The n%Mn-MoS2/rGO (labeled as n%MMS/rGO, where n% = Mn/(Mn + Mo) in mol) composites were successfully prepared by a facile hydrothermal method from the Mn-MoS2 (MMS) and rGO precursors, in which the MMS was obtained by a facile one-step calcination of (NH4)6Mo7O24·4H2O, (NH2)2CS, and Mn(CH3COO)2·4H2O as precursors in N2 gas at 650°C. The samples were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron paramagnetic resonance spectroscopy (EPR), UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and X-ray photoelectron spectroscopy (XPS), which indicates the composites containing nanosheets of Mn-MoS2 and rGO components. The photocatalytic activities of the n%MMS/rGO composite photocatalysts were evaluated through the photodegradation of rhodamine B (RhB) under the visible light irradiation. The enhancement in the photocatalytic performance of the achieved composites was attributed to the synergic effect of Mn doping and rGO matrix. The investigation of photocatalytic mechanism was also conducted.

1. Introduction

Environmental pollution is becoming more serious, leading to a significant negative impact on human health and the ecological environment. Among these challenges, water pollution caused by waste compounds from the discharge of industries such as paper, paint, printing, and textile is concerned as the most serious one [1]. Therefore, the treatment of wastewater for reducing its contamination is one of the urgent issues that attract much attention. There are numerous approaches for eliminating these contaminants, in which the photocatalytic degradation under visible light is the most attractive one [2].

As promising photocatalysts, oxide semiconductors, such as TiO2 [3], ZnO [4], WO3 [5], and Ta2O5 [6], have been widely investigated. However, their optical absorption in the visible light region is hindered by their broad band gap, preventing them from employing in the photocatalytic water treatment process. Recently, the two-dimensional (2D) layered transition-metal dichalcogenides have attracted worldwide attention in the solar cell, environmental, hydrogen evolution catalysts, and energy fields [7]. Among them, molybdenum disulfide (MoS2), a 2D semiconducting material with outstanding electronic and optical properties, is constructed from layers consisting of a plane of Mo atoms sandwiched between two planes of S atoms. Therefore, MoS2 has tailorable band gap depending on the number of MoS2 layers (from 1.2 eV in bulk to 1.9 eV in monolayer) [8]. Thanks to its narrow band gap, MoS2 is capable of strong absorption in the visible light region to create electron-hole pairs, allowing MoS2 to be a promising visible light-driven photocatalyst for practical applications [9]. However, the most common disadvantage of pure MoS2 photocatalyst is the rapid recombination of the photo-generated electron-hole pairs, resulting in low photocatalytic activities [9]. To overcome this problem, combination with other materials and doping strategy are the two most general solutions. The improvement in the photocatalytic activities of the host material via the element doping could be attributed to the enhanced effect in interfacial charge-transfer reactions of dopants [10]. Doping transition metals such as Co, Ni, and Fe into the S-edge of MoS2 could modify the edge site activity for hydrogen evolution reaction [11]. Graphene, a typical well-known material with 2D structure, is applied in various fields such as fuel cells, supercapacitors, sensors, solar cells, batteries, water treatment, adsorbent, and reinforcement for polymer. Recently, graphene and its derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) have emerged as a key matrix material for designing photocatalysts, in which they contribute to enhance the conductivity and reduce the recombination of the photo-generated electron-hole pairs [12]. Therefore, an integration of MoS2 into graphene or its derivatives may lead to a heterostructure with some favorable properties, such as fast electron mobility and increased optical absorbance, facilitating the photocatalytic activities in the visible light region [13].

In this article, to our knowledge, for the first time, photocatalytic degradation of rhodamine B (RhB) on the Mn-MoS2/rGO composites is reported. A photocatalytic mechanism for the photodegradation of RhB was proposed.

2. Materials and Methods

2.1. Materials

All chemicals, including graphite powder (Merck, Germany, ≥99%), L-ascorbic acid (HiMedia Laboratories Pvt. Ltd., India, ≥99%), ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24·4H2O, Mn(CH3COO)2·4H2O, and thiourea (NH2)2CS (Merck Germany, ≥99%), KMnO4, NaNO3, H2SO4, HCl, H2O2, C2H5OH, and rhodamine B (Xilong Chemical Co., Ltd. China, ≥90%), P25 (TiO2, Sigma-Aldrich, ≥99.5%), were used without further purification.

2.2. Preparation of Samples

GO was prepared from graphite powder using the Hummers method [14]. rGO was synthesized by reducing GO with L-ascorbic acid agent. Mn-MoS2 was prepared via a facile method of calcinating the mixture of (NH4)6Mo7O24·4H2O, (NH2)2CS, and Mn(CH3COO)2·4H2O as the precursors. In a typical experiment, [NH4]6Mo7O24·4H2O and (NH2)2CS were dissolved in a solvent of water : ethanol (1 : 1, v/v) under vigorously stirring at room temperature for 30 minutes. A proper amount of Mn(CH3COO)2·4H2O was added and continuously stirred for another 1 h to obtain a homogeneous solution. The resulting solution was heated at 60°C under stirring to evaporate solvents, and then, a solid was obtained. This solid was well ground and calcinated at 650°C for 1 hour in N2 gas. The obtained samples were labeled as n%MMS, where n% = Mn/(Mn + Mo) in mol. For comparison, MoS2 was also prepared in the same procedure without the addition of Mn(CH3COO)2·4H2O and denoted as MS.

The n%MMS/rGO composites were fabricated by a facile hydrothermal method from n%MMS and rGO. In a typical procedure, 4 mg of n%MMS (n = 1, 3, 5, and 7) and 1 mg of rGO were dispersed in a solvent of water : ethanol (1 : 1, v/v) under stirring for 5 hours. This mixture was transferred into 100 mL Teflon-lined autoclave and treated at 180°C for 10 hours. A black precipitate was collected using centrifugation and washed with distilled water and ethanol before being dried at 80°C for 10 hours and labeled as n%MMS/rGO (n = 1, 3, 5, and 7).

2.3. Characterization of Materials

The powder X-ray diffraction (XRD) patterns were recorded on a D8 Advanced Bruker X-ray diffractometer with Cu-Kα (λ = 1.5406 Å) radiation. The Fourier-transform infrared spectra were measured at room temperature using an IRPrestige-21 spectrophotometer (Shimadzu). Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were conducted on a SEM-JEOL-JSM 5410 LV (Japan). The high-resolution transmission electron microscopy (HR-TEM) image, selected area electron diffraction (SAED), and fast Fourier transformation (FFT) patterns were obtained with an operating voltage of 200 kV on a JEOL TEM-2100F microscope. The UV-Vis absorption spectra were recorded using a GBC Instrument-2885 spectrophotometer. The X-ray photoelectron spectroscopy (XPS) spectra were measured using a Theta Probe AR-XPS System (Thermo Fisher Scientific). The electron paramagnetic resonance (EPR) spectra were obtained on a Bruker A300 spectrometer at X-band frequency (9.8492 GHz).

2.4. Photocatalytic Activity

Photocatalytic activities were evaluated through the photodegradation of RhB in aqueous solution under the visible light irradiation. For a typical experiment, 0.1 g of the photocatalyst was dispersed into a 400 mL RhB solution with a concentration of 20 mg/L under stirring at room temperature. Before irradiation, the suspension was stirred in the dark for 2 hours to reach the adsorption-desorption equilibrium of RhB molecules on the surface of the photocatalyst before being illuminated by the compact light (60 W–220 V). After every interval of 30 minutes, 4 mL solution was taken out and centrifuged to remove the photocatalyst. The residual concentration of RhB as a function of investigated time was determined by recording its absorbance at 553 nm using a UV-Vis spectrometer. For comparison, photocatalytic activity of P25 was evaluated with the same procedure as mentioned above.

3. Results and Discussion

Crystal structure of MS, MMS (abbreviation for 3%MMS), and n%MMS/rGO composites was investigated by XRD, and their patterns are shown in Figure 1(a). As can be seen, the XRD patterns of n%MMS/rGO composites, MMS, and MS are analogous with the peaks at 2θ = 14.1, 33.6, 39.84, 58.1, and 68.96° corresponding to (002), (100), (103), (110), and (201) planes of the hexagonal MoS2 phase, respectively [15].

It is worth mentioning here that characteristic diffraction peak due to stacked rGO is absent in n%MMS/rGO composites, which demonstrates that rGO is not stacked in n%MMS/rGO [16]. The strong peak at 2θ = 14.1° (002), corresponding to d-spacing of 0.62 nm, elucidates a well-staked layered structure of MoS2 in the composites [17]. In addition, no peaks for MnS or other impurity phases were observed, suggesting that Mn may be successfully doped into the MoS2 host [18]. Particularly, the peak intensity of the obtained composites decreases for increasing the content of Mn. Furthermore, the enlargement of 2θ range of 10–16° (as shown in Figure 1(b)) shows a slight shift of (002) peak to the lower-angle region, which indicates the increase in interlayer spacing of the doped samples. This expansion could be explained by the larger ionic radius of Mn2+ (0.067 nm) than that of Mo4+ (0.065 nm), according to Shannon ionic radius [19] at a coordination number of 6 [20].

Figure 1(c) presents the FTIR spectra of MS, MMS, and n%MMS/rGO composites. For all the materials, the peaks around 450, 528, and 920 cm−1 are corresponding to Mo-S stretching vibrations [16, 21, 22], which demonstrates the presence of MoS2 in the materials. For rGO, the peaks at 1060, 1619, 1224, and 1720 cm−1 are attributed to C-O (alkoxy) stretching vibrations [23], C=C stretches from the unoxidized graphitic domain [24], C-OH stretching [25], and C=O stretching of carboxyl group [25], while the vibration band around 2350 cm−1 could be ascribed to C=O stretching of CO2 from the air. The nature of bonding found by IR spectra could be a further confirmation of the coexistence of MoS2 and rGO in the achieved composites.

EPR spectroscopy was used to investigate the effect of doping Mn ions on MMS and composite structures. Figure 1(d) shows the room temperature EPR spectra of the MS, MMS, and 3%MMS/rGO. Accordingly, the type of defects presents in the prepared nanoparticles can be studied by the factor calculated by  = hν/BHo [26]. The low pristine intensity for MS with factor  = 2.003 (with magnetic field = 3350 Gauus) is shown in Figure 1(c), which may be due to defects such as vacancy S that serves as the paramagnetic center of the material, while the weak EPR at high field could be assigned for the localized defect related to Mo due to the dislocation and dangling bond [20]. The EPR signals of the doped samples show a hyperfine split into six peaks corresponding to the electron-spin interaction of Mn2+ ion with nuclear-spin of I = 5/2 [27, 28]. The Mn doping-induced defect could serve as a trapping site to reduce the recombination rate of photo-generated charged carriers, which may enhance photocatalytic activities of the doped samples.

The UV-Vis DRS spectra of rGO, MS, MMS, and n%MMS/rGO composites are shown in Figure 1(e). The absorption edges of MS, MMS, and n%MMS/rGO composites extend to 800 nm, suggesting that the absorption of the composites is completely in the visible light region. Compared to the sole MMS, the absorption intensity of n%MMS/rGO composites is significantly enhanced over the entire investigated range of wavelength, which could be attributed to the intrinsic light absorption of rGO. Therefore, the combination with rGO could enhance the visible light harvesting ability of the obtained composites, which is favorable for improving their photocatalytic performance. In addition, due to high electronic conductivity, rGO could offer a good diffusion pathway for electron to extend the lifetime of electron-hole pairs which finally could overcome the intrinsic limitation of high recombination rate of the single MoS2 [14].

The surface morphology of the photocatalysts was investigated by SEM and TEM. As observed in Figure 2(a), rGO is constructed from nanosheets with the thickness of ∼10 nm that is consistent with the TEM image in Figure 2(e) and confirms the successful exfoliation of rGO via the chemical reduction by L-ascorbic acid agent. As shown in Figures 2(b) and 2(c), the surface morphology of MS and MMS is composed of nanosheets aggregated together. There is no significant change in the aggregation of MoS2 nanosheets in both samples, which may lead to less exposed active sites and inferior activity. Normally, MoS2 is a -type semiconductor with low conductivity, easy to agglomerate, and poor in charge transfer capacity, which makes the material has low catalytic activity [29]. Therefore, dispersion in a high specific surface area matrix as rGO could be an effective way to not only reduce the agglomeration but also lessen the charge transfer resistance. According to Figure 2(d), the better dispersion could be observed after combining Mn-doped MoS2 with rGO matrix. This observation is also confirmed by the TEM image in Figure 2(f). Furthermore, the interlayer d(002)-spacing of 0.65 nm, calculated from the line profile (using Image J tools) in HR-TEM image of Figure 2(g), shows a slight expansion compared to the theoretical value [30], which is in good agreement with the aforementioned XRD discussion. The crystal structure of the 3%MMS/rGO composite was further verified by the FFT patterns and the SAED shown in Figures 2(h) and 2(i). The values of the lattice spacing are 2.82 and 1.61 Å, which are corresponding to the (100) and (110) planes of 2H-MoS2, respectively [31], which both are confirmed in SAED pattern (Figure 2(i)). The halo ring and unclear dot arrangement in FFT and SAED patterns illustrate for the low crystallinity of the material.

The XPS measurement was performed on a representative material 3%MMS/rGO. The Mn2p XPS spectrum (Figure 3(a)) can be deconvoluted into six peaks corresponding to Mn2+ at binding energies of 640 and 651.48 eV, Mn3+ at 642.3 and 653.8 eV, and Mn4+ at 645 and 654.8 eV [3234]. This result suggests the coexistence of Mn2+, Mn3+, and Mn4+ ions on the surface of the sample. Figure 3(b) presents Mo3d XPS spectrum with two peaks at 229.16 and 232.43 eV corresponding to the Mo4+3d5/2 and Mo4+3d3/2, respectively. In addition, the peak at 226.9 eV is attributed to S2s of MoS2 [35]. The S2p XPS spectrum in Figure 3(c) contains two peaks at 161.8 and 162.9 eV, which are assigned to the S2p3/2 and S2p1/2 of S2− species in the composite [36]. The C1s XPS spectrum in Figure 3(d) can be split into four peaks, in which peaks at 284.8, 285.7, 287.9, and 288.9 eV are attributed to the C-C/C=C, C-S/C-O, C=O, and O-C=O bonds, respectively, for functional groups in the rGO [37] and from C-S bonds [38].

The photocatalytic activities of MS, MMS, and n%MMS/rGO were evaluated through the photodegradation of RhB in aqueous solution under the visible light irradiation (Figure 4(a)). It can be observed that, after photodegradation for 240 minutes, the concentration of RhB decreases by about 25.9% in the presence of MS; however, the degradation is significantly enhanced by MMS (41%). This indicates that Mn doping in MoS2 results in an increase in the photodegradation efficiency of the obtained material. Remarkably, compared to the sole components, the composites show a much higher photodegradation efficiency. Among them, 3%MMS/rGO exhibits the highest photodegradation efficiency of 90%. This suggests that rGO plays a pivotal role in enhancing the photocatalytic performance of the composites. A comparison between the composites shows that the photocatalytic activity increases from n = 1 to 3 but decreases when n at higher values of 5 and 7. For comparison with other works, photocatalytic activity of P25 was evaluated with the same procedure, and the result is shown in Figures 4(a) and 4(b). It is observed that the composites show the much better photocatalytic performance with the higher rate constants compared to P25 (the commercial TiO2). Accordingly, Mn doping in MoS2 along with combination of rGO is an effective strategy in modification of MoS2 as a photocatalyst with enhanced catalytic performance.

Normally, in order to evaluate the kinetics of the photocatalytic progress, the Langmuir–Hinshelwood model has been applied [39]. Figure 4(b) shows the linear relationship of ln (Co/C) versus the irradiation time, indicating that the photodegradation of RhB fits well with the pseudo-first-order kinetic model according to the equation ln (Co/C) = k·t, where C is the equilibrium concentration of RhB (mg/L), Co is the initial concentration of RhB before irradiation (mg/L), t (min) is the reaction time, and k (min−1) is the reaction rate constant. The obtained data are summarized in Table 1.

Accordingly, the 3%MMS/rGO shows the highest rate constant of 0.00919 min−1, which is 7.6 and 4.16 times higher than MS and MMS, respectively. This may be due to a substantial synergic effect between Mn-MoS2 and rGO on enhancement in the photocatalytic performance of the composite.

In order to clarify the main active species responsible for the degradation of RhB over 3%MMS/rGO, controlled photocatalytic activity experiments were performed with the addition of different radical scavengers at the natural pH of the dye. In this study, four different scavengers, tert-butyl alcohol (TB) scavenger for [40], ammonium oxalate (AO) for H+ [41], 1,4-benzoquinone (BQ) for [42], and dimethyl sulfoxide (DMSO) for e [43], were used. Figure 4(c) shows the photocatalytic activities of 3%MMS/rGO for the degradation of RhB in the presence of different radical scavengers. The obtained data are summarized in Table 2.

These results indicate that h+ and may play a major role, whereas and e are minor active species in the photodegradation of RhB. Based on the effect of scavengers, a mechanism for the photocatalytic degradation of RhB over n%MMS/rGO is proposed in Figure 5. Accordingly, when light is absorbed, photoelectrons (e) from the conduction band of MoS2 are transferred to rGO. The photoelectrons will then react with O2 molecule adsorbed on the photocatalyst surfaces to form radicals. These radicals and active photoholes (h+) degrade RhB. In this process, the rGO acts as a sink for the excited electrons because of its π-conjunction structure. Mn is as acceptors of photoelectrons from MoS2, and the transferred process could restrain the recombination of holes and electrons, facilitating the photocatalytic performance. The photocatalytic degradation of RhB on the n%MMS/rGO composite can be illustrated as follows:

4. Conclusions

n%MMS/rGO composites containing nanosheets of Mn-MoS2 and rGO components were successfully fabricated using the hydrothermal method. Among the prepared composites, 3%MMS/rGO exhibits the highest RhB degradation efficiency (90%) much higher than that of Mn-doped MoS2 (41%) and MoS2 (25.9%). The mechanism for the photocatalytic decomposition of RhB on 3%MMS/rGO was proposed, in which photoelectrons (e) from the conduction band of MoS2 transferring to rGO and photoholes (h+) in the valence band of MoS2 play a pivotal role in the photodegradation of RhB. This work can contribute a facile preparation to design a heterostructured photocatalyst with enhanced photocatalytic performance for the practical applications in the field of the wastewater treatment.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.


This work was financially supported by the Vietnam Ministry of Science and Technology (Grant no. NĐT.52.KR/19).