Preparation of Fe₂O₃/g-C₃N₄ Photocatalysts and the Degradation Mechanism of NOR in Water under Visible Light Irradiation

Abstract

Fe₂O₃/g-C₃N₄ nano-heterostructures for photocatalytic degradation of NOR (norfloxacin) were successfully prepared by combining co-precipitation and calcination methods. The g-C₃N₄, Fe₂O₃, and different composite ratios of Fe₂O₃/g-C₃N₄ (FeCNs) were characterized by XRD, SEM, XPS, UV-vis DRS, PL, and electrochemical tests, and the mechanism of photocatalytic degradation of NOR was analyzed. The results indicated that the semiconductor was attached to the surface of g-C₃N₄ in the form of α-Fe₂O₃ crystal with good crystalline structure. The composite of Fe₂O₃ with g-C₃N₄ increased the specific surface area of the material, effectively reduced the band gap, strengthened the photogenerated e⁻/h⁺ pair separation, and improved the photocatalytic performance of the composite. The photocatalytic degradation of NOR was consistent with the quasi-primary reaction kinetic model. Among them, FeCN-25wt% showed the optimal photocatalytic degradation of NOR (72.3%) with the largest degradation rate (k = 0.00900 min⁻¹). The Fe₂O₃/g-C₃N₄ composite structure is inferred to be a Z-type heterojunction.

1. Introduction

With the rapid development of the pharmaceutical industry, antibiotics have been widely detected in soil and surface water, which has caused many concerns. Norfloxacin (NOR) is widely used as a third-generation fluoroquinolone in the medical field [1]. Due to its difficult degradation, its continuous accumulation in aquatic ecosystems can lead to the formation and spread of bacterial resistance, threatening human health and ecological balance [2,3]. Therefore, there is an urgent need to develop a simple, efficient and low-cost method to degrade and convert NOR.

Photocatalytic oxidation technology has the advantages of being green, efficient, and low cost [4,5,6], and shows good application prospects in the treatment of antibiotics. In the selection of photocatalysts, graphitic carbon nitride (g-C₃N₄) has higher solar energy utilization than traditional TiO₂ and ZnO photocatalysts, and is chemically stable and easy to synthesize [7,8,9,10].

However, unmodified g-C₃N₄ has poor photogenerated carrier mobility, low photogenerated electron (e⁻)–hole (h⁺) pair separation efficiency, and weak photocatalytic ability of the material. Fe₂O₃ is a common semiconductor photocatalytic material with low cost, narrow band gap, and good stability [11,12]. Therefore, it is of great practical significance to couple g-C₃N₄ with other semiconductors to reduce carrier complexation.

In this paper, co-precipitation and calcination methods were combined to prepare Fe₂O₃/g-C₃N₄ nano-heterostructures for photocatalytic degradation of NOR. The g-C₃N₄, Fe₂O₃, and FeCNs were characterized by XRD, SEM, XPS, UV-vis DRS, PL, and electrochemical tests to further analyze the mechanism of photocatalytic degradation of NOR, which can provide a referable idea for the effective removal of antibiotics in water.

2. Materials and Methods

2.1. Materials

The chemical agents used in this article were all analytical grade, including norfloxacin (NOR), urea, dicyandiamide, melamine, ferrous sulfate heptahydrate, oxalate, etc. They were purchased from McLean Biochemical Technology Co., Ltd., in Shanghai, China. The reaction solvents in the experiments were deionized water.

2.2. Preparation of Photocatalyst

2.2.1. Preparation of g-C₃N₄

The urea, dicyandiamide, and melamine were ground and sieved, respectively, and then spread in the crucible to ensure that the agents were heated uniformly during the thermal condensation process, and then the crucible was capped and placed in a muffle furnace. The muffle furnace was programmed to increase the temperature at a rate of 2.5 °C/min for 220 min to 550 °C, and the temperature was maintained at this temperature for 4 h. The reaction products were cooled down to room temperature, and three kinds of g-C₃N₄ were obtained after grinding and sieving, labeled as CNU, CND, and CNM, respectively.

2.2.2. Preparation of Fe₂O₃/g-C₃N₄

Fe₂O₃/g-C₃N₄ was prepared by a combination of co-precipitation and calcination as follows: a certain concentration of oxalic acid solution and ferrous sulfate heptahydrate solution were configured, respectively. A certain amount of g-C₃N₄ (CNM) powder was dispersed in oxalic acid solution by ultrasonic dispersion for 2 h, to which ferrous sulfate heptahydrate solution was added dropwise at a rate of 1 mL/min, stirred magnetically for 2 h, and a yellow precipitate was obtained after 24 h of standing. The above precipitate was centrifuged at high speed, washed alternately with deionized water and anhydrous ethanol until the supernatant was neutral, dried, and laid flat in the crucible, and then placed in a muffle furnace after the crucible was covered. The program of muffle furnace was set as follows: the heating rate was 5 °C/min, the temperature was increased to 450 °C after 90 min, and kept for 2 h. After the reaction, the solid powder of Fe₂O₃/g-C₃N₄ was obtained by cooling to room temperature. The composite products were labeled as FeCN-20 wt%, FeCN-25 wt%, FeCN-30 wt%, FeCN-35 wt%, and FeCN-40 wt% depending on the content (mass fraction) of Fe₂O₃.

2.2.3. Preparation of Fe₂O₃

Ferrous sulfate heptahydrate and oxalic acid solutions were co-precipitated to produce ferrous oxalate precipitation, which was later centrifuged, washed, dried, and calcined to produce Fe₂O₃ red solid powder. In order to avoid the influence of other variables on the experimental results, the preparation conditions of this process were not different from those of Fe₂O₃/g-C₃N₄ except that g-C₃N₄ powder was not added.

2.3. Characterization of Photocatalyst

The crystal structure and composition of the samples were analyzed utilizing an X-ray diffractometer (XRD, Brooke D8 Advance, Bruker Corporation, Karlsruhe, Germany). Scanning electron microscope (SEM, SU8010, Hitachi, Tokyo, Japan) was used to characterize the surface morphology of the samples. The specific surface area and pore size distribution of the material were tested and analyzed using a fully automated physical static analyzer (Autosorb-IQ2-MP, Quantachrome Instruments, Beach, FL, USA). X-ray electron spectroscopy (XPS, D/max-2400, Rigaku, Tokyo, Japan) was conducted to analyze the elemental compositions and chemical valence states on the photocatalysts’ surface. The sample was tested using a UV-vis DRS (UV-2600, Shimadzu, Kyoto, Japan) to attain the maximum absorption of the material at 200–1200 nm. Photoluminescence (PL) of the samples was recorded on a fluorescence spectrometer (F-4600, Hitachi, Japan) with a xenon lamp excitation wavelength of 350 nm and a testing range of 375~420 nm. The photocurrent density and electrochemical impedance spectra (EIS) of the samples were obtained on an electrochemical workstation (Chenhua CHI 660E, Shanghai, China) to measure the electrical properties of the photocatalysts (the electrolyte is sodium sulfate).

2.4. Photocatalytic Performance Testing

The photocatalytic performance of the prepared photocatalysts for NOR under visible light irradiation was measured using a 250 W sodium lamp (590 nm) as a light source. During the experiments, 150 mg of photocatalysts was added to a 200 mL solution containing NOR (20 mg/L) and stirred in the dark for 30 min until an adsorption–desorption equilibrium was reached between the photocatalysts and NOR. Then, the sodium lamp was turned on, air was blown in, and the solution was exposed to light for 150 min. Samples were taken at 30 min intervals and filtered through a 0.45 μm organic filter membrane. The absorbance of the solution was measured at 278 nm to obtain the concentration of NOR.

3. Results

3.1. Characterization of Photocatalysts

The XRD analysis of the photocatalysts prepared in this study are presented in Figure 1. As can be seen from Figure 1a, CNU, CND, and CNM all display the characteristic diffraction peaks at 12.98° and 27.56°, which are analyzed against the g-C₃N₄ standard card (JCPDS No. 87-1526) and discover that the diffraction peak at 12.98° corresponds to the (100) crystallographic plane and the diffraction peak at 27.56° corresponds to the (002) crystallographic plane, indicating that the urea, dicyandiamide, and melamine calcination products are all g-C₃N₄ [13]. From the XRD characterization of the three g-C₃N₄, it is found that CNU, CND, and CNM did not undergo any significant shift of the characteristic diffraction peaks, but the intensity of the diffraction peaks is CND > CNM > CNU. The average diameters of CNU, CND, and CNM are calculated using the Scherrer equation [14], which are 35.2, 39.6, and 42.7 nm, respectively.

As can be seen from Figure 1b, Fe₂O₃ shows distinct characteristic diffraction peaks at 24.14°, 30.26°, 33.15°, 35.65°, 40.89°, 49.46°, 54.09°, 57.35°, 62.44°, and 63.99°. Comparative analysis by α-Fe₂O₃ standard card (JCPDS No. 33-0664) reveal that they correspond to (012), (104), (110), (113), (024), (116), (122), (214), and (300) crystal planes, which belong to the characteristic diffraction peaks of the α-crystalline Fe₂O₃, indicating that the synthesized Fe₂O₃ is α-Fe₂O₃.

Compared with the monomer of g-C₃N₄ and Fe₂O₃, the characteristic diffraction peaks of FeCNs do not display a significant shift and the crystal structure is relatively intact. However, different synthesis ratios can lead to significant differences in peak intensity between the composite material and the monomer.

Following, 0.15 g each of CNU, CND, and CNM in 200 mL of 20 mg/L NOR solution achieved adsorption equilibrium by dark adsorption for 30 min, followed by photocatalytic degradation for 150 min. The results are shown in Figure S1. After 150 min, the degradation rates of NOR by CNM, CND, and CNU are 49.1%, 45.3%, and 42.9%, respectively, while the blank control is only 20.4% (Figure S1a). From Figure S1b, the correlation coefficients R² of CNM, CND, and CNU are all greater than 0.9600, so the photocatalytic degradation of NOR conforms to the quasi-primary reaction kinetic model. The photocatalytic degradation rate of NOR by the three precursors is higher than that of the blank control, and it satisfies CNM > CND > CNU. This is because the condensation process of melamine to produce CNM is shorter than that of urea to produce CNU and melamine to produce CND, resulting in fewer intermediates and better degradation effect on NOR. Therefore, CNM was chosen as the precursor for subsequent experiments.

The SEM images of CNM, Fe₂O₃, and FeCN-25wt% are demonstrated in Figure 2. As can be seen from Figure 2a,b, CNM is a lamellar structure with an irregular surface, but it is difficult to observe its nanosheet structure due to the strong stacking effect of g-C₃N₄ [15]. α-Fe₂O₃ is a rod-like structure, which presents an elongated rectangular shape (Figure 2c,d). From Figure 2e,f, it can be seen that the FeCN-25wt% is loose and the rod-like Fe₂O₃ particles are distributed in the interlayer gaps and surface of the CNM, which increases the specific surface area of the composite.

The N₂ adsorption–desorption isotherms of CNM, Fe₂O₃, and FeCN-25wt% are displayed in Figure S2. In the low-pressure region, CNM, Fe₂O₃, and FeCN-25 wt% all exhibit low N₂ adsorption, mainly mesoporous adsorption. Moreover, the desorption curve is located above the adsorption curve and does not overlap, forming an IV type H3 hysteresis loop. In Table S1, the specific surface area of FeCN-25wt% (165.575 m²/g) is three times that of CNM (47.257 m²/g), which is consistent with the results of SEM testing.

Figure 3 illustrates the results of high-resolution XPS spectra of CNM, Fe₂O₃, and FeCN-25wt%. As can be seen in Figure 3a, C 1s can be categorized into three peaks with binding energies of 284.80 eV, 286.28 eV, and 288.42 eV, which correspond to graphitized carbon (C-C/C=C, C-N) and sp²-bonded carbon (N-C=N), respectively [16]. The N 1s spectra have three peaks at 398.75 eV, 400.14 eV, and 401.32 eV (Figure 3b), originating from the 3-s-triazine ring C-N=C, N-(C)₃, and N in C-N-H, respectively [16]. In Figure 3c, O 1s can be divided into two peaks with binding energies of 529.64 eV and 531.11 eV, stemming from the lattice O and surface -OH of the α-Fe₂O₃, respectively [17]. Figure 3d reveals the Fe 2p with two peaks at 710.38 eV and 723.62 eV, which both originate from the oscillations of Fe³⁺ [18,19].

Comparing CNM and Fe₂O₃, the N 1s, O 1s, and Fe 2p binding energies in FeCN-25wt% are significantly shifted, with positive shifts of 0.2 eV, 0.4 eV, and negative shifts of 0.2 eV, respectively. It is shown that after the composite, an interaction structure is formed between the two components in FeCN-25wt%, which promotes the formation of inter-semiconductor heterojunction, changes the electron–hole pair migration mode, and reduces the electron–hole complexation rate for the purpose of enhancing the catalytic performance.

The optical properties of the prepared CNM, Fe₂O₃, and FeCN-25wt% photocatalysts were analyzed using UV-vis DRS, and the results are shown in Figure 4a. The onset wavelengths of the absorption spectra of g-C₃N₄ and Fe₂O₃ are 464 nm and 632 nm, respectively, which are in agreement with the literature [20,21]. The CNM absorption boundary is 464 nm, Fe₂O₃ exhibits a broad and strong absorption in the UV-visible range of 200–800 nm, and the FeCN-25wt% absorption boundary is about 505 nm. It can be seen that the absorption boundary and response range of the photocatalytic materials have been significantly improved after compositing, which is conducive to the improvement of photocatalytic efficiency.

The energy band gap values of the three photocatalysts were obtained by calculations based on the Kubelka–Munk equation [22], as shown in Figure 4b. The band gap widths of CNM, Fe₂O₃, and FeCN-25wt% are 2.83 eV, 1.95 eV, and 2.74 eV, respectively. It can be seen that g-C₃N₄ combines with Fe₂O₃ to form a Fe₂O₃/g-C₃N₄ heterojunction, effectively reducing the band gap width and facilitating photo excitation, thereby improving photocatalytic performance.

Figure 5 refers to the PL spectra of CNM and FeCN-25wt%. Compared with CNM, the intensity of FeCN-25wt% is greatly weakened. By constructing the heterostructure between Fe₂O₃ and g-C₃N₄, not only the electrical conductivity of the catalyst is improved [23], but also the defects of g-C₃N₄ are regulated, which accelerate the separation and transfer of photogenerated charge carriers on the complexes and enhance the catalytic activity of the photocatalytic materials.

In order to evaluate the separation and complexation of electron–hole pairs and the migration ability of photogenerated carriers of the photocatalysts, transient photocurrent tests and EIS analyses were carried out for CNM, Fe₂O₃, and FeCN-25wt%, respectively, and the results are shown in Figure 6. Compared with CNM and Fe₂O₃, FeCN-25wt% shows relatively higher photocurrent intensity (Figure 6a), which indicates that the loading of Fe₂O₃ promotes charge separation, improves the charge release and transfer ability, and increases the efficiency of photogenerated electron–hole pair separation, and highlights the performance advantage of the material in the photocatalytic system. The impedance arc radius of FeCN-25wt% is smaller compared with that of CNM and Fe₂O₃ (Figure 6b), which indicates that the resistance of the composite material is minimized when the surface charge is transferred. Therefore, by constructing the heterojunction between Fe₂O₃ and CNM, the interfacial charge migration ability of the material is effectively enhanced, which effectively reduces the photocatalytic reaction initiation conditions to a certain extent, makes the reaction easier to occur, and improves the photocatalytic activity of the material.

3.2. Photocatalytic Performance

The photocatalytic degradation effect of CNM, Fe₂O₃, and FeCNs on NOR and the fitting curve of the quasi-first-order reaction kinetics model are depicted in Figure 7. In Figure 7a, compared with CNM and Fe₂O₃, FeCNs have significantly improved the removal rate of NOR. When the Fe₂O₃ loading is 20 wt%, 25 wt%, 30 wt%, 35 wt%, and 40 wt%, the removal rates of NOR after visible light irradiation for 150 min are 66.7%, 72.3%, 68.1%, 61.2% and 54.2%, respectively.

From Figure 7b, the correlation coefficients R² of CNM, Fe₂O₃, and FeCNs are all greater than 0.9600, so the photocatalytic degradation of NOR conforms to the quasi-primary reaction kinetic model. The reaction rate constants (k) for the photocatalytic degradation of NOR by FeCN-20wt%, FeCN-25wt%, FeCN-30wt%, FeCN-35wt%, and FeCN-40wt% are 0.00764 min⁻¹, 0.009 min⁻¹, 0.00828 min⁻¹, 0.00665 min⁻¹, and 0.00541 min⁻¹, while the photocatalytic degradation rates of NOR by blank control, CNM, and Fe₂O₃ are 0.00154 min⁻¹, 0.0045 min⁻¹, and 0.00276 min⁻¹. This indicates that FeCNs increase the photocatalytic degradation reaction rate. And among a series of photocatalytic materials with different composite ratios, FeCN-25wt% exhibits the best degradation effect on NOR. The degradation rate of FeCN-25wt% is 2.3684 times higher than that of CNM and 3.2609 times higher than that of Fe₂O₃. In summary, the composite of CNM and Fe₂O₃ effectively enhance the photocatalytic degradation performance.

3.3. Photocatalytic Mechanism Analysis

The main reactive groups for the photocatalytic degradation of NOR by FeCN-25wt% were explored by reactive group scavenging experiments. The results are shown in Figure S3. According to Figure 8, the photocatalytic degradation efficiency of the Blank can reach 75.34%. The photocatalytic degradation efficiency of the materials was reduced to 47.29%, 50.23%, 37.31%, and 17.62%, respectively, by adding ethylenediaminetetraacetic acid (EDTA) for h⁺ capture and removal, AgNO₃ for e⁻ capture and removal, isopropanol (IPA) for ·OH capture and removal, and benzoquinone (BQ) for ·O₂⁻ capture and removal. Therefore, the role of active groups in the photocatalytic degradation process is in the following order: ·O₂⁻ > ·OH> h⁺ > e⁻.

According to Figure S4, the valence band (VB) of g-C₃N₄ and Fe₂O₃ are 1.71 eV and 2.42 eV, respectively, which is consistent with reference [24]. Meanwhile, the band gap of g-C₃N₄ and Fe₂O₃ are 2.83 eV and 1.95 eV, respectively (Figure 4b). Therefore, the conduction bands (CB) of g-C₃N₄ and Fe₂O₃ are −1.12 eV and 0.47 eV, respectively. Based on the above analysis, we hypothesized a possible mechanism for the photocatalytic degradation of NOR by FeCN-25wt% under visible light conditions (Figure 8). Visible light excitation of Fe₂O₃ and g-C₃N₄ produces e⁻ leaps to form photogenerated e⁻/h⁺ pairs. When the excitation intensity is greater than the band gap, the e⁻ on the VB of Fe₂O₃ and g-C₃N₄ jump to the CB. In addition, the CB of Fe₂O₃ has a higher potential than that of the VB of g-C₃N₄, and the e⁻ of the former tend to move to the latter and complex with the h⁺, and this process improves the photogenerated carrier transfer between Fe₂O₃ and g-C₃N₄. The VB potential of Fe₂O₃ is more positive than the H₂O/OH potential, so h⁺ on the VB of Fe₂O₃ can oxidize H₂O to ·OH. Similarly, the CB potential of g-C₃N₄ is more negative than the O₂/·O₂⁻ potential, so e⁻ on the CB of g-C₃N₄ can reduce O₂ to ·O₂⁻. ·OH and ·O²⁻ conduct redox reactions with NOR to degrade it into small molecule organic compounds or finally complete the mineralization reaction to produce H₂O and CO₂. In summary, it is inferred that the heterojunction formed by the Fe₂O₃/g-C₃N₄ composite operates according to the Z-type heterojunction mechanism.

4. Conclusions

Compared with the CNM and Fe₂O₃, the composite FeCN-25wt% effectively reduces the band gap, broadens the response range, and improves the photogenerated e⁻/h⁺ pair separation efficiency. In addition, FeCN-25wt% displays the optimal photocatalytic degradation of NOR (72.3%) with the largest degradation rate (K = 0.00900 min⁻¹). The main active role in the catalytic degradation process is ·O₂⁻, and the Fe₂O₃/g-C₃N₄ composite structure is inferred to be a Z-type heterojunction. In conclusion, this paper provides referable ideas for the effective removal of antibiotics from water.