J. Phys. Chem.

C 2007, 111, 12779-12785 12779

Efficient Photocatalysis on BaBiO3 Driven by Visible Light

Junwang Tang,†,§ Zhigang Zou,‡ and Jinhua Ye*,†

National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and
Photoreaction Control Research Center (PCRC), National Institute of AdVanced Industrial Science and
Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
ReceiVed: May 1, 2007

A novel photocatalyst BaBiO3 with perovskite structure was prepared by a soft chemical method and
characterized by XRD, UV-visible diffuse reflectance spectroscopy, BET surface area measurement, and
field emission scanning electron microscopy. The material can absorb light with wavelength λ < 650 nm,
which almost covers the region from UV through all strong visible light in the sunlight and an indoor lamp’s
illumination. It was found that the oxide can efficiently decompose organic contaminants, such as acetaldehyde
and methylene blue dye, and yield high photocurrent density whenever under UV-light or visible-light
irradiation. The density of states and band edge of the material were theoretically calculated on the basis of
density functional theory and an atom’s Mulliken electronegativity, respectively. It is revealed that both the
valence band and conduction band of BaBiO3 contain a large portion of Bi 6s orbitals, which results in the
narrow band gap, highly mobile charge carriers, and low barrier for photoelectron excitation.

Introduction Very recently, a family of Bi-based oxides has been found

Photocatalysis using solar energy is highly expected to be to be very active under visible-light irradiation, which is
an ideal “green” technology for sustainable development of attributed to the hybridized valence band (VB) by O 2p and Bi
human beings, where an active photocatalytic material is 6s so as to narrow the band gap.10,12,13,16 In addition, the high
definitely an important key.1 To date, TiO2 is well-known as a activity of the materials is also ascribed to s composition in the
stable, low-cost, and highly efficient photocatalytic material.1-5 VB because the photogenerated charge carriers in s orbital have
But its application is limited at the ultraviolet light region a high mobility due to the dispersive characteristic of the s
(wavelength λ < 400 nm) because of its optical absorption orbital.13,18 The typical examples are BiVO4 for O2 evolution
characteristic. A photocatalytic material active under visible light from water with relevant sacrificial reagent12 and CaBi2O4 for
has long been expected from the viewpoint of efficient utilization some organics’ decomposition.13 In order to further narrow the
of solar irradiation. Recently, numerous attempts have been band gap and greatly increase the activity of a photocatalyst,
made to improve the inherently low efficiency of TiO2 in modification of the conduction band (CB) is expected here while
harvesting sunlight with shifting its spectral response into the keeping the benefit of the hybridized VB. Namely a material is
visible region, mainly by cation or anion doping.6-9 Typically, expected in which both the VB and CB are composed of the
Asahi et al. succeeded in improving its optical absorption from hybridized orbitals containing s composition. From our experi-
the UV region to the visible-light region (λ < 500 nm) by ences in synthesizing the visible-light-driven oxide photocata-
nitrogen doping.6 But its activity is not high enough for practical lysts,5,13 the BaBiO3 semiconductor was prepared herein by a
application, and there is concern for the stability of doped soft chemical method. The theoretical calculation shows that
anions.9,10 the material actually has the hybridized VB and CB containing
Another approach to realize visible-light photocatalysis is to s composition. The oxide can absorb light with wavelength λ
develop a new photocatalytic material independent of TiO2. < 650 nm, which almost covers the region from UV through
Oxide materials are plausible candidates of the new photocata- all strong visible light in the sunlight and an indoor lamp’s
lytic material in relation to their high chemical stability and illumination. To our knowledge, the material represents the
easy preparation compared with non-oxide materials.10-13 Kim narrowest band gap in oxide photocatalysts. Furthermore, the
et al.,10 Kudo et al.,12,14 and our group13,15-17 recently reported novel photochemical properties of the material are demonstrated
several undoped oxide photocatalysts responsive to visible light. by the photocatalytic decomposition of gaseous acetaldehyde
However, their activities for organic decomposition are not and aqueous methylene blue dye as well as the photocurrent
sufficient yet at wavelengths longer than 500 nm. Therefore, to measurement under visible-light irradiation. The possible mech-
develop an active photocatalytic material at a wide range of anism is also discussed based on the theoretical calculation and
visible light is still seriously challenging and more beneficial experimental results.
for practical applications of photocatalytic technology.
Experimental Section
* Corresponding author. E-mail: jinhua.ye@nims.go.jp.
† National Institute for Materials Science (NIMS). Material Preparation and Characterization. The BaBiO3
§ Present address: Department of Chemistry, Imperial College London,
powder photocatalyst was prepared by a soft chemical method.
London SW7 2AZ, U.K.
‡ National Institute of Advanced Industrial Science and Technology A suitable amount of Ba(NO3)2 and a small excess of Bi(NO3)3‚
(AIST). Present address: Department of Materials Science and Engineering, 5H2O were dissolved in water. Then a citric acid and EDTA
Nanjing University, P. R. China. solution in ammonia was added to the aqueous solution. The
10.1021/jp073344l CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/09/2007
12780 J. Phys. Chem. C, Vol. 111, No. 34, 2007 Tang et al.

composite solution was dried at 393 K for 10 h and calcined at

923 K for 5 h in air. The crystal structure of the sample was
determined by X-ray diffraction method (JEOL JDX-3500
Tokyo, Japan). The UV-visible diffuse reflectance spectrum
of the sample was measured with a Shimadzu UV-2500PC
double-beam spectrometer equipped with an integrating sphere
attachment, and BaSO4 was used as the reference material. The
data were transformed into absorbance with the Kubelka-Munk
function. The surface area of the material was measured by BET
measurement on nitrogen adsorption at 77 K (Micromeritics
automatic surface area analyzer Gemini 2360, Shimadzu). The
morphology of the samples was observed with a field emission
scanning electron microscope (FE-SEM) (JSM 6500, JEOL,
Japan) operated at 15 kV.
Photoelectrochemical Property. The photoelectrochemical
property was investigated using BaBiO3 film as an anode. The Figure 1. XRD pattern of prepared BaBiO3.
BaBiO3 film was prepared on a conductive glass FTO (fluorine
doped SnO2) by using a pulsed laser deposition (PLD) technique
in O2 flow. A pulsed laser Nd:YAG at 355 nm was employed,
and the repetition rate was 1 Hz. The O2 (purity > 99.9999%)
partial pressure in the chamber was kept at 2.0 × 10-1 Torr
during the deposition. The deposition time was 40 min. The
thickness of the film was about 1 µm observed by SEM.
Photocurrent density of the BaBiO3 film was measured in a
Pyrex cell with quartz windows using a conventional three-
electrode system containing platinum foil, the prepared film,
and a saturated calomel electrode (SCE) as the counter, working,
and reference electrodes, respectively. As a reference, the
photocurrent of TiO2 (single crystal (110), available com-
mercially) was also measured under visible-light irradiation. The Figure 2. Three-dimensional packing diagrams of BaBiO3. Distorted
electrolyte was 0.1 mol/L KOH aqueous solution. A Xe lamp Bi1O6 octahedra and Bi2O6 octahedra connect by corner-sharing with
each other.
(500 W, Ushio Denki Co.) was employed as the light source.
Before these measurements, the electrolyte was purged with N2
to remove dissolved O2. Results and Discussion
Photocatalytic Reactions. The photocatalytic reaction system
Physical Properties. Figure 1 shows the XRD patterns of
is a gas-closed system equipped with two gas chromatographs
BaBiO3 calcined at 923 K for 5 h, indicating that the material
(GC-8A with TCD detector and GC-14B with FID detector,
Shimadzu). The optical system for the catalytic reaction includes is well crystallized. We also analyzed the samples by ICP-OES
a 300 W Xe arc lamp (focused through a shutter window), a (inductively coupled plasma-optical emission spectroscopy) in
cutoff filter (providing the visible light of different wavelength), the solution containing HCl and HNO3 and found that its
and a water filter (preventing IR irradiation). Photocatalytic chemical composition is close to the nominal BaBiO3 apart from
acetaldehyde decomposition was performed with 0.8 g of small excess of Bi2O3. The crystal structure of the sample is
powdered photocatalyst placed at the bottom of a Pyrex glass determined as the monoclinic structure with space group I12/
cell at room temperature. The reaction gas was 0.5 atm gaseous m1, a ) 6.19 Å, b ) 6.15 Å, c ) 8.68 Å, and β ) 90.16°, by
mixture consisting of 837 ppm CH3CHO, 21% O2, and Ar the X-ray diffraction method. Figure 2 shows the crystal
balance gas. The photocatalytic methylene blue (MB) degrada- structure of the material, where BiO6 octahedra connect by
tion was carried out with 0.3 g of powdered photocatalyst sharing corners with each other, forming a diamond tunnel, and
suspended in 100 mL of MB solution (15.3 mg/L), which was Ba is located in the tunnel. According to Cox et al. report,19
prepared by dissolving MB powder in distilled water in a Pyrex the material’s structure contains two kinds of distorted BiO6
glass cell at room temperature in air. MB degradation was octahedra: Bi1O6 and Bi2O6. In the former octahedral, the
determined by a UV-visible spectrometer (UV-2500, Shi- valence of Bi is +3 and in the latter octahedral it is +5,
madzu). The ionic ingredients in MB solution before and after indicating the material is an ordered perovskite where the
the photocatalytic reaction were qualitatively analyzed by LC- ordered cations are the same element and the two valence states
MS (liquid chromatograph-mass spectrometer). The illuminated
of Bi ions coexist in the composite,19 in agreement with XPS
spectra and light power of the Xe lamp with different cutoff
characterization. The bond length of Bi1O6 (2.2791 Å (4) and
filters were measured by a spectroradiometer (USR-40, USHIO,
2.2898 Å (2)) is longer than that of Bi2O6 (2.1177 Å (4) and
Japan).
2.1140 Å (2)).20 The morphology of the as-synthesized material
Calculation Method. The band structure and partial density
was observed by FE-SEM, shown in Figure 3. It has a porous
of states of the material was calculated based on density
framework, and the original particle size is about 100-200 nm.
functional theory (DFT). The generalized gradient approximation
(GGA-PBE) was applied. The pseudo-atomic calculations were Due to the second sintering of the original particle, the surface
performed for this material with 5s25p66s2 (Ba), 6s26p3 (Bi), area is very low, about 1.2 m2/g.
and 2s22p4 (O). The selected unit cells for the calculations were The optical band gap of BaBiO3 can be evaluated by its
[BaBiO3]2. The kinetic energy cutoff was 400 eV. absorption spectrum shown in Figure 4. The optical absorption
Highly Efficient Photocatalytic Material J. Phys. Chem. C, Vol. 111, No. 34, 2007 12781

Figure 5. (a) Energy band structure of BaBiO3, (b) total density of

states (DOS), and (c) enlarged partial DOS of Bi [solid curve: Bi 6s
orbital; dotted curve: Bi 6p orbital] calculated by the plane-wave-based
density function method. The selected unit cells for the calculations
Figure 3. Picture of BaBiO3 powder observed by FESEM. were [BaBiO3]2. The kinetic energy cutoff was 400 eV.

(VB) and conduction band (CB), and (iii) long lifetime and high
mobility of photogenerated charge carriers. The UV-visible
absorption spectrum of BaBiO3 has shown that the material has
the narrowest band gap among all the reported Bi-based oxide
photocatalysts so far, to our knowledge. We next calculated the
total density of states (DOS) of BaBiO3 by the plane-wave-
based density function theory (DFT) to clarify the composition
of the VB and the CB. Figure 5 represents the calculated band
structure of BaBiO3. As shown in Figure 5a,b, there are four
predominant bands near the Fermi level: Ba 5p orbital, VB
(comprised of O 2p, Bi 6s, and Bi 6p), CB (comprised of Bi
6s, Bi 6p, and O 2p), and a hybridized orbital comprised of Bi
Figure 4. Kubelka-Munk conversion spectrum of BaBiO3. Inset: band
gap energy calculated by the extrapolating method. 6p and Ba 6s, similar to the previous reports.22,23 The photo-
generated electrons usually transfer from the top of VB (HOMO)
near the band edge of a semiconductor often obeys the following to the bottom of the CB (LUMO). Referring to the enlarged
equation:17 partial DOS of bismuth as shown in Figure 5c, it can be
concluded that the top of the VB is mainly composed of Bi 6s
and O 2p, and the bottom of the CB is Bi 6s and Bi 6p. Namely,
Rhν ∝ (hν - Eg)n (1)
the top of the VB and the bottom of the CB both have a large
contribution of the Bi 6s orbitals. In addition, the valence band
where a, ν, and Eg are the absorption coefficient, light frequency, maximum is considered as the Fermi level, and it is located at
and band gap, respectively. Typically, n decides the character- the A point in the first Brillouin zone, while the conduction
istics of the transition in a semiconductor. The value of n is band minimum is located at the V point. This implies that
determined as 2 by the method.17 This means that the optical BaBiO3 is an indirect semiconductor, in good agreement with
transition in the oxide is indirectly allowed, in good agreement the previous result obtained from its optical absorption spectrum
with the later theoretical calculation where the oxide is proved
and publication.23b The band gap energy calculated by the
to be an indirect semiconductor. The value of the band gap for
method is about 0.6 eV, much smaller than the actual value
the photocatalyst is determined as 2.05 eV by the extrapolation
obtained by the optical absorption spectrum, which is due to
method (see inset in Figure 4),17 which is consistent with the
the fact that this method has a large error for band gap
previous result measured by the momentum transfer resolved
calculation.24
electron energy loss spectroscopy.21 The material also shows
weak absorption to light of λ > 600 nm, which is probably Figure 6 shows the density contour maps for the LUMO
arising from the defect level of Bi or O. Our previous results (Figure 6a) and HOMO (Figure 6b) of BaBiO3. Clearly, Bi3+
also show that defects can extend the optical absorption of a and Bi5+ have different contributions to LUMO and HOMO,
semiconductor.13 For a material containing Bi, calcining at respectively. LUMO is composed of Bi5+ 6s and 6p orbitals
higher temperature or with longer time can form defects due to except O 2p orbitals. In contrast, HOMO consists of only Bi3+
Bi evaporation. The as-synthesized BaBiO3 was calcined with 6s and O 2p orbitals. This further confirms both LUMO and
longer time to further improve its absorption. However, it was HOMO have Bi 6s composition. However, Ba has no contribu-
found that the material color changed from red to black and tion to both LUMO and HOMO. In general, the VB of the
became a metal-like material, resulting in a very low activity. oxides with d0 and d10 metal cations (M) consists of O 2p
So controlling defects is an important but complicated issue orbitals, and the CB consists of d ortitals of the metal M. For
for improving photocatalytic activity. many Bi-based oxides, such as CaBi2O4 and BiVO4, the VB
Band Structure. Photocatalytic activity of a semiconductor are composed of Bi 6s and O 2p orbitals and the CB are
is closely relevant to its band structure. For an efficient visible- composed of only d orbitals (BiVO4) or p orbitals (CaBi2O4).
light photocatalyst, there exist three key factors to be satisfied: Compared with them, this material exhibits different band
13 (i) narrow band gap, (ii) suitable potential of valence band structure. Both VB and CB contain Bi 6s orbitals. The unique
12782 J. Phys. Chem. C, Vol. 111, No. 34, 2007 Tang et al.

Figure 7. Suggested band structure of BaBiO3 with respect to the

vacuum level with an error of 0.2 eV.

Figure 8. CO2 yield as a function of irradiation time on BaBiO3 for

Figure 6. (a) Density contour maps of the bottom orbital of the acetaldehyde decomposition with a 440 nm optical cutoff filter. The
conduction band (LUMO) and (b) density contour maps of the top inset is the illuminated spectra of the Xe lamp under full arc (dotted
orbital of the valence band (HOMO) for BaBiO3. line, photon flux: 1.92 × 10-6 mol/s) and using a 440 nm cutoff filter
(solid line, photon flux: 1.63 × 10-6 mol/s). I: intensity. λ: wavelength.
band structure is suggested to have played an important role in
the photocatalytic property of the material.
For electron energy E and its velocity υ, there is a correlation is the electronegativity of the semiconductor which is the
known as υ ) ∆kE(k)/p. If ∆kE(k) is small, namely the energy geometric mean of the Mulliken electronegativity of the
band is flat, υ will be low. In other words, the charge carriers constituent atoms. and Eg is the band gap energy of the
are heavily localized. In many materials, the photoholes are used semiconductor. Referring to the Mulliken electronegativity of
to be localized and the activity of the materials is mainly every atom in BaBiO3,29 we have theoretically speculated the
correlated with the photoelectrons’ mobility.17b,25,26 In BaBiO3, band edge position of the CB of the photocatalyst. It is 4.32
both the VB and CB are very abrupt, suggesting that υ of the eV with respect to the vacuum level. Considering the method
photohole and the photoelectron is high. Subsequently the error of 0.2 eV,28 the CV is just around the H2 evolution
mobilities of both charge carriers are high in BaBiO3, indicating potential. Subsequently the VB edge of the semiconductor is
the material should be very active. 6.37 eV on the basis of its band gap energy, indicating the
The band edge positions of a photocatalyst are of particular material has strongly oxidative ability close to a well-known
importance in the photocatalytic reaction. There are photoelec- oxidant H2O2 (6.27 eV). On the basis of these results, the band
trochemical and spcectroscoptic methods to determine the band potential of BaBiO3 is illustrated in Figure 7.
edge of the CB of a semiconductor. Compared with those, a Photocatalytic Properties. First, we investigated decomposi-
calculation method using Mulliken electronegativity of the tion of volatile organic compound (VOC) on the BaBiO3 powder
constituent atoms is much simpler and useful to investigate some sample. TiO2 was taken as a reference photocatalyst (P-25,
unstable materials. So far considerable success has been commercially available, particle size of nearly 30 nm, surface
achieved in calculating band position and photoelectric thresh- area of 49 m2/g). Acetaldehyde was selected as a typical gaseous
olds for many compounds using the Mulliken electronegativities contaminant in this work. Figure 8 represents acetaldehyde
of the constituent atoms.27-29 Herein the Mulliken electrone- decomposition vs visible-light irradiation time using a 440 nm
gativity of an atom is the arithmetic mean of the atomic electron cutoff filter (here the total photon flux is 1.63 × 10-6 mol of
affinity and the first ionization energy.27 The Mulliken elec- photons/s. Inset in this figure is the used Xe lamp spectra and
tronegativity forms a readily evaluated absolute electronegativity photons flux under different cutoff filters.) It only needs 40 min
based only on measurable physical parameters. In a sense, it is for more than 80% of acetaldehyde to be converted into CO2,
the electrochemical potential of the electron in the neutral atom. and acetaldehyde is nearly oxidized completely after 3.3 h of
The CB edge position of a semiconductor at the point of zero irradiation, although the present material has a very small surface
charge can be expressed empirically by16,27 area.
Figure 9 shows the wavelength dependence of the acetalde-
ECB ) X - 0.5Eg hyde conversion ratio to CO2, whose data points were collected
at 20 min of light irradiation by using different optical cutoff
where ECB is the CB edge potential relative to vacuum level, X filters. It is clearly seen that BaBiO3 shows a high activity under
Highly Efficient Photocatalytic Material J. Phys. Chem. C, Vol. 111, No. 34, 2007 12783

Figure 9. Wavelength dependence of acetaldehyde conversion into

CO2 using different cutoff filters on (a) BaBiO3 and (b) TiO2. Full arc
indicates Xe lamp irradiation without any cutoff filter.

either UV or visible-light irradiation, and the activity still

remains even if a 640 nm cutoff filter is used. TiO2 nearly loses
its activity when a 440 nm cutoff filter is used, in good
agreement with its well-known absorption characteristics.1,6 The
absorption spectrum reveals that the material can intrinsically
absorb visible light to 600 nm, and the absorption at λ > 600
nm is attributed to defects. So the photocatalytic activity after
600 nm is due to the defects, which is much smaller than the
activity caused by intrinsic absorption. Similarly we have found
that the defects produced by high-temperature annealing on
another Bi-based oxide also extends the optical absorption and
then the activity range of that photocatalyst.13 Nick Serpone
very recently underlined the prominent action of oxygen vacancy
in doped TiO2 as well.30 Although the nature of the defects in Figure 10. (a) Methylene blue (MB) degradation on BaBiO3 (9) and
BaBiO3 is not clear at present, the defect is definitely an MB-only photolysis (b) under Xe lamp irradiation with a 420 nm
important issue in exploiting a highly efficient semiconductor optical cutoff filter [the total light power is 55 mW/cm2]. (b) MB
photocatalyst for environmental purification. We further esti- degradation as a function of wavelength at 30 min light radiation at
room temperature in air.
mated the quantum yield using the band-pass filter based on
the number of incident photons according to the process
proposed for acetaldehyde conversion to CO2.3 They were efficiency in a wide visible-light region on the basis of a direct
evaluated to be 0.08% for TiO2 and 0.20% for BaBiO3 at 437.6 photocatalytic effect and the partial effect of MB-dye sensitiza-
nm (an error of 14.4 nm). tion.15 However, the material is not very stable in aqueous
We also carried out the following controlling experiments: solution, but it is relatively stable in organic solvent. The
(I) blank experiment (without any photocatalyst), (II) the dark possible reason for this and the way to improve its stability in
experiment (without light illumination), and (III) without oxidant aqueous solution are underway. We also tried to perform water
O2. In any case, CO2 was hardly detected. So it is worth noting splitting on the material because the calculated CB is close to
that the photocatalyst, the light irradiation, and oxidant O2 are the H2 evolution potential. However no obvious H2 evolution
all indispensable for the acetaldehyde decomposition. The was observed, indicating that probably the CB is lower than
stability of the sample was also checked by an exposure to air the H2 evolution potential or the overpotential of the material
for 2 months and repeated experiments for 10 times (each time is not enough for H2 production.
lasted 2 days). No obvious change in its photocatalytic activity, Photoelectrochemical Characterization. We further inves-
bulk crystal structure, and optical absorption were found. tigated the photoelectrochemical property of the material as
We next investigated the photocatalytic degradation of shown in Figure 11a. Without light irradiation, the photocurrent
methylene blue (MB), being a typical dye contaminant in density is nearly zero. With light irradiation, about 0.2 mA/
wastewater, on BaBiO3 at room temperature in air. Figure 10a cm2 photocurrent density is observed at a bias of 1.0 V (vs SCE).
compares the results of MB photolysis and MB degradation on In particular, under visible-light irradiation (λ > 440 or 480
BaBiO3 under visible-light irradiation (λ > 420 nm), respec- nm), the photocurrent density is still high, nearly 0.1 mA/cm2.
tively. It is known that MB can self-photolyze.6,9,13 However, All these indicate that the catalyst is a semiconductor strongly
the efficiency strongly depends on the utilized light power. responsive to light irradiation, even visible-light irradiation. In
Under the present condition (λ > 420 nm), only 25% of MB is addition, the photocurrent density of the material was also
photolyzed after 2 h. On the contrary, 100% of MB is easily compared with that of TiO2, as shown in Figure 11b. Rutile
degraded on BaBiO3, revealing a high activity of this material TiO2 can absorb a little of visible light (band gap 2.97 eV),31
under visible-light irradiation. Results of the LC-MS measure- so it yields a little bit of photocurrent under visible-light
ment of MB solution before and after the photocatalytic reaction irradiation (λ > 420 nm). Under the same condition, BaBiO3
also confirm that MB is degraded other than simply bleached shows a much larger photocurrent, confirming it is a potential
on BaBiO3 (see Supporting Information). Figure 10b shows MB photofunctional material.
degradation as a function of wavelength. The photocatalyst keeps So far we have developed several Bi-based oxides revealing
a high activity at wavelengths up to 600 nm, verifying the high photocatalytic properties under visible-light irradiation.13,16 Table
12784 J. Phys. Chem. C, Vol. 111, No. 34, 2007 Tang et al.

as well. Therefore the high efficiency of BaBiO3 is possibly

attributed to the large portion of s orbital components in both
the VB and CB, resulting in the narrowest band gap, higher
mobility of photogenerated charge carriers, and lower barrier
to electron transition among all reported Bi-based photocatalysts.
All these suggest some useful information to design an active
photocatalytic material under solar irradiation.

Conclusions
A novel perovskite photocatalyst BaBiO3 was prepared where
Bi3+ and Bi5+ ions coexist. The material can absorb a wide
range of light irradiation up to 650 nm, which almost covers
the region from UV through all strong visible light in the
sunlight and an indoor lamp’s illumination. The theoretical
calculation shows that the material is an indirect semiconductor.
DOS and density contour maps of LUMO and HOMO of
BaBiO3 indicate that both the top of the VB and the bottom of
the CB contain a large portion of Bi 6s orbitals, suggesting there
is s-to-s excitation in the material. A band structure calculation
shows the charge carriers in the CB and VB are highly mobile.
The band edge calculation further verifies the strongly oxidative
potential of the photocatalyst. All these indicate the material
should be an active photocatalyst under visible-light irradiation,
which has been proven by highly efficient decomposition of
various organic contaminants and high photocurrent density at
a wide range of light irradiation. Nevertheless, further efforts
are necessary to increase the surface area as well as to improve
Figure 11. (a) Photocurrent densities (jp: mA/cm2) of BaBiO3 anode, the stability of the material for practical application.
in dark conditions (dotted line), with 480 and 440 nm optical cutoff
filters and full arc irradiation. (b) Comparison of photocurrent densities
on BaBiO3 and TiO2 under visible-light irradiation with a 420 nm cutoff
Acknowledgment. This work was partially supported by the
filter. Global Environment Research Fund and a Grant-in-Aid for
Scientific Research on Priority Areas (417) from the Ministry
TABLE 1: Comparison of Photocatalytic Activity of of Education, Culture, Sports, Science and Technology (MEXT)
Different Bi-Based Oxides under Visible-light Irradiation of the Japanese Government. We thank Prof. Dehua He
acetaldehyde decomposition MB degradation (Chemistry Department, Qinghua Univeristy, China) for the LC-
(initial rate) (initial rate) MS measurement and Dr. S. Fukushima and Dr. T. Kobayashi
materials µmol/ha µmol/hb (Materials Analysis Station, NIMS) for the XPS characterization
CaBi2O4 6.1 × 103 10 and chemical composition analysis. J.T. thanks the Japan Society
ZnBi12O20 2.9 × 103 0 for Promotion of Science (JSPS) for financial support.
BaBiO3 7.1 × 103 81.6
a The initial rate indicates the reaction rate at the initial 20 min with Supporting Information Available: Results of the LC-MS
440 nm cutoff filter. b The initial rate indicates the reaction rate at the measurement of the MB solution before and after the photo-
initial 30 min with 420 nm cutoff filter. catalytic reaction. This material is available free of charge via
the Internet at http://pubs.acs.org.
1 summarizes their activity for acetaldehyde decomposition and
MB degradation. Compared with the other Bi-based photocata- References and Notes
lysts, BaBiO3 shows the highest activity not only for acetalde-
hyde decomposition but also for MB degradation. To be an (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Chem. ReV. 1995, 95, 69.
efficient photocatalyst with a narrow band gap, it is important (2) Zhu, H.; Lan, Y.; Gao, X.; Ringer, S. P.; Zheng, Z.; Song, D.;
to control the position of the VB. The characteristic transition Zhao, J. J. Am. Chem. Soc. 2005, 127, 6730.
of electrons in the 6s orbital of Bi3+ to the CB in several Bi3+- (3) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem.
B 1998, 102, 2699.
containing oxides has been reported to occur at the relatively (4) Karkmaz, M.; Puzenat, E.; Guillard, C.; Herrmann, J. M. Appl.
low energies.10,12,13,16 Thus Bi3+ is regarded as one of predomi- Catal. B 2004, 51 (3), 183.
nant elements controlling the VB position. On the other hand, (5) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625.
empty Bi5+ 6s is a possibly efficient way to control the CB (6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science
2001, 293, 269.
position. It is well-known that s, p, and d orbitals have different (7) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98,
spatial orientation. The photogenerated electron transfer from 13669.
VB to CB will be heavily affected by the spatial orientation of (8) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505.
these orbitals. Among them, the s orbital has spherical sym- (9) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. J.
Phys. Chem. B 2004, 108, 17269.
metry, so the s-s transition might have the lowest barrier. It (10) Kim, H.; Hwang, D.; Lee, J. J. Am. Chem. Soc. 2004, 126, 11459.
has been confirmed that photoelectrons can transfer from the (11) Sayama, K.; Nomura, A.; Zou, Z.; Abe, R.; Abe, Y.; Arakawa, H.
6s orbital of Bi3+ to the 6s orbital of Bi5+ during the light Chem. Commun. 2003, 2908.
excitation in BaBiO3.32 The less localized characteristics of the (12) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459.
(13) Tang, J.; Zou, Z.; Ye, J. Angew. Chem., Int. Ed. 2004, 43, 4463.
s orbital are also beneficial for the migration of the photoge- (14) Kohtani, S.; Koshiko, M.; Kudo, A.; Tokumura, K.; Ishigaki, Y.;
nerated charge carriers,13,18 proved by our calculation results Toriba, A.; Hayakawa, K.; Nakagaki, R. Appl. Catal. B 2003, 46, 573.
Highly Efficient Photocatalytic Material J. Phys. Chem. C, Vol. 111, No. 34, 2007 12785

(15) (a) Tang, J.; Zou, Z.; Ye, J. Chem. Mater. 2004, 16, 1644. (b) Tang, (23) (a) Mattheiss, L. F.; Hamann, D. R. Phys. ReV. Lett. 1988, 60, 2681.
J.; Zou, Z.; Yin, J.; Ye, J. Chem. Phys. Lett. 2003, 382, 175. (b) Mattheiss, L. F.; Hamann, D. R. Phys. ReV. B 1983, 28, 4227.
(16) (a) Tang, J.; Ye, J. Chem. Phys. Lett. 2005, 410, 104. (b) Tang, J.; (24) Please see the manual of the software CASTEP 3.1.
Zou, Z.; Ye, J. Res. Chem. Intermediat. 2005, 31, 499. (25) Ouyang, S.; Zhang, H.; Li, D.; Yu, T.; Ye, J.; Zou, Z. J. Phys.
(17) (a) Tang, J.; Zou, Z.; Ye, J. J. Phys. Chem B 2003, 107, 14265. (b) Chem. B 2006, 110, 11677.
Tang, J.; Ye, J. J. Mater. Chem. 2005, 15, 4246. (26) Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2002, 106,
(18) Sato, J.; Saito, N.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2003, 12441.
107, 7970. (27) Nethercot, A. H. Phys. ReV. Lett. 1974, 33, 1088.
(19) Cox, D. E.; Sleight, A. W. Solid State Commun. 1976, 19, 969. (28) Butler, M. A.; Ginley, D. S. J. Electrochem. Soc. 1978, 125, 228.
(20) Pei, S. Y.; Jorgensen, J. D.; Dabrowski, B.; Hinks, D. G.; Richards, (29) Hotop, H.; Lineberger, W. C. J. Phys. Chem. Ref. Data 1975, 4
D. R.; Mitchell, A. W.; Newsam, J. M.; Sinha, S. K.; Vaknin, D.; Jacobson, (3), 539.
A. J. Phys. ReV. B 1990, 41, 4126. (30) Serpone, N. J. Phys. Chem. B 2006, 110, 24287.
(21) Wang, Y. Y.; Dravid, V. P.; Bulut, N.; Han, P. D.; Klein, M. V.; (31) Tang, H.; Levy, F.; Berger, H.; Schmid, P. E. Phys. ReV. B 1995,
Schnatterly, S. E.; Zhang, F. C. Phys. ReV. Lett. 1995, 75, 2546. 32, 7771.
(22) Papaconstantopoulos, D. A. Phys. ReV. B 1989, 40, 8844. (32) Allen, P. B.; Bischofs, I. B. Phys. ReV. B 2002, 65, 115113.