World Applied Programming, Vol (3), Issue (3), March 2013. 108-112 ISSN: 2222-2510 2013 WAP journal. www.waprogramming.com Effect of Reaction Tempreture on ME Capped ZnS Nanoparticles: Structural and Optical Characterization Abbas Rahdar Department of Physics, Faculty of Science, University of Zabol, Zabol, Iran. a.rahdar@uoz.ac.ir Fereshteh Izadpanah Department of Physics, Faculty of Science, University of Ker-man, Kerman, Iran. Abstract: In this work, we report effect of reaction tempreture on structural and optical properties of nanocrystalline ZnS particle with mercaptoethanol as a stabilizer, which have been synthesized by coprecipitation method. The structural properties of ZnS nanoparticles have been characterized by X-ray diffraction (XRD) analysis. The XRD patterns show fcc structure in nanoparticles. The optical size calculated from the Brus equation has been found in the range of 2.79-2.67 nm with the increase in tempreture of reaction. We also found that optical band gap (E g ) increases with the increase in tempreture of reaction using UV-Vis spectrophotometer. This behavior is related to size quantization effect due to the small size of the particles.tem image shows morphology. Keywords: ZnS nanoparticles, Brus equation,reaction, Capping agent, Optical band gap I. INTRODUCTION A burst of research activities has been emerged in the synthesis and characterization of semiconductor nanoparticles for size dependent optical properties, because they find great applications in photonic and biophotonics [1-2]. The size dependence of the bandgap is the most identified aspect of quantum confinement in semiconductors;the bandgap increases as the size of the particles decreases. When the dimensions of nanocrystalline particles approach the exciton Bohr radius, a blue shift in energy is observed due to the quantum confinement phenomenon.the effective mass model is commonly used to study the size dependence of optical properties of quantum dots (QD) system [3].The tunability of the properties of nanoparticles by controlling their size may provide an advantage in formulating new composite materials with optimized properties for various applications.however, applications would be restricted due to different nonradiative relaxations pathways. One of the most important nanoradiative pathways is surface related defects.to overcome the above mentioned difficulties, organic and inorganic capping agents are used to passivate the free quantum dots. Various wet chemical methods have been developed for the synthesis of sulphide nanoparticles[4-9]. To control the growth of the nanoparticles,organic stabilizers (polymers) e.g. polyethylene oxide (PEO), poly(n-vinyl-2 Pyrrolidone, PVP), polyvinylcarbazole (PVK) are added during the wet-chemical synthesis for capping the surface of the particles ([10-12]. Zinc sulfide (ZnS) is a semiconductor material, which has a band gap of 3.70 ev [13] and is an ideal material for studies of variation in discrete gap energy due to the reduction of physical dimensions into nanoscale regime. Research over past several years revealed that ZnS particles are potential to be implemented as cathode-ray tube, luminescent materials in flat panel displays and IR windows [14-15]. There have been extensive reports in the past few years demonstrating the systematic exploration of growing ZnS nanoparticles with a narrow size distribution in the surfactant system. So in the present paper, we tried to find the structural and optical properties of ZnS nanoparticles with 2-Mercaptoethanol as capping agent by the chemical co-precipitation method that was magnetically stirred for different temperatures were studied. II. MATERIALS AND EXPERIMENTAL PROCEDURE Zinc chloride (ZnCl 2 ), sodium sulfide (Na 2 S) as source materials, mercaptoethanol as capping agent for control particles size and doubledistilled water as dispersing solvent were used to prepare ME Capped ZnS nanoparticles. The ME Capped ZnS nanoparticles were prepared by the chemical co-precipitation method at different temperatures as follows. First, Zinc chloride sodium sulfide was dissolved indouble-distilled water with 0.1 M concentration separately. 108
then obtained solutions was magnetically stirred for 20 min. Afterwards, Na 2 S (50 ml) solution was added drop by drop to the ZnCl 2 solution under vigorous stirring in an N 2 atmosphere separately. Then, an appropriate amount (50 ml) of ME (0.01 M) was added to the reaction medium to control the particle size of ME Capped ZnS. M. E ZnCl 2 Na 2 S ZnS 2 NaCl (1) the resulting solution titrated to ph 9 with 2 M NaOH and then was magnetically stirred for 20 min. The stirring speed was same for all samples. The nanoparticles were collected by centrifugation at 2000 rpm for 10 minutes. In the final step, the obtained precipitate was filtered and dried at room temperature to remove both water and organic capping and other byproducts formed during the reaction process. After sufficient drying, the precipitate was crushed to fine powder with the help of mortar and pestle. It is necessary to mention that different sample of nanoparticles has been obtained at different temperatures namely ME-capped ZnS (A), (B) and (C) as the amount of tempreture of used for reaction medium in the preparation is 15, 25 and 35 respectively. The synthesis process of ZnS nanoparticles by the coprecipitation method is summarized in a flow chart shown in Fig. 1. Fig. 1. The flow chart of preparation ME-capped ZnS nanoparticles by chemical co-precipitation method. III. RESULTS AND DISCUSSION The X-ray diffraction (XRD) patterns of the ME-capped ZnS (A) and (C) nanoparticles are depicted in Fig. 2 and Fig. 3 respectively.the spectrum presents three broaden peaks at about 29.2 o, 48.1 o and 57.3 o in 2θ, which correspond to the Miller index of the reflecting planes for (111), (220) and (311). 109
Fig. 2. XRD pattern of ME-capped ZnS (A) nanoparticles. The broadening of the diffraction peaks suggests that the dimensions of the nanoparticles are very small. Fig. 3. XRD pattern of ME-capped ZnS (C) nanoparticles. The peak broadening at lower angle is more meaningful for the calculation of particle size. The average size (D) of nanoparticles was also estimated using the Scherrer formula (Scherrer 1918) using (111) reflection from the XRD pattern as follows: 0.9λ D =, (2) Bcos θ Where λ, B, and θ are the X-ray wavelength of the radiation used (K α (Cu) = 0.154056 nm), the full width at half maximum (FWHM) of the diffraction peak and the Bragg diffraction angle, respectively. The values of Average crystallite size obtained from XRD (Scherrer formula ) for different tempretures are listed in Table 1. The absorption spectra of the different samples are shown in Table 1. The absorption edge is observed in the range of 320 280 nm, which is blue shifted compared to bulk ZnS (345 nm). Fig. 4. Shows Absorption spectra of of ME-capped ZnS (C) nanoparticles. Fig. 4. Absorption spectra of of ME-capped ZnS (C) nanoparticles. 110
The absorption spectrum of ME-capped ZnS nanoparticles synthesized at 35 c temperature is shown in Fig. 4. It shows an excitonic absorption peak at 302 nm (4.10 ev). There is a clear blue shift of band gap with respect to bulk ZnS, since band gap of bulk ZnS with cubic structure is 3.6 ev (345 nm). This is due to the modification of valence and conduction bands by quantum confinement effects. Even though the nanocrystallites exhibit bulk like crystal structure, they are too small to have bulk like electronic wave functions. Applying the confinement effects, the optical band gap energy of nanocrystallites is given by Brus equation (Brus, 1986): Eg( nano ) Εg( bulk 2 2 2 π 1 1 1.8e ) 2R 2 me mh εr where E g (bulk) in ev is the bandgap energy of bulk, me and mh are electron and hole effective masses and R in nm is the particle size.for cubic ZnS, E g =3.6 ev, m e =0.34 m o and m h =0.23 m o, m o is the free electron rest mass and ε=8.76 is the permittivity of the sample (Landolt., 1987). Taking the band gap of synthesized nanoparticles as 4.10 ev,the substitutionin Eq. (3) gives the particle optical size as 2.67 nm. The discrepancy with XRD (2.50 nm). (3) Table 1. Calculated size of ZnS nanoparticles from Scherrer formula and Brus equation. Samples ME-capped ZnS (B) ME-capped ZnS (C) amount of Tempreture of used for reaction medium (c ) Optical Size (nm) Average crystallite size (nm) Absorption Edge (nm) Optical band gap (ev) 25 2.79 2.72 315 3.93 35 2.67 2.35 302 4.10 The obtained direct optical band gap values for different samples are shown in Table 1. It is necessary to mention that the optical direct band gap values of the ZnS nanoparticles can be determine by Tauc s relation (Tauc, 1966; Liu et al., 2006): α hυ α (4) 1 2 0 ( hυ Eg ) Where hν, α o and Eg are photon energy, a constant and optical band gap of the nanoparticles, respectively. Absorption coefficient (α) of the nanoparticles at different wavelengths can be calculated from the absorption spectra. Finally, the values of E g can be determine by extrapolations of the linear regions of the plot of ( αhυ) 2 versus h. ME-capped ZnS nanoparticles were then subjected to transmission electron microscopy (TEM,JEOLJEM3010) for characterization. Fig. 4 shows TEM image of ME-capped ZnS Semiconductor nanocrystals. Fig. 4. TEM image of ME-capped ZnS (C) nanoparticles. 111
IV. CONCLUSIONS It is possible to produce ZnS nanoparticles using mercaptoethanol as a capping agent in order to control the particle size using a simple chemical method. XRD and optical band gap data have been obtained to confirm nano-size of these materials. The cubic structure of the synthesized nanoparticles has been observed using XRD studies.it is also observed that the particle size depends on reaction tempreture. The optical band gap and optical size values of ZnS nanoparticles have changed from 3.93 to 4.10 ev and 2.79 to 2.67 nm by increasing tempreture of reaction respectively. These values exhibit a blue shift in E g which is related to the size decrease of the particles and to the quantum confinement limit reaching of nanoparticles.the yellowish green emission band at 476 nm for the ME-Capped ZnS nanoparticles is most likely due to the self - activated defect centres formed by the zinc va-cancy inside the lattice.tem image shows clearly that the particles almost are spherical. ACKNOWLEDGEMENT The authors would like to thank Mr. A. Rahdar, Mr. R, Hakimi-Pooya, Mrs. M. Asudeh, Mr. H. Rahdar and Mrs. Heidari Mokarrar for their support and assistance with this project. REFERENCES [1] A. Henglein, Chem. Rev. 89 (1989) 1861. [2] M. Nirmal, L.E. Brus, Science 271 (1996) 933. [3] Gopa Ghosh, Milan Kanti Naskar, Amitava Patra *, Minati Chatterjee (2006) Optical Materials 28,1047 1053. [4] T. Trindade, P. O_Brien, Adv. Mater. 8 (1996) 161. [5] T. Trindade, P. O_Brien, Chem. Mater. 9 (1997) 523. [6] M. Azad Malik, P. O_Brien, N. Revaprasadu, J. Mater. Chem. 11 (2001) 2382. [7] M. Chatterjee, A. Patra, J. Am. Ceram. Soc. 84 (2001) 44. [8] N. Arul Dhas, A. Gedanken, Appl. Phys. Lett. 72 (1998) 2514. [9] J. Nanda, S. Sapra, D.D. Srama, N. Chandrasekharan, G. Hodes,Chem. Mater. 12 (2000) 1018. [10] R. He, X.-F. Qian, J. Yin, H.-a. Xi, L.-J. Bian, Z.-k. Zhe, Colloids Surf. A: Phys. Eng. Aspects 220 (2003) 151. [11] Y. Yang, H. Chen, X. Bao, J. Cryst. Growth 252 (2003) 251. [12] K. Roy Choudhury, M. Samoc, A. Patra, P.N. Prasad, J. Phys.Chem. B 108 (2004) 1556. [13] D. Denzler, M. Olschewski, K. Sattler J. Appl. Phys., 84 (1998), p. 2841 [14] B. Bhattacharjee, D. Ganguli, K. Iakoubovskii, A. Stesmans, S. Chaudhuri Bull. Mater. Sci., 25 (2002), p. 175. [15] S. Wageh, S.L. Zhao, X.R. Xu J. Cryst. Growth, 255 (2003), p. 332 [16] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos,Science 281 (1998) 2013. [17] W.C.W. Chan, S.M. Nie, Science 281 (1998) 2016. [18] L. Brus, Journal of Physical Chemistry 90 (1986) 2555. [19] Dios M de, Barroso F, Tojo C, Blanco M C and Lopez-Quintela M A (2005) Effects of the reaction rate on the size control of nanoparticles synthesized in microemulsions. Colloids and Surfaces A: Physicochem. Eng. Aspects 83, 270 271. [20] Rahdar A, Asnaasahri Eivari H and Sarhaddi R (2012) Study of structural and optical properties of ZnS:Cr nanoparticles synthesized by coprecipitation method. Indian Journal of Science and Technology 5, 1855-1858. [21] Rahdar A, (2012) ZnS: Mn-SPP semiconductor nanocrystals: Synthesis and optical and structural characterization. Indian Journal of Science and Technology 5, 3307-3309. [22] Tauc J, (1966) Optical Properties of Solids Academic Press Inc, New York. [23] A. Khosrav, M. Kundu, L. Jatwa, S.K. Deshpande, V.A. Bhgawat, M. Shastri, S.K. Kulkarni, Applied Physics Letters 67 (1995) 2705. 112