One-pot synthesis of Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures and their photocatalytic properties

Abstract

Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunction photocatalysts were successfully prepared by a facile, one-pot solvothermal method. In the obtained heterojunctions, Bi₄V₂O₁₁ nanoparticles were uniformly immobilized on or inlaid onto the surfaces of the Bi₂₄O₃₁Br₁₀ nanosheets. They exhibited superior visible light photocatalytic activity towards the degradation of rhodamine B (RhB). Among them, the 25% Bi₄V₂O₁₁ sample possessed the highest photocatalytic activity, the degradation rate of which was 4 and 6 times as fast as that of bare Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁, respectively. The enhanced photocatalytic activity could be attributed to the effective separation and transfer of photogenerated carriers. The trapping experiments confirmed that the photogenerated holes and ˙O₂⁻ radicals were the main active species responsible for the photodegradation of RhB.

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1. Introduction

In the past few decades, semiconductor photocatalysts have attracted substantial attention due to their potential applications in environmental remediation.^1,2 TiO₂ was once believed to be a powerful photocatalyst owing to its low cost, strong oxidizing power, non-toxic nature and environmentally friendly features.³ However, TiO₂ can only be activated by UV light due to its wide band gap, which accounts for less than 5% of solar energy compared with 43% for the visible light.⁴ In order to use the solar energy more effectively, many visible light driven photocatalysts have been developed, such as Bi₂WO₆,^5,6 BiVO₄,⁷ NaTaO₃,⁸ YFeO₃,⁹ BiOX (X = Cl, Br and I),^10–12 Bi₂₄Al₂O₃₉,¹³ Bi₁₂TiO₂₀ (ref. 14) and Bi₂MoO₆ (ref. 15).

Bi₂₄O₃₁Br₁₀, as one of the oxygen-rich bismuth oxyhalides, has been known to researchers.^16,17 It has been reported that Bi₂₄O₃₁Br₁₀ possesses a narrower energy band gap compared with the original BiOBr, which enables it to absorb more visible light.¹⁸ Xiao et al. prepared hierarchical Bi₂₄O₃₁Br₁₀ nanoflakes by a microwave heating route and investigated its photocatalytic activity. The results revealed that, in comparison with BiOBr, Bi₂₄O₃₁Br₁₀ possesses higher photocatalytic efficiency.¹⁹ Nevertheless, the photocatalytic activity of Bi₂₄O₃₁Br₁₀ is still far from efficient for practical application and requires further improvement. Construction of Bi₂₄O₃₁Br₁₀-contained heterostructures may be a promising method for developing highly efficient photocatalysts. Up to now, Bi₂₄O₃₁Br₁₀/BiOBr heterostructure photocatalysts have been developed by a controlled calcination of BiOBr. Similar Bi₂₄O₃₁Cl₁₀/BiOCl heterostructure photocatalysts have also been reported, both of which display enhanced photocatalytic activity.^20,21 Recently, another Bi-contained photocatalyst, Bi₄V₂O₁₁, has been widely studied because of its peculiar structural and optical properties.^22,23 Based on the estimated energy band positions, we speculate that the coupling of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ can form type II semiconductor heterostructures, which helps the separation and transfer of photogenerated electron–hole pairs, favoring the enhancement of the photocatalytic activity.

In this paper, the novel Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures were constructed and prepared via a facile, one-pot solvothermal method for the first time. The structural and optical properties of the products were characterized by XRD, FESEM, TEM, XRF, XPS, FT-IR, BET, UV-Vis DRS, EIS and PL. The photocatalytic activities were evaluated by photocatalytic degradation of RhB under visible light irradiation. The results show that Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures exhibit markedly enhanced photocatalytic performance, whose mechanism is investigated and discussed in detail.

2. Experimental

2.1. Preparation of photocatalysts

All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and they were used as received without further purification. Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures were synthesized via a facile one-pot solvothermal method. In a typical process, 2 mmol of Bi(NO₃)₃·5H₂O was first dissolved into 20 mL of ethylene glycol (EG). Second, another 15 mL EG solution containing appropriate stoichiometric amount of cetyltriethylammonium bromide (CTAB) and NH₄VO₃ was added into the above solution and stirred until a transparent solution was obtained. Third, certain volume of 2 M NaOH solution was added dropwise until the pH value of the solution was 8.0. The resultant precursor solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. Finally, the autoclave was sealed and maintained at 160 °C for 16 h and allowed to gradually cool to room temperature. The precipitate was washed three times with absolute ethanol and distilled water, respectively, and dried at 80 °C in air. Based on the abovementioned process, Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures with the Bi₄V₂O₁₁ contents of 12.5 at%, 25 at% and 37.5 at% were obtained, which were named as 12.5% Bi₄V₂O₁₁, 25% Bi₄V₂O₁₁ and 37.5% Bi₄V₂O₁₁, respectively. In addition, pure Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ were also prepared for comparison.

2.2. Characterization

The composition and structure of as-prepared samples were examined by means of X-ray diffraction (XRD, Bruker D8 Advance with Cu Kα radiation at 40 kV and 30 mA). The morphology was investigated by using a field emission scanning electron microscope (SEM, Hitachi S-4800) equipped with an energy dispersive X-ray spectrometer (EDS) and a transmission electron microscope (TEM, Philips, Tecnai 12). X-ray fluorescence (XRF) spectroscopy analysis was conducted on a Bruker S8 Tiger instrument. FT-IR spectra were recorded by using a Vertex70 FTIR spectrometer. The surface properties of the samples were examined by X-ray photoelectron spectroscopy (XPS) with Al Kα X-ray (hν = 1486.6 eV) irradiation operating at 150 W (XPS: Thermo ESCALAB250, USA). The optical property was analyzed by both UV-vis diffuse reflectance spectra (DRS, Varian Cary 300) and photoluminescence spectra (PL, Varian Cary-Eclipse 500). Electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation (CHI 660B Chenhua Instrument Company, Shanghai, China).

2.3. Photocatalytic and electrochemical performances

The photocatalytic activity of as-prepared samples was determined by the degradation of Rhodamine B (RhB) under visible light irradiation. The visible-light source was a 150 W tungsten-halogen lamp (Beijing Institute of Opto-Electronic Technology, light intensity = 200 mW cm⁻²). The short-wavelength components (λ < 400 nm) of the light were cut off by using a cutoff glass filter. Experiments were carried out at 20 ± 3 °C as follows: 0.1 g of photocatalyst was added into 100 mL of 15 mg L⁻¹ RhB solution. The distance between the bottom of the lamp and the top of the solution was 10 cm. Before irradiation, the suspension was stirred for 20 min in darkness to reach the adsorption–desorption equilibrium. At given irradiation time intervals, approximate suspensions of 3 mL were taken and centrifuged to remove the catalyst particles. The concentration of remnant RhB was determined by UV-vis spectroscopy at its characteristic wavelength of 550 nm.

EIS measurements were performed on a CHI 660D electrochemical workstation (Shanghai Chenhua, China) with a standard three-electrode configuration. The counter electrode was a platinum wire. The reference electrode was a saturated Ag/AgCl electrode. The working electrodes were prepared by dip-coating process. To accomplish this, 20 mg of photocatalyst was suspended in 5 mL ethanol to produce slurry, which was dip-coated onto a 1.5 × 1.5 cm fluorine-tin oxide (FTO) glass electrode. After the films were dried under ambient conditions, they were sintered at 350 °C for 2 h. The electrolyte was 0.5 M Na₂SO₄ aqueous solution. EIS were carried out at the open circuit potential, and the amplitude of the sinusoidal wave was 10 mV.

3. Results and discussion

3.1. Characterization

The crystal structure of the as-prepared samples was investigated by means of X-ray powder diffraction (XRD). As shown in Fig. 1, the diffraction peaks of pure Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ samples are in good agreement with the monoclinic phase Bi₂₄O₃₁Br₁₀ (JCPDS card no. 75-0888) and the orthogonal phase Bi₄V₂O₁₁ (JCPDS card no. 42-0135), respectively. Considering that the characteristic peaks of Bi₄V₂O₁₁ coincide with those of Bi₂₄O₃₁Br₁₀ to a larger extent, no special diffraction peaks for Bi₄V₂O₁₁ can be found. Nonetheless, the diffraction peaks of Bi₂₄O₃₁Br₁₀ (2θ = 24.1°, 42.8° and 50.4°) are found to be weaker in intensity with increasing Bi₄V₂O₁₁ content. When the Bi₄V₂O₁₁ content is 37.5 at%, some peaks (2θ = 24.1° and 42.8°) even disappear completely. The results may be attributed to the crystallinity of Bi₄V₂O₁₁ in Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures.

Fig. 1 XRD patterns of the as-synthesized samples.

FT-IR spectra of the Bi₂₄O₃₁Br₁₀, Bi₄V₂O₁₁ and 25% Bi₄V₂O₁₁ samples were measured and shown in Fig. 2. The absorption peak existing at 1390 cm⁻¹ is attributed to the bending vibrations of N–O, which may come from the raw material. The absorption peak at 419 cm⁻¹ is assigned to the Bi–O band, and the peak at 720 cm⁻¹ is related to the asymmetry and symmetric stretching vibration peaks of the Bi–Br band in the Bi₂₄O₃₁Br₁₀ structure.^24,25 The peaks at 609 cm⁻¹ and 1030 cm⁻¹ can be assigned to the vibrations of V–O–V and VO band, respectively. The absorption at about 878 cm⁻¹ is believed to be associated with the coupled vibration between V–O–V and VO.^26,27 It can be seen from Fig. 2 that the 25% Bi₄V₂O₁₁ sample possesses similar FT-IR spectra to that of pure Bi₂₄O₃₁Br₁₀. The difference is that the peak at 1030 cm⁻¹ can also be found in the spectrum of 25% Bi₄V₂O₁₁ sample, which indicates the existence of Bi₄V₂O₁₁.

Fig. 2 FT-IR transmittance spectra of the as-prepared samples.

X-ray photoelectron spectroscopy (XPS) was carried out on the 25 at% Bi₄V₂O₁₁ sample to determine the surface compositions and chemical states of the elements. The shift of the peak position on the charge effect was calibrated by using the binding energy of C1s at 284.78 eV. The survey XPS spectra of 25 at% Bi₄V₂O₁₁ sample are shown in Fig. 3a. The V2p peak around 516.0 eV can be detected in 25 at% Bi₄V₂O₁₁. Fig. 3b–e show high-resolution XPS spectra of the primary elements. Two peaks at 159.16 eV and 164.48 eV (Fig. 3b) are assigned to Bi4f_7/2 and Bi4f_5/2, respectively, which are assigned to Bi³⁺ in the composites.²⁸ The asymmetric XPS peak of O1s (Fig. 3c) indicates that oxygen species are present in the form of lattice oxygen and hydroxyl groups adhered onto the surface.²⁹ The Br3d_5/2 and Br3d_3/2 peaks are associated with the binding energies at 68.63 and 69.57 eV (Fig. 3d), respectively. The binding energies of V2p_3/2 and V2p_1/2 are 516.72 and 524.38 eV (Fig. 3e), which can be characteristic of the V species in Bi₄V₂O₁₁. Therefore, it may be concluded that Bi₄V₂O₁₁/Bi₂₄O₃₁Br₁₀ heterostructure photocatalysts have been successfully synthesized.

Fig. 3 XPS spectra of the 25 at% Bi₄V₂O₁₁ sample.

The morphologies of the as-prepared samples were revealed by SEM, as shown in Fig. 4. It can be seen that pure Bi₂₄O₃₁Br₁₀ has an irregular hierarchical structure, which is further assembled by nanosheets with a thickness of ∼15 nm (Fig. 4a). On the contrary, Bi₄V₂O₁₁ consists of a number of monodispersed nanoparticles with sizes of about dozens of nanometers (Fig. 4e). After the coupling of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁, both 12.5 at% Bi₄V₂O₁₁ and 25 at% Bi₄V₂O₁₁ display a degraded hierarchical structure, in which Bi₄V₂O₁₁ nanoparticles are found to firmly combine with Bi₂₄O₃₁Br₁₀ nanosheets (Fig. 4b and c). However, the 37.5 at% Bi₄V₂O₁₁ sample shows the conglomeration of nanoparticles, and there are no Bi₂₄O₃₁Br₁₀ nanosheets found in it (Fig. 4e), indicating that excess Bi₄V₂O₁₁ hinders the formation of Bi₂₄O₃₁Br₁₀ nanosheets. In addition, the EDS pattern (Fig. 4f) shows that the 25 at% Bi₄V₂O₁₁ sample contains the elements O, V, Br and Bi. This indirectly proves that the prepared sample is the composite of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁. In addition, the elemental analysis of 25% Bi₄V₂O₁₁ sample was carried out by XRF in order to estimate the quantity of Bi₄V₂O₁₁. The atomic fractions of V, Br and Bi are 1.76%, 20.54% and 77.70%, respectively. It is found that the real content of Bi₄V₂O₁₁ (29.9%) is slightly higher, which can be attributed to a preferential precipitation of Bi₄V₂O₁₁.

Fig. 4 SEM images of as-prepared samples: (a) Bi₂₄O₃₁Br₁₀, (b) 12.5 at% Bi₄V₂O₁₁, (c) 25 at% Bi₄V₂O₁₁, (d) 37.5 at% Bi₄V₂O₁₁, (e) Bi₄V₂O₁₁ and (f) EDS pattern of 25 at% Bi₄V₂O₁₁.

The Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructures were further characterized by TEM and HRTEM. Fig. 5a shows the TEM image of the 25 at% Bi₄V₂O₁₁ sample. The grey region with a flake shape is a Bi₂₄O₃₁Br₁₀ nanosheet, whereas the dark region resulted from the Bi₂₄O₃₁Br₁₀ nanoparticles, which is consistent with the SEM result. To further investigate the structural information, the corresponding HRTEM image is shown in Fig. 5b. The (117) plane of Bi₂₄O₃₁Br₁₀ and (200) plane of Bi₄V₂O₁₁ can be well identified based on the accurate measurement of lattice fringes, indicating that Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ have been completely coupled to form Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterostructure.

Fig. 5 (a) TEM and (b) HRTEM images for 25 at% Bi₄V₂O₁₁.

3.2. Optical property

3.2.1. DRS patterns. Fig. 6a shows the UV-vis DRS of the as-prepared samples. It can be seen that all the samples display strong absorption at wavelengths lower than 440–500 nm, indicating the possibility of photocatalytic activity under visible light irradiation. The optical band-gap energy (E_g) can be estimated by extrapolating the straight portion of (αhν)^1/2 against an hν plot to α = 0, as shown in Fig. 6b.²⁹ The E_g values are about 2.52 eV, 2.45 eV, 2.40 eV, 2.33 eV and 2.25 eV for Bi₂₄O₃₁Br₁₀, 12.5% Bi₄V₂O₁₁, 25% Bi₄V₂O₁₁, 37.5% Bi₄V₂O₁₁ and Bi₄V₂O₁₁ sample, respectively. Among these, the E_g values of Bi₂₄O₃₁Br₁₀ and BiOBr are very close to those reported in the literature.^19,23

Fig. 6 (a) UV-vis DRS spectra and (b) the band gap energies of the as-prepared samples.

3.2.2. Photocatalytic degradation. The photocatalytic activity of as-prepared samples was evaluated by the degradation of RhB under visible light irradiation. Fig. 7 displays the temporal evolution of UV-vis absorption spectra of the RhB solution in the presence of 25% Bi₄V₂O₁₁. It can be seen that the intensity of the characteristic absorption peak related to RhB decreases evidently, accompanied by a slight blue shift of the maximum absorption from 550 to 498 nm. The color of the RhB solution also changes correspondingly from red to light green and finally colorless, as shown in the inset of Fig. 7. The experimental results reveal that the degradation of RhB over 25% Bi₄V₂O₁₁ undergoes two processes under visible light irradiation, i.e. the de-ethylation process in a stepwise manner and then the destruction of a conjugated structure. A similar degradation process of RhB has been reported in the RhB/TiO₂ and RhB/Bi₂WO₆ systems.^30,31

Fig. 7 Spectral changes during the degradation of RhB over 25% Bi₄V₂O₁₁ sample.

To further investigate the effects of Bi₄V₂O₁₁/Bi₂₄O₃₁Br₁₀ heterostructures on photocatalytic activity, the photocatalytic degradation experiments of RhB were carried out with Bi₂₄O₃₁Br₁₀, 12.5% Bi₄V₂O₁₁, 25% Bi₄V₂O₁₁, 37.5% Bi₄V₂O₁₁ and Bi₄V₂O₁₁ as the photocatalysts. The experimental results are shown in Fig. 8a. It can be seen that no RhB degradation can be detected in the absence of a catalyst, suggesting that photolysis of RhB can be ignored under visible light. After 120 min of visible light irradiation, RhB with different photocatalysts is degraded in varying degrees. The degradation rates are 76.6%, 98.3%, 99.4%, 91.0% and 63.2% for Bi₂₄O₃₁Br₁₀, 12.5% Bi₄V₂O₁₁, 25% Bi₄V₂O₁₁, 37.5% Bi₄V₂O₁₁ and Bi₄V₂O₁₁, respectively. Clearly, Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunction composites possess higher photocatalytic activity than bare Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁. To quantify the photocatalytic ability of these samples, the apparent pseudo-first-order model, ln(C₀/C) = kt, is used to describe the photocatalytic reaction, where t is the irradiation time and k is the rate constant. As shown in Fig. 8b, ln(C/C₀) exhibits a good linear relationship with the irradiation time, indicating that the photocatalytic reaction belongs to the pseudo-first-order reaction. The reaction rate constants are also concluded in Fig. 8b. This suggests that the Bi₄V₂O₁₁ content has a significant effect on the photocatalytic performance of Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions. The 25% Bi₄V₂O₁₁ sample displays an optimal photocatalytic performance, whose rate constant is 4 and 6 times as fast as that of pure Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁, respectively. Therefore, it can be concluded that the coupling of Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ is highly beneficial for improving photocatalytic activity.

Fig. 8 (a) Photocatalytic degradation of RhB in the presence of various photocatalysts, (b) degradation kinetics of RhB.

3.2.3. Mechanism of the enhanced photocatalytic activity. It is well known that photocatalytic activity is largely affected by photoinduced charge transfer and separation, which determine the final quantum yield. To understand the effect of the coupling of Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ on charge transfer resistance, EIS measurements were carried out over Bi₂₄O₃₁Br₁₀ and 25% Bi₄V₂O₁₁. The resultant EIS Nyquist plots are shown in Fig. 9a. Compared with Bi₂₄O₃₁Br₁₀, the 25% Bi₄V₂O₁₁ sample exhibits a smaller arc radius in the EIS Nyquist plot, which indicates a faster interfacial charge-transfer process. This indicates that the coupling of Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ can remarkably enhance interfacial charge transfer efficiency of photogenerated electron–hole pairs, which then contributes to the enhancement of photocatalytic activity. In addition, the effective separation of photogenerated carriers is also beneficial to the enhancement of photocatalytic activity. The photoluminescence (PL) emission spectra can be used to evaluate the separation capability of the photoinduced carriers. The PL signal results from the radiative recombination process of self-trapped excitations. The lower the PL intensity, the smaller is the probability that the photogenerated electron–hole pairs recombine. The PL emission spectra of the as-prepared samples are shown in Fig. 9b. When excited by a short wavelength light of 250 nm, the shape and position of the resultant PL spectra look similar, and two emission peaks at around 420 and 484 nm can be observed for all of the samples. In contrast, the emission intensity of the PL signal for Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions is evidently lower than that for bare Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁. This suggests that the recombination of photogenerated charge carriers is effectively inhibited. Therefore, the coupling of Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ is highly favorable for the effective separation of photogenerated carriers during the photocatalytic reaction.

Fig. 9 (a) Electrochemical impedance spectroscopy of Bi₂₄O₃₁Br₁₀ and 25% Bi₄V₂O₁₁; and (b) PL spectra of the as-prepared samples.

Based on the abovementioned results, it is evident that the enhanced activity of Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions can be attributed to the high efficiency of charge separation and electron transfer processes. In order to understand the photoinduced charge transfer and separation process in detail, the potentials of the conduction band (CB) and valence band (VB) edges of Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ were evaluated based on the Mulliken electronegativity theory,³² whose equations are listed as follows:

E_VB = X − E₀ + 0.5E_g (1)

E_CB = E_VB − E_g (2)

where E_VB is the VB edge potential, and X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The X values for Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ are calculated as 6.18 eV and 6.33 eV, respectively. E₀ 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. According to eqn (1) and (2), the E_VB and E_CB values of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ are calculated to be 3.09 and 0.57 eV and 2.78 and 0.53 eV, respectively. The E_CB of Bi₄V₂O₁₁ is more negative than that of Bi₂₄O₃₁Br₁₀, and E_VB of Bi₂₄O₃₁Br₁₀ is more positive than that of Bi₄V₂O₁₁. The calculated results indicate that the band potentials of Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀ take on a staggered-band alignment, and an energy bias is naturally generated on the contact interface between Bi₄V₂O₁₁ and Bi₂₄O₃₁Br₁₀, which is advantageous for the separation and transportation of charge carriers.³³ As shown in Fig. 10, under visible light irradiation, both Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ can be activated by visible light due to their narrow band gap energies. The local electrical field at the Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ interface pushes the photogenerated electrons toward the CB of Bi₂₄O₃₁Br₁₀, and the holes on the VB of Bi₂₄O₃₁Br₁₀ toward that of Bi₄V₂O₁₁ at the same time. In such a way, the photoinduced electrons and holes can be efficiently separated, and the recombination of electron−hole pairs is hindered, which is consistent with the result of the PL and EIS. Therefore, higher photocatalytic activity of Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions can be mostly ascribed to the efficient separation of photogenerated carriers.

Fig. 10 Schematic diagrams of band configuration and the charge separation process for Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions.

3.2.4. Main active species. To make the reaction mechanism clear, it is necessary to detect main active oxidative species during the photocatalytic reaction. For this purpose, isopropanol (IPA),³⁴ benzoquinone (BQ) and triethanolamine (TEOA)³⁵ were used as the scavengers of hydroxyl radicals (˙OH), superoxide radicals (˙O₂⁻) and active holes (h⁺), respectively, to examine the effects of reactive species on the photodegradation rate of RhB. The experimental results are shown in Fig. 11. It can be seen that the photocatalytic degradation of RhB is significantly suppressed with the addition of BQ and TEOA, whereas IPA has a negative effect on the photodegradation rate of RhB. Therefore, it can be concluded that h⁺ and ˙O₂⁻, instead of ˙OH, are the main active species of Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions for RhB degradation under visible light irradiation, which is consistent with the previously reported data.¹⁹ In fact, the abovementioned experimental results can also be explained from a theoretical viewpoint. First, it has been widely accepted that ˙OH radicals cannot be generated in the Bi-based photocatalyst system. The standard redox potential of BiV/BiIII is more negative than that of ˙OH/H₂O (+2.27 eV)³⁶ and ˙OH/OH⁻ (+1.99 eV);³⁷ thus, the photogenerated holes on the surface of Bi-based photocatalysts cannot react with H₂O or OH⁻ to form ˙OH. Second, the maximum photon energy of the visible light used in this research is equivalent to 3.1 eV, which is larger than the E_g values of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁. Therefore, the electrons can be excited from the VB of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁ to a higher potential edge of about −0.01 eV and −0.32 eV, respectively. The reformed CB potential of Bi₄V₂O₁₁ is more negative than E(O₂/˙O₂⁻)(−0.28 eV). Therefore, ˙O₂⁻ radicals can be formed from the photogenerated electrons on the new CB of Bi₄V₂O₁₁, which then contribute to the photocatalytic degradation of RhB. Finally, the photogenerated holes often possess very strong oxidizing ability. It has been reported that the photogenerated holes can directly participate in the photodegradation of RhB.³⁸ Based on the abovementioned experiments and discussion, it can be seen that h⁺ and ˙O₂⁻ radicals are the main active species responsible for the photocatalytic degradation of RhB over Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions.

Fig. 11 Degradation rate of RhB in the presence of various radical scavengers with 40 min of irradiation time.

4. Conclusions

Bi₂₄O₃₁Br₁₀/Bi₄V₂O₁₁ heterojunctions have been successfully prepared by a facile, one-pot solvothermal method. They have been investigated to be highly active photocatalysts for the photodegradation of RhB under visible light irradiation. Among them, the 25% Bi₄V₂O₁₁ sample displays the highest degradation efficiency, the reaction rate of which is about 4 and 6 times as fast as that of Bi₂₄O₃₁Br₁₀ and Bi₄V₂O₁₁, respectively. Based on the EIS and PL results, the enhanced photocatalytic activity is attributed to the effective separation and transfer of the photogenerated carriers. In addition, photogenerated holes and ˙O₂⁻ radicals are believed to be the main active species responsible for photocatalytic degradation.

Acknowledgements

This work was supported by the National Natural Science Fund (51304198). We would like to thank the Advanced Analysis and Calculation Center of CUMT for their support.

Notes and references

1. A. Kubacka, M. Fernández García and G. Colón, Chem. Rev., 2012, 112, 1555–1614.
2. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
3. S. A. Fujishima, X. Zhang and D. A. Tryk, Surf. Sci. Rep., 2008, 63, 515–582.
4. Z. Jiang, F. Yang, N. Luo, B. T. T. Chu, D. Sun, H. Shi, T. Xiao and P. P. Edwards, Chem. Commun., 2008, 6372–6374.
5. S. C Zhang, C. Zhang, Y. Man and Y. F. Zhu, J. Solid State Chem., 2006, 179, 62–69.
6. Z. J. Zhang, W. Z. Wang, J. Xu, M. Shang, J. Ren and S. M. Sun, Catal. Commun., 2011, 13, 31–34.
7. S. Eda, M. Fujishima and H. Tada, Appl. Catal., B, 2012, 125, 288–293.
8. D. R. Liu, Y. S. Jiang and G. M. Gao, Chemosphere, 2011, 83, 1546–1552.
9. Y. W. Zhang, J. X. Yang, J. F. Xu, Q. Y. Gao and Z. L. Hong, Mater. Lett., 2012, 81, 1–4.
10. J. Y. Xiong, G. Cheng, G. F. Li, F. Qin and R. Chen, RSC Adv., 2011, 1, 15421553.
11. M. Shang, W. Z. Wang and L. Zhang, J. Hazard. Mater., 2009, 167, 803–809.
12. J. X. Xia, S. Yin, H. M. Li, H. Xu, L. Xu and Q. Zhang, Colloids Surf., A, 2011, 387, 23–28.
13. Z. Wan, G. K. Zhang, J. T. Wang and Y. L. Zhang, RSC Adv., 2013, 3, 19617–19623.
14. J. G. Hou, Y. F. Qu, D. Krsmanovic, C. Ducati, D. Eder and R. V. Kumar, Chem. Commun., 2009, 3937–3939.
15. G. H. Tian, Y. J. Chen, W. Zhou, K. Pan, Y. Z. Dong, C. G. Tian and H. G. Fu, J. Mater. Chem., 2011, 21, 887–892.
16. U. Eggenweiler, E. Keller and V. Kramer, Acta Crystallogr., Sect. B: Struct. Sci., 2000, 56, 431–437.
17. J. W. Wang and Y. D. Li, Chem. Commun., 2003, 2320–2321.
18. X. Xiao, C. Liu, R. P. Hu, X. X. Zuo, J. M. Nan, L. S. Li and L. S. Wang, J. Mater. Chem., 2012, 22, 22840–22843.
19. X. Xiao, R. P. Hu, C. Liu, C. L. Xing, X. X. Zuo, J. M. Nan and L. S. Wang, Chem. Eng. J., 2013, 225, 790–797.
20. C. Yu, W. Zhou, J. Yu, F. Cao and X. Li, Chin. J. Chem., 2012, 30, 721–726.
21. F. T. Li, Q. Wang, X. J. Wang, B. Li, Y. J. Hao, R. H. Liu and D. S. Zhao, Appl. Catal., B, 2014, 150–151, 574–584.
22. V. Thakral and S. Uma, Mater. Res. Bull., 2010, 45, 1250–1254.
23. X. F. Chen, J. B. Liu, H. Wang, Y. L. Ding, Y. X. Sun and H. Yan, J. Phys. Chem. B, 2013, 1, 877–883.
24. V. P. Olstoy and E. V. Telstobrov, Solid State Ionics, 2002, 151, 165–169.
25. G. Cheng, J. Y. Xiong and F. J. Stadler, New J. Chem., 2013, 37, 3207–3213.
26. C. Wen, Q. M. Li and F. P. Jun, J. Mater. Sci., 2004, 39, 2625–2627.
27. F. Prinetto and G. Ghiotti, J. Phys. Chem. B, 1998, 102, 10316–10325.
28. L. Chen, Q. Zhang, R. Huang, S. F. Yin, S. L. Luo and C. T. Au, Dalton Trans., 2012, 41, 9513–9518.
29. X. Zhang, F. Jia and L. Zhang, J. Phys. Chem. C, 2008, 112, 747–753.
30. T. X. Wu, H. Hidaka and N. Serpone, J. Phys. Chem. B, 1998, 102, 5845–5851.
31. X. F. Cao, L. Zhang, X. T. Chen and Z. L. Xue, CrystEngComm, 2011, 13, 306–311.
32. X. P. Lin, J. C. Xing, W. D. Wang, Z. C. Shan, F. F. Xu and F. Q. Huang, J. Phys. Chem. C, 2007, 111, 18288–18293.
33. X. Zong, H. Yan, G. Wu, G. Ma, F. Wen and L. Wang, J. Am. Chem. Soc., 2008, 130, 7176–7177.
34. X. C. Zhang, T. Y. Guo, X. W. Wang, Y. W. Wang, C. M. Fan and H. Zhang, Appl. Catal., B, 2014, 150–151, 486–495.
35. L. Q. Ye, J. Y. Liu, Z. Jiang, T. Y. Peng and L. Zan, Appl. Catal., B, 2013, 142–143, 1–7.
36. A. Fujishima and X. T. Zhang, C. R. Chim., 2006, 9, 750–760.
37. S. Kim and W. Choi, Environ. Sci. Technol., 2002, 36, 2019–2025.
38. Y. Fu, C. Chang, P. Chen, X. L. Chu and L. Y. Zhu, J. Hazard. Mater., 2013, 254–255, 185–192.

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