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Chemical Engineering Journal 183 (2012) 238– 243

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

V-assisted photocatalytic synthesis of ZnO–reduced graphene oxide compositesith enhanced photocatalytic activity in reduction of Cr(VI)

injuan Liua, Likun Pana,∗, Qingfei Zhaob, Tian Lva, Guang Zhua, Taiqiang Chena, Ting Lua,huo Suna, Changqing Sunc

Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, Shanghai 200062, ChinaChemistry Department, Shanghai Normal University, Shanghai 200234, ChinaSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

r t i c l e i n f o

rticle history:eceived 25 July 2011eceived in revised form5 December 2011ccepted 15 December 2011

a b s t r a c t

ZnO–reduced graphene oxide (RGO) composites are successfully synthesized via UV-assisted photocat-alytic reduction of graphite oxide by ZnO nanoparticles in ethanol. Their morphology, structure andphotocatalytic performance in reduction of Cr(VI) are characterized by scanning electron microscopy,transmission electron microscopy, atomic force microscopy, X-ray diffraction spectroscopy, UV–vis

eywords:nOeduced graphene oxidehotocatalysis

absorption spectrophotometer, respectively. The results show that in the composites the RGO nanosheetsare decorated densely by ZnO nanoparticles, which displays a good combination between RGO and ZnO.ZnO–RGO composites exhibit an enhanced photocatalytic performance in reduction of Cr(VI) with a maxi-mum removal rate of 96% under UV light irradiation as compared with pure ZnO (67%) due to the increasedlight absorption intensity and range as well as the reduction of electron–hole pair recombination in ZnOwith the introduction of RGO.

. Introduction

As an emerging carbon material with unique two-dimensionalonjugated chemical structure, graphene has attracted a great dealf attention in recent years owing to its good conductivity, supe-ior chemical stability and high specific surface area [1–3], as wells its potential applications in photocatalysis [4–6], energy stor-ge [7–9], solar cells [10,11], transparent electrodes [12,13], andeld emission [14]. Currently one attractive challenge is to combinehese 2D carbon nanostructures with metal oxide to form hybrid

aterials with good functionalities [15–24]. Investigations haveeen carried out to produce graphene-based materials via reduc-ion of exfoliated graphite oxide (GO), such as chemical reductionsing hydrazine or NaBH4 [4,25,26], and high temperature anneal-

ng reduction [27]. However, in these methods, hazardous reducinggent or high temperature is normally required. Therefore, theevelopment of facile and practical reducing methods still remains

challenging issue.As an environmental-friendly and efficient approach, ultraviolet

UV)-assisted photocatalytic reduction of GO using semiconduc-or oxides, typically TiO2 or ZnO, has been applied to synthesisraphene-based composite materials [28–30]. This one-step strat-

∗ Corresponding author. Tel.: +86 21 62234132; fax: +86 21 62234321.E-mail address: lkpan@phy.ecnu.edu.cn (L. Pan).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.12.068

© 2011 Elsevier B.V. All rights reserved.

egy can be used to fabricate high quality graphene based compositewithout using any stabilizing reagent. Williams et al. [31,32] andAkhavan et al. [33–35] fabricated ZnO–graphene or TiO2–graphenecomposites by carrying out UV-assisted photocatalytic reduction ofGO by ZnO or TiO2 in ethanol solution. Ng et al. [36,37] synthesizedthe composites of graphene and TiO2, WO3, or BiVO4 via a one-stepUV or visible light induced-photocatalytic reduction process andthey exhibited higher photoelectrocatalytic efficiencies than theirpatent materials. Despite the above progress to date, the explo-ration on the synthesis of graphene-based composite materials byUV-assisted photocatalytic reduction method is not nearly enough.Especially the application of the as-synthesized graphene-basedcomposite materials such as ZnO–graphene in photocatalysis is sel-dom reported so far. In ZnO–graphene hybrid materials, ZnO actsas photocatalyst, to excite the electrons from valence band to con-duction band and create electron–hole pairs, which can migrate andinitiate redox reactions with water and oxygen, and then degradeorganic molecules or reduce metal ion absorbed on the surfaceof ZnO [38–44]. Graphene acts as an excellent electron-transportmaterial in the process of photocatalysis, so that the hybridiza-tion of ZnO with graphene can hinder the recombination of chargecarries and increase the photocatalytic performance [4,25].

In this work, we have successfully fabricated ZnO–reducedgraphene oxide (RGO) composites via UV-assisted photocatalyticreduction of GO by ZnO nanoparticles in ethanol and investigatedtheir photocatalytic performances. ZnO–RGO composites exhibit

X. Liu et al. / Chemical Engineering Journal 183 (2012) 238– 243 239

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ig. 1. AFM image of ZG-3: topography image and height profiles obtained from posf the references to color in this figure legend, the reader is referred to the web ver

n enhanced photocatalytic performance in reduction of Cr(VI)nder UV light irradiation as compared with pure ZnO.

. Experimental

The ZnO nanoparticles used in this work were produced by aeaction similar to that described by Yang and Chan [45]. 25 mlf 1.0 M ZnSO4 solution was added dropwise into 30 ml of 2.0 MH4HCO3 solution under vigorous stirring at 60 ◦C in a water bath

or 1 h. The as-synthesized white precipitate was isolated by filtra-ion, washed for three times with distilled water and ethanol, andhen dried in a vacuum oven at 60 ◦C for 24 h. Finally, the productas calcined at 600 ◦C for 1 h to obtain white powder of ZnO.

Commercial graphite powder was used as the starting reagentor the synthesis of GO via modified Hummers method, which haseen described in our previous works [46,47]. A certain amount of.8 mg/ml GO solution and 1 g ZnO nanoparticles were dispersed

n 60 ml ethanol and sonicated for 30 min to produce uniformispersion. UV-assisted photocatalytic reduction of GO by ZnOanoparticles in ethanol was carried out in a 100 ml cylindricaluartz glass vessel using a 500 W high pressure Hg lamp with the

ain wave crest at 365 nm for about 7 h under ambient condi-

ions and magnetic stirring. After UV irradiation, it was obviouslyound that the color of suspension had changed into grayish-black,ndicating the successful chemical reduction of GO sheets [48]. It

indicated by different triangle symbols (green: RGO; red: ZnO). (For interpretationf the article.)

should be noticed that no further reduction of GO under UV irra-diation for longer time is observed by Fourier transform infraredspectroscopy (FTIR) measurement which is normally used to char-acterize the degree of converting GO to RGO. The as-synthesizedZnO–RGO samples with 0.6, 0.8, 1.0, 1.2 wt.% RGO, named as ZG-1,ZG-2, ZG-3, ZG-4 were isolated by filtration, washed for three timeswith distilled water, and finally dried in a vacuum oven at 60 ◦C for24 h.

The surface morphology, structure and composition of the sam-ples were characterized by atomic force microscopy (AFM, VeecoDimension 3100), field-emission scanning electron microscopy(FESEM, Hitachi S-4800), high-resolution transmission electronmicroscopy (HRTEM, JEOL-2010), X-ray diffraction spectroscopy(XRD, Holland Panalytical PRO PW3040/60) with Cu K� radiation(V = 30 kV, I = 25 mA), Renishaw Raman spectrometer (Via + Reflex)system with a 514.5 nm excitation source, and FTIR (NICOLETNEXUS 670), respectively. The UV–vis absorption spectra wererecorded using a Hitachi U-3900 UV–vis spectrophotometer. Pho-toluminescence (PL) spectra were recorded on a HORIBA Jobin Yvonfluoromax-4 fluorescence spectrophotometer, using the 325 nmexcitation line of a Xe lamp as light source.

The photocatalytic performance of the as-prepared samples wasevaluated by photocatalytic reduction of Cr(VI) under UV lightirradiation. The samples (1 g/l) were dispersed in 60 ml Cr(VI) solu-tions (10 mg/l) which were prepared by dissolving K2Cr2O7 into

240 X. Liu et al. / Chemical Engineering Journal 183 (2012) 238– 243

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(1 0 0) peak at 44.5 ) can be observed in the composite, which mightbe due to the low amount and relatively low diffraction intensityof RGO.

ig. 2. Surface morphologies of (a) ZnO nanoparticles and (b) ZG-3 by FESEM meas

eionized water. The mixed suspensions were first magneticallytirred in the dark for 0.5 h to reach the adsorption–desorptionquilibrium. Under stirring, the mixed suspensions were exposedo UV irradiation produced by a 500 W high pressure Hg lamp withhe main wave crest at 365 nm. At certain time intervals, 2 ml ofhe mixed suspensions were extracted and centrifugated to removehe photocatalyst. The filtrates were analyzed by recording UV–vispectra of Cr(VI) using a Hitachi U-3900 UV–vis spectrophotome-er. It is known that the photocorrosion is one of major problems fornO photocatalyst, which significantly decreases its photocatalyticctivity. Therefore, the photo-stability of as-prepared sample wasvaluated for reduction of Cr(VI) under UV light irradiation during

three-cycle experiment.

. Results and discussion

To indicate the acquirement of RGO, an aqueous solution ofnO–RGO was dropped onto the hydrophilic-treated silicon forFM measurement. Fig. 1 shows the AFM image of as-preparedG-3. It is observed that ZnO nanoparticles are attached onto theurface of RGO sheet, which is demonstrated in the height profileiagram containing the sharp peaks with the height of ∼40 nm. Therofile also shows that the thickness of the RGO is 4.16 nm, corre-ponding to the RGO of five or six layers based on theoretical valuesf 0.78 nm for single layer graphene and the thickness contributionorm oxygen-containing groups on the faces [46].

Fig. 2(a) and (b) show the FESEM images of ZnO and ZG-3. Theorphologies of ZG-1, ZG-2, and ZG-4 (not shown here) are sim-

lar as the one of ZG-3. It is clearly observed that the surface ofurled RGO nanosheets is packed densely by ZnO nanoparticles,hich displays a good combination between RGO and ZnO. From

ow-magnification HRTEM image of ZG-3 in Fig. 2(c), it is clearlyeen that the RGO sheets are decorated by ZnO nanoparticles. TheGO sheets act as bridges for the connection between different ZnOanoparticles, which could significantly increase the separation of

nt; (c) low-magnification and (d) high-magnification HRTEM images of ZG-3.

photo-generated carriers, and enhance the photocatalytic perfor-mance. Fig. 2(d) shows a high-magnification HRTEM image of ZG-3.The larger crystallite is identified to be ZnO nanoparticles. The lat-tice fringes with an interplanar distance of 0.28 nm can be assignedto the (1 0 0) plane of the hexagonal ZnO.

Fig. 3 shows the XRD patterns of ZnO, ZG-1, ZG-2, ZG-3 and ZG-4.All of the diffraction peaks match the standard data for a wurtzite-structure (JPCDS 36-1451) of ZnO, which demonstrates that thepresence of RGO does not result in the development of new crys-tal orientations or changes in preferential orientations of ZnO. Notypical diffraction peaks for RGO ((0 0 2) diffraction peak at 26◦ and

◦

Fig. 3. XRD patterns of ZnO, ZG-1, ZG-2, ZG-3, and ZG-4.

X. Liu et al. / Chemical Engineering Journal 183 (2012) 238– 243 241

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be effectively inhibited in the composite. The inhibition effect can

Fig. 4. Raman spectra of GO, ZnO, ZG-1, ZG-2, ZG-3, and ZG-4.

Fig. 4 displays the Raman spectra of GO, ZnO, ZG-1, ZG-2, ZG-, and ZG-4. Two characteristic peaks of ZnO at 295 and 591 cm−1

orrespond to the first and second-order LO phonon modes, respec-ively. The Raman spectrum of GO is dominated by two intensityeaks of carbon materials at 1354 and ∼1599 cm−1, which areeferred as D band corresponding to the breathing mode of �-pointhonons of A1g symmetry and G band assigned to the E2g phononf sp2 bonds of carbon atoms, respectively. For the ZnO–RGO com-osite, the D band moves to 1348 cm−1 and the G band shifts to585 cm−1, indicating the reduction of GO. It should be noticed thato obvious peaks of carbon are observed in ZG-1, which shoulde due to the low amount of RGO in this composite. Further, it

s observed that ZnO–RGO composite display an increased inten-ity ratio of the D peak and G peak (ID/IG) in comparison to that ofure GO, showing a decrease in the average size of the sp2 domainspon the reduction of GO to RGO [5,49,50]. Similar trends are alsobserved in the transformation from TiO2–GO to TiO2–RGO com-osite [19,35].

Fig. 5 shows the FTIR spectra of GO, ZnO, and ZG-3. The broadbsorption band at 3425 cm−1 is assigned to the hydroxyl groups ofbsorbed H2O molecules. The absorption band at 450 cm−1 of ZG-3s similar to that of pure ZnO [51], which is owing to stretching mod-ls of Zn O. There is a obvious decrease in the intensities of C O1627 cm−1), C OH (1183 cm−1) and C O (1068 cm−1) stretch-ng vibration peaks in ZG-3 compared to those in GO, indicating

hat UV-assisted photocatalytic reduction is an effective method toemove oxygen-containing groups of GO.

Fig. 5. FTIR spectra of GO, ZnO, and ZG-3.

Fig. 6. UV–vis absorption spectra of ZnO, ZG-1, ZG-2, ZG-3, and ZG-4.

The UV–vis absorption spectra of ZnO, ZG-1, ZG-2, ZG-3 andZG-4 are shown in Fig. 6. The sharp characteristic absorption peakat 374 nm indicates the presence of good crystalline and impuritysuppressed ZnO nanostructures. It is observed that the absorbanceof ZnO–RGO composite increases even in visible light region withthe increase of RGO content, which is similar to those reportedin hybridization of TiO2 with graphite-like carbon layers or RGO[5,20]. Such an increase in absorbance may be due to the absorp-tion contribution from RGO, the increase of surface electric chargeof the oxides and the modification of the fundamental processof electron–hole pair formation during irradiation [25]. In addi-tion, the red shift in the absorption edge of ZnO–RGO compositeobtained by extrapolating the linear portion of the curve to zeroabsorbance as compared to pure ZnO is ascribed to the chemicalbonding between semiconductor photocatalyst and RGO, whichis similar to the result in the case of TiO2–RGO composite mate-rials [17,18]. Therefore, the presence of RGO in ZnO can increasethe light absorption intensity and range, which is beneficial to thephotocatalytic performance.

Fig. 7 shows PL spectra of ZnO, ZG-1, ZG-2, ZG-3 and ZG-4. Ascompared with the spectrum of pure ZnO, the PL peak of ZnO–RGOcomposite displays a red shift, which is consistent to the resultof UV–vis absorption spectra. Furthermore, the introduction ofRGO decreases excitonic PL intensity, which indicates that therecombination of photo-induced electrons and holes in ZnO can

be explained from a view of stepwise structure of energy levelsconstructed in ZnO–RGO composite, as shown in Fig. 8. The con-duction band of ZnO is −4.05 eV and valence band −7.25 eV (vs.

Fig. 7. PL spectra of ZnO, ZG-1, ZG-2, ZG-3, and ZG-4.

242 X. Liu et al. / Chemical Engineering Journal 183 (2012) 238– 243

vlZpefac

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FUr

Fig. 8. Schematic diagram of energy levels of ZnO and RGO.

acuum) [4]. The work function of RGO is −4.42 eV [52]. Such energyevels are beneficial for photo-induced electrons to transfer fromnO conduction band to RGO, which could efficiently separate thehoto-induced electrons and hinder the charge recombination inlectron-transfer processes, thus enhance the photocatalytic per-ormance [10]. Therefore, the incorporation of RGO into ZnO playsn important role in the photocatalytic performance of ZnO–RGOomposite.

Photocatalytic reduction of Cr(VI) by ZnO, ZG-1, ZG-2, ZG-3 andG-4 was performed under UV irradiation, as shown in Fig. 9. Theormalized temporal concentration changes (C/C0) of Cr(VI) duringhe photocatalytic process are proportional to the normalized max-mum absorbance (A/A0), which can be derived from the change inhe Cr(VI) absorption profile at a given time interval. It is observedhat Cr(VI) cannot be reduced under the dark condition even in theresence of photocatalyst (ZG-3) and ZnO–RGO composites exhibitetter photocatalytic performance than pure ZnO. The photocat-lytic performance of ZnO–RGO composite is dependent on theroportion of RGO in the composite. The removal rate of Cr(VI) forure ZnO is 67%. When RGO is introduced into ZnO, the removalate is increased to 80% and 88% for ZG-1 and ZG-2 and reachesaximum value of 96% for ZG-3. It is known that during photo-

atalysis, the light absorption and the charge transportation andeparation are crucial factors [18]. The enhancement of the photo-atalytic performance should be ascribed to the increase of the lightbsorption intensity and range, and the reduction of electron–holeair recombination in ZnO with the introduction of RGO in theomposite, which can be confirmed from absorption and PL mea-urements. However, when the RGO content is further increased

bove its optimum value, the photocatalytic performance deterio-ates. This is ascribed to the following reasons: (i) RGO may absorbome UV light and thus there exists a light harvesting competition

ig. 9. Photocatalytic reduction of Cr(VI) by ZnO, ZG-1, ZG-2, ZG-3, and ZG-4 underV irradiation. The concentrations of Cr(VI) and photocatalyst are 10 mg/l and 1 g/l,

espectively.

Fig. 10. Photo-stability of ZG-3 by investigating its photocatalytic activity under UVlight irradiation with three times of cycling uses.

between ZnO and RGO with the increase of RGO content, whichlead to the decrease of the photocatalytic performance [25]; (ii) theexcessive RGO can act as a kind of recombination center instead ofproviding an electron pathway and promote the recombination ofelectron–hole pair in RGO [10,52].

The photo-stability of ZG-3 by investigating its photocatalyticperformance under UV light irradiation with three times of cyclinguses was studied, as shown in Fig. 10. It can be seen that the recy-cled use of ZG-3 for three times does not conspicuously affect itsphotocatalytic activity. Apparently, the composite is stable underthe studied conditions and the photocorrosion effect of ZnO waseffectively inhibited by RGO after hybridization [53,54].

4. Conclusions

ZnO–RGO composites are successfully synthesized via UV-assisted photocatalytic reduction of GO by ZnO nanoparticles inethanol to improve the photocatalytic performance of ZnO. Theexperimental results indicate that (i) ZnO–RGO composites exhibita better photocatalytic performance than pure ZnO; (ii) the photo-catalytic performance of ZnO–RGO is dependent on the proportionof RGO in the composite and ZnO–RGO composite with 1.0 wt.%RGO achieves a highest Cr(VI) removal rate of 96%; (iii) theenhanced photocatalytic performance is ascribed to the increasedlight absorption intensity and range as well as the reduction ofphotoelectron–hole pair recombination in ZnO with the introduc-tion of RGO.

Acknowledgements

Financial support from Special Project for Nanotechnology ofShanghai (No. 1052nm02700), the Key laboratory of new ceram-ics and fine processes at Tsinghua University, Project of ShanghaiNormal University (DKL917), and Funds for the PhD academic new-comer award of East China Normal University (No. XRZZ2011017)are gratefully acknowledged.

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