J. Phys. Chem. C 2007, 111, 12779-12785
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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 Photocatalysis using solar energy is highly expected to be an ideal “green” technology for sustainable development of human beings, where an active photocatalytic material is definitely an important key.1 To date, TiO2 is well-known as a stable, low-cost, and highly efficient photocatalytic material.1-5 But its application is limited at the ultraviolet light region (wavelength λ < 400 nm) because of its optical absorption characteristic. A photocatalytic material active under visible light has long been expected from the viewpoint of efficient utilization of solar irradiation. Recently, numerous attempts have been made to improve the inherently low efficiency of TiO2 in harvesting sunlight with shifting its spectral response into the visible region, mainly by cation or anion doping.6-9 Typically, Asahi et al. succeeded in improving its optical absorption from the UV region to the visible-light region (λ < 500 nm) by nitrogen doping.6 But its activity is not high enough for practical application, and there is concern for the stability of doped anions.9,10 Another approach to realize visible-light photocatalysis is to develop a new photocatalytic material independent of TiO2. Oxide materials are plausible candidates of the new photocatalytic material in relation to their high chemical stability and easy preparation compared with non-oxide materials.10-13 Kim et al.,10 Kudo et al.,12,14 and our group13,15-17 recently reported several undoped oxide photocatalysts responsive to visible light. However, their activities for organic decomposition are not sufficient yet at wavelengths longer than 500 nm. Therefore, to develop an active photocatalytic material at a wide range of visible light is still seriously challenging and more beneficial for practical applications of photocatalytic technology. * Corresponding author. E-mail:
[email protected]. † National Institute for Materials Science (NIMS). § Present address: Department of Chemistry, Imperial College London, London SW7 2AZ, U.K. ‡ National Institute of Advanced Industrial Science and Technology (AIST). Present address: Department of Materials Science and Engineering, Nanjing University, P. R. China.
Very recently, a family of Bi-based oxides has been found to be very active under visible-light irradiation, which is attributed to the hybridized valence band (VB) by O 2p and Bi 6s so as to narrow the band gap.10,12,13,16 In addition, the high activity of the materials is also ascribed to s composition in the VB because the photogenerated charge carriers in s orbital have a high mobility due to the dispersive characteristic of the s orbital.13,18 The typical examples are BiVO4 for O2 evolution from water with relevant sacrificial reagent12 and CaBi2O4 for some organics’ decomposition.13 In order to further narrow the band gap and greatly increase the activity of a photocatalyst, modification of the conduction band (CB) is expected here while keeping the benefit of the hybridized VB. Namely a material is expected in which both the VB and CB are composed of the hybridized orbitals containing s composition. From our experiences in synthesizing the visible-light-driven oxide photocatalysts,5,13 the BaBiO3 semiconductor was prepared herein by a soft chemical method. The theoretical calculation shows that the material actually has the hybridized VB and CB containing s composition. The oxide 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. To our knowledge, the material represents the narrowest band gap in oxide photocatalysts. Furthermore, the novel photochemical properties of the material are demonstrated by the photocatalytic decomposition of gaseous acetaldehyde and aqueous methylene blue dye as well as the photocurrent measurement under visible-light irradiation. The possible mechanism is also discussed based on the theoretical calculation and experimental results. Experimental Section Material Preparation and Characterization. The BaBiO3 powder photocatalyst was prepared by a soft chemical method. A suitable amount of Ba(NO3)2 and a small excess of Bi(NO3)3‚ 5H2O were dissolved in water. Then a citric acid and EDTA 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 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 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 threeelectrode 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 commercially) was also measured under visible-light irradiation. The electrolyte was 0.1 mol/L KOH aqueous solution. A Xe lamp (500 W, Ushio Denki Co.) was employed as the light source. Before these measurements, the electrolyte was purged with N2 to remove dissolved O2. Photocatalytic Reactions. The photocatalytic reaction system is a gas-closed system equipped with two gas chromatographs (GC-8A with TCD detector and GC-14B with FID detector, Shimadzu). The optical system for the catalytic reaction includes a 300 W Xe arc lamp (focused through a shutter window), a cutoff filter (providing the visible light of different wavelength), and a water filter (preventing IR irradiation). Photocatalytic acetaldehyde decomposition was performed with 0.8 g of powdered photocatalyst placed at the bottom of a Pyrex glass cell at room temperature. The reaction gas was 0.5 atm gaseous mixture consisting of 837 ppm CH3CHO, 21% O2, and Ar balance gas. The photocatalytic methylene blue (MB) degradation was carried out with 0.3 g of powdered photocatalyst suspended in 100 mL of MB solution (15.3 mg/L), which was prepared by dissolving MB powder in distilled water in a Pyrex glass cell at room temperature in air. MB degradation was determined by a UV-visible spectrometer (UV-2500, Shimadzu). The ionic ingredients in MB solution before and after the photocatalytic reaction were qualitatively analyzed by LCMS (liquid chromatograph-mass spectrometer). The illuminated spectra and light power of the Xe lamp with different cutoff filters were measured by a spectroradiometer (USR-40, USHIO, Japan). Calculation Method. The band structure and partial density of states of the material was calculated based on density functional theory (DFT). The generalized gradient approximation (GGA-PBE) was applied. The pseudo-atomic calculations were performed for this material with 5s25p66s2 (Ba), 6s26p3 (Bi), and 2s22p4 (O). The selected unit cells for the calculations were [BaBiO3]2. The kinetic energy cutoff was 400 eV.
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Figure 1. XRD pattern of prepared BaBiO3.
Figure 2. Three-dimensional packing diagrams of BaBiO3. Distorted Bi1O6 octahedra and Bi2O6 octahedra connect by corner-sharing with each other.
Results and Discussion Physical Properties. Figure 1 shows the XRD patterns of BaBiO3 calcined at 923 K for 5 h, indicating that the material is well crystallized. We also analyzed the samples by ICP-OES (inductively coupled plasma-optical emission spectroscopy) in the solution containing HCl and HNO3 and found that its chemical composition is close to the nominal BaBiO3 apart from small excess of Bi2O3. The crystal structure of the sample is determined as the monoclinic structure with space group I12/ m1, a ) 6.19 Å, b ) 6.15 Å, c ) 8.68 Å, and β ) 90.16°, by the X-ray diffraction method. Figure 2 shows the crystal structure of the material, where BiO6 octahedra connect by sharing corners with each other, forming a diamond tunnel, and Ba is located in the tunnel. According to Cox et al. report,19 the material’s structure contains two kinds of distorted BiO6 octahedra: Bi1O6 and Bi2O6. In the former octahedral, the valence of Bi is +3 and in the latter octahedral it is +5, indicating the material is an ordered perovskite where the ordered cations are the same element and the two valence states of Bi ions coexist in the composite,19 in agreement with XPS characterization. The bond length of Bi1O6 (2.2791 Å (4) and 2.2898 Å (2)) is longer than that of Bi2O6 (2.1177 Å (4) and 2.1140 Å (2)).20 The morphology of the as-synthesized material was observed by FE-SEM, shown in Figure 3. It has a porous framework, and the original particle size is about 100-200 nm. Due to the second sintering of the original particle, the surface area is very low, about 1.2 m2/g. The optical band gap of BaBiO3 can be evaluated by its absorption spectrum shown in Figure 4. The optical absorption
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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 were [BaBiO3]2. The kinetic energy cutoff was 400 eV.
Figure 3. Picture of BaBiO3 powder observed by FESEM.
Figure 4. Kubelka-Munk conversion spectrum of BaBiO3. Inset: band gap energy calculated by the extrapolating method.
near the band edge of a semiconductor often obeys the following equation:17
Rhν ∝ (hν - Eg)n
(1)
where a, ν, and Eg are the absorption coefficient, light frequency, and band gap, respectively. Typically, n decides the characteristics of the transition in a semiconductor. The value of n is determined as 2 by the method.17 This means that the optical transition in the oxide is indirectly allowed, in good agreement with the later theoretical calculation where the oxide is proved to be an indirect semiconductor. The value of the band gap for the photocatalyst is determined as 2.05 eV by the extrapolation method (see inset in Figure 4),17 which is consistent with the previous result measured by the momentum transfer resolved electron energy loss spectroscopy.21 The material also shows weak absorption to light of λ > 600 nm, which is probably arising from the defect level of Bi or O. Our previous results also show that defects can extend the optical absorption of a semiconductor.13 For a material containing Bi, calcining at higher temperature or with longer time can form defects due to Bi evaporation. The as-synthesized BaBiO3 was calcined with longer time to further improve its absorption. However, it was found that the material color changed from red to black and became a metal-like material, resulting in a very low activity. So controlling defects is an important but complicated issue for improving photocatalytic activity. Band Structure. Photocatalytic activity of a semiconductor is closely relevant to its band structure. For an efficient visiblelight photocatalyst, there exist three key factors to be satisfied: 13 (i) narrow band gap, (ii) suitable potential of valence band
(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-wavebased 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 6p and Ba 6s, similar to the previous reports.22,23 The photogenerated electrons usually transfer from the top of VB (HOMO) to the bottom of the CB (LUMO). Referring to the enlarged 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, 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 maximum is considered as the Fermi level, and it is located at the A point in the first Brillouin zone, while the conduction band minimum is located at the V point. This implies that BaBiO3 is an indirect semiconductor, in good agreement with the previous result obtained from its optical absorption spectrum and publication.23b The band gap energy calculated by the method is about 0.6 eV, much smaller than the actual value obtained by the optical absorption spectrum, which is due to the fact that this method has a large error for band gap calculation.24 Figure 6 shows the density contour maps for the LUMO (Figure 6a) and HOMO (Figure 6b) of BaBiO3. Clearly, Bi3+ and Bi5+ have different contributions to LUMO and HOMO, respectively. LUMO is composed of Bi5+ 6s and 6p orbitals except O 2p orbitals. In contrast, HOMO consists of only Bi3+ 6s and O 2p orbitals. This further confirms both LUMO and HOMO have Bi 6s composition. However, Ba has no contribution to both LUMO and HOMO. In general, the VB of the oxides with d0 and d10 metal cations (M) consists of O 2p orbitals, and the CB consists of d ortitals of the metal M. For many Bi-based oxides, such as CaBi2O4 and BiVO4, the VB are composed of Bi 6s and O 2p orbitals and the CB are composed of only d orbitals (BiVO4) or p orbitals (CaBi2O4). Compared with them, this material exhibits different band structure. Both VB and CB contain Bi 6s orbitals. The unique
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Figure 7. Suggested band structure of BaBiO3 with respect to the vacuum level with an error of 0.2 eV.
Figure 6. (a) Density contour maps of the bottom orbital of the conduction band (LUMO) and (b) density contour maps of the top orbital of the valence band (HOMO) for BaBiO3.
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 known as υ ) ∆kE(k)/p. If ∆kE(k) is small, namely the energy band is flat, υ will be low. In other words, the charge carriers are heavily localized. In many materials, the photoholes are used to be localized and the activity of the materials is mainly correlated with the photoelectrons’ mobility.17b,25,26 In BaBiO3, both the VB and CB are very abrupt, suggesting that υ of the photohole and the photoelectron is high. Subsequently the mobilities of both charge carriers are high in BaBiO3, indicating the material should be very active. The band edge positions of a photocatalyst are of particular importance in the photocatalytic reaction. There are photoelectrochemical and spcectroscoptic methods to determine the band edge of the CB of a semiconductor. Compared with those, a calculation method using Mulliken electronegativity of the constituent atoms is much simpler and useful to investigate some unstable materials. So far considerable success has been achieved in calculating band position and photoelectric thresholds for many compounds using the Mulliken electronegativities of the constituent atoms.27-29 Herein the Mulliken electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy.27 The Mulliken electronegativity forms a readily evaluated absolute electronegativity based only on measurable physical parameters. In a sense, it is the electrochemical potential of the electron in the neutral atom. The CB edge position of a semiconductor at the point of zero charge can be expressed empirically by16,27
ECB ) X - 0.5Eg where ECB is the CB edge potential relative to vacuum level, X
Figure 8. CO2 yield as a function of irradiation time on BaBiO3 for acetaldehyde decomposition with a 440 nm optical cutoff filter. The inset is the illuminated spectra of the Xe lamp under full arc (dotted 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.
is the electronegativity of the semiconductor which is the geometric mean of the Mulliken electronegativity of the constituent atoms. and Eg is the band gap energy of the semiconductor. Referring to the Mulliken electronegativity of every atom in BaBiO3,29 we have theoretically speculated the band edge position of the CB of the photocatalyst. It is 4.32 eV with respect to the vacuum level. Considering the method error of 0.2 eV,28 the CV is just around the H2 evolution potential. Subsequently the VB edge of the semiconductor is 6.37 eV on the basis of its band gap energy, indicating the material has strongly oxidative ability close to a well-known oxidant H2O2 (6.27 eV). On the basis of these results, the band potential of BaBiO3 is illustrated in Figure 7. Photocatalytic Properties. First, we investigated decomposition of volatile organic compound (VOC) on the BaBiO3 powder sample. TiO2 was taken as a reference photocatalyst (P-25, commercially available, particle size of nearly 30 nm, surface area of 49 m2/g). Acetaldehyde was selected as a typical gaseous contaminant in this work. Figure 8 represents acetaldehyde decomposition vs visible-light irradiation time using a 440 nm cutoff filter (here the total photon flux is 1.63 × 10-6 mol of photons/s. Inset in this figure is the used Xe lamp spectra and photons flux under different cutoff filters.) It only needs 40 min for more than 80% of acetaldehyde to be converted into CO2, and acetaldehyde is nearly oxidized completely after 3.3 h of irradiation, although the present material has a very small surface area. Figure 9 shows the wavelength dependence of the acetaldehyde conversion ratio to CO2, whose data points were collected at 20 min of light irradiation by using different optical cutoff filters. It is clearly seen that BaBiO3 shows a high activity under
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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 BaBiO3 is not clear at present, the defect is definitely an important issue in exploiting a highly efficient semiconductor photocatalyst for environmental purification. We further estimated 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 evaluated to be 0.08% for TiO2 and 0.20% for BaBiO3 at 437.6 nm (an error of 14.4 nm). We also carried out the following controlling experiments: (I) blank experiment (without any photocatalyst), (II) the dark experiment (without light illumination), and (III) without oxidant O2. In any case, CO2 was hardly detected. So it is worth noting that the photocatalyst, the light irradiation, and oxidant O2 are all indispensable for the acetaldehyde decomposition. The stability of the sample was also checked by an exposure to air for 2 months and repeated experiments for 10 times (each time lasted 2 days). No obvious change in its photocatalytic activity, bulk crystal structure, and optical absorption were found. We next investigated the photocatalytic degradation of methylene blue (MB), being a typical dye contaminant in wastewater, on BaBiO3 at room temperature in air. Figure 10a compares the results of MB photolysis and MB degradation on BaBiO3 under visible-light irradiation (λ > 420 nm), respectively. It is known that MB can self-photolyze.6,9,13 However, the efficiency strongly depends on the utilized light power. Under the present condition (λ > 420 nm), only 25% of MB is photolyzed after 2 h. On the contrary, 100% of MB is easily degraded on BaBiO3, revealing a high activity of this material under visible-light irradiation. Results of the LC-MS measurement of MB solution before and after the photocatalytic reaction also confirm that MB is degraded other than simply bleached on BaBiO3 (see Supporting Information). Figure 10b shows MB degradation as a function of wavelength. The photocatalyst keeps a high activity at wavelengths up to 600 nm, verifying the high
Figure 10. (a) Methylene blue (MB) degradation on BaBiO3 (9) and MB-only photolysis (b) under Xe lamp irradiation with a 420 nm optical cutoff filter [the total light power is 55 mW/cm2]. (b) MB degradation as a function of wavelength at 30 min light radiation at room temperature in air.
efficiency in a wide visible-light region on the basis of a direct photocatalytic effect and the partial effect of MB-dye sensitization.15 However, the material is not very stable in aqueous solution, but it is relatively stable in organic solvent. The possible reason for this and the way to improve its stability in aqueous solution are underway. We also tried to perform water splitting on the material because the calculated CB is close to the H2 evolution potential. However no obvious H2 evolution was observed, indicating that probably the CB is lower than the H2 evolution potential or the overpotential of the material is not enough for H2 production. Photoelectrochemical Characterization. We further investigated the photoelectrochemical property of the material as shown in Figure 11a. Without light irradiation, the photocurrent density is nearly zero. With light irradiation, about 0.2 mA/ cm2 photocurrent density is observed at a bias of 1.0 V (vs SCE). In particular, under visible-light irradiation (λ > 440 or 480 nm), the photocurrent density is still high, nearly 0.1 mA/cm2. All these indicate that the catalyst is a semiconductor strongly responsive to light irradiation, even visible-light irradiation. In addition, the photocurrent density of the material was also compared with that of TiO2, as shown in Figure 11b. Rutile TiO2 can absorb a little of visible light (band gap 2.97 eV),31 so it yields a little bit of photocurrent under visible-light irradiation (λ > 420 nm). Under the same condition, BaBiO3 shows a much larger photocurrent, confirming it is a potential photofunctional material. So far we have developed several Bi-based oxides revealing photocatalytic properties under visible-light irradiation.13,16 Table
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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
Figure 11. (a) Photocurrent densities (jp: mA/cm2) of BaBiO3 anode, 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 filter.
TABLE 1: Comparison of Photocatalytic Activity of Different Bi-Based Oxides under Visible-light Irradiation
materials
acetaldehyde decomposition (initial rate) µmol/ha
MB degradation (initial rate) µmol/hb
CaBi2O4 ZnBi12O20 BaBiO3
6.1 × 103 2.9 × 103 7.1 × 103
10 0 81.6
a The initial rate indicates the reaction rate at the initial 20 min with 440 nm cutoff filter. b The initial rate indicates the reaction rate at the initial 30 min with 420 nm cutoff filter.
1 summarizes their activity for acetaldehyde decomposition and MB degradation. Compared with the other Bi-based photocatalysts, BaBiO3 shows the highest activity not only for acetaldehyde decomposition but also for MB degradation. To be an efficient photocatalyst with a narrow band gap, it is important to control the position of the VB. The characteristic transition of electrons in the 6s orbital of Bi3+ to the CB in several Bi3+containing oxides has been reported to occur at the relatively low energies.10,12,13,16 Thus Bi3+ is regarded as one of predominant elements controlling the VB position. On the other hand, empty Bi5+ 6s is a possibly efficient way to control the CB position. It is well-known that s, p, and d orbitals have different spatial orientation. The photogenerated electron transfer from VB to CB will be heavily affected by the spatial orientation of these orbitals. Among them, the s orbital has spherical symmetry, so the s-s transition might have the lowest barrier. It has been confirmed that photoelectrons can transfer from the 6s orbital of Bi3+ to the 6s orbital of Bi5+ during the light excitation in BaBiO3.32 The less localized characteristics of the s orbital are also beneficial for the migration of the photogenerated charge carriers,13,18 proved by our calculation results
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 the stability of the material for practical application. Acknowledgment. This work was partially supported by the Global Environment Research Fund and a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. We thank Prof. Dehua He (Chemistry Department, Qinghua Univeristy, China) for the LCMS measurement and Dr. S. Fukushima and Dr. T. Kobayashi (Materials Analysis Station, NIMS) for the XPS characterization and chemical composition analysis. J.T. thanks the Japan Society for Promotion of Science (JSPS) for financial support. Supporting Information Available: Results of the LC-MS measurement of the MB solution before and after the photocatalytic reaction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Zhu, H.; Lan, Y.; Gao, X.; Ringer, S. P.; Zheng, Z.; Song, D.; Zhao, J. J. Am. Chem. Soc. 2005, 127, 6730. (3) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 102, 2699. (4) Karkmaz, M.; Puzenat, E.; Guillard, C.; Herrmann, J. M. Appl. Catal. B 2004, 51 (3), 183. (5) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (7) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (8) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505. (9) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2004, 108, 17269. (10) Kim, H.; Hwang, D.; Lee, J. J. Am. Chem. Soc. 2004, 126, 11459. (11) Sayama, K.; Nomura, A.; Zou, Z.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2003, 2908. (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. (14) Kohtani, S.; Koshiko, M.; Kudo, A.; Tokumura, K.; Ishigaki, Y.; Toriba, A.; Hayakawa, K.; Nakagaki, R. Appl. Catal. B 2003, 46, 573.
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