Fabrication of Bi2MoO6/MIL-101 (Fe) Composites for Visible Light Photocatalytic Activity Enhancement

Bi2MoO6/MIL-101 (Fe) composite photocatalyst was prepared by a two-step solvothermal method, the photocatalytic decomposition reaction was carried out using visible light as light source and Congo red (CR) aqueous solution as the simulated dye wastewater. The results showed that the photocatalytic activity of Bi2MoO6/MIL-101 (Fe) composite photocatalyst was better than pure MIL-101(Fe) catalyst for the degradation of CR. Bi2MoO6/MIL-101(Fe) composite photocatalyst could compensate for the slower electron mobility and the easy compounding of photogenerated electron-hole pairs of MIL-101(Fe). The degradation efficiency of Bi2MoO6/MIL-101(Fe) photocatalyst for CR remained high after five cycles, indicating its good stability and reusability.


Introduction
Water is the source of life, closely related to our human beings, and determines the fate of the world.Unfortunately, due to human misconduct, the problem of water pollution is becoming more and more serious.In the study of photocatalysis, although conventional inorganic semiconductors are non-toxic and stable, they suffer from disadvantages such as wide band gaps and limited light absorption in most cases, which limit their application in catalysis.Bismuth molybdate is a typical ternary oxide that enters the picture through its fascinating intrinsic features [1] .Bi2MoO6 is the simplest member of the Aurivillius oxide family, consisting of [Bi2O2] 2+ sheets linked to the angle-sharing structure of the MoO6 octahedron.Also, bismuth is a typical p-bulk transition metal with a d10 configuration.According to density flooding theory (DFT) calculations [2] , the Bi 6s orbital will hybridise the O 2p level to form the appropriate VB, stimulating the dispersion of the VB and thus leading to a narrow band gap (2.5 eV-2.8 eV) with a visible light response extending to 500 nm [3] .Therefore, it is capable of capturing visible light.Bi2MoO6 has photocatalytic activity for water cracking and degradation of organic pollutants [4] .The practical application of Bi2MoO6 is limited by its poor quantum yield, which is caused by the rapid recombination of light-induced carriers [5] .
The greatest advantage of Metal organic skeleton (MOFs) materials over inorganic semiconductor materials is their porosity.MOFs have been widely used in many fields such as adsorption, photocatalysis and catalysis due to their controlled pore size, modified pore surface, low density and high surface area [6] 错误 !未找到引用源。 .To date, scientists have attempted to combine Bi2MoO6 with metal organic frameworks (MOFs) such as UIO-66(Zr), MIL-100(Fe), MIL-88B(Fe) and MIL-125 to enhance photocatalytic performance for the removal of dye wastewater [7] .The integration of MOFs with photocatalysts creates an advanced structure that enhances photocatalytic activity, and Fe-based MOFs are considered an environmentally friendly material due to their magnetic properties and ease of recycling.Some Fe-based MOFs include MIL-53(Fe), MIL-88(Fe), MIL-100(Fe) and MIL-101(Fe) for dye removal, with MIL-101(Fe) having a high photocatalytic efficiency [8] .MIL-101(Fe) has attracted attention due to its good thermal stability and high pollutant removal performance.
In this study, Bi2MoO6 loaded MIL-101(Fe) photocatalytic system was synthesized using a two-step solvothermal method.The photocatalytic activities of the as-prepared materials were investigated by the degradation of CR solution driven by visible light.It can be found that Bi2MoO6/MIL-101(Fe) composite had dramatically enhanced photocatalytic activity as compared to pure MIL-101(Fe), which might be due to accelerating the separation of photogenerated charge carriers after Bi2MoO6 loaded on MIL-101(Fe).The stability and reusability of Bi2MoO6/MIL-101(Fe) were examined.

Preparation of MIL-101(Fe)
4.96 mmol terephthalic acid and 9.989 mmol ferric trichloride hexahydrate were dissolved in 60 mL DMF solution under ultrasonic conditions, stirred magnetically for 30 min and then transferred to a hydrothermal reactor lined with polytetrafluoroethylene and reacted at 180 ℃ for 12 h.After the reaction, the product was cooled to room temperature, washed three times with DMF, anhydrous ethanol and deionised water, centrifuged and collected.The product was collected by centrifugation, dried overnight and the brown powder was obtained as MIL-101(Fe).

Preparation of Bi2MoO6/MIL-101(Fe) Materials
Different amounts of MIL-101(Fe) were uniformly dispersed into 25 mL of deionized water under ultrasonic conditions, 0.2 mmol of sodium molybdate dihydrate was added, and the mixed solution was magnetically stirred for 1 h.0.4 mmol of bismuth nitrate pentahydrate was added to make the molar mass ratios of Bi:Fe 1:1, 3:1, 6:1 and 9:1, respectively, and named 11 BiFe, 31 BiFe, 61 BiFe and 91 BiFe, respectively.Continued to stir magnetically for 30 min and then transferred to a hydrothermal reaction kettle lined with polytetrafluoroethylene and reacted at 180℃ for 12 h.After the reaction, the products were cooled to room temperature, washed three times with anhydrous ethanol and deionized water respectively, centrifuged and collected, and dried overnight to obtain Bi2MoO6/MIL-101(Fe).

Characterisation Methods
The X-ray diffraction (XRD) patterns using to analyze the crystal structures and phase dates of the samples were collected at room temperature on a Rigaku-Dmax 2500 diffractometer with Cu Ka radiation (λ=1.5406nm, 40 kV, 30mA) at a scan rate of 2° min -1 in the range of 5°-60°.The morphology and composition of the samples were observed using a field emission scanning electron microscope (FE-SEM, JSM-7610F) and an energy dispersive x-ray analysis system (EDS, Oxford EDS).UV-Vis absorption spectra of the samples were obtained on a UV-Vis spectrophotometer (UV-2450,Shimadzu Corporation, Japan) with scanning range from 200 to 800 nm.

Photocatalytic Test
Different concentrations of Congo Red (CR) solutions were used as simulated dye wastewater for photocatalytic performance testing.(CR 50 mg/L solution was prepared as an example: 1 g of CR solid particles was weighed, added to deionised water, mixed and dissolved, then transferred to a 1 L volumetric flask, fixed and shaken well to make a stock solution.(Take 12.5 mL of CR stock solution in a 250 mL volumetric flask, fix and shake well to make a 50 mg/L CR solution).In this thesis, a 350 W Xe lamp was used as a simulated light source under a UV cut-off filter ≤ 420 nm.In each experiment, an amount of photocatalyst was added to 50 mL of simulated dye wastewater and placed in a 60 mL test tube.To ensure adsorption-desorption equilibrium between the photocatalyst and the simulated dye wastewater, the photocatalyst was reacted with the simulated dye wastewater in the dark for 30 minutes prior to illumination.Next, the light source was switched on and then the above solution was irradiated under a xenon lamp equipped with a water circulation device.During the photocatalytic process, 5 mL of the dye simulated wastewater supernatant was taken at the same time interval and then the concentration of the dye simulated wastewater solution was measured using the maximum absorption wavelength of a TU-1901 double beam UV-Vis spectrophotometer.At the same time, the same volume of CR solution was taken as a blank control experiment.Finally, the formula for calculating the degradation efficiency of the simulated dye wastewater is: Where C is the initial concentration of the simulated dye wastewater and Ct is the concentration of the simulated dye wastewater at irradiation t.

Results and Discussion
Information on the crystal structure and phase composition of 11 BiFe was investigated by X-ray diffraction (XRD) spectroscopy.As shown in Figure 1, XRD spectrum of 11 BiFe shows four distinct diffraction peaks at 2θ = 9.5°,17.1°,19.2°and21.7, corresponding to the (428), ( 088), ( 7911) and (4814) crystal planes of MIL-101(Fe) (JCPDS 76-2388).Similar peaks have been reported in previous studies and are almost identical in position to the derived peaks of MIL-101(Fe) material, suggesting that the crystal structure of 11 BiFe is isomorphic to that of MIL-101(Fe) and the introduction of Bi2MoO6 did not alter the crystalline structure of original MIL-101(Fe) material.Meanwhile, XRD spectrum of 11 BiFe [9]] shows diffraction peaks at 2θ = 23.3°,28.1°, 32.3°, 34.6°, 35.8°, 46.7°, 55.3°and 58.2°, corresponding to reflections in the (111), (131), ( 200), (060), (151), (062), (311) and (191) planes of Bi2MoO6 , respectively.All the diffraction peaks of Bi2MoO6 are clearly visible, indicatingt hat the crystal structure of Bi2MoO6 is loaded onto MIL-101(Fe).Hence, it can be concluded that Bi2MoO6/MIL-101 (Fe) material is successfully achieved in this study.The morphologies of MIL-101(Fe) and 11 BiFe catalysts were observed by scanning electron microscopy and the results are shown in Figure 2. Figure 2(a) depicts that the morphology of MIL-101(Fe) material prepared by a one-step solvothermal method is regular, and its size is uniform.As displayed in Figure 2(b), the morphology of 11 BiFe composite synthesised by a two-step solvothermal method is clustered and consists of small irregular nanoparticles.11 BiFe was subjected to EDS energy spectroscopy, as described in Figure 2(c), and the elemental composition and content of 11 BiFe were further investigated.Bi, C, O, Mo and Fe elements are all contained in the 11 BiFe composite [10] .The results show that Bi2MoO6 is successfully loaded onto MIL-101(Fe) via a two-step solvothermal process.The optical properties of the photocatalysts were characterised by UV-vis diffuse reflectance spectroscopy (UV-Vis DRS) to evaluate their light absorption capacity.As showed in Figure 3, the loading of Bi2MoO6 onto MIL-101(Fe) significantly enhances MIL-101(Fe) light absorption ability in the visible region.The strong absorption range of MIL-101(Fe) and 11 BiFe catalysts appear at around 300-500 nm, which may be due to the intrinsic band gap transition caused by the electrons moving from the valence band O 2p orbitals to the conduction band Mo 4d orbitals and Bi 6p orbitals [11] .This phenomenon may be related to their bandgap energy values (Eg), which are calculated using the Tauc plot method [12] , as shown in Figure 3(b).The Eg value for 11BiFe is 2.58 eV, which is much lower than the Eg value for MIL-101(Fe) (2.85 eV).The narrow band gap of 11 BiFe allows it to absorb visible light efficiently, promoting visible light-driven photocatalysis [13] .Obviously, Bi2MoO6/MIL-101(Fe) composites have enhanced visible light absorption compared to MIL-101(Fe), which are beneficial to photocatalytic performance.To investigate the photocatalytic activity of the obtained photocatalysts, the degradation for CR solution was performed under visible light.As shown in Figure 4(a), curve 1 indicates that there is rarely any decomposition of CR after 180 min irradiation in the absence of photocatalyst, illustrating that the self-degradation of CR can be neglected in photocatalytic reaction.From curve 2 to 6 in Figure 4(a), it can be seen that all of Bi2MoO6/MIL-101(Fe) composites with different ratios of Bi:Fe display much higher photocatalytic efficiency as compared to that of MIL-101(Fe), which could be primarily due to the enhancing of visible light absorption and accelerating of charge transfer and separation in Bi2MoO6/MIL-101(Fe) composite system.Appreciably, the photocatalytic activity of 11 BiFe is best one among all the samples and its degradation rate reaches 96.1% within 180 min, suggesting that appropriate ratio of Bi:Fe are beneficial for the improvement of the photocatalytic activity of Bi2MoO6/MIL-101(Fe) composites.Figure 4(b) shows the kinetic fitting curves for the photocatalytic degradation of CR by different catalysts.The first-order kinetic equation is: where t is the light time, min; k is the primary reaction rate constant, min -1 .As seen, the fitted curve of ln(C0 /C) shows a good linear relationship with t.The degradation pattern of CR by catalysts can be described by the primary kinetic model.The primary kinetic constants for the degradation of MIL-101(Fe), 11 BiFe, 13 BiFe, 16 BiFe and 19 BiFe are 0.00339, 0.01154, 0.00519, 0.00436 and 0.00251, respectively.11BiFe has the highest degradation kinetic constant, indicating that it is composites with optimal ratio of Bi:Fe.The reusability of photocatalyst is an important factor in assessing its practical application, and the stability of the photocatalyst also affects its activity.To further investigate the stability of composite photocatalyst, five consecutive replicate experiments were carried out and the results are shown in Figure 5.After five cycles of photodegradation, the photocatalytic efficiency of 11 BiFe decreased from 96.1 % to 90.2 % and 11 BiFe composite did not deactivate significantly.The experimental results show tha Bi2MoO6/MIL-101 (Fe) composite photocatalyst has good degradation stability and can be used repeatedly for the treatment of organic dye wastewater.

Conclusion
In summary, Bi2MoO6/MIL-101(Fe) composite photocatalysts have been successfully fabricated by a facile and controlled two-step solvothermal method.It was found that all of Bi2MoO6/MIL-101(Fe) composites with different ratios of Bi:Fe display much higher photocatalytic efficiency as compared to that of MIL-101(Fe) under visible light for 180 min.This indicates that Bi2MoO6/MIL-101(Fe) is a kind of photocatalytic material with good catalytic performance.Moreover，the degradation efficiency of Bi2MoO6/MIL-101(Fe) is still over ninety percent after 5 successive cycle usage, indicating its high stability and reusability.This study indicates that the obtained Bi2MoO6/MIL-101(Fe) composites can be an ideal candidate for the application in the water treatment.