Mn3O4@ZnO Hybrid Material: An Excellent Photocatalyst for the Degradation of Synthetic Dyes including Methylene Blue, Methyl Orange and Malachite Green

In this study, we synthesized hybrid systems based on manganese oxide@zinc oxide (Mn3O4@ZnO), using sol gel and hydrothermal methods. The hybrid materials exhibited hierarchical morphologies and structures characterized by the hexagonal phase of ZnO and the tetragonal phase of Mn3O4. The hybrid materials were tested for degradation of methylene blue (MB), methyl orange (MO), and malachite green (MG) under ultraviolet (UV) light illumination. The aim of this work was to observe the effect of various amounts of Mn3O4 in enhancing the photocatalytic properties of ZnO-based hybrid structures towards the degradation of MB, MO and MG. The ZnO photocatalyst showed better performance with an increasing amount of Mn3O4, and the degradation efficiency for the hybrid material containing the maximum amount of Mn3O4 was found to be 94.59%, 89.99%, and 97.40% for MB, MO and MG, respectively. The improvement in the performance of hybrid materials can be attributed to the high charge separation rate of electron-hole pairs, the co-catalytic role, the large number of catalytic sites, and the synergy for the production of high quantities of oxidizing radicals. The performance obtained from the various Mn3O4@ZnO hybrid materials suggest that Mn3O4 can be considered an effective co-catalyst for a wide range of photocatalytic materials such as titanium dioxide, tin oxide, and carbon-based materials, in developing practical hybrid photocatalysts for the degradation of dyes and for wastewater treatment.


Introduction
Environmental pollution is caused by the extensive use of synthetic dyes, pigments, and other coloring products, in various industries such as food, beverages, cosmetics, pharmaceuticals, clothes and paper [1]. These chemical compounds are characterized by a complicated structure, high stability and highly toxicity to living things. In addition, the

Synthesis of Z Mn 3 O 4 @ZnO Hybrid Materials Using Sol Gel and Hydrothermal Methods
The synthesis of the nanostructured hybrid material was carried out in two steps. The Mn 3 O 4 nanostructures were prepared using the sol gel method, and the typical synthesis process is described below. The Mn 3 O 4 material was reproduced according to the work reported in [41]. Precursor solutions of 0.4 M glycerol and 0.3 M potassium permanganate were prepared. The glycerol and potassium permanganate solutions were then mixed in a ratio of 1:2 by volume, stirred for 2 h, and finally kept for 24 h without disturbance, to obtain the desired gel at room temperature. Afterwards, the gel was heated at 80 • C for 30 min, the product precipitates were obtained, and centrifugation was used to collect the fine brown-colored Mn 3 O 4 powder. Meanwhile, the precursor ZnO was prepared by mixing 0.1 M of zinc acetate dihydrate and 5 mL of 33% aqueous ammonia solution in 100 mL of deionized water. Different amounts of Mn 3 O 4 were used for the deposition of ZnO in the hydrothermal method: 0.3, 0.5, 0.75 and 1.0 g of precipitated Mn 3 O 4 were dissolved in four beakers containing 200 mL of a precursor solution of ZnO, in order to develop hybrid Mn 3 O 4 @ZnO systems. The materials were labeled as sample 1, sample 2, sample 3, and sample 4, respectively. A fifth beaker contained only the ZnO precursor solution, and the pristine ZnO sample was used as a comparison. After being mixed, the solutions were covered very tightly with aluminum foil, and left for hydrothermal reaction in a preheated electric oven for 5 h at 95 • C. After completion of the reaction, the beakers were cooled down to room temperature and the product was collected on filter paper and washed several times with ethanol followed by deionized water. Finally, the product was dried at 60 • C overnight. The experimental layout is given in Scheme 1. Stepwise synthesis of Mn3O4@ZnO hybrid materials using sol gel and hydrothermal methods.

Characterization of Morphology, Crystalline Structure, and Chemical Composition of the Materials
Scanning electron microscopy (Jeol, JSM-6380 A, Tokyo, Japan)was used at 10 kV to evaluate the morphology of the prepared nanostructured materials. Powder X-ray diffraction (Miniflex 600, Rigaku, Austin, TX, USA) was employed at 45 mA and 45 kV to identify the crystal structure and phase of the nanostructured materials. Energy dispersive spectroscopy (Jeol, JSM-6380 A) was used to quantify the elemental composition of each Scheme 1. Stepwise synthesis of Mn 3 O 4 @ZnO hybrid materials using sol gel and hydrothermal methods. Scanning electron microscopy (Jeol, JSM-6380 A, Tokyo, Japan)was used at 10 kV to evaluate the morphology of the prepared nanostructured materials. Powder X-ray diffraction (Miniflex 600, Rigaku, Austin, TX, USA) was employed at 45 mA and 45 kV to identify the crystal structure and phase of the nanostructured materials. Energy dispersive spectroscopy (Jeol, JSM-6380 A) was used to quantify the elemental composition of each synthesized material. Following this, UV-visible spectrophotometry (PE Lamda365, Waltham, MA, USA) was employed to record absorption spectra during dye degradation under UV light illumination. The UV light source consisted of six UV tubes with a wavelength of 365 nm.
In this study, we studied the performance of hybrid materials for the degradation of various dyes (methylene blue, methyl orange, and malachite green), using 5 mg of each catalyst nanomaterial, and a fixed concentration for each dye (1.87 × 10 −5 M MB, 2.44 × 10 −5 M MO, and 5.48 × 10 −5 M MG). Before irradiation with UV light, the dye solution was stirred for 30 min, in order to optimize its homogeneity.
The % degradation efficiency of each dye was estimated from the following equation: where C o and C t are the concentrations of each dye before and after degradation, respectively.  relative intensity decreases, while that of the peaks from the tetragonal phase of Mn3O4 increases (JCPDS sheet: 00-003-1041, in blue at the bottom of the figure). The XRD patterns shown in Figure 1 demonstrate that the Mn3O4@ZnO hybrid was successfully prepared. The nanomaterial is composed of ZnO and Mn3O4, while no amorphous phases or impurities were observed.

Photocatalytic Degradation of MB, MO and MG onto Different Mn3O4@ZnO Hybrid Systems
The photodegradation of MB, MO and MG in aqueous solution using pure ZnO and the different hybrid systems ZnO@Mn3O4 was studied under UV light illumination.

Photocatalytic Degradation of MB, MO and MG onto Different Mn 3 O 4 @ZnO Hybrid Systems
The photodegradation of MB, MO and MG in aqueous solution using pure ZnO and the different hybrid systems ZnO@Mn 3 O 4 was studied under UV light illumination. Figure 4 shows the absorption spectra of pure ZnO during MB degradation over time under UV light illumination. A solution of MB with a concentration of 1.87 × 10 −5 M was used, to which 5 mg of catalyst was added. The spectra show a typical absorption peak around 663 nm which slowly decreases over time, indicating that the degradation rate is limited and therefore takes a long time. This suggests that pure ZnO exhibits a low density of catalytic sites and a high rate of recombination of the electron-hole pair; therefore, the release of oxidizing agents under illumination is limited, thus leading to poor MB degradation performance. The degradation kinetics was also studied as shown in Figure 4b,c. The reaction rate of MB degradation was evaluated by monitoring the change in concentration C t over time from the initial concentration C o , as shown in Figure 4b, and the linear fitting revealed the fitting error of less than 5%. Clearly, the photoactivity of pure ZnO is highly correlated to the illumination time. The rate constant (K) of the photodegradation process was also calculated using the first order reaction equation [42]:

Methylene Blue
The reaction kinetics was also studied as Ln(C o /C t ) over the illumination time, and is shown in Figure 4c. The rate constant for MB degradation using pure ZnO was found to be 1.14 × 10 −3 min, confirming that the reaction follows first-order kinetics.
4b,c. The reaction rate of MB degradation was evaluated by monitoring the change in concentration Ct over time from the initial concentration Co, as shown in Figure 4b, and the linear fitting revealed the fitting error of less than 5%. Clearly, the photoactivity of pure ZnO is highly correlated to the illumination time. The rate constant (K) of the photodegradation process was also calculated using the first order reaction equation [42]: The reaction kinetics was also studied as Ln(Co/Ct) over the illumination time, and is shown in Figure 4c. The rate constant for MB degradation using pure ZnO was found to be 1.14 × 10 −3 min, confirming that the reaction follows first-order kinetics.
The degradation of MB was also performed using the various samples, in order to verify how the degradation rate is affected by the composition of the hybrid material, and the spectra are shown in Figure 5a-d. The four panels show that all samples work better as photocatalysts than pure ZnO. Furthermore, comparing the decrease of the absorption spectrum over time in the four panels, it is clear that a higher amount of Mn3O4 in the hybrid material greatly improves the degradation of MB. This trend could be attributed to the role of Mn3O4 in separating the charges of the electron-hole pairs, resulting in the production of a high amount of oxidizing radicals that play an important role in driving the degradation of MB in aqueous solution. The reaction kinetics was investigated for the hybrid samples of Mn3O4@ZnO, and the ratio between the concentrations of MB at a certain time Ct to the initial concentration Co is shown in Figure 6a. The degradation of MB The degradation of MB was also performed using the various samples, in order to verify how the degradation rate is affected by the composition of the hybrid material, and the spectra are shown in Figure 5a-d. The four panels show that all samples work better as photocatalysts than pure ZnO. Furthermore, comparing the decrease of the absorption spectrum over time in the four panels, it is clear that a higher amount of Mn 3 O 4 in the hybrid material greatly improves the degradation of MB. This trend could be attributed to the role of Mn 3 O 4 in separating the charges of the electron-hole pairs, resulting in the production of a high amount of oxidizing radicals that play an important role in driving the degradation of MB in aqueous solution. The reaction kinetics was investigated for the hybrid samples of Mn 3 O 4 @ZnO, and the ratio between the concentrations of MB at a certain time C t to the initial concentration C o is shown in Figure 6a. The degradation of MB on hybrid materials clearly depends on the illumination time, and the linear fitting revealed a fitting error of less than 5%. The reaction kinetics in terms of order of reaction was studied by calculating Ln(C o /C t ) over time (Figure 6b), and it was found that the rate constant was characterized by first-order kinetics. The rate constant (K) values for samples 1, 2, 3 and 4 were estimated to be 3.33 × 10 −3 min, 4.19 × 10 −3 min, 7.33 × 10 −3 min, and 8.51 × 10 −3 min, respectively. These values of rate constant suggest that a higher amount of Mn 3 O 4 in the hybrid material significantly improves the degradation kinetics of MB in aqueous solution under UV light illumination. The degradation efficiency was calculated from the absorbance spectra and plotted over time for each hybrid system, in Figure 6c. The hybrid Mn 3 O 4 @ZnO system with the highest amount of Mn 3 O 4 (sample 4) was the most efficient, with a degradation efficiency of 94.59% for MB. The degradation efficiency of the other hybrid samples 1, 2, and 3 was 82.07%, 85.28%, and 92.91%, respectively. The degradation efficiency of pure ZnO photocatalyst was found to be 69.60%, as shown in Figure 6d, indicating that the performance of pure ZnO is not sufficient, and the photocatalytic properties of the proposed hybrid system are improved. from the absorbance spectra and plotted over time for each hybrid system, in Figure 6c. The hybrid Mn3O4@ZnO system with the highest amount of Mn3O4 (sample 4) was the most efficient, with a degradation efficiency of 94.59% for MB. The degradation efficiency of the other hybrid samples 1, 2, and 3 was 82.07%, 85.28%, and 92.91%, respectively. The degradation efficiency of pure ZnO photocatalyst was found to be 69.60%, as shown in Figure 6d, indicating that the performance of pure ZnO is not sufficient, and the photocatalytic properties of the proposed hybrid system are improved.

Methyl Orange
The photodegradation of methyl orange was investigated with a solution with a concentration of 2.44 × 10 −5 M and 5 mg of photocatalyst under UV light illumination. In the case of pure ZnO (Figure 7), the degradation was found to be very poor, due to the stable nature of the material, limited number of active sites, and the rapid rate of charge recombination. The absorption trend is very slow, which means that the reaction rate is associated with a high activation energy using pure ZnO as photocatalyst. Poor performance in the degradation of MO by pure ZnO indicates that it is not a suitable material for this application. The reaction kinetics was studied by evaluating the concentration of MO over time, C t , with respect to the initial concentration, C o , as shown in Figure 7a, and indicates that the linear fitting revealed a fitting error of less than 5%. The plot in Figure 7b shows that the degradation is linear, with respect to the irradiation time with UV light. The reaction kinetics with respect to UV light irradiation time turned out to be of the first order, and the rate constant was calculated to be 6.44 × 10 −4 min −1 , as shown in Figure 7c. The degradation efficiency for MO was calculated from the absorption spectra of MO over a time of 320 min, and the calculated value was 73.56%, as shown in Figure 7d. The four different hybrid samples were tested for the degradation of MO, using a solution with a concentration of 2.44 × 10 −5 M and 5 mg of each hybrid catalyst. The degradation experiments lasted 240 min, and an adsorption spectrum was acquired every 30 min, as shown in Figure 8. Comparing the plots in Figure 8 with that in Figure 7a, it is clear, albeit qualitatively, that the degradation is in all cases better than that of pure ZnO. This indicates that the presence of Mn 3 O 4 in the ZnO@Mn 3 O 4 hybrid material increases the density of active sites and decreases the charge recombination rate of electron-hole pairs. In fact, the degradation of MO increases as the quantity of Mn 3 O 4 present in the hybrid photocatalyst increases, i.e., from sample 1 to sample 4. The degradation kinetics of MO was also studied by evaluating how the MO concentration over time, C t , varies with respect to the initial concentration, C o , (Figure 9a), and the linear fitting revealed a fitting error of less than 5%. The Ln of the ratio is instead shown in Figure 9b. As can be seen in both panels, the degradation rate strongly depends on the irradiation time with UV light. The reaction kinetics of the four hybrid photocatalysts are of the first order, and the rate constant values obtained for samples 1, 2, 3 and 4 are respectively 2.32 × 10 −3 min, 2.50 × 10 −3 min, 2.74 × 10 −3 min and 4.58 × 10 −3 min. The percent degradation efficiency of samples 1, 2, 3 and 4 was calculated and found to be 82.78%, 83.38%, 85.12%, and 89.99%, respectively, as shown in Figure 9c. It is therefore clear that the degradation efficiency for MO increases as the quantity of Mn 3 O 4 in the hybrid material increases. The degradation efficiency for MO was calculated from the absorption spectra of MO over a time of 320 min, and the calculated value was 73.56%, as shown in Figure 9d.

Malachite green
Considering the high toxicity of MG to aquatic life and human health, pure ZnO and Mn3O4@ZnO hybrid materials were tested for the degradation of MG under UV light illumination. In the experiments, a solution of MG with a concentration of 5.48 × 10 −5 M and 5 mg of catalyst were used. Figure 10 shows the maximum absorption peak of MG at 617 nm and its evolution over time in the presence of pure ZnO. The adsorption peak decreases, confirming the catalytic activity; however, the degradation process is relatively slow, and the efficiency is not very high, due to the wide band gap of pure ZnO, its fast charge recombination rate and its low density of catalytic sites. The reaction rate of MG with pure ZnO was studied quantitatively by plotting the ratio between the concentration over time, Ct , and the initial concentration, Co, , and it is clear that the degradation of MG strongly depends on the time of irradiation with UV light (Figure 10b). Furthermore, plotting LnCo/Ct as in Figure 10c, it can be seen that the reaction kinetics are of the first order. The percent degradation efficiency was calculated, obtaining a value of 85.37%, as shown in Figure 10d. The performance of pure ZnO was therefore unsatisfactory, demonstrating that the material must be functionalized to efficiently degrade MG under

Malachite Green
Considering the high toxicity of MG to aquatic life and human health, pure ZnO and Mn 3 O 4 @ZnO hybrid materials were tested for the degradation of MG under UV light illumination. In the experiments, a solution of MG with a concentration of 5.48 × 10 −5 M and 5 mg of catalyst were used. Figure 10 shows the maximum absorption peak of MG at 617 nm and its evolution over time in the presence of pure ZnO. The adsorption peak decreases, confirming the catalytic activity; however, the degradation process is relatively slow, and the efficiency is not very high, due to the wide band gap of pure ZnO, its fast charge recombination rate and its low density of catalytic sites. The reaction rate of MG with pure ZnO was studied quantitatively by plotting the ratio between the concentration over time, C t , and the initial concentration, C o , and it is clear that the degradation of MG strongly depends on the time of irradiation with UV light (Figure 10b). Furthermore, plotting LnC o /C t as in Figure 10c, it can be seen that the reaction kinetics are of the first order. The percent degradation efficiency was calculated, obtaining a value of 85.37%, as shown in Figure 10d. The performance of pure ZnO was therefore unsatisfactory, demonstrating that the material must be functionalized to efficiently degrade MG under UV light illumination. In this way, the hybrid materials Mn3O4@ZnO were also tested, with their absorption spectra over time shown in Figure 11. All materials functionalized with Mn3O4 proved to be highly efficient for the degradation of MG, as can be seen from the strong decrease of the absorption peak at 617 nm. Note that the degradation experiment in this case only lasted 70 min. Comparing the four panels (a-d), it can be seen that a greater quantity of Mn3O4 increases degradation efficiency. The excellent performance of the hybrid materials can therefore be reasonably attributed to the functionalization with Mn3O4 , which acts as a co-catalyst and lowers the charge recombination rate of the electron-hole pairs. The synergy of the two materials allows a faster production of oxidizing radicals, which increases the degradation rate of MG in aqueous solution [38][39][40]. The synergy of high surface area due to the nanostructured nature of Mn3O4 and the higher exposure of catalytic sites from ZnO allowed the hybrid material to behave effectively for the degradation of various synthetic dyes [38][39][40].  In this way, the hybrid materials Mn 3 O 4 @ZnO were also tested, with their absorption spectra over time shown in Figure 11. All materials functionalized with Mn 3 O 4 proved to be highly efficient for the degradation of MG, as can be seen from the strong decrease of the absorption peak at 617 nm. Note that the degradation experiment in this case only lasted 70 min. Comparing the four panels (a-d), it can be seen that a greater quantity of Mn 3 O 4 increases degradation efficiency. The excellent performance of the hybrid materials can therefore be reasonably attributed to the functionalization with Mn 3 O 4 , which acts as a co-catalyst and lowers the charge recombination rate of the electron-hole pairs. The synergy of the two materials allows a faster production of oxidizing radicals, which increases the degradation rate of MG in aqueous solution [38][39][40]. The synergy of high surface area due to the nanostructured nature of Mn 3 O 4 and the higher exposure of catalytic sites from ZnO allowed the hybrid material to behave effectively for the degradation of various synthetic dyes [38][39][40].
The reaction rate was quantitatively assessed in terms of the change in MG concentration over time, C t , with respect to its initial concentration, C o , as shown in Figure 12a, and suggested a fitting error of less than 5%. The curves show that the degradation of MG in aqueous solution depends on the time of illumination with UV light. The kinetics of the degradation of MG is first order for all hybrid photocatalysts Mn 3 O 4 @ZnO, and the rate constants for samples 1, 2, 3 and 4 are calculated in 1.72 × 10 −2 min, 2.38 × 10 −2 min, 2.73 × 10 −2 min, and 3.01 × 10 −2 min, respectively, as shown in Figure 12b. The degradation efficiency for the four hybrid materials (samples 1, 2, 3 and 4) was calculated as 93.22%, 95.71%, 96.67%, and 97.40%, respectively, as shown in Figure 12c. The percent degradation efficiency of pure ZnO was calculated, and obtained a value of 85.37%, as shown in Figure 12d.
can therefore be reasonably attributed to the functionalization with Mn3O4 , which acts as a co-catalyst and lowers the charge recombination rate of the electron-hole pairs. The synergy of the two materials allows a faster production of oxidizing radicals, which increases the degradation rate of MG in aqueous solution [38][39][40]. The synergy of high surface area due to the nanostructured nature of Mn3O4 and the higher exposure of catalytic sites from ZnO allowed the hybrid material to behave effectively for the degradation of various synthetic dyes [38][39][40].  The reaction rate was quantitatively assessed in terms of the change in MG concentration over time, Ct ,with respect to its initial concentration, Co, , as shown in Figure  12a , and suggested a fitting error of less than 5%. The curves show that the degradation of MG in aqueous solution depends on the time of illumination with UV light. The kinetics of the degradation of MG is first order for all hybrid photocatalysts Mn3O4@ZnO, and the rate constants for samples 1, 2, 3 and 4 are calculated in 1.72 × 10 −2 min, 2.38 × 10 −2 min, 2.73 × 10 −2 min, and 3.01 × 10 −2 min, respectively, as shown in Figure 12b. The degradation efficiency for the four hybrid materials (samples 1, 2, 3 and 4) was calculated as 93.22%, 95.71%, 96.67%, and 97.40%, respectively, as shown in Figure 12c. The percent degradation efficiency of pure ZnO was calculated, and obtained a value of 85.37%, as shown in Figure  12d. Overall, the degradation measurements of MB, MO and MG dyes demonstrated that Mn3O4@ZnO hybrid nanomaterials are excellent photocatalysts. The photocatalytic activity of the hybrid material with the maximum amount of Mn3O4 was the best for all three dyes, reaching almost 100% for MG. The performance of the various Mn3O4@ZnO hybrid materials for the degradation of MB, MO and MG is summarized in Table 1. The activity of the Mn3O4@ZnO hybrid system towards MB, MO and MG was compared with the recently reported works, as given in Table 2, and it is obvious that the presented photocatalyst is a potential and alternative candidate to utilize within practical wastewater treatment. The advantageous features are its simplicity, significant efficiency, and multiple dye-degradation aspects. However, the understanding of improved performance has to be further characterized by different analytical techniques such as Xray photoelectron spectroscopy, high resolution transmission electron microscopy, and photoluminescence spectroscopy.    Table 2, and it is obvious that the presented photocatalyst is a potential and alternative candidate to utilize within practical wastewater treatment. The advantageous features are its simplicity, significant efficiency, and multiple dye-degradation aspects. However, the understanding of improved performance has to be further characterized by different analytical techniques such as X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and photoluminescence spectroscopy.

Conclusions
In this research work, we used sol gel and hydrothermal methods to synthesize different nanostructured materials based on pristine ZnO and functionalized with different amounts of Mn 3  showed excellent degradation properties for all dyes: 94.59%, 89.99% and 97.40% for MB, MO and MG, respectively. The degradation for all dyes followed first-order kinetics. The rate constant values for the best catalyst were 8.51 × 10 −3 min for MB, 4.58 × 10 −3 min for MO, and 3.01 × 10 −2 min for MG. The improved performance of hybrid materials can be attributed to the synergy between ZnO and Mn 3 O 4 , which leads to a high charge separation rate, a high density of catalytic sites, and the generation of a large amount of oxidizing reactive species. These improvements suggest that Mn 3 O 4 can be used as a dopant to increase the photocatalytic properties of other metal oxides such as TiO 2 , SnO 2 and carbon-based materials.