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Effect of Noble Metal in CdS/M/TiO2 for PhotocatalyticDegradation of Methylene Blue under Visible LightShaohua Shen a b , Liejin Guo a , Xiaobo Chen b , Feng Ren b , Coleman X. Kronawitter b &Samuel S. Mao ba State Key Laboratory of Multiphase Flow in Power Engineering , Xi'an Jiaotong University ,Xi'an, Shaanxi, Chinab Lawrence Berkeley National Laboratory , University of California at Berkeley , Berkeley,California, USAPublished online: 17 Mar 2010.

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/TiO2 for Photocatalytic Degradation of Methylene Blue under Visible Light, InternationalJournal of Green Nanotechnology: Materials Science & Engineering, 1:2, M94-M104, DOI: 10.1080/19430841003684823

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International Journal of Green Nanotechnology:Materials Science & Engineering, 1:M94–M104, 2010Copyright c© Taylor & Francis Group, LLCISSN: 1943-0841 print / 1943-0833 onlineDOI: 10.1080/19430841003684823

Effect of Noble Metal in CdS/M/TiO2 for PhotocatalyticDegradation of Methylene Blue under Visible Light

Shaohua ShenLiejin Guo

Xiaobo ChenFeng Ren

Coleman X. KronawitterSamuel S. Mao

ABSTRACT. The CdS/M/TiO2 (M = Ag, Ru, Au, Pd, and Pt) three-component nanojunction systemswere constructed using a two-step photodeposition method, and evaluated for their photocatalytic activ-ities through the degradation of methylene blue in aqueous solution under visible light irradiation. Theauthors found that the photocatalytic activity of CdS/M/TiO2 (M = Ag, Ru, Au, Pd, Pt) three-componentnanojunctions was superior to that of CdS/TiO2 two-component system. Moreover, the photocatalyticactivity of the three-component nanojunction system was found to be dependent significantly on thetype of the noble metals. The results can be explained by the influence of charge transfer on the basisof the work functions of different noble metals.

[Supplementary materials are available for this article. Go to the publisher’s online edition of theInternational Journal of Green Nanotechnology: Materials Science and Engineering to view the freesupplementary file.]

KEYWORDS. CdS/M/TiO2, heterojunction, noble metal, photocatalysis, Z-scheme

INTRODUCTION

Since the discovery of photo-induced split-ting of water on TiO2 electrodes,[1] wa-

Received 25 January 2010; accepted 6 February 2010.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. 50821064 and No. 90610022), 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 710049, China, E-mail: lj-guo@mail.xjtu.edu.cn; or to Samuel S.Mao, Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, CA 94720,USA. E-mail: ssmao@lbl.gov

ter splitting and environmental clean-up be-come two active fields supported by het-erogeneous photocatalysis.[2,3] Environmentalapplications of heterogeneous photocatalysis

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have been considered among the most ef-fective methods for elimination of manyhazardous organic pollutants from the en-vironment, particularly from the wastewater.Heterogeneous photocatalysis offers a “green”technology for completely decomposing suchcontaminants in the presence of semiconductingcatalyst particles.

Among numerous semiconducting photocat-alysts developed so far, TiO2 is regarded tobe the most efficient and environmentally be-nign, which has been most widely used for pho-todegradation of various pollutants.[4] However,its large band gap along with a fast recombina-tion rate of photogenerated electron-hole pairsin TiO2 hinders its photocatalytic activity, lim-iting further commercialization and industrialapplications. One effective approach to over-come such a hurdle involves coupling of dif-ferent semiconductors with appropriate energylevels. For instance, CdS/TiO2 heterojunctionhas been widely studied for effective decompo-sition of organic compounds[5–7] and for photo-catalytic water splitting.[8–10] In CdS/TiO2 het-erojunction, CdS with narrow band gap acts asa visible-light sensitizer; combined with TiO2,CdS is also responsible for effective charge sep-aration that enables suppression of the recombi-nation process.[11,12]

In the past decade, Z-scheme photocat-alytic systems (two-step photoexcitation) mim-icking photosynthesis in a green plant havebeen investigated to realize a system for over-all water splitting that are composed of H2-and O2-photocatalysts and a suitable electronmediator.[13–17] The electron mediator in so-lution plays an indispensable role in shuttlingthe photo-generated carriers between the H2-and O2-photocatalysts. Recently, Kudo and co-workers succeeded in constructing a simpleZ-scheme photocatalytic system driven by in-terparticle electron transfer without an elec-tron mediator under visible light irradiation.18

In this (Ru/SrTiO3:Rh)-(BiVO4) system, theexcited electrons in SrTiO3:Rh reduce waterto form H2 on Ru co-catalyst, the holes inBiVO4 oxidize water to form O2 to accomplishoverall water splitting, and the reversible Rhspecies at the surface of photocatalyst plays apivotal role for electron transfer between par-

ticles. On the other hand, all solid-state Z-scheme CdS/Au/TiO2 three-component nano-junction systems have been developed to showmuch higher photocatalytic activity than single-and two-component systems for decomposi-tion of organic compounds.[19,20] In such athree-component system, photo-induced elec-trons achieved a vectorial transfer of TiO2 →Au → CdS through a two-step excitation ofCdS, TiO2, and Au as a mediator. Similarly,metal Ag species in the AgBr-Ag-Bi2WO[21]

6 andAgBr-Ag-TiO22

2 Z-schematic nanojunction sys-tems also acted as the electron transfer media-tor, contributing to the enhancement of electron-hole separation and interfacial charge transfer.However, to the best of our knowledge, therehave been few reports on the effects of differ-ent noble metals (such as Au, Ag, etc.) on thephotocatalytic activity of such three-componentZ-scheme systems.

In the present study, we constructedCdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt) three-component Z-scheme nanojunction systemswith different noble metals as the electron trans-fer mediator. These CdS/M/TiO2 nanojunctionsprepared by a two-step photodeposition methodwere used to carry out photocatalytic degrada-tion of methylene blue. In particular, we inves-tigated the effect of noble metals (Ag, Ru, Au,Pd, and Pt) on the photocatalytic activity of suchZ-scheme systems.

EXPERIMENTAL

Sample Preparation

TiO2 (P25) was acquired from NipponAerosil. Cd(NO3)2·4H2O, Na2S·9H2O andethanol were purchased from Sigma-Aldrichand sulfur powder was purchased from Al-lied Chemical. Aqueous solutions of no-ble metal compound were prepared fromH2PtCl6·H2O (Sigma-Aldrich), AgNO3 (J.T.Baker), Na2PdCl4 (Aldrich), RuCl3 (Strem), andHAuCl4·4H2O (Sigma-Aldrich). All chemicalswere used as received.

CdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt)three-component photocatalysts were preparedby the two-step photodeposition approach,[19]

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as described in the Supplementary Material,Scheme 1. In a typical synthesis process, 0.1g P25 TiO2 was dispersed in 100 mL ethanolaqueous solution (v : v = 1:1). To this suspen-sion, a 0.4 mg/mL (noble metal) aqueous solu-tion (0.75 mL) of noble metal compound wasadded. Under stirring, the mixture was bubbledwith argon for 30 min and then irradiated witha 150-W Xe lamp for 5 h to load noble metalon TiO2 (M [0.3 wt%]/TiO2). After the resultingM/TiO2 ethanol/water suspension, added withsulfur powder (0.011 g) and Cd(NO3)2·4H2O(0.154 g), had been bubbled with argon for 30min, irradiation was carried out for a given pe-riod with a 150-W Xe lamp. Then the productsCdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt) were cen-trifuged, washed with water and ethanol, anddried at 65◦C in air. The CdS/TiO2 photocata-lyst without noble metal loading was preparedby photodeposition of CdS on P25 TiO2 insteadof M/TiO2.

CdS/TiO2 and CdS/Ag/TiO2 samples as ref-erence (CdS/TiO2-R and CdS/Ag/TiO2-R) wereprepared by the precipitation method. P25TiO2 (or Ag [0.3 wt%]/TiO2 prepared byphotodeposition), 0.1 g, was dispersed in an100-mL ethanol aqueous solution (v : v = 1:1)containing Cd(NO3)2·4H2O (0.154 g). Understirring, 10 mL aqueous solution containingdouble excess of Na2S was added to the mix-ture, and the resulting yellow suspension wasstirred overnight at room temperature. The prod-ucts CdS/TiO2-R and CdS/Ag/TiO2-R were cen-trifuged, washed with water and ethanol, anddried at 65◦C in air.

In all these photocatalyts, the mass contentsof CdS and M (versus TiO2) were 72.2% and0.3%, respectively.

Characterization

X-ray diffraction (XRD) patterns were ob-tained from a PANalytical X’pert diffractome-ter using Ni-filtered Cu Kα irradiation (wave-length 1.5406 A) with the scanning step of0.05◦/s, the operation voltage and current were45 kV and 40 mA, respectively. Ultraviolet-visible (UV-Vis) absorption spectra of the sam-ples were determined on a Varian Cary 50 UVspectrophotometer with MgO as the reference,

the scanning range was from 200 to 900 nm.Transmission electron microscopy (TEM) stud-ies were carried out on a JEOL JEM 2010 instru-ment. Elemental analysis of the photocatalytstwas conducted by an energy-dispersive X-rayspectrometer (EDS) attached to the transmissionelectron microscope.

Photocatalytic Degradation of MethyleneBlue

Photocatalytic activity of the photocatalytswas determined by measuring the decomposi-tion of methylene blue (MB) using a 150-Wxenon lamp (Newport) as light source at ambientconditions.[23,24] Light was passed through a UVcut-off filter (λ > 400 nm; Newport FS-C S/N147) (Supplementary Material, Figure S1), andguided onto the side window of an open cuvettefilled with 3 mL of MB auqeous solution (opti-cal density ≈1.0) and a given amount of photo-catalyst (60 mg/L). The suspension was stirredcontinuously during the whole process. The con-centration of methylene blue was estimated bymeasuring its maximum absorbance at 664 nmwith a Varian Cary 50 UV spectrophotometerafter irradiation for a period of time (Supple-mentary Material, Figure S2).

RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of CdS/M/TiO2

(M=Ag, Ru, Au, Pd, Pt) prepared byphotodeposition method. Even though the noblemetals (M) in CdS/M/TiO2 were different, theseheterojunctions displayed quite similar crystalstructures. All samples exhibited some diffrac-tion peaks assigned to the anatase and rutilephases, which coexisted in P25 TiO2. 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 (spacegroup F-43m; a = 5.818 A; JSPDS Card No.10-0454), respectively. The diffraction peaks as-signed to noble metals could not be identifiedin these CdS/M/TiO2 heterojunctions becausethe mass content of noble metals (0.3%, ver-sus TiO2) is under the detection limit for XRDanalysis. The XRD patterns of CdS/TiO2 and

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FIGURE 1. XRD patterns of CdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt) prepared by photodepositionmethod. (Figure provided in color online.)

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CdS/Ag/TiO2 prepared by precipitation methodas reference (CdS/TiO2-R and CdS/Ag/TiO2-R)are shown in Figure 2. They also possessed cu-bic CdS, anatase and rutile TiO2 phases, similarto CdS/TiO2 and CdS/Ag/TiO2 prepared by thephotodeposition method. However, as indicatedby the weaker intensity of diffraction peaks as-signed to cubic CdS phase in CdS/TiO2-R andCdS/Ag/TiO2-R, the cubic CdS in CdS/TiO2 andCdS/Ag/TiO2 prepared by the photodepositionmethod had better crystallinity than the samplesprepared by precipitation.

The UV-Vis diffuse reflectance spectra forCdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt) are shownin Figure 3. The single TiO2 (P25) photocata-lyst showed a sharp edge, whereas the CdS/TiO2

composite prepared by photodepostion methodhad two absorption edges; the main edge due toCdS is located at ∼550 nm and second one dueto TiO2 at ∼400 nm. This is in agreement withthe previous report by Jang et al. that the spec-tra of CdS/TiO2 composite photocatalysts pre-pared by precipitation method showed a com-bination of these two spectra.[8,25] When com-

pared to CdS/TiO2, all the CdS/M/TiO2 nano-junctions had two similar absorption edges as-signed to CdS and TiO2. However, additionalabsorption appeared in the range of 600–900 nmin the spectra of CdS/M/TiO2. This could be re-lated to the existence of noble metal particles(M=Ag, Ru, Au, Pd, Pt) in these CdS/M/TiO2

nanojunctions, corresponding to the report byDoremus that small particles of noble met-als showed an optical absorption band in vis-ible light that resulted from collective oscilla-tions of the free electrons in them.[26] The UV-Vis diffuse reflectance spectra of CdS/TiO2-Rand CdS/Ag/TiO2-R were also given in Figure4, which makes a comparison between opti-cal property of the samples prepared by pho-todeposition method and precipitation method.CdS/TiO2 and CdS/Ag/TiO2 prepared bydifferent methods had similar spectra with twoabsorption edges; the increased absorption invisible region (longer than 600 nm) of theCdS/Ag/TiO2 (or CdS/Ag/TiO2-R) spectrumwas attributable to the presence of Ag, as dis-cussed previously.

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FIGURE 2. XRD patterns of CdS/TiO2 and CdS/Ag/TiO2 prepared by photodeposition method,and CdS/TiO2-R and CdS/Ag/TiO2-R prepared by precipitation method. (Figure provided in coloronline.)

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FIGURE 3. UV-Vis diffuse reflectance spectra of TiO2 and CdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt)prepared by photodeposition method. (Figure provided in color online.)

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FIGURE 4. UV-Vis diffuse reflectance spectra of CdS/TiO2 and CdS/Ag/TiO2 prepared by photode-position method, and CdS/TiO2-R and CdS/Ag/TiO2-R prepared by precipitation method. (Figureprovided in color online.)

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To visualize the hybridization of CdSand Ag/TiO2 in CdS/Ag/TiO2 prepared bythe photodeposition and precipitation method,CdS/Ag/TiO2 and CdS/Ag/TiO2-R were inves-tigated by TEM. The representative TEM im-

age of the CdS/Ag/TiO2-R sample is shown inFigure 5A, displaying separated phases of TiO2

and CdS, as confirmed by EDS element anal-ysis (Supplementary Material, Figure S3a, b).The Ag signal could not be identified in the EDS

FIGURE 5. TEM images of (A) CdS/Ag/TiO2-R prepared by precipitation method, and (B)CdS/Ag/TiO2 prepared by photodeposition method.

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spectra, because of the very small amount of Agin CdS/Ag/TiO2-R sample. In contrast, the ob-vious separation of CdS and TiO2 phases doesnot occur in CdS/Ag/TiO2 sample, as shownin Figure 5B. Moreover, the EDS spectra ofCdS/Ag/TiO2 (Supplementary Material, FigureS3c) indicate homogenous dispersion of CdS.Thus, CdS is supposed to deposit on the surfaceof Ag/TiO2 particles and cover the Ag sites dur-ing the photodeposition process (the formationof CdS/Ag/TiO2 Z-scheme nanojunction), as de-duced from the previous studies that Ag acted asthe reduction sites for photocatalytic reductionof S to S2− ions.[19,20]

Photocatalytic activity of the CdS/M/TiO2

(M=Ag, Ru, Au, Pd, Pt) nanojunctions wastested by visible-light photodecomposition ofmethylene blue. Figure 6 shows the normal-ized optical density change of methlyene blue at∼664 nm under visible light irradiation (>400nm) as catalyzed by CdS/M/TiO2 (M=Ag, Ru,Au, Pd, Pt) as a function of time. We foundthat the pure TiO2 (P25), which is a well-known UV-active photocatalyst, had a very low

visible-light photocatalytic activity. This is be-cause only a small part of light passed throughthe UV cut-off filter could be absorbed by TiO2

and utilized for photocatalytic decomposition ofmethylene blue (Supplementary Material, Fig-ure S1). CdS/TiO2 two-component nanojunctionshowed higher photocatalytic activity than pureTiO2 (P25), due to the photosensitization of CdSin visible light region and higher charge sep-aration in a CdS/TiO2 nanojunction system.27

Compared to pure TiO2 and the CdS/TiO2 two-component nanojunction, all the CdS/M/TiO2

(M=Ag, Ru, Au, Pd, Pt) three-component nano-junctions exhibited higher visible-light pho-tocatalytic performance. The charge transfermechanism, which is similar to Z-scheme ingreen plants’ photosynthesis, has been proposedby other researchers.[19–21] The vectorial photo-generated electron transfer of TiO2 → M (noblemetal) → CdS in three-component nanojunctionsystems greatly improves the separation rate ofphoto-induced charges, resulting in higher pho-tocatalytic activity. On the other hand, the pho-tocatalytic activity of the CdS/M/TiO2 (M=Ag,

FIGURE 6. Photodecomposition of methylene blue catalyzed by CdS/M/TiO2 (M=Ag, Ru, Au, Pd,Pt) nanojunctions prepared by photodeposition method, under the irradiation of visible light (>400nm) for up to 90 min. (Figure provided in color online.)

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FIGURE 7. Energy band diagram and charge transfer mechanism in CdS/M/TiO2 (M=Ag, Ru, Au,Pd, Pt) three-component nanojunction systems. (Figure provided in color online.)

Ru, Au, Pd, Pt) three-component nanojunctionsshowed a significant dependence on the type ofnoble metal, and the photocatalytic activity de-creased in the order of Ag > Ru ≈ Au > Pd >

Pt.Based on the energy band diagram, the charge

transfer process in CdS/M/TiO2 (M=Ag, Ru,Au, Pd, Pt) three-component nanojunction sys-tems is illustrated in Figure 7. Consideringthe noble metal acting as a mediator in thevectorial electron-transfer process, their Fermienergy levels are supposed to affect the interfa-cial charge transfer and thus the charge separa-tion in the CdS/M/TiO2 (M=Ag, Ru, Au, Pd,Pt) three-component nanojunctions. In general,the minimum energy needed to move an elec-tron from the Fermi energy level into vacuumis defined as the work function.28 Accordingto the different work functions of these noblemetals,29 we obtain the height of Fermi energylevels in the reducing order of Ag > Ru > Au >

Pd > Pt, as depicted in Figure 7. This trend isin good correlation with the photocatalytic ac-tivity of CdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt)nanojunctions depending on the type noble met-als, i.e., the higher the Fermi level of the noblemetal, the better the photocatalytic activity ofCdS/M/TiO2.

As the charge transfer mechanism proposed inFigure 7, photo-generated electrons in the con-

duction band of TiO2 easily flow into noble metalthrough the Schottky barrier (electron transfer I:TiO2 → M), and the holes left in the valenceband of TiO2 are available for oxidation reac-tion. Simultaneously, the photo-generated holesin the valence band of CdS also easily flow intothe noble metal to recombine with the storedelectrons (electron transfer II: M → CdS), be-cause of the higher Fermi energy levels of noblemetal than the valence band level of CdS, and theelectrons left in the conduction band of CdS areavailable for reduction reaction. Thus, the re-sulted vectorial photo-generated electrons trans-fer of TiO2 → M (noble metal) → CdS real-izes the complete separation of photo-generatedholes in the valence band of TiO2 (VB-hole[TiO2]) and electrons in the conduction band ofCdS (CB-electron [CdS]). Therefore, we can de-duce that the higher Fermi levels of noble metalsin CdS/M/TiO2 nanojunctions make the electrontransfer II (M → CdS) much quicker, because ofthe larger difference between the Fermi level ofnoble metal and the valence band level of CdS.This will lead to more efficient charge separa-tion between VB-hole (TiO2) and CB-electron(CdS), which contributes to the better photocat-alytic activity of CdS/M/TiO2, depending on thenoble metals (Ag > Ru ≈ Au > Pd > Pt).

In order to illustrate the effect of vectorialelectrons transfer of TiO2 → M → CdS on the

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FIGURE 8. Photodecomposition of methylene blue catalyzed by photocatalysts prepared by pho-todeposition method (CdS/TiO2 and CdS/Ag/TiO2), and by precipitation method (CdS/TiO2-R andCdS/Ag/TiO2-R), under the irradiation of visible light (>400 nm) for up to 90 min. (Figure providedin color online.)

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FIGURE 9. Schematic illustration of charge transfer in nanojunction systems. (a) CdS/TiO2

two-component nanojunctions prepared by photodeposition and precipitation (CdS/TiO2 andCdS/TiO2-R); (b) CdS/Ag/TiO2 three-component nanojunction prepared by precipitation method(CdS/Ag/TiO2-R); (c) CdS/Ag/TiO2 three-component nanojunction prepared by photodepositionmethod (CdS/Ag/TiO2). (Figure provided in color online.)

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improved photocatalytic activity of CdS/M/TiO2

three-component nanojunctions, CdS/TiO2 two-component nanojunction (CdS/TiO2-R) andCdS/Ag/TiO2 three-component nanojunction(CdS/Ag/TiO2-R) were also prepared forphotodegradation of methylene blue by precipi-tation method. As shown in Figure 8, CdS/TiO2

(photodeposition method) and CdS/TiO2-R (pre-cipitation method) displayed almost the samephotocatalytic efficiency for methylene bluedegradation under visible light irradiation up to90 min. This is due to the similar hybridizationof CdS with TiO2 and charge transfer inthe CdS/TiO2 two-component nanojucntions(Figure 9a), even though prepared by differentmethods. CdS/Ag/TiO2-R obtained via photode-position of Ag on TiO2 and subsequent precipita-tion of CdS on TiO2 showed higher photoactivitythan CdS/TiO2 two-component nanojunctions,as the Ag metal deposited on TiO2 would capturethe electrons from the conduction band of TiO2

and accelerate charge separation (Figure 9b).Compared to CdS/Ag/TiO2-R, the CdS/Ag/TiO2

three-component nanojunction with differenthybridization of CdS with Ag/TiO2, in whichAg nanoparticles deposited on TiO2 was cov-ered by CdS via photodeposition (Figure 9c),showed much higher photocatalytic activity.

CONCLUSIONS

Z-scheme CdS/M/TiO2 (M=Ag, Ru, Au,Pd, Pt) three-component nanojunction systemshave been successfully constructed via a two-step photodeposition method. Compared toCdS/TiO2 two-component nanojunction system,the CdS/M/TiO2 Z-scheme systems exhibitedhigher photocatalytic activity for methyleneblue degradation under visible light irradiation.This enhancement is mainly due to the vecto-rial electrons transfer of TiO2 → M → CdS,which greatly improves the separation of photo-induced charges. Moreover, the noble metals(M=Ag, Ru, Au, Pd, Pt) make a great effecton the photocatalytic activities of CdS/M/TiO2

nanojunction systems, depending on the workfunctions of the noble metals. The photocatalyticactivity of CdS/M/TiO2 (M=Ag, Ru, Au, Pd, Pt)nanojunction systems decreased in the order of

Ag > Ru ≈ Au > Pd > Pt, corresponding tothe height of Fermi energy levels in the reducingorder of Ag > Ru > Au > Pd > Pt.

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