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Materials Science and Engineering B 177 (2012) 570– 574

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B

j o ur nal homep age: www.elsev ier .com/ locate /mseb

nhanced visible-light-responsive photocatalytic property of CdS and PbSensitized ZnO nanocomposite photocatalysts

hengcheng Liua, Zhifeng Liua,∗, Yabin Lia, Zhichao Liua, Yun Wanga, Lei Ea, Jing Yaa,icola Gargiulob, Domenico Caputob

Department of Materials Science and Engineering, Tianjin Institute of Urban Construction, 300384, Tianjin, ChinaDepartment of Materials and Production Engineering, University of Naples Federico II, Piazzale V. Tecchio 80, 80125, Napoli, Italy

r t i c l e i n f o

rticle history:eceived 14 November 2011eceived in revised form 3 February 2012ccepted 3 March 2012

a b s t r a c t

CdS and PbS nanoparticles sensitized ZnO nanorods were synthesized by successive ionic layer adsorptionand reaction method. The photocatalytic activity of different structures was evaluated by photocatalyticdegeneration yield of methyl orange. Co-sensitization of CdS and PbS nanoparticles on ZnO nanorodsshowed enhanced photocatalytic activity due to its response at visible light area and the stepwise band

vailable online 18 March 2012

eywords:nOanocompositeshotocatalyststepwise band gap

gap constructed in ZnO/CdS/PbS nanostructures.© 2012 Elsevier B.V. All rights reserved.

. Introduction

Recent years, the degradation of organic and inorganic contami-ants in waste water has attracted extensive attention [1]. Since theioneer work of water photolysis with TiO2 semiconductor elec-rodes reported by Fujishima and Honda in 1972 [2], wide bandap metal oxide semiconductors, such as TiO2, ZnO, Fe2O3 and ZnS,ere taken as the attractive photocatalysts for the water remedia-

ion.However, photocatalysts such as TiO2 and ZnO can be stimu-

ated under ultraviolet (UV) light which occupies only 4% in theolar spectra, which greatly impeded the photocatalytic efficiencyf these photocatalysts in the direct use of solar light. Furthermore,here is a lack of suitable photoinstability in organic aqueous solu-ion for the photocatalysts like ZnO. Thus, two major challenges insing this photocatalysts are to broaden the light response regionnd improve the stability of them without sacrificing their effi-iency. Fortunately, the coupling of two or three semiconductorsr particles with different band gap energy could improve the sta-ility necessary for practical applications and extend the energy

ange of photoexcitation. Up to now, lots of studies related tohe ZnO or TiO2 coupled with other semiconductors have beenttended, such as SnO2 [3,4], Fe2O3 [5], ZrO2 [6,7], and CdS [8],

∗ Corresponding author. Tel.: +86 22 23085236; fax: +86 22 23085110.E-mail address: tjulzf@163.com (Z. Liu).

921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2012.03.020

for the purpose of improving the photocatalytic activity of ZnO orTiO2. The physical and optical properties of these coupled pho-tocatalysts are obviously modified correspondingly [9]. Specially,the ZnO or TiO2-based photocatalysts mixed by a narrow bandgap semiconductor may increase the photocatalytic efficiency byimproving the stability and extending the energy range of photoex-citation greatly. Recently, different heterojunction systems such asTiO2/PbS and TiO2/CdS have been studied for their potential appli-cation in photocatalytic degradation of organic pollutants [10–12].High efficiencies suggest that both the heterojunction can playimportant role in the degradation of organic pollutants using visiblelight.

Although TiO2 in the anatase form has been found to havea quite higher efficiency for photocatalytic degradation and hasbeen widely used for environmental applications, ZnO (3.36 eV)is a suitable alternative to TiO2 as long as the similar band gapenergy. And 1D ZnO nanorods/nanotubes have larger surface areaand quick transport ability of photogenerated carrier. Moreover,ZnO is more efficient than TiO2 in the photodegradation of someespecial organic compounds [13,14].

The aim of the present work is to investigate the photo-catalytic degradation of methyl orange (MO), in the presenceof CdS and PbS nanoparticles sensitized ZnO nanorods under

visible light radiation. For this purpose, CdS and PbS nanopar-ticles deposited on the ZnO nanorods via successive ionic layeradsorption and reaction (SILAR) method using their precursorrespectively. CdS was chosen here due to the quantum confinement

nd Engineering B 177 (2012) 570– 574 571

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C. Liu et al. / Materials Science a

ffects, which had been widely studied [15,16], and PbS washosen as the other sulfide for its small band gap and its sim-le preparation method. To illustrate the high efficiency of thenO/CdS/PbS nanorods, a series of nanostructures, both the singlend co-sensitized ZnO photocatalysts were designed. By varyinghe sensitizing order of the CdS and PbS nanoparticles, a system-tic study on the factors to influence the photovoltaic propertiesf ZnO photocatalysts was carried out. It is found that the for-ation of a stepwise band gap in co-sensitizing of CdS and PbS

anoparticles was the key factor of high efficiency of the photocat-lysts.

. Experimental

.1. Preparation of photocatalysts

After synthesizing ZnO nanorods by the hydrothermal methodn the ITO substrate [17,18], CdS and PbS nanoparticles wereeposited successively on them using SILAR method, respectively19]. Typically, nanoparticles were deposited using their precur-ors. For CdS, the ZnO nanorods film was dipped into ethanololution containing Cd(NO3)2 for 1 min, washed with ethanol,hen dipped it into methanol solution containing Na2S for another

min, washed with methanol again and at last dried it in their. The above procedure is defined as one SILAR cycle, andhe amount of CdS can be increased by repeating the assem-ly cycles. PbS nanoparticles were also prepared in the sameay except that the ethanol solution containing Cd(NO3)2 was

eplaced by methanol solution containing Pb(NO3)2. In addi-ion, for the preparation of PbS, the sample was first dippednto the methanol solution containing Na2S, then dipped into

ethanol solution containing Pb(NO3)2 for 1 min respectively, andecessary cycles also can be done. In our experiment, the pre-ursor concentration of CdS and PbS is 0.05 mol/L and 0.02 mol/L,espectively. It should be noticed that the number x in bracketf ZnO/CdS(x)/PbS(x) denotes cycles. In this paper, the samplese prepared were ZnO/CdS(3), ZnO/PbS(4), ZnO/CdS(3)/PbS(4)

nd ZnO/PbS(4)/CdS(3), respectively. Alcohol is used here to getetter wetting and faster drying, which can lead to the forma-ion of better-defined particles in gaps during the ZnO nanorods20].

.2. Characterization

The morphology of the samples was observed using a PHILIPSL-30 scanning electron microscope (SEM). X-ray diffraction (XRD)atterns of the photocatalysts were recorded with a Rigaku/max-2500 using Cu K� radiation (� = 0.154059 nm). UV–vis

ransmittance spectra were examined on a DU-8B UV–vis double-eam spectrophotometer.

.3. Photocatalytic activity evaluation

The liquid-phase photocatalytic degradation of methyl orangeMO) was monitored in a 50 mL watch-glass containing 1 cm × 1 cmf photocatalysts and 20 mL of A. R. Grade MO solution. After plac-ng 30 min in dark until the MO was absorbed into the gaps ofanorods, visible light photocatalysis was carried out by irradi-ting the photocatalysts with a fluorescent lamp located 10 cmway from the watch-glass. In addition, the temperature in theatch-glass was maintained at room temperature during the pho-

ocatalytic process. To monitor the photocatalytic process, 1 mL ofO was taken out from the watch-glass every 30 min, and analyzed

y a UV–vis spectrophotometer (DU-8B) at its characteristic wave-ength (464 nm) to determine the degradation yield. The veracity

Fig. 1. SEM image of bare ZnO nanorods (the inset is the high magnification SEM).

of the results was checked by repeating the experiment at least 3times.

The degradation yield is expressed by (�):

� = A0 − A

A0× 100%

where A0 is the initial absorbency of MO and A is the finalabsorbency of MO.

3. Results and discussion

Fig. 1 shows the SEM image of the bare ZnO nanorods. It can beseen that well-aligned ZnO nanorods arrays in large scale can besuccessfully fabricated on ITO glass substrate and the well-definedcrystallographic planes of the hexagonal nanorods can be clearlyidentified from the high-magnification SEM image (inset image inFig. 1).

In order to improve the light response region of the ZnO pho-tocatalyst, the ZnO/CdS(3), ZnO/PbS(4), ZnO/PbS(4)/CdS(3) andZnO/CdS(3)/PbS(4) photocatalysts were prepared by SILAR method.Fig. 2 gives the SEM and TEM images of the ZnO/CdS(3) andZnO/PbS(4) nanorods. Comparing Fig. 2(a) and (c) to Fig. 2(b)and (d), it can be seen that the amount of the CdS nanoparticleswas more than that of PbS, indicating that the CdS are easier todeposit on ZnO nanorods than PbS. This phenomenon was maybedue to the different lattice parameter of PbS and CdS. The lat-tice parameter of the as-prepared materials can be found fromICDD/JCPDS PDF Retrievals [Level-2 PDF, Sets 1-89 (02-13-09)]:ZnO (wurtzite structure): 3.249 nm; CdS (hexagonal structure):4.421 nm; PbS (cubic phase): 5.936 nm. The mismatch of latticeparameters between ZnO and CdS is smaller than that of ZnO andPbS, so the CdS nanoparticles are easier to deposit on ZnO nanorodsthan PbS ones. Furthermore, the size of CdS and PbS was about10–30 nm which can be examined from the TEM images (Fig. 2(b)and (d)).

Significant morphology change of the CdS/PbS co-sensitized ZnOnanorods can be observed by Fig. 3(a). After co-sensitization byCdS/PbS nanoparticles, the surface of the ZnO nanorods becomerough comparing to the bare ZnO nanorods (Fig. 1). X-ray diffrac-tion measurements were performed to study the crystal structureof the prepared CdS and PbS co-sensitized ZnO nanorods photocat-alysts, as shown in Fig. 3(b). The strongest diffraction peaks (0 0 2)revealed the well-crystallized hexagonal wurtzite phase of ZnO.

The peaks of the CdS and PbS phases are relatively weak due to thelow concentration in the sensitized reaction. The peak at 2� = 50.9◦

is corresponding to the (3 1 1) of PbS, revealing the formation ofcubic phase of PbS. In addition, another weak peak at 2� = 47.3◦

572 C. Liu et al. / Materials Science and Engineering B 177 (2012) 570– 574

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Fig. 2. The SEM and TEM i

an be indexed as typical diffraction peak of the hexagonal CdS.s previously mentioned, CdS nanoparticles are easier to depositn the ZnO nanorods than PbS ones due to the difference of latticearameter. However, from the crystal structure obtained by XRD,nO and CdS have a similar hexagonal structure, which may alsoncrease the deposition of CdS nanoparticles on ZnO nanorods.

To confirm the optical properties of these prepared photocata-ysts, the UV–vis transmittance spectra were also performed andhe results were presented in Fig. 4. It can be seen that the as-rown ZnO nanorods show higher transmittance in the visibleegion (70–90%). The deposition of CdS or PbS nanoparticles onnO nanorods increases the absorption ability and presents redhift phenomena, due to the small band gap of PbS (Eg = 1.2–1.5 eV)19] as well as CdS (Eg = 2.25 eV). Moreover, ZnO/CdS(3) andnO/PbS(4) have a similar transmittance in the visible region. The-

retically, ZnO/PbS(4), which has a smaller band gap, should have

much higher absorbance capacity. This opposite result should bexplained by the different amount of the two nanoparticles cou-led on the ZnO nanorods, as discussed above. ZnO/CdS(3)/PbS(4)

Fig. 3. SEM image (a) and XRD p

of ZnO/CdS and ZnO/PbS.

and ZnO/PbS(4)/CdS(3) showed a lower transmittance compared tothe single nanoparticles sensitized nanorods, which means that theco-sensitized photocatalysts have complementary and enhance-ment effect in the light harvest. Moreover, due to the differencein the amount of CdS and PbS nanoparticles deposited on ZnOnanorods, the transmittance of ZnO/PbS(4)/CdS(3) is higher thanthat of ZnO/CdS(3)/PbS(4).

The degradation yield of MO is used to evaluate the photocat-alytic activity of the samples under the visible light irradiation. Theeffect of initial concentration of the MO on the photocatalytic activ-ity of photocatalysts was investigated by testing the degradationyield. We supposed that the reaction rate only depend on MO con-centration at room temperature under the condition of effectivearea of ZnO nanorods film (photocatalyst) is 1 cm × 1 cm and pHvalue is 7. Fig. 5 shows the concentration of MO versus irradiation

time under visible light irradiation with different photocatalysts.The results showed that all the samples exhibited an obvious pho-tocatalytic activity under visible light irradiation, which reveals theimportant effect of the CdS and PbS nanoparticles. It should be

attern (b) of ZnO/CdS/PbS.

C. Liu et al. / Materials Science and Engineering B 177 (2012) 570– 574 573

Fig. 4. The transmittance spectrum of ZnO nanorods-based photocatalysts.

5432100.0

0.2

0.4

0.6

0.8

1.0

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0

Irradiation time(h)

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ig. 5. Degradation kinetics of MO of different photocatalysts (C0 and C are the equi-ibrium concentrations of MO before and after visible light irradiation, respectively).hotocatalysts’ area: 1 cm × 1 cm; MO: 10 mg L−1.

oted that ZnO/PbS(4) has a lower photocatalytic efficiency (48%)

han that of ZnO/CdS(3), which maybe caused by the charge recom-ination [10–12] and small amount of PbS nanoparticles on ZnOanorods. Moreover, for the co-sensitized ones, the photocatalyticctivity was different when the sensitizing order is reverse, which

Fig. 6. Relative Fermi level alignment due to the contact of CdS and PbS (a), (b) and th

Fig. 7. The schematic diagram of ZnO/PbS/CdS photocatalyst.

was presented in Fig. 5. The highest photocatalytic activity of 83%after 5 h was achieved with ZnO/CdS(3)/PbS(4) photocatalyst, whilethe photocatalytic activity of ZnO/PbS(4)/CdS(3) was only 52%.

Theoretically, since the conduction band (CB) of CdS was higherthan that of ZnO, and the CB of PbS was lower than ZnO, the highphotocatalytic activity of ZnO/CdS(3)/PbS(4) can be ascribed to thestepwise band gap (Fig. 6(a) and (c)) [21]. After the sensitizationof PbS on ZnO/CdS, the stepwise structure formed due to realign-ment of Fermi level between CdS and PbS (Fig. 6(a) and (c)) [21].Just because of this structure, the photogenerated electrons andholes produced in CdS and PbS nanoparticles under visible lightirradiation can be separated effectively. The electric field in thespace-charge region is helpful for the injection of photoelectronsfrom PbS to ZnO ((PbS)e−

CB → (CdS)e−CB → (ZnO)e−

CB), whereas,the holes flow oppositely (Fig. 6(c)). Once the electron–hole pairsseparated, both the electrons and holes can be more involved ininterfacial charge transfer reaction, which decrease the recombi-nation of photogenerated electrons and increase the photocatalyticactivity of the ZnO/CdS/PbS photocatalysts [21].

For the inversed structure (ZnO/PbS/CdS), it is difficult to depositPbS nanoparticles on ZnO nanorods due to the mismatch of lattice

constant and crystal structure between ZnO and PbS, and nonef-fective structure easily formed on the surface of ZnO nanorods.So, to compare these factors, CdS nanoparticles are assembleddirectly on the surface of ZnO nanorods shown in Fig. 7(1). These

e proposed band edges structures for the ZnO/CdS/PbS (c) and ZnO/PbS/CdS (d).

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dS nanoparticles can effectively enhance the absorption of solaright and increase the photogenerated electrons of ZnO/PbS/CdS;esides, some CdS nanoparticles can be assembled on PbS nanopar-icles (Fig. 7(2)). Thus, a reversed stepwise band gap with higheresistance was formed, which worked as a barrier for injecting thehotogenerated electrons from the outer CdS layer and transferringoles from the inner PbS (Fig. 6(b) and (d)). Then recombinationf photogenerated electrons and holes increases in this structure.oth the above two aspects play important roles on the photo-atalytic activity of ZnO/PbS/CdS photocatalysts. Moreover, thehotocatalytic efficiency of ZnO/PbS(4)/CdS(3) is higher than thatf ZnO/PbS(4) and lower than that of ZnO/CdS(3) indicates thathe stepwise band gap structure in ZnO/CdS/PbS has an obviousunction on photocatalytic activity.

Based on the above results, it can be seen that the ZnO/CdS/PbShotocatalyst show remarkable photocatalytic activity thannO/PbS/CdS, ZnO/CdS and ZnO/PbS. It believes that both of theollowing efforts are helpful for the photocatalytic activity ofnO/CdS/PbS. Firstly, a heterojunction system can be obtained byensitizing CdS and PbS nanoparticles on ZnO nanorods, which cantilize more abundant solar light. Secondly, the CdS nanoparticlesre easy to deposit on ZnO nanorods due to the match of lattice con-tant and crystal structure between ZnO and CdS. Furthermore, theecombination of photogenerated electrons and holes is decreasedue to the presence of stepwise band gap structure caused byhanging the sensitizing order of the CdS and PbS nanoparticles.

. Conclusions

CdS/PbS co-sensitized ZnO nanorods photocatalysts werebtained when the precursor concentration of CdS and PbSas 0.05 M and 0.02 M respectively. Analysis indicated the nar-

ow band gap semiconductor sensitized ZnO nanostructures

ad a promising photocatalytic activity in the degradationf MO due to the extended light response at visible lightegion. Under visible light irradiation, the photocatalytic activ-ty of these photocatalysts was showed in the following

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ineering B 177 (2012) 570– 574

order: ZnO/CdS/PbS > ZnO/CdS > ZnO/PbS/CdS > ZnO/PbS. The high-est photocatalytic yield of ZnO/CdS/PbS was mainly due to thestepwise band gap constructed in it. Therefore, the dual narrowband gap nanoparticles co-sensitized ZnO nanorods can be consid-ered as an important nanomaterial in photocatalytic technologies.

Acknowledgements

The authors gratefully acknowledge financial support fromNational Natural Science Foundation of China (no. 51102174) andNatural Science Foundation of Tianjin (11JCYBJC27000).

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