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Chemical Engineering Journal 191 (2012) 571– 578
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
j ourna l ho mepage: www.elsev ier .com/ locate /ce j
arge scale preparing carbon nanotube/zinc oxide hybrid and its application forighly reusable photocatalyst
ai Daia,∗, Graham Dawsonb,∗, Song Yanga, Zheng Chenb, Luhua Lub
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, PR ChinaSuzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, PR China
r t i c l e i n f o
rticle history:eceived 13 November 2011eceived in revised form 2 March 2012ccepted 2 March 2012
eywords:
a b s t r a c t
Multi-walled carbon nanotube (MWCNT)/zinc oxide (ZnO) hybrid were large scale synthesized by thereflux method in ethylene glycol with the aid of polyvinylpolypyrrolidone (PVP). The MWCNT/ZnO hybridwas characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy spec-tra (EDS), X-ray photoelectron spectra (XPS), X-ray diffraction (XRD) and high-resolution transmissionelectron microscopy (HRTEM), which have indicated uniform hybrid structure of 18 nm diameter ZnO
hotocatalystarbon nanotube/ZnOreparationharacterizationctivity
coating on MWCNT surface. The photocatalytic performance of ZnO nanoparticles and the MWCNT/ZnOhybrid were investigated by removal of methylene blue. Consistent with the shift of its UV–vis dif-fuse reflectance spectroscopy (DRS) to shorter wavelengths, the heterostructure of MWCNT/ZnO hybridinduces an improvement in photocatalytic performance. The stability of the hybrid was character-ized through cyclic photocatalytic test. Results indicated no observable performance degradation forMWCNT/ZnO photocatalyst even after ten recycles.
. Introduction
The integration of quasi-one dimensional nanotubes with zeroimensional nanoparticles has received increasing attention due toheir photonic, electrochemical, electromagnetic interfacial prop-rties and structure stability, which are not available to theespective components alone [1]. The high aspect ratio carbonanotubes (CNTs), which are of promising electrical, electrochemi-al and interfacial chemistry features [2–6], favors the constructionf functional heterostructure materials for high efficiency energyransformation in optoelectronics, catalysis, sensing, and superca-acitors [7–11]. Variety of CNT-nanocrystal heterostructures andheir synthesis methods have thus been widely developed in recentears [12,13]. Cooperated with metals or their oxides, multi-walledarbon nanotubes (MWCNTs) can also serve as additional photo-ensitizers in photocatalysts [14,15].
Of all inorganic compounds for MWCNT/metal oxide het-rostructure construction, zinc oxide (ZnO), a direct wide bandap (3.37 eV) semiconductor with a large excitation binding energy
60 meV), has been earlier investigated as a potential photo-atalyst derived from their high specific surface areas, uniqueation exchange ability and non-toxicity [16–20]. More recently,∗ Corresponding authors. Fax: +86 561 3803256.E-mail addresses: [email protected] (K. Dai), [email protected]
G. Dawson).
385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.03.008
© 2012 Elsevier B.V. All rights reserved.
nano-ZnO has received much attention due to high photoactivity inseveral photochemical, UV light response, photoelectron-chemicalprocesses and its low cost production possibility [21,22]. As a newchoice for eliminating environmental pollutants, nano-ZnO medi-ated photocatalysis process has been used to successfully degradeorganic pollutants recently [23–25].
To obtain CNT/metal oxide heterostructure, a mild synthesismethod is of great importance for their obvious advantage in highquality and large scale production for practical applications, suchas electrochemical techniques, sol–gel process, hydrothermal andaerosol techniques, gas-phase deposition [26–29]. Different strate-gies for mild synthesis approach have therefore been put forwardto fabricate MWCNT/inorganic oxide hybrid, which fall into twobasic classes. One main approach involves the prior synthesis ofnanoparticles that subsequently connected to surface function-alized MWCNTs by either covalent or noncovalent interactions[30–32]. The second approach is the one step method involvingdirect deposition of nanoparticles onto MWCNT surface, in situ for-mation of nanoparticles through redox reactions or electrochemicaldeposition on MWCNTs [33–37]. The latter one is believed to bepromising for easily preparing uniform heterostructure nanoma-terials. However, lack of active sites on pristine MWCNT surfaceshas hindered the achievement of the desired advantages. To over-
come this hindrance, supramolecular self assembly of surfactantson MWCNT surface has been employed to obtain active function-alized surfaces of MWCNTs for the formation of uniform hybridstructures and their excellent chemical performances have been
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nitially shown [30,38]. But the achievement of this hybrid struc-ure requires accurate control of the experimental condition, whichs unfavorable for their large scale production.
In this work, we proposed a hard template technique to prepareniform MWCNT/ZnO hybrid structures as a promising method forasy and large scale synthesis of nanomaterials. The large scaleniform heterostructure of MWCNT/ZnO hybrid was confirmed.
mportantly, the MWCNT/ZnO hybrid shows a strong structure-nduced enhancement of photocatalytic performance and exhibits
significantly improved photocatalytic property and promisingong-term cyclic stability in the photodegradation of methylenelue (MB) than that of nanostructured ZnO. This work not onlyives insight into understanding the method of preparation ofniform MWCNT/ZnO in a solution-phase synthetic system, butlso provides a new way to improve the photocatalytic perfor-ance by designing the reusable MWCNT/ZnO hybrid. Such aWCNT-metal oxide hybrids-induced enhancement of photocat-
lytic performance could also be applicable to other photocatalysts.
. Experimental
.1. Treatment of MWCNTs
MWCNTs were purchased from Shenzhen Nano Tech Port CO.,TD. The samples were 40–60 nm in diameter. The raw materialsere refluxed in 68 wt% HNO3 at 180 ◦C for 12 h, and then the mix-
ure was washed several times with double distilled water on aintered glass filter. Finally, treated MWCNTs were obtained aftereing dried in an oven at 100 ◦C for 5 h.
.2. Preparation of MWCNTs/ZnO hybrid and ZnO nanoparticles
Analytical pure 0.05 mol/L Zn(NO3)2·6H2O, 0.6 golyvinylpolypyrrolidone and 1 g treated MWCNTs were added
n 150 mL ethylene glycol, and then ultrasonicated at 30 ◦C for0 min to obtain a uniform mixture. The mixture was then refluxedt 197 ◦C for 24 h. After filtering with double distilled waternd drying at 80 ◦C for 6 h, the hollow structural MWCNTs/ZnOybrid was obtained. ZnO nanoparticles were also prepared byhe above-mentioned method without MWCNTs. All experimentsere carried out under normal atmospheric environments.
.3. Analytical and testing instruments
The structure of the MWCNTs was observed on a JEM 200CXransmission electron microscope (TEM). The structure of the
WCNT/ZnO was observed on a Tecnai G2 F20 S-Twin High-esolution transmission electron microscopy (HRTEM). Scanninglectron microscopy (SEM) of samples was performed using JSM-700F with Inca Energy-dispersive X-ray spectroscopy (EDS). Allltrasound operations were performed in a KQ-100DV ultrasoni-ator with the frequency of 40 kHz. X-ray diffraction (XRD) dataor samples were collected using a Rigaku D/MAX 24000 diffrac-ometer with Cu K� radiation. The Brunauer–Emmett–Teller (BET)pecific surface area values were determined by using nitrogendsorption data at 77 K obtained by a Micromeritics ASAP 2010ystem with multipoint BET method. The X-ray photoelectronpectra (XPS) of the MWCNT/ZnO were measured using a KratosXIS Ultra DLD X-ray photoelectron spectrometer. UV–vis diffuseeflectance spectroscopy (DRS) measurements were carried out
sing a Hitachi UV-3600 UV–vis spectrophotometer equipped withn integrating sphere attachment. The analysis range was from00 to 600 nm, and BaSO4 was used as reflectance standard. Gashromatography–mass spectrometry (GC–MS) was performed onJournal 191 (2012) 571– 578
an Agilent Technologies 7890A GC system equipped with a 5975Cinert MSD with triple axis detector.
2.4. Photocatalytic experiment
The photocatalytic activity of ZnO/MWCNTs and ZnO was evalu-ated by the photocatalytic degradation of MB under 250 W 365 nmHg lamp irradiation. The photocatalytic experiments were carriedout in a reactor containing the 200 mL 10 mg/L aqueous solutionof MB and 0.2 g catalysts. The distance between the lamp and thereactor was 1 cm. Before irradiation, the suspension was magneti-cally stirred in the dark for 1 h to establish an adsorption/desorptionequilibrium under ambient conditions. Then, the mixture wasexposed to the UV light irradiation. At the given irradiation time,the concentration of MB was quantified by the absorbance, and theabsorbance wavelength used in the spectrophotometer analysis todetermine MB concentration was about 664 nm. The degradationefficiency was calculated as follows:
� = C0 − C
C0× 100%
where C0 is the absorbance of original MB solution and C is theabsorbance of the MB solution after UV light irradiation.
According to the Langmuir–Hinshelwood kinetics model, thephotocatalytic process of MB can be expressed as the followingapparent pseudo-first-order kinetics equation:
lnC0
C= kappt
where kapp is the apparent pseudo-first-order rate constant, C0 isoriginal MB concentration and C is MB concentration in aqueoussolution at time t.
3. Results and discussion
SEM images in Fig. 1a and b show the morphology of MWCNTsamples before and after acid treatment. The raw materials wereproduced via the chemical vapor deposition using Ni as catalyst,and Ni catalyst particles cannot be observed after acid treatment.This is shown by the EDS spectra in Fig. 1c and d which relate to thesamples in Fig. 1a and b, respectively. The observation of oxygenin the acid treated MWCNTs indicates the successful oxidation ofthe MWCNT surface. The opened tips of MWCNTs can be clearlyobserved in the TEM image in Fig. 1e.
The SEM image in Fig. 2a shows the overall morphology for ZnO-coated MWCNTs. All of the MWCNTs are covered with a dense layerof ZnO nanoparticles and no free nanoparticles were found. TheZnO nanoparticles deposited on the surface of MWCNTs are sep-arated and their sizes are uniform. The HRTEM image in Fig. 2bshows the clear view of the sidewalls of the MWCNTs and the lat-tice fringes in ZnO particles which corresponds to the [0 0 2] and[1 0 1] planes of reflection. The morphologies of ZnO particles can beclearly observed and most particles are near-spherical in shape anddispersed tightly on the surface of MWCNTs. The binding betweenZnO and the MWCNTs surface is tight enough to resist repeatedrinsing and ultrasonication. The size of the ZnO nanoparticles onthe surface of MWCNTs is about 18 nm. EDS in Fig. 2c shows thepresence of Zn and O on MWCNTs. A comparative SEM image forindependently synthesized ZnO nanoparticles in the same condi-tion has also been shown in Fig. 2d. We found that the size of solelysynthesized ZnO particles is between 50 and 80 nm, larger than
18 nm for ZnO particles that deposited on MWCNT surface.XPS spectra of C1s, Zn2p and O1s for the MWCNT/ZnO to probethe chemical environment of the elements in the near surface rangeare shown in Fig. 3. Fig. 3a shows the high-resolution spectra of
K. Dai et al. / Chemical Engineering Journal 191 (2012) 571– 578 573
of (c)
CidTopabi1FMcTlpcoaa[
Fig. 1. SEM images of (a) untreated and (b) treated MWCNTs, EDS spectrum
1s in which the C1s peaks can be fitted as four peaks at bind-ng energies of 284.22, 285.01, 286.13 and 288.54 eV, implying fourifferent chemical environments of carbon existing in the sample.he peaks at 284.22 and 285.01 eV were assigned to contributionsf C C (sp2) and C C (sp3) bonds, respectively [39,40]. While theeak at 286.13 eV was ascribed to the existence of C OH bonds,nd the peak at 288.54 eV was ascribed to the existence of C OOHonds [41,42]. As indicated in Fig. 3b, it is observed that the bind-
ng energies of Zn 2p3/2 and 2p1/2 are centered at 1021.31 and044.36 eV, respectively, in agreement with those of pure ZnO.ig. 3a shows the high-resolution spectra of O 1s. In the case ofWCNT/ZnO hybrid, the curve fitting of O 1s spectra basically indi-
ates three components centered at 530.80, 532.81 and 534.62 eV.he peak at 530.80 eV is due to oxygen in the ZnO crystal lattice. Theatter two peaks are commonly ascribed to the surface oxygen com-lexes of carbon phase [43,44]. This clearly indicates that all the zincations in the hybrid material are in the oxidized state. Hydroxyl
xygen on the MWCNT/ZnO will lead to an increase of photocat-lytic activity because the surface hydroxyl is an active speciesnd plays an important role in semiconductor photocatalysis45,46].untreated and (d) treated MWCNTs and TEM image of (e) treated MWCNTs.
Fig. 4 shows the XRD pattern of the as-prepared MWCNT/ZnOhybrid and ZnO nanoparticles. All the peaks of hybrid and ZnO weredistinguishable. The diffraction angles at 2� = 31.68◦, 34.42◦, 36.26◦,47.32◦, 56.56◦, 62.86◦, 66.62◦ and 68.80◦, can be assigned to (1 0 0),(0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) crystal planesof pure ZnO with hexagonal wurtzite and the lines match well withthe normal value reported by JCPDS (No. 36-1451), respectively.The broad peak at 26.36◦ is attributed to the characteristic peakof MWCNTs [47]. From the XRD pattern, it can be concluded thatthe obtained MWCNT/ZnO are composed of hexagonal ZnO andMWCNTs. Average crystalline size can be calculated with Scherrer’sformula:
d = K�
cos �
where d is the average size of the particles, K = 0.9, � is the wave-
length of X-ray radiation, is the full width at half maximum of thediffracted peak, and � is the angle of diffraction. The average ZnOnanoparticle size on MWCNT is 18.7 nm. The result accorded withHRTEM and SEM.
574 K. Dai et al. / Chemical Engineering Journal 191 (2012) 571– 578
Fig. 2. SEM image of (a) MWCNT/ZnO, HRTEM image of (c) MWCNT/ZnO, EDS spectrum (c) of selected area of (a), and SEM image of (d) ZnO nanopaticles.
280285290
284.8(a)C1s
Inte
nsi
ty (
arb
. u
nit
s)
Binding energy (eV)
10501045104010351030102510201015
(b)Zn2p
Inte
nsi
ty (
arb
. u
nit
s)
Binding energy (eV)
526528530532534536538540
(c)O1s
Inte
nsi
ty (
arb
. u
nit
s)
Binding energy (eV)
Fig. 3. XPS spectra measured at the surface of MWCNT/ZnO.
K. Dai et al. / Chemical Engineering Journal 191 (2012) 571– 578 575
70656055504540353025
Untreated MWCNT
ZnO
MWCNT/ZnO
(00
2)
Inte
ns
ity
(a
,u)
(10
2)
(20
1)
(11
2)
(20
0)
(10
3)
(11
0)
(10
1)
(00
2)(1
00
)
ZnO
MWCNT
Treated MWCNT
MsreooMntbet
ZoT
f[eit
8007006005004003002001000
0
10
20
30
40
50
60
70
MWCNT/ZnO
ZnO
Vo
lum
e a
dso
rbed
(S
TP
)(cm
3/g
)
Relative pressure(P/P0)
Fig. 6. Isotherms for Nitrogen adsorption–desorption.
Table 1Data of BET surface area, pore specific volume of samples.
Sample BET (m2/g) Pore specificvolume (cm3/g)
2Theta(deg.)
Fig. 4. XRD pattern of samples.
The DRS spectra of ZnO particles and the as-preparedWCNT/ZnO are shown in Fig. 5. As indicated in Fig. 5, pure ZnO
hows an absorption edge at 375 nm corresponding to the indi-ect band gap (3.31 eV). However, the absorption edge of MWCNTsnrobed with ZnO is dominated by the characteristic absorptionf ZnO. An interesting feature for the hybrid is a slight blue shiftf band edge absorption with respect to ZnO particles withoutWCNTs. This is due to the size confinement effect, with the
anopatricles attached to the MCWNT’s having a smaller diame-er than the free ZnO nanoparticles. The attached ZnO with widerand-gap (3.38 eV) would improve the redox abilities of photogen-rated electron–hole pairs to enhance the photocatalytic activity ofhe hybrid [48].
Fig. 6 shows the nitrogen adsorption–desorption isotherms fornO particles and the as-prepared MWCNT/ZnO at 77 K. The dataf BET surface area, pore specific volume of samples were listed inable 1.
The photocatalytic process mainly takes place on the sur-ace of catalysts and involves comprehensive competing reactions
49]. The widely accepted photodegradation mechanism for het-rostructure hybrid has been shown in Fig. 7. When the catalysts irradiated with a photon of sufficient energy, equal or largerhan band gap, MWCNT may absorb the irradiation and inject the450425400375350325
ZnO
MWCNT/ZnO
Ab
so
rba
nc
e
Wavelength (nm)
Fig. 5. DRS spectra of ZnO and MWCNT/ZnO.
ZnO 22 0.034ZnO/MWCNT 45 0.123
photo-induced electron into ZnO conduction band, which can trig-ger the formation of reactive radicals, superoxide radical ion •O2−
and hydroxyl radical •OH, both responsible for the degradationof the organic compound. Furthermore, inhibiting the undesirableelectron–hole pair recombination is important to enhance the pho-tocatalytic activity. Upon UV illumination, the electron–hole pairsare generated and the electrons reach to the conduction band ofZnO and then transfer to the MWCNTs. Since MWCNTs behaveas conducting wire, there is no possibility of accumulation of theelectrons on MWCNT side [50]. This enhances the electron–holeseparation and helps the faster growth of the photocurrent in thehybrid structure as the space charge effect in ZnO side becomesnegligible. Accordingly, it is more reasonable to ascribe this synergyeffect to MWCNT as a photosensitizer [51].
The photocatalytic removal of MB from model aqueous solutionswas investigated using 365 nm UV light irradiation sources. Fig. 8shows the adsorption and photodegradation progress of MB by ZnO,MWCNT/ZnO, Degussa P25 and without photocatalyst. Fig. 9 showsGC–MS spectra of photodegradation intermediates by ZnO andMWCNT/ZnO. The removal of MB over 98% was achieved in 30 minfor MWCNT/ZnO under UV light. With a complete color change of
MB solution from deep blue to colorless observed, the photodegra-dation intermediates of MB in our experiment were identified asdemethylated MB and traces of smaller degradation fragments.However, ZnO nanoparticles can only reach 68% of removal withinFig. 7. Mechanism of photodegradation.
576 K. Dai et al. / Chemical Engineering Journal 191 (2012) 571– 578
3020100-10-20-30-40-50-60
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Adsorption Degradation
Time (min)
C/C
0
without photocatalyst
MWCNT/ZnO
ZnO
P25
Fig. 8. Adsorption and photodegradation progress of MB with ZnO, MWCNT/ZnO,P25 and without photocatalyst.
Table 2Apparent rate constant values for the degradation of MB.
Sample MWCNT/ZnO ZnO P25
tbaBpTbZaFM
airpZtSiTc
Fig. 10. Demethylation of MB dye during photocatalytic degradation process.
kapp (min−1) 0.1517 0.0637 0.0349
he same reaction time, the color of the solution changed to lightlue, and the degradation product are the mixture of unreacted MBnd the same traces observed from the MWCNT/ZnO experiment.ased on the intermediate products found in this work, a possiblehotocatalytic degradation pathway for MB is proposed in Fig. 10.his pathway is similar, but not identical, to the literature proposedy TiO2 as the photocatalyst [52]. The introduction of MWCNT intonO apparently induces a synergy effect on MB removal. The vari-tions in ln(C0/C) as a function of irradiation times are given inig. 11. The calculated apparent rate constant kapp values for ZnO,WCNT/ZnO and P25 in UV light are given in Table 2.Stability of photocatalyst is crucially important for practical
pplications. One of the major drawbacks of ZnO photocatalystss their severe photocorrosion under UV irradiation, which canesult in significant decrease of the photocatalytic activity in reusedrocess. Some studies indicate that the photocatalytic activity ofnO could be improved by tuning their texture and microstruc-ure, but the recycled performance is still not satisfactory [53].ilver, polyaniline and C have also been found to be effective
60n improving the stability of ZnO under light irradiation [54,55].o evaluate the stability of MWCNT/ZnO, we carried out the recy-le experiment under identical conditions. The irradiation time forFig. 11. Kinetics of the photocatalytic degradation of MB with MWCNT/ZnO, ZnOand P25.
Fig. 9. GC–MS spectra of photodegradation intermediates by (a) ZnO and (b) MWCNT/ZnO.
K. Dai et al. / Chemical Engineering
10987654321
0
20
40
60
80
100
120
η(%
)
Cycle (times)
MWCNT/ZnO
ZnO
FM
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ig. 12. Comparison of photodegradation performance within ten cycles forWCNT/ZnO and ZnO.
ach test is 30 min. As shown in Fig. 12, the photocatalytic effi-iency of MWCNT/ZnO has no observable change even after tenecycles. The results indicated that MWCNT/ZnO photocatalyst has
good reusable performance. By comparison, the performance ofnO nanoparticles was also investigated. As indicated in Fig. 12, theegradation efficiency of MB is about 68% in the first cycle for ZnOanoparticle catalyst, and less than 40% after the tenth cycle. Theecrease in the photocatalytic efficiency of ZnO particles should beelated to the adsorption of organic molecules on the catalyst sur-ace. In addition, MWCNT/ZnO is prepared with high crystallinity,hich also contributes to the high durability and activity of theaterial [56,57]. Moreover, the MWCNT/ZnO could be easily col-
ected by low speed centrifugation or simple filtration over a shortime, while ZnO nanoparticles have to be collected by long timeigh speed centrifugation. This is beneficial for the separation andeuse of catalysts. It is obvious that MWCNT/ZnO hybrid is a suit-ble photocatalyst due to its high activity and excellent recyclederformance accompanied by easy separation from the reactionystem.
. Conclusions
Uniform MWCNT/ZnO hybrid was synthesized by an easy andarge scale reflux method. The morphology and structure werearefully investigated through SEM, XPS, HRTEM, EDS and XRD.WCNT/ZnO hybrid shows a slight blue shift compared to pure
nO. The photocatalytic performance of samples was investigated;WCNT/ZnO displayed the highest photocatalytic activity with
app of 0.1517 min−1 under UV light and showed an attractive phto-atalytic performance of high reuse ability. This could be attributedo the synthetic effect of larger surface area, smaller crystalline size,nd the formation of the chemical bond between ZnO and carbonanotube, which improve interface contact. Considering the easef separation, recycling, mass production and high reuse ability ofWCNT/ZnO hybrid, it should be a promising candidate for water
leaning applications.
cknowledgments
This work was supported by the National Natural Science Foun-ation of China (20803051), the key Foundation of Educationalommission of Anhui Province (KJ2012A250), the Youth Founda-ion of Huaibei Normal University (700432), the Huaibei Science
[
[
Journal 191 (2012) 571– 578 577
and Technology Development Funds (20110305) and the NaturalScience Foundation of Jiangsu Province (BK2010258).
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