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Novel rattle-type magnetic Fe3O4@Ag@H-BiOCl photocatalyst with enhanced visible light-driven photocatalytic activity

  • Jianhui Shi
  • Shuting Feng
  • Tian Chen
  • Fan Wu
  • Wenbo Guo
  • Yuzhen Li
  • Peiting Zhao
Article
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Abstract

A novel rattle-type magnetic Fe3O4@Ag@H-BiOCl nanocomposite was prepared successfully by a facile solvothermal method. The structures, morphologies, magnetic and electronic properties and photocatalytic performance of as-prepared products were successfully characterized by multiple techniques. The results show that the composites exhibit an obvious cavity, excellent magnetic properties, largely increased electron–hole separation efficiency and visible-light absorption. Due to the existence of an inner cavity, this novel photocatalyst exhibits excellent adsorption and transfer performance to organic pollutants in aqueous systems. The photocatalytic activity of the materials was evaluated by the photocatalytic degradation of rhodamine B (RhB) and antibiotic agent ciprofloxacin (CIP). It was found that the Fe3O4@Ag@H-BiOCl exhibited much better photocatalytic performance than pure BiOCl, which displayed the degradation rates of RhB and CIP were 99.5 and 98.3% after 120 min under visible light irradiation, respectively. The enhanced photocatalytic activities may result from the synergetic promoting effect of surface plasmon resonance of Ag and facet-dependent oxygen vacancy of BiOCl, which significantly improves visible-light absorption capacity and separation rate of charge carriers. A possible mechanism of the enhancement of visible photocatalytic activity was proposed. This study provides a promising approach to improve visible-light-response and recycle photocatalysts to treat waste water.

1 Introduction

In the course of economic development, a series of problems on environmental pollution emerged, which have prevented the sustainable development of modern human society [1, 2]. The semiconductor-based photocatalytic technique, utilizing the renewable solar energy to mineralise organic pollutants into non-toxic products, is an effective method for degradating most types of environmental contaminants [3, 4]. Therefore, exploiting the novel and high-efficient hybrid photocatalysts is significant for air purification and waste water decontamination [5]. Among various semiconductor photocatalysts, bismuth oxyhalides are being investigated in details in recent years owing to their superior properties and practical applications. Due to the favourable visible-light response of the bismuth oxyhalides, these photocatalysts have been studied by plenty of researchers in varying degrees [6, 7]. However, as a semiconductor with a large band gap, the absorption capacity of a pure bismuth oxyhalide to visible-light is very limited towards removal of organic contaminants in an aqueous system. Therefore, many strategies have been developed for improving functional bismuth oxyhalide based visible-light photo-catalysts by doping with metal ions or constructing heterojunctions with other semiconductors. Noble metals loading may be a relatively ideal method [8, 9, 10]. Recently, the coupling of noble metallic nanoparticles with semiconductors to restrict recombination of photo-induced charge has attracted much attention [11, 12]. By depositing noble metal such as Pt, Au and Ag nanoparticles onto semiconductor surface, it showed remarkable photocatalytic activity under visible light irradiation [13, 14, 15]. The possibility is that the noble metals can strongly absorb visible light because of their surface plasmon resonance effect (SPR) [16, 17, 18]. Comparing with other noble metal, Ag nanoparticles are relative low-cost and widely used, such as Ag@AgCl, Ag/AgX/GO and Ag/g–C3N4 etc., photocatalysts.

To enhance the separation of the photocatalyst from the treated water effectively, preparing the photocatalysts with good magnetic properties is one of the most effective strategies. In particular, good magnetic properties can exhibit good magnetic separation and regeneration of the photocatalysts. The majority of researches have showed that Fe3O4/g–C3N4 can not only be recycled [19], but also be exhibited good photocatalytic performance [20, 21, 22]. It is because Fe3O4 has the remarkable magnetism and a narrow band gap (0.1 eV) [23]. Generally, Fe3O4 is similar to a conductor as its conductivity is as high as 1.9 × 106 Sm−1 [24] so that it can act as the medium to transfer photogenerated electrons rapidly.

Moreover, the excellent adsorption ability of the photo-catalyst to organic pollutant molecules also plays a major role in its degradation performance. To the best of our knowledge, mesoporous structure exhibits very high specific surface area [25, 26]. Integrating the mesoporous structure with a rattle-type structure undoubtedly can successfully enhance the mass transport ability due to the presence of inner cavities and connected mesopores, and it can improve the degradation ability to some extent indirectly.

Based on the above analysis, we designed and fabricated a novel rattle-type mesoporous magnetic Fe3O4@Ag@H-BiOCl photocatalyst successfully by a facile solvothermal synthesis method. Due to the presence of the inner cavity and mesoporous opening structure, this hierarchical photocatalyst showed a good adsorption ability to organic pollutant molecules. Simultaneously, the rattle-type structure also speeded up the interfacial charge transfer rate and reduced the rapid recombination of photogenerated charge carriers. Thus, this novel rattle-type mesoporous magnetic Fe3O4@Ag@H-BiOCl hierarchical photocatalyst showed highly excellent photoactivity for the degradation of RhB and antibiotic agent ciprofloxacin (CIP) in an aqueous system.

2 Experimental section

2.1 Reagents

All reagents are of analytical reagent grade and used without further purification. Ferrous chloride hexahydrate (FeCl3·6H2O), sodium acetate (NaAc), silver nitrate (AgNO3), TEOS, bismuth nitrate (Bi(NO3)3·5H2O), polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) were purchased from Beijing Chemicals Corporation. Ethylene glycol (EG), acetone, ammonia aqueous (NH3·3H2O), Nitric acid (HNO3) and chloroform (CHCl3) were purchased from Beijing Chemical Reagents Corporation. CIP was analytical pure and purchased by Shanghai Shunbo Biological Engineering Co. Deionized water was used in the whole experiments.

2.2 Preparation of magnetic Fe3O4 nanoparticles

Magnetic Fe3O4 nanoparticles were synthesized through a modified solvothermal reaction [27]. Typically, 40 mL of ethylene glycol was added to a mixture consisting of 1.35 g of FeCl3·6H2O and 7.2 g of NaAc and stirred for 30 min, the resulting suspension was transferred into a 50 mL Teflon lined stainless-steel autoclave and heated at 200 °C for 10 h. Then it was cooled down to room temperature naturally. The as-prepared black products (Fe3O4) were thoroughly washed with ethanol and deionized water three times, and then were collected magnetically.

2.3 Preparation of Fe3O4@Ag nanoparticles

Fe3O4@Ag was synthesized via a modified one-pot hydrothermal procedure [28]. Firstly, 0.2 g AgNO3 and 20 mg Fe3O4 was added into water–ammonia mixed solution followed by being sonicated for 30 min. Then a transparent solution was obtained by adding 0.5 g PVP into 30 mL ethanol. Additionally, the above two obtained solutions were mixed together under mechanical stirring and ultrasonic treatment for several minutes. The resultant solution was transferred into a Teflon-lined autoclave followed by being heated at 120 °C and maintained for 8 h. The resultant products were recovered with magnet and washed with deionized water and ethanol several times, and then dried at 60 °C overnight in a vacuum oven for further use.

2.4 Preparation of Fe3O4@Ag@SiO2 nanoparticles

Fe3O4@Ag@SiO2 nanoparticles were fabricated according to the modified Stöber method [29]. Typically, 0.10 g of Fe3O4@Ag nanoparticles were homogeneously dissolved in a mixture including 40 mL of ethanol, 10 mL of deioned water and 1.0 mL of ammonia aqueous solution. And then, 0.10 g of tetraethyl orthosilicate (TEOS) were added into the resultant solution. After being thoroughly stirred at room temperature for 8 h, the obtained Fe3O4@Ag@SiO2 microspheres were collected with a magnet and washed several times with ethanol and deionized water to remove nonmagnetic by products.

2.5 Preparation of Fe3O4@Ag@H-BiOCl photocatalyst

In a typical synthesis, 2.4 g of Bi(NO3)3 was dispersed in 40 mL ethylene glycol under magnetic stirring at room temperature, and then added the different chelating agents and surfactants (tartaric acid-P123 and triethanolamine-PVP) .The PH of the solution was adjusted to 7 through dropwise addition of NH3·H2O aqueous solution, and the mixture was then sealed in a Teflon lined stainless-steel autoclave that was heated to 160 °C and maintained for 3 h. The final BiOCl samples were collected by centrifugation, washed by water and ethanol several times and dried at 60 °C in a vacuum oven for 24 h.

For the Fe3O4@Ag@H-BiOCl nanocomposite, we firstly prepared and obtained Fe3O4@Ag@SiO2@BiOCl under the same condition except introducing the Fe3O4@Ag@SiO2 nanoparticles as the core. Next, Fe3O4@Ag@SiO2@BiOCl was dispersed in water–ammonia mixed solution by ultrasonic treatment for 2 h. Then it was sealed in a Teflon lined stainless-steel autoclave at 150 °C for 12 h (12 h is the optimal etching time according to our previous research [30]).

Fe3O4@BiOCl was also prepared under the same condition for comparison.

2.6 Characterization

Powder XRD patterns were recorded on a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation (Ka = 1.54059 Å). The morphologies and structures of the as synthesized samples were detected using a field emission scanning electron microscope (FESEM, S4800, Hitachi). Transmission electron microscopy (TEM) was performed with a field-emission transmission electronmicroscope (TECNAI G2, 200 kV). The X-ray photoelectronspectra (XPS) were obtained in an ECSALAB 250 spectrometer. Nitrogen adsorption–desorption analysis was operated at liquid nitrogen temperature (77 K) using a Micromeritics ASAP2010M instrument. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. UV–Vis diffuse reflection spectroscopy (DRS) was determined on a Shimadzu UV-2450 spectrophotometer using BaSO4 as the reference. The photocurrent and electrochemical impedance spectroscopy (EIS) were obtained in an electrochemical system (CHI-660B, China), using a conventional three-electrode cell. The PL spectra of the photocatalysts was measured using a spectrophotometer (Hitachi F-4500 spectrophotometer) equipped with a 150 W xenon lamp as the excitation source. Their magnetic properties were done using a VSM at room temperature.

2.7 Photocatalytic experiments

The photocatalytic performances of the Fe3O4@Ag@H-BiOCl composites were evaluated by catalyzing degradation of RhB and CIP in aqueous solution under visible light irradiation with a 300 W Xe lamp with a 420 nm cut off filter placed 20 cm away from the reaction solution. In a typical reaction, 0.05 g of the photocatalyst and an aqueous solution of RhB (10 mg/L) or CIP (10 mg/L) were added into the reactor. Before the light illumination, the suspensions were thoroughly stirred in the dark for 40 min to reach the saturated adsorption equilibrium of pollutants. At certain time intervals, 3 mL degradation solution was extracted with a magnet to extracted to determine the concentration of RhB and CIP in the aqueous solution by UV–Vis spectrophotometer (UV-2450, Shi-madzu) according to its absorbance at 553 and 276 nm, respectively.

3 Results and discussion

3.1 Structural and morphology characterizations

Figure 1 shows the diagrammatic sketch of preparation of the Fe3O4@Ag@H-BiOCl and its SEM images. The synthesis of Fe3O4@Ag@H-BiOCl nanocomposites involves five steps: preparation of the monodisperse Fe3O4 nanoparticles, loading Ag on the Fe3O4 nanoparticles, coating silica layer onto the Fe3O4@Ag nanocomposites, depositing the BiOCl particles on the surface of Fe3O4@Ag@SiO2 and etching the SiO2 layer. Finally, we obtained the magnetic nanocomposite Fe3O4@Ag@H-BiOCl with rattle-type structure. The cavity of the resultant solids can be seen in the SEM image of Fig. 1, which is pointed through the red arrow.

Fig. 1

Schematic illustration of the preparation of Fe3O4@Ag@H-BiOCl composites

The phase structure of the as-prepared samples is investigated by X-ray diffraction. Figure 2 displays XRD patterns of Fe3O4, Fe3O4@Ag, Fe3O4@Ag@SiO2, BiOCl and Fe3O4@Ag@H-BiOCl. The XRD typical diffraction peaks of Fe3O4 at 2θ = 30.11°, 35.81°, 43.11°, 57.31° and 63.01° could be ascribed to [220], [311], [400], [511] and [440] planes of Fe3O4 in a face-centered cubic (fcc) Fe3O4, respectively [31]. The XRD pattern of Fe3O4@Ag is successfully characterized by four characteristic diffraction peaks located at 2θ = 38.2°, 44.3°, 64.5° and 77.5°, corresponding to the [111], [200], [220] and [311] planes of the fcc phase of Ag, respectively [32]. However, the intensity of the four peaks in the Fe3O4@Ag@SiO2 composite significantly weakens because of the silica layer. In the XRD patterns of Fe3O4@Ag@H-BiOCl composite, the obvious diffraction peaks can be ascribed to the tetragonal structure of BiOCl besides of the characteristic diffractions of cubic Fe3O4 [33], indicating the successful crystallization of BiOCl on the surface of the Fe3O4@Ag magnetite core. The specific peaks of Ag are absent because of its low content and the existence of outer BiOCl. Notably, the intensity of the [001] peak of BiOCl and Fe3O4@Ag@H-BiOCl is much stronger than that of the [110] peak, which indicates that the [001] facet was exposed preferentially by as-obtained BiOCl through adding tartaric acid-P123. It will be beneficial to improve the photocatalytic activities of composites. Therefore, BiOCl studied in this article was pointed to adding tartaric acid-P123.

Fig. 2

Wide-angle XRD patterns of Fe3O4, Fe3O4@Ag, Fe3O4@Ag@SiO2, BiOCl and Fe3O4@Ag@H-BiOCl

The surface compositions and chemical states of as-products were further investigated by XPS spectra in the Fig. 3a, which revealed that the surface of the Fe3O4@Ag@H-BiOCl photocatalyst consisted of Fe, O, Bi, Cl and Ag elements. In Fig. 3b, two peaks were located at 710.6 and 724.0 eV in the high-resolution XPS spectra of Fe 2p belong to Fe2p3/2 and Fe 2p1/2, respectively. These binding energies could be attributed to Fe3O4. In Fig. 3c, the O 1 s peaks were divided into three peaks. The peaks located at 530.4 and 531.7 eV were assigned to the Bi–O bonds of the (BiO)22+ slabs of BiOCl and the surface-adsorbed hydroxyl groups, respectively [34]. The binding energy located at 529.7 eV was ascribed to the Fe-O bond [35]. The peak in Fig. 3d at about 201.0 eV could be assign to Cl element. The peaks in Fig. 3e located at 160.2 and 165.5 eV were ascribed to Bi 4f7/2 and Bi 4f5/2, respectively. The lower binding energies centered at 163.2 and 158.1 eV were assigned to lower charged Bi ions, which may be due to the existence of oxygen vacancies [36]. Ag can be proved via the existence of the signals of Ag 3d5/2 and Ag 3d3/2 located at 368.4 and 374.3 eV as shown in Fig. 5f. The XPS profiles further provided insight that the prepared photocatalyst contained Fe3O4, Ag and BiOCl, which is consistent with the XRD analysis.

Fig. 3

XPS spectra of Fe3O4@Ag@H-BiOCl: a survey spectrum, b Fe 2p, c O 1 s, d Cl 2p, e Bi 4f and f Ag 3d

The morphology and size details of samples were investigated by TEM measurement, which was given in Fig. 4.The TEM image of pure Fe3O4 (Fig. 4a) indicates Fe3O4 is spherical and the mean diameter of the particles is about 180 nm. They have the narrow size distribution and excellent monodispersity. In addition, the results revealed that the surface of the Fe3O4 nanoparticles is rough and they are formed of numerous smaller crystals. Figure 4b exhibits the TEM image of Fe3O4@Ag microspheres with a mean diameter of about 200 nm, which obviously shows that the shell is composed of smaller particles. The smaller particles are Ag nanoparticles of about 3 nm. In the Fig. 4c, we can obtain that the as-prepared Fe3O4@Ag@SiO2 microspheres still sustain the morphological properties and the mean size of particle increases to 230 nm, which showed the successful coating of SiO2 layer on the surface of Fe3O4@Ag. So we can observe the obvious core–shell structure of Fe3O4@Ag@SiO2. The black inner layer is the Fe3O4@Ag core and the gray outer layer is the amorphous SiO2 shell, and the silica layer is around 15 nm in thickness. Figure 4d shows the TEM photograph of Fe3O4@Ag@H-BiOCl obtained via etching of SiO2 for Fe3O4@Ag@SiO2@BiOCl in the ammonium solution for 12 h. The results showed the BiOCl nanoparticles have successfully deposited on the surface of Fe3O4@Ag and obtained the typically rattle-type structured Fe3O4@Ag@H-BiOCl nanoparticles with a clear core–shell and a cavity, which is consistent with the SEM results in Fig. 1. The thickness of BiOCl layer is about 10 nm.

Fig. 4

TEM images of a Fe3O4, b Fe3O4@Ag, c Fe3O4@Ag@SiO2 and d Fe3O4@Ag@H-BiOCl

Performing an analysis of N2 adsorption–desorption is necessary to further prove the rattle-type mesoporous structure. In the Fig. 5a, the Fe3O4@Ag@SiO2@BiOCl nanocomposites only display that the mesoporous structure may be attribute to hierarchical structures of the outer BiOCl. As shown in Fig. 5b. The rattle-type structure Fe3O4@Ag@H-BiOCl compound exhibits a remarkable hysteresis loop, indicating the typical ink-bottle-type pores in which large cavities are linked through narrow windows. The surface area, pore volume and average pore size of Fe3O4@Ag@H-BiOCl are 457.6 m2/g, 2.35 cm3/g, and 2.76 nm, respectively. The results indicated the final products still keep integrity and possess rattle-type mesoporous structure, which is consistent with the results of TEM and SEM. The relatively big pore may be ascribed to the inner cavity while small pores could be attributed to the hierarchical structures of the outer BiOCl. The unique rattle-type structure can improve the photoactivity of the BiOCl photocatalyst, which should mainly because of the excellent adsorption performance to the target molecules and the fast interfacial charge transfer. This point also was verified by the photocatalytic experiments.

Fig. 5

Nitrogen-sorption isotherms of a Fe3O4@Ag@SiO2@H-BiOCl, b Fe3O4@Ag@H-BiOCl (inset is BJH pore-size distributions)

3.2 Magnetic property

The magnetic properties of the multifunctional compounds were successfully characterized using a vibrating sample magnetometer (VSM). The magnetization curves of the obtained products registered at 300 K indicate that nearly no residual magnetism is detected (Fig. 6), In other words, the hybrids exhibited paramagnetic characteristics. Magnetic measurements exhibit that Fe3O4, Fe3O4@Ag, Fe3O4@Ag@SiO2 and Fe3O4@Ag@H-BiOCl have magnetization saturation values of 81, 45, 27 and 21 emu/g, respectively. Compared with the Fe3O4 nanoparticles, the special saturation magnetization of the Fe3O4@Ag@H-BiOCl hybrids is decreased significantly, attributed to the diamagnetic contribution of the silica, Ag nanoparticles and the BiOCl shell lead to a low mass fraction of the Fe3O4 magnetic substance. The final products can quickly respond to the external magnetic field (the inset of Fig. 6) Although they only have low saturation magnetization value, which provides their potential application for the photocatalysist reusability.

Fig. 6

The magnetic hysteresis loops of Fe3O4, Fe3O4@Ag, Fe3O4@Ag@SiO2 and Fe3O4@Ag@H-BiOCl

3.3 Optical characterization and PL analysis

The photocatalytic performance of the photocatalysts is closely associated with their light absorption ability. For comparison, the UV–Vis DRS of pure BiOCl, Fe3O4@BiOCl and Fe3O4@Ag@H-BiOCl are recorded and exhibited in Fig. 7a. It is obvious that BiOCl shows absorption wavelengths from the UV to the visible region up to 460 nm. In comparisons, Fe3O4@BiOCl exhibits the more excellent light absorption performance and its light harvest range also displays obvious red-shift reaching to ∼1400 nm, which are mainly due to the narrower band gap of Fe3O4. Furthermore, when the Ag species are deposited on the surface of Fe3O4, Fe3O4@Ag@H-BiOCl exhibits more excellent light absorption in the visible light region. this may be resulted from the surface plasmon resonance (SPR) effect of Ag nanoparticles [37] and the introduction of the inner cavity. The broader light absorption region and the stronger light absorption of Fe3O4@Ag@H-BiOCl are able to make the most of visible light and generate more effectively photogenerated electron–hole pairs, leading to the higher photocatalytic activity.

Fig. 7

a UV–Vis diffuse reflectance spectra, b photoluminescence (PL) spectra of BiOCl, Fe3O4@BiOCl and Fe3O4@Ag@H-BiOCl

Photoluminescence (PL) spectra has been widely used to investigate the charge transfer, migration and recombination behaviour in photocatalysts. As we all know, weaker intensity represents lower recombination probability of photo-induced charge carriers. Figure 7b shows the PL spectra for pure BiOCl, Fe3O4@BiOCl and Fe3O4@Ag@H-BiOCl. It can be observed that BiOCl exhibits more intense fluorescence mission peak at ∼426 nm, while the PL intensity of Fe3O4@BiOCl is obviously lower than that of it. The lower PL intensity is necessarily contributed to the good electrical conductivity of Fe3O4, which demonstrates the efficient interfacial electron transfer from conduction band of BiOCl to that of Fe3O4 nanoclusters, inhibiting the recombination of photoinduced charge carriers. For Fe3O4@Ag@H-BiOCl composite, it can be observed that more seriously quenching phenomenon is further taking place compared with Fe3O4@BiOCl, which should be due to good electrical conductivity and induced SPR effect of Ag nanoparticles. Therefore, Fe3O4@Ag@H-BiOCl hampers the recombination of photo-induced charge carriers more effectively to improve photocatalytic performance. The transfer and recombination behaviour of electron–hole pairs in the interfaces of photocatalyst is one of the most important effect factor on photocatalytic performance. The efficient delivery of charge carriers can enhance the photocatalytic activity of photocatalyst.

3.4 Photocatalytic degradation of contaminants and reusability

Firstly, the photocatalytic activity of as-synthesized composites is evaluated by the photocatalytic degradation to RhB under visible light illumination. The saturated adsorption equilibrium has been achieved between the catalyst and RhB via adsorption process for 40 min. In the Fig. 8a, it can be observed that 3.42, 5.60 and 32.2% of RhB were adsorbed by BiOCl,Fe3O4@Ag@SiO2@BiOCl and Fe3O4@Ag@H-BiOCl respectively within 40 min, which should be resulted from their different adsorption capacities. This is consistent with the N2 adsorption–desorption results. Figure 8b displays the photocatalytic activities of BiOCl, Fe3O4@Ag@SiO2@BiOCl and Fe3O4@Ag@H-BiOCl under the illumination of visible light. The blank test shows that the RhB is slightly degraded without any photocatalyst, elucidating that the photolysis of RhB could be neglected. However, the introduction of Fe3O4 and Ag nanoparticles could improve the photocatalytic activity of BiOCl. Furthermore, we can see that the Fe3O4@Ag@H-BiOCl exhibits the highest photocatalytic activity, which is much better than that of Fe3O4@Ag@SiO2@BiOCl. It may be attributed to the bigger surface area and the existence of the cavity for Fe3O4@Ag@H-BiOCl. Figure 8c shows the time-dependent adsorption spectra of RhB solution in the existence of the Fe3O4@Ag@H-BiOCl photocatalyst under visible light illumination. In addition, the photocatalytic degradation kinetics curves of RhB via different samples were displayed in Fig. 8d. All the photo degradations of RhB by the different samples efficiently obeyed the pseudo-first-order kinetics. It is easy to see that the Fe3O4@Ag@H-BiOCl photocatalyst possesses the highest degradation rate which is about 1.7 times higher than that of pure BiOCl sample. Considering the importance of photocatalyst stability in practical applications, the photocatalytic reusability of theFe3O4@Ag@H-BiOCl photocatalyst for RhB degradation under visible light irradiation was also investigated. After each cycling photocatalytic experiment, the photocatalyst was collected, washed and dried for next recycling. From the Fig. 8e, it is can be seen that the Fe3O4@Ag@H-BiOCl sample exhibits the same photocatalytic degradation activity over five successive cycles for the degradation of RhB. The result suggests that the high-stability of Fe3O4@Ag@H-BiOCl for its practical application.

Fig. 8

a The saturated adsorptions in the dark, b photocatalytic degradation of RhB, c UV–Vis spectrum changes, d kinetic fit for the degradation, e cycling runs for the photodegradation of RhB, f photocatalytic degradation of CIP of pure BiOCl, Fe3O4@Ag@SiO2@BiOCl and Fe3O4@Ag@H-BiOCl

Ciprofloxacin (CIP) has been extensively used for treating bacterial infections, which can generate great harm to the environments and human health. Therefore, it is extremely important for the removal of CIP. The Fig. 8f displays the degraded results for CIP by BiOCl,Fe3O4@Ag@SiO2@BiOCl and Fe3O4@Ag@H-BiOCl under the visible light illumination. The results display that only 30% CIP is photodegraded by pure BiOCl under visible light within 30 min. However, the Fe3O4@Ag@H-BiOCl compounds with the rattle-type structure shows the highest activity. After the illumination for 30 min, 66% CIP could be photodegraded by Fe3O4@Ag@H-BiOCl. And when the irradiation time prolongs to 120 min, the removal efficiency could be up to 98.3%. It possesses a similar results with the degradation results of RhB.

Based on the above results, the possible degradation mechanisms have been discussed in Fig. 9. In the Fe3O4@Ag@H-BiOCl system, we consider that the generation of oxygen vacancy may lead to the formation of the mid-gap states. Firstly, the electrons could be excited up to oxygen vacancy states rather than the conduction band from the valence band of BiOCl under visible light illumination. The electrons in the mid-gap states and conduction band may reduce oxygen to produce ·O2−, at the same time, the valence band generates h+. Then photocatalytic degradation of pollutants can be achieved by ·O2− and h+. In addition, the visible light can irradiate to the Ag species directly because of the rattle-type structure, SPR-excited electrons would first enrich on the surface of Ag and then efficiently transfer to the conduction band of BiOCl, leaving over holes to oxidize RhB on the Ag species. The electrons on the conduction band of BiOCl were trapped by molecular oxygen to produce ·O2−, and then the formed ·O2− would also further react with RhB. Therefore, the synergetic effect of surface plasmon resonance of Ag and facet-dependent oxygen vacancy of BiOCl results in the enhancement of the photocatalytic activities of Fe3O4@Ag@H-BiOCl under visible light.

Fig. 9

Schematic diagrams for the possible photocatalytic mechanism of Fe3O4@Ag@H-BiOCl

4 Conclusions

In summary, A novel magnetic Fe3O4@Ag@H-BiOCl photocatalyst with the rattle-type structure was successfully fabricated. The nanocomposites exhibit excellent adsorption capacity because of the inner obvious cavity. It shows efficient separation and delivery of photo-generated carriers and enhanced photocatalytic activity, which are due to the synergetic effect of surface plasmon resonance of Ag and facet-dependent oxygen vacancy of BiOCl. The 99.5% of RhB and 98.3% of CIP could be potodegraded after 120 min of visible light illumination in the existence of Fe3O4@Ag@H-BiOCl. At the same time, the composites exhibit good magnetic properties, which indicates their potential application for the photocatalysist separation. This work offers a promising approach to enhance visible-light-response and recycle photocatalysts for environmental remediation.

Notes

Acknowledgements

This work was supported by Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2015128), and Qualified Personnel Foundation of Taiyuan University of Technology (QPFT) (No: tyutrc-201326c) and the Shanxi Provincial Key Research and Development Plan (general) Social Development Project (201703D321009-5).

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  • Jianhui Shi
    • 1
  • Shuting Feng
    • 1
  • Tian Chen
    • 1
  • Fan Wu
    • 1
  • Wenbo Guo
    • 1
  • Yuzhen Li
    • 1
  • Peiting Zhao
    • 1
  1. 1.College of Environmental Science and EngineeringTaiyuan University of TechnologyTaiyuanPeople’s Republic of China

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