-
Available online at www.sciencedirect.com
Ceramics International 40 (20
oor
, F
of Tg U
form8 N
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:
[email protected] (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
www.elsevier.com/locate/ceramint
-
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