Enhanced visible light photocatalytic performances of self-assembled hierarchically structured BiVO₄/Bi₂WO₆ heterojunction composites with different morphologies

Abstract

Novel self-assembled hierarchical BiVO₄/Bi₂WO₆ heterostructured composites with different morphologies were controllably synthesized via a facile and template-free solvothermal method. The effects of the molar ratio of V to W on the structure, morphology, photocatalytic activity and photoelectrochemical properties of the BiVO₄/Bi₂WO₆ composite photocatalysts were investigated in detail. The results showed that all the BiVO₄/Bi₂WO₆ composites exhibited much better photocatalytic activities for methylene blue (MB) degradation and photoelectrochemical performances than pure BiVO₄ and Bi₂WO₆. Among the composites, the BiVO₄/Bi₂WO₆ microsphere with the molar ratio of V to W of 2 : 1 exhibited the best photocatalytic performance. The mechanism of enhanced activity was systematically investigated by UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and photo-electrochemical methods including transient photocurrent responses, electrochemical impedance spectroscopy (EIS) and Mott–Schottky plots. The enhanced photocatalytic activity could be ascribed to the tetragonal–monoclinic heterophase structure, high surface oxygen vacancy concentration, narrow band gap energy and mainly the formed heterojunction structure which could effectively promote the separation of photogenerated electron–hole pairs. In addition, a possible photocatalytic mechanism for the enhanced photocatalytic activity was proposed on the basis of the calculated energy band positions of BiVO₄ and Bi₂WO₆.

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1. Introduction

In the past decades, semiconductor photocatalysis as a promising advanced oxidation technology has attracted extensive attention to help solve the urgent energy shortages and environmental crises.^1,2 TiO₂ is considered as an effective photocatalytic material for environmental purification and energy conversion, due to its high photocatalytic activity, good chemical stability, non-toxicity and low cost.^3,4 However, the large band gap energy (3.2 eV), which only responds to ultraviolet (UV) light (accounts for only 4% of the total sunlight), greatly restricts its practical application due to the poor utilization efficiency of solar energy.⁵ As a consequence, many efforts have been made to develop visible-light-responsive photocatalysts with high photocatalytic performance.

To date, bismuth-based oxysalts have been extensively investigated due to their potential applications in visible photocatalysis, gas sensing and biomedicine.^6,7 As a promising visible-light-driven semiconductor photocatalyst, bismuth vanadate (BiVO₄) has attracted considerable attention due to its narrow band gap energy, good chemical and thermal stability, non-toxicity and excellent photocatalytic performance under visible light irradiation.⁸ Nevertheless, the efficiency is far away from the theoretical conversion efficiency, mainly due to the rapid recombination rate of photogenerated electron–hole pairs, poor electronic conductivity and weak adsorptive performance.⁹ Therefore, tremendous efforts have been made to utilize visible light more efficiently and improve the photocatalytic performance, such as element doping,^10,11 morphology control,^12,13 facet engineering,^8,14 as well as composites fabricated by heterojunction construction^15,16 and electron transporting material loading.¹⁷ Worth to be mentioned, the construction of BiVO₄-based heterostructured composite by coupling with another semiconductor with suitable band gap energy is considered as an effective and promising way to enhance the photocatalytic performance. The formed heterojunction structure could extend the spectral responsive range, improve the separation efficiency of photogenerated electron–hole pairs, prolong the lifetime of charge carriers, and thus could remarkably improve the photocatalytic activity.^18,19

As one of the typical Aurivillius oxides with layered structure, Bi₂WO₆ has been considered as an excellent visible light photocatalyst for degradation of organic pollutants and water splitting due to its excellent intrinsic physical and chemical properties.^20–22 What's more, Bi₂WO₆ has suitable band edge potentials matching well with BiVO₄, which makes it to be a suitable material for constructing heterojunction with BiVO₄.²³ Considering the fact that the unique morphology is beneficial to enhance the photocatalytic performance, it can not only facilitate mass transportation and light harvesting, but also can accelerate charge movement and assist the separation of photogenerated electron–hole pairs.²⁴ Therefore, the BiVO₄/Bi₂WO₆ composites with extraordinary morphologies may be supposed to exhibit excellent photocatalytic activities under visible light irradiation.

In this work, novel self-assembled hierarchical BiVO₄/Bi₂WO₆ heterostructured composites with different morphologies were synthesized by a facile solvothermal method without using any surfactant and template. The photocatalytic activities were evaluated by the photocatalytic degradation of methylene blue (MB) under visible light irradiation. The effects of molar ratio of V to W on the structure, morphology, photocatalytic activity and photoelectrochemical property of BiVO₄/Bi₂WO₆ composite photocatalysts were investigated in detail. Furthermore, a possible mechanism for the enhanced photocatalytic activity was proposed according to the estimated energy band positions.

2. Experimental

2.1. Materials

Bi(NO₃)₃·5H₂O, Na₂WO₄·2H₂O, NH₄VO₃, ethylene glycol (EG), NaOH, Na₂SO₄, MB, absolute ethanol were all of analytical grade purity and were used as received without further purification. All of them were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water was used throughout this study.

2.2. Synthesis of BiVO₄/Bi₂WO₆ composite photocatalysts

In a typical synthesis procedure, 8 mmol of Bi(NO₃)₃·5H₂O was dissolved in 30 mL of EG to form transparent solution A. Appropriate amounts of NH₄VO₃ and Na₂WO₄·2H₂O with different molar ratios (molar ratios of V to W were 4 : 1, 2 : 1, 1 : 2, and 1 : 4, respectively) were dissolved in 30 mL of deionized water to form homogeneous mixture B. After mixing solution A and B, the pH value of the mixture solution was adjusted to 5.0 using 2 M NaOH solution, followed by stirring for 30 min to form a homogeneous suspension. The obtained suspension was then transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. After the autoclave cooled to room temperature, the precipitate was collected, washed with deionized water and absolute ethanol, dried at 80 °C for 6 h, and then calcined at 400 °C for 5 h. For the purpose of comparison, pure BiVO₄ and Bi₂WO₆ samples were prepared under the same conditions.

2.3. Characterization

The crystalline phases of the samples were determined by powder X-ray diffraction (XRD) on a Bruker AXS D8-Focus diffractometer equipped with Ni-filtered and Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA in the range of 2θ = 10 to 70° with a step size of 0.01°. Surface morphology was observed with a Hitachi SU8010 field emission scanning electron microscopy (FESEM), equipped with energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. Transmission electron microscopy (TEM) images and high resolution transmission electron microscopy (HRTEM) images were recorded on a FEI FT-30 transmission electron microscope with an accelerating voltage of 300 kV. The surface composition and oxidation state of the sample were characterized by X-ray photoelectron spectroscopy (XPS) using a VG Multilab 2000 X-ray photoelectron spectrometer equipped with a Al Kα X-ray source. The Brunauer–Emmett–Teller (BET) specific surface area and pore size of the sample were analyzed by nitrogen adsorption–desorption in a Micromeritics ASAP 2020 apparatus at 77 K. The UV-vis diffuse reflectance spectra (DRS) were recorded on a Shimadzu UV-2550 UV-vis spectrophotometer equipped with an integrating sphere attachment in the range of 250–800 nm at room temperature. The photoluminescence (PL) spectra were recorded using a Horiba Jobin Yvon Fluoromax-4P fluorescence spectrophotometer with a 450 W xenon lamp at an excitation wavelength of 360 nm.

2.4. Measurement of photocatalytic activity

The photocatalytic activities of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composite photocatalysts were evaluated by the degradation of MB in aqueous solution under visible light irradiation. A 500 W Xe lamp with a 400 nm cutoff filter was used as the light source. In a typical reaction, 60 mg of photocatalyst was dispersed into 60 mL of MB aqueous solution with an initial dye concentration of 10 mg L⁻¹. The experimental temperature was kept at 25 ± 2 °C by circulating water through an external cooling jacket. The solution was stirred magnetically for 1 h in the dark to establish adsorption–desorption equilibrium before visible light irradiation. During the irradiation, the reaction samples were collected at 1 h intervals and centrifuged to remove the photocatalyst particles. The ratio (C/C₀) of the MB concentration was adopted to evaluate the degradation efficiency (i.e., C₀ was the MB concentration at the time of the adsorption–desorption equilibrium, where C was the MB concentration at certain time) by measuring the absorbance value at 664 nm via a UV-1081 UV-vis spectrophotometer.

2.5. Photoelectrochemical measurements

Photoelectrochemical measurements were performed by using a CHI660C electrochemical workstation with a standard three-electrode configuration, which employed a Pt plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The working electrode was prepared on fluoride tin oxide (FTO) conductor glass. Typically, 10 mg sample powder was mixed with 1 mL alcohol to make slurry. The slurry was then spread onto the FTO glass, whose side part was previously protected using Scotch tape. The working electrode was further dried at 100 °C for 3 h to improve adhesion. A 500 W Xe lamp equipped with a 400 nm cutoff filter was used as visible light source. 0.2 M of aqueous Na₂SO₄ solution was used as the supporting electrolyte. Mott–Schottky plots were measured at the potential range of −0.5 V to 0.2 V and the frequency of 5000 Hz with an AC voltage of 5 mV in the dark.

3. Results and discussion

3.1. XRD analysis

The crystalline phases of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios are examined by XRD measurements, as shown in Fig. 1. It can be seen that all the diffraction peaks of pure BiVO₄ could be well indexed to the monoclinic scheelite BiVO₄ (m-BiVO₄) phase (JCPDS card no. 14-0688). In addition, the diffraction peaks of pure Bi₂WO₆ match well with the orthorhombic Bi₂WO₆ phase (JCPDS card no. 73-1126). No peaks of any other phases or impurities are observed, implying the high purity of BiVO₄ and Bi₂WO₆ samples. As for the BiVO₄/Bi₂WO₆ composites, the characteristic diffraction peaks of monoclinic BiVO₄ and orthorhombic Bi₂WO₆ phases could be observed in the XRD patterns, indicating that BiVO₄ and Bi₂WO₆ are successfully jointed in the composites. Furthermore, with the increase in the contents of BiVO₄ in the composites, the relative intensities of monoclinic BiVO₄ phase increase gradually. It is worth to be mentioned that the characteristic diffraction peaks of tetragonal BiVO₄ (t-BiVO₄) phase (JCPDS card no. 14-0133) appear accompanied by the monoclinic BiVO₄ when the molar ratios of V to W are 4 : 1 and 2 : 1. This indicates that the existence of Bi₂WO₆ affects the crystallization of BiVO₄.²⁵ Moreover, the presence of the tetragonal–monoclinic heterophase structure is beneficial to enhance the photocatalytic activity.¹² In addition, all the samples display narrow and sharp diffraction peaks, indicating the high crystallinity of the prepared samples.

Fig. 1 XRD patterns of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios (the tetragonal BiVO₄ is marked as ★).

3.2. Morphology and microstructure

The FESEM images of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios and the corresponding EDX spectra are presented in Fig. 2. As shown in Fig. 2a, the pure BiVO₄ sample presents a pinwheel-like structure with four blades, which is composed of smaller building blocks with a rough surface. The diagonal distance and thickness of the pinwheel are 6–8 μm and 0.2–0.3 μm, respectively. Fig. 2b shows that the pure Bi₂WO₆ sample has irregular plate-like particles with a non-uniform size distribution. As for the BiVO₄/Bi₂WO₆ composites, uniform and three-dimensional (3D) hierarchical structures assembled with many thin two-dimensional (2D) nanoplates can be observed. However, the morphologies of these composites are different from each other due to the different molar ratios of V to W in the composites. The composite that prepared with the molar ratio of V to W of 4 : 1 displays flower-like morphology with many raisin-like secondary structures connected at the centre, which is composed of thin 2D nanoplates (Fig. 2c). When the molar ratio of V to W is 2 : 1, the composite shows well-developed microsphere structure with the diameter of 3–5 μm (Fig. 2e). Further increase in the contents of Bi₂WO₆ in the composites, the composites still have microsphere appearance but with different detailed features, and the sizes of which are decreased gradually. As shown in Fig. 2g, the sample (V/W = 1 : 2) exhibits uniform underdeveloped microsphere structures with relatively small amounts of mutually perpendicular thin nanoplates. However, when the molar ratio of V to W is 1 : 4, more mutually perpendicular thin nanoplates are tightly accumulated to form more perfect microsphere structure with the size of about 1.5 μm (Fig. 2i). Furthermore, EDX is carried out to identify the elemental composition of BiVO₄/Bi₂WO₆ composites with different molar ratios. It can be seen that obvious signals of Bi, V, W, and O elements could be observed from the EDX spectra, confirming the presence of Bi, V, W, and O elements in the composites. Meanwhile, the atomic ratio of V to W in the composites is 1.45, 1.37, 0.52, and 0.30, respectively, which has deviation from the theoretical value, especially for the composites with high V contents. This is consistent with the previous results.^26,27

Fig. 2 FESEM images and the corresponding EDX patterns of the prepared samples: (a) pure BiVO₄, (b) pure Bi₂WO₆, (c and d) BiVO₄/Bi₂WO₆ composite (V/W = 4 : 1), (e and f) BiVO₄/Bi₂WO₆ composite (V/W = 2 : 1), (g and h) BiVO₄/Bi₂WO₆ composite (V/W = 1 : 2), and (i and j) BiVO₄/Bi₂WO₆ composite (V/W = 1 : 4).

To further reveal the chemical composition and element distribution in the composite, elemental mapping is performed on a single BiVO₄/Bi₂WO₆ microsphere in which the molar ratio of V to W is 2 : 1. As shown in Fig. 3b–e, the elemental mapping results clearly reveal that uniform distribution of Bi, V, W, and O elements throughout the single BiVO₄/Bi₂WO₆ microsphere (Fig. 3a), confirming the coexistence of BiVO₄ and Bi₂WO₆ in the BiVO₄/Bi₂WO₆ composite. Therefore, these results give solid evidence that BiVO₄ and Bi₂WO₆ are successfully composited together and have assembled to form 3D hierarchical structures.

Fig. 3 (a) FESEM image of a single representative BiVO₄/Bi₂WO₆ microsphere (V/W = 2 : 1) and the corresponding elemental mappings of (b) Bi, (c) V, (d) W, and (e) O elements.

To obtain more detailed information about the crystalline structure of the BiVO₄/Bi₂WO₆ composite, TEM and HRTEM characterizations are performed. Fig. 4A presents the TEM image of an individual microsphere morphology with the molar ratio of V to W of 2 : 1. It is clearly to see that the microsphere is assembled from many thin nanoplates, which is consistent with the FESEM result. Fig. 4B shows the HRTEM image of the BiVO₄/Bi₂WO₆ microsphere recorded from the yellow square region in Fig. 4A. The three sets of crystal structures with different interplanar spacing are clearly presented together in the HRTEM image. The observed interplanar spacing of 0.291 nm, 0.230 nm, and 0.315 nm agree well with the fringe spacing of the (040) lattice plane of monoclinic BiVO₄, (301) lattice plane of tetragonal BiVO₄ and (113) lattice plane of orthorhombic Bi₂WO₆, respectively.^28–30 Moreover, the HRTEM image reveals the highly crystalline nature of the composite. These results demonstrate that BiVO₄ and Bi₂WO₆ particles contact with each other tightly in the composite, and that the heterojunction structure is formed between the BiVO₄ and Bi₂WO₆.

Fig. 4 TEM image (A) and the corresponding HRTEM image (B) of BiVO₄/Bi₂WO₆ microsphere with the molar ratio of V to W of 2 : 1.

3.3. Surface composition and oxidation state

The surface composition and oxidation state of the BiVO₄/Bi₂WO₆ microsphere with the molar ratio of V to W of 2 : 1 are further investigated by the XPS technique, the results of which are presented in Fig. 5. According to the overall XPS survey, only Bi, V, W, O, and C elements are detected on the surface of the sample (Fig. 5a). The C 1s peak at around 284.5 eV can be attributed to the signal from carbon contained in the instrument and is used for calibration.³¹ From the high resolution XPS spectra of Bi 4f in Fig. 5b, two strong peaks located at about 158.98 eV and 164.32 eV can be assigned to the characteristic spin–orbit split of the Bi 4f_7/2 and Bi 4f_5/2, respectively, confirming that bismuth species in the BiVO₄/Bi₂WO₆ composite are in the form of Bi³⁺ cations.^16,32 In addition, it can be observed that the Bi 4f_7/2 region could be deconvoluted into two peaks at around 156.78 eV and 159.06 eV, and Bi 4f_5/2 region also can be deconvoluted into two peaks at around 162.16 eV and 164.40 eV, indicating that there are two types of Bi ions in the BiVO₄/Bi₂WO₆ composite. Considering the binding energies of pure BiVO₄ and Bi₂WO₆, the peaks at 156.78 eV and 162.16 eV can be attributed to the binding energies of Bi 4f_7/2 and Bi 4f_5/2 in Bi₂WO₆, respectively.³³ The peaks at 159.06 eV and 164.40 eV could originate from the Bi 4f_7/2 and Bi 4f_5/2 in BiVO₄, respectively.³⁴ However, the binding energy values in the BiVO₄/Bi₂WO₆ composite are slightly different from those of pure BiVO₄ and Bi₂WO₆, due to the changes in the local environment and electron density in the interfacial structure.³⁵

Fig. 5 XPS patterns of BiVO₄/Bi₂WO₆ microsphere (V/W = 2 : 1): (a) overall XPS survey, (b) Bi 4f, (c) V 2p, (d) W 4f, and (e) O 1s.

As shown in Fig. 5c, the peaks with binding energies at about 516.73 eV, 520.71 eV, and 522.79 eV can be assigned to V 2p_3/2, O 1s satellite and V 2p_1/2, respectively. In addition, it can be observed that the V 2p_3/2 region could be deconvoluted into two peaks at around 516.74 eV and 514.02 eV, which could be attributed to surface V⁵⁺ and V⁴⁺ species, respectively.^36,37 This implies that vanadium is presented as mixed V⁴⁺ and V⁵⁺ valence states in the BiVO₄/Bi₂WO₆ composite, indicating more oxygen vacancies on the surface of the sample. Furthermore, the presence of more V⁴⁺ suggests that more active sites (i.e., redox site V⁴⁺–OH) are located on the catalyst surface. The presence of mixed V⁴⁺ and V⁵⁺ valence states supports the catalytic surface reaction, because it enables high electronic conductivity.³⁸ Fig. 5d shows the XPS spectra of W 4f. The binding energies of 35.03 eV and 37.25 eV, which are ascribed to W 4f_7/2 and W 4f_5/2 peaks, respectively, are attributed to W⁶⁺ in Bi₂WO₆.³⁹ The asymmetric O 1s signal in Fig. 5e indicates the presence of different oxygen species on the surface of the sample. After Gaussian fitting of the O 1s signal peak, the bands at 529.97 eV and 527.62 eV could be attributed to the surface adsorbed oxygen (O_ads) and lattice oxygen (O_latt) species, respectively.⁴⁰ The area ratio of O_ads/O_latt is calculated to be 2.02. Generally speaking, for an oxygen deficient catalyst, the higher surface O_ads concentration, the larger amount of oxygen vacancies it possesses, and then the better activity it is.⁴¹ The surface adsorbed oxygen species are the active oxygen species that could be converted to oxygen-related free radicals. And then the oxygen-related free radicals could degrade MB to CO₂, H₂O and other intermediates, thus enhancing the photocatalytic activities of the samples.

3.4. Nitrogen adsorption–desorption

The N₂ adsorption–desorption isotherm and the corresponding pore size distribution curve of the BiVO₄/Bi₂WO₆ microsphere with the molar ratio of V to W of 2 : 1 are illustrated in Fig. 6. The isotherm exhibits the type IV adsorption curve along with a H3 hysteresis loop at the relative pressure P/P₀ ranging from 0.8 to 1.0, which is usually ascribed to slit-like mesopores formed by sheet-like particles.⁴² The BJH pore size distribution curve obtained from the desorption branch of nitrogen isotherm indicates that the BET specific surface area and pore volume are 5.16 m² g⁻¹ and 0.05 m³ g⁻¹, respectively. The pores comprise a part of the slit-like pores (about 42 nm), which resulted from randomly assembled nanosheets. Besides, the voids formed by interaggregated microspheres may also contribute to the pores.⁴³

Fig. 6 The nitrogen adsorption–desorption isotherm of the BiVO₄/Bi₂WO₆ microsphere (V/W = 2 : 1); inset is the corresponding pore size distribution.

3.5. Optical properties

It is well known that the optical property of a semiconductor plays an important role in visible light photocatalytic activity.⁴⁴ Fig. 7a shows the UV-vis diffuse reflectance spectra of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios. It can be seen that the pure Bi₂WO₆ exhibits strong absorption in the UV region, while relatively weak absorption in the visible light. The pure BiVO₄ shows strong absorption in UV region as well as in the visible light region. Compared with pure BiVO₄ and Bi₂WO₆, the BiVO₄/Bi₂WO₆ composites show a broader absorption in the visible light region and increased absorption intensity. Furthermore, there is a clear red-shift of the absorption edges of BiVO₄/Bi₂WO₆ composites in comparison with those of pure BiVO₄ and Bi₂WO₆. This indicates that BiVO₄/Bi₂WO₆ composites can absorb and utilize visible light more efficiently, leading to the higher photocatalytic activities. The band gap energy can be obtained from the plot of the modified Kubelka–Munk function [F(R_∞)hν]^1/2 versus the energy of the absorbed light (hν).^19,45 The results are shown in Fig. 7b. It can be seen that the band gap energies are estimated to be 2.38 eV and 2.71 eV for pure BiVO₄ and Bi₂WO₆, respectively. However, the band gap energies of BiVO₄/Bi₂WO₆ composites are estimated to be located from 2.02 to 2.12 eV, which are lower than those of both pure BiVO₄ and Bi₂WO₆. This may be ascribed to the interaction between BiVO₄ and Bi₂WO₆ in the BiVO₄/Bi₂WO₆ composites due to the modified electronic structures.⁴⁶ These results imply that the BiVO₄/Bi₂WO₆ heterojunction structure with a matching energy band position has formed and may be suitable for photocatalytic decomposition of organic contaminants under visible light irradiation.

Fig. 7 UV-vis DRS spectra (a) and the estimated band gap energies (b) of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios.

3.6. PL spectra analysis

The separation efficiency of photogenerated electron–hole pairs is also an important factor that greatly affects the photocatalytic activity of a photocatalyst.⁴⁷ Because PL emission peak mainly results from the recombination of free charge carriers, PL spectrum is commonly employed to study the immigration, transfer and separation efficiency of photogenerated charge carriers in the semiconductor system. The lower PL intensity usually indicates the lower recombination rate of photogenerated electron–hole pairs, thus the higher photocatalytic activity.⁴⁸ Fig. 8 presents the PL spectra of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios. It can be observed that the samples show similar curve shapes but difference in intensities. The PL intensities of BiVO₄/Bi₂WO₆ composites are obviously weaker compared to those of pure BiVO₄ and Bi₂WO₆, which means that the recombination rate of photogenerated carriers in the composite photocatalysts has been greatly suppressed by the heterojunction structures.⁴⁹ Moreover, the BiVO₄/Bi₂WO₆ composite with the molar ratio of V to W of 2 : 1 has the lowest PL intensity, suggesting that it has the lowest recombination rate of photogenerated electron–hole pairs, leading to the highest photocatalytic activity.

Fig. 8 PL spectra of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios.

3.7. Photoelectrochemical properties

To further understand the effect of heterojunction on the separation efficiency of photogenerated electron–hole pairs, the transient photocurrent responses of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios are recorded for several on-off cycles under visible light irradiation. It is widely accepted that the separation efficiency of photogenerated charge carriers plays a vital role in the photocatalytic activity. A higher photocurrent density would mean the presence of longer living photogenerated electrons and holes, and hence the higher photocatalytic activity.⁵⁰ It can be clearly seen from Fig. 9a that the BiVO₄/Bi₂WO₆ composites display obvious enhancement in the photocurrent densities as compared to those of pure BiVO₄ and Bi₂WO₆. Particularly, the BiVO₄/Bi₂WO₆ microsphere with the molar ratio of V to W of 2 : 1 exhibits the highest photocurrent density. The enhanced photocurrent density indicates more efficient separation efficiency and longer lifetime of photogenerated charge carriers, which could be attributed to the formation of heterojunction structure between BiVO₄ and Bi₂WO₆, thus can effectively reduce the recombination rate of photogenerated electron–hole pairs.

Fig. 9 Transient photocurrent responses (a) and the corresponding electrochemical impedance spectroscopy (EIS) Nyquist plots (b) of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios under visible light irradiation.

Electrochemical impedance spectroscopy (EIS) analysis is also employed to investigate the charge transfer resistance and the separation efficiency of photogenerated electron–hole pairs. As displayed in Fig. 9b, all the Nyquist plots show semicircles at high frequencies. Moreover, the hierarchical heterostructured BiVO₄/Bi₂WO₆ composites show smaller arc radii of Nyquist plots than those of pure BiVO₄ and Bi₂WO₆, indicating the decrease of charge transfer resistance, and leading to a fast interfacial charge transfer process as well as an effective separation of photogenerated electron–hole pairs.⁵⁰

In order to investigate the variations in electronic properties of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites, Mott–Schottky plots are carried out to study the relation between capacitance of the space charge region and the applied potential, as shown in Fig. 10. Obviously, the Mott–Schottky plots of all samples show positive slope, indicating that all the photocatalysts possess the nature of an n-type semiconductor. Furthermore, the BiVO₄/Bi₂WO₆ microsphere (V/W = 2 : 1) shows substantially the smallest slope of the Mott–Schottky plot than those of pure BiVO₄ and Bi₂WO₆, suggesting that the BiVO₄/Bi₂WO₆ microsphere (V/W = 2 : 1) has the fastest carrier transfer and the largest density of carriers, and thus the best photoelectrochemical performance.⁵¹

Fig. 10 (a) Mott–Schottky plots of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composites with different molar ratios at a frequency of 5000 Hz in an aqueous solution of Na₂SO₄ (0.2 M).

3.8. Photocatalytic performance

The photocatalytic performances of pure BiVO₄, pure Bi₂WO₆ and BiVO₄/Bi₂WO₆ composite photocatalysts are evaluated by the photocatalytic degradation of MB dye under visible light irradiation. Fig. 11a shows the degradation efficiencies of MB dye by different photocatalysts. It can be seen that the direct photolysis of MB is almost negligible in the absence of the photocatalyst. The photocatalytic degradation efficiencies of MB for the pure BiVO₄ and Bi₂WO₆ are only about 19.03% and 17.09% after 7 h under visible light irradiation, respectively, indicating that the activities of pure BiVO₄ and Bi₂WO₆ samples are poor. As for the BiVO₄/Bi₂WO₆ composites, an obvious enhancement of degradation efficiency is achieved compared with pure BiVO₄ and Bi₂WO₆. This result indicates that the heterojunction structure formed in the composite is favorable for improving the photocatalytic activity.⁵² Furthermore, the molar ratio of V to W plays an important role in the photocatalytic performance. Obviously, the highest photocatalytic activity is achieved when the molar ratio of V to W is 2 : 1.

Fig. 11 (a) Comparison of photocatalytic performance of the prepared samples on the degradation of MB dye under visible light irradiation; (b) kinetics of MB degradation reaction over the different samples.

It is well known that when the pollutant is within the millimolar concentration range, photocatalytic degradation of organic pollutants follows the pseudo-first order reaction kinetics behavior, as shown by the equation: ln(C₀/C) = kt + a, where k is the apparent first order rate constant.⁵³ As shown in Fig. 11b, the reaction rate constants of all the BiVO₄/Bi₂WO₆ composites are higher than those of pure BiVO₄ and Bi₂WO₆. Especially for the composite with molar ratio of V to W of 2 : 1, the reaction rate constant for the degradation of MB dye is the highest, which is about 3.01 and 3.28 times higher than that of pure BiVO₄ and Bi₂WO₆, respectively.

3.9. Photostability

The stability and reusability of the photocatalyst has also been evaluated by the recycle test for the photocatalytic degradation of MB over the BiVO₄/Bi₂WO₆ composite (V/W = 2 : 1). After each run, the catalysts are collected and washed by simple filtration followed by ultrasonic cleaning with deionized water. As shown in Fig. 12, there is no obvious loss in photocatalytic degradation efficiency after four times successive recycle test, suggesting that the BiVO₄/Bi₂WO₆ heterojunction photocatalyst has good stability and no photocorrosion occurred during the photocatalytic degradation of MB under visible light irradiation.

Fig. 12 Recycle experiment in the photocatalytic degradation of MB over the BiVO₄/Bi₂WO₆ composite (V/W = 2 : 1) under visible light irradiation.

3.10. Proposed photocatalytic mechanism of BiVO₄/Bi₂WO₆ composites

It is well known that the matching energy band structure of composite is suitable for efficient separation of photogenerated carriers and thus enhancing the photocatalytic activities. The conduction band (CB) edge potential of a semiconductor at the point of zero charge can be calculated according to the empirical formula:^54,55 E_CB = χ − E^e − 1/2E_g, where χ is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, E^e is the energy of free electrons on the hydrogen scale with the value of about 4.5 eV, and E_g is the band gap energy of the semiconductor. The χ values for BiVO₄ and Bi₂WO₆ are ca. 6.035 and 6.360 eV, respectively.^35,52 The band gap energies of BiVO₄ and Bi₂WO₆ are 2.38 and 2.71 eV, respectively. In addition, the valance band (VB) edge potential can be determined by the equation: E_VB = E_g + E_CB. Therefore, the conduction band bottom and valance band top of BiVO₄ and Bi₂WO₆ are calculated to be 0.35, 2.73 eV and 0.51, 3.22 eV, respectively.

Based on the above band energy level analysis, a possible mechanism for the MB photocatalytic degradation over BiVO₄/Bi₂WO₆ heterostructured photocatalysts under visible light irradiation is proposed in Fig. 13. When the two semiconductor materials (BiVO₄ and Bi₂WO₆) are in contact, the heterojunction structure is formed. Once the BiVO₄/Bi₂WO₆ composite system is irradiated under visible light (>400 nm), electrons are excited from the valance band to the conduction band of BiVO₄, leaving the same amount of holes in the valance band. The same process occurs in Bi₂WO₆, but the quantity of photogenerated charge carriers is less than that in BiVO₄ due to its relatively wide band gap energy. Due to the CB edge potential of BiVO₄ (0.35 eV) being more negative than that of Bi₂WO₆ (0.51 eV), and the VB of Bi₂WO₆ (3.22 eV) being more positive than that of BiVO₄ (2.73 eV), the local electric field at the BiVO₄/Bi₂WO₆ interface pushes the photogenerated electrons on the CB of BiVO₄ transfer to that of Bi₂WO₆, and holes on the VB of Bi₂WO₆ migrate to that of BiVO₄ at the same time. Therefore, the recombination of photogenerated charge carriers can be greatly suppressed, resulting in the enhanced separation efficiency of photogenerated electron–hole pairs. This is consistent with the results of PL spectra, transient photocurrent responses, EIS spectra and Mott–Schottky plots. In addition, the presence of V⁴⁺ species could alter the electronic properties of BiVO₄/Bi₂WO₆ composites. The V⁴⁺/V⁵⁺ pair can form an impurity energy level, which can act as an electron/hole trap center that substantially decreases the recombination rate.⁵⁶ The photogenerated electrons in the CB of BiVO₄ may immediately transfer to V⁵⁺ species, due to the lower Fermi level of V⁵⁺ species on the surface. The V⁴⁺ species, created from V⁵⁺ by electron trapping, easily release and transfer electron to oxygen molecule absorbed on the surface of photocatalysts to produce superoxide radicals ˙O₂⁻. In addition, the photogenerated electrons in the CB of Bi₂WO₆ can also react with O₂ to produce ˙O₂⁻. The absorbed H₂O and OH⁻ can be oxidized to hydroxyl radicals ˙OH by the photogenerated holes. The MB dye adsorbed on the surface of BiVO₄/Bi₂WO₆ composites would be degraded to CO₂, H₂O and other intermediates by activated oxygen species (e.g., ˙OH and ˙O₂⁻) together with the photogenerated holes. On the other hand, the MB molecules can also be photoexcited to generate the MB* under visible light irradiation. The photoexcited MB* radicals adsorbed on the surface of photocatalysts would transfer electrons to the CB of BiVO₄ and then to Bi₂WO₆. Meanwhile, the injected electrons can react with surface adsorbed oxygen molecules to form ˙O₂⁻, which also promote the degradation of MB and enhance the overall photocatalytic performances of BiVO₄/Bi₂WO₆ composites.⁵⁷

Fig. 13 Possible photocatalytic mechanism for the degradation of MB dye over BiVO₄/Bi₂WO₆ composites under visible light irradiation.

4. Conclusions

In conclusion, a series of BiVO₄/Bi₂WO₆ heterojunction composite photocatalysts have been successfully synthesized through a facile solvothermal method without using any surfactant and template. The molar ratio of V to W had a significant influence on the structure, morphology, photocatalytic activity and photoelectrochemical properties. The BiVO₄/Bi₂WO₆ microsphere with the molar ratio of V to W of 2 : 1 exhibited the highest photocatalytic activity for the MB degradation under visible light irradiation, the reaction rate constant k value of which was approximately 3.01 and 3.28 times higher than that of pure BiVO₄ and Bi₂WO₆, respectively. The mechanism of enhanced activity was studied by UV-vis DRS, PL spectra, transient photocurrent response, EIS spectra and Mott–Schottky plots. The enhanced photocatalytic activity could be mainly attributed to the matching energy band structure formed in the heterojunction which could effectively enhance the separation efficiency of photogenerated electron–hole pairs. In addition, the tetragonal–monoclinic heterophase structure, high surface oxygen vacancy concentration and narrow band gap energy were also beneficial to enhance the photocatalytic activity.

Acknowledgements

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 41472042, 41172051 and 21571162) and the National College Students' Innovative Training Program (No. 201410491024).

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