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J. Phys. Chem. C 2008, 112, 18076–18081
New Photocatalyst, Sb2S3, for Degradation of Methyl Orange under Visible-Light Irradiation Meng Sun, Danzhen Li,* Wenjuan Li, Yibin Chen, Zhixin Chen, Yunhui He, and Xianzhi Fu* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou UniVersity, Fuzhou, 350002, People’s Republic of China ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: September 7, 2008
A novel visible-light photocatalyst, Sb2S3, was synthesized with a simple method. The specimen was characterized by X-ray diffraction, transmission electron microscopy, Brunauer-Emmett-Teller (BET) surface area analysis, and UV-vis diffuse reflectance spectroscopy. The results revealed that the as-synthesized sample was orthorhombic phase and consisted of rodlike particles. It possessed a surface area of 15.1 m2 · g-1, and the band gap was about 1.66 eV. The photocatalytic activity of Sb2S3 nanorods was evaluated by the decomposition of methyl orange in aqueous solution under visible-light irradiation. The results demonstrated that the photodegradation ratio of methyl orange was up to 97% after 30 min of irradiation, which was much better than that of CdS and TiO2-xNx under the same condition. Meanwhile, the possible mechanism of the photocatalytic reaction had also been studied by liquid chromatography-mass spectrometry, and the •OH had been detected also by terephthalic acid photoluminescence probing technique. 1. Introduction The photodegradation of several toxic compounds using TiO2 as photocatalyst has been widely studied over the past decade.1-4 TiO2 has been proved to be the most excellent photocatalyst for the oxidative degradation of organic compounds under ultraviolet (UV) irradiation.5,6 However, it is active only under UV irradiation (λ < 387 nm) because of its wide band gap (Eg ≈ 3.2 eV),7 which hinders its further application in the visiblelight region (λ > 400 nm). In order to improve the efficiency of utilizing solar energy, attention has been focused on designing visible-light photocatalysts. The process of designing new photocatalysts may also help us understand the nature of photocatalysis and photoinduced chemical reaction. Through modification of TiO2, including doping TiO2 with transition metals or nonmetal atoms8-11 and coupling TiO2 with a narrow band gap semiconductor, a series of visible-light-responsive photocatalysts have been synthesized, such as TiO2-xNx,12,13 TiO2-xCx,14,15 and Mn/TiO2.16 On the other hand, through exploitation of new materials some non-titania-based catalysts have been synthesized; for example, some sulfides have been found having visible-light-driven catalytic activity, such as Bi2S317 and CdS.18 All these findings may provide new insights for the design of non-titania-based visible-light-driven photocatalysts. Sb2S3 is regarded as a promising material for solar energy due to its band gap (Eg ) 1.64 eV)19 which covers the range of the solar spectrum20,21 and determines its applicability as an optoelectronic material. Sb2S3 thin films have already been applied in microwave devices,22 rechargeable storage cells,23 and various optoelectronic devices.24,25 For the strong absorption of visible light, the large surface area, and its special optical and electronic properties arising from the quantum confinement of electrons, it may be possible for Sb2S3 to be used as photocatalytic material. In the present paper, Sb2S3 is found to have exhibited efficient photocatalytic property in the decomposition of methyl orange (MO) and p-hydroxyazobenzene (p* Corresponding authors. Phone and Fax: (+86) 591-83779256. E-mail:
[email protected] (D.L.);
[email protected] (X.F.).
HAB). To the best of our knowledge, Sb2S3 has been rarely reported as photocatalyst in the photocatalytic field.26 2. Experimental Section 2.1. Synthesis of Samples. Sb2S3 nanorods were synthesized with a simple wet chemical method under refluxing condition.27 Potassium sulfocyanate (KSCN) and antimony trichloride (SbCl3) were used as raw materials, and tartaric acid acted as a complexing agent to prevent the hydrolysis of SbCl3. The sample was prepared according to the following procedure. The reaction was carried out in a 250 mL round-bottom flask, in which a 4.50 g (30.0 mmol) of tartaric acid was added to 150 mL of distilled water followed by 1.88 g (8.1 mmol) of SbCl3 with continuous stirring until a transparent solution was obtained. Then, a 1.50 g (15.0 mmol) of KSCN was added to this solution and stirred for 10 min until the potassium thiocyanate was absolutely dissolved, and the whole solution was refluxed at 115 °C for 24 h. Finally, the dark-brown precipitate obtained was filtered and washed with distilled water and absolute ethanol several times and then dried under vacuum at 60 °C for 3 h. 2.2. Characterizations. The phase constitution of the product was determined by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Nifiltered Cu KR radiation. The crystallite size was calculated from the peak half-width with corrections for instrumental line broadening according to the Scherrer equation: D ) 0.89λ/β cos θ, where D is the average crystal size in nm, λ is the Cu KR wavelength (0.15406 nm), β is the half-width of the peak in radians, and θ is the corresponding diffraction angle. The Brunauer-Emmett-Teller (BET) surface area was measured with an ASAP2020 M (Micromeritics Instrument Corp.). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were measured by JEOL model JEM 2010 EX instrument to observe the microstructures and morphology of the samples. The optical properties were analyzed by UV-vis diffuse reflectance spectroscopy (DRS) using a UV-vis spectrophotometer (Cary 500 scan spectrophotometers, Varian, U.S.A.), and BaSO4 was used
10.1021/jp806496d CCC: $40.75 2008 American Chemical Society Published on Web 10/24/2008
Sb2S3 Photocatalyst
Figure 1. XRD patterns of Sb2S3 nanorods: (a) before photocatalytic reaction; (b) after photocatalytic reaction.
as a reflectance standard. Liquid chromatography-mass spectrometry (LC-MS) was used to monitor the degradation process of MO. 2.3. Photocatalytic Activity Measurements. The photocatalytic activity of Sb2S3 nanorods was evaluated by photodegradation of MO aqueous solution. The visible-light source used in the measurements was a 500 W halogen lamp (Philips Electronics) placed in a cylindrical glass vessel in which cold water was circulating in order to avoid overheating. The temperature of the aqueous solution was maintained at 25 °C using a fan that kept blowing air to the aqueous solution continuously during the photoreaction. Two cutoff filters were equipped to completely remove any radiation below 420 nm, ensuring the Sb2S3/MO aqueous mixture was irradiated only by visible light (420 nm < λ < 800 nm, inset of Figure 6). A 0.04 g of powdered photocatalysts was added into a 100 mL Pyrex glass vessel which contained 80 mL of MO aqueous solution (6.1 × 10-5 mol · L-1). Visible-light irradiation was conducted after the suspension was magnetically stirred in the dark for 60 min to reach adsorption-desorption equilibrium. During irradiation, 3 mL aliquots were sampled at the given time intervals and centrifuged to remove the catalyst. The resulting clear liquor was analyzed on a Varian UV-vis spectrophotometer (model: Cary-50) to record the concentration changes of MO. The percentage of degradation is reported as C/C0. C is the absorption of MO at each irradiated time interval of the main peak of the absorption spectrum at 464 nm, and C0 is the absorption of the starting concentration when adsorptiondesorption equilibrium is achieved. In order to test its chemical stability, the as-prepared Sb2S3 was recycled and reused five times in the decomposition of MO under the same condition. After each photocatalytic reaction, the aqueous solution was centrifuged to recycle the Sb2S3 powders that were then dried at 60 °C for another test. Though the sample was stable and not dissolved in the photodegradation under visible-light irradiation, the weight of the catalyst decreased for the loss in the recycling process, and a small amount of the sample was added to maintain the initial weight of 0.04 g for each test. In addition, the degradation of p-HAB in the presence of Sb2S3 was also performed under the same conditions mentioned above. 3. Results and Discussion 3.1. XRD and TEM Analysis. The photocatalytic activity of catalyst is relative to its crystallinity and particle size. The high crystallinity and large surface area of a photocatalyst can effectively increase its photocatalytic activity. As shown in Figure 1, all these diffraction peaks of the synthesized Sb2S3, including not only the peak position but also their relative intensities, can be indexed into the orthorhombic crystalline
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18077
Figure 2. TEM images of Sb2S3 nanorods: (a) image at low magnification; (b) HRTEM image of an Sb2S3 nanorod.
Figure 3. Calculated energy bands of the Sb2S3 nanorods with Eg ) 1.35 eV.
Figure 4. Calculated partial density of states.
structure, which is in line with the standard spectrum (JCPDS no. 42-1393). Seven distinctive peaks at 24.89°, 25.01°, 29.25°, 32.35°, 17.52°, 47.31°, and 35.52° match well with the (130), (310), (211), (221), (120), (151), and (240) crystal planes of orthorhombic Sb2S3, respectively. The absence of any other peak due to impurities indicates the purity of the product. The calculated lattice parameters a )11.18 Å, b ) 11.32 Å, c ) 3.84 Å are in agreement with the reported values. The morphologies of the as-synthesized Sb2S3 are demonstrated in the TEM images shown in Figure 2. As shown in Figure 2a,
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Figure 5. UV-vis diffuse reflectance spectrum of Sb2S3 and optical band gap energy Eg of Sb2S3 (inset).
Figure 8. UV-vis spectral changes of p-HAB in aqueous Sb2S3 dispersions as a function of irradiation time.
Figure 6. UV-vis spectral changes of MO in aqueous Sb2S3 dispersions as a function of irradiation time, and transmittance of the combined filters (inset).
Figure 9. TEM images of the used Sb2S3 nanorods: (a) image at low magnification; (b) HRTEM image of an Sb2S3 nanorod.
Figure 7. Photocatalytic properties of different catalysts: (a) with Sb2S3 (40 mg) and visible light, (b) with Sb2S3 in dark, (c) with CdS (40 mg) and visible light, (d) with P25-N(40 mg) and visible light, and (e) with P25 (40 mg) and 365 nm UV light.
the as-synthesized Sb2S3 nanorods have diameters in the range of 50–100 nm and lengths of 100–1000 nm. A representative HRTEM image showing clear lattice fringes is shown in Figure 2b. The interlayer spacing of 0.277 nm corresponds to the (221) plane of Sb2S3. 3.2. Band Structure and BET Surface Area. We have performed density functional theory (DFT) calculations with the GGA density functional by the PBE method28 to study its band structure. Figure 3 shows the calculated band structure of the valence band maximum (VBM) and the conduction band minimum (CBM), which are located at the G and Y points, respectively. It means that Sb2S3 is an indirect-gap semiconductor material. A minimum forbidden gap between VBM and CBM is ca. 1.35 eV, which is lower than the experimental result of 1.66 eV. The reason for the difference may be that the discontinuity in the exchange-correlation potential is not taken into account in the theoretical calculation, underestimating the energy gap between unoccupied and occupied orbitals. Figure 4 presents the calculated partial density of states (PDOS). The
Figure 10. Cycling runs in the photodegradation of MO in the presence of Sb2S3 under visible-light irradiation.
conduction band in the range of 1-4 eV is mainly composed of Sb 5p and S 3p orbitals. The valence band at about -6 to 0 eV primarily consists of S 3p and Sb 5p orbitals. The calculated results also show that Sb2S3 is a p-type semiconductor. The optical properties of the nanorods have been studied by UV-vis reflectance spectroscopy, and the corresponding UV-vis reflectance spectrum of the nanorods is shown in Figure 5. It can be seen that Sb2S3 shows a strong photoabsorption property in the visible-light region. It is well-known that the relation between absorption coefficient and band gap energy of an indirect-gap semiconductor can be described by the formula [F(R)E]1/2 ) A(E - Eg), where E and Eg are the photon energy and optical band gap energy, respectively, and A is the characteristic constant of semiconductors. In the equation, [F(R)E]1/2 has a linear relation with E. Extrapolating the linear relation to [F(R)E]1/2 ) 0 gives the band gap Eg of the sample as shown in the inset of Figure 5. The band gap of the synthesized Sb2S3 is approximately 1.66 eV, which is quite comparable to the values reported. Therefore, it can easily be activated by visible light in photocatalytic reactions.
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Figure 11. (a) •OH-trapping PL spectra of Sb2S3/TA solution. (b) Plot of the induced PL intensity (at 426 nm) against irradiation time. (c) Plot of the photocatalytic degradation of MO (10 ppm) by Sb2S3 (0.02 g) particles against irradiation time.
irradiation, and the peak (λ ) 464 nm) nearly disappears after 30 min while a new peak (λ ) 245 nm) emerges.
Figure 12. Chromatogram in the degradation process of MO: (a) the origin solution after adsorption-desorption equilibrium without visiblelight irradiation; (b) after 5 h of irradiation.
In addition, we have also tested its BET surface. The BET specific surface area of the sample is calculated from N2 isotherms at 77 K. It is found to be 15.1 m2 · g-1. 3.3. Photocatalytic Activity. The photocatalytic activities of Sb2S3 nanorods are evaluated by decomposition of MO in aqueous solution under visible-light irradiation. Temporal changes in the concentration of MO are monitored by examining the variations in maximal absorption in UV-vis spectra at 464 nm. Figure 6 shows the temporal evolution of the spectral changes of the MO mediated by Sb2S3. It can be found that the concentration of MO decreases quickly under visible-light
In order to exhibit the high photoactivity of Sb2S3, TiO2-xNx (P25-N) and CdS are used as references, which have been researched relatively a lot as visible-light-responsive catalysts. In addition, TiO2-P25 was generally accepted as a standard under UV light irradiation, and it was also used to compare the photoactivity of Sb2S3. It can be clearly seen from Figure 7 that after just 30 min, MO is almost completely decomposed in the presence of Sb2S3 while there is no degradation in the case of P25-N, and CdS can only exhibit lower activity than that of Sb2S3. In addition, after 50 min of irradiation, the degradation ratio of MO for Sb2S3 is about 97%, while that of P25 is about 52%. That is to say, the photoactivity of Sb2S3 under visible light is even better than that of P25 under UV irradiation. p-HAB is also selected as a model pollutant. The analysis of the degradation process may be much easier for its simple structure. In addition, p-HAB cannot absorb visible light and its peak of light absorption band is at the wavelength of 347 nm. So, the degradation of p-HAB in the presence of Sb2S3 under visible-light irradiation cannot be a photosensitized reaction. Figure 8 shows the concentration changes of p-HAB
Figure 13. Mass spectrum view of MO (m/z ) 304): (a) the origin solution after adsorption-desorption equilibrium without visible-light irradiation; (b) after 5 h of irradiation.
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Figure 14. Mass spectrum view of the main byproduct.
under visible-light irradiation over Sb2S3. The photocatalytic conversion ratio is about 81%. 3.4. Photocatalytic Stability. The stability of photocatalyst is important for its application. The stability of Sb2S3 was researched by XRD and TEM characterizations. Figure 1 shows that the XRD patterns of the fresh and used samples are uniform in peak position and relative intensities, which indicates that the Sb2S3 nanorods are stable in the photocatalytic progress. Figure 9 shows the TEM images of the used sample. In comparison with Figure 2, we can see the morphology of Sb2S3 has no obvious changes. The HRTEM image of the used sample reveals the clear lattice fringes with d ) 0.364 nm, which corresponds to the (101) crystallographic plane of Sb2S3. The combination of the above results and conclusions may prove that the Sb2S3 nanorods are chemically stable in the MO decomposition process. Furthermore, in the lifetime test (shown in Figure 10), Sb2S3 only exhibits slight loss of activity. The good photocatalytic stability proves that Sb2S3 is stable or just etched in a small degree from another point of view. The solubility product constant of Sb2S3 is 2.9 × 10-59, which means it is hardly dissolved in water. In this experiment, the catalyst was separated by centrifugation, and the powders can
Sun et al. be separated completely form the suspension, so the water system will not be polluted for the toxicity of Sb2S3. However, for its practical application, further work needs to be done to make its reuse more convenient. 3.5. Mechanism. Terephthalic acid photoluminescence probing technique (TA-PL) has been widely used in the detection of hydroxyl radicals.29 2-Hydroxyl-terephthalic acid, which is generated when terephthalic acid captures the hydroxyl radicals, performs a strong fluorescence characteristic, so we have detected the hydroxyl radicals indirectly by monitoring the fluorescence intensity changes of Sb2S3/TA solution. Figure 11 shows the fluorescence spectra of Sb2S3/TA solution under visible-light irradiation, and the fluorescence intensity increases steadily with the irradiation time within 12 min. It can be concluded that hydroxyl radicals are indeed generated on Sb2S3 under visible-light irradiation, and the rate of hydroxyl radical generation is in accordance with that of MO degradation, suggesting •OH may be the main reactive species. The introduction of atmospheric pressure ionization sources has transformed LC-MS from a difficult and unreliable technique into a routine analytical tool. It is a suitable analytical approach to study the possible mechanism of the MO decomposition. Figure 12 reports the chromatograms of the MO solution in the presence of Sb2S3. Typical intermediates generated in the degradation of MO are identified from negative ionization mode mass spectra. The significant peaks presented at the various degradation times are labeled with the corresponding m/z values. The mass peak at m/z ) 304.0 of the initial solution appearing at 6.9 min is that of the MO dye. When the adsorption-desorption equilibrium is achieved, the intensity of that peak is about 1.53 × 105 counts as shown in Figure 13a. Then after 5 h of visible-light irradiation, its intensity reduces to 5.14 × 102 counts and some new peaks (m/z ) 335, 143, 255) with strong intensity can be clearly seen in Figure 13b. The intensity of the strongest peak (m/z ) 335) is 3.46 × 103 counts.
Figure 15. Proposed photodegradation pathways for MO in the Sb2S3 suspension.
Sb2S3 Photocatalyst In addition, there are many other fragment ions engendered in the degradation process, and the corresponding peaks are also labeled by m/z values (m/z ) 173, 240, 271, 212) as shown in Figure 14. The m/z values mentioned above may have some deviation from the molecular weight because of losing hydrogen. The emergence of new peaks may prove that it is really a photocatalytic progress rather than adsorption. It suggests that MO has been decomposed but not mineralized completely, though the detailed mechanism of the photocatalytic process on the Sb2S3 surface is still not completely clear. However, according to the m/z values of the byproducts and the structure of MO, we have inferred the possible structures of the degradation products and the pathway of photodegradation visualized in Figure 15. The potential photocatalytic chemical process in the degradation of MO may involve several steps: (a) When Sb2S3 nanorods were irradiated by visible light of energy greater than its band gap, electron-hole pairs would be generated and separated partially. (b) Some of the electrons recombined with holes while others transferred onto the surface of Sb2S3 quickly and were then captured by the adsorbed oxygen molecule to yield •O2- and H2O2; the holes were partially recombined with electrons and partially reacted with water molecule to produce •OH. The produced H2O2 has already been proved to be helpful in the degradation process during the experiment.30,31 (c) The •OH radical which was recognized as the main reactive species responsible for the organic degradation could also be generated when •O2- interacted with H2O2. •O2-, H2O2 and •OH all could oxidize the MO molecule absorbed on the surface of Sb2S3 nanorods in some degree. As analyzed above, MO was partially oxidized into carbon dioxide and water, whereas a little was decomposed into fragmental ions which were detected by LC-MS. The special product C7H11NO42-, which was produced after the NdN double bond of MO is destroyed, has been found by LC-MS. In a word, the high photocatalytic activity of Sb2S3 may be mainly attributed to the formation of •OH. 4. Conclusions In summary, we have exploited a novel photocatalystsSb2S3 nanorods with diameters of 50–100 nm and lengths of 100–1000 nm prepared by a simple wet chemical method. It is an indirectgap material with the intrinsic band gap of 1.66 eV. The results of MO degradation under visible-light irradiation showed that the photocatalytic activity of Sb2S3 was higher than that of P25-N and CdS, and the conversion ratio of MO was up to 97%. The possible pathway of the photocatalytic decomposition of MO has been proposed. The active species, hydroxyl radicals, were detected by TA-PL and proved to be the most possible reason for the photodegradation of dyes. The successful application of Sb2S3 in the photocatalytic field may also provide useful insight for the development of other visiblelight-absorbing semiconductors.
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18081 Acknowledgment. This work was financially supported by the NNSF of China (20537010, 20873023, and 20677010), an “863” Project from the MOST of China (2006AA03Z340), the National Basic Research Program of China (973 Program, 2007CB613306), and the Natural Science Foundation of Fujian, China (2003F004, 2005HZ1007). The authors are indebted to Professor Yongfan Zhang for the discussions of band structure. References and Notes (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Yan, M.; Chen, F.; Zhang, J.; Anpo, M. J. Phys. Chem. B 2005, 109, 8673. (4) Yu, J. G.; Yu, H. G.; Cheng, B.; Zhao, X. J.; Yu, J. C.; Ho, W. K. J. Phys. Chem. B 2003, 107, 13871. (5) Chen, Y.; Dionysiou, D. J. Mol. Catal. A: Chem. 2005, 244, 73. (6) Ge, L.; Xu, M.; Sun, M.; Fang, H. Mater. Res. Bull. 2006, 41, 1596. (7) Gericher, H. Top. Appl. Phys. 1979, 31, 115. (8) Diwald, O.; Thompson, T. L.; Zubkov, T. J. Phys. Chem. B 2004, 108, 6004. (9) Ge, L.; Xu, M.; Fang, H. Mater. Lett. 2007, 61, 63. (10) Li, X. Z.; Li, F. B.; Yang, C. L.; Ge, W. K. J. Photochem. Photobiol., A 2001, 141, 209. (11) Ge, L.; Xu, M.; Fang, H. J. Sol.-Gel Sci. Technol. 2006, 40 (1), 65. (12) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (13) Sakthivel, S.; Kisch, H. ChemPhysChem 2003, 4, 487. (14) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (15) Sakthivel, S.; Kisch, H. Angew. Chem. 2003, 115, 5057. (16) Yamashita, H.; Harada, M. J. Photochem. Photobiol., A 2002, 148, 257. (17) Bessekhouad, Y.; Mohammedi, M.; Trari, M. Sol. Energy Mater. Sol. Cells 2002, 73 (3), 339. (18) Karunakaran, C.; Senthilvelan, S. Sol. Energy 2005, 79, 505. (19) Bube, R. J. Appl. Phys. 1960, 31, 315. (20) Nair, M. S.; Pena, Y.; Campos, J.; Garcia, V. M.; Nair, P. K. J. Electrochem. Soc. 1998, 114, 2113. (21) Savadogo, O.; Mondal, K. C. Sol. Energy Mater. Sol. Cells 1992, 26, 117. (22) Grigas, J.; Meshkauskas, J.; Orliukas, A. Phys. Status Solidi 1976, 37, K39. (23) Rajpure, K. Y.; Bhosale, C. H. Mater. Chem. Phys. 2000, 64, 70. (24) Chockalingam, M. J.; Nagaraja, K.; Rangarajan, N.; Suryanarayana, C. V. J. Phys. D: Appl. Phys. 1970, 3, 1641. (25) George, J.; Radhakrishnan, M. K. Solid State Commun. 1980, 33, 987. (26) Li, K. Q.; Huang, F. Q.; Lin, X. P. Scr. Mater. 2008, 58, 834. (27) Ota, J.; Srivastava, S. K. Cryst. Growth Des. 2007, 7, 343. (28) Perdew, P.; Burke, S.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (29) Barreto, J. C.; Smith, G. S.; Strobel, N. H. P.; McQuillin, P. A.; Miller, T. A. Life Sci. 1994, 56 (4), 89. (30) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. EnViron. Sci. Technol. 1988, 22, 798. (31) Hoffmann, A. J.; Carraway, E. R. EnViron. Sci. Technol. 1994, 28, 776.
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