Oxalic acid assisted synthesis of ZnS nanoparticles and their optical properties

Oxalic acid assisted synthesis of ZnS nanoparticles and their optical properties

Accepted Manuscript Oxalic Acid Assisted Synthesis of ZnS nanoparticles and their Optical Properties S. Sasi Florence, Nurdogan Can PII: DOI: Referenc...

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Accepted Manuscript Oxalic Acid Assisted Synthesis of ZnS nanoparticles and their Optical Properties S. Sasi Florence, Nurdogan Can PII: DOI: Reference:

S2211-3797(18)30601-6 https://doi.org/10.1016/j.rinp.2018.05.041 RINP 1484

To appear in:

Results in Physics

Received Date: Revised Date: Accepted Date:

13 March 2018 25 May 2018 25 May 2018

Please cite this article as: Sasi Florence, S., Can, N., Oxalic Acid Assisted Synthesis of ZnS nanoparticles and their Optical Properties, Results in Physics (2018), doi: https://doi.org/10.1016/j.rinp.2018.05.041

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Oxalic Acid Assisted Synthesis of ZnS nanoparticles and their Optical Properties S.Sasi Florence1 and Nurdogan Can1,2 1

Department of Physics, Jazan University, Gizan, Saudi Arabia

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Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey

Abstract ZnS nanoparticles and Oxalic acid (OA) assisted Zinc sulfide nanoparticles were synthesized by an easy and low-cost aqueous method using double distilled water as solvent. A phase change has been observed while increasing the concentration of OA. Morphological studies by Transmission Electron Microscopy (TEM), Optical studies from Ultra Violet Spectroscopy (UV) and Photoluminescence (PL) and structural studies from X-ray diffraction (XRD) have been done to characterize the samples. UV absorption spectrum confirmed the blue shift and formation of nanostructures. From the PL spectrum, it is observed that both ZnS nanoparticles and OA assisted ZnS nanoparticles excited about 370 nm exhibit a blue-green emission between 420-470 nm. It may be recognized to the recombination of the imperfection of ZnS nanoparticles. The spherical morphology of ZnS nanoparticles and OA assisted ZnS nanoparticles has been shown in TEM micrographs and the size of the particles was measured as 8-20 nm and it is in good agreement with XRD studies.

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1. Introduction Semiconductor nanoparticles are very important materials due to their various potential applications compared to their bulk counterparts [1–4]. Zinc sulfide (ZnS) semiconductor nanoparticles are one of the important compound semiconductors because of its potential applications including solar cells, sensors and optoelectronic applications [5]. These nanomaterials show interesting properties which are widely used for cathodoluminescent devices due to its optical properties [6-9]. Many synthesis techniques have been involved in preparation of the materials such as aqueous method [10–12], hydrothermal treatment [13], wet chemical synthetic route [14], mechanochemical method [15], high energy ball milling [16] etc. Research on ZnS nanoparticles, including synthesis, surface structure and property characterization has been carried out earlier [17]. Lee et al. observed few emission bands together with blue and green in the similar samples, 420 and 520 nm [18] and 460 and 507 nm respectively by Xue, [19]. Anuja Datta et al. [20] reported phase conversion of doped ZnS nanostructures prepared by solvothermal process but the nanorods were in organic solvent and two peaks have been observed in IR and UV range. On other hand, it was also observed at 480 nm and at 415 nm [21] as a single emission peak. OA is a stronger and less dynamic ligand and have comparable chain lengths with a double bond between C9 and C10 but the head group is different. OA ligands induced atomic alignment of wurtzite ZnS bifrustum-shaped nanocrystals have been reported to obtain aligned 2D superstructures [22]. In this study, the phase transformation of ZnS nanoparticles from wurtzite structure to cubic structure while increasing OA concentration where OA as an organic ligand via simple, inexpensive aqueous chemical reaction method has been reported. The influence of OA concentration used in the synthesis on the particle size, 2

structure and luminescence has been investigated. The results endow this kind of material with potential application in the field of solar cells and optoelectronic devices.

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Experimental method The chemical reagents used in the process were first grade chemicals and used as

it is. The overall process was done by a simple process by using distilled water as a solvent. Therefore, it is an environment friendly method and it is a facile synthesis method for obtaining nanomaterials. The whole synthesis process has been carried out at minimum temperature and maintained constant conditions. First, 1:1 ratio of zinc chloride and sodium sulfide solutions has been taken separately. It has been dissolved in deionized water (100 ml) and then Na2S solution was added drop wise. It has been kept stirring at 80 ºC for 1 hour. After 1 hour, ZnS nano colloid has been formed. To synthesis OA assisted ZnS nanomaterials, the required molar ratios (0.1,0.2 and 0.3) of aqueous solutions of OA has been supplied drop wise to the colloid. The ZnS nanoparticles as a colloid have been collected by the centrifugation process at 2000 rpm for 10 minutes. Ultrasonic bath has also been done for further purification. The colloid has been dried using an oven at 100 0C for 3 hours and ground by using an agate mortar manually. The samples have been characterized by different techniques. The structural studies were performed using X’Per pro PANalytical X-ray Diffractometer (CuKα radiation λ =1.5406A˚), with 2θ value 20–80º at 0.05º. UV and PL studies were carried out by using Perkin-ELmer spectrophotometer (Lambda 19) and JobinYuvon Flurolog-3 spectrophotometer in the range 200-800 nm. JEOL2010 transmission electron microscope (TEM) (accelerating voltage of 200 kV) was used for recording TEM images. 3

3. Results and discussion 3.1. XRD Studies The XRD patterns of the undoped and OA assisted ZnS nanoparticles have been shown in Fig.1. The Standard bar diagrams are shown at top of the XRD pattern for wurtzite and cubic ZnS. From the figure, ZnS nanoparticles illustrate strong and pointed reflections which conforms the wurtzite structure of ZnS where these features correspond to the (1 0 0), (0 0 2 and (1 1 0) planes. The planes illustrate some size broadening effects which representing the size of these nanocrystals (JCPDS card no. 89-2349). The entry of OA into ZnS has been proved from the gradual decrease/increase in intensities while increasing OA with few extra peaks due to the accumulation of OA. Anuja Datta et al. have reported thermodynamically controlled phase transformation in doped ZnS nanoparticles because of the presence of organic solvent molecules. S. B. Quadri et al. [23] have reported the phase transformation in ZnS nanocrystals due to the nanosize effect of the particles. Wang et al. [24] have reported morphology-tuned phase transformation in ZnS nanobelts. Here, we state the phase transformation of undoped and OA assisted ZnS in the presence of pure aqueous medium. The particle size was calculated from Scherrer’s formula. The average particle size increased from 8-25 nm with the increase of OA concentration.

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Fig. 1. X-ray pattern of undoped ZnS and OA assisted nanoparticles

3.2. Morphological Studies Fig. 2 shows the TEM images of the as-prepared undoped and OA assisted ZnS nanoparticles. These images illustrated the particle size sharing ranging from 8- 20 nm and the identifiable secluded particles are about 8 nm which is in good agreement with XRD data. These images also confirmed that the agglomeration ratio increases with increasing OA concentration which agrees comparatively with the XRD data.

Fig. 2 TEM images of a) undoped ZnS b) 1% OA:ZnS c) 2% OA:ZnS d) 3% OA:ZnS 5

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Optical Studies The UV-visible absorbance spectra of the ZnS nanoparticles are given in Figure 3.

From the spectra it is observed that the maximum absorption is at 300 nm for ZnS nanoparticles (Fig.3 curve a) and 310, 320, 330 nm for OA assisted nanoparticles (Fig.3 curves b to d). The absorption spectrum exhibited a blue shift compared to the bulk ZnS (340 nm, Energy gap=3.65 eV). Thus, it has been proved that the band gap of OA assisted ZnS nanoparticles has been overlarge in comparison with undoped ZnS nanoparticles. The quantum confinement effect has been occurred within these nanoparticles which are exhibiting a progression of the optical Eg because of the smaller crystallite of OA assisted ZnS nanoparticles.

Fig. 3. UV Spectrum of undoped ZnS and OA assisted nanoparticles

The investigation of the influence of the OA as an organic ligand on the host structure, the PL spectra of undoped ZnS and OA assisted ZnS have been done. Fig. 4 illustrates the emission spectrum of the ZnS nanoparticles. The peak located at 420 nm in the Fig.4 can be attributed to the blue shift. The emission wavelength is unlimited about 420-470 nm to low energy. Usually, few emission peaks are studied for the nanostructures, the exciton and the trapped 6

luminescence [25, 26]. But in this paper, we report a single broad blue emission of ZnS and OA:Zns nanostructures synthesized in aqueous medium. As concentration of OA increases, a gradual red shift of the Eg appears. This is because of the substtution of OA in Zn2+ ions positions which is confirmed by the XRD spectra.

Fig. 4. PL spectra of ZnS nanoparticles a) undoped ZnS b) 1% OA:ZnS c) 2% OA:ZnS d) 3% OA:ZnS

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Conclusion In summary, undoped and OA assisted zinc sulphide nanoparticles have been

analysed by an easy, inexpensive aqueous method. From XRD, wurtzite to cubic structural change is observed while increasing OA concentration in ZnS. TEM analysis indicated the spherical morphologies of ZnS nanoparticles. A strong blue shift is observed from the optical studies because of the quantum confinement effect. PL result shows the improved intensity of emission at 450 nm for the higher concentration of OA comparatively the lower concentration of OA assisted ZnS nanoparticles.

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Acknowledgement The authors thank The Deanship of Scientific Research, Jazan University, Jizan, Saudi Arabia for the financial support under DSR project (Grant no: JUP7/00035).

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Graphical Abstract

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 Room temperature synthesis of ZnS nanoparticles  Oxalic Acid played a determinative role in the formation of ZnS nanoparticles  ZnS nanoparticles exhibit significant broad spectra

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