Facile Synthesis of MoS₂-C₆₀ Nanocomposites and Their Application to Catalytic Reduction and Photocatalytic Degradation

Facile Synthesis of MoS ₂ -C ₆₀ Nanocomposites and Their Application to Catalytic Reduction and Photocatalytic Degradation

Jiulong Li ^* and Weon Bae Ko ^*,**,†

* Department of Convergence Science, Graduate School,

** Department of Chemistry, Sahmyook University, Seoul 139-742, South Korea (Received November 21, 2016, Revised December 9, 2016, Accepted December 13, 2016)

Abstract: MoS

precursors were synthesized by reacting thioacetamide (C

H

NS) with sodium molybdate dihydrate (Na

MoO

·2H

O) in aqueous HCl solution. MoS

nanoparticles were prepared from dried MoS

precursors by calcination in an electric furnace at 700°C for 2 h under an inert argon atmosphere. MoS

-C

nanocomposites were obtained by heating MoS

nanoparticles and fullerene (C

) together in an electric furnace at 700°C for 2 h. Their morphological and the struc- tural properties were characterized by powder X-ray diffraction and scanning electron microscopy. The MoS

nanoparticles and MoS

-C

nanocomposites were used as catalysts in the reductions of 2-, 3-, and 4-nitrophenol in the presence of sodium borohydride. The photocatalytic activities of the MoS

nanoparticles and MoS

-C

nanocomposites were evaluated in the degradation of organic dyes (brilliant green, methylene blue, methyl orange, and rhodamine B) under ultraviolet light (254 nm).

Keywords: MoS

nanoparticles, MoS

-C

nanocomposites, reductions, photocatalytic activities

Introduction

In past decades, water pollution has become a significant environmental problem. Consequently, wastewater treatment techniques are attracting attention worldwide, with extensive research efforts devoted to improving the efficiency of water- treatment. ^1-3 The removal of pigments and synthetic dyes from industrial wastewater is challenging because of their solubility and chemical stability in aqueous solution. ⁴ Aro- matic nitro compounds are one of the severe water pollutants, and so the development of effective decontamination tech- nology is necessary to decrease their presence in the envi- ronment. ⁵

To remove aromatic nitro compounds, reduction using sodium borohydride (NaBH ₄ ) in the presence of a suitable catalyst is considered as an excellent method. ⁶ The reduction of aromatic nitro compounds can be used to produce amin- ophenol, which is an important intermediate in the synthesis of antipyretic and analgesic drugs. ⁷ Therefore, reduction methods could be used not only to reduce pollution, but also to produce useful reactants for the pharmaceutical industry.

Many metal and metal oxide materials, such as Au, Ag, Ni, and CuO, have been used as catalysts in these reduction reac- tions. ^8-11

Photocatalytic degradation using semiconductor photocat- alysts is an effective method for reducing the pollution caused by pigments and synthetic dyes. ^12,13 As the most promising photocatalyst, titanium dioxide (TiO ₂ ) has attracted much attention, and many photocatalysts composed of semi- conductors, such as ZnO, CdS, and WO ₃ , coupled with TiO ₂ have been synthesized. ^14,15 However, TiO ₂ band gap is about 3.2 eV, which limits its photocatalytic activity. Therefore, much effort has been focused on developing highly efficient photocatalysts that are more applicable. ^1,12

Molybdenum disulfide (MoS ₂ ) has a layered graphite-like structure. ¹⁶ Because of its unique optoelectronic, catalytic, photocatalytic, and electronic properties, MoS ₂ has been used in wide-ranging applications in many fields. ^17,18 The unit cell of MoS ₂ contains molybdenum atoms sandwiched between layers of two sulfur atoms. Multi-layer of molybdenum disul- fide is maintained by weak van der Waals interactions between the two-dimensional S–Mo–S structures, and the resultant MoS ₂ thin layers show excellent catalytic and optical adjust- ability. ^19,20 MoS ₂ is a newly emerging semiconductor, with a

†

Corresponding author E-mail: [email protected]

narrow band gap of 1.17 eV, and MoS ₂ nanoparticles can be synthesized using hydrothermal, solvothermal, plasma micro- wave, electrochemical and chemical methods. ^21-25 As a cat- alyst, MoS ₂ has been reported to remove N and S from crude oil, thus improving its catalytic performance. ²⁶ MoS ₂ is always used in a heterojunction structure in combination with other semiconductors that can improve the photocatalytic activity. ^27,28 Recent results have shown that the combination of semiconductors with nano carbon is an effective method for enhancing their catalytic and photocatalytic activity. ^29,30 Graphene-MoS ₂ nanocomposites have been synthesized using a modified hydrothermal method and their catalytic activity was investigated in the hydrodesulfurization of dibenzothio- phene. ³¹ Fullerene (C ₆₀ ), a nano carbon material, can be used as an n-type semiconductor in the photodegradation of organic dyes because of its electron-accepting performance. Many C ₆₀ -modified semiconductors have been synthesized and their nanocomposites have seen excellent applications in catalysis and photocatalysis. ^32,33

In our study, MoS ₂ precursors were synthesized by reacting thioacetamide (C ₂ H ₅ NS) and sodium molybdate dihydrate (Na ₂ MoO ₄ ·2H ₂ O) in aqueous HCl solution. MoS ₂ nanopar- ticles were then obtained by calcining the MoS ₂ precursors.

MoS ₂ -C ₆₀ nanocomposites were fabricated from the MoS ₂ nanoparticles and fullerene (C ₆₀ ) by heating in an electric furnace at 700°C for 2 h. X-ray diffraction (XRD) and scan- ning electron microscopy (SEM) were used to characterize the product. The catalytic activities of the MoS ₂ nanoparti- cles and MoS ₂ -C ₆₀ nanocomposites were assessed by the reduction of 2-, 3-, and 4-nitrophenol in the presence of NaBH ₄ . MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites were then used as photocatalysts for the degradation of organic dyes such as methylene blue (MB), rhodamine B (RhB), brilliant green (BG), and methyl orange (MO) under ultraviolet light irradiation at 254 nm.

Experimental

1. Materials

Sodium molybdate dihydrate (Na ₂ MoO ₄ ·2H ₂ O) was pur- chased from Junsei Chemical Co. Thioacetamide (TAA) and fullerene (C ₆₀ ) were supplied by Tokyo Chemical Industry.

Hydrochloric acid, ethanol, 4-nitrophenol, and 2-nitrophenol were obtained from Sigma-Aldrich. 3-Nitrophenol was sup- plied by Tae Jin Chemical. BG and MO were purchased from

Sigma-Aldrich (China). MB·3H ₂ O, RhB, and tetrahydrofuran (THF) were obtained from Samchun Chemicals (Korea). All chemical materials were used without further purification.

2. Material fabrication

Deionized water (50 mL) and ethanol (7.5 mL) were mixed for use as the reaction solvent. TAA (1 g) and Na ₂ MoO ₄ ·2H ₂ O (0.5 g) were dissolved in this solvent at 82°C. The mixture was stirred at 82°C on a hot-plate magnetic stirrer (MS- 300HS) for 5 min. Adding HCl (11.25 mL, 12 N) to the solu- tion resulted in a dark green precipitate that then quickly turned dark brown. After 10 min stirring, the precipitate was washed with deionized water five times and dried at 100°C for 12 h. MoS ₂ nanoparticles were prepared from the resul- tant dry powder by heating in an electric furnace (Ajeon Heating Industry) at 700°C for 2 h under inert argon atmosphere. MoS ₂ nanoparticles and fullerene (C ₆₀ ) were mixed in a 2:1 mass ratio in THF (10 mL) and then calcined in an electric furnace at 700°C for 2 h to obtain MoS ₂ -C ₆₀ nanocomposites.

3. Characterization

XRD (Bruker, D8 Advance) with Cu Kα radiation (λ = 1.54178 Å) was used to characterize the structures and iden- tify the crystalline phases of the MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites. The surface morphology and par- ticle shapes of these products were examined by SEM (JEOL Ltd, JSM-6510) at an accelerating voltage of 0.5-30 kV. Cat- alytic reduction and photocatalytic degradation were char- acterized by UV-Vis spectrophotometry (Shimazu UV- 1619PC). Photocatalytic degradation was performed using a UV lamp (8 W, 254 nm, 77202 MARNE LA VALLEE-Cedex 1 FRANCE).

4. Evaluation of catalytic reduction

2-, 3-, and 4-Nitrophenol were prepared as 1.0×10 ^–4 M solutions. The appropriate nitrophenol solution (10 mL) was added into a glass vial, followed by NaBH ₄ powder (10 mg).

After the NaBH ₄ had dissolved completely, catalyst (5 mg)

was added to the solution and its catalytic activity was inves-

tigated. The catalytic reduction of nitrophenols was moni-

tored every 20 min using a UV-Vis spectrophotometer. The

catalysts were MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocom-

posites.

5. Evaluation of photocatalytic degradation

In a typical experiment, the photocatalytic activities of MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites were evaluated in the degradation of organic dyes briliant green (BG), methylene blue (MB), methylene orange (MO), and

rhodamine B (RhB). An organic dye solution (10 mL) with an absorbance value of about 1.0 was injected into a glass vial, followed by the photocatalyst (5 mg). To achieve an adsorption-desorption equilibrium for the organic dye, the glass vial was kept in a dark environment for 30 min. A UV lamp at 254 nm was used to provide UV light, with a dis- tance of 1 cm between the lamp and reactor. The photocat- alytic degradation of the organic dyes was analyzed by UV-

Figure 1. XRD patterns of (a) MoS

nanoparticles and (b) MoS

-C

nanocomposites.

Vis spectrophotometry, monitoring every 10 min when using MoS ₂ nanoparticles as photocatalysts, and 5 min intervals when using MoS ₂ -C ₆₀ nanocomposites as photocatalysts.

Results and Discussions

1. Synthesis and characterization

The crystal structures of MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites, obtained from their XRD patterns, are shown in Figure 1. As shown in Figure 1(a), MoS ₂ nanopar- ticle peaks were observed at 32.55°, 39.01°, 43.09°, 49.11°, and 58.25° as 2θ values, corresponding to the (100), (103), (006), (105), and (110) crystal planes, respectively. These peaks could be ascribed to MoS ₂ (JCPDS No. 77-1716). ⁴ The peak at 14.22° with very low intensity represents the distinct

(002) plane, which revealing that few layers might be con- tained in the crystal structure of the MoS ₂ nanoparticles. ^19,34 The XRD patterns of MoS ₂ -C ₆₀ nanocomposites shows peaks corresponding to the (100), (103), (006), (105), and (110) planes of MoS ₂ , observed at 2θ = 33.17°, 39.55°, 43.47°, 49.03°, and 58.54°, respectively (Figure 1(b)). Other char- acteristic peaks at 2θ = 20.5° and 22.59 appeared as a small peak represented the (311) and (222) planes of fullerene (C ₆₀ ), respectively (JCPDS No. 44-0558). ³⁵ Finally, the prominent peak at 14.14° was attributed to the (002) plane due to MoS ₂ nanoparticles. These results suggest the exis- tence of a hexagonal structure in the MoS ₂ -C ₆₀ nanocom- posites. Comparison of Figure 1(a) and 1(b) shows that the crystallinity of the MoS ₂ nanoparticles was lower than that of the MoS ₂ -C ₆₀ nanocomposites. ¹⁷

SEM images for the MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites are shown in Figure 2. Aggregated shape of broken small powder were observed for the MoS ₂ nanopar- ticles (Figure 2(a)), in complete agreement with the results obtained in Figure 1. ³⁶ The SEM image of MoS ₂ -C ₆₀ nano- composites shows that fullerene (C ₆₀ ) has a cubic structure and that MoS ₂ nanoparticles with a splintered structure are distributed unevenly on the fullerene (C ₆₀ ) surface. Com- pared with the morphology of MoS ₂ nanoparticles, the stone plate-like fullerene (C ₆₀ ) assumed a larger morphological size in Figure 2(b). In addition, Figure 2(b) shows better crys- tallinity than Figure 2(a), which strongly supported the results of the X-ray diffraction patterns.

2. Reduction catalytic performance

The catalytic activities of MoS ₂ nanoparticles and MoS ₂ - C ₆₀ nanocomposites were assessed by the reduction of nitro- phenols (2-, 3-, and 4-nitrophenol) in the presence of NaBH ₄ . Figure 3(a) and 4(a) show the reduction of 4-nitrophenol to 4-aminophenol using MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites as catalysts, respectively. After NaBH ₄ was added to the solution of 4-nitrophenol, the 4-nitrophenol peak in the UV spectrum red-shifted from 317 nm to 400 nm, due to the formation of 4-nitrophenolate ions under the alkaline conditions. After the catalyst was added, and then the peak at 400 nm decreased and a new peak appeared at 300 nm simultaneously, which was attributed to the reduction of 4- nitrophenol. Using the same method, 3-nitrophenol and 2- nitrophenol were reduced to 3-aminophenol and 2-amino- phenol, respectively, in the presence of NaBH ₄ by MoS ₂ Figure 2. SEM images of (a) MoS

nanoparticles and (b) MoS

-

C

nanocomposites.

nanoparticles and MoS ₂ -C ₆₀ nanocomposites as catalysts. As shown in Figure 3(b) and 4(b), the peak at 390 nm represents 3-nitrophenolate ions, with the addition of catalyst causing the peak at 390 nm to decrease, the peak at 291 nm to be blue-shifted, and the peak at 251 nm to become unclear and finally disappear. Figure 3(c) and 4(c) show the reduction of

2-nitrophenol, in which the transformation of 2-nitrophenol into 2-aminophenol was indicated by a decreasing peak at 415 nm and the red-shift of a peak at 282 nm. Additionally, the nitrophenol solutions containing NaBH ₄ were a deep yel- low color, while nitrophenol reduction changed this into a light yellow.

Figure 3. UV-vis spectra of (a) 4-nitrophenol, (b) 3-nitrophenol and (c) 2-nitrophenol reduction with MoS

nanoparticles as catalyst in

the present of NaBH

.

3. Photocatalytic performance

In order to test the photocatalytic activities of the synthe- sized samples, solutions of organic dyes, BG, MB, MO, and

RhB, were prepared and their degradation was performed using MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites as photocatalysts under UV lamp irradiation at 254 nm. Before UV irradiation, the reactor was kept in a dark environment Figure 3. Continued.

Figure 4. UV-vis spectra of (a) 4-nitrophenol, (b) 3-nitrophenol and (c) 2-nitrophenol reduction with MoS

-C

nanocomposites as catalyst

in the present of NaBH

.

for 30 min to achieve an adsorption-desorption equilibrium for the organic dyes. Figure 5(a) and 6(a) show the degra- dation of BG using MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nano- composites as photocatalysts, respectively. Figure 5(b) and 6(b) show UV–Vis spectra of the degradation of MB. MB

degraded faster than the other organic dyes at the same con-

ditions. Figure 5(c) and 6(c) show the degradation of MO in

the presence of photocatalysts, which proved that the pho-

tocatalyst MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites

had comparatively poor photocatalytic activities in this reac-

Figure 4. Continued.

tion. The degradation of RhB is shown in Figure 5(d) and 6(d). With increasing irradiation time of UV lamp, the absor- bance values of the organic dyes decreased, with a color change from dark to light also observed.

4. Kinetics study

To investigate the catalytic activities and photocatalytic

activities of the MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nano-

Figure 5. UV-vis spectra of the degradation of (a) BG, (b) MB, (c) MO and (d) RhB with MoS

nanoparticles as photocatalyst under

ultraviolet irradiation at 254 nm.

composites, kinetics studies were undertaken. The results showed that the value of concentration ratio ln(C/C ₀ ) was proportional to the reduction time of nitrophenol and also to the degradation time of organic dye. In many reports, the Langmuir-Hinshelwood model was applied to fit pseudo-

first-order reactions. ^37,38 In our study, we used the Langmuir- Hinshelwood model to investigate the catalytic and photo- catalytic activities of MoS ₂ nanoparticles and MoS ₂ -C ₆₀ nanocomposites. The kinetic equation can be written as fol- lows:

Figure 5. Continued.

ln(C/C ₀ ) = −kt

where C ₀ is the initial concentration of various nitrophenols or organic dyes, C is the concentration at time t, and k is the

reaction rate constant. Figure 7(a) and 8(a) show the fitting

results of nitrophenol reductions using 5 mg of MoS ₂ nano-

particles and MoS ₂ -C ₆₀ nanocomposites as the catalyst,

respectively. This investigation found that the reaction rates

Figure 6. UV-vis spectra of the degradation of (a) BG, (b) MB, (c) MO and (d) RhB with MoS

-C

nanocomposites as photocatalyst

under ultraviolet irradiation at 254 nm.

for nitrophenol reduction with MoS ₂ nanoparticles as the cat- alyst were in the order of 3-nitrophenol > 2-nitrophenol > 4- nitrophenol. The MoS ₂ -C ₆₀ nanocomposites catalyst gave a reaction rate order of 4-nitrophenol > 3-nitrophenol > 2-nitro-

phenol. Figure 7(b) and 8(b) show that the rate of photo- catalytic degradation of the organic dyes in the presence of photocatalysts under UV irradiation at 254 nm were in the order of MB > BG > RhB > MO.

Figure 6. Continued.

Conclusion

In this study, thioacetamide and sodium molybdate dihy- drate were used to prepare MoS ₂ nanoparticles, while MoS ₂ -

C ₆₀ nanocomposites were obtained by heating the MoS ₂

nanoparticles with fullerene (C ₆₀ ) in an electric furnace at

700°C for 2 h. XRD analysis was employed to characterize

the structural properties of the samples. SEM imaging

Figure 7. Kinetics studies for (a) reduction of various nitrophenols using MoS

nanoparticles as catalyst in the present of NaBH

and

(b) photocatalytic degradation of the organic dyes using MoS

nanoparticles as photocatalyst under ultraviolet irradiation at 254 nm.

showed that fullerene (C ₆₀ ) had stone plate-like structure and was surrounded by MoS ₂ nanoparticles with broken small powder shape. As catalysts, the activities of the MoS ₂ nano-

particles and MoS ₂ -C ₆₀ nanocomposites were exhibited in the

reductions of 2-, 3-, and 4-nitrophenol in the presence of

sodium borohydride. The experimental results showed that

Figure 8. Kinetics studies for (a) reduction of various nitrophenols using MoS

-C

nanocomposites as catalyst in the present of NaBH

and (b) photocatalytic degradation of the organic dyes using MoS

-C

nanocomposites as photocatalyst under ultraviolet irradiation at

254 nm.

the order of reactivity for nitrophenol reduction catalyzed by MoS ₂ nanoparticles was 3-nitrophenol > 2-nitrophenol > 4- nitrophenol. The MoS ₂ -C ₆₀ nanocomposites catalyst showed the reaction rate order of 4-nitrophenol > 3-nitrophenol > 2- nitrophenol. The photocatalytic activities of the MoS ₂ nano- particles and MoS ₂ -C ₆₀ nanocomposites were evaluated in the degradation of organic dyes BG, MB, MO and RhB under UV light (254 nm). A kinetics study indicated that the order of photocatalytic activity for MoS ₂ nanoparticles and MoS ₂ - C ₆₀ nanocomposites as catalysts in the photocatalytic deg- radation of organic dyes was MB > BG > RhB > MO.

Acknowledgements

This study was supported by Sahmyook University research funding in Korea.

References

1. H. Liu, T. Lv, C. Zhu, X. Su, and Z. Zhu, “Efficient synthesis of MoS ₂ nanoparticles modified TiO ₂ nanobelts with enhanced visible-light-driven photocatalytic activity”, J. Mol. Catal. A:

Chem., 396, 136 (2015).

2. H. Liu, T. Lv, X. H. Wu, C. K. Zhu, and Z. F. Zhu, “Prepara- tion and enhanced photocatalytic activity of CdS@RGO core- shell structural microspheres”, Appl. Sulf. Sci., 30, 242 (2014).

3. T. Y. Li, C. Yang, X. H. Rao, F. Xiao, J. D. Wang, and X. T.

Su, “Microstructural study of microwave sintered zirconia for dental applications”,Ceram. Int., 41, 1255 (2015).

4. W. Liu, Q. Hu, F. Mo, J. Hu, Y. Feng, H. Tang, H. Ye, and S.

Miao, “Photo-catalytic degradation of methyl orange under visible light by MoS ₂ nanosheets produced by H ₂ SiO ₃ exfoli- ation”, J. Mol. Catal. A: Chem., 395, 322 (2014).

5. S. Ameen, M. S. Akhtar, M. Nazim, and H. S. Shin, “Rapid photocatalytic degradation of crystal violet dye over ZnO flower nanomaterials”, Mater. Lett., 96, 228 (2013).

6. M. Sun, Y. Wang, Y. Fang, S. Sun, and Z. Yu, “Construction of MoS ₂ /CdS/TiO ₂ ternary composites with enhanced photo- catalytic activity and stability”, J. Alloys Comp., 684, 335 (2016).

7. H. J. Fan, C. S. Lu, W. L. W. Lee, M. R. Chiou, and C. C.

Chen, “Mechanistic pathways differences between P25-TiO ₂ and Pt-TiO ₂ mediated CV photodegradation”, J. Hazard.

Mater., 185, 227 (2011).

8. H. W. Kei and J. C. Yu, “Sonochemical synthesis and visible light photocatalytic behavior of CdSe and CdSe/TiO ₂ nanoparticles”, J. Mol. Catal. A: Chem., 247, 268 (2006).

9. Y. B. Chen, L. Z. Wang, G. Q. Lu, X. D. Yao, and L. J. Guo,

“Nanoparticles enwrapped with nanotubes: a unique architec- ture of CdS/titanate nanotubes for efficient photocatalytic hydrogen production from water”, J. Mater. Chem., 21, 5134 (2011).

10. A. Goyal, S. Bansal, and S. Singhal, “Facile reduction of nitrophenols: Comparative catalytic efficiency of MFe ₂ O ₄ (M= Ni, Cu, Zn) nano ferrites”, Int. J. Hydrogen Energy, 39, 4895 (2014).

11. C. V. Rode, M. J. Vaidya, and R. V. Chaudhari, “Synthesis of p-aminophenol by catalytic hydrogenation of nitrobenzene”, Org. Process Res. Dev., 3, 465 (1999).

12. M. Haruta and M. Date, “Advances in the catalysis of Au nanoparticles”, Appl. Catal. A: Gen., 222, 427 (2001).

13. K. S. Shin, Y. K. Cho, J. Y. Choi, and K. Kim, “Facile syn- thesis of silver-deposited silanized magnetite nanoparticles and their application for catalytic reduction of nitrophenols”, Appl. Catal. A: Gen., 413, 170 (2012).

14. S. K. Ghosh, M. Mandal, S. Kundu, S. Nath, and T. Pal,

“Bimetallic Pt-Ni nanoparticles can catalyze reduction of aro- matic nitro compounds by sodium borohydride in aqueous solution”, Appl. Catal. A: Gen., 268, 61 (2004).

15. W. R. Zhao, Y. Wang, Y. Yang, J. Tang, and Y. N. Yang, “Car- bon spheres supported visible-light-driven CuO-BiVO ₄ het- erojunction: preparation, characterization, and photocatalytic properties”, Appl. Catal. B: Environ., 115, 90 (2012).

16. H. J. Song, S. You, X. H. Jia, and J. Yang, “MoS ₂ nanosheets decorated with magnetic Fe ₃ O ₄ nanoparticles and their ultra- fast adsorption for wastewater treatment”, Ceram. Int., 41, 13896 (2015).

17. B. Pourabbas and B. Jamshidi, “Preparation of MoS ₂ nano- particles by a modified hydrothermal method and the photo- catalytic activity of MoS ₂ /TiO ₂ hybrids in photo-oxidation of phenol”, Chem. Eng. J., 138, 55 (2008).

18. X. Z. Wang, S. X. Yang, Q. Yue, F. M. Wu, and J. B. Li,

“Response of MoS ₂ nanosheet field effect transistor under dif- ferent gas environments and its long wavelength photore- sponse characteristics”, J. Alloys Comp., 615, 989 (2014).

19. J. Lei, Z. Jiang, X. Lu, G. Nie, and C. Wang, “Synthesis of Few-Layer MoS ₂ Nanosheets-Wrapped Polyaniline Hierar- chical Nanostructures for Enhanced Electrochemical Capaci- tance Performance”, Electrochim. Acta, 176, 149 (2015).

20. X. Wu, X. Yah, Y. Dai, J. Wang, J. Wang, and X. Cheng,

“Facile synthesis of AgNPs/MoS ₂ nanocomposite with excel- lent electrochemical properties”, Mater. Lett., 152, 128 (2015).

21. J. Zhou, H. Xiao, B. Zhou, F. Huang, S. Zhou, and W. Xiao,

“Hierarchical MoS ₂ -rGO nanosheets with high MoS ₂ loading with enhanced electro-catalytic performance”, Appl. Surf.

Sci., 358, 152 (2015).

22. W. J. Li, E. W. Shi, Z. Z. Chen, H. Ogino, and T. Fukuda,

“Hydrothermal synthesis of MoS ₂ nanowires”, J. Cryst.

Growth, 250, 418 (2003).

23. J. H. Zhan, Z. D. Zhang, X. F. Qian, C. Wang, Y. Xie, and T.

Qian, “Solvothermal synthesis of nanocrystalline MoS ₂ from MoO ₃ and elemental sulfur”, J. Solid State Chem., 141, 270 (1998).

24. D. Vollath and D. V. Szabo, “Synthesis of nanocrystalline MoS ₂ and WS ₂ in a microwave plasma”, Mater. Lett., 35, 236 (1998).

25. Q. Li, E. C. Walter, W. E. van der Veer, B. Murray, J. T. New- berg, E. W. Bohannan, J. A. Switzer, J. C. Hemminger, and R.

M. Penner, “Molybdenum disulfide nanowires and nanorib- bons by electrochemical/chemical synthesis”, J. Phys. Chem.

B, 109, 3169 (2005).

26. J. T. Richardson, “Electronic properties of unsupported cobalt- promoted molybdenum sulfide”, J. Catal., 112, 313 (1988).

27. X. Zong, J. F. Han, G. J. Ma, H. J. Yan, G. P. Wu, and C. Li,

“Enhancement of photocatalytic H ₂ evolution on CdS by loading MoS ₂ as cocatalyst under visible light irradiation”, J.

Am. Chem. Soc., 130, 7176 (2008).

28. Y. Xu and R. Xu, “Nickel-based cocatalysts for photocatalytic hydrogen production”, Appl. Surf. Sci., 351, 779 (2015).

29. D. Hou, W. Zhou, X. Liu, K. Zhou, J. Xie, G. Li, and S. Chen,

“Pt nanoparticles/MoS ₂ nanosheets/carbon fibers as efficient catalyst for the hydrogen evolution reaction”, Elect. Acta, 166, 26 (2015).

30. D. H. Youn, C. Jo, J. Y. Kim, J. Lee, and J. S. Lee, “Ultrafast synthesis of MoS ₂ or WS ₂ -reduced graphene oxide compos- ites via hybrid microwave annealing for anode materials of lithium ion batteries”, J. Power Sources, 295, 228 (2015).

31. M. A. Al-Daous, “Graphene-MoS ₂ composite: Hydrothermal synthesis and catalytic property in hydrodesulfurization of dibenzothiophene”, Catal. Commun., 72, 180 (2015).

32. X. Zhao, H. Liu, Y. Shen, and J. Qu, “Photocatalytic reduction of bromate at C ₆₀ modified Bi ₂ MoO ₆ under visible light irra- diation”, Appl. Catal. B: Environ., 106, 63 (2011).

33. T. Hasobe, H. Imahori, S. Fukuzumi, and P. V. Kamat, “Light energy conversion using mixed molecular nanoclusters. Por- phyrin and C ₆₀ cluster films for efficient photocurrent gener- ation”, J. Phys. Chem. B, 107, 12105 (2003).

34. K. Chang, W. Chen, L. Ma, H. Li, H. Li, F. Huang, Z. Xu, Q.

Zhang, and J. Y. Lee, “Graphene-like MoS ₂ /amorphous car- bon composites with high capacity and excellent stability as anode materials for lithium ion batteries”, J. Mater. Chem., 21, 6251 (2011).

35. K. Miura and M. Ishikawa, “C ₆₀ intercalated graphite as nano- lubricants”, Materials, 3, 4510 (2010).

36. Y. Xu, E. Hu, K. Hu, Y. Xu, and X. Hu, “Formation of an adsorption film of MoS ₂ nanoparticles and dioctyl sebacate on a steel surface for alleviating friction and wear”, Tribol.

Int., 92, 172 (2015).

37. D. James and T. Zubkov, “Photocatalytic properties of free and oxide-supported MoS ₂ and WS ₂ nanoparticles synthe- sized without surfactants”, J. Photochem. Photobiol. A: Chem., 262, 45 (2013).

38. D. Y. Liang, C. Cui, H. H. Hu, Y. P. Wang, S. Xu, B. L. Ying,

P. G. Li, B. Q. Lu, and H. L. Shen, “One-step hydrothermal

synthesis of anatase TiO ₂ /reduced graphene oxide nanocom-

posites with enhanced photocatalytic activity”, J. Alloys

Compd., 582, 236 (2014).