Hydrothermal Synthesis of MoS2/rGO Heterostructures for Photocatalytic Degradation of Rhodamine B under Visible Light

Faculty of Natural Sciences, Quy Nhon University, Quy Nhon 55000, Vietnam Applied Research Institute for Science and Technology, Quy Nhon University, Quy Nhon 55000, Vietnam Institute of Research and Development, Duy Tan University, Da Nang 50000, Vietnam Faculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang 50000, Vietnam School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi 100000, Vietnam


Introduction
The limited source of fresh water in earth crust, which is suffering from pollution caused by the persistent organic pollutants such as dyes, pesticides, and antibiotics, raises a huge challenge for human being to find more sustainable way in use and recycling wastewater [1,2]. A variety of strategies have been investigated in effort to remove those contaminants in wastewater for recycle or discard into environment within harmless effect [3,4]. However, the traditional methods are tough to achieve high efficiency, owing to the sophisticated organic pollutants. The most prominent among applications is photocatalytic technology due to its advantageous features such as high light-harvesting efficiency, environmental safety, low cost, and thorough degradation using naturally available solar energy sources [5][6][7]. In recent years, the transition metal oxide-based semiconductors, including TiO 2 and ZnO, have attracted much attention of scientific community by their promising photocatalytic performances [3,[8][9][10]. However, the nature as large band gap semiconductors limits their practical application due to the fact that they are only active under the UV light. Therefore, MoS 2 with narrow band gap and fully activated under visible light could overcome this limitation. Nevertheless, the high rate of recombination of photoinduced charge carriers in single-phase MoS 2 normally prohibits its photocatalytic performance. Therefore, the integration of two or more photocatalysts to form a composite has been favorably considered [11][12][13][14][15][16][17][18]. One of the most successful strategies in coupling materials toward a better photocatalytic performance is grafting with a highly electronic conductive counterpart such as carbonbased materials. Among this family of materials, graphene, a two-dimensional (2D) material constructed from the sp 2hybridized carbon elements, has attracted interest owing to its unique properties such as large specific surface area, good electrical conductivity, and high optical transmittance [19,20]. These properties allow graphene to be utilized in the broad scope of application, including adsorption, photocatalyst, supercapacitors, and batteries. It is considered as a potential support when combining with a semiconductor material, leading to reduction in the recombination rate of charge carriers [3]. Accordingly, semiconductor/graphene photocatalysts have been widely used in many areas, such as water splitting for H 2 evolution, CO 2 reduction, organic synthesis, disinfection, and advanced oxidation processes in wastewater treatment [21]. The chemical-exfoliated graphene oxide (GO) is reported as low electrical conductivity [22]. By the reduction of oxygen-based functional group on the surface, the reduced graphene oxide (rGO) with recover sp 2 -hybridized framework performs better electronic conduction [23]. Therefore, as an analogue of graphene, rGO is a promising candidate for coupling and improving the conductivity of final composite.
In this current work, we designed and highlighted the interfacial interaction of MoS 2 with rGO using a two-step hydrothermal process, in which rGO nanosheets functioned as a framework for scaffolding MoS 2 , leading to enhanced properties of MoS 2 /rGO hybrid nanostructures. Specifically, the impact of hydrothermal temperature and ratio of precursors has been systematically investigated. It was indicated by RhB degradation activities that the photocatalytic performance of the optimized MoS 2 /rGO conventional hybrid structure was substantially improved.  [24,25]. Firstly, 15 mL of H 2 SO 4 solution was slowly added to a 500 mL flask containing 0.6 g of graphite powder and 0.3 g of NaNO 3 . The suspension was stirred for 24 hours at room temperature. Next, the mixture was cooled down to 3-5°C and kept at this temperature for 2 hours. Then, 1.8 g of KMnO 4 was added to the mixture and stirred for 5 hours at room temperature, which was then heated to 98°C, and kept at this temperature for 30 min. After that, the mixture was cooled down to 40°C in air. Thereafter, 90 mL distilled water and 7.5 mL H 2 O 2 30% were added. Finally, the suspension was centrifuged, washed 3 times in diluted HCl (5%), 3 times in distilled water, 3 times in ethanol, and dried in air at 80°C for 12 hours. After that, ascorbic acid was employed as a reducing agent to achieve rGO.

Synthesis of MoS 2 /rGO
Composites. Firstly, MoS 2 was synthesized via a facile method of annealing the mixture of (NH 4 ) 6 Mo 7 O 24 .4H 2 O and (NH 2 ) 2 CS under Ar gas at 650°C as described in the previous publication [24]. The MoS 2 /rGO composites were fabricated from the two separate components (rGO and MoS 2 ) by the hydrothermal method. To investigate the influence of the hydrothermal temperature on the composite properties, a fixed weight ratio of MoS 2 to rGO (=4/1) was added to the ethanol-water solution before applying the ultrasonication for 1 h and continuously stirring for 5 h to form a homogeneous mixture. After that, the obtained mixture was transferred to a 100 mL Teflon-lined autoclave and heated at different temperatures (140, 160, 180, and 200°C) for 10 h. Finally, the obtained mixture was filtered, washed with water, and centrifuged before being dried at 80°C for 12 h to obtain the samples, which are denoted as MoS 2 /rGO (4/1-T) composites with T = 140, 160, 180, and 200°C, respectively. In order to investigate the effect of the ratio of MoS 2 to rGO, a series of samples with different weight MoS 2 /rGO ratios was prepared following the same procedure for MoS 2 /rGO (4/1-180°C) and denoted as MoS 2 /rGO (x-180°C) with x = 2/1, 4/1, and 6/1, respectively.

Characterization of Materials.
The crystal phase of the synthesized samples was characterized by the X-ray powder diffraction (Brucker D8 Advance with Ni-filtered Cu Kα radiation (λ = 1:5418 Å). The Fourier-transform infrared spectroscopy of the samples was recorded on FT-IR-GX-Per-kinElmerLabRAM HR Evolution (Horiba) with a 647.1 nm laser as an excitation source. The surface morphology and elemental composition were analyzed by scanning electron microscope (SEM-SEM-JEOL-JSM 5410 LV) and energy scattering spectroscope (EDX-JEOL 5410), respectively. The specific surface area obtained by Brunauer-Emmett-Teller (BET) analysis and pore size distribution from Barrett-Joyner-Halenda (BJH) technique were measured on a Tristar II 3202 instrument using N 2 adsorption-desorption at 77 K. The UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) of the samples was analysed using UV-Vis-Cary 5000 spectrophotometer (Varian). The X-ray photoelectron spectroscopy (XPS) was recorded on Theta Probe AR-XPS System (Thermo Fisher Scientific). The Raman spectroscopy was carried out T64000 Raman with a 647.1 nm laser as an excitation source, and detector CCD was cooled by liquid nitrogen.

Evaluation of Photocatalytic
Activity. The photocatalytic activities of the obtained materials were evaluated via the photodegradation of RhB in an aqueous solution under visible light irradiation. For a typical experiment, 100 mg of the catalyst was added into 400 mL of 20 mg/L RhB solution. Before illumination, the reaction mixture was stirred continuously for 2 h in the dark to reach adsorption-desorption equilibrium. Next, the mixture was illuminated by compact lamp (60 W-220 V). After every interval of 30 min, 4 mL of the reaction solution was collected and removed from the photocatalyst by centrifugation for further measurement. The residual concentration of RhB was recorded using a UV-Vis spectrometer.  [26]. These featured peaks in composites were obtuse and lower (peak at 002) than those of pristine MoS 2 due to the overlap of the rGO layers between MoS 2 crystals because of composite formation (as shown in Figure 1(a)). Notably, the intensity of the peaks corresponding to MoS 2 gradually decreases with the increase of temperature, indicating formation of a more solid and stable composite structure. A similar phenomenon was also observed in the previous report of Guo et al. [27].

Results and Discussion
To further evaluate the impact of MoS 2 supported on rGO nanosheet, chemical bonding characteristics of the asprepared materials were revealed by the FTIR spectra (as shown in Figure 1(b)). While the peaks in the wavenumber of about 530, 630, and 920 cm -1 can be assigned to Mo-S bonds [28][29][30], a peak at about 550 cm -1 can be attributed S-S vibration modes [31]. The peaks in the range of 1550-1650 cm -1 represent the bonds in sp 2 hybridization such as C=C and C-O-C [32]. Furthermore, a broad band from 1112 to 1393 cm -1 is associated with the existence of C-O and C-OH bonds [33]. Vibration bands in the region of 3200-3700 cm -1 can be ascribed to the -OH bonds from adsorbed H 2 O molecules of the rGO in the composites. As the hydrothermal temperature increases, the intensity of most characteristic peaks decreases, especially at T = 180°C. At high temperature (T = 200°C), a reduction of functional groups may come from change of rGO structure to almostlike grapheme [27].
UV-Vis-DRS curves of MoS 2 and MoS 2 /rGO-T composites are presented in Figure 2(a). Accordingly, the absorption edges of both the MoS 2 and MoS 2 /rGO composites extend to the wavelength range from 400 to 700 nm. Based on the absorption edge, the bandgap energy of as-synthesized composites obviously shift towards the visible light region, which proves that the combination of MoS 2 and rGO could enhance the light-harvesting ability and expand the optical absorption range to the region of solar energy [12]. As shown in Figure 2(b), the Tauc plots of MoS 2 and MoS 2 /rGO-T composites were present using Kulbeka-Munk equation: in which, hν is incident photoenergy, C is constanst, E g is band gap value, and n is exponent. For direct band gap, n = 1/2 was applied, and the E g value was determined as the intercept of tangent of Tauc plot on x-axis. The SEM images for the MoS 2 /rGO (4/1-T) composites are shown in Figure 3. It is obvious that the hydrothermal temperature exhibits a significant effect on the surface morphology of the MoS 2 /rGO-T composites. For MoS 2 /rGO (4/1-140°C) (Figure 3(a)), its morphology is in nanosheets, similar to rGO (Figure 3(e)), without the clear presence of the structure of MoS 2 . When the hydrothermal temperature increases to 160°C, MoS 2 nanosheets in the form of flakes clearly appear on the material surface but nonuniformly distribute (Figure 3(b)). While at 180°C (Figure 3(c)), the composite displays uniform formation between MoS 2 layers and rGO nanosheets. However, as the hydrothermal temperature increases to 200°C (Figure 3(d)), the MoS 2 layers densely appeared, stacked, and covered the rGO nanosheets. This phenomenon is consistent with the results observed from the XRD patterns.   (Figure 4(a)).
Additionally, in order to evaluate the kinetics of photocatalytic progress, the Langmuir-Hinshelwood model has been applied [34]. Figure 4   Journal of Nanomaterials k:t, where C is the equilibrium concentration of RhB (mg/L), C o is the initial concentration of RhB before irradiation (mg/L), t is the reaction time (min), and k (min −1 ) is the reaction rate constant. The obtained data are summarized in Table 1.
As observed in Table 1, MoS 2 /rGO (4/1-180°C) performs the highest k value (0.00652 min -1 ), which is about 5.3 times higher than that of the pure MoS 2 . This observation is consistent with the above statement that MoS 2 /rGO-180°C has more favourable hybrid structure and morphology as well as better light-harvesting ability than the remaining composites. Therefore, the hydrothermal condition of 180°C is considered to be the most appropriate, allowing the creation of more active MoS 2 /rGO composites, reducing the        Journal of Nanomaterials recombination rate of photogenerated electron-hole pairs in the single MoS 2 .

3.2.
Effect of the Ratio of MoS 2 to rGO. Further investigation on the MoS 2 supported on rGO sheets substrate, the XRD patterns, and FT-IR spectra of selected samples with different mass MoS 2 /rGO ratios are displayed in Figure 5.
As can be illustrated in Figure 5(a), the XRD patterns of the MoS 2 /rGO composites exhibit characteristic peaks at 2θ = 14:1; 33.6; 39.84; and 58.1°corresponding to the (002), (100), (103), and (110) planes, respectively, which are well agreed with the 2H hexagonal phase of MoS 2 [26]. The increase of MoS 2 /rGO mass ratio led to the gradually increase in intensity of MoS 2 -related peaks, which could be clearly observed at the (002) lattice plane. Among the three composites, the pattern of the sample with ratio of 4/1 (MoS 2 /rGO (4/1-180°C)) presents the most obtuse peak, indicating clear integration between graphene and MoS 2 nanosheets toward an uniform heterojunction.
The chemical bonding characteristics of the composites with various MoS 2 /rGO ratios are displayed in Figure 5(b). The typical peaks of Mo-S, S-S, C-OH, CO, OH, and C=C functional groups at different wavenumbers (as mentioned in Section 3.1) all appeared in the composites. For MoS 2 /rGO (4/1-180°C), however, the intensity of these peaks reduced significantly, suggesting a robust structure with intimate contact between the MoS 2 and rGO in the composite.
The optical absorption capacity of MoS 2 and the MoS 2 /rGO composites was investigated by UV-Vis DRS. As shown in Figure 6(a), similar to the results obtained in changing the hydrothermal temperature, changes in the ratios of precursors lead to changes in the optical absorption intensity of the materials. Moreover, a dramatic shift in the absorption peak of the visible light region to around 700 nm appeared in all the composites. Moreover, the main peak systems of both MoS 2 and rGO present in these composites, which is consistent with previous reports [32]. This feature is favorable for increasing the light absorption ability as well as extending the optically active region of the composites to the low energy region (inherently abundant in the      Journal of Nanomaterials sunlight region) (as shown in Figure 6(b)). Consequently, it is proved that the combination of intercalated MoS 2 nanosheets and rGO layers plays an important role in increasing the photosensitivity of the materials.
The morphology of the materials with different ratios of MoS 2 is given in the SEM images. Figures 7(a)-7(c) display different surface structure morphology of the composites. Accordingly, the MoS 2 flakes appeared on rGO relatively little in MoS 2 /rGO (2/1-180°C) (Figure 7(a)). Nevertheless, when increasing the ratio to 4/1 (Figure 7(b)), rGO was covered by a large number of MoS 2 flakes. With further increase in the ratio to 6/1 (Figure 7(c)), the layers of MoS 2 flakes appear more densely and tend to agglomerate together into large bulks. On the other hand, the EDX analysis of MoS 2 /rGO (180°C-4/1) indicates that the atomic ratio of Mo to S is approximately 1 : 2, in accordance with the bonding properties of MoS 2 in the material structure (as shown in Figure 7(d)).
The specific surface area and porosity of the MoS 2 /rGO (180°C-x) composites were confirmed by the BET analysis as illustrated in Figure 8. The hysteresis phenomenon and the hysteresis loops as shown in Figure 8(a) could be assigned to a type-IV isotherm (according to IUPAC classification), suggesting their mesoporous structure. The results in Table 2 show that the specific surface area of MoS 2 /rGO-4/1 was higher than that of MoS 2 /rGO-2/1 and MoS 2 /rGO-6/1. This is in agreement with the aforementioned discussion of the SEM images, which may contribute to a significant enhancement in the photocatalytic efficiency of the material. The appropriate ratio of MoS 2 and rGO in MoS 2 /rGO-2/1 and MoS 2 /rGO-4/1 could provide a sufficient content of rGO framework assuring the better dispersion of MoS 2 nanosheets. This leads to prevent components' nanosheets from recollapse then help enlarge their surface area. However, at higher content of MoS 2 (MoS 2 /rGO-6/1), the insufficient rGO framework may cause the restacking of excess MoS 2 . Therefore, the specific surface area of MoS 2 /rGO-6/1 is the lowest value among three samples.
The Raman spectra of MoS 2 and a representative composite MoS 2 /rGO (180°C-4/1) shown in Figure 9 exhibit two characteristic peaks located at wavenumbers of between 378 and 404 cm -1 , corresponding to the in-plane E 1 2g and out-of-plane A 1g vibration modes of the hexagonal phase of MoS 2 crystal [34]. An energy value difference between them corresponding to 26 cm -1 reflects the multilayer structure of MoS 2 [35]. It is worth mentioning that the intensity of peaks at about 380 cm -1 (internal vibration of Mo-S bond) and 405 cm -1 (out-of-plane vibration of "S" element in Mo-S bond) of MoS 2 /rGO (180°C-4/1) is much lower than that of the single MoS 2 that is suitable for the 2H-MoS 2 phase [36]. The intensity of the E 1 2g peak is lower than that of the A 1g peak, indicating that the crystal structure of the obtained materials contains significant defect sites and side structures [23]. This observation is expected to enhance their photocatalytic activity. In addition, values at 1330 and 1590 cm -1 can be related to D and G breathing vibrations, respectively, corresponding to the defects and sp 2 hybrid carbon atoms in the MoS 2 /rGO composite [37]. This parameter indicates the presence of rGO integrated into MoS 2 to form a composite structure during the hydrothermal process. On the other hand, the I D /I G peak intensity ratio of 1.41 for MoS 2 /rGO (180°C-4/1) is lower than that for rGO (I D /I G = 1:72), indicating a decrease in size and then density of the defect sites on the rGO surface in the composite compared to single rGO [23,37].
To distinguish the existing forms of Mo and S in MoS 2 and MoS 2 /rGO (4/1-180°C), X-ray photoelectron spectroscopy was performed. Figure 10(a) exhibits the binding energy values of S2s, Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 , and Mo 6+ at photoelectron peaks of 226.7, 229.2, 232.5, and 235.5 eV, respectively [38]. In which the peaks of Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 , and S2s reflect the presence of the S-Mo-S bond in MoS 2 . The binding energy of 235.5 eV corresponds to Mo 6+ in the MoO 3 or MoO 4 2compounds. The existence of this peak may come from the oxidation of Mo 4+ to Mo 6+ in the calcination process for preparing MoS 2 . In addition, the results in Figure 10(b) show that the binding energy values at 161.9 and 163 eV can be assigned to S 2-2p 3/2 and S 2-2p 1/2 levels of the S 2species in the composite [39]. All binding energy values at peaks of Mo3d and S2p in the composite shifted compared to that in single MoS 2 material, as described clearly in Figure 10. This shows that, in the hydrothermal process, there is an interaction of interlayer stacking between the  Journal of Nanomaterials MoS 2 layers on the rGO nanosheets in forming composites. Furthermore, the slight shift of Mo3d and S2p signal in the MoS 2 /rGO composite to lower binding energy could be an evidence for the electrons transfer from MoS 2 to rGO which is expected to accelerate the separation of photoinduced electrons and holes. These changes in the composite structure with different MoS 2 /rGO ratios also have significant effects on the photocatalytic activity of the material in the degradation of MB ( Figure 11). The results show that all the MoS 2 /rGO composites exhibit a much higher catalytic activity compared to the sole component, MoS 2 . Upon increasing the MoS 2 loading, the light absorption intensity of the composites increases, leading to boosting catalytic efficiency. Particularly, as the MoS 2 /rGO ratio increased from 2/1 to 4/1, the degradation of RhB increased to approximately 80%. However, when this ratio increased to 6/1, photocatalytic degradation tended to decrease. This is attributed to that when the MoS 2 content overloads, it leads to aggregation on the surface of the rGO (as observed in the SEM image), which reduce the number of active sites and the light-harvesting ability in the visible region as well as the electron-hole transportation process. Therefore, the overloading of MoS 2 could limit the photocatalytic efficiency of the materials. Similar to the process of investigating the effects of temperature, the degradation of RhB in the composites with various ratios fits well with the pseudofirst-order kinetic model of Langmuir-Hinshelwood. The obtained data in Table 3 show that MoS 2 /rGO (180°C-4/1) has a higher constant rate than the other samples. This can be explained by the synergistic effect between MoS 2 and rGO, which enhanced the photocatalytic performance of the materials. Among the composites, MoS 2 /rGO (180°C-4/1) displayed the most excellent efficiency of RhB degradation.

Conclusion
In this work, the MoS 2 /rGO composites were synthesized via a hydrothermal method using MoS 2 and rGO as precursors. The impacts of hydrothermal temperature and mass ratio of MoS 2 to rGO on surface morphology, crystallographic phase, and optical property were investigated. The efficiency for RhB photodegradation of the optimal interfacially assembled MoS 2 /rGO composites can reach approximately 80%, which is three times higher than that of the single component MoS 2 . This observation is due to the reduction in electronhole pair recombination and the enhancement in visible light harvesting. Therefore, this current work revealed that the optimized MoS 2 /rGO composites could be a promising candidate for the photodegradation of persistent toxic organic compounds in aqueous solutions.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.