New Insights into Mn_1−xZn_xFe₂O₄ via Fabricating Magnetic Photocatalyst Material BiVO₄/Mn_1−xZn_xFe₂O₄

Abstract

BiVO₄/Mn_1−xZn_xFe₂O₄ was prepared by the impregnation roasting method. XRD (X-ray Diffractometer) tests showed that the prepared BiVO₄ is monoclinic crystal, and the introduction of Mn_1−xZn_xFe₂O₄ does not change the crystal structure of BiVO₄. The introduction of a soft-magnetic material, Mn_1−xZn_xFe₂O₄, was beneficial to the composite photocatalyst’s separation from the liquid solution using an extra magnet after use. UV-vis spectra analysis indicated that Mn_1−xZn_xFe₂O₄ enhanced the absorption intensity of visible light for BiVO₄. EIS (electrochemical impedance spectroscopy) investigation revealed that the introduction of Mn_1−xZn_xFe₂O₄ enhanced the conductivity of BiVO₄, further decreasing its electron transfer impedance. The photocatalytic efficiency of BiVO₄/Mn_1−xZn_xFe₂O₄ was higher than that of pure BiVO₄. In other words, Mn_1−xZn_xFe₂O₄ could enhance the photocatalytic reaction rate.

1. Introduction

Photocatalysis and photocatalytic technology, which uses semiconductor materials to directly absorb and transmute renewable solar light energy into chemical energy, have been considered as promising methods to resolve environmental and energy problems facing the human population [1]. To date, cleaning up organic compounds via degradation and water splitting to produce H₂ are the two most important applications of photocatalysis and its corresponding technology, which are aimed at environmental pollutant treatment and molecular hydrogen production, respectively [2].

In spite of the promising results regarding solar-light-driven photocatalysts, there are still some challenges inhibiting their practical application. The most pressing problem concerns the photocatalytic reaction kinetics using photocatalysts under solar light irradiation [3]. Most of the degradation reactions under solar light irradiation are very slow (they may take several hours). Therefore, it is very important to exploit new and highly efficient photocatalysts. In addition, the intrinsic relationship between photocatalytic activity and photocatalytic material structure can be elucidated by studying the photocatalytic mechanism, which will guide the synthesis and application of new and more efficient photocatalytic systems.

A monoclinic scheelite structure, BiVO₄, with a better absorption ability of visible light, has attracted a great deal of attention due to its a relatively narrow band gap. The hybridization of Bi^6s-O^2p orbitals upshifted the valence band of monoclinic BiVO₄ to a lower potential at about +2.4 eV [2,4,5,6,7]. Nonetheless, the photocatalytic efficiency of BiVO₄ is generally low because of its poor electron transfer and slow reaction kinetics.

Given that the photocatalyst materials could not be thoroughly recycled from a liquid solution after reaction, secondary pollution would probably be induced by the residual photocatalyst. This prevents it application as a wastewater treatment. Fortunately, a magnetic photocatalyst could solve the above issue [8,9]. The exploitation and application of various magnetic photocatalyst materials have been boosting scientists’ morale.

Here, we use Mn_1−xZn_xFe₂O₄ as a soft-magnetic substrate to prepare magnetic photocatalyst BiVO₄/Mn_1−xZn_xFe₂O₄. In the composite system, the magnetic substrate, Mn_1−xZn_xFe₂O₄, facilitated the recovery of the photocatalyst from the liquid reaction solution using an extra magnet after the reaction. Most importantly, Mn_1−xZn_xFe₂O₄ could enhance the photocatalytic reaction rate of BiVO₄ by enhancing the absorption intensity of visible light for BiVO₄ and heightening the conductivity of BiVO₄. We sincerely hope to extend the application field of Mn_1−xZn_xFe₂O₄ according to this report.

2. Experimental Procedure

All reagents purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) were of analytical grade purity and were used directly without further purification. The water used in all experimental processes was deionized water.

2.1. Preparation of BiVO₄/Mn_1−xZn_xFe₂O₄

Preparation of Mn_1−xZn_xFe₂O₄ [10]. ZnSO₄, MnSO₄, and FeCl₃⋅6H₂O with a given molar ratio of n (ZnO): n (MnO): n (Fe₂O₃) = 13.3:32.8:53.9 was separately weighed and dissolved in water to form three solutions. Subsequently, the prepared ZnSO₄ solution and FeCl₃ solution were added into the MnSO₄ solution under continuous stirring condition, in order to form the mixture solution. A definite amount of (NH₄)₂C₂O₄⋅H₂O was dissolved to form a (NH₄)₂C₂O₄ solution. The prepared mixture solution was slowly added into the (NH₄)₂C₂O₄ solution, and the mixture solution and (NH₄)₂C₂O₄ solution were heated to 80 °C, respectively. The pH value of the whole reaction system was adjusted to 7 by slowly adding NaOH solution, and a large quantity of precipitate was observed. The precipitate was washed using water and dried at 80 °C for 12 h after filtrating to gain the precursor. The precursor was sintered at 1200 °C for 3 h to form the resultant magnetic substrate, Mn_xZn_1−xFe₂O₄.

Preparation of BiVO₄ [11]. First, 0.01 mol Bi(NO₃)₃·5H₂O and 0.02 mol tartaric acid were dissolved using a certain amount of 2 mol/L HNO₃ solution to form a mixture solution A. Then, its pH value was adjusted to 7.5. Subsequently, 0.01 mol NH₄VO₃ was dissolved in 50 mL hot water (70 °C) to form solution B. Solution A and solution B were mixed to form the precursor of BiVO₄. The precursor was dried at 80 °C for 24 h and then sintered at 500 °C for 4 h to obtain pure BiVO₄.

Preparation of BiVO₄/Mn_1−xZn_xFe₂O₄. First, 0.01 mol Bi(NO₃)₃·5H₂O and 0.02 mol tartaric acid were dissolved with a certain amount of 2 mol/L HNO₃ solution to form a mixture solution A. Then, its pH value was adjusted to 7.5. Subsequently, 0.01 mol NH₄VO₃ was dissolved in 50 mL hot water (70 °C) to from solution B. Solution A and solution B were mixed to form C solution. Then, 15.0 wt % Mn_xZn_1−xFe₂O₄ was added to solution C. BiVO₄/Mn_xZn_1−xFe₂O₄ (15.0 wt %) was obtained after the mixture was dried at 80 °C for 24 h and then sintered at 500 °C for 4 h. The composites BiVO₄/Mn_xZn_1−xFe₂O₄ (10.0%, 20.0%, 25.0 wt %) were prepared by adjusting the mass ratio of Mn_xZn_1−xFe₂O₄.

2.2. Materials Characterization

The structure characterization of the samples was determined by X-ray Diffractometer (Shimadzu, XRD-6000, Kyoto, Japan), Fourier transform infrared spectroscopy (FTIR, Perkin-Elmersystem 2000, Akron, OH, USA), and INVIA Raman microprobe (Renishaw Instruments, Wotton-under-Edge, UK). The light absorption properties, magnetization, and surface performances of products were examined by an ultraviolet-visible diffuse reflectance spectrophotometer (UV-vis, DRS, TU1901, Company, Beijing, China), vibrating sample magnetometer (VSM 7410, Lake Shore, Carson, CA, USA), Brunauer−Emmett−Teller (BET, ASAP-2020, Micromeritics, Norcross, GA, USA), and scanning electron microscopy (SEM, EVO-LS15X, ZEISS, Upper Cohen, Germany). The electrochemical workstation (PGSTAT30) was employed to measure the electrochemical impedance spectroscopy (EIS) of samples. The following are the test EIS parameters: K₃[Fe(CN)₆]:K₄[Fe(CN)₆] (1:1)-KCl electrolyte solution was employed. The work electrode content contained the prepared photocatalytic materials, acetylene black, and polytetrafluoroethylene (mass ratio, 85:10:5); the counter electrode was platinum foil; and the reference electrode was a saturated calomel electrode (SCE). Finally, the AC (Alternating Current) voltage amplitude was set at 5 mV and the frequency range was 1 × 10⁵~1 × 10⁻² Hz.

2.3. Photocatalytic Tests

The photocatalytic activity of the prepared photocatalysts was investigated by the degradation of simulated dye wastewater (Rhodamine B, RhB) under visible light irradiation. One hundred milligrams photocatalyst was put into 100 mL RhB solution with a 5 mg/L concentration, then the suspension liquid was placed in the dark for 0.5 h with stirring to reach the adsorption-desorption equilibrium. A 500 W Xe lamp, equipping with a UV cut-off filter, was used as the visible light source (λ ≥ 420 nm). At the given irradiation time intervals, a series of the reaction solution was withdrawn and the absorbance was measured using the UV-vis spectrophotometer (TU-1901).

3. Results and Discussion

Primary analysis of photodegradation revealed that BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) was the most efficient in the RhB degradation process under visible light irradiation.

3.1. Structure and Specific Surface Property

Figure 1 shows the XRD patterns of the as-prepared samples. The characteristic spectra of monoclinic crystal BiVO₄ was well indexed with the standard card (JCPDS file 14-0688), corresponding to the characteristic diffraction phases of (110), (011), (121), (040), (200), (002), (211), (150), (132), and (042) [12,13]. The diffraction pattern of Mn_1-xZn_xFe₂O₄ was fully matched with the standard card (JCPDS file 74-2400), with the characteristic reflection phases (220), (311), (222), (400), (422), (511), (440), (620), and (622) [10].

The diffraction peaks of the Mn_1−xZn_xFe₂O₄ pattern were difficult to observe in the pattern of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %). One the one hand, the intensity of the diffraction peaks of Mn_1−xZn_xFe₂O₄ was relatively weak compared with that of the diffraction peaks of BiVO₄. On the other hand, the diffraction patterns location of Mn_1−xZn_xFe₂O₄ overlapped with the domain diffraction patterns of BiVO₄.

The peaks at (121) for BiVO₄ in both patterns of pure BiVO₄ and BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) were clear, even in terms of their intensities. This phenomenon revealed that the introduction of Mn_1−xZn_xFe₂O₄ did not alter the growth orientation of BiVO₄.

In addition, no impurity phases were found in the BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) sample, confirming that there was no appreciable decomposition reaction for BiVO₄ and Mn_1−xZn_xFe₂O₄ and no perceptible chemical reaction between the two components even though they were sintered at 500 °C.

To further elucidate the structure of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %), we carried out the measurement of Fourier transform infrared spectroscopy. Figure 2 illustrates the FTIR spectra of the as-prepared samples. The vibration absorption peaks of Mn-O, Zn-O, and Fe-O bands of Mn_1−xZn_xFe₂O₄ were at 560.1 cm⁻¹, 473.7 cm⁻¹, and 412.4 cm⁻¹, respectively [14], while the V-O vibration absorption peaks of BiVO₄ were at 734.3 cm⁻¹ and 823.4 cm⁻¹ [15]. This spectrum of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) could confirm the coexistence of Mn_1−xZn_xFe₂O₄ and BiVO₄ in the prepared composite, which indicated that BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) was prepared successfully. In addition, the absorption patterns at 2341.7 cm⁻¹ and 3433.6 cm⁻¹ were attributed to CO₂ and the surface adsorption H₂O [9].

Raman spectroscopy can provide structural information for materials, and is also a sensitive method to study the crystallization, local structure, and electronic properties of materials. The Raman spectra of the synthesized samples are shown in Figure 3. It can be seen from Figure 3 that the Raman band at 120 cm⁻¹, 210 cm⁻¹, 324 cm⁻¹, 366 cm⁻¹, and 826 cm⁻¹ is the typical vibrational band of BiVO₄ [16]. These bands could be also observed in the spectroscopy of BiVO₄/ Mn_1−xZn_xFe₂O₄ (15 wt %), which further revealed that the BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) was synthesized successfully, in agreement with the of XRD and FTIR analysis results. In addition, the intensity of BiVO₄ was much larger than that of Mn_1−xZn_xFe₂O₄. So, Mn_1−xZn_xFe₂O₄ signal was not obvious in the spectra of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %).

In order to further observe the morphology of the sample, we characterized the material using SEM. Figure 4 is the SEM diagram of the prepared sample: (a) BiVO₄, (b) Mn_1−xZn_xFe₂O₄, and (c) BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %). It can be seen from Figure 4a that pure BiVO₄ is a three-dimensional spherical structure, and Figure 4b shows that the prepared Mn_1−xZn_xFe₂O₄ is six-square-like structure. The larger sphere seen in Figure 4c is the core-shell structure of BiVO₄ coated with Mn_1−xZn_xFe₂O₄, which indicated that the introduction of Mn_1−xZn_xFe₂O₄ caused the agglomeration of the resultant composite to some extent.

Figure 5 shows the N₂ adsorption-desorption isotherm and the pore size distribution curve of BiVO₄/Mn_1−xZn_xFe₂O₄ sample. The absorption-desorption isotherm could be categorized as a typical Type III absorption-desorption isotherm [17], indicating that the sample has a porous structure, which was convex to the P/P₀ axis over its entire range, revealing that the as-prepared composite BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) belonged to the nonporous structure. This sharp increase in the adsorption isotherm was attributed to the presence of macropores. The most probable pore size of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) is 6.0 nm. In addition, the specific surface area of BiVO₄/Mn_1−xZn_xFe₂O₄ sample calculated by BET measurement is 2.22 m²/g.

3.2. Magnetic Properties

Figure 6 depicts the hysteresis loops of the magnetic substrate and magnetic photocatalyst BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %). The saturation magnetization (Ms) and remanent magnetization (Ms) of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) were 84.03 emu/g and 7.03 emu/g, respectively. Compared with pure Mn_1−xZn_xFe₂O₄, the Ms of the magnetic photocatalyst decreased by 75.4% and 91.6%, respectively, due to the reduction to the magnetic material content per unit mass of the magnetic photocatalyst [18]. Overall, the magnetic properties of BiVO₄/Mn_1−xZn_xFe₂O₄ (15 wt %) were beneficial to its separation from the liquid solution and its recycling from the reaction solution using an extra magnet after use. In addition, the magnetic photocatalyst showed magnetic properties similar to pure Mn_1−xZn_xFe₂O₄, which indicated that the magnetic photocatalyst included Mn_1−xZn_xFe₂O₄.

3.3. UV-Vis DRS Analysis

The optical absorption properties of semiconductors are considered to be a key factor affecting their photocatalytic activity. Figure 7 shows the UV-Vis diffuse reflectance spectra of BiVO₄, BiVO₄/Mn_1−xZn_xFe₂O₄, and the curve of (Ahv)² vs. hv. It can be seen from the diagram that BiVO₄ and BiVO₄/Mn_1−xZn_xFe₂O₄ mainly absorb light in the wavelength range below 500 nm. Compared with pure BiVO₄, the maximum absorption wavelength of BiVO₄/Mn_1−xZn_xFe₂O₄ (lambda max) increases, and the absorption of visible light is enhanced to some extent. In addition, the band gap plays a very important role in the determination of photocatalytic activity. The relationship between absorbance and incident light intensity hv can be expressed by the following formula [8,9]:

Ahv = C (hv − Eg)^n/2

(1)

In the above equation, A, Eg, h, and v represent the absorption coefficient, band gap width, Planck constant, and incident light frequency, respectively, and C is defined as a constant. The band gap energy of the sample can be obtained from the (Ahv)²~hv curve. The band gaps (Eg) of BiVO₄ and BiVO₄/Mn_1−xZn_xFe₂O₄ were estimated. They have nearly the same Eg values (2.36 eV), which are in agreement with the literature [5,19]. Though the incorporation of Mn_1−xZn_xFe₂O₄ could not extend the range of absorption light, it is worth noting that the anther function of Mn_1−xZn_xFe₂O₄ enhanced the absorption intensity of visible light for BiVO₄ by charge transfer transitions (2Fe³⁺ → Fe²⁺ + Fe⁴⁺) [18].

3.4. Conductivity and Electrochemical Performance

Electrochemical impedance spectroscopy (EIS) is an effective method to evaluate the electron transfer between solid and electrolyte interfaces. Figure 8 shows the Nyquist diagram of the prepared samples. It can be seen from the diagram that the diameter of the semicircle decreases obviously with the introduction of Mn_1−xZn_xFe₂O₄, which indicates that the resistance of the solid interface layer and the surface electron transfer impedance decrease. The BiVO₄/Mn_1−xZn_xFe₂O₄ charge transfer impedance (206 Ω·cm²) is less than pure BiVO₄ (351 Ω·cm²) [20]. This is due to the doping effect of Fe ions in the Mn_1−xZn_xFe₂O₄ crystal lattice system. The charge transfer was easier, probably due to the charge transfer transitions of Fe ions (2Fe³⁺ → Fe²⁺ + Fe⁴⁺) and Mn ions (2Mn⁴⁺ → Mn²⁺ + Mn⁶⁺). The electron transfer impedance of the composite solid /electrolyte interface decreases, which may be because the incorporation of magnetic Mn_1−xZn_xFe₂O₄ could enhance the conductivity of BiVO₄ and heighten the quantum efficiency of BiVO₄. Thus, we can preliminarily predict that the photocatalytic activity of BiVO₄/Mn_1−xZn_xFe₂O₄ must be higher than that of pure BiVO₄ under the same light irradiation conditions.

3.5. Photocatalytic Activity and Stability

The results of the RhB photodegradation by the prepared BiVO₄ and BiVO₄/Mn_1−xZn_xFe₂O₄ samples are shown in Figure 9. Obviously, the photocatalytic efficiency of BiVO₄/Mn_1−xZn_xFe₂O₄ was higher than that of pure BiVO₄. Under the same visible light irradiation, the degradation rate of RhB for pure BiVO₄ reached 97% after 3 h, while BiVO₄/Mn_1−xZn_xFe₂O₄ only took 2 h to achieve the same degradation rate. This result was in good agreement with the EIS analysis. On the one hand, the introduction of Mn_1−xZn_xFe₂O₄ enhanced the conductivity of BiVO₄, further heightening the electron transfer ability. On the other hand, Mn_1−xZn_xFe₂O₄ enhanced the absorption intensity of visible light for BiVO₄. Thus, BiVO₄/Mn_1−xZn_xFe₂O₄ could produce more photoinduced electron-hole pairs under same visible light irradiation. The two factors generate a synergistic effect to boost the quantum efficiency of BiVO₄.

The repeatability of the magnetic photocatalyst was detected by cycling tests. After each cycle, the catalyst was separated by an external magnet, and then was washed and dried for the next cycle. The degradation rate of RhB using the fifth recycled photocatalyst was still more than 93% (see Figure 10). This indicated that the as-prepared magnetic photocatalyst had an excellent stability. In general, the introduction of a magnetic substrate can enhance the stability of a single component photocatalyst.

4. Conclusions

BiVO₄/Mn_1−xZn_xFe₂O₄ was prepared by the impregnation roasting method. The synthesis method was suitable for the mass production of various composites. XRD tests showed that the prepared BiVO₄ is monoclinic crystal, and the introduction of Mn_1−xZn_xFe₂O₄ does not change the crystal structure of BiVO₄. The introduction of a soft-magnetic material, Mn_1−xZn_xFe₂O₄, was beneficial to the composite photocatalyst’s separation from the liquid solution using an extra magnet after the reaction. UV-vis spectra analysis indicated that Mn_1−xZn_xFe₂O₄ enhanced the absorption intensity of visible light for BiVO₄. EIS investigation revealed that the introduction of Mn_1−xZn_xFe₂O₄ could decrease the electron transfer impedance of BiVO₄, further enhancing its conductivity and heightening its quantum efficiency. The photocatalytic efficiency of BiVO₄/Mn_1−xZn_xFe₂O₄ was higher than that of pure BiVO₄.