Photocatalytic activity of BiFeO3/ZnFe2O4 nanocomposites under visible light irradiation

Herein, BiFeO3/ZnFe2O4 nanocomposites were synthesized via a glyoxylate precursor method using a two-pot approach. Phase evolution is investigated by X-ray diffraction and Raman spectroscopy, which confirm that no impurity phases are formed between BiFeO3 and ZnFe2O4 following calcination at 600 °C. The specific surface area characterized by N2 adsorption–desorption isotherms decreases from 30.56 to 13.13 m2 g−1 with the addition of zinc ferrite. In contrast, the magnetization increases from 0.28 to 1.8 emu g−1 with an increase in the amount of ZnFe2O4. The composites show strong absorption in the visible region with the optical band gap calculated from the Tauc's plot in the range from 2.17 to 2.22 eV, as measured by diffuse reflectance spectroscopy. Furthermore, the maximum efficiency for the photodegradation of methylene blue under visible light is displayed by the composite containing 25 wt% ZnFe2O4 due to the synergic effect between BiFeO3 and ZnFe2O4, as confirmed by photoluminescence spectroscopy.


Introduction
In recent years, scientic research has been focused on new visible light photocatalysts based on semiconductors to address the increasing environmental pollution and energy demands by efficient utilization of solar energy. 1,2 To date, various metal oxides (ZnO 3 and TiO 2 4 ) and metal suldes (ZnS 5 ) have been studied to efficiently degrade harmful organic pollutants and for hydrogen production through water splitting under UV light irradiation. 4 However, the UV region spans only 5% of the entire solar spectrum, restricting their applications. As a result of band gap engineering improvement, composites can be fabricated by coupling two narrow band gap semiconductors, which have attracted considerable attention for the development of efficient visible light photocatalysts. [6][7][8] Bismuth ferrite (BiFeO 3 ), which has potential applications in sensors, actuators, and digital memory, is a well-known multiferroic material simultaneously possessing ferroelectric and ferromagnetic ordering at room temperature. 9,10 Furthermore, BiFeO 3 displays a distinct photovoltaic effect with an open circuit voltage of 0.8-0.9 V as a working solar device, which represents a new potential application. 11,12 Due to its relatively narrow band gap of 2.2 eV, BiFeO 3 has been considered as a possible visible light photocatalyst under solar light irradiation for the photodegradation of organic contaminants. 13,14 However, its quantum yield is poor due to the rapid recombination of the photogenerated electron-hole pairs that limits its practical use in photocatalytic applications. 15,16 Therefore, many strategies have been developed to enhance the photocatalytic efficiency of BiFeO 3 by modifying the size and morphology of its particles, cation doping, and coupling with other semiconductors. [17][18][19] For instance, several semiconductors such as g-C 3 N 4 , carbon nanober, graphene, CuO and ZnO have been coupled with BiFeO 3 to improve its photogenerated electronhole separation, thus enhancing its interfacial charge transfer the efficiency. 6,[20][21][22][23][24][25][26][27] Spinel magnetic zinc ferrite (ZnFe 2 O 4 ) with a narrow band gap of 1.92 eV exhibits a signicant photoresponse in the visible light region and has been utilized in gas sensors, catalysts and semiconductor photocatalysts. 1 Furthermore, the magnetic properties of ZnFe 2 O 4 can be used to recycle photocatalysts by the application of a magnetic eld, making it an interesting product in the industrial photodegradation of organic pollutants. 7,28 To the best of our knowledge, there are no reports on the synthesis and application of BiFeO 3 /ZnFe 2 O 4 nanocomposites for pollutant degradation under visible light irradiation. Uniyal and Yadav only reported the dielectric and magnetic properties of BiFeO 3 /ZnFe 2 O 4 composites synthesized via the sol-gel method as a function of annealing temperature. 29 Herein, we report the structure, microstructure, magnetic properties and photocatalytic performances of BiFeO 3 /ZnFe 2 O 4 composites synthesized via the glyoxylate precursor method. The optimum amount of ZnFe 2 O 4 is determined to maximize the photocatalytic activity of BiFeO 3 powder. composites were synthesized via a two-pot approach in which the required amount of previously synthesized BiFeO 3 powder was added to the solution precursor of zinc ferrite, where the dried precursor was calcined at 600 C for 1 hour. Phase evolution was investigated using a PANalytical X'pert X-ray diffractometer (XRD) with monochromatic CuKa radiation. Raman analysis was performed on the powders using a WiTec Alpha 300R instrument (Nd:YAG laser source: l ¼ 532 nm and 0.7 MW power, and range: 100-900 cm À1 ). The morphology and microstructure of the powders were observed using a TESCAN Vega II scanning electron microscope (SEM). The specic surface areas of the as-prepared powders were determined according to the Brunauer-Emmett-Teller (BET) method with nitrogen adsorption at 77 K using a PHS-1020 instrument aer degassing at 250 C for 5 h. The Barrett-Joyner-Halenda (BJH) cumulative pore volume was calculated from the adsorption branch of the isotherms. The equivalent particle size was calculated based on the BET surface area as follows:

Experimental procedure
where, D BET is the equivalent particle size (nm), r is the theoretical density and S BET stands for the BET surface area (m 2 g À1 ).
A vibrating sample magnetometer (Meghnatis Daghigh Kavir Kashan Co., Iran) with a maximum eld of 10 kOe was employed to measure the magnetic properties at room temperature. UV-vis absorption spectra were recorded on a Shimadzu UV-vis-52550 spectrophotometer in the wavelength range of 300À800 nm. Room temperature photoluminescence spectra (PL) were obtained on a uorescence spectrophotometer (F-4600, Hitachi, Japan) at an excitation wavelength of 210 nm. The photocatalytic activity of the BiFeO 3 /ZnFe 2 O 4 nanocomposites was evaluated by the degradation of methylene blue (MB) in aqueous solution under visible light radiation. Two 100 W xenon lamps with a cutoff ultraviolet lter (l ¼ 420 nm) were introduced as the visible light source. In each experiment, 0.1 g of photocatalyst was added to 100 mL of methylene blue solution at a concentration of 15 mg L À1 . In addition, the solution pH was adjusted to 2 by adding HCl to obtain the maximum MB adsorption on the catalyst surface, 14 as shown in the ESI. † The suspension was stirred in the dark for 60 min to establish the adsorption/desorption equilibrium, then the solution was irradiated under visible light. At appropriate time intervals, about 5 mL of suspension was sampled, where the solid phase was separated from the solution via centrifugation at 4000 rpm for 20 min. The concentration of each degraded solution was monitored on a PG Instruments Ltd T80-UV/vis spectrophotometer. , which indicates well crystallized BiFeO 3 nanoparticles were produced by the glyoxylate precursor method. However, some impurity Bi 2 Fe 4 O 9 phases (JCPDS card no. 42-0181) were also observed with BiFeO 3 . The chemical synthesis of BiFeO 3 typically leads to the formation of impurities, may be due to its chemical kinetics. 31 Aer compositing with 25 wt% ZnFe 2 O 4 , a weak diffraction peak at 2q ¼ 35.32 corresponding to the (311) reection peak of ZnFe 2 O 4 appeared. With an increase in the zinc ferrite content, the diffraction peaks of ZnFe 2 O 4 became clearer and stronger, and the impurity peak disappeared. Furthermore, no impurity species were formed between BiFeO 3 and ZnFe 2 O 4 during the calcination process, which indicates that ZnFe 2 O 4 was successfully loaded on the BiFeO 3 particles without destroying its crystal structure. The amount of BiFeO 3 and ZnFe 2 O 4 phases in the composites was calculated by Rietveld renement, which is in agreement with the nominal values, as typically shown in the ESI. †

Results and discussion
The Raman spectra of pure BiFeO 3 , pure ZnFe 2 O 4 and BiFeO 3 -xZnFe 2 O 4 composites are presented in Fig. 2. In the spectrum of pure BiFeO 3 , the Raman active modes with A 1 and E symmetry can be summarized using the following irreducible representation G ¼ 4A 1 + 9E. 32 The two peaks at 173 and 220 cm À1 are assigned as A1 modes, and the peaks at 286, 361 and 481 cm À1 correspond to the E modes. Pure ZnFe 2 O 4 exhibited four peaks at 246, 327, 471 and 648 cm À1 , which are assigned as the T 2g (1), E g , T 2g (2) and A 1g modes for a cubic spinel structure, respectively. 33 The A 1g mode of ZnFe 2 O 4 appears aer 25 wt% ZnFe 2 O 4 was loaded, while the other modes were dominant at higher zinc ferrite contents. Moreover, the purity of the BiFeO 3 -xZnFe 2 O 4 composites is conrmed by the absence of Raman modes of impurity phases.
The N 2 adsorption-desorption isotherms of the BiFeO 3 -50 wt% ZnFe 2 O 4 composite are shown in Fig. 4. Table 1 also presents the specic surface area (S BET ), equivalent particle size (D BET ) and pore volume. The particle agglomerations show a typical type II isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classication. 34 The surface area of pure BiFeO 3 is 30.56 m 2 g À1 and 13.13 m 2 g À1 for pure ZnFe 2 O 4 . The higher specic surface area of pure BiFeO 3 is attributed to more gaseous products being formed during its synthesis, 35 as conrmed by its higher pore volume (0.089 cm 3 g À1 ). The BJH pore size distribution is also depicted in the inset of Fig. 4. The pore size distribution of the BiFeO 3 -50 wt% ZnFe 2 O 4 composite powder exhibits a mesopore spreading of about 3-4 nm. Fig. 5 illustrates the magnetization curves of the BiFeO 3 -xZnFe 2 O 4 composites as well as the pure BiFeO 3 and ZnFe 2 O 4 powders. The pure BiFeO 3 nanoparticles exhibit a ferrimagnetic response with the magnetization of 0.28 emu g À1 at 10 kOe. However, the magnetization increases with an increase in zinc ferrite content since pure ZnFe 2 O 4 has a magnetization of 1.8 emu g À1 . Bulk BiFeO 3 is known to show a G-type antiferromagnetic ordering with a linear eld-dependence of magnetization, while the BiFeO 3 nanoparticles exhibit weak ferrimagnetism due to the interruption of the long-range antiferromagnetic order at the particle surface. 36 The bulk ZnFe 2 O 4 also has a normal spinel structure with antiferromagnetic behavior, while the ZnFe 2 O 4 nanoparticles exhibit a partially inverse spinel structure with some magnetic moment at room temperature. 37 A high surface-to-volume ratio in nanoparticles leads to more uncompensated spins from the surface, inducing an enhancement in magnetization. The BiFeO 3 -xZnFe 2 O 4 composites show higher saturation magnetization than pure bismuth ferrite as a result of the higher magnetization in the zinc ferrite phase. This ferrimagnetism behavior can be exploited for the magnetic recovery of the photocatalyst aer degradation.
The optical properties of the BiFeO 3 -xZnFe 2 O 4 composites, as well as the pure BiFeO 3 and ZnFe 2 O 4 powders were investigated via UV-vis diffuse reectance spectroscopy, which are presented in Fig. 6. The absorption spectra show that the samples absorb a considerable amount of visible light. The direct optical band gap, E g , was determined using the equation (ahn) 2 ¼ A(hn À E g ), where, hn is the photon energy in eV, a is the absorption coefficient and A is a material constant, 38 as shown in the inset of Fig. 6. According to the Tauc plots, the band gaps for x ¼ 0, 25, 50, 75 and 100 wt% were calculated to be 2. 17      on the content of the two components in the composite. [20][21][22]42 For the optimal content of 25 wt% ZnFe 2 O 4 , the most appropriate BiFeO 3 /ZnFe 2 O 4 heterojunctions might be formed, which benet the transfer and separation of photogenerated electrons and holes, as can be inferred from the PL spectra. The suppression of charge recombination in BiFeO 3 by pairing with ZnFe 2 O 4 can be conrmed by photoluminescence (PL) emission spectra, as presented in Fig. 8. As is known, the recombination of excited electrons and holes leads to PL emission, where a lower emission intensity indicates a decrease in recombination probability. Fig. 8 shows the PL emission spectra of the pure BiFeO 3 and BiFeO 3 -25 wt% ZnFe 2 O 4 photocatalysts at an excitation wavelength of 210 nm. The irradiative recombination process of self-trapped excitations results in an emission band at about 423 nm for pure BiFeO 3 . 43 Clearly, the PL emission intensity decreases when zinc ferrite was added, which conrms that the coupling of BiFeO 3 with ZnFe 2 O 4 results in an enhanced ability to capture photoinduced electrons in comparison with pure BiFeO 3 and pure ZnFe 2 O 4 . The lower PL emission intensity of the BiFeO 3 -25 wt% ZnFe 2 O 4 photocatalyst benets a delay in the recombination rate and, thus, higher photocatalytic activity. [44][45][46][47] In addition to the lower recombination rate of electron-hole pairs in the BiFeO 3 -25 wt% ZnFe 2 O 4 catalyst, its higher specic surface area can also adsorb more MB dye on the exterior of its particles, as shown in Fig. 7c, hence facilitating the photodegradation of MB dye.   Based on the above structural characterizations and visible light photocatalytic tests, a possible mechanism for the photodegradation of MB by the BiFeO 3 /ZnFe 2 O 4 photocatalyst under visible light irradiation is proposed. Fig. 9 shows the band positions and transfer path of the photogenerated electronhole pairs between BiFeO 3 and ZnFe 2 O 4 . The conduction (CB) and valence (VB) band positions of BiFeO 3 and ZnFe 2 O 4 at the point of zero charge were obtained from previous reports. 15,48 According to the general p-n heterojunction formation process, 8

Conclusions
A two-pot approach was used for the synthesis of BiFeO 3 / ZnFe 2 O 4 composites without any impurity species formed between BiFeO 3 and ZnFe 2 O 4 . The particle size decreased from 210 nm for pure BiFeO 3 to 80 nm for pure ZnFe 2 O 4 . The pure BiFeO 3 nanoparticles exhibited a higher specic surface area than the pure ZnFe 2 O 4 nanoparticles, which may be due to the greater amount of released gaseous products. The magnetization of the BiFeO 3 /ZnFe 2 O 4 composites increased from 0.28 to 1.8 emu g À1 with an increase in the ZnFe 2 O 4 content. The optical band gaps of composites initially decreased from 2.17 to 2.03 eV and then increased to 2.22 eV as a function of the amount of zinc ferrite. The maximum efficiency ($97%) for the photodegradation of methylene blue under visible light was exhibited for BiFeO 3 -25 wt% ZnFe 2 O 4 aer 30 minutes irradiation due to the synergic effect between BiFeO 3 and ZnFe 2 O 4 .

Conflicts of interest
There are no conicts to declare.   This journal is © The Royal Society of Chemistry 2018