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RGO hybrids prepared by a hydrothermal route as a highly efficient catalytic for sonocatalytic degradation of methylene blue
Results in Physics 14 (2019) 102458
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MoS2/RGO hybrids prepared by a hydrothermal route as a highly efficient catalytic for sonocatalytic degradation of methylene blue
T
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Leideng Zou, Rui Qu, Hong Gao, Xin Guan, Xiaofei Qi, Cheng Liu, Zhiyong Zhang , Xiaoyi Lei School of Information Science and Technology, Northwest University, Xi'an 710127, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2/RGO Hierarchical structure Sonocatalytic-degradation Methylene blue
In this paper, the MoS2/RGO hybrids were successfully prepared via a one-pot hydrothermal. The MoS2/RGO was used for sonocatalytic degradation of methylene blue (MB) for the first time. It exhibited high sonocatalytic performance under ultrasound (US) irradiation than that of pure MoS2. Then the MoS2/RGO hybrids are well analyzed by various spectroscopic techniques such as TGA, X-Ray Diffraction, XPS, TEM, Raman, PL, UV–Vis and DRS etc. Furthermore, the sonocatalytic-degradation mechanism is discussed. The enhanced sonocatalytic performance of MoS2/RGO composites can be put down to that the heterojunction structure of MoS2/RGO can heighten the separation efficiency of the electron-hole pairs. The TEM images of the MoS2/RGO reveal that the RGO nanosheets had inserted into MoS2 layers. Thus, the template growth method based on GO expanded the interlayers of MoS2 so that it exposed a greater fraction of the catalytically active sites to offer a large surface area and improve the electronic conduction efficiency. This work provides an evidence that the MoS2/RGO possesses enormous potential in sonocatalytic applications.
Introduction Currently, numerous researches have concentrated on how to deal with the organic pollutants as their high toxicity, persistence as well as refractory in the ecosystem [1–3]. So that, many researchers have noticed a variety of technologies due to their degradation for pollutants by using Earth-abundant and clean energy source, such as photocatalysis [4], photo-Fenton [5], UV-H2O2 [6], catalytic ozonation [7], sonolysis [8], and sonocatalysis [9]. Among various advanced oxidation technologies, photocatalysis has become an important role in recent years. However, the contaminated water is usually adiaphanous and highlyconcentrated in many areas, so that the light can hardly penetrate wastewater and the photocatalysis ability is limited. Therefore, it has been reported that the semiconductor-based sonocatalysis have excellent advantages in dye removal applications due to the ultrasonic wave possess a property that spread and exist in any water medium [10–12]. Until now, many research groups have focused on sonocatalysis materials (e.g. TiO2 [13], CdS [14], ZnO@Fe3O4 [15], MoS2 [16] and Mo2O3 [17] etc.). Among various materials, nontoxic, chemically stable, Earth-abundant and tunable structure molybdenum disulfide (MoS2) has been extensively studied in recent years that possess a sandwich-like structure [18]. As we all know, during sonolysis, the
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ultrasonic irradiation can produce the light called “sonoluminescence” because of the high localized temperature and pressure [19,20], so that the electron-hole pairs will be generated under ultrasonic irradiation. Nevertheless, the recombination of electron-hole pairs on the bulk MoS2 surface [21] will limit the efficiency of the sonocatalytic reaction. In addition, the poor density and reactivity of active sites of MoS2 have limited its wide application in sonocatalytic field because of the undesirable stacking of multilayer MoS2 deposit products [22]. It is possible to overcome these issues by compositing with carbonaceous materials (carbon cloth [23], carbon sphere [24], graphene [25] etc. to create a more efficient sonocatalyst [26]. Herein, we report a one-pot hydrothermal for preparation nanosized MoS2-reduced graphene oxide (MoS2/RGO) hierarchical structure. In current work, the microstructure of the prepared MoS2/RGO composites was been characterized. The characterization results indicate that the MoS2 nanosheets can be anchored into and well dispersed on RGO two sides, which is beneficial for well-ordered MoS2 layer rather than undesirable stacking of multilayer MoS2. In particular, the template growth method based on GO not only expand the interlayers of MoS2 and expose a greater fraction of the catalytically active sites but also form a hierarchical structure to offer a large surface area and improve the electronic conduction efficiency. Thus, compositing with GO can improve the sonocatalytic conversion efficiency.
Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.rinp.2019.102458 Received 27 February 2019; Received in revised form 3 June 2019; Accepted 16 June 2019 Available online 12 July 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102458
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In order to prove the capacity of the sonocatalyst, we have examined the performances of MoS2 and MoS2/RGO in the degradation of MB under US irradiation and studied the sonocatalytic performance of the hybrids with different GO content. It shows that MoS2/RGO can be an outstanding sonocatalyst for the high efficiency catalytic activity and the performance of MoS2/RGO hybrids is outdistancing the pure MoS2.
kept stirring under dark condition for 30 min to obtain the adsorption equilibrium of MB on the sonocatalysts. Then, the MB solution was placed in an ultrasonic bath (KQ2200DE, 40 kHz, 100 W, Kunshan, China). During the ultrasonic treatment process in the dark, 3 ml solution was removed at regular intervals to study the MB degradation by UV–Vis spectrophotometer (UV-1800PC).
Experimental
Characterization
Chemicals Graphite powdered as a C source of the graphene was provided by Shanghai mountain Chemical Co., Ltd. Concentrated sulfuric acid (H2SO4, 98.08%), hydrochloric acid (HCl, 36.46%), hydrogen peroxide (H2O2, 30%), sodium nitrate (NaNO3, 99%) and potassium permanganate (KMnO4, 99%) were supplied by Shanghai Yiai Chemical Technology Co., Ltd. Ammonium molybdate ((NH4)6Mo7O24·4H2O, ≥99.0%) and thiourea (CH4N2S, ≥99.0%) were analytical grade and provided from Sinopharm Chemical Reagent limited corporation.
The samples were measured using X-ray diffraction (XRD, SHIMADZU 6100, Cu Kα λ = 1.5418 Å) operated at 40 kV and 30 mA. SEM and TEM images were recorded using Zeiss Sigma and FEI Tecnai G2 F20 microscope, respectively. Thermal gravimetric analyzer (TGA, NETZSCH STA 449C) and X-ray photoelectron spectroscopy (XPS, AXIS ULTRA) techniques were used to characterize the relevant properties of the material. And fourier transform infrared spectra (FT-IR) were taken on a Bruker TENSOR 27 spectrometer with KBr pellet. UV–Vis absorption spectra were measured using a UV-1800PC scanning spectrophotometer equipped with Xe-lamp.
Synthesis of graphene oxide (GO)
Result and discussion
GO was synthesized from natural graphite powder by a modified Hummers’ method [27]. First at all, 1 g graphite powder and 1 g sodium nitrate were put in round bottom flask under the condition of ice bath, and this time poured into 26 ml sulfuric acid (H2SO4) slowly with a glass rod. At this time, 16 g potassium permanganate (KMnO4, 99%) was added little by little in round bottom flask and kept stirring. The flask was moved to 40 °C environment and stirred constantly 5 h. Then, the environment temperature was heated to 95 °C and stirring 1 h. Then 60 ml deionized water was added in round bottom flask. Then 5 ml H2O2 was dropped into the solution. In the last, 20 ml diluted hydrochloric acid (5 ml HCl and 15 ml deionized water) was added in the round bottom flask. Then the solution was centrifuged with 10,000 rap·min−1 about 5 min. Then, the sediment was washed using the deionized water and alcohol until it was litmus less. Finally, the solid precipitation was put into the vacuum (60 °C, 12 h) drying oven still it became the graphene oxide powder.
Growth mechanism Fig. 1 is the growth mechanism diagram of the preparation of MoS2 nanoparticles which are anchored to the GO surface using a hydrothermal route. Firstly, there will obtain a sufficiently dispersed GO solution after ultrasonic treatment 1 h. At the same time, the (NH4)2MoS4 and CH4N2S were added into the dispersion solution. Finally, the MoS2/RGO hybrids were obtained after hydrothermal process at 200 °C for 24 h. The reaction process could be explained as followed:
SC (NH2 )2 + 2H2O → CO2 + 2NH3 + H2 S
(1)
H2 S → SH+ + S 2 −
(2)
4MoO42 − + 9S 2 − + 24H+ → 4MoS2 12H2 O + SO42 −
(3)
During the hydrothermal reaction, the GO provides a good platform and template for the existence of molybdenum and sulfur source because of the hydrophilic groups on both sides of the GO (such as the –OH or –COOH groups) [28]. And due to this functional group and the edge defect of GO, the Mo atoms could form bonds with C atoms because of the graphene self-assembling during the hydrothermal process [29]. So that the MoS2 nanoflakes would anchor robustly on the both side of GO, which has been confirmed by the Raman spectra. In this reaction system, the GO had been reduced to RGO as it has been proved that the GO will be reduced under high temperature [30] or via chemical reductants [31]. This approach has two major advantages: the method is simple and safe, and it can reduce the undesirable stacking of multilayer MoS2 and expose more active sites. Moreover, it is possible for future mass production in the factory. In order to evidence the GO have been reduced to RGO after the hydrothermal route, FTIR spectroscopic was performed to record that whether the GO had been reduced or not in the hybrids. The FTIR spectra (Fig. 2) shows the presence of functional groups on GO and MoS2/RGO hybrids. From the spectrum of GO, it appears a strong and broad peak at 3000–3750 cm−1, which indicates the presence of the O–H stretching vibration because of the undried absolutely sample [11,32]. At 1700–1900 cm−1, there is a weak peak consistent with C] O or COOH. Meantime, the peaks at 1550–1650 and 1350 cm−1 confirmed the existence of sp2-hybridized C]C and CeOeC [33]. However, there is not obvious peak around 1700–1900 cm−1 in the MoS2/ RGO hybrids spectrum, indicating that the C]O and COOH groups have disappeared. According to the FTIR spectra, it could be found that the graphene oxide have been effectively reduced to RGO after the hydrothermal reaction. Note that the RGO can enhance the MoS2
Synthesis of nanosized MoS2 In the room temperature, 319 mg thiourea (CH4N2S) powder and 185 mg ammonium molybdate ((NH4)6Mo7O24·4H2O) were added into 30 ml deionized water under rotating to get the dispersion solution A. Put the solution A into a Teflon lined stainless-steel autoclave with a capacity of 60 ml, seal the autoclave and then heat at 200 °C for 24 h. After cooling to room temperature, the sediment were washed with deionized water and ethanol three times, then the products were dried at 60 °C in vacuum over night to obtain nanosized MoS2. Synthesis of MoS2/RGO hybrids GO (10, 15, 20 mg) powdered was added into 30 ml deionized water under ultrasonic dispersion for 60 min to get the dispersion solution at the room temperature. Then 319 mg thiourea (CH4N2S) powder and 185 mg ammonium molybdate ((NH4)6Mo7O24·4H2O) were added into these solution under rotating to get the dispersion solution. The solution was then put in a 60 ml Teflon lined stainless-steel autoclave. Finally, seal the autoclave and then put the autoclave in a drying oven at 200 °C for 24 h. In the same way, the products were collected just like the method above. Sonocatalytic degradation of MB with sonocatalytic materials Typically, the sonocatalyst (1.0 g/L) was dispersed in the methylene blue (0.2 g/L, 50 ml, PH = 9) aqueous solution. Then the solution was 2
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Fig. 1. Growth mechanism diagram of the formation process of MoS2/RGO hybrids.
Fig. 2. FTIR spectra of MoS2, GO and MoS2/RGO hybrids.
Fig. 3. Raman spectra of (a) MoS2/RGO1, (b) MoS2/RGO2, (c) MoS2/RGO3, and (d) GO. (The peaks marked with * indicate the characteristic bonds of Mo−C.)
sonocatalytic ability because of the excellent conductivity, robust mechanical and chemical strength [30].
more powerful electronic conducting and electrolyte penetration [36]. Raman analysis TGA analysis The Raman spectra (Fig. 3) shows two obvious peaks at 379.3 cm−1 and 403.8 cm−1, which was related to the hexagonal MoS2 well, connected with the in-plane E12g and out-of-plane A1g vibrational modes, respectively [34]. More attractively, there are two strong peaks at 850 and 1000 cm−1 which is the characteristic bonds of Mo−C [35]. The , results direct confirmed that the MoS2 have well anchored on the RGO. In addition, we can find the D peak (1350 cm−1) and G peak (1595 cm−1) in every spectrum. As it know, the intensity ratio of D peak to G peak (ID/IG) can be used to describe the degree of defects in graphene-based materials. So that the ID/IG of the sample materials have calculated to be 1.019 (MoS2/RGO1), 1.0289 (MoS2/RGO2) and 1.0910 (MoS2/RGO3), suggesting that the disorder of MoS2/RGO2 is strongest than others. It is well accepted that the MoS2/RGO2 exhibited the high sonocatalytic performance because the higher ID/IG is, the
To confirm the compositions of the products, TGA (N2 condition, 10 °C/min) was performed to confirm the content of MoS2 in the MoS2/ RGO hybrids (Fig. 4a). The MoS2/RGO hybrids with different GO (10 mg, 15 mg, 20 mg) contents were defined as MoS2/RGO1, MoS2/ RGO2, and MoS2/RGO3, respectively. It can be observed that per TG curve displays three stages of decomposition. It is showing that the MoS2/RGO hybrids have an initial mass loss around 100 °C because of the evaporation of water. Peculiarly, compared with other samples, the MoS2/RGO2 weight decreased significantly, which may probably due to the sample was not be dried completely. In the second stage, these results suggest that the weight loss at 100–200 °C is the thermal decomposition of the organic surface groups [37]. In the last stage at 200–450 °C, the MoS2 is burned in the air. Compared the XRD analysis 3
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Fig. 4. (a) TGA profiles of MoS2/RGO hybrids. (b) XRD pattern of MoS2/RGO hybrids after TGA analyzing.
result with JCPDS. Card # 01-074-7912 (Fig. 4b), it can be proved that the powder is MoO3. The reaction can be described as follows: (4)
SMoS2 + 7O2 → 2MoO3 + 4SO2
We assume the weight of MoS2 is X. The MoS2 content has completely transformed to MoO3 (assume as Y) under 450 °C. So that one has the relation: X/160 = Y/144 X = Y/0.9 The RGO ratio = (M100 °C – MMoS2)/M100 °C, So that, the weight of MoS2 is 91.27, 86.18 and 86.18% (Table 1), respectively. Therefore, the RGO ratio of the MoS2/RGO is 7.83%, 9.57% and 12.94%, respectively. This test result is not consistent with the parameters set in our experiment which may due to the MoS2/RGO sample have introduced some impurities (MoO3: Its appearance reduces the content of GO.) after hydrothermal process.
XRD analysis Fig. 5. XRD pattern of MoS2 and MoS2/RGO1, 2, 3 with GO (10, 15, 20 mg).
The X-ray diffraction patterns of the pure MoS2, GO, and MoS2/RGO hybrids with different GO (10, 15, 20 mg) are exhibited in Fig. 5. The XRD pattern of MoS2 is a typical hexagonal structure, and it shows weak diffraction peaks at 14.2°, 33.0° and 58.9°. It could be well assigned to (0 0 2), (1 0 0) and (1 1 0) planes, which is quoted the JCPDS. Card #371492 [38]. Compared with pure MoS2, there is a new diffraction peak for the MoS2/RGO composites at the 18.5° with d spacings of 4.79 Å, consisted with the result of TEM. We all know that the d spacing of pristine 2H-MoS2 is about 6.15 Å. However, the d spacing of the MoS2 have been broadened because a lot of GO scrap inserted the layer of the MoS2 after hydrothermal process. As shown in Fig. 5, the result shows that along with the increase in GO content, the intensity of diffraction peak at 18.5° increases, which could be due to more GO scrap have inserted the layer of the MoS2. The peak has also been reported in some work [39,40]. When the GO nanosheets insert MoS2 layers, the sonogenerated electrons would move from the MoS2 conduction band to the graphene apace. So that it could effectively prevent the charge recombination so as to improve the sonotocatalytic capacity of the MoS2/ RGO.
XPS analysis It can be found in Fig. 6a that the C 1s, O 1s, Mo 3d and S 2p existed in the X-ray photoelectron spectroscopy, which could prove the MoS2/ RGO hybrids have been prepared successfully. The peaks at 284.4, 285.2 and 287.1 eV in the XPS spectra of C 1s (Fig. 6b) could fit on the forms C]C, CeO, C]O, respectively. Furthermore, there was a relatively weak peak at 288.8 eV, which could well assign to the eCOOH functional group [41]. This is because the GO was not be reduced completely and it still has a little functional groups (eCOOH) on its surface. As shown in Fig. 6c and d, the high-resolution XPS spectra of sulfur and molybdenum element were measured so as to further testify the status of Mo 3d and S 2p. The Mo at 232 eV and 228.2 eV were corresponded to Mo4+ of MoS2/RGO in Fig. 6c. The peaks appearing at 235.7 eV was related with Mo (VI) corresponds to MoO3 or MoO42−, and the Mo (VI) has found in about 8% of the total Mo amount (Table 2). In the synthesis of nanosized MoS2, the (NH4)6Mo7O24·4H2O were been used as the existence of molybdenum source. A lot of researches have shown that the (NH4)6Mo7O24·4H2O) will be thermally decomposed into a small number of MoOx at about 200 °C temperature in the hydrothermal process [42–44]. And the peaks appearing at 229.2 eV was related with Mo(V), which may result from the partial reduction of the Mo6+ [45]. Moreover, the XPS peak of 225.7 eV was assigned for S 2s. Fig. 6d shows the high-resolution XPS spectra of S 2p. The peaks appeared at 160.9 and 162.1 eV were attributed to the S 2p3/
Table 1 TGA data of the MoS2/RGO in Fig. 4(a). Sample
M100 °C
M450 °C
m (MoS2)
Rate
MoS2/RGO1 MoS2/RGO2 MoS2/RGO3
99.02% 95.30% 98.99%
82.14% 77.57% 77.60%
91.27% 86.18% 86.18%
7.83% 9.57% 12.94%
4
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Fig. 6. XPS spectra of the MoS2/RGO2 hybrid: (a) survey spectrum, (b, c, d) C 1s, Mo 3d and S 2p spectrum.
results. More importantly, the additional oxidized molybdenum (just like MoO3 [47]) can be more conducive to sonocatalytic degradation.
Table 2 Elemental compositions of the MoS2/RGO2 hybrid XPS spectra in Fig. 6. Mo 3d: Peak Label +4
Mo Mo+5 Mo+4 Mo+6 S 2s
Position (eV)
Area
% Conc.
228.2 229.2 232.0 235.7 225.7
2999 1853 7091 1043 1416
23 14 55 8
Morphology and structure of MoS2/RGO The scanning electron microscopy (SEM) images of the hierarchical MoS2/RGO2 composites with different magnifications have shown in Fig. 7a, b. It can be observe that the MoS2 nanoflowers have made up of nanosheets clearly. Moreover, the MoS2 exposed a lot of active sites which is beneficial to sonocatalytic activity. Moreover, MoS2 nanoparticles are dispersed on the RGO due to the large surface of graphene oxide. Fully it suggests that the MoS2 nanosheets are anchored on the RGO and formed a hierarchical structure. The hierarchical structures not only provide sufficient void space between the neighboring MoS2 nanosheets, but also effectively prevent the charge recombination. So the MoS2 can provide more catalytic edge sites on the RGO. Meantime, the electrical coupling between MoS2 and RGO can make the electrons transfer to RGO [48]. It is beneficial to sonocatalytic activity for the hybrids and the catalytic efficiency is far beyond previous MoS2 catalysts [16]. The TEM, HRTEM images of the GO and MoS2/RGO2 hybrid and the STEM of MoS2/RGO2 hybrid are shown in Fig. 8. The TEM image shows that GO has a crumpled morphology (Fig. 8a). Because of the hydrophilic groups on both sides of the GO (such as the eOH or eCOOH groups), they make it possible for sulfur sources and molybdenum sources to exist. From Fig. 8b we can find that the MoS2 nanoflowers are well interspersed and anchored on both sides of the wrinkling GO, which is looking like a sandwich. Coming into being the heterostructure, electrons can transfer from MoS2 to RGO instead of recombining. HTEM images (Fig. 8c) of the catalyst surface indicated the presence of a 0.23 nm d-spacing values in the MoS2 nanosheets, which corresponded to the (1 0 0) lattice spacing of the MoS2 hexagonal phase
S 3d: Peak Label 2−
S S2− S22− S4+
Position (eV)
Area
160.9 162.1 163.0 168.5
1869 1154 912 1219
2 and S 2p1/2, respectively. Besides, the S at 163 and 168.5 eV were corresponded to S22−and S4+ species in SO32− [42]. The atomic quantification of S and Mo can provide further analysis directly. The atomic quantification of S and Mo can provide further analysis directly. So the stoichiometry have been calculated by the relevant calculation formula [46]. (NA is Normalized Area and ASF is Atomic Sensitivity Factor) Mo:S = NA (Mo):NA (S) NA = Area/ASF The ASF of sulfur and molybdenum atoms is 0.524 and 3.321. From Table 2 and the relevant calculation formula, the estimated atomic ratio of Mo to S has calculated to be 1:1.9. The atomic ratio indicates that the MoS2/RGO2 composite contained a small quantity of oxidized molybdenum after hydrothermal process [45], consistent with the TGA 5
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Fig. 7. SEM images of MoS2/RGO2 hybrids (a and b).
Sonocatalytic degradation of MB with MoS2/RGO hybrids
(Fig. 5), consistent with the XRD results. HTEM presents the image of MoS2 nanosheets on the RGO, revealing stacking of few-layer MoS2 sheets with an interlayer spacing of 0.96 nm. The RGO inserted into two MoS2 layers in the middle region and the MoS2 grown on both sides, consistent with the XRD results. Fig. 8d represents the STEM image of the MoS2/RGO2 sample. Obviously, the MoS2 nanoflowers with a diameter of 200 nm are dispersed on the RGO with a three-dimensional agglomeration. They grew on RGO nanosheets uniformly, indicating a three-dimensional laminate structure. Moreover, it's obvious the MoS2 anchored on both sides of the GO. The MoS2 nanoflowers in the circle anchored on the back of GO and others grew on the front, which may have shortened the diffusion length for sonogenerated carriers and reduced the channel length of the conducting path [48] so that the sonotocatalytic capacity have been improved.
The sonocatalytic ability of the prepared sample was evaluated through methylene blue (MB) degradation in an aqueous system using ultrasound (US) irradiation. Fig. 9a-b reflected the change in the absorption spectrum extracted from the MB solution in the case of original MoS2 and MoS2/RGO2 complexes. It is revealed that the relative intensity of the absorption peak significantly decreases with time. Irradiation for about 30 min, and then it can found that the degradation rate of MB for dispose MoS2 up to 98%, however MoS2/RGO2 only need 10 min, implying that MoS2/RGO hybrids showed the excellent sonodegradation performance than pure MoS2. In order to prove that the results of this experiment were not the adsorption, the FTIR spectra of MoS2/RGO before and after US in the MB solution are shown in Fig. 9c. There is no obvious change in the intensity of MoS2/RGO as compared to the FTIR spectra of MoS2/RGO after US, which indicates that the MB has been fully degraded and the structure of the MoS2/RGO hybrids is
Fig. 8. (a) TEM image of the GO. (b) TEM, (c) HRTEM and (d) STEM images of the MoS2/RGO2 hybrid catalyst. 6
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Fig. 9. (a and b) Sonocatalytic degradation of MB with MoS2 and MoS2/RGO2 under the same condition. (c) FTIR spectra of MoS2/RGO2 before and after adsorption of MB. (d) C/C0 of the MB solution with different sonocatalysts.
tiny microbubbles are forming, growing and finally collapsing because of the high localized temperature and pressure, which leads to the emission of light called sonoluminescence [52–54]. Pure MoS2 has been reported to exhibit a narrow band gap energy of 1.9 eV with the conduction band (CB) and valence band (VB) (−0.12 and 1.78 eV vs. NHE) [55]. Furthermore, the work function of graphene is about −0.08 eV (NHE) [56,57]. Thus, when the MoS2/RGO crystal was exposed on the ultrasonic irradiation wastewater, the sono-generated electrons will move to the conduction band (CB) from the valence band (VB). And the electron will transfer from MoS2 to RGO when they have formed a hierarchical structure. In the MoS2/RGO hierarchical nanostructure, the electrons would be moved fast by means of the RGO due to their different work functions [58], which could expedite powerfully the separation efficiency of the electron-hole pairs. Thus the high
also not damaged. Here, we also have studied the possible catalytic mechanism with UV–Vis absorption spectra and PL spectra in Fig. 10. As shown in Fig. 10(a), the absorption spectrums of the MoS2/RGO2 is stronger than pure MoS2. It is due to that the reflection of light would been reduced when the RGO showed up [49,50]. And in Fig. 10(b), compared with pure MoS2, the PL spectrum of MoS2/RGO2 is obviously weak, which may originated from the separation of the electron-hole pairs because of compounding with the RGO [51]. In a conclusion, the MoS2/RGO2 have the ability to absorb more photons and emit less radiation than pure MoS2, which is more beneficial for sonocatalytic reactions under the light of “sonoluminescence”. As shown in Scheme 1, we have talked about the reaction mechanisms about the sonocatalyst. In a typical sono-degradation process,
Fig. 10. (a) UV–Vis absorption spectra for the MoS2 and MoS2/RGO2. (b) The PL spectra of MoS2 and MoS2/RGO2. 7
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Scheme 1. The reaction mechanisms of sono-degradation activity for MoS2/RGO hybrids.
Fig. 11. Comparison of the MoS2/RGO2 degradation efficiency under different situations: (a) effect of the sonocatalyst dosage (power = 100 w, [MB]0 = 0.2 g/L, PH = 9), (b) effect of incipient MB concentration (power = 100 w, [MoS2/RGO2]0 = 1.0 g/L, PH = 9), (c) effect of PH ([MoS2/RGO2]0 = 1.0 g/L, [MB]0 = 0.2 g/L, PH = 9, power = 100 w), (d) effect of ultrasonic power ([MoS2/RGO2]0 = 1.0 g/L, [MB]0 = 0.2 g/L, PH = 9).
conditions showed the following order: MB (99.99%) > MoS2 (79.9%) > MoS2/RGO1 (18.3%) > MoS2/RGO3 (16.4%) > MoS2/ RGO2 (9.1%). Evidently, MoS2/RGO hybrids systems shown a stronger degradation activity compared to the pure MoS2. Especially, MoS2/ RGO2 hybrids exhibit excellent sonocatalytic performance from the sonocatalytic measurements above. The optimum ratio of MoS2/RGO was found to be obtained using 319 mg thiourea (CH4N2S) and 185 mg ammonium molybdate ((NH4)6Mo7O24·4H2O) solution to 15 mg of GO. This great enhancement can be attributed to these following aspects: Firstly, the sonocatalytic performance of the catalyst increases from
concentration of electrons and hole can possibly produce a lot of reaction process that have been illustrated in Scheme 1. Accordingly, the hierarchical nanostructure here can effectively improve the sonotocatalytic capacity. Therefore, it has been studied that the catalytic efficiency for MB with pure MoS2 and MoS2/RGO hybrids with different GO content systems (Fig. 9d). In these test, the experiment parameter as follow: [MoS2]0 = [MoS2/RGO1]0 = [MoS2/RGO2]0 = [MoS2/ RGO3]0 = 1.0 g/L, power = 100 w, [MB]0 = 0.2 g/L and PH = 9. After the same ten minutes, the MB residual concentration under the dark
8
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Effect of PH The effect of pH on the sonocatalytic degradation of MB was studied (Fig. 11c). The different pH value was changed to 3.0, 9.0, and 11.0 using HCl or NaOH. The outcomes seem that the pH value has a great influence on the degradation efficiency in the US system. On the one hand, the MB degradation efficiency declined because the reactive radicals (eOH) was significantly decreased under alkaline pH [64]. On the other hand, the MB degradation efficiency reduces seriously which possibly due to the MoS2/RGO2 sonocatalyst surface was corrode so that the MoS2/RGO2 catalytic capacity has declined and the radical scavenger (Cl−) have reduced the generation of eOH under acidic pH [65]. Effect of ultrasonic power Fig. 11d indicates the MoS2/RGO2 degradation of MB at the different ultrasonic bath power (25, 50 and 100 W). With the increase of ultrasonic power, the catalytic efficiency is obviously improved, which due to the higher power will provide more energy so that there will produce more reactive radicals (eOH) that benefit for sonocatalysis [66].
Fig. 12. Effect of radical scavengers ([MoS2/RGO2]0 = 1.0 g/L power = 100 w, [MB]0 = 0.2 g/L, PH = 9).
Effect of radical scavengers Herein, NaCl (50 mg/L), Na2SO4 (50 mg/L) and ethanol (100 mg/L) were chosen as the eOH radical scavengers, which negatively influence the performance of the MoS2/RGO2 degradation of MB. As shown in Fig. 12, radical scavengers reduce the degradation efficiency. The possible reactions of these every radical scavengers can be described as follows: [64,67,68]
the MoS2/RGO hybrids to MoS2. On the one hand, the MoS2 is the indirect band gap material except the monolayer MoS2 [59]. Because of the undesirable stacking of multilayer MoS2, it makes the poor density and reactivity of active sites in MoS2 [60]. Moreover, the quick recombination of sonoelectrone hole pair is the common problem for pure MoS2 materials, which have limited its wide application in sonocatalytic field [61]. In this work, we used the graphene oxide as a selective substrate for the molybdenum and sulfur source to synthesize a layered MoS2/RGO hybrids sonocatalyst. By using this method, it can be well control the layers and the stacking way of MoS2. Therefore, compositing with graphene oxide that possess large Brunauer-Emmet-Teller (BET) surface area and the high conductivity can powerfully block the recombination of electron-hole pairs and increase the light absorption. On the other hand, the interlayer distance of the MoS2 sheets was increased up to 0.96 nm because of the intercalation of graphene nano fragments into two S-Mo-S layers. The result not only exposed more active sites, but also much easier for the irradiation light to pass through. The second point, the sonocatalytic performance of the hybrids increases from the MoS2/RGO1 to MoS2/RGO2, but decreases with the further increase of GO (20 mg). For the enhanced sonocatalytic performance of MoS2/RGO2 compared with that of MoS2/RGO1, it can be well understood that more excess GO was given and the interlayer distance of the MoS2 sheet would been expanded well. However, when too much GO was existing in the hybrids, the MoS2 sheets do not have enough capacity to absorb the light [62], so that the sonocatalytic performance have been limited.
Cl− + ·OH → Cl· + OH−
(5)
SO42 − + ·OH → SO4·− + OH−
(6)
C2 H5 OH + ·OH → ·C2 H5 OH + H2 O
(7)
Contrasting with this result, it can find that ethanol is a powerful scavenger than NaCl and Na2SO4. Moreover, it's worth noting that the radicals of ·OH play an important role to degradation of MB in the US system. Reusability of the sonocatalyst To measure the reusability of MoS2/RGO2 hybrids, the sonocatalyst will be centrifuged after sonocatalytic complete. Then, the MoS2/RGO2 sediment was washed by the deionized water and ethanol, and finally dried in vacuum. In order to test the reusability of the MoS2/RGO2 sonocatalyst, Fig. 13 shows the C/C0 of MB solution after four successive sonocatalysis cycles. As can be seen in Fig. 12, the MoS2/RGO hybrids exhibit excellent stability and keep more than 90% catalytic efficiency in four consecutive cycles. This result also proved that the sample was not damaged during the ultrasound process, consistent with the FTIR spectroscopic analysis (Fig. 9c). It indicates the catalyst possess excellent regenerability and reusability.
Effect of the sonocatalyst dosage The effect of MoS2/RGO2 dosage (0.5, 1.0, 1.5 and 2.0 g/L) on the removal of the MB solution was researched. As can be seen in Fig. 11a, the degradation efficiency in the US system has been increased as the concentration of the sonocatalyst increases. It is obvious that the more active sites are provided by enhancing the MoS2/RGO2 dosage, which will come into being the more reactive radicals (eOH) [14]. Therefore, the sonocatalytic degradation of MB was strengthened step by step in the US system (MB concentration = 0.2 g/L, pH = 9, power = 100 W).
Conclusions The MoS2 and MoS2/RGO hybrids with hierarchical structure were successfully synthesized via a one-pot hydrothermal and applied for sonocatalytic degradation of MB. The MoS2 nanoflowers were well dispersed and anchored on both sides of the wrinkling RGO. The catalytic efficiency of the hierarchical MoS2/RGO hybrids was almost three times higher than that of bare MoS2 using ultrasound (US) irradiation, because of the synergistic effect of the MoS2 and RGO. In the comparison system, the MoS2/RGO2 exhibited the best sonocatalytic performance because of different amounts of graphene oxide affected MoS2 differently. This study offered an effective, low-cost, and simple hierarchical sonocatalyst for the degradation of MB in sewage
Effect of incipient MB concentration Fig. 11b describes the MB dosage (0.1, 0.2, 0.5 and 1.0 g/L) impact on the degradation efficiency. It is understandable that the degradation efficiency has been reduced with the increase of the dye concentration. This is due to the procreant active sites are insufficient so that the reactive radicals (eOH) are not enough for degrading the more MB [63]. 9
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Fig. 13. Cycling runs of the sonocatalytic degradation RGO2]0 = 1.0 g/L power = 100 w, [MB]0 = 0.2 g/L, PH = 9).
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treatment. Acknowledgements This work was supported by the Key Program for International Science and Technology Cooperation Projects of Shaanxi Province (2018KWZ-08), the Key Research and Development Program of Shaanxi Province (2018GY-025), the National Natural Science Foundation of China (Grant No. 61701402) and the Hongwu Weng Original Research Foundation of Peking University. References [1] Schäfer H, Chatenet M. Steel: the resurrection of a forgotten water-splitting catalyst. ACS Energy Lett 2018;3(3):574–91. [2] Alimi OS, Farner Budarz J, Hernandez LM, Tufenkji N. Microplastics and nanoplastics in aquatic environments: aggregation, deposition, and enhanced contaminant transport. Environ Sci Technol 2018;52(4):1704–24. [3] Sayama K. Production of high-value-added chemicals on oxide semiconductor photoanodes under visible light for solar chemical-conversion processes. ACS Energy Lett 2018;3(5):1093–101. [4] Hao D, Yang Y, Xu B, Cai Z. Bifunctional fabric with photothermal effect and photocatalysis for highly efficient clean water generation. ACS Sustainable Chem Eng 2018;6(8):10789–97. [5] Conte LO, Schenone AV, Giménez BN, Alfano OM. Photo-Fenton degradation of a herbicide (2,4-D) in groundwater for conditions of natural pH and presence of inorganic anions. J Hazard Mater 2018. [6] Dai C, Tian X, Nie Y, Lin H-M, Yang C, Han B, et al. Surface facet of CuFeO2 nanocatalyst: a key parameter for H2O2 activation in fenton-like reaction and organic pollutant degradation. Environ Sci Technol 2018;52(11):6518–25. [7] Xiao J, Han Q, Xie Y, Yang J, Su Q, Chen Y, et al. Is C3N4 chemically stable toward reactive oxygen species in sunlight-driven water treatment? Environ Sci Technol 2017;51(22):13380–7. [8] Ilyas A, Chen CJ, Ding D, Romeo A, Buell TJ, Wang TR, et al. Magnetic resonanceguided, high-intensity focused ultrasound sonolysis: potential applications for stroke. Neurosurg Focus 2018;44(2):E12. [9] Choi J, Cui M, Lee Y, Kim J, Yoon Y, Jang M, et al. Synthesis, characterization and sonocatalytic applications of nano-structured carbon based TiO2 catalysts. Ultrason Sonochem 2018. [10] Kahkha MRR, Kahkha BR, Faghihi-Zarandi A. Application of zinc sulfide carbon nanotubes nanocatalyst for sonocatalysis degradation of methyl orange. Pollution Res 2018. [11] Areerob Y, Cho JY, Jang WK, Oh WC. Enhanced sonocatalytic degradation of organic dyes from aqueous solutions by novel synthesis of mesoporous Fe3O4-graphene/ZnO@SiO2 nanocomposites. Ultrason Sonochem 2018;41:267. [12] Naghibi S, Gharagozlou M. Solvothermal synthesis of M-doped TiO 2 nanoparticles for sonocatalysis of methylene blue and methyl orange (M = Cd, Ag, Fe, Ce, and Cu): sonocatalytic performance of M-doped TiO 2 NPs. J Chin Chem Soc Taipei 2017. [13] Rad TS, Khataee A, Kayan B, Kalderis D, Akay S. Synthesis of pumice-TiO2 nanoflakes for sonocatalytic degradation of famotidine. J Cleaner Prod 2018;202:853–62. [14] Song L, Li Y, Zhang S. Sonocatalytic degradation of rhodamine B in presence of CdS.
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MoS2/RGO hybrids prepared by a hydrothermal route as a highly efficient catalytic for sonocatalytic degradation of methylene blue
T
⁎
Leideng Zou, Rui Qu, Hong Gao, Xin Guan, Xiaofei Qi, Cheng Liu, Zhiyong Zhang , Xiaoyi Lei School of Information Science and Technology, Northwest University, Xi'an 710127, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2/RGO Hierarchical structure Sonocatalytic-degradation Methylene blue
In this paper, the MoS2/RGO hybrids were successfully prepared via a one-pot hydrothermal. The MoS2/RGO was used for sonocatalytic degradation of methylene blue (MB) for the first time. It exhibited high sonocatalytic performance under ultrasound (US) irradiation than that of pure MoS2. Then the MoS2/RGO hybrids are well analyzed by various spectroscopic techniques such as TGA, X-Ray Diffraction, XPS, TEM, Raman, PL, UV–Vis and DRS etc. Furthermore, the sonocatalytic-degradation mechanism is discussed. The enhanced sonocatalytic performance of MoS2/RGO composites can be put down to that the heterojunction structure of MoS2/RGO can heighten the separation efficiency of the electron-hole pairs. The TEM images of the MoS2/RGO reveal that the RGO nanosheets had inserted into MoS2 layers. Thus, the template growth method based on GO expanded the interlayers of MoS2 so that it exposed a greater fraction of the catalytically active sites to offer a large surface area and improve the electronic conduction efficiency. This work provides an evidence that the MoS2/RGO possesses enormous potential in sonocatalytic applications.
Introduction Currently, numerous researches have concentrated on how to deal with the organic pollutants as their high toxicity, persistence as well as refractory in the ecosystem [1–3]. So that, many researchers have noticed a variety of technologies due to their degradation for pollutants by using Earth-abundant and clean energy source, such as photocatalysis [4], photo-Fenton [5], UV-H2O2 [6], catalytic ozonation [7], sonolysis [8], and sonocatalysis [9]. Among various advanced oxidation technologies, photocatalysis has become an important role in recent years. However, the contaminated water is usually adiaphanous and highlyconcentrated in many areas, so that the light can hardly penetrate wastewater and the photocatalysis ability is limited. Therefore, it has been reported that the semiconductor-based sonocatalysis have excellent advantages in dye removal applications due to the ultrasonic wave possess a property that spread and exist in any water medium [10–12]. Until now, many research groups have focused on sonocatalysis materials (e.g. TiO2 [13], CdS [14], ZnO@Fe3O4 [15], MoS2 [16] and Mo2O3 [17] etc.). Among various materials, nontoxic, chemically stable, Earth-abundant and tunable structure molybdenum disulfide (MoS2) has been extensively studied in recent years that possess a sandwich-like structure [18]. As we all know, during sonolysis, the
⁎
ultrasonic irradiation can produce the light called “sonoluminescence” because of the high localized temperature and pressure [19,20], so that the electron-hole pairs will be generated under ultrasonic irradiation. Nevertheless, the recombination of electron-hole pairs on the bulk MoS2 surface [21] will limit the efficiency of the sonocatalytic reaction. In addition, the poor density and reactivity of active sites of MoS2 have limited its wide application in sonocatalytic field because of the undesirable stacking of multilayer MoS2 deposit products [22]. It is possible to overcome these issues by compositing with carbonaceous materials (carbon cloth [23], carbon sphere [24], graphene [25] etc. to create a more efficient sonocatalyst [26]. Herein, we report a one-pot hydrothermal for preparation nanosized MoS2-reduced graphene oxide (MoS2/RGO) hierarchical structure. In current work, the microstructure of the prepared MoS2/RGO composites was been characterized. The characterization results indicate that the MoS2 nanosheets can be anchored into and well dispersed on RGO two sides, which is beneficial for well-ordered MoS2 layer rather than undesirable stacking of multilayer MoS2. In particular, the template growth method based on GO not only expand the interlayers of MoS2 and expose a greater fraction of the catalytically active sites but also form a hierarchical structure to offer a large surface area and improve the electronic conduction efficiency. Thus, compositing with GO can improve the sonocatalytic conversion efficiency.
Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.rinp.2019.102458 Received 27 February 2019; Received in revised form 3 June 2019; Accepted 16 June 2019 Available online 12 July 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102458
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In order to prove the capacity of the sonocatalyst, we have examined the performances of MoS2 and MoS2/RGO in the degradation of MB under US irradiation and studied the sonocatalytic performance of the hybrids with different GO content. It shows that MoS2/RGO can be an outstanding sonocatalyst for the high efficiency catalytic activity and the performance of MoS2/RGO hybrids is outdistancing the pure MoS2.
kept stirring under dark condition for 30 min to obtain the adsorption equilibrium of MB on the sonocatalysts. Then, the MB solution was placed in an ultrasonic bath (KQ2200DE, 40 kHz, 100 W, Kunshan, China). During the ultrasonic treatment process in the dark, 3 ml solution was removed at regular intervals to study the MB degradation by UV–Vis spectrophotometer (UV-1800PC).
Experimental
Characterization
Chemicals Graphite powdered as a C source of the graphene was provided by Shanghai mountain Chemical Co., Ltd. Concentrated sulfuric acid (H2SO4, 98.08%), hydrochloric acid (HCl, 36.46%), hydrogen peroxide (H2O2, 30%), sodium nitrate (NaNO3, 99%) and potassium permanganate (KMnO4, 99%) were supplied by Shanghai Yiai Chemical Technology Co., Ltd. Ammonium molybdate ((NH4)6Mo7O24·4H2O, ≥99.0%) and thiourea (CH4N2S, ≥99.0%) were analytical grade and provided from Sinopharm Chemical Reagent limited corporation.
The samples were measured using X-ray diffraction (XRD, SHIMADZU 6100, Cu Kα λ = 1.5418 Å) operated at 40 kV and 30 mA. SEM and TEM images were recorded using Zeiss Sigma and FEI Tecnai G2 F20 microscope, respectively. Thermal gravimetric analyzer (TGA, NETZSCH STA 449C) and X-ray photoelectron spectroscopy (XPS, AXIS ULTRA) techniques were used to characterize the relevant properties of the material. And fourier transform infrared spectra (FT-IR) were taken on a Bruker TENSOR 27 spectrometer with KBr pellet. UV–Vis absorption spectra were measured using a UV-1800PC scanning spectrophotometer equipped with Xe-lamp.
Synthesis of graphene oxide (GO)
Result and discussion
GO was synthesized from natural graphite powder by a modified Hummers’ method [27]. First at all, 1 g graphite powder and 1 g sodium nitrate were put in round bottom flask under the condition of ice bath, and this time poured into 26 ml sulfuric acid (H2SO4) slowly with a glass rod. At this time, 16 g potassium permanganate (KMnO4, 99%) was added little by little in round bottom flask and kept stirring. The flask was moved to 40 °C environment and stirred constantly 5 h. Then, the environment temperature was heated to 95 °C and stirring 1 h. Then 60 ml deionized water was added in round bottom flask. Then 5 ml H2O2 was dropped into the solution. In the last, 20 ml diluted hydrochloric acid (5 ml HCl and 15 ml deionized water) was added in the round bottom flask. Then the solution was centrifuged with 10,000 rap·min−1 about 5 min. Then, the sediment was washed using the deionized water and alcohol until it was litmus less. Finally, the solid precipitation was put into the vacuum (60 °C, 12 h) drying oven still it became the graphene oxide powder.
Growth mechanism Fig. 1 is the growth mechanism diagram of the preparation of MoS2 nanoparticles which are anchored to the GO surface using a hydrothermal route. Firstly, there will obtain a sufficiently dispersed GO solution after ultrasonic treatment 1 h. At the same time, the (NH4)2MoS4 and CH4N2S were added into the dispersion solution. Finally, the MoS2/RGO hybrids were obtained after hydrothermal process at 200 °C for 24 h. The reaction process could be explained as followed:
SC (NH2 )2 + 2H2O → CO2 + 2NH3 + H2 S
(1)
H2 S → SH+ + S 2 −
(2)
4MoO42 − + 9S 2 − + 24H+ → 4MoS2 12H2 O + SO42 −
(3)
During the hydrothermal reaction, the GO provides a good platform and template for the existence of molybdenum and sulfur source because of the hydrophilic groups on both sides of the GO (such as the –OH or –COOH groups) [28]. And due to this functional group and the edge defect of GO, the Mo atoms could form bonds with C atoms because of the graphene self-assembling during the hydrothermal process [29]. So that the MoS2 nanoflakes would anchor robustly on the both side of GO, which has been confirmed by the Raman spectra. In this reaction system, the GO had been reduced to RGO as it has been proved that the GO will be reduced under high temperature [30] or via chemical reductants [31]. This approach has two major advantages: the method is simple and safe, and it can reduce the undesirable stacking of multilayer MoS2 and expose more active sites. Moreover, it is possible for future mass production in the factory. In order to evidence the GO have been reduced to RGO after the hydrothermal route, FTIR spectroscopic was performed to record that whether the GO had been reduced or not in the hybrids. The FTIR spectra (Fig. 2) shows the presence of functional groups on GO and MoS2/RGO hybrids. From the spectrum of GO, it appears a strong and broad peak at 3000–3750 cm−1, which indicates the presence of the O–H stretching vibration because of the undried absolutely sample [11,32]. At 1700–1900 cm−1, there is a weak peak consistent with C] O or COOH. Meantime, the peaks at 1550–1650 and 1350 cm−1 confirmed the existence of sp2-hybridized C]C and CeOeC [33]. However, there is not obvious peak around 1700–1900 cm−1 in the MoS2/ RGO hybrids spectrum, indicating that the C]O and COOH groups have disappeared. According to the FTIR spectra, it could be found that the graphene oxide have been effectively reduced to RGO after the hydrothermal reaction. Note that the RGO can enhance the MoS2
Synthesis of nanosized MoS2 In the room temperature, 319 mg thiourea (CH4N2S) powder and 185 mg ammonium molybdate ((NH4)6Mo7O24·4H2O) were added into 30 ml deionized water under rotating to get the dispersion solution A. Put the solution A into a Teflon lined stainless-steel autoclave with a capacity of 60 ml, seal the autoclave and then heat at 200 °C for 24 h. After cooling to room temperature, the sediment were washed with deionized water and ethanol three times, then the products were dried at 60 °C in vacuum over night to obtain nanosized MoS2. Synthesis of MoS2/RGO hybrids GO (10, 15, 20 mg) powdered was added into 30 ml deionized water under ultrasonic dispersion for 60 min to get the dispersion solution at the room temperature. Then 319 mg thiourea (CH4N2S) powder and 185 mg ammonium molybdate ((NH4)6Mo7O24·4H2O) were added into these solution under rotating to get the dispersion solution. The solution was then put in a 60 ml Teflon lined stainless-steel autoclave. Finally, seal the autoclave and then put the autoclave in a drying oven at 200 °C for 24 h. In the same way, the products were collected just like the method above. Sonocatalytic degradation of MB with sonocatalytic materials Typically, the sonocatalyst (1.0 g/L) was dispersed in the methylene blue (0.2 g/L, 50 ml, PH = 9) aqueous solution. Then the solution was 2
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Fig. 1. Growth mechanism diagram of the formation process of MoS2/RGO hybrids.
Fig. 2. FTIR spectra of MoS2, GO and MoS2/RGO hybrids.
Fig. 3. Raman spectra of (a) MoS2/RGO1, (b) MoS2/RGO2, (c) MoS2/RGO3, and (d) GO. (The peaks marked with * indicate the characteristic bonds of Mo−C.)
sonocatalytic ability because of the excellent conductivity, robust mechanical and chemical strength [30].
more powerful electronic conducting and electrolyte penetration [36]. Raman analysis TGA analysis The Raman spectra (Fig. 3) shows two obvious peaks at 379.3 cm−1 and 403.8 cm−1, which was related to the hexagonal MoS2 well, connected with the in-plane E12g and out-of-plane A1g vibrational modes, respectively [34]. More attractively, there are two strong peaks at 850 and 1000 cm−1 which is the characteristic bonds of Mo−C [35]. The , results direct confirmed that the MoS2 have well anchored on the RGO. In addition, we can find the D peak (1350 cm−1) and G peak (1595 cm−1) in every spectrum. As it know, the intensity ratio of D peak to G peak (ID/IG) can be used to describe the degree of defects in graphene-based materials. So that the ID/IG of the sample materials have calculated to be 1.019 (MoS2/RGO1), 1.0289 (MoS2/RGO2) and 1.0910 (MoS2/RGO3), suggesting that the disorder of MoS2/RGO2 is strongest than others. It is well accepted that the MoS2/RGO2 exhibited the high sonocatalytic performance because the higher ID/IG is, the
To confirm the compositions of the products, TGA (N2 condition, 10 °C/min) was performed to confirm the content of MoS2 in the MoS2/ RGO hybrids (Fig. 4a). The MoS2/RGO hybrids with different GO (10 mg, 15 mg, 20 mg) contents were defined as MoS2/RGO1, MoS2/ RGO2, and MoS2/RGO3, respectively. It can be observed that per TG curve displays three stages of decomposition. It is showing that the MoS2/RGO hybrids have an initial mass loss around 100 °C because of the evaporation of water. Peculiarly, compared with other samples, the MoS2/RGO2 weight decreased significantly, which may probably due to the sample was not be dried completely. In the second stage, these results suggest that the weight loss at 100–200 °C is the thermal decomposition of the organic surface groups [37]. In the last stage at 200–450 °C, the MoS2 is burned in the air. Compared the XRD analysis 3
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Fig. 4. (a) TGA profiles of MoS2/RGO hybrids. (b) XRD pattern of MoS2/RGO hybrids after TGA analyzing.
result with JCPDS. Card # 01-074-7912 (Fig. 4b), it can be proved that the powder is MoO3. The reaction can be described as follows: (4)
SMoS2 + 7O2 → 2MoO3 + 4SO2
We assume the weight of MoS2 is X. The MoS2 content has completely transformed to MoO3 (assume as Y) under 450 °C. So that one has the relation: X/160 = Y/144 X = Y/0.9 The RGO ratio = (M100 °C – MMoS2)/M100 °C, So that, the weight of MoS2 is 91.27, 86.18 and 86.18% (Table 1), respectively. Therefore, the RGO ratio of the MoS2/RGO is 7.83%, 9.57% and 12.94%, respectively. This test result is not consistent with the parameters set in our experiment which may due to the MoS2/RGO sample have introduced some impurities (MoO3: Its appearance reduces the content of GO.) after hydrothermal process.
XRD analysis Fig. 5. XRD pattern of MoS2 and MoS2/RGO1, 2, 3 with GO (10, 15, 20 mg).
The X-ray diffraction patterns of the pure MoS2, GO, and MoS2/RGO hybrids with different GO (10, 15, 20 mg) are exhibited in Fig. 5. The XRD pattern of MoS2 is a typical hexagonal structure, and it shows weak diffraction peaks at 14.2°, 33.0° and 58.9°. It could be well assigned to (0 0 2), (1 0 0) and (1 1 0) planes, which is quoted the JCPDS. Card #371492 [38]. Compared with pure MoS2, there is a new diffraction peak for the MoS2/RGO composites at the 18.5° with d spacings of 4.79 Å, consisted with the result of TEM. We all know that the d spacing of pristine 2H-MoS2 is about 6.15 Å. However, the d spacing of the MoS2 have been broadened because a lot of GO scrap inserted the layer of the MoS2 after hydrothermal process. As shown in Fig. 5, the result shows that along with the increase in GO content, the intensity of diffraction peak at 18.5° increases, which could be due to more GO scrap have inserted the layer of the MoS2. The peak has also been reported in some work [39,40]. When the GO nanosheets insert MoS2 layers, the sonogenerated electrons would move from the MoS2 conduction band to the graphene apace. So that it could effectively prevent the charge recombination so as to improve the sonotocatalytic capacity of the MoS2/ RGO.
XPS analysis It can be found in Fig. 6a that the C 1s, O 1s, Mo 3d and S 2p existed in the X-ray photoelectron spectroscopy, which could prove the MoS2/ RGO hybrids have been prepared successfully. The peaks at 284.4, 285.2 and 287.1 eV in the XPS spectra of C 1s (Fig. 6b) could fit on the forms C]C, CeO, C]O, respectively. Furthermore, there was a relatively weak peak at 288.8 eV, which could well assign to the eCOOH functional group [41]. This is because the GO was not be reduced completely and it still has a little functional groups (eCOOH) on its surface. As shown in Fig. 6c and d, the high-resolution XPS spectra of sulfur and molybdenum element were measured so as to further testify the status of Mo 3d and S 2p. The Mo at 232 eV and 228.2 eV were corresponded to Mo4+ of MoS2/RGO in Fig. 6c. The peaks appearing at 235.7 eV was related with Mo (VI) corresponds to MoO3 or MoO42−, and the Mo (VI) has found in about 8% of the total Mo amount (Table 2). In the synthesis of nanosized MoS2, the (NH4)6Mo7O24·4H2O were been used as the existence of molybdenum source. A lot of researches have shown that the (NH4)6Mo7O24·4H2O) will be thermally decomposed into a small number of MoOx at about 200 °C temperature in the hydrothermal process [42–44]. And the peaks appearing at 229.2 eV was related with Mo(V), which may result from the partial reduction of the Mo6+ [45]. Moreover, the XPS peak of 225.7 eV was assigned for S 2s. Fig. 6d shows the high-resolution XPS spectra of S 2p. The peaks appeared at 160.9 and 162.1 eV were attributed to the S 2p3/
Table 1 TGA data of the MoS2/RGO in Fig. 4(a). Sample
M100 °C
M450 °C
m (MoS2)
Rate
MoS2/RGO1 MoS2/RGO2 MoS2/RGO3
99.02% 95.30% 98.99%
82.14% 77.57% 77.60%
91.27% 86.18% 86.18%
7.83% 9.57% 12.94%
4
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Fig. 6. XPS spectra of the MoS2/RGO2 hybrid: (a) survey spectrum, (b, c, d) C 1s, Mo 3d and S 2p spectrum.
results. More importantly, the additional oxidized molybdenum (just like MoO3 [47]) can be more conducive to sonocatalytic degradation.
Table 2 Elemental compositions of the MoS2/RGO2 hybrid XPS spectra in Fig. 6. Mo 3d: Peak Label +4
Mo Mo+5 Mo+4 Mo+6 S 2s
Position (eV)
Area
% Conc.
228.2 229.2 232.0 235.7 225.7
2999 1853 7091 1043 1416
23 14 55 8
Morphology and structure of MoS2/RGO The scanning electron microscopy (SEM) images of the hierarchical MoS2/RGO2 composites with different magnifications have shown in Fig. 7a, b. It can be observe that the MoS2 nanoflowers have made up of nanosheets clearly. Moreover, the MoS2 exposed a lot of active sites which is beneficial to sonocatalytic activity. Moreover, MoS2 nanoparticles are dispersed on the RGO due to the large surface of graphene oxide. Fully it suggests that the MoS2 nanosheets are anchored on the RGO and formed a hierarchical structure. The hierarchical structures not only provide sufficient void space between the neighboring MoS2 nanosheets, but also effectively prevent the charge recombination. So the MoS2 can provide more catalytic edge sites on the RGO. Meantime, the electrical coupling between MoS2 and RGO can make the electrons transfer to RGO [48]. It is beneficial to sonocatalytic activity for the hybrids and the catalytic efficiency is far beyond previous MoS2 catalysts [16]. The TEM, HRTEM images of the GO and MoS2/RGO2 hybrid and the STEM of MoS2/RGO2 hybrid are shown in Fig. 8. The TEM image shows that GO has a crumpled morphology (Fig. 8a). Because of the hydrophilic groups on both sides of the GO (such as the eOH or eCOOH groups), they make it possible for sulfur sources and molybdenum sources to exist. From Fig. 8b we can find that the MoS2 nanoflowers are well interspersed and anchored on both sides of the wrinkling GO, which is looking like a sandwich. Coming into being the heterostructure, electrons can transfer from MoS2 to RGO instead of recombining. HTEM images (Fig. 8c) of the catalyst surface indicated the presence of a 0.23 nm d-spacing values in the MoS2 nanosheets, which corresponded to the (1 0 0) lattice spacing of the MoS2 hexagonal phase
S 3d: Peak Label 2−
S S2− S22− S4+
Position (eV)
Area
160.9 162.1 163.0 168.5
1869 1154 912 1219
2 and S 2p1/2, respectively. Besides, the S at 163 and 168.5 eV were corresponded to S22−and S4+ species in SO32− [42]. The atomic quantification of S and Mo can provide further analysis directly. The atomic quantification of S and Mo can provide further analysis directly. So the stoichiometry have been calculated by the relevant calculation formula [46]. (NA is Normalized Area and ASF is Atomic Sensitivity Factor) Mo:S = NA (Mo):NA (S) NA = Area/ASF The ASF of sulfur and molybdenum atoms is 0.524 and 3.321. From Table 2 and the relevant calculation formula, the estimated atomic ratio of Mo to S has calculated to be 1:1.9. The atomic ratio indicates that the MoS2/RGO2 composite contained a small quantity of oxidized molybdenum after hydrothermal process [45], consistent with the TGA 5
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Fig. 7. SEM images of MoS2/RGO2 hybrids (a and b).
Sonocatalytic degradation of MB with MoS2/RGO hybrids
(Fig. 5), consistent with the XRD results. HTEM presents the image of MoS2 nanosheets on the RGO, revealing stacking of few-layer MoS2 sheets with an interlayer spacing of 0.96 nm. The RGO inserted into two MoS2 layers in the middle region and the MoS2 grown on both sides, consistent with the XRD results. Fig. 8d represents the STEM image of the MoS2/RGO2 sample. Obviously, the MoS2 nanoflowers with a diameter of 200 nm are dispersed on the RGO with a three-dimensional agglomeration. They grew on RGO nanosheets uniformly, indicating a three-dimensional laminate structure. Moreover, it's obvious the MoS2 anchored on both sides of the GO. The MoS2 nanoflowers in the circle anchored on the back of GO and others grew on the front, which may have shortened the diffusion length for sonogenerated carriers and reduced the channel length of the conducting path [48] so that the sonotocatalytic capacity have been improved.
The sonocatalytic ability of the prepared sample was evaluated through methylene blue (MB) degradation in an aqueous system using ultrasound (US) irradiation. Fig. 9a-b reflected the change in the absorption spectrum extracted from the MB solution in the case of original MoS2 and MoS2/RGO2 complexes. It is revealed that the relative intensity of the absorption peak significantly decreases with time. Irradiation for about 30 min, and then it can found that the degradation rate of MB for dispose MoS2 up to 98%, however MoS2/RGO2 only need 10 min, implying that MoS2/RGO hybrids showed the excellent sonodegradation performance than pure MoS2. In order to prove that the results of this experiment were not the adsorption, the FTIR spectra of MoS2/RGO before and after US in the MB solution are shown in Fig. 9c. There is no obvious change in the intensity of MoS2/RGO as compared to the FTIR spectra of MoS2/RGO after US, which indicates that the MB has been fully degraded and the structure of the MoS2/RGO hybrids is
Fig. 8. (a) TEM image of the GO. (b) TEM, (c) HRTEM and (d) STEM images of the MoS2/RGO2 hybrid catalyst. 6
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Fig. 9. (a and b) Sonocatalytic degradation of MB with MoS2 and MoS2/RGO2 under the same condition. (c) FTIR spectra of MoS2/RGO2 before and after adsorption of MB. (d) C/C0 of the MB solution with different sonocatalysts.
tiny microbubbles are forming, growing and finally collapsing because of the high localized temperature and pressure, which leads to the emission of light called sonoluminescence [52–54]. Pure MoS2 has been reported to exhibit a narrow band gap energy of 1.9 eV with the conduction band (CB) and valence band (VB) (−0.12 and 1.78 eV vs. NHE) [55]. Furthermore, the work function of graphene is about −0.08 eV (NHE) [56,57]. Thus, when the MoS2/RGO crystal was exposed on the ultrasonic irradiation wastewater, the sono-generated electrons will move to the conduction band (CB) from the valence band (VB). And the electron will transfer from MoS2 to RGO when they have formed a hierarchical structure. In the MoS2/RGO hierarchical nanostructure, the electrons would be moved fast by means of the RGO due to their different work functions [58], which could expedite powerfully the separation efficiency of the electron-hole pairs. Thus the high
also not damaged. Here, we also have studied the possible catalytic mechanism with UV–Vis absorption spectra and PL spectra in Fig. 10. As shown in Fig. 10(a), the absorption spectrums of the MoS2/RGO2 is stronger than pure MoS2. It is due to that the reflection of light would been reduced when the RGO showed up [49,50]. And in Fig. 10(b), compared with pure MoS2, the PL spectrum of MoS2/RGO2 is obviously weak, which may originated from the separation of the electron-hole pairs because of compounding with the RGO [51]. In a conclusion, the MoS2/RGO2 have the ability to absorb more photons and emit less radiation than pure MoS2, which is more beneficial for sonocatalytic reactions under the light of “sonoluminescence”. As shown in Scheme 1, we have talked about the reaction mechanisms about the sonocatalyst. In a typical sono-degradation process,
Fig. 10. (a) UV–Vis absorption spectra for the MoS2 and MoS2/RGO2. (b) The PL spectra of MoS2 and MoS2/RGO2. 7
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Scheme 1. The reaction mechanisms of sono-degradation activity for MoS2/RGO hybrids.
Fig. 11. Comparison of the MoS2/RGO2 degradation efficiency under different situations: (a) effect of the sonocatalyst dosage (power = 100 w, [MB]0 = 0.2 g/L, PH = 9), (b) effect of incipient MB concentration (power = 100 w, [MoS2/RGO2]0 = 1.0 g/L, PH = 9), (c) effect of PH ([MoS2/RGO2]0 = 1.0 g/L, [MB]0 = 0.2 g/L, PH = 9, power = 100 w), (d) effect of ultrasonic power ([MoS2/RGO2]0 = 1.0 g/L, [MB]0 = 0.2 g/L, PH = 9).
conditions showed the following order: MB (99.99%) > MoS2 (79.9%) > MoS2/RGO1 (18.3%) > MoS2/RGO3 (16.4%) > MoS2/ RGO2 (9.1%). Evidently, MoS2/RGO hybrids systems shown a stronger degradation activity compared to the pure MoS2. Especially, MoS2/ RGO2 hybrids exhibit excellent sonocatalytic performance from the sonocatalytic measurements above. The optimum ratio of MoS2/RGO was found to be obtained using 319 mg thiourea (CH4N2S) and 185 mg ammonium molybdate ((NH4)6Mo7O24·4H2O) solution to 15 mg of GO. This great enhancement can be attributed to these following aspects: Firstly, the sonocatalytic performance of the catalyst increases from
concentration of electrons and hole can possibly produce a lot of reaction process that have been illustrated in Scheme 1. Accordingly, the hierarchical nanostructure here can effectively improve the sonotocatalytic capacity. Therefore, it has been studied that the catalytic efficiency for MB with pure MoS2 and MoS2/RGO hybrids with different GO content systems (Fig. 9d). In these test, the experiment parameter as follow: [MoS2]0 = [MoS2/RGO1]0 = [MoS2/RGO2]0 = [MoS2/ RGO3]0 = 1.0 g/L, power = 100 w, [MB]0 = 0.2 g/L and PH = 9. After the same ten minutes, the MB residual concentration under the dark
8
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Effect of PH The effect of pH on the sonocatalytic degradation of MB was studied (Fig. 11c). The different pH value was changed to 3.0, 9.0, and 11.0 using HCl or NaOH. The outcomes seem that the pH value has a great influence on the degradation efficiency in the US system. On the one hand, the MB degradation efficiency declined because the reactive radicals (eOH) was significantly decreased under alkaline pH [64]. On the other hand, the MB degradation efficiency reduces seriously which possibly due to the MoS2/RGO2 sonocatalyst surface was corrode so that the MoS2/RGO2 catalytic capacity has declined and the radical scavenger (Cl−) have reduced the generation of eOH under acidic pH [65]. Effect of ultrasonic power Fig. 11d indicates the MoS2/RGO2 degradation of MB at the different ultrasonic bath power (25, 50 and 100 W). With the increase of ultrasonic power, the catalytic efficiency is obviously improved, which due to the higher power will provide more energy so that there will produce more reactive radicals (eOH) that benefit for sonocatalysis [66].
Fig. 12. Effect of radical scavengers ([MoS2/RGO2]0 = 1.0 g/L power = 100 w, [MB]0 = 0.2 g/L, PH = 9).
Effect of radical scavengers Herein, NaCl (50 mg/L), Na2SO4 (50 mg/L) and ethanol (100 mg/L) were chosen as the eOH radical scavengers, which negatively influence the performance of the MoS2/RGO2 degradation of MB. As shown in Fig. 12, radical scavengers reduce the degradation efficiency. The possible reactions of these every radical scavengers can be described as follows: [64,67,68]
the MoS2/RGO hybrids to MoS2. On the one hand, the MoS2 is the indirect band gap material except the monolayer MoS2 [59]. Because of the undesirable stacking of multilayer MoS2, it makes the poor density and reactivity of active sites in MoS2 [60]. Moreover, the quick recombination of sonoelectrone hole pair is the common problem for pure MoS2 materials, which have limited its wide application in sonocatalytic field [61]. In this work, we used the graphene oxide as a selective substrate for the molybdenum and sulfur source to synthesize a layered MoS2/RGO hybrids sonocatalyst. By using this method, it can be well control the layers and the stacking way of MoS2. Therefore, compositing with graphene oxide that possess large Brunauer-Emmet-Teller (BET) surface area and the high conductivity can powerfully block the recombination of electron-hole pairs and increase the light absorption. On the other hand, the interlayer distance of the MoS2 sheets was increased up to 0.96 nm because of the intercalation of graphene nano fragments into two S-Mo-S layers. The result not only exposed more active sites, but also much easier for the irradiation light to pass through. The second point, the sonocatalytic performance of the hybrids increases from the MoS2/RGO1 to MoS2/RGO2, but decreases with the further increase of GO (20 mg). For the enhanced sonocatalytic performance of MoS2/RGO2 compared with that of MoS2/RGO1, it can be well understood that more excess GO was given and the interlayer distance of the MoS2 sheet would been expanded well. However, when too much GO was existing in the hybrids, the MoS2 sheets do not have enough capacity to absorb the light [62], so that the sonocatalytic performance have been limited.
Cl− + ·OH → Cl· + OH−
(5)
SO42 − + ·OH → SO4·− + OH−
(6)
C2 H5 OH + ·OH → ·C2 H5 OH + H2 O
(7)
Contrasting with this result, it can find that ethanol is a powerful scavenger than NaCl and Na2SO4. Moreover, it's worth noting that the radicals of ·OH play an important role to degradation of MB in the US system. Reusability of the sonocatalyst To measure the reusability of MoS2/RGO2 hybrids, the sonocatalyst will be centrifuged after sonocatalytic complete. Then, the MoS2/RGO2 sediment was washed by the deionized water and ethanol, and finally dried in vacuum. In order to test the reusability of the MoS2/RGO2 sonocatalyst, Fig. 13 shows the C/C0 of MB solution after four successive sonocatalysis cycles. As can be seen in Fig. 12, the MoS2/RGO hybrids exhibit excellent stability and keep more than 90% catalytic efficiency in four consecutive cycles. This result also proved that the sample was not damaged during the ultrasound process, consistent with the FTIR spectroscopic analysis (Fig. 9c). It indicates the catalyst possess excellent regenerability and reusability.
Effect of the sonocatalyst dosage The effect of MoS2/RGO2 dosage (0.5, 1.0, 1.5 and 2.0 g/L) on the removal of the MB solution was researched. As can be seen in Fig. 11a, the degradation efficiency in the US system has been increased as the concentration of the sonocatalyst increases. It is obvious that the more active sites are provided by enhancing the MoS2/RGO2 dosage, which will come into being the more reactive radicals (eOH) [14]. Therefore, the sonocatalytic degradation of MB was strengthened step by step in the US system (MB concentration = 0.2 g/L, pH = 9, power = 100 W).
Conclusions The MoS2 and MoS2/RGO hybrids with hierarchical structure were successfully synthesized via a one-pot hydrothermal and applied for sonocatalytic degradation of MB. The MoS2 nanoflowers were well dispersed and anchored on both sides of the wrinkling RGO. The catalytic efficiency of the hierarchical MoS2/RGO hybrids was almost three times higher than that of bare MoS2 using ultrasound (US) irradiation, because of the synergistic effect of the MoS2 and RGO. In the comparison system, the MoS2/RGO2 exhibited the best sonocatalytic performance because of different amounts of graphene oxide affected MoS2 differently. This study offered an effective, low-cost, and simple hierarchical sonocatalyst for the degradation of MB in sewage
Effect of incipient MB concentration Fig. 11b describes the MB dosage (0.1, 0.2, 0.5 and 1.0 g/L) impact on the degradation efficiency. It is understandable that the degradation efficiency has been reduced with the increase of the dye concentration. This is due to the procreant active sites are insufficient so that the reactive radicals (eOH) are not enough for degrading the more MB [63]. 9
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Fig. 13. Cycling runs of the sonocatalytic degradation RGO2]0 = 1.0 g/L power = 100 w, [MB]0 = 0.2 g/L, PH = 9).
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