Cu-TiO2 Nanocatalyst for Photodegradation of Acid Red 88 in Aqueous Solution

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Copyright 2010 American Scientic PublishersAll rights reservedPrinted in the United States of America

Science of Advanced Materials

Vol. 2, 5157, 2010

CuO-TiO 2 Nanocatalyst for Photodegradation ofAcid Red 88 in Aqueous Solution

Arumugam Manivel, Selvaraj Naveenraj, Panneer Selvam Sathish Kumar, and Sambandam AnandanNanomaterials and Solar Energy Conversion Lab, Department of Chemistry,

National Institute of Technology, Trichy 620015, India

CuO-TiO 2 nanocatalyst with high photocatalytic activity was successfully synthesized through asimple impregnation method. The as-synthesized samples are characterized by X-ray diffraction,scanning electron microscopy, transmission electron microscopy and diffused reectance UV-visspectroscopy. The results show that the CuO-TiO 2 sample is a composite material composed of

CuO and TiO 2 . The photocatalytic activity of CuO-TiO 2 nanocatalyst for the degradation of AcidRed 88 is much higher than that of bare TiO 2 sample can be attributed due to presence of morenumber of surface active sites in CuO-TiO 2 . Further enhancement in the photodegradation rateby many folds was observed upon addition of electron acceptors [Peroxomonosulphate (PMS),Peroxodisulphate (PDS) and Hydrogen peroxide (H 2O2 ]. The reason for such enhancement is dueto generation of more number of active radicals at the photocatalytic surface upon illumination.

Keywords: CuO-TiO 2 nanoparticles, Acid Red 88, Electron Acceptors, PhotocatalyticDegradation.

1. INTRODUCTION

Azo dyes are widely used in textile industries. Substan-tial amount of dyestuff is lost during the dyeing processin the textile industry, which poses a major problem tothe environment. 15 To prevent the environment from suchpollutants, biodegradation and other conventional non-destructive methods such as adsorption, coagulation, etc.,are adopted, but such processes produces secondary pol-lutants, which needs further treatment. 5 To avoid suchsecondary pollutants, heterogeneous photocatalysis tech-nique is to be adopted which generally includes semicon-

ductor as the photocatalyst is one among the promisingadvanced oxidation processes (AOPs). 6 7 The major advan-tage of heterogeneous photocatalysis is upon illuminationof semiconductor (TiO 2 , the electrons and holes formedwhich are capable of initiating chemical reactions to con-vert pollutants to less harmful products. 810 However, theefciency of photocatalytic reactions is limited due to highrecombination rate of photo-induced electron hole pairformed in photocatalytic microenvironment. 11 Numerousstudies are available in literature regarding TiO 2 semicon-ductor in order to improve the quantum efciency of pho-

tocatalytic degradation of pollutants. However, an intenseresearch activity is seen in recent years in advancing

Author to whom correspondence should be addressed.

the synthesis and functionalization of semiconductor andmetal nanoparticles with various sizes and shapes. Thegoal of these activities is to improve the performance andutilization of nanoparticles in many applications. The sizeand shape dependent optical and electronic properties of these nanoparticles make an interesting case for exploit-ing them in light induced chemical reactions. Hence, theefciencies of metal-doped TiO 2 are higher when com-pared with bare TiO 2. Further, upon comparing the ef-ciencies of metal doped TiO 2 such as gold, silver andcopper, gold and copper-doped TiO 2 shows greater degra-dation rate compared to silver-doped TiO 2 under ultravi-olet irradiation. 12 However, compared to gold, copper hasbeen receiving greater attention because of its good cat-alytic activity and cheap nature of the dopant ion as well.Hence, various researchers extensively utilized Cu-TiO 2 asa better photocatalyst in the oxidation of various organicmolecules 13 such as, dihydroxybenzenes 14 and methyltert-butylether 15 under UV irradiation. That is, in generalphotocatalytic degradation of organics, the holes are keyspecies in the overall activity: the addition of electronto the sink (e.g., metal, molecular oxygen, etc.) reducesthe recombination of photo-induced charges, which is thecrucial factor for the enhancement of the photocatalyticdegradation of organic compounds. Based on the aboveprinciple, here we aimed to prepare CuO doped TiO 2(CuO-TiO 2 by impregnation method in order to (i) hinder

Sci. Adv. Mater. 2010, Vol. 2, No. 1 1947-2935/2010/2/051/007 doi:10.1166/sam.2010.1071 51


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CuO-TiO 2 Nanocatalyst for Photodegradation of Acid Red 88 in Aqueous Solution Manivel et al.

the recombination of electron/hole pairs and (ii) enhancethe photocatalytic degradation of azo dye (Acid Red 88;AR88), an environmental pollutant.

2. EXPERIMENTAL PROCEDURE

2.1. Materials and Methods

TiO2 (Degussa P25, Germany) having a specic surfacearea of 57 m2 g 1 was used as a starting material toprepare CuO-TiO 2. CuSO4 5H2O was purchased fromFluka. Potassium peroxomonosulphate, a triple salt withthe composition 2KHSO 5 KHSO 4 K2SO4 from JanssenChimica (Belgium) was used as received. Peroxodisul-phate (PDS) and Hydrogen peroxide (H 2O2 was analyt-ical grade reagent purchased from E-Merck, India. AcidRed 88 (AR88) was a gift sample from Atul Ltd, India.

Unless otherwise specied, all reagents used were of ana-lytical grade and the solutions were prepared using milli-pore water.

2.2. Instrumental

The diffuse reectance spectra of the samples wererecorded in the wavelength range 200800 nm using aShimadzu UV-vis 2550 spectrophotometer equipped withthe integrating sphere accessory. BaSO 4 was used as areference. Surface morphology, particle size and the vari-ous contours of the photocatalyst powders were analyzedby Scanning Electron Microscope (HITACHI-3000 SHModel). Transmission electron microscopic (TEM) imageswere recorded using a TECNAI G 2 model instrument. Thesurface area of the samples were measured with the assis-tance of Flowsorb II 2300 of Micrometrics, Inc. X-raydiffraction patterns were recorded in the range of 2070(2 using Philips PW1710 diffractometer, Holland. Massspectrometric experiments were performed on a Micro-mass Quattro II electrospray mass spectrometry (ESMS)coupled to a Hewlett Packard series 1100 degasser. Thesolvent ow rate was 0.03 mL/min. The solvent was ace-tonitrile/water (50/50).

2.3. Preparation of the Photocatalyst

Nano-size CuO-TiO 2 photocatalyst was prepared byimpregnation method as described in the literatureearlier. 14 Previously calcined TiO 2 sample (at 400 C for5 h) was mixed with the aqueous solution of CuSO 4 5H2O(8 atomic wt%). The mixture is stirred for 48 hours toallow the penetration of copper ions on the titanium diox-ide crystal matrix. The supernatant liquid (water) is evap-orated by heating at 100 C over a time of 24 hours. Theresultant product is calcined at 500 C for 5 hours.

2.4. Evaluation of Photocatalytic Efciency

The photocatalytic degradation experiments were con-ducted with the desired concentration of AR 88 and cat-alyst (TiO 2 /CuO-TiO 2 under ambient open atmosphericconditions. A 500 ml capacity borosilicate glass photore-actor was used in all experiments. When the UV light

irradiation was required, the aqueous solution was irradi-ated with 6 W UV lamp (Heber Scientic, India, emittinglight radiation of 365 nm as the light source) immersedin a quartz well placed at the middle of the reactor. Theintensity of incident radiation is 9 mW/cm 2, was measuredusing a UV radiometer (Spectroline, DMX-series, NY).The reactor walls were covered with aluminium foil toavoid release of radiation. In order to ensure adsorptionequilibrium, the suspension was stirred for about 45 minin dark, prior to irradiation. The irradiation was carried outfor 60 minutes. 5 ml aliquots are withdrawn at appropriatetime intervals (10 min) and the photocatalyst is removedimmediately by ltration through a PVDF syringe lter(0.45 m). The apparent kinetics of disappearance of thesubstrate AR88 was determined by following the concen-tration of the substrate ( max = 506 nm) using UV-Visspectrophotometer (PG instruments, UK) after a certainperiod of irradiation of the catalyst suspension and thenltered with a 0.45 m PVDF (Polyvinylidene uoride)syringe lter. All experiments were carried out at naturalpH ( 6.0).

3. RESULTS AND DISCUSSION3.1. Characterization of the Photocatalyst

Figure 1 shows the diffused reectance UV-vis spectraof the prepared photocatalyst (CuO-TiO 2 and bare TiO 2.

200 300 400 500 600 700 800

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

(b)

(a)

A b s o r b a n c e

( a . u . )

Wavelength (nm)

2 3 4 5 6 7

0.0

0.10.2

0.3

0.4

0.5

0.6

(b)

(a) (

h ) 1

/ 2 (

a . u . )

Photoenergy (eV)

Fig. 1. Diffused reectance UV-Vis spectra of (a) undoped TiO 2 , and(b) CuO doped TiO 2 nanophotocatalyst. Inset shows Tauc plot of (a)undoped TiO 2 , and (b) CuO doped TiO 2 .

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Manivel et al. CuO-TiO 2 Nanocatalyst for Photodegradation of Acid Red 88 in Aqueous Solution

Bare TiO 2 shows absorption edge cut off at 400 nm con-rms that the band gap is 3.2 eV. While CuO doped TiO 2shows the absorption edge cut off around 420 nm conrmsthat the band gap gets reduced from 3.2 eV. That is, uponanalyzing the absorption coefcients of TiO 2 /CuO-TiO 2 byTauc approach, 16 the direct band gap of the semiconduc-tors were calculated by using the following equation,

=C hv E bulk g

1/ 2

hv

where is the absorption coefcient, C is a constant, his the photon energy and E bulk g is the band gap. From theinset of Figure 1, extrapolation of the linear region gives aband gap of 2.84 eV for CuO-TiO 2 sample and 3.2 eV forpure TiO 2 sample. Such red shift endorsed that incorpora-tion of the CuO dopant in the prepared nano photocatalyst(CuO-TiO 2 . Further, on noticing the region 450800 nmof CuO-TiO 2 absorption spectra following conclusions arearrived. That is,(i) the broad absorption seen around 600800 nm might beattributed to the presence of Cu 2+ species in the preparedCuO-TiO 2 photocatalyst and(ii) the increase in absorption around 450500 nm mightbe due to the presence of some unstable Cu + clusters inthe prepared CuO-TiO 2 photocatalyst. 17

Figure 2(a) shows the typical X-ray diffraction patternof the CuO-TiO2 nano photocatalyst. The XRD patternof the un-doped TiO 2 (Degussa) has also been inserted

(Fig. 2(b)) for comparison. Six distinctive peaks of TiO 2(25.26 , 38.16 , 48.17 , 54.03 , 55.12 and 64.69 cor-responding to anatase reection planes [(1 0 1), (0 0 4),(2 0 0), (1 0 5), (2 1 1) and (2 0 4)] were found in Cu-TiO2sample as per JCPDS 84-1285. In addition, three distinc-tive peaks of TiO 2 [27.3 (1 1 0), 41.24 (1 1 1) and 56.3(2 2 0)] corresponding to rutile phase is also present in

20 30 40 50 60 70

0

500

1000

1500

2000

(b)

C u

O

C u

O

C u

O

C u

O

I n t e n s

i t y

( a . u

)

2 degree

(a)

Fig. 2. X-ray Diffraction pattern of the (a) CuO-TiO 2 nano photocata-lyst and (b) bare TiO2 .

the prepared CuO-TiO 2 sample. There is no characteristicspeak related to Cu 0 is seen in the XRD pattern. However,a less intense peak at 2 value around 35 (0 0 2) showsthe presence of copper dopant in its oxide form (CuO),but not in metallic state. In addition, two more characteris-tic peaks appearing at 2 = 43 3 , 50.4 also supports thatonly CuO is present in the prepared photocatalyst (JCPDS

No. 04-0836) and no evidence for any other copper phases.Thus, XRD analysis conrms that, as synthesized CuO-TiO2 photocatalyst exist both in anatase as well as in rutilephase. The peak broadening indicates that there is a grainrenement occurs due to doping. Scherrer formula wasused to calculate the particle size of the as prepared pho-tocatalyst (CuO-TiO 2 and it was found to be 28 nm.

Figure 3 shows the scanning electron microscopic (SEM)images of CuO doped TiO 2 samples. The surface morphol-ogy clearly shows the presence of the CuO dopants on thesurface of the TiO 2. It shows that the particle structures are

irregular with porous nature. The incorporation of CuO sig-nicantly increases the surface area (60.51 m 2 /g) slightly,which might be attributed to the effect of surface porositycoming from CuO precursor. The white dispersion on theimage is obviously the TiO 2 semiconductor and we can seeCuO dopant as small grey patches. There is possibility forthe dopants to occupy the interstitial position inside the

(a)

(b)

Fig. 3. Scanning electron microscopic images of CuO-TiO 2 nano pho-tocatalyst in (a) 3,000 and (b) 15,000 magnication.

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unit lattices of the semiconductor particles. Literatures sur-vey reveals that, the presence of the Cu in the interstitialposition as CuO will creates higher number of defects mostlikely, oxygen vacancies inside TiO 2 and will induce thechange in crystalline phase during annealing. 18 19 There-fore, the dopant might present in the surface pores of thecatalyst.

To obtain further insight into the nature of the metaldopant and photocatalyst species identied by XRD andSEM, we have conducted a detailed TEM analysis for theprepared CuO-TiO 2 catalyst. The TEM micrograph for theCuO-TiO 2 is shown in Figure 4(a) indicates that all parti-cles are spherical in shape and the average particle diame-ter range between 1550 nm. The bright grey particles inthe TEM micrographs are obviously titania particles. TheCuO dopants have been observed as the dark patches onthe grey surface of titania. The CuO-doped titania particlesappears, different in shapes but at higher magnication

with utmost of small spherical asperities ( < 30 nm) evenlydistributed can be seen which is in good agreement withthe particle size of catalyst calculated from XRD analysis(28 nm). At higher magnication the crystal lattice fringesare visible. This has been shown in Figure 4(b). Moreover,the particle size of CuO is found to be less than 15 nm andthe TiO2 can act as support to adsorb copper precursorsbecause of their large surface area during the preparationprocess. This is clearly evident from the TEM images. Theselected area electron diffraction (SAED) pattern of thesample is shown in Figure 4(c). From SAED, it is clear

that the diffraction patterns of Titania are surrounded bythe several bright diffraction patterns which may corre-sponds to CuO which clearly indicates that the preparedsample is composed of both CuO and TiO 2.

3.2. Photodegradation Studies

The photocatalytic degradation of Acid Red 88 (AR 88)using CuO-TiO 2 photocatalyst under ultraviolet irradiation( max = 365 nm) has often been modeled to the Langmuir-Hinshelwood rate law, which also covers the adsorptionproperties of the substrate on the photocatalytic surface. 14The pseudo-rst order reaction rate constants for the pho-tocatalytic degradation reactions are obtained from theslopes of the linear relationship between lnC/C o versusthe corresponding irradiation time (min) (Fig. 5) for vari-ous catalyst amounts.

Inset of Figure 5 shows the effect of catalyst amounton the rate of photocatalytic degradation of AR88. Thecatalyst amount is varied between 0.61.8 g/L. The rateincreases initially with an increase in the catalyst amountand reaches a maximum for 1.4 g/L and then getsdecreased upon increase in the catalyst amount. This isdue to the fact that upon increasing the catalyst amount,absorption of light by photocatalyst particles increases.That is, increasing the catalyst amount leads to an increase

in the amount of light absorbed by the semiconductor par-ticles, which would increase the degradation efciency.Further observation that the rate constant decreases withan increase in the catalyst amount above 1.4 g/L might bedue to a decrease in the light penetration caused by the

(a)

(b)

(c)

Fig. 4. TEM Micrographs of (a) CuO-TiO 2 nano photocatalyst (b)HRTEM of the CuO-TiO 2 photocatalyst, and (c) the corresponding SAEDpattern.

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60Time (minutes)

l n ( C / C

0 ) 2

2.5

3

3.5

4

4.5

0.5 1 1.5 2Amount of photocatalyst (g/L)

k

1 0

4 s

1

Fig. 5. Plot of photodegradation of Acid Red 88 (5 10 5 M) in thepresence of various amount of CuO-TiO 2 photocatalyst. Inset shows plotof photodegradation rate for various amount of CuO-TiO 2 .

additional catalyst particles. Hence, 1.4 g/L of CuO-TiO 2is found to be the optimum catalyst amount for maximumdegradation of AR88. This optimum amount of catalyst isxed for the further experiments.

Figure 6 shows the rate of degradation for various con-centrations of the dye (AR 88) at a xed amount of the cat-alyst (1.4 g/L). The amount of dye is varied from 3 10 5

to 12 10 5 M. As the concentration of dye increases, therate decreases continuously and then levels off. The reasonfor such behaviour is that only the fraction of the AR88adhering to the CuO-TiO 2 surface is photoactive. If all the

photocatalytic sites are adhered by the dye, then there isno further increase in the rate with increase in the concen-tration of the dye which leads to a saturation limit in therate of degradation of dye. Therefore, dye adsorbed layeradjacent to the CuO-TiO 2 surface is active with respectto the propensity of receiving the electron from the CuO-TiO2(e

CB .

The observed rate constant of photocatalytic degrada-tion of AR88 (5 10 5 M) in the presence of CuO-TiO 2

3

6

9

0 2 4 6 8 10[AR 88] 10 5 M

k

1 0

4 s

1

Fig. 6. Effect of concentration of Acid Red 88 on the degradation rateat xed photocatalyst concentration (1.4 g/L).

nanoparticles (1.4 g/L) is 4 1 10 4 s 1, which is higherthan that observed for bare TiO 2 (6 4 10

6 s 1 . It shouldbe noted that upon irradiation of CuO-TiO 2 using UV lightgenerates electrons and holes, which are sufcient for pho-todegradation of AR88 into low molecular weight organiccompounds. Further, the increased photocatalytic activityof CuO modied TiO 2 is due to the change in the surface

properties such as oxygen vacancies and crystal defects.Thus, the modication of the semiconductor particle sur-face with copper would provide an effective environmentfor increasing the number of surface active sites for dye-semiconductor interaction, which in turn may increase thephotodegradation processes.

Further to enhance the photocatalytic efciency of CuO-TiO2, the external electron acceptors such as PMS, PDSand H2O2 (0.3 mm) were used and their effects on degra-dation rate was monitored with respect to optimized dyeand catalyst concentration. The results showed that the

rate constants increases upon addition of PMS, PDS andH2O2 when compared to photodegradation rate of CuO-TiO2 without any electron acceptors (Fig. 7). The reasonfor the increase in the photodegradation rate may be dueto the immediate trapping of the photogenerated electronsby the electron acceptors (PMS, PDS, H 2O2 , which inturn decreases the recombination of electronhole pairsthere by increasing the oxidation efciency of the organicdye molecules by the holes. Among the three electronacceptors used, PMS shows the highest efciency. Thereason for such observed enhancement in the photocat-

alytic degradation rate is that the electron acceptor PMSmay react with both CuO-TiO 2(e

CB and CuO-TiO 2(h

+

VBto generate highly active radicals whereas PDS and H 2O2may react only with CuO-TiO 2(e

CB .

13 20 However PDSshows poor efciency compared to H 2O2 may be attributeddue to requirement of very high potential ( E = 2 12 eV)for the dissociation of PDS into SO 4 radicals whereas

0

2

4

6

8

10

12

With H 2O2

With PMS

With PDS(Withoutoxidants) k

1 0

4 s

1

Fig. 7. Photocatalytic degradation rate of Acid Red 88 with CuO-TiO 2in the presence and absence of electron acceptors. Concentrations aremaintained as follows: (AR88 = 5 10 5 M; CuO-TiO 2 = 1.4 g/L and[electron acceptors] = 0.3 mM).

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H2O2 requires low potential ( E = 1 76 eV) for its disso-ciation into OH radicals. On comparing PMS with H 2O2,PMS requires slightly higher potential ( E = 1 82 eV) forits dissociation into SO 4 and OH

sradicals. However,PMS shows higher efciency because sulphate radicals aremore selective for oxidation than hydroxyl radicals at nat-ural pH.21

When CuO-TiO 2 nanocatalysts are dispersed in the solu-tion with an organic pollutant, the surface electrons onCuO nanoparticles should eventually transfer to the dyein the dark. However, when these catalysts are radiatedby UV light with photon energy higher or equal to theband gap of TiO 2, electrons (e

in the valence band (VB)can be excited to the conduction band with simultaneousgeneration of same amount of holes (h + in the valenceband. Reactions 13 are suggested reactions responsiblefor the degradation of the dye using CuO-TiO 2 nanophotocatalyst.

CuO-TiO 2h = 365nm

CuO-TiO 2 e

CB + CuO-TiO 2 h+

VB (1)

CuO-TiO 2 e

CB + Dye Products (2)

CuO-TiO 2 h+

VB + Dye Photo bleached Products (3)

Further, the addition of electron acceptors (PMS, PDSand H2O2 can easily trap the photoelectrons to produceactive radicals. Such active radicals dwell at the surfaceactive sites of the photocatalyst and they can interact effec-tively with the target pollutant by continuously producingvarious radical intermediates, which in turn degrade thepollutants effectively. Similarly, the photoinduced holescan be easily trapped by OH to further produce ahydroxyl radical species (OH ), which is an extremelystrong oxidant for the partial or complete mineralizationof organic pollutants. 22 23 Thus at the photocatalyst surfaceradical chain branching occurs which effectively degradethe organic pollutants. 24 Therefore, the photocatalytic reac-tion process can be proposed as follows.

CuO-TiO 2 eCB + HSO5 CuO TiO2 +

OH + SO4(4)

HSO5 + OH/ SO4 SO

5 + OH / SO24 + H

+ (5)

SO5 + Dye Products (6)

To go further in the understanding of the AR88 degra-dation mechanism, we have examined ESI-MS behavior of AR88 and its degradation products before and after irradia-tion (1 h) with UV light in the presence of PMS as electronacceptor and for the catalyst CuO-TiO 2. Mass spectrumobtained for the AR88 molecule before irradiation exhib-ited the [M-Na] peak at (m/z 377). After 1 h irradiation,base AR88 peak (m/z 377) intensity was found decreasesand in addition lot of different m/z peaks were found. The

mechanism proposed below for the formation of peaks atm/z [223, 173] corresponds to the cleavage of azo group. 25

Further, detailed product analysis using mass interpretationis going in our lab in order to nd the perfect degradationpathway.

NH 2

SO 3H

NOOH

NN

SO3H

Acid red 88

+ OH

+ SO 4+ OH /SO 4

2+

1-Nitroso-naphthalen-2-ol4-aminonaphthalene

sulphonic acid(7)

HO

NH 2

SO3H

NOOH

+ OH / SO 4Radical intermediates + OH /SO 4

2

+ OH /SO 42

(8)

Radical intermediates CO 2 + H 2O + Other possible side reactions (9)

4. CONCLUSION

The results presented in this paper indicated that CuO-TiO2 nano photocatalyst could be efciently used todegrade the dye, AR88 using 365 nm UV lamp. The resultsindicate that doped copper were present in its oxide formand acts as a co-catalyst for photodegradation processes.

The main viable reason for the enhanced rate is that therapid inhibition of the photo-induced charge carriers byCuO. The benecial effect of electron acceptors [PMS,PDS & H2O2] is to generate more number of radicals,which in turn degrade the pollutants effectively throughradical chain branching mechanism.

Acknowledgment: The authors thank DST, New Delhiand DEST, Australia for the sanction of INDIA-AUSTRALIAN strategic research fund (INT/AUS/P-1/07dated 19th Sep 2007) for their collaborative research. The

author (Mr. Sathish Kumar), thank AICTE, New Delhi, forthe NDF fellowship.

References and Notes

1. H. Zollinger, ed., Color Chemistry: Synthesis, Properties andApplications of Organic Dyes and Pigments, VCH (1991) .

2. J. Weber and V. C. Stickney, Wat. Res. 27, 63 (1993) .3. C. Rafols and D. Barcelo, J. Chromatogr. A 777, 177 (1997) .4. A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, and J. M.

Hermann, Appl. Catal. B: Environ. 31, 145 (2001) .5. I. K. Konstantinou and T. A. Albanis, Appl. Catal. B: Environ. 49, 1

(2004) .6. M. Hoffmann, M. Martin, W. Choi, and D. Bahnemann, Chem. Rev.95, 69 (1995) .

7. M. R. Dhananjeyan, E. Fine, and J. Kiwi, J. Photochem. Photobiol. A: Chem. 136, 125 (2000) .

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Received: 24 September 2009. Accepted: 24 November 2009.

Sci. Adv. Mater. 2, 5157, 2010 57
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