Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 142–150

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Facile and low temperature route to synthesis of CuS nanostructure in mesoporous material by solvothermal method Sh. Sohrabnezhad a,⇑, M.A. Zanjanchi a, S. Hosseingholizadeh b, R. Rahnama b a b

Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht, Iran Department of Chemistry, Faculty of Science, Payam_Noor University, Sari, Iran

h i g h l i g h t s

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

 We used solvothermal method for

synthesis of CuS/MCM-41 nanocomposite.  CuS material has different phases. The synthesis only one phase of CuS is very hard.  We synthesized one phase of CuS nanostructure by solvothermal method in ethylene glycol.  It is observed different phases for CuS by solvothermal method in water solvent.  For characterization of heterogeneous catalysts we need to novel experimental techniques.

a r t i c l e

i n f o

Article history: Received 9 October 2013 Received in revised form 23 November 2013 Accepted 4 December 2013 Available online 18 December 2013 Keywords: Polyol method Copper sulfide nanostructures Covellite phase Chalcocite phase X-ray diffraction Photocatalysis

CuS

CuS

CuS

MB

MCM-41

a b s t r a c t The synthesis of CuS nanomaterial in MCM-41 matrix has been realized by chemical synthesis between MCM-41, copper sulfate pentahydrate and thiourea via a solvothermal method in ethylene glycol and water, separately. X-ray diffraction analysis (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and fourier transform infrared (FT-IR) were used to characterize the products. At synthesized CuS/MCM-41 sample in ethylene glycol, X-ray diffraction and diffuse reflectance spectroscopy showed pure covellite phase of copper sulfide with high crystality. But prepared CuS/MCM-41 sample in water shows the covellite, chalcocite and the djurleite phase of copper sulfide nanostructures. The formation of CuS nanostructures was confirmed by FT-IR. Photocatalytic activity of CuS/MCM-41 nanocomposites was studied for degradation of Methylene Blue (MB) under visible light. The CuS/MCM-41 nanocomposite is more effective nanocatalyst than synthesized CuS/MCM-41 sample in water for degradation of methylene blue. Several parameters were examined, catalyst amount (0.1–1 g L1), pH (1–13) and initial concentration of MB (0.96–10 ppm). The extent of degradation was estimated from the residual concentration by spectrophotometrically. The support size was obtained in the range 60–145 nm by TEM. In the same way, the average size of copper sulfide in CuSMCM-41E and CuS/MCM-41W nanostructures were obtained about 10 nm and 16 nm, respectively. Ó 2013 Elsevier B.V. All rights reserved.

Introduction Synthetic dyestuffs used by several industries such as textile, dyeing and printing industries are a major source of water ⇑ Corresponding author. Tel.: +98 911 335 8616; fax: +98 131 323 3264. E-mail address: [email protected] (Sh. Sohrabnezhad). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.050

pollution which is visible even in a low concentration of dyes. Removing color from wastewaters is often more important than other colorless organic substances because of their considerable effects on the environmental water [1–4]. Heterogeneous photocatalysis has already been investigated and successfully applied to the degradation of different organic pollutants. Among the various methods, a great deal of attention has paid to the advanced

Sh. Sohrabnezhad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 142–150

oxidation process (AOP) for the treatment of air and water streams [5]. AOP can generate free radicals, such as hydroxyl radicals (OH) which are strong and nonselective oxidant species that react with the majority of organic pollutants. Free radicals such as HO2 and O2 may also be involved in the degradation process, but these radicals are less effective than the hydroxyl radicals [6]. Up to date, remarkable progresses have been made in the photodegradation of dye pollutants under ultraviolet (UV) light, while less efforts have been paid to the visible light. Therefore, effective utilization of visible light to degrade different organic pollutants using the solar energy has been an attractive attempt in recent years [7,8]. Nano-scale semiconductor particles possess higher surface area to volume ratio than their bulk counter parts, and thus allow for greater photon adsorption on the photocatalyst surface [9–11]. As a result, such materials offer potential uses in catalysis, electronics, sensors, light transmission and solar cells applications [12–16]. Many of the current studies are focused on the synthesis of different nanostructured materials and their various applications. CuS is a transparent p-type semiconductor with a band gap 1.27 eV for bulk form. The top of the valence band is primarily composed of well-hybridized states of Cu 3d and S 3p states, while the bottom of the conduction band consists mainly of Cu 4s state. The band gap of CuS was found to be a direct-allowed transition type through the analysis of the symmetry of these states. It was also found that the dispersion of the valence band is relatively large due to the considerable hybridization of Cu 3d and S 3p states. This dispersed valence band is responsible for the emergence of p-type electrical conduction in this material. On the other hand, the dispersion of the conduction band is rather small, probably because of the layered structure, in comparison with typical n-type conducting materials. This small dispersion of the conduction band leads to the wide band gap and high stability of exactions in CuS [17]. Copper sulfides are a particularly interesting class of metal sulfides due to their ability to form with various stoichiometries. The copper–sulfur system can exist in the chalcocite (Cu2S) and covellite (CuS) phases, with several stable and metastable phases of various stoichiometries between the two ideal ones [17]. Copper sulfide is found to exist into two forms at room temperature as ‘‘Copper-rich’’ and ‘‘Copper-poor.’’ Copper-rich phases exist as anilite (Cu1.75S), digenite (Cu1.8S), djurleite (Cu1.94S) and chalcocite (Cu2S). The Copper-poor phase exists as covellite (CuS). Covellite has been known to exist in two forms, brown CuS and green CuS. Recent studies showed that doping a semiconductor onto a suitable support has several advantages in photocatalysis processes. (1) increases the activity of the semiconductor (2) decreases the high turbidity (3) increases the adsorption of pollutants (4) minimize electron/hole recombination (5) prevent uncontrollable growth of particles (6) prevent particle aggregation (7) controls particle size [18,19]. In this context, molecular sieves due to their unique properties including ion exchange, size, charge and shape electivity, are good candidates to support a semiconductor onto them [20–22]. Incorporated and photocatalysis of copper sulfide in different matrix has not been extensively studied. Thus the development of facile, low temperature and surfactant-free approach for the controlled synthesis of CuS nanostructure in matrix is very imperative to explore different structural aspects of materials. In this work, the photo-efficiency of the CuS incorporated into MCM-41 was studied in the degradation of an aqueous solution containing Methylene Blue (MB). Methylene blue is a widely used colored compound in dyeing and printing textiles. CuS/MCM-41 nanocomposite has been prepared by solvothermal method in water and ethylene glycol (EG) (polyol method), separately. CuSO4 and thiourea were as a source of Cu and S ions, respectively and ethylene glycol as a solvent. Efficiency of two nanocomposite was compared in the degradation of aqueous solution of MB. After

143

the selection of CuS/MCM-41 by polyol method as more effective catalyst with respect to other nanocomposite by solvothermal method, some experiments were performed to investigate the effects of various experimental parameters including catalyst loading, pH of the solution and initial dye concentration on the efficiency of photodegradation process. Experimental Materials All the chemical reagents and solvents used in the present work, including copper sulfate, thiourea, ethylene glycol, ethanol, tetraethylorthosilicate (TEOS), HcL, NaOH and hexadecyltrimethylammonium bromide are analytical grade reagents (Merck) and were used as received without further purification. Hydrochloric acid and sodium hydroxide were applied for variation of PH of sample solutions. The dye of methylene blue (C.I. name: Basic Blue 9, C16H18ClN3S:3H2O) (Scheme 1) was purchased from Fluka company. Preparation of MCM-41 matrix The MCM-41 material was synthesized by a room temperature method with some modification in the described procedure in the literature [23]. We used tetraethylorthosilicate (TEOS: Merck, 800658) as a source of silicon and hexadecyltrimethylammonium bromide (HDTMABr; BOH, 103912) as a surfactant template for preparation of the mesoporous material. The molar composition of the reactant mixture is as follows:

SiO2 : 1:6EA : 0:215HDTMABr : 325H2 O where EA stand for ethylamine. The MCM-41 prepared was calcined at 550 °C for 5 h to decompose the surfactant and obtain the white powder. This powder was used for loading the CuS nanostructures. Preparation of CuS/MCM-41 catalysts Preparation of CuS/MCM-41 by polyol method In a typical synthesis, 1 mmol CuSO45H2O (0.31 g) was dissolved in 50 mL ethylene glycol and a green solution was formed. Then 2 mmol thiourea (Tu, SC(NH2)2, 0.19 g) was added into the above mentioned solution under vigorous stirring for 30 min. After that, MCM-41 matrix was treated in above solution for 2 h. Adsorption of sulfate thiourea complex of copper [Cu(SC(NH2)2)2SO4] from aqueous solution on the MCM-41 matrix followed by thermal decomposition of the adsorbed complex at 250 °C with formation of finely dispersed copper sulfides and elimination of gaseous products (Eq. (1)): ½CuðSCðNH2 Þ2 Þ2 SO4 

250  C air

!

CuS þ 2N2 " þ2HcL " þH2 S " þ2CO2 " þ2H2 O "

The black product (CuS) was collected and washed repeatedly with deionized water and ethanol several times to remove the impurities and by products. Finally the product was dried in oven at 70 °C for 4 h. The prepared sample is called CuS/MCM-41E. Preparation of CuS/MCM-41 by solvothermal method in water In this method, CuS nanostructures synthesis was carried out as mentioned above under same conditions only water was used

Scheme 1. Structure of methylene blue.

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instead of ethylene glycol and temperature was 90 °C. Meanwhile, the black product was washed with deionized water several times and was dried in oven at 70 °C for 4 h. The prepared sample is called CuS/MCM-41W. Preparation of CuS nanostructures For the comparing, as reference sample, also CuS composite was synthesized by solvothermal method in ethylene glycol and water separately. In a typical synthesis 1 mmol CuSO45H2O (0.31 g) were dissolved in 50 mL ethylene glycol and water separately and a green and blue solutions were formed respectively. Then 2 mmol thiourea (Tu, SC(NH2)2, 0.19 g) was added into the above mentioned solutions under vigorous stirring for 30 min. Afterwards, the solutions were transferred into a 60 mL Teflon-lined stainless steel autoclaves, sealed, and maintained at 150 °C for 24 h on an oven, separately. The rate of temperature increasing is 3 °C/min. Then the autoclaves were cooled naturally for about 6 h to room temperature. Finally, the products were centrifuged and washed with distilled water and ethanol three times, respectively, and dried under vacuum at 60 °C for 4 h. The reaction conditions were varied to explore the effect of different reaction parameters on the size and morphology of the products. The prepared samples are called CuS/E and CuS/W in solvents ethylene glycol and water, respectively. Characterization Powder X-ray diffraction patterns of the samples were recorded using a X-ray diffractometer (Bruker D8 Advance) with Cu K radiation (k = 1.54 Å). The UV–Vis diffused reflectance spectra (UV–Vis DRS) obtained from UV–Vis Scinco 4100 spectrometer with an integrating sphere reflectance accessory. BaSO4 was used as a reference material UV–Vis absorption spectra were recorded using a shimadzu 1600 pc in the spectral range of 190–900 nm. Chemical analysis of the samples was done by energy dispersive X-ray analysis (EDX) joined a Philips XL30 scanning electron microscopy (SEM). The infrared spectra on KBr pellet were measured on a Bruek spectrophotometer. The transmission electron micrographys (TEM) were recorded with a Philips CM10 microscope, working at a 100 kV accelerating voltage. Samples for TEM were prepared by dispersing the powdered sample in acetone by sonication and then drip drying on a copper grid coated with carbon film. The specific surface areas were calculated according to the Brunauer–Emmett–Teller (BET) method. Photocatalytic performance Photodegrdation experiments were performed with a photocatalytic reactor system. This bench – scale system consisted of cylindrical Pyrex – glass cell with 1.0 L capacity, 10 cm inside diameter and 15 cm height. A 100 w tungsten filament Philips lamp (k > 400 nm) was placed in a 5 diameter quartz tube with one end tightly sealed by a Teflon stop. The lamp and the tube were then immersed in the photoreactor cell with a light path of 3.0 cm. The photoreactor was filled with 50 ml of 0.32–16 ppm dye as pollutant and 0.005–0.1 g/L of CuS/MCM-41 nanocrystal as nanophotocatalyst. The whole reactor was cooled with a water – cooled jacket around its outside and the temperature was kept at 25 °C .All reactants in the reactions were stirred using a magnetic stirrer to ensure that the suspension of the catalyst was uniform during the course of reaction. To determine the percent of the destruction of dyes, the samples were collected at regular intervals, and centrifuged to remove the nanocatalyst particles that exist as undissolved particles in the samples. The maximum absorbance wavelength (kmax) of methylene blue is 664 nm. Therefore, photometric analysis of samples before and

after irradiation can be used for measurement of the %D (degradation efficiency of dye). The absorption of solution and solid samples was measured by a UV–Vis spectro photometer shimadzu model 1600 PC and UV–Vis diffused reflectance spectrometer UV–Vis DRS Scinco model 4100, respectively. The decrease of absorbance value of samples at kmax of dye after irradiation in a certain time interval will show the rate of decolorization and therefore, photodegradation efficiency of the dye as well as the activity of nanostructures as photocatalyst. The decolorization and degradation efficiency have been calculated as:

%D ¼ 100  ½C o  C=C o  where Co is the initial concentration of dye and C is the concentration of dye after irradiation in selected time interval. In order to obtain maximum degradation efficiency, pH, concentration of dye and amount of photocatalyst were studied in amplitudes of 1–13, 0.96–10 ppm and 0.1–1.0 g/L respectively. Results and discussion Characterization of the photocatalyst XRD patterns In order to confirm the crystalline structure of prepared nanocrystal catalysts, powder XRD study was carried out. The low angle X-ray powder diffraction patterns of the prepared matrix are presented in Fig. 1.The XRD pattern of MCM-41 shows typical characteristic three-peak pattern with a very strong one at a low 2h and two peaks at higher 2h values [24,25]. In our previous work we showed that no peak is observed between 2h = 10–70° for matrix [25]. Fig. 2 displays the XRD patterns of CuS/E (a), CuS/MCM-41E (b), CuS/W (c) and CuS/MCM-41W (d) samples. Measurements of the samples were carried out in the 2-theta degrees covering the range of 10–70°, in the condition of 40 kV and 40 mA, at a step size of 2h = 0.02°. Fig. 2a presents the XRD patterns of copper sulfide powders when the solvent of reaction was ethylene glycol. When comparing the results of XRD with the JCPDS card details 060464 corresponding to synthetic covellite, the main peaks coincide with 27.6°, 29.2°, 32.1°, 47.9°, 52.7° and 59.3° of the primitive hexagonal structure of covellite (CuS), similar to those reported on the synthesis of CuS nanostructures by other methods [26–30]. The XRD pattern of CuS/MCM-41E sample is well matched with the pattern of CuS/E (Fig. 2b). This indicates that CuS nanostructures were incorporated in MCM-41 matrix. No diffraction peaks corresponding to other phases of copper sulfide or CuO could be detected in the XRD pattern, which confirms that pure covellite CuS could be obtained under the current synthetic route. The XRD patterns of CuS and CuS/MCM-41 samples by solvothermal method in water have showed in Fig. 2c–d. When water was used as solvent,

Fig. 1. X-ray diffraction pattern of MCM-41.

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370 650-800 450 280

305

365 450

640-680

520

Intensity (a.u)

Absorbance (a.u)

355 280

305

700

340 305 270 650-700 260

20

30

40

50

60

2θ Fig. 2. XRD patterns of CuS samples prepared at different solvents (water and ethylene glycol).

spectra become broader and observed other diffraction peaks. However, the broadened diffraction peaks in Fig. 2c–d imply that the prepared products consist of slight distortions of the crystal lattice occur [31]. Fig. 2c shows peaks at 2h = 19.5°, 23°, 24.5°, 37.5° and other peaks at 2h = 43°, 45° which can be attributed to Cu2S and Cu1.94S phases, respectively [31–33]. The other diffraction peaks in XRD pattern of CuS/W attribute to hexagonal structure of covellite (CuS) [26]. The XRD pattern of CuS/MCM-41W shows one broad peak at 2h = 23° and other peak at 2h = 28° that can be attributed to Cu2S phase [31,32]. The single broad peak into three peaks at 31.5°, 32° and 32.5° that 2h = 31.5° corresponding to Cu1.94S phase [33] and other are attributed to covellite CuS [26–30]. The average crystallite sizes of matrix and CuS/MCM-41 samples were calculates by scherrer’s equation. The MCM-41 matrix has crystal size almost 153 nm. The average crystal size for CuS/ MCM-41E and CuS/MCM-41W is 17 and 22 nm, respectively. The same way, average size for CuS/E and CuS/W was calculated 10 and 19 nm, respectively. Diffuse reflectance spectra To ascertain the capability of each particular composite material to photodegradation organic chemicals in the visible range of spectrum, one needs to analysis the UV–Vis diffuse reflectance spectra. Fig. 3 shows that all of CuS samples can be used as efficient photocatalysts under visible light irradiation (absorption edge in visible region). The MCM-41 matrix shows small broad peaks at 260 and 310 nm. These bands are attributed to a charge transfer transition of framework tetrahedral atoms [34]. The Cu2+ ions have a 3d9 electronic structure. In the presence of a crystal field generated by ligands, d–d transitions appear in the visible or near-IR range which is ascribed to the formation of covellite CuS phase. All CuS samples show broad weak absorption in the visible-light region (640–800 nm) and 270–305 nm. These bands can be attributed to the covellite phase of copper sulfide which is blue shifted

300

310

400

500

600

700

Wavelength (nm) Fig. 3. UV–Vis diffuse reflectance spectra of the MCM-41 and CuS samples.

because of quantum confinement from its bulk in IR region and ligand to copper charge transfer, respectively [25,35–37]. On the other hand, CuS/W and CuS/MCM-41W samples show weak absorbance at 450 nm. This band is due to Cu2S (chalcocite) phase [25,31,36]. The other broad peak is observed at 500–600 nm for CuS/W sample. This band is due to plasmon resonance band copper [38–40]. The kmax at 340, 350, 365 and 370 nm for CuS/E, CuS/ MCM-41E, CuS/W and CuS/MCM-41/W are attributed to the CuS nanostructure, respectively. The optical band gap estimated from the absorption edges of kmax are 3.38 eV, 3.10 eV, 3.00 eV and 2.83 eV for the synthesized CuS nanostructures, respectively (Table 1). The estimated optical band gap energy value is higher than that of bulk value (1.27 eV) [32], which is attributed to the quantum confinement effect arising from smaller crystal size. The results of diffuse reflectance spectra and XRD patterns confirms that synthesized CuS samples in ethylene glycol solvent are pure covellite phase and no peaks corresponding to other phases of copper sulfide. Meanwhile, in processes of photodegradation organic chemicals, CuS/E and CuS/W samples show high turbidity that decreases the radiation flux. The use of mesoporous supported semiconductor has allowed the enhancement of photodegradation rate in comparison with neat semiconductor [41]. SEM images The surface morphology of CuS nanostructures and CuS/MCM41 samples were investigated by SEM and the micrographs are presented in Fig. 4. Fig. 4a shows the morphologies of CuS nanostructures by polyol method. The typical SEM image reveals the formation of hierarchical structures. It can be seen that the product consists of an aggregate of fine microspheres. The microspheres have densely packed arrangement of nanoplates. When H2O was used as solvent, formation of CuS on MCM-41 matrix is very low and random (Fig. 4b). But, when EG was employed as solvent (Fig. 4c), ball-like CuS nanospheres were obtained. In fact,

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Table 1 Specific surface area (BET) and pore volume of prepared nanoparticle samples from MCM-41 and CuS samples. Sample

Abs. edges (nm) Short

Band gap (eV)

Specific surface area (m2/g)

Pore volume (m3/g)

MCM-41 CuS/MCM-41E CuS/MCM-41W CuS/E CuS/W

– 400 436 362 410

– 3.10 2.83 3.38 3.00

1025 1228 1041 23 14

0.72 0.91 0.78 0.051 0.042

and aggregate together to form hierarchical network. In case of water there was no coordinating ability, so direct metal-Tu complexes formed and the morphology tended to be spherical shaped products. The BET results show that, the pore volume of the host materials, which was 0.72 cm3 g1for MCM-41, increased to 0.91 cm3 g1 for CuS/MCM-41E sample. The specific surface areas of the nanocomposite CuS/MCM-41E sample was increased from 1025 m2 g1 for MCM-41 to 1228 m2 g1 for CuS/MCM-41E sample (Table 1). It is apparent that the high surface area CuS/MCM-41E plays a special role in these visible light photocatalysis. High surface area CuS/MCM-41E is better photocatalyst than Degussa P25 TiO2 or other catalyst [44], because CuS/MCM-41E simply facilitates adsorbing organics and allows transfer of these adsorbed compounds to active sites on. Meanwhile, surface area of CuS/ MCM-41W sample is lower than CuS/MCM-41E sample and similar to MCM-41 matrix (Table 1). The typical EDX spectrum of CuS/MCM-4 E sample is shown in Fig. 5. The spectrum shows sharp peaks corresponding to Cu and S, and the atomic ratio of Cu to S is found to be 1:1 within the experimental error. The very weak ‘O’ peak may be originated from the oxidation of the product exposed to the atmosphere since nanocrystalline material exhibits a high surface to- volume ratio. The EDX spectra of CuS/MCM-41E sample reveals information of CuS nanostructures outside of mesoporous material. The EDX spectrum for CuS/MCM-4 W sample is similar to CuS/MCM-41E sample.

morphology of CuS nanosphere in CuS/MCM-41E sample is similar CuS/E sample. The difference in the morphology of CuS incorporated in MCM-41 matrix may be attributed to type of solvent. It is well known that solvent plays an important role in controlling the shape of the materials. Physicochemical properties of solvent such as polarity, viscosity, and softness will strongly influence the solubility, reactivity, collision rate between the reactants molecules and the transporting behavior of the reactants [42]. EG is a bidentate ligand known to have polarity, strong chelating ability and a certain reducing ability, which is important for the reaction [28,43]. There is the strong complexion between Cu2+ions, thiourea (Tu) and EG that leads to the formation of Cu–EG–Tu complex. At elevated temperature the stability of the complex decreases, and at the initial stage, covellite CuS nuclei were formed in EG through the dissociation of the complex. Subsequently, these CuS crystal nuclei preferentially grew in same direction and further turn into nanospheres due to the different surface energies of the hexagonal crystal structure. As the reaction proceed the nanospheres diffuse

Intensity (a.u)

Fig. 4. SEM images of CuS (a), CuS/MCM-41W (b) and CuS/MCM-41E (c).

TEM images TEM images of MCM-41 and CuS/MCM-41 samples are shown in Fig. 6. The morphologies and microstructure of the obtained MCM41 samples are clearly revealed by TEM. The results obtained using TEM are in accordance with those provided by SEM. Fig. 6a shows the typical TEM images of MCM-41sample, where nanosized particles of near sphere or elongated sphere can be consistently seen [45]. The sizes of the particles are in range from 70 to 100 nm. Even darker images have been obtained on MCM-41 reference indicating that the dark aspect can be associated with the presence of water trapped inside MCM-41 grains [46]. The TEM image of CuS/MCM41W samples reveals information of CuS nanostructures outside of mesoporous material (Fig. 6b). The particle size of CuS

1.00

3.00

5.00

7.00

Energy (keV) Fig. 5. EDX spectrum of CuS/MCM-41E sample.

10.00

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on surface MCM-41 material. Increased intensity at 1618– 1650 cm1 and the appearance of intensive bands at 3250, 3163 cm1 in Fig. 7b–c can be attributed adsorption of thiourea on MCM-41or Cu-MCM-41 matirx [49]. In CuS/MCM-41W sample (Fig. 7b) the formation of complex between copper and thiourea from the aqueous solution is confirmed by a shift of intensive band 688 cm1 for free thiorea to 606 cm1 for the complex [Cu(SC(NH2)2)2SO4], which is ascribed to C–S bond [50]. But in CuS/MCM-41E sample (Fig. 7c), the formation of Cu–EG–Tu complex from EG solution is confirmed by a shift of band 608 cm1 for free thiourea in EG solvent to 575 cm1 for the complex. Photocatalytic performance The photooxidation activity of CuS/MCM-41 samples was evaluated by photocatalytic decomposition of methylene blue (Fig. 8). Prior to irradiation, the MB solution over the nanocatalyst was kept in the dark for 40 min to obtain the equilibrium adsorption state. After 40 min of irradiation under visible light in a CuS/MCM-41E nanocomposite suspension, 95% of dye was decomposed and decolorization of solution was observed. Besides, no new bands appear in the UV–Vis region due to the reaction intermediates formed during the degradation process. The effect of visible irradiation, pure MCM-41, CuS/E, CuS/W and CuS/MCM-41 nanocomposite on photodegradation of methylene blue is shown in Fig. 9. This figure indicates that in the presence of mixed photonanocatalyst and visible irradiation, 95% of dye degraded at the irradiation time of 40 min while it was 25%, 12% and 76% for CuS/E, CuS/W and CuS/ MCM-41W nanocomposite, respectively. The degradation observed in the same experiment performed in the absence of CuS material was 21% and in the absence CuS/MCM-41 samples was 2%. These experiments demonstrated that both visible light and a photocatalyst, are needed for the effective degradation of methylene blue. Methylene blue solution and samples were kept in dark for the desired time.

1652

780

906

608 575 460

688 606

790

956

1510 1415 1375

3163

b

1217

1618

nanospheres observed from TEM photographs is around 15 nm. Fig. 6c shows the TEM image of CuS nanostructures produced on surface MCM-41 matrix by polyol method. The image shows nanoplate of CuS with approximately 10 nm in size [28,47].

3380 3250 3163

Fig. 6. TEM images of (a) MCM-41, (b) CuS/MCM-41W and (c) CuS/MCM-41 E samples.

Transmittance %

3380

1407

1120 1050

c

460

Reaction mechanism Based on the results, a plausible mechanism is proposed for photocatalytic degradation of methylene blue in Scheme 2.

3420

462

806

1375

960

a

1625

1510

1080

FT-IR spectra The FT-IR spectra of MCM-41 and CuS/MCM-41 samples at the range of 800–4000 cm1 are shown in Fig. 7. The FT-IR spectrum of MCM-41 matrix shows (Fig. 7a) bands at 1082, 958, 806 and 462 cm1 which can be attributed to Si–O–Si bending and stretching vibrations. The spectrum of OH-groups in the parent MCM41shows bands at 3420 cm1 which belong to acidic bridged hydroxyls [48]. All bands in CuS/MCM-41E and CuS/MCM-41W samples show both shift to lower wave numbers and decreased intensity with respect to the MCM-41 matrix. This shift and decreased intensity reveals that CuS nanostructures could produce

1082

4000

3000

2000

1500

1000

500

cm -1 Fig. 7. Ft-IR spectra of (a) MCM-41, (b) CuS/MCM-41W and (c) CuS/MCM-41E.

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The holes could be trapped by the basic sites to lose the oxidation capacity, since the framework oxygen sites in MCM-41 material act as the electron donation centers and are rich on MCM-41 surface. This may be due to the ability of MCM-41 matrix in promoting the photoelectrons’ transfer process and thus enhancing the separation of electron/hole pairs. Further work is required to elucidate this mechanism.

Abs

t = 0 min

Effect of experimental parameters

t= 40 min

300

400

500

600

700

800

Wavelength (nm) Fig. 8. Spectra change that occur during the photocatalytic degradation of aqueous solution of methylene blue: pH = 8, CuS/MCM-41E nanocatalyst = 0.2 g/L, Co = 1.6 ppm.

It is well documented that the absorption of photons possessing energy equal to or higher than that of the semiconductor (3.1 eV for CuS in MCM-41) causes charge separation: þ

CuS þ hv ! CuSðe Þ þ CuSðh Þ In general, photo-generated electrons are expected to be trapped by O2 molecules in the solution to form superoxide ions (O 2 ) and other reactive oxygen species. On the other hand, the valence band (hVB) potential is positive enough to generate hydroxyl radicals at the surface and the conduction band (eCB) potential is negative enough to reduce molecular oxygen. The hydroxyl radical are powerful oxidizing agents and attack organic pollutants present at or near the surface MCM-41. It causes photooxidation of dye according to the following reactions [50,51]:

CuS þ hv ! CuSðeCB þ hVB Þ

Effect of nanocatalyst amount In order to determine the optimal amount of photocatalyst, some experiments were performed at pH 8 by varying the amount of CuS/MCM-41E nanocatalyst from 0.1 to 1.00 g/L. By varying nanocatalyst amounts, it is observed that 0.2 g/L is found to be optimum. Increasing 0.1–0.8 g/L, the rate of photocatalytic activity is increased of course activity is not found to be encouraging between 0.2 and 0.8 g/L. At higher amounts (0.8 g/L) the activity is decreased. This is due to higher amount of the catalyst makes the solution turbid resulting a control on the penetration of light into solution that intern leads to reduction in hydroxyradical formation. Effect of concentration of dyes In order to determine the optimal amount of dye, some experiments were performed at pH 8 by varying the amount of dye from 0.96 to 10.0 ppm. The degradation efficiency of dye decreased with increasing the initial concentration of dye to more than 1.60 ppm. The decrease of %D with increase of concentration of dye can be due to two reasons. With increasing the amount of dye, more dye molecules will be adsorbed on the surface of the photocatalyst and the active sites of the catalysts will be reduced. Therefore, with increasing occupied spaces of catalyst surface, the generation of hydroxyl radicals will be decreased. Increasing concentration of dye can also be lead to decrease in the number of photons that arrive to the surface of catalysts. The more light is absorbed by molecules of dye and the excitation of photocatalyst particles by photons will be reduced. Thus, photodegradation efficiency diminished [52].

hVB þ H2 OðadsÞ ! Hþ þ  OHðadsÞ hVB þ OHðadsÞ !  OHðadsÞ eCB þ O2ðadsÞ !  O2ðadsÞ 

OHðadsÞ þ dye ! degradation of the dye

Fig. 9. Effect of visible light and different photocatalyst on photocatalytic degradation of methylene blue. Co = 1.6 ppm, CuS/MCM-41 E nanocomposite = 0.20 g/L, pH = 8.

Effect of PH Photodegradation of dye (1.60 ppm) was studied in amplitude pH of 1–13 in the presence of CuS/MCM-41E nanocatalyst (0.2 g/ L). The results for irradiation time of 40 min are shown in Fig. 10. In presence of 0.2 g/L CuS/MCM-41E and in pH 8, degradation efficiency is obtained 95%. Probably, the surface of photocatalyst is positively charged in acidic solutions and negatively charged in alkaline solutions. As a result, it is not surprising to observe increase in the adsorption of dye molecules (with positive charge) on the surface of photocatalyst in alkaline solutions and thus the increasing of degradation efficiency of dye [53]. Demethylation and photo degradation of methylene blue are observed in presence of CuS/MCM-41E nanocrystal and at pHs above 8 [54]. A low pH is associated with a positively charged surface which cannot provide hydroxyl groups needed for hydroxyl radical formation. On the other hand, higher pH value can provide higher concentration of hydroxyle ions to react with the holes form hydroxyl radicals [30]. But, the degradation of dye is inhibited when the pH value is between 8 and 9 because the hydroxyl ion competes with dye molecules in adsorption on the surface of photocatalyst [53]. In other words, at low pH, the adsorption of cationic dyes on the surface of photocatalysts decreases because the photocatalyst surface is positively changed and repulsive forces are due to decreasing adsorption. Thus, the degradation efficiency will decrease in acidic pH.

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149

Scheme 2. Plausible photocatalytic degradation pathway of methelene blue.

and reused; the rate of degradation is restored and is equivalent to fresh catalyst. Thus, the calcinations of the used catalyst are necessary in order to maintain the activity. Conclusions

Fig. 10. Change in decomposition% aqueous solution of methylene blue as a function of pH: CuS/MCM-41E nanocomposite = 0.20 g/L, Co = 1.6 ppm.

Recycling studies Fig. 11 shows the reproducibility of CuS/MCM-41E nanocrystals as nanoparticles for methylene blue photodegradation during a four-cycle experiment. Each experiment was carried out under identical concentration of 1.60 ppm dye, 0.2 g/L of nanocatalyst, pH of 8, irradiation time of 30 min and at room temperature. After each degradation experiment, the concentration of dye was adjusted back to its initial value of 1.60 ppm. As seen in Fig. 10 a small and gradual decrease in the activity of nanocatalyst was observed at the first two cycles. The difference may be due to the accumulation of organic intermediate in the cavities and on surface of the MCM-41 matrix thus affecting the adsorption in turn reducing the activity. Nanocatalyst was calcined at 550 °C for 2 h

120

%Degradation

100 80 60 40 20 0

0

1

2

3

4

5

Number of cycles Fig. 11. Reproducibility of the nanocatalysts for methylene blue photodegradation.

The comparison of photocatalytic activity of different synthesized CuS nanocomposite samples clearly indicated that the CuS nanosphers incorporated in MCM-41 by polyol method is the most active photocatalyst for the decolorization of the mixture of MB under used conditions. The experimental results indicated that the decolorization of dyes was facilitated in the presence of CuS/ MCM-41 and was favorable in the basic conditions. Pure CuS nanostructure observes by polyol method. The observations of these investigations clearly demonstrated the importance of choosing the optimum degradation parameters for obtaining a high degradation rate such as pH, concentration of dye and dosage of nanocompositet, which are essential for any practical applications of photocatalytic oxidation processes. The prepared CuS sample in MCM-41matrix by solvothermal method in water solvent is not good candidates for decolorization of MB. When water was used as solvent, phases different observe for CuS nanostructure. Meanwhile, in prepared products occur slight distortions of the crystal lattice. References [1] R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, A. gostiano, Appl. Catal., B 60 (2005) 1–11. [2] V.K. Gupta, R. Jain, A. Mittal, T.A. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Mater. Sci. Eng., C 32 (2012) 12–17. [3] A. Nezamzadeh-Ejhieh, Z. Shams-Ghahfarokhi, J. Chem. 2013 (2012) 11. [4] M. Krissanasaeranee, S. Wongkasemjit, A.K. Cheetham, D. Eder, Chem. Phys. Lett. 496 (2010) 133–138. [5] W. Zhang, Y. Li, C. Wang, P. Wang, Desalination 266 (2011) 40–45. [6] A. Nezamzadeh-Ejhieh, Mehdi Amiri, Powder Technol. 235 (2013) 279–288. [7] A. Nezamzadeh-Ejhieh, N. Moazzeni, J. Ind. Eng. Chem. 19 (2013) 1433–1442. [8] Sh. Sohrabnezhad, A. Rezaei, Superlattices Microstruct. 55 (2013) 168–179. [9] E. Godocikova, P. Balaz, J.M. Criado, C. Real, E. Gock, Thermochim. Acta 440 (2006) 19–22. [10] Ch. Tan, Y. Zhu, R. Lu, P. Xue, Ch. Bao, X. Liu, Z. Fei, Y. Zhao, Mater. Chem. Phys. 91 (2005) 44–47. [11] M. Tsuji, M. Hashimoto, Y. Nishizawa, T. Tsuji, Mater. Lett. 58 (2004) 2326– 2330. [12] T.Y. Ding, M.S. Wang, S.P. Guo, G.C. Guo, J.S. Huang, Mater. Lett. 62 (2008) 4529–4531. [13] M.A. Yildirim, A. Ates, A. Astam, Physica E 41 (2009) 1365–1372. [14] Y.C. Zhang, T. Qiao, X.Y. Hu, J. Cryst, J. Cryst. Growth 268 (2004) 64–70. [15] Y.B. He, A. Polity, I. Österreicher, D. Pfisterer, R. Gregor, B.K. Meyer, M. Hardt, Phys. B 9 (2009) 1069–1073. [16] L. Gao, E. Wang, S. Lian, S. Kang, Y. Lan, D. Wu, Solid State Commun. 130 (2004) 309–312.

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Facile and low temperature route to synthesis of CuS nanostructure in mesoporous material by solvothermal method.

The synthesis of CuS nanomaterial in MCM-41 matrix has been realized by chemical synthesis between MCM-41, copper sulfate pentahydrate and thiourea vi...
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