ternary graphene–tio2–fe3o4 nanocomposite as a recollectable photocatalyst with enhanced...

Job/Unit: I20454 /KAP1 Date: 21-08-12 14:11:51 Pages: 7

FULL PAPER

DOI: 10.1002/ejic.201200454

Ternary Graphene–TiO2–Fe3O4 Nanocomposite as a RecollectablePhotocatalyst with Enhanced Durability

Yue Lin,[a] Zhigang Geng,[a] Hongbing Cai,[a] Lu Ma,[b] Jia Chen,[c] Jie Zeng,*[a]

Nan Pan,[a] and Xiaoping Wang*[a,d]

Keywords: Photocatalysts / Water pollution / Nanoparticles / Nanocomposites / Graphene / Titanium / Iron

We developed a ternary nanocomposite of graphene–TiO2–Fe3O4 (GTF) as a low-cost, recollectable, and stable photo-catalyst for the degradation of organic dyes. The nanoc-omposite has been successfully prepared by successivelygrowing TiO2 and Fe3O4 nanoparticles on the reducedgraphene oxide (RGO). The as-synthesized GTF nano-composite shows higher photocatalytic activity as comparedwith that of pure TiO2 nanoparticles and can be easily col-lected from water using a magnet. More importantly, benefit-ing from the presence of RGO, GTF can suppress the photo-

Introduction

Water pollution is a major global problem. It is estimatedthat around 10–15% of organic dyes are discharged, leadingto a seriously adverse impact on public health and the envi-ronment.[1] Various methods associated with many adsorb-ents have been developed to treat dye-polluted water.[2–5]

Unfortunately, most of the current strategies developed ac-cording to the adsorption principle still retain several prob-lems, such as low adsorption capacity, nonrecyclability, orcomplex operations for recycling.[2–5] Photocatalysis hasbeen regarded as a green, simple, and low-cost method todegrade dyes.[6–11] In this context, semiconductors includingTiO2 and ZnO were first used to reach the goal.[6,8,12] Anumber of binary photocatalysts involving titania-coatedmagnetite,[13] TiO2–Fe3O4 hollow spheres,[14] and TiO2–Fe3O4 nanocomposites[15] were also developed to realize the

[a] Hefei National Laboratory for Physical Sciences at theMicroscale, University of Science and Technology of China,Hefei 230026, P. R. ChinaFax: +86-551-3606266E-mail: [email protected]

[email protected]: http://www.hfnl.ustc.edu.cn/2012/0709/3363.html

[b] Department of Materials Science and Engineering, Universityof Science and Technology of China,Hefei 230026, P. R. China

[c] School of Nuclear Science and Technology, University ofScience and Technology of China,Hefei 230026, P. R. China

[d] Department of Physics, University of Science and Technologyof China,Hefei 230026, P. R. ChinaSupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201200454.

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dissolution behavior of Fe3O4 nanoparticles that usually oc-currs in TiO2–Fe3O4 binary nanocomposites, rendering it ahighly stable photocatalyst. Furthermore, the GTF nano-composite works well in different pH environments and iscapable of eliminating mixtures of various dyes. In addition,the GTF is also able to degrade the dyes under sunlight.These attractive features make the GTF nanocomposite apromising photocatalyst for practical use in wastewater treat-ment.

recollection of photocatalysts by taking advantage of themagnetic properties of Fe3O4. However, these binary com-posites always suffered a serious decrease of photocatalyticefficiency after several uses,[14,15] possibly due to the chemi-cal instability of Fe3O4 induced by the photogenerated elec-trons transferred from TiO2.[13] In this case, it is particularlyimportant to increase the durability of such recollectablephotocatalysts for practical use.[9]

Recently, researchers have developed several ways to im-prove the activity of photocatalysts, such as carbon-dopedTiO2, carbon-coated TiO2, carbon–nanotube–TiO2, andgraphene–TiO2 nanocomposites,[16–21] among which graph-ene–TiO2 nanocomposites showed fantastic activity.Graphene, since its discovery in 2004,[22] has attracted ex-tensive attention due to its excellent mechanical, thermal,optical, and electronic properties.[23,24] In graphene–TiO2

nanocomposites, the graphene is proposed to serve as anefficient acceptor for the photogenerated electrons, thus sig-nificantly suppressing charge recombination and enhancingthe photocatalytic rate of the nanocomposite as comparedto that of pure TiO2 nanoparticles (NPs).[17–19,25,26] How-ever, for practical applications, graphene–TiO2 nanocompo-sites still suffer from difficult recollection. It is noted thatin previous studies on graphene-based composites, the focushas been on binary nanocomposites, including graphene–metal (e.g. graphene–Au[27] and graphene–Pt[28]), graphene–magnetic (e.g. graphene–Fe3O4

[3,29] and graphene–Co3O4

[30]), and graphene–semiconductor (e.g. graphene–TiO2,[17–19,25,26] graphene–ZnO,[31] graphene–CdS,[32] andgraphene–CdSe[33,34]) composites. However, graphene-based multicomposites are of great interest for their inte-


J. Zeng, X. Wang et al.FULL PAPERgrated multiple functions but are quite challenging becauseof difficulties in the precise control of their synthesis, eventhough the synthesis of Fe3O4–graphene–TiO2 has been re-ported recently.[35]

Herein, we report a ternary, hybrid graphene–semicon-ductor–magnetic nanocomposite, specifically referring tographene–TiO2–Fe3O4 (GTF), which possesses the inte-grated functions as depicted in Figure 1: i. TiO2 NPs as asemiconductor photocatalyst to degrade the dye, ii. graph-ene as an effective electron pathway to suppress the chargerecombination in TiO2 and enhance its photocatalytic ac-tivity, and iii. Fe3O4 NPs as a magnetic material for mag-netic separation. By employing the degradation of Rhod-amine B (RhB) as a model reaction, we systematically in-vestigated the photocatalytic properties of GTF. Besides theintegrated functions, we found that the ternary GTF nano-composite exhibited an improved durability for successivephotocatalytic reactions as compared to the TiO2–Fe3O4 bi-nary nanocomposite. Furthermore, GTF is capable of de-grading a mixture of different dyes efficiently.

Figure 1. Schematic illustration of the structure and electron trans-fer in GTF. The inset is a photograph indicating the photodegrad-ation of RhB aqueous solution using GTF as the catalyst. The RhBmolecules were eliminated with the help of GTF under UV light,and the GTF can be efficiently recollected with a small magnet.

Results and Discussion

In this study, graphene oxide (GO) was produced usingHummers’ method.[36] Graphene–TiO2 was synthesizedthrough the hydrolysis of Ti(BuO)4 in an ethanol/watersolution containing GO at 80 °C for 12 h.[19] GTF was pre-pared by depositing Fe3O4 NPs onto graphene–TiO2 byusing chemical precipitation methods,[29] in whichFeCl3·6H2O and FeCl2·4H2O reacted with NH4OH in anaqueous solution containing graphene–TiO2. Figure 2 (a)shows a TEM image of the obtained GTF nanocomposite,in which Fe3O4 and TiO2 NPs can be easily distinguishedbased on the differences in their size and contrast. Theaverage size of TiO2 and Fe3O4 NPs is estimated to beca. 20 and ca. 80 nm, respectively. Figure S1 (see Support-ing Information) shows a selected area electron diffraction(SAED) pattern of the GTF, in which the diffraction ringsof Fe3O4 and TiO2 NPs can be observed. The powder XRD

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pattern of the GTF confirmed the cubic magnetite crystalstructure (JCPDS No. 89–3854) for Fe3O4 and the anatasephase with tetragonal crystal structure (JCPDS No. 89-4921) for TiO2 (Figure 2, b). The C 1s spectra (Figure S2)of GO, graphene–TiO2, and GTF were determined by X-ray photoelectron spectroscopy (XPS). Each spectrum con-tains three peaks at ca. 284.8 eV (assigned to graphitic car-bon atoms), ca. 286.7 eV (assigned to epoxy/ether), and ca.288.7 eV (assigned to C=O).[37] As compared to those ofGO, the intensities of peaks at ca. 286.7 and ca. 288.7 eVdropped for both graphene–TiO2 and GTF, indicating thatGO was reduced in the synthesis of graphene–TiO2. To de-termine the hybridization efficiency of GO, we charac-terized the mass of carbon in the following way: GTF wasimmersed in aqua regia and aged at 80 °C for 2 h to decom-pose TiO2 and Fe3O4 and leave RGO in solution. The RGOwas then washed three times with water, dried, andweighed. It was found that the amount of carbon was onlyreduced by 3–4% compared to that of the feed materials,demonstrating the high efficiency of hybridization.

Figure 2. TEM image (a) and XRD pattern (b) of the GTF nano-composite.

A scanning electron microscope (SEM) image of theGTF nanocomposite (Figure S3a) reveals the coating ofTiO2 and Fe3O4 NPs on the RGO sheets. Due to the differ-ence of the mean atomic weight between Fe3O4 and TiO2,the corresponding NPs can be distinguished in a back scat-tering electron image (Figure S3b), in which the Fe3O4 NPsappear to be brighter than the TiO2 NPs. The enlarged im-age (Figure S3c) and the element mappings of GTF (Fig-ures S3d–f) obtained by scanning transmission electron mi-croscopy (STEM) show that the strong iron and titaniumsignals come from the Fe3O4 NPs (ca. 80 nm) and the TiO2

NPs (ca. 20 nm), respectively, whereas the oxygen signal oc-curs in both Fe3O4 and TiO2 NPs. Moreover, the room-temperature magnetic hysteresis loop (Figure S4) confirmsthe magnetic properties of GTF. An experiment to separatethe GTF from the solution with a magnet is shown in Fig-ure 1.

The photocatalytic capability of GTF was evaluated bythe photodegradation of RhB in an aqueous solution irra-diated by UV lamps. We chose this model reaction to moni-tor the performance of our catalysts as it has been wellstudied and the intermediates of this reaction are clear.[38]

The reaction mechanism involved in this study is almost the


Recollectable Nanocomposite Photocatalyst

same as that for traditional semiconductor photocatalysts(such as TiO2, ZnO, CdS, and CdSe).[9] The differencemight be the presence of RGO, which contributed to chargetransfer.[17,19] For a traditional semiconductor photocata-lyst, both electrons and holes generated in the photocatalystcould decompose pollutants directly. Alternatively, elec-trons could also react with oxygen to produce oxy radicals,which finally convert into hydrogen peroxide. In this case,holes would react with hydroxyl ions to produce hydroxylradicals. Hydroxyl radicals are the most active species todecompose pollutants in the photodegradation process.[9] Inour experiments, RGO could act as an acceptor of the elec-trons generated in TiO2 particles, supposedly suppressingthe recombination of charges and enhancing the photocata-lytic activity. As to the reactive oxy radicals, we believe theymay be generated on the surface of RGO as electrons havebeen efficiently transferred onto RGO.

Figure 3 a shows the normalized concentration of RhBvs. UV irradiation time with various GTF loadings, inwhich 5 mL of an aqueous solution containing 7.7�10–6 m

(C0) RhB was used as the starting reagent. According tothe Beer–Lambert law, the concentration of RhB is linearlyrelated to the absorbance of the solution when the concen-tration is low. In this case, the concentration of RhB can bedetermined from the UV/Vis spectra.[5,19] As can be seenfrom Figure 3 (a), the concentration of RhB decreases ap-parently with the time of UV irradiation, indicating the oc-currence of photodegradation of RhB. In addition, thephotodegradation becomes more obvious with increasingGTF loading. The time consumed to degrade RhB com-pletely decreases from 55 to 25 min when the GTF loadingincreases from 20 to 250 μL (0.79 g/L).

Figure 3. Photocatalytic degradation of an aqueous solution ofRhB with various GTF loadings under the UV irradiation. Theconcentration of GTF is 0.79 g/L. (a) The variation of normalizedconcentration of RhB C/C0 with the irradiation time t. The insetshows –ln (C/C0) vs. t. (b) A plot of reaction rate constant k vs.GTF loading. The inset is a plot of ln (k) vs. ln (GTF loading).

In order to further decipher the catalytic kinetics, plotsof –ln (C/C0) vs. t for various GTF loadings are given in theinset of Figure 3 (a). As seen, all curves show good linearbehavior, revealing that the kinetics of the degradation reac-tion is controlled dominantly by a pseudo-first-order reac-tion, which is consistent with previous results.[12,19,39] Fromthe curves in the inset of Figure 3 (a), we calculated thereaction rate constant k for various GTF loadings. The re-sults are shown in Figure 3 (b). Interestingly, k is found todepend on the GTF loading. A plot demonstrating the lin-ear relationship between k and [GTF loading]n is shown in

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the inset of Figure 3 (b). This relationship was described asthe screen effect[12,13,40] and has also been observed in thesystem with pure TiO2 as the catalyst.[12,40,41] Based on theinset of Figure 3 (b), the exponent n for the GTF nano-composite is estimated to be ca. 0.35, which is smaller thanthat of pure TiO2 NPs (0.67),[41] implying that a strongerscreen effect exists in our GTF photocatalyst as comparedwith pure TiO2. This is reasonable because the Fe3O4 NPswill screen and weaken the UV light needed to irradiatethe TiO2 NPs in GTF. We also prepared a series of GTFnanocomposites with various Fe3O4 loadings while keepingthe amount of TiO2 NPs the same. Figures S5–7 show thestructural analysis of this series of catalysts. As expected, itwas found that the photocatalytic activities of these GTFnanocomposites towards RhB decrease with increasingFe3O4 loading (Figure S8), possibly because i) Fe3O4 par-ticles blocked some of the TiO2 active sites and ii) some UVlight was absorbed by Fe3O4 particles. As such, it is nothard to understand why the photocatalytic activity of GTFis a little lower than that of binary graphene–TiO2.

The aforementioned results indicate that the photocata-lytic activity was determined by the active sites wherecharges were generated and/or separated. As a counterpartcritical for the control of charge separation, the RGOshould play an important role. To address this issue, weprepared a series of GTFs (Figure S9) containing differentamounts of RGO (from 0 to 0.875, 1.75, 3.5, and 7 mg)with a fixed molar ratio (0.14) of Fe3O4/TiO2. Figure S10ashows the kinetic curves of all the GTFs; and Figure S10bshows the plot of reaction rate constant vs. the amount ofRGO. It is clear that the introduction of RGO (0.875 mg)can significantly increase the reaction rate. The reaction ratecontinuously increased as the amount of RGO was furtherincreased to 1.75, 3.5, and 7.0 mg. These RGO-amount-de-pendent experiments further confirmed the mechanism weproposed above, in which RGO acted as an acceptor of theelectrons generated in the TiO2 particles,[17,19,42] suppressedthe recombination of charges, and ultimately enhanced thephotocatalytic activity of GTF.

As a recyclable photocatalyst, renewable photocatalyticactivity is important. Among reported systems such asTiO2–Fe3O4 composite, the photocatalytic activity is alwaysdramatically reduced after use for a few cycles,[14,15,43] thushindering its practical application severely. Figure S11shows the durability test of TiO2–Fe3O4 composite. It isclear that its activity decreased by �15% after running forfive cycles. The poor durability of this set of photocatalystsis believed to be caused by the photodissolution of Fe3O4

NPs.[13] Specifically, Fe3+ of Fe3O4 NPs was reduced to Fe2+

by the photogenerated electrons, which transferred from theTiO2 portion to the Fe3O4 portion. Fe2+ is then dissolvedin the solution.[13] In this case, the active electrons were con-sumed and Fe3O4 NPs were gradually decomposed. As forthe graphene–TiO2 system, the quick and efficient electrontransfer from TiO2 NPs to RGO leads to the suppressionof electron–hole recombination in TiO2 NPs and an en-hancement of the photocatalytic activity of the compos-ite.[17–19] In this context, we consider that, for the GTF nano-


J. Zeng, X. Wang et al.FULL PAPERcomposite, the RGO can shunt most of the photo-generated electrons from the TiO2 NPs and thus decreasethe opportunity of electron transfer to the Fe3O4 NPs. Fig-ure 1 schematically illustrates this mechanism. It is not un-reasonable to expect that the GTF would suppress thephotodissolution of Fe3O4 NPs as compared with theTiO2–Fe3O4 binary system, thus enhancing its durability forthe photodegradation of dyes.

To verify the above argument, we investigated the cyclicphotocatalytic activity of the GTF. As seen in Figure 4, noobvious decrease of photocatalytic reaction rate was ob-served after five cycles of successive photodegradation ofRhB, indicating that our GTF is renewable for photocata-lytic reactions. The inset of Figure 4 shows the plots of–ln (C/C0) against t for each cycle, further confirming thepreservation of photocatalytic activity. It is worth notingthat there is no elemental Fe detected by inductively cou-pled plasma mass spectrometry (ICP-MS) in the residualsolution after photodegradation, suggesting that the photo-dissolution of Fe3O4 NPs has been effectively suppressed.

Figure 4. The cyclic photocatalysis of GTF. 100 μL of the GTFstock suspension was used to degrade RhB (5 mL, 7.7�10–6 m) ina quartz vial. The inset shows the curves of –ln (C/C0) vs. t for eachcycle.

Considering the actual dye solution may not be neutral,we also investigated the photocatalytic degradation of RhBby GTF at different pH values (5.0, 7.0, 9.6, and 11.5). Asshown in Figure 5 (a), compared to the neutral solution,the photocatalytic activity of GTF increased slightly whenthe pH was 9.6 and 11.5, whereas it remained almost thesame when the pH was 5.0. This can be understood by con-sidering the role of OH–. Usually, the degradation of dyesis mainly due to hydroxyl radical attack, direct oxidationby the positive hole, and direct reduction by the electron inthe conduction band.[12] When the pH is above the point ofzero charge of TiO2 (ca. 6.8),[40] more OH– ions arise in thesolution and make the TiO2 surface negatively charged. Thenegative modification of the TiO2 surface can not only en-hance the adsorption of cationic RhB, but also degrade thedye molecular effectively through the hydroxyl radical OH·,which comes from the combination of OH– with the photo-generated hole in the TiO2 NPs.[9]

To simulate real polluted water, we mixed three kinds ofdyes involving RhB, Orange Pure, and Acid Blue 92. Thephotocatalytic degradation of this mixture by GTF (Fig-ure 5b) shows the universality of GTF as a photocatalyst,indicating the potential application in practical use. In ad-


Figure 5. (a) Effect of the initial pH on photocatalytic degradationof an aqueous solution of RhB. (b) Photocatalytic degradation ofan aqueous solution containing a mixture of RhB, Orange Pure,and Acid Blue 92. Specifically, 3 μL of RhB (5 � 10–3 m), 20 μL ofOrange Pure (5�10–3 m), and 20 μL of Acid Blue 92 (5�10–3 m)were dissolved in 4.957 mL of deionized water, with the additionof 100 μL of GTF suspension as the catalyst.

dition, we found that GTF was also capable of degradingthe organic dye even under the illumination of sunshine(Figure S12). Although the degradation rate under sunshineis much slower than that under the UV irradiation, thiscapability makes the GTF nanocomposite a promising can-didate for wide applications in the field of photocatalysis.

It should be mentioned that the depositing sequence ofTiO2 and Fe3O4 NPs on the graphene can significantly af-fect the photocatalytic activity of the nanocomposite. Forcomparison, we have also synthesized a graphene–Fe3O4–TiO2 (GFT) nanocomposite. The ternary structure was alsoconfirmed by TEM (Figure S13), SEM (Figure S14a), andback scattering electron (Figure S14b) images. However, thephotocatalytic activity of GFT (Figure S15) was muchlower than that of GTF. The exact mechanism is still notclear. We speculate that the interface between RGO andTiO2 plays an important role.

Conclusions

We have demonstrated the synthesis of a ternary, hybridnanocomposite consisting of TiO2 and Fe3O4 NPs sup-ported on graphene by a simple, aqueous-phase method.This nanocomposite, also denoted as GTF, exhibited ahigher photocatalytic activity during the degradation ofRhB as compared to the unsupported TiO2–Fe3O4 compos-ite and pure TiO2 NPs. The photodissolution of Fe3O4 inGTF has been suppressed to a comparatively low level bythe presence of graphene, which insures the stability of GTFin photocatalytic reactions. The photocatalytic activity ofGTF is almost unchanged after reusage during five cycles.The electron transfer between the TiO2 NPs and RGO playsa key role in achieving the high durability of GTF. In ad-dition, the GTF can be easily recollected from water usinga magnet. Due to its attractive features such as easy recol-lection and reusability, the GTF hybrid nanocomposite ob-tained in this work may find use in many applications, espe-cially in wastewater treatment. Additionally, our approachis expected to be extendible to other hybrid systems.



Experimental SectionChemicals: Titanium isopropoxide [Ti(BuO)4, �98%] was pur-chased from Aros Organics. Ethanol (�99.7%) was purchasedfrom Sinopharm Chemical Reagent Co., Ltd. H2SO4 was pur-chased from Shanghai Lingfeng chemical Reagent Co., Ltd. Rho-damine B, Acid blue 92, and Orange Pure were purchased fromSigma. N,N-Dimethylformamide (DMF) was purchased fromShanghai Meixing Chemical Co., Ltd. The deionized water wasproduced by using a Millpore Milli-Q grade, with a resistivity of18.2 MΩ cm–1. All chemicals were used without further purifica-tion.

Synthesis of Nanocomposites: Graphene–TiO2. Hummers’ methodwas adopted to obtain GO.[36] In a typical synthesis of graphene–TiO2, an aqueous solution of NaOH (1 m) was added dropwiseinto a GO solution (7 mL, 0.5 mg/mL) until the pH of the solutionreached 12. The solution was then dialyzed until the solution be-come neutral and heated to 80 °C with the addition of ethanol(65 mL). Then, Ti(BuO)4 (180 μL), H2SO4 (70 μL), and ethanol(5 mL) were added. The mixture was kept at 80 °C for 12 h withstirring and then cooled to room temperature.[19] The products werewashed with water and redispersed in water/DMF (25 mL/0.5 mL).The mixture was heated to 200 °C in a 50 mL Teflon-lined auto-clave and kept at 200 °C for 20 h. The graphene–TiO2 was collectedby centrifugation, washed three times with water, and dried beforeusing as the seeds for the growth of Fe3O4.

Graphene–TiO2–Fe3O4 (GTF): In a typical synthesis of GTF,graphene–TiO2 (43 mg) was suspended in deionized water (30 mL)in a round-bottomed flask under N2. An aqueous solution(1.56 mL) containing FeCl3·6H2O (3.95 mg) and FeCl2·4H2O(60 mg) was then injected into the flask using a pipette. After stir-ring for 5 h under N2, the flask was sealed after a NH4OH aqueoussolution was added (4.5 mL, 1.5 m). The reaction was allowed tocontinue at 65 °C for 2.5 h. The product was collected by centrifu-gation and washed three times with water to remove excess ionsbefore characterization. To obtain GTF with different ratios ofFe3O4 to TiO2, the amount of graphene–TiO2 precursor employedwas kept the same and the amounts of FeCl3·6H2O andFeCl2·4H2O were varied.

Graphene–Fe3O4–TiO2 (GFT): In a typical synthesis of graphene–Fe3O4, GO solution (7 mL, 0.5 mg/mL) was placed in a round-bottomed flask under N2. An aqueous solution (1.56 mL) contain-ing FeCl3·6H2O (3.95 mg) and FeCl2·4H2O (60 mg) was then in-jected into the flask by using a pipette. After stirring for 5 h underN2, the flask was sealed after a NH4OH aqueous solution (4.5 mL,1.5 m) was added. The reaction was allowed to continue at 65 °Cfor 2.5 h. The graphene–Fe3O4 was collected by centrifugation andwashed three times with water to remove excess ions before usingas the seeds for the growth of TiO2. GFT was prepared in the sameway as that for graphene–TiO2, except that graphene–Fe3O4 wasemployed as the seed instead of GO.

Photocatalytic Degradation of Dye Pollutants: GTF (56.4 mg) wasimmersed an RhB aqueous solution (5�10–3 m) for 12 h to allowfor sufficient adsorption of RhB onto the GTF. The immersedGTF was recollected, washed, and suspended in deionized water(71 mL) as a GTF stock suspension. After being vigorously shaken,the GTF suspension (100 μL) was added into an aqueous solution(5 mL) containing RhB (7.7�10–6 m) in a quartz vial. The stirredsuspension was illuminated with two germicidal UV lamps (Philip-TUV/15W). After 5 min of irradiation, the quartz vial was put neara magnet to separate the GTF. The UV/Vis spectrum of the solu-tion was then measured to determine the concentration of RhB.


Characterization: Scanning electron microscopy (SEM) imageswere obtained with a Sirion 200 FE-SEM at an acceleration voltageof 10 kV. Transmission electron microscopy (TEM), high-resolu-tion TEM (HRTEM), selected area electron diffraction (SAED),and energy dispersive X-ray (EDX) analysis were performed witha JEOL-2010 microscope operated at 200 kV. High angle annulardark field (HAADF) scanning transmission electron microscopy(STEM) was conducted with a JEM 2100F microscope. EDX map-ping was recorded with an X-Max Silicon Drift Detector (SDD)from Oxford. XRD was carried out on a MAC MXPAHF X-raydiffractometer with Cu-Kα radiation. X-ray photoelectron spec-troscopy (XPS) was performed with an ESCALAB 250 instrument(Thermo-VG Scientific). UV/Vis spectra were measured with an U-4100 Spectrophotometer (HITACHI). Inductively coupled plasmamass spectrometry (ICP-MS) was conducted with an Optima 7300DV instrument.

Supporting Information (see footnote on the first page of this arti-cle): Figure S1: SAED pattern of GTF corresponding to the TEMimage shown in Figure S1a. Figure S2: C1s XPS spectrum of GO,graphene–TiO2 (GT), and GTF. Figure S3: (a) SEM and (b) backscattering electron images of GTF nanocomposite (molar ratio ofFe3O4/TiO2 = 0.14). (c) High angle annular dark field STEM imageand the corresponding EDX mappings for (d) oxygen, (e) iron, and(f) titanium. Figure S4: Room temperature M-H curve for GTF(molar ratio of Fe3O4/TiO2 = 0.14). Figure S5: EDX of GTF withvarious Fe3O4/TiO2 ratios. Figure S6: TEM images of GTF withvarious Fe3O4/TiO2 ratios corresponding to Figure S5. Figure S7:XRD patterns of GTF with various atomic molar ratios of Fe3O4/TiO2. Figure S8: Photocatalytic degradation of RhB with pureTiO2 NPs, graphene–TiO2 (GT), and GTF with various Fe3O4/TiO2 ratios. Figure S9. TEM images of GTF with the amount ofRGO varied from (a) 0, (b) 0.875, (c) 1.75, (d) 3.5, and (e) 7 mgand the molar ratio of Fe3O4/TiO2 fixed at 0.14. Figure S10. Photo-catalytic degradation of an aqueous solution of RhB with variousamounts of RGO under the irradiation of UV light. (a) The varia-tion of the normalized concentration of RhB C/C0 with the irradia-tion time t. (b) A plot of reaction rate constant k vs. the amountof GO used for preparation of the GTF. Figure S11. The cyclicphotocatalysis of TiO2–Fe3O4 composite. 100 μL of the suspensionstock of TiO2–Fe3O4 composite was used to degrade RhB (5 mL,7.7 �10–6 m) in a quartz vial. Figure S12: Degradation of RhB(5 mL, 7.7�10–6 m) under sunlight with the addition of 100 μL ofGTF suspension stock as the catalyst. Figure S13: TEM image ofgraphene–Fe3O4–TiO2 nanocomposite. Figure S14: (a) SEM and(b) back scattering electron images of graphene–Fe3O4–TiO2. Fig-ure S15: Degradation of RhB (5 mL, 7.7�10–6 m) under UV irradi-ation with the addition of 100 μL of graphene–Fe3O4–TiO2 suspen-sion stock as the catalyst.

Acknowledgments

We thank Mr. Cheng-Hao Wu (University of California, Berkeley)for helpful discussions. This work is supported by MOST of China(2011CB921403) and the National Natural Science Foundation ofChina (NSFC) under grant numbers 21121003, 90921013,11074231, and 11004179.

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Nanocomposite Photocatalyst

We report a facile method for achieving a Y. Lin, Z. Geng, H. Cai, L. Ma, J. Chen,ternary, hybrid nanocomposite consisting J. Zeng,* N. Pan, X. Wang* ............. 1–7of TiO2 and Fe3O4 nanoparticles (NPs)supported on graphene. This nanocompos- Ternary Graphene–TiO2–Fe3O4 Nano-ite (GTF) exhibited a higher photocatalytic composite as a Recollectable Photocatalystactivity during the degradation of Rhod- with Enhanced Durabilityamine B compared to the unsupportedTiO2–Fe3O4 composite and pure TiO2 NPs. Keywords: Photocatalysts / Water pol-The photocatalytic activity of GTF is al- lution / Nanoparticles / Nanocomposites /most unchanged after repeated use. Graphene / Titanium / Iron


ternary graphene–tio2–fe3o4 nanocomposite as a recollectable photocatalyst with enhanced...

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