Structural characterization and magnetic properties of ...

Appl. Phys. A (2016)122:122 DOI 10.1007/s00339-016-9655-0

Structural characterization and magnetic properties of Co co-doped Ni/ZnO nanoparticles G. Vijayaprasath1 • R. Murugan1 • S. Asaithambi1 • G. Anandha Babu1 P. Sakthivel1 • T. Mahalingam2 • Y. Hayakawa3 • G. Ravi1

•

Received: 7 October 2015 / Accepted: 20 January 2016 Ó Springer-Verlag Berlin Heidelberg 2016

Abstract In this paper, we present the structural, morphological, optical and magnetic properties of Zn1-xAxO (A = Ni, Co and x = 0.20 mol%) and Zn0.80Ni0.10Co0.10O nanoparticles synthesized by a chemical co-precipitation method. Powder X-ray diffraction data confirm the formation of a single-phase wurtzite-type ZnO structure for all the samples. FTIR and EDS measurements ensure that the divalent Ni and Co ions are incorporated in the wurtzite host matrix without any impurity phase. Photoluminescence and Raman spectra indicate the presence of donor defects and oxygen vacancies in the prepared samples. In VSM analysis, undoped ZnO nanoparticles exhibit diamagnetic behavior at room temperature. A systematic increase in ferromagnetic moment (*0.70 emu/g) is observed for Ni-, Co-doped and Co co-doped Ni/ZnO at 300 K. The exchange interaction between delocalized carriers and the localized ‘d’ spins of Ni and Co ions is predicted as the cause of the room temperature ferromagnetism.

1 Introduction Nano materials have fascinated researchers in recent years, because these materials exhibit unusual physical properties such as optical, electrical and magnetic properties, as & G. Ravi [email protected] 1

School of Physics, Alagappa University, Karaikudi, Tamil Nadu 630 004, India

2

Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, South Korea

3

Research Institute of Electronics, Shizuoka University, Hamamatsu 432-8011, Japan

compared to their bulk counterparts. Among various nanomaterials, zinc oxide (ZnO) is one of the important candidates which has a wide band gap of 3.34 eV and large exciton binding energy of 60 meV. In general, ZnO is a suitable material which finds various applications in spintronics [1], sensors [2], fabric industries [3] etc. Recently, there is a growing interest in semiconductor nanocrystals considering the issue of size miniaturization in the semiconductor industry [4, 5]. ZnO nanostructures provide several opportunities for researchers to explore the unique properties and applications. For semiconductors, doping of metal elements is a powerful tool to tailor the electrical and optical properties for the fabrication of optoelectronic devices. More interestingly, the results from previous theoretical and experimental studies have shown that ZnO doped with appropriate transition metals were considered as diluted magnetic semiconductors [6, 7] which attracted a great deal of interest due to their ferromagnetism at room temperature. The large number of studies and investigations on various transition metal (TM) doped ZnO nanoparticles have been carried out by various research groups [8]. Many researchers are fascinated toward diluted magnetic semiconductor (DMS), a promising material for application in spintronic devices. Moreover, Sluiter et al. [9] have shown that the doping of TM into ZnO matrix led to fine tuning of the band gap. In the past few years, few studies of transition metal (Fe, Mn, Cr, Co, Ni and Cu)-doped ZnO bulk or film with high Curie temperature (Tc) were reported [10– 12]. Earlier, a good amount of work was reported on single transition metal-doped ZnO prepared by various techniques [13, 14]. Most of the literature reports suggested that the co-doping of TMs with ZnO enhances room temperature ferromagnetism, as the co-dopant provide extra positive carriers in the host material [15–17]. Huawei Cao et al.

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have observed the ferromagnetic state for Mn, Fe co-doped ZnO [18]. There are also reports on co-doping of two magnetic elements like Ni, Mn, Fe, Co and Cu with ZnO [19, 20]. In the present work, an attempt has been made to synthesize a room temperature ferromagnetic DMS material Zn0.80Ni0.10Co0.10O by co-doping Ni and Co ions into the ZnO lattice. Magnetic measurements indicated that the undoped ZnO was diamagnetic in nature, whereas Ni- and Co co-doped ZnO samples exhibited weak ferromagnetic behavior at room temperature, which is possibly related to the substitution of Ni and Co ions in Zn ions. The synthesized nanoparticles were subjected to various studies such as surface morphology, particle size distribution, elemental and spectroscopic analyses.

2 Experimental details The analytical grade zinc acetate dihydrate [Zn (CH3COO)22H2O], nickel acetate dihydrate [Ni (CH3COO)22H2O], cobalt acetate dihydrate [Co (CH3COO)22H2O] and sodium hydroxide (NaOH) chemicals were purchased [Merck chemicals] and used in as-received condition. The undoped, (Ni and Co) doped ZnO and Co co-doped Ni/ ZnO was prepared by using chemical co-precipitation method. In the synthesis process, a required amount of zinc acetate was completely dissolved in deionized water and aqueous NaOH solution was added drop by drop to the zinc acetate solution. Later, the resultant solution was stirred at room temperature for 30 min, and then the solution was kept at 80 °C for 5 h. The white precipitate was observed and washed several times with double distilled water and ethanol. Later, the precipitate was dried at 120 °C for an hour. Finally, the prepared samples were annealed at 400 °C in air for 2 h to obtain undoped ZnO nanopowder. For the synthesis of Zn0.80Ni0.20O (Ni/ZnO) nanoparticles, a calculated amount (0.20 mol%) of nickel acetate in water was mixed with zinc acetate solution separately. Aqueous NaOH solution was added drop by drop to the above homogenous mixture to get a white precipitate with pale green color. Then, the precipitate was dried at 120 °C for an hour. Finally, the prepared sample was annealed at 400 °C in air for 2 h to obtain Ni-doped ZnO nanopowder. The above procedure for the preparation of Ni-doped ZnO was followed to synthesize Zn0.80Co0.20O (Co/ZnO) and Co co-doped Ni/ZnO NPs. The calculated amount 0.10 mol% of cobalt acetate, 0.10 mol% nickel acetate and 0.80 mol% of zinc acetate was used for the synthesis of Co co-doped Ni/ZnO (Co–Ni/ZnO) nanopowder.

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3 Characterization of samples The structural analysis of the synthesized samples was carried out using a powder X-ray diffractometer (PANalytical X’Pert Pro) with Cu-Ka radiation source (wave˚ ) with a 0.05° step increment per minute. length: 1.5418 A The samples were functionally characterized by FourierTransform Infra Red (FT-IR) spectroscopy using Thermo Nicolet 380 with KBr pellet method at room temperature in the range 4000–400 cm-1. The morphology of the samples was examined by powder technique using FEI-QUANDA 200F Field Emission SEM (FESEM) operating at 30 kV. Energy dispersive X-ray analysis was performed using an inbuilt EDS (Model: AMETEK) analyzer to measure elemental analysis of the NPs. A Cary Eclipse PL spectrograph was used to record photoluminescence (PL) spectra of the samples. Raman spectrum was recorded using imaging spectrograph STR 500 mm focal length laser Raman spectrometer (SEKI Japan). The magnetic measurements of the NPs were carried out using a vibrating sample magnetometer (Lake Shore Model-7404, USA).

4 Results and discussion 4.1 Powder X-ray diffraction The XRD pattern reveals that the doping did not disturb the wurtzite structure of host ZnO (Fig. 1A). All the diffraction peaks are well fitted with the standard data of undoped ZnO (space group P63mc) JCPDS data card no. 36-1451 [21]. It is seen that the dominant crystal phase of the samples is matched with wurtzite hexagonal structure. The absence of secondary phase or clusters in diffraction pattern confirms the single-phase formation of ZnO. The intense XRD peaks obtained for undoped ZnO sample indicate a better crystallinity. The peak intensity for Co co-doped Ni/ZnO sample is lesser than undoped ZnO, Zn0.80Ni0.20O and Zn0.80Co0.20O samples. The incorporation of dopant ion deteriorates the crystallinity of the samples due to the ionic radius difference between Zn, Ni and Co. Furthermore, the doping causes the decrease in the intensity and broadening in the width of diffraction peaks. This, in turn, results in the formation of crystallites with smaller diameters due to the increase in lattice disorder with co-doping. The inset of Fig. 1B shows that the Bragg angle of the intense (101) diffraction plane which is slightly shifted toward higher values from 36.38° (without dopant) to 36.40° for Zn0.80Ni0.20O sample and lower values for Zn0.80Co0.20O and Zn 0.80Ni0.10Co0.10O samples. This is an evidence for the creation of internal compressive micro-stress in the doped

Structural characterization and magnetic properties of Co co-doped Ni/ZnO nanoparticles

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Fig. 1 A XRD patterns of (a) undoped, (b) Ni/ZnO (c) Co/ZnO and (d) Co–Ni/ZnO NPs, B (101) peak shift toward lower angle side and decreasing intensity with the doping of the samples

samples. According to Bragg’s law of scattering, the scattering angle is inversely proportional to the interplanar distance. Therefore, shift of peaks position toward higher angle manifests that the interplanar distance become short when Co is doped with Zn–Ni–O. This compression of interplanar distance creates the stress inside the ZnO lattice. In addition, a line broadening was also observed in the XRD spectra. It is commonly known that the peak shift and line broadening might be the result of size or micro-strain, or both. The average crystallite size of the samples is calculated using Debye–Scherrer’s formula [22]. The crystallite size of undoped ZnO NPs is found to be 28.3 nm. The crystallite size for Ni-, Co-doped ZnO, Co co-doped Ni/ZnO NPs decreases compared to undoped ZnO NPs as shown in Table 1. The reduction of crystallite size is mainly due to the distortion created in the host ZnO lattice by the foreign impurities and the presence of Ni2?, Co2? decreases the nucleation and subsequent growth rate of ZnO NPs. The dislocation density and micro-strain was calculated using equation in Ref. [22]. The observation of small changes in 2h values of diffraction peaks and the peak broadening are due to the increase in micro-strain [23] and the line broadening effect is mainly caused by the size and micro-strain [24] of nanoparticles. The calculated crystallite size and micro-strain of undoped and doped ZnO nanoparticles are given in Table 1. As compared to undoped ZnO NPs, the strain value

increases for Ni2?- and Co2?- doped ZnO NPs due to the relaxation of strain in the respective unit cells which causes major changes in the size and shape of the particles. The lattice parameters of a semiconductor depend on the foreign atoms, defects and the difference in ionic radii with respect to the substituted matrix ions. The difference in the ionic radii of Ni2? and Co2? with respect to Zn2? gives the variation of the lattice constants. For undoped ZnO NPs, the lattice parameters ‘a’ and ‘c’ are estimated to be 3.23 ˚ , respectively. The substitution of Ni2? and and 5.19 A 2? Co ions in the place of Zn2? ions increases the lattice constants ‘a’ and ‘c’ and the interplanar distance ‘d’. The unit cell volume of ZnO also increases with the substitution of Ni and Co metals. The estimated values are ˚ 3 for ZnO, Zn0.80Ni0.20O, 47.1, 47.3, 47.6 and 47.7 A Zn0.80Co0.20O and Zn0.80Ni0.10Co0.10O, respectively. This indicates that the dopant ions reside partially in tetrahedral Zn positions. Further, the incorporation of Ni and Co ions causes prominent changes in bond length of Zn–O. The bond lengths are calculated using the relation [25], ffi vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 ! u a2 1 L¼t u c2 ð2Þ þ 2 3 where ‘a’ and ‘c’ are the lattice parameters and ‘u’ is a positional parameter. The positional parameter ‘u’ can be calculated by the relation,

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Table 1 Average crystallite size, dislocation density and micro-strain of undoped and Nidoped ZnO (Ni/ZnO), Co-doped ZnO (Co/ZnO) and Co codoped Ni/ZnO (Co–Ni/ZnO) nanoparticles

u¼

Samples

Crystallite size (nm)

Dislocation density (lines/m2)

Micro-strain 910-3

Undoped

38.3

8.03156E ? 14

0.958

Ni/ZnO

32.1

2.76541E ? 15

1.513

Co/ZnO

28.6

1.33865E ? 15

1.249

Co–Ni/ZnO

21.9

4.82017E ? 15

2.098

a2 þ 0:25 3c2

ð3Þ

There is a strong correlation between c/a ratio and ‘u’. The c/a ratio is constant with increasing ‘u’ manifests that four tetrahedral distances remain nearly constant through a distortion of the tetrahedral angles, which was attributed to the long-range polar interaction in ZnO lattice structures. In our present work, the c/a ratio is constant for Ni, Co and Co co-doped Ni/ZnO NPs. The values of c/a and ‘u’ parameters are given in Table 2. The Zn–O bond length of ˚ . The bond length values are undoped ZnO NPs is 1.970 A found to increase with doped samples due to the replacement of Zn2? ions in ZnO lattice. The atomic packing factor (c/ a) was constant for all the samples with respect to the lattice parameter ‘a’ and ‘c’. The increase in lattice parameter is observed in Co-doped ZnO and Co co-doped Ni/ZnO when compared to undoped and Ni-doped ZnO which is attributed to the formation of CoZn–2VZn complex (Co occupies Zn site and spontaneously inducing two Zn vacancies) [26]. The observed shift in XRD peak position and peak intensity change, d value, cell parameters, bond length, volume and stress in Zn0.80Ni0.10Co0.10O sample confirmed the substitution of Co into Zn–Ni–O lattice. The observed constant c/ a ratio reveals that the incorporation of Co does not modify the hexagonal wurtzite structure of ZnO. 4.2 Fourier-transform infrared (FTIR) spectroscopic analysis FTIR is a useful technique to obtain the information of chemical bonding in a material. The vibrating band

position and number of absorption peaks depend on crystalline structure and chemical composition of the samples. The characteristic peaks exhibited by FTIR spectra of ZnO, Zn0.80Ni0.20O, Zn0.80Co0.20O and Zn0.80Ni0.10Co0.10O nanoparticles are shown in Fig. 2A. The important absorption bands around 2700–3500 and 1092 cm-1 are attributed to normal polymeric O–H stretching vibration of H2O in Zn–O lattice [27]. The peak found at 1696 cm-1 is attributed to H–O–H bending vibration which is assigned to a small amount of H2O in the ZnO nanoparticles. The absorption peak exhibited in between 2344 and 2352 cm-1 is due to the presence of CO2 molecules in the air [28]. Since the measurement was carried out at room temperature under air atmosphere, the absorption of H2O from moisture content and CO2 from the atmosphere is unavoidable. The peak appeared at 1492 cm-1 represents the asymmetrical stretching of nitrate (NO2) compounds. The absorption peak appeared below 480 cm-1 could be attributed to the metal–oxygen peak of Zn–O stretching mode in ZnO matrix [29]. The characteristic IR peaks below 1000 cm-1 are very important to study the presence of Zn–O/Ni–O/Co–O/Zn– Ni–Co–O bonds and their functional groups. The enlarged spectrum in the wave number range below 1000 cm-1 is shown in Fig. 2B. Absorption bands observed in the range 420–680 cm-1 are attributed to the stretching modes of Zn–Ni–O, Zn–Co–O and Zn–Ni–Co–O for the corresponding Zn0.80Ni0.20O, Zn0.80Co0.20O and Zn0.80Ni0.10Co0.10O samples, respectively. A weak absorption peak at 500 cm-1 corresponds to the octahedral coordination and a strong peak around 600 cm-1 is due to tetrahedral

Table 2 Lattice parameters, cell volume and bond length of undoped and Ni-doped ZnO (Ni/ZnO), Co-doped ZnO (Co/ZnO) and Co co-doped Ni/ZnO (Co–Ni/ZnO) nanoparticles S. no.

Samples

Lattice parameter ˚) values (A

Error in Lattice parameter values

a

c

a

c

Atomic packing factor (c/a)

Cell Volume ˚ )3 (V) (A

Positional parameter (u)

Bond length ˚) (Zn–O) (L) (A

1

Undoped

3.237

5.188

±0.012

±0.018

1.602

47.098

0.379

1.970

2

Ni/ZnO

3.242

5.195

±0.007

±0.011

1.602

47.312

0.379

1.973

3

Co/ZnO

3.249

5.208

±0.000

±0.002

1.602

47.623

0.379

1.977

4

Co–Ni/ZnO

3.248

5.206

±0.001

±0.000

1.602

47.716

0.380

1.979

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Fig. 2 FTIR spectra of A undoped, (a) Ni/ZnO, (b) Co/ZnO (c) and (d) Co–Ni/ZnO NPs and B enlarged view of the wave number range below 1000 cm-1

coordination [30]. It reveals the strong tetrahedral orientation of Zn–Ni–O and Zn–Co–O bonds. The band occurs at 651 cm-1 is due to the vibration of Zn–Ni–O–Co local bond [31]. The gradual shifts in the absorption frequency with Ni doping and Co doping are caused by the difference in the bond lengths that occurs when Ni2? and Co2? ions replace Zn2? ions and confirm the incorporation of Ni and Co into ZnO lattice structure. The change in intensity corresponding to the frequency around 550–700 cm-1 represents a change in density of defect states surrounding to Ni/Co ions in Zn–O. When Co is co-doped into Zn–Ni–O, the broad band corresponding to Zn–O bond is shifted to lower side compared to Codoped ZnO [32]. In addition to the band shift, the intensity of band is also decreased. The intrinsic host lattice defects and activated impurities cause this shift in the vibrational mode in cobalt co-doped Ni/ZnO. 4.3 Morphological analysis The surface morphologies of undoped, Ni-doped, Codoped and Co co-doped Ni/ZnO nanoparticles are shown in Fig. 3A–D. Figure 3A shows the morphology of ZnO sample which presents a random form with spherical and

hexagonal shapes with relatively uneven sizes. Figure 3B, C shows the surface morphology of Ni, Co-doped ZnO sample which has lower grain size than the undoped ZnO sample. In addition to the suppression of grain size, the grown particles agglomerated with one another. The increase in surface area to volume ratio would increase the attractive force between the nanoparticles and the particles tend to agglomerate with one another. The small grains with hexagonal particles are improved morphologies for Ni and Co co-doped ZnO as shown in Fig. 3D. 4.4 Compositional analysis The elemental analysis of the samples to quantify Zn, Ni, Co and O was carried out using EDX analysis. EDX spectra of ZnO, Zn0.80Ni0.20O, Zn0.80Co0.20O and Zn0.80Ni0.10Co0.10O nanoparticles are shown in Fig. 4A–D. The atomic percentage of the compositional elements such as Zn, Ni, Co and O are present in Zn0.80Ni0.10Co0.10O nanoparticles, which is inserted in Fig. 4. The appearance of C peak in the spectrum is attributed from the emission of carbon tape used during the EDX measurement. The EDX analysis confirms the presence of Ni and Co in ZnO in nominal percentage with chemical purity.

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Fig. 3 SEM images of (A) undoped, (B) Ni/ZnO, (C) Co/ZnO and (D) Co–Ni/ZnO NPs

The atomic percentage of zinc atom decreases for Niand Co-doped ZnO and Co co-doped Ni/ZnO samples than in undoped ZnO which confirms the substitution of dopants into Zn–O as shown in the inset of Fig. 4. The increase in oxygen percentage depends on the dopant and co-dopant concentration. The difference between the actual and the nominal concentration is probably due to the dilution of Ni, Co ions in the ZnO host matrix [33]. 4.5 Photoluminescence (PL) analysis PL is a sensitive study to identify the quality of crystal structure and the presence of defects in the prepared samples. The band-to-band excitation of ZnO promotes electrons from the valence band to the conduction band, leaving holes in the valence band. The holes migrate from the valence band to deep levels and recombination occurs between electrons from either the conduction band or

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shallow donor levels and trapped holes on deep levels [34– 37]. The PL spectra of undoped and doped ZnO nanoparticles are shown in Fig. 5A. The first deconvoluted peak around 385 nm corresponds to near band edge (NBE) emission which shifts into blue region with the doping of Ni and Co ions. In general, oxygen vacancy (VO) and zinc interstitial (Zni) are donors, while zinc vacancy (VZn) and interstitial oxygen (Oi) are acceptors in ZnO. Among these point defects, only Zni is a shallow donor and the corresponding defect level could be located slightly below the conduction band edge. Lin et al. [38] pointed out that the energy gap from the Zni level to the valance band was about 2.9 eV. Zeng et al. [37] experimentally assigned the violet shoulder at 406 nm in ZnO nanoparticles to Zni. In our work, the emission around 406 nm is probably associated with electron transition from a shallow donor level of Zni to top level of the valence band [34, 36]. Also, this peak is blue shifted with doping of Ni and Co content. The


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Fig. 4 Compositional analysis of (A) undoped, (B) Ni/ZnO, (C) Co/ZnO and (D) Co–Ni/ZnO NPs

blue emission around 435 and 455 nm can be attributed to the transition from extended Zni states, which are slightly below the simple Zni state to the valence band. These extended states might be formed during the annealing process due to defect ionization reaction which results defect localization coupled with a disordered lattice [36]. A broad emission band centered at 489 nm originates from recombination of the photo-excited holes with the electrons occupying the single ionized oxygen vacancies [34]. The green emission exhibits at 520 nm in our samples is the most controversial one. Vanheusden et al. [39] have assigned the green emission in ZnO phosphor powders due to the single ionized oxygen vacancy, and this emission was originated from the recombination of a photo-generated hole with the single ionized charge state. Zhang et al. [40] reported that the defects of single ionized oxygen vacancies were the main reason for the origin of visible emission (Eg = 2.2 eV) of ZnO quantum dots. On the

other hand, there are reports available for similar emissions assigned to Zni and/or VZn [41–43]. The PL spectra of undoped and doped ZnO NPs show blue and green emission between 430 and 550 nm. The blue and green emission bands are originated from the intrinsic defects and donor–acceptor pair recombination involving Ni and Co impurity acceptors. Green emission bands at 520 and 541 nm arise from the oxygen vacancies in Zn–Ni–Co–O lattices due to the increase in the impurity levels in the samples. Hence, the co-doping tends to increase the distortion centers in the lattice of the samples. 4.6 Raman analysis Raman scattering is a versatile technique for detecting the incorporation of dopants, defects and lattice disorder in the host lattice [44]. The zone-center optical phonon of the wurtzite structure of ZnO can be classified according to the

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Fig. 5 PL emission spectra of (A) undoped, (B) Ni/ZnO, (C) Co/ZnO and (D) Co–Ni/ZnO NPs using an excitation wavelength at 325 nm

following irreducible representations: Copt = A1 ? E1 ?2E2 ?2B1. The B1 modes are silent in Raman scattering, whereas A1 and E1 modes are polar and hence exhibit different frequencies for the transverse-optical (TO) and longitudinal-optical (LO) phonons [45]. The room temperature Raman spectra of Ni-, Co-doped ZnO and Co codoped Ni/ZnO nanoparticles ranging from 200 to 800 cm-1 are shown in Fig. 6. The six first-order Ramanactive phonon modes of undoped ZnO observed at 148 cm-1 (E2 low), 257 cm-1 (A1TO), 414 cm-1 (E1TO), 429 cm-1 (E2 high), 504 cm-1 (A1 LO) and 549 cm-1 (E1 LO) are well matched with the literature values [46]. The mode observed at 429 cm-1 can be assigned to the high frequency branch of E2H mode, which involves oxygen motion characteristic of wurtzite structure [47]. The presence of E2H mode in all the four samples indicates the hexagonal wurtzite structures, which are consistence with the XRD studies. In comparison with the Raman spectrum

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Fig. 6 Micro-Raman spectra of (a) undoped, (b) Ni/ZnO, (c) Co/ZnO and (d) Co–Ni/ZnO NPs


of undoped ZnO, the additional mode exhibits at 525 cm-1 for Ni-, Co- and Co co-doped Ni/ZnO samples can be assigned to the quasi-LO phonon mode due to the abundant shallow donor defects such as zinc interstitials Zni and/or oxygen vacancies VO, bounded on the tetrahedral Ni/Co sites. The existence of the quasi-LO phonon mode manifests the incorporation of Ni/Co in the ZnO lattice. Similarly, the additional mode at 525 cm-1 in Fig. 6D, is also induced by the host lattice defects triggered by the Co codoping in Ni/ZnO. It is clearly noted that the defect induced with TM and co-doped ZnO samples increases the peak intensity of the Raman spectrum. However, the integrated intensity ratio of defect peak (induced by Ni and Co) to E2H modes decreased with Ni and Co doping elements [48]. 4.7 Magnetic measurements The magnetization versus magnetic field (M–H) for undoped, Ni-doped, Co-doped and Co co-doped Ni/ZnO nanoparticles were measured at room temperature using vibrating sample magnetometer (VSM) in the field range from -15 to 15 kOe. The hysteresis curves (Fig. 7A) exhibit ferromagnetic (FM) nature with small hysteresis loop for doped ZnO at room temperature. The undoped ZnO sample exhibits diamagnetic behavior. The FM behavior can be attributed to the incorporation of Ni and Co ions in ZnO structure. The room temperature ferromagnetism (RTFM) arises as a result of exchange interaction between delocalized charge carriers initiating to form oxygen vacancies and zinc interstitial in the crystal structure [49]. Further co-doping of Co content with Ni/ ZnO sample increases the oxygen vacancy concentration that eventually change the magnetic ordering of the sample which shows increased magnetization than other samples

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[49]. RTFM observed in TM-doped ZnO nanoparticles is originated from two factors; (1) the number of defects and oxygen vacancies; (2) the exchange interactions between the TM ions and the O ion spin moment [50]. The existence of ferromagnetism in Zn0.80Ni0.20O and Zn0.80Co0.20O may be due to the clustering of metallic Ni/Co and/or intrinsic ferromagnetism from the charge carriers [51]. XRD results clearly indicate the absence of metallic Ni and Co phase in the sample, and therefore, the observed ferromagnetism at room temperature is an intrinsic property of Ni- and Codoped ZnO. It is believed that the existence of ferromagnetism in Ni:ZnO and Co:ZnO could be due to the exchange coupling between localized ‘d’ spins on the Ni and Co ions mediated by delocalized carriers [52]. In these samples, the magnetization is not found to be saturated magnetization even at the maximum applied field of 15 kOe. The non-saturation magnetization behavior is the characteristic of weak FM ordering of the spins in the samples. Even though, M–H loop exhibits non-saturation, the presence of small hysteresis loop in all the samples represents weak ferromagnetic nature in ZnO NPs. In recent reports, the occurrence of ferromagnetism in oxide based DMS are predicted by the defect mediated ferromagnetism model based on the bound magnetic polaron (BMP) theory [53]. The ferromagnetism in our samples is likely arises from an intrinsic factor which is highly correlated with the structural defects [54]. During the thermal annealing process, the structural defects such as VZn and VO would likely be generated, which could be proved from PL measurement. The defects can overlap many dopant ions as well as adjacent defects induced a ferromagnetic coupling between dopant spins. Thus, the magnetic coupling between Co ions with ZnO is FM mediated by VZn and VO, and this may account for the observed RTFM [55]. It indicates that defects do play a part to get a FM coupling of the magnetic spin.

Fig. 7 A Magnetic behavior of (a) undoped, (b) Ni/ZnO, (c) Co/ZnO and (d) Co–Ni/ZnO NPs. B Enlarged hysteresis loop of respective NPs

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Also, it is observed that when additional Co was codoped in Zn0.90Ni0.10O, the magnetic behavior is found to change in the sample. The incorporation of the element Co is conducive to the formation of FM order. The changes in the magnetic behavior can be attributed to the co-doped Co which would create additional carriers. In addition, the defects present in the samples are also expected to enhance magnetic behavior in Co-doped ZnO samples than in Nidoped ZnO. Hence, we predict that in the present work, the introduction of Co element change the formation of FM ordering in the doped ZnO NPs. The PL peaks observed at 490, 520 and 540 cm-1 for the respective Ni-, Co-doped ZnO and Co co-doped Ni/ ZnO NPs correspond to the blue and green emissions due to the existing zinc interstitial and oxygen vacancies. Both blue and green emissions are relatively strong with suitable excitations and also with the stabilities of oxygen vacancies and Zn interstitials. Moreover, Raman spectra of Ni-, Co-doped and Co codoped Ni/ZnO NPs reveal that there is a remarkable vibration of E1L mode, which can be assigned to the local vibration mode related to Ni, Co and Ni/Co bound with the donor defects. Moreover, from Raman spectra of Ni-, Codoped and Co co-doped Ni/ZnO, a remarkable vibration of E1L mode is observed which could also be assigned to the local vibration mode related to Ni, Co and Ni/Co bound with the donor defects. These donor defects can be assigned to doubly occupied oxygen vacancies and zinc interstitials.

5 Conclusion Magnetic properties of undoped and doped ZnO nanoparticles synthesized by co-precipitation method are investigated. X-ray diffraction and Raman measurements indicate the wurtzite structure for undoped and doped ZnO with substitution of Co2? and Ni2? in host lattice sites. The average crystallite size between 38 and 21 nm is observed from X-ray diffraction analysis. The observed shift in XRD peak position and changes in d value, cell parameters, bond length, volume and stress confirmed the substitution of Co into Zn–Ni–O lattice. Presence of Zn–O and Zn–Ni–Co–O bonds was confirmed by FTIR analysis. The surface morphology of the samples was studied by HRSEM analysis. Room temperature magnetic measurements indicate that the Co- and Ni-doped with ZnO induces the ferromagnetism, whereas co-doping of Co with Ni/ZnO might change the magnetic moment of ferromagnetic to weak ferromagnetic behavior. The prepared nanoparticles of Co co-doped Ni/ZnO may potentially be used for magnetic energy storage device applications.

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G. Vijayaprasath et al. Acknowledgments The authors G. Ravi and G. Vijayaprasath gratefully acknowledge the UGC (Ref. No. F. 41-933/2012 (SR)), India, for providing financial support to carry out this work. Also, the authors acknowledge Department of Industrial Chemistry, Alagappa University, for extending HRSEM (Funded by DST-PURSE) facility. The work was carried out as the co-operative research projects of the Research Institute of Electronics, Shizuoka University, Japan.

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