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MoS2 heterojunction with nanosheets-on-microrods structure for enhanced photocatalytic activity towards tetracycline degradation
Applied Surface Science 491 (2019) 88–94
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Fabrication of p-n CuBi2O4/MoS2 heterojunction with nanosheets-onmicrorods structure for enhanced photocatalytic activity towards tetracycline degradation
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Feng Guoa, Mingyang Lib, Hongji Renb, Xiliu Huangb, Wenxiu Houa, Chao Wanga, Weilong Shic, , ⁎ Changyu Lud, a
School of Energy and Power, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, PR China School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China School of Material science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China d Hebei Province Key Laboratory of Sustained Utilization & Development of Water Recourse, Hebei Province Collaborative Innovation Center for Sustainable Utilization of Water Resources and Optimization of Industrial Structure, Department of Water Resource and Environment, Hebei Geo University, Shijiazhuang 050031, Hebei, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2 CuBi2O4 P-n heterojunction Photocatalyst Tetracycline
Semiconductor-based photocatalytic oxidation technology represents a promising strategy for clean, low-cost, and environmental friendly to solve the water pollution problems by utilizing solar energy. Herein, p-CuBi2O4/nMoS2 as a noble-metal-free efficient visible-light driven composite was successfully synthesized via a simply onepot hydrothermal method, which possessed the unique nanosheets-on-microrods heterostructure. Based on the experimental data analysis, with the assistance of MoS2, the absorption band of the prepared heterojunction becomes broader and covers the entire visible region. And the highest photocatalytic activity for the degradation of tetracycline was achieved when introducing ~5% weight ratio of MoS2 in the composite (76%, 120 min), which is about 2.79 and 2.96 times than that of MoS2 and CuBi2O4, respectively. And the improvement of the photocatalytic activity of the p-CuBi2O4/n-MoS2 derives from the energy band matching and the formation of the built-in electric field. This work will open opportunities for designing the high-efficient CuBi2O4-based p-n heterojunction photocatalysts and would be of great significance to satisfy ever-increasing environmental demands in the future.
1. Introduction In recent years, with the development of the economy, environmental pollution and energy shortage are becoming more and more serious, which are directly related to the daily life of human beings [1–3]. Among the environmental problems, the contamination of antibiotic wastewater causes the widespread concern of the world. Hundreds of antibiotics have been detected in seawater, groundwater, surface water and even drinking water deriving from the severe abuse phenomenon [4,5]. Owing to the fact that antibiotic wastewater exists the characteristics of high biotoxicity, difficult degradability and drug resistance [6], people persistently drinks water containing tetracycline antibiotics, which can change their composition of the intestinal flora, further resulting in the fatty liver, metabolic syndrome, autism and other diseases to extremely harm the human health. Therefore, it is of great urgency to search for an ideal way to remove the antibiotics from
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water bodies. As an economic, clean and safe technology, semiconductor photocatalysis can directly utilize solar energy as a driving force to participate in various photocatalytic reactions such as photocatalytic hydrogen production, photocatalytic carbon dioxide reduction and photocatalytic degradation of organic pollutants in water, etc. [7–15]. In the past few decades, a large number of UV or visible light-driven photocatalysts have been reported, such as TiO2 [16], ZnO [17], WO3 [18], AgBr [19], Bi2WO6 [20], and CdS [21], CuInZnS [22], Cu2O [23], etc. Among these semiconductors, CuBi2O4, as a p-type semiconductor material, exhibits excellent photoelectric properties under visible light irradiation due to its narrow band gap (Eg = 1.6–1.8 eV), which has been widely applied in various fields. For example, Zhu et al. reported the CuBi2O4 semiconductor, as a photocatalyst, possessing high antibacterial effect on Escherichia coli [24]. Geetu Sharma et al. used CuBi2O4 for photocatalytic generation of hydrogen fuel from water to
Corresponding authors. E-mail addresses: [email protected] (W. Shi), [email protected] (C. Lu).
https://doi.org/10.1016/j.apsusc.2019.06.158 Received 21 April 2019; Received in revised form 10 June 2019; Accepted 14 June 2019 Available online 16 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration of the synthesis procedure to obtain CBO/MS heterojunction.
2. Experimental section
alleviate the shortages of energy [25]. However, pure CuBi2O4 photocatalyst still exhibits low photocatalytic efficiency because of the fast recombination of the photo-generated electron-hole pairs. Up to now, some strategies have been proposed to enhance the photocatalytic performance of CuBi2O4, such as integration with carbon materials [26], constructed the heterojunctions [27], and so on. Significantly, it is proven that the immobilization of co-catalysts onto the surface of semiconductors is one of the most efficient strategies, which can not only effectively promote the separation of the photogenerated charges, but also be benefit for the surface catalytic reaction by reducing the activation energy [28,29]. In this regard, noble metals (e.g. Pt, Ag and Au) frequently serve as highly efficient co-catalysts to boost the photocatalytic performance of the single semiconductor [30–32]. Regrettably, because of the rarity and expensive property, the application of noble metals is restricted. Consequently, it is highly desired to explore non-noble metal co-catalysts to modify CuBi2O4 for improving its photocatalytic activity. Molybdenum disulfide (MoS2), as an n-type semiconductor, has a layered structure, and its atoms are covalently bonded to form twodimensional layer structure, which are superimposed by a weak van der Waals interaction. MoS2 with the narrow band gap of around 1.7 eV has a strong absorption capacity in the visible region [33]. For its features of earth-abundant, low-cost and high stability, MoS2 is mostly used as a co-catalyst for enhancing the photocatalytic activity of semiconductors [34,35]. Recently, it is reported that MoS2 can activate O2 to produce superoxide radicals (%O2−) that play a crucial role in photooxidation process [36]. For example, Zhou et al. prepared MoS2/GO composite by a one-step hydrothermal method, which exhibited improved photocatalytic activities to degrade 99% MB within 60 min [37]. Wen et al. synthesized MoS2/g-C3N4 nanocomposites showed enhanced visiblelight photocatalytic activity for the removal of nitric oxide (NO) [38]. Thus, we try to use n-MoS2 as a co-catalyst to couple p-CuBi2O4 to form p-CuBi2O4/n-MoS2 for improved the photocatalytic photocatalytic degradation activity of organic pollutants. Moreover, to the best of our knowledge, the study on the synthesis and photocatalytic activity of p-n CuBi2O4/MoS2 heterojunction has not yet been reported. Herein, p-n CuBi2O4/MoS2 heterojunction photocatalyst with nanosheets-on-microrods structure was prepared by a facile one-step hydrothermal method to degrade tetracycline hydrochloride (TC) under the visible light irradiation. The results showed that the photogenerated carriers of heterojunction were effectively separated and the photocatalytic activity was significantly improved. Moreover, the possible photocatalytic mechanism of p-n CuBi2O4/MoS2 was proposed and discussed.
Briefly, 2.42 g of Bi(NO3)3·5H2O, 0.6 g Cu(NO3)2·2.5H2O and 0.87 g NaOH were mixed in 80 mL of distill water and stirred for 3 h. Then, the homogeneous precursor was transferred into 100 mL Teflon-lined stainless-steel autoclave, and kept at 180 °C for 24 h. After cooling to room temperature, the product was collected, washed and dried, and then the pure CuBi2O4 was prepared and labeled as CBO. For the CuBi2O4/MoS2 heterojunctions, different weight percentages MoS2 nanosheets were added to the fabricated process of CuBi2O4 microrods. As the CuBi2O4/MoS2-5% for example, in a typical procedure, 0.143 g commercial MoS2, 2.42 g of Bi(NO3)3·5H2O, 0.6 g Cu(NO3)2·2.5H2O and 0.87 g NaOH were mixed in the 80 mL distill water and followed by vigorous stirring 3 h for a uniform suspension. The mixed solution was transferred into a Teflon-lined steel autoclave, which was heated in an oven at 180 °C for 24 h. At last, the obtained CuBi2O4/MoS2-5% composite was collected and washed with ethanol and distilled water several times, and dried at 70 °C for 6 h. On this basis, different mass ratios of CuBi2O4/MoS2 composites at 1 wt%, 3 wt%, 5 wt%, 7 wt% and 10 wt % were prepared and denoted as CBO/MS-1%, CBO/MS-3%, CBO/MS5%, CBO/MS-7% and CBO/MS-10%, respectively. The following discussion will focus on the sample (CuBi2O4/MoS2-5%) with highest photocatalytic performance. Thus, this condition was selected to synthesize composite and the product was abbreviated as CBO/MS. In addition, pure MoS2 was abbreviated as MS. The characterizations and photocatalytic degradation parameters of TC can be observed in the Supporting Information. 3. Results and discussion The synthesis route of CBO/MS heterojunction photocatalyst was illustrated in Fig. 1. Initially, MS was put into the precursor mixed solution of Bi(NO3)3·5H2O, Cu(NO3)2·3H2O and NaOH, stirred and mixed uniformly. Then, the above mixture was transferred into Teflonlined steel autoclave for 24 h hydrothermal reaction (180 °C). Through the facilely one-step hydrothermal method, the MS nanosheets were coated onto the CBO microrods to successfully form CBO/MS heterojunction with nanosheets-on-microrods structure. The crystalline structure of MS, CBO and CBO/MS composite was clarified by X-ray diffraction (XRD) analysis and presented in Fig. 2a. The characteristic diffraction peaks of CBO and MS can perfectly indexed to JCPDS No. 72-493 and 2-132, respectively [27,36]. For the CBO/MS heterojunction, CBO maintains the monoclinic phase, and the extra diffraction peaks from MS nanosheets can also be found, suggesting the existence of MS in the composite. In order to further prove 89
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Fig. 2. (a) XRD patterns of MS, CBO and CBO/MS heterojunction. (b–d) TEM and HRTEM analysis of CBO/MS heterojunction. (e–i) TEM-EDX element mapping of the CBO/MS heterojunction for Cu, Bi, O, Mo and S elements.
centered at 159 eV and 164 eV are pertained to Bi 4f7/2 and Bi 4f5/2, respectively, signifying the presence of Bi3+ in CBO [41,42]. Apart from this, an obscure peak was found adjacent to Bi 4f5/2, corresponding to the 2p peak of S2− [43]. The relatively weak intensity can be ascribed to the low content of MS in composite and the peak of S 2p binding energy was overlapped with that of Bi 4f5/2. In Fig. 3c, the O 1s spectrum can be deconvoluted into two peaks centered at the binding energies of 529.4 eV and 531.6 eV, respectively. The stronger peak at 529.4 eV is the feature of lattice oxygen (OL), and the lower one is originated from the chemisorbed oxygen (OC) [43]. As shown in Fig. 3d, on the one hand, the peak of Mo4+ locates at 229 eV and 232 eV (assigned to the Mo 3d5/2 and Mo 3d3/2 peak, respectively) [44]. On the other hand, the peak at 235.6 eV is caused by the presence of Mo6+ [45], which is probably due to oxidation in air. Notably, a weak S 2s peak of MS can be also observed from Fig. 3d. These results indicate the CBO/MS heterojunction was successfully formed. UV–vis diffuse reflectance spectra were performed to investigate the optical properties of as-prepared photocatalysts. Fig. 4a shows that both pristine CBO and MS possess strong photo-absorption wavelengths from ultraviolet to visible-light region, and the absorption band edges of CBO and MS are approximately located at 700 and 750 nm, respectively. After MS loading, the as-fabricated CBO/MS heterojunction exhibits an obvious red shift compared with bare CBO, indicating the efficient photosensitization of MS. As shown in Fig. 4b, according to the Tauc plots converted curve, the band gaps of pure CBO and MS were estimated to be 1.76 and 1.65 eV, respectively, which are very closed to previous reports [40,45]. Electrochemical Mott-Schottky plots of pure CBO and MS were analyzed at a frequency of 1.0 kHz in 0.5 M Na2SO4 aqueous solution at room temperature (Fig. 4c and d). The results reveal that CBO is a p-type semiconductor character with the negative
the actual content of each element in the composite sample, the ICPAES element analysis of the CBO/MS sample (5 wt% MoS2 in composite) was carried out. The test results are as shown in the Table S1, the actual element content ratio of Cu, Bi and Mo element is 19.09:38.57:1.00, which is close to the stoichiometric ratio of 19:38:1 in the composite. This result is consistent with the theoretical value within the allowable error range. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and scanning TEM-energy-dispersive X-ray (TEM-EDX) element mapping were carried out to observe the morphologies and to detect the composition of as-prepared photocatalyst. Based on the SEM observation (Fig. S1), pure CBO presents uniform rod-like structure with average diameter about 100–150 nm, which is agreement with our previous report [39]. After hydrothermal process, the surface of CBO microrods is evenly covered with ultrathin MS nanosheets (Fig. 2b and c). The HRTEM image (Fig. 2d) further affirms the coexistence of CBO and MS. Concretely, the lattice fringes spacing of 0.244 nm and 0.27 nm are corresponded to the (211) and (100) planes of CBO and MS, respectively [36,40]. Moreover, the TEM-EDX element mapping images (Fig. 2e–i) of Cu, Bi, O, Mo and S demonstrated the homogenous distribution and intimate contact of MS and CBO in heterojunction. XPS analysis was conducted to confirm the elemental states and chemical composition of CBO/MS. XPS survey spectrum (Fig. S2) revealed that the main elements of Bi, Mo, O, and Cu are in the obtained composite. In the Cu 2p XPS spectrum (Fig. 3a), the binding energy peaks at 934 eV and 954 eV are corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. Additionally, there are two peaks at 943 eV and 963 eV can be assigned to the satellite peaks of Cu 2p, implying the existence of divalent copper in the heterojunction, which is consistent with the previous research [27]. From Fig. 3b, the peaks of Bi 4f XPS spectrum 90
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Fig. 3. High-resolution XPS spectra of CBO/MS: (a) Cu 2p; (b) Bi 4f (S 2p); (c) O 1s and (d) Mo 3d.
Fig. 4. (a) UV–vis absorption spectra of CBO, MS and CBO/MS. (b) Plots of (αhv)2 vs. hv for the band gap energies of CBO and MS. Mott-Schottky plots for (c) CBO and (d) MS. 91
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Fig. 5. (a) Absorption properties of TC over as-synthesized samples and blank test for TC degradation. (b) Photodegradations of TC over as-prepared products under visible irradiation (λ > 420 nm). (c) Apparent rate constants for the prepared samples for TC degradation. (d) Cycling runs for the photocatalytic TC degradation in the presence of CBO/MS (5% of MS contents).
120 min). Meanwhile, all as-prepared photocatalysts can be well accorded with the first order kinetics during the TC degradation process (Fig. S4). By comparing the apparent rate constants of prepared photocatalysts (Fig. 5c), CBO/MS-5% displays the fastest degradation rate (0.0095 min−1), which is about 2.79 and 2.96 times than that of MS (0.0034 min−1) and CBO (0.0032 min−1), respectively. Moreover, in order to further reveal the mineralization degree of TC in photocatalysis, the evolution of total organic carbon (TOC) under visible light irradiation was studied. As shown in Fig. S5, in the absence of any photocatalyst, only 0.4% of TOC was removed, while CBO/MS-5% photocatalyst reduced TOC by 48.3%. This indicates that CBO/MS composites have good mineralization efficiency for TC photodegradation. The cycling experiment was carried out to detect the reusability of the CBO/MS photocatalyst (Fig. 5d). To further rule out the photosensitization effect under light irradiation, the photodegradation of colorless phenol was investigated and shown in Fig.S6. The as-prepared CBO/MS composite showed enhanced photocatalytic activity compared with CBO and MS, and its effective visible light catalytic performance was confirmed. After 3 cycles, although the degradation rate decreased slightly, it still demonstrated the high efficiency and stability of CBO/ MS composite. The XRD patterns (Fig. S7) of CBO/MS before and after three photocatalytic cycles were compared, which showed that there was no obvious change in CBO/MS. In addition, the TEM image (inset of Fig. S7) of CBO/MS after photocatalysis was taken, demonstrating that the heterojunction still maintained the stable interface between MS nanosheets and CBO microrods. The reproducible transient photocurrent responses of CBO, MS and CBO/MS heterojunction were recorded at open circuit potential in 0.5 M Na2SO4 electrolyte. As shown from Fig. 6a, it is obvious that both pure CBO and MS showed lower photocurrent intensities due to the high recombination rate of photo-generated carriers. Notably, the photocurrent density increase greatly after the modification of MS on
slope of the linear plot, and MS is an n-type semiconductor with the positive slope of the linear plot. It can be found that the flat band (FB) potentials of CBO and MS are determined to be 1.02 and −0.07 V (vs. NHE). Note that the FB potentials is 0.1 V higher than the conduction band (CB) potentials in the n-type semiconductors; however, the FB potentials is 0.1 V are considered to be about 0.1 V below than the valence band (VB) potentials in the p-type semiconductors [46]. Consequently, the CB and VB potentials of MS and CBO are speculated to be −0.17 and 1.12 V (vs. NHE). Furthermore, considering their band gaps, the VB and CB potentials of MS and CBO are calculated to be 1.48 and −0.64 V (vs. NHE), respectively. Therefore, both above p-type and ntype characteristic semiconductors of CBO and MS satisfy the necessary requirement for p-n heterojunction. TC was selected as the target pollutant to test the photocatalytic activity of as-prepared photocatalysts under visible light irradiation (λ > 420 nm). Fig. 5a shows the dark adsorption process of all samples before the degradation of TC solution. Typically, photocatalysts were suspended in TC solution and stirred for 30 min to reach adsorption equilibrium. Compared with pure MS and CBO, the adsorption content of the composite photocatalysts improved slightly, indicating that the surface adsorption capacity of the photocatalyst is enhanced by the combination of the two materials, as verified by nitrogen sorption isotherms of as-prepared CBO/MS composite samples (Fig. S3). Furthermore, it is obtained that in the absence of photocatalyst, and there is no self-degradation of TC under visible light irradiation. From Fig. 5b, the degradation efficiency of TC are only 39% and 36% for MS and CBO, respectively. The photocatalytic activity of photocatalysts are evidently improved after loading MS nanosheets on the surface of CBO microrods, which can be largely attributed to the efficiency separation of photogenerated electron-hole pairs by forming p-n heterojunction. The CBO/MS heterojunction with containing 5% MS mass ratio exhibits the highest photocatalytic activity (76% degradation of TC within 92
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Fig. 6. Photoelectrochemical (a) and electrochemical impedance spectroscopy measurements (b) of MS, CBO and CBO/MS heterojunction.
p-n heterojunction is constructed and a built-in electric field is generated in the space charge region (Fig. 8b). Under visible light irradiation, electrons are excited from the VBs of CBO and MS to their respective CBs, leaving holes in the VBs. Then, the internal electric field drives the electrons and holes flow to the opposite direction, that is, the electrons flows from CB of CBO to that of MS, holes from VB of MS to that of CBO, thus effectively inhibiting the recombination of photogenerated electron-hole pairs, which more photogenerated electrons and holes on the surface of CBO and MS can participate in the photocatalytic reaction process, therefore the photocatalytic activity of CBO/MS heterojunction is significantly improved.
CBO. This indicates the faster transfer rate of photo-generated electrons of CBO/MS p-n heterojunction, leading to more efficient separation of electron-hole pairs. Again, the electrochemical impedance spectroscopy (EIS) of as-prepared samples was displayed in Fig. 6b. The smaller radius of the arc represents the lower charge-transfer impedance [47]. It can be seen that CBO/MS displays the smallest arc radius, which can further testify photogenerated electrons-holes pairs can be efficiently separated by loading MS nanosheets onto the CBO microrods. These above results reveal two positive roles (as co-catalyst and forming p-n heterojunction) of loading MS nanosheets in heterojunction for enhancing photocatalytic activity. The trapping experiment of active species was performed to investigate the photocatalytic mechanism of CBO/MS heterojunction in detail. Disodium ethylenediamine tetra-acetic acid (EDTA), 2-propanol (IPA) and Nitrogen gas (N2) as the scavengers of h+, %OH and %O2− were separately added into the TC solution system of containing photocatalysts. As shown in Fig. 7a, the introduction of EDTA and N2 were significantly suppressed the photocatalytic degradation efficiency, which indicated the h+ and %O2− were main participants in this photocatalytic system. In order to further verify the existence of %O2− species during the photocatalysis, the experiment of electron-spin resonance (ESR) is illustrated in Fig. 7b. There is no characteristic peaks of active species can be observed at the condition of dark. With the exposure of light irradiation, the obvious signals of DMPO-%O2− can be observed, demonstrating that more %O2− radicals were generated and participated in photocatalysis. Based on the above analysis, the discussion for the photocatalytic mechanism of p-CBO/n-MS is revealed in Fig. 8. As speculated by the Mott-Schottky plots in Fig. 4b and d, the corresponding positions of CB and VB for p-type CBO and n-type MS before contact have given in the Fig. 8a. As reported, when p-type CBO and n-type MS were in contact,
4. Conclusions In this work, CBO/MS p-n heterojunction photocatalyst with nanosheets-on-microrods structure was successfully designed to degrade TC under visible light for environmental restoration. Compare to the pristine MS and CBO, CBO/MS heterojunction displayed more excellent photocatalytic performances, particularly the CBO/MS-5% showed the highest photocatalytic performance (TC degradation rate of 76% within 120 min), which could correspond to the inhibition of photogenerated electron-hole pairs and the improvement of light absorption. Our work can pave the way in favor of the more efficient p-n heterojunction composite photocatalysts designation to meet the demand of environment for the better future development. Acknowledgements This work is supported by Postgraduate Research & Practice Innovation Program of Jiangsu Province (China) (KYCX18_2340, KYCX19_1677 and KYCX19_1706), the Doctoral Scientific Research
Fig. 7. (a) Effects of a series of scavengers on the TC over CBO/MS. (b) ESR spectra of CBO/MS in methanol aqueous dispersion with/without visible light irradiation. 93
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Fig. 8. The schematic of charge transfer between p-type CuBi2O4 and n-type MoS2: (a) before contact and (b) after the formation of the p-n heterojunction.
Foundation of Jiangsu University of Science and Technology (China) (1062931806 and 1142931803) and General Project of basic Research Plan of Natural Science Foundation of Shaanxi Province (2019JQ-382).
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Fabrication of p-n CuBi2O4/MoS2 heterojunction with nanosheets-onmicrorods structure for enhanced photocatalytic activity towards tetracycline degradation
T
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Feng Guoa, Mingyang Lib, Hongji Renb, Xiliu Huangb, Wenxiu Houa, Chao Wanga, Weilong Shic, , ⁎ Changyu Lud, a
School of Energy and Power, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, PR China School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China School of Material science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China d Hebei Province Key Laboratory of Sustained Utilization & Development of Water Recourse, Hebei Province Collaborative Innovation Center for Sustainable Utilization of Water Resources and Optimization of Industrial Structure, Department of Water Resource and Environment, Hebei Geo University, Shijiazhuang 050031, Hebei, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2 CuBi2O4 P-n heterojunction Photocatalyst Tetracycline
Semiconductor-based photocatalytic oxidation technology represents a promising strategy for clean, low-cost, and environmental friendly to solve the water pollution problems by utilizing solar energy. Herein, p-CuBi2O4/nMoS2 as a noble-metal-free efficient visible-light driven composite was successfully synthesized via a simply onepot hydrothermal method, which possessed the unique nanosheets-on-microrods heterostructure. Based on the experimental data analysis, with the assistance of MoS2, the absorption band of the prepared heterojunction becomes broader and covers the entire visible region. And the highest photocatalytic activity for the degradation of tetracycline was achieved when introducing ~5% weight ratio of MoS2 in the composite (76%, 120 min), which is about 2.79 and 2.96 times than that of MoS2 and CuBi2O4, respectively. And the improvement of the photocatalytic activity of the p-CuBi2O4/n-MoS2 derives from the energy band matching and the formation of the built-in electric field. This work will open opportunities for designing the high-efficient CuBi2O4-based p-n heterojunction photocatalysts and would be of great significance to satisfy ever-increasing environmental demands in the future.
1. Introduction In recent years, with the development of the economy, environmental pollution and energy shortage are becoming more and more serious, which are directly related to the daily life of human beings [1–3]. Among the environmental problems, the contamination of antibiotic wastewater causes the widespread concern of the world. Hundreds of antibiotics have been detected in seawater, groundwater, surface water and even drinking water deriving from the severe abuse phenomenon [4,5]. Owing to the fact that antibiotic wastewater exists the characteristics of high biotoxicity, difficult degradability and drug resistance [6], people persistently drinks water containing tetracycline antibiotics, which can change their composition of the intestinal flora, further resulting in the fatty liver, metabolic syndrome, autism and other diseases to extremely harm the human health. Therefore, it is of great urgency to search for an ideal way to remove the antibiotics from
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water bodies. As an economic, clean and safe technology, semiconductor photocatalysis can directly utilize solar energy as a driving force to participate in various photocatalytic reactions such as photocatalytic hydrogen production, photocatalytic carbon dioxide reduction and photocatalytic degradation of organic pollutants in water, etc. [7–15]. In the past few decades, a large number of UV or visible light-driven photocatalysts have been reported, such as TiO2 [16], ZnO [17], WO3 [18], AgBr [19], Bi2WO6 [20], and CdS [21], CuInZnS [22], Cu2O [23], etc. Among these semiconductors, CuBi2O4, as a p-type semiconductor material, exhibits excellent photoelectric properties under visible light irradiation due to its narrow band gap (Eg = 1.6–1.8 eV), which has been widely applied in various fields. For example, Zhu et al. reported the CuBi2O4 semiconductor, as a photocatalyst, possessing high antibacterial effect on Escherichia coli [24]. Geetu Sharma et al. used CuBi2O4 for photocatalytic generation of hydrogen fuel from water to
Corresponding authors. E-mail addresses: [email protected] (W. Shi), [email protected] (C. Lu).
https://doi.org/10.1016/j.apsusc.2019.06.158 Received 21 April 2019; Received in revised form 10 June 2019; Accepted 14 June 2019 Available online 16 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration of the synthesis procedure to obtain CBO/MS heterojunction.
2. Experimental section
alleviate the shortages of energy [25]. However, pure CuBi2O4 photocatalyst still exhibits low photocatalytic efficiency because of the fast recombination of the photo-generated electron-hole pairs. Up to now, some strategies have been proposed to enhance the photocatalytic performance of CuBi2O4, such as integration with carbon materials [26], constructed the heterojunctions [27], and so on. Significantly, it is proven that the immobilization of co-catalysts onto the surface of semiconductors is one of the most efficient strategies, which can not only effectively promote the separation of the photogenerated charges, but also be benefit for the surface catalytic reaction by reducing the activation energy [28,29]. In this regard, noble metals (e.g. Pt, Ag and Au) frequently serve as highly efficient co-catalysts to boost the photocatalytic performance of the single semiconductor [30–32]. Regrettably, because of the rarity and expensive property, the application of noble metals is restricted. Consequently, it is highly desired to explore non-noble metal co-catalysts to modify CuBi2O4 for improving its photocatalytic activity. Molybdenum disulfide (MoS2), as an n-type semiconductor, has a layered structure, and its atoms are covalently bonded to form twodimensional layer structure, which are superimposed by a weak van der Waals interaction. MoS2 with the narrow band gap of around 1.7 eV has a strong absorption capacity in the visible region [33]. For its features of earth-abundant, low-cost and high stability, MoS2 is mostly used as a co-catalyst for enhancing the photocatalytic activity of semiconductors [34,35]. Recently, it is reported that MoS2 can activate O2 to produce superoxide radicals (%O2−) that play a crucial role in photooxidation process [36]. For example, Zhou et al. prepared MoS2/GO composite by a one-step hydrothermal method, which exhibited improved photocatalytic activities to degrade 99% MB within 60 min [37]. Wen et al. synthesized MoS2/g-C3N4 nanocomposites showed enhanced visiblelight photocatalytic activity for the removal of nitric oxide (NO) [38]. Thus, we try to use n-MoS2 as a co-catalyst to couple p-CuBi2O4 to form p-CuBi2O4/n-MoS2 for improved the photocatalytic photocatalytic degradation activity of organic pollutants. Moreover, to the best of our knowledge, the study on the synthesis and photocatalytic activity of p-n CuBi2O4/MoS2 heterojunction has not yet been reported. Herein, p-n CuBi2O4/MoS2 heterojunction photocatalyst with nanosheets-on-microrods structure was prepared by a facile one-step hydrothermal method to degrade tetracycline hydrochloride (TC) under the visible light irradiation. The results showed that the photogenerated carriers of heterojunction were effectively separated and the photocatalytic activity was significantly improved. Moreover, the possible photocatalytic mechanism of p-n CuBi2O4/MoS2 was proposed and discussed.
Briefly, 2.42 g of Bi(NO3)3·5H2O, 0.6 g Cu(NO3)2·2.5H2O and 0.87 g NaOH were mixed in 80 mL of distill water and stirred for 3 h. Then, the homogeneous precursor was transferred into 100 mL Teflon-lined stainless-steel autoclave, and kept at 180 °C for 24 h. After cooling to room temperature, the product was collected, washed and dried, and then the pure CuBi2O4 was prepared and labeled as CBO. For the CuBi2O4/MoS2 heterojunctions, different weight percentages MoS2 nanosheets were added to the fabricated process of CuBi2O4 microrods. As the CuBi2O4/MoS2-5% for example, in a typical procedure, 0.143 g commercial MoS2, 2.42 g of Bi(NO3)3·5H2O, 0.6 g Cu(NO3)2·2.5H2O and 0.87 g NaOH were mixed in the 80 mL distill water and followed by vigorous stirring 3 h for a uniform suspension. The mixed solution was transferred into a Teflon-lined steel autoclave, which was heated in an oven at 180 °C for 24 h. At last, the obtained CuBi2O4/MoS2-5% composite was collected and washed with ethanol and distilled water several times, and dried at 70 °C for 6 h. On this basis, different mass ratios of CuBi2O4/MoS2 composites at 1 wt%, 3 wt%, 5 wt%, 7 wt% and 10 wt % were prepared and denoted as CBO/MS-1%, CBO/MS-3%, CBO/MS5%, CBO/MS-7% and CBO/MS-10%, respectively. The following discussion will focus on the sample (CuBi2O4/MoS2-5%) with highest photocatalytic performance. Thus, this condition was selected to synthesize composite and the product was abbreviated as CBO/MS. In addition, pure MoS2 was abbreviated as MS. The characterizations and photocatalytic degradation parameters of TC can be observed in the Supporting Information. 3. Results and discussion The synthesis route of CBO/MS heterojunction photocatalyst was illustrated in Fig. 1. Initially, MS was put into the precursor mixed solution of Bi(NO3)3·5H2O, Cu(NO3)2·3H2O and NaOH, stirred and mixed uniformly. Then, the above mixture was transferred into Teflonlined steel autoclave for 24 h hydrothermal reaction (180 °C). Through the facilely one-step hydrothermal method, the MS nanosheets were coated onto the CBO microrods to successfully form CBO/MS heterojunction with nanosheets-on-microrods structure. The crystalline structure of MS, CBO and CBO/MS composite was clarified by X-ray diffraction (XRD) analysis and presented in Fig. 2a. The characteristic diffraction peaks of CBO and MS can perfectly indexed to JCPDS No. 72-493 and 2-132, respectively [27,36]. For the CBO/MS heterojunction, CBO maintains the monoclinic phase, and the extra diffraction peaks from MS nanosheets can also be found, suggesting the existence of MS in the composite. In order to further prove 89
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Fig. 2. (a) XRD patterns of MS, CBO and CBO/MS heterojunction. (b–d) TEM and HRTEM analysis of CBO/MS heterojunction. (e–i) TEM-EDX element mapping of the CBO/MS heterojunction for Cu, Bi, O, Mo and S elements.
centered at 159 eV and 164 eV are pertained to Bi 4f7/2 and Bi 4f5/2, respectively, signifying the presence of Bi3+ in CBO [41,42]. Apart from this, an obscure peak was found adjacent to Bi 4f5/2, corresponding to the 2p peak of S2− [43]. The relatively weak intensity can be ascribed to the low content of MS in composite and the peak of S 2p binding energy was overlapped with that of Bi 4f5/2. In Fig. 3c, the O 1s spectrum can be deconvoluted into two peaks centered at the binding energies of 529.4 eV and 531.6 eV, respectively. The stronger peak at 529.4 eV is the feature of lattice oxygen (OL), and the lower one is originated from the chemisorbed oxygen (OC) [43]. As shown in Fig. 3d, on the one hand, the peak of Mo4+ locates at 229 eV and 232 eV (assigned to the Mo 3d5/2 and Mo 3d3/2 peak, respectively) [44]. On the other hand, the peak at 235.6 eV is caused by the presence of Mo6+ [45], which is probably due to oxidation in air. Notably, a weak S 2s peak of MS can be also observed from Fig. 3d. These results indicate the CBO/MS heterojunction was successfully formed. UV–vis diffuse reflectance spectra were performed to investigate the optical properties of as-prepared photocatalysts. Fig. 4a shows that both pristine CBO and MS possess strong photo-absorption wavelengths from ultraviolet to visible-light region, and the absorption band edges of CBO and MS are approximately located at 700 and 750 nm, respectively. After MS loading, the as-fabricated CBO/MS heterojunction exhibits an obvious red shift compared with bare CBO, indicating the efficient photosensitization of MS. As shown in Fig. 4b, according to the Tauc plots converted curve, the band gaps of pure CBO and MS were estimated to be 1.76 and 1.65 eV, respectively, which are very closed to previous reports [40,45]. Electrochemical Mott-Schottky plots of pure CBO and MS were analyzed at a frequency of 1.0 kHz in 0.5 M Na2SO4 aqueous solution at room temperature (Fig. 4c and d). The results reveal that CBO is a p-type semiconductor character with the negative
the actual content of each element in the composite sample, the ICPAES element analysis of the CBO/MS sample (5 wt% MoS2 in composite) was carried out. The test results are as shown in the Table S1, the actual element content ratio of Cu, Bi and Mo element is 19.09:38.57:1.00, which is close to the stoichiometric ratio of 19:38:1 in the composite. This result is consistent with the theoretical value within the allowable error range. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and scanning TEM-energy-dispersive X-ray (TEM-EDX) element mapping were carried out to observe the morphologies and to detect the composition of as-prepared photocatalyst. Based on the SEM observation (Fig. S1), pure CBO presents uniform rod-like structure with average diameter about 100–150 nm, which is agreement with our previous report [39]. After hydrothermal process, the surface of CBO microrods is evenly covered with ultrathin MS nanosheets (Fig. 2b and c). The HRTEM image (Fig. 2d) further affirms the coexistence of CBO and MS. Concretely, the lattice fringes spacing of 0.244 nm and 0.27 nm are corresponded to the (211) and (100) planes of CBO and MS, respectively [36,40]. Moreover, the TEM-EDX element mapping images (Fig. 2e–i) of Cu, Bi, O, Mo and S demonstrated the homogenous distribution and intimate contact of MS and CBO in heterojunction. XPS analysis was conducted to confirm the elemental states and chemical composition of CBO/MS. XPS survey spectrum (Fig. S2) revealed that the main elements of Bi, Mo, O, and Cu are in the obtained composite. In the Cu 2p XPS spectrum (Fig. 3a), the binding energy peaks at 934 eV and 954 eV are corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. Additionally, there are two peaks at 943 eV and 963 eV can be assigned to the satellite peaks of Cu 2p, implying the existence of divalent copper in the heterojunction, which is consistent with the previous research [27]. From Fig. 3b, the peaks of Bi 4f XPS spectrum 90
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Fig. 3. High-resolution XPS spectra of CBO/MS: (a) Cu 2p; (b) Bi 4f (S 2p); (c) O 1s and (d) Mo 3d.
Fig. 4. (a) UV–vis absorption spectra of CBO, MS and CBO/MS. (b) Plots of (αhv)2 vs. hv for the band gap energies of CBO and MS. Mott-Schottky plots for (c) CBO and (d) MS. 91
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Fig. 5. (a) Absorption properties of TC over as-synthesized samples and blank test for TC degradation. (b) Photodegradations of TC over as-prepared products under visible irradiation (λ > 420 nm). (c) Apparent rate constants for the prepared samples for TC degradation. (d) Cycling runs for the photocatalytic TC degradation in the presence of CBO/MS (5% of MS contents).
120 min). Meanwhile, all as-prepared photocatalysts can be well accorded with the first order kinetics during the TC degradation process (Fig. S4). By comparing the apparent rate constants of prepared photocatalysts (Fig. 5c), CBO/MS-5% displays the fastest degradation rate (0.0095 min−1), which is about 2.79 and 2.96 times than that of MS (0.0034 min−1) and CBO (0.0032 min−1), respectively. Moreover, in order to further reveal the mineralization degree of TC in photocatalysis, the evolution of total organic carbon (TOC) under visible light irradiation was studied. As shown in Fig. S5, in the absence of any photocatalyst, only 0.4% of TOC was removed, while CBO/MS-5% photocatalyst reduced TOC by 48.3%. This indicates that CBO/MS composites have good mineralization efficiency for TC photodegradation. The cycling experiment was carried out to detect the reusability of the CBO/MS photocatalyst (Fig. 5d). To further rule out the photosensitization effect under light irradiation, the photodegradation of colorless phenol was investigated and shown in Fig.S6. The as-prepared CBO/MS composite showed enhanced photocatalytic activity compared with CBO and MS, and its effective visible light catalytic performance was confirmed. After 3 cycles, although the degradation rate decreased slightly, it still demonstrated the high efficiency and stability of CBO/ MS composite. The XRD patterns (Fig. S7) of CBO/MS before and after three photocatalytic cycles were compared, which showed that there was no obvious change in CBO/MS. In addition, the TEM image (inset of Fig. S7) of CBO/MS after photocatalysis was taken, demonstrating that the heterojunction still maintained the stable interface between MS nanosheets and CBO microrods. The reproducible transient photocurrent responses of CBO, MS and CBO/MS heterojunction were recorded at open circuit potential in 0.5 M Na2SO4 electrolyte. As shown from Fig. 6a, it is obvious that both pure CBO and MS showed lower photocurrent intensities due to the high recombination rate of photo-generated carriers. Notably, the photocurrent density increase greatly after the modification of MS on
slope of the linear plot, and MS is an n-type semiconductor with the positive slope of the linear plot. It can be found that the flat band (FB) potentials of CBO and MS are determined to be 1.02 and −0.07 V (vs. NHE). Note that the FB potentials is 0.1 V higher than the conduction band (CB) potentials in the n-type semiconductors; however, the FB potentials is 0.1 V are considered to be about 0.1 V below than the valence band (VB) potentials in the p-type semiconductors [46]. Consequently, the CB and VB potentials of MS and CBO are speculated to be −0.17 and 1.12 V (vs. NHE). Furthermore, considering their band gaps, the VB and CB potentials of MS and CBO are calculated to be 1.48 and −0.64 V (vs. NHE), respectively. Therefore, both above p-type and ntype characteristic semiconductors of CBO and MS satisfy the necessary requirement for p-n heterojunction. TC was selected as the target pollutant to test the photocatalytic activity of as-prepared photocatalysts under visible light irradiation (λ > 420 nm). Fig. 5a shows the dark adsorption process of all samples before the degradation of TC solution. Typically, photocatalysts were suspended in TC solution and stirred for 30 min to reach adsorption equilibrium. Compared with pure MS and CBO, the adsorption content of the composite photocatalysts improved slightly, indicating that the surface adsorption capacity of the photocatalyst is enhanced by the combination of the two materials, as verified by nitrogen sorption isotherms of as-prepared CBO/MS composite samples (Fig. S3). Furthermore, it is obtained that in the absence of photocatalyst, and there is no self-degradation of TC under visible light irradiation. From Fig. 5b, the degradation efficiency of TC are only 39% and 36% for MS and CBO, respectively. The photocatalytic activity of photocatalysts are evidently improved after loading MS nanosheets on the surface of CBO microrods, which can be largely attributed to the efficiency separation of photogenerated electron-hole pairs by forming p-n heterojunction. The CBO/MS heterojunction with containing 5% MS mass ratio exhibits the highest photocatalytic activity (76% degradation of TC within 92
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Fig. 6. Photoelectrochemical (a) and electrochemical impedance spectroscopy measurements (b) of MS, CBO and CBO/MS heterojunction.
p-n heterojunction is constructed and a built-in electric field is generated in the space charge region (Fig. 8b). Under visible light irradiation, electrons are excited from the VBs of CBO and MS to their respective CBs, leaving holes in the VBs. Then, the internal electric field drives the electrons and holes flow to the opposite direction, that is, the electrons flows from CB of CBO to that of MS, holes from VB of MS to that of CBO, thus effectively inhibiting the recombination of photogenerated electron-hole pairs, which more photogenerated electrons and holes on the surface of CBO and MS can participate in the photocatalytic reaction process, therefore the photocatalytic activity of CBO/MS heterojunction is significantly improved.
CBO. This indicates the faster transfer rate of photo-generated electrons of CBO/MS p-n heterojunction, leading to more efficient separation of electron-hole pairs. Again, the electrochemical impedance spectroscopy (EIS) of as-prepared samples was displayed in Fig. 6b. The smaller radius of the arc represents the lower charge-transfer impedance [47]. It can be seen that CBO/MS displays the smallest arc radius, which can further testify photogenerated electrons-holes pairs can be efficiently separated by loading MS nanosheets onto the CBO microrods. These above results reveal two positive roles (as co-catalyst and forming p-n heterojunction) of loading MS nanosheets in heterojunction for enhancing photocatalytic activity. The trapping experiment of active species was performed to investigate the photocatalytic mechanism of CBO/MS heterojunction in detail. Disodium ethylenediamine tetra-acetic acid (EDTA), 2-propanol (IPA) and Nitrogen gas (N2) as the scavengers of h+, %OH and %O2− were separately added into the TC solution system of containing photocatalysts. As shown in Fig. 7a, the introduction of EDTA and N2 were significantly suppressed the photocatalytic degradation efficiency, which indicated the h+ and %O2− were main participants in this photocatalytic system. In order to further verify the existence of %O2− species during the photocatalysis, the experiment of electron-spin resonance (ESR) is illustrated in Fig. 7b. There is no characteristic peaks of active species can be observed at the condition of dark. With the exposure of light irradiation, the obvious signals of DMPO-%O2− can be observed, demonstrating that more %O2− radicals were generated and participated in photocatalysis. Based on the above analysis, the discussion for the photocatalytic mechanism of p-CBO/n-MS is revealed in Fig. 8. As speculated by the Mott-Schottky plots in Fig. 4b and d, the corresponding positions of CB and VB for p-type CBO and n-type MS before contact have given in the Fig. 8a. As reported, when p-type CBO and n-type MS were in contact,
4. Conclusions In this work, CBO/MS p-n heterojunction photocatalyst with nanosheets-on-microrods structure was successfully designed to degrade TC under visible light for environmental restoration. Compare to the pristine MS and CBO, CBO/MS heterojunction displayed more excellent photocatalytic performances, particularly the CBO/MS-5% showed the highest photocatalytic performance (TC degradation rate of 76% within 120 min), which could correspond to the inhibition of photogenerated electron-hole pairs and the improvement of light absorption. Our work can pave the way in favor of the more efficient p-n heterojunction composite photocatalysts designation to meet the demand of environment for the better future development. Acknowledgements This work is supported by Postgraduate Research & Practice Innovation Program of Jiangsu Province (China) (KYCX18_2340, KYCX19_1677 and KYCX19_1706), the Doctoral Scientific Research
Fig. 7. (a) Effects of a series of scavengers on the TC over CBO/MS. (b) ESR spectra of CBO/MS in methanol aqueous dispersion with/without visible light irradiation. 93
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Fig. 8. The schematic of charge transfer between p-type CuBi2O4 and n-type MoS2: (a) before contact and (b) after the formation of the p-n heterojunction.
Foundation of Jiangsu University of Science and Technology (China) (1062931806 and 1142931803) and General Project of basic Research Plan of Natural Science Foundation of Shaanxi Province (2019JQ-382).
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