Nanoporous Au–TiMCM-41—An inorganic hybrid photocatalyst toward visible photooxidation of methyl orange

Solar Energy Materials & Solar Cells 94 (2010) 1783–1789

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Nanoporous Au–TiMCM-41—An inorganic hybrid photocatalyst towardvisible photooxidation of methyl orange

Panneer Selvam Sathish Kumar, Michael Ruby Raj, Sambandam Anandan n

Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy 620 015, India

a r t i c l e i n f o

Article history:

Received 1 October 2009

Accepted 22 May 2010Available online 9 June 2010

Keywords:

Au–TiMCM-41

Nanocomposite

MeOr

Electron acceptor

Photocatalytic degradation

Visible light

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.05.046

esponding author. Tel.: +91 44 2503639; fax

ail addresses: [email protected], sanand@

a b s t r a c t

Modification of MCM-41 nanopore materials was found to improve the catalyst activity because it

would provide an effective environment for increasing the number of active surface sites for molecular

interaction. Hence in this article, with an attempt to extend light absorption of TiMCM-41 based

photocatalyst toward the visible light range and to eliminate the rapid recombination of excited

electrons/holes during photoreaction, a new type of photocatalyst Au/TiMCM-41 was prepared. The

crystal phase composition and surface morphology of the Au/TiMCM-41 photocatalyst were

comprehensively examined by X-ray differential detection and transmission electron microscopy

analysis. The analytical results of UV–visible diffused reflectance spectra indicated that the absorbance

in the visible region above 500 nm for Au/TiMCM-41 photocatalyst made it possible to be excited by

visible light. The experiment demonstrated that the photocatalytic degradation of methyl orange using

Au/TiMCM-41 photocatalyst was significantly higher than that using TiMCM-41 photocatalyst.

Possibilities to enhance catalyst performance further by using electron acceptors are also discussed.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Nanoporous materials play an important role in many aspectsof science and technology, of which molecular recognition isbecoming increasingly important for environmental protection [1].Extremely specific molecular selectivity is required for theremoval of highly diluted pollutants in water; otherwise manycoexisting compounds with higher concentrations will saturatethe adsorption capacity [2]. So, in recent years the intenseworldwide activity in applications of newly developed simplegeometry templated porous media (MCM-41) in which the porediameter can be accurately tuned in the range from 2 to 25 nm byMobil researchers [3,4] has led to renewed interest in this subject.However, pure MCM-41 showed very limited catalytic activity dueto the lack of lattice defect, the redox properties, the basicity andacidity [5]. The introduction of guests (transition metals) intonanoporous silicates may increase the active sites and thusimprove the catalytic activity [6–9]. This is because in MCM-41,pores are not interconnected and the surface contains hydroxylgroups, which may experience specific interactions with mole-cules susceptible of having hydrogen bonds. So, specific surfaceinteraction effects on the properties of the confined compoundsmay be interest to everyone.

ll rights reserved.

: +91 431 2500133.

nitt.edu (S. Anandan).

To these ends, TiO2 is the most widely used photocatalyst inthe area of photocatalysis [10]. However, TiO2 has its owndrawbacks like fast recombination of photoinduced chargecarriers (electrons and holes) and it can only be excited byultraviolet irradiation (r365 nm; Eg¼3.2 eV). In order to avoidthese drawbacks of using bare TiO2 as a photocatalytic material, itis necessary to substitute titania into the structure of MCM-41nanochannels, a member of new family of nanoporous materialsmay lead to new catalysts. These hybrid nanocrystalline nanopor-ous materials are expected to show very high active surfacearea and enhanced photocatalytic efficiencies [11]. Earlier studiesshowed that MCM-41 materials can achieve stable photoinducedcharge separation [12,13]. Further, incorporation of reducibletransition metal ions into nanoporous materials impedesback electron transfer by acting as more stable electron acceptors[12–15].

Gold was recognized to be poorly active as a catalyst; however,when Au nanoparticles are highly dispersed on semiconductormetal oxides or hydroxides, they exhibit good catalytic activity.Such unusually high activity of gold strongly interacting withthe support has later been confirmed by many research groups[16–20]. Further, it is expected that the encapsulation of goldnanoparticles into the MCM-41/TiMCM-41 surface can tailor thephotoresponsiveness into the visible region (r400 nm).

Hence in the present work, we have studied the influence ofgold deposition in the nanoporous TiMCM-41 (prepared by twomodes of incorporation of Ti into MCM-41 nanochannel) for thevisible light assisted photocatalytic degradation of azo dye

P.S. Sathish Kumar et al. / Solar Energy Materials & Solar Cells 94 (2010) 1783–17891784

(methyl orange). The gold nanoparticles were introduced into theTiMCM-41 nanochannels through the deposition-precipitationmethod (�2 atomic weight %). In addition to this externalelectron acceptors such as peroxomonosulphate (PMS), perox-odisulphate (PDS) and hydrogen peroxide (H2O2) were added andtheir superiority in degrading the target pollutant was observed inorder to get effective outcome in the field of environmentalresearch.

2. Experimental

2.1. Materials

Chloroauric acid trihydrate (HAuCl4 �3H2O), hexadecyltri-methylammonium bromide (CTAB), tetraethyl orthosilicate(TEOS), titanium isopropoxide and ammonium hydroxide(NH4OH) were purchased from Sigma-Aldrich and used as astarting material to prepare Au–TiMCM-41 nanocomposites.Methyl orange (MeOr; dye content 85%) was purchased fromSigma-Aldrich and used without further purification. Potassiumperoxomonosulphate, a triple salt with the composition of2KHSO5 �KHSO4 �K2SO4 (commercially known as ‘‘Oxone’’) fromJanssen Chimica (Belgium), was used as received. Unless other-wise specified, all the reagents used were of analytical grade andthe solutions were prepared using millipore water.

2.2. Preparation of the photocatalyst

2.2.1. Preparation of TiMCM-41 by hydrothermal crystallization

method

TiMCM-41 hybrid material was prepared by the hydrothermalcrystallization method [21] as follows: about 2.2 g of CTAB wasdissolved in 52 ml of water at 40 1C. 26 ml of ammoniumhydroxide (28%) was then added slowly under stirring. Additionof 10 ml of TEOS and 0.03 g of titanium isopropoxide to thesolution was followed by continued stirring for 3 h at roomtemperature. The resulting gel was then transferred into a Teflon-lined autoclave and set aside for 48 h at 110 1C. Filtration andwashing with distilled water until the mother liquor reaches theneutral pH gave TiMCM-41. Template removal was typicallyconducted by heating at 100 1C for 12 h followed by calcination at550 1C for 5 h. Similarly, MCM-41 is prepared without titaniumisopropoxide for comparison.

2.2.2. Preparation of TiMCM-41 by grafting technique

As-synthesized MCM-41 materials were dehydrated invacuum (10�3 Torr) for 48 h. Then titanium grafting wasperformed as follows: 0.5 g of the vacuum-dehydrated MCM-41sample was added into a solution of 0.045 g titanium isoprop-oxide in 10 g of anhydrous hexane under vigorous stirring in N2

atmosphere. The mixture stood for 1 h and was then filtered andwashed with anhydrous hexane. The grafted material wascalcined at 550 1C for 5 h in oxygen atmosphere to convertunreacted alkoxide ligands into Ti–OH groups and to removeresidual isopropyl alcohol and hexane.

2.2.3. Deposition of gold nanoparticles at the TiMCM-41

nanochannels

Encapsulation of gold nanoparticles at the TiMCM-41 nano-channels were performed by deposition-precipitation methodwith NaOH [22,23] as follows: 100 ml of an aqueous solution ofHAuCl4 �3H2O (1.05�10�3 M) was heated to 80 1C. The pH of thesolution was adjusted to 7 by dropwise addition of NaOH (1 M),then 1 g of TiMCM-41 was dispersed in the solution and the pH of

the suspension was readjusted to 7 by the dropwise addition ofNaOH (1 M). The suspension was then thermostated at 80 1C withvigorous stirring for 2 h and the solids were gathered bycentrifugation (12,000 rpm for 10 min) and washed with 100 mlof distilled water under stirring for 10 min at 50 1C. The aboveprocedure was repeated several times and the samples were driedunder vacuum at 100 1C for 2 h. The final yield of gold loading was�2 atomic weight % on TiMCM-41.

2.3. Characterization techniques

Surface morphology, particle size and the various contours ofthe photocatalyst powders were analyzed by XRD (measuredusing Rigaku diffractometer, Cu-Ka radiation, Japan) and trans-mission electron microscopy (recorded using TECNAI G2 model),respectively. Diffuse reflectance UV–vis spectra of the sampleswere recorded using a Shimadzu UV–vis 2550 spectrophotometer.The surface area, pore volume and pore diameter of the samplewere measured with the assistance of Flowsorb II 2300 ofMicrometrics, Inc. Infrared spectra were recorded using aShimadzu FT-IR 8400 S spectrophotometer by employing KBrpellet technique.

2.4. Evaluation of photocatalytic activity

The photocatalytic experiments were conducted under ambi-ent atmospheric conditions and at natural pH (�6.0) using 150 Wtungsten halogen lamp (lZ400 nm; intensity of incident radia-tion is 80600710 Lux measured using Extec, USA) as the lightsource. In order to ensure adsorption/desorption equilibrium, thesolution was stirred for about 45 min in dark, prior to irradiation.The apparent kinetics of disappearance of the substrate, MeOr,was determined by following the concentration of the substrate(lmax¼466 nm) using a UV–vis spectrophotometer (PG Instru-ments, UK) after a certain period of irradiation of the photo-catalyst suspension. Prior to the analysis, the solid catalyst wasremoved from samples by filtration through a 0.45 mm poly-vinylidene fluoride (PVDF) filter.

3. Results and discussion

3.1. Characterization of the photocatalyst

The surface morphology of the as-prepared Au/TiMCM-41nanocatalyst was identified from transmission electron microscopy(TEM). The ordered hexagonal pore arrangements of pure MCM-41and TiMCM-41 prepared via hydrothermal method and graftingmethod are clearly visible from Fig. 1(A)–(C). Fig. 1(D)–(F) clearlydepicts that the Au nanoparticles were found to be uniformlydispersed on the support surfaces without affecting the MCM-41morphology [24]. The corresponding selected area electrondiffraction (SAED) pattern is shown in the inset of Fig. 1(E) and (F)confirming the formation of the single crystalline Au nanoparticlesas well. The size of the as-prepared gold nanoparticles was 5–15 nm.

Upon formation of MCM-41 by the condensation reactionsbetween the surfactant and organoalkoxysilane the followingtrend is generally observed with respect to nanochannels: that is,pore sizes of the channels are significantly reduced or at the sametime it is expected that the wall thickness may be increased[25,26]. Hence, it is important to measure the pore diameter, porevolume and specific surface area of all the prepared samplesusing nitrogen adsorption/desorption isotherm and multi-pointBET analysis [27,28]. The observed results for the preparedphotocatalysts were tabulated in Table 1. The surface area of

Fig. 1. High resolution TEM micrograph of unmodified MCM-41 (A), titania modified MCM-41 via hydrothermal method (B), titania modified MCM-41 via grafting method

(C), gold supported titania modified MCM-41 via hydrothermal method (D, E) and gold supported titania modified MCM-41 via grafting method (F). Inset of E and F shows

SAED pattern of gold supported titania modified MCM-41 via hydrothermal and grafting methods.

P.S. Sathish Kumar et al. / Solar Energy Materials & Solar Cells 94 (2010) 1783–1789 1785

as-prepared MCM-41 is 1029 m2/g and the pore diameter is3.0 nm. However for encapsulation of titania into MCM-41 adecrease in surface area and pore volume was observed either

prepared via hydrothermal method (911 m2/g; 2.82 nm) orgrafting method (938 m2/g; 2.90 nm). Further deposition of goldnanoparticles, surface area and pore volume are reduced for both

Table 1Physicochemical characteristics of modified photocatalysts.

Catalyst Surface area [SBET] (m2 g�1) Pore volume (cm3 g�1) Pore diameter (nm)

MCM-41 1029 0.72 3.00

Ti-MCM-41 (hydrothermal method) 911 0.64 2.82

Ti-MCM-41 (grafting method) 938 0.66 2.90

Au/Ti-MCM-41 (hydrothermal method) 825 0.60 2.62

Au/Ti–MCM-41 (grafting method) 771 0.62 2.78

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

6000

7000

8000

9000

10000

11000

E

D

C

Inte

nsity

(arb

. uni

ts)

2 (θ) degree

TiMCM-41 (Grafting method) Au/TiMCM-41 (Hydrothermal method) Au/TiMCM-41 (Grafting method)

0 2 4 6 8 10 12 14

0

2000

4000

6000

8000

10000

B

A

Inte

nsity

(arb

. uni

ts)

2 (θ) degree

MCM-41 Ti-MCM-41 (Hydrothermal method)

Fig. 2. XRD pattern of unmodified MCM-41 (A), titania modified MCM-41 via hydrothermal method (B), titania modified MCM-41 via grafting method (C), gold supported

titania modified MCM-41 via hydrothermal method (D) and gold supported titania modified MCM-41 via grafting method (E).

200 300 400 500 600 700 800

0.1

0.2

0.3

0.4

0.5

D

C

B

A

Abs

orba

nce

Wavelength (nm)

Fig. 3. Diffuse reflectance UV–vis spectra of titania modified MCM-41 via

hydrothermal method (A), titania modified MCM-41 via grafting method (B), gold

supported titania modified MCM-41 via hydrothermal method (C) and gold

supported titania modified MCM-41 via grafting method (D).

P.S. Sathish Kumar et al. / Solar Energy Materials & Solar Cells 94 (2010) 1783–17891786

the samples (see Table 1). This indicates that pore volumedecreases upon addition of transition metals or in other wordsthickness of pore wall increases.

Fig. 2 shows the X-ray diffraction pattern of Au–TiMCM-41,TiMCM-41 and MCM-41 samples at low angle measurements. Onecan easily find XRD peaks at low angle 1.8–101, which indicate thewell ordered nanostructure of MCM-41 is retained even after theinsertion of titania and gold nanoparticles into the MCM-41framework. Moreover additional peak at a higher angle of 251indicates the characteristic of titania and it is present in theanatase form (80%) at the nanochannels [29] (figure not given).However characteristic of gold peak at higher angle (38.41 and44.61) may overlap with MCM-41 peaks and it is not visualizedclearly in XRD patterns (figure not given).

Fig. 3 displays the diffuse reflectance UV–vis spectra ofAu–TiMCM-41, and TiMCM-41. For Au–TiMCM-41, the absorptionband centered at about �550 nm may be due to the attribution ofsurface plasmon resonance of gold nanoparticles. TiMCM-41samples show a broad peak in the UV region, which may be dueto the presence of titania in tetrahedral coordination [30,31] inMCM-41 nanochannels.

Fig. 4 shows FT-IR spectra of the as-prepared Au–TiMCM-41,TiMCM-41 and MCM-41 samples. All the samples show bands at

1400 1200 1000 800 600 400

E

D

C

B

A

% T

rans

mitt

ance

Wave number ( cm-1 )

Fig. 4. FT-IR spectra of unmodified MCM-41 (A), titania modified MCM-41 via

hydrothermal method (B), titania modified MCM-41 via grafting method (C), gold

supported titania modified MCM-41 via hydrothermal method (D) and gold

supported titania modified MCM-41 via grafting method (E).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30 40 50 60 70Time (min)

-ln (

C/C

0)

EAB

C

D

Fig. 5. Plot of photodegradation of MeOr in the presence of titania modified

MCM-41 via hydrothermal method (A), titania modified MCM-41 via grafting

method (B), gold supported titania modified MCM-41 via hydrothermal method

(C), gold supported titania modified MCM-41 via grafting method (D) and in the

presence of colloidal TiO2 (E). CMeOr¼5�10�5 M; CCat¼1 g L�1.

P.S. Sathish Kumar et al. / Solar Energy Materials & Solar Cells 94 (2010) 1783–1789 1787

1080 and 810 cm�1 correspond to asymmetric and symmetricSi–O stretching vibrations. The bands at 963 cm�1 were due tothe stretching and bending vibration of surface Si–O� groups.Further, this is also indicative of the formation of Si–O–Ti linkage.As the heteroatoms were introduced to MCM-41, a slight red shiftwas observed for almost all the bands. It may be because thereis a strong interaction between the atoms of gold, titaniumand silicon, i.e., the heteroatoms were incorporated into theframework of MCM-41 [32].

4. Photocatalytic degradation of methyl orange

The catalytic properties of Au–TiO2 and related systemshave attracted chemical industries [33–35] as well as academia[36–39]. But no work has been reported on removal of dyes usingAu–TiMCM-41 nanoporous materials. Hence we studied herephotocatalytic degradation of methyl orange using Au–TiMCM-41nanoporous material and visible light. After checking that there isno appreciable degradation of methyl orange with visible lightirradiation alone, the photocatalytic degradation of methyl orangewas studied in the presence of Au–TiMCM-41 nanoporousmaterials. But for unmodified MCM-41, no photodegradation ratewas observed.

First, the effect of Au–TiMCM-41 (1.0 g L�1) on the photo-catalytic degradation of MeOr (5�10�5 M) was studied inaqueous solution (natural pH; �6.0) and the results werecompared with respect to the same amount of TiMCM-41. Anincrease in � ln(C/Co) corresponds to a decrease in the concentra-tion of the dye. It can be seen that the degradation of MeOroccurred under irradiation in the presence of Au–TiMCM-41. Thelinear relationship between � ln(C/Co) vs irradiation time withrespect to Au–TiMCM-41 indicates that the photocatalyticdegradation follows a pseudo first-order kinetics (Fig. 5). Theobserved rate constant for Au–TiMCM-41 (Table 2) wassignificantly higher compared to that of TiMCM-41 as well aswith that of pure colloidal TiO2. Modification of MCM-41nanopore materials, i.e., Au on titanosilicate supports, was foundto improve the catalyst activity because it would providean effective environment for increasing the number of activesurface sites for dye–semiconductor interaction, which in turn

increases the photodegradation processes. Further, the reason forenhanced efficiency may be due to the following cooperativeeffects: (i) enhanced adsorption of MeOr on the Au–TiMCM-41surface, leading to more injection of the photoexcited electronfrom MeOrn to the conduction band of semiconductor in self-photosensitization pathway under visible light irradiation[40–42]; (ii) the surface plasmon resonance of Au nanoparticleson TiMCM-41 is excited by visible light, enhancing the surfaceelectron excitation and electron–hole separation [43–45]; (iii) Aunanoparticles acting as electron traps to impede electron/holerecombination [46,47]; and (iv) Fermi level equilibration betweenAu nanoparticles and semiconductor may decrease the band gapof the semiconductor and in turn diminish the rapidrecombination of electron–hole pairs [48–50].

Further, it is also known that the photocatalytic activities arelimited by the recombination of photogenerated hole–electronpairs [46]. Hence, in order to improve the efficiencies ofphotodegradation of MeOr, experiments were carried out in thepresence of external electron acceptors (PMS, PDS, H2O2). Further,the use of these electron acceptors alone in the absence of thephotocatalyst did not degrade the methyl orange. In order tocompare the photocatalytic activities, experiments were per-formed using a fixed amount of Au–TiMCM-41 (1.0 g L�1),MeOr (5�10�5 M) and electron acceptors (8�10�5 M). ForAu–TiMCM-41 prepared by the grafting method, a completedecolorization (100%) of the dye was observed in the presence ofPMS (k¼21.27�10�4 s�1), 3.9% of decolorization was achievedwith PDS (k¼0.82�10�4 s�1) and 1.07% of decolorization wasachieved with H2O2 (k¼0.229�10�4 s�1) while only 0.6% ofdecolorization was effected in the absence of any oxidants in60 min, under the same experimental conditions as depictedin Fig. 6. Compared to the rate constant observed in the absenceof the electron acceptors, about 200-fold increase in therate constant is observed for Au–TiMCM-41 nanocompositesin the presence of PMS. The reason for the increase in thephotodegradation rate may be due to the immediate trapping ofthe photogenerated electrons by electron acceptors, which in turndecreases the recombination of electron–hole pairs [51]. Similartrend is also observed in the case of Au–TiMCM-41 prepared bythe hydrothermal method. This is because the distance of theperoxo bond (–O–O–) in electron acceptors is comparable amongthe symmetrical peroxides (H2O2, PDS) and non-symmetricalperoxide (PMS), and it is believed that non-symmetrical peroxidemay be more easily activated than the symmetrical peroxides

Table 2Comparison of photocatalytic degradation rate of MeOr (5�10�5 M) using different types of catalyst in the presence and absence of PMS (8�10�5 M). Amount of catalyst

maintained for the experimentis 1 gL�1.

Catalyst [1 g L�1]

Rate Colloidal Ti-MCM-41 Ti-MCM-41 Au–Ti-MCM-41 Au–Ti-MCM-41

constant (s�1) TiO2 (H.M.)a (G.M.)b (H.M.)a (G.M.)b

Without PMS 1.13�10�9 1.34�10�9 1.95�10�9 1.15�10�5 1.43�10�5

With PMS 9.21�10�9 1.71�10�8 1.81�10�8 15.53�10�4 21.27�10�4

a Hydrothermal method.b Grafting method.

With out Electron

acceptors

With PMS

With PDSWith H2O2

0

5

10

15

20

25

kx10

4 s-1

Fig. 6. Comparison of photodegradation of MeOr using gold supported titania

modified MCM-41 prepared via grafting method in the presence and absence of

electron acceptors. Concentrations are maintained as follows: MeOr [5�10�5 M],

PMS¼ PDS¼H2O2 0.08 mM, Au–TiMCM-41 (1.0 g L�1).

A B

C

D

0

5

10

15

20

25

kX

10

s

Fig. 7. Comparison of photodegradation of MeOr in the presence of PMS using

titania modified MCM-41 via hydrothermal method (A), titania modified MCM-41

via grafting method (B), gold supported titania modified MCM-41 via hydro-

thermal method (C) and gold supported titania modified MCM-41 via grafting

method (D). Concentrations are maintained as follows: MeOr (5�10�5 M),

PMS (0.08 mM), TiMCM-41/Au–TiMCM-41 (1.0 g L�1).

P.S. Sathish Kumar et al. / Solar Energy Materials & Solar Cells 94 (2010) 1783–17891788

[52,53]. Hence PMS is more active than PDS (Eqs. (1)–(3)). But theobserved activity with respect to H2O2 is very low compared toPDS. This may be due to the fact that in H2O2 although theformation of hydroxyl radical is increased, the scavengingreaction of the hydroxyl radicals predominates and may resultin a reduced dye degradation rate (Eqs. (4)–(7)) [15,54]

�O3S–O–O–SO3� +e�-SO4

2� +SO4�� (1)

(2,3)

H2O2+e�-OH�+OH� (4)

H2O2+h+-H+ +HO2� (5)

HO2�+HO2

�-H2O2+O2 (6)

OH�+H2O2-H2O+HO2� (7)

The observed rate constant of photooxidation of methyl orangein the presence of TiMCM-41/Au–TiMCM-41 prepared by graftingmethod is approximately equivalent to TiMCM-41/Au–TiMCM-41prepared by hydrothermal method in the presence and absence ofPMS (Fig. 7; Table 2). This is because modification of the MCM-41surface through either method would provide an effectiveenvironment for increasing the number of surface active sitesfor dye–semiconductor interaction. However, the slight increase

in rate constant with respect to TiMCM-41/Au–TiMCM-41prepared by the grafting method might be due to higher TiO2

concentration used in the preparation procedure in the case ofgrafting method compared to the hydrothermal method.

5. Conclusions

In the present study, we found that an otherwise inactiveMCM-41 or less active Ti-MCM-41 upon modification with Ausupport became a new type of inorganic hybrid effectivephotocatalyst for the visible light photodegradation of methylorange. The results suggest that the photocatalytic efficiency ofTiMCM-41 could be significantly enhanced by Au loading, whichincreased the visible light absorption of the catalyst, decreasedthe recombination of the photogenerated charge carriers, in-creased the electron injection from MeOrn and subsequent chargetrapping by the titania matrix. Thus, a combination of increasedavailability of charge carriers due to enhanced photosensitizationeffect and visible light absorption might have played a major rolein enhancing the efficiency of the photocatalyst. It has also beenshown that the dye degradation efficiency could be furtherenhanced in the presence of external electron acceptors, such asPMS.

Acknowledgements

SA thanks CSIR, New Delhi, for the sanction of major researchfund (CSIR reference no. 01(2197)/07/EMR-II). Mr. Sathish Kumarthanks AICTE, New Delhi, for the NDF fellowship.

P.S. Sathish Kumar et al. / Solar Energy Materials & Solar Cells 94 (2010) 1783–1789 1789

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