Carbon dot-assisted hydrothermal synthesis of flower-like MoS₂ nanospheres constructed by few-layered multiphase MoS₂ nanosheets for supercapacitors

Abstract

Molybdenum disulfide (MoS₂) has emerged as a promising electrode material for supercapacitors. Elaboration of MoS₂ with desired structures, morphologies and compositions as well as fabrication of MoS₂-based hybrids are current research directions. Herein, we demonstrate engineering MoS₂ with a multiphase structure including 2H and 1T phases as well as edge-rich nanospherical morphology via a hydrothermal route with the assistance of carbon dots (CDs) for the first time. The resultant MoS₂ 3D nanospheres are formed through the self-assembly of MoS₂ 2D nanosheets consisting of a few atomic layers stacked along the (002) direction with an enlarged interlayer spacing. The introduced CDs not only involve the growth of the few-layered multiphase MoS₂ nanosheets but also mediate the formation of MoS₂ nanospheres, whilst the residual CDs may intersperse onto the surfaces of MoS₂ nanospheres. The novel MoS₂ nanospheres-based electrode exhibits favorable electrochemical responses in an aqueous electrolyte, such as high specific capacitance (145 F g⁻¹), good rate capability and excellent cyclic stability (90% capacity retention after 2000 cycles) owing to enhanced ionic intercalation and improved electrical conductivity associated with the specific structures and morphologies. This work would pave a new pathway through the structural, morphological and compositional design for improving the electrochemical properties of transition metal dichalcogenides (TMDs) applicable as alternative energy storage materials.

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1. Introduction

Supercapacitors are a new type of electrochemical energy storage device which can store a large amount of charge and deliver it at high power ratings. Among three types of supercapacitors, pseudocapacitances possess much higher specific capacitance than electrical double-layer capacitances and thus have attracted tremendous attention in recent years. Efficient ions intercalation and transportation in the electrode materials are essential for the electrochemical storage performances of the supercapacitors.¹

Two-dimensional (2D) atomically-thick materials, exemplified by graphene and few-layered transition metal dichalcogenides (TMDs), have shown promise as new-generation energy materials.^2–6 For instance, high volumetric specific capacitances of ∼300 F cm⁻³ have been obtained for restacked graphene nanosheets.³ Huang et al. reported that a layered MoSe₂/graphene composite presents superior electrochemical performance as a supercapacitor electrode material.⁴

The bulk MoS₂ is similar to graphite, in which one Mo layer is sandwiched between two S layers through strong covalent bonding while the resulting S–Mo–S trilayers are stacked by weak van der Waals forces. Due to its anisotropic structure, MoS₂ is prone to form 2D morphology with large surface area and permeable channels for ions adsorption, intercalation and transportation especially in ultrathin forms.^7,8 However, the poor intrinsic electrical conductivity of the bulk MoS₂ associated with the indirect bandgap (1.2 eV) significantly limits its practical applications as an electrode material.⁷ To enhance the electrical conductivity, MoS₂-based hybrids composing of conductive materials have been fabricated.^9–14 Theoretical calculation can predict transition from indirect to direct bandgap when MoS₂ becomes thin enough, rendering improved electrical conductivity from the pristine semiconductivity.¹⁵ Recent studies reveal that the single- and few-layered MoS₂ nanosheets exhibit significantly improved electronic activity compared with the bulk MoS₂, such as high charge-carrier mobility, tunable charge-carrier types and high on/off ratio.^8,16 To prepare these ultrathin MoS₂ nanosheets, several strategies have been developed.¹⁷

Generally, the bulk MoS₂ possesses two polymorph structures of 2H phase and 1T phase depending on the arrangement of Mo and S atoms. In the 2H structure, Mo atom coordination is trigonal prismatic and the corresponding material is semiconductive. In the 1T structure, Mo atom coordination is octahedral offering the material a metal character. Most recently, the metallic 1T phase MoS₂ was obtained from the semiconductive 2H phase MoS₂,¹⁸ which is 10⁷ times more conductive than the 2H counterpart. In addition, the minor 1T phase does not change the distributions of Mo atoms in the 2H phase MoS₂ host and thus does not hamper its practical applications.

Carbon dots (CDs) are novel carbon nanomaterials and possess unique properties such as hydrophilicity, stability, low cytotoxicity, and photoluminescence.¹⁹ Recently, CDs have been used as stabilizers to facilitate the exfoliation of graphite to graphene in an aqueous medium.²⁰ Herein, with the assistance of CDs, we successfully introduced the metallic 1T phase into the host of the few-layered 2H phase MoS₂ in a simple one-pot hydrothermal route. Without further exfoliation and restack treatment, the obtained MoS₂ nanostructures were directly used as an electrode for the supercapacitor and exhibit improved electrochemical characteristics, which can be ascribed to the synergistic contribution of the ultrathin 2D lamellar nanostructure with the expanded interlayer distance, edge-rich 3D nanospherical morphology, incorporation of the metallic 1T phase and semiconductive 2H phase, and the anchored CDs.

2. Experimental

2.1 Hydrothermal synthesis of CDs

Carbon dots (CDs) were synthesized by a hydrothermal method. In a typical route, 0.1 g of folic acid was added into 15 ml of deionized water, and then 1.0 ml of glycerol was dropped under vigorous stirring for 30 min at room temperature. The resulting solution was loaded into a Teflon-lined stainless steel autoclave (50 ml capacity) and heated at 200 °C for 12 h. A yellow-brownish CDs colloidal solution was obtained and characterized by UV-Vis absorption spectroscope (Hitachi U-4100), photoluminescence spectroscope (PL, GangDong F-280 Fluorospectrophotometer) and X-ray diffractometer (XRD, Rigaku D/max-r B diffractometer with Cu K_α radiation) as shown in Fig. S1.†

2.2 CDs-assisted hydrothermal synthesis of MoS₂

The synthetic procedure was designed as follows: 0.6 mmol of ammonium molybdate and 18.0 mmol of thiourea were dissolved in 20 ml of deionized water, followed by addition of the purified CDs after dialysis treatment (i.e., 5 mg, 10 mg and 20 mg), and the resulting solution was transferred into a Teflon-lined stainless steel autoclave (50 ml capacity). The hydrothermal reaction was carried out at 150 °C for 12 h. The resultant black precipitates were collected, washed respectively with deionized water and absolute ethanol, and dried at 60 °C overnight. For comparison, MoS₂ was synthesized by the same route in the absence of CDs, so-called pure MoS₂. A series of reactions are involved as follows:

(NH₄)₆Mo₇O₂₄(aq) + 3H₂O(l) → 7MoO₃(s) + 6NH₃·H₂O(l) (1)

CS(NH₂)₂(aq) + 2H₂O(l) → 2NH₃(g) + H₂S(g) + CO₂(g)

CS(NH₂)₂(aq) + H₂O(l) → CH₃CONH₂(l) + H₂S(g)

4MoO₃(s) + 2H₂S(g) → 4MoO₂(s) + S(s) + SO₂(g) + 2H₂O(l)

MoO₂(s) + 2H₂S(g) → MoS₂(s) + 2H₂O(l)

2.3 Characterizations

The morphologies, phase structures and chemical compositions of the resultant MoS₂ nanostructures were analyzed using field emission scanning electron microscope (FE-SEM, Hitachi SU-8010), transmission electron microscope (TEM) and high resolution TEM (HRTEM, Hitachi H-7650), X-ray diffractometer, Raman spectroscope (Renishaw inVia with 532 nm laser), energy dispersive spectrometer (EDS, Hitachi H-7650), and X-ray photoelectron spectroscope (XPS, PHI 5000 Versa Probe with Mg K_α radiation).

2.4 Electrochemical measurements

The electrochemical properties were examined by an electrochemical workstation (CHI660D). Typically, 20.0 mg of the as-prepared MoS₂, 2.5 mg of polytetrafluoroethylene solution (10 wt%) and 2.5 mg of acetylene black were dispersed in 10 ml of alcohol and then ground for 2 h to produce paste. The resulting paste was then pressed as a film (0.5 × 0.5 cm²). Cyclic voltammetry (CV) within a scan rate range of 5 to 300 mV s⁻¹ was conducted in 0.5 M Na₂SO₄ using a Ag/AgCl reference electrode (in 3 M KCl solution), a platinum wire (99.99%) counter electrode, and the above film working electrode. Nyquist plots were measured within a frequency range of 1000 kHz to 0.001 Hz at an overpotential of 50 mV. Galvanostatic charge/discharge (GCD) was measured at current densities from 0.5 to 8 A g⁻¹.

3. Results and discussion

XRD technique was used to explore the structural information of the as-prepared MoS₂. The obvious evolution of XRD patterns of different MoS₂ is displayed (Fig. 1). For the pure MoS₂ (S0) obtained without CDs, all diffraction peaks are well indexed to the standard bulk 2H phase MoS₂ (JCPDS 37-1492). With the assistance of 5 mg of CDs, the synthesized MoS₂ (S1) exhibits improved crystallinity in comparison to the pure MoS₂, as evidenced by intensified characteristic peaks at 14.4°, 32.7° and 58.3° corresponding to (002), (100) and (110) planes of the 2H phase MoS₂, respectively. Further addition of CDs (10 mg (S2) and 20 mg (S3)), however, leads to a newly emerged peak at 9.1°, the obviously weakened peak of the (002) plane and almost unchanged peaks of the (100) and (110) planes compared with the pure MoS₂. Generally, the (002) reflection in lamellar materials can be ascribed to atomic layers stacked along the (002) direction. The newly emerged peak can be assigned to the (002) plane but with an enlarged interlayer spacing of 9.4 Å instead of pristine 6.2 Å. At present, much effort has been devoted to tailor the dominant (002) plane of MoS₂.^7,21 From our findings, a novel MoS₂ with the expanded (002) interlayer distance is identified, which could be manipulated by CDs introduced in the synthetic solution.

Fig. 1 XRD patterns of MoS₂ synthesized in the absence of CDs (S0) and with the assistance of CDs of 5 mg (S1), 10 mg (S2) and 20 mg (S3) compared with standard bulk MoS₂ (JCPDS 37-1492) shown at the bottom.

FE-SEM micrographs of different MoS₂ were captured. The morphologies of the pure MoS₂ demonstrate that it is disordered agglomerate built up with uneven MoS₂ nanosheets randomly overlapped (Fig. 2(a)). In addition, these MoS₂ nanosheets present obscure and rough edges, indicating insufficient crystallization (Fig. 2(b)). However, the as-prepared MoS₂ with the assistance of CDs (10 mg) exhibits uniform flower-like nanospheric morphologies with dense stretching petal-like edges (Fig. 2(c)), which correspond to the ultrathin MoS₂ nanosheets with the lateral sizes of ∼10 nm (Fig. 2(d)). EDS elemental maps of the latter MoS₂ nanospheres were acquired (Fig. 2(e)–(g)). The calculated atomic ratio of Mo and S is 1 : 2.03, close to the theoretical stoichiometric ratio of a MoS₂ molecule (1 : 2). According to the relatively sparse and uneven distribution of C compared with the full and homogeneous spread of Mo and S, we hypothesize that the introduced CDs probably play multiple roles in the synthesis of MoS₂ nanospheres because of their unique structures and properties. CDs may not only involve the growth of the 2D MoS₂ nanosheets but also tailor the construction of the 3D MoS₂ nanospheres, or some may just be absorbed into MoS₂ nanospheres.

Fig. 2 FE-SEM micrographs of MoS₂ synthesized without CDs (a and b) and with 10 mg of CDs (c and d). EDS mapping images of the latter, showing C (e), Mo (f) and S (g).

TEM image (Fig. 3(a)) of the as-prepared MoS₂ in the presence of CDs (10 mg) clearly displays sponge-like nanostructures constructed by the ultrathin nanosheets, well consistent with the above FE-SEM observation. The incorporated CDs were distinguishable when observing under high magnification (Fig. 3(a) inset). HRTEM image (Fig. 3(b)) shows the clear lattice fringes of the lamellar MoS₂ nanosheets with the interlayer spacing of 9.4 Å, well in accordance with the above XRD result. HRTEM observation also reveals that the current MoS₂ nanosheets consist of few layers. Raman spectra further confirm that the most MoS₂ nanosheets compose of about 4 layers (Fig. S2†). This is derived from the fact that the difference between representative E¹_2g Raman shift at 381 cm⁻¹ and A_1g Raman shift at 405 cm⁻¹ is close to that (24.3 cm⁻¹) of the quadri-layered MoS₂ nanosheets.^22–24 It is well recognized that the difference between characteristic Raman shifts could be indicative of the layer numbers of the few-layered MoS₂ nanosheets.^22–24 Furthermore, the intensities of both E¹_2g and A_1g peaks were decreased with increasing CDs contents, which could be due to the reduced interlayer interactions and dielectric screening of long-range Coulomb forces with decrease of the layer numbers.^25,26 SAED pattern (Fig. 3(c)) reveals the polycrystalline nature of the as-prepared MoS₂ with CDs. Although CDs were not actually verified by XRD and Raman analysis owing to their tiny amount and small sizes, their existence and distribution were clearly verified by EDS elementary analysis and TEM microscopic observation.

Fig. 3 (a) TEM and (b) HRTEM images of MoS₂ nanospheres obtained with CDs (10 mg). The inset in (a) shows an enlarged TEM micrograph under high magnification. (c) Selected area electron diffraction (SAED) pattern of a representative MoS₂.

Currently, there are two manipulative growth and evolution mechanisms of the crystalline nanoflowers: the initial structures are nanosheets as petals which curl and tangle to form nanoflowers, or the final nanoflowers are based on nuclei, followed by an outward growth of the petal-like nanosheets.^27–29 To elucidate the effects of CDs on the evolution of MoS₂ nanostructures, individual time-dependent synthesis of MoS₂ in the presence or absence of CDs were carried out as shown in Fig. S3.† For the synthesis with CDs, after 1.5 h, MoS₂ nanosheets were initially formed owing to the intrinsic anisotropic property of MoS₂ (Fig. S3(a)†). After 2.0 h, more and more nanosheets were produced, tangling each other (Fig. S3(c)†). After 2.5 h, the flower-like MoS₂ nanospheres with dense petals were constructed (Fig. S3(e)†). Comparatively, for the synthesis without CDs, after 1.5 h, amorphous clusters were firstly formed (Fig. S3(b)†). After 2.0 h, some MoS₂ nanosheets with uneven dimensional and lateral sizes were obtained (Fig. S3(d)†). After 2.5 h, more and more nanosheets were produced, which randomly stack (Fig. S3(f)†). The above findings reveal that CDs effectively facilitate growth of 2D MoS₂ nanosheets and construction of 3D MoS₂ nanospheres within short synthetic time. A schematic evolution process of MoS₂ obtained in the presence of CDs is thus proposed (Fig. 4). Initially, MoS₂ prefers to grow into 2D nanosheets, in which the zero-dimensional CDs (ca. 2.5 nm in size) with high surface energy readily function as exogenous nuclei/seeds, besides the resultant MoS₂ micelles as endogenous nuclei. Consequently, both synthetic energy and time are reduced. According to the sponge-like MoS₂ nanostructures without a dense core (Fig. 3(a)), we suppose that the final 3D nanoflowers are assembled by the 2D nanosheets.³⁰ Similar proposal for formation mechanism of Mo(S_xSe_1−x)₂ nanoflowers has been reported.²⁷ Moreover, the remaining CDs with high surface energy readily adhere on MoS₂ nanospheres as observed by TEM (Fig. 3(a) inset).

Fig. 4 The schematic illustration of the formation mechanism of the flower-like MoS₂ nanospheres.

In order to examine the chemical compositions and valence states of MoS₂ nanospheres synthesized with CDs (10 mg), XPS analysis was performed (Fig. 5). From the survey spectrum (Fig. 5(a)), the elements of Mo, S, C and O are identified. The high-resolution scan of Mo 3d electrons depicts two major peaks arising from Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2 orbitals located at 229.1 and 232.1 eV, respectively (Fig. 5(b)), suggesting the existence of the dominant 2H phase.³¹ Meanwhile, the 1T components with the binding energies of ∼0.8 eV lower than the 2H counterparts appear (Fig. 5(b)),^16,32 together with a minor peak at 225.7 eV corresponding to S 2s of MoS₂. Overall, the obtained MoS₂ exclusively consists of the multiphase of the major 2H phase and minor 1T phase. It has been well known that the metallic 1T phase MoS₂ is 10⁷ times more electrically conductive than the semiconductive 2H phase MoS₂.¹⁸ Therefore, the incorporation of the 1T phase into the 2H phase could be beneficial to the electrochemical performances of MoS₂ as an electrode material. The S 2p XPS spectrum (Fig. 6(c)) shows typical S 2p_3/2 and S 2p_1/2 doublet of S²⁻.⁷ All the XPS findings suggest the successful synthesis of the multiphase MoS₂. Moreover, according to C 1s peak shape as shown in Fig. 5(d), the carbon atom may exist in different forms, including a major C–C bond centered at 284.5 eV and few oxidized C-containing bonds at the higher binding energies over 284.5 eV.

Fig. 5 XPS survey spectrum (a) and high-resolution XPS Mo 3d (b), S 2p (c) and C 1s spectra (d) of MoS₂ obtained with the assistance of 10 mg of CDs.

Fig. 6 (a) Cyclic voltammogram (CV) curves of the pure MoS₂ obtained without CDs and MoS₂ nanospheres synthesized with CDs (10 mg) at a scan rate of 5 mV s⁻¹; (b) CV curves of MoS₂ nanospheres with different scan rates. (c) Galvanostatic charge/discharge (GCD) profiles of MoS₂ nanospheres at various current densities. (d) Specific capacitance of different MoS₂ as a function of current density. (e) Cyclic stability at 1 A g⁻¹ of different MoS₂. (f) Nyquist plots of different MoS₂. Insets in (f) are the corresponding Nyquist plots at high and medium frequencies and the proposed equivalent electrical circuit.

Nanostructures offering the advantages of large surface-to-volume ratio, favorable transport property, and high feasibility for volume change upon ion insertion/extraction and other reactions, show promising as potential electrode materials for next-generation supercapacitors.³³ Fig. 6(a) compares the CV curves of different MoS₂ obtained without CDs or with 10 mg of CDs at a low scan rate of 5 mV s⁻¹ over a potential window of −0.65 to 0.35 V. Redox peaks are clearly distinguishable for the current MoS₂ nanospheres instead of the pure MoS₂, suggesting MoS₂ nanospheres possess typical pseudocapacitive characteristics associated with the variable valences of Mo especially at the edges during the electrochemical processes.³⁴ In addition, the more symmetric rectangular shape and larger integral area of the CV curves of MoS₂ nanospheres in comparison to the pure MoS₂ reveal that they have good typical electrostatic double-layer capacitive behavior with small contact resistance. The CV curves for MoS₂ nanospheres at different scan rates were examined (Fig. 6(b)). It could be found that all the CV curves retain similar rectangular shapes within the experimental scan rates, indicating MoS₂ nanospheres possess high rate capability and ideal supercapacitor behavior.

Fig. 6(c) displays the GCD curves of MoS₂ nanospheres with different current densities. The symmetric triangular-shape charge/discharge circles especially at high current densities further confirm the ideal supercapacitive characteristics and superior reversible redox property, which is in good agreement with the above CV curves. Accordingly, the gravimetric specific capacitance was then calculated and plotted (Fig. 6(d)) using the discharge portion of the individual GCD profiles. At 0.5 A g⁻¹, MoS₂ nanospheres deliver a high gravimetric specific capacitance of 145 F g⁻¹, which is larger than that (120 F g⁻¹) of the pure MoS₂ and comparable to 148 F g⁻¹ for a MoS₂–graphene hybrid.⁷ With increase of the current density to 8 A g⁻¹, MoS₂ nanospheres can preserve about 50% specific capacitance, which further confirms the high rate capability and is comparable to other ideal electrostatic double-layer capacitive.^33,35 The good rate capability and high specific capacitance can be ascribed to the high specific surface area (18.08 m² g⁻¹) associated with the flower-like and edge-rich morphology as well as improved electrical conductivity of MoS₂ nanospheres due to the incorporated metallic 1T phase.

MoS₂ nanospheres exhibit excellent cyclic stability in comparison to the pure MoS₂; giving about 90% capacity retention after 2000 charge/discharge cycles (Fig. 6(e)). The remarkable cycling stability and desirable reversible capacity of the current MoS₂ nanospheres are comparable to the reported MoS₂-based hybrids with excellent electrochemical performances.^4–6,36 The underlying reason can be attributed to the synergistic effects of the special 2D microstructure with enlarged lattice spacing and few layer numbers allowing more efficient ions intercalation and transportation,³⁷ unique 3D morphologies with more edges enabling the reversible electrochemical reactions, and 1T phase incorporation enhancing the electrical conductivity.

Fig. 6(f) and inset (up) compare Nyquist plots of the pure MoS₂ and MoS₂ nanospheres. It can be seen that both Nyquist plots consist of a semicircle arc at high frequencies and straight lines at medium and low frequencies. Compared with the pure MoS₂, MoS₂ nanospheres demonstrate a slightly reduced semicircle at the high frequencies, which can be an indicator for the enhanced kinetics of the electrode reaction (Fig. 6(f) inset (up)). Furthermore, the angle of the straight line at the low frequencies for MoS₂ nanospheres is 75° in comparison to 60° for the pure MoS₂, close to ideal supercapacitor (phase angle is about 90°), further suggesting MoS₂ nanospheres possess typical supercapacitive behavior. The corresponding equivalent electric circuit was established (Fig. 6(f) inset (down)). Here, R₁ represents liquid phase resistance, R_ct and Z_w correspond to the charge transfer resistance at the electrolyte/electrolyte interface and solid-state diffusion resistance, while C_d denotes double-layer capacitance. According to the diameter of the semicircle arc, the R_ct value for MoS₂ nanospheres is about 15 Ω. The small R_ct value means the good electrical conductivity and low resistance of the electrode material. Overall, all the electrochemical measurements reveal that the current MoS₂ nanospheres possess the excellent structural stability and efficient electronic/ionic transportation capacity, and in return, high storage capacities and stable cycling life.

To further clarify the reason behind the electrochemical performances of different MoS₂, textural properties were investigated. N₂ adsorption and desorption isotherms and Barrette Joynere Halenda (BJH) cumulative pore-size distributions of different MoS₂ obtained without or with CDs (10 mg) are displayed in Fig. S4(a) and (b),† respectively. Both samples present type IV isotherms according to IUPAC nomenclature (Fig. S4(a)†), indicating the existence of micropores and/or mesopores. The hysteresis loop of MoS₂ nanospheres shifts to the lower pressure region compared with the pure MoS₂, suggesting the presence of relatively more small pores. The pore size distribution curves as shown in Fig. S4(b)† are well consistent with the above isothermal behavior. As can be seen, the pure MoS₂ mainly has mesopores of 2.8 nm pore size, while MoS₂ nanospheres are of complicated pore structures including different mesopores (2.2, 3.5 nm and the larger sizes). In addition, MoS₂ nanospheres possess the larger surface area (18.08 m² g⁻¹) than the pure MoS₂ (16.06 m² g⁻¹). The larger surface area and complicated pore structure of MoS₂ nanospheres associated with the specific morphologies and multiphase structure can be beneficial for the improved electrochemical performances.

4. Conclusions

In this study, the novel flower-like MoS₂ nanospheres constructed by the few-layered 2H MoS₂ nanosheets incorporated with 1T phase were successfully synthesized via a facile CDs-assisted hydrothermal route within short synthetic time for the first time. The obtained MoS₂ achieved remarkable electrochemical performances including high specific capacitance of 145 F g⁻¹, superior rate capability, and good cycling stability, enabling them potential as a pseudocapacitive electrode material. The synergistic effects of the incorporated metallic 1T phase, the ultrathin 2D nanosheets with the enlarged interlayer spacing, the embedded/anchored CDs, the edge-rich 3D nanoflowers, and the large surface area and complex pore structure contribute to the improved electrochemical performances of MoS₂ nanospheres obtained with the assistance of CDs. It is believed that such a simple and efficient strategy would open up a new pathway for the large-scale production of various TMDs applicable in the fields of the energy storage.

Acknowledgements

This work was financially supported by the Scientific Research Foundation for the Postdoctorals, Heilongjiang Province (AUGA4110005410), the Fundamental Research Funds for the Central Universities (Grant No. HIT. IBRSEM. 201331), and the Fundamental Research Funds for the Central Universities and Program for Innovation Research of Science in Harbin Institute of Technology (Grant No. PIRS of HIT 201411).

References

1. P. Simon and Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater., 2008, 7(11), 845–854.
2. M. Chhowalla, Z. Liu and H. Zhang, Two-dimensional transition metal dichalcogenide (TMD) nanosheets, Chem. Soc. Rev., 2015, 44(9), 2584–2586.
3. X. W. Yang, C. Cheng, Y. F. Wang, L. Qiu and D. Li, Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage, Science, 2013, 341(6145), 534–537.
4. K. J. Huang, J. Z. Zhang and J. L. Cai, Preparation of porous layered molybdenum selenide–graphene composites on Ni foam for high-performance supercapacitor and electrochemical sensing, Electrochim. Acta, 2015, 180, 770–777.
5. K. J. Huang, H. L. Shuai and J. Z. Zhang, Ultrasensitive sensing platform for platelet-derived growth factor BB detection based on layered molybdenum selenide–graphene composites and Exonuclease III assisted signal amplification, Biosens. Bioelectron., 2016, 77, 69–75.
6. H. L. Shuai, K. J. Huang and Y. X. Chen, A layered tungsten disulfide/acetylene black composite based DNA biosensing platform coupled with hybridization chain reaction for signal amplification, J. Mater. Chem. B, 2016, 4(6), 1186–1196.
7. J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang and H. Zhang, et al., Controllable disorder engineering in oxygen-incorporated MoS₂ ultrathin nanosheets for efficient hydrogen evolution, J. Am. Chem. Soc., 2013, 135(47), 17881–17888.
8. C. N. R. Rao, U. Maitra and U. V. Washmare, Extraordinary attributes of 2-dimensional MoS₂ nanosheets, Chem. Phys. Lett., 2014, 609(11), 172–183.
9. J. Guo, F. Li, Y. Sun, X. Zhang and L. Tang, Oxygen-incorporated MoS₂ ultrathin nanosheets grown on graphene for efficient electrochemical hydrogen evolution, J. Power Sources, 2015, 291, 195–200.
10. L. Hu, Y. Ren, H. Yang and Q. Xu, Fabrication of 3D hierarchical MoS(2)/polyaniline and MoS(2)/C architectures for lithium-ion battery applications, ACS Appl. Mater. Interfaces, 2014, 6(16), 14644–14652.
11. E. G. D. Firmiano, A. C. Rabelo, C. J. Dalmaschio, A. N. Pinheiro, E. C. Pereira and W. H. Schreiner, et al., Supercapacitor Electrodes Obtained by Directly Bonding 2D MoS₂ on Reduced Graphene Oxide, Adv. Energy Mater., 2014, 4(6), 175–178.
12. K. J. Huang, L. Wang, Y. J. Liu, H. B. Wang, Y. M. Liu and L. L. Wang, Synthesis of polyaniline/2-dimensional graphene analog MoS₂ composites for high-performance supercapacitor, Electrochim. Acta, 2013, 109(11), 587–594.
13. K. J. Huang, L. Wang, J. Z. Zhang, L. L. Wang and Y. P. Mo, One-step preparation of layered molybdenum disulfide/multi-walled carbon nanotube composites for enhanced performance supercapacitor, Energy, 2014, 67(4), 234–240.
14. C. N. R. Rao, K. Gopalakrishnan and U. Maitra, Comparative study of potential applications of Graphene, MoS₂, and other two-dimensional materials in energy devices, sensors, and related areas, ACS Appl. Mater. Interfaces, 2015, 7(15), 7809–7832.
15. K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Atomically Thin MoS2: A New Direct-Gap Semiconductor, Phys. Rev. Lett., 2010, 105(13), 474–479.
16. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen and M. Chhowalla, Photoluminescence from Chemically Exfoliated MoS₂, Nano Lett., 2011, 11(12), 5111–5116.
17. X. Huang, Z. Zeng and H. Zhang, Metal dichalcogenide nanosheets: preparation, properties and applications, Chem. Soc. Rev., 2013, 42(5), 1934–1946.
18. M. Acerce, D. Voiry and M. Chhowalla, Metallic 1T phase MoS₂ nanosheets as supercapacitor electrode materials, Nat. Nanotechnol., 2015, 10(4), 313–318.
19. A. D. Zhao, Z. W. Chen and C. Q. Zhao, Recent advances in bioapplications of C-dots, Carbon, 2015, 85, 309–327.
20. M. H. Xu, W. Zhang, Z. Yang, F. Yu, Y. J. Ma and N. T. Hu, et al., One-pot liquid-phase exfoliation from graphite to graphene with carbon quantum dots, Nanoscale, 2015, 7(23), 10527–10534.
21. M. R. Gao, M. K. Chan and Y. Sun, Edge-terminated molybdenum disulfide with a 9.4-A interlayer spacing for electrochemical hydrogen production, Nat. Commun., 2015, 6, 1–8.
22. Y. Zhao, X. Luo, H. Li, J. Zhang, P. T. Araujo and C. K. Gan, et al., Interlayer breathing and shear modes in few-trilayer MoS₂ and WSe₂, Nano Lett., 2013, 13(3), 1007–1015.
23. X. Fan, P. Xu, D. Zhou, Y. Sun, Y. C. Li and M. A. Nguyen, et al., Fast and Efficient Preparation of Exfoliated 2H MoS₂ Nanosheets by Sonication-Assisted Lithium Intercalation and Infrared Laser-Induced 1T to 2H Phase Reversion, Nano Lett., 2015, 15(9), 5956–5960.
24. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin and A. Olivier, et al., From Bulk to Monolayer MoS₂: Evolution of Raman Scattering, Adv. Funct. Mater., 2012, 22(7), 1385–1390.
25. S. Y. Tai, C. J. Liu, S. W. Chou, F. S. S. Chien, J. Y. Lin and T. W. Lin, Few-layer MoS₂ nanosheets coated onto multi-walled carbon nanotubes as a low-cost and highly electrocatalytic counter electrode for dye-sensitized solar cells, J. Mater. Chem., 2012, 22(47), 24753–24759.
26. Y. X. Fang, S. J. Guo, D. Li, C. Z. Zhu, W. Ren and S. J. Dong, et al., Easy Synthesis and Imaging Applications of Cross-Linked Green Fluorescent Hollow Carbon Nanoparticles, ACS Nano, 2012, 6(1), 400–409.
27. O. E. Meiron, L. Houben and M. Bar-Sadan, Understanding the formation mechanism and the 3D structure of Mo(S_xSe_1−x)(2) nanoflowers, RSC Adv., 2015, 5(107), 88108–88114.
28. X. L. Zhou, J. Jiang, T. Ding, J. J. Zhang, B. C. Pan and J. Zuo, et al., Fast colloidal synthesis of scalable Mo-rich hierarchical ultrathin MoSe_2−x nanosheets for high-performance hydrogen evolution, Nanoscale, 2014, 6(19), 11046–11051.
29. D. Sun, S. M. Feng, M. Terrones and R. E. Schaak, Formation and Interlayer Decoupling of Colloidal MoSe₂ Nanoflowers, Chem. Mater., 2015, 27(8), 3167–3175.
30. P. Ilanchezhiyan, G. Mohan Kumar and T. W. Kang, Electrochemical studies of spherically clustered MoS₂ nanostructures for electrode applications, J. Alloys Compd., 2015, 634, 104–108.
31. Q. Weng, X. Wang, X. Wang, C. Zhang, X. Jiang and Y. Bando, et al., Supercapacitive energy storage performance of molybdenum disulfide nanosheets wrapped with microporous carbons, J. Mater. Chem. A, 2015, 3(6), 3097–3102.
32. C. A. Papageorgopoulos and W. Jaegermann, Li intercalation across and along the van-der-waals surfaces of MoS₂ (0001), Surf. Sci., 1995, 338(1–3), 83–93.
33. X. Wang, J. Ding, S. Yao, X. Wu, Q. Feng and Z. Wang, et al., High supercapacitor and adsorption behaviors of flower-like MoS₂ nanostructures, J. Mater. Chem. A, 2014, 2(38), 15958–15963.
34. J. Wu, H. Li, Z. Y. Yin, H. Li, J. Q. Liu and X. H. Cao, et al., Layer thinning and etching of mechanically exfoliated MoS₂ nanosheets by thermal annealing in air, Small, 2013, 9(19), 3314–3319.
35. B. Lei, G. R. Li and X. P. Gao, Morphology dependence of molybdenum disulfide transparent counter electrode in dye-sensitized solar cells, J. Mater. Chem. A, 2014, 2(11), 3919–3925.
36. C. Hao, F. Wen, J. Xiang, L. Wang, H. Hou and Z. Su, et al., Controlled Incorporation of Ni(OH)₂ Nanoplates Into Flowerlike MoS₂ Nanosheets for Flexible All-Solid-State Supercapacitors, Adv. Funct. Mater., 2014, 24(42), 6700–6707.
37. X. Rui, H. Tan and Q. Yan, Nanostructured metal sulfides for energy storage, Nanoscale, 2014, 6(17), 9889–9924.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15074h

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