Journal of Colloid and Interface Science 377 (2012) 114–121

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ZnO/graphene-oxide nanocomposite with remarkably enhancedvisible-light-driven photocatalytic performance

Benxia Li ⇑, Tongxuan Liu, Yanfen Wang, Zhoufeng WangSchool of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, People’s Republic of China

a r t i c l e i n f o

Article history:Received 30 December 2011Accepted 20 March 2012Available online 28 March 2012

Keywords:ZnOGraphene oxideNanocompositeSynthesisPhotocatalysisVisible light

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.03.060

⇑ Corresponding author. Fax: +86 554 6668643.E-mail address: libx@mail.ustc.edu.cn (B. Li).

a b s t r a c t

In this work, a high-performance photocatalyst of ZnO/graphene-oxide (ZnO/GO) nanocomposite wassynthesized via a facile chemical deposition route and used for the photodegradation of organic dye fromwater under visible light. The nanocomposite was characterized by X-ray diffraction, X-ray photoelectronspectroscopy, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller N2 adsorption–desorption analysis, and UV–Vis diffusion reflectance spectroscopy. The ZnO/GOnanocomposite consisting of flower-like ZnO nanoparticles anchored on graphene-oxide sheets has ahigh surface area and hierarchical porosity, which is benefit to the adsorption and mass transfer of dyeand oxygen species. For the photodegradation of organic dyes under visible light, ZnO/GO nanocompositeexhibited remarkably enhanced photocatalytic efficiency than graphene-oxide sheets and flower-likeZnO particles. Moreover, the photocatalytic efficiency of ZnO/GO nanocomposite could be furtherimproved by annealing the product in N2 atmosphere. The outstanding photocatalytic performancewas ascribed to the efficient photosensitized electron injection and repressed charge carriers recombina-tion in the composite with GO as electron collector and transporter, thus leading to continuous genera-tion of reactive oxygen species for the degradation of methylene blue.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Currently, organic dyes and their effluents have become one ofthe main sources of water pollution due to the greater demand inindustry such as textile, paper, and plastic. These organic dyes con-taminate environment by the release of the toxic, cancerogenic,and colored wastewater [1–5]. Most of these dyes escape from tra-ditional wastewater treatment and persist in water because oftheir high stability against light, temperature, chemicals, andmicrobial attack [3–5], whereas photocatalytic degradation of theorganic pollutants has opened a new door for the elimination oforganic dyes in wastewater [6–10]. The photodegradation processutilizes cheaply available semiconductors and leads to completemineralization of organic compounds to CO2, water, and mineralacids [11]. Thus, semiconductor photocatalysis is deemed to analternative technique to remove various organic pollutants. Untilnow, ZnO and TiO2 photocatalysts have been widely used becauseof its strong oxidizing power, non-toxic nature, and low cost [12–16]. ZnO is a wide band-gap semiconductor oxide (3.37 eV) with aconduction band edge located at approximately the same level asthat of TiO2. More attractively, the electron mobility of ZnO hasbeen proven to be higher than that of TiO2 [17]. However, despite

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its great potential, the photocatalytic efficiency remains very lowbecause of the fast recombination of the photogenerated elec-tron–hole pairs in the single phase semiconductor.

The performance of semiconductor photocatalysts is often en-hanced by means of noble metal loading [18–20], ion doping[21,22], and incorporation of electron-accepting materials [23–25]. These actions are used to extend the light absorption rangeor suppress the electron–hole recombination. As a rising star ofcarbon family, graphene has become the focus of considerableinterest because of its unique electronic properties and otherexcellent attributes, such as the large theoretical specific surfacearea and the high transparency [26–29]. Meanwhile, grapheneoxide (GO) is receiving increasing attention because it possessesthe similar properties to graphene as well as the special surfacestructures with the introduced hydroxyl and carboxyl groups forsynthesis of GO-containing nanocomposites [30–38]. Particularly,the fabrication of semiconductor/GO composites has attracted sub-stantial research efforts motivated by the desire to improve thephotocatalytic efficiency [34–38]. The recent studies have revealedthat the composites simultaneously covered three excellent attri-butes: the increasing adsorptivity of pollutants, extended lightabsorption range, and efficient charge transportation and separa-tion [25,39], which are the ideal traits of a photocatalyst we havebeen pursuing for. Therefore, it is believed that anchoring well-organized ZnO nanostructures on GO sheets can efficiently utilize

B. Li et al. / Journal of Colloid and Interface Science 377 (2012) 114–121 115

the combinative merits of ZnO and GO to obtain a photocatalystwith superior performance.

This work demonstrated a facile strategy to synthesize ZnO/GOnanocomposite consisting of flower-like ZnO nanoparticles an-chored on GO sheets and its use for photocatalytic degradation oforganic dye in water under visible light. The composition, mor-phology, and microstructure of the as-obtained ZnO/GO nanocom-posite were characterized. The photocatalytic performance of ZnO/GO nanocomposite was evaluated by the photodegradation ofmethylene blue in water and compared with that of pure flower-like ZnO nanoparticles and GO sheets, to highlight the importanceof the anchoring of ZnO nanoparticles on GO sheets for maximumutilization of ZnO photocatalyst and GO as electron collector andtransporter in photocatalytic degradation of organic pollutants.

2. Experimental

2.1. Materials

Graphite flakes (200 mesh) were purchased from QingdaoTianyuan Company. 37% hydrochloric acid (HCl), 98% sulfuric acid(H2SO4), 85% phosphoric acid (H3PO4), 30% hydrogen peroxide(H2O2), potassium permanganate (KMnO4), zinc chloride (ZnCl2),sodium hydroxide (NaOH), and absolute ethanol were purchasedfrom Shanghai Sinopharm Chemical Reagent Co. Ltd. All chemicalswere analytical grade without further purification. Deionizedwater was used throughout this study.

2.2. Preparation of graphene oxide (GO) sheets

The graphene oxide was synthesized by chemical oxidation andexfoliation of graphite flakes using an improved Hummers’ method[40]. Graphite flakes (1.5 g) and KMnO4 (9.0 g) were successivelyadded into a 9:1 mixture of concentrated H2SO4/H3PO4

(180:20 mL) under continuous stirring. The reaction was heatedat 50 �C and stirred for 12 h. Then, the reaction was cooled to roomtemperature and poured onto ice (�200 mL) with 30% H2O2 (3 mL).The mixture was then repeatedly centrifuged and washed in suc-cession with water, 30% HCl solution, and ethanol. The obtained so-lid product of GO was finally dried at 80 �C.

2.3. Preparation of ZnO/GO nanocomposite

To synthesize ZnO/GO composite, 0.1 g dried GO precipitatewas dispersed in 40 mL of water to form GO suspension by ultra-sonication, in which a further exfoliation of GO was achieved. Then,zinc chloride (ZnCl2, 1.0 mmol, 0.1364 g) and sodium hydroxide(NaOH, 10.0 mmol, 0.4000 g) were successively dissolved in theabove GO suspension. The mixture was sealed in a glass bottle(60 mL), kept static at 90 �C for 6 h, and then cooled to room tem-perature naturally. Finally, the composite was filtered, washed sev-eral times with distilled water and ethanol, and dried at 80 �C for24 h. For comparison, flower-like ZnO nanoparticles were synthe-sized by a similar method without adding GO. In addition, theZnO/GO product prepared via aqueous-solution route at 90 �Cwas further annealed at 400 �C and in N2 for 2 h.

2.4. Characterizations of the samples

The products were characterized by X-ray diffraction (XRD, Shi-madzu XRD-6000, CuKa radiation), field-emission scanning elec-tron microscopy (FE-SEM, Sirion200), and transmission electronmicroscopy/high resolution transmission electron microscopy(TEM/HRTEM, H-7650 and JEOL-2010). X-ray photoelectron spec-troscopy (XPS) measurements were performed on a VGESCALAB

MKII X-ray photoelectron spectrometer with an exciting sourceof MgKa. The nitrogen adsorption and desorption isotherms at77 K were measured using a Micromeritics ASAP 2000 system afterthe sample was degassed in a vacuum at 130 �C overnight. The UV–Vis diffuse reflectance spectra (DRS) were recorded on a recordingspectrophotometer of Perkin Elmer Lambda 950.

2.5. Photocatalytic property test

The photocatalytic properties of the samples were evaluated byphotodegradation of methylene blue (MB) in water under visible-light irradiation from a 300 W Xe light equipped with a 420 nmcutoff filter (CEL-HXF300/CEL-HXUV300, China). In every experi-ment, 80 mg of photocatalyst was suspended in 100 mL of a5.0 � 10�5 M aqueous solution of MB. Prior to irradiation, the sus-pension was stirred in the dark for 2 h to achieve an adsorption–desorption equilibrium between the photocatalyst and MB mole-cules. After that, the solution was exposed to the visible-light irra-diation under magnetic stirring. At given time intervals, 3 mL ofsolutions was sampled for analysis of the MB concentration. Thephotocatalytic degradation process was monitored using a UV–Vis spectrophotometer (Shimadzu UV2550) to record the charac-teristic absorption at 665 nm.

3. Results and discussion

3.1. The formation mechanism of ZnO/GO composite

In this work, ZnO/GO nanocomposite was fabricated by a two-step aqueous-solution route. GO sheets were firstly prepared usingan improved Hummers’ method, and ZnO nanoparticles were thenanchored on GO sheets via a facile reaction between Zn2+ and OH�

ions in aqueous solution. Fig.1 illustrates the fabrication processand formation mechanism of ZnO/GO composite. It has beenproved that the surfaces of the chemically exfoliated GO sheetsare covered by a large number of hydroxyl, carboxyl, and epoxygroups that are introduced on GO sheets due to oxidation proce-dures [41,42]. These functional groups can act as anchor sites toenable the subsequent in situ formation of ZnO nanoparticles onGO sheets. The formation of ZnO/GO composite undergoes the fol-lowing two distinctive stages: (i) when dissolving ZnCl2 into GOsuspension, Zn2+ ions will be adsorbed onto the surfaces of GOsheets due to their bonding with the O atoms of the negativelycharged oxygen-containing functional groups via electrostaticforce. (ii) After the addition of NaOH, the crystal growth units ofZnðOHÞ2�4 and ZnO2� may combine with the functional groups ofGO sheets via intermolecular hydrogen bonds or coordinationbonds, acting as anchor sites for ZnO nanoparticles. With heatingat 90 �C, a large number of ZnO nuclei are formed in a short timedue to the hydrolysis reaction of ZnðOHÞ2�4 . Finally, ZnO/GO nano-composite is obtained. The in situ formation of ZnO nanoparticlesin return caused the exfoliation of the lamellar GO.

3.2. XRD and XPS analysis

Fig. 2 shows the XRD patterns of ZnO nanoparticles, ZnO/GOnanocomposite, and GO sheets, respectively. All the diffractionpeaks in the XRD pattern of ZnO nanoparticles are consistent withthe hexagonal phase wurtzite ZnO (JCPDS No. 36-1451). The dif-fraction peak at around 2h = 8� in the XRD pattern of GO sheets be-longs to the (001) reflection of GO, and the interlayer spacing(0.95 nm) is much larger than that of graphite (about 0.37 nm) ow-ing to the introduction of oxygen-containing functional groups onthe graphite sheet surfaces [43,44]. The diffraction peaks of ZnO/GO nanocomposite are similar to those of hexagonal (wurtzite)

Fig. 1. Illustration of the fabrication process and formation mechanism for ZnO/GO nanocomposite. (1) Oxidation of graphite to graphite oxide with larger interlayer spacing.(2) GO sheets from the ultrasonic exfoliation of graphite oxide. (3) Adsorption and bonding of Zn2+ ions onto the GO sheets. (3) The nucleation and growth of ZnO crystallites,resulting in ZnO/GO nanocomposite.

Fig. 2. (a) XRD patterns of ZnO nanoparticles, ZnO/GO nanocomposite and GO; (b–d) XPS spectra of ZnO/GO nanocomposite.

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ZnO. Nevertheless, the (001) diffraction peak at 8� of GO droppedprofoundly to an almost undetectable level, which may indicatethat the regular layered structure of GO has been destroyed andexfoliated GO sheets are formed due to the growth of ZnO nano-crystals [31]. The XRD patterns demonstrate that the ZnO/GOnanocomposite can be obtained in the present system. Further evi-dence for the composition of the ZnO/GO nanocomposite was ob-tained by the X-ray photoelectron spectra (XPS, Fig. 2b–d). The

survey spectrum (Fig. 2b) shows the presence of Zn and O as wellas C. The higher-resolution spectrum of Zn2p3/2 in Fig. 2c showsthat the level of Zn2p3/2 is 1021.68 eV. The C1s XPS spectrum ofZnO/GO (Fig. 2d) clearly indicates a considerable degree of graph-ene oxide with three main components that correspond to carbonatoms in different functional groups: the non-oxygenated CACbond (284.79 eV), the CAO of epoxy and hydroxyl (286.15 eV),and the carbonyl C@O (288.35 eV) [35–37]. The small peak at

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290.71 eV can be ascribed to the p–p� satellite of the aromatic con-jugated domains modified with carboxylic acid, hydroxyl, andepoxide groups in graphene oxide [45,46]. Therefore, the peak at290.71 eV suggests the existence of a partially modified p structurein the ZnO/GO nanocomposite.

3.3. SEM and TEM images

FESEM images in Fig. 3a–d show the morphologies and micro-structures of GO sheets, pure ZnO nanoparticles, and ZnO/GOnanocomposite, respectively. From the FESEM image in Fig. 3a,the layered structure of the stacked GO sheets can be seen, andthere are many wrinkles through all the surfaces of GO sheets.The solid GO sample is severely agglomerated because of its highspecific surface area. Fig. 3b shows an FESEM image of pure ZnOnanoparticles, and it reveals that the detailed morphology of ZnOis flower-like aggregates with diameters about 1 lm, assembledby many densely arranged nanosheets. In the presence of GO mate-rial, ZnO/GO nanocomposite with flower-on-sheet morphology(Fig. 3c and d) was obtained under similar experimental conditionfor preparing flower-like ZnO nanoparticles. As shown in Fig. 3c,GO nanosheets were well decorated by the flower-like ZnO parti-cles. Moreover, some ZnO particles entered into the interlayers ofGO sheets to form a sandwich-like composite structure (Fig. 3d),which can prevent the stacking of GO sheets, and thus avoid theloss of their high active surface area.

Fig. 4 shows TEM images of GO sheets and ZnO/GO composite. Itis observed from Fig. 4a that the GO sheets present a crumpled sur-face, which is consistent with the observation from FESEM image.Fig. 4b and c show TEM images with different sizes of ZnO/GOnanocomposite. The light-gray thin films are the GO sheets, andthe dark regions on the GO background are due to the presenceof ZnO particles. It can be clearly seen in Fig. 4b that the exfoliatedGO sheet was decorated with ZnO aggregates with sizes of 0.5–1 lm, whereas some of the flower-like ZnO microstructures onGO sheets were crushed into fragments because of the ultrasonictreatment before TEM observation. Moreover, the loaded

Fig. 3. FESEM images of (a) GO sheets, (b) pur

flower-like ZnO particles were mainly located at the edge of theGO sheets, which might result from the aggregation of ZnO parti-cles that confined their efficient dispersion on GO sheets. Fig. 4dshows HRTEM image of a ZnO fragment marked with the circlein Fig. 4c, which exhibits well-resolved two-dimensional latticefringes with the spacings of 0.52 nm and 0.28 nm, in good agree-ment with the interplanar spacings of {0001} and {0110} planesof hexagonal (wurtzite) ZnO.

3.4. N2 adsorption/desorption measurement

Due to the high surface area of graphene sheets (calculated va-lue: 2630 m2 g�1), a higher surface area of ZnO/GO nanocompositethan pure ZnO is expected. The N2 adsorption–desorption iso-therms at 77 K and pore-size distribution plot calculated by theBarrett–Joyner–Halenda (BJH) method of ZnO/GO nanocompositeare shown in Fig. 5. The isotherms (Fig. 5a) present a reverse ‘‘S’’shape, which is identified as type IV and characteristic of mesopor-ous structures [47,48] From the adsorption branch of the iso-therms, the specific surface area of 234.0561 m2 g�1 is calculatedfor ZnO/GO nanocomposite through a multi-point Brunauer–Em-mett–Teller (BET) method [49]. The distributions of pore sizesshown in Fig. 5b indicate that there are two types of pores in theZnO/GO nanocomposite. One type of them is concentrated in therange of typical mesoporous structure of 2.8 nm, which is probablyattributed to the interspaces between the layers of GO sheets. Theother type of pores with larger sizes distributes around 65.8 nm,which presumably arises from the spaces between nanosheets inthe flower-like ZnO particles [12]. The average pore diameter of16.6949 nm is calculated for ZnO/GO nanocomposite using a Bar-ret–Joyner–Halenda (BJH) model. The cumulative volume of poreswith diameters between 1.7 and 300 nm is 0.9743 cm3 g�1. Forcomparison, the BET surface area of pure ZnO with flower-likemorphology was calculated to be 25.1617 m2 g�1. The higher spe-cific surface area and hierarchical porosity of ZnO/GO nanocom-posite provide the possibility for the efficient adsorption andmass transfer of the degradable organic molecules and hydroxyl

e ZnO, (c and d) ZnO/GO nanocomposite.

Fig. 4. TEM images of (a) GO sheets, (b and c) ZnO/GO nanocomposite, (d) HRTEM image of a ZnO fragment marked with the circle in (c).

Fig. 5. (a) N2 adsorption–desorption isotherms at 77 K and (b) pore diameter distribution of ZnO/GO nanocomposite.

118 B. Li et al. / Journal of Colloid and Interface Science 377 (2012) 114–121

radicals in photochemical reaction, and a superior photocatalyticperformance of ZnO/GO nanocomposite should be expected.

Fig. 6. UV–Vis diffuse reflection spectra of pure ZnO and ZnO/GO nanocomposite.

3.5. UV–Vis diffuse reflectance spectra and photocatalytic performance

Fig. 6 shows UV–Vis diffuse reflectance spectra of pure ZnO andZnO/GO nanocomposite. ZnO sample shows the characteristicspectrum with its fundamental absorption edge rising at 400 nm,while the ZnO/GO composite shows absorption in the whole visibleregion. Because ZnO and GO in the ZnO/GO composite are differenttwo phases, their band-gap energies were not changed [50]. Theabsorption edge of ZnO can also be detected in the UV–Vis spec-trum of ZnO/GO sample. Besides, an intense and broad backgroundabsorption in visible region was observed due to the presence ofGO.

The photocatalytic performance of ZnO/GO nanocomposite wasevaluated by examining the photodegradation of methylene blue(MB) as a representative pollutant under visible-light irradiationfrom a 300 W Xe lamp. Prior to irradiation, the photocatalytic

Fig. 7. (a) The time-dependent absorption spectra of MB solution (5.0 � 10�5 mol/L, 100 mL) in the presence of ZnO/GO nanocomposite (80 mg) and under visible-lightirradiation, and the inset is the color-change sequence of MB solution during this process. (b) Photodegradation of MB over photocatalyst-free solution (blank), GO sheets,flower-like ZnO particles, ZnO/GO nanocomposite, and annealed ZnO/GO, respectively.

Fig. 8. (a) Kinetic linear simulation curves and (b) the reaction rates of photocatalytic degradation of MB over photocatalyst-free solution (blank), GO sheets, flower-like ZnOparticles, ZnO/GO nanocomposite, and annealed ZnO/GO, respectively.

Fig. 9. Cycling runs in the photocatalytic degradation of MB in the presence of ZnO/GO nanocomposite (80 mg) and under visible-light irradiation.

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reaction system was magnetically stirred in the dark for 2 h toreach the adsorption/desorption equilibrium of MB on the surfaceof the photocatalyst. Fig. 7a shows the UV–Vis absorption spec-trum of the aqueous solution of MB (initial concentration,5.0 � 10�5 mol/L; 100 mL) with 80 mg of ZnO/GO nanocompositeobtained at low temperature as photocatalyst and exposure tothe visible light for various durations. The characteristic absorptionof MB at 665 nm decreases rapidly with extension of the exposuretime, and almost disappears after about 60 min. Further exposureleads to no absorption peak in the whole spectrum. The color-change sequence in the MB solution during this process is shownin the inset of Fig. 7a, from which it is clear that the intense bluecolor of the initial solution gradually disappears with increasinglylonger exposure times. To demonstrate the synergy-inducedenhancement of the photocatalytic efficiency of ZnO/GO nanocom-posite, contrastive experiments were performed using flower-likeZnO particles and GO sheets as photocatalyst for the photodegra-dation of MB, respectively. The results of the MB degradation in aseries of experimental conditions are summarized in Fig. 7b, whereC0 and Ct are the initial concentration after the equilibrium adsorp-tion and the residual concentration of MB, respectively. Withoutany photocatalyst (blank), there was hardly any degradation ofMB solution under visible-light irradiation. In the presence of GOsheets, MB solution was just slightly degraded after 100 min. Bycomparison, the degradation rate of MB in the presence of flow-er-like ZnO particles is 54.3% after 100 min of irradiation. However,ZnO/GO nanocomposite was found to exhibit very prominent

photocatalytic efficiency, and 98.1% of MB was photodegradedfrom the aqueous solution after visible-light irradiation for60 min. Moreover, the photocatalytic efficiency of ZnO/GO nano-composite could be further improved by annealing the product inN2 atmosphere. The time-dependent absorption spectra of MBsolution in the presence of annealed ZnO/GO (80 mg) and undervisible-light irradiation are shown in Fig. S1 (Supporting informa-tion), which demonstrates that the characteristic absorption of

Fig. 10. Possible mechanism of photosensitized degradation of dye over ZnO/GO nanocomposite under visible light.

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MB almost disappears after 40 min of the exposure time. As shownin Fig. 7b, 97.9% of MB was photodegraded from the aqueous solu-tion in the presence of the annealed ZnO/GO after only 40 min ofvisible-light irradiation. The kinetic linear simulation curves ofphotocatalytic degradation of MB with photocatalyst-free solution(blank), GO sheets, flower-like ZnO particles, ZnO/GO nanocom-posite, and annealed ZnO/GO are presented in Fig. 8a, respectively.It is clear that the curve with irradiation time as abscissa and ln(Ct/C0) as the vertical ordinate is close to a linear curve. The values of k(Fig. 8b) for photocatalyst-free solution (blank), GO sheets, flower-like ZnO particles, ZnO/GO nanocomposite, and annealed ZnO/GOare 0.00072, 0.0024, 0.0077, 0.064, and 0.099 min�1, respectively,indicating that the ZnO/GO nanocomposite has remarkably en-hanced photocatalytic activity, which may be attributed to theinteractions between the GO sheets and ZnO nanoparticles. Never-theless, the photocatalytic activity of the ZnO/GO nanocompositeobtained via aqueous-solution route at low temperature has notachieved the best result because the surface of the product waspresented by a number of OH groups, which is believed to quenchphotogenerated chargers. After annealed at 400 �C and in N2, theOH groups were eliminated. Therefore, the annealed ZnO/GOexhibited further improved photocatalytic efficiency. To demon-strate the photocatalytic effect of ZnO/GO nanocomposite towarda more stable dye, the photodegradation of methyl orange (MO)in water under the visible-light irradiation was carried out. Thetime-dependent absorption spectra of MO solution in the presenceof ZnO/GO nanocomposite (Fig. S2, Supporting information) indi-cate that the characteristic absorption of MO almost disappearedafter about 80 min, and the color of MO solution changed graduallyfrom yellow to colorless after irradiation for 80 min.

The stability of photocatalyst during photocatalytic reaction is acrucial factor for the practical applications. To test the reusabilityof ZnO/GO nanocomposite in MB photodegradation, five cycles ofthe photocatalytic experiment for ZnO/GO nanocomposite werecarried out. As shown in Fig. 9, MB could be totally decomposedin each cycle and the ZnO/GO photocatalyst exhibited almost nochange in its photocatalytic activity during the repeated photocat-alytic experiments. Thus, the higher photocatalytic activity andreusability of ZnO/GO nanocomposite were beneficial for its appli-cation as a photocatalyst.

The photocatalytic degradation of organic dyes by semiconduc-tor under visible-light irradiation generally involves two mecha-nisms. The first is based on the excitation of the semiconductor,which involves excitation of the semiconductor by light irradiationto form photogenerated electrons in the conduction band andholes in the valence band, and the subsequent chemical reactionswith the surrounding media after the photogenerated chargesmove to the particle surface [51]. The other mechanism is basedon the excitation of dye [52–54], in which the dye acts as a sensi-tizer of visible light as well as injects excited electrons to an elec-tron acceptor to become a cationic dye radical (dye�+), followed by

self-degradation or degradation by the reactive oxidation species.Such a photocatalytic degradation process for dyes has been dem-onstrated to be an efficient approach to remove textile dyestufffrom aquatic environments [37,38,55,56]. Considering the wideband-gap (3.37 eV) of ZnO, degradation of MB or MO under visiblelight in the present case should be conducted following the dye-excitation mechanism, as illustrated in Fig. 10. GO sheets withlarge specific surface area and numerous oxygenic groups allowgood access of dye molecules to their surfaces in photocatalyticsystem. For example, MB was firstly excited to MB�, followed byelectron transfer to the conduction band of ZnO via the transporta-tion in GO sheets, then the photoelectrons were captured by sur-face-adsorbed O2 to generate the reactive oxidation species (suchas �OH;O�2� ). Finally, the MB�+ radical was degraded itself or bythe formed reactive oxidation species. Such an efficient electron-transfer process with GO as electron collector and transporter isresponsible for the enhanced photocatalytic performance undervisible light. As for GO sheets used as photocatalyst, although theefficient electron-transfer process can be occurred between MB�

and GO, a very slow reaction rate is caused by the electron accu-mulation on GO surface and the rapid electron-MB�+ recombina-tion [56].

4. Conclusion

In conclusion, the ZnO/GO nanocomposite was successfully syn-thesized via a facile chemical deposition route at low temperatureand its use for the photodegradation of organic dye from water un-der visible light was investigated. The ZnO/GO nanocomposite iscomposed of flower-like ZnO nanoparticles anchored on graph-ene-oxide sheets. For the photodegradation of organic dyes fromwater under visible light, ZnO/GO nanocomposite exhibits muchhigher photocatalytic efficiency than GO sheets and flower-likeZnO particles. The enhanced photocatalytic performance of ZnO/GO nanocomposite can be attributed to the efficient photosensi-tized electron injection and repressed electron recombinationdue to the electron-transfer process with GO as electron collectorand transporter. These features make the ZnO/GO composite anexcellent candidate for applications relating to a number of envi-ronmental issues. The preparation method may be extended to fab-ricate more graphene-based composites for a variety ofapplications, such as catalysts, gas sensors, and nanoelectronicdevices.

Acknowledgments

Financial supports from the National Natural Science Founda-tion of China (21001003), Natural Science Foundation of AnhuiProvince of China (10040606Q15), Natural Science Research forColleges, and Universities of Anhui Province of China (KJ2010A101) are acknowledged.

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Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2012.03.060.

References

[1] I. Arslan, I.A. Balcioglu, T. Tuhkanen, D. Bahnemann, J. Environ. Eng. 126 (2000)903–911.

[2] T. Robinson, G. Mcmullan, R. Marchant, P. Nigam, Bioresour. Technol. 77 (2001)247–255.

[3] E. Forgacs, T. Cserhati, G. Oros, Environ. Int. 30 (2004) 953–971.[4] H.S. Rai, M.S. Bhattacharyya, J. Singh, T.K. Bansal, P. Vats, U.C. Banerjee, Crit.

Rev. Environ. Sci. Technol. 35 (2005) 219–238.[5] V.K. Gupta, Suhas, J. Environ. Manage. 90 (2009) 2313–2342.[6] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemannt, Chem. Rev. 95

(1995) 69–96.[7] P.A. Pekakis, N.P. Xekoukoulotakis, D. Mantzavinos, Water Res. 40 (2006)

1276–1286.[8] C.C. Chen, W.H. Ma, J.C. Zhao, Chem. Soc. Rev. 39 (2010) 4206–4219.[9] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997–3027.

[10] J.G. Hou, Z. Wang, S.Q. Jiao, H.M. Zhu, J. Hazard. Mater. 192 (2011) 1772–1779.[11] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735–758.[12] B.X. Li, Y.F. Wang, J. Phys. Chem. C 114 (2010) 890–896.[13] A. Umar, M.S. Chauhan, S. Chauhan, R. Kumar, G. Kumar, S.A. Al-Sayari, S.W.

Hwang, A. Al-Hajry, J. Colloid Interface Sci. 363 (2011) 521–528.[14] J. Becker, K.R. Raghupathi, J.S. Pierre, D. Zhao, R.T. Koodali, J. Phys. Chem. C 115

(2011) 13844–13850.[15] J.Q. Wang, Y.Y. He, J. Tao, J. He, W.J. Zhang, S.J. Niu, Z.Y. Yan, Chem. Commun.

46 (2010) 5250–5252.[16] L.W. Zhu, L. Gu, Y. Zhou, S.L. Cao, X.B. Cao, J. Mater. Chem. 21 (2011) 12503–

12510.[17] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo, H.

Siegbahn, J. Photochem. Photobiol., A 148 (2002) 57–64.[18] Y. Liu, L.F. Chen, J.C. Hu, J.L. Li, R. Richards, J. Phys. Chem. C 114 (2010) 1641–

1645.[19] L.L. Sun, D.X. Zhao, Z.M. Song, C.X. Shan, Z.Z. Zhang, B.H. Li, D.Z. Shen, J. Colloid

Interface Sci. 363 (2011) 175–181.[20] J.W. Chiou, S.C. Ray, H.M. Tsai, C.W. Pao, F.Z. Chien, W.F. Pong, C.H. Tseng, J.J.

Wu, M.H. Tsai, C.H. Chen, H.J. Lin, J.F. Lee, J.H. Guo, J. Phys. Chem. C 115 (2011)2650–2655.

[21] L. Yu, S. Yuan, L.Y. Shi, Y. Zhao, J.H. Fang, Microporous Mesoporous Mater. 134(2010) 108–114.

[22] H.C. Qin, W.Y. Li, Y.J. Xia, T. He, ACS Appl. Mater. Interfaces 3 (2011) 3152–3156.

[23] K. Woan, G. Pyrgiotakis, W. Sigmund, Adv. Mater. 21 (2009) 2233–2239.

[24] J.B. Mu, C.L. Shao, Z.C. Guo, Z.Y. Zhang, M.Y. Zhang, P. Zhang, B. Chen, Y.C. Liu,ACS Appl. Mater. Interfaces 3 (2011) 590–596.

[25] H. Zhang, X.J. Lv, Y.M. Li, Y. Wang, J.H. Li, ACS Nano 4 (2010) 380–386.[26] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191.[27] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385–388.[28] L.Y. Jiao, L. Zhang, X.R. Wang, G. Diankov, H.J. Dai, Nature 458 (2009) 877–880.[29] M.J. Allen, V.C. Tung, R.B. Kaner, Chem. Rev. 110 (2010) 132–145.[30] Y.Y. Liang, Y.G. Li, H.L. Wang, J.G. Zhou, J. Wang, T. Regier, H.J. Dai, Nat. Mater.

10 (2011) 780–786.[31] J.W. Zhu, G.Y. Zeng, F.D. Nie, X.M. Xu, S. Chen, Q.F. Han, X. Wang, Nanoscale 2

(2010) 988–994.[32] J. Wang, Z. Gao, Z.S. Li, B. Wang, Y.X. Yan, Q. Liu, T. Mann, M.L. Zhang, Z.H. Jiang,

J. Solid State Chem. 184 (2011) 1421–1427.[33] Y.L. Chen, Z.A. Hu, Y.Q. Chang, H.W. Wang, Z.Y. Zhang, Y.Y. Yang, H.Y. Wu, J.

Phys. Chem. C 115 (2011) 2563–2571.[34] Y. Yang, L.L. Ren, C. Zhang, S. Huang, T.X. Liu, ACS Appl. Mater. Interfaces 3

(2011) 2779–2785.[35] G.D. Jiang, Z.F. Lin, C. Chen, L.H. Zhu, Q. Chang, N. Wang, W. Wei, H.Q. Tang,

Carbon 49 (2011) 2693–2701.[36] Y.H. Ng, A. Iwase, A. Kudo, R. Amal, J. Phys. Chem. Lett. 1 (2010) 2607–2612.[37] B.J. Li, H.Q. Cao, J. Mater. Chem. 21 (2011) 3346–3349.[38] Z.G. Xiong, L.L. Zhang, X.S. Zhao, Chem. Eur. J. 17 (2011) 2428–2434.[39] N.L. Yang, J. Zhai, D. Wang, Y.S. Chen, L. Jiang, ACS Nano 4 (2010) 887–894.[40] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z.Z. Sun, A. Slesarev, L.

Alemany, W. Lu, J.M. Tour, ACS Nano 4 (2010) 4806–4814.[41] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A.

Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282–286.[42] J.T. Paci, T. Belytschko, G.C. Schatz, J. Phys. Chem. C 111 (2007) 18099–18111.[43] J.F. Shen, M. Shi, N. Li, B. Yan, H.W. Ma, Y.Z. Hu, M.X. Ye, Nano Res. 3 (2010)

339–349.[44] Y. Yang, T.X. Liu, Appl. Surf. Sci. 257 (2011) 8950–8954.[45] E.C. Onyiriuka, Chem. Mater. 5 (1993) 798–801.[46] X.Z. Zhou, X. Huang, X.Y. Qi, S.X. Wu, C. Xue, F.Y.C. Boey, Q.Y. Yan, P. Chen, H.

Zhang, J. Phys. Chem. C 113 (2009) 10842–10846.[47] A. Vinu, D.P. Sawant, K. Ariga, M. Hartmann, S.B. Halligudi, Microporous

Mesoporous Mater. 80 (2005) 195–203.[48] Y.Y. Li, J.P. Liu, X.T. Huang, G.Y. Li, Cryst. Growth Des. 7 (2007) 1350–1355.[49] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309–319.[50] T.G. Xu, L.W. Zhang, H.Y. Cheng, Y.F. Zhu, Appl. Catal., B 101 (2011) 382–387.[51] A. Fujishima, X.T. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582.[52] P.V. Kamat, Chem. Rev. 93 (1993) 267–300.[53] J.N. Clifford, E. Palomares, M.K. Nazeeruddin, R. Thampi, M. Gratzel, J.R.

Durrant, J. Am. Chem. Soc. 126 (2004) 5670–5671.[54] M. Zhang, C.C. Chen, W.H. Ma, J.C. Zhao, Angew. Chem., Int. Ed. 47 (2008)

9730–9733.[55] D. Zhao, C.C. Chen, Y.F. Wang, W.H. Ma, J.C. Zhao, T. Rajh, L. Zang, Environ. Sci.

Technol. 42 (2008) 308–314.[56] Z.G. Xiong, L.L. Zhang, J.Z. Ma, X.S. Zhao, Chem. Commun. 46 (2010) 6099–

6101.