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MoS2 heterojunctions with highly efficient ultrasonic catalytic degradation for levofloxacin and methylene blue
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Distinctive binary g-C3N4/MoS2 heterojunctions with highly efficient ultrasonic catalytic degradation for levofloxacin and methylene blue Yangqing Hea,∗, Zhanying Mab, Lucas Binnah Juniora a b
Department of Applied Chemistry, Xi'an University of Technology, Xi'an, 710048, China Department of Chemistry, Xianyang Normal University, Xianyang, 712000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 Ultrasonic catalysis MoS2 Levofloxacin degradation
Sonocatalytic degradation was considered as a new and advanced strategy for the elimination of hazardous organic pollutants from wastewater. Until now, the synergy of employing ultrasonic irradiation with g–C3N4–based catalysts for the removal of antibiotic residues is rarely reported. In this paper, a binary g-C3N4/ MoS2 heterojunctions was successfully fabricated by using a facile hydrothermal strategy and applied to investigate the degradation efficiency of levofloxacin and methylene blue (MB) under ultrasonic condition for the first time. Physicochemical characterizations indicated that MoS2 particles presented irregular shapes with 80 nm and adhered onto the surface of g-C3N4 micro-rods with different lengths ranging from 2 to 5 μm. The ultrasonic degradation experiments indicated that g-C3N4/MoS2 could eliminate 75.81% of levofloxacin within 140 min and 98.43% of MB in 14 min with excellent recyclability. The excellent activity and stability could mainly be attributed to synergetic effects including (1) highly efficient separation and transfer system of sonogenerated charge from g-C3N4 to MoS2; (2) typical sonoluminescence and transient cavitation effects in ultrasonic system are apt to increase the radical generation and (3) keeping the interspaces and reactive sites clean by continuous strong surface cleaning. The reactive species detecting experiments revealed that the •OH and •O2played key roles in MB degradation. Furthermore, a possible mechanism for ultrasonic catalytic performance of g-C3N4 enhanced by MoS2 was proposed based on the detail analysis of the band gaps of g-C3N4 and MoS2, together with the “hot spot” theory of ultrasonication. This work gives an effective and alternative approach for the removal of antibiotic residues and organic dyes in wastewater.
1. Introduction The pollution of water cycle systems by organic dyes and antibiotic residues is an increasingly serious threat to human life and modern society especially when the effluents are directly released into the environmental water without proper treatment. As much as possible, numerous methods have been developed for the removal of these pollutants from wastewater in order to reduce the negative effects they have on the environment. In recent times, semiconductor-based photocatalytic oxidation technology [1,2] is considered as a burgeoning strategy to remove the environmental contaminants with the help of photoinduced reactive oxidative species, such as HO•, O2•- and HO2•. Owing to their promising intrinsic merits, including high efficiency, simple equipment, mild reaction conditions, convenient operation and eco-friendly nature, research onto the design and construction of photocatalysts have intrigued tremendous attention. Wide range of semiconductors such as TiO2-based, Bi-based and C3N4-based materials,
∗
have been used to degrade organic pollutants such as MB [3], RhB [4], AO7 [5], phenol [6], ciprofloxacin [7], tetracycline [8], and so on. Among them, as a promising non-metal semiconductor, g-C3N4 has gained wide-spread interest due to its non-toxicity, outstanding chemical stability and excellent electronic features [9,10]. Nevertheless, one chief drawback which is its low quantum yield, severely blocks its industrial practicability. To overcome this challenge, constructing heterojunction structural materials with suitable band gap matched other semiconductors e.g. Bi2O3 [11], BiVO4 [12], BiCOOH [13], Ag3PO4 [14] and MoS2 [15] have been widely employed to boost the photoinduced electron-hole pairs separation, so as to enhance the visible-light photocatalytic capability [16]. For example, Lan et al. modified g-C3N4 with MoS2 via a water exfolination method, and the results indicated that the separation efficiency of photogenerated charge carriers were obviously increased [17]. Therefore, the MoS2/g-C3N4 composites displayed better visible-light driven performance during the degradation of HCHO than pure g-C3N4. Zhang et al. reported that improved
Corresponding author. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.ceramint.2020.01.287 Received 22 January 2020; Received in revised form 30 January 2020; Accepted 31 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yangqing He, Zhanying Ma and Lucas Binnah Junior, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.287
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photocatalytic capability for RhB elimination and Cr6+ reduction was achieved by boosting e−-h+ pairs separation through inlaying MoS2 in g-C3N4 [15]. More recently, some scholars have employed excellent technologies to improve g-C3N4 heterojunctions. For example, Liao et al. constructed H2-treated g-C3N4 [18], Dong et al. fabricated Sr-intercalated g-C3N4 [19] and Cui et al. came up with a photocatalytic mechanism of activating molecules with spatially localized charge centers using (BiO)2CO3 nanospheres [20]. All these g-C3N4 hybrid heterostructures demonstrated very promising photocatalytic performance in the conversion of NO into NO2− and NO3−. Although g-C3N4–based heterostructure exhibited enhanced catalytic performance, its comprehensive degradation efficiency is still limited in the practical application and the reasons might be concluded as follows: (1) the rapid recombination of photogenerated charge carriers, (2) catalyst surface would be poisoned by intermediates, and (3) stirring or bubbling alone during photocatalysis was insufficient for catalyst particles well dispersed in reaction solution, which reduced their surface activity and dispersibility. The application of ultrasonic (US) has opened a new window for overcoming these defects. The reason being that, US is an attractive technique which scatters catalysts in the reaction system, making the catalyst surface unstained and promoting mass transfer [21]. What is more, US irradiation could provoke cavitation effect in aqueous phase and large amounts of energy would be generated as the micro-bubbles are imploded. This is sufficient to destruct the strong chemical bonds of pollutants as they penetrate into the bubbles, resulting in the pyrolysis of H2O molecules and successive formation of •OH [22]. For example, Pandit et al. reported that TiO2 catalyst could degrade about 82.12% of 2,4,6-trichlorophenol within 150 min [23]. Also, Jamalluddin et al. considered that 90% of Reactive blue 4 could be degraded by Fe (III)/ TiO2 catalyst under ultrasonic irradiation [24]. Again, Li et al. pointed out that ultrasonication played a key role in the decomposition of bisphenol A (BPA) and the degradation efficiency could reach 98.0% [25]. Even though many great researches have affirmed that MoS2 is an excellent cocatalyst for the improvement of the artificial photocatalytic capability of g-C3N4 in hydrogen evolution [26–29] and organic pollutants elimination [15,17] as mentioned above, no previous investigation has reported the photocatalytic performance in levofloxacin degradation using MoS2 as the cocatalyst of g-C3N4, specifically under the ultrasonic wave assisted condition. Herein, g-C3N4/MoS2 heterojunction was successfully synthesized and comprehensively characterized to understand its morphology, phase structure, surface composition and so on. More importantly, the catalytic property of g-C3N4/MoS2 for the removal of antibiotic levofloxacin and the organic dye, methylene blue, was evaluated under ultrasonic irradiation. The detailed dominating reactive species were also studied. Additionally, the possible mechanism for increased sonocatalytic degradation performance was proposed according to the UV–vis, photoluminescence (PL) and detailed band gap analysis.
of Mo and S always at 1:2) was dissolved in obtained g-C3N4 dispersion, followed by continuous ultra-sonication. After ultrasound treatment for 30 min, the mixture was transferred into a 100 mL Teflon stainless steel reactor and heated at 180 °C for 18 h. After that, the precipitates were obtained by filtration, washed with deionized water and dried to yield g-C3N4/MoS2 sample, denoted as g-C3N4/x-MoS2, where x is the molar of Na2MoO4·2H2O added in the reaction. For comparison, pure MoS2, g-C3N4/0.5-MoS2, g-C3N4/1.0-MoS2, gC3N4/1.5-MoS2, g-C3N4/2.0-MoS2 and g-C3N4/3.0-MoS2 samples were also synthesized by changing the molar of Na2MoO4·2H2O while keeping the other parameters as described above. 2.3. Characterization The crystal phase of as-prepared materials was analyzed using an Xray diffractometer (D8 ADVANCE A25 with Cu Kα radiation at 30 kV and 40 mA). The X-ray photoelectron spectroscopy (XPS), UV–vis DRS, field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET) were employed to characterize the samples as reported previously [30]. The photoluminescence data for the samples were recorded on a Fluorescence spectrophotometer (RF-6000). 2.4. Ultrasonic catalytic experiment The ultrasonic catalytic activity was carried out in a 150 mL glass reactor covered with aluminum foil in order to shield the indoor light, and placed in an ultrasonic bath, continuously emitting ultrasonic wave at 30 kHz. Typically, the 60 mg g-C3N4 and the composites, g-C3N4/ MoS2 were dispersed in solutions of 60 mL MB (10 mg/L) and levofloxacin (10 mg/L), respectively. At each given time intervals, 4 mL reaction solution were collected and filtered for further analysis. The MB concentration was examined by UV-3200 spectrophotometer and its absorbance obtained at 664 nm. The levofloxacin concentration was determined on a HPLC instrument (Shimadzu LC-20AT) adopting the solution containing 0.1% phosphoric acid and acetonitrile (volume ration: 87/13) as the mobile phase (0.4 mL/min, monitored at λmax = 290 nm). The sonocatalytic efficiency was evaluated according to the equation of D% = (C0-Ct)/C0 × 100%, where C0 is the original absorbance value of MB and original peak area of levofloxacin, and Ct is the absorbance value for MB and peak area for levofloxacin at any moment. 3. Results and discussion 3.1. Structural and compositional analysis X-ray diffraction (XRD) pattern was employed to study the crystal phase and constituents of pure Mo1-xS2, g-C3N4 and g-C3N4/MoS2 heterojunctions. As depicted in Fig. 1a, the XRD patterns of MoS2 displayed diffraction peaks similar to hexagonal MoS2 (JCPDS No. 37–1492) [30]. Three weak intense peaks with 2θ at 10.36°, 33.87° and 58.70° corresponded with (002), (100) and (110) planes of MoS2, respectively. From Fig. 1b, two diffraction signals with 2θ at 13.29° and 27.54° of pure gC3N4 were in correspondence with the (100) and (002) planes of g-C3N4 (JCPDS No. 87–1526), and this was in compliance with our previous report [30]. In the case of g-C3N4/MoS2 heterojunction samples, all the reflection peaks can be well matched with the starting materials of MoS2 and g-C3N4, indicating both of them is present in the composites. The diffraction peaks of MoS2 in g-C3N4/MoS2 were only small and weak, which might be due to its high dispersion [31] or low content [32]. The enlarged XRD patterns of the g-C3N4/MoS2 between 25° and 31° were revealed in Fig. 1c. It is interesting to note that the peak with 2θ at 27.54° of pure g-C3N4 shifted to lower 2θ angles as loading various amount of MoS2, suggesting that coupling with MoS2 could affect the crystal structure of g-C3N4.
2. Experimental 2.1. Synthesis of pristine g-C3N4 Pristine g-C3N4 was fabricated through melamine thermal polymerization in Ar atmosphere at 550 °C in a tube furnace for 4 h at 2 °C/ min heating rate. This was followed by the grinding of the yellow products into powder in a quartz mortar and stored for further use. 2.2. Synthesis of g-C3N4/MoS2 heterojunction The heterojunction g-C3N4/MoS2 with different molar MoS2 was synthesized via hydrothermal method. In a typical process, 0.5 g g-C3N4 was dispersed in 70 mL deionized water and ultra-sonicated for 30 min. Then, a specified amount of Na2MoO4·2H2O and thiourea (molar ratio 2
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Fig. 1. XRD patterns of pure MoS2, g-C3N4 and g-C3N4/MoS2 heterojunctions.
electronic interaction between g-C3N4 and MoS2.
3.2. Microstructure analysis The typical microstructure of g-C3N4/2.0-MoS2 heterojunctions was examined by SEM, TEM, EDS and elemental mapping. Fig. 2a depicts SEM profile of g-C3N4/MoS2. It was clear that MoS2 presented irregular shape with 80 nm and adhered onto the surface of micro-rods with different lengths ranging from 2 to 5 μm, in which yellow and red arrows represent the MoS2 nanoparticles and g-C3N4 micro-rods, respectively. The results indicated the existence of both g-C3N4 and MoS2. Fig. 2b shows the TEM image of the sample. It indicated a large number of irregular particles united and deposited on the surface of g-C3N4, in accordance with Fig. 2a. To further identify the distribution of both gC3N4 and MoS2 in g-C3N4/2.0-MoS2 heterojunction, EDS and elemental mapping were tested (Fig. 2c–d) and the results confirmed the elements of C, N, S and Mo distributed evenly in g-C3N4/2.0-MoS2 heterojunctions.
3.4. BET analysis The BET surface area and relevant curves of the pore size distributions of pure g-C3N4 and g-C3N4/2.0-MoS2 composite were characterized using N2 adsorption-desorption isotherms measurement. Fig. 4a displays the typical N2 adsorption-desorption curves of pure g-C3N4 and g-C3N4/2.0-MoS2 composite, which indicates a similar type IV isotherms with H4 typical hysteresis loop for g-C3N4 and H2 for g-C3N4/ 2.0-MoS2 composite. While the SBET of pure g-C3N4 and g-C3N4/2.0MoS2 composite are 30.535 and 23.351 m2/g, respectively, it was clear that g-C3N4/2.0-MoS2 composite exhibited low BET surface area compared with that of pure g-C3N4. This may be due to the covering of the g-C3N4 surface by MoS2 particles. Also, Fig. 4b displays the corresponding pore size distribution curves which were obtained using desorption data by the Barrett-Joyner-Halenda (BJH) method. It was noticeable that g-C3N4/2.0-MoS2 composite had a poor pore distribution compared with g-C3N4. The results suggested that the ultrasonic catalytic activity of g-C3N4/2.0-MoS2 composite could not be correlated with the BET surface area and pore size distribution. Similar results had been reported in literature [35].
3.3. XPS analysis Fig. 3a shows the survey XPS spectra of samples, indicating that gC3N4 is composed of C and N elements, while g-C3N4/2.0-MoS2 composite is made up of C, N, Mo and S elements. The Mo 3d XPS narrow spectrum in Fig. 3b showed double peaks at 234.3 and 231.2 eV, which is associated with Mo4+ (3d3/2) and Mo4+ (3d5/2), respectively, an affirmation that the major oxidation state of Mo element in g-C3N4/2.0MoS2 composite is Mo4+ ions [33]. A single peak at around 162.1 eV in S 2p XPS spectrum shown in Fig. 3c is attributed to S2− in g-C3N4/2.0MoS2 [34]. In Fig. 3d, the binding energies at 292.5 and 288.9 eV of C 1s for pure g-C3N4 could be attributed to the sp2 hybridized carbon in N-containing aromatic ring (−N–C]N units) and the sp2-bonded carbons (C–NH2 units). C 1s peaks in g-C3N4/2.0-MoS2 were shifted to 289.9 and 286.1 with lower binding energies compared with pure gC3N4. The N 1s peak at 400.78 eV which is attributed to the C–N groups, shifted to lower binding energies relative to pure g-C3N4. These notable shifts of the C 1s and N 1s orbits could be ascribed to the strong
3.5. Catalytic performance The catalytic performance of pure g-C3N4 and C3N4/MoS2 composite was investigated by degradation of MB and the results are illustrated in Fig. 5. From Fig. 5a, pure g-C3N4 could only degrade 59.40% of MB in 14 min. Also, the g-C3N4/1.5-MoS2 sample exhibited rapid degradation and reached its maximum degradation efficiency of 96.21% in 14 min. In contrast, g-C3N4/2.0-MoS2 heterojunction was very effective for fast degradation of MB with 80.21% in 2 min and 98.43% in 14 min. Therefore, taking the enhancement of catalytic activity into consideration, it is quite obvious that the optimum catalyst is the g-C3N4/2.0-MoS2 sample. 3
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Fig. 2. SEM (a), TEM (b) and corresponding EDS (c), elemental mapping (d) images of g-C3N4/2.0-MoS2 sample.
chromatography (HPLC) technique. Fig. 6b displays the intensity changes of levofloxacin HPLC spectra during the degradation process over g-C3N4/2.0-MoS2 composites. As expected, the intensity of the peak observed at 14.8 min, which was consistent with the retention time of original levofloxacin, gradually decreased with reaction time. Meanwhile, a new peak emerged at around 12.4 min and its intensity increased gradually as reaction proceeded (marked as small black rectangle), suggesting that the degradation of levofloxacin was not simply physical adsorption but forceful breaking of its molecules, and the formation of new by-products. In order to confirm the stability of g-C3N4/2.0-MoS2 composite, recycling degradation tests were conducted by successive sonocatalytic decomposition of MB and levofloxacin as revealed in Fig. 7. It was clear that after six reaction cycles, g-C3N4/2.0-MoS2 could still degrade 94.11% MB within 14 min (Fig. 7a) and degrade 69.58% levofloxacin within 140 min after 3 cycles (Fig. 7b) under ultrasonic condition. In recent years, the coupling of g-C3N4 with MoS2 has proven to be an excellent and promising scientific strategy which have been employed by many researchers to explore efficient heterogeneous nano-sized photocatalysts [17,27,28]. A detailed comparison between g-C3N4/2.0MoS2 ultrasonic-induced catalysts and other reported g-C3N4/MoS2 photo-induced catalysts [9,38,39] and their usage in the degradation of organic dyes is listed in Table 1. Considering the recycle times, g-C3N4/ 2.0-MoS2 possesses superior stability for MB and levofloxacin
The change of UV–vis absorption spectra of MB solution degraded over g-C3N4/2.0-MoS2 composite as a function of regular time intervals is illustrated in Fig. 5b. It is clearly observed that the characteristic absorption peaks of MB at 664 and 292 nm decreased gradually, and disappeared completely within 14 min. This shows that the molecular structure of MB was destroyed by g-C3N4/2.0-MoS2. In addition, the color of MB solution diminished increasingly from the initial blue to colorless (as inset in Fig. 5b). This further supports the gradual elimination of MB solution. It was worth noting that new absorption peaks emerged at about 240 nm, revealing the stable intermediates or byproducts formed during catalysis. What is more, this fast degradation can be reached under ultrasonic condition without any other additives such as peroxide. Similar phenomenon was also reported in literature [36,37]. To further investigate the potential catalytic performance of gC3N4/2.0-MoS2 composite for removing antibiotic residues, the degradation experiments for levofloxacin were also conducted, and the results are depicted in Fig. 6. Pure g-C3N4 exhibited very low catalytic activity and could only achieve 48.22% of decomposition rate for levofloxacin after 140 min. Meanwhile, the g-C3N4/2.0-MoS2 composite exhibited considerably enhanced catalytic activity with 75.81% of levofloxacin degradation rate, in comparison with that of g-C3N4 under the same condition. After ultrasonic catalytic reaction, the decomposed intermediates were detected using high-performance liquid
4
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Fig. 3. XPS spectra of g-C3N4 and g-C3N4/2.0-MoS2 composite (a) full spectra, (b) Mo 3d, (c) S 2p, (d) C 1s and (e) N 1s.
Fig. 4. N2 adsorption and desorption isotherms (a) and the BJH pore size distribution of pure g-C3N4 and g-C3N4/2.0-MoS2 composite. 5
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Fig. 5. (a) Decomposition of MB under ultrasonic condition by pure g-C3N4 and g-C3N4/MoS2 composites, (b) UV–vis absorption spectra of MB during the degradation process by g-C3N4/2.0-MoS2 composites.
Fig. 6. (a) Decomposition of levofloxacin under ultrasonic condition by pure g-C3N4 and g-C3N4/2.0-MoS2composite, (b) high-resolution liquid chromatograph spectra of levofloxacin during the degradation process by g-C3N4/2.0-MoS2 composites.
Fig. 7. Stability of g-C3N4/2.0-MoS2 composites over methylene blue (a) and levofloxacin (b) degradation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Comparison of photo-induced and ultrasonic-induced catalysis of g-C3N4/MoS2 composite. Reaction condition
Reaction time
Degradation degree(D%) or residual solution concentration(C/C0)
Recycle times
Ref.
Visible light Visible light Sunlight Ultrasonic
120 min 60min for RhB 60 min for RhB 14 min for MB 140 min for levofloxacin
98%(D%)-RhB(20 mg/L) 100% (D/%)-RhB(10 mg/L) 32.50%(C/C0)–RhB(20 mg/L) 98.43% (D/%)- MB(10 mg/L) 75.81%(D/%)- levofloxacin(10 mg/L)
– 4 3 6 3
[38] [39] [9] This work
6
for for for for
RhB RhB MB levofloxacin
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Fig. 8. (a) Catalytic degradation of MB over g-C3N4/2.0-MoS2 with different quenchers, PL spectra of the terephthalic acid solution ultrasonic irradiated at different time with (b) and without (c) g-C3N4/2.0-MoS2 heterojunctions, PL spectra of the 2′,7′-dichlorofluoresce in-diacetate (DCF-DA) solution ultrasonic irradiated at different time with (c) and without (d) g-C3N4/2.0-MoS2.
benzoquinone (BQ), sodium ethylene di-amine tetra acetic acid (Na2EDTA) and potassium dichromate (K2Cr2O7) were introduced as effective reactive radical scavengers for ·OH, ·O2-, h+ and e−, respectively. As indicated in Fig. 8a, MB removal efficiency was remarkably suppressed to 54.37% and 60.39% as IPA and BQ were introduced, suggesting that ·OH radicals were produced when the holes were trapped by either H2O molecules or by OH− anions, as well as the formation of ·O2- radicals when the electrons were trapped by the molecules of O2 adsorbed on g-C3N4/2.0-MoS2 surface, respectively. In contrast, the introduction of Na2EDTA and K2Cr2O7 only slightly decreased MB degradation efficiency. These results approved that ·OH and ·O2- are the dominant reactive radicals in MB ultrasonic catalytic decomposing process. To further confirm this conclusion, terephthalic acid and 2′,7′-
elimination under ultrasonic condition. Such higher recyclability in our work can be ascribed to the contribution of ultrasonic strong surface cleaning function, which could keep the interspaces and active reaction sites of g-C3N4/2.0-MoS2 clean, and also avoid the covering or enrichment of intermediates or final products.
3.6. Detection of reactive radicals It is usually considered that degradation of organic pollutants (such as dyes and antibiotics) is a catalytic oxidation and reduction process, whereby reactive species such as (·OH, ·O2-, h+ and e−) may play an important role. To confirm the reactive radicals associated with the ultrasonic degradation of MB over g-C3N4/2.0-MoS2 composite and further explore the catalytic mechanism, isopropanol (IPA), 7
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growth and crack of transient cavitation bubbles with high pressure and temperature, which is in favor of breaking strong chemical bonds of H2O and O2 molecules present in the bubbles. During this process, highly reactive •OH and •O radicals were generated as demonstrated by Eqs (1) and (2). On the basis of sonoluminescence effect, the broader range of light and higher energy would be produced which could excite g-C3N4/2.0MoS2 catalysts to yield charge carriers as shown in Eq. (3). According to our previous reports, the valence band (VB) edge potentials of g-C3N4 and MoS2 are approximated to be +1.65 and + 1.33 eV, while their conduction band (CB) are −1.03 and 0.33 eV, respectively [42]. This well-matched band energy of g-C3N4 and MoS2 endows them to form ideal type-I heterojunctions. Considering the lower position of the VB of MoS2, the holes in the VB of g-C3N4 are partly transferred to that of MoS2 and accumulated on the VB of MoS2. Meanwhile, the electrons on the CB of g-C3N4 can partly migrate onto the CB of MoS2, leading to the accumulation of electrons on it. Unfortunately, those electrons cannot reduce the oxygen attached on the surface of the catalysts to form •O2due to its more positive edge potential than that of O2/•O2- (−0.33 V vs NHE). Meanwhile, the radicals detection experiments in Fig. 8 indicate that they play an important role in the degradation of MB. Therefore, the •O2- radicals getting involved in the sonocatalytic reaction might partly come from the CB of g-C3N4, which can attack the O2 molecules adsorbed on the surface of the catalysts as shown in Eq. (4). Similarly, the holes on the VB of g-C3N4 or MoS2 also cannot oxidize either OH− anions or H2O molecules to generate •OH radicals (2.27 V vs NHE) because of its much lower potential than that of •OH/H2O and •OH/ OH− (2.27 and 1.99 V vs NHE), respectively. The radicals of •OH formed in the transient cavitation process and •O2- generated from sonoluminescence are considerably strong oxidants, which can completely break down the molecules of organic pollutants into other biodegradable compounds or small molecules as shown in Eqs. (5) and (6). Thus, according to the above data analysis, we are positive in drawing the conclusion that the ultrasonic-induced e−−h+ pairs were remarkably separated via type-I heterojuncted structural strategy. This conclusion is further confirmed by the significantly reduced photoluminescence intensity of g-C3N4/MoS2 composite as depicted in Fig. 9.
Fig. 9. Photoluminescence spectra of pure g-C3N4 and g-C3N4/2.0-MoS2 composite.
dichlorodihydrofluoresce in diacetate (DCF-DA) were used as the probe molecules to identify the generation of ·OH and ·O2- radicals respectively during ultrasonic catalysis by fluorimetry. Fig. 8b and c illustrate the fluorescence intensity variation of the terephthalic acid solution corresponding with and without g-C3N4/2.0-MoS2 composite at different ultrasonic times. As expected, the fluorescence intensity at 426 nm gradually increased with prolonging ultrasonic time, indicating the generation of numerous ·OH radicals in g-C3N4/2.0-MoS2 system. On the contrary, Fig. 8c indicated relatively low fluorescence intensity at 426 nm, which means that less ·OH radicals were formed in the absence of g-C3N4/2.0-MoS2. The ·O2- radicals generation ability of gC3N4/2.0-MoS2 was also evaluated by the fluorescence spectra of 2′,7′dichlorofluoresce (in DCF, the oxidation products of DCFH2 by·O2- radicals) [40]. Similar results were observed as that in terephthalic acid solution fluorescence as shown in Fig. 8d and e. The fluorescence spectra of DCF-DA solution with g-C3N4/2.0-MoS2 are gradually increased along with increasing of ultrasonic irradiation times, while it is apparently less ·O2- radicals generated in the absence of g-C3N4/2.0MoS2. The above data further supported the fact that the ·OH and ·O2radicals charged the ultrasonic degradation process of MB in g-C3N4/ 2.0-MoS2 system.
H2O + US → •OH + H•
(1)
O2 + US → 2O•
(2)
g-C3N4/MoS2 + US → g-C3N4(e
3.7. Mechanism of ultrasonic catalytic performance
−
g-C3N4(e ) + O2 →
Ultrasonic catalysis mechanism was universally recognized as “hot spot” and sonoluminescence effect [41]. According to the “hot spot” theory, ultrasonication of aqueous solution can elicit the generation,
−
+
+ h ) + MoS2 (e
−
+h )
•O2-
•OH + methylene blue → intermediates = CO2 + H2O
Fig. 10. Schematic illustration of the ultrasonic degradation mechanism in g-C3N4/MoS2 system. 8
+
(3) (4) (5)
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•O2- + methylene blue → intermediates = CO2 + H2O
(6)
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In combination with the above analysis, a possible ultrasonic-catalytic mechanism associated with the reactive radicals was proposed as illustrated in Fig. 10. 4. Conclusion In this work, the MoS2 incorporated g-C3N4 heterostructure was successfully fabricated via a hydrothermal method and adopted as the ultrasonic catalyst to remove the organic dye, methylene blue, and antibiotic, levofloxacin, for the first time. Owing to the typical sonoluminescence and transient cavitation effects in ultrasonic system and the type-I heterojunction structure, the separation efficiency of ultrasonic-induced charge carriers was greatly enhanced. Thus, g-C3N4/ MoS2 composites displayed promising ultrasonic catalytic activity for methylene blue and levofloxacin degradation. Furthermore, due to the ultrasonic strong surface cleaning ability, the g-C3N4/MoS2 ultrasonic catalysts exhibited higher stability and recyclability for both methylene blue and levofloxacin. This work not only provides a promising catalyst for the elimination of various hazardous dyes and antibiotic residues from wastewater, but also affords new insights into the broad applications of g-C3N4–based heterojunctions. Acknowledgements This work was supported by the Natural Science Foundation of Shaanxi Province in China (2019JQ-221). References [1] J.H. Carey, J. Lawrence, H.M. Tosine, Photodechlorination of PCBs in the presence of TiO2 in aqueous suspensions, Bull. Environ. Contam. Toxicol. 16 (1976) 697–700. [2] J.F. Niu, P.X. Dai, B.H. Yao, X.J. Yu, Q. Zhang, Microwave-assisted solvothermal synthesis of novel hierarchical BiOI/rGO composites for efficient photocatalytic degardation of organic pollutants, Appl. Surf. Sci. 430 (2018) 165–175. [3] M. Suresh, A. Sivasamy, Bismuth oxide nanoparticles decorated Graphene layers for the degradation of methylene blue dye under visible light irradiations and antimicrobial activities, J. Envion. Chem. Eng. 6 (2018) 3745–3756. [4] J. Jin, Q. Liang, C.Y. Ding, Z.Y. Li, S. Xu, Simultaneous synthesis-immobilization of Ag nanoparticles functionalized 2D g-C3N4 nanosheets with improved photocatalytic activity, J. Alloys Compd. 691 (2017) 763–771. [5] S.M. Aghdam, M. Haghighi, S. Allahyari, Precipitation dispersion of various ratios of BiOI/BiCl nanocomposite over g-C3N4 for promoted visible light nanophotocatalyst used in removal of acid orange 7 from water, J. Photochem. Photobiol., A 338 (2017) 201–212. [6] H.J. Lu, Q. Hao, T. Chen, L.H. Zhang, D.M. Chen, C. Ma, W.Q. Yao, Y.F. Zhu, A high performance Bi2O3/Bi2SiO5 p-n heterojunction photocatalyst induced by phase transition of Bi2O3, Appl. Catal., B 237 (2018) 59–67. [7] X.J. Yu, J. Zhang, J. Zhang, J.F. Niu, J. Zhao, Y.C. Wei, B.H. Yao, Photocatalytic degradation of ciprofloxacin using Zn-doped Cu2O particles: analysis of degradation pathways and intermediates, Chem. Eng. J. 374 (2019) 316–327. [8] S. Adhikari, H.H. Lee, D.H. Kim, Efficient visible-light induced electron-transfer in z-scheme MoO3/Ag/C3N4 for excellent photocatalytic removal of antibiotics of both ofloxacin and tetracycline, Chem. Eng. J. (2019), https://doi.org/10.1016/j. cej.2019.123504. [9] S. Asadzadeh-Khaneghah, A. Habibi-Yangjeh, D. Seifzadeh, Graphitic carbon nitride nanosheets coupled with carbon dots and BiOI nanoparticles: boosting visible-lightdriven photocatalytic activity, J. Taiwan Inst. Chem. E. 87 (2018) 98–111. [10] A. Akhundi, A. Habibi-Yangjeh, Novel g-C3N4/Ag2SO4 nanocomposites: fast microwave-assisted preparation and enhanced photocatalytic performance towards degradation of organic pollutants under visible light, J. Colloid Interface Sci. 482 (2016) 165–174. [11] Y.Q. Cui, X.Y. Zhang, R.N. Guo, H.X. Zhang, B. Li, M.Z. Xie, Q.F. Cheng, X.W. Cheng, Construction of Bi2O3/g-C3N4 composite photocatalyst and its enhancedvisible light photocatalytic performance and mechanism, Separ. Purif. Technol. 203 (2018) 301–309. [12] Z.C. Sun, Z.Q. Yu, Y.Y. Liu, C. Shi, M.S. Zhu, A.J. Wang, Construction of 2D/2D BiVO4/g-C3N4 nanosheet heterostructures with improved photocatalytic activity, J. Colloid Interface Sci. 533 (2019) 251–258. [13] Y.Q. Cui, X.Y. Zhang, H.X. Zhang, Q.F. Cheng, X.W. Cheng, Construction of BiOCOOH/g-C3N4 composite photocatalyst and its enhanced visible light photocatalytic degradation of amido black 10B, Separ. Purif. Technol. 210 (2019) 125–134. [14] L. Ghalamchi, S. Aber, V. Vatanpour, M. Kian, A novel antibacterial mixed matrixed PES membrane fabricated from embedding aminated Ag3PO4/g-C3N4 nanocomposite for use in the membrane bioreactor, J. Ind. Eng. Chem. 70 (2019) 412–426.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Distinctive binary g-C3N4/MoS2 heterojunctions with highly efficient ultrasonic catalytic degradation for levofloxacin and methylene blue Yangqing Hea,∗, Zhanying Mab, Lucas Binnah Juniora a b
Department of Applied Chemistry, Xi'an University of Technology, Xi'an, 710048, China Department of Chemistry, Xianyang Normal University, Xianyang, 712000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 Ultrasonic catalysis MoS2 Levofloxacin degradation
Sonocatalytic degradation was considered as a new and advanced strategy for the elimination of hazardous organic pollutants from wastewater. Until now, the synergy of employing ultrasonic irradiation with g–C3N4–based catalysts for the removal of antibiotic residues is rarely reported. In this paper, a binary g-C3N4/ MoS2 heterojunctions was successfully fabricated by using a facile hydrothermal strategy and applied to investigate the degradation efficiency of levofloxacin and methylene blue (MB) under ultrasonic condition for the first time. Physicochemical characterizations indicated that MoS2 particles presented irregular shapes with 80 nm and adhered onto the surface of g-C3N4 micro-rods with different lengths ranging from 2 to 5 μm. The ultrasonic degradation experiments indicated that g-C3N4/MoS2 could eliminate 75.81% of levofloxacin within 140 min and 98.43% of MB in 14 min with excellent recyclability. The excellent activity and stability could mainly be attributed to synergetic effects including (1) highly efficient separation and transfer system of sonogenerated charge from g-C3N4 to MoS2; (2) typical sonoluminescence and transient cavitation effects in ultrasonic system are apt to increase the radical generation and (3) keeping the interspaces and reactive sites clean by continuous strong surface cleaning. The reactive species detecting experiments revealed that the •OH and •O2played key roles in MB degradation. Furthermore, a possible mechanism for ultrasonic catalytic performance of g-C3N4 enhanced by MoS2 was proposed based on the detail analysis of the band gaps of g-C3N4 and MoS2, together with the “hot spot” theory of ultrasonication. This work gives an effective and alternative approach for the removal of antibiotic residues and organic dyes in wastewater.
1. Introduction The pollution of water cycle systems by organic dyes and antibiotic residues is an increasingly serious threat to human life and modern society especially when the effluents are directly released into the environmental water without proper treatment. As much as possible, numerous methods have been developed for the removal of these pollutants from wastewater in order to reduce the negative effects they have on the environment. In recent times, semiconductor-based photocatalytic oxidation technology [1,2] is considered as a burgeoning strategy to remove the environmental contaminants with the help of photoinduced reactive oxidative species, such as HO•, O2•- and HO2•. Owing to their promising intrinsic merits, including high efficiency, simple equipment, mild reaction conditions, convenient operation and eco-friendly nature, research onto the design and construction of photocatalysts have intrigued tremendous attention. Wide range of semiconductors such as TiO2-based, Bi-based and C3N4-based materials,
∗
have been used to degrade organic pollutants such as MB [3], RhB [4], AO7 [5], phenol [6], ciprofloxacin [7], tetracycline [8], and so on. Among them, as a promising non-metal semiconductor, g-C3N4 has gained wide-spread interest due to its non-toxicity, outstanding chemical stability and excellent electronic features [9,10]. Nevertheless, one chief drawback which is its low quantum yield, severely blocks its industrial practicability. To overcome this challenge, constructing heterojunction structural materials with suitable band gap matched other semiconductors e.g. Bi2O3 [11], BiVO4 [12], BiCOOH [13], Ag3PO4 [14] and MoS2 [15] have been widely employed to boost the photoinduced electron-hole pairs separation, so as to enhance the visible-light photocatalytic capability [16]. For example, Lan et al. modified g-C3N4 with MoS2 via a water exfolination method, and the results indicated that the separation efficiency of photogenerated charge carriers were obviously increased [17]. Therefore, the MoS2/g-C3N4 composites displayed better visible-light driven performance during the degradation of HCHO than pure g-C3N4. Zhang et al. reported that improved
Corresponding author. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.ceramint.2020.01.287 Received 22 January 2020; Received in revised form 30 January 2020; Accepted 31 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yangqing He, Zhanying Ma and Lucas Binnah Junior, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.287
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photocatalytic capability for RhB elimination and Cr6+ reduction was achieved by boosting e−-h+ pairs separation through inlaying MoS2 in g-C3N4 [15]. More recently, some scholars have employed excellent technologies to improve g-C3N4 heterojunctions. For example, Liao et al. constructed H2-treated g-C3N4 [18], Dong et al. fabricated Sr-intercalated g-C3N4 [19] and Cui et al. came up with a photocatalytic mechanism of activating molecules with spatially localized charge centers using (BiO)2CO3 nanospheres [20]. All these g-C3N4 hybrid heterostructures demonstrated very promising photocatalytic performance in the conversion of NO into NO2− and NO3−. Although g-C3N4–based heterostructure exhibited enhanced catalytic performance, its comprehensive degradation efficiency is still limited in the practical application and the reasons might be concluded as follows: (1) the rapid recombination of photogenerated charge carriers, (2) catalyst surface would be poisoned by intermediates, and (3) stirring or bubbling alone during photocatalysis was insufficient for catalyst particles well dispersed in reaction solution, which reduced their surface activity and dispersibility. The application of ultrasonic (US) has opened a new window for overcoming these defects. The reason being that, US is an attractive technique which scatters catalysts in the reaction system, making the catalyst surface unstained and promoting mass transfer [21]. What is more, US irradiation could provoke cavitation effect in aqueous phase and large amounts of energy would be generated as the micro-bubbles are imploded. This is sufficient to destruct the strong chemical bonds of pollutants as they penetrate into the bubbles, resulting in the pyrolysis of H2O molecules and successive formation of •OH [22]. For example, Pandit et al. reported that TiO2 catalyst could degrade about 82.12% of 2,4,6-trichlorophenol within 150 min [23]. Also, Jamalluddin et al. considered that 90% of Reactive blue 4 could be degraded by Fe (III)/ TiO2 catalyst under ultrasonic irradiation [24]. Again, Li et al. pointed out that ultrasonication played a key role in the decomposition of bisphenol A (BPA) and the degradation efficiency could reach 98.0% [25]. Even though many great researches have affirmed that MoS2 is an excellent cocatalyst for the improvement of the artificial photocatalytic capability of g-C3N4 in hydrogen evolution [26–29] and organic pollutants elimination [15,17] as mentioned above, no previous investigation has reported the photocatalytic performance in levofloxacin degradation using MoS2 as the cocatalyst of g-C3N4, specifically under the ultrasonic wave assisted condition. Herein, g-C3N4/MoS2 heterojunction was successfully synthesized and comprehensively characterized to understand its morphology, phase structure, surface composition and so on. More importantly, the catalytic property of g-C3N4/MoS2 for the removal of antibiotic levofloxacin and the organic dye, methylene blue, was evaluated under ultrasonic irradiation. The detailed dominating reactive species were also studied. Additionally, the possible mechanism for increased sonocatalytic degradation performance was proposed according to the UV–vis, photoluminescence (PL) and detailed band gap analysis.
of Mo and S always at 1:2) was dissolved in obtained g-C3N4 dispersion, followed by continuous ultra-sonication. After ultrasound treatment for 30 min, the mixture was transferred into a 100 mL Teflon stainless steel reactor and heated at 180 °C for 18 h. After that, the precipitates were obtained by filtration, washed with deionized water and dried to yield g-C3N4/MoS2 sample, denoted as g-C3N4/x-MoS2, where x is the molar of Na2MoO4·2H2O added in the reaction. For comparison, pure MoS2, g-C3N4/0.5-MoS2, g-C3N4/1.0-MoS2, gC3N4/1.5-MoS2, g-C3N4/2.0-MoS2 and g-C3N4/3.0-MoS2 samples were also synthesized by changing the molar of Na2MoO4·2H2O while keeping the other parameters as described above. 2.3. Characterization The crystal phase of as-prepared materials was analyzed using an Xray diffractometer (D8 ADVANCE A25 with Cu Kα radiation at 30 kV and 40 mA). The X-ray photoelectron spectroscopy (XPS), UV–vis DRS, field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET) were employed to characterize the samples as reported previously [30]. The photoluminescence data for the samples were recorded on a Fluorescence spectrophotometer (RF-6000). 2.4. Ultrasonic catalytic experiment The ultrasonic catalytic activity was carried out in a 150 mL glass reactor covered with aluminum foil in order to shield the indoor light, and placed in an ultrasonic bath, continuously emitting ultrasonic wave at 30 kHz. Typically, the 60 mg g-C3N4 and the composites, g-C3N4/ MoS2 were dispersed in solutions of 60 mL MB (10 mg/L) and levofloxacin (10 mg/L), respectively. At each given time intervals, 4 mL reaction solution were collected and filtered for further analysis. The MB concentration was examined by UV-3200 spectrophotometer and its absorbance obtained at 664 nm. The levofloxacin concentration was determined on a HPLC instrument (Shimadzu LC-20AT) adopting the solution containing 0.1% phosphoric acid and acetonitrile (volume ration: 87/13) as the mobile phase (0.4 mL/min, monitored at λmax = 290 nm). The sonocatalytic efficiency was evaluated according to the equation of D% = (C0-Ct)/C0 × 100%, where C0 is the original absorbance value of MB and original peak area of levofloxacin, and Ct is the absorbance value for MB and peak area for levofloxacin at any moment. 3. Results and discussion 3.1. Structural and compositional analysis X-ray diffraction (XRD) pattern was employed to study the crystal phase and constituents of pure Mo1-xS2, g-C3N4 and g-C3N4/MoS2 heterojunctions. As depicted in Fig. 1a, the XRD patterns of MoS2 displayed diffraction peaks similar to hexagonal MoS2 (JCPDS No. 37–1492) [30]. Three weak intense peaks with 2θ at 10.36°, 33.87° and 58.70° corresponded with (002), (100) and (110) planes of MoS2, respectively. From Fig. 1b, two diffraction signals with 2θ at 13.29° and 27.54° of pure gC3N4 were in correspondence with the (100) and (002) planes of g-C3N4 (JCPDS No. 87–1526), and this was in compliance with our previous report [30]. In the case of g-C3N4/MoS2 heterojunction samples, all the reflection peaks can be well matched with the starting materials of MoS2 and g-C3N4, indicating both of them is present in the composites. The diffraction peaks of MoS2 in g-C3N4/MoS2 were only small and weak, which might be due to its high dispersion [31] or low content [32]. The enlarged XRD patterns of the g-C3N4/MoS2 between 25° and 31° were revealed in Fig. 1c. It is interesting to note that the peak with 2θ at 27.54° of pure g-C3N4 shifted to lower 2θ angles as loading various amount of MoS2, suggesting that coupling with MoS2 could affect the crystal structure of g-C3N4.
2. Experimental 2.1. Synthesis of pristine g-C3N4 Pristine g-C3N4 was fabricated through melamine thermal polymerization in Ar atmosphere at 550 °C in a tube furnace for 4 h at 2 °C/ min heating rate. This was followed by the grinding of the yellow products into powder in a quartz mortar and stored for further use. 2.2. Synthesis of g-C3N4/MoS2 heterojunction The heterojunction g-C3N4/MoS2 with different molar MoS2 was synthesized via hydrothermal method. In a typical process, 0.5 g g-C3N4 was dispersed in 70 mL deionized water and ultra-sonicated for 30 min. Then, a specified amount of Na2MoO4·2H2O and thiourea (molar ratio 2
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Fig. 1. XRD patterns of pure MoS2, g-C3N4 and g-C3N4/MoS2 heterojunctions.
electronic interaction between g-C3N4 and MoS2.
3.2. Microstructure analysis The typical microstructure of g-C3N4/2.0-MoS2 heterojunctions was examined by SEM, TEM, EDS and elemental mapping. Fig. 2a depicts SEM profile of g-C3N4/MoS2. It was clear that MoS2 presented irregular shape with 80 nm and adhered onto the surface of micro-rods with different lengths ranging from 2 to 5 μm, in which yellow and red arrows represent the MoS2 nanoparticles and g-C3N4 micro-rods, respectively. The results indicated the existence of both g-C3N4 and MoS2. Fig. 2b shows the TEM image of the sample. It indicated a large number of irregular particles united and deposited on the surface of g-C3N4, in accordance with Fig. 2a. To further identify the distribution of both gC3N4 and MoS2 in g-C3N4/2.0-MoS2 heterojunction, EDS and elemental mapping were tested (Fig. 2c–d) and the results confirmed the elements of C, N, S and Mo distributed evenly in g-C3N4/2.0-MoS2 heterojunctions.
3.4. BET analysis The BET surface area and relevant curves of the pore size distributions of pure g-C3N4 and g-C3N4/2.0-MoS2 composite were characterized using N2 adsorption-desorption isotherms measurement. Fig. 4a displays the typical N2 adsorption-desorption curves of pure g-C3N4 and g-C3N4/2.0-MoS2 composite, which indicates a similar type IV isotherms with H4 typical hysteresis loop for g-C3N4 and H2 for g-C3N4/ 2.0-MoS2 composite. While the SBET of pure g-C3N4 and g-C3N4/2.0MoS2 composite are 30.535 and 23.351 m2/g, respectively, it was clear that g-C3N4/2.0-MoS2 composite exhibited low BET surface area compared with that of pure g-C3N4. This may be due to the covering of the g-C3N4 surface by MoS2 particles. Also, Fig. 4b displays the corresponding pore size distribution curves which were obtained using desorption data by the Barrett-Joyner-Halenda (BJH) method. It was noticeable that g-C3N4/2.0-MoS2 composite had a poor pore distribution compared with g-C3N4. The results suggested that the ultrasonic catalytic activity of g-C3N4/2.0-MoS2 composite could not be correlated with the BET surface area and pore size distribution. Similar results had been reported in literature [35].
3.3. XPS analysis Fig. 3a shows the survey XPS spectra of samples, indicating that gC3N4 is composed of C and N elements, while g-C3N4/2.0-MoS2 composite is made up of C, N, Mo and S elements. The Mo 3d XPS narrow spectrum in Fig. 3b showed double peaks at 234.3 and 231.2 eV, which is associated with Mo4+ (3d3/2) and Mo4+ (3d5/2), respectively, an affirmation that the major oxidation state of Mo element in g-C3N4/2.0MoS2 composite is Mo4+ ions [33]. A single peak at around 162.1 eV in S 2p XPS spectrum shown in Fig. 3c is attributed to S2− in g-C3N4/2.0MoS2 [34]. In Fig. 3d, the binding energies at 292.5 and 288.9 eV of C 1s for pure g-C3N4 could be attributed to the sp2 hybridized carbon in N-containing aromatic ring (−N–C]N units) and the sp2-bonded carbons (C–NH2 units). C 1s peaks in g-C3N4/2.0-MoS2 were shifted to 289.9 and 286.1 with lower binding energies compared with pure gC3N4. The N 1s peak at 400.78 eV which is attributed to the C–N groups, shifted to lower binding energies relative to pure g-C3N4. These notable shifts of the C 1s and N 1s orbits could be ascribed to the strong
3.5. Catalytic performance The catalytic performance of pure g-C3N4 and C3N4/MoS2 composite was investigated by degradation of MB and the results are illustrated in Fig. 5. From Fig. 5a, pure g-C3N4 could only degrade 59.40% of MB in 14 min. Also, the g-C3N4/1.5-MoS2 sample exhibited rapid degradation and reached its maximum degradation efficiency of 96.21% in 14 min. In contrast, g-C3N4/2.0-MoS2 heterojunction was very effective for fast degradation of MB with 80.21% in 2 min and 98.43% in 14 min. Therefore, taking the enhancement of catalytic activity into consideration, it is quite obvious that the optimum catalyst is the g-C3N4/2.0-MoS2 sample. 3
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Fig. 2. SEM (a), TEM (b) and corresponding EDS (c), elemental mapping (d) images of g-C3N4/2.0-MoS2 sample.
chromatography (HPLC) technique. Fig. 6b displays the intensity changes of levofloxacin HPLC spectra during the degradation process over g-C3N4/2.0-MoS2 composites. As expected, the intensity of the peak observed at 14.8 min, which was consistent with the retention time of original levofloxacin, gradually decreased with reaction time. Meanwhile, a new peak emerged at around 12.4 min and its intensity increased gradually as reaction proceeded (marked as small black rectangle), suggesting that the degradation of levofloxacin was not simply physical adsorption but forceful breaking of its molecules, and the formation of new by-products. In order to confirm the stability of g-C3N4/2.0-MoS2 composite, recycling degradation tests were conducted by successive sonocatalytic decomposition of MB and levofloxacin as revealed in Fig. 7. It was clear that after six reaction cycles, g-C3N4/2.0-MoS2 could still degrade 94.11% MB within 14 min (Fig. 7a) and degrade 69.58% levofloxacin within 140 min after 3 cycles (Fig. 7b) under ultrasonic condition. In recent years, the coupling of g-C3N4 with MoS2 has proven to be an excellent and promising scientific strategy which have been employed by many researchers to explore efficient heterogeneous nano-sized photocatalysts [17,27,28]. A detailed comparison between g-C3N4/2.0MoS2 ultrasonic-induced catalysts and other reported g-C3N4/MoS2 photo-induced catalysts [9,38,39] and their usage in the degradation of organic dyes is listed in Table 1. Considering the recycle times, g-C3N4/ 2.0-MoS2 possesses superior stability for MB and levofloxacin
The change of UV–vis absorption spectra of MB solution degraded over g-C3N4/2.0-MoS2 composite as a function of regular time intervals is illustrated in Fig. 5b. It is clearly observed that the characteristic absorption peaks of MB at 664 and 292 nm decreased gradually, and disappeared completely within 14 min. This shows that the molecular structure of MB was destroyed by g-C3N4/2.0-MoS2. In addition, the color of MB solution diminished increasingly from the initial blue to colorless (as inset in Fig. 5b). This further supports the gradual elimination of MB solution. It was worth noting that new absorption peaks emerged at about 240 nm, revealing the stable intermediates or byproducts formed during catalysis. What is more, this fast degradation can be reached under ultrasonic condition without any other additives such as peroxide. Similar phenomenon was also reported in literature [36,37]. To further investigate the potential catalytic performance of gC3N4/2.0-MoS2 composite for removing antibiotic residues, the degradation experiments for levofloxacin were also conducted, and the results are depicted in Fig. 6. Pure g-C3N4 exhibited very low catalytic activity and could only achieve 48.22% of decomposition rate for levofloxacin after 140 min. Meanwhile, the g-C3N4/2.0-MoS2 composite exhibited considerably enhanced catalytic activity with 75.81% of levofloxacin degradation rate, in comparison with that of g-C3N4 under the same condition. After ultrasonic catalytic reaction, the decomposed intermediates were detected using high-performance liquid
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Fig. 3. XPS spectra of g-C3N4 and g-C3N4/2.0-MoS2 composite (a) full spectra, (b) Mo 3d, (c) S 2p, (d) C 1s and (e) N 1s.
Fig. 4. N2 adsorption and desorption isotherms (a) and the BJH pore size distribution of pure g-C3N4 and g-C3N4/2.0-MoS2 composite. 5
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Fig. 5. (a) Decomposition of MB under ultrasonic condition by pure g-C3N4 and g-C3N4/MoS2 composites, (b) UV–vis absorption spectra of MB during the degradation process by g-C3N4/2.0-MoS2 composites.
Fig. 6. (a) Decomposition of levofloxacin under ultrasonic condition by pure g-C3N4 and g-C3N4/2.0-MoS2composite, (b) high-resolution liquid chromatograph spectra of levofloxacin during the degradation process by g-C3N4/2.0-MoS2 composites.
Fig. 7. Stability of g-C3N4/2.0-MoS2 composites over methylene blue (a) and levofloxacin (b) degradation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Comparison of photo-induced and ultrasonic-induced catalysis of g-C3N4/MoS2 composite. Reaction condition
Reaction time
Degradation degree(D%) or residual solution concentration(C/C0)
Recycle times
Ref.
Visible light Visible light Sunlight Ultrasonic
120 min 60min for RhB 60 min for RhB 14 min for MB 140 min for levofloxacin
98%(D%)-RhB(20 mg/L) 100% (D/%)-RhB(10 mg/L) 32.50%(C/C0)–RhB(20 mg/L) 98.43% (D/%)- MB(10 mg/L) 75.81%(D/%)- levofloxacin(10 mg/L)
– 4 3 6 3
[38] [39] [9] This work
6
for for for for
RhB RhB MB levofloxacin
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Fig. 8. (a) Catalytic degradation of MB over g-C3N4/2.0-MoS2 with different quenchers, PL spectra of the terephthalic acid solution ultrasonic irradiated at different time with (b) and without (c) g-C3N4/2.0-MoS2 heterojunctions, PL spectra of the 2′,7′-dichlorofluoresce in-diacetate (DCF-DA) solution ultrasonic irradiated at different time with (c) and without (d) g-C3N4/2.0-MoS2.
benzoquinone (BQ), sodium ethylene di-amine tetra acetic acid (Na2EDTA) and potassium dichromate (K2Cr2O7) were introduced as effective reactive radical scavengers for ·OH, ·O2-, h+ and e−, respectively. As indicated in Fig. 8a, MB removal efficiency was remarkably suppressed to 54.37% and 60.39% as IPA and BQ were introduced, suggesting that ·OH radicals were produced when the holes were trapped by either H2O molecules or by OH− anions, as well as the formation of ·O2- radicals when the electrons were trapped by the molecules of O2 adsorbed on g-C3N4/2.0-MoS2 surface, respectively. In contrast, the introduction of Na2EDTA and K2Cr2O7 only slightly decreased MB degradation efficiency. These results approved that ·OH and ·O2- are the dominant reactive radicals in MB ultrasonic catalytic decomposing process. To further confirm this conclusion, terephthalic acid and 2′,7′-
elimination under ultrasonic condition. Such higher recyclability in our work can be ascribed to the contribution of ultrasonic strong surface cleaning function, which could keep the interspaces and active reaction sites of g-C3N4/2.0-MoS2 clean, and also avoid the covering or enrichment of intermediates or final products.
3.6. Detection of reactive radicals It is usually considered that degradation of organic pollutants (such as dyes and antibiotics) is a catalytic oxidation and reduction process, whereby reactive species such as (·OH, ·O2-, h+ and e−) may play an important role. To confirm the reactive radicals associated with the ultrasonic degradation of MB over g-C3N4/2.0-MoS2 composite and further explore the catalytic mechanism, isopropanol (IPA), 7
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growth and crack of transient cavitation bubbles with high pressure and temperature, which is in favor of breaking strong chemical bonds of H2O and O2 molecules present in the bubbles. During this process, highly reactive •OH and •O radicals were generated as demonstrated by Eqs (1) and (2). On the basis of sonoluminescence effect, the broader range of light and higher energy would be produced which could excite g-C3N4/2.0MoS2 catalysts to yield charge carriers as shown in Eq. (3). According to our previous reports, the valence band (VB) edge potentials of g-C3N4 and MoS2 are approximated to be +1.65 and + 1.33 eV, while their conduction band (CB) are −1.03 and 0.33 eV, respectively [42]. This well-matched band energy of g-C3N4 and MoS2 endows them to form ideal type-I heterojunctions. Considering the lower position of the VB of MoS2, the holes in the VB of g-C3N4 are partly transferred to that of MoS2 and accumulated on the VB of MoS2. Meanwhile, the electrons on the CB of g-C3N4 can partly migrate onto the CB of MoS2, leading to the accumulation of electrons on it. Unfortunately, those electrons cannot reduce the oxygen attached on the surface of the catalysts to form •O2due to its more positive edge potential than that of O2/•O2- (−0.33 V vs NHE). Meanwhile, the radicals detection experiments in Fig. 8 indicate that they play an important role in the degradation of MB. Therefore, the •O2- radicals getting involved in the sonocatalytic reaction might partly come from the CB of g-C3N4, which can attack the O2 molecules adsorbed on the surface of the catalysts as shown in Eq. (4). Similarly, the holes on the VB of g-C3N4 or MoS2 also cannot oxidize either OH− anions or H2O molecules to generate •OH radicals (2.27 V vs NHE) because of its much lower potential than that of •OH/H2O and •OH/ OH− (2.27 and 1.99 V vs NHE), respectively. The radicals of •OH formed in the transient cavitation process and •O2- generated from sonoluminescence are considerably strong oxidants, which can completely break down the molecules of organic pollutants into other biodegradable compounds or small molecules as shown in Eqs. (5) and (6). Thus, according to the above data analysis, we are positive in drawing the conclusion that the ultrasonic-induced e−−h+ pairs were remarkably separated via type-I heterojuncted structural strategy. This conclusion is further confirmed by the significantly reduced photoluminescence intensity of g-C3N4/MoS2 composite as depicted in Fig. 9.
Fig. 9. Photoluminescence spectra of pure g-C3N4 and g-C3N4/2.0-MoS2 composite.
dichlorodihydrofluoresce in diacetate (DCF-DA) were used as the probe molecules to identify the generation of ·OH and ·O2- radicals respectively during ultrasonic catalysis by fluorimetry. Fig. 8b and c illustrate the fluorescence intensity variation of the terephthalic acid solution corresponding with and without g-C3N4/2.0-MoS2 composite at different ultrasonic times. As expected, the fluorescence intensity at 426 nm gradually increased with prolonging ultrasonic time, indicating the generation of numerous ·OH radicals in g-C3N4/2.0-MoS2 system. On the contrary, Fig. 8c indicated relatively low fluorescence intensity at 426 nm, which means that less ·OH radicals were formed in the absence of g-C3N4/2.0-MoS2. The ·O2- radicals generation ability of gC3N4/2.0-MoS2 was also evaluated by the fluorescence spectra of 2′,7′dichlorofluoresce (in DCF, the oxidation products of DCFH2 by·O2- radicals) [40]. Similar results were observed as that in terephthalic acid solution fluorescence as shown in Fig. 8d and e. The fluorescence spectra of DCF-DA solution with g-C3N4/2.0-MoS2 are gradually increased along with increasing of ultrasonic irradiation times, while it is apparently less ·O2- radicals generated in the absence of g-C3N4/2.0MoS2. The above data further supported the fact that the ·OH and ·O2radicals charged the ultrasonic degradation process of MB in g-C3N4/ 2.0-MoS2 system.
H2O + US → •OH + H•
(1)
O2 + US → 2O•
(2)
g-C3N4/MoS2 + US → g-C3N4(e
3.7. Mechanism of ultrasonic catalytic performance
−
g-C3N4(e ) + O2 →
Ultrasonic catalysis mechanism was universally recognized as “hot spot” and sonoluminescence effect [41]. According to the “hot spot” theory, ultrasonication of aqueous solution can elicit the generation,
−
+
+ h ) + MoS2 (e
−
+h )
•O2-
•OH + methylene blue → intermediates = CO2 + H2O
Fig. 10. Schematic illustration of the ultrasonic degradation mechanism in g-C3N4/MoS2 system. 8
+
(3) (4) (5)
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•O2- + methylene blue → intermediates = CO2 + H2O
(6)
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In combination with the above analysis, a possible ultrasonic-catalytic mechanism associated with the reactive radicals was proposed as illustrated in Fig. 10. 4. Conclusion In this work, the MoS2 incorporated g-C3N4 heterostructure was successfully fabricated via a hydrothermal method and adopted as the ultrasonic catalyst to remove the organic dye, methylene blue, and antibiotic, levofloxacin, for the first time. Owing to the typical sonoluminescence and transient cavitation effects in ultrasonic system and the type-I heterojunction structure, the separation efficiency of ultrasonic-induced charge carriers was greatly enhanced. Thus, g-C3N4/ MoS2 composites displayed promising ultrasonic catalytic activity for methylene blue and levofloxacin degradation. Furthermore, due to the ultrasonic strong surface cleaning ability, the g-C3N4/MoS2 ultrasonic catalysts exhibited higher stability and recyclability for both methylene blue and levofloxacin. This work not only provides a promising catalyst for the elimination of various hazardous dyes and antibiotic residues from wastewater, but also affords new insights into the broad applications of g-C3N4–based heterojunctions. Acknowledgements This work was supported by the Natural Science Foundation of Shaanxi Province in China (2019JQ-221). References [1] J.H. Carey, J. Lawrence, H.M. Tosine, Photodechlorination of PCBs in the presence of TiO2 in aqueous suspensions, Bull. Environ. Contam. Toxicol. 16 (1976) 697–700. [2] J.F. Niu, P.X. Dai, B.H. Yao, X.J. Yu, Q. Zhang, Microwave-assisted solvothermal synthesis of novel hierarchical BiOI/rGO composites for efficient photocatalytic degardation of organic pollutants, Appl. Surf. Sci. 430 (2018) 165–175. [3] M. Suresh, A. Sivasamy, Bismuth oxide nanoparticles decorated Graphene layers for the degradation of methylene blue dye under visible light irradiations and antimicrobial activities, J. Envion. Chem. Eng. 6 (2018) 3745–3756. [4] J. Jin, Q. Liang, C.Y. Ding, Z.Y. Li, S. Xu, Simultaneous synthesis-immobilization of Ag nanoparticles functionalized 2D g-C3N4 nanosheets with improved photocatalytic activity, J. Alloys Compd. 691 (2017) 763–771. [5] S.M. Aghdam, M. Haghighi, S. Allahyari, Precipitation dispersion of various ratios of BiOI/BiCl nanocomposite over g-C3N4 for promoted visible light nanophotocatalyst used in removal of acid orange 7 from water, J. Photochem. Photobiol., A 338 (2017) 201–212. [6] H.J. Lu, Q. Hao, T. Chen, L.H. Zhang, D.M. Chen, C. Ma, W.Q. Yao, Y.F. Zhu, A high performance Bi2O3/Bi2SiO5 p-n heterojunction photocatalyst induced by phase transition of Bi2O3, Appl. Catal., B 237 (2018) 59–67. [7] X.J. Yu, J. Zhang, J. Zhang, J.F. Niu, J. Zhao, Y.C. Wei, B.H. Yao, Photocatalytic degradation of ciprofloxacin using Zn-doped Cu2O particles: analysis of degradation pathways and intermediates, Chem. Eng. J. 374 (2019) 316–327. [8] S. Adhikari, H.H. Lee, D.H. Kim, Efficient visible-light induced electron-transfer in z-scheme MoO3/Ag/C3N4 for excellent photocatalytic removal of antibiotics of both ofloxacin and tetracycline, Chem. Eng. J. (2019), https://doi.org/10.1016/j. cej.2019.123504. [9] S. Asadzadeh-Khaneghah, A. Habibi-Yangjeh, D. Seifzadeh, Graphitic carbon nitride nanosheets coupled with carbon dots and BiOI nanoparticles: boosting visible-lightdriven photocatalytic activity, J. Taiwan Inst. Chem. E. 87 (2018) 98–111. [10] A. Akhundi, A. Habibi-Yangjeh, Novel g-C3N4/Ag2SO4 nanocomposites: fast microwave-assisted preparation and enhanced photocatalytic performance towards degradation of organic pollutants under visible light, J. Colloid Interface Sci. 482 (2016) 165–174. [11] Y.Q. Cui, X.Y. Zhang, R.N. Guo, H.X. Zhang, B. Li, M.Z. Xie, Q.F. Cheng, X.W. Cheng, Construction of Bi2O3/g-C3N4 composite photocatalyst and its enhancedvisible light photocatalytic performance and mechanism, Separ. Purif. Technol. 203 (2018) 301–309. [12] Z.C. Sun, Z.Q. Yu, Y.Y. Liu, C. Shi, M.S. Zhu, A.J. Wang, Construction of 2D/2D BiVO4/g-C3N4 nanosheet heterostructures with improved photocatalytic activity, J. Colloid Interface Sci. 533 (2019) 251–258. [13] Y.Q. Cui, X.Y. Zhang, H.X. Zhang, Q.F. Cheng, X.W. Cheng, Construction of BiOCOOH/g-C3N4 composite photocatalyst and its enhanced visible light photocatalytic degradation of amido black 10B, Separ. Purif. Technol. 210 (2019) 125–134. [14] L. Ghalamchi, S. Aber, V. Vatanpour, M. Kian, A novel antibacterial mixed matrixed PES membrane fabricated from embedding aminated Ag3PO4/g-C3N4 nanocomposite for use in the membrane bioreactor, J. Ind. Eng. Chem. 70 (2019) 412–426.
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