A novel Z-scheme Bi₂WO₆-based photocatalyst with enhanced dye degradation activity

Abstract

To overcome the drawback caused by rapid recombination of photogenerated electron-hole pairs, a novel visible-light-driven Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ Z-scheme photocatalyst was fabricated through the hydrothermal method. The as-prepared sample was analyzed by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), electrochemical impedance spectra (EIS), and X-ray photoelectron spectrometer (XPS). At the same time, the photocatalytic performance of ternary material was estimated by degradation of methylene blue (MB) aqueous solution under visible-light irradiation. Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ displayed a superior photocatalytic performance compared with pure Bi₂WO₆. A dye degradation rate of 98.1% was appeared in ternary material under a 30-min photocatalytic experiment. r-GO plays an important role in ternary material; it acted as a charge-transfer bridge to accelerate electron transfer from Bi₂WO₆ to Bi₂₅FeO₄₀. Therefore, the recombination of the photogenerated electron holes of the Bi₂WO₆ itself was effectively suppressed. Most important, the possible photocatalytic mechanism was evaluated on the basis of UV–vis and PL analysis. This work provides a new idea for the synthesis of new photocatalytic materials with enhanced photocatalytic properties.

Introduction

Methylene blue (MB), a water-soluble polycyclic aromatic hydrocarbon, is commonly used in industrial, pharmaceutical, aquaculture, and skin care areas (Liu et al. 2012). However, causing serious pollution to water due to increased production and wide application, the polluted water with MB has the characteristics of high concentration, high chroma, high pH, and hard to degradation (Saravanan et al. 2013). Researchers have tried many methods to solve the above problem, such as adsorption, photocatalytic technology, and membrane separation technology; photocatalytic technology has been widely concerned owing to its low energy consumption, mild conditions, simple operation, and no secondary pollution.

Over the past few decades, Bi₂WO₆, a typical narrow band gap semiconductor (E_g ≈ 2.7 eV), has been widely used for photodegradation of organic contaminants due to its unique structure and stable chemical properties (And and Zhu 2010). However, the photoactivity of pure Bi₂WO₆ is still not satisfactory attributing to the rapid recombination of photogenerated electron-hole pairs, so designing a new photocatalyst to improve its photocatalytic efficiency is imperative. Heterostructure photocatalyst has been proved to be a good choice to enhance the photocatalytic performance in comparation to single-component photocatalyst, with successful cases like Fe₃O₄/Bi₂WO₆ (Zhou et al. 2015), Cd/Bi₂WO₆ (Xu et al. 2015), AgBr/Ag/Bi₂WO₆ (Zhang et al. 2009), g-C₃N₄/Bi₂WO₆ (Lei et al. 2011), Bi₂S₃/Bi₂WO₆ (Sangeeta and Do-Heyoung 2018), and CdS/Bi₂WO₆ (Lei GE, Liu J 2011). These composite materials display an enhanced photocatalytic performance owing to the fast transfer photogenerated electrons-hole pair.

Bi₂₅FeO₄₀, a typical soft bismuth ore material, with a relatively narrow band gap (2.1–2.7 eV) (Ji et al. 2017), has been regarded as a promising photocatalyst attributing to its special structure, a crystal structure with a large number of oxygen vacancies and ion vacancies, which can greatly improve the photocatalytic performance of material (Köferstein et al. 2014). Unfortunately, the photocatalytic performance of pure Bi₂₅FeO₄₀ can only be enhanced by the addition of an electron scavenger such as H₂O₂, which can surpass the recombination of photogenerated electrons-hole pairs (Wisedsri et al. 2011; Hou et al. 2010). Until now, researchers have been tried many ways to improve its photocatalytic performance, such as Bi₂₅FeO₄₀/RGO (Basith et al. 2018), Bi₂₅FeO₄₀/Bi₂Fe₄O₉ (Wang et al. 2019), and Bi₂₅FeO₄₀/Fe₃O₄/Fe₂O₃ (de Góis et al. 2019). In light of this, the emergence of an all-solid-state Z-scheme heterojunction could address the problem to some extent. In this Z-scheme system, the photogenerated electrons in the CB of PSII could recombine with the holes in the VB of PSI; thus, holes with strong oxidizing ability are left in the VB of PSII and photogenerated electrons with strong reducing ability are left in the CB of PSI(Hu et al. 2017; Zhou et al. 2014). Also, conductive medium plays an important role in Z-scheme system, like Au (Bai et al. 2016), Ag (Li et al. 2017), and graphene oxide (GO) (Jiang et al. 2017; Miao et al. 2017; Rakibuddin et al. 2017) or reduced graphene oxide (r-GO) (Li et al. 2014; Jo and Selvam 2017; Chen et al. 2017); among them, r-GO stands out due to its large specific surface area and strong electrical conductivity. Ma et al. (2016) fabricated a Z-scheme catalyst named g-C₃N₄/r-GO/Bi₂WO₆; the excellent photocatalytic performance had been verified by degrading TCP. Zhang et al. (2016a, b) constructed a Bi₂WO₆/MoS₂/RGO Z-scheme catalyst to remove Cr⁶⁺; the r-GO could effectively improve the photocatalytic performance of Bi₂WO₆. Wu et al. (2017) synthesized a g-C₃N₄-r-GO-TiO₂ Z-scheme system and further revealed the multi-functional roles of r-GO in enhancing the photodegradation rate.

In summary, a novel Z-scheme photocatalyst Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ was fabricated, and Bi₂WO₆ (E_CB = + 0.6 V, E_VB = + 3.26 V vs. NHE) and Bi₂₅FeO₄₀ (E_CB = − 1.3 V, E_VB = + 1.13 V vs. NHE) were matched well. Firstly, nanosheet Bi₂WO₆ was synthesized in the presence of CTAB, then Bi₂WO₆ was used as a precursor template, and Fe(NO₃)₃·9H₂O and r-GO were added to obtain a ternary material under hydrothermal condition of 180 °C. The photocatalytic performance was evaluated by degrading RhB solution under visible-light irradiation. Ternary material exhibited better photocatalytic performance than binary and single materials. At the same time, a possible photo-degradation mechanism of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ has been discussed in detail on the basis of UV–vis and PL results. Such distinctive Z-scheme photocatalyst has greatly overcome the shortcomings of Bi₂WO₆ and successfully achieved the separation of photogenerated electron-hole pairs.

Experiment

Materials

All reagents used in this experiment were of analytical grade and were not further purified prior to use. All of them were purchased from Chengdu Kelong Co., Ltd.

Preparation of Bi₂WO₆ nanosheet

The nanosheet Bi₂WO₆ was constructed through the hydrothermal method. Typically, 1.2125 g of Bi(NO₃)₃·5H₂O and 0.1 g of hexadecyl trimethyl ammonium bromide (CTAB) were firstly dispersed into 50 mL of deionized water under ultrasonic treatment until obtaining a clear solution, subsequently adding 0.4125 g of Na₂WO₄·2H₂O to above solution and keeping vigorous magnetic stirring at room temperature for 30 min. Transferring the mixed solution to a 100-mL Teflon-lined autoclave and keeping at 160 °C for 12 h. The precipitate was washed with ethanol and deionized water for several times after completion of the hydrothermal reaction. Bi₂WO₆ nanosheet was obtained after drying at 60 °C for 12 h.

Preparation of Bi₂₅FeO₄₀, Bi₂WO₆/Bi₂₅FeO₄₀, and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀

At first, graphene oxide (GO) was fabricated through modified Hummer’s method (Shi et al. 2016). Then, 0.5 g of Bi₂WO₆ was dispersed in deionized water, followed by a certain amount of Fe(NO₃)₃·9H₂O was added to the above solution, adjusting the pH of the solution to 10 using NaOH solution (10 M). In addition, 10 mg of GO and 1 mg of L(+)-ascorbic acid (L-AA) were dissolved in 10 mL of deionized water and ultra-sonicated for 30 min to obtain even solution, mixing the above two solutions and stirring for 1 h, then transferring the mixed solution to a 100-mL Teflon-lined autoclave, and keeping at 180 °C for 12 h. The precipitate was washed with ethanol and deionized water for several times after the hydrothermal reaction was completed. Ternary material was obtained after drying at 100 °C for 10 h. For comparison, the pristine Bi₂₅FeO₄₀ and Bi₂WO₆/Bi₂₅FeO₄₀ were prepared under identical condition without adding Bi₂WO₆ or GO.

Characterization

The crystalline phases and purity of catalysts were determined by an X-ray diffractometer (XRD, PANalytical B.V., X Pert PRO MPD) with a Cu-Kα radiation (λ = 1.54060 Å, 40 kV, 40 mA) in the range of 5° ≤ 2θ ≤ 80°, operated at a scanning rate of 1°/min. Raman spectra (Oceanoptics, IDRaman micro IM-52) was used to analyze the molecular structure of materials. The morphology structures, element composition, and distribution of as-prepared samples were observed by scanning electron microscopy (SEM, Carl Zeiss AG, ZEISS EV0 MA15) equipped with an energy dispersive X-ray (EDX) spectroscopy. The nanostructures of catalysts were tested using a transmission electron microscope (TEM, Japan, Jeol 2100F). The UV–vis absorption spectra were recorded by a UV–vis spectrometer (Platinum Elmer Instrument Co., Ltd., PerkinEImerLambda850), choosing BaSO₄ as background. The surface chemical composition and chemical states of solid surface were carried out using an X-ray photoelectron spectrometer (XPS, America, Thermo ESCALAB 250XI) with Al Kα source. Photoluminescence (PL) spectra of samples were implemented using a PerKinEImer LS55 spectrofluorometer with an excitation wavelength of 270 nm.

Electrochemical measurements

The electrochemical impedance spectra (EIS) and photocurrent response intensity curve were obtained through CHI604D (Shanghai) electrochemical workstation equipped with a standard three-electrode cell: A Pt counter electrode, a saturated calomel reference electrode, and a FTO glass working electrode in the experiment; the electrolyte is Na₂SO₄ aqueous solution (0.5 M) and a 200 W LED lamp is used as a light source. Specific steps are as follows: a 5 mg of sample was dissolved in a mixed solution consisting of 50 μL of 5% Nafion solution and 1 mL absolute ethanol, and then ultra-sonicated for 30 min to obtain a uniform solution. The suspension was coated on the FTO glass working electrode to make an effective exposed area of 1 cm², measuring after ventilation overnight.

Photocatalytic experiment

The photocatalytic performance of all samples was evaluated by degrading MB aqueous solution under visible-light irradiation. There are same steps in each sample: 40 mg of the photocatalyst was dispersed into 50 mL of MB aqueous solution (20 mg/L). Before lighting, the mixed solution was stirred for 30 min in dark to ensure good dispersion and establish an adsorption–desorption equilibrium and then exposed to 200 W of visible light to begin photocatalytic experiment. The solution of aliquots volume was centrifuged to remove photocatalytic powders every 5 min, and then, the concentration of MB molecule at different time was measured by a UV–visible spectrophotometer (UV-1800, Shimadzu, Japan) at λ = 664 nm; deionized water was reference solution. Generally, calculating the degradation rate by the following formula: η =$\frac{C_0-{C}_t}{C_0}\times 100\%$, where C₀ is the initial concentration of MB and C_t is the concentration of MB at time t (min). In addition, cyclic stability of photocatalytic material is a critical performance too, which was recycled four times to ensure its reliability. The photocatalytic powders were washed with deionized water and ethanol for several times at the end of each experiment, and then, put it in fresh MB solution to start next recycle under identical condition.

Results and discussion

Crystal structure and morphology

The XRD patterns of GO, Bi₂WO₆, Bi₂₅FeO₄₀, Bi₂WO₆/Bi₂₅FeO₄₀, and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ are shown in Fig. 1. The characteristic peak located at 2θ = 9.6° in the GO XRD pattern is assigned to the (001) plane of GO (Zhu et al. 2018). For pure Bi₂WO₆, the conspicuous peaks located at 2θ = 28°, 34°, 47.5°, 56°, 59°, 69°, 76°, and 79° are assigned to (113), (006), (220), (313), (226), (400), (139), and (420) respectively (Zhang et al. 2016a, b). The sharp peak patterns of Bi₂₅FeO₄₀ are in good assignment with the standard Joint Committee on Powder Diffraction Standards (JCPDS); the peaks at 24°, 27°, 30°, 33°, 39°, 52°, 66°, and 78° are related to (220), (310), (222), (321), (332), (530), (640), and (653) respectively (Zhang et al. 2015). However, the charactiristic peaks of Bi₂₅ FeO₄₀ have been not found in binary composite and ternary composite, only the charactiristic peaks of Bi₆ WO₆ appeared. Probably because the small dosage of Fe(NO₃)₃·9H₂O (6% by Bi₂WO₆) is wrapped with Bi₂WO₆ to make it difficult to detect. Also, the characteristic peak of GO in the XRD pattern of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ cannot be appeared, which can be considered as the relatively low amount of GO and the appearance of RGO. No other impurity phases appear, proving the high purity of Bi₂WO₆/r-GO/Bi₂₅FeO_40.

Fig. 1

XRD patterns of GO, Bi₂WO₆, Bi₂₅FeO₄₀, Bi₂WO₆/Bi₂₅FeO₄₀, and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites

To discuss the morphology and detailed structure of as-prepared products, SEM images are displayed in Fig. 2a–c. The pure Bi₂WO₆ is constructed from a large number of regular square nanosheets. GO displays a flake-like structure with a wrinkled surface (Fig. 2b). For Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ (Fig. 2c), the r-GO and Bi₂₅FeO₄₀ do not cover the square structure of Bi₂WO₆ due to the small dosage. The Bi₂₅FeO₄₀ particles load on the Bi₂WO₆ square nanosheet, and they are in close contact with r-GO, which can promote the effective separation of photogenerated electron-hole pairs and further improve the photocatalytic activity of ternary composite. Moreover, the EDS pattern suggests that there are five elements of Bi, W, Fe, O, and C in the ternary composite. In addition, TEM technology is operated to further characterize the morphology of Bi₂WO₆ and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀. As shown in Fig. 3a, Bi₂WO₆ has a square structure. The ternary composite reveals an irregular morphology after the introduction of r-GO and Bi₂₅FeO₄₀, which is in agreement with SEM analysis that Bi₂₅FeO₄₀, Bi₂WO₆, and r-GO are in good contact with each other.

Fig. 2

SEM images of a Bi₂WO₆, b GO, and c Bi₂WO₆/r-GO/Bi₂₅FeO₄₀, and the EDS pattern of d Bi₂WO₆/r-GO/Bi₂₅FeO₄₀

Fig. 3

TEM images of a Bi₂WO₆ and b Bi₂WO₆/r-GO/Bi₂₅FeO₄₀

Surface structure and properties

Raman spectroscopy was employed to provide further evidence of the local structure of materials. As shown in Fig. 4, the Raman peaks at approximately 304, 712, and 791 cm⁻¹ were appeared in pure Bi₂WO₆ which were in assignment with the bending vibration of WO₆, antisymmetric bridging mode of tungstate chains, and stretching vibration (Ma et al. 2016). For single Bi₂₅FeO₄₀, the peaks mainly concentrate on 273 and 528 cm⁻¹. Two distinct peaks at 1323 and 1528 cm⁻¹ are related to D (the symmetry A1g k-point phonon) and G (the E2g phonon of sp2 carbon atoms) bands of GO (Chen et al. 2017). In general, the intensity ratio (I_D/I_G) of the D band and the G band can indicate the reduction degree of GO (Shen et al. 2017), the larger the I_D/I_G, the higher the reduction degree of GO. Similarly, the characteristic peaks of GO and Bi₂WO₆ in the Raman spectra are clearly visible; however, the peaks of Bi₂₅FeO₄₀ are indistinguishable, indicating that Bi₂₅FeO₄₀ is highly dispersed on the surface of ternary composite. Furthermore, the value of I_D/I_G (1.48) in Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ is increased compared with GO (I_D/I_G = 1.30), indicating successful reduction of GO during the hydrothermal process.

Fig. 4

Raman spectra of GO, Bi₂WO₆, Bi₂₅FeO₄₀, Bi₂WO₆/Bi₂₅FeO₄₀, and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites

The chemical states of the ternary composite are studied by means of XPS technique. As the full survey spectra of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ are shown in Fig. 5a, the signals of Bi, W, Fe, O, and C elements are found in ternary composite, indicating the presence of Bi₂WO₆, Bi₂₅FeO₄₀, and r-GO. Figure 5 b exhibits the Bi 4f spectra of ternary composite in which the peaks at 159.4 and 164.7 eV are assigned to Bi 4f_7/2 and Bi 4f_5/2 respectively, suggesting that the elemental state of bismuth is Bi³⁺ (Wu et al. 2007) in the catalyst. For the XPS result of Fe 2p, the signal has been divided into Fe 2p_1/2 and Fe 2p_3/2 attributing to the spin–orbit coupling (Zhang et al. 2016a, b), the peak centers located at 709.8 and 723.4 eV respectively. Figure 5 d shows the XPS result of W; the two typical peaks at 35.78 and 37.9 eV are assigned to W 4f_7/2 and W 4f_5/2, which can be ascribed to the oxidation state of W⁶⁺ (Fu et al. 2005). As shown in Fig. 5e, the characteristic orbital of the O 1s is observed with peak locations at 531.9, 529.8, and 531.0 eV, which are corresponding to the r-GO (Zhu et al. 2018), Fe–O–Bi, and Bi–O–W (Zhang et al. 2016a, b). The binding energy peaks in Fig. 5f at 284.9, 286.8, 287.7, and 288.7 eV are well in accordance with C 1s state in r-GO, which are assigned to C–C, C–O, C=O, and O–C=O (Zhu et al. 2018) respectively. All binding energies are consistent with previous reports.

Fig. 5

XPS spectra of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composite, a survey, b Bi 4f, c Fe 2p, d W 4F, e O 1s, and f C 1s

The optical absorption properties of as-prepared samples are investigated via the UV–vis DRS technique. All samples exhibit visible-light absorption in the wavenumber range of 200 to 700 cm⁻¹ in Fig. 6. Ternary composite establishes a wide absorption edge after the introduction of Bi₂₅FeO₄₀ and r-GO; the absorption edges of Bi₂WO₆, Bi₂₅FeO₄₀, Bi₂WO₆/Bi₂₅FeO₄₀, and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ are 470, 520, 610, and 680 cm⁻¹ respectively, indicating the ternary material can make full use of visible light and further improve the photocatalytic performance. Meanwhile, the band gap energy is calculated according to the following formula:

$$ {\left(\upalpha \mathrm{h}\upnu \right)}^{\mathrm{n}}=A\left(\mathrm{h}\upnu -{E}_{\mathrm{g}}\right) $$

where α, ν, A, and E_g are absorption coefficient, optical frequency, constant, and band gap respectively; the value of n depends on the type of semiconductor; there is 1/2 in direct band gap and 2 in indirect band gap (López and Gómez 2012). The value of n in Bi₂WO₆ and Bi₂₅FeO₄₀ are both 2 due to they are indirect gap semiconductor. As shown in Fig. 3, the band gap of Bi₂WO₆ and Bi₂₅FeO₄₀ is 2.66 and 2.42 eV respectively, which are similar to previous reports.

Fig. 6

UV–vis DRS spectra of Bi₂WO₆, Bi₂₅FeO₄₀, and the Bi₂WO₆/Bi₂₅FeO₄₀ and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites, and the plot of (αhν)² vs. hν (inset) for Bi₂WO₆ and Bi₂₅FeO₄₀

Photoelectrochemical performance

The electrochemical impedance spectroscopy (EIS) test is an effective method for analyzing the separation efficiency of photogenerated electron-hole pairs. Generally, the smaller the arc radius of the EIS diagram, the higher the photogenerated electron-hole pairs separation efficiency (Shi et al. 2016). As shown in Fig. 7a, the arc radius of all samples is in the order Bi₂₅FeO₄₀ > Bi₂WO₆ > Bi₂WO₆/Bi₂₅FeO₄₀ > Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites. Obviously, ternary material with the smallest arc radius has the highest photogenerated electron-hole pairs’ separation efficiency, which is consistent with the PL response. The photocurrent response intensity curve of all samples was conducted to the same electrolyte (Na₂SO₄ solution, 0.5 M) under the condition of alternating light and dark. As seen in Fig. 7b, modified Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ exhibits a higher photocurrent response intensity than single Bi₂WO₆; its value can reach 2.38 μA/cm² which is about 6.4 times than that of pristine Bi₂WO₆. Such Z-scheme photocatalyst will show excellent potential in water treatment and other fields.

Fig. 7

a EIS Nyquist plots and b photocurrent response spectra of Bi₂WO₆, Bi₂₅FeO₄₀, and the Bi₂WO₆/Bi₂₅FeO₄₀ and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites

Photocatalytic degradation of RhB

The photoluminescence (PL) spectra are investigated to evaluate the migration and recombination processes of photogenerated electrons-hole pairs of the photocatalyst. Fluorescence comes from the recombination of free carriers inside the semiconductor, the faster the recombination of photogenerated electron-hole pairs, the higher the fluorescence excitation intensity of the material. As shown in Fig. 8, the emission peaks of all samples focus on 450 nm; Bi₂WO₆ exhibits a strong PL intensity owing to the rapid recombination of photogenerated electron-hole pairs on its surface. The PL strength of Bi₂WO₆/Bi₂₅FeO₄₀ is lower than that of Bi₂WO₆ and Bi₂₅FeO₄₀, indicating the combination of the two can suppress the recombination of photogenerated electron-hole pairs. More importantly, Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ displays a weaker PL intensity than Bi₂WO₆/Bi₂₅FeO₄, revealing r-GO as an electronic modifier in the Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ enables higher separation of photogenerated electron-hole pairs, and then, more active substances generated in the photocatalytic process. The result indicates the successful construction of Z-scheme photocatalyst.

Fig. 8

Photoluminescence spectra for pure Bi₂WO₆, Bi₂₅FeO₄₀, Bi₂WO₆/Bi₂₅FeO₄₀, and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites

To demonstrate the photocatalytic activity of all samples, the photodegradation of MB aqueous solution (20 mg/L) was operated under visible-light irradiation at room temperature. The dynamic degradation curve of all samples is shown in Fig. 9a. Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ displays superior photocatalytic performance in 30-min irradiation compared with pure Bi₂WO₆. Furthermore, the blank experiment without photocatalyst proves that the property of MB is stable, thus the self-photolysis of it can be ignored. The degradation rate of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ is close to 98.1% that is about 2.3 times than that of pure Bi₂WO₆ (42%). Furthermore, the UV–vis absorption spectrum of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ is shown in Fig. 9b, which further explains the change in MB concentration during the irradiation time. In addition, the kinetic of the photocatalytic reaction is in line with first-order reaction that can be described as: ln (C_t/C₀) = kt. Here, C₀ is the initial concentration of MB, C_t is the concentration of MB at irradiation time t (min), and k (min⁻¹) is apparent reaction rate constant. Obviously, Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ shows a highest reaction rate constant (0.10155 min⁻¹) which is about 9.2 and 11.4 times than that of Bi₂WO₆ and Bi₂₅FeO₄₀, respectively. Meanwhile, the stability of Bi₂WO₆ and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ was studied via recycling experiment; all conditions are the same as mentioned above. The catalyst was centrifuged and washed with deionized water at the end of each cycle. It is obvious that Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ exhibits a better stability than Bi₂WO₆ after four cycles.

Fig. 9

a Dynamic degradation curve of MB with different catalysts under visible-light irradiation. b Absorption changes of MB solution in the presence of 6% Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composite. c Reaction rate constants (k) of the as-prepared samples. d Cycling performances of photocatalytic degradation of MB over Bi₂WO₆ and Bi₂WO₆/r-GO/Bi₂₅FeO₄₀

Photocatalytic mechanism

In order to clearly elaborate the possible degradation mechanism of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀, the Mott–Schottky plots and UV–vis spectra were carried out to estimate the CB and VB potentials of Bi₂WO₆ and Bi₂₅FeO₄₀. As shown in Fig. 10, the flat band potential (U_fb) values of Bi₂WO₆ and Bi₂₅FeO₄₀ are + 0.56 and − 1.34 V (vs. SCE). Then, the values of CB are 0.36 and − 1.54 V (vs. SCE, CB ≈ U_fb − 0.2 V), and they are 0.6 and − 1.3 V vs. normal hydrogen electrode (NHE, NHE = SCE + 0.24 V) (Xu et al. 2018). In addition, the VB potentials of Bi₂WO₆ and Bi₂₅FeO₄₀ could calculate through E_VB = E_CB + E_g (Lin et al. 2017), and the E_g values for Bi₂WO₆ and Bi₂₅FeO₄₀ are 2.66 and 2.43 V according to UV–vis results. Therefore, the VB potentials of Bi₂WO₆ and Bi₂₅FeO₄₀ are determined to be 3.26 and 1.13 V. The band structures of them are shown in Fig. 11.

Fig. 10

Mott–Schottky plots of Bi₂WO₆ (a) and Bi₂₅FeO₄₀ (b)

Fig. 11

Energy band structure of Bi₂WO₆ and Bi₂₅FeO₄₀

Zhang et al. (2016a, b) had analyzed the double transfer theory of Bi₂WO₆/Bi₂₅FeO₄₀. The photogenerated electrons in the CB of Bi₂₅FeO₄₀ transfer to the VB of Bi₂WO₆, while the holes deposited on VB of Bi₂₅FeO₄₀ and electrons were left at CB of Bi₂WO₆. The photogenerated electrons and holes are effectively separated to improve the photocatalytic performance of the material. It is obvious that the double transfer theory cannot explain the mechanism of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ in MB dye degradation under visible light. So, we propose the Z-scheme mechanism of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ followed the previous reports and illustrated in Fig. 12. The band gap of Bi₂WO₆ and Bi₂₅FeO₄₀ is 2.66 and 2.43 eV respectively, while the CB and VB potentials of Bi₂WO₆ are estimated to be + 0.6 and + 3.26 eV, and for Bi₂₅FeO₄₀, they are − 1.3 and 1.13 eV. As shown in Fig. 12, after irradiated with visible light, the photogenerated electrons in the CB of Bi₂WO₆ could rapidly transfer to the VB of Bi₂₅FeO₄₀ through r-GO and recombine with holes, and the remaining photogenerated electrons and holes are accumulated on the CB of Bi₂₅FeO₄₀ and the VB of Bi₂WO₆, respectively. Notably, the CB potential (− 1.3 eV) on Bi₂₅FeO₄₀ is more negative than that of O₂/ ̇O₂⁻ (− 0.46 eV) (Wu et al. 1998), which can reduce O₂ to ̇O₂⁻ radicals. Meanwhile, the VB potential (3.26 eV) on Bi₂WO₆ is more positive than that of H₂O/ ̇OH (2.85 eV) (Li et al. 2015), thus the H₂O could be oxidized to ̇OH, and the holes with excellent oxidation ability on VB can directly degrade the dye in the photocatalytic process. r-GO plays two important roles in this photocatalytic system, on the one hand, increasing the contact area and reaction space between catalyst and contaminant, and, on the other hand, accelerating the transfer of electrons between Bi₂WO₆ and Bi₂₅FeO₄₀, effectively suppressing the recombination of photogenerated electron-hole pairs. In summary, the photocatalytic activity of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ is greatly improved. In addition, the photocatalytic process of Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ Z-scheme can be described as follows:

$$ {\displaystyle \begin{array}{l}{\mathrm{Bi}}_2{\mathrm{WO}}_6+\mathrm{h}\upnu \to {\mathrm{Bi}}_2{\mathrm{WO}}_6\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)\\ {}{\mathrm{Bi}}_{25}{\mathrm{FeO}}_{40}+\mathrm{h}\upnu \to {\mathrm{Bi}}_{25}{\mathrm{FeO}}_{40}\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)\\ {}{\mathrm{Bi}}_2{\mathrm{WO}}_6\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)+{\mathrm{Bi}}_{25}{\mathrm{FeO}}_{40}\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)\to {\mathrm{Bi}}_2{\mathrm{WO}}_6\left({\mathrm{h}}^{+}\right)+{\mathrm{Bi}}_{25}{\mathrm{FeO}}_{40}\left({\mathrm{e}}^{-}\right)\\ {}{\mathrm{Bi}}_{25}{\mathrm{FeO}}_{40}\left({\mathrm{e}}^{-}\right)+{\mathrm{O}}_2\to {\mathrm{Bi}}_{25}{\mathrm{FeO}}_{40}+\dot{\mkern6mu}{{\mathrm{O}}_2}^{-}\\ {}{\mathrm{Bi}}_2{\mathrm{WO}}_6\left({\mathrm{h}}^{+}\right)+{\mathrm{H}}_2\mathrm{O}\to \dot{\mkern6mu}\mathrm{OH}+{\mathrm{Bi}}_2{\mathrm{WO}}_6\\ {}{\mathrm{Bi}}_2{\mathrm{WO}}_6\left({\mathrm{h}}^{+}\right)+\dot{\mkern6mu}{{\mathrm{O}}_2}^{-}+\dot{\mkern6mu}\mathrm{OH}+\mathrm{MB}\to {\mathrm{Bi}}_2{\mathrm{WO}}_6+\mathrm{Degradation}\ \mathrm{products}\end{array}} $$

Fig. 12

Z-Scheme charge transfer process in the Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ composites

Conclusion

In summary, the Z-scheme Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ photocatalyst is successfully prepared by the hydrothermal method, which exhibits high photocatalytic activity in the degradation of MB solution under visible light compared with single Bi₂WO₆ and Bi₂₅FeO₄₀. Furthermore, Bi₂WO₆/r-GO/Bi₂₅FeO₄₀ still maintains a high degradation rate for MB solution after four consecutive reuses. In this photocatalytic process, the r-GO is regarded as an electron transport bridge that could rapidly accumulate h⁺ in VB of Bi₂WO₆ and e⁻ in CB of Bi₂₅FeO₄₀, both of them are the main active substances in dye degradation; therefore, the photocatalytic activity of the ternary composite is improved. We believe this work may provide new insights and perspectives for the preparation of new Z-scheme photocatalysts and be successfully applied in the environmental field.