Enhanced Photocatalytic Degradation of Methylene Blue Using ZnFe 2 O 4 /MWCNT Composite Synthesized by Hydrothermal Method

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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 30 (2015) 321–329

http://d1369-80

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journal homepage: www.elsevier.com/locate/mssp

Enhanced photocatalytic degradation of methylene blueby reduced graphene-oxide/titanium dioxide/zinc oxideternary nanocomposites

Nivea Raghavan, Sakthivel Thangavel, Gunasekaran Venugopal n

Nanomaterials Research Lab (NmRL), Department of Nanosciences and Technology, Karunya University, Coimbatore 641 114, Tamil Nadu, India

a r t i c l e i n f o

Keywords:Photocatalytic degradationReduced graphene-oxide nanosheetsZnO nanorods

x.doi.org/10.1016/j.mssp.2014.09.01901/& 2014 Elsevier Ltd. All rights reserved.

esponding author. Tel.: þ91 9894789648 (mail addresses: [email protected],@gmail.com (G. Venugopal).

a b s t r a c t

In this report, an attempt has been made to prepare reduced graphene-oxide/Titaniumdioxide/Zinc oxide (rGO/TiO2/ZnO) ternary photocatalyst system via a facile two stepsolvothermal method and their results were compared with rGO/TiO2 and TiO2. Thestructural, morphological and optical properties were explored using X-Ray diffraction(XRD), Scanning electron microscope (SEM), Energy Dispersive Spectra (EDS), Raman andPhotoluminescence (PL). SEM images noticeably present the 2D sheet morphology of GO,irregular spherical morphology of TiO2 and nanorods morphology of ZnO. XRD patterndepicted the formation of TiO2 anatase and wurtzite hexagonal structure of ZnO, which ishighly desirable for photocatalysis application. Further, the results of Raman spectra are ingood agreement with the XRD data. The PL spectra evidently revealed the quenchingeffect of electron–hole recombination process. The photocatalytic degradation of thesystem was investigated using a model dye methylene blue (MB). The efficiency of theternary system was evaluated and compared using rGO/TiO2 and TiO2. The degradationefficiency of rGO/TiO2/ZnO, rGO/TiO2 and TiO2 was found to be 92%, 68% and 47% within120 min respectively. Our results will pave the way for the development of futuristic rGObased ternary nanocomposites for photocatalytic applications.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Water is the most essential resource for the existence ofall beings; nevertheless nowadays water pose a seriousthreat to all beings, from aquatic to human being. Aroundthe globe, it has become the chief carrier of all contaminantssuch as carcinogenic dye [1] effluent, heavy metals [2], andPharmaceutical waste [3] despite carrying essential naturalminerals such as sodium, magnesium, calcium etc. It ismainly due to the reckless discharge of untreated industrialeffluents into the water bodies. Synthetic dye production hasbeen increased tremendously to meet the demand of textile

obile).

industries. On the other hand, environmentalists are strivinghard to combat the dye effluents effects. It is high time; theindustrial effluents treatment has to be carried out forcibly,in order to curtail the harmful effects of pollutants in water,eventually to save the planet earth and to prevent diseases.There are several approaches to treat industrial effluents,such as Precipitation Ion–Exchange, Evaporation, Reverseosmosis, Ultrafiltration, Microfiltration, Solvent extraction[4,5]. Each approach has its own prospective and constraints.With intense literature review of each method, photocataly-tic degradation technique was employed, because othermethods merely transfer the industrial waste from onephase to another phase, i.e. detrimental sludge remains evenafter the treatment. Therefore the photocatalytic technique isundoubtedly the finest method to convert harmful organiccontaminants into carbonaceous products [6].

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329322

TiO2 is a paramount photocatalyst; it was first exploredby Fujisima et al., in the year 1972 [7]; then it started torevolutionize the scientific world in the field of photocata-lysis and hydrogen fuel production, owing to its simple,abundant, strong oxidizing and non-toxic nature [8,9]. Yetthere are few drawbacks in TiO2, such as recombinationeffect and its low photoresponse towards visible light.These two bottlenecks make it an undesirable candidatefor effective photocatalysis. Therefore numerous appro-aches were attempted to develop a TiO2 hybrid basedphotocatalyst by either doping it with rare earth metal[10], nitrogen [11], sulfur [12], etc. or by preparing nano-composites utilizing activated carbon [13], CNT [14], gra-phene [15] based TiO2 hybrid, in order to effectivelydegrade the dye effluents. Despite TiO2, ZnO is a promisingphotocatalyst [16] and it has been widely used for photo-catalytic degradation of pollutant; furthermore researchersalso prepared TiO2/ZnO binary nanocomposite in order toacquire the best from each metal oxide [17,18]. Apart fromthis duo, recently graphene and graphene-oxide is yetanother excellent entrant in the photocatalysis field, whichsupports the duo in attaining effective degradation as itscavenges and shuttles electron; thus recombination issuecan be minimized tremendously [15]. It is a recent incred-ible 2D nanosheets, which has astounding electrical [19],mechanical [20] and thermal properties [21]. Reports affirmthat metal-oxide nanoparticles can be anchored on the GOsheets via the abundant oxygen-containing functionalgroups such as epoxy, hydroxyl, carbonyl, and carboxylgroups which are present on the GO sheets [22–28].Further, GO itself acts as a photocatalyst by itself [29].Recently, several ZnO/rGO [22–24] and TiO2/rGO [25–28]systems have been reported in efforts to obtain a compositewith superior photocatalytic performance, arising from thesynergistic effects between the metal oxide and rGO.Therefore, the incorporation of another photocatalyst activemetal oxide (ZnO) into graphene–TiO2 composites to formternary composites should be a promising method to designadvanced photocatalyst materials for dye degradation. TherGO plays a vital role in the photocatalysis, where it inhibitsthe undesirable recombination effect. Recently researchersare more interested in the preparation of rGO based ternarycomposite, solely to deliver a synergistic effect. Ping et al.,and Hem Raj et al., prepared Ag–ZnO/rGO and Ag–Ag–Br/TiO2/rGO [30,31] respectively and found successful in theirventure. Yet, Ag is the prime element in the composite;apparently it is not an economic photocatalytic ternarycomposite. Hence, we proposed a cost-effective rGO/TiO2/ZnO (RGTZ) ternary system, where each material is abun-dant and cost-effective. Therefore, herein we attempted toprepare RGTZ nanocomposite via a simple solvothermalprocess in order to obtain synergistic effect towards theeffective degradation of model dye, MB. Eventually theternary nanocomposite was evaluated with bare TiO2 (T)and rGO/TiO2 (RGT).

2. Materials and methods

High quality expandable graphite powder was purchasedfrom Merck. High grade H2O2 and HCl were purchased fromRankem, India. Titanium (IV) isopropoxide (TTIP, 98%), Zinc

nitrate and Hexamethylenetetramine (HMT) were purchasedfrom Sigma Aldrich.

2.1. Preparation of graphene-oxide

Graphene-oxide was prepared by a universal method,i.e. Modified Hummer's method [19]. Briefly, the expand-able graphite powder (2 g) was stirred in 50 mL of con-centrated H2SO4 acid for 30 min. Subsequently 6 g ofKMnO4 was gradually added to the solution, then stirredfurther and to it 90 mL of deionized water was added.Eventually 5 mL of H2O2 was added to the above solution.5% HCl aqueous solution was added finally in order forwashing. Later, repeated centrifugation was done usingdeionized water until the pH of the solution reachesneutral. Soon after, it was kept at 60 1C for overnight fordrying. A homogeneous dispersion of GO nanosheets wasobtained after sonication.

2.2. Preparation of rGO/TiO2/ZnO ternary nanocomposite

Two step solvothermal approach was carried out toprepare rGO/TiO2/ZnO. In brief, 0.5 wt% of GO was exfo-liated in 75 mL ethanol via sonication; consequently 1 mLof TTIP was added into the GO dispersed solution andsonicated for 45 min in order to obtain homogenousdispersed solution. This solution was transferred to aStainless steel based Teflon autoclave and kept at 180 1Cfor 18 h. Later the sample was washed with deionizedwater several times in order to remove the looselybounded TiO2 and it was dried at 80 1C for 12 h. ThusrGO–TiO2 nanocomposite was prepared. Subsequently forthe preparation of rGO/TiO2/ZnO (RGTZ) ternary nanocom-posite, 40 mM of zinc nitrate and HMT of equi-molarconcentration was prepared, and as prepared and rGO–TiO2(RGT) was added to the solution and stirred vigor-ously. Later the solution was transferred to the stainlesssteel based Teflon autoclave and kept at 90 1C for 5 h anddried at 80 1C overnight. Thus rGO/TiO2/ZnO ternarynanocomposite was prepared. TiO2 anatase powder wasalso prepared using the same method without the additionof GO and ZnO2 precursor.

2.3. Characterization

The sample thus synthesized was characterized usingXRD, SEM, Raman and PL spectroscopy. Powder X-raydiffraction studies were carried out using XRD instrumentShimadzu X-600 Japan, with CuKα radiation, to identify thecrystalline phase and also to assess the structural integrityof sample. The XRD patterns of GO, TiO2, RGT and RGTZnanocomposite were obtained in the scanning range of2θ¼10–901 and with scanning speed of 10 (deg/min). SEMimages were taken using JEOL 6390, Japan, whereby thesamples were dispersed on a carbon tape. The Ramanspectra were obtained in order to investigate the carbonbased sample; it was done using HORIBA Jobin Yvon Ramanspectrophotometer system. The PL spectra were obtainedby using a FLUOROLOG, HORIBA Jobin Yvon spectrophot-ometer in order to study the transfer behavior of photo-generated electrons and holes.

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329 323

2.4. Photocatalytic studies

Photocatalytic investigation was carried out in a closedchamber illuminated with visible light. The reactionsolution was irradiated with a 300 W Xenon lamp. Mea-sured amount (0.1 g/L) of the photocatalyst was sus-pended in 100 mL of 0.3 mg L�1 concentrated MBsolution. Before irradiation, the suspensions were stirredin the dark for 30 min in order to establish adsorption–desorption equilibrium. Subsequently irradiated cease-lessly for 120 min, about 3 mL of the reaction solutionwas taken out at a regular time interval. Consequently,the clear supernatant was taken into a quartz cell (pathlength 1.0 cm), then analyzed using a UV–visible Spectro-meter (JASCO V-60 spectrophotometer) at λmax¼663 nm,absorption maximum of MB.

3. Results and discussion

The SEM images are presented in Fig. 1. The SEM image ofGO (Fig. 1(a)) depicts the sheet-like morphology of GO. The 2Dstructure of GO is obviously seen. The SEM of RGT is shown inFig. 1(b), in which the irregular spherical morphology of TiO2

presented on rGO nanosheet is seen. It is inhomogenouslyscattered on 2D layer of reduced graphene-oxide sheet. TheSEM image of RGTZ ternary nanocomposite is shown in Fig. 1(c) in which the presence of ZnO nanorods is seen clearly andit is superficially presented over the 2D GO. The irregularspherical shaped TiO2 and rod-shaped ZnO are vividly seen inFig. 1(c), and they are scattered over the 2D sheet. The TiO2

particles are agglomerated over GO sheet. EDS mapping wasemployed to further confirm the element distribution. The

T

Fig. 1. SEM image of (a) GO,

element mapping of Ti, Zn, and O in Fig. 2(a) directly confirmsthe formation of TiO2 and ZnO. It is very obvious that thesample is free from elemental impurities. In Table 1, theelement atomic % and weight % are given. The content of Tiand Zn is around 11.79 and 21.04 at% on the surface of GOrespectively. The color mapping of each element is given inFig. 2(b)–(e).

The XRD pattern of graphite, GO, TiO2, RGT, and RGTZ isshown in Fig. 3(a). For graphite, a sharp peak was observedat 26.51, which represents the characteristic peak of the(002) plane in hexagonal graphite. However, GO shows asharp peak at 11.631, corresponding to a d-spacing of7.60 Å, which is quite higher than the d-spacing of pre-cursor graphite (3.36 Å). This change indicates that gra-phite is oxidized to GO. Fig. 3(b) shows XRD pattern ofreduced graphene-oxide. After solvothermal treatment,peak at 11.631 disappeared and a new peak centered at2θ¼23.611, appeared with the d-spacing of 0.32 nm,indicating the reduction of GO to rGO. Further, as shownin the diffractogram, pure titania prepared by the sol-vothermal route possess good crystallinity. The 2θ peakvalues at 25.31, 37.81, 48.111, 541, 62.61, and 68.91 can beindexed to (101), (004), (200), (105), (211), and (204)planes of anatase titania respectively. It is in good agree-ment with JCPDS no. 21-1272. In XRD pattern of RGT andRGTZ nanocomposite, the characteristic peak of GO is notseen, indicating the reduction of GO to rGO. It might bealso due to the low amount and low intensity of rGO.Furthermore the Raman studies confirm the rGO forma-tion, which is discussed in later section. Further, thecharacteristic peak of rGO might be screened by the mainpeak of anatase TiO2 at 25.31. In RGT nanocomposite, the

TiO2

ZnO

iO2

(b) RGT and (c) RGTZ.

Fig. 2. (a) EDS elemental mapping of RGTZ system. Color mapping of element in RGZT system (b) C, (c) Zn, (d) Ti, and (e) O.

Table 1Elemental composition in RGTZ system.

Elements At % Wt %

C 57.16 38.16O 32.63 29.02Zn 5.79 21.04Ti 4.43 11.79

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329324

peak of pure TiO2 is exactly present. In XRD pattern ofRGTZ nanocomposite, all the diffraction peaks of TiO2 andZnO characteristic peaks are present. ZnO peaks are

consistent with the hexagonal phase wurtzite structure(JCPDS number 80-0074).

Raman spectroscopy is a powerful tool and non-invasivemethod to examine graphene and graphene based materials.The characteristic Raman spectra of GO and RGTZ nanocom-posite are shown in Fig. 4. The GO Raman spectra have twocharacteristic modes at 1356 and 1586 cm�1, primarilyknown as D and G bands, which are ascribed to the breathingmode of the k point photons of the A1g symmetry involvingphonons near the K boundary and the first order scatteringof the E2g phonon of the sp2 carbon atoms, respectively[32,33]. The D mode is active in the presence of disorder. InRGTZ nanocomposite, the positions of G band and D-band

Fig. 3. (a) XRD pattern of graphite, GO, TiO2, RGT and RGTZ system and (b) XRD pattern of reduced graphene-oxide.

Fig. 4. Raman spectra of GO and RGTZ system. Fig. 5. Normalized PL spectra of TiO2, RGT and RGTZ system.

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329 325

remained unaltered. The ID/IG intensity ratio of GO is 0.99;this value is further increased to 1.21 in RGTZ nanocompo-site. Cancado et al. established a relation between theintegrated intensities of the disorder-induced Raman bands(ID/IG) in nanographite samples with different crystallite size(La) using the following equation [34]:

La ¼ 2:4� 10�10λ4lIDIG

� ��1

ð1Þ

where λl is excitation wavelength, which is 514.5 nm and Lais sp2 domain size. The average sizes of GO, rGO/TiO2/ZnOare 16.98 nm and 13.89 nm respectively. This size reductionis mainly due to the reduction of surface epoxy groups andthus GO to rGO conversion has occurred. Consequentlyincrease in the intensity of ID/IG ratio is owing to thedecrease in the average size of the sp2 domains formedduring a solvothermal assisted reduction. RGTZ nanocompo-site Raman spectrum shows the presence of TiO2 and ZnOdespite GO characteristic mode. The inset shows a magnified

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329326

image of Raman spectra, where peaks of ZnO and TiO2 canbe seen vividly. The TiO2 peaks are at 151, 394, 510, and632 cm�1, which are assigned to the Eg, B1g, B1gþA1g, and Egmodes of the anatase phase respectively, consequentlyconfirming the formation of anatase phase [28]. In addition,ZnO peaks are seen at 119, 436 and 1146 cm�1, whichcorrespond to 2E2

low, E2

highand 2LO respectively [35,36]. The

presence of anatase TiO2 and ZnO is confirmed by means ofRaman analysis and the Raman peaks are consistent with theprevious report.

The PL spectra are used to study the transfer behavior ofphoto-generated electron and holes in semiconductor mate-rials. The room temperature PL spectra of TiO2, RGT andRGTZ nanocomposite are shown in Fig. 5. The spectra of TiO2

and RGT nanocomposite are broad centered at 400–440 nm.The low intensity peak of RGT nanocomposite, comparedto bare TiO2, depicts the reduction of holes and elec-tron recombination, where rGO scavenges and shuttles the

Fig. 6. Time dependent photocatalytic degradation U

electron. The photocatalytic activity is enhanced remarkablydue to the vital role of rGO. Unexpectedly, for RGTZ nan-ocomposite, the ZnO PL spectra are only seen; at first site,the reason could not be predicted; later it was understoodfrom a recent report of Chun Cheng et al., where heobserved and reported the same phenomenon for TiO2–

ZnO hybrid [17]; thus we conclude that this strong emissionobserved in the PL spectrum of RGTZ nanocompositenanostructures may be attributed due to enhanced chargetransfer/separation process. The PL spectra of RGTZ withvisible broadband emission were induced by structure-dependent defects in ZnO nanorods. In RGTZ nanocompo-site, the excitation wavelength observed in the range340–450 nm represents the absorbance energies corre-sponding to emission of blue–yellow light and yellow–greenextinction wavelength between (450–700 nm) which arewell matched with previous report [36]. The intensity of thevisible broad band in the region of 530 nm originated from

V–vis spectra of (a) TiO2, (b) RGT and (c) RGTZ.

Fig. 7. The plot for relative concentration versus irradiation time graph.

Fig. 8. The kinetic fitting curve of MB photodegradation.

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329 327

the emission of point defects. The photocatalytic studiesdemonstrated high efficiency in RGTZ nanocomposite,which could be solely due to the charge transfer interactionof ZnO, TiO2 and defect sites of rGO.

The photocatalytic effect of TiO2, RGT and RTGZnanocomposite was evaluated using MB dye. The timedependent UV–vis absorption spectra are shown inFig. 6. After irradiation for 120 min, the percentages ofdegradation for TiO2, RGT and RGTZ nanocomposite are92%, 68% and 47% respectively. As expected, the ternarynanocomposite presented remarkable performance thanTiO2 and RTG nanocomposite. The anatase bare TiO2

exhibited normal photocatalytic activity owing to therecombination issue. RGT has shown better performancedue to the contribution of reduced graphene 2D struc-ture. The delocalized conjugated π structures in rGOallow charge carriers to achieve high mobility andrelatively slow down charge recombination. The RGTZternary nanocomposite exhibited better performancethan TiO2 and RGT nanocomposites. The hexagonalwurtzite ZnO nanorods actively participated in thephotocatalysis and eventually increased the photocata-lytic activity immensely. In addition, graphene-oxide is agood absorbent of organic compounds. It is due to theπ–π conjugation between dye and aromatic region [37].Recently several reports highlighted about its extraor-dinary performance as an absorber of toxic dyes [38],heavy metal removal [39]. This is an additional benefit,as it traps the organic species towards its high surfacearea 2D sheets; thus TiO2 and ZnO in the vicinity caneffectively fight against the organic dye. The holes andelectron initiate an oxidative pathway; subsequently theadsorbed dye can be oxidized. Thus the photoactiveradicals generated during the reaction produce carbo-naceous products such as CO2, H2O and other inter-mediates. The relative concentration versus irradiationtime graph is shown in Fig. 7. Fig. 8 shows the kineticfitting curves of MB photodegradation using TiO2, RGTand RGTZ nanocomposite. The photocatalysis reactionobeys Pseudo first order reaction kinetic equationaccording to Langmuir–Hinshelwood model, which isgiven as follows [40]:

C ¼ C0e�kt ð2Þ

ln C=C0 ¼ kt ð3Þwhere C0, C, and k are initial concentration, final con-centration and Pseudo first order kinetic constantrespectively. The degradation kinetic order was fittedby a first order equation. The as-prepared RGTZ nano-composite is 1.6 and 1.1 times efficient than bare TiO2

and RGT nanocomposite respectively. The reaction con-stants of RGTZ, RGT and TiO2 are found to be 0.32, 0.27,and 0.014 min�1 respectively. Thus RGTZ nanocompositeis a premium photocatalyst compared to other nano-composites, exclusively due to the synergistic effect oftrio, i.e. rGO, ZnO, and TiO2. The possible mechanism ofMB degradation using RGTZ nanocomposite is shown inFig. 9. When RGTZ nanocomposites are irradiated, thephotogenerated electron can be excited from the valenceband of TiO2 to the conduction band of ZnO, whereas the

photogenerated holes will be left in the valence band ofTiO2. The electron can react with O2 to generate O2

∙, and

the holes migrate to the surface and react with OH� orH2O to generate �OH radicals. The brief mechanism isgiven as follows:

rGO/TiO2/ZnOþhʋ-rGO/TiO2 (hþ , e�)�ZnO (4)

rGO/TiO2 (hþ)-ZnO (e�) (5)

e�þO2-O2�

(6)

hþþOH�-OH (7)

hþþH2O-OHþHþ (8)

Fig. 9. Mechanism of photocatalytic degradation.

N. Raghavan et al. / Materials Science in Semiconductor Processing 30 (2015) 321–329328

4. Conclusion

rGO/TiO2/ZnO ternary photocatalyst was prepared via thetwo step facile solvothermal method. It is a recommendedmethod, as it reduces GO to rGO without addition of toxicreductant and further allows charge transfer interactionbetween ZnO, TiO2 and defect sites of rGO. The anataseTiO2 and wurtzite ZnO structure in RGTZ ternary nanocom-posite was confirmed via XRD and rod morphology of ZnO isclearly seen in SEM image. Raman spectra results presentedsignificant reduction of GO to rGO in RGTZ system and fur-ther confirmed the presence of TiO2 and ZnO. The en-hanced charge transfer/separation process resulting fromthe novel RGTZ nanocomposite is due to the charge transferinteraction between TiO2, ZnO and rGO and it is wellsupported by the PL spectra. The ternary nanocompositephotocatalytic performance was evaluated against TiO2 andRGT nanocomposite using MB. RGTZ outperformed othertwo; this predominant photocatalytic activity is due to thesynergistic contribution of TiO2, ZnO and GO. Hence ourresults confirm that rGO/TiO2/ZnO ternary nanocompositecould be an excellent photocatalyst for further developmentsin the photocatalytic degradation of organic dyes and can bea suitable candidate for various eco-friendly environmentalapplications.

Acknowledgement

The authors acknowledge the Silver Jubilee Fellowship(SJF) from Karunya University, Coimbatore, India, forproviding financial support to carry out this research work.Also, all the authors extend their sincere thanks to Mr. A.

Raja and Mr. M.B.S. Pravin at Center for Research inNanotechnology (CRN) at Karunya University for the assis-tance in doing sample characterization.

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