Tuning the structural, optical and electrical properties of NiO nanoparticles prepared by wet chemical route

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Tuning the structural, optical and electrical properties of NiO nanoparticles prepared by wet chemical route Sheraz Yousafa, Sonia Zulfiqarb, Mahrzadi Noureen Shahic, Muhammad Farooq Warsia, Najeeb Fuad Al-Khallid, Mohamed F. Aly Aboude, Imran Shakire,∗ a

Department of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur, 63100, Pakistan Department of Chemistry, School of Sciences & Engineering, The American University in Cairo, New Cairo, 11835, Egypt c Department of Chemistry, University of Okara, Okara, 56130, Pakistan d Department of Electrical Engineering, King Saud University, Riyadh, 11421, Saudi Arabia e Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: NiO Substitution XRD Electrical properties Band gap tuning FTIR UV–Visible

Unsubstituted NiO and its derivatives with Cu+2 and Zn+2 were synthesized by co-precipitation method. Structural characterizations were done by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopic and scanning electron microscopic (SEM) techniques. XRD results confirmed the cubic phase of NiO and all its derivatives. The crystallite size for all compositions of NiO nanoparticles was found 10–20 nm. XRD data was also used to calculate other physical parameters like lattice constant, unit cell volume, micro-strain, dislocation density, X-rays density and porosity. It was found that the synthesized nanomaterials have single phase. XRD data was supported by FTIR results. Morphological studies confirmed the spherical nature of NiO particles with size < 100 nm. Optical characterization was done by UV–Visible spectroscopic technique. The indirect band gap energy of NiO was ∼3.42 eV and band gap tuning was noticed in the case of transition metal cations substitution. Zn+2 substituted NiO nano-crystallites exhibited lowest band gap energy (3.06 eV). The nanoparticles with lower band gap may have better photocatalytic and other related properties. DC electrical properties were recorded by two probe technique. An increase in electrical conductivity was noticed as a result of divalent transition metal cations substitution. Cu+2 substituted NiO exhibited highest conductivity due to more conductive inherent nature of copper. Thus the Dc conductivity values for various compositions of NiO nanoparticles were observed in range 4.39 × 10-10 to 4.50 × 10-8 Scm-1.

1. Introduction Over recent years, transition metal oxides at nano-scale level are attaining extensive attention of researchers due to their outstanding performance in a number of applications [1–4]. Among all of these metal oxides, NiO is more popular due to its little cost, stability, superb electrical, optical such as optoelectronic properties and several manufacturing prospects. It is p-type semiconducting material with wide indirect band gap energy ranges from 3.4 eV to 4 eV [5,6]. NiO found itself in a variety of applications such as gas sensing [7], water splitting [8], catalysis [9], lithium ion batteries [10] and electrochemical cells such as fuel cell [11] etc. The wide band gap energy has limited its optical and optoelectronic applications only in narrow ultraviolet region of light. As sunlight mainly comprised of visible light and a little portion is of UV light. It was considered as inactive in visible region of

∗

light. Consequently, its band gap energy needs to be tailored. Various modifications were done in order to overcome these limitations such as doping with metals [12], non-metals [13], synthesizing ferrites [14] and nano-composites [15] with multiple properties. Cosubstitution with other metals was also found to be beneficial as compared to corresponding unsubstituted metal oxides [16]. Therefore, in order to decrease band gap energy, substitution by transition metal cations in crystal lattice of NiO may be beneficial as substitution caused mid gap defects in the host crystal lattice. This will change its electrical, magnetic and optical properties. Several transition metal doped nickel oxide nano-crystallites were synthesized by various researchers. Mishra et al. have tried to enhance magnetic properties of NiO nanoparticles. For this purpose, they used iron as a dopant material due to its inherent magnetic nature and successfully synthesized iron doped NiO nanoparticles with different

Corresponding author. E-mail addresses: [email protected], [email protected] (I. Shakir).

https://doi.org/10.1016/j.ceramint.2019.10.097 Received 1 August 2019; Received in revised form 12 September 2019; Accepted 10 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Sheraz Yousaf, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.097

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dopant contents. They found that NiO nanomaterials with size 14 nm and iron-doped NiO nanomaterials with size 7 nm showed ferromagnetism. However, the NiO with 31 nm size and iron-doped NiO nanomaterials with 25 nm were found antiferromagnetic at room temperature [17]. Sanker et al. have synthesized Mn+2 doped NiO spherical nanoparticles by sol gel route. They found that with the increase in Mn+2 doping; the grain size decreased and band gap energy increased from 3.79 to 3.95 eV. Photocatalytic degradation efficiency of highest and lowest dopant concentration was also tested by methylene blue as a source of pollutant under ultraviolet light illumination. It was observed that highest doped NiO nanoparticles exhibited maximum (92.3%) degradation due to availability of more active sites [18]. Ponnusamy et al. have synthesized Fe+3 doped NiO dilute magnetic nanoparticles by wet chemical route and found that decrease in crystalline size from 11.97 to 8.23 nm and increase in band gap energy from 3.958 to 4.025 eV. The synthesized nanoparticles showed ferromagnetic behavior at room temperature which has been ascribed owing to particle size effect [19]. Sheena et al. reported microstructural characterization and modified spectral response of cobalt doped NiO nanoparticles. Cobalt doped NiO nanoparticles were synthesized by co-precipitation method. The band gap energy of NiO decreased from 3.71 eV to 3.24 eV at higher doping contents [20]. Mn+2, Fe+3 and Zn+2 doped NiO nanoparticles were synthesized by Boukhari and his coworkers by co-precipitation method. They investigate and assure that the crystallite size, lattice parameters and band gap energy directly depends upon nature of dopant material. In general overall decrease in band gap energy was noticed as a result of Mn+2, Fe+3 and Zn+2 doping [21]. Structural, morphological and optical characterization of Co+2 doped NiO film was also investigated by Taşköprü et al and found that band gap energy decreased with the increase in doping content [22]. A number of techniques are available to synthesize NiO nanoparticles such as chemical precipitation [23], co-precipitation [24], sol gel [25], solvothermal and hydrothermal method [5]. Among these methods, co-precipitation method was preferred due to low cost, easily adoptable, controlled crystallite size and morphology of synthesized material [26]. The structural, optical and electrical properties of NiO have not been idealized. In order to link the current gap in the literature, we are reporting comprehensive structural electrical and optical studies of transition metal cations doped NiO nanoparticles. As ionic radius of Ni+2 is 0.69 Å is comparable with Cu+2 (0.73 Å) and Zn+2 (0.74 Å), this will facilitate their substitution in NiO crystal lattice, lower its band gap energy and enhanced its electrical properties. This will make it suitable for various applications.

Table 1 Synthesis composition of pure and transition metal cation doped NiO nanocrystallites. Sr. No.

Ni(0.1 M)

Cu(0.1 M)

Zn(0.1 M)

Composition

1. 2. 3. 4.

100 cm3 80 80 80

20 10

20 10

NiO Cu0·2Ni0·8O Zn0·2Ni0·8O Zn0.1 Cu0·1Ni0·8O

further stirred for half an hour at 100 °C and then it was cooled down to room temperature. The obtained precipitates were filtered and washed using deionized water in order to neutralize pH and then were dried in oven at about 100 °C. Finally, precipitates were grinded to fine powdered form and annealed in muffle furnace for 2 h at 500 °C. For the synthesis of Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO nanocrystallites, similar process was adopted by using their composition as given in Table 1 separately [23]. The whole synthesis process, step by step is shown in Fig. 1. Possible reactions taking place during synthesis of NiO have been presented in equations (2.1)–(2.5). NaOH Na+2 + OHNi(NO3)2·6H2O Ni Ni

+2

+2

2.1 +2

NO3-

+ 2 H2O

-

+ 2 OH + x H2O Ni(OH)2. xH2O

Ni(OH)2. xH2O Ni(OH)2

500 ᵒC

100 ᵒC

Ni(OH)2 + xH2O

NiO + H2O

2.2 2.3 2.4 2.5

2.3. Characterizations Crystalline nature and structural validation of pure and transition metal cation substituted NiO nanoparticles were done by using X-ray diffraction (XRD) technique. The instrument used for this purpose was Philips X'Pert (PRO 3040/60) diffractometer using Cu-Kα radiation as source of incident radiations with 0.15402 nm wavelength. Furthermore, molecular structure and nature of chemical bonding were examined by Fourier transform infrared (FTIR) spectroscopy using Alpha Bruker ATR spectrometer. The surface morphology of NiO and its derivatives was investigated using ZEISS LEO SUPRA 55 field emission scanning electron microscope (FESEM). UV–Visible analysis was carried out by means of Carry 60 UV–Visible–NIR dual beam spectrophotometer. The current voltage (I–V) measurements were performed using Keithley 6487 ammeter.

2. Experimental work 3. Results and discussion 2.1. Chemicals 3.1. X-rays diffraction analysis Pure and transition metal cation substituted NiO nanoparticles were prepared by consuming following precursor materials. Nickel nitrate hexahydrate (Ni (NO3)2·6H2O, 99% Sigma-Aldrich); cupper chloride dihydrate (CuCl2·2H2O, 99.99% Sigma-Aldrich); zinc nitrate hexahydrate (Zn (NO3)2·6H2O, 98.00% Sigma-Aldrich) and sodium hydroxide pellets (NaOH, 98% Sigma-Aldrich). All the chemicals were used as such without any further purification.

Fig. 2 shows XRD patterns of NiO and divalent transition metal cations substituted NiO nanoparticles. The patterns exhibited main diffractions peaks at 37.2°, 43.23°, 62.82°, 75.31° and 79.82°. These peaks corresponded to (111), (200), (220), (311) and (222) diffraction planes respectively. This data was match with ICDD # 01-078-0423. Thus FCC structure of NiO nanoparticles was exhibited which might remain unchanged while substitution with transition metal cations. XRD results of all the substituted samples did not show formation of any secondary phase. It was observed that, peak corresponded to (200) hkl value, shifted towards lower 2θ values in the case of Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO nanoparticles as depicted in Fig. 2 (b). It was due to substitution of larger sized transition metal cations in NiO crystal lattice [20]. Cell software was used to calculate unit cell parameters (lattice constant). Increases in unit cell length and cell volume were noticed which was due to substitution with larger size metallic cations. The

2.2. Synthesis process Aqueous solution of nickel nitrate (0.1 M) was prepared in deionized water at room temperature. Nickel nitrate solution (100 cm3) was taken in a beaker and placed on hotplate for magnetic stirring. The process of stirring was continued at 50 °C for half an hour. After half an hour, 5 mM standardized solution (50 cm3) of sodium hydroxide was added dropwise into the nickel nitrate solution. This resulted in color change from greenish blue to dark green. The reaction mixture was 2

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Fig. 1. Schematic illustration for synthesis of pure and transition metal cations substituted NiO nano-crystallites.

Fig. 2. (a) XRD diffraction pattern of pure and transition metal cations substituted NiO nano-crystallites and (b) XRD peak shifting as a result of substitution. Table 2 X-rays diffraction results of physical parameters of pure, Cu doped, Zn doped and Cu, Zn co doped NiO. Nanomaterials Pure NiO Cu0·2Ni0·8O Zn0·2Ni0·8O Cu0·1Zn0·1Ni0·8O

Unit cell length (Å) 4.180 4.182 4.196 4.186

Unit Cell volume (Å)3 74.03 73.14 73.88 73.34

Strain (με) −0.00026 0.00171 0.00059 0.00116

Dislocation Density (nm)-2 0.00290 0.01051 0.00334 0.00863

Table 3 X-rays diffraction results of physical parameters of pure, Cu doped, Zn doped and Cu, Zn co doped NiO. Nanomaterials

Pure NiO Cu0·2Ni0·8O Zn0·2Ni0·8O Cu0·1Zn0·1Ni0·8O

Fig. 3. Williamson-Hall plot of pure and transition metal cations substituted NiO nano-crystallites.

Crystallite Size (nm) Sherrer

W–H

Bulk density (gcm-1)

19.38 9.75 17.30 10.76

16.49 9.46 15.54 9.99

3.588 3.256 3.460 2.865

X-rays density (gcm-1)

Porosity (%)

6.88 6.86 6.80 6.86

47.90 52.60 49.10 58.20

crystallite size of NiO and transition metal cations substituted NiO was calculated by using scherrer method [27] as given in equation (3.1).

D=

3

Kλ β cos θ

3.1

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The crystallite size of NiO was 18.55 nm and decreased upon substitution with Cu+2 and Zn+2–9.5 nm and 17.3 nm respectively. In the case of Cu+2 and Zn+2 co-substituted NiO, crystallite size decreased to 10.76 nm. It was due to substitutional replacement of Ni+2 ions with Cu+2 and Zn+2 cations. Besides this, crystallite size and micro-strain were also calculated by using another method known as Williamsonhall plot method as given in equation (3.2) to (3.3) [28].

βhkl cos θ =

Kλ + 4ε sin θ D

βhkl = βD +βs

3.2

3.3

According to this method, the slope of the graph plotted against “4sinθ” (x-axes) and “βhklcosθ” (y-axes) gave the value of micro-strain whereas average crystallite size was obtained from y-intercept as given in Fig. 3. It was found that crystallite size of NiO was 14.23 nm whereas for Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO were 9.46 nm, 15.54 nm and 9.99 nm respectively Table 3. The crystallite size obtained from two different methods gave nearly the similar results whereas scherrer method gave to some extent larger size. It was due to existence of diverse geometries of particles. Further, it was observed that NiO exhibited negative micro-strain (−0.00026), as given in Table 2, that was due to contraction of its crystal lattice whereas lattice strain in case of Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO was

Fig. 4. FTIR spectra of pure and transition metal cations substituted NiO nanparticles.

In equation (3.1), D = crystallite size of nano-materials, K = 0.9 and corresponded to Scherrer's constant, λ = wavelength of X-rays used (Cu Kα 1.5406 Å), θ = Bragg's angle and β = full width at half maxima.

Fig. 5. SEM images for pure NiO (a), Cu0·2Ni0·8O (b), Zn0·2Ni0·8O (c), Cu0·1Zn0·1Ni0·8O (d). 4

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Fig. 6. UV–Visible spectra of (a) pure NiO, (b) Cu0·2Ni0·8O, (c) Zn0·2Ni0·8O,(d) Cu0·1Zn0·1Ni0·8O.

positive that further supported substitution of Ni+2 by metal cations. Micro-strain in the case of Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO nano-crystallites was positive. It was due to expansion of unit cell due to substitution of smaller sized Ni+2 (0.69 Å) by larger sized Cu+2 (0.73 Å) and Zn+2 (0.74 Å) in NiO crystal lattice. The irregularity incorporated in crystal lattice due to substitution in NiO was calculated in terms of dislocation density by equation (3.4) [29,30].

δ=

1 D2

equation (3.7) [34,35].

Porosity =

It was observed that NiO nanoparticles exhibited lowest dislocation density (0.0029) and increases in the case of Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO nano-crystallites as given in Table 2. This increment in dislocation density was due to increase in irregularity and crystal defects incorporated by substitution of transition metal cations in NiO crystal lattice as lower values of dislocation density governs to regularity in crystal system [31]. The X-ray density values of NiO and transition metal cations substituted NiO nanoparticles were also calculated using following mathematical relation equation (3.5).

4M ×V N

m v

3.7

3.1.1. FTIR analysis Understandings about molecular structure and nature of chemical bonding in NiO and transition metal cation substituted NiO nanocrystallites were done by using FTIR analysis as shown in Fig. 4. The number of absorption peaks and their positions are attributed to chemical composition, crystalline nature and morphology of material. The absorption bands below 1000 cm-1 are considered to be essential for studying presence of metal–oxygen bonds in the material. In FTIR spectra, sharp IR absorption bands, ranges 496 to 423 cm-1 were observed due to stretching vibrations of octahedral NiO6 group in FCC structure of NiO [36]. A blue shifting was observed in the absorption bands of transition metal cations substituted NiO. This blue shift was observed due to distortion of Ni–O–Ni bond. Substitution of Cu+2 and Zn+2 to NiO crystal lattice increased in bond length due to their larger size and less electronegativity. This increase in bond length is responsible for shifting absorption bands to lower wavenumber. According to Hook's law, vibrational frequency inversely depends upon mass of vibrating molecules. Thus shifting of absorption bands confirmed successful substitution of Cu+2 or Zn+2 to NiO crystal lattice in its derivatives [37].

3.5

where, ‘M’ is molecular weight, ‘N’ is Avogadro's number and ‘V’ is volume of unit cell and ρx−ray is theoretical or X-rays density [32]. The experimental or bulk densities of NiO and transition metal cations substituted NiO nanoparticles were also calculated by using equation (3.6).

ρbulk =

ρX − ray

Where, ρ bulk is experimental density and ρx−ray is theoretical density. Calculated values of all the parameters are provided in Table 3. It was found that Cu+2, Zn+2 co-substituted NiO possessed maximum porosity (58.2%).

3.4

ρX − ray =

1 − ρbulk

3.6

In equation (3.6), ‘m’ is the mass of palette, ‘v’ is the volume of pellet which is equal to πr2h (h = radius and r = thickness of pallet) [33]. Percent porosity of nano-crystallites was also calculated by relation 5

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Fig. 7. Bandgap energy of (a) pure NiO, (b) Cu0·2Ni0·8O, (c) Zn0·2Ni0·8O, (d) Cu0·1Zn0·1Ni0·8O.

morphology. Similarly, FESEM results of Cu+2, Zn+2 co-substituted NiO nanoparticles are given in Fig. 5 (d). It was noticed that co-substitution resulted aggregated nano-flakes like morphology. The morphological changes incorporated as a result of transition metal cations substitution to NiO nanoparticles.

Table 4 Bandgap energy of pure, Cu doped, Zn doped and Cu, Zn co doped NiO. Nanomaterials

Direct Bandgap (eV)

Indirect Bandgap (eV)

Pure NiO Cu0·2Ni0·8O Zn0·2Ni0·8O Cu0·1Zn0·1Ni0·8O

2.58 1.93 1.25 1.60

3.42 3.34 3.06 3.14

3.3. UV–visible analysis UV–Visible absorption spectra of semiconductor materials have been considered as very important tool to judge fundamental information related to its optical band gap and composition. It was found that optical spectra of such materials are additionally divided into three main regions. These are (1) weak absorption region, (2) absorption edge region and (3) strong absorption region. Weak absorption region in UV–visible spectrum corresponds to impurities whereas strong absorption region is due to pure material and corresponded to optical band gap energy of material. The absorption edge region appears due to structure disorder incorporated as a result of doping or substitution in the material [40]. Fig. 6 (a) shows UV–Visible spectra of NiO, it was found that NiO gave sharp absorption at 297 nm [41]. Fig. 6 (b) shows UV–visible spectra of Cu+2 substituted NiO nanoparticles. It was observed that with the substitution of Cu+2 cations, absorption bands were shifted to higher wavelength. Fig. 6 (c) shows UV–Visible spectra of Zn+2 substituted NiO nanoparticles. It was also found that with the doping of Zn+2 in NiO, absorption bands were shifted to higher wavelength. Fig. 6 (d) shows UV–Visible absorption spectra of Cu+2, Zn+2 co-substituted NiO nanoparticles. Similarly, again redshift in absorption spectrum was noted. The shifting of absorption bands is related to particle size of nanomaterials. From FESEM results, it was found that particle size of NiO nanoparticles increased as a result of substitution by Cu+2 and Zn+2 cations. This increase in particle size might be

Table 5 Electrical properties of pure, Cu doped, Zn doped and Cu, Zn co doped NiO. Nanomaterials

Resistivity (Ωcm)

Conductivity (Scm-1)

Pure NiO Cu0·2Ni0·8O Zn0·2Ni0·8O Cu0·1Zn0·1Ni0·8O

6.83 × 109 2.22 × 107 2.27 × 109 7.19 × 108

1.46377 × 10-10 4.50294 × 10-8 4.3982 × 10-10 1.39088 × 10-9

3.2. SEM analysis Morphological studies of NiO and its transition metal cations substituted derivatives were done by field emission scanning electron microscope. Fig. 5 (a) shows FESEM image of NiO nanoparticles. It was found that NiO nanoparticles were in spherical shape [38]. Its particle size ranged from 50 nm to 110 nm. Similarly, Cu+2 substituted NiO nanoparticles showed aggregation in the final product as given in Fig. 5 (b). Metal oxide nanoparticles shows aggregation in order to compensate high surface energy induced by high surface to volume ratio [39]. Fig. 5 (c) shows FESEM image for Zn+2 substituted NiO nanomaterials. It was noted that Zn+2 substitution resulted nano-flakes like

6

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Fig. 8. Current-Voltage measurements of (a) pure NiO, (b) Cu0·2Ni0·8O, (c) Zn0·2Ni0·8O, (d) Cu0·1Zn0·1Ni0·8O.

band gap energy of NiO reduced upon substitution with transition metal cations (Table 4). Substitution by transition metal cations to NiO crystal lattice introduces several extra energy levels in the band gap of NiO near to valence and conduction band edge. Therefore, sub-band states of transition metal cations are responsible for narrowing of band gap energy of NiO [20]. The energy of conduction band edge (ECB) and valence band edge (EVB) of pure, Cu+2, Zn+2 and Cu+2, Zn+2 cosubstituted NiO nano-particles, was calculated by equations (3.9) and (3.10) [46].

responsible for shifting of absorption bands towards higher wavelength [42]. It was noticed that, no absorption peak was observed in visible region for NiO nanoparticles whereas in the case of Cu+2 and Zn+2 substituted NiO nano-crystallites, weak absorption at 968 nm, 1078 was also observed. Similarly, in the case of Cu+2, Zn+2 co-substituted NiO, two weak absorption bands at 966 nm and 1089 nm were also observed. These weak absorption peaks were considered to be due to mid gap defects state incorporated as a result of substituting phenomenon [43]. 3.3.1. Optical band gap analysis The bandgap of a material has been considered as a fundamental tool to judge either incident photons can excite it or not. The optical band gap energy was calculated by well-known relation given by Tauc, using equation (3.8) [44]. 1

(αhυ) n = A (hυ − Eg )

ECB = X− Ee −

1 Eg 2

ECB = EVB - Eg

3.9

3.10

Where, X = electronegativity (Pure NiO is 5.970 eV, Cu0·2Ni0·8O is 5.978 eV, Zn0·2Ni0·8O is 5.971 eV) and Cu0·1Zn0·1Ni0·8O is 5.974 eV), Ee = energy of free electrons (4.5 eV) on hydrogen scale and Eg = Band gap. The calculated value of “EVB” for Pure, Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO nano-crystallites were 3.18 eV, 3.148 eV, 3.00 eV and 3.044 eV. The “ECB” for Pure, Cu+2, Zn+2 and Cu+2, Zn+2 co-substituted NiO nano-crystallites was −0.240 eV, −0.192 eV, −0.058 eV and −0.096 eV, respectively. Thus lowering in band gap energy was observed due to overlapping of positive and negative band edge energies in valence and conduction band of NiO and its derivatives [47].

3.8

In equation (3.8), α = molar absorptivity, A = measured absorbance, h = Plank's constant, ʋ = frequency of light, n is constant and related to mode of transition and Eg = band gap energy. Value of n is 1 2 for direct band gap and 2 for indirect band gap. Band gap energy was calculated by extrapolating the straight region of the graph, as shown in Fig. 7, plotted between “hʋ” and (αhʋ) 1/n. The bandgap energy of NiO is very high and found to be 2.58 eV and 3.42 eV for direct and indirect allowed transitions respectively [45]. The direct and indirect bandgap energy of Cu+2 substituted NiO was found to be 1.93 eV and 3.34 eV respectively. Similarly, the direct and indirect bandgap energy of Zn+2 substituted NiO was found to be 1.25 eV and 3.06 eV respectively. Whereas, in the case of Cu+2, Zn+2 co-substituted NiO nano-crystallites due to combine effect of both transition metal cations, band gap energy decreased to 1.60 for direct allowed and 3.14 for indirect allowed electronic transitions. Generally, it was noticed that direct and indirect

3.4. I–V measurement Current-voltage measurements were done in order to evaluate electrical properties of NiO and transition metal cations substituted NiO nanoparticles. The resistivity of NiO and transition metal cations 7

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substituted NiO nanoparticles was calculated by equation (3.11).

A ρ=R × l

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3.11

In this expression, ρ is resistivity, R is resistance, A is surface area of the pellet and l is the thickness of pellet. Furthermore, the resistivity values obtained from equation 3.11 is converted to conductivity. The DC electrical conductivity of pure, Cu+2, Zn+2 and Cu+2, Zn+2 cosubstituted NiO nanoparticles are given in Table 5. NiO due to high band gap energy is poor conductor as its DC conductivity calculated by I–V measurement is very low that was 1.46377 × 10-10 Scm-1 [48]. Its DC conductivity was further enhanced by substitution with transition metal cations and found to be increased. In the case of Cu+2 substituted NiO nanoparticles, maximum conductivity (4.50294 × 10-8 Scm-1) was observed because of substitution of more conductive Cu+2 ions. In the case of copper atoms, the valence 4s energy levels are half filled, therefore numerous electrons are capable to transfer electric current [49]. Fig. 8 (a) shows I–V characteristic curves of NiO. From this it was clear that NiO exhibited ohmic behavior. Similar to this, I–V characteristics curve for Cu+2, Zn+2 co-substituted NiO nanoparticles transforming from ohmic behavior to non-ohmic. This observed electrical transformation was due to addition of some additional energy levels in band gap of material [50]. 4. Conclusion Wet chemical route successfully yielded nanoparticles of NiO and its derivatives exhibited nano-flaks like morphology. XRD data confirmed the cubic phase of NiO and its derivatives. The substitution of Cu+2 and Zn+2 was confirmed by peak shifting in XRD data. Gradually change in lattice parameters was also observed in substituted NiO nanoparticles. The Cu+2 and Zn+2 substitutions affected the physical, optical and electrical properties of NiO nanoparticles. The maximum change in bandgap of NiO was observed for Zn+2 substituted NiO nanoparticles. Similarly, the DC electrical conductivity was also affected by substitution of Cu+2 and Zn+2 in NiO crystal lattice. The maximum increase in DC conductivity was noticed in the case of Cu+2 substituted NiO nanoparticles. This increase in electrical conductivity was attributed due to inherent conductive nature of copper. The obtained data revealed that the physical, electrical, optical and many other properties of NiO nanoparticles can be tailored easily by substitution of judicially selected metal cations. Acknowledgement We are thankful to The Islamia University of Bahawalpur (Pakistan) and Higher Education Commission (HEC) of Pakistan. Authors from King Saud University (Riyadh, Saudi Arabia) sincerely appreciate the deanship of scientific research for their contribution in this research through research grant (RGP-VPP-312). Dr. Sonia Zulfiqar is highly grateful to American University in Cairo (AUC) for financial support through STRC mini-grant and research project No. SSE-CHEM-S.Z.FY19-FY20-FY21-RG (1-19)-2018-Oct-01-17-53-22. References [1] Y.L. Pang, S. Lim, H.C. Ong, W.T. Chong, Research progress on iron oxide-based magnetic materials: synthesis techniques and photocatalytic applications, Ceram. Int. 42 (2016) 9–34. [2] B. Raveau, Transition metal oxides: promising functional materials, J. Eur. Ceram. Soc. 25 (2005) 1965–1969. [3] P. Pascariu, M. Homocianu, ZnO-based ceramic nanofibers: preparation, properties and applications, Ceram. Int. 45 (2019) 11158–11173. [4] Ç. Oruç, A. Altındal, Structural and dielectric properties of CuO nanoparticles, Ceram. Int. 43 (2017) 10708–10714. [5] M. Hashem, E. Saion, N.M. Al-Hada, H.M. Kamari, A.H. Shaari, Z.A. Talib, S.B. Paiman, M.A. Kamarudeen, Fabrication and characterization of semiconductor nickel oxide (NiO) nanoparticles manufactured using a facile thermal treatment, Results in Physics 6 (2016) 1024–1030.

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