Visible-Light Photocatalytic Degradation of BiTaO4 Photocatalyst and ...

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Visible-Light Photocatalytic Degradation of BiTaO4 Photocatalyst and Mechanism of Photocorrosion Suppression Rui Shi, Jie Lin, Yajun Wang, Jing Xu, and Yongfa Zhu* Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, China ReceiVed: October 25, 2009; ReVised Manuscript ReceiVed: March 7, 2010

BiTaO4 photocatalyst for methylene blue (MB) degradation, which worked under visible-light irradiation, was systematically investigated for the first time. BiTaO4 possess similar degradation ability of TiO2-XNX under visible-light irradiation (λ > 420 nm). Density functional calculations revealed that Bi 6s orbitals contributed to the formation of valence band, resulting in narrowing the band gap. Moreover, comparing with BiNbO4, we found BiTaO4 showed photocorrosion suppression performance during the photocatalytic process. The differences in the photocorrosion suppression between BiTaO4 and BiNbO4 could be attributed to the conduction curvature and bandwidth, leading to differences in the mobility of photogenerated electrons formed in the conduction bands, which mainly consisted of Ta 5d and Nb 4d orbitals, respectively. 1. Introduction Development of photocatalysts which can efficiently decompose organic contaminants has extensively been explored.1-5 Unfortunately, the number of photocatalysts which are active for contaminants degradation under visible-light irradiation is still limited. In some cases, doping of a foreign element into active photocatalysts with wide band gaps is the well-studied way to design the visible-light-driven photocatalyst.5 In another way, developing new types of photocatalysts with tunable electronic structures of particular interest is a good strategy.6-8 Metal oxide compounds containing lead and bismuth, such as Pb3Nb4O13, PbBi2Nb2O9, Bi2WO6, BiVO4, Bi2MoO6, etc. are good visible-light driven photocatalysts candidates.9-16 The visible-light response of these materials arises due to the presence of the Pb2+ and Bi3+ lone pair electrons. It was clarified that the band gap narrowing was due to the contribution of Pb and Bi 6s orbitals to the valence band formation. However, the use of lead will result in environmental problems. Recently, many metal oxides consisting of TaO6 octahedron have been proven to be able to degrade organic contaminants or decompose water into H2 and O2 under UV or visible-light irradiation.17,18 Therefore, this interests us in the investigation into the band structure and the visible-light photocatalytic properties of BiTaO4. The photocatalytic water-splitting and degradation of Orange G (OG) and Alizarin green (AG) over BiTaO4 under UV irradiation have been reported.17-19 To the best of our knowledge, there has been no report regarding the application of BiTaO4 in the degradation of organic contaminants under visible-light irradiation. In the present study, we reported BiTaO4 in photocatalytic oxidative decomposition of methylene blue (MB) under visible-light irradiation. In detail, the morphology, physical properties, and band structure of BiTaO4 were systematically investigated. And density functional calculations were introduced to clarify the role of Bi3+ in the valence band. Moreover, comparing with BiNbO4, we found BiTaO4 showed * To whom correspondence should be addressed. E-mail: zhuyf@ tsinghua.edu.cn. Fax: +86-10-62787601. Phone: +86-10-62787601.

relatively stable performance for MB degradation. No photocorrosion phenomenon appeared during the photocatalytic process. 2. Experimental Section 2.1. Synthesis of Sample and Characterization. BiTaO4 was synthesized by a solid-state reaction process. All chemicals used were analytic grade reagents without further purification. The reaction mixtures of Bi2O3 and Ta2O5 were ground well in an agate mortar and calcined at 600 °C for 4 h, then at 1150 °C for 20 h. TiO2-XNX, known for its good photocatalytic activity in decomposition of the pollutants under visible-light irradiation, was also prepared as a reference.20 Purity and crystallinity of the as-prepared sample was characterized by X-ray diffraction (XRD) on a Bruker D8-advance diffractometer, using Cu KR radiation (λ ) 1.5418 Å). The XRD data for indexing and cellparameter calculation were collected in a scanning mode with a step length of 0.02° and a preset time of 5.6 s/step. Morphologies and size of the prepared samples were further examined with scanning electron micrographs (SEM KYKY2800). The Brunauer-Emmett-Teller (BET) surface area was measured by ASAP 2010 V5.02H. The absorbed gas was nitrogen. Diffuse reflection spectra (DRS) were obtained on a Hitachi U-3010 UV-vis spectrophotometer. Total organic carbon (TOC) was measured with a Tekmar Dohrmann Apollo 9000 TOC analyzer. Chemical characterization of the sample surface was recorded with X-ray photoelectron spectroscopy (XPS ULVAC-PHI, Quantera). The charge effect was calibrated by using the binding energy of C1s. 2.2. Photocatalytic Degradation. The photocatalytic activities of the BiTaO4 were evaluated by degradation of MB under an 11 W bactericidal lamp with 254 nm and a 500 W Xe lamp with different cutoff filters. The average light intensity was 28 and 0.8 mW cm-2 for the Xe lamp and the bactericidal lamp, respectively. The reaction cell was placed in a sealed black box of which the top was opened and the varied cutoff filters were placed to provide visible-light irradiation. In each run, 50 mg of the BiTaO4 catalyst was added to 100 mL of the MB solution. After the suspension was stirred for 60 min, the light was turned

10.1021/jp9101866  2010 American Chemical Society Published on Web 03/18/2010

Photocatalytic Degradation of BiTaO4 Photocatalyst

Figure 1. XRD spectra of BiTaO4.

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Figure 3. UV-visible diffuse reflectance spectra of BiTaO4 and TiO2-XNX.

Figure 4. Electronic band structure of BiTaO4.

Figure 2. SEM image of BiTaO4.

on to initiate the reaction. The concentration of the MB solution was monitored with a Hitachi UV-vis spectrophotometer each 30 min. 2.3. Calculation of Band Structure. The quantum-mechanical calculations performed here were based on density functional theory (DFT). Exchange-correlation effects were taken into account by using the generalized gradient approximation (GGA).21,22 The total energy code CASTEP was used, which utilized pseudopotentials to describe electron-ion interactions and represents electronic wave functions using a plane-wave basis set. The kinetic energy cutoff was set at 340 eV. 3. Results and Discussion 3.1. Structure and Optical Properties. It was reported that BiTaO4 could be crystallized in two forms: triclinic (hightemperature β-type) and orthorhombic (low-temperature R-type) systems. The powder XRD analysis showed that BiTaO4 was a triclinic system with space group P1 in our sample (JCPDS 00016-0906) (Figure 1). The sample was well crystallized, and all of the diffraction peaks can be indexed with lattice parameters a ) 5.927 Å, b ) 7.674 Å, c ) 7.779 Å, R ) 102.98°, β ) 89.95°, γ ) 93.47°. The surface area of the BiTaO4 powder was 0.26 m2/g. Figure 2 shows the SEM morphology of BiTaO4 powder. The BiTaO4 particles were distributed dispersively, and the average particle size of BiTaO4 was about 3 µm. The photoabsorption ability of the material was detected by UV-vis DRS, as shown in Figure 3. BiTaO4 presented the photoabsorption properties from the UV-light region to visible light shorter than 470 nm. The value of the band gap for BiTaO4 was determined as 2.75 eV by the extrapolation method (see

the inset in Figure 3). The BiTaO4 sample showed a sharp edge, while TiO2-XNX showed a long tail: the main edge due to the oxide at 390 nm and a long tail due to the nitride at about 450 nm. The colors of both materials were pale yellow, indicating that these materials indeed absorbed the visible light. 3.2. Band Structure. The calculated electronic band structure of BiTaO4 was shown in Figure 4. One of the respective highsymmetry points is the G point (0, 0, 0) in terms of the reciprocal basis vectors. In addition, the valence band maximum is considered as the Fermi level, and it is located at the G point in the first Brillouin zone, while the conduction band minimum is also located at the G point. Therefore, a direct optical transition can occur with no significant change in the wave vector for this compound. This direct band gap is one of the desirable properties of photocatalyst materials because light absorption for this kind of material can occur more efficiently as compared to that for material with an indirect band gap. The band gap of BiTaO4 was estimated to be 2.3 eV. Generally, the band gap calculated by DFT was smaller than that obtained experimentally, which was frequently pointed out as a common feature of DFT calculations.23 Figure 5 shows the densities of states (DOS) for BiTaO4. In an effort to obtain more exact information on the atom-specific character of each band, the DOS was further decomposed into the atom orbital (AO)-projected DOS (PDOS) in terms of atomic and angular momentum contributions. The occupied band on the highest energy side (VB) consisted of the hybrid orbital of Bi 6s and O 2p. In contrast, the contribution of Bi 6s was not observed for the bottom of conduction band (CB), and it mainly consisted of Ta 5d orbitals, as shown in Figure 5. In general, the other tantalates, such as NaTaO3, CaTa2O6, and BaTa2O6 whose valence band (VB) only consisted of O 2p orbitals were active under UV-light irradiation,24-26 but were not active under visible-light irradiation. In contrast to them, BiTaO4 could absorb

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Figure 5. Densities of states of BiTaO4.

Figure 7. Changes in the MB concentration and the decrease of TOC over BiTaO4 and TiO2-XNX under visible-light irradiation (λ > 420 nm).

Figure 6. Band structure of BiTaO4 photocatalyst. The VB and CB levels of BiTaO4 are roughly estimated by eq 1.

visible light because Bi 6s orbitals were attributed to the VB on the basis of DFT calculations. Furthermore, the hybridization of the Bi 6s and O 2p level made the VB largely dispersed, which favored the mobility of the photogenerated holes in the VB and was beneficial to the oxidation reaction. Thus, Bi is a potential candidate for such a VB-control element.27 The band edge positions of a photocatalyst are of particular importance in the photocatalytic reaction. There are photoelectrochemical and spectroscoptic methods to determine the band edge of the CB of a semiconductor. Compared with those, eq 1 reported by Scaife28 for the oxides not containing partly filled d-levels can be applied for the approximate determination of the flat band potential

Vfb(NHE) ) 2.94 - Eg

(1)

where Vfb and Eg represent a flat band potential and a band gap, respectively. According to this expression, the rough CB edge potential of BiTaO4 was 0.19 eV with respect to the normal hydrogen electrode. Subsequently the edge position of the VB of BiTaO4 was determined as 2.94 eV. It is well-known that H2O2 and O3 can oxide many organics because of their strong oxidative potential: 1.77 eV (H2O2) and 2.07 eV (O3).27 Compared with them, the present photocatalyst has a much stronger property of oxidation. On the basis of these results, the band potential of BiTaO4 was illustrated in Figure 6. 3.3. Photodegradation Activity. The photocatalytic activity of BiTaO4 was characterized by the experiment of photocatalytically decomposing MB. Figure 7 showed time profiles of C/C0 under visible-light irradiation (λ > 420 nm). When the suspensions were magnetically stirred in the dark to ensure establishment of an adsorption/desorption equilibrium of MB on the sample surface, the MB solution concentration decreased only a little, but after the Xe lamp was turned on, the MB

Figure 8. Wavelength dependence of MB degradation constant with different cutoff filter after light irradiation for 10 h over BiTaO4.

solution concentration decreased markedly. The MB solution decomposed about 78% after 10 h of visible-light irradiation. It was found that the self-photolysis decomposition rate of MB solution was about 15% in 10 h in the same condition. As a comparison, the result of MB decomposition over TiO2-XNX was also given. TiO2-XNX exhibited the high value of photodegradation of MB by 87% in 10 h. Assuming that the decomposing reaction precedes the pseudo-first-order reaction, we estimated the rates of MB decomposed over BiTaO4 and TiO2-XNX to be 0.00243 and 0.00326 min-1. In the catalytic reaction, the surface area is an important factor for the catalytic activity. The surface area of the BiTaO4 sample was only 0.26 m2/g and was about 2 orders smaller than that of TiO2-XNX (29.70 m2/g). Since an efficient photocatalytic reaction process occurs on the photocatalyst surface, the smaller surface area of BiTaO4 might be a barrier to effective photocatalytic reaction. The approach to improve the surface area of BiTaO4 is under investigation and is expected to increase significantly the photocatalytic activity of this material. The decrease of TOC is also shown in Figure 7. Clearly, TOC reduction was slower than the loss of MB, suggesting that the intermediates occurred during the photocatalytic process. When 78% of MB was transformed, about 35% of the mineralized degree was reached under visible-light irradiation (λ > 420 nm) in 10 h. The wavelength dependence of the photocatalytic activity of a semiconductor is often used to distinguish if the reaction is really driven by light. Here, the light wavelength dependence of MB degradation was observed from full arc (without filter) to λ > 510 nm by using different cutoff filters (Figure 8). BiTaO4 showed a photocatalytic activity of 0.00368 min-1 under full arc irradiation. With an increase in the wavelength of cutoff filters applied, the activity decreased, and the activity still

Photocatalytic Degradation of BiTaO4 Photocatalyst

Figure 9. The photostability of BiTaO4 and BiNbO4 samples. Inset: The photocatalytic action of BiTaO4 and BiNbO4 before and after 100 h of photocatalytic reaction.

remained even if a 510 nm cutoff filter was used (the cutoff filter could not prohibit all the visible light below the limitation wavelength). In the present work, it is obvious that the variation of the photocatalytic properties over BiTaO4 was closely relevant to that of light wavelength, suggesting that MB catalytic degradation over BiTaO4 was truly driven by light. To further clarify the effect of catalysis and photolysis, we carried out the decomposition experiments in the dark with BiTaO4 and selfphotolysis decomposition under full arc light irradiation without BiTaO4. But the decomposition of MB in these two conditions was hardly observed and even lower than that over BiTaO4 under light irradiation with the 510 nm cutoff wavelength. All these results confirmed that the MB degradation was inherently the result of a photocatalytic reaction. 3.4. Photocorrosion Suppression. Previous studies by Suslick and co-workers have shown BiNbO4 to be an active photocatalyst for H2 evolution from aqueous methanol under UV irradiation.29 However, they also found that UV irradiation of the BiNbO4 caused reduction of the metal centers, leading to the formation of metallic bismuth and reduced Nb oxide species. So how about photocatalytic degradation MB of BiTaO4 powder under UV-light irradiation? To evaluate the photostability of the catalyst, the recycled experiments for the photodegradation of MB under UV irradiation were performed, and the results were shown in Figure 9. When BiTaO4 was used for the first time, 93% of MB could be degraded in 4 h (reaction constant k ) 0.01075 min-1). After 20 h of photocatalytic reaction, photocatalytic activity did not change. Long photocatalytic reaction times did not affect the photocatalytic activity. Even after irradiation for 100 h, more than 90% of the photocatalytic activity of the initial BiTaO4 was preserved. The bulk and surface of the sample after photocatalysis were also observed, shown in Figures 10 and 11. After 100 h of photocatalysis reaction, the XRD pattern of BiTaO4 varied negligibly, indicating that the structure was well preserved. There was no significant difference in the binding energies of the Bi 4f and Ta 4d peaks before and after reaction. These XPS and XRD results indicated that the surface and bulk of BiTaO4 were unchanged during the reaction and that BiTaO4 functioned as a stable photocatalyst for the degradation of MB. According to reference, we synthesized BiNbO4 using standard solid state reaction techniques. The results of repeated experiments for the durability of MB degradation on BiNbO4 are also shown in Figure 9. After 20 h of photocatalytic reaction, a significant decrease of photocatalytic activity for BiNbO4 was found: the reaction rate constant decreased from 0.00982 min-1 to 0.00443 min-1, namely, only 55% of MB was degraded for

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Figure 10. XRD patterns of BiTaO4 before and after MB degradation: (a) before reaction and (b) after 100 h for the repeatable degradation of MB.

Figure 11. XPS spectra (a) of Bi 4f and (b) of Ta 4d for BiTaO4 photocatalysts before and after 100 h of reaction.

4 h. After exposure under UV light for 100 h, photocatalytic degradation of MB was hardly evident. The drastic decrease of photocatalytic activity for BiNbO4 was related to the photocorrosion. This result was consistent with Suslick’s conclusion. Given the dramatic change in photocatalytic activity, XRD and XPS studies were performed to determine if there was a change in the crystal structure or surface of BiNbO4 after use as a photocatalyst (see the Supporting Information, Figures S1 and S2). The XRD pattern of BiNbO4 varied negligibly after 100 h of photocatalysis. However, in the XPS spectrum, Nb 3d and Bi 4f peaks were shifted to lower binging energy compared to the unused material, indicating that Bi3+ and Nb5+ have been reduced. It can be inferred that substantial photoreduction of BiNbO4 occurred in the surface during use. When a semiconductor photocatalyst is irradiated with light of equal or greater energy than the material’s band gap, an electron-hole pair is created. Ideally, in MB degradation photocatalysis the pairs of charge carriers are transported to the

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Shi et al. electron mobility, most of the electrons would effectively migrate to the surface active sites and participate in the photocatalytic reaction. However, for BiNbO4 powder, the probability of surface reaction of electrons with MB was decreased because of smaller electron mobility. Then a portion of the electrons were consumed in reduction of Bi3+ and Ta5+, which would compete with photocatalysis process. Therefore, it is important to note that the mobility of photoinduce electrons is enhanced due to larger curvature and wider bandwidth of conduction, which will remarkably increase the stability of the BiTaO4 powder. 4. Conclusion BiTaO4 was found as an effective photocatalyst for MB degradation under visible-light irradiation. The band gap of BiTaO4 was estimated to be 2.75 eV from the onset of the absorption edge. It was indicated from the electronic structure study of BiTaO4 by the density functional method that the hybrid of Bi 6s and O 2p orbitals resulted in a decrease in the band gap compared with typical tantalates. Importantly, BiTaO4 powder was very stable during the photocatalytic process. Although the photocatalytic activity of BiTaO4 under visible light was not very high at present, it may be made a promising visible-light-driven photocatalyst by modifying its surface conditions and increasing its surface area.

Figure 12. Electronic band structures of BiTaO4 and BiNbO4: (a) BiTaO4 and (b) BiNbO4.

catalyst surface and participate in the oxidation and reduction of MB. This process is strongly associated with the electronic structure of the semiconductor.30 With BiNbO4 photocatalysts, considering the data presented, it appeared a portion of the electrons were reducing Bi3+and Nb5+ as well as performing the desired reactions. That is to say that a portion of the electrons in the conductor band reduced the photocatalyst itself rather than MB in the surface. However, a reverse phenomenon has been observed in BiTaO4 material where photocorrosion was suppressed. On the basis of the consideration of the electronic structure, here the band structures of BiTaO4 and BiNbO4 were compared to explore the reason for photocorrosion suppression for BiTaO4. The calculated electronic band structures of BiTaO4 and BiNbO4 were shown in Figure 12. The conduction band of BiTaO4 and BiNbO4 mainly consisted of Ta 5d and Nb 4d orbitals, respectively. For the BiTaO4 sample, the conduction band was from 2.31 to 4.19 eV. On the other hand, for the BiNbO4 sample, the conduction band was from 2.56 to 4.06 eV. The energy levels of DOS obtained by DFT calculations did not indicate the absolute values. However, we can compare the degree of dispersion of the bands of BiTaO4 and BiNbO4.31 It was noteworthy that a larger curvature was observed in the conduction band of BiTaO4 than that of BiNbO4. As a consequence, the bandwidth of conduction of BiTaO4 was 1.88 eV, which was wider than that of BiNbO4 (1.50 eV). The mobility of the electronic carrier is proportional to the reciprocal effective mass of the carrier that is in proportion with the curvature and bandwidth.31,32 This means that BiTaO4 has larger electron mobility than BiNbO4. In the present study, the BiNbO4 powder suffered surface photocorrosion during the photocatalysis process. However, as the Nb5+ was substituted by Ta5+, the photocorrsion was efficiently suppressed. The reasons were then proposed as follows: It is known that the photocorrosion and photocatalytic reaction are two competition processes. For BiTaO4 powder, due to larger

Acknowledgment. This work was partly supported by the National Natural Science Foundation of China (20925725 and 50972070) and the National Basic Research Program of China (2007CB613303). Supporting Information Available: XRD and XPS patterns of BiNbO4 before and after MB degradation (Figures S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoffmann, M.; Martin, S.; Choi, W.; Bahnemann, D. Chem. ReV. 1995, 95, 69. (2) Tao, X.; Ma, W.; Zhang, T.; Zhao, J. Angew. Chem., Int. Ed. 2001, 40, 3014. (3) Belhekar, A.; Awate, S.; Anand, R. Catal. Commun. 2002, 3, 453. (4) Falconer, J.; Magrini-Bair, K. J. Catal. 1998, 179, 171. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (6) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (7) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2005, 109, 7323. (8) Wang, D.; Kako, T.; Ye, J. J. Am. Chem. Soc. 2008, 130, 2724. (9) Li, X.; Ye, J. J. Phys. Chem. C 2007, 111, 13109. (10) Kim, H.; Hwang, D.; Lee, J. J. Am. Chem. Soc. 2004, 126, 8912. (11) Fu, H.; Pan, C.; Yao, W.; Zhu, Y. J. Phys. Chem. B 2005, 109, 22432. (12) Tang, J.; Zou, Z.; Ye, J. Catal. Lett. 2004, 92, 53. (13) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459. (14) Man, Y.; Zong, R.; Zhu, Y. Acta Phys. Chim. Sin. 2007, 23 (11), 1671. (15) Fu, H.; Zhang, L.; Yao, W.; Zhu, Y. Appl. Catal. B 2006, 66, 100. (16) Fu, H.; Zhang, S.; Xu, T.; Zhu, Y.; Chen, J. EnViron. Sci. Technol. 2008, 42, 2085. (17) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Chem. Phys. Lett. 2001, 343, 303. (18) Zou, Z.; Ye, J.; Arakawa, H. Solid. State. Commun. 2001, 119, 471. (19) Muktha, B.; Darriet, J.; Madras, G.; Row, T. J. Solid State Chem. 2006, 179, 3919. (20) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384. (21) Payne, M.; Teter, M.; Allan, D.; Arias, T.; Joannopoulos, J. ReV. Mod. Phys. 1992, 65, 1045.

Photocatalytic Degradation of BiTaO4 Photocatalyst (22) Segall, M.; Lina, P.; Probert, M.; Pickard, C.; Hasnip, P.; Clark, S.; Payne, M. J. Phys.: Condens. Matter 2002, 14, 2717. (23) Dreizler, R. M.; Gross, E. K. Density Functional Theory: An Approach to the Quantum Many-Body Problem; Springer-Verlag: Berlin, Germany, 1990. (24) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (25) Zhang, L.; Fu, H.; Zhang, C.; Zhu, Y. J. Phys. Chem. C 2008, 112, 3126. (26) Xu, T.; Zhao, X.; Zhu, Y. J. Phys. Chem. B 2006, 110, 25825. (27) Tang, J.; Zou, Z.; Ye, J. J. Phys. Chem. C 2007, 111, 12779.

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6477 (28) Scaife, D. Sol. Energy 1980, 25, 41. (29) Dunkle, S.; Suslick, K. J. Phys. Chem. C 2009, 113, 10341. (30) Sato, J.; Saito, N.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2001, 105, 6061. (31) Hosogi, Y.; Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. Chem. Mater. 2008, 20, 1299. (32) Matsushima, S.; Nakamura, H.; Arai, M.; Xu, C. Chem. Lett. 2002, 700.

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