Effect of PVA Capping on the Optical and Structural Properties of hydrothermally synthesized ZnS Nanocrystals

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1 20 Effect of PVA Capping on the Optical and Structural Properties of hydrothermally synthesized ZnS Nanocrystals Mandeep kaur, M. Tech. (NST) Student, Department of Nano Science & Technology, DAVIET Jalandhar, Punjab Kanchan L. Singh, Associate professor, Department of Nano Science & Technology, DAVIET, Jalandhar, Punjab,India Praveen Kumar, Assistant professor, Nanotechnology Research Centre, DAVIET, Jalandhar , India ABSTRACT The pure and PVA capped ZnS nanocrystals were synthesized by hydrothermal method using zinc nitrate and thioacetamide as the zinc source and sulphur source respectively. The XRD patterns confirmed the formation of cubic phase ZnS nanoparticles with particle sizes 32.6nm and 30.6nm respectively for the pure and PVA capped nanocrystals. The SEM micrographs show the improvement in the particle size distribution upon PVA capping. The presence of characteristic bands for the PVA in the FTIR spectrum reveals capping of ZnS nanocrystals. The strong absorption in wavelength range nm along with a blue shift in the optical absorption edge also indicates the formation of nanocrystals. The optical gap of pure and PVA capped ZnS has been calculated using the Tauc s relationship and blue shift in the optical gap has been reported. These results have been explained with the improvement in the passivation of surface defects for the PVA ZnS nanocrystals. Keywords: Hydrothermal synthesis, ZnS nanocrystals, UV-Vis spectroscopy, FTIR 1. INTRODUCTION Zinc sulfide is a well known photo- and electroluminescent material belongs to II-VI group of semiconductor materials having large band gap (3.75 ev) and large excitonic energy (~37 mev) along with low Bohr exciton radius (2.5 nm) makes it suitable as host material for large variety of dopants for the fabrication of various solid state devices as well as small biomolecular probes for fluorescence [1-3]. It has also remarkable chemical stability against oxidation and hydrolysis resulting in its applicability as photo-catalyst [4]. A covalently bonded solid, zinc sulfide crystallizes in two different forms: zinc blende (cubic) and wurtzite phases (hexagonal) where Zinc blende (ZnS) is generally used as luminescent materials[5]. Due to its luminescence properties it is widely used in electronic industries in flat display, solar cells, sensors, lasers and also as catalyst in pollution treatment [6]. It is also used in latent fingerprints [7] ZnS nanoparticles can be synthesis by different method: Aqueous chemical method, chemical deposition method, Simple soft chemical method, Chemical precipitation method, and hydrothermal method [8-12]. Over the years, attempts have been made to prepare, stabilize and isolate homogeneously dispersed transition metal doped ZnS nanoparticles with various capping agents by using co-precipitation methods, chemical precipitation methods, aqueous (micellar) solution method etc [8-15,]. The surface modification of crystal is done to prevent self agglomeration of particles, size control, and homogeneous size distribution of particles [12, 15]. The hydrothermal is simple economical and cost effective method for the synthesis of nanocrystals at high temperatures and high vapour pressures along with different reaction parameters of the sample. The purity of hydrothermally synthesized powders significantly exceeds the purity of the starting materials because of the self-purifying property of the hydrothermal crystallization, during which the growing crystals/crystallites tend to reject impurities present in the growth environment [13-14]. The use of different surface passivating agents have resulted in the controlled growth, size distribution along the improvement in the quality of the crystallites synthesized using different techniques. Tamakar et al. have shown the linear decrease in the crystallite size with the increase in the capping agent concentration (mercaptoethanol solution) [12]. The recent literature also highlights the control of surface stabilization and agglomeration on the nanocrystallites on the surfactant head groups for tailoring their optical properties by Mehta et al. [15]. Panda et al. have also reported the controlled growth due to the steric effect of the PVP molecule to prevent the agglomeration of ZnS nanoparticles inside the spherical aggregates [6]. The Borah et al. have studied the effect of the ZnS nanoparticle formation within the PVA polymer matrix with particle sizes 5-7 nm range [10].

2 21 The present work reports the effect of the PVA capping agent on the hydrothermally synthesized ZnS nanoparticles. The effect of capping agent on the particle size, optical absorption, size distribution has been investigated using XRD, SEM, FTIR and UV-Vis spectroscopy techniques. 2. EXPERIMENTAL DETAILS The ZnS nanoparticles were synthesis by using reagents zinc nitrate hexahydrate (GR) (loba Chemie, 98%), thioacetamide (CDH, 99.0%) and polyvinyl alcohol (Loba Chemie). In the typical process 4gm of capping agent was added in 200 ml deionized water and stirred the solution for few minute using magnetic stirrer until it dissolve completely. To this solution, 0.1M Zinc Nitrate Hexahydrate was mixed and finally the precursor solution for the sulfur, 0.1M thioacetamide was added to solution to yield the transparent solution. This solution was further left on the magnetic stirring for 30 min. for homogeneous mixing of the precursors at room temperature which does not favor the formation of precipitates. After this, the precursor solution was taken in an air tight pyrex glass bottle (250ml) and placed in oven maintained at C for 3 h for the completion of the hydrothermal process. End products solution containing white colour were then centrifuged (Labspin TC 450D, Eltek Instruments) and repeatedly washed with de-ionized water and then with ethanol for 4-5 times to remove the impurities. The final product was dried at 80 0 C in hot air oven for 6 h. The overall process is depicted in figure 1. scanning electron microscope (JSM-6610, Jeol). The IR absorption spectra were performed on the dried samples using the KBr pallet method in cm -1 at room temperature by using the FTIR spectrophotometer (RZX, Perkin Elmer, USA). The optical absorption studies were performed using the UV-Vis spectrophotometer (Evolution 300, Thermo Scientific, USA). 3. RESULTS AND DISCUSSION 3.1 X-ray diffraction study The XRD patterns of pure and PVA capped ZnS nanopowder samples are shown in figure 2. The prominent diffraction peaks at 28.6 o, 47.5 o and 56.5 o for (111), (220) and (311) planes respectively of the cubic zinc blende structure of the ZnS [8].The crystalline size Figure 2: X-ray Diffraction of the pure and PVA capped ZnS nanocrystals. Figure 1: Hydrothermal synthesis process Further, this powder was used for the characterization using XRD, SEM, FTIR and UV-Vis spectroscopic techniques. The x-ray diffraction study of the powder samples was carried using x-ray diffractometer (X Pert Pro, Panalytical Instruments) and the microscopic analysis of the samples was performed using the of the nanocrystals can be calculated by using Debyescherrer s formula: L =, where λ is the X-ray wavelength ( Å), β is the full width half maximum value of diffraction peak, θ is the degree of the diffraction peak corresponding to the plane [13]. The calculated estimated values are 32.6 nm and 30.6 nm respectively for the pure and PVA capped ZnS nanoparticles. The lattice parameter for both the particles is nm. The similar range particle size reported by yaun et al. and Hoa et al.[5,8]

3 22 characteristic bands for the PVA in the IR absorption spectra reveals the formation of PVA capped ZnS nanoparticles. Figure 4: The FTIR Spectra of pure and PVA capped ZnS nanocrystals. Figure 3: SEM images for (A) pure ZnS nanocrystals and (B) PVA capped ZnS nanocrystals respectively. 3.2 Microscopic study The SEM images have been performed to study the particle morphology and size distribution for the synthesized ZnS nanoparticles. Figure 3 shows the SEM images for the pure and PVA capped ZnS nanoparticles. It is clearly observed that the particles are spherical in shape for both of the samples and an improvement in the size distribution of the nanoparticles has been observed with the PVA capped sample. This may be attributed to the restriction to the formation/growth of nanoparticles with the use of PVA capping agent during the synthesis of ZnS nanoparticles as compared to the pure nanoparticles [16]. 3.3 FTIR spectroscopy study The FTIR absorption spectra for pure and PVA capped ZnS nanoparticles is shown in Figure 4. The FTIR spectra of major peaks of pure ZnS appeared at 1906 cm -1,1542 cm -1, 1461 cm -1, 1406 cm -1 with O-H bending ( at 1618 cm -1 ) of water (John et al. (2010) [9]. On the other hand, the PVA capped ZnS nanoparticles shows the absorption bands for O-H stretching peaks at 2369 cm -1, C=C(CH 2 ) at 1621 cm - 1, 1543 cm -1 due to stretching and bending vibration, C-H (Sp 2 ) at 1462 cm -1 has been observed. Similar results has also been reported for the PVA capped ZnS nanoparticles synthesized by using the co-precipitation technique by Sharma et al. [17]. The observation of 3.4 Optical properties Figure 5 shows the optical absorption spectra for the pure and PVA capped ZnS nanoparticles. The strong absorption in the wavelength range nm and the excitonic peaks at 287 and 317 nm respectively corresponding to pure and capped ZnS nanoparticles. The blue shift in the excitonic band edge has been observed which confirms the formation of nanoparticles. The similar results are reported by Tamrakar et al. [12], Pathak et al.[18]. The optical gap of synthesized ZnS nanoparticles has been determined by using the Tauc s relation: 1/m ( - g), where C is the constant E g is the bandgap of the nanoparticles and m depend on type of transition near Figure 5: The Optical absorption spectra for the PVA capped ZnS nanocrystals and pure ZnS nanocrystal.

4 23 the fundamental absorption edge. The value of m (i.e. m=1/2 for direct allowed, n=2 for indirect allowed transition, n=3/2 for direct forbidden and when n=3 for indirect forbidden transitions [18]) suggests the type of transition occurred. The plot of ( h ) 2 versus photon energy h gives the value of direct band gap of the samples. Figure 6 shows the variation of ( h ) 2 versus photon energy h for the pure and capped ZnS nanoparticles. From the extrapolation near the fundamental absorption edge, the optical gap was calculated which found to be 4.86 ev for pure and 5.13 ev for PVA capped ZnS nanocrystals respectively. Similar result has been reported by Liu et al. (2011) for the ZnS nanoparticles [19]. homogeneity upon capping. The observation of characteristic IR absorption bands for PVA in PVA capped ZnS nanoparticles confirms the capping of the nanoparticles. The strong absorption at nm wavelength region along with the strong excitonic peaks at 287 nm and 313 nm respectively for the capped and pure ZnS nanoparticles. The calculated optical gap is 4.86 ev and 5.13 ev for the pure and PVA capped ZnS nanocrystals. ACKNOWLEDGEMENT The authors are thankful to Dr. Rajeev Kumar Sharma, DAV College, Jalandhar for his help in taking the optical absorption and FTIR studies. REFERENCES Figure 6: Plot of (αhν) 2 versus photon energy hν for the pure and PVA capped ZnS nanocrystals. calculated which found to be 4.86 ev for pure and 5.13 ev for PVA capped ZnS nanocrystals respectively. Similar result has been reported by Liu et al. (2011) for the ZnS nanoparticles [19]. The blue shift in the optical gap has been observed from its bulk value of 3.77 ev which reveals the effect of the quantum confinement in the synthesized surface passivated ZnS nanocrystals. 4. CONCLUSIONS The hydrothermal method has been successfully used for the synthesis of pure and PVA capped ZnS nanocrystals. The diffraction data was used to calculate the particle size using Scherrer formula and found to be 32.6 nm and 30.6 nm respectively for the pure and PVA capped zns nanocrystals. The spherical shape of the nanoparticles has been observed form the microscopic study for both the pure and PVA capped ZnS nanocrystals with the improvement in the size [1] Biswas, S., Kar, K., & Chaudhuri, S Optical and Magnetic Properties of Manganese- Incorporated Zinc Sulfide Nanorods Synthesized by a Solvothermal Process, J. Phys. Chem. B. 109: [2] Kuzuya, T., Tai,Y., Yamamuro,S., & Sumiyama, K Synthesis of copper and zinc sulfide nanocrystals via thermolysis of the polymetallic thiolate cage, Science and Technology of Advanced Materials. 6: [3] Borah, J.P, & Sarma, K.C Optical and Optoelectronic Properties of ZnS Nanostructured Thin Film. Acta physica polonica A. 114(4). [4] Dean, J., Dick, J., Doyon, G., Fan, R., Izmailov, S.,.Tsz, K. & Ratanalert, S. Crystal Chemistry. Available online: Crystal.pdf. [5] Yuan-yuan, S., Juan, Y., Ke-qiang, Q. (2010), Synthesis of ZnS nanoparticles by solid-liquid chemical reaction with ZnO and Na2S under ultrasonic, Trans. Nonferrous Mat. Soc. China, 20, s211-s215. [6] Panda, S. K.,, Anuja D.,and Chaudhuri S.(2007), Nearly monodispersed ZnS nanospheres: Synthesis anoptical properties, Chemical Physics Letters,440, [7] Sametband M., Shweky I.,Banin U., Mandler D. and Almog J. (2007) Application of nanoparticles for the enhancement of latent fingerprints. chem comm [8] Hoa, T. T. Q., Vu, L. V., Canh, T. D. & Long, N. N. (2009) Preparation of ZnS nanoparticles by hydrothermal method,journal of Physics: Conference Series. 187: [9] John, R., & Florence, S. S. (2010). Optical, structural and morphological studies of bean- like

5 24 ZnS nanostructures by aqueous chemical method, chalcogenide letters. 7(4): [10] Borah, J. P., Barman, J. & Sharma K. C. (2008) Structural and optical properties of ZnS nanoparticles, Chalcogenide Letters. 5(9): [11] Ghosh, G., Naskar, M. K., Patra, A. & Chatterjee, M. (2006) Synthesis and characterization of PVPencapsulated ZnS nanoparticles., Optical Materials. 28: [12] Tamrakar, R., Ramrakhiani, M. & Chandra, B. P. (2008) Effect of Capping Agent Concentration on Photophysical Properties of Zinc Sulfide Nanocrystals, The Open Nanoscience Journal. 2: [13] Wang, L.S. and Hong, R.Y. Synthesis, Surface Modification and Characterisation of Nanoparticles, Advances 290 in Nanocomposites - Synthesis, Characterization and Industrial Applications. [14] Skandan, G. and Singhal, A. (2006) Perspectives on the Science and Technology of Nanoparticle Synthesis Taylor & Francis Group. [15] Mehta, S. K., Kumar, S., Chaudhary, S. and Bhasin, K. K. (2009), Effect of Cationic Surfactant Head Groups on Synthesis, Growth and Agglomeration Behavior of ZnS Nanoparticles, Nanoscale Res Lett, 4, [16] Pol S., Pol V. Moreno C., Gedanken A.,& Cheylan S. (2008) Facile synthesis of photoluminescent ZnS and Znse nanopowders. Langmuir 24, [17] Sharma, M., Tripathi, S. (2012), Preparation and nonlinear characterization of zinc selenide nanoparticles embedded in polymer matrix, J. Phys. Chem. Sol. 73, [18] Pathak C., pathak P., Kumar P. & Mandal M (2012) Characterization and optical properties of Ni 2+ Doped ZnS nanoparticles, Journal of Ovonic research 8(1) [19] Liu, L., Xie, R., Yang, L., Xiao, D. & Zhu. J et.al Synthesis structural, and optical properties of core /shell ZnS: Fe/ZnS nanocrystals. Phys. Stat. Sol. A. 1-5.