preparation of zns spherical nanoparticles

Journal of Applied Chemical Research, 7, 4, 63-70 (2013) Journal of Applied Chemical Research www.jacr.kiau.ac.ir Development of a Mild Hydrothermal Method toward Preparation of ZnS Spherical Nanoparticles Leila Vafayi 1 , Soodabe Gharibe 1 , Shahrara Afshar 2 1 Department of Science, Islamic Azad University, Firoozkooh Branch, Iran. 2 Department of Chemistry, Iran University of Science and Technology, Tehran, Iran. Received 13 Jul. 2013; Final version received 11 Aug.2013 Abstract In this work, ZnS nanostructures of various morphologies have been synthesized via a mild hydrothermal method. The effects of surfactants such as sodium dodecyl sulfate (SDS) (anionic) and cetyl-trimethylammonium bromide (CTAB) (cationic), the type of source of Zn and S on the morphology of ZnS were studied. The products were characterized by XRD, SEM, EDX, TEM analysis and UV-vis spectroscopy. The XRD patterns showed that the ZnS samples have a zinc blende structure. The results showed that the ZnS samples are in spherical, sheet, nano and submicrorod forms. Keywords: ZnS, Surfactant, Nanosphere, Hydrothermal, TEM. *Corresponding author: Leila Vafayi, Department of Science, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran. Email: leilavafayi@yahoo.com. Tel: +98 21 7644 3869, Fax: +98 21 7644 2868. Introduction Semiconductors are a class of materials dened primarily by their electronic properties. During the past two decades, Nanocrystalline semiconductor particles have attracted considerable because of their novel properties, such as large surface-to-volume ratio and the three dimensional connement of the electrons [1-5]. In metals and other conductors, the conduction and valence bands overlap, without a signicant energy barrier for promoting electrons from the valence to the conduction band. In insulators, there is a large energy barrier for promoting electrons from the valence to the conduction band, essentially eliminating conduction. In semiconductors, however, the energy barrier for conduction is intermediate between conductors and insulators. Typically, the band gaps (Eg) for metals, semiconductors, and insulators are less than 0.1 eV, between 0.5 and 3.5 eV, and greater than 4 eV, respectively. L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 64 II-VI semiconductor nanocrystals attract much attention because of their size dependent photo- and electro- luminescence properties and promising applications in optoelectronics [6- 9]. Among the family of II–VI semiconductors, zinc sulde semiconductor is an important member of this family because of their favorable electronic and optical properties for optoelectronic applications. ZnS can have two different crystal structures (zinc blende and wurtzite); both have the same band gap at 340 nm (3.66 eV) and the direct band structure. ZnS has been used widely as an important phosphor for photoluminescence (PL), electroluminescence (EL) and cathodoluminescence (CL) devices due to its better chemical stability compared to other chalcogenides such as ZnSe. In optoelectronics, it nds use as light emitting diode, reector, dielectric lter and window material [10-13]. Nanocrystalline ZnS can be prepared by various methods such as sonochemical preparation [14], coevaporation [15], wet chemical process [16], sol-gel [17] solid state [18], micro-wave irradiation [19], ultrasonic irradiation [20] or synthesis under high-gravity environment [21]. The properties and applications of nano structured semiconductors are strongly dependant on their crystal phase, size, composition and shape. Therefore synthesizing of highly tuned nanocrystals has been a challenging topic. Here in, we describe a mild hydrothermal method for the synthesis of spherical, sheet, nano and submicrorod ZnS structures. Effects of the zinc and sulfur sources and surfactant on the morphology and size of ZnS nanostructures have been investigated. Experimental All reagents were analytical grade and purchased from Merck Company. Reagents were used without any further purication. In order to obtain zinc sulphide particles with various morphologies the reaction factors such as: Zinc sources, sulfur sources and type of the surfactant have been investigated. The synthesis conditions are reported in the following sections. It is noteworthy the Zn to S molar ratio has been set to be 1 in all experiments. Preparation of zinc sulde by keeping the type of the zinc source In this section, zinc chloride as the zinc source and thiourea, thioacetamide, sodium sulde as the sulfur source were employed in the reactions. In three different experiments, a solution of 3 mmol ZnCl 2 in 20 mL deionized distilled water was added to a solution of 3 mmol of sulfur salt in 20 mL deionized distilled water under stirring. The mixture was transferred into an autoclave, which was lled with distilled water up to 70% of the total volume. The autoclave was sealed and kept at 120 °C for 5 h. After L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 65 cooling the system to room temperature, the product was separated by centrifugation, washed with absolute ethanol and deionized water for several times, and then dried under vacuum at 70 °C for 10 h. Preparation of zinc sulde by keeping the type of the sulfur source In this experimental section, sodium sulde as the sulfur source and zinc nitrate, zinc acetate as the zinc source were employed in the previous section reactions in the same condition. Preparation of zinc sulde nanospheres using SDS and CTAB surfactants In a typical synthesis, a solution of 3 mmol ZnCl2 in 20 mL deionized distilled water was added to a solution of surfactants with ZnCl 2 to SDS or CTAB ratio equal to 1 prepared in 20 mL of deionized water under stirring. Then, 3 mol of Na 2 S in 20 ml deionized distilled water was added to the above mixture under vigorous stirring. The mixture was transferred into an autoclave, sealed and kept at 120 °C for 5 h. After cooling the system to room temperature, the product was separated by centrifugation, washed with absolute ethanol and deionized water for several times, and then dried under vacuum at 70 °C for 10 h. Characterization The crystal phase and particle size of the synthesized products, after purication, were characterized by X-ray diffraction (XRD) using FK60-04 with Cu Kα radiation (λ= 1.54 Å) in 2q ranges from 5° to 70°, and with instrumental setting of 35 kV and 20 mA. The morphology of the nanostructures was observed by emission scanning electron microscopy (SEM, Philips- XLφ30) and transmission electron microscopy (TEM, Philips-CM120). UV-vis spectra of the ZnS nanosphere were recorded by MPC-2200, UV2550 UV-vis spectrophotometer in the wavelength range of 200 to 800 nm. Results and discussion The XRD patterns of prepared ZnS samples are shown in Figure 1. All peaks can be well indexed to cubic zinc blend with lattice constant of a=b=c=5.406 Å (JCPDS, No.05- 0566). No other crystalline phase was found in the XRD patterns, indicating the high purity of the products. L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 66 Figure 1. XRD patterns of the prepared ZnS samples using: (a) ZnCl 2 and Na 2 S , (b) ZnCl 2 and thioacetamide, (c) Zn(Ac) 2 and Na 2 S, (d) Zn(NO3) 2 and Na 2 S, (e) ZnCl 2 and thiourea, (f) ZnCl 2 and Na 2 S with SDS surfactant, (g) ZnCl 2 and Na 2 S with CTAB surfactant, as Zn and S sources, respectively. The morphology of the products was observed by SEM and TEM images. The SEM images of the synthesized products are shown in Figure 2 (a-g). These images clearly demonstrate that the products are in different morphologies. Figure 2 (a-b, e) are related to the prepared samples using ZnCl 2 as of the zinc source and Na 2 S, thioacetamide and thiourea as of the sulfur sources, respectively. These images clearly indicate that the products are in different morphologies. Since the experimental condition is the same for all of them; the only difference between the samples is the source of the sulfur. Figure 2 (a, c and d) are related to the prepared samples using Na 2 S as of the sulfur source and ZnCl 2 , Zn(Ac) 2 and Zn(NO 3 ) 2 as of the zinc sources, respectively. These images clearly indicate that the products are in different morphologies. Also, the only difference between these samples is the source of the Zinc in the same of the experimental condition. The results clearly indicate that when the anions of the metal salts are a multidentate, the products are not formed in the shape of spherical. The SEM images of the synthesized products using ZnCl 2 as source of Zn and Na 2 S as of the sulfur with SDS and CTAB surfactants are shown in Figure 2 (f and g), respectively. The SEM observations show that the samples prepared with surfactants are less aggregated. This can be due to coating of inorganic core by the surfactant, which prevents the nanoparticles to aggregate. It is proved that the prevention of aggregation of nanoparticles in the presence of the surfactant is more effective when the surfactant has a long and branched chain structure [22, 23]. Figure 1. XRD patterns of the prepared ZnS samples using: (a) ZnCl 2 and Na 2 S , (b) ZnCl 2 and thioacetamide, (c) Zn(Ac) 2 and Na 2 S, (d) Zn(NO 3 ) 2 and Na 2 S, (e) ZnCl 2 and thiourea, (f) ZnCl 2 and Na 2 S with SDS surfactant, (g) ZnCl 2 and Na 2 S with CTAB surfactant, as Zn and S sources, respectively. L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 67 Figure 2. SEM images of ZnS samples prepared using: (a) ZnCl 2 and Na 2 S , (b) ZnCl 2 and thioacetamide, (c) Zn(Ac) 2 and Na 2 S, (d) Zn(NO3) 2 and Na 2 S, (e) ZnCl 2 and thiourea, (f) ZnCl 2 and Na 2 S with SDS surfactant, (g) ZnCl 2 and Na 2 S with CTAB surfactant, as Zn and S sources, respectively. L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 68 The SEM images show that the ZnS prepared with CTAB surfactant is less aggregated because of its longer and branched chain structure. The energy dispersive X-ray (EDAX) analyses of prepared samples conrm that the Zn and S elemental ratio is 1:1 for all ZnS samples (Figure 3). Figure 4 shows the TEM image of the prepared ZnS nanoparticles using CTAB surfactant. It is clear that ZnS nanoparticles are spherical with average diameter of 40 nm. The optical properties of the semiconductor nanomaterials depend on the size and the shape of the particles. To investigate the optical properties of the as-prepared ZnS samples, the UV–vis absorption spectra were recorded, as shown in Figure 5. The optical absorption spectrum can be used to estimate the bandgap of the semiconductors. The bandgap for the prepared ZnS samples was calculated to be 3.76 (330nm) and 3.81 eV (323nm), using ZnCl 2 and Na 2 S as starting materials without and with CTAB surfactant, respectively. The L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 69 absorption edge shows a blue shift to higher energies as compared with the bulk ZnS (3.66 eV). This is due to the fact that smaller particles have larger band gaps and absorb at shorter wavelengths [24, 25]. Investigation of the spectra reveals that the synthesized ZnS using CTAB surfactant (Figure 5b) has a sharper absorption edge. This is in agreement with the SEM images and indicates that the synthesized particles can be considered to be monodispersed. Conclusion ZnS nanostructures have been synthesized using mild hydrothermal method. The results show that by changing of the sources of zinc and sulfur from multidentate to monodentate, the obtained pure ZnS nanocrystals show a tendency towards ZnS nanospherical structures. Also, it is concluded that the ZnS nanospheres prepared using CTAB surfactant are spherical and can be considered as monodispersed particles. The nanoparticles with monodispersed nanosphere characteristics are appropriate templates for hollow nanostructures. Therefore, these hollow nanostructures via removal of ZnS attract the attention of many researchers in applying them in drug delivery and catalysis. Acknowledgments This article was extracted from the project entitled: “Synthesis and characterization of ZnS@SiO 2 core-shell nanocomposite for drug delivery application”. Financial support for this project was provided by Islamic Azad University Firoozkooh branch. References [1] N.Q. L. Yin, Liu, J.M. Lei, Y.S. Liu, M.G. Gong, Y.Z. Wu, L.X. Zhu, X.L. Xu, Chin. L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013) 70 Phys. B., 21(11), 116101 (2012). [2] J. Rita, S. Sasi Florenc, Chalcogenide. Lett., 7(4), 269 (2010). [3] C. Dumbrava, A. Badea, G. Prodan, I. Poppvici, V. Ciupin, Chalcogenide. Lett., 6(9), 437 (2009). [4] F. Mollaamin, S. Gharibe, M. Monajjemi, Int. J. Phys. Sci., 6(6), 1496 (2011). [5] P. S. Beer, P. S. Sunder, V. Shweta, K. J. Rakesh, Optoelectron. Biomed. Mater., 4, 29 (2012). [6] P.B. Jyoti, J. Barman, K.C. 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It is clear that ZnS nanoparticles are spherical with average diameter of 40. to coating of inorganic core by the surfactant, which prevents the nanoparticles to aggregate. It is proved that the prevention of aggregation of nanoparticles in the presence of the surfactant