Magnetic photocatalyst Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄: Facile synthesis and its photocatalytic activity

2.1 Experimental materials

Chengdu Kelong Chemical Co., Ltd. (Chengdu, China) purchased Bi(NO₃)₃·5H₂O, NaCl, NaOH, ZnSO₄·7H₂O, MnSO₄·H₂O, Fe₂(SO₄)₃, and C₂H₆O₂. It is noteworthy that all chemical reagents were of analytical grade and can be used directly without any purification.

2.2 Preparation of Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄

Herein, by the application of hydrothermal and roasting methods, bismuth oxychloride/manganese and zinc ferrite (Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄) composite photocatalysts were synthesized by preparing Bi(NO₃)₃·5H₂O and NaCl as raw materials and preparing Mn_xZn_1-xFe₂O₄ as an additive. It was synthesized by the following procedures presented in Fig. 1. First, a certain amount of ZnSO₄·7H₂O, MnSO₄·H₂O, and Fe₂(SO₄)₃ mixed solution was dissolved in distilled water and stirred ultrasonically for 10 min. Then 2 mol/L sodium hydroxide solution was added dropwise. The pH of the solution was adjusted to 13 and stirred for 10 min. Then the mixed solution was placed in a high-pressure reactor at 200 °C for 5 h. After washing and drying, the Mn_xZn_1-xFe₂O₄ magnetic matrix was obtained.Then, 20 mL of ethylene glycol, 0.97 g of Bi(NO₃)₃·5H₂O were mixed into a beaker and sonicated for 10 min. Mn_xZn_1-xFe₂O₄ accounting for 5 %∼25 % of the total mass were added into the levitated solution and mechanically stirred for 10 min. After that, 50 mL of NaCl (0.0134 mol/L) was added dropwise to the levitated solution. After stirred for 10 min again, the mixture was calcined at 160 °C for 12 h to obtain the intermediate product in the reaction at room temperature. The specimens were scrubbed with anhydrous ethanol and aqua destillata many times and desiccated at 75 °C for 12 h. The samples were ground into 100 mL ceramic crucible, continuously roasted at 500 °C for 2 h and ground to obtain Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ at room temperature. In addition, 5 % ∼ 25 % Mn_xZn_1-xFe₂O₄/Bi₃O₄Cl were named MB-5, MB-10, MB-15, MB-20, and MB-25, respectively.

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Fig. 1

Preparation flow of Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄.

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2.3 Characterization

The phase structure and crystallographic structure of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst were obtained by X-ray diffraction (Shimadzu XRD 6000, Japan) with monochromated high-intensity Cu Ka radiation. The surface appearance of the composites was characterized by SEM (S4800, Japan). Through TEM (Tecnai G2 F20, USA), the appearance and microstructure of the samples were further displayed. A specific surface and aperture analyser (Quadrasorb 2 MP, USA), which was applied to analyse the composites, exhibited the specific surface area (BET). In addition, UV–vis diffuse reflectance spectra (DRS) were acquired on a Shimadzu UV-2600 UV–Vis spectrophotometer (TU-1901, China). With excitation at a wavelength of 380 nm, photoluminescence (PL) spectra of the powder samples were recorded with a fluorescence spectrophotometer (F-4700, Japan). Furthermore, electrochemical impedance spectroscopy (EIS, CHI660E, CHI Shanghai, Inc.) were utilized to confirm the separation efficiency and interfacial charge transfer of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst.

2.4 Photocatalytic evaluation

RhB was applied as a simulated dye wastewater, which evaluated the degradation activity of the photocatalytic composite under irradiation with visible light. The simulated pollutant (10 mg/L) was prepared with the addition of 100 mg photocatalyst and placed into darkness for 30 min with stirring to achieve adsorption equilibrium. At ten-minute intervals, the absorbance was measured during the photodegradation process under a xenon lamp (CEL-HXF300-T3, China). The photodegradation process could be presented via a pseudofirst-order reaction under low RhB conditions, which follows the formula,

3.1 Photocatalytic activity

Photocatalytic performance of the photocatalyst was measured under simulated solar light irradiation, as illustrated in Fig. 2(a). In 100 min, the degradation rates of MB-5, MB-10, MB-15, MB-20, and MB-25 for RhB were 99.6 %, 99.5 %, 98.9 %, 93.3 % and 78.5 %, respectively. The kinetic model analysis confirmed that the degradation of RhB by the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite magnetic photocatalyst met the first-order reaction kinetics model. MB-5 exhibited perfect photocatalytic degradation of RhB with 0.0195 min⁻¹. MB-10 displayed weaker catalytic degradation but outperformed MB-5 in magnetic recovery, which was chosen as the best candidate to be test analysed.

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Fig. 2

(a) Photocatalytic degradation; (b) kinetic linear simulation curves of RhB over Bi₃O₄Cl and Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄; (c) values of reaction rate constants.

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3.2 Structure characteristics

The crystalline formation of the composites could be investigated via X-ray diffraction (XRD) patterns shown in Fig. 3. The different peaks of Bi₃O₄Cl and Mn_xZn_1-xFe₂O₄ were indexed to monoclinic Bi₃O₄Cl (JCPDS No.: 36-0760) and cubic Mn_xZn_1-xFe₂O₄ (JCPDS No.: 74-2399), respectively. The diffraction peaks at 2θ = 24.31°, 29.13°, 29.68°, 31.65°, 38.77°, 43.36° and 45.25° could be perfectly indexed to monoclinic Bi₃O₄Cl. By the Scherrer formula, the mean grain size of Bi₃O₄Cl was determined to be 29.9 nm. Compared with pristine Bi₃O₄Cl, the appearance of characteristic peaks associated with Bi₃O₄Cl in Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ and the characteristic peaks at 2θ = 29.91°, 42.94° and 62.01° corresponding to Mn_xZn_1-xFe₂O₄ could still be recognized, confirming the successful incorporation of Mn_xZn_1-xFe₂O₄ onto Bi₃O₄Cl.

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Fig. 3

XRD patterns of Bi₃O₄Cl, Mn_xZn_1-xFe₂O₄ and MB-10.

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The surface morphologies of the samples are observed via SEM and EDS mapping, as presented in Fig. 4. Compared with granular Mn_xZn_1-xFe₂O_4, Bi₃O₄Cl showed irregular nanosheets. SEM and EDS mapping of the results revealed the combination of Bi₃O₄Cl and Mn_xZn_1-xFe₂O_4, corresponding to XRD. Only Bi, Cl, O, Mn, Zn and Fe were uniformly distributed in the composite. These results indicate that the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalysts obtained were pure.

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Fig. 4

(a)-(c) SEM images of Bi₃O₄Cl, Mn_xZn_1-xFe₂O₄ and MB-10; (d) EDS spectrum of MB-10; (e)-(j) EDS elemental mapping of Bi, Cl, O, Mn, Zn and Fe; (k) TEM image of MB-10; (l) HRTEM image of MB-10.

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Transmission electron microscopy (TEM) is used to express the surface morphologies of Bi₃O₄Cl/Mn_xZn_1-xFe₂O_4. The TEM images of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst investigated in Fig. 4(k) confirm the doping of Mn_xZn_1-xFe₂O₄ onto Bi₃O₄Cl shown in SEM. The HRTEM exhibited in Fig. 4(l) shows a spacing of 0.26 nm and a crystal lattice fringe spacing of 0.30 nm, identified with (4 1 1) crystal Bi₃O₄Cl and (3 1 1) crystal Mn_xZn_1-xFe₂O₄, respectively. This clearly exhibits the differences between each crystal plane and confirms the successful preparation of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ compound.

The chemical composition of support surface of MB-10, which was surveyed by XPS survey spectra, proving the constitution and chemical shift of of Bi₃O₄Cl and Mn_xZn_1-xFe₂O₄. According to Fig. 5(a), MB-10 exhibits all elements of Bi, Cl, Mn, O, Zn, C, and Fe, which are in consistent with the HRTEM and EDS outcomes. The spectrum of Cl 2P could be separated into two characteristic peaks at 199.2 eV (Cl 2P_3/2) and 197.7 eV (Cl 2P_1/2), indicating the existence of Cl^– (Zhou et al., 2018). In the meantime, the characteristic peaks at 163.9 eV (Bi 4f_5/2) and 158.6 eV (Bi 4f_7/2) in the Bi 4f spectrum confirmed the existent form of Bi³⁺ (Li et al., 2018). The O 1s spectrum can fit the characteristic peak at 529.2 eV illustrated in Fig. 5(d). The major peaks at 651.5 eV, 709.2 eV, and 1017.5 eV in Mn 2p, Fe 2p and Zn 2p spectrum are attributed to Mn_xZn_1-xFe₂O₄ presented in Fig. 5(e-g). Moreover, the peaks of O 1s, Bi 4f and Br 3d in MB-10 slightly shift to the weaker binding energy direction through the intense interaction between Bi₃O₄Cl and Mn_xZn_1-xFe₂O₄ (compared to pure Bi₃O₄Cl). In summary, the interface between Bi₃O₄Cl and Mn_xZn_1-xFe₂O₄ is formed by chemical interactions rather than simple physical mixing.

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Fig. 5

(a) X-ray photoelectron spectroscopy (XPS) survey spectra; (b–g) corresponding high-resolution XPS spectra of each element.

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The aperture partition and Brunauer–Emmett–Teller surface area (S_BET) of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ magnetic composite photocatalyst were investigated by N₂ adsorption and desorption isotherms, as exhibited in Fig. 6. The whole specimens were presented typical Type IV isotherms, indicating the presence of mesoporous within the production. At higher relative pressures P/P₀, type H₃ hysteresis loop was observed, suggesting the existence of slit-shaped macropores. Fig. 6 illustrates the pore size distribution of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst, and the most likely pore size distribution is 21.2 nm. The total pore volume of Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ was 0.0203 cm³/g. The S_BET values of Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ and Bi₃O₄Cl were 7.27 and 5.82 m²/g, respectively. Typically, the superior specific surface area of the photocatalysts was essential to higher adsorption of RhB, since more active loci could be furnished.

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Fig. 6

The N₂ adsorption-desorption isotherms of MB-10 (Inset is pore diameter distribution curve).

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3.3 Optical properties

Upon UV–vis diffuse reflection spectra, the photoresponsibility of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst and Bi₃O₄Cl are investigated in Fig. 7(a). UV and visible light could be absorbed by both. In comparison with pristine Bi₃O₄Cl, the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst absorbed a smaller wavelength of 420 nm, which demonstrated a narrower response range for visible light of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst leading to weaker photocatalytic activity.

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Fig. 7

(a) UV–vis DRS of Bi₃O₄Cl and MB-10; (b) The (ahv)^1/2-hv curve of Bi₃O₄Cl and MB-10.

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The (ahv)^1/2-hv images are obtained in Fig. 7(b) since Bi₃O₄Cl is an indirect band gap semiconductor (Lin et al., 2006). Band gap energy (Eg) is shown at the intersection point of the curve tangent extension line and the abscissa axis. The calculated results indicated that the E_{g values} of Bi₃O₄Cl and Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ were 3.05 eV and 3.18 eV, respectively. Compared with Bi₃O₄Cl, Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalysts exhibited a higher band gap energy, which indicates that the greater difficulty in yielding electrons and holes than Bi₃O₄Cl was responsible for the weaker photocatalytic activity of Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄.

3.4 Photoelectrochemical properties

Typically, the weaker the PL intensity is, the less the compound probability of photogenerated h⁺ and e⁻ is, which results in the longer life period of photogenerated carriers. Steady-state photoluminescence (PL) is applied to explore the migration efficiency and segregation of photoinduced electron-hole pairs investigated in Fig. 8. Under photoexcitation, the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst showed a greater photocurrent response intensity than Bi₃O₄Cl, which indicates the higher photogenerated carrier recombination rate of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst than Bi₃O₄Cl. Furthermore, the greater efficiency of electron-hole separation after coupling decreases the lifetime of light-generated holes and electrons, which confirmed the slightly weaker photocatalytic activity of the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite magnetic photocatalyst compared with pure Bi₃O₄Cl.

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Fig. 8

PL emission spectra of Bi₃O₄Cl和MB-10.

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Fig. 9 shows the electrochemical impedance spectroscopy (EIS) analysis for Bi₃O₄Cl and the prepared Bi₃O₄Cl/Mn_xZn_1-xFe₂O_4. Bi₃O₄Cl presents a smaller curvature radius than the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalyst, which demonstrates a better interfacial charge transfer and separation efficiency and exhibits a greater conductivity.

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Fig. 9

EIS of Bi₃O₄Cl and MB-10.

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The TPR results of pristine Bi₃O₄Cl and Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ are investigated in Fig. 10 by an electrochemical workstation. Irradiated by simulated sunlight and obtained energy, the electrons transformed, resulting to an instantaneous increase in the photocurrent density of the reaction system and reaching the peak. Notably, the lowest photocurrent density was observed in the absence of light. Furthermore, the transient photocurrent density of pristine Bi₃O₄Cl is 0.56 µA·cm⁻², which was nearly 2 times that of Bi₃O₄Cl/Mn_xZn_1-xFe₂O_4, proving the superior election mobility performance.

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Fig. 10

Transient photocurrent response (I-t) of Bi₃O₄Cl and MB-10.

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3.5 Magnetic properties

Fig. 11 illustrates the hysteresis loop diagrams of Mn_xZn_1-xFe₂O₄ and Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite photocatalysts. Mn_xZn_1-xFe₂O₄ exhibited a greater hysteresis loop and coercivity, which can foster magnetic induction intensity and a lower hysteresis loss. Herein, the coercivity (Hc) of Mn_xZn_1-xFe₂O₄ was 42 G, and the residual magnetization (Mr) was 6.8 emu/g. Compared with the saturation magnetization (Ms) of 70 emu/g owing to Mn_xZn_1-xFe₂O_4, Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ displayed a weaker Ms of 6.42 emu/g. The Mr of Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ retained 0.52 emu/g, which indicates a lower Mn_xZn_1-xFe₂O_4, and the calculation process was responsible for the weaker residual magnetization (Mr). Given an external magnetic field, Bi₃O₄Cl exhibited no magnetism, while Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ displayed an ideal magnetism sharply. Hence, Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ could be regarded as a composite photocatalyst with excellent magnetic properties for recycling and magnetism.

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Fig. 11

(a)-(b) The magnetic hysteresis loops; (c) The suspension of Bi₃O₄Cl and MB-10; (d) The suspension of Bi₃O₄Cland MB-10 under the magnetic field.

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3.6 Stability and recycling ability

The stability of the composite is proven by circular photodecomposition of RhB solution under the same conditions as presented in Fig. 12. Given an accurate weighing, the recovery of each catalyst was 91.3 %. After 5 cycles, no distinct deterioration could be observed in the degradation curves, and the recovery of the catalyst was 82.2 %, which verifies the excellent stability of the composites in RhB degradation.

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Fig. 12

Degradation rate of RhB on MB-10 after being recycled.

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3.7 Photocatalytic mechanism

Free radical capture experiments are prepared to reveal the main active substances by the samples investigated in Fig. 13. IPA (Isopropyl alcohol), BQ (benzoquinone) and AO (ammonium oxalate) were prepared in the experiment to detect hydroxyl radicals (·OH), photogenerated holes (h⁺) and superoxide anion rabical ( ${·\text{O}}_{\text{2}}^{\text{-}}$ ), respectively. Confirming their effects on catalytic degradation in the reaction system. After adding AO, IPA and BQ, the degradation rate of RhB retained 44.5 %, 32.6 % and 8.2 %, respectively. This indicates the importance of h⁺, ·OH and ${}^{\bullet }{\text{O}}_{\text{2}}^{\text{-}}$ .

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Fig. 13

Effect of radical scavengers on RhB photocatalytic degradation of MB-10.

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In addition, the band gap energies of the specimens could be initially evaluated under UV–vis DRS. The fitted E_g of Bi₃O₄Cl was estimated to be 3.05 eV. In addition, the E_VB values of Bi₃O₄Cl and Mn_xZn_1-xFe₂O₄ were 3.43 eV and 1.17 eV, respectively. However, with E_VB higher than 2.3 eV_, Bi₃O₄Cl fostered OH^– oxidation to ·OH(OH^–/·OH, 2.3 eV vs. NHE) by photogenerated holes (h⁺) (Li et al., 2017). Mn_xZn_1-xFe₂O₄ with weaker E_VB negatively contributed to the inability to oxidize OH^– to ·OH. Furthermore, compared with −0.33 eV, the lower E_CB of Mn_xZn_1-xFe₂O₄ displayed perfect O₂ reduction to ${}^{\bullet }{\text{O}}_{\text{2}}^{\text{-}}$ (O₂/ ${}^{\bullet }{\text{O}}_{\text{2}}^{\text{-}}$ , −0.33 eV vs. NHE) (Fu et al., 2012), while Bi₃O₄Cl with a higher E_CB exhibited nothing on it. The Z-type heterojunction (Zhao et al., 2023; Zhao et al., 2023) Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ is investigated in Fig. 14, which reveals the possible pathway to catalyse the mechanism. The intensive combination of e⁻ on Bi₃O₄Cl and h⁺ on Mn_xZn_1-xFe₂O₄ contributes to the Z-type heterojunction. The h⁺ on Bi₃O₄Cl had two pathways, and some of them degraded RhB directly. Others effectively promoted OH^– oxidation to ·OH, which eliminated RhB later. Concurrently, the e⁻ on Mn_xZn_1-xFe₂O₄ reduced O₂ to ${{}^{\bullet }\text{O}}_{\text{2}}^{\text{-}}$ , confirming the duration of RhB under ${}^{\bullet }{\text{O}}_{\text{2}}^{\text{-}}$ . The purpose and discussion of RhB degradation by the Bi₃O₄Cl/Mn_xZn_1-xFe₂O₄ composite magnetic photocatalyst were exhibited under the reaction mechanism, which matches the results of free radical capture.

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Fig. 14

Photocatalytic mechanism of RhB dye degradation over Bi₃O₄Cl/Mn_xZn_1-xFe₂O_4.

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