A core–shell structured magnetic Ag/AgBr@Fe₂O₃ composite with enhanced photocatalytic activity for organic pollutant degradation and antibacterium

Abstract

A core–shell structured magnetic Ag/AgBr@Fe₂O₃ composite was synthesized through a facile solvothermal method. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible absorption spectroscopy (UV-vis) were applied to characterize the structures and properties of the as-prepared samples. The results indicate that Fe₂O₃ was coated on the surface of Ag/AgBr and heterostructures were formed. Electrochemistry analysis and photoluminescence (PL) spectra analysis indicate that the introduction of Fe₂O₃ could improve electron and hole separation efficiency. The photocatalytic activity of the Ag/AgBr@Fe₂O₃ composites was evaluated by using organic dye methyl orange (MO), endocrine disrupting chemical bisphenol A (BPA) and Escherichia coli (E. coli) as the target pollutants. The as-prepared Ag/AgBr@Fe₂O₃ composites exhibited much higher photocatalytic activities than pure Ag/AgBr, which was attributed to the effective charge separation of the Ag/AgBr@Fe₂O₃ composite. In addition, the as-prepared Ag/AgBr@Fe₂O₃ composite has magnetic properties, therefore after the photocatalytic reaction, it can be quickly separated from solution by an extra magnetic field. Trapping experiments and ESR analysis indicate that the h⁺ and ˙O₂⁻ are the main active species for the photocatalytic degradation. A possible Z-scheme pathway photocatalytic mechanism was proposed.

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1. Introduction

In recent years, environmental issues caused by hazardous wastes and water pollution have aroused wide concern. Among these, water pollution which is engendered by chemical pollution and pathogenic microorganisms has been one of the most serious issues in modern society.¹ As a promising technology, the semiconductor photocatalysis technique provides a “green” method for environmental purification.^2–4 After the discovery of photocatalytic splitting of water on a TiO₂ electrode,⁵ plentiful semiconductor materials have been established.^6–13 Among these photocatalysts, plasmonic photocatalysts have been found as an alternative and promising substitute for developing highly efficient visible-light photocatalysts. The plasmonic photocatalysts could dramatically enhance the absorption of visible light and efficiently separate photogenerated electrons and holes because of the surface plasmon resonance (SPR) effect. Generally, noble metal nanoparticles (such as Au and Ag) can photo-induce collective oscillations of the conduction electrons with a resonant frequency, this phenomenon is the surface plasmon resonance effect, which can significantly amplify the absorption of visible light.¹⁴ So, the plasmonic photocatalysts provide new opportunities for developing visible light photocatalysts. In recent years, much interest has been paid to the plasmonic photocatalysts based on silver/silver halides (Ag/AgX, X = Cl, Br, I) composite materials due to the surface plasmon resonance of noble metallic Ag.^15–21 The silver/silver halides composite photocatalysts have been confirmed that they are not only a photo stable photocatalyst with high visible light photoactivity but also can be used for coupling with other semiconductors to improve the photoactivity. Numerous strategies have been reported for synthesizing Ag/AgX (X = Cl, Br) based photocatalysts, including texturization or morphology control to increase the surface area and band alignment by coupling with other semiconductors. For example, Shen et al.²² reported that Ag/AgCl spherical and sheet like nanoarchitecture morphologies could be controllably fabricated by means of tuning the concentration of cetyltrimethylammonium chloride (CTAC) and AgNO₃ aqueous solution. The sheet like Ag/AgCl displayed distinctly boosted catalytic performances toward the photodegradation of methyl orange (MO). Li et al.²³ synthesized 1D AgBr@Ag nanostructures by a facile wet chemical method. These AgBr@Ag nanorods have a high surface area and provide more active sites. Many Ag/AgX based composite photocatalysts have been fabricated, such as Ag/AgCl/W₁₈O₄₉, Ag/AgBr/TiO₂, and AgCl@Ag@TiO₂ etc. These composites not only exhibited very high photocatalytic activity in the photodegradation of organic pollutant but also show very high antibacterial ability under visible light irradiation.^24–28 Up to now, this kind of materials has been widely studied in the degradations of non-biodegradable dyes, alcohols, phenols and killing pathogenic bacteria in waste water.^29,30 However, the practical applications of these photocatalysts are often limited by the easy loss of the suspended particulate catalysts in the process of photocatalytic reaction and separation. So, finding out an easy way to recover the Ag-based photocatalyst is very necessary. To this end, some strategies have been established, one of them is combining Ag/AgX with a magnetic material. For instance, Tian et al.³¹ fabricated core–shell structured γ-Fe₂O₃@SiO₂@AgBr:Ag composite microspheres. Dai et al.³² prepared core–shell Ag–AgI/Fe₃O₄@SiO₂ plasmonic photocatalyst. An et al.³³ constructed ferromagnetic Fe₃O₄@SiO₂@AgCl:Ag plasmonic nanophotocatalysts, etc. These strategies have achieved great advantages in the development of magnetic plasmonic nanophotocatalysts, but the synthesis route could become cumbersome. Our previous work has attempted to construct ferromagnetic plasmonic nanophotocatalysts by coupling Ag/AgCl with magnetic material CoFe₂O₄.³⁴ This synthesis route is simple and the photoactivity of Ag/AgCl/CoFe₂O₄ was enhanced. So, it is necessary to continue investigating high efficient magnetic plasmonic nanophotocatalysts for a long and sustained environment protection strategy.

Semiconductor Fe₂O₃, a kind of magnetic material with band gap 2.0–2.2 eV, can absorb a large fraction of visible light.³⁵ It has lots of advantages in the photocatalytic field, such as environmental-friendly, good chemical stability and low cost.³⁶ Despite these advantages, Fe₂O₃ suffers from a miserably short excited state lifetime, high recombination rate, poor conductivity and difficulty to separate the photo-induced electron–hole pairs.^37–39 Thus pure Fe₂O₃ may not be a good candidate for photocatalytic reaction. However, as a narrow band gap semiconductor, Fe₂O₃ could be used for modifying other semiconductors to extend the absorb range for visible light, such as Fe₂O₃/TiO₂,⁴⁰ Fe₂O₃/ZnO,⁴¹ α-Fe₂O₃/CdS.⁴² These results reveal that the introduction of Fe₂O₃ could enhance their photocatalytic abilities. Furthermore, Fe₂O₃ has magnetic property, which provides a new sight for fabricating magnetic photocatalysts. Ye et al.⁴³ fabricated magnetically separable polymeric carbon nitride photocatalysts Fe₂O₃/g-C₃N₄ and the synergistic effect between Fe₂O₃ and g-C₃N₄ enhanced the photocatalytic activity for the degradation of organic dye under visible light. All in all, these results reveal that the Fe₂O₃ nanoparticles may have potential applications in photocatalysis and magnetic field systems.

Depending on the analysis above, if the hybrid material of Fe₂O₃ and Ag/AgBr is prepared, it is likely that this hybrid structure with higher photocatalytic performance and magnetic property can be obtained. To the best of our knowledge, there are no existing reports on the preparation of such a core–shell structured Ag/AgBr@Fe₂O₃ nanoparticle system. In this work, we fabricated magnetic photocatalyst core–shell Ag/AgBr@Fe₂O₃ composites by a facile solvothermal method. The as-prepared photocatalyst exhibited excellent photocatalytic activity in eliminating methyl orange (MO), bisphenol A (BPA) and Escherichia coli (E. coli) in water under visible light irradiation. This study may provide new insights in the fabrication of high efficient and magnetic plasmonic photocatalysts by a facile method.

2. Experimental

2.1 Materials

All the reagents were analytical grade and used without further purification. They were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2 Preparation of photocatalysts

2.2.1. Preparation of Fe₂O₃. Fe₂O₃ was prepared by solvothermal method and followed by calcination method. Firstly, the preparation of Fe₂O₃ precursor: 8.08 g Fe(NO₃)₃·9H₂O was dissolved in 80 mL ethylene glycol under magnetic stirring and then 7.20 g CH₃COONa and 2.00 g polyethylene glycol-600 (PEG-600) were added into the solution and kept stirring for 1.5 h. The result homogeneous suspension was transferred into several 25 mL Teflon-lined autoclaves, followed by solvothermal treatment at 200 °C for 22 h. After cooled down naturally, the products were washed three times with pure water and ethanol then dried at 60 °C overnight. Secondly, the as-prepared Fe₂O₃ precursor was calcined at air atmosphere at temperature of 500 °C for 2 h.

2.2.2. Synthesis of Ag/AgBr@Fe₂O₃ composites. Samples with different mass fraction were synthesized by a facile solvothermal method. In a typical procedure: 100 mL of ethylene glycol (EG) and 100 mL of water were mixed via stirring and ultrasonic processing (solution A). 0.06 g as-prepared Fe₂O₃ was distributed in 15 mL solution A by ultrasonic treatment and then 0.194 g AgNO₃ was added under vigorous mechanical stirring for 0.5 h at room temperature to facilitate the adsorption of Ag⁺ on the surfaces of the Fe₂O₃ particulates. After that, 1.14 mM potassium bromide (the solvent is solution A) was added drop by drop to the above solution and the mixture was keeping stirring for further 30 min in room temperature. At last, the suspension was transferred to 25 mL Teflon-lined stainless-steel autoclaves and kept for 12 h at 140 °C. After cooling to room temperature, the precipitate was washed with deionized water and ethanol for three times, and then dried under 60 °C for 12 h. According to this method, different mass fraction of Fe₂O₃ in the total weight of Fe₂O₃ and AgNO₃ were 1%, 3%, 5% and 10%, respectively. And the final products were named as 1% Ag/AgBr@Fe₂O₃, 3% Ag/AgBr@Fe₂O₃, 5% Ag/AgBr@Fe₂O₃ and 10% Ag/AgBr@Fe₂O₃, respectively.

2.3 Characterization

The X-ray diffraction (XRD) patterns of the crystal phase was collected on a Bruker D8 diffractometer with high-intensity Cu-Kα (λ = 1.5418 Å) in the 2θ range of 10–80°.The SEM images of the samples were obtained on a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV, which was equipped with an energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. X-ray photoelectron spectroscopy (XPS) was measured on a PHI5300 with a monochromatic Mg Kα source to explore the elements on the surface. The UV-vis diffuse reflectance spectra of the samples were carried out on a Shimadzu UV-2450 UV-vis spectrophotometer (Shimadzu Corporation, Japan) in solid state with the BaSO₄ powder was used as the substrate. To diminish the analysis error, a certain amount of samples (about 0.001 g) was mixed with BaSO₄ (about 0.009 g) before the UV-vis absorption spectra test. The magnetic properties of the Fe₂O₃ and Ag/AgBr@Fe₂O₃ composites were measured in a vibrating sample magnetometer (Quantum Design Corporation, USA) with a maximum applied field of ±2 T. Photocurrent measurements was performed on an electrochemical workstation (CHI 660B, Chenhua Instrument Company, Shanghai, China).

2.4 Photocatalytic activity

The photocatalytic activities of the as-prepared samples were assessed by degrading 10 mg L⁻¹ organic dye MO and 10 mg L⁻¹ BPA under 300 W Xe lamp with a UV cutoff filter (λ > 420 nm). And all photocatalytic experiments were performed at 30 °C by using a circulating water system to prevent thermal catalytic effects. In addition, an air pump was used to ensure a constant supply of oxygen during the photoreactions. In a typical procedure, 0.07 g as-prepared samples were dispersed in 70 mL above MO or BPA solution. The mixture was magnetically stirred for about 30 min in the dark to ensure the saturated adsorption of organic pollutant on the surface of catalysts. At regular intervals, approximately 4 mL of the suspension was extracted and then centrifuged to remove essentially all the catalysts and the remaining liquid were collected for further test. The remaining MO degradations were analyzed by a UV-vis spectrophotometer (UV-2450, Shimadzu) at wavelength 463 nm. The remaining BPA degradations were analyzed by the two Varian ProStar 210 pumps, an Agilent TC-C18 column, and a Varian ProStar 325 UV-vis Detector at 230 nm. The mobile phase was 1 mL min⁻¹ with a solution of methanol and H₂O with the volume ratio 75 : 25 and 20 μL of the sample solution was injected.

2.5 Antibacterial activity

Prior to experiment, all the glassware and the culture medium solution were sterilized by autoclaving at 121 °C for 20 min. And all the experiments were processed under sterile conditions. The antibacterial activities of Ag/AgBr@Fe₂O₃ composites were evaluated via the inactivation of Escherichia coli (E. coli) bacteria. The zone of inhibition test was carried out to identify the antibacterial activities of the prepared photocatalysts in dark. Powder photocatalysts were placed on the nutrient agar medium which has been inoculated with 20 μL of the prepared Escherichia coli (E. coli) bacteria suspension. Then the plates were incubated at 37 °C for 16 h in dark.

Photocatalytic disinfection experiments were also performed. In a typical procedure, 10 mg of the as-prepared sample was suspended in 20 mL culture medium solution which contains 1/(2 × 10⁴) concentration of the original E. coli concentration. Then the mixture was magnetically stirred in the dark for 10 min, subsequently, the light was switched on to start irradiation. 20 μL of the solution was sampled at the time intervals of 0 min, 5 min, 10 min and 13 min. Each solution was dispersed on the nutrient agar medium and incubated at 37 °C for 16 h in dark.

3. Results and discussion

3.1 XRD analysis

The X-ray diffraction (XRD) pattern of AgBr, Fe₂O₃ and Ag/AgBr@Fe₂O₃ composites are shown in Fig. 1A. In the patterns of AgBr (Fig. 1a), it is obvious that all the diffraction peak at 2θ = 26.7°, 31.0°, 44.3°, 52.5°, 55.0°, 64.5°and 73.2° (marked with “♠”) appeared, which matched exactly with the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) and (4 2 0) crystal planes of cubic phase AgBr (JCPDS No. 06-0438).⁴⁴ For the pattern of Fe₂O₃ (Fig. 1f), the diffraction peaks at 2θ = 24.2°, 33.2°, 35.6°, 40.9°, 43.3°, 49.5°, 54.1°, 57.6°, 62.5° and 64.1° (marked with “♦”) are assigned to the (012), (104), (110), (113), (202), (024), (116), (018), (214) and (300) crystal planes of hematite structure α-Fe₂O₃ (JCPDS No. 33-0664).⁴⁵ These peaks at 30.3°, 35.6°, 43.3° and 63.1° (marked with “♣”) cannot be ignored in this pattern, and they could be indexed to the (220), (311), (400) and (440) crystal planes of cubic phase γ-Fe₂O₃ (JCPDS No. 39-1346).⁴⁶ This result suggests that the as-prepared Fe₂O₃ contains both hematite structure α-Fe₂O₃ and cubic phase γ-Fe₂O₃. The strong magnetic property of the as-prepared Fe₂O₃ is due to the γ-Fe₂O₃ phase (maghemite). It is difficult to find the peaks of Fe₂O₃ in the patterns of Ag/AgBr@Fe₂O₃ composites, when the mass fraction of Fe₂O₃ is less than 15%. There are two possible reasons. The first one is that the peaks of Fe₂O₃ are covered because the peaks of AgBr are very strong. The second one is that the Fe₂O₃ loaded on the surface of AgBr and were well dispersed. Similar result can also be found in other systems.⁴⁷ However, we magnified the patterns of 1% Ag/AgBr@Fe₂O₃, 3% Ag/AgBr@Fe₂O₃ and 5% Ag/AgBr@Fe₂O₃ samples by narrow-scan diffraction patterns, as showed in Fig. 1B. The peaks of Fe₂O₃ still could not be noticed. So, we believe the later is the main reason and this conjecture has also been confirmed by the followed SEM, EDS and XPS analyses.

Fig. 1 XRD patterns of the as-prepared photocatalysts series. (A) Wide-scan diffraction patterns: (a) pure Ag/AgBr, (b) 1% Ag/AgBr@Fe₂O₃, (c) 3% Ag/AgBr@Fe₂O₃, (d) 5% Ag/AgBr@Fe₂O₃, (e) 15% Ag/AgBr@Fe₂O₃ and (f) Fe₂O₃. (B) Narrow-scan diffraction patterns.

3.2 SEM and EDS analysis

Fig. 2 shows the morphology of Fe₂O₃, Ag/AgBr and the dispersion state of Fe₂O₃ on the surface of Ag/AgBr, which were examined by SEM. Fig. 2a and b are the morphology image of the pure Fe₂O₃, it can be seen that the Fe₂O₃ particles are granular like with diameters of ca. 20–50 nm. Fig. 2c shows the morphology image of the pure Ag/AgBr, the large particles with diameters of ca. 7–15 μm are AgBr particles and some of the small particles attached on their surface are Ag NPs.⁴⁸ By close observation (Fig. 2d), these Ag NPs are scattering on the surface of AgBr sparsely. This may be the reason why the peaks of Ag⁰ couldn't be found in the XRD patterns. Fig. 2e and f are low and high magnification SEM images of 1% Ag/AgBr@Fe₂O₃ composites, respectively. In the low magnification SEM image, it can be seen that the introduction of Fe₂O₃ didn't change the shape of Ag/AgBr largely, while in high magnification SEM image (Fig. 2f), it can be clearly seen that Fe₂O₃ NPs were highly distributed throughout the surface of Ag/AgBr. Similarly, the SEM images of 3% Ag/AgBr@Fe₂O₃ and 5% Ag/AgBr@Fe₂O₃ are shown in Fig. 2g, h and i, j, respectively. It can be seen that with the increase of Fe₂O₃ content more and more holes can be found on the surface of Ag/AgBr, indicating that some of the Fe₂O₃ are wrapped inside of AgBr particles. Similar results can also be found in our previous work.⁴⁹ In addition, it can be noticed that an agglomeration of Fe₂O₃ particle was formed when the content of Fe₂O₃ in a high level (>3%), this may decreases the photocatalytic activity of Ag/AgBr@Fe₂O₃ composites. At the same time, EDS of the pure Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ have also been taken out and shown in Fig. 2k and l, respectively. Only Ag and Br can be found in the image of Ag/AgBr, while in the image of 3% Ag/AgBr@Fe₂O₃ composites Ag, Br, Fe and O were directly observed. Combining the results of XRD, SEM and EDS, it can be confirmed that Ag/AgBr@Fe₂O₃ has been successfully synthesized.

Fig. 2 Low and high magnification SEM images of as-prepared samples: Fe₂O₃ (a and b), Ag/AgBr (c and d), 1% Ag/AgBr@Fe₂O₃ (e and f), 3% Ag/AgBr@Fe₂O₃ (g and h), 5% Ag/AgBr@Fe₂O₃ (i and j), EDS of Ag/AgBr (k) and 3% Ag/AgBr@Fe₂O₃ (l).

3.3 XPS analysis

XPS analysis was carried out to further determine the elemental compositions and chemical status of the as-prepared Ag/AgBr and Ag/AgBr@Fe₂O₃ composite. The results were shown in Fig. 3. The full scan survey XPS spectrum of the Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite indicate that the product consists of Ag, Br, Fe, O and C elements (Fig. 3a). The C element is from the XPS instrument itself. Fig. 3b shows the high-resolution XPS spectrum of Ag 3d, it consists two individual peaks at about 366.00–367.00 eV and 372.00–373.00 eV, which are attributed to Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively.⁵⁰ In contrast to Ag/AgBr, the Ag 3d spectra of Ag/AgBr@Fe₂O₃ composite show a little shift, and the decrease in the binding energy with an amount of ∼0.4 eV might be attributed to the interaction of Ag⁺ ions of the composite with the Fe₂O₃ nanoparticles. It also suggests that the interaction between Fe₂O₃ and Ag/AgBr existed and the combination sites are Ag elements.^51,80 The spectra of Br 3d is shown in Fig. 3c. The binding energies of Br 3d_5/2 and Br 3d_3/2 are about 67.69 eV and 68.58 eV, respectively, which can be attributed to the Br atoms existing in a Br⁻ state.⁵² In addition, it is worth noting that both the intensity of Ag 3d and Br 3d of Ag/AgBr@Fe₂O₃ composite are much weaker than that of Ag/AgBr, respectively. Considering that the main components of the composites are Ag/AgBr, this result confirmed that the Fe₂O₃ NPs were covered on the surface of Ag/AgBr again. Fig. 3d shows the spectra of Fe 2p, the two peaks at about 710.54 eV and 724.15 eV are attributed to the binding energies of Fe 2p_3/2 and Fe 2p_1/2, respectively. The high-resolution O 1s XPS spectra of Ag/AgBr and Ag/AgBr@Fe₂O₃ samples are shown in Fig. 3e. The peak at about 529.71 eV can be assigned to the oxygen in Fe₂O₃ crystals,⁵³ and the peak at about 531.52 eV is considered to be the enthetic O, such as ambient adsorption.⁵⁴

Fig. 3 XPS spectra of the pure Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite. (a) Survey of the sample; (b) Ag 3d; (c) Br 3d; (d) Fe 2p and (e) O 1s.

3.4 UV-vis absorption spectra analysis

Fig. 4 shows the UV-vis absorption spectra (in the diffuse reflectance spectra mode) and corresponding Tauc's plots of (αhv)² vs. (hv) of as-prepared samples.⁵⁵ From the UV-vis absorption spectra (Fig. 4a), it can be seen that the bare Ag/AgBr has a main absorption region from 200 to 450 nm. And the absorption in the range 500–700 nm is ascribed to the SPR effect of Ag nanoparticles on the surface of the AgBr, which indicates the existence of Ag⁰ in the composite catalysts. This result is in accordance with our previous work.⁴⁸ For the Ag/AgBr@Fe₂O₃ composites, an apparent absorption enhancement for light is observed compared to the pure Ag/AgBr, which is thought to result from Ag/AgBr covered by Fe₂O₃. This result is consistent with the above SEM and XPS analyses. Furthermore, the light absorption ability was enhanced with the increased Fe₂O₃ contents. The enhanced light absorption may lead to forming more electron–hole pairs.⁵⁶ In addition, it shouldn't be neglected that the light absorption ability of pure Fe₂O₃ is less than that of Ag/AgBr@Fe₂O₃ composites in the range 600–800 nm, which suggests a synergistic effect between Ag/AgBr and Fe₂O₃.⁵⁷ The band gap energy E_g of the as-prepared Fe₂O₃ and Ag/AgBr was estimated from the corresponding Tauc's plots of (αhv)² vs. (hv),⁵⁴ and the result is shown in the Fig. 4b. The band gap energy of as-prepared Fe₂O₃ and Ag/AgBr was estimated to be 2.1 eV and 2.6 eV, respectively, which is similar to the reported literatures.^51,58 In addition, to study the flowchart of photo-excited charge carriers, potentials of the conduction band (CB) and valence band (VB) edges of as-prepared Fe₂O₃ was evaluated by Mulliken electronegativity theory: E_CB = X − E_C − 0.5(E_g), where X was the absolute electronegativity of the atom semiconductor, E_C was the energy of free electrons with the hydrogen scale (4.5 eV); E_g was the band gap of the semiconductor. Therefore, the conduction band (CB) of Fe₂O₃ and AgBr was determined to be 0.3 eV and 0 eV, respectively, which are similar to the reported literatures.^59,60 Then the valence band (VB) can be determined by E_VB = E_CB + E_g,⁶¹ and the values are 2.4 eV and 2.6 eV for Fe₂O₃ and AgBr, respectively.

Fig. 4 (a) UV-vis absorption spectra of as-prepared samples and (b) corresponding Tauc's plots of (αhv)² vs. (hv) of Fe₂O₃ and Ag/AgBr.

3.5 Electrochemistry analysis

It is widely accepted that the higher photocurrent was, the better electron and hole separation efficiency would be.^62,63 Fig. 5 shows 5 on–off cycles visible light irradiation transient photocurrent responses of Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ hybrid material. Obviously, the 3% Ag/AgBr@Fe₂O₃ hybrid material have higher photocurrent response than that of the Ag/AgBr. It indicates that the 3% Ag/AgBr@Fe₂O₃ hybrid material has a higher separation rate of photoexcited electrons and holes under the irradiation of visible light.^64,65 In addition, it can be seen that the photocurrent value rapidly decreased and increased to a constant value when the light was off and on. This result demonstrated that the recombination of electrons and holes was inhibited greatly, which is good for photocatalytic reaction.⁶⁶

Fig. 5 Transient photocurrent response for pure Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite.

3.6 PL analysis

To further confirm that the introduction of Fe₂O₃ could reduce the electrons and holes recombination rate. PL spectra for Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ hybrid material were presented in Fig. 6. It is obvious that PL intensity of 3% Ag/AgBr@Fe₂O₃ decreased significantly compared to that of Ag/AgBr, which means efficient transfer of photoexcited electrons between Fe₂O₃ and Ag/AgBr. This result indicates that the introduction of Fe₂O₃ favors the effective charge separation of Ag/AgBr and improve the photocatalytic activities.⁶⁷

Fig. 6 PL spectra of pure Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite.

3.7 VSM analysis

Fig. 7 shows the magnetic behavior of as-prepared Fe₂O₃ and 3% Ag/AgBr@Fe₂O₃ composite, which is investigated by a vibrating sample magnetometer at room temperature. The curve presents almost no hysteresis loop, which suggests that the as-prepared Fe₂O₃ and 3% Ag/AgBr@Fe₂O₃ composite have ferromagnetic behavior.⁶⁸ The magnetization saturation (M_s) values are 34.9 and 3.3 emu g⁻¹ for Fe₂O₃ and 3% Ag/AgBr@Fe₂O₃ composite, respectively. The magnetic separation ability of the 3% Ag/AgBr@Fe₂O₃ composite was tested in water by placing a magnet near the glass bottle. The red particles were attracted toward the magnet within a short time (Fig. 6 inset graph). Therefore, this will provide an easy and efficient way to separate the photocatalysts from water under an external magnetic field.

Fig. 7 The hysteresis loops of pure Fe₂O₃ and 3% Ag/AgBr@Fe₂O₃ composite (the inset graph shows the sample dispersed in water (left) and separated by external magnet (right)).

3.8 Photocatalytic activity

Photodegradation of MO under visible light irradiation was carried out to evaluate the photocatalytic ability of the as-prepared samples. As shown in Fig. 8a, it can be seen that MO self-decomposition is negligible. The pure Ag/AgBr could degrade MO by 55.1% after 12 min under visible light irradiation. The Ag/AgBr@Fe₂O₃ composites show much better photocatalytic activity than Ag/AgBr alone. After visible light irradiation for 12 min, the degradation efficiency of MO over 1% Ag/AgBr@Fe₂O₃, 3% Ag/AgBr@Fe₂O₃ and 5% Ag/AgBr@Fe₂O₃ are 91.7%, 94.4% and 82.7%, respectively. It can be noticed that the photocatalytic ability of Ag/AgBr@Fe₂O₃ composites is greatly influenced by the content of Fe₂O₃. When the Fe₂O₃ content is 3%, it has the best photocatalytic activity. Whatever the Fe₂O₃ content is high or less than 3% (such as 1% and 5%), its photocatalytic activity decreased. The linear relationship of −ln(C/C₀) versus time is shown in Fig. 8b and the pseudo-first-order constants and relative coefficients are summarized in Table 1. The results indicate that the photocatalytic activity was improved by the introduction of Fe₂O₃ compared to pure Ag/AgBr. Especially, the 3% Ag/AgBr@Fe₂O₃ composite exhibit about 3.81 times degradation rate as high as that of pure Ag/AgBr. Fig. 8c shows the time-dependent absorption spectra of MO solution in the presence of 3% Ag/AgBr@Fe₂O₃ composite. It can be seen that the color of MO solution changed from yellow to light yellow and then colourless during the reaction (Fig. 8c inset graph). An evident decrease in MO absorption at λ = 463 nm was observed, indicating that the chromophoric structure was destroyed.

Fig. 8 (a) Photodegradation of MO by as-prepared samples. (b) Kinetic fit for the degradation of MO with the as-prepared samples. (c) Time-dependent absorption spectra of MO solution in the presence of 3% Ag/AgBr@Fe₂O₃ composite.

Table 1 Kinetic constants and regression coefficients of MO degradation under visible-light irradiation

Sample	Kinetic constant (k, min^−1)	R^2
Ag/AgBr	0.06078	0.98979
1% Ag/AgBr@Fe2O3	0.13290	0.98047
5% Ag/AgBr@Fe2O3	0.19153	0.98513
3% Ag/AgBr@Fe2O3	0.23217	0.98847

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Bisphenol-A (2,2-bis(4-hydroxyphenyl)propane, BPA), which is a compounds termed endocrine disruptor, has frequently been detected in surface water due to it is widely used in the production of polycarbonate plastics and epoxy resins. The toxicity tests revealed that it may cause various adverse effects on aquatic organisms even at low exposure levels.⁶⁹ Therefore, the degradation of BPA is important. In this work, the degradation of BPA under visible light with the as-prepared samples also has been studied. As shown in Fig. 9a, it can be seen that the BPA couldn't self-decomposition without catalysts. The pure Ag/AgBr has very low degradation efficiency for BPA, and only 26.0% BPA were degraded within 3 h. While the Ag/AgBr@Fe₂O₃ composites exhibit a relative higher photodegradation for BPA, as same as the photodegradation for MO, the 3% Ag/AgBr@Fe₂O₃ composite exhibit the best photocatalytic activity, it can degrade BPA up to 69.0% within 3 h. The HPLC analysis of BPA solution in the presence of 3% Ag/AgBr@Fe₂O₃ composite has been taken out. The results showed in Fig. 9b, and it is obvious that the characteristic peak of BPA decreased with the photoreaction process, which confirmed that BPA was indeed degraded by 3% Ag/AgBr@Fe₂O₃ composite.

Fig. 9 (a) Photodegradation of BPA by as-prepared samples. (b) The HPLC of the BPA degraded solution for different times in the presence of 3% Ag/AgBr@Fe₂O₃.

3.9 Photocatalytic antibacterial activity

Fig. 10a shows the results of the antibacterial experiment of 3% Ag/AgBr@Fe₂O₃ in the dark. It is obvious that the 3% Ag/AgBr@Fe₂O₃ composite has a clear inhibition zone of about 16 mm against E. coli. This result indicates 3% Ag/AgBr@Fe₂O₃ has a good antibacterial ability in the dark. Fig. 10b shows the results of the photocatalytic antibacterial experiments. It can be seen that the amount of E. coli was not obviously decrease under the visible light irradiation, indicating that the E. coli could survive under visible light irradiation. In the presence of 3% Ag/AgBr@Fe₂O₃ composite, numerous E. coli were still alive without light irradiation. However, when the system was irradiated by visible light for 5 min, more than half of the E. coli were killed. When the system was irradiated for 10 min, only a small number of E. coli were survived. When the system was irradiated for 13 min all the E. coli were killed. This result indicates that 3% Ag/AgBr@Fe₂O₃ possesses antibacterial ability under visible light irradiation.

Fig. 10 The representative inhibition zone of 3% Ag/AgBr@Fe₂O₃ disks against E. coli after 16 h incubation in the dark (a) and the photocatalytic antibacterial activities of light only and 3% Ag/AgBr@Fe₂O₃ against E. coli at different irradiation times (b).

3.10 Cycling runs

The recycle ability of magnetic photocatalyst is an important property for practical applications. To evaluate the reusability of the Ag/AgBr@Fe₂O₃ hybrid material, recycling reactions for degrading MO over 3% Ag/AgBr@Fe₂O₃ under visible light were carried out. As shown in Fig. 11, the MO still can be degraded up to 89.1% after five consecutive cycles, which implies the high recycle ability of Ag/AgBr@Fe₂O₃ hybrid material.

Fig. 11 Cycling runs of 3% Ag/AgBr@Fe₂O₃ composite.

3.11 Mechanism of pollutant photodegradation

To study the photodegradation mechanism, the trapping experiments of active species were taken out by using 2-propanol as the hydroxyl radical (˙OH) scavenger, N₂-bubbling for inhibiting the superoxide radical (˙O₂⁻), and triethanolamine (TEOA) as the hole (h⁺) radical scavenger.⁷⁰ The results are shown in Fig. 12. It can be seen that the degradation efficiency of MO was hardly affected by adding 1 mM 2-propanol, indicating that the ˙OH is not the main reactive species. The degradation rate of MO was decreased under an N₂ atmosphere, suggesting that the ˙O₂⁻ plays an important role in photodegradation process. When 1 mM TEOA was added, the degradation rate of MO was greatly inhibited, which reveals that the h⁺ was a dominant reactive species. To further confirm the active species in the photodegradation process, the electron spin resonance (ESR) spin-trap technique were carried out. The Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite were dispersed with DMPO in water and irradiated for 8 min by a Quanta-Ray Nd:YAG pulsed laser system, and the results are shown in Fig. 13. Under visible light irradiation, the characteristic signals of both the DMPO-superoxide radical (˙O₂⁻) and the DMPO-hydroxyl radical (˙OH) could be observed. In addition, it is obviously that the signal intensity of the 3% Ag/AgBr@Fe₂O₃ is much higher than that of Ag/AgBr for both the DMPO-superoxide radical (˙O₂⁻) and the DMPO-hydroxyl radical (˙OH). This result reveals a synergistic effect between AgBr and Fe₂O₃, which could enhance the formation of reactive species thus improve photocatalytic performance. The ESR analysis indicate that the ˙O₂⁻ and ˙OH are formed, and this result is a little different from the trapping experiments result. Both ESR analysis and trapping experiments revealed ˙O₂⁻ plays an important role in photodegradation process, but it is very different for ˙OH. Trapping experiments revealed ˙OH did not obvious affect the degradation rate of MO, however, ESR analysis revealed the characteristic signals of DMPO-hydroxyl radical (˙OH). It is well known that the ˙OH could oxide MO in solution.^71,72 So, if the ˙OH is actually formed in the MO degradation process, the trapping experiment of ˙OH will reveal the according result, but in fact it didn't. Since the ˙OH mainly come from oxidizing OH⁻ or H₂O by the photogenerated holes. Considering that the ESR experiments and the trapping experiments were performed under different solutions, suggesting a very little amount of ˙OH were formed in the MO degradation process due to the holes have directly oxidized MO. But the ˙OH will formed in the ESR experiment. It is well known that photocatalyst adsorption ability for dye also play a critical role in the photo-degradation process. Fig. 14 shows the comparison of MO adsorption ability of pure Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite in 15 min. It can be seen that at a same origin MO concentration, the MO adsorption ability of 3% Ag/AgBr@Fe₂O₃ composite is much higher than that of pure Ag/AgBr. Therefore, an adsorption oxidation mechanism was proposed, as discussed below.

Fig. 12 Trapping experiments of active species in the photocatalytic process.

Fig. 13 ESR spectra of radical adducts trapped by DMPO in Ag/AgBr and 3% Ag/AgBr@Fe₂O₃ composite aqueous dispersion under visible light irradiation. ESR spectra of (a) superoxide radical and (b) hydroxyl radical.

Fig. 14 The MO adsorption results of pure Ag/AgBr and Ag/AgBr@Fe₂O₃ composite in 15 min.

Based on the results of above researches, a possible Z-scheme pathway for the adsorption and photocatalytic degradation of MO over Ag/AgBr@Fe₂O₃ was proposed, as shown in Fig. 15. A uniquely hierarchical nanostructure was formed when the Fe₂O₃ nanocrystals were introduced on the surfaces of Ag/AgBr, which would provide a high surface area and numerous active sites for the photocatalytic degradation of organic substances. Under visible light, both Fe₂O₃ and Ag/AgBr were excited. Metallic Ag nanoparticles can absorb visible light, and generated electron–hole pairs.⁷³ On the one hand, the electrons will transfer to the CB of AgBr or are captured by the O₂ in the solution to form ˙O₂⁻.^74–78 On the other hand, the remained holes on the metallic Ag nanoparticles will recombine with the electrons that has been excited on the CB of Fe₂O₃ due to the high Schottky barrier at the metal–semiconductor interface.⁷⁹ For the VB holes, since the VB of AgBr is 2.6 eV, which is higher than that of the Fe₂O₃ (2.4 eV). So, the holes on the VB of AgBr will transfer to the VB of Fe₂O₃ and directly oxidize organic substances that have been adsorbed in the layer of Fe₂O₃, but neither oxidize OH⁻ nor H₂O into ˙OH. In sum, the Fe₂O₃ on the surface of the photocatalysts could improve the adsorbability for organic substances and the interaction between Fe₂O₃, metallic Ag nanoparticles and AgBr greatly improved the separation efficiency of electron–hole pairs and thus enhanced the photocatalytic performance.

Fig. 15 The proposed mechanism of the reaction process.

4. Conclusions

In summary, core–shell structured Ag/AgBr@Fe₂O₃ magnetic photocatalyst was prepared successfully via a facile solvothermal process. The Ag/AgBr were covered by Fe₂O₃ and formed a uniquely core–shell nanostructure, which would provide a high surface area and numerous active sites for the photocatalytic reaction. After the introduction of Fe₂O₃, the Ag/AgBr@Fe₂O₃ composites exhibit increased photocatalytic activity in the degradation of MO and BPA. The 3% Ag/AgBr@Fe₂O₃ composite exhibited the optimal photocatalytic performance. This enhancement could be attributed to the unique CB positions of Fe₂O₃ and Ag/AgBr result in a Z-scheme pathway photocatalytic mechanism, which could improve the separation efficiency of electron–hole pairs. Furthermore, the Ag/AgBr@Fe₂O₃ composite has antibacterial and magnetic ability, which have greatly extended the applications of Ag/AgBr based photocatalysts.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China for Youths (No. 21407065), Natural Science Foundation of Jiangsu Province for Youths (BK20140533, BK2012717), China Postdoctoral Science Foundation (No. 2014M551520, 2014M560399), Jiangsu Postdoctoral Science Foundation (1401143C) ​and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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