Dramatic activity of a Bi₂WO₆@g-C₃N₄ photocatalyst with a core@shell structure

Abstract

Here we report a Bi₂WO₆@g-C₃N₄ core@shell structure which was prepared by a combined ultrasonication–chemisorption method with enhanced photocatalytic degradation. The composites were extensively characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectroscopy (DRS). Compared with bare Bi₂WO₆ and g-C₃N₄, the Bi₂WO₆@g-C₃N₄ composites exhibited significantly enhanced photocatalytic activity for methylene blue (MB) degradation under visible light irradiation. The 3 wt% Bi₂WO₆@g-C₃N₄ showed the highest photocatalytic activity under visible light irradiation, which was about 1.97 times higher than Bi₂WO₆. In addition, the quenching effects of different scavengers displayed that the reactive h⁺ and ˙O₂⁻ play the major role in the MB decolorization. The core@shell hybrid photocatalyst exhibited dramatically enhanced photo-induced electron–hole separation efficiency, which was confirmed by the results of photocurrent and EIS measurements. On the basis of the experimental results and estimated energy band positions, a mechanism for the enhanced photocatalytic activity was proposed.

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1. Introduction

Recently, the ever-growing emission of dye wastewater from various industries such as textiles, printing, food and cosmetics has become a major threat to humans and ecologies owing to the toxicity and non-biodegradability. Heterogeneous photocatalysis appears to be one of the most efficient and economic techniques for the remediation of a contaminated environment.^1,2 However, traditional photocatalysts still cannot fully meet the requirements of practical application in environmental cleaning and hydrogen production driven by solar energy due to a wide band gap and rapid recombination of photo-generated electrons and holes.^3,4 Up to now, the development of visible-light-driven semiconductor photocatalysts that can directly degrade organic compounds in water remains a great challenge.

Bismuth tungstate (Bi₂WO₆), with a narrow band gap of 2.8 eV, has been confirmed to present amusing solar or visible-light photocatalytic performance in water-splitting and organic pollutants degradation.⁵ Various morphology and structure of Bi₂WO₆ photocatalysts have also been found to improve the photocatalytic activity.^6–9 Although the strategy of shape control can improve the photocatalytic performance of Bi₂WO₆, high recombination of photogenerated carriers between the hybrid orbital of Bi 6s and O 2p to the empty W 5d orbital results in low photo quantum efficiency of Bi₂WO₆.^10–12 Recent studies have shown that coupling of Bi₂WO₆ with other semiconductors improve the photocatalytic performance of Bi₂WO₆ to a substantial extent by promoting the effective separation of photoinduced charge carriers and broadening the visible light responsive range, such as BiOI,¹³ TiO₂,¹⁴ AgBr¹⁵ and CdS.¹⁶ These new composite photocatalysts greatly improved the practical application in the utilization of solar energy. However, the point contact caused by these bulk phase recombination could lead to low conjunction of Bi₂WO₆ with π-conjugated material, which could limit the photo-induced charge separation.

Photocatalysts with core@shell nanostructures possess excellent visible-light-driven photocatalytic activity and many advantages.^17,18 Compared with point contact, the core@shell architecture maximizes interaction area between the core and the semiconductor shell, facilitates the separation of the photogenerated charge carriers.¹⁹ Moreover, coupling with different semiconductor can introduce a strong interface electric field by band-edge offset, and accelerate the separation of photogenerated charge carriers. Recently, a polymeric photocatalyst made of graphitic carbon nitride (g-C₃N₄) has attracted much attention.^20–23 The well-crystallized g-C₃N₄ is a well-known π-conjugated material in the improvement of the photogenerated electron–hole pair separation. More importantly, the s-triazine ring structure and the high condensation of g-C₃N₄ make the polymer highly stable to temperature (up to 600 °C in air) and chemical exposure (e.g., acid, base, and organic solvents).²⁴ In addition, g-C₃N₄ is a soft polymer and it can be easily used for coating. Many materials have been coupled with g-C₃N₄ to inform the core@shell structure, such as CdS,²⁵ Cu₂O,²⁶ ZnO²⁷ or BiPO₄.¹⁷ And also, by comparing the energy levels of g-C₃N₄ with Bi₂WO₆, it is fortunate to find that the energy levels of g-C₃N₄ and Bi₂WO₆ are well-matched overlapping band-structures, favoring the charge transfer generated by semiconductors under light irradiation, prolonging the lifetime of electron–hole pairs.

In this study, we report the novel core@shell composites comprising of Bi₂WO₆ and g-C₃N₄. The photocatalytic activity of as-obtained Bi₂WO₆@g-C₃N₄ nanocomposites has been investigated under artificial solar light irradiation by using several probe reactions, including the oxidative degradation of MB, RhB MO and phenol. The obtained composites show significantly enhanced photocatalytic activities for the degradation of organic compounds under visible light irradiation. According to optical and photoelectrical analysis, the reason is mainly attributed to the extended visible-light response and the prolonged lifetime of photogenerated carriers in Bi₂WO₆@g-C₃N₄ interfaces. In addition, the possible mechanism of Bi₂WO₆@g-C₃N₄ photocatalysis is discussed based on main oxidative species detection experiments, transient photocurrent and electrochemical impedance spectroscopy technique.

2. Experimental

2.1. Synthesis of photocatalysts

Synthesis of g-C₃N₄ and g-C₃N₄ nanosheets. All chemicals were reagent grade and used without further purification. Typically, 10 g of melamine was put into an alumina crucible with a cover and heated at a rate of 2 °C min⁻¹ to 550 °C in a muffle furnace and then kept at this temperature for 4 h. All the experiments were performed under air conditions. The resulting yellow product was collected and ground into powder for further use. In brief, 0.05 g of g-C₃N₄ was ultrasonicated in 50 mL of water for 24 h and centrifuged at about 2000 rpm to remove the unexfoliated g-C₃N₄. Finally, the suspension of ultrathin g-C₃N₄ nanosheets was used for further study.

Synthesis of Bi₂WO₆. Bi₂WO₆ was prepared by a precipitation method. Briefly, 1.46 g amount of Bi(NO₃)₃·5H₂O and 0.50 g of Na₂WO₄·2H₂O were dissolved in 40 mL of ethylene alcohol separately under magnetic stirring at room temperature. After mixing the two solutions, a transparent mixture (80 mL) was obtained transferred into a 100 mL Teflon-lined autoclave and subsequently heated at 160 °C for 13 h. The reactor was then allowed to cool to room temperature naturally. The precipitate was collected and washed with distilled water, and the final products dried at 60 °C for 12 h.

Synthesis of Bi₂WO₆@g-C₃N₄. The concentration of ultra-thin g-C₃N₄ nanosheets suspension was estimated to be about 0.12 mg mL⁻¹. A certain amount of Bi₂WO₆ was added to the ultrathin g-C₃N₄ nanosheets dispersion (100 mL) and stirred for 48 h. The nominal weight ratios of g-C₃N₄ to Bi₂WO₆ were 1, 2, 3, 5 and 10 wt%, and weight of Bi₂WO₆ was 1.2 g, 0.6 g, 0.4 g, 0.24 g and 0.12 g, respectively. The water was evaporated and the residue was dried at 60 °C for 24 h to obtain a Bi₂WO₆@g-C₃N₄ composite powder.

2.2. Characterization of the photocatalysts

The crystal structures and phase states of Bi₂WO₆@g-C₃N₄ composites were determined by X-ray diffractometry (XRD) using a Rigaku D/MAX2500 PC diffractometer with Cu Kα radiation at an operating voltage of 40 kV and an operating current of 100 mA. The Fourier transform infrared spectra (FTIR) of the samples were recorded on an IR Vertex70 FTIR spectrometer. The morphologies of the samples were imaged with a scanning electron microscopy (SEM) (Hitachi, s-4800) and a transmission electron microscopy (TEM) (JEOL Ltd, JEM-2010). UV-visible light (UV-Vis) diffuse reflectance spectra were recorded on a UV-Vis spectrometer (Puxi, UV1901). Electrochemical and photoelectrochemical measurements were performed in 0.1 M Na₂SO₄ electrolyte solution in a three-electrode quartz cell. Pt sheet was used as a counter electrode and Hg/Hg₂Cl₂/sat. KCl was used as a reference electrode. The Bi₂WO₆@g-C₃N₄ composite thin film on indium-tin oxide (ITO) was used as the working electrode for investigation. The photoelectrochemical response was recorded with a CHI 660B electrochemical system.

2.3. Photocatalytic activity

The photocatalytic activities of Bi₂WO₆@g-C₃N₄ composites were evaluated with its catalytic degradation of MB under irradiation of visible light. A 250 W halide lamp (Philips) with a 420 nm cutoff filter was located at a distance of 10 cm from an unsealed beaker for the first test group. A glass reactor with 25 ± 2 °C circulating water flowing outside was employed for the secondary test group. For each test, 0.25 g catalyst powder was added into 50 mL 10 mg L⁻¹ MB solution. The test solutions were stirred in the dark for 30 min before irradiated under the visible light. During the irradiation, a 3 mL sample of the reaction suspension was taken every 5 minutes and centrifuged at 10 000 rpm for 6 min. The supernatant was collected and analyzed on the UV-vis spectrophotometer.

Photocatalytic degradations of MB in the dark in the presence of the photocatalyst and under visible-light irradiation in the absence of the photocatalyst were also used as negative controls. In addition, the degradation of RhB and MO was investigated with the same procedure.

3. Result and discussion

3.1. Characterization of catalysts

The crystal structure of the product was characterized by X-ray diffraction (XRD). The typical XRD patterns of Bi₂WO₆ and Bi₂WO₆@g-C₃N₄ composite are shown in Fig. 1. Two characteristic peaks are observed for bulk g-C₃N₄. The peak observed at 13.1° corresponds to the in-plane structural packing motif of tristriazine units and is indexed as the (100) peak. Another intense peak at 27.5° corresponds to the interlayer stacking of aromatic segments with a distance of 0.324 nm, which is indexed as the (002) peak of the stacking of the conjugated aromatic system.²⁸ According to Fig. 1, the diffraction peaks at 28.3°, 32.8°, 47.1° and 55.8° corresponding to the (131), (200), (202) and (331) crystallographic planes of orthorhombic Bi₂WO₆.²⁹ Compared with diffraction pattern of bare Bi₂WO₆, no significant diffraction peaks of any other phase or impurity was observed on Bi₂WO₆@g-C₃N₄ composites indicating that the characteristic peaks associated with g-C₃N₄ are not obviously detected, which due to the limit use of g-C₃N₄. Nevertheless, the presence of g-C₃N₄ could be confirmed by XPS analyses when its content was increased to 3 wt%.

Fig. 1 XRD patterns of g-C₃N₄, Bi₂WO₆, Bi₂WO₆@g-C₃N₄ photocatalysts.

The composition of Bi₂WO₆@g-C₃N₄ was further characterized by FTIR spectroscopy (Fig. 2). The characteristic absorption bands of Bi₂WO₆ appeared in 730 cm⁻¹, which corresponded to W–O stretching in the Bi₂WO₆.^30,31 No characteristic absorption peak of the ionic liquids is found in the FTIR spectra, it shows that the ionic liquid can be easily removed from the surface of the material by washing with deionized water and alcohol. For the pure g-C₃N₄, three main absorption regions can be observed clearly. The broad peak at 3000–3500 cm⁻¹ is ascribed to the stretching vibration of N–H and the stretching vibration of O–H of the physically adsorbed water.^32–34 The strong bands between 1200–1700 cm⁻¹, with the characteristic peaks at 1240, 1320, 1407, 1567, and 1640 cm⁻¹, are attributed to the typical stretching vibration of CN heterocycles.³² In addition, the peak at 808 cm⁻¹ corresponds to the breathing mode of triazine units.^32,34 The characteristic bands of Bi₂WO₆ still remained in the Bi₂WO₆@g-C₃N₄ composite, but the typical absorption peaks of g-C₃N₄ decreased dramatically in intensity or even disappeared as compared with those of pure g-C₃N₄, indicating the reduction of g-C₃N₄.

Fig. 2 FTIR spectra of the prepared g-C₃N₄, Bi₂WO₆ and Bi₂WO₆@g-C₃N₄ photocatalysts.

The morphologies of the g-C₃N₄, Bi₂WO₆ and Bi₂WO₆@g-C₃N₄ (3 wt%) composite were revealed by SEM and TEM. Fig. 3a shows the SEM micrographs of pure g-C₃N₄ sample. Bulk g-C₃N₄ are solid agglomerates of several micrometers in size. Flakes with laminar morphology were observed of g-C₃N₄ nanosheets after ultrasonic exfoliation. The TEM image of the product after ultrasonic exfoliation as presented in Fig. 3b, which shows that the displayed layer possesses chiffon-like ripples and wrinkles. The morphology of g-C₃N₄ suggests that g-C₃N₄ nanosheets were successful obtained in the reaction of water solution. Ultrathin nanosheets maintain the stable suspension over a long time in water due to its matching surface energy with bulk g-C₃N₄ and its highest polarity among solvents.³⁵ In addition, the ultrathin nanosheets are so soft that in can easily coated the surface of Bi₂WO₆ nanospheres. Fig. 3c indicates that the sample of Bi₂WO₆ is shaped into uniform sphere morphology with average size of 30–40 nm. Fig. 3d shows the SEM images of the Bi₂WO₆@g-C₃N₄. Compared with Bi₂WO₆ nanospheres (inset in Fig. 3d), Bi₂WO₆@g-C₃N₄ could be obtained whose surface is distinctly enwrapped with gauze-like g-C₃N₄ nanosheets. However, the SEM images have not enough resolution for analysis of the core@shell structure. The morphological and structural features of samples were further examined by TEM/HRTEM. g-C₃N₄ was successfully coated over Bi₂WO₆ nanospheres with intimate contact in a spontaneous adsorption process, and the core@shell structures can be clearly observed in TEM image (Fig. 3e) due to the different electron penetrability between the cores and the shells. From the HRTEM image of Bi₂WO₆@g-C₃N₄ nanocomposite (Fig. 3f), the clear lattice fringes with a spacing of 0.315 nm can be assigned to (131) lattice planes of orthorhombic Bi₂WO₆, which is in good agreement with XRD results. The outer layer of the as-prepared Bi₂WO₆@g-C₃N₄ sample is distinctly different from the Bi₂WO₆ core. TEM image indicated that the exfoliated g-C₃N₄ sheets were preferentially coated on the lateral surface of the nanospheres to achieve a minimum surface energy. When the morphology of the pure Bi₂WO₆ is compared with that of Bi₂WO₆@g-C₃N₄, it is obvious the g-C₃N₄ sheets can coat with Bi₂WO₆ to prevent their aggregation. The homogeneous attachment of Bi₂WO₆ nanospheres with g-C₃N₄ sheets is beneficial for the separation of photogenerated charge, which can prolong the lifetime of electron–hole pairs. In order to measure the amount of g-C₃N₄ in the composite, X-ray fluorescence spectrometer (XRF) analysis for the Bi₂WO₆@g-C₃N₄ (3 wt%) was performed. The elemental contents of C and N are found to be 1.02 wt% and 1.78 wt%, respectively, the calculated value of g-C₃N₄ content in the composite was about 3 (mass%).

Fig. 3 SEM image of g-C₃N₄ (a), TEM images of g-C₃N₄ nanosheets (b) and Bi₂WO₆ (c), SEM (d) image of Bi₂WO₆@g-C₃N₄ (inset in (d) SEM image of Bi₂WO₆), TEM (e) and HRTEM (f) images of Bi₂WO₆@g-C₃N₄.

Fig. 4 displays the UV-vis diffuse reflectance spectra of bare Bi₂WO₆ and Bi₂WO₆@g-C₃N₄ composite. The absorption edge of pure g-C₃N₄ photocatalysts is about 460 nm, with band gap (E_g) calculated to be 2.7 eV.³⁶ The absorption threshold of Bi₂WO₆ is located at around 450 nm due to the intrinsic band–gap transition corresponding to the E_g of 2.8 eV.^5,37 Further Bi₂WO₆@g-C₃N₄ shows the broader absorption edge and extends to the visible region as compared to that of Bi₂WO₆ due to the presence of g-C₃N₄ on the Bi₂WO₆ surface. It is notable that compared with Bi₂WO₆ surface, the absorption edge of Bi₂WO₆@g-C₃N₄ experiences a red shift of about 10–20 nm. This observation was mainly attributed to the light shielding of Bi₂WO₆ covered by g-C₃N₄. The absorption intensity of the prepared composites varied with the content of g-C₃N₄.

Fig. 4 UV-vis diffuse reflectance spectra of prepared photocatalysts.

The surface elemental composites and chemical environment variation of samples were analyzed by XPS. Fig. 5a is XPS survey spectra of 3 wt% Bi₂WO₆@g-C₃N₄. Typical survey XPS spectrum of Bi₂WO₆@g-C₃N₄ indicates that C, N, Bi, W and O elements could all be detected as shown in Fig. 5a. In Fig. 5b, the C 1s spectrum can be found in three peaks of 284.4, 285.4 and 288.2 eV. The peak at around 284.4 eV is regarded as graphitic carbon, whereas the peak at 285.4 eV is ascribed to C–NH₂ species on the g-C₃N₄.²⁷ The peak at 288.2 eV corresponded to the sp²-hybridized carbon in N–C=N coordination.^38,39 XPS spectrum of N 1s can be deconvoluted into two peaks centered at 399.7 eV and 398.5 eV, respectively, corresponding to the nitrogen atoms in the aromatic rings (C–N=C) and tertiary nitrogen (N–(C)₃).^40,41 As shown in Fig. 5d, the peaks at 164.3 and 158.9 eV, corresponding to Bi 4f_5/2 and Bi 4f_7/2, can be assigned to Bi³⁺ of bare Bi₂WO₆.⁴² The peaks at 37.6 and 35.2 eV can be assigned to W 4f_5/2 and W 4f_7/2, respectively (Fig. 5e), which represent a W⁶⁺ oxidation state.⁴³ The O 1s spectrum could be fitted by three peaks at binding energies of 531.7, 530.6 and 529.7 eV respectively, which are shown in Fig. 5f. The peak at 531.7 and 529.8 eV are consistent with the different chemical environments of oxygen element in Bi–O and W–O. The peaks at 530.6 correspond to the hydroxyl groups on the surface of Bi₂WO₆.⁴⁴

Fig. 5 XPS spectra of (a) survey spectrum, (b) C, (c) N, (d) Bi, (e) W and (f) O of the sample 3 wt% Bi₂WO₆@g-C₃N₄.

3.2. Photocatalytic activity

The photocatalytic performance of the as synthesized Bi₂WO₆@g-C₃N₄ product was evaluated by degradation of MB under simulated sunlight irradiation. Fig. 6a displays the changes of the MB concentration versus the reaction time over the Bi₂WO₆@g-C₃N₄ photocatalysts. Simple photolysis of MB was also carried out for comparison. No MB was photodegraded by the visible light in the absence of photocatalyst, indicating that the contribution of photolysis of MB to the photoactivity was negligible. As shown in Fig. 6a, it is clear that the concentration of the MB solution gradually decreases during the photodegradation. Only 21.2% MB can be photodegraded by bulk g-C₃N₄ in 30 min. g-C₃N₄ nanosheets possess the bigger of BET specific surface areas. In that case, the g-C₃N₄ nanosheets show improved photocatalytic activity and degrade nearly 33.6% of MB.⁴⁵ The use of Bi₂WO₆ leads to 46.2% photodegradation of MB. Nearly 90.8% of MB was degraded after 30 min irradiation in the presence of Bi₂WO₆@g-C₃N₄ photocatalyst, indicating its excellent photocatalytic activity. Besides the degradation of pollutant MB, the photocatalytic performance of Bi₂WO₆@g-C₃N₄ was further studied by degradation of pollutant RhB and MO. Similar to the photocatalytic degradation of MB, the Bi₂WO₆@g-C₃N₄ also exhibited evidently improved photocatalytic activity compared with the pure Bi₂WO₆ and g-C₃N₄ on the degradation of RhB and MB. Phenol is a colorless organic compound and was also chosen as a model contaminant. In the present work, the degradation of phenol over Bi₂WO₆@g-C₃N₄ under visible light irradiation (>420 nm) was also studied to further evaluate the photocatalytic performance of Bi₂WO₆@g-C₃N₄ composites. Under visible-light irradiation, the results of the photocatalytic evaluation of as-prepared samples are shown in Fig. 6d. Phenol was degraded 39.1% over Bi₂WO₆ under visual light irradiation for 60 min. The degradation ratio was significantly increased to 62.6% over the Bi₂WO₆@g-C₃N₄ composite photocatalyst with a 60 min visible light irradiation. By comparison of the phenol degradation rates, the photocatalytic activity of Bi₂WO₆@g-C₃N₄ was confirmed for the degradation of phenol, and affirmed that the disappearance of phenol molecules was due to photocatalytic degradation instead of only to adsorption. Well-aligned band-structures and intimately contact interface enhance charge separation, which result in excellent photocatalytic activity.

Fig. 6 Dynamic curves of photodegradation for MB (a), RhB (b) MO (c) and phenol (d) solutions over different samples.

g-C₃N₄ content has a significant influence on the photocatalytic activity of the composite. Photocatalytic degradation kinetic curve was investigated by the first-order simplification of Langmuir–Hinshelwood (L–H) kinetics, which is well established for photocatalysis at low initial pollutant concentrations. The relevant equation is as follows:

ln(C₀/C) = kt (1)

where C₀/C is the ratio of the concentration of the dyes at adsorption–desorption equilibrium and after various intervals of time, and k is the apparent first-order rate constant (min⁻¹). The value of k value is obtained from the gradient of the graph of ln(C₀/C) versus time (t), which is shown in the insert of Fig. 7. Obviously, the reaction rates constant k of Bi₂WO₆@g-C₃N₄ composites containing 1 wt%, 2 wt%, 3 wt%, 5 wt% and 10 wt% g-C₃N₄ were 0.03816, 0.05469, 0.08143, 0.04838 and 0.03174 min⁻¹, respectively. The photocatalytic activity of the Bi₂WO₆@g-C₃N₄ composites increased with the increase of g-C₃N₄ content from 1 to 3 wt%. When the g-C₃N₄ content was relatively low (<3.0 wt%), the contact area gradually increased with the increment of g-C₃N₄. In this case, an effective charge separation can be achieved, resulting in enhancement of photocatalytic activity and inhibition of photocorrosion. At g-C₃N₄ content higher than the optimized content, excessive g-C₃N₄ covered on the surface of Bi₂WO₆ could form a dense shell. The dense shell suppress the electrons on the VB of Bi₂WO₆ transfer to surface of g-C₃N₄ and reduces the number of superoxide radicals which can act a main reactive species in the photocatalytic degradation. At the same time, this can facilitate the recombination of photoinduced electron–hole pairs. Consequently, the photocatalytic activity will decrease rapidly with further increasing of the g-C₃N₄ content. Therefore, it is important to make a balance between the active trapping sites favoring the inhibition of the recombination of electron–hole pairs and fewer trapped parts leading to a lower capacity for the separation of interfacial charge transfer.

Fig. 7 Comparison of degradation of MB over Bi₂WO₆@g-C₃N₄ composite with various g-C₃N₄ contents of 1.0 wt%, 2.0 wt%, 3.0 wt%, 5.0 wt% and 10.0 wt%.

To evaluate the stability and reusability of the Bi₂WO₆@g-C₃N₄ hybrid photocatalyst, we carried out the additional experiments to degrade MB under visible light cycled for five times (Fig. 8). After every run of photocatalytic reaction, the concentrated MB solution was injected and the separated photocatalysts were washed back into the reactor in order to keep the initial concentration of MB and photocatalysts constant. The recycling runs of the photocatalytic degradation of MB by the Bi₂WO₆@g-C₃N₄ (3 wt%) sample under visible-light were studied (Fig. 8). As shown in Fig. 10, the photocatalytic activity of Bi₂WO₆@g-C₃N₄ for MB degradation is effectively maintained from 90.8% to 77.5% after five cycling runs, which indicates that Bi₂WO₆@g-C₃N₄ has high stability in the photodegradation process under visible light. This decrease was thought to be due to some catalyst washout during the recovery steps, which could be minimized through the use of centrifugation between runs. Therefore, the results obtained illustrate that the incorporation of g-C₃N₄ into the Bi₂WO₆ matrix not only enhances the visible light photocatalytic performance of Bi₂WO₆, but also inhibits the photo-corrosion, thus giving rise to a stable durability of photocatalytic activity.

Fig. 8 Recycling runs of the degradation of MB over Bi₂WO₆@g-C₃N₄ (3 wt%) composite under visible-light irradiation (λ > 420 nm).

The electrons and holes produced by photocatalysis have strong reduction and oxidation abilities. However, they usually do not react with the organic dyes directly. Instead, some active species (such as ˙OH and ˙O₂⁻) are first formed through the reaction of charges and adsorbed H₂O or O₂. So, it is important to detect main oxidative species in the photocatalytic process for elucidating the photocatalytic mechanism. The main oxidative species in photocatalytic process are detected through the trapping experiments of radicals using IPA as hydroxyl radical scavenger and EDTA-2Na as holes radical scavenger, and purging N₂ as ˙O₂⁻ scavenger, respectively.⁴⁶ If the free radical scavenged played acted an important role in the photocatalytic degradation of MB, the degradation rate would be reduced greatly in the presence of the appropriate scavenger. Before irradiation, the scavenger (10 mmol L⁻¹) was added to the MB solutions together with the catalyst. As is clear from Fig. 9, the addition of IPA did not affect the decolorization rate of MB over Bi₂WO₆@g-C₃N₄ (3 wt%), suggesting that ˙OH was not main reactive species in the photocatalytic process. To further prove the existence of ˙O₂⁻, a control experiment was carried out under N₂ atmosphere, where N₂ was purged through the solution to remove the dissolved O₂ in water in order to reduce radicals. And as showed in Fig. 9, the degradation rate decreases obviously to 17.8% in the presence of N₂ (˙O₂⁻ scavenger) and the degradation rate was 90.8% in the absence of scavengers, which suggests that ˙O₂⁻ is the main reactive species for MB degradation. Meanwhile, EDTA-2Na had a significant effect on the degradation rate compared to the runs performed in the absence of scavenger, suggesting that h⁺ was also a dominant reactive species.

Fig. 9 Photocatalytic degradation of MB over Bi₂WO₆@g-C₃N₄ (3 wt%) in the presence of IPA, EDTA-2Na, N₂ and in the absence scavengers.

It is well-known that the photocatalytic redox reactions are intimately relevant to the separation efficiencies of photoinduced electron–hole pairs arisen from the excited semiconductor materials.⁴⁷ Photocurrent can be produced from the photo-generated electrons in the conducting bands of semiconductor photocatalysts with leaving holes in their valence bands. Therefore, higher photocurrents are indicative of better electron and hole separation efficiencies, and thus higher photocatalytic activities.⁴⁸ Fig. 10 shows the current–voltage curves for these three samples under several on/off visible-light irradiation cycles. It is clear that uniform and reversible photocurrent responses are observed in all electrodes, indicating effective charge transfer and successful electron collection for these three samples within the photoelectrochemical cell. Experimental results reveal that Bi₂WO₆@g-C₃N₄ exhibit higher current density than the single-component semiconductors, which demonstrate that a more effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer occur in the core@shell structure of Bi₂WO₆@g-C₃N₄.

Fig. 10 Transient photocurrent of g-C₃N₄, Bi₂WO₆ and Bi₂WO₆@g-C₃N₄ under visible light irradiation.

As a powerful tool to explore the electrochemical process, EIS has been widely employed in testing the electro-catalytic activity for the regeneration of a redox couple.^39,49 Fig. 11 reveals that the Nyquist plots diameter of 3 wt% Bi₂WO₆@g-C₃N₄ composite is much smaller over pristine Bi₂WO₆ and g-C₃N₄, suggesting the lower resistance and the faster interfacial charge transfer. This is consistent with the photocurrent results. This result demonstrates that the introduction of g-C₃N₄ into Bi₂WO₆ can enhance the separation and transfer efficiency of photogenerated electron–hole pairs, which is favorable for enhancing photocatalytic activity.

Fig. 11 Electrochemical impedance spectroscopy of g-C₃N₄, Bi₂WO₆ and Bi₂WO₆@g-C₃N₄.

To approach the mechanism of the enhanced photocatalytic activity of the Bi₂WO₆@g-C₃N₄ composite, the relative band positions of the two semiconductors were investigated, since the band-edge potential levels play a crucial role in determining the flowchart of photoexcited charge carriers in the composite. As shown in the Fig. 12, the conduction band (CB) and valence band (VB) potentials of g-C₃N₄ (−1.3 and 1.4 eV) are more negative than those of Bi₂WO₆ (0.46 and 3.26 eV), suggesting that g-C₃N₄ and Bi₂WO₆ match the band potentials in the Bi₂WO₆@g-C₃N₄ composite.^36,50 Under visible-light irradiation, both g-C₃N₄ and Bi₂WO₆ can absorb visible-light photons to produce photo-generated electrons and holes. The electrons were promoted from the VB to the CB of g-C₃N₄ and Bi₂WO₆, leaving the holes behind. Obviously, the difference between the CB edge potentials of g-C₃N₄ and Bi₂WO₆ allowed the electron transfer from the CB of g-C₃N₄ to that of Bi₂WO₆. Subsequently, simultaneous holes on the VB of Bi₂WO₆ migrated to that of g-C₃N₄ because of the enjoined electric fields of the two materials. In such a way, the photogenerated electrons can be effectively collected by Bi₂WO₆ and holes can be effectively collected by g-C₃N₄. Therefore, the efficiently separation of photogenerated electrons and holes can be achieved, and the recombination process of electron–hole pairs can be hindered, in accordance with the result of photocurrent and EIS. The electrons in Bi₂WO₆ crystals are good reductants that could capture the adsorbed O₂ onto the composite catalyst surface and reduce it to ˙O₂⁻. The highly oxidative species ˙OH is produced as a consequence of the reduction of oxygen. Under the action of substantial strong oxidizing species, the structure of MB was destroyed and finally decomposed into degradation products.

Fig. 12 Schematic illustration of the mechanism for the high photocatalytic performance of Bi₂WO₆@g-C₃N₄ composite.

4. Conclusions

In summary, g-C₃N₄ can be successfully coated onto the surface of Bi₂WO₆ and a series of Bi₂WO₆@g-C₃N₄ photocatalysts with different varying compositions of g-C₃N₄ and Bi₂WO₆ were prepared. The g-C₃N₄ coated Bi₂WO₆ exhibits enhanced photocatalytic activity in the degradation of MB under visible light irradiation, which is higher than that of pure Bi₂WO₆, and the content of g-C₃N₄ impacts the catalytic activity of Bi₂WO₆@g-C₃N₄. The photocatalytic activity of as-prepared 3.0 wt% Bi₂WO₆@g-C₃N₄ exhibited the best photocatalytic activity. The mechanism has been discussed by energy band positions and photocurrent and EIS measurements. The enhanced photocatalytic activity can be attributed to the efficient separation and recombination preventing of electron–hole pairs. Both ˙O₂⁻ and photogenerated holes are the main reactive species responsible for the degradation of MB. Furthermore, Bi₂WO₆@g-C₃N₄ has high stability and are easily recyclable, suggesting that the photocatalyst is a promising photocatalytic material which has good potential for application to pollutants purification.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51372068), Hebei Natural Science Funds for Distinguished Young Scholar (No. B2014209304), Hebei Natural Science Funds for the Joint Research of Iron and Steel (grant No. B2014209314).

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