Construction of the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction for highly efficient visible-light-driven photocatalytic reduction of Cr(VI)

Abstract

Herein, a Bi₂WO₆/Bi₄V₂O₁₁ heterostructured photocatalyst was fabricated through a facile one-pot solvothermal method. In this heterojunction system, highly homogeneous-dispersed Bi₄V₂O₁₁ nanocrystals were anchored onto Bi₂WO₆ nanoflakes, endowing the heterojunction with nanosized interfacial contact. Because of the favorable interfacial contact and band alignment, photoinduced charge carrier transfer is facilitated in the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction, which can be verified by photocurrent and photoluminescence measurements. In contrast with pristine Bi₂WO₆ and Bi₄V₂O₁₁ samples, the Bi₂WO₆/Bi₄V₂O₁₁ heterostructured photocatalyst displays drastically improved photocatalytic Cr(VI) reduction activity under visible-light irradiation. More significantly, the as-fabricated heterojunction possesses superior activity for Cr(VI) relative to the counterpart obtained by mechanical mixing, indicating that the interfacial contact plays a determinant role in promoting charge carrier transfer. Moreover, Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions exhibited good cycling stability even after 5 cycles. A plausible photocatalytic Cr(VI) reduction reaction mechanism over the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction is proposed on the basis of band alignment. This work reports a high-efficiency photocatalyst for Cr(VI) and might shed light on the decisive factor for optimizing the heterostructure.

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1. Introduction

Nowadays, energy shortage and environment pollution are two prominent global issues. Photocatalysis as a ‘‘green’’ technology, possessing bright potential for the development of renewable energy and the elimination of contaminants, has attracted substantial attention in the past few decades. The fabrication of visible-light-driven photocatalysts with outstanding catalytic activity and stability has drawn considerable attention.^1–3

In the past few decades, Bi₂WO₆ with a layered structure consisting of alternating (Bi₂O₂)²⁺ layers and (WO₄)²⁻ octahedral layers has been considered as a promising photocatalyst, which possesses visible-light harvesting ability for catalysis reactions because of its narrow band gap (2.6–2.8 eV).^4–6 Unfortunately, pristine Bi₂WO₆ can only harvest light with a wavelength less than approximately 450 nm, which takes up a tiny portion of solar light.^7,8 Besides a rapid recombination of photoinduced electron–hole pairs, sluggish charge transport in pristine Bi₂WO₆ will greatly restrict its expectable photocatalytic activity.^9–11 In order to overcome the obstacles of limited light absorption and a rapid recombination of charge carriers, many strategies have been developed, such as doping,⁷ controlling morphology¹² and nanostructuring.¹³

Among them, the method of constructing heterojunctions stands out since it can efficiently boost charge carrier separation and transfer stemming from the as-introduced built-in electric field at the heterostructured interfaces.^14–16 For example, Liu's group reported the Ag₂O modified Bi₂WO₆ heterojunction photocatalysts synthesized by the solution precipitation strategy and the results showed that the Ag₂O/Bi₂WO₆ composite photocatalyst exhibited obviously enhanced photocatalytic activity compared with pure Ag₂O and Bi₂WO₆ for the degradation of organic contaminants.¹⁷ To date, different types of Bi₂WO₆-based composite photocatalysts have been fabricated with increased photocatalytic efficiency, which can be roughly classified into the composites of Bi₂WO₆ with oxides such as Bi₂O₃/Bi₂WO₆,¹⁸ Co₃O₄/Bi₂WO₆,¹⁹ WO₃/Bi₂WO₆,²⁰ ZnO/Bi₂WO₆,²¹ TiO₂/Bi₂WO₆,²² the composites of Bi₂WO₆ with sulphides such as Bi₂S₃/Bi₂WO₆,²³ Bi₂WO₆/CdS,²⁴ the composites of Bi₂WO₆ with BiOX such as BiOCl/Bi₂WO₆,²⁵ BiOBr/Bi₂WO₆,²⁶ BiOI/Bi₂WO₆,²⁷ the composites of Bi₂WO₆ with carbonaceous materials such as C₃N₄/Bi₂WO₆,^28,29 Ag₂CO₃/Bi₂WO₆,³⁰ Bi₂O₂CO₃/Bi₂WO₆,³¹ the Bi-based compounds such as Bi₂WO₆/BiVO₄³² and Bi₂MoO₆/Bi₂WO₆,³³ and the composites of Bi₂WO₆ with noble metals such as Ag loaded Bi₂WO₆,³⁴etc.

Very recently, Bi₄V₂O₁₁ with a peculiar layered structure which has intrinsic oxygen vacancies in perovskite (VO_3.5□_0.5)²⁻ (□ represents intrinsic oxygen vacancies) slabs sandwiched between (Bi₂O₂)²⁺ layers is considered as an excellent photocatalyst for oxygen evolution and water purification.^35–39 Because of the narrow band gap (∼2.1 eV) and excellent charge carrier transport stemming from its unique crystalline structure, Bi₄V₂O₁₁ has been employed to construct heterojunction photocatalysts. Lv and co-workers fabricated the BiVO₄/Bi₄V₂O₁₁ heterojunction nanofibres via a facile electrospinning process and evaluated their photocatalytic activity for the degradation of RhB. The results indicated that the photocatalytic performance of BiVO₄/Bi₄V₂O₁₁ heterojunction nanofibres was markedly enhanced compared with that of pure BiVO4 nanofibres.⁴⁰ Liu et al. reported the preparation of Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunction photocatalysts using a facile one-pot solvothermal method and the photocatalysis experimental results revealed that Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions exhibited high photocatalytic activity towards the degradation of RhB compared with bare Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁.⁴¹

All of the above heterojunctions realize dramatically enhanced photocatalytic activity due to the effective separation and transfer of photoinduced charge carriers. Nevertheless, to the best of our knowledge, the construction of the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction has not been reported, which might promote the photocatalytic properties of Bi₂WO₆ dramatically.

In this study, the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction was fabricated using a one-pot solvothermal method. This heterojunction is constructed from Bi₂WO₆ nanoflakes and anchored Bi₄V₂O₁₁ nanocrystals, which achieves a nanosized interfacial contact. The photocatalytic performances of the as-fabricated pristine Bi₂WO₆, Bi₄V₂O₁₁ and different Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions were evaluated by photocatalytic Cr(VI) reduction under visible-light illumination, and the photocatalytic experimental results indicated that the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction exhibited a higher photocatalytic activity than pristine Bi₂WO₆ and Bi₄V₂O₁₁. Finally, the enhanced photocatalytic mechanism of the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction has also been discussed.

2. Experimental

2.1 Photocatalysts preparation

All chemical reagents were received from Aladdin Chemical Co., Ltd, and used without further purification.

2.0 mmol of Bi(NO₃)₃·5H₂O was dissolved in 16 mL of ethylene glycol (EG) in a water bath at 80 °C under magnetic stirring. Then, appropriate stoichiometric amounts of Na₂WO₄·2H₂O and NH₄VO₃ were added to the above Bi(NO₃)₃ solution, which was subsequently continuously stirred for 20 min in a water bath at 80 °C to obtain a transparent solution. In order to fixed the pH value of 8, 2 mL of 2 M NaOH solution was slowly dropped into the above solution and subsequently 16 mL of absolute ethanol was added. Then, the above solution was poured into a 50 mL Teflon-lined stainless autoclave, which was subsequently sealed and maintained at 160 °C for 16 h in an oven. After the reactor cooled down naturally to room temperature, the as-obtained product was centrifuged using water and absolute ethanol three times, respectively. Finally, the photocatalyst was obtained after drying treatment at 80 °C in air for 4 h. According to the abovementioned method, Bi₂WO₆/Bi₄V₂O₁₁ heterojunction photocatalysts with different mole amounts of Na₂WO₄·2H₂O and NH₄VO₃ at 0.8 mmol and 0.2 mmol, respectively (denoted as BWV-82), 0.6 mmol and 0.4 mmol, respectively (denoted as BWV-64), 0.4 mmol and 0.6 mmol, respectively (denoted as BWV-46) were prepared, in which the mole amount of Bi(NO₃)₃·5H₂O was fixed as 2 mmol, and the total mole amounts of Na₂WO₄·2H₂O and NH₄VO₃ are 1 mmol. In addition, pristine Bi₂WO₆, Bi₄V₂O₁₁ and mechanically mixed Bi₂WO₆/Bi₄V₂O₁₁ (W : V = 8 : 2, denoted as BWV-82-MM) were also fabricated for the purpose of comparison.

2.2 Characterizations

The crystalline phases of the as-fabricated samples were analyzed by X-ray diffraction (XRD) on a Rigaku D/max-2000 diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range of 2θ = 20°–90° at a scanning rate of 4 °C min⁻¹ with a scan width of 0.02°. The morphology of the samples was observed using a field emission scanning electron microscope (FE-SEM, HELIOS NanoLab 600i) with an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses were carried out on a JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. X-Ray photoelectron spectroscopy (XPS) was conducted using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer coupled with a pass energy of 20.00 eV and an Al Kα excitation source (1486.6 eV). UV-vis diffuse reflectance spectra (DRS) were recorded using a spectrophotometer (HITACHI UH-4150) with BaSO₄ as the reflectance standard. The photoluminescence (PL) spectra were recorded on HORIBA FluoroMax-4.

2.3 Photocatalytic activity and photoelectrochemical measurements

The photocatalytic performance of the as-fabricated samples was evaluated by the reduction of Cr(VI) with citric acid as a hole scavenger under visible-light illumination using a 300 W Xe lamp (Trusttech PLS-SXE 300, Beijing) with a UV cutoff filter (λ ≥ 400 nm). In a typical photocatalytic measurement process, 0.05 g of the as-fabricated sample was added to 100 mL of Cr(VI) solution (10 mg L⁻¹, which was based on Cr in a dilute K₂Cr₂O₇ solution). The photocatalyst was dispersed in the Cr(VI) solution under ultrasonic treatment for 10 min, and the mixed solution was magnetically stirred in the dark for 30 min to reach adsorption–desorption equilibrium between the photocatalyst and the degrading pollutants, then adding 0.05 g of citric acid to the reactor before irradiation. When the photodegradation experiment began, 4 mL of the solution was collected from the suspension at fixed-time intervals, subsequently centrifuged at 10⁴ rpm for 5 min to remove catalyst powders, and the Cr(VI) concentration was determined at 352 nm by using the HITACHI UH-5300 UV-vis spectrometer.

The photoelectrochemical measurements were carried out on a CHI604C electrochemical working station using a standard three-compartment cell with Bi₄V₂O₁₁, Bi₂WO₆ and Bi₂WO₆/Bi₄V₂O₁₁ samples coated at FTO glass as the working electrodes, a piece of Pt sheet as a counter electrode and standard Ag/AgCl in saturated KCl as a reference electrode, a 0.5 M Na₂SO₄ aqueous solution as an electrolyte. Conversion between the measured potential (vs. Ag/AgCl) and NHE is achieved through the following equation.⁴²

E_NHE = E_Ag/AgCl + 0.1976 (25 °C)

The light source employed was a 300 W Xe lamp. The Mott–Schottky measurements were carried out at a frequency of 100 Hz with 10 mV amplitude.

3. Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns of the as-prepared pristine Bi₂WO₆, Bi₄V₂O₁₁ and Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions. The diffraction peaks of the pristine Bi₂WO₆ and Bi₄V₂O₁₁ samples can be well indexed to the orthorhombic phase Bi₂WO₆ (JCPDS No. 73-1126) and the orthorhombic phase Bi₄V₂O₁₁ (JCPDS No. 42-0135), respectively. The XRD patterns of Bi₂WO₆/Bi₄V₂O₁₁ heterostructured samples are similar to those of pristine Bi₂WO₆, while no characteristic peak belonging to Bi₄V₂O₁₁ could be observed. The absence of XRD peaks of Bi₄V₂O₁₁ might be due to their tiny size and uniform dispersion on the surface of Bi₂WO₆,^43–45 which is displayed as below. The morphology and microstructure of samples are investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Pristine Bi₂WO₆ (Fig. 2a) exhibits irregular flake-like morphologies with a thickness of about 30 nm. In contrast, pure Bi₄V₂O₁₁ (Fig. 2b) displays uniform and monodispersed hierarchical flower-like microspheres with a diameter of approximately 1 μm. When the heterojunction is constructed by Bi₂WO₆ with Bi₄V₂O₁₁, a representative sample of BWV-82 possesses irregular nanoflake morphology, whose size is smaller than that of pristine Bi₂WO₆ (Fig. 2c). The TEM image of BWV-82 (Fig. 2d and Fig. S1, ESI†) further illustrates that the nanoflakes are ca. 5 nm in thickness and 100 nm in length. Interestingly, as observed from the magnified TEM image (Fig. 2e) of Fig. 2d, nanocrystals with a size of approximately 6 nm are homogeneously dispersed and tightly adhered to the surface of the nanoflakes.

Fig. 1 XRD patterns of pristine Bi₂WO₆, Bi₄V₂O₁₁ and Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions.

Fig. 2 FESEM images of (a) Bi₂WO₆, (b) Bi₄V₂O₁₁ and (c) BWV-82; (d), (e) TEM images and (f) HRTEM image of BWV-82.

As vividly elucidated in a high-resolution TEM (HRTEM) image (Fig. 2f), the interplanar spacing of the nanoflakes is 0.273 nm which could correspond to the (002) plane of the orthorhombic Bi₂WO₆, while the interplanar spacing of the as-anchored nanocrystals is 0.225 nm, assigned to the (204) plane of orthorhombic Bi₄V₂O₁₁. Thus, the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction is successfully constructed by Bi₂WO₆ nanoflakes with the as-anchored Bi₄V₂O₁₁ nanocrystals. Moreover, attributed to this unique configuration, nanosized interfacial contact is also achieved due to the tiny size of Bi₄V₂O₁₁ nanocrystals. In addition, in the morphology of the BWV-82 heterojunction, no flower-like hierarchical structure of Bi₄V₂O₁₁ microspheres was observed and Bi₄V₂O₁₁ became tiny nanocrystals, which could be attributed to the restricted crystal growth of Bi₄V₂O₁₁ due to the confined space effect of two-dimensional Bi₂WO₆ nanoflakes.⁴⁶ In addition, the SEM images of Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions with various mole amounts of Na₂WO₄·2H₂O and NH₄VO₃ are shown in the ESI,† Fig. S2. The optical absorption properties of as-prepared photocatalysts are analyzed by UV-vis diffuse reflection spectroscopy (DRS). From the observation of UV-vis DRS spectra (Fig. 3), it can be clearly seen that the absorption edges of pristine Bi₂WO₆ and Bi₄V₂O₁₁ are located at ca. 440 nm and 612 nm, respectively. In contrast, the absorption spectra of Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions exhibited the combined absorption properties of pristine Bi₂WO₆ and Bi₄V₂O₁₁. This result further provided the evidence for constructing the heterojunction between Bi₂WO₆ and Bi₄V₂O₁₁. Moreover, the Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions exhibit an enhanced light absorption ability and a red shift of the absorption edge compared to pristine Bi₂WO₆. It indicates that the introduction of Bi₄V₂O₁₁ is beneficial for light-harvesting of Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions. With the purpose of demonstrating the surface electronic states and chemical compositions of the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction photocatalysts, X-ray photoelectron spectroscopy (XPS) is carried out. Fig. 4 shows the XPS spectra of the as-prepared BWV-82 heterojunction photocatalyst. The survey spectra and C ls spectra of BWV-82 are shown in Fig. S3 (ESI†). The peak positions in all of the XPS spectra are calibrated using those of C 1s (284.6 eV) spectra. As displayed in Bi 4f spectra (Fig. 4a), two photoelectron peaks located at 164.2 and 158.9 eV are assigned to Bi 4f_5/2 and Bi 4f_7/2, respectively, which corresponds to Bi³⁺.⁴⁷ In W 4f XPS spectra (Fig. 4b), the binding energies for W 4f_5/2 and W 4f_7/2 are 37.4 and 35.2 eV, respectively, which are assigned to W⁶⁺ in Bi₂WO₆.⁴⁸ For V 2p peaks (Fig. 4c), two peaks located at 524.1 and 516.5 eV are assigned to V 2p_1/2 and V 2p_3/2, respectively, which belong to V⁵⁺ in Bi₄V₂O₁₁.^37,41 Moreover, Fig. 4d shows the O 1s XPS, which can be divided into three peaks. The binding energies of 529.5, 530.1 and 531.0 eV could be assigned to the lattice oxygen in Bi–O,⁴⁹ the lattice oxygen in W–O,²⁵ and intrinsic oxygen vacancies (V_O) in Bi₄V₂O₁₁,^50,51 respectively. The XPS results further demonstrate the presence of Bi₂WO₆ and Bi₄V₂O₁₁ species in BWV-82. The photocatalytic activity of the as-prepared photocatalysts is evaluated by photocatalytic reduction of a toxic heavy metal ion Cr(VI) in an aqueous solution. As shown in Fig. 5a, the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction photocatalysts exhibit higher photocatalytic activity for the reduction of Cr(VI) relative to pristine Bi₂WO₆ and Bi₄V₂O₁₁, indicating the superiority of the constructed heterojunction.

Fig. 3 UV-vis diffuse reflectance spectra (DRS) of pristine Bi₂WO₆, Bi₄V₂O₁₁ and Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions.

Fig. 4 XPS spectra of the BWV-82 sample: (a) Bi 4f, (b) W 4f, (c) V 2p, (d) O 1s.

Fig. 5 (a) Photocatalytic reduction curves of Cr(VI) aqueous solution in the presence of various catalysts under visible-light illumination, (b) photocatalytic reduction kinetics of Cr(VI).

In particular, the BWV-82 sample possesses the most standout photocatalytic properties among the samples and almost completely reduces Cr(VI) to Cr(III) within 20 min. Furthermore, the reaction rate of the photocatalytic reduction of Cr(VI) is evaluated by the apparent pseudo-first-order model according to the Langmuir–Hinshelwood (L–H) kinetics model.⁵²

ln(C₀/C) = kt

where C₀ and C are the initial Cr(VI) concentrations (mg L⁻¹) before light irradiation and the Cr(VI) concentration (mg L⁻¹) at time t after light irradiation, respectively, k is the apparent first-order rate constant (min⁻¹). Fig. 5b shows the photocatalytic reduction rate constants, in which the k values of the pristine Bi₂WO, Bi₄V₂O₁₁, BWV-46, BWV-64 and BWV-82 are 0.02159 min⁻¹, 0.02923 min⁻¹, 0.03127 min⁻¹, 0.11511 min⁻¹, and 0.22783 min⁻¹, respectively. This result reveals that the BWV-82 heterojunction exhibits the highest photocatalytic efficiency among the samples, which is approximately 10 times in contrast with pristine Bi₂WO₆ and Bi₄V₂O₁₁. The promoted photocatalytic performance could be ascribed to the facilitated photoinduced charge carriers stemming from the construction of heterojunction, which could endow the catalyst with a built-in electric field at the interface. Furthermore, for comparison, the photocatalysis experiment towards reduction of Cr(VI) over the BWV-82 heterojunction without citric acid as a hole scavenger is performed. As shown in Fig. S5 (ESI†), the photocatalytic activity of the heterojunction photocatalyst without citric acid (denoted as BWV-82-NS) is not as high as that of BWV-82 with a scavenger, indicating that the addition of citric acid allows the photocatalytic activity of the BWV-82 heterojunction to be further improved, which could be attributed to the more effective separation of carriers stemming from the scavenging hole by citric acid.

However, even when citric acid was not added, the photocatalytic performance of BWV-82-NS was found to be superior to that of pristine Bi₂WO₆ and Bi₄V₂O₁₁. With the aim to confirm the viewpoint, transient photocurrent analysis was conducted, which is widely recognized as a typical method used to evaluate the separation effect of photoinduced charge carriers for a photocatalyst.⁵³Fig. 6a presents the photocurrent intensities of pristine Bi₂WO₆, Bi₄V₂O₁₁ and the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction (BWV-82) from 0.4 to 0.8 eV (vs. Ag/AgCl) under visible-light illumination. Obviously, the BWV-82 sample exhibits a significantly enhanced photocurrent response, and approximately 4 and 10-fold promotions are realized in sharp contrast with pristine Bi₂WO₆ and Bi₄V₂O₁₁, respectively. It reveals that BWV-82 possesses a much higher separation and transport efficiency of photoinduced electrons and holes. In order to further verify the enhanced separation efficiency of photoinduced charge carriers in BWV-82, photoluminescence (PL) spectra analysis is carried out, since recording the PL spectra is an effective method for monitoring the recombination efficiency of photoinduced electrons and holes. Generally, there is a strong correlation between the photocatalytic activity and the PL intensity, that is, a lower recombination rate leads to a lower PL intensity and a higher photocatalytic activity.⁵⁴ As illustrated in the PL spectra of the pristine Bi₂WO₆, Bi₄V₂O₁₁ and BWV-82 excited at a wavelength of 320 nm (Fig. 6b), BWV-82 displays a lower PL emission intensity than those of pristine Bi₂WO₆ and Bi₄V₂O₁₁, indicating that the recombination rate of photoinduced electron–hole pairs is effectively hindered in the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction (BWV-82). This result is quite consistent with the abovementioned transient photocurrent response analysis. Thereby, it can be concluded that the construction of a heterojunction by combining Bi₂WO₆ nanoflakes and Bi₄V₂O₁₁ nanocrystals is extremely beneficial to the separation and transfer of photoinduced charge carriers, thus highly enhancing the photocatalytic activity.⁴⁶ Attributed to the unique configuration, nanosized interfacial contact is achieved in the as-constructed Bi₂WO₆/Bi₄V₂O₁₁ heterojunction, which might play a determinant role in promoting the separation and transfer of photoinduced charge carriers. In order to confirm the above argument, the photocatalytic Cr(VI) reduction experiment over the mechanically mixed Bi₂WO₆/Bi₄V₂O₁₁ (W : V = 8 : 2, denoted as BWV-82-MM) is also performed and illustrated in Fig. S4 (ESI†). The mechanically mixed BWV-82-MM sample shows much lower photocatalytic activity than the BWV-82 heterojunction prepared by a one-pot solvothermal approach, which only reaches 50% reduction rate of Cr(VI) in 20 min under visible-light irradiation. This could result from the significant difference in the heterostructured interfaces of the two samples. Through the simple mechanical mixing process, only loose and diffuse interfaces are introduced due to the large size of Bi₄V₂O₁₁ microspheres instead of nanocrystals with little contact retardation. Such an insufficient contact interface could suppress the separation and transfer of photoinduced charge carriers. However, the as-constructed BWV-82 heterojunction photocatalyst possesses interfaces with nanosized close contact, which could promote interfacial carrier transfer, resulting in drastically promoted photocatalytic Cr(VI) reduction performance. This result proves the superiority of the as-constructed heterojunction and implies that the intimate interface between Bi₂WO₆ and Bi₄V₂O₁₁ plays a significant role in enhancing the photocatalytic activity.

Fig. 6 (a) Transient photocurrent responses of the pristine Bi₂WO₆, Bi₄V₂O₁₁ and BWV-82 electrodes under visible-light irradiation, (b) photoluminescence (PL) spectra of the pristine Bi₂WO₆, Bi₄V₂O₁₁ and BWV-82 under an excitation wavelength of 320 nm.

From the point of view of practical application, cycling stability of photocatalysts is one very important factor for evaluating photocatalytic performances. The circulation runs for the photocatalytic Cr(VI) reduction with the BWV-82 heterojunction are carried out to evaluate its cycling stability. Prior to the next cycle experiment, the previously used photocatalyst is subjected to ultrasonic cleaning with distilled water and drying at 60 °C. Fig. 7 displays the results of the recycling experiment with 5 cycles. After 5 cycles, the photocatalytic Cr(VI) reduction rate still remains above 90%, which indicates that the as-fabricated BWV-82 heterojunction photocatalyst possesses good cycling stability. With the aim of understanding the mechanism of the enhanced photocatalytic activity in the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction, the energy band structure is investigated by determining the optical band gaps and flat band potentials of pristine Bi₂WO₆ and Bi₄V₂O₁₁. The band gap of the semiconductor can be calculated by the Kubelka–Munk function:⁵⁵

αhν = A(hν − E_g)^n/2

where α, h, ν, A, and E_g represent the absorption coefficient, Planck's constant, light frequency, a constant and the band gap, respectively. In this function, n is determined by the type of optical transition of a semiconductor: Generally, n = 1 for direct transition and n = 4 for indirect transition. Both Bi₂WO₆⁵⁶ and Bi₄V₂O₁₁⁵⁷ belong to the direct transition semiconductor and the value n is 1. Fig. 8a shows the plots of (αhν)²vs. photon energy (hν) of pristine Bi₂WO₆ and Bi₄V₂O₁₁ samples. Thus, the band gaps of pristine Bi₂WO₆ and Bi₄V₂O₁₁ are calculated to be 2.84 and 2.15 eV, respectively. Moreover, the flat-band potentials of pristine Bi₂WO₆ and Bi₄V₂O₁₁ are determined by the Mott–Schottky analysis. As shown in Fig. 8b, both samples show positive slopes, indicating that they are n-type semiconductors. And the individual flat-band potentials of Bi₂WO₆ and Bi₄V₂O₁₁ are estimated to be 0.06 and 0.26 V vs. Ag/AgCl (0.26 and 0.46 V vs. NHE), which indicated that the flat-band potential of Bi₂WO₆ is more negative than that of Bi₄V₂O₁₁ by 0.2 V. According to the semiconductor theory, the conduction band potential (E_CB) of n-type semiconductors is very close to that of (0.1–0.2 V more negative) the flat-band potential.⁵⁸ Therefore, it can be concluded that the CB position of Bi₂WO₆ is more negative than that of Bi₄V₂O₁₁. Based on the above results, a possible photocatalysis mechanism over the Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions can be depicted as shown in Fig. 9. Both the VB and CB edges of Bi₄V₂O₁₁ are localized within the energy gap of Bi₂WO₆, forming a straddling band alignment which belongs to the “type I” heterojunction.⁵⁹ Since the CB edge potential of Bi₂WO₆ is more negative than that of Bi₄V₂O₁₁, during the photocatalytic Cr(VI) reduction process, the photoinduced electrons transfer easily from the CB of Bi₂WO₆ to the CB of Bi₄V₂O₁₁ under visible-light irradiation, realizing the reduction from Cr(VI) to Cr(III). Similarly, photoinduced hole transfer from the VB of Bi₂WO₆ to the VB of Bi₄V₂O₁₁ is quenched by citric acid as a hole scavenger. Furthermore, because of the nanosized interface contact, the photoinduced electrons and holes in Bi₂WO₆ can transfer to Bi₄V₂O₁₁ rapidly, which participate in the subsequent catalysis reactions timely due to the tiny size of Bi₄V₂O₁₁ nanocrystals via a short migration pathway. Thereby, the photoinduced electrons and holes can be rapidly separated and transferred over the Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions, which results in the enhanced photocatalytic activity upon Cr(VI) reduction.

Fig. 7 Circulation runs for the photocatalytic Cr(VI) reduction over the BWV-82 heterojunction.

Fig. 8 (a) Plots of (αhν)²vs. hν of pristine Bi₂WO₆ and Bi₄V₂O₁₁ samples, (b) Mott–Schottky plots of pristine Bi₂WO₆ and Bi₄V₂O₁₁.

Fig. 9 Schematic illustrations of the photoinduced charge carrier separation and the photocatalytic Cr(VI) reduction reaction mechanism over the Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions.

4. Conclusions

In summary, a novel visible-light responsive Bi₂WO₆/Bi₄V₂O₁₁ heterojunction photocatalyst with nanosized interfacial contact has been successfully fabricated using a facile one-pot solvothermal method. The Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions show excellent photocatalytic Cr(VI) reduction activity under visible-light illumination. In particular, the sample of the BWV-82 heterojunction exhibits the highest photocatalytic Cr(VI) reduction efficiency which is approximately 10 times higher than those of pristine Bi₂WO₆ and Bi₄V₂O₁₁. In addition, the Bi₂WO₆/Bi₄V₂O₁₁ heterojunction also displays enhanced transient photocurrent response and lower PL intensity, which indicated that the photoinduced charge carriers are more effectively separated and transferred to the Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions.

The remarkably enhanced photocatalytic activity and the separation effect of the charge carriers are attributed to the formation of a heterojunction with a nanosized interfacial contact between Bi₂WO₆ nanoflakes and Bi₄V₂O₁₁ nanocrystals. Moreover, Bi₂WO₆/Bi₄V₂O₁₁ heterojunctions exhibit good cycling stability even after 5 cycles. This work may be further extended to the research and development of a novel semiconductor heterojunction containing a new kind of Bi₄V₂O₁₁ photocatalyst for water purification and related applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (21471040).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj03413j

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