Facile synthesis of Bi₂MoO₆–MIL-100(Fe) metal–organic framework composites with enhanced photocatalytic performance

Abstract

A novel Bi₂MoO₆–MIL-100(Fe) heterostructure photocatalyst was synthesized by a facile solvothermal method for the first time, and characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectra, transmission electron microscopy, and photoluminescence spectra. The as-prepared Bi₂MoO₆–MIL-100(Fe) composites exhibited excellent photocatalytic activity and stability towards the degradation of Rhodamine B (RhB) under visible light irradiation. The introduction of MIL-100(Fe) into semiconductor Bi₂MoO₆ improved the photocatalytic performance remarkably, and the optimal composite with 9% MIL-100(Fe) content showed the highest photocatalytic activity. According to the active radical trapping experiments, the h⁺ and ˙O₂⁻ radicals were the two main active species in the photocatalytic reaction. The enhanced photocatalytic performance is mainly attributed to the formation of the Bi₂MoO₆–MIL-100(Fe) heterostructure, which is beneficial to the transfer and separation of photo-generated electron–hole pairs. The possible mechanism of the enhanced photocatalytic performance of the Bi₂MoO₆–MIL-100(Fe) composites was discussed.

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1. Introduction

The semiconductor photocatalysis technique has attracted extensive attention in the splitting of water and the degradation of organic pollutants using sunlight as the energy source.¹ Among the various photocatalysts, TiO₂ is an attractive photocatalyst, because of its low toxicity, high photocatalytic activity and long term chemical stability.^2–4 However, the applications of TiO₂ are severely limited, owing to it only being excited by ultraviolet (UV) light which accounts for less than 5% of the solar spectrum. Therefore, many reports are focused on developing visible light responsive photocatalytic materials to replace TiO₂, such as Bi₂MoO₆,⁵ In₂O₃,⁶ Ag₃PO₄,⁷ and g-C₃N₄ (ref. 8) photocatalysts.

Bi₂MoO₆ is a typical Aurivillius oxide, exhibited excellent photocatalytic activity under the visible light irradiation.⁹ Compared with TiO₂, Bi₂MoO₆ has a lower band gap (2.5–2.8 eV), which makes it utilize more sunlight and displays remarkable photocatalytic performance for organic pollutant degradation under visible light irradiation.¹⁰ However, the practical application of the pure phase Bi₂MoO₆ is limited by its poor quantum yield, which is caused by the rapid recombination of photoinduced charge carriers. To further improve the photocatalytic activity of Bi₂MoO₆, many feasible methods have been explored, including designing textural properties, doping composites^11,12 and combining with other semiconductors,^13–15 carbon materials¹⁶ or noble metals.¹⁷ Nevertheless, there is still plenty of space to explore unfulfilled potentials for fabricating Bi₂MoO₆-based photocatalysts.

Metal–organic frameworks (MOFs) are an extensive class of crystalline materials formed by the self-assembly between metal ions and organic ligands.¹⁸ Recently, they have obtained extensive attention owing to their huge potential applications in sensing,¹⁹ gas storage,²⁰ drug delivery,²¹ optical properties²² and catalysis. These MOFs composites can act as catalysts, using unsaturated metal sites and catalytically active organic likers.^23,24 Additionally, MOFs exhibit tunable photo-physical and photo-chemical properties, it can work well as excellent candidates for photocatalysis. For example, Dan-Hardi et al. synthesized a new photoactive crystalline titanium dicarboxylate (MIL-125(Ti)), which exhibits high photonic sensitivity under UV-visible irradiation.²⁵ Hou Wang et al. had reported that In₂S₃@MIL-125(Ti) can remove the tetracycline by adsorption and photocatalysis. These reports showed that some MOFs can be applied in photocatalysis.²⁶ Compared with traditional semiconductor photocatalysts, the superiority of MOFs is their ultrahigh surface area and narrow micropore distribution which may lead to the formation of monodisperse photoactive species supported on MOFs.²⁷ Recently, several semiconductor@MOF heterostructures like CdS@MIL-100(Fe),²⁸ CdS@UiO-66(NH₂),¹⁸ ZnO@ZIF-8,²⁹ and GaN@ZIF-8 (ref. 30) have been synthesized and exhibited excellent catalytic performance due to their synergistic effect. However, the photocatalytic performance of Bi₂MoO₆–MOF composites, to the best of our knowledge, has not been investigated.

Herein, we report a novel Bi₂MoO₆–MIL-100(Fe) composite that shows excellent photocatalytic activity in degradation organic pollutants. A series of Bi₂MoO₆–MIL-100(Fe) composites with different weight addition ratios of MIL-100(Fe) have been successfully synthesized via a facile solvothermal method. The photocatalytic performance was evaluated by the degradation of Rhodamine B under visible light irradiation. The combination of MIL-100(Fe) remarkably improved the photostability and the photocatalytic efficiency of the photocatalyst for degradation RhB. Furthermore, the possible mechanism of the enhanced photocatalytic performance of the Bi₂MoO₆–MIL-100(Fe) composite was discussed.

2. Experimental

2.1 Materials

Bismuth nitrate pentahydrate, sodium molybdate dehydrate were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China), benzene-1,3,5-tricarboxylic acid (H₃btc) and Fe powder were purchased from Aladdin Industrial Corporation (Shanghai, China). Ethylene glycol (EG), and ethanol (EtOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were analytic grade and used without further purification.

2.2 Preparation of MIL-100(Fe)

According to the previous literature,^31,32 MIL-100(Fe) was prepared by solvothermal method. In detail, a mixture of Fe powder with H₃btc, HF, HNO₃ and H₂O with a molar ratio of 1 : 0.67 : 2 : 0.6 : 2.77 was synthesized in a Teflon-lined stainless steel autoclave at 150 °C for 24 h. Then the orange solid was centrifuged, washed with ethanol and hot water for several times, and finally dried overnight 60 °C under vacuum.

2.3 Synthesis of Bi₂MoO₆–MIL-100(Fe) composite

The as-prepared MIL-100(Fe) with the desired weight addition was dispersed in 10 mL ethanol by ultrasonication to gain the homogeneous MIL-100(Fe)–EtOH dispersion. Next, 2 mmol Bi(NO₃)₃·5H₂O and 1 mmol Na₂MoO₄·2H₂O dissolved in 20 mL ethylene glycol were dropwised into MIL-100(Fe)–EtOH dispersion. After being stirred for 30 min at room temperature, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 160 °C for 12 h. Finally, the obtain samples were collected, washed with deionized water and ethanol, dried overnight at 60 °C under vacuum. According to the method, as-synthesized composites with expected MIL-100(Fe) contents of 3, 6, 9 and 12% were synthesized and named as Bi₂MoO₆–MIL-100(Fe)-3, Bi₂MoO₆–MIL-100(Fe)-6, Bi₂MoO₆–MIL-100(Fe)-9 and Bi₂MoO₆–MIL-100(Fe)-12, respectively. For comparison, the Bi₂MoO₆–MIL-100(Fe) physical mixture composites were synthesized by simple mechanical mixture, and named as Bi₂MoO₆–MIL-100(Fe)-9(M). As a control, pure Bi₂MoO₆ was also prepared in ethanol under the same reaction conditions as that of Bi₂MoO₆–MIL-100(Fe) composites except for the addition of MIL-100(Fe).

2.4 Characterization

X-ray diffraction (XRD) patterns of all samples were carried out using an Empyrean diffractometer (PANalytical B.V., Netherlands), with Cu Kα radiation (λ = 1.54184 Å) in the 2θ range of 10–80°. The morphologies and microstructures of the products were characterized by a Merlin field emission scanning electron microscope (FE-SEM, Zeiss). Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained by FEI Tecnai G2 F20 microscope at an accelerating voltage of 200 kV. The Brunauer–Emmett–Teller (BET) surface area was examined by nitrogen adsorption–desorption isotherm at 77 K on Micromeritics Instrument 3Flex adsorption analyzer. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Escalb 250Xi spectrometer with a monochromatic Al (Kα) radiation. The UV-vis diffuse reflectance spectra (DRS) of samples were measured with Hitachi U-4100 UV-vis spectrophotometer. The photoluminescence (PL) spectra were tested with an FLS 920 FluorescencLe spectrometer at room temperature.

2.5 Photocatalytic evaluation

The photocatalytic activity of samples was evaluated by the degradation of Rhodamine B (RhB). In experiments, 0.1 g samples were added to 100 mL aqueous solution of RhB (10 mg L⁻¹) in a BL-GHX photochemical reactor (Shanghai Bilon Instrument. ltd. China). A 500 W Xe lamp was used as the visible-light source with 420 nm cut-off filter. Before irradiation, the suspension was kept in the dark under stirring for 30 min to ensure an adsorption–desorption equilibrium. After a given irradiation, about 4 mL suspension was collected, centrifuged and analyzed immediately. The RhB concentration were analyzed by a UV-vis spectrometer (Agilent 8453, USA) at 554. The TOC concentration was analyzed by Shimadzu TOC-LCPN analyzer. The degradation efficiency (%) was calculated as follows:where C₀ is the initial concentration of RhB, and C is the time-dependent concentration of RhB upon irradiation.

3. Results and discussion

3.1 XRD analyses

The crystal structures of the as-synthesized samples were examined by XRD patterns. Fig. 1 shows the XRD patterns of Bi₂MoO₆, and Bi₂MoO₆–MIL-100(Fe) composites with different contents of MIL-100(Fe). For comparison, the patterns of pure MIL-100(Fe), Bi₂MoO₆ as well as simulated MIL-100(Fe) are also shown. These distinct diffraction peaks at 2θ = 10.85, 28.2, 32.5, 46.7 and 55.5°, can be attributed to the (020), (131), (200), (062) and (133) crystal planes of pure Bi₂MoO₆ (JCPDS 76-2388). The diffraction peaks of pure MIL-100(Fe) sample can match with simulated MIL-100(Fe), the XRD result indicated the structure of pure MIL-100(Fe) sample is intact. For the Bi₂MoO₆–MIL-100(Fe) samples (Fig. 1d–g), all diffraction peaks indexed to the orthorhombic Bi₂MoO₆ phase are clearly observed. However, no peak in samples assigned to MIL-100(Fe) was observed, possibly due to the low content and high dispersion of MIL-100(Fe). In addition, no impurity was observed. With the increase of MIL-100(Fe) contents, the intensities of peaks for Bi₂MoO₆ get reduced.

Fig. 1 XRD patterns of (a) MIL-100(Fe), (b) MIL-100(Fe)-simulated, (c) Bi₂MoO₆, (d) Bi₂MoO₆–MIL-100(Fe)-3, (e) Bi₂MoO₆–MIL-100(Fe)-6, (f) Bi₂MoO₆–MIL-100(Fe)-9, (g) Bi₂MoO₆–MIL-100(Fe)-12.

3.2 Morphology

Fig. 2 shows the SEM images of Bi₂MoO₆, MIL-100(Fe) and their composites Bi₂MoO₆–MIL-100(Fe)-9, respectively. The rough microspheres composed of nanoparticles with dozens of nanometers are observed for Bi₂MoO₆ (Fig. 2a). And the diameter of the microsphere is 1–3 μm. In the Fig. 2b, the pure MIL-100(Fe) exhibited regular octahedron morphology with diameter ranging from 2 to 4 μm. As for the Bi₂MoO₆–MIL-100(Fe)-9 composite (Fig. 2c and d), it can be observed that MIL-100(Fe) serves as a support and the nanoparticles of Bi₂MoO₆ grow on the surface of MIL-100(Fe). Meanwhile, there is no aggregation of Bi₂MoO₆ nanoparticles on MIL-100(Fe), indicating that the interaction between Bi₂MoO₆ and MIL-100(Fe) is strong. Besides, the regular octahedron morphology of MIL-100(Fe) was not changed in the composite, the result demonstrated the structure of MIL-100(Fe) was not destroyed in the synthesized process of Bi₂MoO₆–MIL-100(Fe) composite. The Energy Dispersive Spectroscopy (EDS) patterns of Bi₂MoO₆–MIL-100(Fe)-9 composite revealed the existence of Bi, Mo, C, O and Fe. The tight interaction between the Bi₂MoO₆ and MIL-100(Fe) can enhance the photocatalytic activity.

Fig. 2 SEM images of (a) pure Bi₂MoO₆, (b) pure MIL-100(Fe), (c) and (d) Bi₂MoO₆–MIL-100(Fe)-9 composites, and (e) EDS patterns of the Bi₂MoO₆–MIL-100(Fe)-9 composites.

The TEM and HRTEM images of Bi₂MoO₆–MIL-100(Fe)-9 sample were shown in Fig. 3a and b, respectively. The TEM image shows the Bi₂MoO₆ nanoparticles on the MIL-100(Fe) surface, and the microstructures of the Bi₂MoO₆–MIL-100(Fe)-9 sample. The HRTEM images clearly indicated the resolved lattice fringes of 0.315, 0.406 and 0.192 nm, which correspond to the (131), (040) and (062) planes of orthorhombic phase of Bi₂MoO₆, respectively. The results indicate that the formation of composites of Bi₂MoO₆ and MIL-100(Fe) can promote faster separation of photoinduced charge carriers and improve photocatalytic activities.

Fig. 3 (a) TEM and (b) HRTEM images of Bi₂MoO₆–MIL-100(Fe)-9.

3.3 XPS analysis

The surface chemical composition of Bi₂MoO₆–MIL-100(Fe)-9 was analyzed by XPS. Fig. 4 shows XPS survey spectra and high-resolution XPS spectra of the Bi 4f, Mo 3d, O 1s, Fe 2p and C 1s regions for the sample of Bi₂MoO₆–MIL-100(Fe)-9. The survey spectra indicated that Bi, Mo, O, Fe and C exist on the surface of Bi₂MoO₆–MIL-100(Fe)-9, and no impurities were detected, in accordance with EDS result. In the Bi 4f spectrum, two peaks at 159.2 eV and 164.5 eV are attributed to Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺, respectively.³³ In the Mo 3d spectrum, peaks at 232.5 eV and 235.6 eV were ascribed to Mo 3d_5/2 and Mo 3d_3/2 of Mo⁶⁺, respectively.³⁴ The O 1s peak of the composite at 530.1 eV is observed, which was relate to Mo–O.³⁵ The Fe 2p peak was associated with the binding energy at 712.1 eV, which are characteristic of Fe(III) in MIL-100(Fe).³¹ In addition, the C 1s peaks at 288.8 and 284.7 eV arises from MIL-100(Fe) and external carbon contamination, respectively.³⁶ In summary, the XPS spectra analysis confirms that Bi₂MoO₆–MIL-100(Fe) composites have been synthesized successfully.

Fig. 4 XPS survey spectra of Bi₂MoO₆–MIL-100(Fe)-9 and high-resolution XPS spectra of the Bi 4f, Mo 3d, O 1s, Fe 2p and C 1s.

3.4 BET analysis

Brunauer–Emmett–Teller (BET) surface area of the as-synthesized samples were examined by nitrogen adsorption–desorption isotherms at 77 K. It can be found that the introduction of MIL-100(Fe) can significantly affect the BET surface area of samples. Compared with pure Bi₂MoO₆, the BET surface area (S_BET) of Bi₂MoO₆–MIL-100(Fe) composites obviously increased, from 9.3172 m² g⁻¹ to 109.6347 m² g⁻¹. The larger surface area of photocatalysts can provide more surface active sites and promote charge carriers transport easily, contributing to an enhanced photocatalytic performance.³⁷ Therefore, MIL-100(Fe) in synthesized composites may play the key role in enhancing the photocatalytic activity.

3.5 PL analysis

Photoluminescence (PL) spectra were used to investigate the transfer and separation efficiency of photogenerated electron–hole pairs.³⁸ As we all known, the lower PL intensity indicated a lower recombination of photoinduced charge carriers. The PL spectra of Bi₂MoO₆, MIL-100(Fe) and Bi₂MoO₆–MIL-100(Fe)-9 for excitation wavelength 605 nm are shown in Fig. 5. The main emission peaks around 460–500 nm appear for three photocatalysts, and the emission peak of Bi₂MoO₆–MIL-100(Fe)-9 is lower than the Bi₂MoO₆ and MIL-100(Fe). The resulting of the PL spectra indicated the addition of MIL-100(Fe) into semiconductor Bi₂MoO₆ is contributed to the separation of photo-generated electron–hole pairs.

Fig. 5 PL spectra of Bi₂MoO₆, MIL-100(Fe) and Bi₂MoO₆–MIL-100(Fe)-9, recorded at an excitation wavelength of 605 nm.

3.6 UV-vis DRS

The UV-vis diffusive reflectance spectra (UV-vis DRS) of Bi₂MoO₆, MIL-100(Fe) and Bi₂MoO₆–MIL-100(Fe)-9 photocatalysts were shown in Fig. 6a. The adsorption of three photocatalytic samples are located in visible light region. The absorption edge of Bi₂MoO₆ is 485 nm, and MIL-100(Fe) exhibits absorption edge in 550 nm. As the composites of two different semiconductors, the Bi₂MoO₆–MIL-100(Fe)-9 exhibits absorption edges at 510 nm. Furthermore, the band gap energies of Bi₂MoO₆ and MIL-100(Fe) could be calculated by the following equation:³⁹

α(hv) = A(hv − E_g)^1/2 (2)

where α, v, E_g, and A are the absorption coefficient, the light frequency, the energy band gap of semiconductor, and a constant, respectively. According to eqn (2), the band-gap of photocatalysts can be estimated from plots of (αhv)² versus energy (hv). In the Fig. 6b, the band gaps of as-synthesized Bi₂MoO₆, MIL-100(Fe) and Bi₂MoO₆–MIL-100(Fe)-9 are estimated to be 2.58, 2.50 and 2.38 eV, respectively. The resulting confirmed that Bi₂MoO₆ can match up with MIL-100(Fe) perfectly, which contributes to the separation of the electron–hole pairs.

Fig. 6 (a) UV-vis diffuse reflectance spectra (DRS) of Bi₂MoO₆, MIL-100(Fe) and Bi₂MoO₆–MIL-100(Fe)-9; (b) the plots of (ahv)^n/2 vs. photon energy (hv) for the band-gap energies of Bi₂MoO₆, MIL-100(Fe) and Bi₂MoO₆–MIL-100(Fe)-9.

3.7 Photocatalytic activity

The photocatalytic activities of as-prepared photocatalysts were evaluated by the degradation of RhB under visible light irradiation. The RhB adsorption and photodegradation process of different photocatalysts are shown in Fig. 7a. Before the visible light irradiation, the absorption–desorption balance has been reached between the catalyst and RhB for 30 min absorption process. Fig. 7a shows that RhB was hardly decomposed by photolysis without any catalysts under visible light irradiation. The adsorption rate of pure Bi₂MoO₆ and MIL-100(Fe) are 15.36% and 44.39% in the dark, respectively. The adsorption rate of MIL-100(Fe) is very high, but it has low photocatalytic performance for RhB under the visible-light irradiation. Meanwhile, pure Bi₂MoO₆ shows low photocatalytic activity for RhB degradation, the photodegradation rate is 62.56%. However, the addition of MIL-100(Fe) improved the photocatalytic activities of Bi₂MoO₆–MIL-100(Fe) composites significantly. The photocatalytic activities of Bi₂MoO₆–MIL-100(Fe) composites enhanced, with the increase of content of MIL-100(Fe). When the addition of MIL-100(Fe) reached 9%, Bi₂MoO₆–MIL-100(Fe) composites demonstrated greater degradation performance, up to 88.46% RhB removed in 120 min. Meanwhile, the absorption of RhB in Bi₂MoO₆–MIL-100(Fe)-9 increased to 22.25% compared with pure Bi₂MoO₆, indicating that the introduction of MIL-100(Fe) promoted the increase of adsorption ability of catalysts. Besides, the physical mixture Bi₂MoO₆–MIL-100(Fe)-9(M) composite exhibited lower activity than Bi₂MoO₆–MIL-100(Fe)-9, it may be due to the instable Bi₂MoO₆–MIL-100(Fe) heterojunctions. The result indicated that the presence of MIL-100(Fe) enhanced light absorption, separation of photo-generated electron–hole pairs, and increased surface area. The enhanced photocatalytic performance is contributed to the heterostructure between Bi₂MoO₆ and MIL-100(Fe). In addition, Fig. 7b shows the corresponding first-order kinetics plot by the equation:⁴⁰

−ln(C/C₀) = kt (3)

where C₀ and C are the RhB concentrations in solution at times 0 and t, respectively, and k is the apparent first-order rate constant. According to the equation, the rates of RhB degradation are 0.00881 min⁻¹, 0.01248 min⁻¹, 0.01314 min⁻¹, 0.02174 min⁻¹, 0.01494 min⁻¹ and 0.01201 min⁻¹ for pure Bi₂MoO₆, Bi₂MoO₆–MIL-100(Fe)-3, Bi₂MoO₆–MIL-100(Fe)-6, Bi₂MoO₆–MIL-100(Fe)-9, Bi₂MoO₆–MIL-100(Fe)-12 and Bi₂MoO₆–MIL-100(Fe)-9(M). The rate of RhB degradation of Bi₂MoO₆–MIL-100(Fe)-9 is higher than other photocatalysts. The result indicated that the interaction between Bi₂MoO₆ and MIL-100(Fe) can improve the photocatalytic performance, and the photocatalytic activity is significantly enhancing with the increase of the mass ratio of MIL-100(Fe). The photocatalytic activity reached the maximum value, when the mass ratio of MIL-100(Fe) is 9%.

Fig. 7 (a) Photodegradation of as-prepared samples for RhB. (b) The kinetic constants (k) of photocatalytic for RhB.

The mineralization rate of RhB over photocatalysts was detected by Total Organic Carbon (TOC) test. The TOC results (Fig. 8) indicated that only 67.42% of RhB is mineralized over Bi₂MoO₆–MIL-100(Fe)-9 after 120 min and more irradiation time is needed to make the substrate completely mineralized. After 210 min, the mineralization rate of RhB over Bi₂MoO₆–MIL-100(Fe)-9 can only reach 87.29%. But the mineralization rate of RhB over Bi₂MoO₆–MIL-100(Fe)-9 is higher than that over Bi₂MoO₆ and Bi₂MoO₆–MIL-100(Fe)-9(M). The above results show that Bi₂MoO₆–MIL-100(Fe) composites exhibited much higher photocatalytic performance than pure Bi₂MoO₆ under visible light irradiation.

Fig. 8 TOC removal process of RhB solution.

3.8 The proposed photocatalytic mechanism

3.8.1 Analysis of active species. The active species including holes (h⁺), superoxide radical (˙O₂⁻), and hydroxyl radicals (˙OH) are possibly impact factors for the photodegradation of organic pollutants. It is important to carry out the trapping experiments for confirming the main active species generated over Bi₂MoO₆–MIL-100(Fe) composites. In the detection, EDTA-2Na (ref. 33) and tert-butyl alcohol (TBA)⁴¹ are used as the scavengers of holes (h⁺) and hydroxyl radical (˙OH). The p-benzoquinone (BQ)⁴² and N₂ (ref. 43) are used as the scavenger of superoxide radical (˙O₂⁻). As shown in Fig. 9, the photocatalytic degradation of RhB over Bi₂MoO₆–MIL-100(Fe)-9 was slightly affected by the addition of TBA (1 mL). However, the photocatalytic activity of Bi₂MoO₆–MIL-100(Fe)-9 was significantly inhibited by the addition of EDTA-2Na (0.5 mmol). Similarly, the degradation rate was obviously decreased after the addition of BQ (0.5 mmol). In the word, active species including h⁺ and ˙O₂⁻ play the crucial roles in the photodegradation of pollutants.

Fig. 9 Photocatalytic degradation of RhB over Bi₂MoO₆–MIL-100(Fe)-9 with different scavengers.

To test the role of dissolved O₂ in the reaction process, enough nitrogen (N₂) was bubbled into the suspension to ensure that the degradation system was operated under anoxic conditions. In the presence of N₂, the photocatalytic activity of catalyst was inhibited under the same condition. This result means that dissolved O₂ is a vital factor in RhB removal over Bi₂MoO₆–MIL-100(Fe) composites, which affects the formation of ˙O₂⁻ via direct reduction of O₂ and the formation of ˙OH via indirect reduction of O₂.⁴⁴

3.8.2 The proposed photocatalytic mechanism. It is well-known that the photocatalytic performances of catalysts rely on the transfer, separation and generation of electron–hole pairs. In our study, the interaction between Bi₂MoO₆ and MIL-100(Fe) can promote the separation of photogenerated charge carries and improve the photodegradation rate under visible light. In order to understand the photocatalytic mechanism of the Bi₂MoO₆–MIL-100(Fe) composites in depth, the initial energy band structures of Bi₂MoO₆ and MIL-100(Fe) were estimated. The band edges of Bi₂MoO₆ and MIL-100(Fe) were calculated by the following empirical formulas:⁴⁵

E_VB = X − E_e + 0.5E_g (4)

E_CB = E_VB − E_g (5)

where E_VB is the valence band (VB) edge potential, E_CB is the conduction band edge potential, E_g is the band gap energy of the semiconductor, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, and the X values for Bi₂MoO₆ and MIL-100(Fe) were calculated to be 5.55 and 5.10 eV, respectively. E_e is the energy of free electrons on the hydrogen scale (about 4.5 eV). The E_g of Bi₂MoO₆ and MIL-100(Fe) are estimated as 2.58 and 2.50 eV. According to above formulas, the VB and CB edge potentials of Bi₂MoO₆ are located at −0.24 and 2.34 eV, respectively. The VB and CB edge potentials of MIL-100(Fe) are determined to be −0.65 and 1.85 eV, respectively.

On the basis of above band energy level analysis and active species trapping detection, the possible proposed mechanism was discussed as illustrated in Scheme 1. Under the visible light irradiation, both Bi₂MoO₆ and MIL-100(Fe) can be excited for generation of electrons (e⁻) and holes (h⁺) in their CB and VB. The photo-generated electrons in the CB of MIL-100(Fe) transfer to the CB of Bi₂MoO₆ via the heterojunction structure. At the same, the holes in the VB of Bi₂MoO₆ can migrate to the VB of MIL-100(Fe), hence promoting the separation of photoinduced electron–holes pairs. O₂ molecules in solution adsorbed onto catalysts form superoxide radical (˙O₂⁻) via electron transfer from MIL-100(Fe) to O₂, since the conduction band potential of MIL-100(Fe) (−0.65 eV versus NHE) was more negative than the standard redox potentials of (E(O₂/˙O₂⁻) = −0.33 eV versus NHE). Similarly, the standard redox potentials of (E(H₂O/˙OH) = +2.8 eV versus NHE) was more positive than the VB of the Bi₂MoO₆ (2.34 eV) and MIL-100(Fe) (1.85 eV), thus it is difficult to oxidize the adsorbed H₂O into ˙OH for photo-generated. The part h⁺ radicals reacted with H₂O molecules in solution to generate reactive hydroxyl (˙OH) and H⁺ ions. Moreover, the adsorbed RhB onto Bi₂MoO₆–MIL-100(Fe) can be excited to RhB* under visible-light irradiation.⁴⁶ Then the electrons from RhB* are transformed into photocatalyst to form RhB⁺˙. The RhB (or RhB⁺˙) is finally photodegraded by the reactive species (O₂, ˙OH) or h⁺ to generate H₂O and CO₂. According to above discussion, the process can be described as follows:

MIL-100(Fe) + hv → MIL-100(Fe)(e⁻ + h⁺) (6)

RhB + hv → RhB* → RhB⁺˙ + e⁻ (7)

Bi₂MoO₂ + e⁻ → Bi₂MoO₂(e⁻ + h⁺) (8)

MIL-100(Fe)(e⁻) + O₂ → ˙O₂⁻ + MIL-100(Fe) (9)

˙O₂⁻ + RhB/RhB⁺˙ → other products → CO₂ + H₂O (10)

˙OH + RhB/RhB⁺˙ → other products → CO₂ + H₂O (11)

h⁺ + RhB/RhB⁺˙ → other products → CO₂ + H₂O (12)

Scheme 1 Photocatalytic degradation mechanism over Bi₂MoO₆–MIL-100(Fe) heterostructure under visible light irradiation.

3.9 Regeneration and reusability

It is significant to evaluate the reusable performance and stability of photocatalysts for practical applications.⁴⁷ To investigate the stability of Bi₂MoO₆–MIL-100(Fe)-9, we carried out 4 times recycle experiment to degrade RhB under the same conditions. After each cycle, the photocatalyst was centrifuged, filtered, washed and dried at 60 °C for 12 h. As shown in Fig. 10a, after four cycles, the degradation rate of RhB decreased from 88.46% to 84.91%, the photocatalyst removed organic pollutants with only a slight loss of activity. After being used four times, as shown in Fig. 10b, the XRD patterns of Bi₂MoO₆–MIL-100(Fe)-9 composite before and after catalytic reaction further demonstrated that the structure of the Bi₂MoO₆–MIL-100(Fe) composites remain intact. Besides, the ICP-AES analysis showed that the leaching Fe and Bi ions from the photocatalysts in the reaction solution were 0.143 mg L⁻¹ and less than 0.01 mg L⁻¹ after cycles reaction respectively. The leaching ions concentration is very low compared with the concentration of catalysts (1000 mg L⁻¹), which is negligible. Therefore, the as-prepared Bi₂MoO₆–MIL-100(Fe)-9 composites show excellent stability and reusability in the visible light photochemical applications.

Fig. 10 (a) Cycling runs for photocatalytic degradation of RhB over Bi₂MoO₆–MIL-100(Fe)-9 under visible light irradiation; (b) XRD patterns of Bi₂MoO₆–MIL-100(Fe)-9 before and after reactions.

4. Conclusions

In summary, Bi₂MoO₆–MIL-100(Fe) photocatalysts have been successfully fabricated by facile solvothermal method in this work for the first time. The characterizations of Bi₂MoO₆–MIL-100(Fe) composites were confirmed by XRD, SEM/EDS, TEM, and XPS. The as-synthesized Bi₂MoO₆–MIL-100(Fe) photocatalyst exhibited excellent photocatalytic performance in degradation of RhB under visible light. The optimal mass ratio of MIL-100(Fe) in the composite is 9% for the excellent synergistic effect between Bi₂MoO₆ and MIL-100(Fe). The presence of MIL-100(Fe) in the composites enhanced light absorption, and increased surface area. The photo-generated h⁺ and ˙O₂⁻ in Bi₂MoO₆–MIL-100(Fe) composites are proved to be the main active species over the photodegradation. The enhanced photocatalytic performance is mainly attributed to perfect matched energy band of heterostructure, improved photoinduced charge carriers transfer efficiency, and inhibited photo-generated electron–hole recombination. Our study confirmed that the introduction of MIL-100(Fe) into semiconductor Bi₂MoO₆ can improve the photocatalytic activity and stability of Bi₂MoO₆-based photocatalysts, it may be a promising candidate in synthesis of novel catalysts.

Acknowledgements

This work was supported by the State Key Laboratory of Lake Science and Environment (2016SKL011).

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