Introduction

Due to their unique optical, electrical, thermal, photocatalytic, mechanical, adsorbent and structural properties, metal oxide (MO) nanocomposites (NCs) have attracted much attention in recent years1,2,3,4,5. The NCs are composed of two or more nano-oxides and possessing properties which depend on the concentration of each constituent oxide in the mixture6,7,8. They are useful in a variety of applications, including solar cells, photovoltaic instruments, battery materials, gas sensors, and fuel cells9,10,11,12,13,14,15. Copper oxide (CuO) is a p-type semiconductor with a narrow bandgap of 1.2 eV8. 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 supercapacitor16,17. Furthermore, it is a non-toxic and readily available semiconductor18,19. 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 properties17,20,21,22. Iron(III) oxide (Fe2O3) is a narrow bandgap of nearly 2 eV. It is associated with certain features, like the low toxicity, low cost, magnetic behavior and high solubility23,24. Hence, it is engaged in various applications involving biomedicine, cosmetics, diagnostics, sensors, radiology, and vaccines9,23,25,26.

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 applications26,27. Mixed metal oxide NCs can be fabricated via different approaches such as the co-precipitation28, sonochemical7, solution combustion29, microwave technique10, ultrasonic-assisted30 and green methods2,11.

In this work, tri-phase Fe2O3–MgO–CuO NCs and pure CuO, Fe2O3 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.

Materials and methods

Materials

Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O; 97%), Iron nitrate nonahydrate (Fe(NO3)3·9H2O; 97%), copper nitrate trihydrate (Cu(NO3)2·3(H2O; 98%) and absolute ethanol were purchased from BDH and used as received without additional treatment.

Synthesis

The sol–gel method20,31 was used to fabricate the Fe2O3–MgO–CuO NCs, which involves the following steps: Cu(NO3)2·3(H2O) (3.382 g in 20 mL ethanol), Fe(NO3)3·9(H2O) (5.65 g in 20 mL ethanol) and Mg(NO3)2·6(H2O) (3.589 g in 20 mL ethanol) with constant molar ratio (1:1:1) were synthesized as three separate solutions. Each solution was stirred for 10 min at 23 ± 2 °C to obtain a homogeneous solution. The solutions were mixed under constant stirring for 70 min at 80 °C until gel was obtained. After that, the gel burns to create xerogel, which grinded to fine powder and annealed at 800 °C for 90 min. The individual pure oxides (Fe2O3, CuO, and MgO) were separately prepared following similar steps as composite, using the corresponding salt.

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 elsewhere32. 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 shows the XRD pattern of the fabricated Fe2O3–MgO–CuO NCs. The observed diffraction peaks of pure oxides are close to the diffraction patterns reported in the X-ray database of JCPDS CuO (45-0937), Fe2O3 (33-0664) and MgO (45-0946). Similarly, in Fe2O3–MgO–CuO NCs, the diffraction patterns of CuO (48-1548), Fe2O3 (39-1346), and MgO (45-0946) match well with their respective standard reference cards. The diffracted peaks in composite were assigned for MgO (cubic), Fe2O3 (cubic) and CuO (monoclinic) phases. The characteristic diffraction peaks of CuO, Fe2O3 and MgO are well specified with no peaks relating to secondary or impurity segments or hydroxide in the sample, confirming the successful growth of Fe2O3–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 Fe2O3 hexagonal phase and Fe2O3–MgO–CuO nanocomposite were calculated22,33,34,35,36 and listed in Table 1.

Figure 1
figure 1

XRD patterns of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO nanocomposites.

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Table 1 Geometric parameters of CuO, Fe2O3 and MgO in grown Fe2O3–MgO–CuO nanocomposites determined from XRD analysis.
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The Scherrer equation37 was utilized to compute the crystallite size (D) of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO NCs. Then, their dislocation density was also calculated1,38,39, 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 Fe2O3. The Fe2O3 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 Fe2O3 in the NCs leads to an increase in the particle size of CuO and MgO.

Table 2 Structural parameters of CuO, Fe2O3 and MgO in grown Fe2O3–MgO–CuO nanocomposites determined from XRD analysis.
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SEM analysis

Figure 2 represents the SEM images of grown pure CuO, Fe2O3, MgO and Fe2O3–MgO–CuO 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 literature40,41, the SEM-based particle size is often larger than those measured by other techniques like XRD, the case that can be seen herein.

Figure 2
figure 2

SEM images of (A) CuO, (B) Fe2O3, (C) MgO, (D) Fe2O3–MgO–CuO nanocomposites. Inserts are magnification of the shown selected area.

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UV–Vis spectroscopy

The optical properties of the Fe2O3–MgO–CuO NCs were studied by UV–visible spectroscopy. Figure 3 displays the absorption spectrum of Fe2O3–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.

Figure 3
figure 3

Absorption spectra of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO (FMC) nanocomposites.

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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 Fe2O3–MgO–CuO NCs. The absorption coefficient (α) value can be computed via the following equation \(\mathrm{\alpha }=\frac{2.303A}{t}\)34.

Figure 4
figure 4

Transmission spectra of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO (CFM) nanocomposites.

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The change in α (λ) for CuO, Fe2O3, MgO and Fe2O3–MgO–CuO NCs is presented in Fig. 5. From this Figure, it can be seen that α decreases as the wavelength (\(\lambda \)) of the incident photon increases. The extinction coefficient (α) value can be calculated via the following equation \(k=\frac{\mathrm{\alpha }\lambda }{4\pi }\)42,43.

Figure 5
figure 5

Absorption coefficient versus wavelength of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO (FMC) nanocomposites.

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The change in \(k\) (λ) for CuO, Fe2O3, MgO and Fe2O3–MgO–CuO NCs is presented in Fig. 6. It can be observed that \(k\) increases as the wavelength of the incident photon increases.

Figure 6
figure 6

Extinction coefficient (k) versus wavelength of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO (FMC) nanocomposites.

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The energy bandgap (Eg) values of CuO, Fe2O3, MgO and Fe2O3–MgO–CuO NCs for the direct electronic transition between the valence band (VB) and conduction band (CB) can be computed via Tauc's relation38,44,45 as shown in Fig. 7. The \({E}_{g}\) values of CuO, Fe2O3, MgO NPs were calculated to about 2.13, 2.29, and 5.43 eV, respectively. However, Fe2O3–MgO–CuO NCs displayed the \({E}_{g}\) of 2.96 eV. In comparison to individual CuO, Fe2O3, MgO NPs, Fe2O3–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 literature46, which showed the possibility of using the prepared materials in some optical application.

Figure 7
figure 7

Optical bandgap plots for CuO, Fe2O3, MgO and CuO–Fe2O3–MgO (FMC) nanocomposite.

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Conclusion

In Summary, tri-phase Fe2O3–MgO–CuO NCs and pure CuO, Fe2O3 and MgO NPs were successfully fabricated using a sol–gel approach. The XRD emphasized the formation of pure CuO, Fe2O3 and MgO NPs and CuO–Fe2O3–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, Fe2O3 and Fe2O3–MgO–CuO NCs, respectively.