J. Phys. Chem. C 2007, 111, 14799-14803
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Fe@Fe2O3 Core-Shell Nanowires as an Iron Reagent. 3. Their Combination with CNTs as an Effective Oxygen-Fed Gas Diffusion Electrode in a Neutral Electro-Fenton System Zhihui Ai,† Tao Mei,† Juan Liu,† Jinpo Li,† Falong Jia,† Lizhi Zhang,*,† and Jianrong Qiu‡ Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, and National Coal Combustion Laboratory, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China ReceiVed: May 11, 2007; In Final Form: July 29, 2007
In this study, core-shell Fe@Fe2O3 nanowires and multiwall carbon nanotubes (CNTs) were combined with poly(tetrafluoroethylene) (PTFE) and then used as an oxygen-fed gas diffusion electrode. The resulting Fe@Fe2O3/CNT composite electrode was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM). A novel electroFenton (E-Fenton) system was established with the resulting gas diffusion cathode, where hydrogen peroxide was electrogenerated by the reduction of O2 adsorbed on the cathode and iron ions were produced by the slow leakage of Fe@Fe2O3 nanowires, simultaneously. The degradation of rhodamine B (RhB) in this E-Fenton system reached 91.5% in 120 min at neutral pH, indicating this system is very promising for wastewater treatment. Moreover, cyclic voltammetry (CV) experiments were performed to probe the reactions on the Fe@Fe2O3/CNT electrode.
Introduction The classic Fenton reactions, consisting of the ferrous iron and hydrogen peroxide, have proven to be effective methods to treat organic pollutants in wastewater, and the generally accepted free radical chain mechanism for the Fenton reaction is shown in eqs 1-5.1
Fe2+ + H2O2 f Fe3+ + •OH + OH-
(1)
Fe3+ + H2O2 f Fe2+ + •HO2 + H+
(2)
Fe
2+
+ •OH f Fe
3+
+ OH
-
Meanwhile, iron ions (Fe2+ or Fe3+, Fen+) is added to the contaminated water to catalyze electrogenerated H2O2 to produce the oxidizing agent •OH via Fenton reactions. In the E-Fenton system, the regeneration of Fe2+ can occur either by a direct cathodic reaction (eq 7), by the oxidation of an organic (eq 8), or by the reaction with H2O2 (eq 9).9 Compared with classic Fenton systems, the E-Fenton system can avoid the addition of expensive H2O2 and maintain an almost constant concentration of H2O2 by electrogeneration during the whole pollutant removal process
O2(g) + 2H+ + 2e- f H2O2
(6)
Fe3+ + e- f Fe2+
(7)
Fe3+ + •R f Fe2+ + R+
(8)
(3)
‚OH + H2O2 f •HO2 + H2O
(4)
•HO2 + •HO2 f H2O2 + O2
(5)
Although the classic Fenton system is very simple, the high cost of H2O2 and the undesirable recycle of ferrous iron are found to be the main drawbacks for its large-scale application in wastewater treatment engineering.2 Since Casado and his coworkers reported an in situ electrogenerated H2O2 process using gas diffusion cathodes in 1994, the E-Fenton process, on the basis of the combined use of cathodically generated hydrogen peroxide and iron ions as the catalyst, was developed in order to overcome the high cost of H2O2.2,3 Recently, an increasing number of papers have been published to deal with the destruction of toxic and refractory organic pollutants in waters by means of E-Fenton methods.3-8 The E-Fenton system can continuously supply H2O2 to the contaminated water through the two-electron reduction of oxygen gas given by eq 6. * To whom correspondence should be addressed.E-mail: zhanglz@ mail.ccnu.edu.cn. Tel/Fax: +86-27-6786 7535. † Central China Normal University. ‡ Huazhong University of Science and Technology.
Fe3+ + H2O2 rf [Fe-O2H]2+ + H+ rf Fe2+ + •HO2 (9) Ideal electrocatalysts for H2O2 electrogeneration should have the following characteristics: a high electrical conductivity, easy accessibility for the reactant gas to the electrocatalyst, and good corrosion resistance, especially under the highly oxidizing conditions.10 Because of the low oxygen solubility in the aqueous solution and slow mass transport, the reduction of oxygen to produce hydrogen peroxide with a high yield happens only on certain cathodic materials, such as mercury, gold, or carbon. Carbon materials are desirable cathodic electrodes for the E-Fenton system because of their relative stability, conductivity, high surface area, and chemical resistance. Most studied carbon-based electrodes mainly focus on traditional forms of carbon such as reticulated vitreous carbon,11,12 carbon felt,13 activated carbon fiber,14,15 and carbon-PTFE O2-diffusion cathodes.7,16-18 Much less attention has been paid to a new
10.1021/jp073617c CCC: $37.00 © 2007 American Chemical Society Published on Web 09/15/2007
14800 J. Phys. Chem. C, Vol. 111, No. 40, 2007 carbon material, carbon nanotubes (CNTs), as the cathode for H2O2 electrogeneration, although CNTs have good electrical conductivity and mechanical strength, as well as relative chemical inertness to most electrolyte solutions, high surface activity, and a wide operational potential window.19-24 Recently, we synthesized Fe@Fe2O3 core-shell nanowires through the reduction of Fe3+ ions with sodium borohydride in aqueous solution in an ambient atmosphere.25 These Fe@Fe2O3 core-shell nanowires could be used as an iron reagent to efficiently degrade RhB in a sono-Fenton system at pH ) 2 or at a neutral pH value.26,27 In view of large-scale wastewater treatment and low operation costs, the E-Fenton system may be more attractive than the sono-Fenton one because ultrasonic devices are often expensive and difficult to be scaled up enough for industrial wastewater treatment. Therefore, recently, we designed a neutral E-Fenton system with an Fe@Fe2O3/ACF composite cathode for the treatment of wastewater.15 In this study, we plan to establish a new and scalable E-Fenton system on the basis of a novel oxygen-fed gas diffusion electrode fabricated through combining Fe@Fe2O3 core-shell nanowires and CNTs with poly(tetrafluoroethylene) (PTFE). In this new E-Fenton system, H2O2 with a constant concentration can be continuously electrogenerated on the Fe@Fe2O3/CNT composite electrode. Meanwhile, the recycle of iron reagent can be realized because of the utilization of Fe@Fe2O3 nanowires at neutral pH.26,27 The effective RhB degradation in this new E-Fenton system suggests that this system is very promising for wastewater treatment. Experimental Section Chemicals. All chemicals in this study were of commercially available analytical grade. Deionized water was used in all experiments. Synthesis of Fe@Fe2O3 Nanowires. The Fe@Fe2O3 nanowires were synthesized by the reaction between ferric chloride and sodium borohydride described previously.25,26 Preparation of the Fe@Fe2O3/CNT Electrodes. The Fe@Fe2O3/CNT cathodes were prepared as follow. CNTs with widths of 40-60 nm and lengths of 3-5 µm (Shenzhen NanoHarbor Co.), PTFE suspension (60 wt %), and Fe@Fe2O3 nanowires were first mixed in ethanol, ground, and then dried at 80 °C to form a dough-like paste. The paste was finally rolled to be a thin layer of 0.2 mm thickness. This layer of catalyst was fixed between two pieces of nickel mesh current collectors and then dried. The final thickness of the electrode was controlled at about 0.50 mm. The optimal mass ratio of CNTs/ Fe@Fe2O3/PTFE was found to be 10:30:1 (Supporting Information, Figure S1). The Fe@Fe2O3/graphite cathode was made with a similar method by using commercial graphite powder instead of CNTs, and the blank CNT cathode was prepared by mixing CNTs and PTFE suspension with a mass ratios of 10:1 in the absence of Fe@Fe2O3 nanowires. The Fe@Fe2O3/ACF cathode was prepared by loading Fe@Fe2O3 nanoparticles on ACF, described previously.15 Characterization of the Fe@Fe2O3/CNT Electrodes. X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation (λ ) 1.54178 Å). Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were performed on a LEO 1450VP scanning electron microscope. Transmission electron microscopy (TEM) images were recorded on a Tecnai 20 FEG transmission electron microscope. Degradation of Rhodamine B. Degradation of Rhodamine B by E-Fenton processes was preformed in a divided thermo-
Ai et al.
Figure 1. XRD pattern of the as-prepared Fe@Fe2O3/CNT electrode.
static cell of 150 mL volume by using a CHI-660B (Shanghai, China) as the potentiostat.28 During our experiments, the potential between the anode and the cathode was controlled at 1.2 V (∆E ) 1.2 V). The anode was a Pt sheet (purity: 99.99%) of 2.0 cm2 area obtained from Beijing Academy of Steel Service (China), also serving as a pseudoreference electrode. The asprepared Fe@Fe2O3/CNT electrode was used as the working cathode (3.0 cm2 in area). The initial concentration of RhB was 1.044 × 10-5 M. A 0.05 mol‚L-1 Na2SO4 aqueous solution was used as the electrolyte to increase the conductivity. The initial pH of the dye solution was about 6. In some cases, the initial pH value of the RhB solution was adjusted to 3 or 8 by the addition of 0.10 M H2SO4 or 0.10 M NaOH, respectively; 5 L‚min-1 of fresh air was fed to the cathode. The solution was magnetically stirred and maintained at room temperature during the whole degradation reaction. Before degradation, the system was kept in the dark for 30 min to establish adsorption/ desorption equilibrium between the solution and electrode in the cathodic cell. Analytical Methods. An UV-visible spectrophotometer (S3100, Scinco) was used to monitor the concentration of RhB in water at its maximum absorption wavelength of 555 nm. The analysis of hydrogen peroxide was carried out using the UVvis spectrum and off-line sampling.29 The concentrations of total iron ions (Fe2+ and Fe3+, Fen+) in the solution were measured by atom absorption spectrometry (WFX-1F2, China). Cyclic voltammetry (CV) was performed on a computer-controlled CHI-660B electrochemical workstation in the cathodic cell with three-electrodes, with the Fe@Fe2O3/CNT electrode as the working electrode, a Ag/AgCl electrode (saturated with KCl) as the reference electrode, and a platinum electrode as the auxiliary electrode between -1.80 and +1.80 V. The scan rate was 50 mV/s. Results and Discussion Characterization of the Fe@Fe2O3/CNT Electrodes. X-ray diffraction was used to characterize the phase structure of the electrodes. Figure 1 displays XRD patterns of the as-prepared Fe@Fe2O3/CNT electrode. The diffraction peaks at a 2θ value of 26.5° are ascribed to the (002) reflection of the CNTs (Joint Committee on Powder Diffraction Standards (JCPDS), file No. 74-444).30 The diffraction peaks at a 2θ value of 18.1° belong to the PTFE (JCPDS, file No. 472217). The diffraction peaks at 2θ values of 35.6 and 44.3° can be assigned to the reflection of the (110) reflection of Fe2O3 (JCPDS, file No. 73-603) and the (110) reflection of Fe (JCPDS, file No. 3-1050), respectively.
Fe@Fe2O3 Core-Shell Nanowires as an Iron Reagent
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14801
Figure 2. The SEM image of the as-prepared Fe@Fe2O3/CNT electrode. The inset is the EDX pattern of the electrode.
Figure 5. Degradation of RhB in the E-Fenton system with a Fe@Fe2O3/CNT cathode at different pH values. The inset is the UVvis spectral changes of RhB with reaction time at neutral pH.
Figure 3. TEM image of the as-prepared Fe@Fe2O3/CNT electrode.
Figure 6. Concentration changes of iron ions as a function of the reaction time in the E-Fenton system with a Fe@Fe2O3/CNT cathode at neutral pH.
Figure 4. The degradation curves of RhB in E-Fenton systems with different cathodes at neutral pH.
These XRD results suggest that PTFE successfully combined Fe@Fe2O3 nanowires together with CNTs in the resulting electrodes. Figure 2 presents the SEM image of the prepared Fe@Fe2O3/ CNT electrodes. The surface layer of the electrode was flat.
Figure 7. Concentration changes of hydrogen peroxide as a function of the reaction time in the E-Fenton system with a Fe@Fe2O3/CNT cathode at neutral pH.
Many macro or mesopores were observed on the surface, suggesting that the electrode may be easily penetrated by the electrolyte and gas components in the solution and provide many
14802 J. Phys. Chem. C, Vol. 111, No. 40, 2007
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Figure 8. Cyclic voltammetry of the Fe@Fe2O3/CNT cathodes in a 1.044 × 10-5 M RhB solution at neutral pH. The inset shows cyclic voltammograms of the Fe@Fe2O3/CNT cathodes in the background electrolyte (a 0.05 mol·L-1 Na2SO4 aqueous solution) at neutral pH under deoxygenated (black curve) and oxygenated (red curve) conditions.
sites for oxygen adsorption. Energy-dispersive X-ray analysis (EDX) reveals that carbon, iron, and oxygen elements coexist in the Fe@Fe2O3/CNT electrodes (inset of Figure 2), which is consistent with the result of XRD. The morphology of the as-prepared electrodes was further examined by TEM measurements. The TEM image confirms that both CNTs and Fe@Fe2O3 nanowires coexisted in the electrode (Figure 3). Obviously, they were separated by ultrasound irradiation during the TEM sample preparation. Degradation of RhB in the E-Fenton System. Degradation of RhB in water was used to test the efficiency of the E-Fenton system with the Fe@Fe2O3/CNT cathode in this study because RhB was found to be stable at different pH values. In order to investigate the effect of different carbon materials on the E-Fenton reaction, an Fe@Fe2O3/CNT cathode, an Fe@Fe2O3/ graphite cathode, an Fe@Fe2O3/ACF cathode, and a blank CNT cathode without binding Fe@Fe2O3 nanowires were compared (Figure 4). After 120 min of reaction, no obvious RhB degradation was observed on the blank CNT cathode. However, 91.5, 74.1, and 60.1% of RhB were degraded on the Fe@Fe2O3/ CNT, Fe@Fe2O3/ACF, and Fe@Fe2O3/graphite cathodes, respectively. The standard deviations for the % degradation were less than 5%. This confirms that the presence of Fe@Fe2O3 nanowires is crucial for the proceeding of the E-Fenton reaction. The nanowires could serve as an effective heterogeneous Fenton reagent to catalytically produce •OH radicals at neutral pH.26,27 The comparisons reveal that the Fe@Fe2O3/CNT cathode is superior to the Fe@Fe2O3/ACF and Fe@Fe2O3/graphite ones, which may be attributed to better diffusion conditions offered by the porous Fe@Fe2O3/CNT cathode where two-dimensional electrochemistry is converted to “quasi-three-dimensional” behavior due to the large distributed area.31 It is known that oxygen reduction at various carbon electrodes is crucial for electrogeneration of H2O2. In order to understand the changes in the overpotential for oxygen reduction at various carbon electrodes, we studied the voltammograms for the oxygen
reduction at different carbon electrodes in a 0.05 mol‚L-1 Na2SO4 aqueous solution (Supporting Information, Figure S2). The reduction peak that appeared at -0.50 V was ascribed to the oxygen reduction. We found that this peak current around the CNT cathode was significantly larger than those at the ACF and graphite cathodes, suggesting that the overpotentials for oxygen reduction at these three electrodes are different and the oxygen reduction is easier at the CNT cathode than at the other two cathodes. This result confirms the result shown in Figure 4. The influence of the initial pH on the degradation efficiency of the E-Fenton system with the Fe@Fe2O3/CNT cathode was also investigated (Figure 5). It was found that the degradation ability of the Fe@Fe2O3/CNT cathode changed with varying pH value. After 120 min of reaction, the degradation ratio at neutral pH was slightly lower than that (99.6%) at pH 3 but higher than that (79.9%) at pH 8. These comparisons reveal that the new E-Fenton system with the Fe@Fe2O3/CNT cathode can effectively work under different pH values. However, neutral pH is better for the recycling of Fe@Fe2O3 nanowires according to our previous studies.26,27 Therefore, we studied the E-Fenton system with the Fe@Fe2O3/CNT cathode at neutral pH in detail. UV-vis spectral changes of the RhB dye solution as a function of reaction time can be used to clarify the changes of molecular and structural characteristics of RhB during E-Fenton oxidation at neutral pH values (inset of Figure 5). The absorption spectrum of the RhB solution was characterized by its maximum absorption at 555 nm in the visible region, which was attributed to the chromophore-containing azo linkage (conjugated xanthene ring) of the dye molecules.32 The absorption peak at 555 nm diminished with increasing E-Fenton reaction time, indicating that the rapid degradation of RhB was attributed to the decomposition of the conjugated xanthene ring in RhB.28 This is reasonable because the NdN bond of the azo dye is a most active site for attack by •OH radicals.33
Fe@Fe2O3 Core-Shell Nanowires as an Iron Reagent To understand high degradation efficiency of this E-Fenton process at neutral pH, the two main species involved in the Fenton reactions, iron ions (Fen+) and hydrogen peroxide (H2O2), were monitored (Figure 6 and 7). As shown in Figure 6, the Fen+ concentration did not increase linearly with reaction time. After approximately 90 min, the Fen+ leaching from the electrode reached steady-state concentrations and remained almost constant between 22.6 × 10-3 and 24.4 × 10-3 M. This result suggests that the Fe@Fe2O3/CNT composite cathode can be reused. Figure 7 presents the concentration of H2O2 generated in the solution during the E-Fenton process at pH 6.0, and the applied potential in our present two-electrode system is 1.2 V (∆E )1.2 V). As illustrated in Figure 7, the H2O2 concentration progressively grows and also reaches a balance after 90 min. After 120 min of the E-Fenton reaction, 1.65 × 10-2 mM H2O2 was generated on the Fe@Fe2O3/CNT cathode. These results indicate that the Fe@Fe2O3/CNT electrode is a favorable cathode for H2O2 electrogeneration. This phenomenon could be likely explained by the fact that the as-prepared Fe@Fe2O3/CNT electrode had numerous macro- or mesopores (Figure 2) so that O2 could easily be electroreduced on the cathode surface to form more H2O2. Obviously, more H2O2 results in more •OH radicals via Fenton reactions. Therefore, the simultaneous and effective formation of Fen+ and H2O2 during the reaction is the reason for the high efficiency of this E-Fenton system with the Fe@Fe2O3/CNT cathode. In order to further understand the oxidation and reduction reactions on the Fe@Fe2O3/CNT cathode in the E-Fenton process at neutral pH, cyclic voltammetric experiments were performed. Figure 8 is the typical cyclic voltammograms of the Fe@Fe2O3/CNT electrode in the 1.044 × 10-5 M RhB solution at neutral pH. A couple of well-defined peaks were observed at ∼0.62 V, which were ascribed to the overall redox potentials of the Fe3+/Fe2+ couple. This means that in this E-Fenton process, Fe2+ could be easily electroregenerated from Fe3+. This pathway to regenerate Fe2+ is very important in the Fenton reaction system. Meanwhile, the peaks at about -0.52 and 1.16 V are assigned to the redox processes of O2 and RhB, respectively.34,35 In addition, cyclic voltammograms of the Fe@Fe2O3/CNT cathode in the background electrolyte (a 0.05 mol‚L-1 Na2SO4 aqueous solution) at neutral pH under the deoxygenated and oxygenated conditions are shown in the inset of Figure 8. It was found that the peak at about -0.52 V under the oxygenated condition was stronger than that under the deoxygenated one, confirming the Fe@Fe2O3/CNT cathode was effective in adsorbing and electrochemically reducing O2 to produce H2O2. Conclusions In summary, we developed a novel E-Fenton system with a Fe@Fe2O3/CNT oxygen-fed gas diffusion cathode. The cathode was fabricated by combining core-shell Fe@Fe2O3 nanowires and multiwall carbon nanotubes with poly(tetrafluoroethylene). This E-Fenton system produced iron ions in situ from coreshell Fe@Fe2O3 nanowires and simultaneously reduced oxygen into hydrogen peroxide. These two Fenton reagents further reacted together to produce hydroxyl radicals to degrade RhB effectively at neutral pH. The high efficiency of this system makes it very promising for wastewater treatment. Acknowledgment. This work was supported by the National Basic Research Program of China (973 Program) (Grant 2007CB613301), the National Science Foundation of China (Grants 20673041, 20503009, and 20777026), the Outstanding
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14803 Young Research Award of the National Natural Science Foundation of China (Grant 50525619), the Open Fund of the Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, the Hubei Province (Grants CHCL0508 and CHCL06012), and the Postdoctors Foundation of China (Grant 20070410935). Supporting Information Available: The degradation curves of RhB in E-Fenton systems with Fe@Fe2O3/CNT cathodes with different mass ratios of CNTs/Fe@Fe2O3/PTFE at neutral pH and the cyclic voltammetry of various carbon cathodes in a 0.05 mol‚L-1 Na2SO4 aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ma, J. H.; Song, W. J.; Chen, C. C.; Ma, W. H.; Zhao, J. C.; Tang, Y. L. EnViron. Sci. Technol. 2005, 39, 5810. (2) Ventura, A.; Jacquet, G.; Bermond, A.; Camel, V. Water Res. 2002, 36, 3517. (3) Brillas, E.; Casado, J. Chemosphere 2002, 47, 241. (4) Panizza, M.; Cerisola, G. Water Res. 2001, 35, 3987. (5) Brillas, E.; Mur, E.; Sauleda, R.; Sanchez, L.; Peral, J.; Domenech, X.; Casado, J. Appl. Catal., B 1998, 16, 31. (6) Brillas, E.; Boye, B.; Banos, M. A.; Calpe, J. C.; Garrido, J. A. Chemosphere 2003, 51, 227. (7) Boye, B.; Dieng, M. M.; Brillas, E. EnViron. Sci. Technol. 2002, 36, 3030. (8) Oturan, M. A.; Peiroten, J.; Chartrin, P.; Acher, A. J. EnViron. Sci. Technol. 2000, 34, 3474. (9) Fockedey, E.; Lierde, A. V. Water Res. 2002, 36, 4169. (10) Villers, D.; Sun, S. H.; Serventi, A. M.; Dodelet, J. P. J. Phys. Chem. B 2006, 110, 25916. (11) Alverez-Gallegos, A.; Pletcher, D. Electrochim. Acta 1999, 44, 2483. (12) Xie, Y. B.; Li, X. Z. Mater. Chem. Phys. 2006, 95, 39. (13) Irmak, S.; Yavuz, H. I.; Erbatur, O. Appl. Catal., B 2006, 63, 243. (14) Wang, A.; Qu, J.; Ru, J.; Liu, H.; Ge, J. Dyes Pigm. 2005, 65, 227. (15) Li, J. P.; Ai, Z. H.; Zhang, L. Z. EnViron. Sci. Technol. 2007, submitted. (16) Brillas, E.; Boye, B.; Sire´s, I.; Garrido, J. A.; Rodrı´guez, R. M.; Arias, C.; Cabot, P. L.; Comninellis, C. Electrochim. Acta 2004, 49, 4487. (17) Brillas, E.; Ban˜os, M. A Ä .; Garrido, J. A. Electrochim. Acta 2003, 48, 1697. (18) Flox, C.; Ammar, S.; Arias, C.; Brillas, E.; Vargas-Zavala, A. V.; Abdelhedi, R. Appl. Catal., B 2006, 67, 93. (19) Sljukic, B.; Banks, C. E.; Compton, R. G. Nano Lett. 2006, 6, 1556. (20) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 829. (21) Lu, X.; Chen, Z. F. Chem. ReV. 2005, 105, 3643. (22) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105. (23) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (24) Zhang, M. G.; Gorski, W. J. Am. Chem. Soc. 2005, 127, 2058. (25) Lu, L. R.; Ai, Z. H.; Li, J. P.; Zheng, Z.; Li, Q.; Zhang, L. Z. Cryst. Growth Des. 2007, 7, 459. (26) Ai, Z. H.; Lu, L. R.; Li, J. P.; Zhang, L. Z.; Qiu, J. R.; Wu, M. H. J. Phys. Chem. C 2007, 111, 4087. (27) Ai, Z. H.; Lu, L. R.; Li, J. P.; Zhang, L. Z.; Qiu, J. R.; Wu, M. H. J. Phys. Chem. C 2007, 111, 7430. (28) Li, J. P.; Ai, Z. H.; Jia, F. L.; Zhang, X.; Zhang, L. Z.; Lin, J. J. Phys. Chem. C 2007, 111, 6832. (29) Konnann, C.; Bahnemann, D.; Hofmann, M. R. EnViron. Sci. Technol. 1988, 22, 798. (30) Sun, Z.; Liu, Z.; Han, B.; Wang, Y.; Du, J.; Xie, Z.; Han, G. AdV. Mater. 2005, 17, 928. (31) Conway, B. E.; Ayranci, E.; HAl-Maznai, H. Electrochim. Acta 2001, 47, 705. (32) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Appl. Catal., B 2003, 40, 271. (33) Daneshvar, N.; Salari, D.; Khataee, A. R. J. Photochem. Photobiol., A 2003, 157, 111. (34) Zhao, X.; Zhu, Y. F. EnViron. Sci. Technol. 2006, 40, 3367. (35) Gong, K. P.; Yu, P.; Su, L.; Xiong, S. X.; Mao, L. Q. J. Phys. Chem. C 2007, 111, 1882.
