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Ceramics International 40 (20

oor

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of Tg U

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ompdatatThe

photo-generated electrons from ZnO to MWNTs, leading to low recombination rate of photo-induced electronhole pairs.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

oxide and has been widely used to degrade organic dyes in

be a good electron acceptor [13], which have been demon-strated to greatly improve the photocatalytic activity ofTiO2 for the degradation of azo dyes, phenol, etc. [1416].

MWNT nanocomposites have been investigated in previous

2. Experimental

All the chemical reagents are of analytical grade and used

without further purication. The ZnO/MWNT nanocompositeshave been synthesized by one-step hydrothermal method.In a typical experiment, 2 mmol ZnCl2 (Sinopharm Chemical

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.http://dx.doi.org/10.1016/j.ceramint.2013.10.157

nCorresponding author. Tel./fax: 86 371 62506070.E-mail address: liuping1980@gmail.com (P. Liu).water under UV irradiation due to their strong oxidizingpower, and being non-toxic, and low cost [57]. However,the recombination rate of the photo-induced electronholepairs in ZnO is usually faster than surface redox reactions[8], resulting in the low photocatalytic efciency. To enhancethe photocatalytic efciency, many efforts have been made tosuppress the electronhole recombination by means of noblemetal loading [9,10] and incorporation of electron-acceptingmaterials [7,11,12].Multi-walled carbon nanotubes (MWNTs) are considered to

researches [13,17], the photocatalytic efciency still remainslow. In this paper, we have demonstrated a one-step synthesisof the ZnO/MWNT nanocomposites by hydrothermal method,which has been widely applied to synthesize nanomaterials dueto the tunable control of reactive parameters [18]. Thephotocatalytic properties of the as-synthesized ZnO/MWNTnanocomposites have also been investigated through thedegradation of methylene blue (MB) and Rhodamine 6G(Rh6G) under UV irradiation.Keywords: B. Nanocomposites; D. Carbon; D. ZnO; E. Functional applications

1. Introduction

The photocatalytic degradation of dye wastewaters bysemiconductor nanostructures has attracted much attentiondue to its high efciency, low-cost and nontoxic end products[14]. Zinc oxide (ZnO) is a wide band-gap semiconductor

In semiconductor oxide/MWNT nanocomposites, MWNTs actas an excellent electron-acceptor/transport material to remark-ably enhance the migration of photo-induced electrons in thephotocatalytic process, suppressing the recombination rate ofelectronhole pairs, which will enhance the photocatalyticperformance. Although the photocatalytic properties of ZnO/Enhanced photocatalytic performancenanocomposites f

Ping Liua,n, Yanqing Guoa, Qingwei Xua

aSchool of Electric and Information Engineering, Zhongyuan UniversitybCollege of Electrical and Electronics Engineering, Huazhon

Received 13 October 2013; received in revisedAvailable online

Abstract

The ZnO nanowire/multi-walled carbon nanotube (MWNT) nanocmethod using zinc chloride as Zn source. Their photocatalytic degrainvestigated under UV irradiation. Experimental results show thnanocomposites is 3 times higher than that of pure ZnO nanowires.CERAMICSINTERNATIONAL

14) 56295633

f ZnO/multi-walled carbon nanotubedye degradation

engge Wanga, Yanbin Lia, Keran Shaob

echnology, No. 41 Middle Zhongyuan Road, Zhengzhou 450007, Chinaniversity of Science and Technology, Wuhan 430074, China

30 October 2013; accepted 31 October 2013ovember 2013

osites have been successfully synthesized by one-step hydrothermalion performances on methylene blue and Rhodamine 6G have beenthe photocatalytic efciency of the as-synthesized ZnO/MWNTenhanced photocatalytic activity is attributed to the fast transfer of

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Reagent Co., Ltd.) was rst dispersed into 30 ml deionizedwater, and then 2 ml MWNT aqueous solution (10 wt%,Chengdu Organic Chemistry Co., Ltd.) was added into theabove-mentioned Zn2 solution under ultrasonication. The pHvalue of the mixed solution was adjusted to 11 using NaOH.After ultrasonication for 30 min, the mixed aqueous solutionwas transferred to a 50-ml Teon-lined autoclave and heated at120 1C for 12 h. Finally, the ZnO/MWNT nanocompositeswere collected after washing, centrifugation and drying.The microstructural morphologies of the as-synthesized ZnO/

MWNT nanocomposites were characterized by eld emissionscanning electron microscope (FESEM, JEOL-JSM-6700F, Japan)and energy dispersive spectrometer (EDS). The crystal structureidentication was performed by X-ray diffraction (XRD, Philips1700X) with Cu-K radiation (0.154178 nm) after dryingnanocomposite lm on silicon substrate. The UVvis absorptionspectra of the aqueous ZnO/MWNT nanocomposite solu-tion were measured on a UV/vis spectrophotometer (Hitachi,

U-2000). The photocatalytic properties of the as-synthesizedZnO/MWNT nanocomposites (10 mg) were evaluated bydecomposing MB and Rh6G solutions (50 ml, 10 mg/l) atambient temperature under UV irradiation (365 nm, 150 W)after 30 min adsorption equilibrium. The absorption spectra of MBand Rh6G solutions were recorded using UV/vis spectro-photometer.

3. Results and discussion

The micro-morphologies of the as-synthesized ZnO/MWNTnanocomposites were characterized by SEM, as shown inFig. 1a. It can be seen that the diameter of as-synthesized ZnOnanowires (NWs) is 200300 nm and the length is 510 mm,which is similar to previous reports [19,20]. One can see thatZnO NWs are plugged into the MWNTs closely. From low-resolution SEM image in Fig. 1b, the ZnO NWs are well-separated from each other and well-mingled with MWNTs in

P. Liu et al. / Ceramics International 40 (2014) 562956335630Fig. 1. (a) A typical SEM image, (b) low-resolution SEM image and (c) EDX mapping of the as-synthesized ZnO/MWNT nanocomposites.

WN

rnatFig. 2. (a) XRD patterns and (b) UVvis spectra of M

P. Liu et al. / Ceramics Intethe nanocomposites. EDS mapping has also been performed tocharacterize this uniformity, as shown in Fig. 1c. One can seeclearly that both MWNTs and ZnO NWs are well dispersed inthe nanocomposites, which will be benecial for charge carriertransfer between ZnO NWs and MWNTs.Fig. 2a shows typical XRD patterns of MWNTs, pure ZnO

and the as-synthesized ZnO/MWNT nanocomposites. FromXRD pattern of pure ZnO NWs, ve diffraction peaks at 32.51,34.91, 36.71, 47.91and 56.41 can be observed clearly, corre-sponding to the (100), (002), (101), (102) and (110) planes ofthe hexagonal wurtzite ZnO phase (JCPDS 65-3411), respec-tively [21]. Only one diffraction peak around 226.51 can beseen from XRD pattern of MWNTs, which is attributed to(002) peak of MWNTs. One can see clearly that all diffractionpeaks in the ZnO/MWNT nanocomposites are in good agree-ment with ZnO NWs and MWNTs [13].The UVvis absorption spectra of MWNTs, pure ZnO, and

the ZnO/MWNT nanocomposites are shown in Fig. 2b. It canbe seen that the MWNTs exhibit a very broad and featurelessabsorption spectrum, while the pure ZnO and ZnO/MWNTnanocomposites display sharp characteristic absorption peaksat ca. 370 nm, indicating the existence of good crystalline andimpurity-suppressed ZnO NWs [5,22]. Compared with pure

Fig. 3. (a) UVvis absorption spectra of the MB solution during the photod(b) Decomposition rate of MB solution with no catalysts, pure ZnO and ZnO/MWTs, pure ZnO and the ZnO/MWNT nanocomposites.

ional 40 (2014) 56295633 5631ZnO, the ZnO/MWNT nanocomposites exhibit an absorptionspectrum with smaller slope in the visible zone, which couldbe attributed to the carbon doping effect [23].To investigate the photocatalytic property of the as-

synthesized ZnO/MWNT composites, the photocatalytic degra-dations of MB solution (10 mg/L) have been tested under365 nm using pure ZnO and ZnO/MWNT nanocomposites.Fig. 3 shows typical absorption spectra of the MB solution andthe degradation rates with pure ZnO, ZnO/MWNT nanocompo-sites and no catalysts. From Fig. 4a, it can be clearly seen thatthe absorption peaks of the MB solution diminished gradually asthe exposure time increased. Above 82% MB can be decom-posed within 15 min using the nanocomposites. After UVirradiation for 30 min, the relative intensity of the absorptionpeaks disappears completely and the MB solution becomescolorless, as illustrated by optical photograph (inset of Fig. 3a).We have also measured the photodegradation rates of the MBsolution with pure ZnO NWs, ZnO/MWNT nanocompositesand no catalysts, as shown in Fig. 3b. It can be seen clearly thatthe degradation time using pure ZnO is 3 times longer than thatof ZnO/MWNT nanocomposites, conrming remarkablyenhanced photocatalytic efciency of the nanocomposites, thisresult is also much better than previous report [13].

egradation. Inset is optical images before and after the photodegradation.NT nanocomposites.

photo-induced electrons (e ) will transit from valence band(VB) to conduction band (CB) and leave positive holes (h ) in

atiosts,

rnatFig. 4. (a) UVvis absorption spectra of Rh6G solution during the photodegradthe photodegradation. (b) Decomposition rate of Rh6G solution with no cataly

P. Liu et al. / Ceramics Inte5632To further characterize the photocatalytic property of theas-synthesized ZnO/MWNT nanocomposites, the photocataly-tic degradations of Rh6G solution (10 mg/L) have also beentested under UV irradiation. After UV irradiation for 35 min,the absorption peaks of Rh6G disappear completely and theRh6G solution become colorless, as illustrated by opticalphotograph (inset of Fig. 4a). Fig. 4b shows the photodegrada-tion rates of Rh6G solution with pure ZnO NWs, ZnO/MWNTnanocomposites and no catalysts. It is observed that thephotodegradation rate using pure ZnO is much lower thanthat of ZnO/MWNT nanocomposites, which further conrmsthe enhanced photocatalytic efciency.Since the ZnO NW/MWNT nanocomposites exhibit much

higher photocatalytic activities than pure ZnO, the incorpora-tion of MWNTs plays an important role in enhancing thephotocatalytic efciency. According to the photocatalyticmechanism based on the excitation of semiconductor [24],Fig. 5 shows the enhanced photocatalytic degradation mechan-ism of the ZnO NW/MWNT nanocomposites, which includesthe generation of reactive oxygen species (ROSs) by excitingsemiconductor using UV irradiation and the oxidation of dyemolecules by these ROSs [24]. Under UV irradiation, the

Fig. 5. Photocatalytic mechanism of the ZnO/MWNT nanocomposites.VB, forming the electronhole pairs in ZnO NWs. Consideringthe potential of the conduction band (4.05 eV) and thevalence band (7.25 eV) of ZnO [22] and MWNTs (4.55.0 eV) [25,26], direct electron transfer from ZnO NWs toMWNT surface is thermodynamically favorable, which willresult in low recombination rate of the photo-induced e/h

pairs [17,27]. These photo-generated electrons on MWNTsurface react easily with the dissolved oxygen (O2) adsorbedon nanocomposite surface to form superoxide radical (dO2

),and the hydroxyl ions (OH) will be oxidized into hydroxylradicals (dOH) by photo-induced holes. The continuousgeneration of these ROSs results in the degradation processthat the dye molecules decomposed into simple organics andfurther converted into CO2 and H2O. Therefore, the enhancedphotocatalytic degradation of the ZnO NW/MWNT nanocom-posites should be attributed to the electron transfer betweenZnO NWs and MWNTs under UV irradiation.

n by the ZnO/MWNT nanocomposites. Inset is optical images before and afterpure ZnO and the ZnO/MWNT nanocomposites.ional 40 (2014) 562956334. Conclusions

We have demonstrated a facile and one-step hydrothermalapproach to synthesize the ZnO NW/MWNT nanocomposites.Their photocatalytic performances on MB and Rh6G havebeen investigated under UV irradiation. Our results suggestthat the photocatalytic efciency of the as-synthesized ZnONW/MWNT nanocomposites is 3 times higher than that ofpure ZnO NWs. This enhanced photocatalytic activity can beattributed to the fact that the recombination process of photo-induced electronhole pairs is suppressed due to the fasttransfer of photo-induced electrons from ZnO to MWNTs.

Acknowledgments

The authors thank the Foundation of Henan EducationalCommittee (No. 2011A470015) and National Natural ScienceFunds of China (No. 51177054).

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Enhanced photocatalytic performance of ZnO/multi-walled carbon nanotube nanocomposites for dye degradationIntroductionExperimentalResults and discussionConclusionsAcknowledgmentsReferences