Evolution of structural and magnetic properties of

Journal of Magnetism and Magnetic Materials 469 (2019) 383–390

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Research articles

Evolution of structural and magnetic properties of nickel oxide nanoparticles: Influence of annealing ambient and temperature

T

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Atefeh Jafaria, Siamak Pilban Jahromib, , Komail Boustanic, Boon Tong Gohd, Nay Ming Huange a

Nanostructure Lab, Physics Department, University of Guilan, Rasht, Iran Fars Science & Technology Park, Chavosh Green Technology Com, Jahrom, Iran c Department of Physics, Payame Noor University, Tehran, Iran d Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia e New Energy Science & Engineering Programme, University of Xiamen Malaysia, Jalan SunSuria, Bandar SunSuria, 43900 Sepang, Selangor Darul Ehsan, Malaysia b

A R T I C LE I N FO

A B S T R A C T

Keywords: NiO nanoparticle Annealing Air O2 Magnetic properties

Nickel oxide (NiO) nanoparticles (NPs) are synthesized using the sol-gel method. The effect of annealing atmosphere type, including air, O2 and annealing temperature (TA) on the structural, morphological and magnetic properties of NiO NPs was investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) analysis. For this purpose, NiO NPs are annealed at 400, 500, 600, 700 and 800 °C. The magnetic properties of the NPs change as a result of increasing crystallite size in terms of TA in both atmospheres. These variations are different according to the type of the annealing atmosphere. At lower temperatures (TA < 600 °C), the NPs that annealed in the O2 atmosphere have higher crystallinity, larger lattice constant, and lower magnetization and coercivity. At higher temperatures, the structural and magnetic properties of the samples in both atmospheres are more similar.

1. Introduction Recently, nanosized metallic oxide of ferromagnetic metals such as Fe, Co, and Ni, are being regarded because of both the richness of their physical and chemical properties and their potential applications in many research areas, such as magnetic recording media, gas sensors, catalysts and biomedical applications [1]. NiO in bulk is an antiferromagnetic material. In 1961, Néel suggested that the small particles of an antiferromagnetic material such as NiO should exhibit superparamagnetism and weak ferromagnetism [2] as a result of uncompensated spins in the two sublattices [3]. Therefore, NiO nanoparticles (NPs) with various sizes exhibit different magnetic behavior which is a very important factor in various applications. Hence, the fabrication of NiO NPs with desired magnetic properties at progressively decreasing size, is one of the most important challenges in order to obtain high magnetization or coercivity [4]. The first investigation of size dependent magnetic properties in NiO was reported by Richardson et al. [5]. Kodama et al. (1997), suggested that anomalous magnetic properties in an antiferromagnetic NiO NPs such as large moments and coercive forces can contribute to a new finite size effect, in which the reduced coordination of surface spins and causes a fundamental change in the magnetic order throughout the

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particle [6]. Wang et al. in 2006 compared the effect of annealing in N2 and O2 atmosphere on the aging of nickel ferrite NPs [7]. Thota et al. in 2007 [8] investigated the effect of calcination temperature on the magnetic properties of NiO NPs, synthesized using the sol-gel method. Karthik et al. (2011) [9] investigated the particle size effect on the magnetic properties of NiO NPs prepared using a precipitation method. Kisan et al. (2015) [2] reported the effect of annealing temperature (TA) on the magnetic properties of ball milled NiO powders. The effect of TA on physical properties of NiO thin films and Ni/NiO core-shell nanowire was investigated by Martínez-Gil et al. [10] and Xiang et al. [11], respectively. Several methods have been employed to synthesize NiO nanostructures such as pulse laser deposition (PLD), sol–gel, solvothermal, hydrothermal, precipitation, sonochemical, anodic plasma, microemulsion, and thermal decomposition [7,8,12]. The preparation of NiO nanostructures is a complicated process and a wide range of synthetic parameters affect the properties of the final products. NiO nanoparticle with a porous structure and appropriate high specific surface area can facilitate a very short diffusion pathway for ions to connect with the external and internal electroactive sites and enhance the rate of reaction. The Sol–gel technique is one of the most simple, promising, and low-cost methods for preparing NiO NPs [13], as well as its advantages

Corresponding author. E-mail address: [email protected] (S. Pilban Jahromi).

https://doi.org/10.1016/j.jmmm.2018.08.005 Received 28 March 2018; Received in revised form 24 July 2018; Accepted 3 August 2018 Available online 04 August 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved.


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corresponding to impurities, proving that the final product is pure NiO. The structural parameters including average crystallite size (D), lattice constant (a), unit cell volume (V) and lattice strain (ε), are summarized in Table 1. Two methods are used to calculate the crystallite size of the NPs including Scherrer equation and average size-strain plot (SSP)

such as producing a uniform morphology, high crystallinity and purity, and controllable particle size which makes it capable of improving the structural, magnetic and electrochemical properties of NiO NPs. According to the authors' knowledge, there is no report on investigation of magnetic properties variation in NiO NPs resulting from annealing in air and oxygen atmosphere in 400–800 °C. So, here, we have synthesized NiO NPs using the sol–gel method and then annealed them at different temperatures 400, 500, 600, 700 and 800 °C in two atmospheres including air and O2.

method. In, Scherrer equation D =

(

Kλ β cos θ

), D is the crystallite size

(nm), k is a constant equal to 0.94, λ is the X-ray wavelength (1.54056 Å for Cu-Kα radiation), β is the peak width at half-maximum (FWHM) intensity, and θ is the peak position [12]. Parameter β needs to be corrected, according to Scherrer equation, since the width of broadening peaks is the result of both crystalline size and microstructure strain. In order to correct β for βD, the Gaussian equation 1 2 ) 2 ) is utilized to set apart the effects of crystalline size (βD = (β 2−βstandard and micro-structure strain. In SSP method, the crystalline size profile and the strain profile are explained using a Lorentzian and a Gaussian function, respectively. The total peak broadening is calculated from Eq. (1) [12]:

2. Experimental details 2.1. Materials Nickel nitrate was used as the precursor, gelatine as the polymerization agent, and distilled water as solvent. Nickel nitrate (Ni (NO3)2·6H2O) was purchased from Acros organic at a purity of 99%. Gelatine (Type B from bovine skin) was purchased from Sigma Aldrich. All chemicals were used as received and without any further purification.

(dhkl βhkl cos θ)2 =

A 2 ε 2 (dhkl \;βhkl cos θ) + ⎛ ⎞ D ⎝2⎠

(1)

where, ε is the lattice strain and constant A depends on the particle A 2 shape. The term (dhkl βhkl cosθ)2 is plotted as a function of D (dhkl βhkl cosθ) for all of the diffraction peaks of NiO NPs after annealing from 2 θ of 20° to 80°. In this case, the crystalline size and strain are extracted from the slop of linearly fitted data and the root of Y-intercept, respectively. A small shift towards the lower angles shown in Fig. 2 is due to the existence of a strain in the lattice. The effect of strain can be uniform or nonuniform. The uniform strain affect the peak position and the nonuniform strain affect the peak broadening and intensity [14,15]. The variations of structural parameters as a function of TA are shown in Fig. 3. It can be seen that the structural properties of the NiO NPs are greatly affected by the annealing temperature and atmosphere. With the increase of TA in both atmospheres, the reflection peaks clearly became sharper and FWHM became narrower, indicating an enhancement of crystallinity (Fig. 1). The crystallite size of the NPs had approximately increased by increments of TA up to 600 °C in both atmospheres, and a slight decrease is observed with further increase to 800 °C (Fig. 3(a-b)). On the contrary, with increasing temperature up to 700 °C, the lattice strain sharply decreased and then increased slightly at 800 °C. These results indicated that the increase of TA improved crystallinity, which is accompanied by a decrease in the lattice strain, similar to kisan et al. results [2]. The lattice constant of 4.178 and 4.181 Å for NiO NPs annealed at 400 °C respectively in air and O2 are in a good agreement with the reported value for NiO (4.178 Å) [3]. When the TA is increased, the lattice constant became larger (at 600 °C) and then smaller (at 800 °C) for samples annealed in O2, while opposite variations are observed in the air-annealed samples (Fig. 3(c-d)). It is found that the crystallite size, lattice constant and unit cell volume of the samples annealed in O2 are somewhat larger than that annealed in air as shown in Fig. 3, while lattice strain for the O2-annealed samples are somewhat smaller than air-annealed. It is clearly seen that in Fig. 1 and its insets, the intensity of peaks for the samples annealed in air is weaker than the O2-annealed, while by increasing temperature in both atmospheres; the intensity of peaks is more similar. It is shown that annealing in O2 atmosphere improved crystallinity due to the reduction of O-vacancy in the lattice. The presence of O-vacancy as a defect in the lattice can produce strain and disorder which results in weak crystallinity. Annealing in a higher oxygen partial pressure (in the pure O2 atmosphere) promotes oxygen incorporation into the structure and removes O-vacancy and therefore reduces strains in the lattice. It seems that 600 °C is a critical temperature for annealing of NiO NPs in the air and O2 atmospheres. It may be due to the fact that, in the O2 atmosphere, most of O-vacancies are removed from the lattice at TA = 600 °C, and as a result, the growth of crystalline, the improvement of crystallinity and expansion of the lattice are occurs. By increasing the

2.2. Preparation of NiO NPs NiO NPs were synthesized using a sol–gel method as follows. Typically, 5 g of Ni(NO3)2·6H2O (Acros, 99%) was dissolved in 20 mL of deionized water and then stirred for 30 min. Meanwhile, 2 g of gelatine (Type-B from bovine skin, Sigma Aldrich) was separately dissolved in 40 mL of deionized water, and then stirred for 30 min at 60 °C to obtain a clear gelatine solution. After that, the nickel nitrate solution was added to the gelatine solution and heated in a water bath at 80 °C under stirring. The stirring was continued for 12 h to obtain a green gel. This gel was rubbed on the inner side of a crucible. The green gel was then placed into a horizontal tube furnace (100 cm in length, 5 cm in diameter). Finally, the horizontal tube furnace at different atmospheres was used (Air and O2). They were fed at about 50 Sccm into the furnace tube at 1 atm of pressure, separately. The furnace was heated from room temperature to 400, 500, 600, 700 and 800 °C at a rate of 30 °C min−1. The synthesis of NiO NPs has been carried out according to literature procedure and our previous works [12,13]. 2.3. Characterization The structural properties of the samples were investigated using Xray diffraction (XRD, Philips PW 1800, Netherlands) and Cu–Kα radiation (λ = 1.5406 Å). The synthesized powder was placed in a sample holder (1 cm × 1 cm × 1 mm) and then exposed to X-ray radiation. The magnetic properties of the samples were measured through vibrating sample magnetometer (VSM, Megnetic Daghigh Daneshpajouh Co., Iran) in varied magnetic field at room temperature. The synthesized powder was placed in a cylindrical sample holder and then placed in varied magnetic field ( ± 10 kOe) at room temperature. Transmission electron microscope (TEM, Hitachi-7100) was used to investigate the nanoparticle size and morphology. The samples were dispersed in ethanol using an ultrasonic bath before placing them onto a coppercoated grid. 3. Results and discussion 3.1. Studying the structure and morphology In general, XRD can be used to characterize the crystal structure of NPs. The XRD patterns of the prepared NiO NPs are shown in Fig. 1 after annealing in air and O2 atmospheres. All of the XRD patterns clearly show the diffraction peaks of (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes, corresponding to face-centered-cubic (FCC) structured NiO (PDF card No. 01-078-0423). No peaks are detected 384


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Fig. 1. XRD pattern of NiO NPs after annealing in air and O2 atmosphere at different temperatures (a) 400 °C, (b) 500 °C, (c) 600 °C, (d) 700 °C, and (e) 800 °C. The inset shows the (2 0 0) peak magnified from the XRD pattern. Table 1 The structural parameters of NiO NPs after annealing in air and O2 atmosphere at different temperatures. Annealing temperature (oC)

400 500 600 700 800

Average Particle size ± 0.2(nm)

size-strain plot ± 0.1(nm)

Constant lattice ± 0.001(Å)

Unit cell volume ± 0.001(Å3)

Strain ± 0.001

Air

O2

Air

O2

Air

O2

Air

O2

Air

O2

9.4 16.8 29.0 39.2 75.6

8.2 16.0 24.2 39.8 65.2

3.7 6.3 21.8 13.4 13.8

9.9 11.9 14.5 12.8 13.4

4.178 4.178 4.177 4.180 4.181

4.181 4.183 4.184 4.182 4.181

72.929 72.929 72.877 73.034 73.087

73.087 73.191 73.244 73.139 73.087

0.035 0.028 0.017 0.006 0.009

0.019 0.006 0.003 0.003 0.005

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Fig. 2. The (2 0 0) peak from the XRD patterns of the annealed NiO NPs in air and O2 atmosphere. The width of the broadening peak are decreased and the intensity are increased with increment of annealing temperature.

annealing in air and O2 atmospheres at different temperatures are shown in Figs. 4 and 5, respectively. According to the normal function, average particle size of the NPs annealed in air at 400, 500, 600, 700 and 800 °C is 9.4, 16.8, 29.0, 39.2 and 75.6 nm, respectively, while in O2-annealed NPs is 8.2, 16.0, 24.2, 39.8 and 65.2 nm, respectively [12,13]. Since, the NiO NPs are irregular shapes, and their particle size cannot be directly defined. Therefore, was measured the shortest diameter of particles in all samples. The average size of NPs is increased with respect to the increase in TA in both atmospheres. At lower

temperature (TA > 600 °C), the presence of O2 can cause lattice constriction as a result of lattice distortion and strain. Annealing of samples in air and at a temperature below 600 °C didn’t have much effect on the unit cell volume, while it expanded by increasing the temperature up to 800 °C. This behavior can arise from the fact that at high temperatures, the effect of temperature is dominant on the type of the ambient atmosphere. These results are in a good agreement with the reported results by Wang et al. [7] and Hwang et al. [16]. The TEM images and size distribution histograms of NiO NPs after

Fig. 3. Variation of average crystallite size calculated by (a) Scherrer equation, and (b) average size-strain plot (SSP) method, (c) lattice strain, and (d) lattice parameter as a function of annealing temperature in air and O2 atmosphere. 386


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Fig. 4. TEM morphological structures of NiO NPs after annealing in air and O2 atmosphere, respectively at (a-b) 400 °C, (c-d) 500 °C, (e-f) 600 °C, (g-h) 700 °C and (i-j) 800 °C.

temperatures (at 400–500 °C), the average particle size is approximately the same based on the crystallite size, while these two sizes was become different as a result of agglomeration at higher temperatures. 3.2. Studying the magnetic properties The magnetization (M) of an antiferromagnetic material in a magnetic field B is calculated using the modified Langevin equation: M = (M0 L (x ) + χa B ) , where χa is the susceptibility of randomly oriented antiferromagnetic particles, L(x) is Langevin equation and B is the magnetic field [17]. Fig. 6(a-e) shows the magnetization curves of the prepared NiO NPs after annealing in air and O2 atmospheres at different temperatures, as a function of applied field H. The inset is a diagram of the low-field interval of the magnetization curve of the samples. The magnetic properties including the maximum magnetization (Mm), the coercivity, the remanent magnetization (Mr) and the initial magnetic susceptibility (χi) are summarized in Table 2, and their variations with TA are shown in Fig. 7(a-d). As can be seen (i) the Mm

Fig. 5. The size distribution of NiO NPs after annealing in air and O2 atmosphere, respectively at (a-b) 400 °C, (c-d) 500 °C, (e-f) 600 °C, (g-h) 700 °C and (i-j) 800 °C extracted from Fig. 4.

value has decreased by the increase in TA (and, in turn, particle size) in both atmospheres. This result is in a good agreement with the experimental results of Kisan et al. [2], Thota and Kumar [8], Nikolic et al. [18], and the theoretical results of Wesselinowa [19]. They investigated the effect of annealing in air on the physical properties of NiO NPs. (ii) the Mm and Mr values of the O2-annealed NiO NPs are smaller than the 387


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Fig. 6. Room-temperature hysteresis loops of the synthesized NiO NPs after annealing in air and O2 atmosphere at different temperatures (a) 400 °C, (b) 500 °C, (c) 600 °C, (d) 700 °C, and (e) 800 °C. Insets: low-field interval of magnetization curve.

Table 2 The reported magnetic parameter values (maximum magnetization (Mm), remanent magnetization (Mr), coercivity (Hc) and initial magnetic susceptibility (χi)) of the NiO NPs after annealing in air and O2 atmosphere at 400, 500, 600, 700 and 800 °C. Annealing temperature (oC)

400 500 600 700 800

χi ± 0.01 (×10−4)

Mm ± 0.001 (emu/g)

Mr ± 0.001 (emu/g)

Hc ± 0.01 (Oe)

Air

O2

Air

O2

Air

O2

Air

O2

0.2394 0.1389 0.1268 0.0869 0.0895

0.1148 0.1089 0.0960 0.0800 0.0777

0.0272 0.0046 0.0038 0.0002 0.0019

0.0018 0.0011 0.0012 0.0007 0.0008

123.48 45.60 53.03 47.20 54.32

59.58 47.35 59.70 50.70 55.44

2.59 1.04 7.38 4.52 3.48

3.20 2.44 2.08 1.44 1.54

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Fig. 7. Variation of (a) maximum magnetization, (b) remanent magnetization, (c) coercivity, and (d) susceptibility respect to annealing temperature in air and O2 atmosphere.

(D ≤ 24 nm) and spin glass behavior (D ≤ 10 nm) can be exhibited in an antiferromagnetic material such as NiO when the size of the particles is reduced to nano-scale [6,22]. Therefore, the magnetic behavior correlates to structural properties especially size, strain, crystallinity, shape and so on. According to Tables 1 and 2, we can conclude that the air-annealed NPs with respect to the size of 12.4 nm and 123.48 Oe coercivity exhibit weak ferromagnetic behavior after annealing at 400 °C and other annealed NPs in both atmospheres exhibit superparamagnetic-like behavior at higher temperatures until 800 °C. The origin of the ferromagnetic and superparamagnetic-like behavior in NiO can be attributed to the finite size effects, surface effects and interparticle interactions. These effects are related to the reduced number of exchange-coupled spins within NPs and the reduced symmetry of surface atoms due to incomplete compensation between antiferromagnetic sub-lattices [6,18]. The fraction of surface to total spins in antiferromagnetic NPs depends on the particle size, and hence the coercivity and magnetic moment should vary with particle size [9]. Moreover, the surface magnetic anisotropy can be significantly affected by the shape of the hysteresis curve [19]. In an antiferromagnetic system which has two sub-lattices with opposite spins in each sub-lattice, the net magnetic momentum changes according to the order or disorder in a system. Lattice strain and weak crystallinity can reduce the order of the system and uncompensated spines produce magnetization. At TA below 600 °C, the small values of Mm and Mr in O2-annealed samples as compared to air-annealed can be due to the higher crystallinity and lower lattice strain in O2-annealed samples. The presence of less coercivity in the O2-annealed NPs at 400 °C can also be due to this fact. The Mm and Mr values became more similar at a temperature above 600 °C, due to the domination of annealing temperature effect on the annealing ambient effect. The observed magnetic behavior is in a good agreement

air-annealed NPs. Magnetization in NiO nanoparticle is related to the presence of metallic nickel ions [20]. In a medium with a high (in pure O2) or low oxygen partial pressure (in air), the contribution of one Ni atom is destroyed by every oxygen atom, and a strong decrease in magnetization is observed [1]. (iii) Coercivity of the air-annealed NPs is decreased considerably from 123.38 Oe for 400 °C to 45.60 Oe for 500 °C, and decreased gradually for O2-annealed NPs followed by a slight variation at higher temperature (TA > 500 °C) in both atmospheres. Karthik et al. [9], reported that the value of Hc decreased as a result of the increase in particle size. (iv) The susceptibility of the O2annealed NPs decreased as a result of the increase in TA while in air annealing, the susceptibility varied several times by the increase in temperature. Tiwari and Rajeev expressed that susceptibility in NiO NPs varies as a function of 1/d, where d is the particle diameter [21]. (v) The shape of the hysteresis is changed during annealing. The magnetic properties of NiO NPs change as follows: Bulk NiO is an antiferromagnetic material. In this structure, there are linear atomic chains of Ni-O-Ni with dominating super-exchange interactions and opposite spins along the (1 1 1) direction [22]. The spins are compensated and had no response to the applied magnetic field. Super-exchange in NiO structure is the exchange interaction between two neighboring Ni+2 ions, which is mediated by an oxygen ion. It is noted that, the super-exchange is sensitive to bond lengths and bond angles. When an oxygen ion is missing from the surface as a result of the size reduction from bulk to nano-scale, the exchange bond would be broken and could give rise to surface spin disorder and frustration. As the particle size decreases, the fraction of atoms lying on the surface of the particles increases, hence surface spin disorder and uncompensated surface spins increase [23]. Different magnetic behaviors like, superparamagnetism (particles size D ≤ 31.5 nm), ferromagnetism 389


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with the change of structural properties.

[6] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Finite size effects in antiferromagnetic NiO nanoparticles, Phys. Rev. Lett. 79 (1997) 1393. [7] Z. Wang, C. Zhao, P. Yang, L. Winnubst, C. Chen, Effect of annealing in O2 or N2 on the aging of Fe0. 5Mn1. 84Ni0. 66O4 NTC-ceramics, Solid state ionics 177 (2006) 2191–2194. [8] S. Thota, J. Kumar, Sol–gel synthesis and anomalous magnetic behaviour of NiO nanoparticles, J. Phys. Chem. Solids 68 (2007) 1951–1964. [9] K. Karthik, G.K. Selvan, M. Kanagaraj, S. Arumugam, N.V. Jaya, Particle size effect on the magnetic properties of NiO nanoparticles prepared by a precipitation method, J. Alloy. Compd. 509 (2011) 181–184. [10] M. Martínez-Gil, M. Pintor-Monroy, M. Cota-Leal, D. Cabrera-German, A. GarzonFontecha, M. Quevedo-López, et al., Influence of annealing temperature on nickel oxide thin films grown by chemical bath deposition, Mater. Sci. Semicond. Process. 72 (2017) 37–45. [11] W. Xiang, Y. Liu, J. Yao, R. Sun, Influence of annealing temperature on the microstructure and magnetic properties of Ni/NiO core-shell nanowires, Physica E: Low-dimensional Syst, Nanostruct, 97 (2018) 363–367. [12] S.P. Jahromi, A. Pandikumar, B.T. Goh, Y.S. Lim, W.J. Basirun, H.N. Lim, et al., Influence of particle size on performance of a nickel oxide nanoparticle-based supercapacitor, RSC Adv. 5 (2015) 14010–14019. [13] S.P. Jahromi, N. Huang, M. Muhamad, H. Lim, Green gelatine-assisted sol–gel synthesis of ultrasmall nickel oxide nanoparticles, Ceram. Int. 39 (2013) 3909–3914. [14] A.K. Zak, W.A. Majid, M.E. Abrishami, R. Yousefi, X-ray analysis of ZnO nanoparticles by Williamson-Hall and size–strain plot methods, Solid State Sci. 13 (2011) 251–256. [15] K. Maniammal, G. Madhu, V. Biju, X-ray diffraction line profile analysis of nanostructured nickel oxide: Shape factor and convolution of crystallite size and microstrain contributions, Physica E: Low-dimensional Syst. Nanostruct. 85 (2017) 214–222. [16] D.-K. Hwang, M.-S. Oh, J.-H. Lim, C.-G. Kang, S.-J. Park, Effect of annealing temperature and ambient gas on phosphorus doped p-type ZnO, Appl. Phys. Lett. 90 (2007) 021106. [17] S. Tiwari, K. Rajeev, Effect of distributed particle magnetic moments on the magnetization of NiO nanoparticles, Solid State Commun. 152 (2012) 1080–1083. [18] D. Nikolić, M. Panjan, G.R. Blake, M. Tadić, Annealing-dependent structural and magnetic properties of nickel oxide (NiO) nanoparticles in a silica matrix, J. Eur. Ceram. Soc. 35 (2015) 3843–3852. [19] J. Wesselinowa, Size and anisotropy effects on magnetic properties of antiferromagnetic nanoparticles, J. Magn. Magn. Mater. 322 (2010) 234–237. [20] S.A. Makhlouf, F. Parker, F. Spada, A. Berkowitz, Magnetic anomalies in NiO nanoparticles, J. Appl. Phys. 81 (1997) 5561–5563. [21] S. Tiwari, K. Rajeev, Signatures of spin-glass freezing in NiO nanoparticles, Phys. Rev. B 72 (2005) 104433. [22] K. Anandan, V. Rajendran, Effects of Mn on the magnetic and optical properties and photocatalytic activities of NiO nanoparticles synthesized via the simple precipitation process, Mater. Sci. Eng., B 199 (2015) 48–56. [23] S. Tiwari, K. Rajeev, Magnetic properties of NiO nanoparticles, Thin Solid Films 505 (2006) 113–117.

4. Conclusion High crystalline NiO NPs have been successfully synthesized using the sol–gel method. The properties of these NPs have been investigated in terms of different annealing temperatures and atmospheres. Weak ferromagnetic and superparamagnetic-like behaviors were observed by increasing the annealing temperature. This is attributed to the finite size effect and uncompensated spins at the surface. Magnetization of the air-annealed NiO NPs has been found to be higher than the O2-annealed NPs as a result of the larger lattice strain and weaker crystallinity. At the temperature below 600 °C, in both atmospheres, crystallite siz increased by the increase in TA, while lattice strain, Mm and Mr decreased. At temperatures above 600 °C, the values of crystallite size, strain, lattice constant, magnetization and coercivity in both atmospheres were very similar. Acknowledgements This work was financially supported by a High Impact Research Grant (UM.C/625/1/HIR/MOHE/05) from the Ministry of Higher Education Malaysia and the SATU Joint Research Scheme 2016 of RU018F-2016 from the University of Malaya. References: [1] A. Roy, V. Srinivas, S. Ram, J. De Toro, U. Mizutani, Structure and magnetic properties of oxygen-stabilized tetragonal Ni nanoparticles prepared by borohydride reduction method, Phys. Rev. B 71 (2005) 184443. [2] B. Kisan, P. Saravanan, S. Layek, H. Verma, D. Hesp, V. Dhanak, et al., Effect of annealing on the magnetic properties of ball milled NiO powders, J. Magn. Magn. Mater. 384 (2015) 296–301. [3] M.P. Proenca, C.T. Sousa, A.M. Pereira, P.B. Tavares, J. Ventura, M. Vazquez, et al., Size and surface effects on the magnetic properties of NiO nanoparticles, PCCP 13 (2011) 9561–9567. [4] S. D’Addato, M. Spadaro, P. Luches, V. Grillo, S. Frabboni, S. Valeri, et al., Controlled growth of Ni/NiO core–shell nanoparticles: structure, morphology and tuning of magnetic properties, Appl. Surf. Sci. 306 (2014) 2–6. [5] J.T. Richardson, W. Milligan, Magnetic properties of colloidal nickelous oxide, Phys. Rev. 102 (1956) 1289.

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