L. Lee, S.R. Hall, T.
Kobayashi, M.H. Mohamad Kassim, K.N.H. Mohd, Ceram.
Int. 45, 15964–15973 (2019)
42. L. Wang, H. Gu, J. He, T. Zhao, X. Zhang, C. Xiao, H. Liu,
X. Zhang, Y. Li, J. Alloys Compd. 695, 599–606 (2017)
Publisher’s Note Springer Nature remains neutral withregard to jurisdictional claims in published maps andinstitutional affiliations.
Springer Nature or its licensor (e.g. a society or otherpartner) holds exclusive rights to this article under apublishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript
version of this article is solely governed by the terms ofsuch publishing agreement and applicable law.