Novel Approach To Enhance Photosensitized Degradation of

Mar 1, 2011 - ... State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, ... The samples were characterized by X-ray diffraction, tr...
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Novel Approach To Enhance Photosensitized Degradation of Rhodamine B under Visible Light Irradiation by the ZnxCd1-xS/TiO2 Nanocomposites Wenjuan Li, Danzhen Li,* Sugang Meng, Wei Chen, Xianzhi Fu, and Yu Shao Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, P. R. China

bS Supporting Information ABSTRACT: In order to exploit efficient photosensitizers with appropriate electronic states to enhance the transfer of electrons, ZnxCd1-xS/TiO2 nanocomposites were first synthesized by a simple hydrothermal method. The samples were characterized by X-ray diffraction, transmission electron microscopy, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, electron spin resonance, and photoluminescence techniques. The results showed that the composite of the two inorganic semiconductors largely enhanced the photosensitized degradation of rhodamine B (RhB) under visible light irradiation (420 nm < λ < 800 nm). These photocatalytic reactions were driven mainly by the light absorption of RhB molecules and to a lesser extent by the excitation of ZnxCd1-xS. They were supposed to arise mainly from the electron transferred from the adsorbed dye in its singlet excited state to the conduction band of ZnxCd1-xS and TiO2. Such a heterogeneous photocatalytic reaction has much significance in the degradation of organic pollutants in ordinary photocatalysis.

’ INTRODUCTION A large number of various toxic pollutants are composed of synthetic textile dyes and other industrial dyestuffs, which are harmful to the environment, hazardous to human health, and difficult to degrade by natural means.1 Most of the dyestuffs are azo dyes and fluorine dyes, which probably have the least desirable consequences in terms of the surroundings. Some azo dyes, fluorone dyes, and their degradation products such as aromatic amines are highly carcinogenic. As international environmental standards are becoming more stringent (ISO 14001, October 1996), technological systems for the removal of organic pollutants such as dyes have been recently developed.2,3 In recent years, the use of semiconductor materials as photocatalysts has attracted great attention for the removal of organic and inorganic pollutants from aqueous phase. TiO2 has been extensively investigated as an efficient photocatalyst because of its stability and nontoxicity.4 However, it requires UV light which is not the optimal candidate as a radiation source to induce the photodegradation of dyes. To exploit effective visible light active photocatalysts with high quantum efficiency and high stability, some intense research activities have been devoted to the synthesis of titania materials doped with S, C, N, etc. atoms.5-7 Others have invested a lot of manpower and material resources for exploiting new nontitania photocatalyst.8,9 However, few of them gave satisfactory results. The use of catalytic oxidation with hydrogen peroxide (H2O2) such as homogeneous Fenton oxidation is r 2011 American Chemical Society

another solution to cope with the problem of decomposing synthetic dyes or other organic compounds.10 But there are some disadvantages for its application such as the requirement of low pH and a significant amount of ferric hydroxide sludge formed in the course of homogeneous Fenton treatment.11 Photosensitization as another approach can extend the photosensitivity of TiO2 into the visible region. Excited by visible light, the sensitizer adsorbed on TiO2 injects electrons into the conduction band (CB) of TiO2, which acts as a mediator for transferring electrons from the excited sensitizer to the electron acceptors on TiO2 surface.12 Traditional sensitizers are small band gap semiconductors or organic dyes. Some of these systems have achieved high quantum efficiencies.13,14 Based on the above mechanism, the transfer of electrons between the dyes and semiconductors is of importance in the degradation of dyes. In other words, enhancing the transfer of electrons would increase the activity. Therefore, the key challenge in sensitization-type photocatalysis is finding efficient sensitizers with appropriate electronic states to enhance the transfer of electrons. The composite of two inorganic semiconductors with appropriate oxidation reduction energy levels could enhance the Received: September 5, 2010 Accepted: February 14, 2011 Revised: February 10, 2011 Published: March 01, 2011 2987

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Environmental Science & Technology charge separation and extend its absorption response to the visible region. It would be an effective appropriate solution to solve the problem of decomposing organic compounds. ZnxCd1-xS nanocrystals have attracted much attention due to their special properties of tunable band gap-width. The photocatalytic activity of ZnxCd1-xS has been reported in our previous papers.15,16 The band gap of ZnxCd1-xS solid solutions can be tuned to appropriate oxidation reduction energy level to couple with TiO2. Through changing the Cd/Zn molar ratio, ZnxCd1-xS/TiO2 composites with good photosensitized activity can be exploited. Herein, we first applied the inorganic semiconductors composites in the photosensitized reactions of dyes. In this paper, ZnxCd1-xS/TiO2 composites were prepared through one step. Their properties in the degradation of the cationic dye rhodamine B (RhB) were investigated in detail.

’ EXPERIMENTAL SECTION Materials. Cadmium acetate (Cd(Ac)2 3 2H2O), zinc acetate (Zn(Ac)2 3 2H2O), thiourea, and polyethylene glycol 2000 (PEG) were analytical grade. They were purchased from the Shanghai Chemical Co. Horseradish peroxidase (POD) and N, N-diethyl-p-phenylenediamine (DPD) were purchased from J&K Chemical Ltd. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was obtained from Sigma Co.. Deionized water was used for the preparation of all solutions. All chemicals were used without further treatment. Titania sol was provided by Research Institute of Photocatalysis of Fuzhou University. One Step Preparation of ZnxCd1-xS/TiO2 Nanocomposites. First, 0.001 mol of Cd(Ac)2 3 2H2O and Zn(Ac)2 3 2H2O according to a certain ratio were dissolved in ethanol to form a mixture solution (A). 0.001 mol thiourea and 0.001 mol PEG were dissolved in deionized water to form solution (B). Then solution (B) was added dropwise to solution (A) with continuous stirring. After 0.5 h of stirring, a certain amount of titania sol was added. The procedure was operated in a Teflon liner with 100-mL capacity. After 1 h of stirring at room temperature, the Teflon liner was sealed in a stainless steel autoclave and maintained at 200 °C for 12 h. As the autoclave was naturally cooled to ambient temperature, the resultant products were washed with water/ ethanol for several times and then dried in air at 60 °C for 3 h. When Cd/Zn = 3:1, ZnxCd1-xS/TiO2 nanocomposites with different ZnxCd1-xS/TiO2 molar ratios were first prepared and their photodegradation activities of RhB were compared. Here we chose three kinds of ZnxCd1-xS/TiO2 molar ratios: 8%, 12%, and 20% (Cd/Zn = 3:1). As the ZnxCd1-xS/TiO2 molar ratio (12%) with the best activity was selected and fixed, different nanocomposites were then prepared by changing the molar ratio of Cd/Zn in the stock solutions. In the experiments mentioned in this paper, the ZnxCd1-xS/TiO2 molar ratio in the nanocomposites was 12%. The as-prepared ZnxCd1-xS/TiO2 samples were marked as ZCT. For comparison, the ZnxCd1-xS with different Cd/Zn molar ratios (marked as ZCS) and TiO2 were also prepared. The detailed procedures were shown in the Supporting Information (SI). Characterization of Samples. Transmission electron microscopy (TEM) images were collected by using a JEOL JEM 2010F microscope working at 200 kV and equipped with an energydispersive X-ray analyzer (Phoenix). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. The UV-vis spectra of various liquid samples and diffuse reflectance spectroscopy (DRS) of the

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prepared samples were performed on Varian Cary 50 UV-vis spectrophotometer and Varian Cary 500 UV-vis spectrophotometer with an integrating sphere attachment ranging from 200 to 800 nm, respectively. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific Inc.) at 3.0  10-10 mbar with monochromatic AlKR Radiation (E = 1486.2 eV). The generation of hydroxyl radicals was investigated by the method of photoluminescence technique with terephthalic acid (PL-TA).17 The photoluminescence spectra were surveyed by Edinburgh FL/FS900 spectrometer. Electron spin resonance (ESR) spectra were obtained using a Bruker model A300 spectrometer with a 500W Xe-arc lamp equipped with an IR-cutoff filter (λ < 800 nm) and an UV-cutoff (λ > 420 nm) as a visible light source. The settings were center field, 3512 G; microwave frequency, 9.86 GHz; power, 20 mW. The flat band potential (Vfb) of TiO2 was determined by the electrochemical method,18 which was carried out in conventional three electrode cells using a PAR VMP3 Multi Potentiotat apparatus. The catalyst was deposited as a film form on a 1 cm  1 cm indium-tin-oxide conducting glass served as working electrode, the saturated calomel electrode (SCE) as the reference electrode, and Pt as the counter electrode. The electrolyte was 0.3 M aqueous LiClO4 at pH 3.0. The Mott-Schottky plots to evaluate the Vfb of the semiconductor space charge region were obtained by measuring impedance spectra at fixed frequency of 1 kHz. Tests of Photocatalytic Activity. The visible light source was a 500 W halogen lamp (Philips Electronics) positioned beside a cylindrical reaction vessel with a plane side. The system was cooled by wind and water to maintain the room temperature. The 420 and 800 nm cutoff filters were placed before the vessel to ensure the irradiation of the RhB/ZCT system occurred only by visible light. A 0.04 g portion of catalysts was added to 80 mL of RhB solution (10-5 M) in a 100-mL Pyrex glass vessel. Prior to irradiation, the suspensions were magnetically stirred for 1 h to ensure the equilibrium of the working solution. After irradiation, at given time intervals, 3-mL aliquots were sampled and centrifuged to remove the catalysts. The degraded solution was analyzed using a Varian Cary 50 Scan UV-vis spectrophotometer and the absorption peak at 552 nm was monitored. The percentage of degradation is reported as C/C0. C is the absorption of pollutants at each irradiated time interval of the main peak of the absorption spectrum. C0 is the absorption of the initial concentration when adsorption-desorption equilibrium was achieved.

’ RESULTS AND DISCUSSION Physicochemical Properties of ZCT Composites. Figure 1a shows XRD patterns of ZCT samples (Cd/Zn = 1:0, 4:1, 3:1, 1:1), ZCS (Cd/Zn = 3:1), and TiO2. The magnified XRD patterns are shown in Figure S1 (S1-S16 shown in the Supporting Information). From the two figures, we can see that the composites of ZCT are composed of ZCS and TiO2. The peaks of ZCS shifted certain angle by increasing the Cd/Zn molar ratio in ZCT samples. The phase of TiO2 existed in the composites was the anatase. Figure 1b shows the UV-vis DRS of ZCT (Cd/Zn = 3:1), ZCS (Cd/Zn = 3:1), and TiO2. The visible light absorption of the ZCT nanocomposite was clearly stronger than that of pure TiO2, which was due to the contribution of ZCS. Moreover, a comparison of F(R) of ZCT with different Cd/Zn ratios and TiO2 is shown in Figure S2. 2988

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Figure 1. (a) XRD patterns of ZCT, ZCS, and TiO2 and (b) DRS of ZCT (Cd/Zn = 3:1), ZCS (Cd/Zn = 3:1), and TiO2.

Figure 2. (a) The absorption spectra of RhB in the presence of ZCT (Cd/Zn = 3:1) under exposure to visible light (420 nm < λ < 800 nm) and (b) comparison of photodegradation of RhB on ZCT (Cd/Zn = 3:1), TiO2, ZCS (Cd/Zn = 3:1), and TiO2-xNx under identical conditions.

To obtain information about the structure of the samples, ZCT (Cd/Zn = 3:1) nanocomposite was characterized by TEM. Figure S3 shows that the elements of Zn, Cd, S, Ti, and O are exactly existed in the composite. ZCS nanoparticles dispersed evenly in the composite. XPS results also verify the structure of the composites (Figure S4 and S5). The chemical valences of Cd, Zn, and S for ZCS and ZCT samples were the same. Photocatalytic Activity of ZCT Nanocomposites. On the basis of the above analysis, we can see the composites were exactly composed of ZCS and TiO2. The photocatalytic activity of the ZCT nanocomposites was evaluated by the degradation of a RhB solution under visible light irradiation (420 nm < λ < 800 nm). Temporal changes in the concentration of RhB were monitored by examining the variations in maximal absorption in UV-vis spectra at 552 nm. Figure 2a shows the temporal evolution of the spectral changes of the RhB mediated by ZCT. It can be seen that after 120 min of irradiation, the absorption peak at 552 nm is blue-shift and simultaneously broaden. It agreed well with the report of Zhao et al. about the TiO2/RhB process.19,20 According to their report, the blue-shift of the absorption band was caused by de-ethylation of RhB because of the attack by one of the active oxygen species on the N-ethyl group. When the de-ethylated process was fully completed, the absorption band shifted to 498 nm and RhB was turned to rhodamine. Rhodamine was then gradually decomposed due to the further destruction of the conjugated structure. The results shown in Figure 2a are analogous to that of the RhB/TiO2 system and the Cu2þ/RhB/ TiO2 heterogeneous system in refs 19 and 20. Therefore, after 2 h

of irradiation, the end product of RhB degradation in the ZCT/ RhB system was rhodamine. In the following irradiation, there were no new shifted peaks observed. When the de-ethylation reactions were complete, we think that the aromatic chromophore would continue to be attacked by the active species leading to the degradation of rhodamine. Therefore, though the mineralization ratio of RhB was low, RhB can be degradated into many intermediate products, and its degradation was actually increased. Some comparative experiments were made to investigate the photocatalytic activity of the nanocomposites under identical conditions. Figure S6 shows the photocatalytic activity of ZCT samples with different Cd/Zn molar ratios. From the comparison, the sample with Cd/Zn = 3:1 showed the best activity. Furthermore, from Figure 2b, it can be clearly seen that under identical conditions, ZCT (Cd/Zn = 3:1) sample shows much greater activity than that of TiO2, ZCS (Cd/Zn = 3:1), and TiO2-xNx (prepared according to ref 21). We also tested the stability of ZCT (Cd/Zn = 3:1) sample through the detection of the degradation rate of cycling runs for the photodegradation of RhB. In Figure S7, the conversion ratios of five cycling runs for the photodegradation do not change much. Before and after degradation of RhB, the samples were also compared and investigated by XRD, TEM, and XPS. Figure S8 shows the XRD patterns of the samples before and after degradation. Although the intensity of the patterns after degradation decreased a little, the position and the ratio of the peaks were nearly the same. The particle morphology, size, and lattice fringes after degradation are 2989

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Figure 3. Comparison of photodegradation of RhB on ZCT (Cd/Zn = 3:1) and TiO2 after light was filtered by a 550/800 or 420/800 nm combined filter.

Figure 4. Photocatalytic degradation of RhB over ZCT (Cd/Zn = 3:1) particles under different conditions with exposure to visible light (420 nm < λ < 800 nm): in air-equilibrated, N2-saturated, O2-saturated solutions, or adding AO.

shown in Figure S9. The fringes of d = 0.33 and 0.35 nm belonged to ZCS and TiO2 respectively can be also found in it. From the detection of XPS (Figure S10), the binding energies of Cd3d, Zn2p, S2p, Ti2p, and O1s before and after degradation are nearly the same in the full survey spectrum and the detailed spectra. It indicates that the ZCT sample possesses good chemical and mechanical stability. In order to distinguish the degradation of RhB driven by the initial excitation of RhB or the ZCT, two groups of combined filters (550/800 and 420/800 nm, Figure S11) were applied in the degradation system to compare the activities. Based on the fact that the light absorption edge of ZCT was at λ = 536 nm and the maximum absorption peak of RhB was at λ = 552 nm,22 the light was filtered by a 550/800 nm combined filter to avoid the light absorption of ZCT. If the photocatalytic reaction was driven by the excitation of ZCT, as light was filtered by the 550/800 nm combined filter, there would be no degradation of RhB. Instead, if the degradation was driven by the excitation of RhB, the photocatalytic activity should be the same under the two conditions. From the results shown in Figure 3, after the light is filtered by a 550/800 nm combined filter, although the photodegradation rate of the main absorption peak in the ZCT/RhB system decreases a few by comparing with that under 420/800 nm combined filter, the RhB is still largely degradated. And under the two conditions, they both showed a great wavelength shift for RhB. After 120 min of irradiation, the maximum absorption peaks shifted from 552 to 498 nm. However, in the degradation process, the wavelength shift for RhB in the 550/800 nm system was still smaller than that in the 420/800 nm system. Therefore, from the comparison, it was believed that the degradation process was mainly driven by the excitation of RhB and to a lesser extent by the excitation of ZCS. Furthermore, the degradation of RhB on TiO2 was also compared after the light was filtered by a 550/800 or 420/800 nm combined filter. For the TiO2/RhB system, after light was filtered by the 550/800 nm combined filter, the photodegradation rate decreased much more than that under 420/800 nm combined filter. The decrease was more than that shown in the ZCT/RhB system. As reported in ref 19, under visible light irradiation, the degradation of RhB on TiO2 was actually a photosensitized reaction driven by the excitation of RhB. As shown in Figure 3, under identical conditions, the TiO2 shows less activity, less wavelength shift for RhB

than those found for the ZCT composite. So the degradation processes were different in the ZCT/RhB and TiO2/RhB systems. The degradation process in the presence of ZCT was a photosensitized reaction not only driven by the excitation of RhB but also affected by the existence of ZCS. The detailed mechanism would be discussed in the following part. Mechanism of the Degradation Process. The transfer of electrons in the photocatalytic process is very important, which would inhibit the recombination of electrons and holes and increase the activity. The separation of electrons and holes was always recognized to be the initial step in the photodegradation mechanism. Therefore, to investigate the mechanism of the process, scavengers for electrons and holes were employed to determine the specific reactive species that played important roles in the photodegradation process. Figure 4 indicates time courses of RhB photodegradation by ZCT (Cd/Zn = 3:1) under different conditions with exposure to visible light (420 nm < λ < 800 nm). For the absorption peak at 552 nm, the photodegradation conversion ratio (PCR) under the air-equilibrated condition was 96% after 120 min of irradiation. Dissolved O2 as an efficient electron scavenger was excluded by N2 in the degradation process. Figure 4 shows that the PCR of RhB is decreased to 93% in the presence of N2. When O2 was bubbled, the PCR was increased. Therefore, the electrons or the active species generated by electrons played an important role in the photocatalytic reaction. Ammonium oxalate (AO) is an effective scavenger of holes.23 After 0.1 g of AO was added to the reaction system, the PCR of RhB was decreased to 81%. The addition of AO seriously inhibited the photocatalytic degradation process. Therefore, the transfer of electrons was further confirmed to be responsible for the degradation. The photogenerated holes and the active species generated by electrons were the major responsibility in the degradation of RhB. ESR spin-trap technique (with DMPO) was employed to probe the nature of the reactive oxygen species generated during the irradiation of the present system.24 As shown in Figure S12, from ESR spectra, the intensity ratio of main peaks engendered in ZCT (Cd/Zn = 3:1)-RhB-water or ZCT (Cd/Zn = 3:1)-water system is about 1:1:1:1 under visible light irradiation. The weak signals were different from the ones reported previously of the DMPO- 3 OH signal, and there is no report about these signals of ZCT yet. As reported,25-27 in the liquid phase, the majority of 2990

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Figure 5. (a) 3 OH-trapping PL spectra of suspensions containing ZCT (Cd/Zn = 3:1) and TA and (b) DMPO spin-trapping ESR spectra in ZCT (Cd/ Zn = 3:1)-RhB-methanol dispersion for DMPO-O2 3 -.

radicals are 3 OH, O2 3 - radicals and hydrated electrons (e-aq), although smaller quantities of other radicals such as 3 H are also formed. The signals shown in the ESR spectra do not belong to 3 OH species. It may be assigned to hydroperoxyl radical (HO2 3 ), e-aq, or 3 H28,29 which should be further investigated. To further investigate the generation of 3 OH, the PL-TA technique was used in the detection. Figure 5a shows the fluorescence spectra of suspensions containing ZCT (Cd/Zn = 3:1) and TA under visible light irradiation. It can be seen that the fluorescence intensity increased steadily with the irradiation time. Therefore, 3 OH radicals were indeed generated on ZCT. Because of the generation of 3 OH radicals, the ZCT would possess the general applicability to degrade organic compounds other than dyes, such as p-hydroxyazobenzene (Figure S13). In ESR spectra, the reason that 3 OH was not detected may be due to the strong signal intensity of other species. The signal intensity of other species was so strong that the signal of 3 OH may be covered. O2 3 - radicals as another important intermediate species were also tested by ESR technique. As shown in Figure 5b and Figure S14, under visible light irradiation, six strong and obvious peaks for DMPO-O2 3 - species are observed in ZCT (Cd/Zn = 3:1)RhB-methanolic dispersions and ZCT methanolic dispersions. In Figure 4, in the absence of O2, the PCR of RhB is decreased in this system. The strong intensity of DMPO-O2 3 - species in ESR spectra proves the transfer of electrons and the effect of O2 again. O2 3 - species would further induce the generation of H2O2 which is another important intermediate species in the photocatalytic processes. The DPD method30 employed for peroxide measurements was used for the detection of H2O2. In the presence of ZCT (Cd/Zn = 3:1) under visible light irradiation (420 nm < λ < 800 nm) (Figure S15), when DPD and POD are added in the system, the oxidized DPD shows two absorption maxima (one at 510 nm and the other at 551 nm). It shows that in the degradation of RhB, the intermediate H2O2 species are generated. Therefore, it not only verified the existence of O2 3 species again but also provided another inevitable source for 3 OH species. The Vfb of the semiconductors was determined by an electrochemical method.31 According to our previous report,16 ZCS (Cd/Zn = 3:1) was n-type semiconductor and its Vfb was -0.8 V. The conduction band potential (VCB) of ZCS was about -0.9 V, which was more negative than the standard redox potential of O2/O2 3 - (-0.33 V vs NHE).32 And the valence band potential (VVB) of ZCS was about 1.51 V, which was more negative than

the standard redox potential of 3 OH/OH- (2.38 V vs NHE).33 As a result, the photogenerated electrons on irradiated ZCS can reduce O2 to give O2 3 -, while the photogenerated holes can not oxidize OH- to give 3 OH. From Figure S16, it has been indicated from Mott-Schottky plots that TiO2 is also n-type semiconductor and its Vfb is -0.6 V. Thereby, it was deduced that the VCB of TiO2 was about -0.7 V, which was more negative than the standard redox potential of O2/O2 3 -. Then the VVB of ZCS was more negative than that of TiO2. When the condition that the satisfactory overlap between the energy level of the excitated dye RhB* (E(RhB*)) and the CB of ZCT was fulfilled, the photocatalytic degradation of RhB can proceed. The relative positions of the standard oxidation redox potential of the RhB (θU(RhB*|RhB 3 þ) = -1.09 V, θU(RhB| RhB 3 þ) = 1.46 V vs NHE),34,35 and ZCT (Cd/Zn = 3:1) just fulfilled this condition. Therefore, under visible light irradiation, electrons can transfer from the adsorbed dye in its singlet excited state to the CB of ZCT. Furthermore, electrons and holes generated by ZCS were separated. Some electrons were injected into TiO2 nanoparticles quickly since the CB of ZCS was more negative than that of TiO2. The transferred electrons can be trapped by surface oxygen to give O2 3 -. And active species such as H2O2 and 3 OH radicals would be generated in the following reactions. At the same time, holes generated in ZCS can directly oxide adsorbed dyes. In addition, there may be e-aq or 3 H species generated in the photocatalytic process. They can also react with the dyes. The detailed mechanism is shown in Figure 6. In our ZCT-RhB system, the advantages to increase the photocatalytic activity of the degradation of RhB can be explained in the three facets: First, the composite increased the visible light absorption and made the absorption edge shift to long wavelength region. In this region, both RhB molecules and ZCS were excitated. And photogenerated electrons can transfer through more approaches between the RhB and ZCT. Therefore, the essential role of the ZCT semiconductors must be the transfer of electrons, injected by the excitated dye and ZCS, to a suitable oxidizing agent (O2) at a separate position on the surface. Thus, the recombination of the excitated electrons and holes were effectively inhibited, and RhB 3 þ could undergo N-dealkylation with high efficiency.35 Second, considered the relative positions of the oxidation redox potential of ZCT, the heterojunction structure may be formed in the composite. Under visible light irradiation, the photogenerated electrons and holes were separated in the build-in field of ZCS and TiO2. There may be more electrons gathered on the 2991

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Figure 6. Scheme diagram of the photocatalytic reaction of RhB on ZCT (Cd/Zn = 3:1).

surface. The transfer rate of electrons was increased at the same time. Third, for TiO2-RhB system, the TiO2 was just the transfer of electrons. The TiO2 cannot be excitated and holes cannot be generated. But for our ZCT system, the photogenerated holes played much more role which had been verified in Figure 4. Therefore, the photocatalytic reaction took place mainly by the light absorption of adsorbed RhB molecules and to a lesser extent by the excitation of ZCT.

’ ASSOCIATED CONTENT

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Supporting Information. Text and Figures S1-S16. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone/Fax: (þ86)591-83779256. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21073036, and 20873023), National Basic Research Program of China (973 Program, 2007CB613306). ’ REFERENCES (1) Brown, M. A.; De-Vito, S. C. Predicting azo dye toxicity. Crit. Rev. Environ. Sci. Technol. 1993, 23, 249–324. (2) Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J. M. Photocatalytic degradation of various types of dyes (alizarin S, crocein orange G, methyl red, congo red, methylene blue) in water by UV-irradiated titania. Appl. Catal., B 2002, 39, 75–90. (3) Chen, Z. X.; Li, D. Z.; Zhang, W. J.; Shao, Y.; Chen, T. W.; Sun, M.; Fu, X. Z. Photocatalytic degradation of dyes by ZnIn2S4 microspheres under visible light irradiation. J. Phys. Chem. C 2009, 113, 4433–4440. (4) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1–21.

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dx.doi.org/10.1021/es103041f |Environ. Sci. Technol. 2011, 45, 2987–2993