Self-assembled MoS₂/rGO nanocomposites with tunable UV-IR absorption

Abstract

MoS₂/reduced graphene oxide (rGO) nanocomposites were synthesized using an ultrasonic pretreatment with a single-stage hydrothermal and reduction process. Self-assembled MoS₂ layers in the rGO matrix were obtained. The effect of quantum confinement in the structure, controlled by the degree of reduction of graphene oxide and the size of the 2D MoS₂ nanocrystals, made it possible to obtain tunable optical absorption. MoS₂/rGO layered nanocomposites exhibit a wide UV-IR absorption in the wavelength range from 280 nm to 973 nm, which is attractive for highly efficient multiband solar cells and advanced photonics.

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Introduction

Layered two-dimensional materials have attracted considerable interest owing to their outstanding physical and chemical properties. The first and most investigated two-dimensional crystal graphene has excellent mechanical, thermal, electrical, and optical properties^1–5 and is promising in various fields, such as electronics,^6,7 optoelectronics,^8,9 supercapacitors,^10,11 photocatalysis,¹² and biochemical sensors.¹³

Recently, molybdenum disulfide (MoS₂), a layered inorganic compound that is a semiconducting analog of graphene, has attracted much attention due to its noticeable bandgap, suitable for logic applications in electronics, and photonics. The MoS₂ crystal consists of covalently bonded S–Mo–S atoms in the plane. The adjacent layers with a spacing of ∼6.5 Å are weakly stacked on the top of each other by van der Waals interactions. The indirect band gap of 1.3 eV for bulk crystal MoS₂ is transformed into a direct band gap of ∼1.9 eV for monolayer MoS₂, which is attributed to quantum confinement resulting in changes in hybridization states d- and p_z-electrons of Mo and S atoms, respectively.¹⁴ Remarkable mechanical^15–17 and opto-electrical properties¹⁴ of MoS₂ make it attractive in various fields of energy harvesting and storage such as hydrogen storage,^18,19 photocatalysis,²⁰ solar cells²¹ and ion batteries.^22–25 Weak van der Waals interactions of neighboring MoS₂ layers allow the creation of new composites with unique properties.²⁶ Hydrothermal/solvothermal synthesis is a cost effective way of growing MoS₂ due to low growth temperature, a simple inexpensive process and mild synthetic conditions suitable for varies substrates, including flexible, large-scale, and selectively structured. The morphology of the MoS₂ crystals can be controlled by pretreating the substrate prior to the growth process by adding an organic solvent such as i-propanol (IPA), acetic acid, L-cysteine, and oleylamine,^22,23,27,28 which play the role of buffering. Structural and morphological compatibility of graphene and MoS₂ allows the use of graphene-like molecules, such as reduced graphene oxide (rGO), carbon nanotubes, etc. as a precursor platform in the hydrothermal method for producing layered MoS₂.^23,25,27

In this work, self-assembled MoS₂/rGO nanocomposites with various structures were obtained using ultrasonic pretreatment with liquid-phase reaction and reduction, which allow controlling the optical absorption of nanocomposites in a wide range of wavelengths.

Experimental

Synthesis of graphene oxide

Graphene oxide (GO) was obtained by a modified Hummer's method.^29,30 In this method, oxidation was combined with a highly efficient method of purifying reaction products. Concentrated sulphuric acid (H₂SO₄, purity 96%, 360 mL) and phosphoric acid (H₃PO₄, purity 69.2%, 40 mL) were placed in a flask with mild stirring. Natural graphite flakes (Aldrich graphite, 3.0 g, purity 99.995 wt%, high density) were added and dispersed in the mixture, followed by the gradual addition of potassium permanganate (KMnO₄, 18.0 g, purity 99%) used as the reducing agent during reaction process. The above mixture was maintained for 36 h at room temperature to release the gas under continuous stirring. The resulting solution is terminated by the addition of an aqueous solution of hydrogen peroxide (H₂O₂, 3 mL, 30 wt%), followed by continuous stirring for 30 minutes. The resulting mixture was then added to an aqueous solution of H₂SO₄ (500 mL, 5 wt%) with stirring for 20 minutes, followed by stirring for 1 hour. The mixture was sifted through two metal standard testing sieves with diameters of 320 μm and 30 μm. The filtrate was centrifuged with 9000 rpm for 20 minutes, and the supernatant was decanted away. The remaining solid material was then washed successively with 400 mL of 30% HCl, 1000 mL of distilled water, and 1000 mL of ethanol. The resulting GO suspension was dried in vacuum at 60 °C.

Synthesis of MoS₂/rGO composite and characterization

MoS₂/rGO nanocomposites with MoS₂ 2D crystals in the rGO matrix were prepared by ultrasonic pretreatment with a soft liquid-phase reaction and reduction using sodium molybdate (Na₂MoO₄·2H₂O), thioacetamide (CH₃CSNH₂, TAA), and GO as starting materials. The CH₃CSNH₂ (TAA, 0.06 g) and 0.5 mg mL⁻¹ GO suspension, TAA : GO = 1 : 1 (wt%) (0 mg mL⁻¹; 1 mg mL⁻¹, 1 : 2 (wt%), 1.5 mg mL⁻¹, 1 : 3 (wt%)) were dissolved in a beaker with 20 mL of deionized (DI) water. After ultrasonication for 1 h, Na₂MoO₄·2H₂O (0.03 g) was added into the solution. GO sheets containing abundant oxygen functional groups give rise to good dispersion due to the interaction between cations and GO sheets and at the same time lead to a decrease in the interaction between GO sheets and inorganic species owing to their negative charge.³¹ After vigorous stirring and ultrasonication of obtained dispersion both for 30 min, the pH value of the mixture was mildly modified to neutral by adding 1 M NaOH drop by drop. The obtained solution was used to grow MoS₂/rGO composite by the hydrothermal technique. The blended solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, followed by heating at 220 °C for 24 h. During hydrothermal process, GO was reduced using NaOH as a reducing agent to rGO with simultaneous dispersion of MoS₂ and rGO nanosheets. The black precipitates of MoS₂/rGO composites were first retrieved from the mixed solution after natural cooling down to room temperature and then were washed by centrifugation (9000 rpm for 20 min) with 200 mL of acetone, 200 mL of methanol, and 200 mL of DI water for more than five times decanting away the supernatant. Then obtained product was dried at 80 °C for 6 h in a vacuum oven. The black powder of MoS₂/rGO composite of different composition was dispersed in IPA by vigorously sonication for 1 h to produce the homogenous solution. Ultrasonication was carried out through a point probe (Vibra cell) and a high-speed centrifuge (Eppendorf 5804). The suspension was separated out by different centrifugation speed. The obtained suspension of MoS₂/rGO composite was spin-coated on the SiO₂/Si substrate. The obtained composites were annealed with a rapid thermal annealing at 800 °C for 5 min in a gas stream of 200 sccm (H₂/N₂ = 1 : 4).

The crystalline structure of the synthesized MoS₂/rGO composites was analyzed by field emission scanning electron microscopy (FESEM, S4800), X-ray diffraction techniques (XRD, Bruker D8 Advance) using a Cu Kα radiation (λ = 0.154 nm), high resolution transmission electron microscopy (HRTEM, JEM-2010FEF), and micro-Raman spectrometry (LabRAM HR800, He–Ne laser, 488 nm). The chemical composition of the composites was investigated using X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Mg Kα excitation), and the XPS spectra were fitted using Gaussian–Lorentzian peak shape and Shirley background subtraction. Optical absorption spectra were recorded using a Carry UV-vis spectrophotometer at room temperature.

Results and discussion

Fig. 1 shows SEM images of MoS₂/rGO nanocomposites with different ratios. The as-grown MoS₂ (TAA : GO = 1 : 0) shows significantly aggregated spherical particles with a diameter of ∼700 nm (Fig. 1(a)). However the addition of GO leads to a decrease in the diameter and degree of aggregation of the particles (TAA : GO = 1 : 1) (Fig. 1(b)). MoS₂ spheres are not observed for MoS₂/rGO dispersions with TAA : GO = 1 : 2 and 1 : 3, indicating that rGO derived from GO acts as a dispersant platform, efficiently reducing the aggregation of MoS₂ and forming during the hydrothermal process, self-assembled MoS₂/rGO layered structures. Annealing changes slightly the morphology of the as-grown structure (Fig. 1S†).

Fig. 1 SEM images of the grown MoS₂/rGO with the ratio (a) (1 : 0), (b) (1 : 1), (c) (1 : 2) and (d) (1 : 3).

Fig. 2a shows X-ray diffraction patterns of MoS₂/rGO composites with different TAA/GO ratios. The diffraction peaks at 2θ = 14.1, 29.5, 32.9, 33.8, 39.5, 43.3, 49.4, 58.4, and 59.2 degree are in good agreement with the (002), (004), (100), (101), (103), (006), (105), (110) and (008) planes of the hexagonal MoS₂ phase (JCPDS 75-1539), respectively.^32,33 The (002) peak, detected in the grown MoS₂, indicates well-stacked layers along the (002) direction.³¹ A wide diffraction peak is also observed at 2θ = 24.3 degrees, which is explained by the (002) plane of graphene layers corresponding to d-spacing of about 0.34 nm.^22,24,25 The peak (002) of MoS₂ gradually decreases and, finally, disappears in MoS₂/rGO composites with increasing rGO/MoS₂ ratio. This indicates a reduction of MoS₂ in the structure stacked along the c-axis to the MoS₂ monolayer in composites with a high GO/TAA ratio (TAA : GO = 1 : 3).

Fig. 2 XRD patterns (a), Raman (b) and XPS (c, d) spectra of MoS₂/rGO with different MoS₂/GO ratios for Mo 3d (c) and S 2p (d).

The composites were also examined using high-resolution transmission electron microscopy (HRTEM). Typical HRTEM images of the MoS₂ and MoS₂/rGO composite (1 : 2) are shown in Fig. 3. MoS₂ is well-stacked into more than 5 layers with interlayer distances of 0.62 and 0.22 nm, which are consistent with (002) and (103) 2H-MoS₂ planes, respectively. However, the MoS₂/rGO composite (1 : 2) shows self-assembling 1–2 layers of MoS₂ on the rGO. The distances between the MoS₂ layers of 0.316 and 0.208 nm correspond to the (004) and (006) planes, respectively.

Fig. 3 Typical HRTEM images of MoS₂ (a and b) and MoS₂/rGO (1 : 2) (c and d) composite.

Raman spectra of the nanocomposites (Fig. 2(b) and S2†) show the peaks at ∼382 and ∼405 cm⁻¹, which correspond to E¹_2g and A_1g modes of H-MoS₂ and characterize the vibration of sulfides in the in-planar and the vertical-plane in the c-axis directions, respectively.^15,32,34 Two additional peaks at ∼1345 and ∼1578 cm⁻¹ are assigned to the D and G bands of the rGO, which are attributed to the phonon vibration on defects and the sublattice A against the sublattice B (E_2g symmetry) in the graphitic layers,³⁵ respectively, confirming the coexistence of the rGO and MoS₂ in the composites.^36,37 The Raman features of the samples are summarized in Table 1 and S1.† In addition to the blue-shift of the E¹_2g and red-shift of the A_1g, intriguing degenerate characteristics in full width at half maximum (FWHM) and striking down-shifting of area ratio of E¹_2g and A_1g modes (I_E/I_A) of MoS₂ were observed with increasing GO. This can be attributed to weak van der Waals forces between the adjacent layers, a decrease in the restoring forces in the vertical direction, the long-range Coulomb interlayer interactions and stacking-induced changes of the intralayer bonding.³⁴ Elevated temperature of annealing also can effect on crystal reconstruction, intralayer bonding and lattice dynamics. The thickness of MoS₂ layers can be quantitatively confirmed by shifting of E¹_2g and A_1g modes in frequency (ΔE).^14,34 The values of ΔE decrease with decreasing TAA/GO ratio, which are in good agreement with the data of XRD and HRTEM confirming that the one two layer MoS₂ is self-organizes on rGO. Shifting the G and D bands for composites with increasing GO, could be caused by the stabilizing sulfide in the hydrothermal process and the interaction between MoS₂ and rGO (Table S2†). The intensity ratio G and D (I_G/I_D), used as a factor in determining the quality of the graphene-like material is in the range from 0.5 to 0.6, indicating significant lattice distortions after the reduction of GO in the hydrothermal process. After annealing, the I_G/I_D ratio is increased by further reducing the GO.

Table 1 XPS and Raman data of MoS₂ and MoS₂/rGO composites

Specimen		XPS (at%)				Raman (cm^−1)
		S/Mo	S–Mo–S	S–Mo–O	O–Mo–O	IE/IA	ΔE	IG/ID
MoS2	As-grown	1.5	69.7	18.6	11.7	1.18	26.5	—
	Annealed	2.1	61.1	1.2	21.9	0.97	26.2	—
MoS2/rGO (1 : 1)	As-grown	1.4	72.8	9.3	17.9	1.01	24.7	0.6
	Annealed	1.5	75.0	5.9	19.1	0.83	23.2	0.5
MoS2/rGO (1 : 2)	As-grown	1.5	64.8	17.1	18.1	0.87	22.5	0.6
	Annealed	1.6	72.7	5.5	21.8	0.64	21.9	0.6
MoS2/rGO (1 : 3)	As-grown	1.3	58.6	12.3	29.1	0.8	21.8	0.6
	Annealed	1.8	59.3	5.3	35.4	0.66	21.4	0.6

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The XPS spectra of Mo 3d and S 2p (Fig. 2(c) and (d)) show the peaks centered at ∼229.3 and ∼232.5 eV, which correspond to the doublet of Mo 3d_5/2 and Mo 3d_3/2, and reveal that typical Mo⁴⁺ state is predominant in prepared samples. A slight amount of Mo⁶⁺ (∼236.0 eV) state still appears and increases with addition of GO, which is due to the oxidization of MoS₂ by oxygen resulted from reduction of GO. The Mo 3d_5/2 peak can be divided into three peaks centered at 229, 230, and 232 eV, corresponding to Mo in S–Mo–S, S–Mo–O, and O–Mo–O, respectively. The XPS spectra of S display the binding energy at 226.4, 162.1, and 163.3 eV, which are assigned as S 2s, S 2p_3/2 and S 2p_1/2, respectively. The S 2p_3/2 peak can be divided into two peaks centered at 162 and 163 eV, corresponding to S–Mo–S and S–Mo–O bonds (Table 1 and S2†). Reduced GO plays a role of a scalable precursor platform for adjusting the synthesis conditions for the self-assembly of MoS₂ layers in GO. Properly used amount of rGO increases the intensity of the S–Mo–S bands. At high oxygen concentration the intensities of the S–Mo–S bands decrease, which results in indicating a lower yield of MoS₂. The C 1s spectra prove the process of reducing the GO supplying oxygen (Fig. S3b and Table S3†). After annealing, the intensities of O–Mo–O and S–Mo–O bands increased and decreased, respectively, while the intensities of the S–Mo–S bands changed slightly, which can be explained by a phase transition from Mo₂S_xO_1−x to MoO₃. Annealing improved the quality of the crystals and removed the reaction residues. The atomic ratio of S and Mo after annealing of composites was in the range from 1.5 to 2.1.

The mechanism of formation of the self-assembled MoS₂/rGO composite is illustrated in Fig. 4. Cations (CSNH₂)⁺ (as TA⁺) are adsorbed onto the surface of GO sheets during the mixing process and ultrasonic treatment through strong electrostatic forces, resulting in the changing the GO charge to positive. Positive charge sheets of GO ensure overcoming the repulsion of the charges between GO and MoO₄²⁻. Thus, a suspension of positively charged sheets of GO with adsorbed TA⁺ was obtained. When Na₂MoO₄·2H₂O is added to the resulting solution, MoO₄²⁻ can be adsorbed on the TA⁺–GO single sheets surface to reconstitute the MoO₄²⁻–TA⁺–GO sheet complex.^38,39 The hydrothermal process results in the GO being reduced to rGO, forming a self-assembled MoS₂/rGO composite.

Fig. 4 Schematic illustration of the formation of a self-assembled MoS₂/rGO composite by sonication-assisted hydrothermal method.

Control of the degree of reduction of graphene oxide allows the creation of super lattices structures with quantum confinement of electrons. Reduction of graphene oxide to rGO with a small band gap (<1.3 eV) would confined the electrons in rGO between the layers of molybdenum disulfide E_g = 1.3 eV. Reducing GO to rGO with a bandgap larger than the bandgap of a single layer of molybdenum disulfide (1.8 eV) or its multilayers (1.3 eV), leads to localization of electrons in the layers of molybdenum disulfide. Nanocomposites with tunnel barriers may form local super lattices with different bandgap. Thus, the size effect can adjust the wavelengths of absorption of the nanocomposite.

To confirm the tunability of the optical properties of the MoS₂/rGO nanocomposite, UV absorption spectroscopy was used. The absorption spectrum of rGO shows a peak at about 280 nm, which originates from the π–π* transitions in the aromatic C–C bonds of the reduced graphene oxide^40–42 (Fig. 5a). The MoS₂ absorption spectrum shows four characteristics peaks of around 400–500 nm and 600–700 nm (Fig. 5b). The peaks at 608 and 670 nm corresponds to the exciton peaks of the MoS₂ arising from the K point of the Brillouin zone.⁴³ The peaks at 405 and 452 nm indicate a reduction in the 2H phase of MoS₂.⁴⁴ The absorption spectra of MoS₂/rGO composites obtained at different formation conditions show six characteristic peaks located at 250–300 nm, 400–500 nm, 600–700 nm and 973 nm (Fig. 6a). It is interesting to note that in the case of MoS₂/rGO nanocomposites, the peak at 280 nm associated with rGO red-shifts to 285 nm gradually with increasing centrifugation rate, indicating an increase in the π-electron concentration with reduction of sp³ graphene oxide to sp²-hybridization of carbon atoms.^40–42 For a smaller size of nanocrystals, the self-assembly process results in a more efficient reduction of GO due to possible charge transfer in MoS₂/rGO nanocomposites. In addition, with increasing the centrifugation speed the exciton peaks related to MoS₂ at around 679 nm and 620 nm show a blue shift followed by a decrease in intensity. This behavior in good agreement with the quantum confinement effect,⁴⁵ since the size of MoS₂ decreases with increasing centrifugation speed. Interestingly, the absorption spectra of the nanocomposites showed the peak of about 973 nm (1.27 eV), which could be related to the strongly reduced GO (G). This peak is also blue shifted with increasing centrifugation speed and can be explained by the quantum confinement effect in G quantum dots. The obtained results show that the absorption of the MoS₂/rGO nanocomposites can be adjusted over a wide range from ultraviolet to infrared light by using differently reduced GO and MoS₂ flakes of various sizes and thickness. MoS₂/GO nanocomposites with a wide adjustable absorption of light have a great potential for their use in high performance multi-band solar cells and advanced photonics.⁴⁶

Fig. 5 Absorption spectra of rGO (a) and few layer MoS₂ nanosheets (b).

Fig. 6 The absorption spectra of MoS₂/rGO composites from ultraviolet to infrared region (a) and from red to infrared region (b), obtained at different centrifugation speeds: a-3500 rpm, b-5000 rpm, c-6500 rpm, d-8000 rpm, e-9500 rpm, f-11 000 rpm.

Conclusions

MoS₂/rGO nanocomposites were synthesized using ultrasonic pretreatment with a single-stage hydrothermal and reduction process. Self-assembled MoS₂ single layers in the rGO matrix were obtained. The nanocomposites, controlled by the degree of reduction of graphene oxide and the size of MoS₂ nanocrystals, exibit tunable optical absorption in a wide UV-IR wavelength range from 280 nm to 973 nm.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under grant 11405117, the International Program of the Ministry of Science and Technology of China under grant 2015DFR00720 and the National Research Foundation (NRF) of Korea grant funded by the Korea government (MSIP) (No. 2015-066177) and under the framework of international cooperation program managed by NRF of Korea (No. 2015K2A1C2067880 and S-2015-A0173-00005), as well as by Basic Science Research Program through NRF funded by the Ministry of Education (No. 2017R1D1A1B03035102) and the Russian Foundation for Basic Research (RFBR No. 16-33-60229).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra12455d

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