Sol–Gel Synthesis and Photocatalytic Activity of Graphene Oxide/ZnFe2O4-Based Composite Photocatalysts

ZnO (ZO), Fe₂O₃ (FO), and graphene oxide (GO)/ZO/FO/ZnFe₂O₄ (ZFO) composite photocatalysts have been synthesized successfully via a simple sol–gel method and low-temperature technology. The phase structure and microstructural analysis confirmed that the GO/ZO/FO/ZFO magnetic separation photocatalyst is composed of GO, hexagonal ZnO, rhombohedral Fe₂O₃, and spinel ZnFe₂O₄ without any other impurities. The GO/ZO/FO/ZFO composite photocatalysts have a high visible light optical absorption coefficient and photocatalytic activity for degrading dyes, refractory pollutants, and antibiotics. The degradation percentages of methyl orange, tetrabromobisphenol A, and oxytetracycline hydrochloride by the GO/ZO/FO/ZFO magnetic separation photocatalyst were 98% for 180 min, 99% for 150 min, and 85% for 180 min, respectively. The special synthesis path leads to the formation of a special heterojunction between GO, ZnO, Fe₂O_3, and ZnFe₂O₄, which does not change the optical band gap value of the main lattice Fe₂O₃, and enhances the surface defects of the GO/ZO/FO/ZFO magnetic separation photocatalyst, resulting in high charge carrier transfer and separation efficiency of the catalyst and then enhanced the photocatalytic activity of the GO/ZO/FO/ZFO magnetic separation photocatalyst.

Introduction

Environmental pollution has always been a coexisting problem with human development (Rasheed, 2022). Dyes, persistent organic pollutants, and antibiotics are all powerful environmental killers (Cheng et al., 2022; He et al., 2022; Li et al., 2022). The degradation of these pollutants has become a necessary means to protect the environment. As countries around the world attach importance to environmental pollution, different technological means have been developed to degrade these pollutants (He et al., 2019a; He et al., 2021a; Wang et al., 2021a; He et al., 2021b; Wang et al., 2022a; Zhao et al., 2022). These technologies mainly include the photocatalytic technology, thermal catalytic technology, piezoelectric catalytic technology, ultrasonic catalytic technology, electrocatalytic technology, biodegradation technology, and a variety of technologies combined together to degrade pollutants (Sadrameli, 2016; Xiong et al., 2018; He et al., 2019b; He et al., 2020; Cheng et al., 2021; Dadashzadeh et al., 2021). Among these technologies, photocatalysis is a semiconductor green technology driven by light energy, which can effectively degrade pollutants, and has attracted extensive attention from researchers all over the world (Selli et al., 2008). The key aspect of the semiconductor photocatalysis technology is to develop semiconductor materials that can respond to visible light and use sunlight efficiently (Ren et al., 2016). Therefore, the development of a semiconductor photocatalyst that can respond to visible light becomes the key to solving this problem.

The photocatalytic activity of photocatalyst also depends on its charge carrier transfer and separation efficiency (Tang et al., 2018; Chen et al., 2019a; Cao et al., 2019; Luo et al., 2019; Xiao et al., 2019; Lin et al., 2020; Tang et al., 2020; Wang et al., 2022b). Even if some semiconductor materials can respond to visible light, the transfer and separation efficiency of charge carriers is not high, and Fe₂O₃ is such a material (Kuang et al., 2017; Chong et al., 2021; Fu et al., 2021). In order to enhance the visible light response capability of semiconductor materials and the transfer and separation efficiency of charge carriers at the same time, the excellent properties of various semiconductor materials can be combined to form a new composite semiconductor photocatalyst (Chen et al., 2019b; Shanavas et al., 2019; Kormányos et al., 2020). ZnO (ZO)/Fe₂O₃(FO)/ZnFe₂O₄(ZFO) is considered to be such a composite that its visible light response is improved compared with any single component materials (Valenzuela et al., 2002; Karpova et al., 2013a; Karpova et al., 2013b). However, due to the lack of carriers for charge carrier transfer and separation among the three, the transfer and separation efficiency of the charge carrier is low. Graphene oxide (GO) is a common carrier of charge carrier transfer and transport and is often used to enhance the charge transfer and separation efficiency of semiconductor materials (Hosseini et al., 2019; Rahmani et al., 2020; Pei et al., 2021). Therefore, the development of GO/ZO/FO/ZFO composite photocatalysts and the study of their degradation of dyes, POPS, and antibiotics are expected to show high photocatalytic activity.

In this study, we proposed the synthesis of GO/ZO/FO/ZFO composite photocatalysts with different GO contents by a low-temperature sintering technique. The effect of GO content on the phase purity, microstructure, optical properties, and photocatalytic activity of the GO/ZO/FO/ZFO composite photocatalyst was systematically studied. Using methyl orange, tetrabromobisphenol A, and oxytetracycline hydrochloride as degradation materials, the photocatalytic activity of the GO/ZO/FO/ZFO composite photocatalyst was studied. Based on the energy band theory and experimental results, a reasonable photocatalytic mechanism of the GO/ZO/FO/ZFO composite photocatalyst is proposed.

Experimental Section

Synthesis of ZnO and Fe₂O₃

According to the stoichiometric ratio of ZnO and Fe₂O₃, zinc nitrate and ferric nitrate were dissolved in 25 ml deionized water, respectively. Solutions containing zinc nitrate and ferric nitrate are labeled as solutions A and B, respectively. After zinc nitrate and ferric nitrate were dissolved, 5 g of citric acid was added to both A and B solutions to make the citric acid react with Zn ions or Fe ions. Then, the A and B solutions were transferred to an oil bath at 300°C for 12 h to obtain a black gel. A and B gels were ground into fine powder in a mortar, and after grinding with urea for half an hour, they were transferred to the combustion network for spontaneous combustion for 2 h to obtain ZnO and Fe₂O₃ powders.

One-Step Low-Temperature Sintering Synthesis of Graphene Oxide/ZnO/Fe₂O₃/ZnFe₂O₄ Photocatalysts

Graphene oxide (GO) and ZnFe₂O₄ (ZFO) were purchased from Aladdin Reagent LTD. According to Guskos et al. (2010), ZO/FO/ZFO with the mass ratio of ZO: FO: ZFO = 20 wt%: 10 wt%: 70 wt% was obtained. Then, according to the GO mass ratios of 5 wt%, 10 wt%, 15 wt%, and 20 wt% to ZO/FO/ZFO, they are named GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4, respectively. Finally, the samples were placed in a box furnace and sintered at 200°C for 2 h to obtain the final sample. Figure 1 shows the preparation flowchart of GO/ZO/FO/ZFO.

FIGURE 1

FIGURE 1. Preparation of the flowchart of GO/ZO/FO/ZFO.

Material Characterization

The phase structures of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 were measured by means of X-ray powder diffraction (XRD) with Cu Kα radiation. The Fourier transform infrared (FTIR) spectrophotometer with Bruker IFS 66v/S was used to study the samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 in the wave-number range of 400–4000 cm⁻¹. The microstructures of GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 were investigated by field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ultraviolet-visible (UV-Vis) diffuse reflectance spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 were measured by using a UV1901 UV-Visible spectrophotometer.

Photocatalytic Experiments for the Degradation of Methyl Orange, Tetrabromobisphenol A, and Oxytetracycline Hydrochloride

The photocatalytic activity of GO/ZO/FO/ZFO composite photocatalysts was studied using methyl orange, tetrabromobisphenol A, and oxytetracycline hydrochloride as degradation materials. The light source, initial pollutant concentration, and catalyst concentration were xenon lamps with 500 W, 20 mg/L for methyl orange, 50 mg/L for tetrabromobisphenol A, and 1 g/L for oxytetracycline hydrochloride, respectively. Including adsorption experiments, samples were taken every 30 min, and each photocatalytic experiment was performed for 210 min. The removed solution was centrifuged, and its absorbance was measured by using a UV-Vis spectrophotometer. The photocatalytic degradation rate, first-order kinetic curve, and degradation percentage of the GO/ZO/FO/ZFO magnetic separation photocatalyst can be calculated based on the measured absorbance value.

Results and Discussion

X-Ray Powder Diffraction Analysis

The phase structure and purity of semiconductor materials are measured by using an X-ray powder diffraction (XRD) instrument to determine whether the synthesized product is the target product. Figure 2 shows the XRD patterns of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. The XRD pattern of sample GO (Figure 2A) shows the XRD characteristic peaks located at 10.905, 42.193, and 77.772° related to (111), (200), and (123), respectively, which is identical to the JCPDS No. 89-8489. Similarly, ZnO (Figure 2B) displays an XRD pattern identical to the hexagonal structural phase with the space group P63mc (186), and the XRD characteristic peaks at 31.736°, 34.378°, 36.214°, 47.483°, 56.534°, 62.775°, 66.302°, 67.886°, 69.007°, 72.462°, 76.865°, 81.268°, and 89.489° related to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202), (104), and (203) crystal plane ZnO corresponds to the JCPDS No. 76-0704. The sample of Fe₂O₃ (Figure 2C) exhibits several characteristic peaks located at 2θ of 24.149°, 33.158°, 35.631°, 39.283°, 40.862°, 43.508°, 49.462°, 54.069°, 56.159°, 57.448°, 57.597°, 62.436°, 64.000°, 66.031°, 69.595°, 71.947°, 72.281°, 75.191°, 75.455°, 77.738°, 78.776°, 79.486°, 80.582°, 80.709°, 82.953°, 84.485°, 84.934°, and 88.557°, corresponding to the (012), (104), (110), (006), (113), (202), (024), (116), (211), (122), (018), (214), (300), (125), (208), (1010), (119), (217), (220), (036), (223), (131), (312), (128), (0210), (0012), (134), and (226) crystal surfaces of rhombohedral structural Fe₂O₃ with the JCPDS No. 87-1164. When GO, ZnO, Fe₂O₃ and ZnFe₂O₄ are combined together, ZnO, Fe₂O₃ and ZnFe₂O₄ peaks appear in GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4, as shown in Figure 2D. However, the diffraction peak of GO is difficult to observe. As the GO content increased to 20%, only a weak diffraction peak was observed at 10.905° of the sample GO/ZO/FO/ZFO4, further confirming that GO exists in the sample GO/ZO/FO/ZFO4.

FIGURE 2

FIGURE 2. XRD patterns of samples (A) GO, (B) ZnO, (C) Fe₂O₃, (D) GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4.

Fourier Transform Infrared Analysis

In order to further study the phase structure and purity of semiconductor materials, FTIR was performed. Figure 3 shows the FTIR spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. For all samples, the peaks at 3453 and 1616 cm⁻¹ can be ascribed to the absorbed water due to the use of KBr in the tablet pressing process (Gao et al., 2021; Li et al., 2021; Liu et al., 2022). For the sample GO, the peaks at 1731 and 1065 cm⁻¹ can be assigned to the graphene oxide (Dat et al., 2022; Hwa et al., 2022). For ZnO, the peaks at 1113, 545, and 418 cm⁻¹ are attributed to the characteristic peak of C-O and the Zn–O and Zn-O-Zn vibrations, respectively (Prabhu et al., 2018; Rammah et al., 2020; Wang et al., 2021b; Nadeem et al., 2021; Wang et al., 2022c; Tang et al., 2022). For Fe₂O₃, the peaks at 550 and 467 cm⁻¹ are assigned to the Fe-O and Fe-O-Fe vibrations, respectively (Lv et al., 2010; Ratep and Kashif, 2021). For the samples GO/ZO/FO/ZFO, the intensity of the characteristic peak of GO increases with the increase in the GO content, and the characteristic peak of Fe₂O₃ is slightly shifted, especially due to the influence of the Fe-O functional group in ZnFe₂O₄ (can be written as ZnO•Fe₂O₃), and the heterojunction between GO, ZnO, Fe₂O₃, and ZnFe₂O₄ is formed.

FIGURE 3

FIGURE 3. FTIR spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4.

XPS Analysis

Figure 4A shows the XPS survey spectrum of the sample GO/ZO/FO/ZFO3. The survey spectra of the samples GO/ZO/FO/ZFO3 exhibit sharp peaks of C, Zn, Fe, and O elements, and no other elements are observed, indicating that the sample GO/ZO/FO/ZFO3 does not contain any impurities. Figure 4B shows the high-resolution XPS spectrum of Zn 2p for the sample GO/ZO/FO/ZFO3. The two bands of Zn 2p1/2 and 2p3/2 were recorded at 1044.61 and 1021.42 eV, respectively, in ZnFe₂O₄, while the peaks at 1024.74 and 1047.77 eV can be ascribed to Zn 2p1/2 and 2p3/2 in ZnO, respectively. The Fe 2p spectrum showed obvious peaks at 711.83 (Fe 2p3/2) and 727.24 (Fe 2p1/2) eV which may be attributed to Fe₂O₃, and the peaks at 714.03 (Fe 2p3/2) and 733.85 (Fe 2p1/2) eV can be assigned to ZnFe₂O₄, as shown in Figure 4C. The O 1s spectrum of the samples GO/ZO/FO/ZFO3 is displayed in Figure 4D. The O1s peak could be convoluted into five peaks at 535.75, 533.51, 531.65, 530.70, and 530.21 eV, respectively. These peaks can be ascribed to C-O, the lattice oxygen of ZnO (O-Zn-ZO), the lattice oxygen of ZnFe₂O₄ (O-Zn-ZFO), the lattice oxygen of Fe₂O₃ (O-Zn-FO), and the lattice oxygen of ZnFe₂O₄ (O-Fe-ZFO), respectively. Figure 4E shows the high-resolution XPS spectra of C1s for the sample GO/ZO/FO/ZFO3. Three peaks at 287.72, 285.32, and 283.49 eV can be assigned C-H, C-O, and C1s for the XPS calibration peak, respectively. The results further confirmed the successful synthesis of the GO/ZO/FO/ZFO photocatalyst by low-temperature sintering technology.

FIGURE 4

FIGURE 4. (A) XPS survey spectrum and high-resolution XPS spectra of (B) Zn 2p, (C) Fe 2p, (D) O 1s, and (E) C 1s for the sample GO/ZO/FO/ZFO3.

Microstructural Analysis

The microstructure of semiconductor material has a great influence on its photocatalytic activity. Figures 5A–D show the SEM images of samples GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. When the GO content is 5%, the samples are mainly composed of fine nanoparticles, with lamellar particles faintly visible, as shown in Figure 5A. When the content of GO increased to 10%, the agglomeration of fine particles became obvious, and the lamellar structure gradually increased, as shown in Figure 5B. When the content of GO further increased to 15%, fine nanoparticles gradually deposited on GO, and the lamellar structure became more obvious, as shown in Figure 5C. With the GO content reaching 20%, lamellar structures appear in the samples in large quantities, as shown in Figure 5D. Figures 5E,F show the TEM and HRTEM images of the sample GO/ZO/FO/ZFO3, respectively. It can be seen from Figure 5E that fine nanoparticles are deposited on the lamellar GO, which is consistent with the SEM observation result. The lattice planes of ZnO, Fe₂O_3, and ZnFe₂O₄ were (100) and (101) with a lattice space of 0.2827 and 0.2486 nm, respectively, (104) and (110) with a lattice space of 0.2699 and 0.2517 nm, respectively, and (311) with a lattice space of 0.2536 nm, respectively. The results further confirmed that the GO/ZO/FO/ZFO3 samples contained GO, ZnO, Fe₂O₃, and ZnFe₂O₄.

FIGURE 5

FIGURE 5. SEM images of samples (A) GO/ZO/FO/ZFO1, (B) GO/ZO/FO/ZFO2, (C) GO/ZO/FO/ZFO3, and (D) GO/ZO/FO/ZFO4. (E) TEM and (F) HRTEM images of the sample GO/ZO/FO/ZFO3.

Optical Properties

The optical properties can be used to determine whether the semiconductor material has a high optical absorption coefficient and further verify whether it has high photocatalytic activity. Figure 6A shows the UV-Vis diffuse reflectance spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. Fe₂O₃ has the lowest reflectivity, and ZnO has the highest reflectivity in the wavelength range of 400–800 nm, while GO tends to be almost constant in this range. When GO, ZnO, Fe₂O₃, and ZnFe₂O₄ are coupled, the reflectivity in the visible light range changes significantly compared with ZnO and is greatly improved. After 850 nm, the reflectivity of GO/ZO/FO/ZFO samples increases sharply with the increase in wavelength.

FIGURE 6

FIGURE 6. (A) UV-Vis diffuse reflectance spectra and (B) UV-Vis absorption spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. First-order differential curves of UV-Vis diffuse reflectance spectra of samples (C) GO, (D) ZnO, (E) Fe₂O₃, (F) GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4.

According to Kubelka–Munk's (K–M) formula, the UV-Vis diffuse reflectance spectrum can be transformed into the UV-Vis absorption spectrum.

$F\left(R\right)=\frac{\alpha }{S}=\frac{{\left(1-R_{\infty }\right)}^2}{2R}(1)$

where R is the reflectance of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4, α is the optical absorption coefficient, and S is the scattering coefficient of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. Figure 6B shows the UV-Vis absorption spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. ZnO exhibits a high UV absorption coefficient, indicating that it has high UV photocatalytic activity. Fe₂O₃ has the highest light absorption coefficient in the wavelength range of 190–900 nm, but its photocatalytic activity is not high because of its high charge carrier recombination rate. When GO, ZnO, Fe₂O₃ and ZnFe₂O₄ are coupled to form heterojunctions, the absorption coefficient of the GO/ZO/FO/ZFO sample is greatly improved in the visible light range compared with ZnO, indicating that GO/ZO/FO/ZFO samples have high visible-light photocatalytic activity.

The first-order differential curve is calculated based on the UV-vis diffuse reflectance spectrum. The peak of the curve can be used to obtain the optical band gap (Eg) value of the semiconductor material (Gao et al., 2019; Gao et al., 2022a).

$E_{\text{g}}\left(\text{e}\text{V}\right)=\frac{hc}{{\lambda }_0\left(\mathrm{nm}\right)}\approx \frac{1240}{{\lambda }_0\left(\mathrm{nm}\right)}(2)$

where λ₀, h, and c are the peak of the first-order differential curve, the Plank constant, and the velocity of light, respectively. Figures 6C–F show the first-order differential curves of UV-Vis diffuse reflectance spectra of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4. The Eg values of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 are 1.44, 3.18, 2.12, 2.11, 2.11, 2.11, and 2.11 eV, respectively. As can be seen from Figure 5F, when GO, ZnO, Fe₂O₃ and ZnFe₂O₄ are coupled, the Eg value does not change at all. According to Xue et al. (2019), Kelaidis et al. (2020), Wang et al. (2021c), Gao et al. (2022b), and Gao et al. (2022c), only defects or vacancies can be introduced, and their Eg values will not be changed after a variety of semiconductor photocatalysts are coupled to form heterojunctions. This result is consistent with the literature reports.

Photocatalytic Activity for the Degradation of Dyes

The photocatalytic activity of the sample GO/ZO/FO/ZFO was studied with dyes, pollutants, and antibiotics as target degradation materials. Figure 7A shows the time-dependent degradation of methyl orange in the presence of the sample GO/ZO/FO/ZFO3 under simulated solar irradiation. Before the photocatalytic degradation experiment, methyl orange was adsorbed for half an hour to exclude the influence of adsorption on photocatalytic activity. After half an hour of adsorption, the adsorption of methyl orange by GO, ZnO, Fe₂O_3, and GO/ZO/FO/ZFO is small, less than 10%. The degradation rate of all samples increased with the increase in illumination time. Under visible light irradiation, the photocatalytic degradation percentage of GO, ZnO, and Fe₂O₃ is less than 30%. The sample GO/ZO/FO/ZFO3 showed the highest photocatalytic activity, and the degradation percentage reached 98% after 3 h of degradation.

FIGURE 7

FIGURE 7. (A) Time-dependent characteristic, (B) plots of ln (A_t/A₀) vs. irradiation time (t), (C) first-order kinetic constant (k), and (D) degradation percentage of degradation of methyl orange in the presence of samples GO, ZnO, Fe₂O₃, and GO/ZO/FO/ZFO under simulated solar irradiation.

The first-order kinetic curve can directly reflect the photocatalytic activity of semiconductor materials. The first-order dynamics equation can be described as follows:

$\mathrm{In}\left({\text{A}}_{\text{t}}/{\text{A}}_0\right)=-\mathrm{Kt},(3)$

where A_t is the absorbance of the pollutant at time t, A₀ is the absorbance of the pollutant at the initial time, k is the first-order kinetic constant, and t is the irradiation time. Figure 7B shows the plots of ln (A_t/A₀) vs. irradiation time of degradation of methyl orange in the presence of the sample GO/ZO/FO/ZFO3 under simulated solar irradiation. ln (A_t/A₀) for samples GO, ZnO, Fe₂O_3, and GO/ZO/FO/ZFO shows a good linear relationship with the irradiation time. Figure 7C shows the k values of degradation of methyl orange in the presence of samples GO, ZnO, Fe₂O₃, and GO/ZO/FO/ZFO under simulated solar irradiation. The k values of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 are 0.00299, 0.00179, 0.00251, 0.00580, 0.00743, 0.02072, and 0.00550 min⁻¹, respectively. The photocatalytic degradation rate of GO/ZO/FO/ZFO3 is 11.58 times that of ZnO. Figure 7D shows the degradation percentage of degradation of methyl orange in the presence of samples GO, ZnO, Fe₂O₃, and GO/ZO/FO/ZFO under simulated solar irradiation. The degradation percentages of samples GO, ZnO, Fe₂O₃, GO/ZO/FO/ZFO1, GO/ZO/FO/ZFO2, GO/ZO/FO/ZFO3, and GO/ZO/FO/ZFO4 are 43%, 30%, 38%, 65%, 75%, 98%, and 62%, respectively. With the increase in the GO content, the photocatalytic velocity of the GO/ZO/FO/ZFO photocatalyst first increases and then decreases. The results further confirmed that the sample GO/ZO/FO/ZFO3 had the highest photocatalytic activity.

Stability and Trapping Experiments

Figure 8A shows the emission spectra (λex = 300 nm) of the GO/ZO/FO/ZFO photocatalyst. The GO/ZO/FO/ZFO photocatalyst showed a narrow fluorescence emission peak at 485 nm, which may be caused by GO (Iliut et al., 2013; El-Hnayn et al., 2020). The maximum emission intensity at 485 nm of the GO/ZO/FO/ZFO photocatalyst is shown in Figure 8B. Compared with the photocatalytic activity, the emission intensity decreased with the increase in the photocatalytic activity. It can be seen from the fluorescence emission spectrum that the photocatalyst exhibits poor transfer and separation rates of photogenerated carriers with higher emission intensity. To study the stability of the GO/ZO/FO/ZFO photocatalyst, Figure 8C shows the stability experiments of the GO/ZO/FO/ZFO3 photocatalyst. After five cycles, the photocatalytic activity of the GO/ZO/FO/ZFO3 photocatalyst decreased by less than 6%, indicating that the GO/ZO/FO/ZFO3 photocatalyst has high stability and recycling ability. The trapping experiments of the GO/ZO/FO/ZFO3 photocatalyst are shown in Figure 8D. Trapping agents include the disodium ethylenediamine tetraacetic acid (EDTA-2Na), 2-propanol (IPA), and 1,4-benzoquinone (BQ), which have been used to detect the hole (h_VB⁺), hydroxyl radical (•OH), and superoxide radical (•O₂⁻), respectively. During each photocatalytic experiment, the amount of the trapping agent added to the reaction solution was 1 mmol/L. When EDTA-2Na, IPA, and BQ were added to the reaction solution, the photocatalytic activity of the GO/ZO/FO/ZFO3 photocatalyst decreased to 32%, 20%, and 11%, respectively. The results showed that holes, hydroxyl radicals, and superoxide radicals play important roles in the photocatalytic reaction. This will provide strong evidence for the subsequent analysis of the photocatalytic mechanism.

FIGURE 8

FIGURE 8. (A) Emission spectra (λex = 300 nm) of the GO/ZO/FO/ZFO photocatalyst. (B) Maximum emission intensity at 485 nm of the GO/ZO/FO/ZFO photocatalyst. The (C) stability and (D) trapping experiments of the GO/ZO/FO/ZFO3 photocatalyst.

Photocatalytic Activity for the Degradation of Pollutants and Antibiotics

Figures 9A,B show the time-dependent degradation of tetrabromobisphenol A and oxytetracycline hydrochloride in the presence of the sample GO/ZO/FO/ZFO3 under simulated solar irradiation, respectively. The degradation percentage of GO/ZO/FO/ZFO3 to tetrabromobisphenol A reached 99% after 150 min and that of oxytetracycline hydrochloride reached 85% after 180 min. The results showed that the sample GO/ZO/FO/ZFO3 had a high photocatalytic activity for the degradation of refractory pollutants and antibiotics. Figures 9C,D show the plots of ln (A_t/A₀) vs. irradiation time (t) for the degradation of tetrabromobisphenol A and oxytetracycline hydrochloride in the presence of the sample GO/ZO/FO/ZFO3, respectively. The k-values of the sample GO/ZO/FO/ZFO3 for the degradation of tetrabromobisphenol A and oxytetracycline hydrochloride are 0.02687 and 0.01022 min⁻¹, respectively. The results showed that the sample GO/ZO/FO/ZFO3 had the highest photocatalytic activity for the degradation of tetrabrombisphenol A.

FIGURE 9

FIGURE 9. Time-dependent degradation of (A) tetrabromobisphenol A and (B) oxytetracycline hydrochloride in the presence of the sample GO/ZO/FO/ZFO3 under simulated solar irradiation. Plots of ln (A_t/A₀) vs. irradiation time (t) for the degradation of (C) tetrabromobisphenol A and (D) oxytetracycline hydrochloride in the presence of the sample GO/ZO/FO/ZFO3.

Photocatalytic Mechanism

In order to gain insight into the photocatalytic mechanism of GO/ZO/FO/ZFO, the conduction band potential (E_CB) and valence band (E_VB) potential of GO, ZnO, Fe₂O₃ and ZnFe₂O₄ were calculated based on the band theory. Table 1 shows the conduction band, valence band potential, electronegativity, and Eg values of GO, ZnO, Fe₂O₃, and ZnFe₂O₄.

$E_{CB}=X-E^{\text{e}}-0.5E_{\text{g}}(4)$

$E_{\mathrm{VB}}=E_{\mathrm{CB}}+E_{\text{g}}(5)$

where the Eg values of GO, ZnO,Fe₂O_3, and ZnFe₂O₄ are 1.44, 3.18, 2.12, and 1.71 eV (Wang et al., 2019), respectively. E^e is 4.5 eV. The absolute electronegativity (X) values of GO, ZnO, Fe₂O_3, and ZnFe₂O₄ are 6.91, 5.79, 5.89, and 5.86 eV, respectively, and can be evaluated by Eqs. 4, 5.

$X\left(C_4{O}_{2-x}{\left(OH\right)}_{2x}\left(x=0.1\right)\right)=\sqrt[6.3]{X^4\left(C\right)\ast X^{2.1}\left(\text{O}\right)\ast X^{0.2}\left(\text{H}\right)}(6)$

$X\left(\mathrm{ZnO}\right)=\sqrt[2]{X\left(Zn\right)\ast X\left(O\right)}(7)$

$X\left(\text{F}{\text{e}}_2{O}_3\right)=\sqrt[5]{X^2\left(Fe\right)\ast X^3\left(O\right)}(8)$

$X\left(\mathrm{Zn}\text{F}{\text{e}}_2{O}_4\right)=\sqrt[7]{X\left(Zn\right)\ast X^2\left(Fe\right)\ast X^4\left(O\right)}(9)$

where X (C) = 6.27 eV, X (H) = 7.18 eV, X (Zn) = 4.45 eV, X (Fe) = 4.06 eV, and X (O) = 7.54 eV.

TABLE 1

TABLE 1. Conduction band, valence band potential, electronegativity, and Eg values of GO, ZnO and Fe₂O₃, and ZnFe₂O₄.

Based on the aforementioned calculations, the energy level diagram of GO/ZO/FO/ZFO is shown in Figure 10. As can be seen from the figure, GO acts as a carrier of charge transfer and transmission between ZnO, Fe₂O₃, and ZnFe₂O₄. When light is illuminated on the surface of the GO/ZO/FO/ZFO composite, the electrons in the respective valence band undergo transition to the conduction band. Since the conduction band of GO is more negative than ZnO, Fe₂O₃ and ZnFe₂O₄, the electrons will relax to the conduction band of GO. The valence bands of GO are more positive than those of ZnO, Fe₂O_3, and ZnFe₂O₄, and the valence band holes will undergo transition to the valence bands of ZnO, Fe₂O₃, and ZnFe₂O₄. Such relaxation and transition will promote the separation of electron and hole pairs of the whole complex, resulting in a large increase in its photocatalytic activity. The conduction electrons will react with oxygen in the reactant solution to form superoxide radicals and with O₂/2H⁺ to form hydroxyl radicals. Valence band holes will react with H₂O/OH⁻ to form hydroxyl radicals. Subsequently, hydroxyl radicals or superoxide radicals react with pollutants to form non-toxic and harmless small organic molecules. Meanwhile, the valence band holes may react directly with pollutants to form non-toxic and harmless small molecular organics. The related chemical reactions can be expressed as follows:

$\mathrm{GO}/\mathrm{ZO}/\mathrm{FO}/\mathrm{ZFO}+\mathrm{hv}\to {\text{e}}^{-}+{\text{h}}^{+}(10)$

$h^{+}+\text{O}{\text{H}}^{-}\to ·\mathrm{OH}(11)$

$e^{-}+{\text{O}}_2\to ·{\text{O}}^{2-}(12)$

$·{\text{O}}^{2-}+2{\text{H}}^{+}+e^{-}\to {\text{H}}_2{\text{O}}_2(13)$

$2e^{-}+{\text{O}}_2+2{\text{H}}^{+}\to {\text{H}}_2{\text{O}}_2(14)$

$e^{-}+{\text{H}}_2{\text{O}}_2\to ·\mathrm{OH}+\text{O}{\text{H}}^{-}(15)$

$·{\text{O}}^{2-}+{\text{H}}_2{\text{O}}_2\to ·\mathrm{OH}+\text{O}{\text{H}}^{-}+{\text{O}}_2(16)$

$·\mathrm{OH}/·{\text{O}}^{2-}+\mathrm{dye}/\mathrm{Pollu}\mathrm{tant}/\mathrm{antibiotics}\to \mathrm{degradation}\mathrm{\hspace{0.17em}products}(17)$

${\text{h}}^{+}+\mathrm{dye}/\mathrm{pollu}\mathrm{tant}/\mathrm{antibiotics}\to \mathrm{degradation}\mathrm{\hspace{0.17em}products}(18)$

FIGURE 10

FIGURE 10. Photocatalytic mechanism of GO/ZO/FO/ZFO.

It can be seen that the holes, hydroxyl radicals, and superoxide radicals play important roles in the whole photocatalytic process.

Conclusion

GO/ZO/FO/ZFO composite photocatalysts with different GO contents were synthesized by a low-temperature sintering technique. The effects of the GO content on the phase purity, functional groups, microstructure, optical properties, and photocatalytic activity of GO/ZO/FO/ZFO composite photocatalysts were studied in detail. XRD, FTIR, and microstructural analysis confirmed that the GO/ZO/FO/ZFO magnetic separation photocatalyst contains four components, GO, ZnO, Fe₂O_3, and ZnFe₂O₄, without any other impurities. The optical property analysis shows that the GO/ZO/FO/ZFO magnetic separation photocatalyst has a high optical absorption coefficient, suggesting that it has a high visible-light photocatalytic activity, and the quadruple recombination does not change the Eg value of the main lattice of Fe₂O₃. The photocatalytic experiments confirmed that the GO/ZO/FO/ZFO magnetic separation photocatalyst had high photocatalytic activity for dyes, refractory pollutants, and antibiotics, and the degradation percentages were 98% for 180 min, 99% for 150 min, and 85% for 180 min, respectively. The special heterojunction structure promotes the transfer and separation of electrons and holes in the GO/ZO/FO/ZFO magnetic separation photocatalyst, showing high photocatalytic activity under the combined action of holes, hydroxyl radicals, and superoxide radicals.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Funding

This work was supported by the Core Teacher Cultivating Program of the Yellow River Conservancy Technical Institute (No. 2020HYGG04).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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