Synthesis of a hierarchical BiOBr nanodots/Bi₂WO₆ p–n heterostructure with enhanced photoinduced electric and photocatalytic degradation performance

Abstract

Three-dimensional composites of flower-like Bi₂WO₆ decorated with BiOBr nanodots (designated BiOBr nanodots/Bi₂WO₆) with varying BiOBr content have been prepared by a simple method. The BiOBr nanodots, with average diameters of 50 nm, adhered tightly to the surface of Bi₂WO₆ and formed p–n heterojunctions between BiOBr and Bi₂WO₆, as evidenced by the characterization of its structure and composition. Compared to pure Bi₂WO₆ and BiOBr, BiOBr/Bi₂WO₆ showed a lower charge-transfer resistance, higher photocurrent and enhanced photoelectric properties. The photocurrent of the 15% BiOBr/Bi₂WO₆ composite was 17.2 and 2.39 times higher than that of pure Bi₂WO₆ and BiOBr, respectively. Meanwhile, this composite showed the highest degradation rate for methylene blue (MB), which was 1.7 and 2.4 times that of pure Bi₂WO₆ and BiOBr, respectively. The enhanced photoelectric and photocatalytic degradation performances were ascribed to the introduction of BiOBr nanodots and the formation of p–n heterojunctions, which could greatly accelerate the separation of photogenerated charge carriers. In addition, the roles of the radical species were investigated, and ·O₂⁻ and h⁺ are thought to dominate the photocatalytic process. Based on the experimental results, a possible photocatalytic mechanism was proposed.

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1. Introduction

Semiconductor photocatalysis has been considered as a potentially promising approach for the abatement of environmental pollutants.^1–3 Among the various studied photocatalysts, Bi₂WO₆ with a band gap of 2.6–2.8 eV has attracted great attention due to its high photocatalytic performance, nontoxicity and wide solar response.^4–6 Unfortunately, the high recombination of photogenerated electron–hole pairs seriously limits the photoelectric conversion efficiency of Bi₂WO₆. Therefore, it is still an urgent issue to take effective efforts to accelerate the separation of the photogenerated charge carriers to advance solar energy use.

Noble metal decorated Bi₂WO₆ could accelerate the migration of interfacial charges through the Schottky barrier, formed between the semiconductor and metal, since the Fermi level of noble metal is more positive than that of semiconductors.^7–10 Additionally, ion doping is an alternative method of resolving the problem,^11–13 as the doped atoms could act as electron traps and facilitate the separation of photogenerated electron–hole pairs. Actually, semiconductor-modified Bi₂WO₆ composites with appropriate band positions, such as Bi₂S₃,¹⁴ ZnWO₄,¹⁵ Ag–AgBr,¹⁶ C₃N₄ (ref. 17 and 18) etc., could greatly improve the charge separation due to the synergetic effect between the coupled semiconductors, where the coupled energy levels provide good internal potential driving forces to separate the photogenerated charges.^19,20 In particular, p–n heterostructure composites, such as the Bi₂O₃–Bi₂WO₆ p–n hierarchical heterojunction,^21,22 could more effectively facilitate the migration of photogenerated electrons through the p–n junctions due to the intimate contact of these interfaces and the internal electric field.

BiOBr is a p-type semiconductor with a band gap of 2.8 eV, and it has been demonstrated to be a promising photocatalyst for pollutant decomposition under visible light irradiation.^23,24 Interestingly, BiOBr and Bi₂WO₆ exhibited suitable band gaps, seemingly separating the photogenerated charges efficiently. Recently, BiOBr–Bi₂WO₆ microsphere and mesoporous nanosheet composites have been synthesized, and these exhibited higher activity than BiOBr and Bi₂WO₆. Li²⁵ reported a facile hydrothermal route for the preparation of BiOBr–Bi₂WO₆ mesoporous nanosheet composites in the presence of titanium isopropoxide, where the titanium alkoxide was not only strongly involved in the growth of BiOBr (001) facets, but also played a critical role in the pore evolution of the product, thus resulting in its large specific surface area, which benefited its photocatalytic activity. Xia²⁶ also presented Bi₂WO₆/BiOBr composite photocatalysts that were prepared using a one-pot EG-assisted solvothermal process and the composites exhibited enhanced photocatalytic activity under visible light irradiation. The strong coupling between Bi₂WO₆ and BiOBr facilitated interfacial charge transfer and inhibited electron–hole recombination. Meanwhile, Zhang²⁷ synthesized BiOBr–Bi₂WO₆ heterojunctions with an “egg-shell” structure and higher photocatalytic degradation efficiency. All of these results confirmed the great prospects for BiOBr–Bi₂WO₆ composites in addressing environmental issues. Therefore, more effort should be devoted to further increasing the charge separation between BiOBr and Bi₂WO₆. In fact, using BiOBr nanodots to decorate the flower-like Bi₂WO₆ could result in a larger specific surface area and contact area, promote the charge transfer to the surface in the composites and increase the number of active sites as well. Moreover, the formation of the p–n heterojunctions between these components could greatly facilitate the separation of photogenerated charge carriers, because of the intimate contact interface and the well-matched overlapping band-structures. Compared with the above BiOBr–Bi₂WO₆ composites, the BiOBr nanodots/Bi₂WO₆ p–n heterostructure composites have aroused great interest because of the additional reactive sites brought by its fascinating morphology, and the remarkable and directional electron–hole transport characteristics, all leading to the enhancement of its photocatalytic activity. Meanwhile, no attempt has been made to control the morphology of the BiOBr nanodots/Bi₂WO₆ p–n heterostructure flower-like composite photocatalysts. This article reports BiOBr nanodots for the decoration of flower-like Bi₂WO₆ to fabricate p–n heterostructure composites via a simple method, and the heterostructure exhibited superior photocatalytic activity for MB and phenol degradation under visible light irradiation.

2. Experimental

2.1 Synthesis of Bi₂WO₆

All of the reagents used for synthesis were commercially available and used without further purification.

Synthesis of Bi₂WO₆ hierarchical architectures: the Bi₂WO₆ hierarchical architectures were synthesized according to the hydrothermal method. Bi(NO₃)₃·5H₂O (2.425 g, 5 mmol) was dissolved in 40 mL of distilled water and stirred vigorously. Then, Na₂WO₄·2H₂O (0.825 g, 2.5 mmol) in 40 mL distilled water was added slowly to the solution with continuous stirring for 30 min. Subsequently, the resulting reaction mixture was transferred to a 150 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. After that, the reactor was allowed to cool to room temperature naturally. The resulting precipitates were collected and washed with de-ionized water and absolute ethanol, then dried at 80 °C for 10 h.

2.2 Fabrication of BiOBr/Bi₂WO₆ composites

Fabrication of BiOBr nanodots/Bi₂WO₆ composites: first, the Bi₂WO₆ powder was dissolved in a 60.0 mL ethylene glycol solution and a certain amount of Bi(NO₃)₃·5H₂O was then added and stirred for 30 min to fully disperse Bi₂WO₆. A desired amount of cetyltrimethylammonium bromide (CTAB) was then added and stirred until a transparent solution was obtained. Subsequently, the solution was transferred into a 90 mL Teflon-lined stainless steel autoclave and maintained at 140 °C for 8 h, then allowed to cool to room temperature naturally. The resulting precipitates were collected and washed with de-ionized water and absolute ethanol, then dried at 80 °C for 10 h. For comparison, pure BiOBr particles were synthesized using the same procedure without the addition of Bi₂WO₆.

2.3 Photocatalyst characterization

The crystal phases of the catalysts were evaluated using powder X-ray diffraction (XRD, D/MAX2500PC, Cu Kα, 40 kV, 100 mA) scanning over the two-theta range of 5–80°. The morphologies of the catalysts were examined using scanning electron microscopy (SEM, Hitachi, s-4800) and transmission electron microscopy (TEM, JEM-2010, 200 kv). UV-vis diffuse reflectance spectra were obtained using a Puxi UV1901 spectrometer with BaSO₄ as a reference and the chemical compositions of the catalysts were studied by energy dispersive X-ray spectroscopy (EDS, Thermo Noran 7). X-ray photoelectron spectroscopy (XPS) measurements were performed on Kratos Axis Ultra XPS systems using monochromatic Al Kα radiation to examine the elements on the surface of the samples. To study the recombination of photoinduced charge carriers, electrochemical impedance spectroscopy (EIS) and photocurrent measurements were taken on an electrochemical system (CHI-660E, China).

2.4 Photocurrent and EIS measurements

The photocurrent and EIS measurements were conducted using an electrochemical analyzer (CHI660E, Chen Hua Instruments, Shanghai, China) with a standard three-electrode configuration. A standard three-electrode cell was used in the photoelectric studies, with a working electrode (as-prepared photocatalyst), a platinum wire as the counter electrode, and a standard calomel electrode (SCE) as the reference electrode. 0.1 M Na₂SO₄ was used as the electrolyte solution. The visible light irradiation was obtained from a 500 W Xe lamp with a 420 nm cutoff filter.

2.5 Photocatalytic activity measurement

The photocatalytic activities of the BiOBr nanodots/Bi₂WO₆ catalysts were evaluated in the photocatalytic degradation of MB and phenol in an aqueous solution under visible light irradiation. Visible light was produced by a 250 W halide lamp (Philips) equipped with a 400 nm cutoff filter. In the MB photodegradation test, 0.1 g catalyst was added into a 150 mL (20 mg L⁻¹) solution, and in the photodegradation of phenol, 0.2 g catalyst was added into a 100 mL (15 mg L⁻¹) solution. Prior to irradiation, the dispersion was stirred for 60 min in the dark to achieve adsorption–desorption equilibrium. During the photocatalytic process, a 3 mL sample of the reaction suspension was withdrawn every 15 minutes and centrifuged at 10 000 rpm for 6 min to remove the suspended catalyst particles. The supernatant solutions were then analyzed using a spectrometer (TU-1901 UV-vis) at the characteristic absorption peaks for MB and phenol of 664 nm and 270 nm, respectively.

The degradation efficiency (%) was calculated as follows:

where C₀ is the initial concentration and C is the time-dependent concentration of dye upon irradiation.

3. Results and discussion

3.1 Characterization of catalysts

Fig. 1 shows the XRD patterns of the Bi₂WO₆, BiOBr and BiOBr nanodots/Bi₂WO₆ p–n heterojunction composites. For the spectra of pure Bi₂WO₆, characteristic diffraction peaks were detected at 2θ angles 28.29°, 32.79°, 47.14°, 55.82°, and 58.54°, attributed to the (131), (200), (202), (331), and (262) crystal planes of the orthorhombic Bi₂WO₆ crystal, respectively, as indexed by (JCPDS 39-0256). The characteristic diffraction peaks for BiOBr were detected at 2θ angles 25.2°, 31.7°, 32.2°, 46.2° and 57.1°, attributed to the (101), (102), (110), (200), and (212) crystal planes of the BiOBr crystal, respectively, and indexed to BiOBr according to (JCPDS 09-0393). Both pure Bi₂WO₆ and BiOBr possessed a high degree of crystallinity based on the intensity of the diffraction peaks obtained. No typical patterns of BiOBr were observed in the 5% BiOBr/Bi₂WO₆ composites, due to the low BiOBr content. However, with an increasing amount of BiOBr in the samples, the intensity of several additional peaks at 2θ angles 25.2, 31.7, 32.2 and 57.1° changed in the BiOBr/Bi₂WO₆ composites, corresponding to the (101), (102), (110) and (212) planes of BiOBr, respectively. Moreover, the intensities of the BiOBr peaks gradually increased with an increase in the BiOBr content of BiOBr/Bi₂WO₆. No traces of other phases were detected, indicating the high purity of the samples, and confirming that Bi₂WO₆ and BiOBr coupled together successfully and without other phases.

Fig. 1 XRD patterns of Bi₂WO₆, BiOBr and BiOBr/Bi₂WO₆.

The surface morphologies of the Bi₂WO₆, BiOBr and BiOBr nanodots/Bi₂WO₆ composites were investigated using SEM, as seen in Fig. 2. It can be seen that pure Bi₂WO₆ and BiOBr exhibit similar morphologies and sizes. They are uniform three-dimensional microspheres with diameters of 3–5 μm (Fig. 2a and b), composed of numerous two-dimensional (2D) interlaced nanosheets. These interlaced nanosheets are aligned from the sphere center to the surface to form hierarchical microspheres with an open porous structure. Interestingly, Bi₂WO₆ exhibited more opening petals, which favors the deposition of the BiOBr nanodots. Fig. 2c shows the SEM micrographs of the 15% BiOBr/Bi₂WO₆ composites, the BiOBr nanodots were uniformly attached on the surface of the Bi₂WO₆ nanosheets with intimate contact, did not affect the flower-like Bi₂WO₆ microspheres. The nanosheet building blocks of the Bi₂WO₆ microspheres acted as clapboards to separate BiOBr nanodots and could endow the photocatalysts with a higher surface-to-volume ratio and more reactive sites, which was beneficial for their photocatalytic activity.²⁸ Interestingly, BiOBr loading was observed to reduce the grain size of the particles, as the deposited BiOBr possessed much smaller grain sizes than the pure BiOBr particles. This may be due to the nanosheets of Bi₂WO₆ greatly inhibiting the growth of the BiOBr particles in the synthesis process. As a result, the BiOBr particles were uniformly assembled on the surface of Bi₂WO₆ and possessed smaller average diameters. A similar observation has previously been reported in our group.²⁹

Fig. 2 SEM images of prepared photocatalysts (a) Bi₂WO₆; (b) BiOBr; (c) 15% BiOBr/Bi₂WO₆; (d and e) TEM and HRTEM images of 15% BiOBr/Bi₂WO₆; (f) EDS image of 15% BiOBr/Bi₂WO₆.

To further investigate the morphology and detailed structural information of the BiOBr nanodots/Bi₂WO₆ heterostructure catalyst, TEM and HRTEM were performed on the 15% BiOBr/Bi₂WO₆ composite. From the low-magnification TEM image in Fig. 2d, it was found that the BiOBr nanodots uniformly dispersed on the surface of Bi₂WO₆, with an average size of about 50 nm, were homogeneously coated on the surface of Bi₂WO₆ and in intimate contact with Bi₂WO₆. The HRTEM image (Fig. 2e) clearly demonstrated the detailed heterostructure and the lattice fringes and the interface of BiOBr and Bi₂WO₆. The spacing of the BiOBr nanodots’ lattice fringes of 0.27 nm and 0.28 nm, coupled with the XRD analysis, matched the (110) and (102) crystallographic planes of the BiOBr crystal. Similarly, the interplanar spacing of 0.27 nm corresponded to the (200) plane of Bi₂WO₆. The above observation suggested that a surface dispersive heterojunction of the BiOBr/Bi₂WO₆ composites was obtained. EDS analysis was performed and the results are shown in Fig. 2f. Peaks attributed to W, Bi, Br, O were observed, which were indicative of the BiOBr and Bi₂WO₆ components present in the system.

The optical properties of the BiOBr/Bi₂WO₆ composites, as well as the pure Bi₂WO₆ and BiOBr samples, were examined using the DRS technique and the results are shown in Fig. 3. According to the spectra, the prepared Bi₂WO₆ and BiOBr exhibited absorption up to 430 and 450 nm, respectively, resulting in visible light induced photocatalytic activity. Compared with bare Bi₂WO₆ and BiOBr, the BiOBr/Bi₂WO₆ composites exhibit a slight deviation in the visible light region, caused by the synergistic effect of the Bi₂WO₆ and BiOBr crystals.²⁴

Fig. 3 UV-vis diffuse reflectance spectra of pure Bi₂WO₆, BiOBr and BiOBr/Bi₂WO₆ samples.

Additionally, the band gap of the semiconductor can be calculated according to the formula:³⁰

αhν = A(hν − E_g)^n/2,

where α, h, ν, A, and E_g represent the absorption coefficient, Planck constant, light frequency, a constant and band gap energy, respectively.³¹ In the equation, n decided the characteristics of the transition in a semiconductor, here n = 1 for direct transition (Bi₂WO₆) and n = 4 (ref. 32) for indirect transition (BiOBr).³³ Thus, the band gaps of pure BiOBr and Bi₂WO₆ were calculated to be 2.85 eV (Fig. 4a) and 2.7 eV (Fig. 4b), respectively.

Fig. 4 Plots of (αhν)^1/2 versus energy (hν) for the band gap energy of BiOBr; (b) plots of (αhν)² versus energy (hν) for the band gap energy of Bi₂WO₆.

XPS spectra for the 15% BiOBr/Bi₂WO₆ heterojunction were presented to determine the oxidation state and elemental composition for each member of the heterojunction. From the Bi 4f XPS spectra shown in Fig. 5a, two strong peaks centered at 159.2 eV and 164.6 eV were observed, attributable to Bi 4f_7/2 and Bi 4f_5/2, respectively. This implied that Bi was mainly present in its tri-valent chemical state.³⁴ The Bi 4f_7/2 peak was divided into two different peaks at 158.8 eV and 159.7 eV, and the Bi 4f_5/2 was divided into 164 eV and 165 eV, respectively. As shown in Fig. 5b, the peak for Br 3d at 68.2 eV consisted of two individual Br 3d_5/2 and Br 3d_3/2 peaks, with binding energies of 67.6 eV and 68.8 eV, respectively.³⁵ These results indicated that the Br elements were mainly in the form of Br⁻. The XPS spectrum of W 4f (Fig. 5c) shows a binding energy at 34.9 eV for W 4f_7/2 and at 37.1 eV for W 4f_5/2, suggesting that W exists in the chemical state of W⁶⁺.³⁶ As shown in Fig. 5d, the peak for O 1s appeared at 530.2 eV and was resolved into two peaks at 529.7 eV and 531 eV, which were assigned to the lattice oxygen and hydroxyl oxygen, respectively.³⁷

Fig. 5 High-resolution XPS spectra of the 15% BiOBr/Bi₂WO₆ sample: (a) Bi 4f, (b) Br 3d, (c) W 4f, and (d) O 1s.

The photocurrent could directly display the mobility capability of the photogenerated electrons, with a higher peak intensity representing a higher separation efficiency of the photogenerated carriers,³⁸ which has a direct positive correlation with the photocatalytic activity of the photocatalyst.³⁹ The photoresponses of Bi₂WO₆, BiOBr and 15% BiOBr/Bi₂WO₆ p–n heterojunction samples over several on/off sunlight irradiation cycles are shown in Fig. 6a. It was noted that the photocurrent increased sharply upon light irradiation. Subsequently, it rapidly decreased to zero at the end of the irradiation. The 15% BiOBr/Bi₂WO₆ composites exhibited the highest photocurrent intensity of the three samples, which was about 17.2 and 2.39 times that of pure Bi₂WO₆ and BiOBr, indicating that the recombination of electrons and holes was greatly inhibited and the separation of photogenerated charges at the interface between Bi₂WO₆ and BiOBr was more effectively realized, thus improving the photocatalytic activity. Moreover, the photocurrent was relatively stable, because the photocurrent for all of the composites did not decrease over the course of 1200 s.

Fig. 6 (a) Transient photocurrent response for pure BiOBr, Bi₂WO₆ and 15% BiOBr/Bi₂WO₆ composites at 0 V, (b) the photocurrent response of the 15% BiOBr/Bi₂WO₆ composites with varying applied potential.

The applied potential changed not only the recombination rate constant but also the charge-transfer rate constant.⁴⁰ The appropriate potential applied to the semiconductor anode could effectively minimize recombination of the photogenerated charges. Fig. 6b presents the photocurrent at various potentials under visible illumination. As the photocurrent observed with applied potential was higher than at the applied potential of 0 V, it is obvious that the applied potential could significantly promote the charge separation. In addition, as can be seen from Fig. 6b, the photocurrent increased with an increase in the applied potential from 0 to 0.9 V, meaning that a larger applied potential could promote the charge separation significantly.

Photoelectrochemical measurements were performed to study the role of interfacial contact in the Bi₂WO₆, BiOBr and 15% BiOBr/Bi₂WO₆ composites. EIS is a significant tool for investigating charge transfer processes on the electrode and at the contact interface between the electrode and electrolyte, and can be used to probe the separation efficiency of charge carriers. The smaller the arc radius of an EIS Nyquist plot, the higher the efficiency of interfacial charge transfer and the more effective the separation of photogenerated electron–hole pairs.⁴¹ Fig. 7a shows the EIS response of the 15% BiOBr/Bi₂WO₆ composite under dark and photo-irradiation conditions. It can be seen that the diameter of the semicircle loop on the EIS Nyquist plot is greatly decreased with photo-irradiation, which indicates that a large number of carriers are generated on the composite photocatalyst under visible light irradiation.

Fig. 7 (a) Effect of photo-irradiation on the EIS data for 15% BiOBr/Bi₂WO₆, (b) EIS data for the pure Bi₂WO₆, BiOBr and 15% BiOBr/Bi₂WO₆ samples, inset: equivalent circuit used to fit the data, (c) the Bode phase plot for the three electrodes.

Fig. 7b shows the typical EIS data of the prepared Bi₂WO₆, BiOBr and 15% BiOBr/Bi₂WO₆ samples, which are presented as Nyquist plots (Zim vs. Zre). Compared with Bi₂WO₆ and BiOBr, the semicircle diameter of 15% BiOBr/Bi₂WO₆ decreased, indicating an improved interfacial charge transfer efficiency and the effective separation of photogenerated charge carriers. The equivalent circuit (inset of Fig. 7b) of the devices was constructed to analyze the impedance spectra, and the impedance spectra were fitted using ZSimpWin software. As shown in the inset of Fig. 7b, R1 is the series resistance of the system. The R2//CPE1components represent the resistance of the semiconductor depletion (space charge) layer and the chemical capacitance. R3//CPE2 represent the charge transfer resistance in the Helmholtz layer (double layer) and the recharged Helmholtz layer. The arc radius in the EIS Nyquist plot of 15% BiOBr/Bi₂WO₆ is smaller than those of the Bi₂WO₆ and BiOBr samples, which means that the effective separation of photogenerated electron–hole pairs and fast interfacial charge transfer to the electron donor/electron acceptor occurred as suggested.⁴² This can be attributed to the 15% BiOBr/Bi₂WO₆ p–n heterojunction composites possessing improved interfacial charge transfer efficiencies and more effectively separating photogenerated charge carriers, in comparison with Bi₂WO₆ and BiOBr.

As shown in Fig. 7c, the characteristic frequency peaks of Bi₂WO₆, BiOBr and 15% BiOBr/Bi₂WO₆ were 17 800 Hz, 8250 Hz and 1780 Hz, respectively. The peak shift from high frequency to low frequency reveals a more rapid electron transport process, because the frequency (f) is closely related to the lifetime (τ) of the injected electrons as follows: τ ≈ 1/(2πf).⁶ From this equation, the electron lifetime of the 15% BiOBr/Bi₂WO₆ (89 μs) is estimated to be about 10 and 4.7 times greater than that of the pure Bi₂WO₆ sample (8.9 μs) and BiOBr (19 μs). The fast charge transfer could effectively suppress the charge recombination and improve the photo-conversion efficiency. The above results of the photoelectric properties were consistent, indicating that coupling the BiOBr nanoparticles and Bi₂WO₆ could effectively facilitate the separation of photogenerated electron–hole pairs and thus result in an enhanced photocatalytic performance.

3.2 Photocatalytic activity

MB was selected as a probe molecule to assess the photocatalytic activity of the various catalysts under visible light. Fig. 8a shows the temporal absorption spectral changes of MB in the absence of 15% BiOBr/Bi₂WO₆ p–n heterojunction photocatalysts. During the photodegradation process, the intensity of the absorption peak at about 664 nm decreased gradually, and about 96% of MB could be degraded after 120 min. Fig. 8b displays the photocatalytic activity of the different catalysts in the MB degradation. Control experiments show that no degradation of MB was detected in the absence of either photocatalyst or light irradiation, indicating that a photocatalytic reaction mechanism was responsible for MB degradation. Meanwhile, it was clearly observed that 15% BiOBr/Bi₂WO₆ composites exhibited enhanced photocatalytic activities relative to bare Bi₂WO₆ and BiOBr. The composite’s photocatalytic activity was about 1.7 and 2.4 times that of Bi₂WO₆ (62%) and BiOBr (43%), respectively, indicating that decorating the three-dimensional flower-like Bi₂WO₆ microspheres with BiOBr nanodots could greatly increase the photocatalytic activity, due to the improved transfer and separation efficiencies of the photogenerated charge carriers at the p–n junctions. To further investigate the heterojunction effect of the present BiOBr/Bi₂WO₆ system, the photodegradation of MB over a mechanically mixed 15% BiOBr + 75% Bi₂WO₆ sample was also performed, as shown in Fig. 8b. The obtained activity (75%) of the mechanically mixed sample is much lower than that of 15% BiOBr/BiPO₄. This can be ascribed to the significant difference in the interfaces of the samples, as the heterojunction photocatalyst shows interfaces with close contact, whereas only a diffuse interface can be formed after a simple mixing process.

Fig. 8 (a) Changes in the temporal UV-vis absorption spectrum during the photodegradation of MB in aqueous solution in the presence of 15% BiOBr/Bi₂WO₆; (b) degradation rate of MB under different conditions with BiOBr, Bi₂WO₆ and 15% BiOBr/Bi₂WO₆.

The photocatalytic degradation kinetics were investigated by the first-order simplification of Langmuir–Hinshelwood (L–H) kinetics, which is well established for photocatalysis at low initial pollutant concentrations.⁴³ The relevant equation is as follows:

where C₀ and C are the concentrations of dye in solution at times 0 and t, respectively, and k_app is the apparent first-order rate constant (min⁻¹). The k value is obtained from the gradient of the graph of ln(C/C₀) versus time (t), as shown in Fig. 9b. The plots of irradiation time (t) versus the ln(C/C₀) displayed nearly linear trends for various amounts of BiOBr content (5–25%) and the loading amounts were observed to strongly influence the activity obtained. The observed activity increased with increasing content up to 15%, exhibiting the highest apparent rate constant of about 0.026 min⁻¹. This was about 5.9 and 3.5 times higher than the values for BiOBr and Bi₂WO₆ (k_app of 0.0044 min⁻¹ and 0.0074 min⁻¹, respectively), but beyond this value the photocatalytic activity decreased. This behavior was thought to be due to the introduction of BiOBr nanoparticles, which could greatly facilitate the migration of photogenerated electron–holes specifically at the p–n junctions. A suitable BiOBr content was necessary to result in a fine particle dispersion on the surface of the flower-like Bi₂WO₆ and the suitable contact junctions. To increase the BiOBr nanodots content would result in a degree of agglomeration and cover the surface of Bi₂WO₆. In addition, overloading with BiOBr would create an unsuitable ratio and decrease the p–n junctions in the BiOBr and Bi₂WO₆ heterostructure. Higher BiOBr deposits could also conversely behave as recombination centers, encouraging the recombination of charge carriers,⁴⁴ which is unfavorable for the composite’s photocatalytic activity. Therefore, an optimal BiOBr loading amount exists, and the 15% BiOBr/Bi₂WO₆ p–n heterojunctions composite was thought to possess effective contact between its components, resulting in the efficient separation of electrons and holes and the best photocatalytic activity.

Fig. 9 (a) First-order rate constant of Bi₂WO₆, BiOBr/Bi₂WO₆ with varying BiOBr content, and BiOBr catalysts, (b) linear transform ln(C/C₀) = f(t) of the MB degradation kinetics curves.

In order to further study the photocatalytic performance of the 15% BiOBr/Bi₂WO₆ composite, phenol, a colorless organic compound, was chosen as a representative model pollutant. For the photocatalysis experiments, 0.2 g of the photocatalyst was dispersed into a 100 mL phenol (15 mg L⁻¹) solution. As seen in Fig. 10, the blank and dark controls indicated that the direct photolysis of phenol could almost be neglected. It was clearly observed that the 15% BiOBr/Bi₂WO₆ p–n heterostructure also exhibited a higher photocatalytic activity and about 50% of phenol could be degraded under visible light irradiation for 6 h, which was 3.6 and 2.3 times more than was degraded by BiOBr and Bi₂WO₆. All of these results confirmed that the BiOBr/Bi₂WO₆ p–n heterojunction composite was an efficient photocatalyst for the degradation of organic contaminants.

Fig. 10 Photocatalytic degradation of phenol over BiOBr, Bi₂WO₆ and 15% BiOBr/Bi₂WO₆ samples under visible light irradiation.

Catalyst stability and reusability are also very important from the viewpoint of the catalyst’s practical applications. The photocatalytic activity of the 15% BiOBr/Bi₂WO₆ composites in the degradation of MB was studied in consecutive cycles under the same conditions (Fig. 11). It can be seen from the results obtained, that the photocatalytic activity decreases slightly over the five consecutive cycles and the degradation in the final run was ∼85%. Therefore, the BiOBr nanoparticles decorated Bi₂WO₆ photocatalyst could see use in environmental protection due to its stable and high performance.

Fig. 11 Cycling runs for the photocatalytic degradation of MB in the presence of 15% BiOBr/Bi₂WO₆.

In order to reveal the photocatalytic mechanism of the 15% BiOBr/Bi₂WO₆ p–n heterojunction composites in MB photodegradation, an investigation of the major active species involved in the process could be carried out via radicals and holes trapping experiments (Fig. 12).⁴⁵ In the reaction system, isopropanol (IPA) was used as a scavenger of radicals,⁴⁶ and EDTA-2Na was used as a holes scavenger.⁴⁷ Fig. 12a presents the photocatalytic activity of the 15% BiOBr/Bi₂WO₆ composite without the addition of any scavengers. The photocatalytic activity was greatly suppressed by the addition of IPA and EDTA-2Na (as seen in Fig. 12b and c). Fig. 12d clearly displays the changes in the apparent rate constant with various scavengers under visible light, indicating that the introduction both of EDTA and IPA had a significant effect in decreasing k_app compared with the runs performed in the absence of scavengers, suggesting that photogenerated holes and radicals were all of the oxidative species in the BiOBr/Bi₂WO₆ composites system.

Fig. 12 Photocatalytic degradation of MB with the 15% BiOBr/Bi₂WO₆ composite in the presence of various scavengers under visible light; (a) no scavenger, (b) IPA, (c) EDTA-2Na, (d) comparison of the visible light photocatalytic activity obtained using the rate constant k_app, inset: linear transform ln(C/C₀) = f(t) of the MB degradation kinetics curves.

It is generally accepted that the photocatalytic activity is mainly governed by the generation, transfer and separation of photogenerated electron–hole pairs. In the 15% BiOBr/Bi₂WO₆ architecture, the p–n heterojunction formed in the BiOBr/Bi₂WO₆ composite and played a major role in accelerating the separation of photogenerated charge carriers. In order to understand the formation of the p-BiOBr/n-Bi₂WO₆ heterojunction clearly, the band edge positions of BiOBr and Bi₂WO₆ were estimated on the basis of the following empirical formulas:⁴⁸

E_VB = X − E^e + 0.5E_g

E_CB = E_VB − E_g

where E_VB is the valence band (VB) edge potential, E_CB is the conduction band (CB) edge potential, and X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms (6.18 eV for BiOBr and 6.36 eV for Bi₂WO₆). E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV), and E_g is the band gap energy of the semiconductor. Based on the band gap positions, the E_VB values of BiOBr and Bi₂WO₆ were calculated to be 3.10 and 3.26 eV, and their homologous E_CB values were estimated to be 0.25 and 0.46 eV, respectively.

The energy band structure diagram is shown in Fig. 13, the band structure of BiOBr and Bi₂WO₆ is a well-coupled band structure (as seen in Fig. 13a), which favors the separation of photogenerated carriers. Meanwhile, when p-type BiOBr nanodots were coupled to the surface of n-type Bi₂WO₆, a novel p–n heterojunction would be formed. According to the general p–n junctions formation process reported in the literature,^49,50 the Fermi level of p-type BiOBr moved up, while that of n-type Bi₂WO₆ moved down until the equilibrium state was formed after contact, as presented in Fig. 13b. Under visible light illumination, both BiOBr and Bi₂WO₆ could be excited to generate electron–holes. Due to the role of the inner electric field between BiOBr and Bi₂WO₆, the photogenerated electrons on the CB bottom of p-BiOBr efficiently transferred to that of n-Bi₂WO₆, while the photogenerated holes simultaneously effectively migrated from n-Bi₂WO₆ to p-BiOBr. The transfer of photogenerated carriers could be promoted through the p–n heterojunction interface between BiOBr and Bi₂WO₆, leading to an improvement in the photocatalytic activity, in accordance with the results of the photocurrent and EIS measurements. Furthermore, the photogenerated electron in the CB of Bi₂WO₆ could be captured by adsorbed O₂ to generate reactive ·O₂⁻ radicals, which could break down the organic contaminant.^19,22 Meanwhile, the photogenerated holes could serve as active sites responsible for organic contaminant photocatalytic degradation.²² Therefore, the photocatalytic process of the BiOBr/Bi₂WO₆ p–n heterojunction composite was mainly associated with the ·O₂⁻ radicals and the photogenerated holes, in accord with the results of the radical scavenger experiments. In a word, the efficient separation of the photogenerated charge carriers achieved in the p–n junctions between BiOBr and Bi₂WO₆, and the participation of the ·O₂⁻ radicals and photogenerated holes caused the BiOBr/Bi₂WO₆ composites to possess increased photocatalytic activity.

Fig. 13 Schematic diagrams for (a) the energy bands of BiOBr and Bi₂WO₆ before contact and (b) the formation of a junction and transfer of photogenerated electrons from BiOBr to Bi₂WO₆ under visible-light irradiation.

4. Conclusions

In summary, BiOBr nanodots decorated flower-like Bi₂WO₆ p–n heterojunction photocatalysts were successfully synthesized through a two-step hydrothermal process and BiOBr/Bi₂WO₆ showed a lower charge-transfer resistance, higher photocurrent intensity and enhanced photoelectric properties. The enhanced photocatalytic activity is ascribed to its decoration with BiOBr nanodots and the formation of the p–n heterojunction between BiOBr and Bi₂WO₆, which could greatly accelerate the separation of photogenerated charge carriers at the p–n junctions, as confirmed by the photocurrent and EIS measurements. Additionally, BiOBr/Bi₂WO₆ showed high stability in the photodegradation process, indicating that the BiOBr nanodots/Bi₂WO₆ p–n heterojunction photocatalysts could be promising candidate catalysts for poisonous wastewater treatment.

Acknowledgements

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

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00794e

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