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UiO-66 composites with enhanced visible-light photocatalytic performance
Inorganic Chemistry Communications 104 (2019) 223–228
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis of flower-like CuS/UiO-66 composites with enhanced visible-light photocatalytic performance
T
Jinxi Chen , Fufang Chao, Xiaoyue Ma, Qiong Zhu, Jifei Jiang, Jiaojiao Ren, Yu Guo, Yongbing Lou ⁎
School of Chemistry and Chemical Engineering, Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device, Southeast University, Nanjing 211189, Jiangsu, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: CuS/UiO-66 composites Solvothermal method Photocatalytic Degradation Rhodamine B
For the first time, a series of flower-like CuS/UiO-66 composites with different UiO-66 contents were fabricated via a facile solvothermal method. The as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis optical absorption spectroscopy and photoluminescence (PL) spectra. The CuS/UiO-66 composites exhibited remarkable performance for the degradation of Rhodamine B (RhB) as compared to pure UiO-66 under visible-light irradiation. Moreover, the catalytic activity was related to the weight ratio of CuS to UiO-66 in the composites, and the CuS/ UiO-66 (weight ratio 2.4:1) composites exhibited the highest photocatalytic activities. The improved photocatalytic activity contributed to the synergetic effect between CuS and UiO-66, which facilitated the separation efficiency of photogenerated electron-hole pairs. The possible mechanism of the enhanced photocatalytic performance of the CuS/UiO-66 composites was also discussed. This work demonstrates that the CuS/UiO-66 composite is a promising photocatalyst for removing of organic pollutants.
1. Introduction With the ever-increasing energy shortage and environmental pollution, extensive attention has been focused on how to use green energy. Recently, developing efficient photocatalysts has got a huge ⁎
interest for making use of solar energy [1–4]. Photocatalysis has already become a clean and effective way to remediate contaminated environments [5–7]. In particular, heterogeneous photocatalysts directly utilized the solar energy, promoted the catalytic performance via a synergistic effect, which could be served as an emerging renewable
Corresponding author. E-mail address: [email protected] (J. Chen).
https://doi.org/10.1016/j.inoche.2019.04.022 Received 19 February 2019; Received in revised form 3 April 2019; Accepted 14 April 2019 Available online 15 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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material in photodegradation of organic dyes from polluted water [8–10]. However, many important challenges, the limited varieties of heterogeneous photocatalysts, low photocatalytic efficiency, complicated synthesis process and so on, need to be addressed before commercial application [11]. Therefore, it is urgent to design heterogeneous photocatalysts with high-performance and low-price through environmentally friendly route [12]. In recent years, metal-organic frameworks (MOFs) have received considerable attention due to the direct response in separation, catalysis, drug delivery, and chemical sensors [13–15]. More specifically, several MOFs attracted great interest in photocatalysis field because of the semiconductor behavior [16–18]. It is obvious that MOFs could be served as a promising material for heterogeneous photocatalysts [19–23]. Hence, MOFs-based hetero-structure photocatalysts were applicable to decompose the organic pollutants for taking synergistic effect of different components [24–26]. Furthermore, the photocatalysis activity of MOFs/semiconductors composites was drastically enhanced compared to the single component. This phenomenon further revealed that MOFs could be employed as stable supporting materials [27–37]. Among various MOFs, UiO-66, a zirconium based MOFs, presented large specific surface area, high stability in aqueous solution, and effective adsorption of organic compounds. These properties make UiO66 a promising porous material for the removal of organic pollutants [38–40]. On the other hand, metal sulfides, such as CuS, CdS, ZnS, In2S3, SnS2, have been extensively investigated as the photocatalysts due to their unique physical and chemical properties, especially good photocatalytic activity [41–43]. In our previous work, ZnIn2S4/UiO-66-(SH)2 composites were used as visible-light photocatalyst for RhB degradation. The photocatalytic activities of the composites were significantly improved comparing to each component alone [44]. Besides, In2S3/ UiO-66 hybrid with enhanced photocatalytic activity was developed to effectively decompose methyl orange and tetracycline hydrochloride [45]. As one of the important II-VI semiconductor family, CuS possesses visible light response and appropriate energy band gap, which is regarded as a competitive candidate in solar cell devices, thermoelectric devices, catalysts [46–48]. However, pure CuS shows high recombination rate of photogenerated electron-hole pairs and low catalytic efficiency. Some effective strategies have been investigated to further promote the activity and stability of CuS, such as doping with noble metal, preparing CuS quantum dots, and coupling with other components (metal sulfides, polymer and graphene) to construct heterogeneous photocatalysts [49–53]. To the best of our knowledge, the coupling between CuS and MOFs to construct hetero-structure composites were rare. Herein, the flower-like CuS/UiO-66 composites have been prepared by the solvothermal strategy. The as-obtained CuS/UiO-66 composite shows remarkable photocatalytic activity and stability in degradation of Rhodamine B (RhB) under visible-light irradiation. More significantly, we also proposed the possible mechanism for the enhanced photocatalytic performance of CuS/UiO-66 composites. We hope that our current research could inspire growing interest on the design of semiconductor-MOF composites photocatalysts by using CuS and MOFs.
2.2. Synthesis of UiO-66 In a typical synthesis, ZrCl4 (116.6 mg, 0.5 mmol) and H2BDC (83.1 mg, 0.5 mmol) were added into 25 mL DMF. The above mixture was stirred for 30 min at room temperature, followed by adding 500 μL acetic acid. After stirring for another 30 min, the obtained homogeneous mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 120 °C for 24 h. After cooling to room temperature, the whitecolor product was recovered by centrifugation, washed with DMF and anhydrous methanol three times respectively, and dried in a vacuum oven at 60 °C for 12 h. 2.3. Synthesis of CuS/UiO-66 composites The CuS/UiO-66 composites were synthesized by a facile solvothermal process. In detail, CuCl2·2H2O (173 mg, 1.0 mmol) was dispersed into the mixture of ET (7.5 mL) and EG (7.5 mL) under ultrasonication. Then, a certain amount of as-prepared UiO-66 powder was added into the mixture and stirred for 1 h at room temperature. Next, the Na2S2O3 solution prepared by dissolving the Na2S2O3·5H2O (173 mg, 1.1 mmol) in the mixture solution of ET (7.5 mL) and EG (7.5 mL) was added in above solution and stirred for another 1 h. The obtained suspension was transferred into a 50 mL Teflon-lined autoclave and maintained at 150 °C for 24 h. After cooling to the room temperature, the resultant solid products were washed with ethanol for several times, and dried at 60 °C for 12 h in a vacuum. The CuS/UiO-66 composites with different weight ratios of CuS to UiO-66 could be fabricated by changing the mass of UiO-66 from 40 mg to 60 mg and 80 mg, and the obtained composites were named as CuS/UiO-66(2.4:1), CuS/UiO-66(1.6:1) and CuS/UiO-66(1.2:1), respectively. For comparison, bare CuS was also prepared by same solvothermal method without addition of UiO-66. 2.4. Characterization The crystallographic information of the as-synthesized products were obtained using powder X-ray diffraction (XRD, Ultima IV, Japan), with Cu-Kα radiation (λ = 1.5406 Å), in a scanning 2θ range of 5–70°. The morphologies and structure analysis of various samples were conducted by scanning electron microscopy (SEM, FEI Inspect F50) and transmission electron microscope (TEM, JEOL JEM-2010F). The Ultravioletvisible spectra (UV–vis) were measured using a Shimadzu UV-2600 in a wavelength range of 200 nm to 800 nm. Photoluminescence (PL) spectra were carried out in a fluorescence spectrophotometer (Fluoromax-4). 2.5. Evaluation of the photocatalytic activity The photocatalytic degradation experiments were performed at room temperature in a 500 mL round-bottomed flask with a circulation water system. A 300 W Xenon lamp equipped with a UV cut-off filter (λ > 420 nm) was used as the visible light source. Typically, 15 mg assynthesized photocatalyst was added into 100 mL RhB solution (10 mg/ L). Prior to light irradiation, the mixture was stirred for about 12 h in dark room, then 0.5 mL H2O2 (30 wt%) was added into the resulted mixture. Afterwards, the suspension was exposed to visible light with stirring. Then, at every 10 min time interval, 4 mL solution mixtures were sampled by a syringe and filtered through a 0.22 μm PTFE syringe filter to isolate the photocatalyst. And the photocatalytic activity was measured by recording the RhB concentration at 554 nm by using a UV–vis spectrophotometer. The photodegradation experiments of MO were performed by the similar procedure except that RhB was replaced by MO.
2. Experimental section 2.1. Materials Zirconium tetrachloride (ZrCl4), N,N-dimethylformamide (DMF), Rhodamine B (RhB), Sodium thiosulfate (Na2S2O3·5H2O) were purchased from Shanghai Macklin Biochemical Co., Ltd. Copper(II) chloride (CuCl2·2H2O), 1, 4-benzenedicarboxylic acid (H2BDC) and Methyl orange (MO) were purchased from J&K Scientific Co., Ltd. Acetic acid, methanol, ethylene glycol (EG) and ethanol (ET) were obtained from Sinopharm Chemical Reagent Co., Ltd. All of materials were obtained from commercial suppliers, and used as received.
3. Results and discussion The XRD patterns of the as-synthesized UiO-66, CuS and CuS/UiO224
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Fig. 1. XRD patterns of series of as-prepared CuS/UiO-66 composites.
66 composites were illustrated in Figs. 1 and S1. The diffraction peaks at 2θ = 27.68°, 29.55°, 31.70°, 32.90°, 47.83°, 52.55° and 59.17°, which could be indexed to (101), (102), (103), (006), (110), (108) and (116) crystal planes of the hexagonal CuS (JCPDS No.06–0464). The XRD pattern of the as-obtained UiO-66 was well matched with the simulated XRD pattern (Fig. S1) [54]. For the CuS/UiO-66 composites, the characteristic diffraction peaks of hexagonal phase CuS were observed, and two additional peaks centered at 2θ value of 6.4°and 15.0°could be indexed to UiO-66. Furthermore, as the UiO-66 amount increasing, the characteristic diffraction peaks of UiO-66 increased. The XRD results indicated the coexistence of CuS and UiO-66 in the composites. The detailed morphology and surface features of the pristine UiO66, CuS and UiO-66/CuS composites were analyzed by SEM and TEM. As shown in Fig. 2, the pure CuS comprised microsphere with diameter approximately 5 μm. The as-prepared UiO-66 displayed a spherical morphology with smooth surfaces, the average diameter was about 100 nm (Fig. S2). As for the UiO-66/CuS composites, the surfaces of microspheres were decorated with many nanoparticles while remaining the original morphology of CuS (Fig. 2b–d). TEM images further confirmed that UiO-66 nanoparticles distributed uniformly on the surface of CuS (Fig. 2e). On the other side, the HRTEM image of CuS/UiO66(2.4:1) composite revealed a lattice spacing of around 1.9 Å, which could be indexed to the (110) plane of the CuS phase (Fig. 2f). The above results confirmed that the UiO-66/CuS composites were successfully synthesized by the solvothermal method. Based on the above preparation procedures and characterization results, the formation mechanism of the pure CuS and the UiO-66/CuS composites was illustrated in Scheme 1. For the pure CuS, as shown in Scheme 1a, the CuS nuclei produced in the presence of the copper ions and sulfur ions at the high temperature and pressure. Next, the CuS nuclei tend to grow larger to form CuS nanosheets in the solvothermal process. Then, the CuS nanosheets could self-assemble to construct CuS microsphere with the reaction time increased. Similarly, the grown mechanism of the UiO-66/CuS composites was shown in Scheme 1b. A certain amount of UiO-66 was added into the CuCl2·2H2O suspension, and the uniform suspension could be obtained after ultrasonication. The copper ions uniformly were anchored on the surface of UiO-66 with continuous stirring. Then, the Na2S2O3·5H2O solution acted as sulfide source to combine copper ions to form CuS nanoparticles on the surface of UiO-66. Then, the UiO-66/CuS nuclei were produced, and then continue to grow to form UiO-66/CuS nanosheets. Finally, the UiO-66/ CuS nanosheets self-assembled into UiO-66/CuS microspheres as the reaction time proceeded. The UV–vis absorption spectra of the as-obtained UiO-66, CuS and UiO-66/CuS composites were shown in Fig. 3a. Pure UiO-66 showed significant absorption in the UV-light region, and pure CuS showed a
Fig. 2. SEM images of (a) UiO-66, (b) CuS/UiO-66(2.4:1), (c) CuS/UiO66(1.6:1), (d) CuS/UiO-66(1.2:1), (e) TEM and (f) HRTEM images of CuS/UiO66(2.4:1).
Scheme 1. Schematic illustration of the preparation process of the CuS/UiO-66 composites.
broad and intensive absorption in visible-light region. Surprisingly, all UiO-66/CuS composites possessed notable absorptions in the visible light region which were different from pure UiO-66. The obviously enhanced absorption in visible light region was credited to the presence of CuS in the composites, which was consistent with their colors change from white to black (Fig. S3). The Photoluminescence (PL) spectra show the photogenerated charge separation efficiency of all materials. In general, the lower PL intensity means the low recombination efficiency of electron-hole pairs, which contributes to the better photocatalytic ability. For pure UiO-66, an obvious wide emission peak at 400–500 nm could be found (Fig. 3b). It could be seen that the PL spectra of UiO-66/ CuS composites were similar to that of pure UiO-66. However, the PL intensity of UiO-66/CuS composites obviously decreased compared to 225
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Fig. 3. (a) UV–vis absorption and (b) photoluminescence spectra of UiO-66, CuS and series of CuS/UiO-66 composites.
Fig. 4. (a) Photocatalytic degradation of RhB over various photocatalysts under visible-light irradiation (λ > 420 nm); (b) Photocatalytic degradation compare of using CuS/UiO-66(2.4:1) and mechanical mixture of pristine UiO-66 and pristine CuS as example, the amount of pristine UiO-66 or CuS was equal to the actual amount of that in CuS/UiO-66(2.4:1).
decomposed after irradiation for 1 h when pure UiO-66 was used as photocatalyst. Strikingly, CuS/UiO-66 composites greatly improved photodegradation efficiency of RhB. All of the CuS/UiO-66 composites displayed superior photocatalytic activities than pure CuS and UiO-66. What's more, the CuS/UiO-66(2.4:1) composite exhibited the best photocatalytic activity, over 90% RhB was decomposed in 1 h. To further investigate the synergetic effect between CuS and UiO-66, the photocatalytic activity of CuS and UiO-66 physically mixed according to the weight ratio of CuS/UiO-66(2.4:1) composites with the best photocatalytic activity was also studied. As shown in Fig. 4b, the physical mixture CuS and UiO-66(2.4:1) composites exhibited an obviously decreased photocatalytic performance. It could be inferred that CuS and UiO-66 in CuS/UiO-66(2.4:1) composites were in close contact. It could be greatly improved the separation rate of photo-excited electron-hole pairs. This result demonstrated that the synergistic effect between UiO66 and CuS made it more likely to separate electrons and holes, so the photocatalytic activities of the composite were higher than that of pure materials alone. The photodegradation of the methyl orange (MO) dyes on CuS/UiO-66 composites were further studied under visible light irradiation. Moreover, CuS/UiO-66(2.4:1) composites showed an obviously better photocatalytic activity for the cationic dyes (RhB) than for anionic dyes (MO) (Fig. S4). This phenomenon was ascribed to the electrostatic repulsion between UiO-66 with a negative potential and MO led to partial desorption in the catalytic process [55]. The reusability and stability of the CuS/UiO-66 composites for degradation of RhB under visible light irradiation were also investigated. As shown in Fig. 5, the efficiency of RhB photodegradation declined to
Fig. 5. Recyclability of the photocatalytic decomposition of RhB for bare UiO66, bare CuS, and CuS/UiO-66(2.4:1) composites under visible light.
pure UiO-66. The above results further confirmed that CuS/UiO-66 composites could transfer electrons effectively and possess a lower rate of electron-hole recombination. The outstanding optical properties of the composites were featured to enhance photocatalytic activity. The photocatalytic activity for the degradation of RhB over different photocatalysts under visible light irradiation was investigated, and the results were displayed in Fig. 4. About 50% of RhB dye could be 226
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semiconductors and could be excited under visible light and produce more active species (%O2−, %OH). Furthermore, the existence of energy band cross between UiO-66 and CuS enabled the efficient transfer of electrons and holes at the interface of the composites. It also reduced the recombination of electron-hole pair. Therefore, the RhB degradation reaction could be effectively catalyzed. 4. Conclusions In summary, flower-like CuS/UiO-66 composites were prepared by a facile solvothermal method. Remarkably, all of the CuS/UiO-66 composites displayed better photocatalytic activity than that of the individual UiO-66 and CuS for degradation of RhB under visible-light irradiation. The improved photocatalytic performance was mainly attributed to the synergistic effect between CuS and UiO-66. This result provides a new outlook for application of MOF-CuS composite for removal of pollutants.
Fig. 6. XRD patterns of fresh and used CuS/UiO-66(2.4:1) samples.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21475021 and 21427807) and the Fundamental Research Funds for the Central Universities (2242017K41023). Conflict of interest The authors declare no conflict of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.04.022. References Scheme 2. Mechanism illustration of visible-light-induced RhB photodegradation process on CuS/UiO-66 composites.
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about 73.5% after three cycles, which was still higher than that of bare CuS and UiO-66. The decay of degradation rate during cycling was attributed to the following reasons. First, it is inevitable to lose certain catalysts in recovery process; on the other hand, the catalyst adsorbs a certain amount of RhB from the solution in the first catalytic process. However, the adsorption capacity of the catalyst is basically saturated in the subsequent experiments. Therefore, there is no doubt that the degradation rate decreases slightly during cycling. Furthermore, the XRD pattern revealed that the structure of CuS/UiO-66(2.4:1) composites were still kept intact after photodegradation (Fig. 6). This result suggested that the CuS/UiO-66 composites possessed remarkable catalytic activity and stability in catalytic reaction. A possible mechanism of the enhanced photocatalytic performance of the CuS/UiO-66 composites was proposed in Scheme 2. The band gaps of CuS and UiO-66 were 2.01 eV and 3.50 eV. [46,55] The CuS in the CuS/UiO-66 composites acted as a sensitizer to absorb visible light. Then, the RhB molecular excited to generate photoinduced electrons, and the photoinduced electrons on the LUMO of RhB could transfer to the LUMO of UiO-66 or the CB of CuS via the dye sensitization effect. Furthermore, the photoinduced electrons on the LUMO of UiO-66 could easily transferred to the CB of CuS by the closed contact interfaces. It is helpful to improve the photocatalytic ability of the CuS/UiO-66 composites. In addition, the electrons could rapidly react with H2O2 to produce %OH, and then degrade the RhB molecular. Based on the above results, we have proposed the following mechanism to explain the enhanced catalytic activity after coupling CuS with UiO-66. First, the high surface area of UiO-66 increased the contact interface with CuS. Both UiO-66 and CuS are narrow bandgap 227
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis of flower-like CuS/UiO-66 composites with enhanced visible-light photocatalytic performance
T
Jinxi Chen , Fufang Chao, Xiaoyue Ma, Qiong Zhu, Jifei Jiang, Jiaojiao Ren, Yu Guo, Yongbing Lou ⁎
School of Chemistry and Chemical Engineering, Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device, Southeast University, Nanjing 211189, Jiangsu, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: CuS/UiO-66 composites Solvothermal method Photocatalytic Degradation Rhodamine B
For the first time, a series of flower-like CuS/UiO-66 composites with different UiO-66 contents were fabricated via a facile solvothermal method. The as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis optical absorption spectroscopy and photoluminescence (PL) spectra. The CuS/UiO-66 composites exhibited remarkable performance for the degradation of Rhodamine B (RhB) as compared to pure UiO-66 under visible-light irradiation. Moreover, the catalytic activity was related to the weight ratio of CuS to UiO-66 in the composites, and the CuS/ UiO-66 (weight ratio 2.4:1) composites exhibited the highest photocatalytic activities. The improved photocatalytic activity contributed to the synergetic effect between CuS and UiO-66, which facilitated the separation efficiency of photogenerated electron-hole pairs. The possible mechanism of the enhanced photocatalytic performance of the CuS/UiO-66 composites was also discussed. This work demonstrates that the CuS/UiO-66 composite is a promising photocatalyst for removing of organic pollutants.
1. Introduction With the ever-increasing energy shortage and environmental pollution, extensive attention has been focused on how to use green energy. Recently, developing efficient photocatalysts has got a huge ⁎
interest for making use of solar energy [1–4]. Photocatalysis has already become a clean and effective way to remediate contaminated environments [5–7]. In particular, heterogeneous photocatalysts directly utilized the solar energy, promoted the catalytic performance via a synergistic effect, which could be served as an emerging renewable
Corresponding author. E-mail address: [email protected] (J. Chen).
https://doi.org/10.1016/j.inoche.2019.04.022 Received 19 February 2019; Received in revised form 3 April 2019; Accepted 14 April 2019 Available online 15 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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material in photodegradation of organic dyes from polluted water [8–10]. However, many important challenges, the limited varieties of heterogeneous photocatalysts, low photocatalytic efficiency, complicated synthesis process and so on, need to be addressed before commercial application [11]. Therefore, it is urgent to design heterogeneous photocatalysts with high-performance and low-price through environmentally friendly route [12]. In recent years, metal-organic frameworks (MOFs) have received considerable attention due to the direct response in separation, catalysis, drug delivery, and chemical sensors [13–15]. More specifically, several MOFs attracted great interest in photocatalysis field because of the semiconductor behavior [16–18]. It is obvious that MOFs could be served as a promising material for heterogeneous photocatalysts [19–23]. Hence, MOFs-based hetero-structure photocatalysts were applicable to decompose the organic pollutants for taking synergistic effect of different components [24–26]. Furthermore, the photocatalysis activity of MOFs/semiconductors composites was drastically enhanced compared to the single component. This phenomenon further revealed that MOFs could be employed as stable supporting materials [27–37]. Among various MOFs, UiO-66, a zirconium based MOFs, presented large specific surface area, high stability in aqueous solution, and effective adsorption of organic compounds. These properties make UiO66 a promising porous material for the removal of organic pollutants [38–40]. On the other hand, metal sulfides, such as CuS, CdS, ZnS, In2S3, SnS2, have been extensively investigated as the photocatalysts due to their unique physical and chemical properties, especially good photocatalytic activity [41–43]. In our previous work, ZnIn2S4/UiO-66-(SH)2 composites were used as visible-light photocatalyst for RhB degradation. The photocatalytic activities of the composites were significantly improved comparing to each component alone [44]. Besides, In2S3/ UiO-66 hybrid with enhanced photocatalytic activity was developed to effectively decompose methyl orange and tetracycline hydrochloride [45]. As one of the important II-VI semiconductor family, CuS possesses visible light response and appropriate energy band gap, which is regarded as a competitive candidate in solar cell devices, thermoelectric devices, catalysts [46–48]. However, pure CuS shows high recombination rate of photogenerated electron-hole pairs and low catalytic efficiency. Some effective strategies have been investigated to further promote the activity and stability of CuS, such as doping with noble metal, preparing CuS quantum dots, and coupling with other components (metal sulfides, polymer and graphene) to construct heterogeneous photocatalysts [49–53]. To the best of our knowledge, the coupling between CuS and MOFs to construct hetero-structure composites were rare. Herein, the flower-like CuS/UiO-66 composites have been prepared by the solvothermal strategy. The as-obtained CuS/UiO-66 composite shows remarkable photocatalytic activity and stability in degradation of Rhodamine B (RhB) under visible-light irradiation. More significantly, we also proposed the possible mechanism for the enhanced photocatalytic performance of CuS/UiO-66 composites. We hope that our current research could inspire growing interest on the design of semiconductor-MOF composites photocatalysts by using CuS and MOFs.
2.2. Synthesis of UiO-66 In a typical synthesis, ZrCl4 (116.6 mg, 0.5 mmol) and H2BDC (83.1 mg, 0.5 mmol) were added into 25 mL DMF. The above mixture was stirred for 30 min at room temperature, followed by adding 500 μL acetic acid. After stirring for another 30 min, the obtained homogeneous mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 120 °C for 24 h. After cooling to room temperature, the whitecolor product was recovered by centrifugation, washed with DMF and anhydrous methanol three times respectively, and dried in a vacuum oven at 60 °C for 12 h. 2.3. Synthesis of CuS/UiO-66 composites The CuS/UiO-66 composites were synthesized by a facile solvothermal process. In detail, CuCl2·2H2O (173 mg, 1.0 mmol) was dispersed into the mixture of ET (7.5 mL) and EG (7.5 mL) under ultrasonication. Then, a certain amount of as-prepared UiO-66 powder was added into the mixture and stirred for 1 h at room temperature. Next, the Na2S2O3 solution prepared by dissolving the Na2S2O3·5H2O (173 mg, 1.1 mmol) in the mixture solution of ET (7.5 mL) and EG (7.5 mL) was added in above solution and stirred for another 1 h. The obtained suspension was transferred into a 50 mL Teflon-lined autoclave and maintained at 150 °C for 24 h. After cooling to the room temperature, the resultant solid products were washed with ethanol for several times, and dried at 60 °C for 12 h in a vacuum. The CuS/UiO-66 composites with different weight ratios of CuS to UiO-66 could be fabricated by changing the mass of UiO-66 from 40 mg to 60 mg and 80 mg, and the obtained composites were named as CuS/UiO-66(2.4:1), CuS/UiO-66(1.6:1) and CuS/UiO-66(1.2:1), respectively. For comparison, bare CuS was also prepared by same solvothermal method without addition of UiO-66. 2.4. Characterization The crystallographic information of the as-synthesized products were obtained using powder X-ray diffraction (XRD, Ultima IV, Japan), with Cu-Kα radiation (λ = 1.5406 Å), in a scanning 2θ range of 5–70°. The morphologies and structure analysis of various samples were conducted by scanning electron microscopy (SEM, FEI Inspect F50) and transmission electron microscope (TEM, JEOL JEM-2010F). The Ultravioletvisible spectra (UV–vis) were measured using a Shimadzu UV-2600 in a wavelength range of 200 nm to 800 nm. Photoluminescence (PL) spectra were carried out in a fluorescence spectrophotometer (Fluoromax-4). 2.5. Evaluation of the photocatalytic activity The photocatalytic degradation experiments were performed at room temperature in a 500 mL round-bottomed flask with a circulation water system. A 300 W Xenon lamp equipped with a UV cut-off filter (λ > 420 nm) was used as the visible light source. Typically, 15 mg assynthesized photocatalyst was added into 100 mL RhB solution (10 mg/ L). Prior to light irradiation, the mixture was stirred for about 12 h in dark room, then 0.5 mL H2O2 (30 wt%) was added into the resulted mixture. Afterwards, the suspension was exposed to visible light with stirring. Then, at every 10 min time interval, 4 mL solution mixtures were sampled by a syringe and filtered through a 0.22 μm PTFE syringe filter to isolate the photocatalyst. And the photocatalytic activity was measured by recording the RhB concentration at 554 nm by using a UV–vis spectrophotometer. The photodegradation experiments of MO were performed by the similar procedure except that RhB was replaced by MO.
2. Experimental section 2.1. Materials Zirconium tetrachloride (ZrCl4), N,N-dimethylformamide (DMF), Rhodamine B (RhB), Sodium thiosulfate (Na2S2O3·5H2O) were purchased from Shanghai Macklin Biochemical Co., Ltd. Copper(II) chloride (CuCl2·2H2O), 1, 4-benzenedicarboxylic acid (H2BDC) and Methyl orange (MO) were purchased from J&K Scientific Co., Ltd. Acetic acid, methanol, ethylene glycol (EG) and ethanol (ET) were obtained from Sinopharm Chemical Reagent Co., Ltd. All of materials were obtained from commercial suppliers, and used as received.
3. Results and discussion The XRD patterns of the as-synthesized UiO-66, CuS and CuS/UiO224
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Fig. 1. XRD patterns of series of as-prepared CuS/UiO-66 composites.
66 composites were illustrated in Figs. 1 and S1. The diffraction peaks at 2θ = 27.68°, 29.55°, 31.70°, 32.90°, 47.83°, 52.55° and 59.17°, which could be indexed to (101), (102), (103), (006), (110), (108) and (116) crystal planes of the hexagonal CuS (JCPDS No.06–0464). The XRD pattern of the as-obtained UiO-66 was well matched with the simulated XRD pattern (Fig. S1) [54]. For the CuS/UiO-66 composites, the characteristic diffraction peaks of hexagonal phase CuS were observed, and two additional peaks centered at 2θ value of 6.4°and 15.0°could be indexed to UiO-66. Furthermore, as the UiO-66 amount increasing, the characteristic diffraction peaks of UiO-66 increased. The XRD results indicated the coexistence of CuS and UiO-66 in the composites. The detailed morphology and surface features of the pristine UiO66, CuS and UiO-66/CuS composites were analyzed by SEM and TEM. As shown in Fig. 2, the pure CuS comprised microsphere with diameter approximately 5 μm. The as-prepared UiO-66 displayed a spherical morphology with smooth surfaces, the average diameter was about 100 nm (Fig. S2). As for the UiO-66/CuS composites, the surfaces of microspheres were decorated with many nanoparticles while remaining the original morphology of CuS (Fig. 2b–d). TEM images further confirmed that UiO-66 nanoparticles distributed uniformly on the surface of CuS (Fig. 2e). On the other side, the HRTEM image of CuS/UiO66(2.4:1) composite revealed a lattice spacing of around 1.9 Å, which could be indexed to the (110) plane of the CuS phase (Fig. 2f). The above results confirmed that the UiO-66/CuS composites were successfully synthesized by the solvothermal method. Based on the above preparation procedures and characterization results, the formation mechanism of the pure CuS and the UiO-66/CuS composites was illustrated in Scheme 1. For the pure CuS, as shown in Scheme 1a, the CuS nuclei produced in the presence of the copper ions and sulfur ions at the high temperature and pressure. Next, the CuS nuclei tend to grow larger to form CuS nanosheets in the solvothermal process. Then, the CuS nanosheets could self-assemble to construct CuS microsphere with the reaction time increased. Similarly, the grown mechanism of the UiO-66/CuS composites was shown in Scheme 1b. A certain amount of UiO-66 was added into the CuCl2·2H2O suspension, and the uniform suspension could be obtained after ultrasonication. The copper ions uniformly were anchored on the surface of UiO-66 with continuous stirring. Then, the Na2S2O3·5H2O solution acted as sulfide source to combine copper ions to form CuS nanoparticles on the surface of UiO-66. Then, the UiO-66/CuS nuclei were produced, and then continue to grow to form UiO-66/CuS nanosheets. Finally, the UiO-66/ CuS nanosheets self-assembled into UiO-66/CuS microspheres as the reaction time proceeded. The UV–vis absorption spectra of the as-obtained UiO-66, CuS and UiO-66/CuS composites were shown in Fig. 3a. Pure UiO-66 showed significant absorption in the UV-light region, and pure CuS showed a
Fig. 2. SEM images of (a) UiO-66, (b) CuS/UiO-66(2.4:1), (c) CuS/UiO66(1.6:1), (d) CuS/UiO-66(1.2:1), (e) TEM and (f) HRTEM images of CuS/UiO66(2.4:1).
Scheme 1. Schematic illustration of the preparation process of the CuS/UiO-66 composites.
broad and intensive absorption in visible-light region. Surprisingly, all UiO-66/CuS composites possessed notable absorptions in the visible light region which were different from pure UiO-66. The obviously enhanced absorption in visible light region was credited to the presence of CuS in the composites, which was consistent with their colors change from white to black (Fig. S3). The Photoluminescence (PL) spectra show the photogenerated charge separation efficiency of all materials. In general, the lower PL intensity means the low recombination efficiency of electron-hole pairs, which contributes to the better photocatalytic ability. For pure UiO-66, an obvious wide emission peak at 400–500 nm could be found (Fig. 3b). It could be seen that the PL spectra of UiO-66/ CuS composites were similar to that of pure UiO-66. However, the PL intensity of UiO-66/CuS composites obviously decreased compared to 225
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Fig. 3. (a) UV–vis absorption and (b) photoluminescence spectra of UiO-66, CuS and series of CuS/UiO-66 composites.
Fig. 4. (a) Photocatalytic degradation of RhB over various photocatalysts under visible-light irradiation (λ > 420 nm); (b) Photocatalytic degradation compare of using CuS/UiO-66(2.4:1) and mechanical mixture of pristine UiO-66 and pristine CuS as example, the amount of pristine UiO-66 or CuS was equal to the actual amount of that in CuS/UiO-66(2.4:1).
decomposed after irradiation for 1 h when pure UiO-66 was used as photocatalyst. Strikingly, CuS/UiO-66 composites greatly improved photodegradation efficiency of RhB. All of the CuS/UiO-66 composites displayed superior photocatalytic activities than pure CuS and UiO-66. What's more, the CuS/UiO-66(2.4:1) composite exhibited the best photocatalytic activity, over 90% RhB was decomposed in 1 h. To further investigate the synergetic effect between CuS and UiO-66, the photocatalytic activity of CuS and UiO-66 physically mixed according to the weight ratio of CuS/UiO-66(2.4:1) composites with the best photocatalytic activity was also studied. As shown in Fig. 4b, the physical mixture CuS and UiO-66(2.4:1) composites exhibited an obviously decreased photocatalytic performance. It could be inferred that CuS and UiO-66 in CuS/UiO-66(2.4:1) composites were in close contact. It could be greatly improved the separation rate of photo-excited electron-hole pairs. This result demonstrated that the synergistic effect between UiO66 and CuS made it more likely to separate electrons and holes, so the photocatalytic activities of the composite were higher than that of pure materials alone. The photodegradation of the methyl orange (MO) dyes on CuS/UiO-66 composites were further studied under visible light irradiation. Moreover, CuS/UiO-66(2.4:1) composites showed an obviously better photocatalytic activity for the cationic dyes (RhB) than for anionic dyes (MO) (Fig. S4). This phenomenon was ascribed to the electrostatic repulsion between UiO-66 with a negative potential and MO led to partial desorption in the catalytic process [55]. The reusability and stability of the CuS/UiO-66 composites for degradation of RhB under visible light irradiation were also investigated. As shown in Fig. 5, the efficiency of RhB photodegradation declined to
Fig. 5. Recyclability of the photocatalytic decomposition of RhB for bare UiO66, bare CuS, and CuS/UiO-66(2.4:1) composites under visible light.
pure UiO-66. The above results further confirmed that CuS/UiO-66 composites could transfer electrons effectively and possess a lower rate of electron-hole recombination. The outstanding optical properties of the composites were featured to enhance photocatalytic activity. The photocatalytic activity for the degradation of RhB over different photocatalysts under visible light irradiation was investigated, and the results were displayed in Fig. 4. About 50% of RhB dye could be 226
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semiconductors and could be excited under visible light and produce more active species (%O2−, %OH). Furthermore, the existence of energy band cross between UiO-66 and CuS enabled the efficient transfer of electrons and holes at the interface of the composites. It also reduced the recombination of electron-hole pair. Therefore, the RhB degradation reaction could be effectively catalyzed. 4. Conclusions In summary, flower-like CuS/UiO-66 composites were prepared by a facile solvothermal method. Remarkably, all of the CuS/UiO-66 composites displayed better photocatalytic activity than that of the individual UiO-66 and CuS for degradation of RhB under visible-light irradiation. The improved photocatalytic performance was mainly attributed to the synergistic effect between CuS and UiO-66. This result provides a new outlook for application of MOF-CuS composite for removal of pollutants.
Fig. 6. XRD patterns of fresh and used CuS/UiO-66(2.4:1) samples.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21475021 and 21427807) and the Fundamental Research Funds for the Central Universities (2242017K41023). Conflict of interest The authors declare no conflict of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.04.022. References Scheme 2. Mechanism illustration of visible-light-induced RhB photodegradation process on CuS/UiO-66 composites.
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about 73.5% after three cycles, which was still higher than that of bare CuS and UiO-66. The decay of degradation rate during cycling was attributed to the following reasons. First, it is inevitable to lose certain catalysts in recovery process; on the other hand, the catalyst adsorbs a certain amount of RhB from the solution in the first catalytic process. However, the adsorption capacity of the catalyst is basically saturated in the subsequent experiments. Therefore, there is no doubt that the degradation rate decreases slightly during cycling. Furthermore, the XRD pattern revealed that the structure of CuS/UiO-66(2.4:1) composites were still kept intact after photodegradation (Fig. 6). This result suggested that the CuS/UiO-66 composites possessed remarkable catalytic activity and stability in catalytic reaction. A possible mechanism of the enhanced photocatalytic performance of the CuS/UiO-66 composites was proposed in Scheme 2. The band gaps of CuS and UiO-66 were 2.01 eV and 3.50 eV. [46,55] The CuS in the CuS/UiO-66 composites acted as a sensitizer to absorb visible light. Then, the RhB molecular excited to generate photoinduced electrons, and the photoinduced electrons on the LUMO of RhB could transfer to the LUMO of UiO-66 or the CB of CuS via the dye sensitization effect. Furthermore, the photoinduced electrons on the LUMO of UiO-66 could easily transferred to the CB of CuS by the closed contact interfaces. It is helpful to improve the photocatalytic ability of the CuS/UiO-66 composites. In addition, the electrons could rapidly react with H2O2 to produce %OH, and then degrade the RhB molecular. Based on the above results, we have proposed the following mechanism to explain the enhanced catalytic activity after coupling CuS with UiO-66. First, the high surface area of UiO-66 increased the contact interface with CuS. Both UiO-66 and CuS are narrow bandgap 227
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