Synthesis of trimetallic oxide (Fe2O3–MgO–CuO) nanocomposites and evaluation of their structural and optical properties

In this paper, tri-phase Fe2O3–MgO–CuO nanocomposites (NCs) and pure CuO, Fe2O3 and MgO nanoparticles (NPs) were prepared using sol–gel technique. The physical properties of the prepared products were examined using SEM, XRD, and UV–visible. The XRD data indicated the formation of pure CuO, Fe2O3 and MgO NPs, as well as nanocomposite formation with Fe2O3 (cubic), MgO (cubic), and CuO (monoclinic). The crystallite size of all the prepared samples was calculated via Scherrer's formula. The energy bandgap of CuO, Fe2O3 and MgO and Fe2O3–MgO–CuO NCs were computed from UV–visible spectroscopy as following 2.13, 2.29, 5.43 and 2.96 eV, respectively. The results showed that Fe2O3–MgO–CuO NCs is an alternative material for a wide range of applications as optoelectronics devices due to their outstanding properties.

Due to their unique optical, electrical, thermal, photocatalytic, mechanical, adsorbent and structural properties, metal oxide (MO) nanocomposites (NCs) have attracted much attention in recent years [1][2][3][4][5] .The NCs are composed of two or more nano-oxides and possessing properties which depend on the concentration of each constituent oxide in the mixture [6][7][8] .They are useful in a variety of applications, including solar cells, photovoltaic instruments, battery materials, gas sensors, and fuel cells [9][10][11][12][13][14][15] .Copper oxide (CuO) is a p-type semiconductor with a narrow bandgap of 1.2 eV 8 .It has unique optical and structural properties with low-cost preparation.It has attracted considerable attention due to its potential applications in superconductivity, gas sensing, solar cell and supercapacitor 16,17 .Furthermore, it is a non-toxic and readily available semiconductor 18,19 .Magnesium oxide (MgO), with a direct bandgap of 5.2-7 eV, is an n-type semiconductor that displays noticeable structural, catalytic, optical, and chemical properties 17,[20][21][22] .Iron(III) oxide (Fe 2 O 3 ) is a narrow bandgap of nearly 2 eV.It is associated with certain features, like the low toxicity, low cost, magnetic behavior and high solubility 23,24 .Hence, it is engaged in various applications involving biomedicine, cosmetics, diagnostics, sensors, radiology, and vaccines 9,23,25,26 .
By combining the different metal oxides (MOs) to form new NCs, various properties of individual oxide could significantly enhanced and, consequently, open up a new avenue of research for optoelectronics, electrical, thermal, photo-catalysis, and biological applications 26,27 .Mixed metal oxide NCs can be fabricated via different approaches such as the co-precipitation 28 , sonochemical 7 , solution combustion 29 , microwave technique 10 , ultrasonic-assisted 30 and green methods 2,11 .
In this work, tri-phase Fe 2 O 3 -MgO-CuO NCs and pure CuO, Fe 2 O 3 and MgO NPs were prepared using sol-gel method.It has the advantages of being environmentally friendly, simple, cheap and fast to perform without any special equipment.Herein, the novelty lies in the designed combination of the three metal oxides in one NC, which supposedly could lead to enhanced properties and potential applications.The obtained oxides were characterized for their structural and optical properties using XRD, UV-visible, and SEM.
Instruments.The optical properties of the synthesized materials were investigated using UV-Vis spectrophotometer (Hitachi U3900 with a software of Varian Cary 50).The structural properties were investigated by X-ray diffraction (XRD) using a Shimadzu EDX-720 (China) with CuKα radiation (λ = 0.154 nm).Morphological properties were assessed using SEM machine from JEOL (Jeol Ltd., Tokyo, Japan).

Results and discussion
The structural integrity of the synthesized metal oxides is confirmed via powder X-ray crystallography.The targeted substances were obtained via sol-gel route followed by calcination at 800 °C.The annealing temperature of 800 °C suggests high crystalline products as reported elsewhere 32 .However, such high temperature could stimulate production of pure substances with better performance.
X-ray diffraction.The crystalline arrangements and phase of the prepared nanopowder are estimated by XRD. Figure 1  diffracted peaks in composite were assigned for MgO (cubic), Fe 2 O 3 (cubic) and CuO (monoclinic) phases.The characteristic diffraction peaks of CuO, Fe 2 O 3 and MgO are well specified with no peaks relating to secondary or impurity segments or hydroxide in the sample, confirming the successful growth of Fe 2 O 3 -MgO-CuO NCs.The crystalline nature of the sample is assessed based on the sharp and strong diffraction peaks in Fig. 1.The cell volume (v), lattice constants (a, b, c) and d-spacing for pure CuO monoclinic, MgO cubic and Fe 2 O 3 hexagonal phase and Fe 2 O 3 -MgO-CuO nanocomposite were calculated 22,[33][34][35][36] and listed in Table 1.
The Scherrer equation 37 was utilized to compute the crystallite size (D) of CuO, Fe 2 O 3 , MgO and Fe 2 O 3 -MgO-CuO NCs.Then, their dislocation density was also calculated 1,38,39 , Table 2.As can be seen, the average D values of CuO, and MgO were larger than in the NCs as compared with individual oxides, due to the agglomeration of particles caused by the presence of Fe 2 O 3 .The Fe 2 O 3 particles act as nucleation sites for the CuO and MgO particles, resulting in aggregation into larger clusters.This phenomenon is known as the Ostwald ripening effect, where smaller particles dissolve and re-deposit on larger particles, resulting in an increase in their size.Hence, the presence of Fe 2 O 3 in the NCs leads to an increase in the particle size of CuO and MgO.NCs.It is seen that the formed nanostructures have spherical shapes with hardly distinct morphology.Furthermore, due to the low resolution of the presented SEM images, the non-size and thus, particle sizes and distribution are difficult to be counted.Nevertheless, the XRD data supported the claimed nanostructures.To improve the seen, and thus the suggested nanostructures, a higher magnification of the SEM image was presented as an insert within the corresponding image.The resulting magnified view is simply support that the particles are in nanometer range.In addition, some nanoparticles are well separated and thus could be counted.For example, by counting of the obviously countable particles of the composite image (Fig. 2D), it is found that the averaged particle size is 153 ± 30 nm, which is higher than that calculated from XRD (56 ± 4 nm) shown in Table 2.According to literature 40,41 , the SEM-based particle size is often larger than those measured by other techniques like XRD, the case that can be seen herein.

SEM analysis.
UV-Vis spectroscopy.The optical properties of the Fe 2 O 3 -MgO-CuO NCs were studied by UV-visible spectroscopy.Figure 3 displays the absorption spectrum of Fe 2 O 3 -MgO-CuO NCs within 200-1000 nm.The absorption spectrum of the scattering radiation is observed in the longer wavelength region, and a larger-tail is seen due to the mixing of different oxides.
The transmission spectra of all the synthesized materials showed almost an opposite behavior to that seen in Fig. 4. Obviously, the optical transmission increased in the visible region for all the synthesized materials and possesses maximum value for Fe 2 O 3 -MgO-CuO NCs.The absorption coefficient (α) value can be computed via the following equation α = 2.303A  The change in k (λ) for CuO, Fe 2 O 3 , MgO and Fe 2 O 3 -MgO-CuO NCs is presented in Fig. 6.It can be observed that k increases as the wavelength of the incident photon increases.
The energy bandgap (Eg) values of CuO, Fe 2 O 3 , MgO and Fe 2 O 3 -MgO-CuO NCs for the direct electronic transition between the valence band (VB) and conduction band (CB) can be computed via Tauc's relation 38,44,45 as shown in Fig. 7.The E g values of CuO, Fe 2 O 3 , MgO NPs were calculated to about 2.13, 2.29, and 5.43 eV, respec- tively.However, Fe 2 O 3 -MgO-CuO NCs displayed the E g of 2.96 eV.In comparison to individual CuO, Fe 2 O 3 , MgO NPs, Fe 2 O 3 -MgO-CuO displayed significant increased absorbance in the visible region due to incorporation of three metal oxide.The reason for the change in the bandgap energy of the NCs compared to the individual metal oxides is likely due to the formation of new energy states at the interfaces between the different metal    oxides.This can result in a shift in the electronic structure and a change in the bandgap energy.Additionally, the presence of multiple metal oxides in the NCs can also lead to increased electron-hole separation and improved charge transport properties, which can further affect the bandgap energy.This result was in good agreement with the literature 46 , which showed the possibility of using the prepared materials in some optical application.

Conclusion
In Summary, tri-phase Fe

Figure 2
represents the SEM images of grown pure CuO, Fe 2 O 3 , MgO and Fe 2 O 3 -MgO-CuO

t 34 .
The change in α (λ) for CuO, Fe 2 O 3 , MgO and Fe 2 O 3 -MgO-CuO NCs is presented in Fig. 5. From this Figure, it can be seen that α decreases as the wavelength ( ) of the incident photon increases.The extinction coefficient (α) value can be calculated via the following equation k = α 4π 42,43 .
2 O 3 -MgO-CuO NCs and pure CuO, Fe 2 O 3 and MgO NPs were successfully fabricated using a sol-gel approach.The XRD emphasized the formation of pure CuO, Fe 2 O 3 and MgO NPs and CuO-Fe 2 O 3 -MgO NCs.The variation in the average crystallite size (D) and lattice constant were observed due to the interaction of the corresponding metal oxides.The optical bandgap was reached 2.13, 5.43, 2.29 and 2.96 eV for CuO, MgO, Fe 2 O 3 and Fe 2 O 3 -MgO-CuO NCs, respectively.

Table 1 .
Geometric parameters of CuO, Fe 2 O 3 and MgO in grown Fe 2 O 3 -MgO-CuO nanocomposites determined from XRD analysis.

Table 2 .
Structural parameters of CuO, Fe 2 O 3 and MgO in grown Fe 2 O 3 -MgO-CuO nanocomposites determined from XRD analysis.