Facile Synthesis and Characterizations of Mixed Metal Oxide Nanoparticles for the Efficient Photocatalytic Degradation of Rhodamine B and Congo Red Dyes

Photocatalytic degradation has been suggested to be a cheap and efficient way to dispose of organic pollutants, such as dyes. Therefore, our research team strives to produce nanophotocatalysts in a simple and inexpensive way. In this work, the Pechini sol–gel technique was employed for the facile synthesis of Mn0.5Zn0.5Fe2O4/Fe2O3 and Fe0.5Mn0.5Co2O4/Fe2O3 as mixed metal oxide nanoparticles for the efficient photocatalytic degradation of Rhodamine B and Congo Red dyes. XRD, FT-IR, a N2 adsorption/desorption analyzer, EDS, FE-SEM, and an UV–Vis diffuse reflectance spectrophotometer were used to characterize the produced samples. The XRD patterns revealed that the average crystallite size of the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples is 90.25 and 80.62 nm, respectively. The FE-SEM images revealed that the Fe0.5Mn0.5Co2O4/Fe2O3 sample consists of cubic and irregular shapes with an average diameter of 1.71 µm. Additionally, the Mn0.5Zn0.5Fe2O4/Fe2O3 sample consists of spherical shapes with an average diameter of 0.26 µm. The energy gaps of the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples are 3.50 and 4.3 eV and 3.52 and 4.20 eV, respectively. In the presence of hydrogen peroxide, the complete degradation of 100 mL of 20 mg/L of Rhodamine B and Congo Red dyes occurred at pH = 8 and 3, respectively, within 50 min, using 0.1 g of the synthesized samples.


Introduction
Organic dyes are present in water sources due to their many industrial uses, such as in paper, textiles, food, cosmetics, and plastics. Because they can cause cancer and make cells change, these dye molecules are harmful to living things. Throwing liquid waste containing dyes into water leads to severe health risks for humans [1][2][3][4][5]. Therefore, effective strategies must be found to dispose of these pollutants.
Photocatalytic degradation has been suggested to be a cheap and efficient way to dispose of dye molecules. The absorption of photons by a photocatalyst results in the transmission of some electrons from the valence band to the conduction band. Hence, this simultaneously generates electrons and holes in the conduction and valence bands, respectively. Electrons and holes can produce hydroxyl free radicals when reacting with water. Organic dye pollutants can be quickly degraded by hydroxyl free radicals and converted into volatile gases, such as CO 2 and H 2 O [19,20]. In 60 mL of distilled water, 8.33 g of FeCl 3 .6H 2 O, 4.43 g of ZnSO 4 .7H 2 O, 3.79 g of Mn(CH 3 COO) 2 .4H 2 O, and 1.47 g of CoCl 2 .6H 2 O were dissolved separately. Additionally, 13.25 g of tartaric acid was dissolved in 60 mL of distilled water. Fe(III), Zn(II), and Mn(II) solutions were mixed and stirred for 20 min to create the Mn 0.5 Zn 0.5 Fe 2 O 4 /Fe 2 O 3 product. Moreover, Fe(III), Co(II), and Mn(II) solutions were mixed and stirred for 20 min to create the Fe 0.5 Mn 0.5 Co 2 O 4 /Fe 2 O 3 product. Afterward, a tartaric acid solution was added to each system with continuous stirring for 20 min. Additionally, 3.33 mL of ethylene glycol was added to each system with continuous stirring. Furthermore, each system was heated at 150 • C until the solution was dried. Finally, the formed powder was ignited at 800 • C for 5 h. Scheme 1 summarizes the synthesizing steps of the nanomaterials.

Instrumentation
An X-ray diffraction (XRD) instrument was used to study the crystalline structure of the synthesized nanomaterials. The diffractograms were collected using a D8 Advance Xray diffractometer equipped with KαCu radiation (λ = 0.15 nm). A Thermo Scientific Nicolet iS50 Fourier-transform infrared spectrometer (FT-IR) was used to study the functional groups of the synthesized nanomaterials. A Jasco V-750 UV-Vis diffuse reflectance spectrophotometer (DRS) and an integrating sphere, calibrated with barium sulfate, were used to determine the band gap of the synthesized nanomaterials. A Quantachrome NOVA Touch LX2 nitrogen-gas-sorption analyzer was used to study the surface textures (BET surface area, total pore volume, and average pore radius) of the synthesized nanomaterials. The synthesized nanomaterials were degassed at 110 °C for 24 h before analyses. A Quanta 250 FEG scanning electron microscope (SEM) attached with an energy dispersive X-ray unit was used to study the surface morphology and elemental analysis of the synthesized nanomaterials. The morphologies of the nanomaterials were obtained using a Talos F200iS transmission electron microscope (TEM). The concentration of Rhodamine B and Congo Red dyes was determined using a Jasco V-750 UV-Vis spectrophotometer. The maximum wavelengths of the Rhodamine B and Congo Red dyes were 554 and 497 nm, respectively.

Photocatalytic Degradation of Rhodamine B and Congo Red Dyes
For every experiment, a specified amount of the Mn0.5Zn0.5Fe2O4/Fe2O3 or Fe0.5Mn0.5Co2O4/Fe2O3 samples was dispersed in a 100 mL aqueous solution of Rhodamine B or Congo Red dyes. The suspension was then agitated magnetically in the dark for 60 min. The solution was then irradiated with three UV lamps (30 cm, 8 watt, and 225 nm) located 8 cm away from the dye solution. In addition, the nanomaterials were separated by centrifugation, and the remaining concentration of the Rhodamine B or Congo Red Scheme 1. The synthesizing steps of the nanomaterials.

Instrumentation
An X-ray diffraction (XRD) instrument was used to study the crystalline structure of the synthesized nanomaterials. The diffractograms were collected using a D8 Advance X-ray diffractometer equipped with K α Cu radiation (λ = 0.15 nm). A Thermo Scientific Nicolet iS50 Fourier-transform infrared spectrometer (FT-IR) was used to study the functional groups of the synthesized nanomaterials. A Jasco V-750 UV-Vis diffuse reflectance spectrophotometer (DRS) and an integrating sphere, calibrated with barium sulfate, were used to determine the band gap of the synthesized nanomaterials. A Quantachrome NOVA Touch LX2 nitrogen-gas-sorption analyzer was used to study the surface textures (BET surface area, total pore volume, and average pore radius) of the synthesized nanomaterials. The synthesized nanomaterials were degassed at 110 • C for 24 h before analyses. A Quanta 250 FEG scanning electron microscope (SEM) attached with an energy dispersive X-ray unit was used to study the surface morphology and elemental analysis of the synthesized nanomaterials. The morphologies of the nanomaterials were obtained using a Talos F200iS transmission electron microscope (TEM). The concentration of Rhodamine B and Congo Red dyes was determined using a Jasco V-750 UV-Vis spectrophotometer. The maximum wavelengths of the Rhodamine B and Congo Red dyes were 554 and 497 nm, respectively.

Photocatalytic Degradation of Rhodamine B and Congo Red Dyes
For every experiment, a specified amount of the Mn 0.5 Zn 0.5 Fe 2 O 4 /Fe 2 O 3 or Fe 0.5 Mn 0.5 Co 2 O 4 /Fe 2 O 3 samples was dispersed in a 100 mL aqueous solution of Rhodamine B or Congo Red dyes. The suspension was then agitated magnetically in the dark for 60 min. The solution was then irradiated with three UV lamps (30 cm, 8 watt, and 225 nm) located 8 cm away from the dye solution. In addition, the nanomaterials were separated by centrifugation, and the remaining concentration of the Rhodamine B or Congo Red dyes in the filtrate was measured using a Jasco V-750 UV-Vis spectrophotometer. The same tests were conducted again, but this time 2 mL of 2 M hydrogen peroxide was added. The photodegradation efficiency (% D) of the nanomaterials against Rhodamine B or Congo Red dyes was determined using Equation (1).
X d (mg/L) is the remaining concentration of the Rhodamine B or Congo Red dyes after the process of stirring in the dark. X e (mg/L) is the remaining concentration of the Rhodamine B or Congo Red dyes after exposure to ultraviolet rays.

Results and Discussion
3.1. Characterization of the Synthesized Nanocomposites 3.1.1. X-ray Diffraction X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for the phase identification of a crystalline material, and it can provide information on the average crystallite size. Figure 1A In this method, an aqueous solution of metal salts is mixed with tartaric acid. Chelation, or the formation of complex ring-shaped compounds around the metal cations, takes place in the solution. Ethylene glycol is then added, and the liquid is heated to 150 • C to allow the chelates to polymerize, or form large, cross-linked networks. As excess water is removed by heating, a solid polymeric resin is achieved. Eventually, at a higher temperature of 800 • C for 5 h, the resin is decomposed, and ultimately, mixed metal oxides are obtained. Hence, this explains the proportions of the elements in Table 1 [33].

Energy Dispersive X-ray Spectroscopy
Energy dispersive X-ray spectroscopy (EDX) is an analytical method which yields a spectrum that displays the peaks correlated to the elemental composition of the investigated sample. Figure 2A,B displays the energy dispersive X-ray patterns of the Energy dispersive X-ray spectroscopy (EDX) is an analytical method which yields a spectrum that displays the peaks correlated to the elemental composition of the investigated sample. Figure 2A,B displays the energy dispersive X-ray patterns of the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples, respectively. The results reveal that the Fe0.5Mn0.5Co2O4/Fe2O3 sample consisted of manganese, oxygen, cobalt, and iron as displayed in Table 1. Additionally, the Mn0.5Zn0.5Fe2O4/Fe2O3 sample consisted of manganese, oxygen, zinc, and iron as displayed in Table 1. Hence, the absence of other elements confirms the success of the method in obtaining pure nano-oxides. The high percentage of iron in the Fe0.5Mn0.5Co2O4/Fe2O3 sample confirms the high percentage of Fe2O3 compared to Fe0.5Mn0.5Co2O4.  The results reveal that all curves belong to the IV types [37][38][39]. In addition, Table 2 displays the surface parameters, such as the BET surface area, total pore volume, and average pore size, of the produced samples. Moreover, the BET surface area of the Mn 0.5 Zn 0. 5      be bigger than the crystallite size obtained from XRD. Figure 5A,B displays the transmission electron microscopy images of the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples, respectively. The results reveal that the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples consisted of cubic and irregular shapes with an average diameter of 100.27 and 84.29 nm, respectively. The average particle size determined from the TEM images is slightly larger than that estimated from the XRD technique as a result of the presence of the agglomeration.

Energy Gap
The energy gap (E g ) was determined using the diffuse reflectance spectra of the F(R) is a constant, while K is the Kubelka-Munk function. Z is an integer based on the transition type. Z = 2 for the direct transitions that are permitted, while Z = 1/2 for the indirect transitions that are permitted. Figure 7A,B displays the plot of (F(R)hυ) 2 against hυ for the Fe 0.5 Mn 0.5 Co 2 O 4 /Fe 2 O 3 and Mn 0.5 Zn 0.5 Fe 2 O 4 /Fe 2 O 3 samples, respectively. Therefore, the transitions that were most abundant in the synthesized nanocomposites were direct allowed transitions. The energy gap (E g ) is determined by extrapolating each graph until (F(R)hν) 2   samples, respectively. It is noticeable that by increasing the pH, the degradation efficiency of the synthesized samples toward Rhodamine B dye increased, while the degradation efficiency of the synthesized samples toward Congo Red dye decreased. Rhodamine B dye is a cationic dye whose adsorption is increased in alkaline media, and thus the efficiency of its degradation in alkaline media is increased [19]. Congo Red dye is an anionic dye whose adsorption is increased in acidic media, and thus the efficiency of its degradation in acidic media is increased [19]. In the case of using the Fe 0. 5 Figure 8A,B displays the plot of % D against pH for the degradation of Rhodamine B and Congo Red dyes using the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples, respectively. It is noticeable that by increasing the pH, the degradation efficiency of the synthesized samples toward Rhodamine B dye increased, while the degradation efficiency of the synthesized samples toward Congo Red dye decreased. Rhodamine B dye is a cationic dye whose adsorption is increased in alkaline media, and thus the efficiency of its degradation in alkaline media is increased [19]. Congo Red dye is an anionic dye whose adsorption is increased in acidic media, and thus the efficiency of its degradation in acidic media is increased [19]. In the case of using the Fe0.5Mn0.5Co2O4/Fe2O3 sample, the degradation efficiency of the sample toward Rhodamine B dye without H2O2 (pH = 8), Rhodamine B dye with H2O2 (pH = 8), Congo Red dye without H2O2 (pH = 3), and Congo Red dye with H2O2 (pH = 3) was equal to 46.02, 100, 36.11, and 100 %, respectively. In the case of using the Mn0.5Zn0.5Fe2O4/Fe2O3 sample, the degradation efficiency of the sample toward Rhodamine B dye without H2O2 (pH = 8), Rhodamine B dye with H2O2 (pH = 8), Congo Red dye without H2O2 (pH = 3), and Congo Red dye with H2O2 (pH = 3) was equal to 47.14, 100, 47.53, and 100 %, respectively.   Figure 9A,B displays the plot of % D against time for the degradation of Rhodamine B and Congo Red dyes using the Fe 0.5 Mn 0.5 Co 2 O 4 /Fe 2 O 3 and Mn 0.5 Zn 0.5 Fe 2 O 4 /Fe 2 O 3 samples, respectively. In the absence of hydrogen peroxide, it was observed that by increasing the time from 10 to 80 min, the degradation efficiency of the synthesized samples toward Rhodamine B and Congo Red dyes increased. Additionally, in the case of increasing the time from 80 to 120 min, there was no significant change in the degradation efficiency of the synthesized samples toward Rhodamine B and Congo Red dyes due to the saturation of the active sites of the samples [19]. In the presence of hydrogen peroxide, the complete degradation of Rhodamine B and Congo Red dyes occurred within 50 min. The Rhodamine B and Congo Red dyes were completely degraded under UV light using only hydrogen peroxide in the absence of the synthesized samples within 5 h, which is much larger than the consumed time (50 min) in the presence of the synthesized samples.

Effect of Time
The degradation of Rhodamine B and Congo Red dyes using the synthesized samples is compatible with the first-order kinetic model as indicated by Equation (3) [19].
k (1/min) represents the first-order constant. Figure 10A Figure 11A,B displays the plot of % D against the quantity of the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples for the degradation of the Red dyes, respectively. It has been observed that by increasing the quantity of samples from 0.025 to 0.1 g, the degradation efficiency of the synthesized samples toward Rhodamine B and Congo Red dyes increases because of the increase in active sites [19]. Additionally, when the quantity of the samples was increased from 0.1 to 0.2 g, there was a significant decrease in the degradation efficiency of the synthesized samples toward Rhodamine B and Congo Red dyes because of the turbidity caused by the particles of the catalyst, which impedes the arrival of light to it [19]. Rhodamine B and Congo Red dyes, respectively. It has been observed that by increasing the quantity of samples from 0.025 to 0.1 g, the degradation efficiency of the synthesized samples toward Rhodamine B and Congo Red dyes increases because of the increase in active sites [19]. Additionally, when the quantity of the samples was increased from 0.1 to 0.2 g, there was a significant decrease in the degradation efficiency of the synthesized samples toward Rhodamine B and Congo Red dyes because of the turbidity caused by the particles of the catalyst, which impedes the arrival of light to it [19].   Figure 12A,B displays the plot of the % D of the Rhodamine B and Congo Red dyes against the concentration of the dyes, using the Fe 0.5 Mn 0.5 Co 2 O 4 /Fe 2 O 3 and Mn 0.5 Zn 0.5 Fe 2 O 4 / Fe 2 O 3 samples, respectively. It has been observed that by increasing the concentration of the Rhodamine B and Congo Red dyes from 10 to 30 mg/L, the degradation efficiency of the synthesized nanocomposites toward the Rhodamine B and Congo Red dyes decreases because the high concentration makes the dye particles block light from reaching the samples [19].  Figure 12A,B displays the plot of the % D of the Rhodamine B and Congo Red dyes against the concentration of the dyes, using the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples, respectively. It has been observed that by increasing the concentration of the Rhodamine B and Congo Red dyes from 10 to 30 mg/L, the degradation efficiency of the synthesized nanocomposites toward the Rhodamine B and Congo Red dyes decreases because the high concentration makes the dye particles block light from reaching the samples [19].     Figure 14 displays the suggested mechanism for the degradation of Rhodamine B and Congo Red dyes using the Fe 0. 5 Figure 14 displays the suggested mechanism for the degradation of Rhodamine B and Congo Red dyes using the Fe0.5Mn0.5Co2O4/Fe2O3 and Mn0.5Zn0.5Fe2O4/Fe2O3 samples. The absorption of photons by a photocatalyst results in the transmission of some electrons from the valence band to the conduction band. Hence, this simultaneously generates electrons and holes in the conduction and valence bands, respectively. Electrons and holes can produce hydroxyl free radicals when reacting with water. Rhodamine B and Congo Red dyes can be quickly degraded by hydroxyl free radicals and converted into volatile gases, such as CO2 and H2O [19,20].    [53,54]. Feng et al. utilized a new dual-modedriven micromotor based on foam-like carbon nitride (f-C 3 N 4 ) with precipitated Fe 3 O 4 nanoparticles, namely Fe 3 O 4 /f-C 3 N 4 , powered by chemical/magnetic stimuli for a rapid reduction in organic pollutants [55]. Li et al. prepared an ordered Schottky heterojunction of heptazine-based crystalline carbon nitride (HCN) and Ti 3 C 2 MXene through the ionothermal method. The HCN/Ti 3 C 2 composites exhibit higher photocatalytic performance than pristine HCN [56].

Conclusions
The