In recent times nano structured semiconductor metal oxides are gradually gaining attention due to its extraordinary optical properties [1, 2]. Graphene is an allotropic modification of carbon, a two-dimensional material with hexagonal crystal lattice, with unique mechanical, electrical, optical and other properties and therefore a material of great interest of researchers. Among the various potential technological applications of graphene, one is its use as a component of nanocomposites. Composites containing metal-oxide compounds, has variety of application in energy storage devices as a material for lithium-ion batteries and super capacitors, in water purification systems and for removal of pollutants from the environment, in electrochemical sensors etc. The excellent mechanical strength, outstanding electrical properties and good thermal stability of graphene oxide (GO) based nanocomposites have increased their use in electronic applications such as capacitors, energy storage devices, sensors, etc. However, reduced graphene oxide (rGO) is two-dimensional (2D) single layer structure of sp2 carbon atoms which exhibits extremely high specific surface areas. Owing to the higher carrier motilities and specific surface areas reduced graphene oxide (rGO) coupled with abundant defects and carboxyl, hydroxyl, and epoxy groups on its surface facilitates great scope in the designing of various nanoelectronics. Furthermore, the metal oxides can be incorporated into carbonbased structures which effectively improves their application in the field of permittivity and absorbing performance. It also modifies the atomic arrangement of the graphene and produces improved synergistic interaction between the constituents. The improved configuration after the additions of metal oxides leads to improvement in its chemical, physical and mechanical properties for various applications [3]. Various studies have been used to fabricate the metal oxide-GO nanocomposites (NCs) for improvement in the dielectric, optical and electrical properties. Some of the common metal oxides/graphene NCs such as ZnO/GO [4], SnO2/GO [5], rGO/Coo/Ag [6], and MgO/rGO [7] have been investigated and reported as improvements in the optical, dielectric and electric properties. Among the metal oxide materials, zinc oxide (ZnO) is a favorable ntype semiconducting material used in various applications such as ultraviolet (UV) detection [8–10], light-emitting diodes [11,12], gas detection [13,14], nano-power generators [15], optoelectronic devices [16] etc. ZnO material also possesses numerous properties such as direct wide band gap (3.37 eV) [17], optical [18], chemical stability [19], dielectric, electrical [20] and piezoelectric properties [21,22]. Additionally, the dopant addition also contributes to improving conductivity and the charge carrier mobilities of ZnO [23]. Ni is expected to have a suitable dopant due to its larger solubility due to the relatively larger Zn as compared to Ni ion and the same oxidation state [24,25].

Many researchers are working in divalent metal doped nickel oxide (NiO) nanoparticles for a wide range of applications. Certainly, NiO is one of the promising and environment friendly p-type semiconductors with wide band gap energy ranging from 3.6 to 4 eV (bulk form) [26]. It is suitable for many technological applications such as sensing devices, chemical micro sensors, gas sensors, agriculture, spintronic devices, superconductors, solar cells and anti-ferromagnetic layers, quantum tunneling, exchange coupled dynamics, and optical coating [27,28]. In addition, NiO nanomaterials are having excellent thermal stability, which makes them suitable for microelectronics and electro chromic material for display [29]. Stoichiometric NiO is an Mott insulator, but one can increase its p-type conductivity by doping an ions with positive charge [30] and also by thermal treatment few Ni2+ ions becomes Ni3+ oxidation compensation and thus acquires on excess oxygen becomes slightly non-stoichiometric and Ni vacancies are trapped Ni3+ ions in the ground state [31]. There are few reports available on the synthesis and characterization of Zn doped NiO nanoparticles. For instance, Sathishkumar et al [32] has investigated the structural, magnetic and electrochemical properties of Zn doped NiO nanocrystals by chemical precipitation method at room temperature. They observed that Zn doped NiO can be used in electrode material for the super capacitor application. Karthik et al [33] has found that the energy gap (Eg) of NiO nanoparticles is 2.93 eV. The increase in energy gap of NiO nanoparticles is an indicative of the quantum confinement effect and arising from the tiny crystallites. Further they noticed that by increasing the dopant concentration, activation energy can be increased due to solubility restrictions. The fundamental property of nanoparticles is seeing that their structure, particle size, distribution, and morphology are nearly related to the preparation techniques. Various synthesis methods have been developed to prepare nanoparticles. For example, sol-gel method, solvothermal method, electrochemical routes, hydrothermal reaction, and chemical precipitation method are the most common method of preparing the nanoparticles. Here, we have chosen the chemical precipitation method for the present study. Compared to other method this is very simplest method, low cost, easy to prepare the nanoparticles with low temperatures, easy to produce the particle and to control the particle size. This paper mainly focuses the synthesis and characterization of undoped and Ni doped ZnO/rGO nanocomposites via chemical precipitation method at different Ni concentration. The main objective of this work is to study the structural, optical and morphological of the prepared samples. Doping of NiO with ZnO is done which is believed to be an excellent dopant for tuning its optical and morphological properties due to its unique stability at Zn+2 sites. Ni2+ (0.69Å) have the same valence compared to Zn2+ and its radius is close to Zn2+ (0.74Å), so it is very easy for Ni2+ sub-lattice to replace Zn2+ in ZnO lattice. Some researches on Ni doped ZnO have been reported and several results showed that the various properties of ZnO were changed after inserting Ni into ZnO matrix [34,35]. By doping Ni into ZnO, a composite material with magnetic and optical properties could be obtained. Magnetic material could be used in magnetic therapy and fluorescence material could be applied in phototherapy agents, so the Ni-doped ZnO would be a new material in medical field. The transition metal doped nanostructure is an effective method to adjust the energy levels and surface states of ZnO, which can further introduce changes in its physical and especially optical properties [36]. Until now, zinc oxide with various shapes was prepared by various methods. Out of these methods of ZnO synthesis, we have used Co-precipitation method chemical synthesis to prepare the nano-particles. However, it is still a great challenge to synthesize ZnO structures doped with the transition metal element using a simple process with a low cost. The high quality nano-crystalline powders of Ni_xZn_1-XO/rGO (x= 0.00, 0.01, 0.03 and 0.05) are successfully synthesized and their structural and optical absorption and compositional properties are investigated. The present synthesis method is reproducible and ensures the large scale production at a low temperature. CO-precipitation advantages: Simple and rapid preparation, Easy control of particle size and composition, Low temperature, Energy efficient. Disadvantages, Trace impurities may also get precipitated with a product, Time consuming, Batch-to-batch reproducibility problems.

Synthesis of Graphene Oxide (GO) and Reduced Graphene Oxide (rGO)

For the graphene oxide preparation, the modified Hummers’ method [37] was followed.3g of natural graphite powder, 1.5g of NaNO₃ (99%, Sigma Aldrich) and 100 ml of H₂SO₄ (99%, Sigma Aldrich) were mixed and the mixture was stirred for 5 hours at 450 rpm in an ice bath. Then, 6g of KMnO₄ (99%, Sigma Aldrich) was added slowly to the above mixture and stirred at room temperature for 36h. The large amount of heat evolution was observed during this reaction. After stirring, 250ml of double distilled water was added below 25 degrees. Then, 5ml of H₂O₂ was added to the solution. The color of the solution was changed from dark brownish to light yellow color. Then the combined mixture was stirred for 6h and aged for 24 h .The slurry was washed using 5% HCL to remove impurities. Then they obtained product was washed with DI water several times to obtain the neutral pH.The slurry was washed with ethanol for 2-3 times. The washed product was dried in a vacuum oven at 333 K for 24 pH.The dried powder was ground using an agate mortar to get fine powder of graphene oxide (GO).A mixture of 1.5g of GO powder and 500ml of DI water was prepared. The mixture was stirred for 30 minutes to form the GO dispersion. Next 1ml of hydrazine hydrate was added in to the dispersion. The mixture was heated to 80 degrees and stirred for 72 hours. After 72 hours the mixture was transferred in to 6 different 50ml centrifuge tubes for cleaning. The cleaning was done by adding DI water and centrifuged at 5000rpm for 15 minutes. These steps were repeated twice before dry in an oven at 80 degrees for 24 hours. Finally the resultant product was obtained (rGO).

Synthesis of NiZnO/rGO Nanocomposites

The Nickel zinc oxide/reduced graphene oxide nanocomposites were prepared by the co-precipitation process. All the chemical reagents were purchased from sigma Aldrich and used directly without further purification.Ni_xZn_1-xO powders with (x=0, 0.03, 0.06, 0.09) grams were prepared using 100 mL of distilled water, metal nitrates and sodium hydroxide. 100?mL of zinc nitrate mixed with Nickel nitrate solution was stirred well for 30 minutes at 70 degrees .Further 3.8 grams of sodium hydroxide was dissolved in 100 mL of distilled water and added to the metal nitrate solution slowly till it reaches the pH ~8 and stirred at 70 degrees at 550 rpm for 3 hours, the obtained solution was washed and filtered and dried in an oven at 160 degrees for 3 hours and calcinated at 250 degrees. The obtained powder was ground using mortar and pestle and collected. In the same procedure the other samples of different compositions were prepared. The obtained Ni_xZnO_(1-x) (x=0, 0.03, 0.06 and 0.09) nanocomposites are added to the GO solution (0.02 grams of reduced graphene oxide is added in a 100 ml of distilled water) and stirred at 30 minutes on a magnetic stirrer, the obtained solutions were washed and filtered and dried in an oven at 160 degrees for 3 hours .Then, NiZnO/rGO samples was obtained, was dried and calcined at 250ºC for 3h. The same procedure was followed for the preparation of Ni_xZnO_(1-x)/rGO nanocomposites in different compositions. Here, we can observe the structural, morphological and optical changes by changing the Nickel doping concentrations.

Characterizations

The characterization of metal oxide nanocomposites is essential for understanding of their structural and optical properties. Development of novel tools and instruments are one of the greater challenges in nanotechnology. The prepared samples were characterized by using various physicochemical methods namely XRD, SEM, EDAX, FTIR, and UV-vis. The prepared NiZnO/rGO samples were characterized by using the powder X-ray (λ=0.15496 nm). The structures of the samples were studied by using the FESEM .The presence of elements in the compound was recorded using the ZIESS ULTRATM-55 Instrument attached to the FESEM. The FTIR spectrum of the prepared sample was recorded, with Cuk resolution of 4 cm–1 over the range 4000–400 cm–1. The absorption study of the prepared samples has been carried out using the UV-vis spectrophotometer.

The crystal structure and phase purity of prepared undoped ZnO and different composition of Ni-doped ZnO/rGO nanocomposites were characterized using X-ray diffraction. Figure 1 shows a typical XRD spectra of pure ZnO and Ni_xZn_1-xO/rGO (x = 0.00, 0.03, 0.06 and 0.09) nano composites. XRD pattern reveals that the diffraction peaks of undoped and nickel doped ZnO/rGO nanocomposites can be indexed to hexagonal wurtzite structure of ZnO which is in good agreement with the standard JCPDS file for ZnO (JCPDS36-1451). In all doped samples, the nickel traces were observed at (200) plane. This new phase emerges at (2θ = 43.3°) as shown in Figure 1. Such an additional diffraction peak corresponds to the secondary phase of NiO (200) (matched with JCPDS No 22-1189). The intensity of NiO peak increases with increasing nickel amount indicating that phase segregation has occurred and such structural degradation in the ZnO lattice may be attributed to introduction of a foreign impurity. Fig. 1 respects the X-ray diffraction pattern taken from the different concentrations of Ni doped with ZnO/rGO obtained in this work. All the diffraction peaks (1 0 0), (0 0 2), (1 0 1) recorded for polycrystalline ZnO were confirmed that the wurtzite structure, having hexagonal phase in good agreement with the reference pattern JCPDA 36-1451. From the other side, the peaks in the diffraction patterns recorded for Ni dopant with different concentration were shown reflections from (1 1 1), (2 0 0) at 36.8 and 43.3 2 theta values according to the reference pattern JCPDS No 22-1189. No other peaks corresponding to additional phases were observed demonstrating high purity of the prepared nanoparticles and suggesting that Ni dopant occupy chemical substitution sites. It is note worthy that Ni dopant slightly sharpen the peaks which signify that the increase in the crystallite size. This is due to the fact that the ionic radius of Ni2+ (0.69 Å) is slightly lower than Zn2+ (0.74 Å) ions. The crystallite size can be obtained either by direct computer simulation of the X-ray diffraction pattern or from the Full Width at Half Maximum (FWHM) of the diffraction peaks using the Debye-Scherer’s formula [38].

D = Κλ/β cos θ

Where, λ - Wavelength of X-rays, or β - FWHM in radian, q - Peak angle.The average D-size was calculated and as 61.47nm, 52.50nm, 43.58nm and 41.42nm for the doping concentration x of Ni in Ni_xZn_1-xO/rGO (x = 0.00, 0.03, 0.06, 0.09) nanocomposites. The size of the crystallites lies in the range of 61.47nm to 41.42 nm and it decreases with the higher nickel concentration. The reduction in the particle size is mainly due to distortion in the ZnO lattice by Ni⁺² impurities decreasing the nucleation and subsequent growth rates by the addition of Ni concentration [39]. The maximum D-size was observed at x=0.02 and reveals that D-size decreasing according to increase of x values. The lower D-size indicates that Ni ions have fitted well into the ZnO lost lattice, confirms the stability of the lattice has improved. For each of the samples, the strain has decreased substantially for x = 0.00, 0.03, 0.06, 0.09 from that of the undoped ZnO. The decrease in strain indicates a better stable lattice structure. An increase in the lattice strain with increasing amount of Ni incorporated in ZnO/rGO. This increase is attributed to the stronger tensile and more O₂ vacancies are introduced in ZnO matrix with incorporation of Ni. These trends reverse suddenly on further increase in dopant concentration [33].

Figure 1: Shows the X-ray diffraction spectra of pure ZnO and Ni_xZn_1-xO/rGO (x = 0.00, 0.03, 0.06 and 0.09) nano composites.

Figure 2: Shows the extended 2Theta range of XRD patterns for highlighting the NiO peaks at (200) (111) peak intensity towards lower 2 theta values in NiZnO/rGO nanocomposites.

For the compound with x = 0.03, with the substitution of the smaller Ni²⁺ at the Zn²⁺ at the tetrahedral sites in the host lattice the degree of distortion and the lattice strain increases . For X = 0.06, with further increase in the number of larger radius Ni²⁺ ions in the octahedral sites, the lattice strain increases. For x = 0.09, a part of the Ni²⁺ ions occupy tetrahedral sites replacing the Zn²⁺ ions and a few occupy octahedral sites with a larger ionic radius. As a result the degree of distortion improves and the lattice strain is found to be decreased. It has been found that the value of crystallite size (Table 1) decreases from 85 to 41 nm as Ni²⁺ doping increases from 0% to 6%. This decrease may be due to the suppression of nucleation and subsequent growth of ZnO by Ni²⁺ doping. More defects such as interstitials and vacancies in the lattice are usually created when doped by foreign impurity. The nucleation is been suppressed by these defects on the grain surface/boundaries and hence prevents the subsequent grain growth. But as Ni²⁺ doping increases from 6% to 9% the crystallite size increases from 41.42 to 43.58 nm. This may be due to the distortion produced around the dopant ions because of mismatch between ionic radii of Zn²⁺ and Ni²⁺. This created distortion may be low for initial concentrations but high for higher concentrations of Ni²⁺. Therefore, for higher Ni²⁺ concentration, the distortion centers increase largely resulting in the increase of average crystallite size [40]. The small changes in 2theta values and the peak broadening are due to the size, or micro strain or size and micro strain and defects or dislocations of nanoparticles [41].

Table 1: shows Crystallite size, strain and dislocation intensity of the Ni_xZn_1-xO/rGO (x = 0.00, 0.03, 0.06 and 0.09) nano composites.

We have synthesized a series of undoped ZnO/rGO and nickel (Ni) doped ZnO/rGO Nanocomposites- using Co-precipitation method. The crystal structure of the Ni_xZnO_(1-x) /rGO nanocomposites with x=0.00, 0.03, 0.06 and 0.09 has been studied. From XRD data, it is confirmed that all samples are in the wurtzite hexagonal structure. No secondary phases have been observed in the present work for the NiZnO/rGO samples and it is to be reported that the doping effect of Ni present at (200) plane. The crystallite size was found to be decreased which results in bandgap decreases respectively. From the UV-vis studies we can observe that when the Ni composition is increased we can observe the change in energy band gap, as the Ni composition increases the band gap energy first increases and then decreases. This reduction in band gap energy of NiZnO/rGO nanocomposites can be attributed to strain induced in crystal lattice that indicates substitution of Ni²⁺ ions with Zn²⁺ ions within ZnO lattice. The FTIR spectroscopy has been done for complete structural analysis for all the samples which assigned prominent peaks for different species and it is confirmed that the Zn-O, C-C, C-O-O bands as major and hydroxyl group as a minor component presents in powders. A red shift in the band gap has been observed from the room temperature optical absorption and PL spectra of NiZnO/rGO nanocomposites. Such a red shift in band gap may be attributed to the sp–d exchange interactions between the band electrons and the localized d electrons of the Ni⁺² ions substituting Zn ions clearly indicating the incorporation of Ni ions into the Zn site of the ZnO lattice. Henceforth, we infer that all the above studies have been analyzed for NiZnO/rGO nanocomposites which may provide the fundamental understanding for many application purposes

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