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Molybdenum disulfide and graphitic carbon nitride (MoS2-g-C3N4) nanocomposites with visible-light induced photocatalytic activity were successfully synthesized by a facile ultrasonic dispersion method. The crystalline structure and morphology of the MoS2-g-C3N4 nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microcopy (TEM), high-resolution TEM (HRTEM) and scanning electron microscopy (SEM). The optical property of the as-prepared nanocomposites was studied by ultraviolet visible diffusion reflection (UV-vis) and photoluminescence(PL) spectrum. It could be observed from the TEM image that the MoS2 nanosheets and g-C3N4 nanoparticles were well combined together. Moreover, the photocatalytic activity of MoS2-g-C3N4 composites was evaluated by the removal of nitric oxide under visible light irradiation (>400nm). The experimental results demonstrated that the nanocomposites with the MoS2 content of 1.5 wt% exhibited optimal photocatalytic activity and the corresponding removal rate of NO achieved 51.67%, higher than that of pure g-C3N4 nanoparticles. A possible photocatalytic mechanism for the MoS2-g-C3N4 nanocomposites with enhanced photocatalytic activity could be ascribed to the hetero-structure of MoS2 and g-C3N4.
© 2016 Optical Society of America
1. Introduction
Photocatalytic degradation of organic contaminants and noxious gas under visible-light has attracted increasing attention due to its potential application in solving the issues of environmental pollutions. Thus, the researchers have been focused on the exploitation and synthesis of highly efficient visible-light responsible photocatalyst which could meet the requirement of practical applications [1,2].
Recently, graphitic carbon nitride(g-C3N4), as a typical metal-free material [3], has become one of the hot topics in the field of photocatalysis due to its appealing electronic structure with a medium band gap(2.7eV)and excellent physicochemistry stability [4,5].As one of the most promising two dimensional (2D) materials, g-C3N4 with C and N elements is non-toxic, low cost, abundant and environment-friendly as well as not only could be used for hydrogen evolution but also for application towards the degradation of organic pollutants [6–8]. However, the photocatalytic efficiency of pure g-C3N4 suffers from several weakness such as a small specific surface area, high combination rate of photo-generated electron–hole pairs and low utilization efficiency of the visible light [9]. In order to enhance the photocatalytic performance of g-C3N4, methods like noble metal doping have been adopted [10,18]. Although noble metals can improve the separation rate of photo-generated electrons and holes [11], the noble metals (i.e. Pt) or their oxides are rare and expensive [12]. Other effective strategies such as copolymerization, semiconductor coupling and nanostructured design have also been proposed [13], but the enhanced photocatalytic efficiency is limited. Therefore, it is still urgent to further enhance the photocatalytic performance.
Molybdenum disulfide (MoS2) is a well-known layered structure material, similarly to graphene, a monolayer or few-layered MoS2 can be obtained through disordered way, which shows very distinct physical and chemical properties [14]. MoS2 is considered as a potential photocatalyst because of its narrow band gap [15]. The unique band structure of MoS2 can form a well energy band alignment with g-C3N4, resulting a type-II band alignment. So it can effectively promote the transfer of photo-generated electrons and holes [16]. MoS2 could also play a similar role to that of the noble metal. The few-layer MoS2 nanosheets could act as effective electron collectors, which was favorable to the separation of electron-hole pairs in g-C3N4. Moreover, the two-dimensional layered structure has a high specific surface area and can be attached well with g-C3N4 to form intimate heterojunction [17]. Ge et. al [18] reported MoS2-g-C3N4 hybrid photocatalysts via facile impregnation method for photocatalytic H2 evolution under visible light irradiation. Li et. al [19]investigated the photocatalytic degradation performance of the MoS2-g-C3N4 by varying the heating rate of the prepared g-C3N4. Herein, we used a ultrasonic dispersion method for the synthesis of MoS2-g-C3N4 nanocomposites. We believe that ultrasound can produce alternating low-pressure and high-pressure waves in solution, resulting in the formation of small vacuum bubbles. This ultrasonic cavitation effect causes high speed impinging liquid jets, deagglomeration and strong hydrodynamic shear-forces [20, 21].Thus the energy of ultrasonic waves can fine the structure of the composites and make the particlse well dispersed. Also the high-speed motion of the particles and mechanical stirring in the sound field can modify the conditions of dissolution and reprecipitation, leading to the close combination of the two materials and the formation of the hetero-structure [22]. To the best of our knowledge, MoS2-g-C3N4 nanocomposites by using the ultrasonic dispersion method for enhancing the removal of nitric oxide (NO) has not yet been reported. Moreover, the repotted method here is facile, economic and environmental benign compared with others [23,24], it is highly attractive for large scale environmental and energetic applications.
In this contribution, to effectively enhance the photocatalytic performance of g-C3N4 under the visible light irradiation, we designed and synthesized MoS2-g-C3N4 nanocomposites by using the ultrasonic dispersion method and modifiedg-C3N4 with different concentration of MoS2. The performance of the MoS2-g-C3N4 nanocomposites was characterized in detail. The photocatalytic activity of the MoS2-g-C3N4 nanocomposites for the removal of NO under visible light irradiation has been measured and investigated. The results demonstrated that the MoS2-g-C3N4 nanocomposites with MoS2 content of 1.5wt% exhibited optimal photocatalytic activity, and the corresponding NO removal rate is 51.672%, which is enhanced by 65.5% compared to the pure g-C3N4 under the same visible-light irradiation.
2. Experimental
2.1 Synthetic procedures
During the experiment process, all of the chemicals were used without further purification. Thioacetamide (TAA, 99% purity) was obtained from Adamas (CH). Urea powder (CH4N2O,99% purity) and sodium molybdate dehydrate (Na2MoO4, 99% purity) were obtained from KESHI (CN). The solutions used in this work were deionized (DI) water. The detailed preparation of polymeric g-C3N4 could be found in our previous report [25]. The MoS2 was synthesized by a hydrothermal method. 250 mg of Na2MoO4 and 200 mg of TAA were mixed into 30mL of DI water, followed by a stirring for 30 min, and then the mixture was dispersed by ultrasonic until the chemicals dissolved completely. The mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 200 °C for 16 h. The cooled black precipitates were washed with DI water and ethanol for three times, and then were dried in a vacuum drying oven at 70 °C for 10h. The dried g-C3N4 and MoS2 solids were ground into powder. The synthesis of MoS2-g-C3N4 hybrid photocatalyst by ultrasonic dispersion method was described as follows. Firstly, 300 mg of the prepared carbon nitride powder was dissolved in 30 ml of ethanol. Secondly, the appropriate amount of MoS2 powder was mixed and immersed into the solution. The mixture was stirred for 2 h by a magnetic stirrer, and followed by a ultrasonic dispersion for 2 h. Finally, the obtained samples were washed 3 times by ethanol and dried in vacuum at 70°C. The MoS2-g-C3N4 nanocomposites with different MoS2 contents of 0.5 wt %, 1wt %, 1.5wt%, 3wt% and 5wt% were fabricated by the above method.
2.2 Photocatalysis activity evaluation
The photocatalytic performance of the as-prepared samples was investigated by removing ppb-level NO in a continuous flow reactor at room temperature. The reaction vessel (30cm × 15cm × 10cm) was made of polymeric glass and covered with saint-glass. For the light irradiation, a 150 W commercial tungsten halogen lamp (General Electric) with a UV cut-off filter (420 nm) was vertically placed outside the reactor. During the test, 0.2 g sample was dispersed uniformly in 50 ml DI water by ultrasonic dispersion, and transferred to two glass dish (12.0 cm in diameter) and dried at 60 °C, then put in the reactor with sealed tight. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance). The initial concentration of NO was diluted to about 600 ppb. The flow rates of the air stream and NO were controlled at 2.4 L/min and 15 mL/min, respectively. The two gas streams were then premixed in a three-way valve. The relative humidity is controlled at 50% in the air stream. When the adsorption-desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was measured every one min by using an NOx analyzer (Thermo Scientific, 42i-TL), which also monitored the concentration of NO2 and NOx (NOx represents NO + NO2). The removal rate η(%) was calculated as: .
where C0 and C are the concentrations of NO in the feeding steam and the outlet steam, respectively [26,27].2.3 Characterization
The crystal structure was characterized by X-ray diffraction with Cu Ka radiation (XRD-6100, SHIMADZU, Japan). The surface morphology was studied by a scanning electron microscope (SEM:JSM-7800F,Japan). A transmission electron microscopy (TEM) and high-resolution transmission electron microscopy were applied to examine the chemical composition. A scan UV-vis spectrophotometer (UV-vis: UV-2100, Shimadzu, Japan) equipped with an integrating sphere was applied to study absorption spectra. The photoluminescence spectra were measured by a fluorescence spectrophotometer (PL: Agilent Cary Eclipse, Australia).
3. Results and discussion
The XRD patterns of the synthesized MoS2, pure g-C3N4 and MoS2-g-C3N4 nanocomposites are shown in Fig. 1. Nearly all the diffraction peaks of the synthetic layered MoS2 could be observed and were very consistent with the standard hexagonal phase of MoS2 (JCPDS 87-2416,Molybdenum disulfide 2H). Three distinct diffraction peaks could be observed at 2θ = 11.1°, 32.4°, and 56.9°, which corresponds to (002), (100), and (110) crystal planes of MoS2, respectively. For the XRD pattern of pure g-C3N4, the typical diffraction peaks at 13.2° and 27.5°, assigned to the (100) and (002) crystal planes of g-C3N4, respectively. The XRD pattern of MoS2-g-C3N4 nanocomposites is quite similar with the pure g-C3N4, which indicates the MoS2 modification has no influence on the crystal structure of g-C3N4. No MoS2 peaks are observed in the composites, mainly because the MoS2 contents are quite low and highly dispersion in g-C3N4 photocatalysts.
Fig. 1 XRD patterns of the synthesized MoS2 nanoflakes, pure g-C3N4 and 1.5wt% MoS2-g-C3N4 nanocomposites.
The morphologies of MoS2, pure g-C3N4 and 1.5 wt% MoS2-g-C3N4 composites were observed by SEM. In the Fig. 2(a), the MoS2 shows a flower-like structure with large specific surface area, which may promote the photocatalysis obviously. An agglomeration of MoS2 particles with a diameter of 200-250nm are observed. The pure g-C3N4 sample appears to have aggregated sheet microstructures, as shown in Fig. 2(b). From the image of Fig. 2(c), MoS2 particles disperse on the surface of the layered g-C3N4. Although the composites was synthesized by ultrasonic dispersion, it seems that the MoS2 naonoparticles could attached well on the layered g-C3N4. This will contribute to the absorption of light as well as its easy contact reaction with pollutants. TEM was further used to study the morphology and microstructures of the MoS2 and MoS2-g-C3N4 composites. Figure 3(a) displays the MoS2 with nanosheet structure. By analyzing the HRTEM image from Fig. 3(b), we found the MoS2 had a layered structure with a interlayer space of 0.645 nm, which could be assigned to the (002) plane. From Fig. 3(c), it clearly shows that MoS2 naonoparticles are dispersed well and wrapped by the layered g-C3N4, which causes the overlap of crystalline lattice. From the HRTEM images of 1.5wt% MoS2-g-C3N4 composites, we can clearly observe the interface of hetero-structure, as shown in Fig. 3(d). Energy dispersive spectroscopy (EDS) and mapping image results further demonstrate the existence of Mo and S elements on the surface nanocomposites,as shown in Fig. 3(e).
Fig. 2 SEM images of different samples. (a) MoS2, (b) pure g-C3N4, and (c) 1.5wt% MoS2-g-C3N4 nanocomposites.
Fig. 3 (a) TEM images and (b) HRTEM, FFT and IFFT images of MoS2, (c) TEM images of 1.5wt% MoS2-g-C3N4 nanocomposites, (d) HRTEM images of 1.5wt% MoS2-g-C3N4 nanocomposites and (e) EDS mapping of 1.5wt% MoS2-g-C3N4 nanocomposites.
The optical properties of the MoS2-g-C3N4 composites with various MoS2 contents were investigated by UV−vis diffuse reflectance spectroscopy, as shown in Fig. 4(a). MoS2 shows a wide light absorption among full spectrum range due to its intrinsic narrow band gap of around 1.9 eV and the multilayer structure [28]. For the pure g-C3N4, a significant strong absorption from UV through the visible range up to 460 nm can be assigned to the intrinsic band gap of g-C3N4 (≈2.7 eV). With the increase of MoS2 content, the absorption intensity of the MoS2-g-C3N4 composites increases obviously in visible light region, which can be attributed to the presence of MoS2 in the composites. Increased visible light absorption generally results in high visible light photocatalytic performance. Thus, the combination ofMoS2 with g-C3N4 is considered to be effective for the visible-light response. In addition, all the MoS2-g-C3N4 composites show the similar absorption edge as compared to pure g-C3N4.
Fig. 4 (a) UV−vis absorption of the as-prepared samples and (b) Photocatalytic activity of the samples for the removal NO gas under visible light irradiation.
The photocatalytic activity of the MoS2-g-C3N4 nanocomposites was evaluated by photocatalytic removal of NO under visible light. As shown in Fig. 4(b), the MoS2 content has a significant influence on the photocatalytic activity of g-C3N4. Due to the moderate band gap and unique electronic structure of g-C3N4, the photocatalytic activity of pure g-C3N4 is about 31.22%. With a small amount of MoS2 content (0.5 wt%), the activity of the MoS2-g-C3N4 nanocomposites is remarkably enhanced. Increasing MoS2 content from 0.5 wt% to 1 wt%, the photocatalytic activity is further improved. The highest photocatalytic activity is obtained for 1.5wt% MoS2-g-C3N4 nanocomposites and the corresponding removal rate of NO is about 51.67%, which has improved about 65.5%, compared to that of pure g-C3N4. However, withthe further increasing of loading contents of MoS2, the photocatalytic activity presents a declining trend, which is because of the increase in the opacity and light scattering. It causes a decrease of visible light source passing through the samples. As also shown in Fig. 4(b), the activity of the samples with 3 and 5wt% MoS2 shows a decline tendency with the increase of irradiation time. When excess amount of MoS2 was added, much more intermediates (HNO2 and HNO3) generated during the photocatalysis process may easily attach to the surface of catalyst [29], which may hinder the photocatalytic activity.
As the photocatalytic activity is related with the efficiency of photogenerated electrons and holes, we investigate the PL spectra for all samples, as shown in Fig. 5(b) The PL spectrum of the nanocomposites are similar to that of pure g-C3N4 and all of them exhibited a broad emission peak centered at around 460 nm, corresponding to the band gap of g-C3N4. The pure g-C3N4 has a strong PL emission peak. Once MoS2 was added, the PL intensity of of the composites drops remarkably. The 1.5wt% MoS2-g-C3N4 nanocomposites shows the lowest PL emission peak, which is correspondence with the photocatalytic activity test. This means that the photogenerated carrier recombination is greatly inhibited, as the electrons are excited from the valence band to the conduction band and then transfer to MoS2 sheets rapidly. And the pure MoS2 shows a significant quenching effect, due to the fast transfer of photogenerated electron-hole pairs.
Figure 6 shows the illustration of the possible enhanced photocatalytic activities mechanism. Under visible-light irradiation, electrons (e−) are excited from the valence band (VB) to the conduction band (CB) in g-C3N4. The photoexcited electrons would transfer from g-C3N4 to MoS2 sheets due to the lower CB positions, the separated electrons on the surface of MoS2 would combine with adsorbed oxygen to formed O2- radicals, which will finally change into hydroxyl radicals so the holes and electrons would be efficiently separated. Then these photoinduced charge carriers induce active species to oxidize the adsorbed NO to form NO3− [20].
Fig. 6 Mechanism illustration for the photocatalytic removal of NO using MoS2-g-C3N4 nanocomposites.
4. Conclusions
In conclusion, high performance of MoS2-g-C3N4 hybrid photocatalyst was successfully fabricated via a simple ultrasound method, which were used for the removal of NO. Under visible-light irradiation, the synthesized 1.5wt% MoS2-g-C3N4 composites shows the best photocatalytic activity with a high removal rate of 51.67%, corresponding to a enhancement of 65%, compared to the pure g-C3N4. The significant enhancement of photocatalytic activity is due to the energy band gap alignment between MoS2 and g-C3N4, which can effectively promote the separation of the photogenerated electrons and holes. The synthesized hybrid photocatalyst may have a potential application in pollutants removal.
Acknowledgments
This work is supported by National Natural Science Foundation of China (61574024, 61404017, 61520106012), Natural Science Foundation of Chongqing (cstc2015jcyjA1055,cstc2015jcyjA90007), Fundamental Research Funds for the Central Universities, (106112015CDJZR125511, 106112015CDJXY120001).
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