Effect of low Co-doping on structural, optical, and magnetic performance of ZnO nanoparticles

Journal Pre-proof Effect of low Co-doping on structural, optical, and magnetic performance of ZnO nanoparticles Gyanendra Pratap Singh, Abhay Kumar Aman, Rakesh Kumar Singh, M.K. Roy

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S0030-4026(19)31864-9

DOI:

https://doi.org/10.1016/j.ijleo.2019.163966

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IJLEO 163966

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Optik

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9 October 2019

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2 December 2019

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Effect of low Co-doping on structural, optical, and magnetic performance of ZnO nanoparticles Gyanendra Pratap Singha , Abhay Kumar Amanb , Rakesh Kumar Singhb , M.K. Roya* a

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Department of Natural Science, PDPM Indian Institute of Information Technology Design and Manufacturing Jabalpur, Dumna Airport Road, 482005, India b Aryabhatta Centre for Nanoscience and Nanotechnology Aryabhatta Knowledge University, Patna 800001, India

Abstract

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In the present work thrust has been given to understand different properties of Co-doped ZnO nanopowders. Pure ZnO and Co doped ZnO semiconductor nanopowders (Zn1−x Cox O, x = 0.01, 0.02, 0.03 and 0.04) were synthesized by using solution method. The X-ray diffraction revealed the formation of single crystalline phase and also indicated successful incorporation of cobalt ions into ZnO lattice without formation of secondary phase of cobalt. Scanning electron micrographs showed change in surface morphology with Co doping. X-ray photoelectron spectroscopy also indicated presence of Co2+ ions into ZnO matrix. A blue shift in the band gap, which increases with increasing Co-doping, was found from the Tauc’s plots. Fourier transform infrared spectrum indicated formation of wurtzite structure of ZnO. Photoluminescence spectrum showed emergence of near band edge emission (NBE) with Co doping. In addition, the dilute magnetic semiconductor (DMS) nature was also observed for doped semiconducting ZnO nanoparticles.

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1. Introduction

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In recent years, there is a continuous effort to develop multifunctional materials to meet the technological challenges associated with changing dimensions and level of functionality of materials. For better application and understanding of these materials, the science of these materials has drawn a great attention. Among the different available materials, dilute magnetic semiconductors (DMSs) nanomaterial have attracted increasing attention due to presence of charge and spin degree of freedom in a single compound. These DMSs have promising technological applications in the field of spintronic and optoelectronics devices [1, 2, 3]. Particularly, ZnO has many unique properties such as large band gap (3.37 eV), large exciton binding energy (60 meV) and high refractive index at room temperature [4, 5]. Due to their unique properties, ZnO has been used at several places such as gas sensor, catalyst, photocatalyst, UV absorber, antibacterial treatment, transparent conducting oxide, solar cell, etc. [6, 7, 8, 9, 10, 11, 12, 13]. There are many reports related to transition metals (Fe, Co, Ni, etc.) doped ZnO which behave as DMS for spintronic application [14, 15, 16, 17]. Among these transition metals, Co as a doping element is important because, (1) it behaves Preprint submitted to Optik

December 3, 2019

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as luminescence activator that increases luminescence property of ZnO nanostructures by creating localized impurities levels across the band [18], (2) it also manipulates the optical behavior of ZnO crystal [19], (3) both (Co and Zn) have nearly similar ionic radii [20, 21]. Various methods such as sol-gel, precipitation, hydrothermal, solution method, thermal decomposition, combustion method, chemical deposition, etc. are available in literature for synthesis of Co doped ZnO nanostructures [22, 23, 24, 25, 26, 27, 28, 29]. Solution method is a, facile, cost-effective, environment friendly, repeatable and low temperature process to prepare ZnO nanocrystals. This method provides a good control of nanoparticles size and shape during synthesis. It has been observed that Co doped ZnO nanoparticles prepared by different methods show different magnetic behavior ranging from paramagnetic to ferromagnetic at room temperature [30, 31, 32, 33, 34]. Shatnawi et al. [30] used solid state reaction method to prepare Co doped ZnO nanoparticles exhibiting ferromagnetic nature at room temperature. Yang et al. [31] synthesized Zn1−x Cox O (0.03≤ x ≤0.09) by using sol-gel method and observed paramagnetic as well as ferromagnetic behavior for different x values. Basith et al. [32] also observed ferromagnetic nature in Co doped ZnO nanoparticles synthesized by microwave-assisted combustion method. There are various mechanism that are used to explain room temperature ferromagnetism in Co doped ZnO nanoparticles such as, secondary phase formation, defects, sp-d exchange interaction and carrier mediated exchange interaction but all these mechanism are still debatable. It has also been observed that Co doping into ZnO has significant effect on photoluminescence properties of ZnO. Basith et al. [32] observed UV emission peak at 400 nm and visible emission peak at 536 nm. While Bhargava et al. [35] did not find any obvious UV emission peak for undoped and Co doped ZnO nanostructures but deep level emissions were found at 415 nm, 450 nm, and 480 nm. Hence, Co doping into ZnO not only changes magnetic nature but also modifies optical properties of ZnO nanoparticles. This means, that in the area of nano-manufacturing, change in the preparation method can result in different associated properties of the nanoparticles. So, the objective of this work is to analyze and study the change in structural, morphological, optical and magnetic performance of ZnO nanoparticles by introducing low concentration of Co using various characterization techniques.

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2. Experiment Details

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2.1. Materials Zinc acetate (Zn(CH3 COO)2 .2H2 O) and cobalt nitrate (Co(N O3 )2 ) were used as source of Zn and Co respectively. Methanol (CH3 OH) and distilled water were used as solvents. Sodium hydroxide (NaOH) was used to maintain the pH of the solution and precipitate formation. Finally, ethanol (C2 H5 OH) was used for washing the precipitates. In this experiment, all the chemical reagents were of analytical grade. 2.2. Synthesis of nanopowder Pure ZnO and Co doped ZnO nanoparticles were prepared by solution method. In this method, at first, we prepared aqueous solution of zinc acetate in methanol and aqueous solution of cobalt nitrate in distilled water in two separate beakers. We took zinc acetate 2

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and cobalt nitrate according to stoichiometric ratio. Zinc acetate solution was added in cobalt nitrate solution gradually and stirred on magnetic stirrer at room temperature for getting clear and homogeneous solution. Then, in the obtained solution, we added NaOH for making the pH of solution 9-10. After adding NaOH, milky white thick solution was obtained which was stirred further for one hour at room temperature. The solution was then centrifuged and white precipitates were obtained. White precipitates were washed multiple times with ethanol and distilled water. The precipitates were dried in hot air oven at 90◦ C. After drying, precipitate were grounded in agate mortar and pestle for getting fine powder. This fine powder was calcined in muffle furnace at 400◦ C for two hour to get Co doped ZnO (Zn1−x Cox O, x = 0.01, 0.02, 0.03 and 0.04) nanoparticles. For pure ZnO nanoparticles, we followed the same above synthesis process without including aqueous solution of cobalt nitrate.

3. Result and discussion

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2.3. Characterization The structure and phase analysis of synthesized samples was done by using D8 Advance, Bruker x-ray diffractometer with CuKα radiation (λ = 0.15418 nm). To study microstructure, surface morphology and to confirm doping, SEM and EDAX were performed using FEI Quanta 200 microscope equipped with EDAX. XPS characterizations were done by using VSW ESCA machine with AlKα radiation. The optical properties were examined with UV-vis and FTIR spectroscopy by using UV-Vis-NIR-Lamda 950, PerkinElmer Spectrophotometer and FTIR-Frontier, PerkinElmer Spectrometer. Photoluminescence property of as prepared samples was analyzed with LS-55, Fluorescence Spectrometer from, Perkin Elmer. Magnetic properties were analyzed by using VSM technique using VSM-7410, Lakeshore magnetometer.

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3.1. X-ray diffraction The XRD pattern for synthesized samples Zn1−x Cox O, (x = 0, 0.01, 0.02, 0.03 and 0.04) are shown in Fig. 1(a). The diffraction peaks for all samples show the formation of only wurtzite hexagonal structure of ZnO (JCPDS card no. 36-1451). The diffraction patterns do not show formation of any secondary phases like Co3 O4 , Zn(OH)2 , Co, Zn, Co(OH)2 within the sensitivity of XRD technique. This is because of successfully substitution of Co2+ ions into ZnO matrix [36]. In Fig. 1(b), peak corresponding to plane (101) of doped samples are slightly shifted relative to undoped sample. Apart from this, Fig. 1(b) also shows variation in peak intensity and broadening for diffraction peak (101) with Co content. It has also been observed that all doped samples show more crystallinity with respect to undoped sample and this degree of crystallinity is decreased as Co molar concentration is increased. As Co concentration in ZnO increases, broadening and peak intensity corresponding to plane (101) also increases which indicates small change in lattice parameter of ZnO with Co doping. This small change in lattice parameter of ZnO is expected due to nearly same ionic radii of Zn2+ (0.06 nm) and Co2+ (0.058 nm) ions [37]. We have used Scherrer’s formula [38] for calculating average crystallite size of nanoparticles. 3

0.9λ (1) β cos θ Where, D is average crystallite size, λ is X-ray wavelength, β is full width at half maximum (FWHM) and θ is Bragg’s angle. Fig. 2(a) shows that variation in D and FWHM with Co concentration. The values of FWHM of diffraction peak (101) for all the doped samples are smaller than undoped ZnO which indicate larger crystallite size of doped samples with respect to undoped sample. The crystallite size was found to be 24 nm for undoped sample and 36-30 nm for doped samples shown in Table 1. The value of D increased for doped samples relative to undoped sample, similar results are also reported in literature [36]. A minute change was observed in 2θ value of diffraction peaks. Peak broadening for doped samples is due to increase in micro strain in nanoparticles, therefore the particle size is decreased as cobalt concentration is increased in ZnO. The micro strain can be calculated by using following formula.

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D=

β cos θ (2) 4 Where  is micro strain, β is Full width at half maximum (FWHM) and θ is Bragg’s angle. The lattice parameters for all hexagonal wurtzite structured samples are calculated by using following equation.   4 h2 + hk + k 2 1 l2 = (3) + 2 d2 3 a2 c

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Where, d is interplanar spacing, h, k and l are Miller indices, and a and c are lattice parameters. Calculated values of lattice parameters (a and c) for all samples are mentioned in Table 1. The variation of lattice parameters a and c with Co molar concentration is shown in Fig. 2(b). There is very small change observed in lattice parameters of ZnO with increasing Co content. This result is attributed to systematic substitution of Zn2+ by Co2+ ions without disturbing crystal structure of ZnO [36]. This may also be due to nearly same ionic radii of Zn2+ (0.060 nm) and Co2+ ions (0.058 nm). The volume of unit cell was calculated by using following equation. √ 2 3a c (4) V = 2

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Here, V is volume of unit cell and a and c are lattice parameters. Volume of unit cell is decreased for doped samples relative to undoped sample because lattice parameters for doped samples are decreased [39]. After Co doping in ZnO, all samples indicate single phase formation of ZnO therefore, we can infer that the crystal structure of ZnO has no visible change due to Co2+ ions. 3.2. Surface morphology investigation SEM is a characterization technique that is used to analysis the surface morphology for microstructure of samples. Since, XRD results were not able to give elemental information 4

for as the prepared samples therefore, we took EDX images for investigation of surface elemental composition. Fig. 3(a-b) shows, low magnification SEM micrographs for pure ZnO and x = 0.01 Co doped ZnO sample. Fig. 3(a) shows irregular shaped surface morphology for pure ZnO sample. While, Fig. 3(b) for x = 0.01 Co doped ZnO sample shows different surface morphology from pure ZnO sample. So, the effect of Co doping on the surface morphology of ZnO can easily be seen by comparing Fig. 3(a) and 3(b). In doped samples EDX spectrum confirms the presence of Co in the samples. For the quantitative amount of Co, data is collected at several places on the sample and found to be in well agreement of the amount of atomic % of Co ion taken at the start of the experiment and only Co as cluster was not visible.

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3.3. X-ray photoelectron spectroscopy (XPS) EDX spectrum already confirm the existence of cobalt ions for as the prepared sample but to know about oxidation state of cobalt ions and its local structure into ZnO lattice, we have performed XPS characterization. XPS spectrum for as prepared sample with x = 0.04, is shown in Fig. 4(a-b). Fig. 4(a) shows full scan and Fig. 4(b) shows Co 2p high resolution XPS spectrum for same sample. Co 2p3/2 and 2p1/2 peaks are centered at binding energy 782 eV and 797.7 eV along with their satellites as shown in Fig. 4(b). These energies (782 eV and 797.7 eV) are similar to the binding energy of photoelectrons for Co2+ in Co3 O4 and CoO [40]. Therefore, these peak positions (782 eV and 797.7 eV) indicate the presence of Co ions in sample x = 0.04 with oxidation state 2+ . The binding energy difference for peaks Co 2p3/2 and Co 2p1/2 is calculated to be 15.5 eV which is similar to that reported in literature [40, 41]. This binding energy difference (15.5 eV) excludes the formation of Co cluster in prepared sample because if Co is present as metal cluster in our sample then binding energy difference should be 15.05 eV [41]. This binding energy difference (15.5 eV) also confirms the existence of Co ions in sample with x = 0.04 in the form of oxidized state with oxidation number +2 [42]. This result also supports the inference drawn from SEM/EDX study

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3.4. UV-Visible spectroscopy This is very useful technique to analysis the optical properties like as band gap, absorption etc. of semiconductor nanoparticles. Light absorb by nanoparticles is depend on several factors such as size of nanoparticles, surface morphology, energy band gap, and impurity centers. The UV-vis absorption spectra for all samples are shown in Fig. 5, as a function of wavelength at room temperature. In Fig. 5, all samples show strong absorption edge below 390 nm. For doped samples, absorption edge is shifted towards lower wavelength as Co concentration is increased in ZnO lattice. It has been found that three new absorption peaks in visible region located at 565, 611, and 660 nm in doped samples. For better visualization of these three peaks, an inset figure for doped sample x = 0.01 is shown in Fig. 5. These peaks may correspond to d-d transitions of the high spin Co2+ 3d7 -4 F ion in tetrahedral oxygen sites and can be ascribed to the transitions from 4 A2 (F) to 2 A1 (G),4 A2 (F) to 2 E(2 G), and 4 A2 (F) to 4 T1 (P), respectively. These results also suggest that Co2+ ions have partially substituted Zn2+ ions in the tetrahedral sites in hexagonal wurtzite structure [43]. Optical band gap for all samples were calculated by using Tauc’s relation [44] as given below. 5

αhν = B(hν − Eg )n

(5)

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Where, α is absorption coefficient, h is plank constant, B is proportionality constant, ν is frequency of incident photon, Eg is optical band gap and value of n equals 1/2, 1, 3/2, and 2 depending on the nature of semiconductor material. Since, ZnO is direct band gap semiconductor therefore we took n equals 1/2 in Tauc’s equation (5). The optical band gap is estimated by plotting a curve between (αhν)2 on y-axis and hν on x-axis. The extrapolation of the tangent passing through point of inflection of the curve intercept on x-axis and give optical band gap as shown in Fig. 6(a-e). The variation in optical energy band gap with Co content for all samples are shown in Fig. 6(f), and tabulated in Table 1. It has been observed that a small and linear change in band gap occurs with Co content. It has been remarked that band gap increases as Co concentration increases. Arshad et al. [45] also showed increase in optical band gap for Co doped ZnO nanoparticles. Band gap for pure ZnO was found to be 3.22 eV which is similar to literature [46]. For doped samples, it was increased from 3.25 to 3.28 eV. This small change in band gap depends on several factors such as lattice parameters, grain size, defects and carrier concentration, etc. Generally, the excess carriers that are introduced through doping (in our case Co) are responsible for blue shift in optical band-to-band transitions for doped samples. This is known as BursteinMoss (BM) effect [47], where a large number of localized carrier density is essential for blue shifting in absorption peaks. The blue shift in absorption peak indicate that more impurity levels are appeared due to Co doping between conduction and valence band so band gap is increased [20]. Variation in band gap from the UV-vis clearly indicates that these samples may be used for wide band gap engineering applications.

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3.5. Fourier transform infrared spectroscopy Fourier transform infrared spectroscopy is method that is used to confirm wurtzite structure formation and to get information about functional group in material. The FTIR spectra was observed for all samples in the region 400-4000 cm−1 as shown in Fig. 7. The FTIR spectra in the range 4000-1800 cm−1 is called functional group region and in the range 1800400 cm−1 is called finger print region for that material. The broad absorption peak near at 3434 cm−1 indicate O-H stretching vibration mode of water for undoped ZnO nanoparticles [48]. The presence of O-H group shows the absorption of water from environment on the surface of sample [49]. O-H stretching mode is shifted towards higher wavenumber for doped samples. The O-H group is extreme important for photocatalytic activity of a material. The absorption peaks are found around at 2482 cm−1 and 2496 cm−1 show the presence of O-C-O molecule. The presence of carbon may be due to use of acetate group as a precursor. Similar results are reported in [50]. A strong absorption band near 1447 cm−1 indicate existence of C=O band that is known as Brownsted acidity. For doped samples this band is changed in dip peak and shifted toward higher wavenumber. A strong dip peak for all samples at 882 cm−1 corresponds to C-OH vibration modes. The absorption band nearly at 471 cm−1 for undoped sample shows the vibration mode of Zn-O and also confirms the formation of wurtzite structure of ZnO [51]. Absorption band position of Zn-O vibration for all doped 6

samples is observed in the region 475 cm−1 to 509 cm−1 . This slight shift in Zn-O vibration modes for doped samples may be because of incorporation of Co2+ into ZnO lattice.

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3.6. Photoluminescence (PL) For further study of optical properties, we carried out photoluminescence (PL) measurement for all samples at room temperature using a laser wavelength 325 nm as shown in Fig. 8. Optical properties of semiconductor materials are significantly affected by density of defects. Photoluminescence spectroscopy is effective way to find out the defects in semiconductor materials. In general, there are two emission bands that appear in Photoluminescence spectrum of ZnO. One lies in UV region and the other in the visible region. The emission band in UV region is called near band edge emission (NBE) and visible region band is named as deep level emission (DLE). The visible emission bands for pure ZnO are located at 415 nm, 448 nm, 480 nm, and 527 nm. These visible emission bands arise due mainly to structural defects and impurities in the sample [52]. But for doped samples, UV emission bands were observed at 383 nm, 385 nm, 388 nm, and 388 nm for Zn1−x Cox O (x = 0.01, 0.02, 0.03 and 0.04) respectively. These UV emission bands are NBE which originate due to free exciton recombination [20]. These NBE are also known as characteristic emission for ZnO. All visible emission bands for doped samples are located around at 415 nm, 418 nm, 420 nm, 440 nm, 454 nm, 482 nm, 522 nm, 524 nm, and 527 nm. The bands at 415 nm, 418 nm, 420 nm and 440 nm, belong to violet emission attributed to transition of electrons from shallow donor level of Zn interstitials to valence band [53]. The blue emission bands are observed at 448 nm, 455 nm, 480 nm and 482 nm. The reason behind blue emission bands is creation of defects levels such as shallow donor levels between valence and conduction band arising from Zn defects such as Zn interstitials position [54]. Some researchers have reported that presence of OH− ions on the surface of ZnO is responsible for blue emission band [54]. The existence of OH− ions on the surface of ZnO is already confirmed by FTIR spectrum. The green emission bands are appeared at 522 nm, 524 nm, and 527 nm for all samples. These green emission bands are observed because of transition of electron to valence band from deep donor level of ionized oxygen vacancies [55]. PL spectrum show that Co doping enhances NBE in ZnO [56]. So we can infer that room temperature PL behavior observed from the as prepared samples can play an important role in optoelectronics.

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3.7. Magnetic measurement The magnetization for all samples was measured in the range of applied magnetic field ±20000 (Oe). The field dependent magnetization curves for all the prepared samples are shown in Fig. 9. Pure ZnO sample shows diamagnetic behavior as described in [57, 58, 59]. M-H curve for sample (x = 0.01) also shows diamagnetic nature. For doped sample (x = 0.01), Co2+ concentration is very small which is unable to change the magnetic behavior of pure ZnO. Hence, diamagnetic nature of ZnO is dominated for the x = 0.01 Co doped ZnO sample. Another reason behind diamagnetic nature in doped sample (x = 0.01) may be the presence of point defects introduced by Co2+ into ZnO lattice [60]. For doped samples (x = 0.02, 0.03, 0.04), M-H curve indicates ferromagnetic behavior with small hysteresis loop near the field. The center of the hysteresis loop is shifted (downward along magnetization axis 7

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and rightward along the magnetic field). This means that the sample is showing spin canting behavior along with exchange bias interaction. There are many reasons for observation of ferromagnetism in Co2+ doped ZnO nanoparticles. Some of them are, (1) formation of secondary phase of Co metal cluster in the ZnO lattice, (2) Increasing of free charge carrier density due to doping of Co2+ ions in ZnO [61], and (3) Producing of oxygen vacancies and defects due to impurity such as doped element into ZnO matrix [62]. But in our Co doped samples, any formation of secondary phase of Co metal cluster has not been seen (as confirmed from XRD, XPS and FTIR results). XRD and FTIR results show formation of only single phase of wurtzite structure of ZnO. But in the absence of secondary phase of Co metal cluster, doped samples (x = 0.02, 0.03, 0.04) show ferromagnetic behavior. The origin of ferromagnetism in Co-doped samples is due to incorporation of Co2+ ions into Zn2+ site in ZnO lattice instead of forming Co metal clusters [63]. Another reason for appearing of ferromagnetism in Co doped ZnO nanoparticles, is presence of defects such as O and Zn vacancy or interstitials [60]. The existence of defects in the as prepared samples is already confirmed by PL outcomes. The saturation magnetization increases as Co doping increases into ZnO as shown in Fig. 9. The exchange interaction between the doped element Co and host element Zn or O is also responsible for room temperature ferromagnetism in Co doped ZnO nanostructures [56]. As, Co concentration increases in ZnO, stronger interaction take place between Co2+ ions and Zn2+ ions. So, saturation magnetization is increased as Co concentration is increased into ZnO lattice. It can be concluded that pure ZnO and x = 0.01 Co doped ZnO show diamagnetic nature while other doped samples (x = 0.02, 0.03, 0.04) show ferromagnetic behavior due to existence of oxygen and zinc related defects. Room temperature ferromagnetic behavior observed for as prepared samples can be exploited for spintronic applications. 4. Conclusion

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We have successfully prepared pure and Co doped ZnO nanoparticles with easy and costeffective solution method. The XRD and FTIR outcomes confirm single phase formation of wurtzite structure of ZnO. SEM images show the change in surface morphology of ZnO with Co doping. XPS results confirm doping of Co into ZnO lattice with oxidation state 2+ . The UV-vis results indicate blue shift in band gap of ZnO. This blue shift in band gap is because of incorporation of Co2+ ions into ZnO lattice. PL findings show only DLE for pure ZnO sample. For doped samples NBE and DLE both are present. DLEs consist of violet, blue, and green emission bands. VSM measurements show diamagnetic nature for pure ZnO and x = 0.01 Co doped ZnO sample. While, ferromagnetic behavior has been observed for Zn1−x Cox O (x = 0.02, 0.03 and 0.04) samples. On the basis of these finding, Co doped ZnO nanoparticles may be used in the field of spintronics, optoelectronics, and photocatalyst etc. 5. Acknowledgements The Authors acknowledge to Dr. S. R. Barman, Scientist-G in Surface Physics Lab, UGC-DAE Consortium for Scientific Research, Indore, India, for providing XPS facility. 8

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Figure captions: Fig.1 (a) XRD pattern for undoped ZnO and Co-doped ZnO nanopowders and (b) Magnified view of (101) diffraction peak for all prepared samples.

Fig.2 Variation in (a) crystal size and FWHM (b) lattice constants for different molar concentration of Co2+ ions into ZnO lattice.

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Fig.3 Low magnification SEM micrographs for (a) pure ZnO and (b) Co-doped ZnO (x = 0.01) nanopowder sample.

Fig.4 (a-b) Full scan XPS spectrum and high resolution Co 2p XPS spectrum for x = 0.04 Co doped ZnO sample.

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Fig.5 UV-Visible absorption spectra of undoped ZnO and Co-doped ZnO nanopowders.

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Fig.6 (a-e) Tauc’s plots for the as prepared samples and (f) band gap variation with cobalt molar concentration.

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Fig.7 FTIR spectra for pure ZnO and Co doped ZnO with different molar concentration of dopant.

Fig.8 (a-e) PL spectra for undoped ZnO and doped ZnO samples.

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Fig.9 Magnetization versus magnetic field for pure ZnO and Co doped ZnO nanopowder samples.

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Fig. 1(a)

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Figures and table:

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Fig. 3

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Fig. 5

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Fig. 7

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Fig. 8

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Fig. 9

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Table 1: Calculated structural and optical parameters for prepared samples

a (nm)

c (nm)

Crystal size (nm)

Micro strain ( 10−3 )

Volume of unit cell (nm)3

Eg (eV)

Pure ZnO

0.3254

0.5212

24.83

1.39

0.04779

3.22

Zn0.99 Co0.01 O

0.3240

0.5196

36.54

0.948

0.04723

3.25

Zn0.98 Co0.02 O

0.3248

0.5204

32.05

1.08

0.04754

3.26

Zn0.97 Co0.03 O

0.3258

0.5218

31.80

1.09

0.04796

3.27

Zn0.96 Co0.04 O

0.3246

0.5202

30.20

1.14

0.04746

3.28

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