key: cord-0073001-lqbr7rsc
authors: Xue, Limei; Liang, Ermiao; Wang, Jiwen
title: Fabrication of magnetic ZnO/ZnFe(2)O(4)/diatomite composites: improved photocatalytic efficiency under visible light irradiation
date: 2022-01-06
journal: J Mater Sci: Mater Electron
DOI: 10.1007/s10854-021-07568-w
sha: 2deae9ecbcd27723948d9249af90256497254e29
doc_id: 73001
cord_uid: lqbr7rsc

The magnetic recoverable ZnO/ZnFe(2)O(4)/diatomite (ZZFDT) composite was synthesized by hydrothermal-precipitation method. The structure, optical properties and magnetic properties of the composites were characterized by different analytical instruments. ZZFDT-1 is composed of cubic spinel, hexagonal wurtzite, tetrahedron structure. SEM and TEM showed that ZnO and ZnFe(2)O(4) particles were loaded onto the surface of diatomite, and the particle size was uniform. In addition, ZZFDT-1 is a typical mesoporous material with a specific surface area of 65.3 m(2)/g and pore size of about 12 nm. The response range of ZZFDT-1 is extended to visible light, and the band gap is 1.5 eV. Moreover, the M–H hysteretic curves of ZZFDT-1 exhibited superparamagnetic properties. The photocatalytic activity of different samples was evaluated by the conversion rate of oxytetracycline (OTC) under visible light. ZZFDT-1 has the best photocatalytic activity and the conversion is up to 95%. Because of its magnetic nature, it can be easily separated from the solution. The results showed that the ZZFDT composite has good photocatalytic activity under visible light. After being reused six times, it still has good stability.

In recent years, antibiotics have been widely produced and used in medicine, food processing, livestock breeding, aquaculture and other fields. But the antibiotic itself is difficult to be degraded, the residual antibiotic will enter the water body, will cause considerable harm to the human and the ecological environment [1, 2] . Thus, we need to find an effective and green way to deal with it. Photocatalysis technology has attracted much attention because it can effectively degrade antibiotics to small molecules without secondary pollution [3] [4] [5] [6] . ZnO has attracted much attention because of its low cost, non-toxicity and high quantum efficiency. Because ZnO has a band gap of 3.2 eV, it can only absorb ultraviolet light. However, ultraviolet light only accounts for 3-5% of solar energy, and the insufficient utilization of solar energy seriously hinders the practical application of ZnO [7] [8] [9] [10] [11] .

Many scholars have proposed many methods to modify ZnO. The most typical method is to combine with other narrow band gap semiconductors to form a heterostructure. The energy band potential of the narrow band gap semiconductor matches the energy band potential of ZnO, and a uniform FERMI energy level is formed between the two semiconductors, this leads to the generation of an internal electronic field and the diffusion of electrons at the conduction and valence band positions. This charge transfer effectively impedes the recombination rate of photogenerated electron-hole pairs and improves the optical quantum efficiency such as in ZnO/Bi 2 O 3 [12] , MoO 3 /ZnO [13] , Ag 2 O/ZnO [14] , ZnO/AgI [15] .

Magnetic nanomaterials are widely used in biomedicine, sensors, catalysts, optics and other fields [16] [17] [18] [19] because of their excellent biocompatibility, adaptability, physical chemistry stability, excellent magnetic properties [20] . Yang et al. [21] prepared ZnFe 2 O 4 hollow spheres by one-step hydrothermal method using carbon plate as a template. The results showed that the response range of ZnFe 2 O 4 Hollow Sphere Sensor to ethylene glycol is 35.5 * 100 ppm. The ZnFe 2 O 4 hollow ball Sensor still has a good gas response during the measurement within eight weeks, showing long-term stability. Patade et al. [22] prepared superparamagnetic MnFe 2 O 4 nanoparticles (NPS) were synthesized by the co-precipitation method. Under the condition of 4 kA/m amplitude and 280 kHz frequency, the induction heating of MnFe 2 O 4 magnetic NPS was studied. The results showed that 0.4 g/mL MnFe 2 O 4 magnetic NPS could reach the thermotherapy temperature (42°C) within 260 s, proving that the material can be used as a heating agent for magnetic heat treatment. However, magnetic nanoparticles in solution is easy aggregation and toxicity. To solve these problems, Kharat et al. [23] coated CoFe 2 O 4 particles with Surfactant (oleic acid), which has anti-inflammatory and complementary anticancerous effects, improved the stability of the nanoparticles, and reduced the accumulation. CoFe 2 O 4 nanoparticles coated with 1 mg/mL oleic acid have great potential for magnetic fluid hyperthermia through biocompatibility research and ongoing clinical trials. Somvanshi et al. [24] prepared ZnFe 2 O 4 nanoparticles by combustion synthesis method and their surface were functionalized by silica and carboxyl-modified polyvinyl alcohol. They proposed a novel approach to extract COVID-19 virus RNA using surface-functionalized ZnFe 2 O 4 . The method is simple and can reduce the diagnostic time.

In the family of magnetic nanomaterials, spinel ferrite nanomaterials are characterized by their high electrical resistivity, high saturation magnetization, low eddy current, good physical chemistry properties, and low cost [25, 26] , which is widely used in memories storage, a drug targeted delivery, magnetic hyperthermia, magnetic resonance imaging, photocatalysis and other fields [27] [28] [29] [30] . The general formula of Spinel ferrite material is MFe 2 O 4 , where M is Co 2? , Ni 2? , Mn 2? , Zn 2? and other divalent metal ions, which are in tetrahedral position, while trivalent Fe ions are in octahedral place. Many ferrite semiconductors also exhibit good photocatalytic activity under visible light. ZnFe 2 O 4 (ZF) is a p-type semiconductor photocatalyst, which can be used as an alternative material because of its relatively small band gap (about 1.8 eV), non-toxic, stable photochemical properties and low cost [31] [32] [33] [34] [35] . In recent years, ZF has been reported to photodegrade pollutants [36] . In addition, because of its magnetic properties, it can be recovered from solution by an external magnet, which makes it have great potential in practical application. The formation of ZnO/ZF heterojunction can inhibit the recombination of photogenerated electron-hole pairs and improve photocatalytic performance [37] [38] [39] [40] [41] [43] reported ZnFe 2 O 4 /Bi 0 -Bi 2 MoO 6 heterojunction photocatalyst can be used for photocatalytic reduction of CO 2 to methanol. Under visible light irradiation, when the coupling ratio of ZnFe 2 O 4 /TiO 2 is 1:1 and the calcination temperature is 900°C, the maximum methanol yield (141.22 lmol/gcat h) is obtained. Now, there is been another research interest on the immobilization of photocatalyst particles onto various substrates. Compared with activated carbon and Graphene, diatomite (DT) is a mineral-rich natural material, which has the advantages of graded porous structure, excellent adsorption capacity, high chemical stability and low price [44] [45] [46] [47] . Diatomite-based photocatalysis composite materials often have excellent photocatalytic activity, which is attributed to the diatomite can effectively improve the contact probability between catalysts and pollutants, leading to the increase of degradation rate such as in F-TiO 2 /DT [48] .

ZnFe 2 O 4 and ZnO can form n-n homotype heterojunction, which hinders the recombination of photogenerated electron-hole pairs and improves the utilization ratio of carriers. Because ZnFe 2 O 4 can absorb visible light, the response range of ZnO/ ZnFe 2 O 4 can be extended to the visible light region. Diatomite as a carrier of ZnO/ZnFe 2 O 4 can improve the contact probability between the photocatalyst and antibiotics because of its excellent adsorption property. In addition, ZZFDT is magnetic and easy to recycle. In a word, the ZnO/ZnFe 2 O 4 /diatomite composite has a good synergistic effect. Moreover, ZnO/ZnFe 2 O 4 /diatomite ternary composites have not been reported and have high research value.

This research aims to prepare ZnO/ZnFe 2 O 4 /diatomite (ZZFDT) composites by a hydrothermalprecipitation method. The structure, morphology and optical properties of ZZFDT were analyzed. The photocatalytic activity of ZZFDT was evaluated by the degradation of oxytetracycline (OTC) under visible light, and the mechanism of photocatalytic degradation of OTC was proposed. O, NaOH and (C 6 H 9-NO) n (PVP) were purchased from Sinopharm Group Chemical Reagent Co., Ltd and are all analytical grade. The crude diatomite was purchased from Yuan-Tong Mineral Co., Ltd (Jilin Province, PRC), and pretreated by pickling [49] . Deionized water was homemade.

ZnFe 2 O 4 /diatomite (ZFDT) was synthesized by the hydrothermal method. The pretreated diatomite (0.7 g) was mixed with 20 mL deionized water, and C 4 H 6 O 4 ZnÁ2H 2 O (0.66 g), Fe(NO 3 ) 3 Á9H 2 O (2.424 g), PVP (0.1 g) were added to the above mixture, which was kept under constant stirring. The NaOH solution was added into the mixed solution to pH 11 and stirred for 30 min. Then, the hybrid solution was transferred to a 100 mL reactor and heated at 180°C for 24 h. After the autoclave was cooled to room temperature, the solid product in the reaction solution was collected by a magnet and washed three times with water and ethanol. The solid product was dried at 80°C for 6 h and calcined at 500°C for two h to afford ZFDT. Pure ZF was prepared by the above method without adding diatomite.

ZZFDT was described synthesized by precipitation method. ZFDT (0.5 g) and C 4 H 6 O 4 ZnÁ2H 2 O (0.68 g) were added into absolute ethanol (20 mL), and the mixture was kept under constant stirring. After 0.5 h, NaOH (0.25 g) was added dropwise into the mixture. The mixed solution was then kept in a water bath at 80°C for two h, during which 5 mL deionized water was added twice. The solid product in the reaction solution was collected by a magnet and washed three times with water and ethanol. The solid product was dried at 120°C for 6 h to afford ZZFDT (ZZFDT-1) composite. Adjusting to the mass ratios of ZnO, ZF, and DT, the obtained products were defined as ZZFDT-2 (1:1:1), ZZFDT-2 (1:1:2), ZZFDT-3 (1:1:3), ZZFDT-4 (1:2:1), ZZFDT-5 (1:3:1), ZZFDT-6 (2:1:1), and ZZFDT-7 (3:1:1). For comparison, ZnO, ZnO/diatomite (ZDT) and ZnO/ZnFe 2 O 4 (ZZF) were prepared by the above methods. Figure 1 shows the schematic illustration of the synthesis process of ZZFDT. The chemical reactions involved in the synthesized of ZZFDT:

The Crystal Structure of the sample was determined by a Bruker 8-Advance X-ray diffractometer at 2h = 10°to 80°under the following conditions: Cu Ka = 0.15406 nm, 40 kV, 40 mA, 5°/min. The absorption peaks in the wavenumber range of 400-4000 cm -1 were measured by FT-IR spectrometer (NiCOLETIS5). The XPS spectra of the samples were measured by X-ray photoelectron spectroscopy (TFSK-250XI) at the vacuum of 5 9 10 -10 Pa. The charge correction is carried out with C 1s = 284.8 eV as the energy standard. The UV-Vis DRS of the samples were measured by ultraviolet-visible Spectrometer (UV-2500, Shimadzu, Japan), with Barium Sulfate as reference. The morphology of the samples was tested by FE-SEM (MX2600FE, U.K.) and TEM (JEM2100F, Japan). The DLS and zeta potential of the samples were measured, and the particle size and potential were obtained. The specific surface area and pore size distribution of the samples were measured using a particular surface area and pore size analyzer (V-SorbX800) at 77 K. The magnetic properties of the samples were analyzed by VSM (EV-9 Hitachi, Japan).

The photocatalytic activity of the composite was evaluated by degrading OTC solution (10 mg/L) under visible light irradiation (300 W Xenon lamp). The mixture of 10 mg/L OTC solution (100 mL) and 0.1 g catalyst sample was stirred in darkness for 30 min to establish adsorption-desorption equilibrium between OTC molecule and photocatalyst. After the illumination was turned on, 3 mL of the suspension was removed at regular intervals. The absorbance of the sample was determined by a UV-Vis spectrophotometer (SPECORD S600, Germany) at 362 nm. Combined with the standard curve of OTC solution, the concentration variation of different samples was studied. The OTC conversion rate is calculated by the following formula:

where C 0 is the initial concentration of OTC solution, C is the concentration of OTC solution after a reaction period.

3 Results and discussion 112) and (201) crystal planes of hexagonal-wurtzite structure ZnO, respectively [51] . In addition, the diffraction peaks appearing at 2h = 21.89°, 35.9°were indexed according to ''Bragg's X-ray diffraction'' law and matched with the standard diffraction data PDF data card sheet No. 76-0973. The indexed results showed that these peaks correspond to the diffraction peaks of crystal quartz SiO 2 [52] , indicating the existence of crystal quartz in DT. The main component of DT is amorphous silicon dioxide, which XRD cannot detect. ZZFDT-1 contains diffraction peaks of pure ZnO, ZF, and DT, proving that the ZZFDT ternary composite has been successfully synthesized.

As can be seen from the illustration in Fig. 2b , the diffraction peak of ZZFDT-1 on the (220) crystal plane is shifted to the left compared with pure ZF, this may be due to the formation of Si-O-Zn, Si-O-Fe bonds (FT-IR can be demonstrated) by the hybridization between DT and ZF, ZnO, resulting in lattice distortion. Figure 3a is the XRD pattern of ZZFDT with different mass ratios. With the increase of the content of one component in ZZFDT, the intensity of its diffraction peak increases gradually, without changing the crystal structure of any component. The diffraction peaks in ZZFDT with different mass ratios widen slightly and the intensity decreases, which indicates that the hybridization between DT and ZF, ZnO impedes the growth of crystal particles [53] .

The crystallite size of the ZF in pure ZF and various composite ratios ZZFDT was calculated using the Debye-Scherrer formula [54] :

where D is the crystallite size (nm), K is the constant (0.9), k is X-ray wavelength (1.541 nm), h is diffraction angle, b is half-peak width of XRD diffraction peak. The average crystallite size of the prepared ZF was 18 * 24 nm. The lattice constant can be calculated from the following formula [55] :

where d is the interplanar spacing and (h k l) is the Miller exponent of the plane. With the increase of DT, the constant lattice decreases from 8.600 to 8.545 Å , and the crystallite size decreases from 21 to 18 nm, because of the interaction between ZF and DT resulted in the formation of new chemical bonds. The X-ray density of ZF was determined by molecular weight and unit cell volume and was calculated using the following formula [56] :

where N A is the Avogadro number and a is a lattice parameter. The values of the lattice parameter are listed in Table 1 .

To verify the crystallite size of ZF in ZZFDT-1 which was calculated by Debye-Scherrer formula, it was recalculated by the Williamson-Hall (W-H) method [36] . The Williamson-Hall (W-H) method is an elegant method used to define the crystallite size and the microstrain from the reflection broadening. The strain has a direct effect on the physical broadening of the XRD spectrum, and the broadening peak of the strain is:

Equation (11) is given by Eq. (7):

Equation (12) can be obtained by combining Eqs. (10) and (11)

where D and e are the crystal size value and microstrain value, respectively. For prepared ZZFDT-1, a plot is drawn with 4sinh as x-axis and bcosh as y-axis, as shown in Fig. 3b . The crystallite size is calculated by the y-axis intercept of linear fitting, and the strain e is estimated by the slope of the fitting.

The crystallite size of ZF in ZZFDT-1 calculated by Eq. (13) is 20 nm, which is a little different from 21 nm calculated by the Debye Scheler formula.

To further study the morphology and microstructure, SEM of ZZFDT-1 were also observed. Figure 4a shows that diatomite has a disc-like appearance and is rich in the pore structure. The smaller pieces around the disk come from skeletons of different shapes, cylindrical, elliptical, or fragmented [57] . This structure makes diatomite have some adsorption ability, which provides a good condition for the loading of photocatalytic materials and the occurrence of photocatalytic reaction. Figure 4b illustrates the coagulation and deposition of ZnO and ZF particles on the pore edge and surface of the diatom disk. The interaction between ZnO and ZF at the interface can enhance the charge transfer ability and thus improve the photodegradation efficiency. Figure 4c and d are TEM diagrams of ZZFDT-1 with different magnification. It can be seen that ZnO and ZF are loaded on DT surfaces with uniform sizes. Figure 4e shows the particle size distribution obtained by fitting the TEM diagram with the Gaussian function. The crystallite size of ZF in ZZFDT-1 is about 15 nm, which is consistent with the average crystallite size of XRD analysis. 

The composition and combination state of elements in the ZZFDT-1 composite was analyzed by XPS spectra. Figure 6a shows the survey spectrum of the ZZFDT, including the Zn, Fe, O, Si, and C elements. Among these, carbon element comes from the carbon contaminant of the XPS analytical instrument. Figure 6b is the spectrum of Zn 2p, and two peaks are located at 1021.1 eV and 1044.1 eV, belonging to Zn 2p 3/2 and Zn 2p 1/2 , indicating the existence of Zn as Zn 2? [64] . Figure 6c shows the spectrum of Fe 2p, two peaks are located at 710.9 and 724.3 eV, corresponding to Fe 2p 3/2 and Fe 2p 1/2 , indicating the existence of Fe as Fe 2? [65] . Figure 6d is the XPS spectrum of O 1s. Three characteristic peaks can be obtained by software processing. The peaks at 527.4 eV is attributed to O in the silanol group on the surface of the sample. The peak at 530.6 eV is attributed to O in the Lattice [66] , and the peak at 532.2 eV is attributed to [67, 68] . These bonds can be used as electron transport channels to enhance the charge transfer ability and thus improve the efficiency of photocatalysis. Figure 7 shows the N 2 adsorption-desorption isotherms and pore size distribution curves of ZnO and ZZFDT-1. As shown in Fig. 7a , the N 2 adsorptiondesorption isotherms of ZZFDT-1 exhibit the classical type IV curves according to the International Union of Pure and Applied Chemistry (IUPAC) classification. With the increase of the relative pressure, the adsorption amount of N 2 increases slowly, which indicates that the interaction between the adsorbate and the adsorbent is weak. Subsequently, near the saturation pressure, the adsorbate molecules congregate at the most favorable active sites on the surface of macropores of diatomite, leading to the adsorption amount of N 2 increasing significantly. The curve-shaped hysteresis loop belongs to the H3 type, related to the capillary condensation with slit-like pores in the material. Because as the capillaries condense, they merge to form narrow slit-like pores. ZZFDT-1 is a typical mesoporous material [69] . Compared with ZnO, the adsorption volume of ZZFDT-1 on nitrogen increased slightly in the wide range of P/P 0 relative pressure. The specific surface area of ZZFDT-1 is more significant than that of ZnO. This is due to the introduction of a special skeleton DT, so that ZnO and ZF are loaded to the surface, playing a certain role in dispersion. The addition of DT can inhibit the growth of ZnO and ZF particles, resulting in smaller grains, which can also improve the surface condition. ZZFDT-1 has a high specific surface area, which is helpful to adsorb pollutants and increase the reaction site, thus improving the photocatalytic activity [70] . The surface pore size distribution was investigated using the BJH method, and the results are shown in Fig. 7b . The average pore diameters of ZZFDT-1 and ZnO are 11.94 nm and 32.52 nm. The total pore volume (0.25 cm 3 /g) of ZZFDT-1 is much larger than that of ZnO. The mesoporous volume of ZZFDT-1 is more significant than that of ZnO, which may be related to the morphology of diatomite (confirmed by SEM). 

The particle size distribution of the sample was studied by dynamic light scattering (DLS) [71] . Figure 8a shows the cumulative crystallite size curve of ZnO, ZZFDT-1. The particle sizes of 90% ZnO and 90% ZZFDT-1 are smaller than 43.57 nm and 29.49 nm, respectively. Because ZnO is insoluble and agglomerative easily in colloidal suspension, its particle size is larger. The decrease of ZZFDT-1 hydrodynamic diameter may be related to the reduction of the crystallite size of ZF. The reduction of particle size of ZZFDT-1 shows that the degree of particle aggregation is reduced, which means that the addition of DT improved the stability of ZZFDT-1. To understand the stability of ZZFDT-1 in the fluid medium, zeta potential was also measured. Figure 8b shows the zeta potential of ZnO and ZZFDT-1 as a function of PH. If the zeta potential of the particle is higher than ± 30 mV, the electrostatic stability of the particle is better [72] . Zeta potentials of ZnO and ZZFDT-1 were ± 35 mV and ± 48 mV, which were significantly higher than ± 30 mV. The isoelectric point (Ip) of ZnO and ZZFDT-1 are 5.8 and 6.8, respectively. The isoelectric point of ZZFDT-1 is higher than that of ZnO, which indicates that the composite has higher colloidal stability. Figure 9a displays the UV-Vis DRS of the ZnO and ZZFDT-1 photocatalysts. The absorption edge of ZnO is about 400 nm, and there is only weak absorption in the visible region. Compared with ZnO, the absorption edge of the ZZFDT-1 composite exhibits a red shift, which indicates that the light response range of the composite can be widened by introducing a third component ZF. Figure 9c shows the UV-Vis DRS of different mass ratios of ZZFDT. ZZFDT-1 exhibits superior visible light absorption. Figure 9b and d show that the band gap of ZnO, ZZFDT-1, ZZFDT-2, ZZFDT-3, ZZFDT-4, ZZFDT-5, ZZFDT-6 and ZZFDT-7 are estimated to be 3. 17, 1.5, 1.68, 1.85, 1.8, 1.85, 1. 72, 1.71 eV, respectively, based on the Kubelkae-Munk function. It is reported that the band gap of ZF is about 1.8 eV [73] , and that of the ZZFDT composite is less than 1.8 eV, which indicates that ZF and ZnO have a good synergistic effect, which can reduce the band gap and improve the quantum efficiency of the composite [74] .

The magnetic properties of ZFDT and ZZFDT-1 were measured using VSM, as shown in Fig. 10a . The magnetic hysteresis loops of ZFDT and ZZFDT-1 were observed at room temperature. The saturation magnetization (M) of ZFDT (3.12 emu/g) is greater than ZZFDT-1 (1.82 emu/g). This may be due to the addition of ZnO, which reduces the ferromagnetism of the composite. The ternary ZZFDT-1 composite can be effectively separated from the OTC solution by an external magnet, although its magnetism is reduced compared with ZFDT (Fig. 10b) . Figure 11a is the photocatalytic degradation curve of the OTC solution by DT, ZnO, ZF, ZDT, ZFDT, ZZF, ZZFDT-1, without catalyst. To study the adsorbability of different samples, the whole apparatus was operated in darkness for 30 min before illumination. As can be seen from the diagram, ZZFDT-1 has the strongest adsorption capacity, which can be attributed to its large specific surface area. In addition, the adsorption-desorption equilibrium of all samples can be established within 30 min because the solution concentration hardly changes between 15 and 30 min of dark adsorption. In the absence of catalyst and no light, the concentration of OTC did not change, indicating that the photocatalytic reaction did not take place. The photocatalysis activity of ZZF is higher than that of ZnO and ZF, which means that ZnO and ZF interact with each other and can inhibit the recombination of photogenerated electron-hole pairs, leading to the enhancement of photoactivity. The photocatalytic activity of ZDT and ZFDT is higher than that of ZnO and ZF, because of the adsorption capacity of DT. DT can adsorb OTC molecules near catalyst particles, thus improving the contact rate between catalyst particles and OTC, accelerating the photocatalytic rate [75] . As shown in Fig. 11b and c, the conversion rate of ZZFDT-1 to OTC is 95%, which is 4.13, 1.90, 1.40, 1.38, 1.36, and 1.27 times higher than that of DT, ZnO, ZF, ZDT, ZFDT and ZZF, respectively. Compared with ZZFDT-2, ZZFDT-3, ZZFDT-4, ZZFDT-5, ZZFDT-6 and ZZFDT-7, the conversion of OTC is increased by 21%, 12%, 15%, 13%, 1% and 4%, respectively. When the doping amount of ZnO or ZF exceeds a certain limit, the excess components may become the composite center of photocarriers, thus reducing the photoactivity of the composite. Conversion efficiency refers to how many grams of OTC can be converted per gram of catalyst/per hour. It can be seen from Fig. 11e that ZZFDT-1 has the highest conversion efficiency (4.75 mg/g/h) for OTC, indicating that there is synergy among the three components.

To prove the photocatalytic activity of the ZZFDT composite, the total organic carbon (TOC) of the OTC solution was analyzed. 10 mg/L OTC solution (100 mL) TOC is 13.12 mg/L. After two hours of photocatalytic reaction, the remaining TOC was 1.72 mg/L, indicating mineralization in the course of OTC degradation.

By applying the first-order model of dynamics to acquire reaction rate constant:

where C t represents the concentration of OTC solution after 30 min in darkness, when the adsorptiondesorption equilibrium has been established; k represents the reaction rate constant; Because the concentration of OTC is unchanged after 105 min, so t is 30-105 min). The fitted kinetic plots are shown in Fig. 11d , and the rate constants are indicated in Table 2 . Figure 12 is the reusability test results of ZZFDT-1.

After completing the experiment of photocatalytic degradation of OTC, the catalyst samples were collected by an external magnet and washed three times with deionized water and ethanol. Then, it was dried at 80°C for the next degradation test. After five repeated tests, ZZFDT-1 kept the removal rate of OTC at 90%, which meant that ZZFDT-1 had good stability and repeatability in the photodegradation process. The results of metal leaching experiments are shown in Table 3 . Assuming that the added ZZFDT-1 (0.1 g) is completely dissolved in 100 mL OTC solution, the total amounts of Zn 2? and Fe 3? in the solution are 360 and 160 lg/mL. The leaching amounts of Zn 2? and Fe 3? are 8.527 and 3.386 lg/mL after being used six times, which shows that ZZFDT-1 has good stability.

It is well known that hole (h ? ), superoxide ions (ÁO 2 -), hydroxyl radicals (ÁOH) and other active compounds play a significant role in photocatalytic activity. To verify which kind of active substance is dominant in the photocatalysis reaction, the photocatalysis degradation experiment with an active group trapping agent was designed. The additives are benzoquinone (BQ), ethylenediamine tetraacetic acid disodium salt (EDTA) and isopropyl alcohol (IPA), which are scavengers of superoxide radical (ÁO 2 -), hole (h ? ) and hydroxyl radical (ÁOH), respectively. As shown in Fig. 13 , IPA showed the highest inhibitory effect, which meant that ÁOH was Mott-Schottky (MS) was used to study the flat band potential of the semiconductor materials, as shown in Fig. 14 . The linear portion of the curve extends to the potential axis, and the value of the intersection point is the flat-band potential. The slope of the linear portion of all the curves in Fig. 14 is positive, which indicates that ZnO, ZF, ZZFDT are ntype semiconductors. The slope of the linear part of the MS curve can be used to calculate the carrier density. The formula is as follows:

where N is the carrier density, C is the capacitance, e 0 is the electron charge (1.602 9 10 -19 C), e 0 is the dielectric constant of the vacuum (8.854 9 10 -12 F/m), e is the dielectric constant of the sample and V is the applied potential. The results showed that the carrier densities of ZnO and ZZFDT-1 are 9.22 9 10 18 and 4.35 9 10 19 , respectively. The DC resistivity of ZnO and ZZFDT-1 was 2.65 9 10 6 and 1.27 9 10 6 X cm. When there are defects or structural vacancies in the Lattice, oxygen ions are diffused. The Diffusion Coefficient of oxygen vacancy is calculated using formula (16):

where r is the reciprocal of the resistivity, k B is the Ludwig Boltzmann constant, T is the temperature, D is the diffusion coefficient, N is the number of atoms/cm 3 , e is the electronic charge. At 298 K, the diffusion coefficients of ZnO and ZZFDT-1 are 2.90 9 10 -8 and 6.05 9 10 -8 cm 2 /s. The diffusion Coefficient of ZZFDT-1 is larger than ZnO, which indicates that more oxygen vacancies are diffused in ZZFDT-1. Fig. 12 The reusability test results of ZZFDT-1 

As can be seen from the illustration in Fig. 14b , the band gap of ZF is 1.67 eV, so the E VB potential of ZnO, ZF is 2.28, 1.11 eV. Moreover, we know that the work function of ZnO is about 5.20 eV [76] and that of ZF is about 5.38 eV [77] . ZF has a more excellent function than ZnO, so ZF has a lower Fermi level than ZnO. To achieve a uniform Fermi level for both semiconductor materials, the electrons in the ZnO conduction band are transferred to the conduction band of ZF until the whole system reaches a uniform Fermi level. This will cause the overall fermi level of the ZZFDT to move to a more negative potential, thus forming a new flat band potential and staggered n-n heterostruction. Compared with ZnO, the flat band potential of ZZFDT-1 shifted negatively to -0.90 eV.

In summary, the mechanism of OTC degradation by ZZFDT-1 composite is proposed, as shown in Fig. 15 . The carrier DT adsorbs the OTC molecule to the surface of ZZFDT-1. The electrons in the valence band are excited to transition into the conduction band (CB) after ZnO absorbs UV light, leaving holes in the valence band (VB). In addition, because ZF has a narrow band gap, it absorbs visible light to create electrons (on the CB) and holes (on the VB). Since the newly formed CB potential in ZZFDT-1 is higher than that of ZnO, and ZnO has a lower VB potential than ZF, the electrons from ZF On CB are transferred to ZF's CB, and the holes from ZnO on VB are transferred to ZF's VB [78] . Similarly, the holes on the VB of ZnO could migrate to the VB of ZF. This migration can promote the separation of photogenerated electron-hole pairs and hinder recombination, which means that more charge carriers react with O 2 or H 2 O to produce more ÁO 2 or ÁOH, resulting in higher photodegradation efficiency. The degradation process of ZZFDT-1 to OTC is as follows [79, 80] :

ZnO þ e À ZnFe 2 O 4 ð Þ!ZnO e À ð Þ ð20Þ 

In summary, this work presented a novel magnetic ternary ZZFDT composite with visible light activated by the hydrothermal-precipitation method. XRD, FT-IR, FE-SEM, TEM and XPS analysis showed that the ZZFDT ternary composite was successfully prepared. The crystallite size of ZF in ZZFDT-1 calculated from Debye Scherrer and the W-H method are nearly the same and exhibit nanocrystalline nature. The magnetic properties of ZZFDT-1 show that it has superparamagnetic nature. The photocatalytic activity of different samples was studied by degrading OTC solution under visible light. All the results showed that ZZFDT has good photocatalytic activity and reusability. Because of its high activity, low cost and convenient recycling, it is a promising wastewater treatment material.

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A review of metal organic framework (MOFs)-based materials for antibiotics removal via adsorption and photocatalysis

Preparation of ZnFe 2 O 4 /ZnO composite: Effect of operational parameters for photocatalytic degradation of dyes under UV and visible illumination

Visible light-induced degradation of indigo carmine over ZnFe 2 O 4 /Tannin/ZnO: role of tannin as a modifier and its degradation mechanism

Posttreatment with ZnFe 2 O 4 nanoparticles to improve photoelectrochemical performance of ZnO nanorods based photoelectrodes

Anisotropic ZnO nanostructures and their nanocomposites as an advanced platform for photocatalytic remediation

Synthesis, characterization, and magnetic properties of ZnO-ZnFe 2 O 4 nanoparticles with high photocatalytic activity

Synergistic effect of supported ZnO/ Bi 2 O 3 heterojunctions for photocatalysis under visible light

Hydrothermal synthesis of MoO 3 /ZnO heterostructure with highly enhanced photocatalysis and their environmental interest

Rational design of novel three-dimensional reticulated Ag 2 O/ZnO Z-scheme heterojunction on Ni foam for promising practical photocatalysis

Enhanced visible-light photocatalytic performances of ZnO through loading AgI and coupling piezo-photocatalysis

Exploration of thermoacoustics behavior of water based nickel ferrite nanofluids by ultrasonic velocity method

Effect of zinc doping on water-based manganese ferrite nanofluids for magnetic hyperthermia application

Multifunctional magnetic nano-platforms for advanced biomedical applications: a brief review

Temperature dependent viscosity of cobalt ferrite/ethylene glycol ferrofluids

Investigations of structural, magnetic and induction heating properties of surface functionalized zinc ferrite nanoparticles for hyperthermia applications

A simple gas sensor based on zinc ferrite hollow spheres: highly sensitivity, excellent selectivity and long-term stability

Self-heating evaluation of superparamagnetic MnFe 2 O 4 nanoparticles for magnetic fluid hyperthermia application towards cancer treatment

Induction heating analysis of surface-functionalized nanoscale CoFe 2 O 4 for magnetic fluid hyperthermia toward noninvasive cancer treatment

Multifunctional nano-magnetic particles assisted viral RNA-extraction protocol for potential detection of COVID-19

Enhancement in surface area and magnetization of CoFe 2 O 4 nanoparticles for targeted dru delivery application

Spinel zinc ferrite nanoparticles: an active nanocatalyst for microwave irradiated solvent free synthesis of chalcones

Tuning of physical properties of multifunctional Mg-Zn spinel ferrite nanocrystals: a comparative investigations manufactured via conventional ceramic versus green approach sol-gel combustion route

Nanocatalyst: a brief review on synthesis to applications

Structural and chemical properties of ZnFe 2 O 4 nanoparticles synthesised by chemical co-precipitation technique

Eco-friendly green synthesis and characterizations of CoFe 2-x Al x O 4 nanocrystals: analysis of structural, magnetic, electrical, and dielectric properties

Fabrication and magnetic transformation from paramagnetic to ferrimagnetic of ZnFe 2 O 4 hollow spheres

Posttreatment with ZnFe 2 O 4 nanoparticles to improve photoelectrochemical performance of ZnO nanorods based photoelectrodes

Photoelectrochemical enhancement of ZnO/BiVO 4 /ZnFe 2 O 4 /rare earth oxide hetero-nanostructures

ZnO/ZnFe 2 O 4 /Ag hollow nanofibers with multicomponent heterojunctions for highly efficient photocatalytic water pollutants removal

Synthesis of ZnO/ZnFe 2 O 4 /Pt nanoparticles heterojunction photocatalysts with superior photocatalytic activity

Magnetically retrievable nanoscale nickel ferrites: an active photocatalyst for toxic dye removal applications

Synthesis of octahedral-like ZnO/ZnFe 2 O 4 heterojunction photocatalysts with superior photocatalytic activity

Efficient visiblelight photocatalysis of ZIF-derived mesoporous ZnFe 2 O 4 / ZnO nanocomposite prepared by a two-step calcination method

Visible-light-driven photocatalytic properties of ZnO/ZnFe 2 O 4 core/shell nanocable arrays

Visible light photocatalytic activities of ZnFe 2 O 4 /ZnO nanoparticles for the degradation of organic pollutants

Hierarchical architectures of wrinkle-like ZnFe 2 O 4 nanosheet-enwrapped ZnO nanotube arrays for remarkably photoelectrochemical water splitting to produce hydrogen

Synthesis and characterization of magnetic ZnFe 2 O 4 /Bi 0 -Bi 2 MoO 6 with Z-scheme heterojunction for antibiotics degradation under visible light

Experimental study of CO 2 conversion into methanol by synthesized photocatalyst (ZnFe 2 O 4 /TiO 2 ) using visible light as an energy source

Synthesis of the Ag/Ag 3 PO 4 /diatomite composites and their enhanced photocatalytic activity driven by visible light

Structure and properties of Ag 3 PO 4 /diatomite photocatalysts for the degradation of organic dyes under visible light irradiation

Synthesis and photocatalytic activity of BiOCl/diatomite composite photocatalysts: natural porous diatomite as photocatalyst support and dominant facets regulator

Preparation of visible light-responsive photocatalytic paper containing BiVO 4 @diatomite/MCC/PVBCFs for degradation of organic pollutants

Fluorine doped anatase TiO 2 with exposed reactive (001) facets supported on porous diatomite for enhanced visible-light photocatalytic activity

Preparation of granulated N-doped TiO 2 /-diatomite composite and its applications of visible light degradation and disinfection

Magnetically separable core-shell ZnFe 2 O 4 @ZnO nanoparticles for visible light photodegradation of methyl orange

Multifunctional environmental applications of ZnO nanostructures synthesized by the microwaveassisted hydrothermal technique

Effect of calcination on structure and photocatalytic property of N-TiO 2 /g-C 3 N 4 @diatomite hybrid photocatalyst for improving reduction of Cr(VI)

The effect of support on the structure and photocatalytic activity of ternary ZnO-ZnFe 2 O 4 /palygorskite composite photocatalysts

Influential diamagnetic magnesium (Mg 2? ) ion substitution in nano-spinel zinc ferrite (ZnFe 2 O 4 ): thermal, structural, spectral, optical and physisorption analysis

Influence of trivalent Al-Cr cosubstitution on the structural, morphological and Mossbauer properties of nickel ferrite nanoparticles

Jadhav, Influential incorporation of RE metal ion (Dy 3? ) in yttrium iron garnet (YIG) nanoparticles: magnetic, electrical and dielectric behavior

Mohammadzadeh, Evaluation of adsorption kinetics and equilibrium for the removal of benzene by modified diatomite

Synthesis of ZnO nanoparticles supported on mesoporous SBA-15 with coordination effect-assist for antibacterial assessment

Synthesis of Fe 3 O 4 /SiO 2 /polypyrrole magnetic nanocomposite polymer powder: Investigation of structural properties and ability to purify of edible sea salts

Study of structural and magnetic properties of silica and polyethylene glycol (PEG-4000)-encapsulated magnesium nickel ferrite (Mg 0.5-Ni 0.5 Fe 2 O 4 ) nanoparticles

Lychee-like ZnO/ZnFe 2 O 4 core-shell hollow microsphere for improving acetone gas sensing performance

Facile synthesis of ordered magnetic mesoporous c-Fe 2 O 3 /SiO 2 nanocomposites with diverse mesostructures

Estimation of the surface interaction mechanism of ZnO nanoparticles modified with organosilane groups by Raman Spectroscopy

MOF-derived hierarchical ZnO/ZnFe 2 O 4 hollow cubes for enhanced acetone gas-sensing performance

Facile microwave-assisted green synthesis of Ag-ZnFe 2 O 4 @rGO nanocomposites for efficient removal of organicdyes under UV-and visible-light irradiation

Abnormal n-p-n type conductivity transition of hollow ZnO/ ZnFe 2 O 4 nanostructures during gas sensing process: the role of ZnO-ZnFe 2 O 4 hetero-interface

Fabrication of magnetically recyclable Ce/N co-doped TiO 2 /NiFe 2 O 4 /diatomite ternary hybrid: improved photocatalytic efficiency under visible light irradiation

In situ synthesis of magnetic MnFe 2 O 4 /diatomite nanocomposite adsorbent and its efficient removal of cationic dyes

Interface actions between TiO 2 and porous diatomite on the structure and photocatalytic activity of TiO 2 -diatomite

Preparation of a La/N co-doped TiO 2 film electrode with visible light response and its photoelectrocatalytic activity on a Ni substrate

Hydrophobic to hydrophilic surface transformation of nanoscale zinc ferrite via oleic acid coating: magnetic hyperthermia study towards biomedical applications

Hyperthermic evaluation of oleic acid coated nano-spinel magnesium ferrite: Enhancement via hydrophobic-to-hydrophilic surface transformation

Hierarchical ZnO nanorod/ZnFe 2 O 4 nanosheet core/ shell nanoarray decorated with PbS quantum dots for efficient photoelectrochemical water splitting

ZnFe 2 O 4 heterostructures for superior triethylamine detection

Characterization and improved solar light activity of vanadium doped TiO 2 /diatomite hybrid catalysts

Synthesis of ternary Ag/ZnO/ZnFe 2 O 4 porous and hollow nanostructures with enhanced photocatalytic activity

Bahnemann, Effect of the degree of inversion on the photoelectrochemical activity of spinel ZnFe 2 O 4 . Catalysts

A magnetic ZnFe 2 O 4 /ZnO/perlite nanocomposite for photocatalytic degradation of organic pollutants under LED visible light irradiation

Magneto-structural and photocatalytic behavior of mixed Ni-Zn nano-spinel ferrites: visible light-enabled active photodegradation of rhodamine B

Visible light driven photocatalytic activity of TiO 2 nanoparticles prepared via gel-combustion process

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We acknowledge the financial support of the Heilongjiang natural fund Project [B2017012].