Effect of Noble Metal in CdS/M/TiO 2 for Photocatalytic Degradation of Methylene Blue under Visible Light

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1 International Journal of Green Nanotechnology: Materials Science & Engineering ISSN: (Print) (Online) Journal homepage: Effect of Noble Metal in CdS/M/TiO 2 for Photocatalytic Degradation of Methylene Blue under Visible Light Shaohua Shen, Liejin Guo, Xiaobo Chen, Feng Ren, Coleman X. Kronawitter & Samuel S. Mao To cite this article: Shaohua Shen, Liejin Guo, Xiaobo Chen, Feng Ren, Coleman X. Kronawitter & Samuel S. Mao (2010) Effect of Noble Metal in CdS/M/TiO 2 for Photocatalytic Degradation of Methylene Blue under Visible Light, International Journal of Green Nanotechnology: Materials Science & Engineering, 1:2, M94-M104, DOI: / To link to this article: View supplementary material Published online: 17 Mar Submit your article to this journal Article views: 1025 View related articles Citing articles: 17 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 29 November 2017, At: 06:47

2 International Journal of Green Nanotechnology: Materials Science & Engineering, 1:M94 M104, 2010 Copyright c Taylor & Francis Group, LLC ISSN: print / online DOI: / Effect of Noble Metal in CdS/M/TiO 2 for Photocatalytic Degradation of Methylene Blue under Visible Light Shaohua Shen Liejin Guo Xiaobo Chen Feng Ren Coleman X. Kronawitter Samuel S. Mao ABSTRACT. The CdS/M/TiO 2 (M = Ag, Ru, Au, Pd, and Pt) three-component nanojunction systems were constructed using a two-step photodeposition method, and evaluated for their photocatalytic activities through the degradation of methylene blue in aqueous solution under visible light irradiation. The authors found that the photocatalytic activity of CdS/M/TiO 2 (M = Ag, Ru, Au, Pd, Pt) three-component nanojunctions was superior to that of CdS/TiO 2 two-component system. Moreover, the photocatalytic activity of the three-component nanojunction system was found to be dependent significantly on the type of the noble metals. The results can be explained by the influence of charge transfer on the basis of the work functions of different noble metals. [Supplementary materials are available for this article. Go to the publisher s online edition of the International Journal of Green Nanotechnology: Materials Science and Engineering to view the free supplementary file.] KEYWORDS. CdS/M/TiO 2, heterojunction, noble metal, photocatalysis, Z-scheme INTRODUCTION Since the discovery of photo-induced splitting of water on TiO 2 electrodes, [1] wa- ter splitting and environmental clean-up become two active fields supported by heterogeneous photocatalysis. [2,3] Environmental applications of heterogeneous photocatalysis Received 25 January 2010; accepted 6 February Shaohua Shen and Leijin Guo are affiliated with the State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, Xi an, Shaanxi, China. Shaohua Shen, Xiaobo Chen, Feng Ren, Coleman X. Kronawitter, and Samuel S. Mao are affiliated with the Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, California, USA. The authors acknowledge the support by the National Basic Research Program of China (No. 2009CB220000), Natural Science Foundation of China (No and No ), and the U.S. Department of Energy. One of the authors (S.S.) also thanks the support from the China Scholarship Council. Address correspondence to Liejin Guo, State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, Xi an, Shaanxi , China, lj-guo@mail.xjtu.edu.cn; or to Samuel S. Mao, Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA. ssmao@lbl.gov M94

3 S. Shen et al. M95 have been considered among the most effective methods for elimination of many hazardous organic pollutants from the environment, particularly from the wastewater. Heterogeneous photocatalysis offers a green technology for completely decomposing such contaminants in the presence of semiconducting catalyst particles. Among numerous semiconducting photocatalysts developed so far, TiO 2 is regarded to be the most efficient and environmentally benign, which has been most widely used for photodegradation of various pollutants. [4] However, its large band gap along with a fast recombination rate of photogenerated electron-hole pairs in TiO 2 hinders its photocatalytic activity, limiting further commercialization and industrial applications. One effective approach to overcome such a hurdle involves coupling of different semiconductors with appropriate energy levels. For instance, CdS/TiO 2 heterojunction has been widely studied for effective decomposition of organic compounds [5 7] and for photocatalytic water splitting. [8 10] In CdS/TiO 2 heterojunction, CdS with narrow band gap acts as a visible-light sensitizer; combined with TiO 2, CdS is also responsible for effective charge separation that enables suppression of the recombination process. [11,12] In the past decade, Z-scheme photocatalytic systems (two-step photoexcitation) mimicking photosynthesis in a green plant have been investigated to realize a system for overall water splitting that are composed of H 2 - and O 2 -photocatalysts and a suitable electron mediator. [13 17] The electron mediator in solution plays an indispensable role in shuttling the photo-generated carriers between the H 2 - and O 2 -photocatalysts. Recently, Kudo and coworkers succeeded in constructing a simple Z-scheme photocatalytic system driven by interparticle electron transfer without an electron mediator under visible light irradiation. 18 In this (Ru/SrTiO 3 :Rh)-(BiVO 4 ) system, the excited electrons in SrTiO 3 :Rh reduce water to form H 2 on Ru co-catalyst, the holes in BiVO 4 oxidize water to form O 2 to accomplish overall water splitting, and the reversible Rh species at the surface of photocatalyst plays a pivotal role for electron transfer between particles. On the other hand, all solid-state Z- scheme CdS/Au/TiO 2 three-component nanojunction systems have been developed to show much higher photocatalytic activity than singleand two-component systems for decomposition of organic compounds. [19,20] In such a three-component system, photo-induced electrons achieved a vectorial transfer of TiO 2 Au CdS through a two-step excitation of CdS, TiO 2, and Au as a mediator. Similarly, metal Ag species in the AgBr-Ag-Bi 2 WO [21] 6 and AgBr-Ag-TiO 22 2 Z-schematic nanojunction systems also acted as the electron transfer mediator, contributing to the enhancement of electronhole separation and interfacial charge transfer. However, to the best of our knowledge, there have been few reports on the effects of different noble metals (such as Au, Ag, etc.) on the photocatalytic activity of such three-component Z-scheme systems. In the present study, we constructed CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) threecomponent Z-scheme nanojunction systems with different noble metals as the electron transfer mediator. These CdS/M/TiO 2 nanojunctions prepared by a two-step photodeposition method were used to carry out photocatalytic degradation of methylene blue. In particular, we investigated the effect of noble metals (Ag, Ru, Au, Pd, and Pt) on the photocatalytic activity of such Z-scheme systems. Sample Preparation EXPERIMENTAL TiO 2 (P25) was acquired from Nippon Aerosil. Cd(NO 3 ) 2 4H 2 O, Na 2 S 9H 2 O and ethanol were purchased from Sigma-Aldrich and sulfur powder was purchased from Allied Chemical. Aqueous solutions of noble metal compound were prepared from H 2 PtCl 6 H 2 O (Sigma-Aldrich), AgNO 3 (J.T. Baker), Na 2 PdCl 4 (Aldrich), RuCl 3 (Strem), and HAuCl 4 4H 2 O (Sigma-Aldrich). All chemicals were used as received. CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) three-component photocatalysts were prepared by the two-step photodeposition approach, [19]

4 M96 International Journal of Green Nanotechnology: Materials Science and Engineering as described in the Supplementary Material, Scheme 1. In a typical synthesis process, 0.1 gp25tio 2 was dispersed in 100 ml ethanol aqueous solution (v : v = 1:1). To this suspension, a 0.4 mg/ml (noble metal) aqueous solution (0.75 ml) of noble metal compound was added. Under stirring, the mixture was bubbled with argon for 30 min and then irradiated with a 150-W Xe lamp for 5 h to load noble metal on TiO 2 (M [0.3 wt%]/tio 2 ). After the resulting M/TiO 2 ethanol/water suspension, added with sulfur powder (0.011 g) and Cd(NO 3 ) 2 4H 2 O (0.154 g), had been bubbled with argon for 30 min, irradiation was carried out for a given period with a 150-W Xe lamp. Then the products CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) were centrifuged, washed with water and ethanol, and dried at 65 C in air. The CdS/TiO 2 photocatalyst without noble metal loading was prepared by photodeposition of CdS on P25 TiO 2 instead of M/TiO 2. CdS/TiO 2 and CdS/Ag/TiO 2 samples as reference (CdS/TiO 2 -R and CdS/Ag/TiO 2 -R) were prepared by the precipitation method. P25 TiO 2 (or Ag [0.3 wt%]/tio 2 prepared by photodeposition), 0.1 g, was dispersed in an 100-mL ethanol aqueous solution (v : v = 1:1) containing Cd(NO 3 ) 2 4H 2 O (0.154 g). Under stirring, 10 ml aqueous solution containing double excess of Na 2 S was added to the mixture, and the resulting yellow suspension was stirred overnight at room temperature. The products CdS/TiO 2 -R and CdS/Ag/TiO 2 -R were centrifuged, washed with water and ethanol, and dried at 65 Cinair. In all these photocatalyts, the mass contents of CdS and M (versus TiO 2 ) were 72.2% and 0.3%, respectively. Characterization X-ray diffraction (XRD) patterns were obtained from a PANalytical X pert diffractometer using Ni-filtered Cu Kα irradiation (wavelength Å) with the scanning step of 0.05 /s, the operation voltage and current were 45 kv and 40 ma, respectively. Ultravioletvisible (UV-Vis) absorption spectra of the samples were determined on a Varian Cary 50 UV spectrophotometer with MgO as the reference, the scanning range was from 200 to 900 nm. Transmission electron microscopy (TEM) studies were carried out on a JEOL JEM 2010 instrument. Elemental analysis of the photocatalytst was conducted by an energy-dispersive X-ray spectrometer (EDS) attached to the transmission electron microscope. Photocatalytic Degradation of Methylene Blue Photocatalytic activity of the photocatalyts was determined by measuring the decomposition of methylene blue (MB) using a 150-W xenon lamp (Newport) as light source at ambient conditions. [23,24] Light was passed through a UV cut-off filter (λ >400 nm; Newport FS-C S/N 147) (Supplementary Material, Figure S1), and guided onto the side window of an open cuvette filled with 3 ml of MB auqeous solution (optical density 1.0) and a given amount of photocatalyst (60 mg/l). The suspension was stirred continuously during the whole process. The concentration of methylene blue was estimated by measuring its maximum absorbance at 664 nm with a Varian Cary 50 UV spectrophotometer after irradiation for a period of time (Supplementary Material, Figure S2). RESULTS AND DISCUSSION Figure 1 shows XRD patterns of CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) prepared by photodeposition method. Even though the noble metals (M) in CdS/M/TiO 2 were different, these heterojunctions displayed quite similar crystal structures. All samples exhibited some diffraction peaks assigned to the anatase and rutile phases, which coexisted in P25 TiO 2. In addition, there were three peaks with 2θ values of 26.5, 44.0, and 52.1, corresponding to (111), (220), and (311) crystal planes of cubic CdS (space group F-43m; a = Å; JSPDS Card No ), respectively. The diffraction peaks assigned to noble metals could not be identified in these CdS/M/TiO 2 heterojunctions because the mass content of noble metals (0.3%, versus TiO 2 ) is under the detection limit for XRD analysis. The XRD patterns of CdS/TiO 2 and

5 S. Shen et al. M97 FIGURE 1. XRD patterns of CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) prepared by photodeposition method. (Figure provided in color online.) 2000 Intensity/a.u x x x 600 c 400 b a : anatase TiO2 x : rutile TiO2 : cubic CdS 2 Theta/degree a: CdS/Ru/TiO2 b: CdS/Pd/TiO2 c: CdS/Au/TiO2 d: CdS/Pt/TiO2 e: CdS/Ag/TiO2 x CdS/Ag/TiO 2 prepared by precipitation method as reference (CdS/TiO 2 -R and CdS/Ag/TiO 2 -R) are shown in Figure 2. They also possessed cubic CdS, anatase and rutile TiO 2 phases, similar to CdS/TiO 2 and CdS/Ag/TiO 2 prepared by the photodeposition method. However, as indicated by the weaker intensity of diffraction peaks assigned to cubic CdS phase in CdS/TiO 2 -R and CdS/Ag/TiO 2 -R, the cubic CdS in CdS/TiO 2 and CdS/Ag/TiO 2 prepared by the photodeposition method had better crystallinity than the samples prepared by precipitation. The UV-Vis diffuse reflectance spectra for CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) are shown in Figure 3. The single TiO 2 (P25) photocatalyst showed a sharp edge, whereas the CdS/TiO 2 composite prepared by photodepostion method had two absorption edges; the main edge due to CdS is located at 550 nm and second one due to TiO 2 at 400 nm. This is in agreement with the previous report by Jang et al. that the spectra of CdS/TiO 2 composite photocatalysts prepared by precipitation method showed a combination of these two spectra. [8,25] When compared to CdS/TiO 2, all the CdS/M/TiO 2 nanojunctions had two similar absorption edges assigned to CdS and TiO 2. However, additional absorption appeared in the range of nm in the spectra of CdS/M/TiO 2. This could be related to the existence of noble metal particles (M=Ag, Ru, Au, Pd, Pt) in these CdS/M/TiO 2 nanojunctions, corresponding to the report by Doremus that small particles of noble metals showed an optical absorption band in visible light that resulted from collective oscillations of the free electrons in them. [26] The UV- Vis diffuse reflectance spectra of CdS/TiO 2 -R and CdS/Ag/TiO 2 -R were also given in Figure 4, which makes a comparison between optical property of the samples prepared by photodeposition method and precipitation method. CdS/TiO 2 and CdS/Ag/TiO 2 prepared by different methods had similar spectra with two absorption edges; the increased absorption in visible region (longer than 600 nm) of the CdS/Ag/TiO 2 (or CdS/Ag/TiO 2 -R) spectrum was attributable to the presence of Ag, as discussed previously. e d

6 M98 International Journal of Green Nanotechnology: Materials Science and Engineering FIGURE 2. XRD patterns of CdS/TiO 2 and CdS/Ag/TiO 2 prepared by photodeposition method, and CdS/TiO 2 -R and CdS/Ag/TiO 2 -R prepared by precipitation method. (Figure provided in color online.) Intensity/a.u x : anatase TiO2 x : rutile TiO2 : cubic CdS x x x Theta/degree a: CdS/TiO2 b: CdS/TiO2-R c: CdS/Ag/TiO2 d: CdS/Ag/TiO2-R FIGURE 3. UV-Vis diffuse reflectance spectra of TiO 2 and CdS/M/TiO 2 (M=Ag,Ru,Au,Pd,Pt) prepared by photodeposition method. (Figure provided in color online.) Abs/a.u g a f d e b a: CdS/Pt/TiO2 b: CdS/Ru/TiO2 c: CdS/Pd/TiO2 d: CdS/Au/TiO2 e: CdS/Ag/TiO2 f: CdS/TiO2 g: TiO2 (P25) c d c b a Wavelength/nm

7 S. Shen et al. M99 FIGURE 4. UV-Vis diffuse reflectance spectra of CdS/TiO 2 and CdS/Ag/TiO 2 prepared by photodeposition method, and CdS/TiO 2 -R and CdS/Ag/TiO 2 -R prepared by precipitation method. (Figure provided in color online.) Abs/a.u b a Wavelength/nm a: CdS/TiO2 b: CdS/TiO2-R c: CdS/Ag/TiO2 d: CdS/Ag/TiO2-R c d To visualize the hybridization of CdS and Ag/TiO 2 in CdS/Ag/TiO 2 prepared by the photodeposition and precipitation method, CdS/Ag/TiO 2 and CdS/Ag/TiO 2 -R were investigated by TEM. The representative TEM image of the CdS/Ag/TiO 2 -Rsampleisshownin Figure 5A, displaying separated phases of TiO 2 and CdS, as confirmed by EDS element analysis (Supplementary Material, Figure S3a, b). The Ag signal could not be identified in the EDS FIGURE 5. TEM images of (A) CdS/Ag/TiO 2 -R prepared by precipitation method, and (B) CdS/Ag/TiO 2 prepared by photodeposition method.

8 M100 International Journal of Green Nanotechnology: Materials Science and Engineering spectra, because of the very small amount of Ag in CdS/Ag/TiO 2 -R sample. In contrast, the obvious separation of CdS and TiO 2 phases does not occur in CdS/Ag/TiO 2 sample, as shown in Figure 5B. Moreover, the EDS spectra of CdS/Ag/TiO 2 (Supplementary Material, Figure S3c) indicate homogenous dispersion of CdS. Thus, CdS is supposed to deposit on the surface of Ag/TiO 2 particles and cover the Ag sites during the photodeposition process (the formation of CdS/Ag/TiO 2 Z-scheme nanojunction), as deduced from the previous studies that Ag acted as the reduction sites for photocatalytic reduction of S to S 2 ions. [19,20] Photocatalytic activity of the CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) nanojunctions was tested by visible-light photodecomposition of methylene blue. Figure 6 shows the normalized optical density change of methlyene blue at 664 nm under visible light irradiation (>400 nm) as catalyzed by CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) as a function of time. We found that the pure TiO 2 (P25), which is a wellknown UV-active photocatalyst, had a very low visible-light photocatalytic activity. This is because only a small part of light passed through the UV cut-off filter could be absorbed by TiO 2 and utilized for photocatalytic decomposition of methylene blue (Supplementary Material, Figure S1). CdS/TiO 2 two-component nanojunction showed higher photocatalytic activity than pure TiO 2 (P25), due to the photosensitization of CdS in visible light region and higher charge separation in a CdS/TiO 2 nanojunction system. 27 Compared to pure TiO 2 and the CdS/TiO 2 twocomponent nanojunction, all the CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) three-component nanojunctions exhibited higher visible-light photocatalytic performance. The charge transfer mechanism, which is similar to Z-scheme in green plants photosynthesis, has been proposed by other researchers. [19 21] The vectorial photogenerated electron transfer of TiO 2 M (noble metal) CdS in three-component nanojunction systems greatly improves the separation rate of photo-induced charges, resulting in higher photocatalytic activity. On the other hand, the photocatalytic activity of the CdS/M/TiO 2 (M=Ag, FIGURE 6. Photodecomposition of methylene blue catalyzed by CdS/M/TiO 2 (M=Ag,Ru,Au,Pd, Pt) nanojunctions prepared by photodeposition method, under the irradiation of visible light (>400 nm) for up to 90 min. (Figure provided in color online.) C/C CdS/TiO2 CdS/Ag/TiO2 CdS/Ru/TiO2 CdS/Au/TiO2 CdS/Pd/TiO2 CdS/Pt/TiO2 P25 TiO Degradation time/min

9 S. Shen et al. M101 FIGURE 7. Energy band diagram and charge transfer mechanism in CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) three-component nanojunction systems. (Figure provided in color online.) Ru, Au, Pd, Pt) three-component nanojunctions showed a significant dependence on the type of noble metal, and the photocatalytic activity decreased in the order of Ag > Ru Au > Pd > Pt. Based on the energy band diagram, the charge transfer process in CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) three-component nanojunction systems is illustrated in Figure 7. Considering the noble metal acting as a mediator in the vectorial electron-transfer process, their Fermi energy levels are supposed to affect the interfacial charge transfer and thus the charge separation in the CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) three-component nanojunctions. In general, the minimum energy needed to move an electron from the Fermi energy level into vacuum is defined as the work function. 28 According to the different work functions of these noble metals, 29 we obtain the height of Fermi energy levels in the reducing order of Ag > Ru > Au > Pd > Pt, as depicted in Figure 7. This trend is in good correlation with the photocatalytic activity of CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) nanojunctions depending on the type noble metals, i.e., the higher the Fermi level of the noble metal, the better the photocatalytic activity of CdS/M/TiO 2. As the charge transfer mechanism proposed in Figure 7, photo-generated electrons in the conduction band of TiO 2 easily flow into noble metal through the Schottky barrier (electron transfer I: TiO 2 M), and the holes left in the valence band of TiO 2 are available for oxidation reaction. Simultaneously, the photo-generated holes in the valence band of CdS also easily flow into the noble metal to recombine with the stored electrons (electron transfer II: M CdS), because of the higher Fermi energy levels of noble metal than the valence band level of CdS, and the electrons left in the conduction band of CdS are available for reduction reaction. Thus, the resulted vectorial photo-generated electrons transfer of TiO 2 M (noble metal) CdS realizes the complete separation of photo-generated holes in the valence band of TiO 2 (VB-hole [TiO 2 ]) and electrons in the conduction band of CdS (CB-electron [CdS]). Therefore, we can deduce that the higher Fermi levels of noble metals in CdS/M/TiO 2 nanojunctions make the electron transfer II (M CdS) much quicker, because of the larger difference between the Fermi level of noble metal and the valence band level of CdS. This will lead to more efficient charge separation between VB-hole (TiO 2 ) and CB-electron (CdS), which contributes to the better photocatalytic activity of CdS/M/TiO 2, depending on the noble metals (Ag > Ru Au > Pd > Pt). In order to illustrate the effect of vectorial electrons transfer of TiO 2 M CdS on the

10 M102 International Journal of Green Nanotechnology: Materials Science and Engineering FIGURE 8. Photodecomposition of methylene blue catalyzed by photocatalysts prepared by photodeposition method (CdS/TiO 2 and CdS/Ag/TiO 2 ), and by precipitation method (CdS/TiO 2 -R and CdS/Ag/TiO 2 -R), under the irradiation of visible light (>400 nm) for up to 90 min. (Figure provided in color online.) CdS/TiO2 CdS/TiO2-R CdS/Ag/TiO2 CdS/Ag/TiO2-R C/C Degradation time/min FIGURE 9. Schematic illustration of charge transfer in nanojunction systems. (a) CdS/TiO 2 two-component nanojunctions prepared by photodeposition and precipitation (CdS/TiO 2 and CdS/TiO 2 -R); (b) CdS/Ag/TiO 2 three-component nanojunction prepared by precipitation method (CdS/Ag/TiO 2 -R); (c) CdS/Ag/TiO 2 three-component nanojunction prepared by photodeposition method (CdS/Ag/TiO 2 ). (Figure provided in color online.)

11 S. Shen et al. M103 improved photocatalytic activity of CdS/M/TiO 2 three-component nanojunctions, CdS/TiO 2 twocomponent nanojunction (CdS/TiO 2 -R) and CdS/Ag/TiO 2 three-component nanojunction (CdS/Ag/TiO 2 -R) were also prepared for photodegradation of methylene blue by precipitation method. As shown in Figure 8, CdS/TiO 2 (photodeposition method) and CdS/TiO 2 -R (precipitation method) displayed almost the same photocatalytic efficiency for methylene blue degradation under visible light irradiation up to 90 min. This is due to the similar hybridization of CdS with TiO 2 and charge transfer in the CdS/TiO 2 two-component nanojucntions (Figure 9a), even though prepared by different methods. CdS/Ag/TiO 2 -R obtained via photodeposition of Ag on TiO 2 and subsequent precipitation of CdS on TiO 2 showed higher photoactivity than CdS/TiO 2 two-component nanojunctions, as the Ag metal deposited on TiO 2 would capture the electrons from the conduction band of TiO 2 and accelerate charge separation (Figure 9b). Compared to CdS/Ag/TiO 2 -R, the CdS/Ag/TiO 2 three-component nanojunction with different hybridization of CdS with Ag/TiO 2,inwhich Ag nanoparticles deposited on TiO 2 was covered by CdS via photodeposition (Figure 9c), showed much higher photocatalytic activity. CONCLUSIONS Z-scheme CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) three-component nanojunction systems have been successfully constructed via a twostep photodeposition method. Compared to CdS/TiO 2 two-component nanojunction system, the CdS/M/TiO 2 Z-scheme systems exhibited higher photocatalytic activity for methylene blue degradation under visible light irradiation. This enhancement is mainly due to the vectorial electrons transfer of TiO 2 M CdS, which greatly improves the separation of photoinduced charges. Moreover, the noble metals (M=Ag, Ru, Au, Pd, Pt) make a great effect on the photocatalytic activities of CdS/M/TiO 2 nanojunction systems, depending on the work functions of the noble metals. The photocatalytic activity of CdS/M/TiO 2 (M=Ag, Ru, Au, Pd, Pt) nanojunction systems decreased in the order of Ag > Ru Au > Pd > Pt, corresponding to the height of Fermi energy levels in the reducing order of Ag > Ru > Au > Pd > Pt. REFERENCES 1. Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, Fujishima, A.; Zhang, X.; Tryk, D. A. Heterogeneous photocatalysis: Fromwater photolysis to applications in environmental cleanup. Int. J. Hydrogen Energy 2007, 32, Mao, S. S.; Chen, X. Selected nanotechnologies for renewable energy applications. Int. J. Energy Res. 2007, 31, Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, Wu, L.; Yu, J. C.; Fu, X. Characterization and photocatalytic mechanism of nanosized CdS coupled TiO 2 nanocrystals under visible light irradiation. J. Mol. Catal. A Chem. 2006, 244, Bessekhouad, Y.; Robert, D.; Weber, J. V. Bi 2 S 3 /TiO 2 and CdS/TiO 2 heterojunctions as an available configuration for photocatalytic degradation of organic pollutant, J. Photochem. Photobio. A Chem. 2004, 163, Zhu, J. H.; Yang, D.; Geng, J. Q.; Chen, D. M.; Jiang, Z. Y. Synthesis and characterization of bamboo-like CdS/TiO 2 nanotubes composites with enhanced visiblelight photocatalytic activity. J. Nanoparticle Res. 2008, 10, Jang, J. S.; Ji, S. M.; Bae, S. W.; Son, H. C.; Lee, J. S. Optimization of CdS/TiO 2 nano-bulk composite photocatalysts for hydrogen production from Na 2 S/Na 2 SO 3 aqueous electrolyte solution under visible light (λ 420 nm). J. Photochem. Photobio. A Chem. 2007, 188, Park, H.; Choi, W.; Hoffmann, M. R. Effects of the preparation method of the ternary CdS/TiO 2 /Pt hybrid photocatalysts on visible light-induced hydrogen production. J. Mater. Chem. 2008, 18, Ogisu, K.; Takanabe, K.; Lu, D. L.; Saruyama, M.; Ikeda, T.; Kanehara, M.; Teranishi, T.; Domen, K. CdS nanoparticles exhibiting quantum size effect by dispersion on TiO 2 : photocatalytic H 2 evolution and photoelectrochemical measurements. Bull. Chem. Soc. Jpn. 2009, 82, Kim, J. C.; Choi, J.; Lee, Y. B.; Hong, J. H.; Lee, J. I.; Yang, J. W.; Lee, W. I.; Hur, N. H. Enhanced photocatalytic activity in composites of TiO 2 nanotubes and CdS nanoparticles. Chem. Commun. 2006, 48, Sant, P. A.; Kamat, P. V. Interparticle electron transfer between size-quantized CdS and TiO 2 semiconductor

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