Structural, Conductivity, and Dielectric Properties of Nickel Oxide and Copper (Cu) or Chromium (Cr) Doped Nickel Oxide Nanoparticles

In this study, NiO NPs and doped NiO NPs were prepared in a two-step process and they were analyzed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and nitrogen adsorption desorption. X-ray diffraction outcome clarified that all the patterns present the similar peaks that are attributed to the cubic variety of the NiO and all doping metals could be inserted into the NiO structure. TEM result showed that the NiO NPs are assembled side by side and aligned along the same direction to form small array shaped nano-agglomerates of 500 nm in length and 50-70 nm in diameter. Adsorption-desorption isotherms for N 2 showed that NiO NPs represents type IV isotherms containing a hysteresis loop at relative pressure between 0.5 and 1.0. Hysteresis loop’s shape was of H 2 type which is characteristic of inckbottle shaped pores. It was also in the shape of solids composed by small spherical particles. The dielectric properties and electrical conductivity for these samples were also studied within the temperature range 25 o C – 110 o C and within the frequency range 100 Hz to 0.3 MHz.


Introduction
The material properties performance in developing the characteristics of reliable gas sensors is receiving a lot of attention from many scientists in the last few decades 1 .This is mainly because such sensors are widely used in several fields and in many applications particularly in hazardous environmental sensing applications as well as in chemical controlling, industrial monitoring, and home safety 2 .Well-known materials used in sensors application are semiconductors made from metal oxides.The electrical conductivity changes of these semi-conductors is a very important characteristic especially when interacting with gas molecules of the surrounding atmosphere 2,3 .Wide research has been conducted on the application of semiconductor materials including environmental, electronic and energy applications 4 .This application has contributed to the increasing importance of developing and improving the physical properties of semi-conductors.Investigations have mainly focused on nanoparticles especially their physical properties.These materials are known to be used in chemical and bio-applications 5 .It is worth noting that the properties of nanomaterials are different from the bulk counterpart owing to the quantum size effects 6,7 .Metal oxides are one such type of nanomaterials, which are gaining significance across several sectors.Metal oxides are known for their unique chemical and physical properties which allow them to be used in a diverse range of optical and electronic applications 8 .Transition metal oxide, nickel Oxide (NiO) known for its novel morphologies owing to a number of spins and surface atoms.NiO demonstrates interesting optical, electronic and magnetic properties 9,10,11,12 .It is a p-type semiconductor, therefore NiO nanoparticles are considered a varied band-gap semicondoctor in the range of 3.6 eV up to 4.0 eV 13,14 .Its structure is rock-salt and displays a normal antiferromagnetic property with magnetic moments residing on Ni 2+ ions.NiO has low electrical conductivity, at room temperature, of less than 10 −13 ohm −1 cm −1 and is considered as a 'Mott-Hubbard Insulator 15 .Moreover, NiO, is an eco-friendly material for super-capacitor terminals and is cost-effective compared to noble metal 16 .NiO is also known for its low resistivity which is the reason for many researchers to investigate 17 .One of the methods utilized to overcome this low resistivity is doping NiO nanomaterials.Doping NiO with (Cu, Fe, Au or Li) as alkali or transition metals will affect its structural, optical, electronic and magnetic properties.
Consequently this will create a Diluted Magnetic Semiconductor because of spin degrees of freedom and holes content that improves its functionality in spintronic 9,13,18 .Cu-doped NiO has been investigated by Manouchehri et

Synthesis of NiO NPs and Doped NiO NPs
The preparation of the NiO NPs was conducted in two steps.In the first step a precursor was synthesized using solid reaction by heating a well ground mixture of the oxalic acid and the nickel nitrate in the molar ration of 3/1, respectively.The nitrates ions were reduced by the oxalic acid (equations 1 and 2).Consequently, the metal ions were coordinated by the oxalate anion existing in the mixture (equation 3).
In the second step, the oxalate precursor was thermally decomposed in air at 400°C in a muffle furnace for 2 hours to produce the NiO NPs.
The same process was conducted to synthesize the doped NiO NPs.The initial mixture was completed by oxalic acid, nickel nitrate and copper nitrate or chromium nitrate with a molar ratio of 3/0.9/0.1, respectively.This suggests the formation of Ni0.9 Cu0.1O noted as NiO:Cu and Ni0.857Cr0.095□0.0475Onoted as NiO:Cr.

Characterization
To investigate the phase structure of the prepared materials, an X-ray diffractometer was used (XRD; Shimadzu 6000, Japan) CuKα radiation λ=1.5406Ǻ and an Ni filter.
Crystallites size could be estimated from the obtained XRD patterns using Scherer formula: DXRD=0.9λ/Bcosθwhere λ is the wavelength 1.5406Ǻ, B is the full width at half maximum (FWHM) and θ is the Bragg angle.The powders morphology, microstructure and composition were investigated through a transmission electron microscope (TEM: JEOL JEM 1400) and a Quanta FEG 250 scanning electron microscope equipped with an energy dispersive analyzer (SEM; Thermo Fischer Scientific, Hillsboro, OR, USA).Nitrogen adsorption-desorption isotherms were recorded on adsorption desorption analyzer (ASAP 2020, USA).From these curves, the specific surface area S of the powders could be found by Brunauer-Emmet-Teller (BET) technique and the particles size DBET was calculated by mean of the formula DBET=6000/dS, where d is the density of the powder and S is its measured specific surface area.Additionally, the peaks broadness increases by doping, indicating the decrease of the crystallites size that was estimated by Scherer formula.In Table 1 these calculated values will be compared to those found by calculations from the N2 adsorption desorption studies.

SEM and TEM Characterizations
Representative images of the prepared materials are shown in Figure 2. On the right side of the images on Figure 2, the EDS spectra of these materials confirm the presence or the absence of the doping element by means of the characteristic X-ray emission.All the powders are formed from agglomerates where voids are observed because of the evolving gas (mainly CO2 and H2O) formed during calcination of the oxalate precursors at 400°C.NiO: Cr seems to be less agglomerated.

N 2 Adsorption Desorption Studies
Nitrogen adsorption-desorption isotherms permit to get additional information about the microstructure of the prepared materials (Figure 4).NiO NPs represent type IV isotherms with a hysteresis loop at high relative pressure between 0.5 and 1.0, indicating the presence of mesopores (2-50 nm) and macropores (>50nm).The shape of the hysteresis loop was of H2 type which is characteristic of solids composed by small spherical particles also inckbottle shaped pores.This result confirms findings observed in the TEM study.The particle size was also calculated using the specific surface area values obtained from the adsorption-desorption isotherms and compared to those obtained from XRD patterns and assembled in Table 1 where they are given micro-structural information too.There is a good homogeneity in the results of the particles size.The values of DBET used to be higher than those of DXRD due to the agglomeration, the particles are stacked together, thus the adsorption of N2 is decreased.For less agglomerated particles, specific surface area will be higher and the resulting calculated particles size will be nearer of that found in XRD.

Dielectric Permittivity and ac Conductivity
The dielectric permittivity (ε`) for pure NiO NPs, NiO:Cu NPs and NiO:Cr NPs versus frequency at altered temperatures is displayed in Figure 5.  Apparently at low frequency relaxation peaks are detected and shifted to higher frequency by increasing the temperature, the shifted peaks can be ascribed to the translation ionic dynamics and the conductivity relaxation of the mobile ions.The maximum peak frequency, fmax, for the dielectric modulus as a function of temperature is shown in Figure 6 d. fmax behavior is explained by Arrhenius equation   =   exp( −    ⁄ ) 23 where f0 is a characteristic constant parameter for a particular relaxation process, k is the Boltzmann's constant, and Er is the relaxation energy.The calculated value of Er were about 0.29 eV, 0.83 eV and 0.51 eV for NiO, NiO:Cr and NiO:Cu, respectively.The values of Er suggest that the ionic conduction mechanism is suitable for the deliberate sample.

Ac Conductivity (σ ac )
The values of ac conductivity (  ) as a function of frequency, can be set by the following equation 24 : In this expression, σt is the total conductivity, σdc is the low-frequency conductivity, ω is the angular frequency,  0 is the permittivity of the free space, tan δ is loss tangent.Frequency dependence of σac for pure NiO NPs, NiO:Cr NPs and NiO:Cu NPs at different fixed temperatures is shown in Figure 7.The constant electrical conductivity at low frequency, could be attributed to the dc conductivity.By increasing frequency, the conductivity rises gradually which shows that the charge carriers are transported by hopping via defect sites along the chains.
This property is because of the increase of the absorbed energy which consequently results in increasing the number of the charge carriers that contribute to the conduction process.The strong increase of the conductivity observed at high frequencies in an indicator of the power-law dependence of the conductivity on the frequency which may suggest variable range hopping conduction mechanism 25 .

Conclusion
NiO NPs, NiO:Cu NPs and (c) NiO:Cr NPs were prepared successfully in two steps.
X-ray diffraction results showed that all the patterns present the same peaks belonging to the cubic phase of NiO.The peaks broadness increases when doping indicating the decrease of the crystallites size that were estimated by Scherer formula.TEM result showed that the NiO NPs are aligned along the same direction to form arrays of about 500 nm in length assembled side by side to give small nano-agglomerates of about 50-70 nm in diameter.N2 adsorption-desorption isotherms showed that NiO NPs is a type IV isotherm with a hysteresis loop at high relative pressure between 0.5 and 1.0, signifying the presence of mesopores (2-50 nm) and macropores (>50nm).The shape of hysteresis loop was of type H2 which is characteristic of inckbottle shaped pores and also solids composed by small spherical particles.The values of ε` decrease when increasing the frequency.The values of ε ` increases by the addition of Cr ions to NiO and decreases by the addition of Cu ions to NiO.A low frequency relaxation peak is observed for pure NiO NPs, NiO:Cr NPs and NiO:Cu NPs and shifted to higher frequency with increasing temperature.The electrical conductivity could be constant at low frequency, which was attributed to the dc conductivity.By increasing frequency, the conductivity also increases steadily which shows that the charge carriers are transported through hopping via defect sites across the chains.
Conflicts of Interest: "The authors declare no conflict of interest."

Figure 1
Figure 1 displays the XRD image s of the undoped-NiO, NiO:Cu and NiO:Cr.It can be seen that all the patterns present the same peaks that are attributed to the cubic variety of the NiO.The peaks were indexed based on the JCPDS file #47-1049 and no other peaks were observed.

Figure 1 :
Figure 1: Compared XRD patterns of NiO NPs pure and 10% molar doped with copper or chromium NiO:Cu and NiO:Cr prepared at 400°C in air.

Figure 2 :
Figure 2: SEM micrographs (left) and EDS spectra (right) of the (a) pure NiO NPs (b) NiO:Cu NPs and (c) NiO:Cr NPs prepared at 400°C in air.

Figure 3 :
Figure 3: TEM micrographs of the pure NiO NPs prepared at 400°C in air.

Figure 5 :
Figure 5: The dielectric constant as a function of temperature for (a) pure NiO NPs (b) NiO:Cr NPs and (c) NiO:Cu NPs.

Figures Figure 1 Compared
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Figure 5 The
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Figure 6 The
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Figure 7 Frequency
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