Ultrasonics Sonochemistry 20 (2013) 478–484

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Ultrasonics Sonochemistry

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Characterization and relative sonocatalytic efficiencies of a new MWCNTand CdS modified TiO2 catalysts and their applicationin the sonocatalytic degradation of rhodamine B

Lei Zhu, Ze-Da Meng, Chong-Yeon Park, Trisha Ghosh, Won-Chun Oh ⇑Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356-706, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 December 2010Received in revised form 4 March 2012Accepted 1 August 2012Available online 11 August 2012

Keywords:CdSMWCNTTiO2

Ultrasound irradiationSonocatalytic activityRhodamine B

1350-4177/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.ultsonch.2012.08.005

⇑ Corresponding author. Tel.: +82 41 660 1337; faxE-mail address: wc_oh@hanseo.ac.kr (W.-C. Oh).

TiO2 nanoparticles modified with MWCNTs and CdS were synthesized by the sol–gel method followed bysolvothermal treatment at low temperature. The chemical composition and surface structure of the CdS/CNT–TiO2 composites were investigated by X-ray diffraction, specific surface area measurements, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and scanning electron microscopy.Then a series of sonocatalytic degradation experiments were carried out under ultrasonic irradiation inthe presence of CNT/TiO2 and the CdS/CNT–TiO2 composites. It was found that RhB was quickly and effec-tively degraded under different ultrasonic conditions. As expected, the nanosized CdS/CNT–TiO2 photo-catalyst showed enhanced activity compared with the non CdS treated CNT/TiO2 material in thesonocatalytic degradation of RhB. The sonocatalyst CCTb with 34.68% contents of Ti heat treated at500 �C for 1 h showed the highest sonocatalytic activity. The synergistic effect of the greater surface areaand catalytic activities of the composite catalysts was examined in terms of their strong adsorption abil-ity and interphase interaction by comparing the effects of different amounts of MWCNTs and CdS in thecatalysts and their roles. The mechanism of sonocatalytic degradation over the CdS/CNT modified TiO2

composites under different ultrasonic conditions was also discussed.� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Colored wastewater is released in textile effluents and containspotentially environmentally hazardous components. Within theecosystem, this colored wastewater is a dramatic source of pollu-tion, eutrophication, and perturbations in aquatic life. Moreover,a variety of organic chemicals are produced during the dyeing pro-cess and some have been shown to be carcinogenic [1]. With thegrowing awareness of the decreasing amount of available water re-sources, many methods, including physical, chemical, and biologi-cal methods, are being used in wastewater treatment andrecycling. Among them, there are two well-known methods thathave been reported. One is the use of a TiO2 photocatalyst, whichis the most widely used because of its good activity, chemical sta-bility, commercial availability, and inexpensiveness. TiO2 has threemain crystal structures: anatase, which tends to be more stable atlow temperature; brookite, which is usually found in minerals andhas an orthorhombic crystal structure; and rutile, which is the sta-ble form at higher temperature. Anatase has higher photocatalyticactivity and has been studied more than the other two forms ofTiO2 [2]. When irradiated with ultraviolet light with a wavelength

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shorter than 380 nm (the band-gap energy of TiO2 is 3.2 eV), theactivated TiO2 generates �OH radicals which oxidize organic com-pounds in water.

The other method relies on high-amplitude ultrasound to irradi-ate polluted water [3,4]. A powerful acoustic field produces manythousands of bubbles in water, which results in repeated growthand collapse cycles every acoustic cycle. The temperature withinthe bubbles is so high that the solute substances vaporized insidethem are immediately decomposed. The temperature near the bub-bles is still high and the resulting thermal dissociation of water re-sults in the formation of hydroxyl radicals �OH which oxidize thesubstances near them and make them innocuous [3]. On the otherhand, research in this field has led to some new properties beingdiscovered. According to the theoretical and experimental investi-gations, sonochemistry is related to sonoluminescence, becauseboth of them originate from the high temperature condition insidethe collapsing bubbles [5]. The spectrum of sonoluminescence has afairly wide range of wavelengths in a liquid and a high intensity ofUV emission [6–8]. Accordingly, sonoluminescence can be a practi-cable method of degrading organic dyes by adding a photocatalyst,because the intense UV flashes of light caused by sonoluminescencecan activate TiO2 effectively. Moreover, some researchers havebegun to use nano-sized TiO2 powder and other sonocatalysts todegrade organic pollutants under ultrasonic irradiation [9–13].

L. Zhu et al. / Ultrasonics Sonochemistry 20 (2013) 478–484 479

However, the use of TiO2 as a photocatalyst is limited mainly bythe recombination of the generated photo-holes and photo-elec-trons. Photocatalysis involves the oxidation of a chemical by photoholes from the semiconductor, so every recombination event in-volves the loss of holes that might otherwise have promoted deg-radation. Therefore, the Victoria transfer of photogeneratedelectrons and holes between the valence and conduction bandsof semiconductors is important to photocatalysis. Various methodshave been reported to improve the photocatalytic efficiency by themodification of the surface or bulk properties, i.e. the doping, code-position of metals, surface chelating, mixing of two semiconduc-tors, coating of an insulating oxide layer, etc. [14–18]. In ourprevious works [19,20], the photocatalytic efficiency of TiO2 wasincreased by using multi-walled carbon nanotubes (MWCNTs) toprepare MWCNT/TiO2 composites. MWCNTs are 1D carbon-basedideal molecules with a nano cylindrical structure, which can con-duct electricity at room temperature with essentially no resistance.This phenomenon is known as ballistic transport [21,22] by whichthe electrons are considered to move freely through the structure,without any scattering from atoms or defects. While the electronsformed by UV irradiation migrate to the surface of the MWCNTs,they are easily transported into the conduction band (CB) of TiO2

which is bound with them. Hence, the increased amount of gener-ated photo-electrons can decrease the high rate of electron/holepair recombination, which otherwise reduces the quantum yieldof the TiO2 process.

CdS is one of the most important II–IV group semiconductors,which are extensively applied in optical and electronic fields, suchas biological labeling, light-emitting diodes, photoelectric conver-sion devices, solar cells, photocatalysis and environmental sensors[23–30].

CdS constitutes a very desirable window layer for many photo-voltaic solar cells, because of its optical and electrical properties[31–33]. CdS has been the subject of intensive research, becauseof its band gap, high absorption coefficient, reasonable conversionefficiency, stability and low cost [34]. In recent years, the doping ofCdS nanostructures has attracted intensive attention. It was dis-covered that, by controlling the particle size and microstructureof CdS or by combining it with some other layer compound or solidporous material, such as titanate nanotubes [35] mesoporous silica[36] and titanosilicate zeolite [37], the preparation and character-ization of layered metal oxides could be achieved [38–40] and [41]some novel physical and chemical properties of CdS were discov-ered. The transportation of photogenerated carriers between theenergy band of CdS and TiO2NTs could prevent the recombinationof the charges and improve the photocatalysis activity. Xiao et al.reported [35] that CdS nanoparticles decorated with titanate nano-tubes were effective for the degradation of RhB, suggesting thatcombining CdS particles and TiO2NTs might result in synergetic ef-fects in the photocatalytic reaction.

However, as alternative powder photocatalysts, CdS/TiO2

nanoparticles have some disadvantages, such as difficult recov-ery, easy cohesion and a low utilization rate in practical applica-tions. Some alternative methods are immobilizing nanoparticlesonto an inert and porous supporting matrix; through the accu-mulation of carriers, the adsorption mass transfer rate and effi-ciency of photocatalytic degradation are effectively improved[42–45].

In this report, we combined the advantages of introducingMWCNTs and CdS to design an effective catalyst. To improve thecatalysis activity of TiO2, CdS/CNT as a doped composite was pre-pared by the sol–gel process. The prepared catalysts were charac-terized by BET, XRD, EDX, SEM and TEM techniques. Thesecatalysts were irradiated with ultrasonic waves with different irra-diation intensities and their catalytic activity was compared withthat of CdS/CNT–TiO2 particles.

2. Experimental

2.1. Materials and reagents

The titanium n-butoxide (99%) used as the titanium alkoxideprecursor to form TiO2 was purchased from Acros Organics, NewJersey, USA. Crystalline MWCNT (95.9%) powder with a diameterof 20 nm and length of 5 lm was purchased from Carbon Nano-material Technology Co., Ltd., Korea. For the oxidization of thesurface of the MWCNTs, m-chlorperbenzoic acid, used as anoxidized reagent, was also purchased from Acros Organics, NewJersey, USA.

Benzene (99.5%), used as a solvent, was purchased from Samc-hun Pure Chemical Co., Ltd., Korea. Cadmium chloride�1-hydrate(CdCl2�H2O) and sodium sulfide�5-hydrate (Na2S�5H2O) purchasedfrom Junsei Chemical Co., Ltd., Japan and Yakuri Pure ChemicalsCo., Ltd., Japan, respectively, were used as cadmium and sulfur pre-cursors. The Rhodamine B used was of analytical grade and waspurchased from Samchun Pure Chemical Co., Ltd., Korea.

2.2. Preparation of CdS/CNT and CdS/CNT–TiO2 composite sonocatalyst

2.2.1. Synthesis of CNT-supported CdS compositesBecause the MWCNTs are very stable, they need to be treated

with strong acids to introduce active functional groups on theirsurface. In this experiment, 2.0 g MCPBA was suspended in 80 mLof benzene as a solvent. Then 1 g of MWCNTs powder was put intothe solution and the mixture was treated by magnetic stirring for6 h at 353 K. The resultant solution was filtered and continuouslywashed with deionized water and ethanol 5 times. Then, the sam-ple was dried at 393 K and fully milled.

The functionalized MWCNTs were then used to synthesize thenanosized CdS/CNT composite. 2.59 mmol of CdCl2�H2O was dis-solved in 50 mL of deionized water. Then, 0.3 g of oxidizedMWCNTs was added to the solution and stirred further for45 min in order to sufficiently adsorb the Cd2+ ions. A solution ofNa2S (30 mL) was prepared separately and added dropwise to thesolution with constant stirring for 8 h at 353 K. Under these condi-tions, the S2� ions react with Cd2+, resulting in the deposition ofCdS on the surface of the MWCNTs. The mixture was transformedinto a black green color. After completion, the black green solutionwas filtered, washed with deionized water and ethanol 5 times andthen dried at 373 K. Finally, the CdS/CNT composite was obtained.

2.2.2. Synthesis of CdS/CNT–TiO2 compositeBefore being used, two beakers containing 40 mL of benzene

were prepared separately. Three, 4 mL of TNB was then added tothe solution, followed by magnetic stirring for 5 min. 0.4 g of theas-prepared CdS/CNT composites were added to the solution withconstant stirring for 6 h, and the power mixtures of CdS/CNT thatreacted with TNB were dried at 393 K for 12 h. Finally, the sampleswere heated to 773 K for 1 h. These photocatalyst composites werenamed CCTa and CCTb. For comparison, CNT/TiO2 photocatalystwas synthesized using similar procedures except for heating at773 K for 1 h and named CT.

2.3. Characterizations of CdS/CNT–TiO2 composite

The synthesized powders were characterized using a variety oftechniques. The crystal phases of the composite photocatalystswere obtained by X-ray diffraction (XRD, Shimata XD-D1, Japan)at room temperature using CuKa radiation. The Brunauer–Emett–Teller (BET) surface area was determined by N2 adsorption mea-sured at 77 K using a BET specific surface area analyzer (Monosorb,USA). Scanning electron microscopy (SEM, JSM-5200 JOEL, Japan)

Fig. 1. Schematic diagram of the sonocatalytic reactor.

10 20 30 40 50 60 70 800

100

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300

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500

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900

Inte

nsity

(cou

nts)

2 theta(0)

CT

CCTa

CCTb SAAAS

ASA S

S

S

S

AA:Anatase S: CdS

Fig. 2. XRD patterns of CNT/TiO2 and CdS/CNT–TiO2 composites prepared withdifferent contents of TiO2.

480 L. Zhu et al. / Ultrasonics Sonochemistry 20 (2013) 478–484

was used to observe the surface morphology and structure of theCdS/CNT–TiO2 composites treated with TNB. The EDX spectra werealso obtained to determine the elemental composition of the syn-thesized composites. Transmission electron microscopy (TEM,JEOL, JEM-2010, Japan) at an acceleration voltage of 200 kV wasused to examine the size and distribution of the CdS depositedon the MWCNT surface and TiO2 attached to the surface of theCdS/CNT composite. The light absorption of the samples was re-corded using a UV–vis spectrophotometer (Optizen POP, MecasysCo., Ltd., Korea) in the range of 200–750 nm.

2.4. Measurement of sonocatalytic activities

The sonocatalytic activities of the CdS/CNT–TiO2 compositeswere determined by the decomposition of RhB in aqueous solutionwith ultrasonic generators (Ultrasonic Processor VCX 750, Korea)operated at a minimum frequency of 20 kHz and output power of750 W through manual adjusting. The catalysts (0.01 g) were sus-pended in 100 mL of RhB solution with a concentration of1.0 � 10�5 M in a glass vessel. Prior to irradiation, the suspensionswere magnetically stirred in the dark for 30 min to ensure theestablishment of an adsorption/desorption equilibrium amongthe sonocatalyst, Rhodamine B and atmospheric oxygen, whichwas hereafter considered as the initial concentration (c0).

For the process of degradation of RhB, a plastic container (diam-eter = 20 cm, height = 6 cm) filled with ice was used to make a lowtemperature environment at around 25 �C. A schematic diagram ofthe sonocatalytic is shown in Fig. 1. It consists of four parts: anultrasonic generator, constant temperature controller, ice waterbath, and reactor. Sonocatalytic degradation was tested using theCNT–TiO2, CdS/CNT–TiO2 a and CdS/CNT–TiO2 b catalysts withultrasonic generators operated at frequencies of 20, 25, and30 kHz, respectively. The reactions were carried out in an opencylindrical stainless glass vessel. The ultrasonic irradiation of thereactor was done for 30, 60, 90, and 120 min and the removal ofthe dispersed powders through centrifuge. The clean transparentsolution was analyzed using a UV–vis spectrophotometer. Thespectra (550–750 nm) for each sample were recorded and theabsorbance was determined at the characteristic wavelength of554 nm [46] for the degraded RhB solution. As the catalytic prop-erties of CdS/CNT–TiO2 composition absorption and degradationthe red color of the solution faded gradually with time.

3. Results and discussion

3.1. Structural properties by XRD patterns

The XRD technique was used to determine the crystallographicstructure of the inorganic part of the composite. The XRD results ofthe CNT/TiO2 and CdS/CNT–TiO2 composites are compared in Fig. 2.For the CNT/TiO2 composite, after heat treatment at 773 K, six

distinctive TiO2 peaks are found at 25.3�, 37.8�, 48.0�, 53.8�,54.9�, and 62.5� corresponding to the (101), (004), (200), (105),(211), and (204) planes of anatase, respectively, indicating thatthe TiO2 in the prepared CNT/TiO2 composite existed in the anatasephase. However, it is worth noticing that the characteristic peaks ofthe MWCNTs could hardly be identified in any of the patterns ofthe composite catalysts, which was further supported by theobservation via SEM, EDX and TEM elemental microanalysis ofthe CNT/TiO2 composites.

From the XRD results of the CdS/CNT–TiO2 composites, it wasconcluded that the TiO2 also existed in the anatase phase. Mean-while, the additional prominent peaks at angles (2h) of 24.7�,26.5�, 28.3�, 36.6�, 43.8�, 48.1�, 51.8�, and 66.8� were indexed tothe reflections from the (100), (002), (101), (102), (110), (103),(112), and (203) planes of the hexagonal structure of CdS, corre-sponding to the work by Ghows et al. [47].

3.2. SEM and EDX analysis

The SEM images of the CNT/TiO2 and CdS/CNT–TiO2 compositesprepared with different concentrations of TiO2 are shown in Fig. 3.In Fig. 3(a), we can clearly observe that the MWCNTs were homog-enously decorated by well-dispersed particles with only a few TiO2

aggregates. In Fig. 3(b) and (c), the porous structure can be ob-served and the TiO2 particles are uniformly dispersed on the wholesurface of the CdS/CNT composite. Also, we can easily see that withincreasing concentration of TiO2, there were some significant dif-ferences in the particle size distribution in these two images.Fig. 3(c) shows a great particle size distribution of sample CCTb.Very large amounts of CdS grains and TiO2 grains were regularlyadsorbed and coated on the surface of the MWCNTs and this spe-cial structure greatly prevented the CdS grains and TiO2 grainsfrom agglomerating. Thus improving the rate of light adsorptionand enhancing the photocatalytic efficiency of the catalyst.

The EDX microanalyses (wt.%) of the CNT/TiO2 and CdS/CNT–TiO2 composites prepared with different contents of TiO2 areshown in Table 1. It can be clearly seen that the CNT/TiO2 compos-ite contained three main elements, viz. C, O, and Ti. In the CdS/CNT–TiO2 composites, the amount of C element decreased andthe amount of Ti element increased with increasing volume ratioof TNB in the benzene used for the formation of the TiO2 grains.The EDX spectra of the CNT/TiO2 and CdS/CNT–TiO2 compositesare shown in Fig. 4. In the spectra, all of the samples showed thepeaks of O and Ti, though some impurity elements, such as Cl,Zn, and Cu, existed in some of the CCTb samples (which might have

Fig. 3. SEM images of CNT/TiO2 and CdS/CNT–TiO2 composites prepared withdifferent contents of TiO2: (a) CT, (b) CCTa, and (c) CCTb.

Table 1EDX elemental microanalysis (wt.%) of CNT/TiO2 and CdS/CNT–TiO2 compositesprepared with different contents of TiO2.

Samples Elements

C O Ti Cd S Impurity

CT 39.84 25.11 35.05 0 0 0CCTa 29.91 38.41 25.06 6.21 0.41 0CCTb 12.06 42.98 34.68 6.51 0.35 3.42

Fig. 4. TEM images of CNT/TiO2 and CdS/CNT–TiO2 composites prepared withdifferent contents of TiO2: (a) CT, (b) CCTa, and (c) CCTb.

L. Zhu et al. / Ultrasonics Sonochemistry 20 (2013) 478–484 481

been introduced from the experimental procedure). Cd and S ele-ments with the same peak (wavelengths) existed in Fig. 4(b) and(c), so it can be attested that the CdS/CNT–TiO2 composites wereformed.

3.3. BET analysis

The BET surface areas of the CNT/TiO2 and CdS/CNT–TiO2 com-posites prepared with different contents of TiO2 are summarized inTable 2. In comparison with the BET surface area of the pristineMWCNTs, which is 211.43 m2/g, the BET surface areas of theCNT/TiO2 composite decreased greatly to 103.51 m2/g, while sam-ples CCTa and CCTb were 123.53 and 102 m2/g, respectively. Theseresults indicate that there was large change in the micropore sizedistribution for the CNT/TiO2 and CdS/CNT–TiO2 composites com-pared with that of the corresponding MWCNTs. It can be consid-ered that after being treated with TNB, the TiO2 particles filledthe pores of the MWCNTs and, thus, decreased the BET surfacearea.

3.4. TEM analysis

The TEM images of the CNT/TiO2 and CdS/CNT–TiO2 compositesare shown in Fig. 5. The results of the TEM analysis corresponded

Table 2BET surface areas of pristine MWCNT and CNT/TiO2, CdS/CNT–TiO2 composites.

Samples SBET (m2/g)

Pristine MWCNT 211.428CT 103.51CCTa 123.53CCTb 102

Fig. 5. EDX elemental microanalysis of the CNT/TiO2 and CdS/CNT–TiO2 compos-ites: (a) CT, (b) CCTa, and (c) CCTb.

0.00

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0.35

CCTbCCTa CT

Ads

orpt

ion

capa

bilit

ies

of R

h.B

η/1

00(%

)

Fig. 6. Adsorption capabilities of RhB solution for different samples under magneticstirring for 30 min.

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Different ultrasonic intensity30%

30%30%

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20%

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degr

adat

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CT CCTa CCTb

Fig. 7. Sonocatalytic activity of different samples evaluated by the decompositionof RhB solution under different ultrasonic intensities for 120 min.

482 L. Zhu et al. / Ultrasonics Sonochemistry 20 (2013) 478–484

with those of the SEM analysis. From Fig. 5(a), it could be observedthat the TiO2 particles were well dispersed on the wall of theMWCNTs with a few TiO2 particles agglomerated together due tothe formation of big grains. The difference in the particle size dis-tribution between the images in Fig. 5(b) and (c) was clearly ob-served. A few regular black dots were observed, whichcorrespond to CdS particles. The mean size of the CdS nanoparticleswas approximately 8–10 nm, as obtained from the image. In otherwords, the CdS particles with a small size were attached uniformlyto the surface of the CNT tubes. The size of the TiO2 particles wasapproximately 10–20 nm and they were distributed uniformly onthe surface of the CNTs. A generally precipitate-free and smoothinterface was observed among the CdS, TiO2 and the CNT matrix.In addition, with increasing contents of TiO2, there was no appar-ent agglomeration of the CdS /CNT–TiO2 nanoparticles. This sug-gests that the presence of the MWCNTs can efficiently inhibit theagglomeration of CdS–TiO2 and improve the dispersion of thenanoparticles.

3.5. Degradation procedure

3.5.1. Adsorption abilityTo obtain an accurate degradation data of the CNT/TiO2 and

CdS/CNT–TiO2 composites, pure adsorption experiments were

performed under dark conditions and the results are showngraphically in Fig. 6. From the figure, the level of RhB adsorptionby CCTa shows that it has the best adsorption ability amongthe three samples, because it has the biggest BET surface area.Meanwhile, CT and CCTb show nearly equal adsorption abilitiesbecause they have similar BET surface areas.

3.5.2. Sonocatalytic activitiesSonocatalytic degradation as a novel decomposing technology

for water treatment has attracted much attention in recent years.It is worth noting that using high-amplitude ultrasound to irradi-ate polluted water can generate OH� radicals for use in the oxida-tion process of dyes. On the other hand, sonoluminescenceinvolves the use of intense UV-light, which excites the TiO2 parti-cles and causes them to act as a photocatalyst during sonication.After irradiation for 120 min, all of the samples exhibit good degra-dation efficiency of RhB. The sonocatalytic efficiency of RhB in-creased with increasing concentration of TiO2, with 17.1%, 28.1%and 38.8% of the RhB solution being degraded for samples CT, CCTaand CCTb under an intensity of 20%, respectively. With increasingintensity from 20% to 25% and 30%, the sonocatalytic efficiency ofRhB was also accordingly increased. A comparison of the degrada-tion reduction ratio between the different ultrasonic irradiationintensities is shown in Fig. 7. Fig. 8 shows an independent diagram

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B

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CT CCTa CCTb

Adsorption effect

Fig. 8. Relative concentration of the RhB solution degraded by the different samplesunder ultrasonic irradiation with 30% intensity for 120 min.

L. Zhu et al. / Ultrasonics Sonochemistry 20 (2013) 478–484 483

of the sonocatalytic degradation of the synthesized samples underultrasonic irradiation with 30% intensity. Overall, from the resultsshown in Figs. 6, 7 and 8, the CCTb catalyst showed the best abilitynot only in terms of its adsorption, but also its sonocatalyticdegradation.

3.6. Sonodegradation mechanism

Up till now, there has been no ready-made mechanism and sat-isfying explanation for the sonocatalytic degradation of organicpollutants in the presence of various semiconductor materials.Two mechanisms referred to as sonoluminescence and ‘‘hot spot’’have been suggested to explain the process of sonocatalytic degra-dation [11]. Firstly, it is well known that ultrasonic irradiation canresult in the formation of light with a comparatively wide wave-length range below 375 nm. These very intense flashed lights canexcite the semiconductor catalyst acting as a photocatalyst andcause a great deal of �OH radicals with high oxidative activity toform on the surface of the semiconductor particles. Secondly, asis well known, the temperature of the ‘‘hot spot’’ produced byultrasonic cavitation in water medium can reach 105 or 106 �C,and this high temperature generates many holes, producing �OHradicals on the surface of the semiconductor catalyst. Therefore,in this study, in order to eliminate the impact of this high

Fig. 9. Simple mechanism of RhB degrad

temperature, the experiment was carried out under lowtemperature conditions. A schematic illustration of the formationof hydroxyl radicals on CNT/TiO2 and CdS/CNT–TiO2 underultrasonic irradiation is shown in Fig. 9.

It is well known that the band-gap of CdS/TiO2 nanocompositesis smaller than that of pure TiO2, due to the special optical proper-ties of CdS. This narrow band-gap allows CdS/TiO2 to adsorb morephotons and this will enhance the catalytic efficiency of TiO2 underlight irradiation. When the CdS/TiO2 composite was irradiated un-der ultrasonic radiation, the photogenerated electrons can be ex-cited from the VB of CdS to the CB of TiO2 through the MWCNTs,whereas the photogenerated holes would be left in the valenceband of CdS. The electrons can react with O2 to generate �O2

�,and the holes theoretically migrate to the surface and react withOH– or H2O to generate �OH. These radicals can then react withthe adsorbed pollutants. The reactions can be expressed as follows:

CdS=TiO2 þ ultrasonic radiation! CdS ðhþ; e�Þ=TiO2 ð1Þ

CdSðhþ; e�Þ=TiO2 ! CdSðhþÞ=TiO2ðe�Þ ð2Þ

e� þ O2 ! �O2� ð3Þ

hþ þ OH� ! �OH ð4Þ

hþ þH2O! �OHþ hþ ð5Þ

When introduced with MWCNTs, it can enhance the adsorptionability, it can also absorb light to create photo-induced electrons(e�) which are transferred into the conduction band (CB) of theTiO2 and CdS particles, thus increasing the amount of electrons.On the other hand, the presence of MWCNTs can enhance the rateof migration of photogenerated electrons to the surfaces and de-crease the recombination probability of photo-electron–hole pairs[48].

In addition, with the substitution of the oxygen atoms by theCNTs and CdS in the anatase crystal structure of TiO2, new levelsare introduced between the conduction and valence bands ofTiO2, the electrons generated by TiO2 can be promoted from the va-lence band to the CNT level introduced by CdS or from lower to thehigher CNT levels, which can increase the quantity of electrons.Therefore, the CdS/CNT–TiO2 composites have a narrower bandgap and can increase the level of sonodegradation under differentultrasonic intensities. This whole process is clearly described inFig. 9.

ation by CdS/CNT–TiO2 composite.

484 L. Zhu et al. / Ultrasonics Sonochemistry 20 (2013) 478–484

4. Conclusions

Novel CdS/CNT–TiO2 composites for synergetic degradationwith different concentrations of titanium (IV) n-butoxide (TNB)were prepared via a simple sol–gel method. EDX analysis showedthat the elemental contents of CdS/CNT–TiO2 were mainly C, O,and Ti with a small quantity of S and Cd. The CdS and TiO2 particleswere uniformly loaded on the wall of the MWCNTs in the form ofsmall spots. Compared with the pristine MWCNTs, though the sur-face areas of CNT–TiO2 and CdS/CNT–TiO2 were greatly decreased,they showed good adsorption effects. Under ultrasonic irradiation,RhB, which is a typical textile dyestuff, was easily and efficientlydegraded by the CdS/CNT–TiO2 composites. Meanwhile, the son-odegradation rate increased with increasing ultrasonic irradiationintensity. Sample CCTb showed higher sonocatalytic activity thanthe other catalysts.

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