Zhang Dan; Li Rong; Xu Qian; Zhang Jing; Wang Lili

(1. School of Metallurgy, Northeastern University, Shenyang 110004;2. School of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113001;3. Xinjiang Institute of Light Industry Technology, Urumqi 830021;4. State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072)

Synthesis of MIL-53(Fe)/MWCNTs Hybrid Material with Enhanced Efficiency for Photocatalytic Degradation of Rhodamine B

Zhang Dan1,2; Li Rong3; Xu Qian4; Zhang Jing2; Wang Lili1

(1. School of Metallurgy, Northeastern University, Shenyang 110004;2. School of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113001;3. Xinjiang Institute of Light Industry Technology, Urumqi 830021;4. State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072)

BMIL-53(Fe)/MWCNTs hybrid material was prepared via the solvethermal synthesis method. The resulting samples were characterized by X-ray diffraction, FT-IR spectroscopy, scanning electron microscopy, UV-Vis absorption spectroscopy,the Brunauer-Emmet-Teller method, and photoluminescence spectroscopy. The result showed that the introduction of multiwalled carbon nanotubes to the MIL-53(Fe) can increase the surface area of the composites, suppress the recombination of photogenerated electron-hole pairs and promote the electron transfer process. The hybrid material showed optimal photocatalytic performance in the degradation of Rhodamine B under the irradiation of ultraviolet and natural light.

MIL-53(Fe); multi-walled carbon nanotubes; photocatalysis; Rhodamine B

1 Introduction

Dyes pose considerable threat to the environment and human body when they are directly released to the environment without further processing[1-3]. Photocatalysis can effectively solve environmental issues by degrading toxic pollutants[4-6]. The metal-organic frameworks(MOFs) materials, which exhibit the designed and controlled structure and function, have been attracting an increasing attention in recent years[7-8]. MOFs materials with semiconductor properties can function photocatalytically under light irradiation[9-12]. MIL-53(Fe), an MOFs material, displays good photocatalytic activity to degrade the organic dyes[13-14]. However, the photocatalytic degradation efficiency of MIL-53(Fe) is low, owing to its fast rate of electron-hole pair recombination.Zhang, et al. prepared MIL-53(Fe)-graphene hybrid materials that showed high photocatalytic performance in the degradation of dye[15]. Graphene can improve the photocatalytic activity, since it can reduce the electronhole pair recombination.

Carbon nanotubes possess outstanding mechanical,electronic and optical properties because of their unique tubular structure. Moreover, the carbon nanotubes exhibit better electrical conductivity and are more stable than graphene. Carbon nanotubes have been added to the photocatalytic systems, as they can increase the surface area of the material, while acting as an electron-transfer channel. It can enhance the absorbance of the visible light and improve the photocatalytic activity of the system[16-19].In the present work, the MIL-53(Fe)/MWCNT (multiwalled carbon nanotubes) hybrid material was prepared through in-situ introduction of multi-walled carbon nanotubes and applied in the field of photocatalytic degradation of Rhodamine B (RhB). It is interesting to note that the hybrid material showed higher photocatalytic efficiency than the neat MIL-53(Fe).

2 Experimental

2.1 Materials

Multi-walled carbon nanotubes (10—20 nm inoutside diameter and 10—30 μm in length) were purchased from the Beijing Dk Nano Technology Co., Ltd. Iron (III) chloride hexahydrate, ethanol and N,N-dimethylformamide (DMF) were supplied by the Sinopharm Chemical Reagent Co., Ltd.1,4-Benzenedicarboxylic acid (H2BDC) was purchased from the Aladdin Industrial Corporation..

2.2 Synthesis of MIL-53(Fe) and MIL-53(Fe)/MWCNT hybrid material

MIL-53(Fe) was prepared according to the literature report[20]. Typically, FeCl3·6H2O (1.35 g),1,4-benzenedicarboxylic acid (H2BDC) (0.83 g), and N,N-dimethylformamide (DMF) (112 mL) were mixed and stirred at room temperature until it became clear, then the reaction mixture was transferred into a 200-mL Teflon lined vessel and heated at 150 °C for 15 h. The resultant suspension was filtered and the filter residue was washed under ultrasonic condition with C2H5OH for 1 h. The orange powder was collected under vacuum after being heated at 150 °C for 24 h.

50 mg of MWCNTs were dispersed into 50 mL of DMF prior to being subjected to sonication for 30 min, and then the MWCNTs suspension was added to the solution of MIL-53(Fe) precursor. The rest methods of preparation and post-treatment were the same as those for preparation of MIL-53(Fe).

2.2 Characterization

The powder X-ray diffraction (XRD) was carried on a Rigaku D/Max-2500 diffractometer with Cu Kα radiation.The morphology of the samples was characterized by a JSM-7500ffield emission scanning electron microscope. The Fourier transform-infrared spectra (FTIR) were recorded on a Nicolet NEXUS 6700 infrared spectrometer. The UV-vis diffuse reflectance spectra were obtained by a Lamda 650 spectrophotometer. The photoluminescence spectra (PL) were conducted on a Cary Eclipse fluorescence spectrophotometer. The nitrogen adsorption-desorption isotherms of the samples were measured using an ASAP 2020 surface area analyzer(Micromeritics, USA) operating at 77 K.

2.3 Photocatalytic activity test

The photocatalytic activity of MIL-53 (Fe) and MIL-53(Fe)/MWCNTs was evaluated by the photodegradation of RhB under irradiation with a highpressure Hg lamp (ML250W, Philips) and natural light at room temperature. 20 mg of MIL-53(Fe) and MIL-53(Fe)/MWCNT photocatalyst were put in a 250-mL beaker containing 100 mL of RhB aqueous solution (20 mg/L), respectively. The distance between the outer edge of beaker and the light source was 20 cm. The suspension was stirred in the dark for 60 min to reach the adsorption/desorption equilibrium. The mixture was kept under stirring during the photodegradation reaction.5 mL of suspension were withdrawn at given time intervals and were immediately centrifuged to remove the catalyst. The RhB concentration was monitored by measuring the absorption intensity at its maximum absorbance wavelength of 553 nm with a UV-Vis spectrophotometer (721 type spectrometer, made by the Shanghai Jinghua Instruments Co., Ltd.).

3 Results and Discussion

3.1 Characterization

Figure 1 XRD patterns of MIL-53(Fe) and MIL-53(Fe)/MWCNTs

Figure 2 FT-IR spectra of MIL-53(Fe) and MIL-53(Fe)/MWCNTs

Figure 1 shows the XRD patterns of MIL-53(Fe) and MIL-53(Fe)/MWCNTs hybrid material. Intense peaks at 2θ = 9.1°, 12.3°, 18.6° were observed, which were consistent with the literature report[21]. The XRD patterns of MIL-53(Fe) and MIL-53(Fe)/MWCNTs hybrid material were similar. This result denoted that the introduction of MWCNTs had no effect on the crystal structure of MIL-53(Fe). The results of fT-IR spectroscopic analysis of MIL-53(Fe) and MIL-53(Fe)/MWCNTs hybrid material are shown in Figure 2. The absorption bands of carboxyl groups are visible at 1 596 cm-1(asymmetric) and 1 390 cm-1(symmetric), respectively. The peak at 750 cm-1corresponds to the C-H bonding vibrations of the benzene rings. The peak of fe-O at 555 cm-1indicates the formation of a metal-oxo bond between the carboxylic group of terephthalic acid and the Fe (III)[22].The hybrid material shows the infrared spectra that are similar to the neat MIL-53(Fe), which means that the MWCNTs do not influence the formation of the MIL-53(Fe)/MWCNTs.Figure 3 shows the SEM images of the MIL-53(Fe) and MIL-53(Fe)/MWCNTs. It can be seen from Figure 3a that the MIL-53(Fe) had spindle and irregular polyhedral shapes. As for the hybrid materials, MWCNTs showed tubular shape and the MIL-53(Fe) still had the spindle and irregular polyhedral shapes. It can be seen that the MWCNTs and MIL-53(Fe) were bonded together.Moreover, the MWCNTs did not change the shape and size of the MIL-53(Fe), indicating that the addition of MWCNTs via the solvethermal treatment did not influence the formation of MIL-53(Fe)/MWCNTs. The pore structures of MIL-53(Fe) and MIL-53(Fe)/MWCNTs were investigated using the nitrogen adsorption-desorption analysis at 77K, with the results shown in Figure 4. The surface area and pore volume data are listed in Table 1.As for the MIL-53(Fe) sample, the N2adsorption is a type I isotherm, indicating to the microporous solids. The N2adsorption isotherm of MIL-53(Fe)/MWCNT showed a type IV isotherm, indicating to the mesoporous solids.The surface area of MIL-53(Fe) was very low (16.7 m2),whereas the surface area of MIL-53(Fe)/MWCNT was found to be increased (164.9 m2). The increase in surface area could be caused by the increase of the pore volume after the introduction of MWCNTs. With an enhanced surface area and pore volume of the material, the MIL-53(Fe)/MWCNTs material could adsorb more organic dye molecules, which would accelerate the reaction rate, thus improving the photocatalytic efficiency.

Figure 3 SEM images of (a): MIL-53(Fe) and (b): MIL-53(Fe)/MWCNTs

Figure 4 N2 adsorption -desorption isotherms of MIL-53(Fe) and MIL-53(Fe)/MWCNTs

Table 1 Pore characteristics of MIL-53(Fe) and MIL-53(Fe)-MWCNTs

The light-absorption characteristics of the samples were determined using the UV-Vis diffuse reflectance spectroscopy. As shown in Figure 5, the absorbance intensity of the sample was enhanced significantly with the addition of MWCNTs, especially in the visible light region. The major absorption edges of the MIL-53(Fe) and MIL-53(Fe)/MWCNTs were around 490 nm and 540 nm, respectively, which corresponded to a band gap energy (Eg) of 2.53eV and 2.30eV(Eg= 1 240/wavelength), respectively. Figure 6 illustrates the fluorescence spectra of MIL-53(Fe) and MIL-53(Fe)/MWCNTs. MIL-53(Fe) had a strong broad emission peak at around 300 nm—500 nm and three peaks at 390 nm,430 nm, and 470 nm. It can be seen that the PL spectrum of MIL-53(Fe)/MWCNTs was similar to that of MIL-53(Fe). However, the intensity of the hybrid material was much lower than that of MIL-53(Fe) because MWCNTs were introduced into the MIL-53(Fe), which could prolong the lifetime of the electron-hole pairs.

Figure 5 UV-Vis diffuse re flectance spectra of MIL-53(Fe)and MIL-53(Fe)/MWCNTs

Figure 6 Photoluminescence spectra of MIL-53(Fe) and MIL-53(Fe)/MWCNTs

3.2 Photocatalytic activity

The photocatalytic degradation of RhB was performed to investigate the photocatalytic activity of the samples.As presented in Figure 7, the neat MIL-53(Fe) could degrade 42.4% of RhB after 60 min of high-pressure Hg light irradiation, whereas the MIL-53(Fe)/MWCNTs could degrade 95.1% of RhB. This fact illustrates that the photocatalytic degradation rate of RhB can be increased significantly with the addition of MWCNTs to MIL-53(Fe). However, pure MIL-53(Fe) degraded 43.2% of RhB, whereas MIL-53(Fe)/MWCNTs degraded 13.2%of RhB under natural light. This means the degradation rate of dyes under UV light was higher than that under natural light. The degradation rate of RhB in the presence of MIL-53(Fe)/MWCNTs and MIL-53(Fe) in the dark reached 52.1% and 26.3%, respectively, after 60 min.These results showed that the MOF samples had certain adsorption affinity to the RhB molecules. This result is related to the pore structure of the materials (16.7 m2for MIL-53(Fe), and 164.9 m2for MIL-53(Fe)/MWCNTs).

Figure 7 Efficiency of MOF and MOF/MWCNTs for degradation of RhB under irradiation with different light source in 60 min

3.3. Mechanism for the degradation of RhB using MIL-53(Fe)/MWCNTs

The possible mechanism for the degradation of RhB using MIL-53(Fe)/MWCNTs was deduced (Figure 8).Under light irradiation, the electrons in the valence band(VB) of MIL-53(Fe) were excited to the conduction band (CB). The photogenerated electrons moved to the surface of the MIL-53(Fe), with some of them being transferred to the MWCNTs, which were in close contact with the MIL-53(Fe). The MWCNTs, as electron acceptors, quickly exported the photogenerated electrons to the surface of MWCNTs. Therefore, the introduction of MWCNTs effectively improved the electron-hole pair separation, which led to higher photocatalytic decomposition efficiency. The electrons on the surface of MIL-53(Fe) and MWCNT reacted with the O2in the solution to produce superoxide radical anions [·O2-][23-24].Meanwhile, the holes in the valence band migrated to the surface of the MIL-53(Fe) and trapped water molecules or hydroxyl ions (OH-) to form hydroxyl radicals (·OH).The ·OH and ·O2- produced thereby can degrade the RhB molecules to CO2, H2O, or other products. The related reactions are as follows：

Moreover, the surface area and pore volume of the composites were increased because of the doping of the MWCNTs. The hybrid materials MIL-53(Fe)/MWCNTs with much greater surface area and pore volume could provide more space and more unsaturated metal sites for the adsorption of organic dye molecules. Furthermore,the porous structure of the MIL-53(Fe)/MWCNTs could facilitate the migration of the photogenerated electrons[6],which could also suppress the recombination of the electron-hole pairs, thus enhancing the photocatalytic degradation efficiency of RhB.

Figure 8 The proposed mechanism for photodegradation of RhB by MIL-53(Fe)/MWCNTs

4 Conclusions

In this study, the MIL-53(Fe)/MWCNTs hybrid material was prepared via a solvothermal method. The addition of MWCNTs did not affect the crystallization phase structure of MIL-53(Fe), which was verified through XRD, SEM,and IR characterizations. The existence of MWCNTs increased the surface area of the material and suppressed the electron-hole pairs recombination. Consequently, the composites showed higher photocatalytic activity than pure MIL-53(Fe). This study showed that MWCNTs could be combined with MOFs materials for application in the field of photocatalysis.

Acknowledgement: This work was financially supported by the National Natural Science Foundation of China (21573101,20903054), the Liaoning Provincial Natural Science Foundation (2014020107), the Program for Liaoning Excellent Talents in University (LJQ2014041), the Scientific Research Foundation for the Returned Overseas Chinese Scholars,State Education Ministry ([2013]1792), the Support Plan for Distinguished Professor of Liaoning Province ([2015]153),and the Open Project of Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences(N-15-10).

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date: 2017-03-01; Accepted date: 2017-04-10.

Dr. Zhang Dan, E-mail： 495553468@qq.com.