Synthesis and Characterization of ZnO–MgO Nanocomposite by Co-precipitation Method

Abstract Co-precipitation along with aging at 80 °C has been used to synthesize zinc-magnesium nanocomposite. Obtained materials were characterized by scanning electron micrograph, point energy dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy, X-ray diffraction analysis (XRD) and UV–visible diffuse reflectance spectra (UV-DRS) so as to determine its various physico-chemical characteristics. EDX analysis confirmed the presence of Zn, Mg, and O elements within ZnO–MgO nanocomposite. Formation of magnesium oxide along with zinc oxide nanocomposite has been confirmed by XRD analysis which has been reaffirmed by point EDX analysis. Optical properties investigated by UV-DRS showed decrease in maximum reflectance (~25%) due to incorporation of MgO within ZnO nanoparticle. Electrochemical study showed higher electrochemical activity of ZnO–MgO nanocomposite than bare ZnO nanoparticle.

glass conical flux at 90 °C. The precipitate was centrifuged at 8000 rpm at room temperature and repeatedly washed with de-ionized water. Obtained white color semi-solid samples were dried for 12 h at 80 °C. In a similar way, by using homogeneous precipitation method, individual ZnO and MgO nanoparticles were prepared, where 0.58 g of ZnSO4·7H 2 O (or 0.49 g of MgSO 4 ·7H 2 O) and 0.75 g of NaOH were added in 60 ml of DI water. Finally, white color powder nanoparticles were formed. Nanoparticles formation can be described by the following equations: Pure ZnO and MgO nanoparticles were formed via reaction (1) and (2), and (3) and (4), respectively. ZnO-MgO nanocomposite was formed via reaction (5) and (6).
Powder X-ray diffraction data of nanocomposites were obtained using an X-ray diffractometer (Bruker AXS, Diffractometer D8, Germany) diffraction unit. Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet 6700, NEXUS, USA) was used to obtain FTIR spectra using KBr pellet technique. To understand the morphology of nanocomposite, a scanning electron micrograph (SEM) (QUANTA, Model 200 FEG, Netherland) was used. Conductive sample were prepared by gold coating using a sputter coater (Edwards S150). After gold sputtering, SEM and energy dispersive X-ray analysis was done using an EDX spectrometry. Shimadzu UV-2100 spectrometer was used to determine the UV-visible diffuse reflectance spectra (BaSO 4 reference material) within range of 200−800 nm.
Electrochemical experiments were carried out using CHI-760c, USA Potentiostat with three electrode single compartment cell setup. Pt wire, Hg/HgCl 2 and glassy carbon electrode (GCE) were used as counter, reference, and working electrodes, respectively; and all the potentials were measured vs. GCE. Electrode modification by obtained nanoparticles was an important part of the experiments. GCE electrode was polished with 0.3 micron alumina slurry and washed with DI several times. Nanoparticles modified electrodes were prepared (1)

Introduction
A wide range of various metal oxide semiconductors have a great potential in practical applications. [1] Nanocrystalline zinc oxide (ZnO), an n-type metal oxide semiconductor, is one of the metal oxide which comprises wide band gap energy of 3.37 eV, large excitation binding energy of 60 meV along with good optical, electrical, and piezoelectric responses. [2] ZnO has a lot of important applications in the field of solar cell, optoelectronics, microelectronics, light emitting devices, gas sensing device transistor, and medical sectors. [3][4][5] Tuning of excitation wavelength of this semiconductor is essential so as to tune its optical, electrical, and magnetic properties which play important roles during practical applications. Doping (or mixing) of one metal oxide semiconductor with other helps in tuning of the properties. Doping of ZnO with magnesium (bandgap of 7.3 eV) could enhance the UV luminescence intensity by adjusting its wavelength. ZnO-MgO nanocomposite exhibit enhanced optical properties that results partly from the different crystallites or electronic coupling between ZnO and MgO which enhances the bandgap. [6][7][8][9][10] Several synthesis techniques such as solid state mixing, [11] electrochemical method, [12] hydrothermal growth, [4] sonochemical, [13] sol gel technique, [14] high pressure synthesis technique, [15] glycine-nitrate combustion route, [16] and thermal evaporation techniques [5,[17][18][19] have already been reported for ZnO-MgO synthesis. Among all, co-precipitation method is the simplest and easy for the production of ZnO-MgO nanocomposite. In one study, ZnO-MgO has been prepared from alcoholic solution by consecutive precipitation or co-precipitation method followed by high temperature annealing. [20] In this work, ZnO-MgO nanocomposite have been prepared by simple single-step co-precipitation method. Products were characterized by various sophisticated characterization techniques. Electrochemical performance of nanoparticles was studied in bare sodium hydroxide solution using three electrode single compartment cell setup.

Experimental details
Magnesium sulfate, heptahydrate and zinc sulfate, heptahydrate was purchased from HiMedia Laboratories Pvt. Ltd, India. Sodium hydroxide and distilled water (DI) was used as purchased from local sources. All chemicals used in this study were of analytical grade and were used without further purification.
Co-precipitation method was used to prepare ZnO-MgO nanocomposite by dissolving 0.58 g of ZnSO 4 ·7H 2 O, 0.49 g of MgSO 4 ·7H 2 O , and 0.75 g of NaOH in DI water (60 ml) under constant agitation in a 100-ml glass beaker. Resultant solution was fasten for 24 h within a 100-ml by rubbing the ethanol wetted sample over the polished GCE surface and dried at room temperature. Above treatment adhered some amount of produced nanoparticle sample on GCE electrode surface.

Results and discussion
SEM micrographs (shown in Figure 1) were used to analyze the morphology of nanoparticles prepared by co-precipitation method. Figure 1  the nanoparticles has been calculated using the following equations [26,27]: where, hν, A, α, and E g is photon energy, proportional constant, absorption co-efficient, and band gap energy, respectively. Exponent n has a values of 3, 2, 1.5, and 0.5 which represent indirect forbidden, indirect allowed, direct forbidden, and direct allowed transitions, respectively. [27,28] Here, the exponent value has been taken as 0.5 as the nature of direct allowed transition in ZnO. Further, the Equation (7) was modified with Kubelka-Munk function F(R) = (1 − R) 2 /2R (where, R is the diffuse reflectance). [28] Hence, the equation has been written as: Using Equation (8), Tauc plot has been drawn to calculate band gap energy values of nanomaterial as shown in Figure 4(b). Analysis shows that ZnO-MgO nanocomposite exhibited band gap energy of 2.9 eV which is in agreement with literature. [29] The results show that the E g value of pure ZnO has been increased from 2.75 [30] to 2.9 eV with metal doping. [31] The cyclic voltammetry curve of pure ZnO electrode and ZnO-MgO nanocomposite electrode in 0.1 M NaOH electrolyte solution at a scan rate of 50 mV/s in the potential range -2 to +2 V has been shown in Figure 5. ZnO-MgO electrode showed reduction peak at around + 1.7 V whereas, pure ZnO electrode has almost negligible reduction peak. As ZnO has no specific property of redox reaction like MgO, therefore, the pure ZnO has no response. [31,32] At the positive end of the potential range, the peak current intensity and  [23] whereas in ZnO-MgO, the peak has been shift at 491 cm −1 stipulating the incorporation of MgO crystal within ZnO nanoparticles. [3] In Figure 3(a) (I), broad peak at 470 cm −1 is due to pure ZnO, [24] whereas in Figure 3(a) (II), the peak has been shifted to 491 cm −1 indicating incorporation of MgO within ZnO nanoparticles. [3] Composition and purity of nanoparticles (ZnO and ZnO-MgO) were determined by X-ray powder diffraction analysis (Figure 3(b)). All the diffraction peaks in Figure 3(b) (I) match with standard patterns of hexagonal structure of pure ZnO (JCPDS No. 01-079-0207) which is in agreement with the previously reported 2θ values. [24] Figure 3(b) (II) exhibits peak of cubic MgO (JCPDS No. 00-004-0821) along with ZnO, illustrating incorporation of MgO within ZnO. [11] XRD studies revealed a high degree of purity and crystallinity of nanoparticles.
Optical properties of pure ZnO and mixed metal oxide (ZnO-MgO) were analyzed by determining UV-visible diffuse reflectance spectra. Figure 4(a) shows the UV-visible absorption spectra of nanomaterials. Pure ZnO nanoparticle showed maximum ~55% reflectance in the visible region (410-800 nm) which is in agreement with previous literature. [25] Reflectance decreased by ~25% after incorporation of MgO with ZnO. The band gap energy (E g ) of  been observed for ZnO-MgO electrode than pure ZnO electrode, in other word, ZnO-MgO electrode shows better electrochemical performance.

Conclusion
In the present study, co-precipitation method has been used for the synthesis of