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ScienceDirect Materials Today: Proceedings 5 (2018) 9144–9147

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NCNN 2017

Optical properties of ZnO nanoparticles synthesized by coprecipitation method using LiOH Anu Katiyar, Nishant Kumar and Anchal Srivastava* Department of Physics, University of Lucknow, Lucknow-226007, India

Abstract ZnO NPs have been successfully synthesized at room temperature by the co-precipitation method using LiOH. All the detected X-ray diffraction peaks along (100), (002), (101), (102), (110) and (103) planes confirm the formation of hexagonal wurtzite structure of ZnO. The crystallite size lies between 18-39 nm. The sharpness of the diffraction peaks indicates good crystallinity of ZnO NPs. The UV-Vis absorption spectrum shows strong absorption peak at 382nm and the optical band gap of is determined to be 3.14 eV. The photoluminescence emission spectrum recorded with an excitation wavelength of 325 nm shows peaks centered at 408 & 426nm, 453nm, 488 and 530nm with a shoulder at 582nm. FESEM shows some cauliflower-like structures which are combination of quasi-spherical grain with average grain size 57nm. .

© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th NATIONAL CONFERENCE ON NANOMATERIALS AND NANOTECHNOLOGY (NCNN VI - 2017 ).

Keywords: Zinc oxide, Nanoparticles, LiOH, Co-precipitation

* E-mail: [email protected]

2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th NATIONAL CONFERENCE ON NANOMATERIALS AND NANOTECHNOLOGY (NCNN VI - 2017 )

Anu katiyar and Anchal Srivastava / Materials Today: Proceedings 5 (2018) 9144–9147

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1. Introduction ZnO nanoparticles have received intensive attention due to their wide application such as solar cells, photo detectors, light emitting diodes, various sensors, biological sensing, nanomedicines[1,2] etc. Zinc oxide is n-type compound semiconductor with a wide band gap of 3.37 eV and large exciton binding energy (60 meV) at room temperature [2]. It is chemically stable and environment friendly [3]. ZnO is a good photoluminescent semiconductor due to its ability to emit visible light [4]. Many researchers have reported the synthesis of ZnO NPs by different methods such as sol–gel [5], hydrothermal [6], solid state reaction [7], solvothermal [8], co-precipitation [9] etc. Synthesis of ZnO NPs by co-precipitation method is simple, inexpensive and high yield providing. The role of various alkali hydroxides such as LiOH, KOH, NaOH etc in the synthesis of ZnO were investigated and effect on structure and morphology were reported [4,8,10,11]. The impact of the addition of a hydroxide on ZnO particle size and morphology [11] were studied and suggested that alkalescent environment providing the hydroxyl group (OH-) facilitates the synthesis of nanostructures [8]. In the present work we have reported the synthesis of ZnO NPs by coprecipitation method along with its structural, morphological, photoluminescence and optical properties. 2. Experimental Method ZnO NPs were synthesized from Zinc Nitrate using LiOH in ethanol by co-precipitation method. 0.4M solution of Zinc nitrate hexahydrate in ethanol and 0.8M solution of LiOH in ethanol were prepared under constant stirring for one hour at room temperature. Afterwards, 0.8M solution of LiOH was added slowly drop by drop to 0.4M solution of zinc nitrate, along the walls of the beaker under constant stirring. After complete addition of LiOH solution the reaction solution was further kept under constant stirring for 2 hours at room temperature. The resulting solution was allowed to settle in a sealed beaker for 15 hours at room temperature. The ZnO NPs settled at the bottom of the beaker and the supernatant solution was separated carefully. The remaining solution was centrifuged for 10 min and the precipitate was washed three times with deionised water and ethanol and then dried in air at 60°C. The crystal phase and crystallinity of the sample has been investigated using X−Ray diffractometer (Model – Rigaku Ultima IV). The surface morphology is obtained using FESEM (Model− JEOL). The absorption and photoluminescence spectra have been recorded using UV−Vis spectrophotometer (Model− V670, Jasco) and fluorescence spectrometer (Model LS−55, Perkin Elmer) respectively. The excitation wavelength is 325 nm coming from a 20 kW Xe discharge lamp. All these measurements have been done at room temperature. 3. Results and Discussion The X-ray diffraction pattern of ZnO NPs obtained by co-precipitation method, Fig.1, shows diffraction peaks along (100), (002), (101), (102), (110), (103), (200), (112) and (004) planes confirming the formation of hexagonal wurtzite structure of ZnO (PDF Card No. 00-005-0664). The major peaks are along (100), (002) and (101) planes and preferred orientation is along (101) plane. The sharpness of the diffraction peaks indicates good crystallinity of ZnO NPs.

Fig. 1: XRD pattern of ZnO NPs.

The lattice constants a and c are calculated using the following expression [12]

1 4 h 2  hk  k 2 I2  [ ]  2 3 d hkl a2 c2

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and are obtained as a=3.256 Å, c=5.204 Å with the ratio c/a= 1.598. The crystallite size of ZnO NPs is estimated by Scherrer formula[13] 0.9

t DS 

 cos 

where tDS is the crystallite size, K is the Scherrer constant which is taken equal to 0.9, λ is the wavelength of the X−rays and β is the full width at half maximum (FWHM) of X−ray diffraction peaks. The crystallite size of ZnO NPs is found to lie between 18-39 nm. The surface morphology of the ZnO NPs was studied by FESEM, Fig.2, which shows the agglomerated irregular morphology of ZnO NPs and the growth is not exactly uniform. Some flake-like as well as some cauliflower-like structures were observed. The cauliflower-like structures are combination of quasi-spherical grain with average grain size 57nm. As expected, these measured grain size is larger than the crystallite size estimated from the DS formula.

Fig. 2: FESEM image of ZnO NPs at different magnifications a) X 30,000 and b) X 50, 000

Figure 3 shows the UV- Vis absorption spectrum of ZnO NPs dispersed in ethanol. A strong absorption occurs around 382 nm which is a characteristic absorption peak of pure ZnO [14]. The Tauc’s plot ((αhν) 2 vs hν) for determining the band gap is shown in the inset of Fig.3. The optical band gap is obtained as 3.14 eV by extrapolating the linear portion of Tauc’s plot which is lower than that of bulk ZnO i.e. 3.37eV.

Fig. 3: UV- Vis absorption spectrum of ZnO NPs. Inset shows the corresponding Tauc’s plot.

Figure 4 shows the Photoluminescence (PL) emission spectrum, of prepared ZnO NPs, which was recorded with an excitation wavelength of 325 nm. Weak peaks at 408nm & 426nm (violet emission); sharp peaks at 453nm (blue emission) and 488 (blue-green emission) in visible region were observed. These peaks occur because of defect states created during synthesis of ZnO NPs. An additional peak at 530nm (green emission) with a shoulder at 582nm (yellow emission) was also observed.


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Fig. 4: PL emission spectrum of prepared ZnO NPs

ZnO exhibits visible emissions are attributed to different intrinsic defects such as oxygen vacancies (VO), oxygen interstitials (Oi), oxygen antisites (OZn), zinc vacancies (VZn) and zinc interstitials (Zni) [15]. The violet emission due to an electron transition from a shallow donor level of neutral Zni to the top level of the valence band [15]. The blue emission may be assigned to singly ionized VZn .The blue-green emission may be related to radiative transition of an electron from the shallow donor level of Zni to an acceptor level of neutral VZn [16]. The green emission may be due to oxygen vacancies [17, 18]. The yellow emission may be attributed to oxygen interstitial defects [19]. 4.

Conclusions

The NPs synthesized at room temperature show the hexagonal wurtzite structure of ZnO with good crystallinity. The crystallite size lies in the range 18-39 nm as obtained by X-ray diffraction and DS method. The ZnO NPs show quasi-spherical and agglomerated cauliflower-like morphology with high absorption in UV region at 382nm and low absorption in visible region. PL spectrum shows emission in visible region ranging from 426 to 531nm, due to defect states. The synthesized ZnO NPs are good UV absorber and may find cosmetic and biomedical applications. Acknowledgements Authors are thankful to Centre of Excellence Scheme of U.P. State Government for providing XRD facility at the Department of Physics, University of Lucknow. References: [1] A. Srivastava, N. Kumar and S. Khare, Opto−Electronics Review 22 (2014) 68–76. [2] R.K. Shukla, A. Srivastava, and K.C. Dubey, J. Cryst. Growth 294 (2006) 427. [3] Z. L. Wang; J. Phys.: Condens. Matter 16 (2004) 829–858. [4] Y. Lv, Wen Xiao, Weiyan Li, Junmin Xue and Jun Ding , Nanotechnology 24 (2013) 175702. [5] Li-Li Han, Lan Cui1, Wei-Hua Wang, Jiang-LongWang and Xi-Wen Du, Semicond. Sci. Technol. 27 (2012) 065020. [6] T. Wirunmongkol, N. O-Charoen and S. Pavasupree, Energy Procedia 34 (2013) 801-807. [7] Yan Zhu and Yingxue Zhou; Appl. Phys. A 92, (2008) 275–278. [8] Q.R. Hu, S.L. Wang, W.H. Tang, Materials Letters 64 (2010) 1822–1824. [9] S. Chakraborty, A.K. Kole, P. Kumbhakar; Materials Letters 67 362–364 (2012). [10] J. Singh , B. Mittu , A. Chauhan, A. Sharma, M.L. Singla,Int. J. Fundamental Applied Sci. 1 (2012) 91-93. [11] A. Anzlovar, K. Kogej, Z. Crnjak Orel, and M. Zigon, Cryst. Growth Des. 14 (2014) 4262−4269. [12] A. Srivastava, N. Kumar, K. P. Misra and S. Khare, Electron. Mater. Lett. 10 (2014) 703-711. [13] A. Srivastava, N. Kumar, K. P. Misra and SanjayKhare, Materials Science in Semiconductor Processing 26 (2014) 259–266. [14] Chieng BW, Loo YY, Materials Letters 73 (2012) 78–82. [15] S. K. Mishra, S. Srivastava, Rajneesh K. Srivastava, A.C. Panday, S.G. Prakash, Adv. Mat. Lett. 2 (2011) 298-302. [16] N. Kumar and A. Srivastava, Journal of Alloys and Compounds 706 (2017) 438-446. [17] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, and B. E. Gnade, J. Appl. Phys 79 (1996) 7983. [18] A. Srivastava, R. K. Shukla, and K. P. Misra, Cryst. Res. Technol. 46 (2011) 949 – 955. [19] A. B. Djurisi, Y H Leung, K H Tam, Y F Hsu, L Ding, W K Ge, Y C Zhong , K S Wong, W K Chan, H L Tam, K W Cheah, W M Kwok and D L Phillips, Nanotechnology 18 (2007) 095702