Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 193–198

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Nanocomposite prepared from ZnS nanoparticles and molecular sievesnanoparticles by ion exchange method: Characterization and its photocatalyticactivity

Afshin Pourahmad ⇑Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran

h i g h l i g h t s

" Synthesis ZnS/MCM-41nanocomposite by ion exchangemethod.

" Degradation of MB by photocatalyst." The kinetics of photocatalytic

degradation is of the pseudo-firstorder.

1386-1425/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.saa.2012.10.053

⇑ Tel.: +98 911 333 2448; fax: +98 131 422 3621.E-mail address: pourahmad@iaurasht.ac.ir

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 September 2012Received in revised form 6 October 2012Accepted 10 October 2012Available online 7 November 2012

Keywords:PhotodegradationMesoporous materialSemiconductorTransmission electron microscopy

a b s t r a c t

In this article, we have reported synthesis of ZnS/MCM-41 nanocomposite and its photocatalytic activity.The photocatalytic activity was evaluated using basic blue 9 or methylene blue (MB) as model pollutantunder UV light irradiation. The catalyst is characterized by transmission electron microscopy (TEM), UV–vis diffused reflectance spectra (UV–vis DRS), X-ray diffraction (XRD), and scanning electron microscopy(SEM) techniques. The effect of ZnS, MCM-41 support and different wt% of ZnS over the support on thephotocatalytic degradation and influence of parameters such as ZnS loading, catalyst a mount, pH and ini-tial concentration of dye on degradation are evaluated. The degradation reaction follows pseudo-firstorder kinetics. The effect of dosage of photocatalyst was studied in the range 0.02–5 g/L. It was seen that0.4 g/L of photocatalyst is an optimum value for the dosage of photocatalyst. The degradation efficiencywas decreased in dye concentration above 3.2 ppm for dye. In the best conditions, the degradation effi-ciency was obtained 0.32 ppm for methylene blue.

� 2012 Elsevier B.V. All rights reserved.

Introduction

Dye wastewaters are discharged by a wide variety of sources,such as textiles, printing, dyeing, dyestuff manufacturing, and foodplants [1]. They are the important sources of water pollution due tosome dyes and their degradation products may be carcinogens andtoxic to mammals [2]. Moreover, the color produced by organicdyes in water is of great concern because the color in water is aes-

ll rights reserved.

thetically unpleasant. Synthesis and understanding the growth ofnano/micro structured photocatalysts and study their applicationsare at the leading edge of today’s research. Nanocrystalline semi-conductor materials are the subject of intense research due to theirremarkable size, shape, and surface dependent physical and chem-ical properties [3,4]. Optical properties of nanosized semiconductorcrystallites could be changed and are different from their corre-sponding bulk materials. Zinc sulfide is a kind of wide band gapII–VI compound semiconductor material (Eg � 3.6 eV) togetherwith its distinguished energy band properties, it becomes goodhost material and possesses photoluminescence and electrolumi-

194 A. Pourahmad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 193–198

nescence characteristics. It facilitates wide applications in thefields of displays, sensors and lasers. Moreover, ZnS is widely usedas photocatalyst, non-linear optical devices, light emitting diodewhen doped, and has potential applications in constructing nano-meter scale electric and optoelectronic devices. In addition, zincsulfide behaves as an effective catalyst and has attracted the atten-tions in the fields of photosynthesis and pollutant treatment.Therefore, the synthesis of various ZnS nanostructures has been avery popular research area in recent years. Semiconductor-loadedzeolite and mesoporous materials such as MCM-41 have drawnattention as potential photocatalysts due to their unique porestructure and adsorption properties. The advantages using zeoliteor mesoporous support for semiconductor photocatalysis includeformation of ultrafine semiconductor particles during sol–geldeposition, increased adsorption in the pores, surface acidity whichenhances electron-abstraction and decreased UV-light scatteringas the main component of zeolite is silica [5]. The other problemsthat have motivated the development of zeolite or mesoporousphotocatalyst are related to complex filtration procedures andthe high turbidity that decrease the radiation flux. The use of zeo-lite or mesoporous supported semiconductor has allowed theenhancement of photodegradation rate in comparison with neatsemiconductor [6]. This paper reports a simple route for the prep-aration of nanoparticles of ZnS into mesoporous MCM-41 nanopar-ticles. The Zinc sulfide nanoparticles were synthesized by ion-exchange of the MCM-41 in aqueous suspension. The ZnS/MCM-41 nanocomposites are used as photocatalyst in degradation ofmethylene blue as organic dyes are studied to achieve degradationefficiency of dye near 99%. Methylene blue (MB) is a heterocyclicaromatic chemical compound with molecular formula C16H18ClN3-

S. It has many uses in a range of different fields, such as biology orchemistry. It appears as a solid, odorless, dark green powder thatyields a blue solution when dissolved in water at room tempera-ture. This dye is stable, incompatible with bases, reducting agentsand strong oxidizing agents. It is harmful if swallowed. It may beharmful if inhaled and in contact with skin as well as causes severeeye irritation and investigated as a mutagen.

Experimental

Materials

The acetate salts of zinc (Zn (CH3COO)2), from Merck, were usedas source of metal ions and Na2S. 9H2O (Merck) was used as sourceof sulfide ion. Hydrochloric acid and sodium hydroxide were ap-plied for variation of pH of sample solutions. The dye of methyleneblue (C.I. name: Basic Blue 9, C16H18ClN3S:3H2O) (see Inline Sup-plementary scheme 1) was purchased from Fluka company.

Preparation of molecular sieves nanoparticles

Nano-sized mesoporous MCM-41 silica with particle size�90 nm was synthesized by a room temperature method withsome modification to the procedure described in the literature[7]. We used tetraethylorthosilicate (TEOS: Merck, 800658) as asource of silicon and hexadecyltrimethylammonium bromide(HDTMABr; BOH, 103912) as a surfactant template for preparationof the mesoporous material. The molar composition of the reactantmixture is as follows:

TEOS : 0:31NaOH : 0:125HDTMABr : 1197H2O

The prepared nano-sized MCM-41 was calcined at 550 �C for 5 hto decompose the surfactant and obtain the white powder. Surfac-tant-free MCM-41 nanoparticles were used for loading the zincsulfide nanoparticles.

Preparation of ZnS/MCM-41 nanocomposite

As a precursor of ZnS semiconductors, solution of Zn (CH3COO)2

(0.1 mol l�1) was prepared. To 50 ml of Zn (CH3COO)2 solution, 1 gof MCM-41 powder was added and the mixture stirred for 12 h at25 �C. The sample was then washed to remove nonexchanged Zn2+

and air-dried. Finally, sulphurizing of the Zn2+ ions was carried outwith 0.1 M Na2S solution. To make the reaction with the S2� ion,1 g Zn2+ – exchanged zeolite was added to 50 mL of 0.1 M solutionof Na2S at a fixed temperature and magnetically stirred for 2 h.Samples were washed with deionized water and collected by filtra-tion. The obtained samples were in fine white colored powderform. The samples were stable at ambient condition and their colordid not change when exposed to atmospheric moisture. The ZnSparticles prepared from MCM-41 are described as ZnS/MCM-41,in subsequent discussions.

Characterization

X-ray diffraction (XRD) pattern was recorded on a Seisert Argon3003 PTC using nickel-filtered XD-3a Cu Ka radiations(k = 1.5418 Å). The UV–vis diffused reflectance spectra (UV–visDRS) were obtained from UV–vis Scinco 4100 spectrometer withan integrating sphere reflectance accessory. BaSO4 was used as areference material. UV–vis absorption spectra were recorded usinga Shimadzu 1600 PC in the spectral range of 190–900 nm. Trans-mission electron microscopy (TEM) was performed on a PhilipsCM10 and microscope operated at 100 kV. Samples were preparedby dispersing the powder in ethanol. Imaging was enabled bydepositing few drops of suspension on a carbon coated 400 meshCu grid. The solvent was left to evaporate before imaging. Scanningelectron microscopy (SEM) images of fabricated ZnS nanoparticleswere obtained using LEO 440i electron microscope. The specific sur-face area and pore volume of the samples were calculated accordingto the Brunauer–Emmett–Teller (BET) method. Infrared spectra onKBr pellet were measured on a Bruek spectrophotometer.

Procedures of photodegradation of methylene blue

Photodegrdation experiments were performed with a photocat-alytic reactor system. This bench-scale system consisted of cylin-drical Pyrex-glass cell with 1.0 L capacity, 10 cm inside diameterand 15 cm height. A UV lamp was placed in a 5 cm diameter quartztube with one end tightly sealed by a Teflon stop the lamp and thetube were then immersed in the photoreactor cell with a light pathof 3.0 cm. The photoreactor was filled with 25 ml of 0.16–3.2 ppmof dye as pollutant and 0.02–5.00 g/L of ZnS/MCM-41 as nano-photocatalyst. The whole reactor was cooled with a water-cooledjacket on its outside and the temperature was kept at 25 �C. Allreactants in the reactions were stirred using a magnetic stirrer toensure that the suspension of the catalyst was uniform duringthe course of the reaction. To determine the percent of destructionof dyes, the samples were collected at regular intervals, and centri-fuged to remove the nanocatalyst particles that exist as undis-solved particles in the samples.

The wavelengths absorbance maximum (kmax) of methyleneblue is 664 nm. Therefore, photometric analysis of samples beforeand after of irradiation can be used for measurement of the %D(degradation efficiency of dye). The absorption of solution and so-lid samples was measured by a UV–vis spectro photometer shima-dzu model 1600 PC and UV–vis diffused reflectance spectrometerUV–vis DRS Scinco model 4100, respectively. The decrease ofabsorbance value of samples at kmax of dye after irradiation in acertain time interval will be shown rate of decolorization andtherefore, photodegradation efficiency of the dye as well as the

A. Pourahmad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 193–198 195

activity of nanoparticles as photocatalyst. The decolorization anddegradation efficiency have been calculated as:

%D ¼ 100� ½Co � C=Co�

where Co is the initial concentration of dye and C is the concentra-tion of dye after irradiation in selected time interval.

In order to obtain maximum degradation efficiency, pH, concen-tration of dye and amount of photocatalyst were studied in ampli-tudes of 2–12, 0.16–3.2 ppm and 0.02–5 g/L respectively.

Results and discussion

Characterization of ZnS nanoparticles in molecular sievesnanoparticles

Fig. 1 shows the XRD patterns of MCM-41 template and ZnS load-ing on the template. Measurements of the samples were carried outin different 2h ranges (2h = 2–10� and 2h = 20–60�), in the conditionof 40 kV and 40 mA, at a step size of 2h = 0.02�. Fig. 1a shows low

2 3 4 5 6 7 8 9 10 2θ

Inte

nsit

y (a

u)

(100)

(110) (200)

(210) MCM-41

Inte

nsit

y (a

u)

ZnS/MCM-41

(100)

(110) (200)

2θ

20 30 40 50 60

2θ

ZnS/MCM-41

MCM-41

(111)

(220) (311)

Inte

nsit

y (a

u)

(a)

(b)

Fig. 1. (a) X-ray diffraction patterns of MCM-41 and ZnS/MCM-41 in range 2h = 2–10� and (b) X-ray diffraction patterns of MCM-41 and ZnS/MCM-41 in range2h = 20–60�.

Table 1Band gap, specific surface area, particle size, pore volume and absorption edge of samples

Sample Band gap (eV) Specific surface area (m2/g)

MCM-41 – 850Bulk ZnS 3.6 –ZnS/MCM-41 4.04 730

angle X-ray diffraction patterns of calcined MCM-41 nanoparticlesand ZnS/MCM-41. Pattern of the product clearly exhibits fourwell-defined peaks which can be indexed with (100), (110),(200), and (210) planes on hexagonal unit cell, indicating mesopor-ous structure of MCM-41. The peak appearing at low angle (2h = 2.2)corresponds to (100) plane of MCM-41 indicating ordered porestructure of MCM-41, which can be attributed to quasi-two dimen-sional hexagonal lattice of MCM-41 [8]. In this figure, we observedthat all X-ray patterns are very similar. However, some differences,such as the broadening and slight shifting to higher angles of the dif-fraction peaks as well as the decrease of their intensity can be ob-served in the spectra. This should be attributed to the porefillingeffects that can reduce the scattering contrast between the poresand the framework MCM-41 materials. These decreases of the peakintensities, are in agreement with the reported results for other zeo-lites [9]. High angle X-ray diffraction pattern (2h = 20–60), Fig. 1b,further supports presence of ZnS. The diffraction peaks located at(2h = 28.5), (2h = 47.7) and (2h = 56.3), coincide with those of b-ZnS (111), (220), and (311), respectively. This means that ZnS crys-tals prepared in this study have zinc blended structure. We havedetermined the size of nanoparticles at the mentioned 2h by usingthe Debye–Scherrer formula, d = 0.9k/b cosh, where d is the averagediameter of the crystalline, k the wavelength of X-ray, b the excessline width of the diffraction peak in radians and h the Bragg angle.Based on this analysis the average size of ZnS nanoparticles has beenfound to be 4 nm (Table 1).

The surface morphology of MCM-41 nanoparticles and ZnS/MCM-41 is investigated by SEM and the micrographs are presentedin Fig. 2 (see Inline Supplementary). Fig. 2a shows the SEM imageof MCM-41 nanoparticles where small spherical particles of nano-mesoporous silica MCM-41 with diameters of �90 nm are evident.There is no considerable change in morphology of ZnS/MCM-41.

The UV–vis diffused reflectance spectra (UV–vis DRS) for ZnSnanoparticles prepared from MCM-41 matrices, bulk ZnS areshown in Fig. 3. Bulk ZnS (Fig. 3) gave absorption below 400 nm.Comparing the absorption edge of bulk ZnS to that of ZnS/MCM-41 sample prepared from mesoporous materials, it is seen that ablue shift in the onset of absorption has occurred in this sample.This blue shift indicates that ZnS exists as small clusters in the ma-trix. This was supported by a significant decrease in the surfacearea of ZnS/MCM-41, compared to the parent zeolite (Table 1). Thisphenomenon of blue shift of absorption edge has been ascribed to adecrease in particle size. It is well known that in case of semicon-ductors the band gap between the valence and conduction band in-creases as the size of the particle decreases in the nanosize range.This results in a shift in the absorption edge to a lower wavelengthregion. The magnitude of the shift depends on the particle size ofthe semiconductor. In present study, the ZnS/MCM-41 samplesprepared from the MCM-41 matrix showed a blue shift of approx-imately 30 nm compared to the bulk particles. From the onset ofthe absorption edge, the band gap of the ZnS particles was calcu-lated using the method of Tandon and Gupta [10] (Table 1). Thesize of ZnS nanoparticles, estimated based on the results of Welleret al. [11] was 4 nm for ZnS/MCM-41 sample.

The results of the specific surface area and pore volume mea-surements (BET measurements) for MCM-41 and ZnS/MCM-41,show that the pore volume of the host mesoporous material, which

.

Particle size (nm) Pore volume (cc/g) Absorption edge (nm)

90 1.05 –– – 3754 0.89 345

600Wavelength (nm)

bulk ZnS

ZnS/MCM-41

1.6

1.2

0.8

0.4

0.0

Abs

orba

nce

(au)

300 350 400 450 500 550

Fig. 3. UV–vis absorption spectrum of bulk ZnS and ZnS/MCM-41.

196 A. Pourahmad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 193–198

was 1.05 ml g�1 for MCM-41, decreased to 0.89 ml g�1 for ZnS/MCM-41materials. Similarly, the specific surface area of the com-posite materials was decreased from 850 m2 g�1 for MCM-41 to730 m2 g�1 for ZnS/MCM-41. Decreasing in the volume of the poresand the specific surface area of the mesoporous material demon-strates that the guests are located in the matrix (Table 1).

The IR spectra of MCM-41 and ZnS/MCM-41 samples in therange of 400–4000 cm�1 are shown in Fig. 4. The broad absorption

3500 3000 2500

Wav

Tra

nsm

itta

nce

[%]

6080

100

4020

0

60

8010

040

20

0

Tra

nsm

itta

nce

[%]

MCM-41

ZnS/MCM-41

Fig. 4. IR spectra of MCM-

band in the region 3765–3055 cm�1 can be attributed to thestretching of the framework Si–OH group with the defective sitesand physically adsorbed water molecules. The vibrations of Si–O–Si can be seen at 1091 cm�1 (asymmetric stretching), 805 cm�1

(symmetric stretching) and 454 cm�1 (bending). In present work,all bands in ZnS/MCM-41 sample show shift to higher wave num-bers with respect to the MCM-41 zeolite. This shift reveals thatnanoparticles could incorporate in MCM-41 zeolite. The increasedintensity is observed in nanoparticle sample with respect to MCM-41 zeolite. This increase relates to extent of perturbation of T–O–Tvibrations of the zeolite lattice (bands 400–1700 cm�1) and in-crease in acidic bridged hydroxyls vibrations (3423 cm�1 band)[12]. These kinds of differences are related to presence of ZnSnanoparticles in MCM-41.

Transmission electron microscopy along with the textural prop-erties of the samples discussed above bring us important informa-tion regarding whether the ZnS particles are located inside oroutside the pore structures used in this work. TEM image of ZnS/MCM-41 sample is shown in Fig. 5 (see Inline Supplementary). Itwas recorded under parallel direction to the pore axis and it is pos-sible to observe typical MCM-41 morphology in the micrograph.Although it is very difficult to identify ZnS nanoparticles withinthe pores of MCM-41 materials by using TEM techniques [13],the higher contrast in the image of the ZnS/MCM-41 sample canbe associated with the presence of ZnS nanoparticles inside thepores of this sample.

2000 1500 1000 500

enumber (cm-1)

41 and ZnS/MCM-41.

Fig. 6. Spectra change that occur during the photocatalytic degradation of aqueoussolution of methylene blue: pH = 7, [15 wt% ZnS/MCM-41] = 0.4 g/L, Co = 0.32 ppm.

A. Pourahmad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 193–198 197

Photodegradation of methylene blue using ZnS/MCM-41nanocomposite

The time dependent electronic absorption spectrum of dye dur-ing photo irradiation was presented in Fig. 6. After 15 min of irra-diation under UV light in a ZnS/MCM-41 suspension, 95% of dyewas decomposed and decolorization of solution was observed. Be-sides, no new bands appear in the UV–vis region due to the reac-tion intermediates formed during the degradation process. Theeffect of UV irradiation, ZnS, MCM-41 and ZnS/MCM-41 materialon photodegradation of methylene blue are shown in Fig. 7. Thisfigure indicates that in the presence of mixed photonanocatalystand UV irradiation, 95% of dye degraded at the irradiation timeof 15 min while it was 15% for MCM-41 and UV irradiation and45% for ZnS (without MCM-41) at the irradiation time of 60 min.It is 3% degradation in the presence of UV light alone. These exper-iments demonstrated that both UV light and a photocatalyst, areneeded for the effective degradation of dye. It is well documentedthat the absorption of photons possessing energy equal to or higherthan that of the semiconductor (4.04 eV for ZnS in MCM-41) causescharge separation:

ZnSþ hv ! ZnSðe�Þ þ ZnSðhþÞ

The photogenerated holes may then react with adsorbed dyeand oxidize the dye molecule by the formation of hydroxyl radi-cals. The photo-produced electrons in the conduction band reactwith the adsorbed oxygen to produced reactive radicals to yieldthe reactive oxygen species.

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70

Time/min

Dec

omp

osit

ion

(D

%) ZnS/MCM-41

ZnS nanoparticle

MCM-41 nanoparticle

UV

Fig. 7. Effect of UV light and different photocatalyst on photocatalytic degradationof methylene blue. Co = 0.32 ppm, [15 wt% ZnS/MCM-41] = 0.4 g/L, pH = 7.

To understand the role of support during the photocatalyticdegradation of dye, the amount of ZnS nanoparticles available over15 wt% ZnS supported system is considered for the degradationactivity and the studies are carried out using 0.4 g/L catalyst in0.32 ppm methylene blue. From Figs. 6 and 7, it is observed thatZnS supported system is showing higher rate of degradation thanZnS or MCM-41 alone. This is due to the higher adsorption capacityand also OH radicals’ availability. The 95% degradation of methy-lene blue on 15 wt% ZnS/MCM-41 is observed within 60 min andit is 45% for ZnS nanoparticles. The adsorption capacity of MCM-41 mesoporous enhances the change of OH radicals attack on theadsorbed methylene blue molecules resulting faster degradationrates. Furthermore, ZnS nanoparticles dispersion over MCM-41mesoporous avoids particle–particle aggregation and light scatter-ing by ZnS. Apart from all, the delocalizing capacity of MCM-41mesoporous can effectively separate the electrons and holes pro-duced during photo excitation of ZnS, thus enhancing the photo-catalytic efficiency [14].

Effect of variables influence on degradation efficiency

Effect of nanocatalyst amountIn order to determine the optimal amount of photocatalyst,

some experiments were performed at pH 7 by varying the amountof catalyst from 0.02 to 5 g/L. The effect of catalyst mount on therate of degradation is shown in Fig. 8 (see Inline Supplementary).As seen, the optimum nanocatalyst amount for degradation of ba-sic dye is 0.4 g/L. It is observed that rate increases with increase incatalyst amount from 0.02 to 5 g/L .This is probably due to increasein the number of ZnS nanoparticles, that increases the number ofphoton absorbed and dye molecule absorbed. Increase of the cata-lyst amount more than 0.4 g/L results in the decrease of degrada-tion rate. This phenomenon may be explained by aggregation ofZnS nanoparticles at high concentrations causing a decrease inthe number of surface active sites and increase in opacity and lightscattering of ZnS nanoparticles at high concentration loading to de-crease in the passage of irradiation through the sample.

Effect of the composition of the supported photocatalystThe effect of ZnS loading on MCM-41 material is investigated

with 2–20 wt% content and the results is depicted in Fig. 9 (see In-line Supplementary). The effective decomposition of dye after15 min irradiation time was observed when the photocatalyst con-tained 15 wt% ZnS, prepared by using ion-exchange method. Forcomment of this result, we propose that the hydroxyl radical onthe surface of nanoparticle ZnS is easily transferred onto the sur-face of nanomesoporous material. This means the organic pollu-tants, which have already been adsorbed on the mesoporousmaterials, have a chance to be degraded due to the appearance ofhydroxyl radical, resulting in the enhancement of photodegrada-tion performance of ZnS/MCM-41. Experimental results show thatabout 15 wt% of ZnS is the best condition to achieve the synergismbetween ZnS and MCM-41. This synergetic effect may be due to thefact that the presence MCM-41 maintaining the molecules of dyenear the photocatalyst as depicted in Fig. 10 (see Inline Supple-mentary). The enhanced phtocatalytic activity over the compositeZnS/MCM-41 is reflecting the beneficial adsorption properties ofMCM-41. If decrease the ZnS in composition of photocatalyst (lessthan 15 wt%) the rate of production reaction of the hydroxyl radicalby ZnS under UV irradiation is not enough to react with all the mol-ecules of dye that are absorbed on the surface of MCM-41 and if in-crease the ZnS in composition of photocatalyst, the adsorptionability of MCM-41 in compared with the rate of production reac-tion of hydroxyl radical with ZnS under UV irradiation decrease.

198 A. Pourahmad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 193–198

Effect of concentration of dyesThe degradation efficiency of dye decreased with increasing the

initial concentration of dye to more than 0.32 ppm. The results areshown in Fig. 11 (see Inline Supplementary). The decrease of %Dwith increase of concentration of dye can be due to two reasons.With increasing the amounts of dye, the more of dye moleculeswill be adsorbed on the surface of the photocatalyst and the activesites of the catalysts will be reduced. Therefore, with increasingoccupied space of catalyst surface, the generation of hydroxyl rad-icals will be decreased. Also, increasing concentration of dye can belead to decreasing the number of photons that is arrived to the sur-face of catalysts. The more light are adsorbed by molecules of dyeand the excition of photocatalyst particles by photons will be re-duced. Thus, photodegradation efficiency diminished [15].

Effect of pHPhotodegradation of dye (0.32 ppm) was studied in amplitude

pH of 2.0–12 in the presence of ZnS/MCM-41 catalyst (0.4 g/L).The results for irradiation time of 15 min are shown in Fig. 12(see Inline Supplementary). In all case, the maximum degradationefficiency was obtained in alkaline pH 8 for methylene blue. Inpresence of ZnS/MCM-41 and in pH 8, degradation efficiency 99%is obtained. Probably, the surface, of photocatalyst is positivelycharged in acidic solutions and negatively charged in alkaline solu-tion. As a result, it is not surprise the increasing of the adsorptionof dye molecules (with positive charge) on the surface of photocat-alyst in alkaline solutions and thus the increasing of degradationefficiency of dye [16].

A low pH is associated with a positively charged surface whichcan not provide hydroxyl group which are needed for hydroxylradical formation. On the other hand, higher pH value can providehigher concentration of hydroxyl ions to react with the holes formhydroxyl radicals [17]. But, the degradation of dye is inhibitedwhen the pH value is so high (pH > 8) because the hydroxyl ioncompete with dye molecules in adsorption on the surface of photo-catalyst [18].

As the other words, at low pH, the adsorption of cationic dyeson the surface of photocatalysts decreased because the photocata-lysts surface will be positively changed and repulsive forces is dueto decreasing adsorption. Thus, the degradation efficiency will bedecreased in acidic pH.

Kinetics of photocatalytical degradation of dye

Several experimental results indicated that the degradationrates of photocatalytic oxidation of various dyes over different cat-alyst fitted by first order kinetic model [19]. Fig. 13 (see Inline Sup-plementary) shows the plot of ln([dye]o/[dye]) vs. irradiation timefor methylene blue. The linearity of plot suggests that the photo-degradation reaction approximately follows the pseudo-first orderkinetics with k = 0.036.

Recycling studies

Fig. 14 (see Inline Supplementary) shows the reproducibility ofZnS/MCM-41 as nanoparticles for basic blue 9 photodegradationduring a four cycles experiment. Each experiment was carriedout under identical concentration of 0.32 ppm of dye, 0.4 g/L ofnanocatalyst, pH of 8, irradiation time of 15 min and at room tem-perature. After each degradation experiment, the concentration ofdye was adjusted back to its initial value of 0.32 ppm. As seen fromFig. 14a small and gradual decrease in the activity of nanocatalyst

was observed at the first two cycles. The difference may be due tothe accumulation of organic intermediate in the cavities and onsurface of the mesoporous materials thus affecting the adsorptionin turn reducing the activity. Nanocatalyst is calcined at 550 �C for2 h and reused; the rate of degradation is restored and is equiva-lent to fresh catalyst. Thus, the calcinations of the used catalystare necessary in order to maintain the activity.

Conclusions

1. The ion-exchange method is an effective method for support ofZnS on MCM-41.

2. A photocatalyst containing 15 wt% ZnS has the maximum effi-ciency on photodegradation of methylene blue.

3. The photodegradation conversion of dye decreases with anincrease in the initial concentration methylene blue.

4. pH is one of the main effecting factors and the optimum pH wasobtained about 8.

5. The kinetics of photocatalytic degradation of basic blue 9 is ofthe pseudo-first order with k = 0.036.

6. ZnS/MCM-41 can be used as an efficient photocatalyst for deg-radation of dyes under UV light irradiation.

Acknowledgment

We thank the Research Vice Presidency of Islamic Azad Univer-sity, Rasht Branch for their encouragement, permission and finan-cial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2012.10.053.

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