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Chemical Physics Letters 634 (2015) 140–145

Contents lists available at ScienceDirect

Chemical Physics Letters

jou rn al h om epa ge: www.elsev ier .com/ locate /cp le t t

dS/ZnO core/shell nano-heterostructure coupled with reducedraphene oxide towards enhanced photocatalytic activity andhotostability

imrjit Singh, Neeraj Khare ∗

epartment of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

r t i c l e i n f o

rticle history:eceived 16 March 2015

n final form 15 May 2015

a b s t r a c t

Reduced graphene oxide (RGO) coupled CdS/ZnO core/shell nanorods have been synthesized using asoft chemical route. The photocatalytic activity of CdS/ZnO/RGO nanostructure has been examined for

vailable online 16 June 2015the degradation of methylene blue dye under solar irradiation at room temperature. The CdS/ZnO/RGOnanostructure exhibits five times enhanced photocatalytic activity as compared to CdS nanostructureswhich is attributed to the formation of heterojunction between the CdS and ZnO and its coupling withthe reduced graphene oxide which facilitates the efficient transport of photogenerated charge carriers.Also, CdS/ZnO core/shell nanostructure exhibits very good photostability due to the presence of stableZnO as cap layer.

. Introduction

Organic dyes which are used in many industrial processes haveecome an integral part of industrial waste water and are creatingevere environmental problems. Photocatalysis using semicon-uctor materials can be an advanced chemical process for theegradation of these organic dyes [1–3]. Various semiconductoraterials such as TiO2 [4], ZnO [5], CdS [6], and Ag3PO4 [7] have

een used for photocatalytic degradation of dyes. However, thehotogenerated charge carriers recombine very easily resulting

n low efficiency of the photocatalytic process. To improve thehotocatalytic activity of the semiconductor materials, recentlyraphene based semiconductor nanocomposites have attracted aot of attention due to high specific surface area and electron

obility properties of the graphene which results in faster trans-ort of the photoinduced charge carriers from the semiconductorso the graphene and thereby reduces the charge carriers recom-ination process [8,9]. Oxide semiconductors such as TiO2, ZnOre regarded as suitable photocatalytic material due to their hightability, low cost and strong oxidizing power, but these semicon-

uctor materials are active in UV light only and thereby limits these of whole solar spectrum. Several approaches such as dopingith cations [10] or anions [11] and decoration of plasmonicetal nanoparticles onto semiconductor materials [12,13] have

∗ Corresponding author.E-mail address: nkhare@physics.iitd.ernet.in (N. Khare).

ttp://dx.doi.org/10.1016/j.cplett.2015.05.074009-2614/© 2015 Published by Elsevier B.V.

© 2015 Published by Elsevier B.V.

been adopted to extend the light absorption range of these oxidesemiconductors into visible light region. The coupling of two semi-conductor materials with suitable positions of their conduction andvalence bands is another novel strategy for efficient separation ofthe photogenerated electron-hole pairs under solar irradiation [14].Nanoheterostructures of various semiconductor materials such asTiO2/MoS2 [15], TiO2/Ag3PO4 [16], CdS/ZnS [17], ZnO/SnO2 [18],and ZnO/CdS [19] have been tried to increase the photocatalyticactivity. Among them, CdS has been proven to be a very efficientvisible light responsive sensitizer for wide band gap semiconductormaterials such as ZnO due to its high optical absorption coefficient[19]. Although this exhibit enhanced photocatalytic activity, but theCdS undergoes photocorrosion [20] in aqueous solution in the pres-ence of light, thereby making the stability of the heterostructurematerial very poor.

In this work, ZnO is coupled with CdS in a core/shell nano-structure by forming ZnO shell over the CdS nanorods. The surfaceof the CdS nanorods was carefully functionalized to get uni-form thickness of the ZnO shell. This, coupled nanostructure isdesigned in such a way that the presence of stable ZnO as a caplayer avoids the photocorrosion of CdS in aqueous solution. Fur-ther, the synthesized core/shell CdS/ZnO nanostructure is coupledwith reduced graphene oxide (RGO) to improve the photocatalytic

activity of the CdS/ZnO core/shell nanostructure. The key ideabehind the designing of ternary CdS/ZnO/RGO nanostructure is tosimultaneously achieve the enhanced photosensitization of wide-band-gap ZnO with narrow band gap CdS for covering a broaderrange of solar spectra and efficient transfer of electrons within

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S. Singh, N. Khare / Chemical P

he coupled components for enhancing the photocatalytic activity.t is demonstrated that this trio-coupled nanostructure not onlyxhibits enhanced photocatalysis but also exhibits very good sta-ility for repeated operations of the photocatalytic process.

. Experimental

.1. Materials preparation

.1.1. Synthesis of graphene oxide (GO)Graphene oxide (GO) was synthesized using a modified Hum-

er’s method [21]. Firstly, 1 g of graphite powder and 0.5 g ofaNO3 were mixed with 30 ml of H2SO4 in an ice bath and stirred

or 30 min. Afterwards, 3 g of KMnO4 was added very slowly to thebove solution and it was stirred for 1 h. The temperature of theolution was raised to 35 ◦C and a mixture of 40 ml deionized waternd 10 ml H2O2 was added to the above brown colour solution toeduce the residual permanganate and manganese dioxide to man-anese sulfates. The colour of the solution changes to bright yellowonfirming the formation of GO. The suspension was centrifugedeveral times to get the GO powder.

.1.2. Synthesis of CdS/ZnO core/shell nanostructureCdS/ZnO core/shell nanostructures were synthesized using a

hemical solution method. Firstly, CdS nanorods were synthesizedsing the hydrothermal method by taking cadmium nitrate andhiourea in the molar ratio of 1:3 in 50 ml of ethylenediamine.fter stirring for 30 min, the mixture was transferred into a 50 mleflon lined stainless steel autoclave. The autoclave was sealed andeated at 200 ◦C for 10 h and cooled naturally to room tempera-ure. The resulting yellow paste was washed and dried to obtain thedS nanorods. For the synthesis of CdS/ZnO core/shell nanostruc-ures, 0.3 g of CdS nanorods were dispersed in 50 ml of deionizedater and stirred for 2 h. Afterward, 0.1 g of citric acid was added to

he above solution to functionalize the CdS nanorods with citrateons. For the synthesis of ZnO shell over CdS nanorods, zinc acetateolution (7 mM) was added drop by drop to the above solution andtirred continuously for few hours. The mixture was heated at 80 ◦Cntil the solution evaporated and thick paste was formed. Finally,he obtained thick paste was heated in the presence of oxygen at300 ◦C to obtain the core/shell nanostructure.

.1.3. Synthesis of CdS/ZnO/reduced graphene oxide (RGO)anostructure

For the synthesis of CdS/ZnO/RGO ternary nanocomposite,00 mg of the CdS/ZnO core/shell nanorods were mixed with 30 mlf ethylene glycol and ultrasonicated for 3 h. The GO suspension

igure 1. (a) XRD patterns of the ZnO, CdS, CdS/ZnO and CdS/ZnO/RGO nanostructures. In

arked with (�) corresponds to CdS. (b) XRD patterns showing the shift in the peak positi

Letters 634 (2015) 140–145 141

(0.3 mg ml−1) was added into the above CdS/ZnO suspension undercontinuous stirring and then 4–5 ml of ammonia solution wasadded to the resulting solution to maintain the pH ∼13. For thereduction of GO to RGO, 2 ml of hydrazine hydrate was added tothe above solution and was kept in an oil bath at 80 ◦C for 6 h. Theresulting black suspension was centrifuged and dried in an oven at60 ◦C for 4 h to obtain the resulting CdS/ZnO/RGO nanostructure.

2.1.4. Synthesis of ZnO nanorodsZnO nanostructures were also synthesized by hydrothermal

technique. For the synthesis of ZnO nanorods, 1.4 g of zinc acetateand 4 g of sodium hydroxide were dissolved in 25 ml deionizedwater separately and then the zinc precursor solution was mixedto sodium hydroxide solution drop by drop under constant stir-ring. After half an hour of stirring, the mixture was subjected tohydrothermal treatment. The hydrothermal reaction was carriedout in a Teflon lined stainless steel autoclave at 80 ◦C for 10 h.

2.2. Characterization methods

The crystal structure of the synthesized nanostructures wasanalyzed using X-ray diffractometer (XRD, Rigaku Ultima-IV) withCuK� (� = 1.54 A) irradiation operating at 40 kV and 40 mA. Themorphology of the nanostructure was studied by using Transmis-sion electron microscopy (TEM, JEOL, JEM-2200-FS). The opticalabsorption spectra were recorded using UV–Vis spectrophotome-ter (Perkin Elmer, LAMBDA-1050). Photoluminescence and Ramanspectra were recorded at room temperature by using (Horiba JobinVyon, LabRam) Raman Spectrometer.

2.3. Photocatalytic measurements

Photocatalytic performances of ZnO, CdS, CdS/ZnO core/shellnanorods and CdS/ZnO/RGO nanostructure were measured bythe degradation of the methylene blue (MB) organic dye undersimulated solar radiation at room temperature. 100 mg of the semi-conductor nanostructures was dispersed in an aqueous solution ofMB (10 mg L−1) and this solution was vigorously stirred for 1 h in thedark to establish the adsorption–desorption equilibrium betweenthe dye and semiconducting nanoparticles. Then, the mixture wasexposed to simulated solar radiation with continuous stirring. At

regular intervals of time, a 5 ml quantity of the MB solution wastaken out and centrifuged (3000 rpm, 5 min) to remove the sus-pended nanoparticles and UV-visible absorption spectrum wasrecorded using a LAMBDA-1050 spectrophotometer (Perkin Elmer)to determine the decomposition of MB in the solution.

the core/shell nanostructures, peaks marked with (*) corresponds to ZnO and peaksons of the most intense peaks of CdS in CdS/ZnO and CdS/ZnO/RGO nanostructures.

1 hysics Letters 634 (2015) 140–145

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Figure 2. (a) TEM image of ZnO nanorods (b) high resolution TEM image of ZnOnanorods (c) TEM image of CdS nanorods (d) high resolution TEM image of CdS

42 S. Singh, N. Khare / Chemical P

. Results and discussion

Figure 1a shows the X-ray diffraction (XRD) patterns of the ZnO,dS, CdS/ZnO and CdS/ZnO/RGO nanostructures. The diffractioneaks at 2� = 31.7◦, 34.3◦, 36.2◦, 47.6◦, 56.6◦, 62.8◦ corresponds to1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) and (1 0 3) crystal planes of thenO wurtzite phase (JCPDS card no. 36-1451). The sharp diffrac-ion peaks show good crystallinity of the sample. The XRD patternf CdS exhibits peaks at 2� values of 24.7◦, 26.5◦, 28.1◦, 36.6◦, 43.8◦,7.9◦, 50.9◦, 51.8◦, 52.9◦, 58.4◦ corresponding to (1 0 0), (0 0 2),1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (2 0 2) crystallanes of the hexagonal phase of CdS (JCPDS card no. 41-1049).he XRD patterns of the CdS/ZnO and CdS/ZnO/RGO nanostruc-ures exhibit characteristic diffraction peaks of both CdS and ZnOhases. However, only the most intense diffraction peaks corre-ponding to (1 0 0), (0 0 2) and (1 0 1) planes of ZnO are presentn the composite nanostructures. The intensity of the peaks cor-esponding to ZnO phase in the CdS/ZnO core/shell nanorods isuch smaller which may be due to a very thin coating of ZnO

round the CdS nanorods. No diffraction peaks corresponding tony secondary phase were observed, which confirms the formationf single phases of ZnO, CdS, CdS/ZnO and CdS/ZnO/RGO nanostruc-ures.

Figure 1b shows that the diffraction peaks corresponding to CdSn the CdS/ZnO core/shell and CdS/ZnO/RGO nanostructures shiftsowards higher Bragg’s angles. The observed shift in the peaksowards the higher diffraction angles directly reflects the latticehrinkage of CdS due to the growth of ZnO shell and the couplingf reduced graphene oxide with CdS/ZnO core/shell nanostructurehich induces strain in the composite nanostructures.

The presence of numerous oxygen containing functional groupsuch as epoxy, hydroxyl, carboxyl, and carbonyl groups on the car-on basal planes of RGO provide reactive sites for the attachment ofdS/ZnO nanostructure on the RGO sheets to prepare CdS/ZnO/RGOanocomposites.

Figure 2 shows the transmission electron microscopy (TEM) andigh resolution transmission electron microscopy (HRTEM) imagesf ZnO, CdS, CdS/ZnO and CdS/ZnO/RGO nanostructures. The TEMmage of ZnO (Figure 2a) shows the rod like structure of ZnO with aiameter of rods ∼500 nm. The HRTEM image of ZnO (Figure 2b)hows well resolved lattice fringes of spacing of 0.25 nm whichorresponds to the interplanar spacing of (1 0 1) plane of wurtzitenO phase. CdS (Figure 2c) also grows into nanorods with a lengthf around 500 nm and a diameter of ∼30 nm. The HRTEM imagef CdS (Figure 2d) shows lattice fringes of spacing 0.34 nm corre-ponding to interplanar spacing of (0 0 2) plane of hexagonal CdS.he TEM image of CdS/ZnO (Figure. 2e) clearly shows the formationf ZnO shell of thickness ∼8 nm around CdS nanorods. The HRTEMmage of CdS/ZnO core/shell nanostructure (Figure 2f) shows thelear interface between the ZnO and the CdS nanorods. The wellesolved lattice fringes with spacing 0.34 nm and 0.25 nm corre-ponds to (0 0 2) plane of CdS and (1 0 1) plane of ZnO, respectively.EM image of GO sheets (Figure 2g) shows that the GO consistsf thin stacked flakes. The well defined few layer structure cane clearly seen at the edges. The high resolution TEM image ofdS/ZnO/RGO nanocomposite (Figure 2h) shows that the CdS/ZnOore/shell nanostructures are well decorated on the RGO sheetshich implies a good coupling of RGO with CdS/ZnO core/shellanostructures.

Figure 3 shows the UV–vis absorption spectra of CdS, CdS/ZnOnd CdS/ZnO/RGO nanostructures. The band gap (Eg) can be deter-

ined from the Tauc’s plots using the relation (˛h�)2 = A(h� − Eg)

22]. Inset of Figure 3 shows the Tauc’s plots for all the three nano-tructures. The bandgap value for CdS is estimated as 2.40 eV. Theandgap of CdS in CdS/ZnO and CdS/ZnO/RGO nanostructures islso ≈2.40 eV.

nanorods (e) TEM image of CdS/ZnO core/shell nanorods (f) high resolution TEMimage of CdS/ZnO nanorods (g) TEM image of GO (h) high resolution TEM image ofCdS/ZnO/RGO nanostructure.

Figure 4 shows the Raman spectra of the RGO, CdS, CdS/ZnOand CdS/ZnO/RGO nanostructures. The Raman spectrum of reducedgraphene oxide (RGO) exhibits peaks at wavenumbers 1348 and1596 cm−1 corresponding to D- and G-bands and are usuallyassigned to structural defects and E2g phonon of C sp2 atoms respec-tively [23]. In the Raman spectrum of CdS, the observed three sharppeaks at wavenumbers 302, 601 and 909 cm−1 are assigned to thefundamental optical phonon mode (LO), the first overtone mode(2LO) and the second overtone mode (3LO) respectively [24]. TheRaman spectrum of CdS/ZnO exhibits all the characteristic peakscorresponding to CdS. However, no Raman signal correspondingto ZnO is observed. This may be due to very thin layer of ZnO on

CdS. The Raman spectrum for CdS/ZnO/RGO exhibits Raman signalscorresponding to CdS/ZnO and also the presence of D and G bandscorresponding to RGO which confirms the coupling of CdS/ZnO withRGO.

S. Singh, N. Khare / Chemical Physics Letters 634 (2015) 140–145 143

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igure 3. UV–vis absorption spectra of the CdS, CdS/ZnO and CdS/ZnO/RGO nano-tructures. Inset of the figure shows the Tauc plots for the three nanostrucutres.

Figure 5a shows the absorption spectra of MB solution whent was exposed to simulated solar radiation for different time inhe presence of CdS/ZnO/RGO nanostructure. The intensity of thebsorption peak is proportional to the concentration of MB presentn the solution. We have observed that the intensity of the absorp-ion peak decreases with the irradiation time, which is due to theecomposition of MB in aqueous solution. The inset of the Figure 5ahows a photograph of MB solution. After solar light exposure, thelue colour of the solution started getting faded and it becamelmost transparent after 80 min of the irradiation which showsB was fully decomposed. In order to compare the efficiency of

hotodegradation of MB in the presence of CdS/ZnO/RGO nano-tructures with the ZnO, CdS or CdS/ZnO core/shell nanostructures,e have recorded the absorption spectra of MB solution for differ-

nt duration of solar light exposure when ZnO, CdS or CdS/ZnOore/shell nanorods were present.

igure 5. (a) UV–vis absorption spectra of MB in aqueous solution in the presence of Cdhotographs of the MB solution after different duration of UV–vis exposure, (b) comparishotodegradation of MB under UV-visible irradiation, (c) bar graph showing the values o

Figure 4. Raman spectra of the RGO, CdS, CdS/ZnO core/shell nanorods andCdS/ZnO/RGO nanostructure at room temperature.

Figure 5b shows the variation of normalized concentration(C/Co) of MB remained in the aqueous solution when the solutionwas exposed to solar light irradiation in the presence of ZnO, CdS,CdS/ZnO core/shell nanorods or CdS/ZnO/RGO nanostructure. It isevident that CdS/ZnO nanostructure has the higher rate of degra-dation of MB as compared to ZnO and CdS nanostructures and thedegradation rate is further enhanced with the coupling of RGO withCdS/ZnO core/shell nanorods. It is observed that the overall pho-tocatalytic activity of the CdS/ZnO heterostructure depends uponthe formation of clear interface between the CdS and ZnO and alsoon the thickness of the ZnO outer layer. The photocatalytic activ-

ity increases (∼10%) with the increase in the ZnO shell thicknessfrom 8 nm to 30 nm but for larger thickness of ZnO (>100 nm) thephotocatalytic activity has been found to decrease. It seems thatfor higher ZnO outer layer, the recombination of charge carriersin ZnO increases, which causes less photogenerated electrons and

S/ZnO/RGO core/shell nanostructure as a function of irradiation time, inset showson of the efficiency of ZnO, CdS, CdS/ZnO and CdS/ZnO/RGO nanostructures for thef rate constants for ZnO, CdS, CdS/ZnO and CdS/ZnO/RGO nanostructures.

144 S. Singh, N. Khare / Chemical Physics Letters 634 (2015) 140–145

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igure 6. Photoluminescence spectra of the CdS, CdS/ZnO core/shell nanorods anddS/ZnO/RGO nanostructure at room temperature.

oles from CdS to come out through ZnO for the degradation of MBye.

The reaction kinetics of the nanostructures for the photodegra-ation of MB are evaluated using the relation;

nCo

C= kt (1)

By fitting the experimental observations with a straight line, thealue of the rate constant (k) was obtained. Figure 5c shows the val-es of rate constants for the ZnO, CdS, CdS/ZnO and CdS/ZnO/RGOanostructures.

The CdS/ZnO/RGO nanostructure has the highest rate con-tant value of 0.024 min−1 as compared to CdS/ZnO (0.013 min−1),dS (0.005 min−1) and ZnO (0.007 min−1) nanorods. The reasonor the enhanced photocatalytic activity for the CdS/ZnO anddS/ZnO/RGO nanostructure can be attributed to the formationf the heterojunction between CdS and ZnO and the coupling ofeduced graphene oxide with CdS/ZnO core/shell nanorods whichesults in faster transport of the charge carriers thereby lower downhe recombination rate of the photogenerated charge carriers.

The addition of RGO with CdS/ZnO nanostructure will alsoncrease the composite surface area which will provide more sur-ace active sites for specific binding of dye molecules and willesult in an increase in the photocatalytic activity. But, the mostrucial factor for the observed significant enhancement in the pho-ocatalytic activity is the efficient separation of the photogeneratedharge carriers in the ternary CdS/ZnO/RGO nanocomposite whichill provide more number of reactive species for the degradation

f methylene blue dye and this higher separation of the photo-enerated charge carriers in the CdS/ZnO/RGO nanocomposite ischieved due to the excellent charge carrier mobility property ofhe RGO.

Photoluminescence (PL) is a powerful technique to probe theigration and the recombination processes of photogenerated

lectron-hole pairs in the semiconductors. Figure 6 shows PL spec-ra of CdS, CdS/ZnO and CdS/ZnO/RGO nanostructures at roomemperature. For the comparison, we have plotted together thepectra of bare CdS nanorods and the PL peak corresponding todS in the CdS/ZnO and CdS/ZnO/RGO nanostructure. The intensityf the peak corresponding to CdS in the CdS/ZnO and CdS/ZnO/RGOanostructure decreases as compared to the peak intensity of the

are CdS nanorods. The decrease in the peak intensity of CdS clearlyeveals that the direct contact between the ZnO and CdS in CdS/ZnOore/shell nanostructure and the coupling of RGO with CdS/ZnO inhe CdS/ZnO/RGO nanocomposite provides an additional route forhe transfer of the charge carriers from CdS to ZnO and then further

Figure 7. Schematic of band diagram of CdS/ZnO core/shell heterostructure andthe coupling of CdS/ZnO with reduced graphene oxide showing the transport ofphotogenerated charge carriers.

to reduced graphene oxide which prevents the recombination ofelectrons and holes generated under solar irradiation in CdS andthereby reducing the PL intensity corresponding to CdS in CdS/ZnOand CdS/ZnO/RGO nanostructures.

The schematic of the band diagram for the heterojunctionbetween CdS and ZnO and the mechanism for the transfer of thecharge carriers from the CdS/ZnO to the reduced graphene oxide isshown in Figure 7. The ZnO and CdS have a type-II band-edge align-ment, where the conduction band edge of ZnO (−0.1 eV vs. NHE)lies below the conduction band position (−0.6 eV vs. NHE) of CdS.Due to the band alignment between CdS and ZnO at the interfacethe photogenerated electrons can easily move from CdS conductionband to ZnO conduction band and the presence of reduced grapheneoxide (−0.08 eV vs. NHE) results in further transport of the elec-trons from CdS/ZnO to reduced graphene oxide. The electrons reactwith oxygen molecules to generate superoxide anion radicals [E0

(O2/O2•−) = −0.04 eV vs. VNHE)] which further undergoes secondary

reactions to generate hydroxyl radicals. Also, due to the small thick-ness of the ZnO cap layer the holes accumulated in the valence bandof CdS are able to tunnel out to react with water molecules (H2O) togenerate hydroxyl radicals. This reduces the recombination of pho-togenerated electron-hole pairs and thus enhances the efficiency ofphotocatalytic process. The possible reaction for degradation of MBcan be described as;

CdS/ZnO/RGO + h� → CdS(h+VB)/ZnO(e−

CB)/RGO(e−)

RGO(e−) + O2 → O2•−

O2•− + H2O → HO2

• + OH−

HO2• + H2O → H2O2 + OH•

H2O2 → 2OH•

CdS (h+ ) + H2O → OH•

OH• + MB → CO2 + H2O

After performing photocatalytic activities with CdS and CdS/ZnOcore/shell nanorods for longer times, we again performed the

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igure 8. XRD patterns of the CdS and CdS/ZnO core/shell nanorods before pho-ocatalytic reaction and after photocatalytic reaction. The peaks marked with (�)orresponds to CdS and peaks marked with (*) corresponds to ZnO.

RD studies (Figure 8) to check the stability of the semiconduc-or nanostructures and found that CdS gets decomposed due tohotocorrosion when it is used alone but the CdS in the CdS/ZnOore/shell nanostructure did not show any sign of photocorrosion.his shows that the stability of CdS is improved when ZnO is used as

shell material in the CdS/ZnO and CdS/ZnO/RGO nanocomposites.In the present case of CdS/ZnO/RGO nanostructures, we have

btained approximately five times higher improvement in thehotocatalytic activity as compared to the photocatalytic activityf CdS alone. Our present approach of combining two semi-onductors with appropriate band positioning through core-shellanostructures and coupling it with RGO indeed shows significant

mprovement in the photocatalytic activity.

. Conclusions

The CdS/ZnO core/shell nanostructures with a ZnO shell arounddS nanorods were prepared to improve the overall stability andhotocatalytic activity of the photocatalyst material. To further

mprove the photocatalytic activity, the CdS/ZnO core/shell nano-tructure is attached to reduced graphene oxide The high resolutionEM image confirmed the formation of ZnO shell around CdS

anorods. The PL results confirmed that the recombination of thehotogenerated electron-hole pairs is inhibited greatly in CdS dueo the transfer of the charge carriers from CdS to ZnO in thedS/ZnO core/shell nanorods and further from CdS/ZnO to reducedraphene oxide in the CdS/ZnO/RGO nanostructure. Substantial

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Letters 634 (2015) 140–145 145

enhancement in the photocatalytic activity of the CdS/ZnO/RGOnanostructure was observed as compared to ZnO and CdS nano-structures. Finally, a mechanism for the enhanced photocatalyticactivity has been proposed to the formation of heterojunction atthe CdS/ZnO interface from the schematic band diagram. The pres-ence of ZnO as a top surface layer in the CdS/ZnO is demonstratedto protect photocorrosion of CdS. Thus, our modified CdS/ZnO/RGOnanostructure can be used as an efficient photocatalyst materialwith improved stability.

Acknowledgements

The authors thank Prof. D. Sakthi Kumar from Toyo Universityfor the kind support for the TEM measurements of the nano-structures. The financial support from DeitY (Govt. of India) andDST-UKIERI project is gratefully acknowledged.

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