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Photocatalytic degradation of an azo dye X6G in water A compartive study using nanostructured indiu

Photocatalytic degradation of an azo dye X6G in water A compartive study using nanostructured indiu
Photocatalytic degradation of an azo dye X6G in water A compartive study using nanostructured indiu

Photocatalytic degradation of an azo dye X6G in water:A comparative study using nanostructured indium

tin oxide and titanium oxide thin ?lms

Mohammad Hossein Habibi *,Nasrin Talebian

Department of Chemistry,University of Isfahan,Isfahan 81746-73441,Iran

Received 19May 2005;received in revised form 11August 2005;accepted 14November 2005

Available online 24January 2006

Abstract

Indium tin oxide (ITO)and titanium oxide (TiO 2)thin ?lms were deposited on glass substrates using electron beam evaporation.Amorphous and crystalline ?lms can be obtained at different annealing temperatures.The thin ?lms are characterized by UV e vis,XRD spectra and AFM analysis.The degradation of X6G (C.I.Reactive Yellow 2),commonly used as textile dye,can be photocatalyzed by ITO and TiO 2thin ?lms.The degradation can be completed in the order of minutes at optimal operational https://www.sodocs.net/doc/9d12670005.html,ing advanced oxidation processes (AOPs)and comparison between photoactivity of both ?lms reveal that,indium tin oxide can be used as a suitable alternative to TiO 2thin ?lms for water treatment.

ó2005Elsevier Ltd.All rights reserved.

Keywords:ITO thin ?lm;TiO 2thin ?lm;Photodegradation;Azo dye;Nanostructure

1.Introduction

Azo dyes are the largest group of the dyes used for dyeing cotton fabrics in the industry [1].Cotton is the most widely used fabric among all textile materials,hence azo dyes are dis-charged frequently and in large quantities into the environ-ment.The term azo dye is applied to synthetic organic colorants that are characterized by a nitrogen-to-nitrogen dou-ble bond [2].Due to azo dyes’poor exhaustion properties as much as 30%of the initial dye applied remains un?xed and end up in ef?uents.A necessary criterion for the use these dyes is that they must be highly stable in light and during washing.They must also be resistant to microbial attack.Therefore,they are not readily degradable and are typically not removed from water by conventional chemical wastewater

treatment systems.In the past mainly chemical coagulation followed by activated sludge process was adopted to treat tex-tile wastewaters.However,azo dyes due to their hydrophilic property are not removed by chemical coagulation.In general,physicochemical methods (coagulation and ?occulation)produce large amounts of sludge which pose handling and disposal problems.On the other hand,due to the electron with-drawing nature of the azo bonds,they are not susceptible to aerobic oxidative catabolism [3]and hence are not removed in aerobic processes.Removing color from wastes is often more important than other colorless organic substances,be-cause the presence of small amounts of dyes (below 1ppm)is clearly visible and in?uences the water environment consid-erably.Therefore,it is necessary to ?nd an effective method of wastewater treatment in order to remove color from textile ef?uents.In recent years an alternative to conventional methods,is ‘‘advanced oxidation processes’’(AOPs),based on the chemical,photochemical and photocatalytic production

of hydroxyl radicals (

OH),which act as strong oxidizing agents,which have emerged as a promising technology for

*Corresponding author.Tel.:t983117932401;fax:t983116680066/t983116687396.

E-mail addresses:habibi@sci.ui.ac.ir ,habibi@chem.ui.ac.ir (M.H.Habibi).

0143-7208/$-see front matter ó2005Elsevier Ltd.All rights reserved.

doi:10.1016/j.dyepig.2005.11.006

Dyes and Pigments 73(2007)186e

194

https://www.sodocs.net/doc/9d12670005.html,/locate/dyepig

the degradation of organic pollutants.AOPs include heteroge-neous photocatalytic systems such as a combination of large band gap semiconductor particles(TiO2,ZnO,etc.),either dis-persed in slurries or immobilized on?lms and UV light[4e8]. The advanced oxidation processes are costly in terms of instal-lation,operation and maintenance but decolorization of azo dyes is achieved with the cleavage of the azo bond,thus ren-dering the dye colorless,with the formation of corresponding aromatic amines,which can be toxic and carcinogenic[9].Be-cause of unique properties,a large number of semiconductor oxide?lms have been investigated during the past60years; among these titanium dioxide(TiO2)has gained much atten-tion because of the strong photocatalytic abilities to purify pol-lutants in air and water under UV irradiation[10,11].However, In2O3:SnO2(known as indium tin oxide or ITO)?lms are used as transparent electrodes in?at panel displays and solar cell, surface heaters for automobile windows,camera lenses,photo-electrodes,storage-type cathode ray tubes,electrochromic devices,liquid crystal displays,electroluminescent devices,bio-logical devices as well as transparent heat re?ecting window material for building[12e18].So far,there is little work done on ITO?lms used as photocatalyst.Due to the wide range of applications of ITO transparent conductive?lms,various techniques for the growth of these?lms have been intensively investigated,such as reactive thermal evaporation deposition [19],magnetron sputtering[20],electron beam evaporation [21],spray pyrolysis[22],chemical vapor deposition[23],vac-uum evaporation[24],ion-assisted deposition techniques[25], electroless chemical growth techniques[26],sol-gel technique [27],and laser-assisted deposition techniques[28].

In this study,thin?lms of indium tin oxide and titanium oxide have been prepared on glass using an electron beam evaporation technique and the photocatalytic properties of both?lms toward Light Yellow X6G(C.I.Reactive Yellow2) degradation are investigated and compared.We examined the effects of some parameters such as?lm thickness,dye concen-tration,solution pH,the presence of H2O2and ethanol on the photodegradation rate.

2.Experimental

2.1.Catalysts preparation

An e-beam evaporation system was used for?lm deposition which is one of the most versatile techniques for the deposi-tion of transparent https://www.sodocs.net/doc/9d12670005.html,pared with other deposi-tion techniques,the e-beam evaporation process produces ?lms with higher purity and better-controlled composition.It also provides the?lms with greater adhesive strength and ho-mogeneity and permits better control of the layer thickness. The glass substrates were?rst cleaned in propanol and meth-anol solutions in an ultrasonic bath for10min.The substrates were then rinsed thoroughly in deionised water and dried in a high purity N2gas stream just before they were loaded into the system for?lm deposition.ITO and TiO2thin?lms were deposited on glass substrates,whose temperature was maintained at25 C during the deposition process.The proportion of indium oxide to tin oxide in the coating source is typically90:10by mass.The thickness of the deposited ?lms was controlled using a quartz crystal thickness monitor, resulting in?lm thicknesses that varied in the range of 50e300nm.

2.2.Catalysts characterization

Surface roughness and morphologies of TiO2and ITO thin ?lms were evaluated by atomic force microscopy.The X-ray diffraction(XRD)patterns obtained on a Philips D8Advanced Bruker X-ray diffractometer using Cu K a radiation at a scan rate of0.03 2q s?1were used to determine the identity of many phases present and their crystallite size.The accelerating voltage and the applied current were40kV and35mA,re-spectively.The crystallite size was calculated by X-ray line broadening analysis by Scherrer formula.UV e vis spectra of ?lms were obtained using a UV e vis spectrophotometer (Cary500Scan Spectrophotometers,Varian).

2.3.Photocatalytic activity measurements

A high surface area of nanocrystalline TiO2and ITO thin ?lms was prepared on microscopy glass slides.To evaluate the catalytic activity,the photodegradation of a well-known or-ganic azo dye X6G(C.I.Reactive Yellow2),is investigated as a simple model compound,Fig.1,under UV irradiation. Photocatalytic experiments were carried out in air at ambient temperature by immersing samples of15cm2into40ml of different concentration aqueous X6G solutions.The solution was stirred continuously.The UV light(three8W mercury lamps)was irradiated perpendicularly to the surface of the ?lms and the distance between the UV source and the?lm was5cm.The change of X6G concentration in accordance with the irradiation time was measured using Cary500Scan Spectrophotometers,Varian.

NH

N

N

N NH N

HO

SO3Na

Cl

Cl

SO3Na

N

3

N

N

NaO3S

Fig.1.Light Yellow X6G(C.I.Reactive Yellow2).187

M.H.Habibi,N.Talebian/Dyes and Pigments73(2007)186e194

3.Results and discussion

3.1.Characterizations of thin?lms

Fig.2shows the UV e vis absorption spectra of TiO2(A) and ITO(B)?lms,annealed at350 C(a),450 C(b)and 550 C(c).It can be seen from Fig.2that the transmittance of?lms at550 C is>90%over the entire visible light spec-tral region.The fast decrease below380nm is due to absorp-tion of light caused by the excitation of electrons from the valence band to the conduction band of both metal oxides. The oscillation of the curves between800and380nm is due to the interference between the?lm and the glass https://www.sodocs.net/doc/9d12670005.html,pared with the transmittance spectra of TiO2?lm,the absorption edge of ITO?lms shows a slight pseudo-blue shift.During annealing,different temperature in?uenced the crystalline particle size and then in?uenced the absorp-tion edge of the thin?lm.With smaller particle size,the electron con?ned in the nanomaterial exhibits different be-haviors from that in the bulk materials.The properties of the electrons in small particles should be dependent upon the crystallite size and the shape due to quantized motion of the electron and hole in a con?ned space.As a result of the con?nement,the band gap increases and the band edges shift[5].The smaller particle the size,the larger is the blue shift;ITO?lm contains smaller particles,and a quantum size effect appears,resulting in a more pseudo-blue shift of the absorption edge.According to[29],the absorption coef?cient a can be expressed as the following:

a?1?d lne1=TTe1Twhere T is the transmittance,and d is the thickness of the ?lm.The absorption coef?cient can be determined by a n?C(h n?E g),where n depends on the type of electron tran-sition.In case of allowed direct and indirect transition semi-conductor n is2and1/2,respectively[30,31].The linear nature of the plots of(a h n)1/2for ITO and of(a h n)2for TiO2 thin?lms above the absorption edge indicates that the funda-mental optical transitions in these?lms are indirect and direct, respectively.The values of(a h n)n for?lms annealed at differ-ent temperatures are plotted vs.the photon energy(not

shown 0

20

40

60

80

100

20

40

60

80

100

300400500600700800

λ (nm)

300400500600700

λ (nm)

T

%

T

%

(a)

(b)

(c)

(A)

(B)

Fig.2.UV e vis spectra of TiO2?lms(A)and ITO?lms(B)of350nm thick-

ness annealed at350 C(a),450 C(b)and550 C

(c).

Fig.3.(A)XRD pattern for TiO2thin?lm and(B)for ITO thin?lm on glass

annealed at550 C,thickness300nm.

188M.H.Habibi,N.Talebian/Dyes and Pigments73(2007)186e194

here).Extrapolating the linear region towards the h n axis gives the effective direct transition energies for TiO2thin?lm, 3.59eV and indirect transition energies for ITO thin?lm, 3.67eV(both having300nm thickness),respectively.

Fig.3(A)and(B)shows XRD patterns of TiO2and ITO ?lms,deposited on glass and heat-treated at550 C.At 450e550 C,TiO2?lm exhibits anatase phase structure,pref-erential orientation in(101)and at550 C,ITO?lm shows cu-bic lattice crystal structure,preferential orientation in(222). The average crystallite size of TiO2and ITO?lms annealed at550 C,ca.45and36nm,respectively,was calculated us-ing the line broadening methods and the equation proposed by Scherrer[32].

Fig.4shows two-dimensional(a e d)and three-dimensional (e e h)AFM images of the TiO2(A,C)and ITO(B,D)thin ?lms deposited on glass at450 C(a,c)and550 C(b,d). The surface morphologies and roughness of TiO2and ITO ?lms are obviously different.However,both particle size and roughness of thin?lms are changed with annealing tem-perature.Fig.4(A)(b)and(B)(d)shows that TiO2and ITO ?lms annealed at550 C are composed of particles of about 50and34nm diameters,respectively,which are nearly in agreement with those obtained from XRD spectra.In addition to particle diameter,AFM image analysis also gives the values of surface roughness.The root mean square roughness values (R rms)of TiO2and ITO are1.23and16.4nm at500 C,re-spectively.Fig.4(C)(f)and(D)(h)also shows that the surface morphology of ITO?lm is rougher than that of TiO2?lm.ITO ?lm contains obvious pore structures between particles;hence the roughness and R rms value of ITO?lms at550 C are larger than that of TiO2?lm at550 C.

3.2.Photocatalytic activity

The loss in absorbance at l max of X6G(406nm)was followed during the photodegradation reaction as a function of irradiation time.The color of X6G solution changes from yellow to colorless with increasing irradiation time which in-dicates that there must be some chemical reaction occurring. No signi?cant color removal was observed in the test experi-ments,i.e.in the presence of UV illumination(without cata-lysts)and in the presence of thin?lm catalysts(in the dark). The apparent rate constant and degradation rate of ITO?lm are greater than those of TiO2?lm.Characterization results of AFM,UV e vis and XRD spectra can explain the enhance-ment in photocatalytic activity.Firstly,AFM results show that the particle size in ITO thin?lms is smaller in TiO2?lms. Therefore,ITO thin?lms possess larger surface area.

Under Fig.4.Two-dimensional(a e d)and three-dimensional(e e h)AFM images of the TiO2(A,C)and ITO(B,D)thin?lms deposited on glass at(a)450 C and(b) 550 C.

189

M.H.Habibi,N.Talebian/Dyes and Pigments73(2007)186e194

the experimental conditions used,the photocatalytic curves fol-low ?rst-order reaction kinetics.Generally,the photodegrada-tion rates of chemical compounds on semiconductor surfaces follow the Langmuir e Hinshelwood model [5,33e 35].R ?

d C

d t

?k r q ?k r KC =e1tKC Te2T

where k r is the reaction rate constant,K is the adsorption coef?cient of the reactant,and C is the reactant concentration.The reaction rate R is proportional to the surface coverage q ;hence a high coverage q or a large adsorption constant K would result in a high photocatalytic activity.Secondly,UV e vis spectra show that the absorption edge of ITO ?lms is at a shorter wavelength range than that of TiO 2?lm.This is because ITO ?lms contain smaller crystallites of slightly higher band gap energy and a stronger oxidation power.XRD measurements further con?rm that the crystallite size of ITO ?lms is smaller than that of TiO 2?lms.All these fac-tors can enhance the photocatalytic activity of ITO thin ?lms.

3.3.Factors affecting color removal

3.3.1.Effect of dye concentration

Fig.5(A)and (B)shows a typical time-dependent UV e vis spectrum of X6G solution during photoirradiation on ITO and TiO 2thin ?lms,respectively.The spectrum of X6G in the vis-ible region exhibits a main band with a maximum at 406nm.The absorption peaks,corresponding to color of dye at l max ?406and another,corresponding to P e P *transition of aromatic rings of dye at 265nm were diminished and ?nally disappeared under reaction which indicated that the dye had been degraded completely.No new absorption bands appear in the visible or ultraviolet regions.As indicated by the absor-bance at 406nm,the X6G dye was decolorized by >90%within the ?rst 60s at pH 1.25.However,the peak at 265nm slightly changed within the same period of time.These results indicate that the fast decolorization of the dye was followed by a much slower mineralization of intermedi-ates formed

subsequently.

Fig.4.(continued).

190M.H.Habibi,N.Talebian /Dyes and Pigments 73(2007)186e 194

In Eq.(2)when C is very small,the KC product is negligi-ble with respect to unity so that Eq.(2)describes ?rst-order kinetics.The integration of Eq.(2)with the limit condition at the start of irradiation,t ?0,the concentration is the initial one,C ?C 0,gives ln eC =C 0T?kt

e3T

where k is the apparent ?rst-order reaction constant.Kinetic parameters resulting from the application of Eq.(3),are re-ported for different dye concentrations at pH ?1.03on ITO and TiO 2thin ?lms in Table 1.The observed results in Table 1reveal that the initial dye concentration in?uences the rate of degradation of the dye.The major portion of degradation occurs in the region near to the irradiated side (termed as re-action zone)where the irradiation intensity is much higher than in the other side [36].Thus at higher dye concentration,degradation decreases at suf?ciently long distances from the light source or the reaction zone due to the retardation in the penetration of light.Hence,it is concluded that as initial con-centration of the dye increases,the requirement of catalyst surface needed for the degradation also increases [37].It is important to note that for dye concentrations below 50mg/l,

which are typically observed at wastewaters,complete degra-dation of X6G takes place in the order of minutes for acidic medium (Fig.6).

3.3.2.Effect of pH

Despite the large number of accurate studies,the effect of solution pH during photocatalytic process is still not clear.The ef?ciencies of photocatalytic processes strongly depend upon the pH of the reaction solution.It was due to the ampho-teric behavior of semiconductor ITO and TiO 2.The surface charge property of ITO and TiO 2thin ?lms changes with the change of solution pH.For nanosized TiO 2,the pH zpc (zero point charge)is known to be 5.1[38],so that the surface is positively or negatively charged at low or high pH,respec-tively.This behavior can be expected to primarily in?uence the adsorption of the dye on the catalyst,thus affecting the overall photocatalytic process.In the case of sulfonated dyes [39,40],a model has been proposed to account for the dramatic role played by pH in determining the critical step of the

A b s o r b a n c e

0.00

0.02

0.04

0.06

0.08

0.10

(B)

λ (nm)

250

300350400450500

A b s o r b a n c e

0.00

0.02

0.04

0.06

0.08

0.10

(A)

Fig.5.Time-dependent UV e vis spectrum of X6G solution during photoirra-diation on (A)TiO 2and (B)ITO thin ?lms;pH, 1.03;?lm thickness,300nm;dye concentration,1?10?6mol l ?1.

Table 1

Effect of dye concentrations on photodegradation rate of X6G Dye concentration (?10?6mol l ?1)ITO

TiO 2

Rate constant (min ?1)Rate constant (min ?1)1 2.94 2.592 1.240.7340.450.4260.380.3080.290.21100.170.1112

0.09

0.06

pH 1.03,?lm thickness,300nm.

0123456t (min)

l n (C o /C )

(A)

0102030405060

t (min)

l n (C o /C )

(B)

Fig.6.Effect of dye concentrations on photodegradation rate of X6G using (A)TiO 2thin ?lm and (B)ITO thin ?lm;pH,1.03;?lm thickness,300nm;(a)e (g)dye concentration 1e 12?10?6mol l ?1.

191

M.H.Habibi,N.Talebian /Dyes and Pigments 73(2007)186e 194

decomposition.The description considers the charge status of both the target substrate and the catalyst surface by applying a simple electrostatic reasoning.Accordingly,the increase of decolorizing recorded at acid pH was attributed to strong dye adsorption,through the deprotonated e SO 3?moiety,on the catalyst.This favorable interaction would enhance the en-counter probability of nascent

OH radicals with the organic dye.As opposed,at alkaline pH,the columbic repulsion aris-ing between e SO 3?and the negative oxide surface would make the dye access to the catalyst a diffusion-controlled process.In

this case,the

OH radicals generated at the catalyst surface would hardly attack the target molecule.Consequently,a com-paratively slower decomposition rate was measured.The same trends were observed for ITO thin ?lm.The results of the pres-ent work for X6G are in good agreement with this model.For ITO and TiO 2thin ?lms,the highest percentage of dye decol-orizing was actually recorded at pH 1.03(Fig.7,Table 2).3.3.3.Effect of ?lm thickness

It is seen from Fig.8and Table 3that decomposition rate constants depend on the ?lm thickness.As expected the rate constants increase with increasing ?lm thickness which is at-tributed to two factors:(a)increase in amount of semiconduc-tor dioxide to participate in the photocatalytic reaction,and (b)increase in the charge carrier concentrations of TiO 2and ITO thin ?lms through crystallinity improvement.However,a limit-ing value can be observed at thick ?lms due to:(a)aggregation of TiO 2and ITO particles in the interior region of thick ?lms,

causing a decrease in the number of surface active sites and (b)increase in opacity and light scattering leading to a decrease in the passage of irradiation through the ?lm.

3.3.

4.Effect of hydrogen peroxide

The effect of the addition of H 2O 2on the decolorization of azo dye was studied at different hydrogen peroxide concentra-tions.Results are given in Fig.9.The decolorization rate of X6G increased with increasing H 2O 2concentration up to 8mmol l ?1,but above it,the degradation rate decreased.The higher constant rates after the addition of peroxide were attrib-uted to the increase in the concentration of hydroxyl radical.According to Eq.(4),at low concentration,hydrogen peroxide inhibits the electron e hole recombination as the better electron acceptor than molecular oxygen [41,42].

On the other hand,hydrogen peroxide may be split photo-catalytically by UV irradiation [43,44]to produce hydroxyl radical directly,(Eq.(5)).But at high concentration,H 2O 2is

a powerful

OH scavenger (Eq.(6))[41,45].

e ?CB tH 2O 2/OH ?

t

OH

e4TH 2O 2th n /2

OH e5TH 2O 2t

OH /HO

2tH 2O

e6T

Furthermore,H 2O 2can be adsorbed onto semiconductor particles to modify their surfaces and subsequently decrease its catalytic activity [46].Therefore,the proper amount of hydrogen peroxide could accelerate the photodegradation of X6G

dye.

00.51.522.5

3t (min)

l n (C o /C )

(A)

00.51.522.5

33.5t (min)

l n (C o /C )

(B)

Fig.7.pH dependence of photodegradation of X6G dye using (A)TiO 2thin

?lm and (B)ITO thin ?lm.pH:(a)1.03,(b)2.34,(c)4.52,(d)6.12,(e)7.68,and (f)9.75;?lm thickness,300nm;dye concentration,1?10?6mol l ?1.

Table 2

Effect of pH solution on degradation rate of X6G pH

ITO

TiO 2

Rate constant (min ?1)Rate constant (min ?1)1.03 2.941 2.5932.340.0980.0763.120.0170.0254.520.0320.0297.680.0220.0199.75

0.013

0.015

Dye concentration,1?10?6mol l ?1;?lm thickness,300

nm.

0.51.522.533.550

100

200

300

Film thickness (nm)

D e g r a d a t i o n r a t e (m i n -1)

Fig.8.The photodegradation rate variation with ?lm thickness:(A)TiO 2and (B)ITO ?lms;pH,1.03;dye concentration,1?10?6mol l ?1.

192M.H.Habibi,N.Talebian /Dyes and Pigments 73(2007)186e 194

3.3.5.Effect of ethanol

The inhibitive in?uence of ethanol,commonly used to quench hydroxyl radicals,provides information on reactive species involved in the reaction.According to the previous lit-erature [47,48],alcohols such as ethanol,are commonly used to quench hydroxyl radicals.The rate constant of reaction be-tween hydroxyl radical and ethanol is 1.9?109M ?1s ?1[42].It was observed that small amounts of ethanol inhibited the photodegradation of X6G which means that,hydroxyl radicals play a major role in photocatalytic process.The decolorization rate decreases with an increase in the amount of ethanol (for example in the presence of 1%(v/v)ethanol,0.005and 0.01min ?1for TiO 2and ITO thin ?lms,respectively).4.Conclusion

Effective degradation of azo dyes is possible by photocatal-ysis in the presence of ITO and TiO 2thin ?lms prepared by e-beam evaporation technique and UV light.The thin ?lms were characterized by XRD,AFM,and UV e vis.The photoca-talytic activity of ITO thin ?lms at 500 C is obviously higher than those of TiO 2thin ?lms.This is attributed to the fact that ITO thin ?lm is composed of smaller particles.ITO thin ?lms with higher surface area and roughness are also bene?cial to the enhancement of photocatalytic https://www.sodocs.net/doc/9d12670005.html,plete color removal was achieved at acidic pH and low concentration of X6G dye.The kinetics of the photocatalytic process follows a Langmuir e Hinshelwood model and depends on several fac-tors such as,dye concentration,?lm thicknesses,pH solution and the addition of hydrogen peroxide and ethanol.The photo-degradation of X6G was enhanced by the addition of proper

amount of hydrogen peroxide,but it was inhibited by ethanol.From the inhibitive effect of ethanol it was detected that hydroxyl radicals played a signi?cant role in the photodegra-dation of https://www.sodocs.net/doc/9d12670005.html,parison between photoactivity of both thin ?lms reveals that indium tin oxide can be used as a suit-able alternative to TiO 2thin ?lms for water treatment.Acknowledgements

We are grateful to Isfahan University Graduate School for ?nancial support of this work.References

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Table 3

Effect of ?lm thickness on degradation rate of X6G Film thickness ITO

TiO 2

Rate constant (min ?1)Rate constant (min ?1)50 1.46 1.25100 1.98 1.54200 2.62 1.95300

2.94

2.59

Dye concentration,1?10?6mol l ?1,pH

1.03.

2

2.53

3.54

4.50

2

4

6

8

10

12

14

16

18

H 2O 2 (mmol/l)

D e c o l o r i s a t i o n r a t e (m i n -1)

Fig.9.Effect of H 2O 2on decolorization rate of X6G dye:(A)TiO 2and (B)ITO thin ?lms of thickness 300nm;dye concentration,1?10?6mol l ?1;pH,1.03.

193

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