High efficiency shear exfoliation for producing high-quality, few-layered MoS₂ nanosheets in a green ethanol/water system

Abstract

This work presented a feasible strategy to generate molybdenum disulfide (MoS₂) nanosheets by a direct liquid shear exfoliation technique in a green mixed solvent system of ethanol/water. The volume ratio of ethanol/water and the initial concentration of the bulk MoS₂ powders were found to have significant impacts on the final exfoliation yield. The best ratio and initial concentration for optimal shear exfoliation were finally selected as 45 vol% and 10 mg mL⁻¹, respectively. According to the proper shear strength and matching viscosity, systematic analysis on the exfoliation mechanism indicated that shear and collision effects would play the major role when the shearing occurred at a high speed of 10 000 rpm. The higher cumulative yield and better quality (up to 30% after 10 times of shear cycling, comparatively large sizes d ∼ 4 μm) were confirmed by detailed experimental characterizations. Moreover, the photothermal performance was also investigated and the prepared MoS₂ nanosheets exhibited great efficiency in transforming near-infrared light into heat. It is expected to be a promising strategy for large-scale production of MoS₂ nanosheets with excellent properties in a low-cost and green solvent system.

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1. Introduction

Studies on the layered transition metal dichalcogenides (TMDs, such as MoS₂, WS₂, and WSe₂) have been extensively carried out since the 1960s due to their unique structures and distinctive properties when compared with their bulk forms.^1–4 One of the most prominent two-dimensional (2D) materials, molybdenum disulfide (MoS₂) has drawn great increasing attention in recent years for its superior electronic and structural properties, which can find broad applications in electronics, catalysis, energy-storage devices, electrochemistry, optoelectronics, etc.^5–9 Superior to its bulk counterpart, MoS₂ nanosheets enjoy more favourable and distinguishing features, including large specific surface area, strong absorbing ability of visible light and extreme flexibility.^10,11 It has been reported that the monolayer or few-layered MoS₂ nanosheets have been placed under the limelight due to more superior electronic and structural properties than the bulk material. For instance, the exceptional lamella morphology is useful in energy storage applications as electrode material for Li-ion battery and supercapacitors.¹² Additionally, MoS₂ nanoflakes present innovative features as solid lubricant or oil additive compared with its bulk material which has been extensively employed in various lubricating oils and greases.^13–15 More recently, the biomedical applications of MoS₂ have been explored rapidly due to its good biocompatibility and high photothermal performance. Therefore, it is of great interest to develop novel MoS₂-based photothermal agents with high photothermal conversion efficiency and excellent biocompatibility.^16,17

Analogous to those employed for graphene preparations, there are several strategies available for the production of 2D TMDs materials, which are generally classified into top-down exfoliations or bottom-up synthesis approaches.^18,19 The bottom-up approaches mainly include chemical vapor deposition (CVD) growth, pulsed laser deposition (PLD), hydrothermal reaction, and so forth.^20–22 The top-down methods rely on the exfoliation of layered bulk crystals, such as the mechanical cleavage, chemical Li-intercalation exfoliation, liquid phase sonication exfoliation, and mechanical ball milling technique.^1,3,23–25 However, these strategies have some unavoidable drawbacks, such as low yield, high cost, inducing structure defect and hard to scale up, which impede their practical applications in various fields.^26,27 In order to overcome these limitations and obtain high-quality MoS₂ nanosheets, Coleman et al. have developed the direct liquid shear exfoliation of layered materials, such as graphene, MoS₂, WS₂, MoSe₂, BN and GaS in organic solvents, surfactants or polymer solutions.^28–32 This method is extremely intriguing and provides a new direction for large-scale and low-cost production of layered compounds.

In shear exfoliation process, the commonly used dispersion systems can be classified into organic solvents, including N-methyl-2-pyrrolidone (NMP), isopropanol (IPA), N,N-dimethyl formamide (DMF), and aqueous surfactant solutions of sodium cholate solution (NaC) and sodium dodecyl benzene sulfonate (SDBS). However, this technique is not mature due to several unavoidable drawbacks caused by above-mentioned organic solvents: high cost, poor dispersion, hard to remove and potential negative impact on the environment.^29,33 Hence, it is imperative to develop an environment-friendly solvent system to replace the traditional organic solvents for large-scale production of MoS₂ nanosheets so as to promote their practical applications.

The main purpose of this work is to improve the liquid shear exfoliation technique for mass production of MoS₂ nanosheets with higher yield and better quality. Scale up is more likely to be achieved if the exfoliation process is relatively insensitive to the details of how to design the compatible exfoliation parameters and choose suitable solvents. Initial trials involved the selection of suitable green and pollution-free solvents for shear mixing the bulk MoS₂ powders. Ethanol introduced aqueous solution was previously proved to be a favorable and green solvent system for liquid phase sonication exfoliation.^34,35 However, there are no systematic reports referred to using the ethanol/water mixed solvent over direct liquid phase shearing exfoliation system. On this basis, we carefully optimize shear parameters to generate MoS₂ nanosheets in the mixed solvent of ethanol/water. Previous studies have reported the large-scale shear-exfoliation of graphene nanosheets using a high shear mixer at a rotating speed of 6000 rpm.²⁸ In the case of MoS₂ with a higher surface energy, a faster speed of 10 000 rpm was applied to shear exfoliation by the same high shear mixer. The wettability between MoS₂ and solvent could be controlled by modifying the ratio of ethanol to water in order to make MoS₂ better dispersed in solution. The following structural and chemical characterizations also proved that almost no basal plane defects or impurities were introduced during the shear exfoliation process. This work may establish a new paradigm in fabrications of MoS₂ nanosheets and other inorganic graphene analogues with low-cost and high-performance features.

2. Experimental section

2.1 Materials

All chemicals and reagents employed in the experimental process were analytical grade. MoS₂ powders (6–40 μm) were purchased from Sigma-Aldrich at the highest available purity, the ethanol obtained from Tianjin Chemical Reagent Company was applied throughout this shear exfoliation study, ultrapure water (>18 MΩ cm) was used for preparation and rinsing. All chemicals and reagents were used as received without any further purification.

2.2 Shear exfoliation

Initially, the required amount of MoS₂ powders was weighed and added into the glass beaker with the capacity of 1 L, and the corresponding ethanol aqueous solutions with different volume ratios were subsequently added into the beaker. Then, the MoS₂ powders were dispersed in ethanol aqueous solution to form 500 mL MoS₂ dispersion. After that, such dispersion was further treated by 1 h of turbulence which was generated by a four-rotating blade Silverson Laboratory Mixer (Model L5, Silverson Machines Ltd., England) with a rotating speed of 10 000 rpm. To avoid the influence of temperature, the ice bath was used among the exfoliation process and the temperature of dispersions was controlled at about 15 °C. This process was recycled for certain times (1–10) to investigate its influence on the morphology and productivity of the MoS₂ nanosheets. The dispersion was settled aside for a period of time before centrifugal separation, which could reduce plenty of centrifugal work and greatly improve the product quality and production efficiency. Following this, the upper dispersion was centrifuged at 1500 rpm with a high speed refrigerated centrifuge (H2050R, Xiangyi Instrument Co., Hunan, China) for 30 min to remove the large sediments. The top two-thirds of supernatant liquid was collected to obtain MoS₂ dispersion. Subsequently, the treated dispersions were vacuum-filtrated through the porous organic membrane with a nominal pore size of 0.45 μm. Finally, the vacuum freeze-drying equipment was applied to dry the sample which was then extracted by pipette to obtain the MoS₂ nanosheets and retained for further characterization. The sediment was collected and dried for the next circle of shear exfoliation.

2.3 Characterization techniques

The surface morphology and microstructure of MoS₂ samples were observed by field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL, Japan) and transmission electron microscopy (TEM, FEI, Tecnai, G2TF20). The Nanoscope IIIa multimode atomic force microscope (AFM, Veeco, USA) was also employed to characterize morphology and thickness of the nanosheets in tapping mode. The X-ray diffraction (XRD, Rigaku D/max-2400 X-ray diffractometer with Cu-Kα radiation, λ = 1.5406 Å, 40 kV, 150 mA over the 2θ range of 10–80°) was used to investigate the microstructure and phase purity of the as-prepared samples. Raman spectroscopy (in Via, LabRAM HR800, 532 nm laser excitation) was employed to characterize the structural features of the samples under different exfoliated conditions. The optical characterization was analyzed by utilizing UV-Vis spectrophotometry (UV-2700, Shimadzu) with a scanning speed of 200 nm min⁻¹ and a bandwidth of 0.1 nm using the 1 cm quartz cuvette. X-ray photoelectron spectroscopy (XPS) was performed by a PHI-5702 (Physical Electronics, USA) spectrometer with monochromatic Al-Kα irradiation and the chamber pressure of ∼3 × 10⁻⁸ Torr to detect the element and binding energy state of the exfoliated MoS₂ nanosheets.

3. Results and discussion

3.1 Selection of exfoliation parameters

We initially tailored the shearing time from 1 to 10 h combining with the subsequent centrifugation in order to achieve a high dispersion concentration by adjusting the volume ratio of ethanol to water as well as the initial bulk MoS₂ concentration. The UV-Vis spectra was used to select the optimal processing parameters, which had crucial influences on the concentration and the production rate of the nanosheets. The effect of ethanol/water volume ratios on the exfoliation yield of MoS₂ nanosheets was explored by UV-Vis absorption spectroscopy, and the spectra were recorded at room temperature in the wavelength region of 200–800 nm, and the exfoliated suspensions were left in specimen tubes over a few days before being investigated and the measurement for each sample was repeated four times. As shown in Fig. 1(a) and (b), the two absorption peaks located at around 627 nm and 672 nm could be attributed to the characteristic A1 and B1 direct excitonic transitions of MoS₂ with the energy split from valence band spin-orbital coupling. In addition, the optical absorbance at 627 nm increased with improving the concentration, which could indirectly reflect the concentration of the nanosheets to a certain extent.^35–40 The absorbance of MoS₂ at 627 nm with different ethanol/water volume ratios was also investigated as follows. When the volume ratio was below 35%, the spectra displayed weak absorption peak at 627 nm because the MoS₂ raw material aggregated into blocks and floated on the surface of the high-shear mixer equipment at the high rotational speed of 10 000 rpm, which inevitably prevented the occurrence of shearing process. When the volume ratio reached 40%, the absorbance increased while the whole mixing system was inclined to stabilize the dispersion at the intensive rotational speed. At the volume ratio of around 45 vol%, the most effective exfoliation appeared and the dispersion has the highest absorbance at 627 nm. Absorbance decreased gradually with higher volume ratios (≥60 vol%), which may cause various viscosity and surface energy accompanied with more energy dissipation and unnecessary ethanol consumption.

Fig. 1 Influences of ethanol/water volume ratio and initial concentration of bulk MoS₂ powders on the exfoliation yield: (a) the UV-Vis spectra of MoS₂ dispersions exfoliated in different ethanol/water solutions; (b) the absorbance intensity curve of MoS₂ in ethanol/water solutions at 627 nm; (c) absorption spectra of MoS₂ dispersions with different MoS₂ initial concentrations; (d) the influence of initial MoS₂ quantities on the exfoliated MoS₂ dispersion concentration; (e) concentration of MoS₂ dispersion as a function of cycle times; (f) cumulative productivity as a function of cycle times.

Moreover, the relationship between the initial concentration of bulk MoS₂ (C_i) and the resulted dispersion concentration of nanosheets (C) was also explored and provided in Fig. 1(c) and (d). Apparently, the absorbance of MoS₂ at 627 nm increased with raising initial bulk concentration gradient, which reflected a higher concentration of nanosheets. It was obvious that a higher dispersion concentration could be achieved and kept approximately invariant at 0.32 mg mL⁻¹ when C_i was above 10 mg mL⁻¹, thus a typical initial concentration of 10 mg mL⁻¹ was selected during the whole shear process. At the same time, the concentrations of MoS₂ dispersions under each shear cycle were measured and summarized in Fig. 1(e). At the initial concentration of 10 mg mL⁻¹, the highest concentration of exfoliated MoS₂ dispersion was approximate 0.38 mg mL⁻¹, while the average concentration of the dispersion is approximately stable at 0.35 mg mL⁻¹ as the cycle times reached 10, which is higher than those reported.^29,41,42 As shown in Fig. 1(f), the cumulative yield is plotted as a function of cycle times, and the yield could reach to nearly 30% when the shear cycles arrived at 10 times. This would hopefully make it a convenient and economical way to prepare the MoS₂ nanosheets.

3.2 Structure and morphology characterizations

The typical morphologies of pristine MoS₂ powders and the exfoliated MoS₂ nanosheets obtained from 45 vol% ethanol aqueous solution were characterized by FE-SEM and TEM observations, as shown in Fig. 2(a)–(c). More FE-SEM morphological images were presented in Fig. S1 of ESI.† Compared with pristine MoS₂ powders (Fig. 2(a)), the morphologies of MoS₂ nanosheets had well defined laminar structure with lateral size from several hundreds of nanometers to several microns (Fig. 2(b)). Moreover, local areas of wrinkled sheets or small amount of residues could be observed, and these were consistent with those reported in other articles for exfoliated layered materials.^26,28,43 FE-SEM and TEM images of the MoS₂ nanosheets illustrated that transparent and 2D ultrathin nanosheets could be obtained by this strategy. More TEM morphologies at different magnifications were available in Fig. S2 of ESI.† The corresponding selected area electron diffraction (SAED) pattern in Fig. 2(c) further displayed the hexagonal symmetry of the MoS₂ nanosheets with typical (100), (110), (103), and (002) planes. FE-SEM based statistical results for flakes' dimensions were analyzed and summarized in Fig. 2(d). The result demonstrated that the typical lateral size was in the range of 0.2–6 μm which mainly depended on the lateral size of bulk powders (6–40 μm).

Fig. 2 FE-SEM images of (a) pristine MoS₂ powders and (b) few-layered MoS₂ nanosheets obtained under a treating time of 1 h at a magnification of ×10 000; (c) TEM images of the exfoliated MoS₂ nanosheets with a treating time of 1 h; (d) size distribution of MoS₂ nanosheets over 200 sheets were counted according to FE-SEM observation.

Further investigations on the thickness of the MoS₂ nanosheets were performed by AFM characterization with tapping mode (Fig. 3). Prior to observation, the exfoliated MoS₂ dispersions were dripped on a silicon wafer substrate. The thickness distribution of the nanosheets could be roughly estimated by additional AFM-based statistical images at different magnifications (ESI, Fig. S3†). As a result, the thickness of the exfoliated MoS₂ nanosheets was mainly 2 nm or so, and its local thickness was close to the theoretical value of single layer MoS₂. Liquid phase shear exfoliation tended to produce nanosheets with a very broad size distribution; meanwhile, it was inevitably observed that some particles were deposited on the surface of MoS₂ nanosheets. Subsequent centrifugation could effectively reduce the aggregation of large particles. On the basis of the above characterizations, the TEM and AFM analyses of the products both illuminated few layered MoS₂ nanosheets could be successfully prepared through this high shear exfoliation technique.

Fig. 3 (a and b) AFM images of MoS₂ nanosheets with a treating time of 1 h deposited on a silicon substrate; (c) the height profile of the products along the line.

Raman spectroscopy, a powerful nondestructive characterization technique, has been applied to study different crystalline structures of exfoliated MoS₂ nanosheets. In terms of MoS₂ raw material (Fig. 4(a)), the appearances of E¹_2g mode at ∼381 cm⁻¹ and the A_1g mode at ∼409 cm⁻¹ are in good agreement with those reported in bulk MoS₂, where the in-plane E¹_2g mode results from opposite vibration of two S atoms with respect to the Mo atoms while the A_1g mode is associated with the out-of-plane vibration of only S atoms in opposite directions.^44,45 By contrast, the spectra of MoS₂ nanosheets presented in Fig. 4(a) displayed additional characteristic peaks at 283 and 454 cm⁻¹. Furthermore, the E¹_2g and A_1g vibrations slightly shifted to 397 and 404 cm⁻¹ with the increase of shear circulation time, which arose from the E_1g, E¹_2g, A_1g, and longitudinal acoustic phonon modes for 2H-MoS₂ nanosheets.^35,46 In addition, E_2g and A_1g modes exhibited well-defined thickness dependence, with the two modes shifting away from each other in frequency with increasing thickness. The shift suggested that the stacking induced structure changes or long-range Coulombic interlayer interactions in few layered MoS₂.^35,45–48

Fig. 4 The corresponding Raman mapping spectra (a) and XRD patterns (b) of bulk MoS₂ and as-prepared MoS₂ nanosheets at different exfoliation times; (c) XPS spectra of Mo 3d and S 2s peaks; (d) XPS spectrum of S 2p peak.

As shown in Fig. 4(b), the XRD studies were carried out to analyze the crystal structure of the MoS₂ nanosheets. The as-prepared MoS₂ samples exhibited the crystallite nature of MoS₂ materials with some typical peaks indexed at 14.2°, 32.6°, 39.5°, 44.2°, 49.8° and 58.3°, corresponding to the (002), (100), (103), (006), (105), and (110) crystal planes of the 2H-MoS₂ structure.³⁶ These values were consistent with the standard card (JCPDS card no. 37-1492) and no impurity peaks or other phases were distinguished. Based on the theory of the X-ray diffraction, the broken crystal particles could lead to widen diffraction peaks. The results indicated that the average grain sizes of the MoS₂ nanosheets were gradually decreased with the continuous shearing mixing, and the intensity decrease of (002) peak suggested the reduction in thickness of bulk MoS₂ flakes when treated for longer shear time.^49,50 To some extent, the widened diffraction peaks also confirmed the reduction of the initial size of the MoS₂ to nanoscale size, which was caused by both turbulence kinetic energy and damage of shear forces.¹⁰ Compared with MoS₂ bulk structure, the XRD patterns and Raman spectra of the exfoliated MoS₂ nanosheets also validated this point.

Furthermore, chemical configuration of the as-prepared MoS₂ samples was analyzed and confirmed by XPS characterization. The survey spectrum analysis of the as-prepared MoS₂ nanosheets was available in Fig. S4 (see ESI†). In Fig. 4(c), the XPS spectra of Mo 3d presented two peaks at 229.2 eV and 232.4 eV, which corresponded to the binding energies of Mo 3d_5/2 and Mo 3d_3/2 for the Mo⁴⁺ oxidation state, while the weak peak at 226.7 eV was assigned to the S 2s. In addition, the S 2p spectrum in Fig. 4(d) also exhibited two characteristic peaks at binding energies of 162.3 eV and 163.6 eV, respectively corresponding to the S 2p_3/2 and S 2p_1/2 orbits of divalent sulfide ions (S²⁻). These peak positions were consistent with the 2H-MoS₂ crystal structure reported in other articles, and none oxidized Mo or S was found after the examination of Mo and S peaks.^51–54 It is promising that this low-cost and environmental-friendly production of MoS₂ nanosheets will provide a new platform for scale-up fabrication of other 2D layered nanomaterials.

3.3 Mechanical exfoliation mechanism

Some groups have performed the exfoliation kinetic analysis on graphene to illuminate the possible affecting factors that might influence the exfoliation process.^5,33,55 The main affecting parameters of shear instrument are generally divided into the following aspects, including rotation rate, shape and diameter of the mixing head, appropriate solubility, compatible surface energy, etc.^28,29,33 The surface energy of exfoliated solvent needs to match with that of layered material. Meanwhile, the exfoliation energy should be minimized, which facilitates shear exfoliation at low shear rates.^28,29 It has been proven that surface energy and Reynolds number (Re) are closely related to the solubility of the layered nanomaterials in different solvents.^56,57 Furthermore, the soluble compatibility between solvent and solute can be reasonably described in the framework of Hansen Solubility Parameters (HSP) theory.^57,58

R_a = [4(δ_D,solv − δ_D,solu)² + (δ_P,solv − δ_P,solu)² + (δ_H,solv − δ_H,solu)²]^1/2 (1)

where R_a is the HSP distance, while δ_D, δ_P, and δ_H are the HSP for the dispersion, polar, and hydrogen bonding interactions, respectively. The predictable solubility is negatively correlated with R_a value under the same conditions. This expression predicts that the concentration is maximized only when all three solubility parameters match well with solvent and compatible solute.⁵⁸ Besides single-component solvent, HSP theory also can be applied to solvent mixtures:

δ_blend = ∑Ø_n,compδ_n,comp (2)

where Ø is the volume fraction for each composition, in which each of the three HSP parameters for a solvent mixture is a linear function of composition. Eqn (1) and (2) can offer a significant support to instruct the solubility of different nanomaterials in various solvent mixtures, which effectively allows us to design ideal solvent systems^26,34,59.

The exfoliation mechanism of MoS₂ is similar to that of graphene, and the ideal case is that the MoS₂ nanosheets can be exfoliated layer by layer from the bulk MoS₂ powders by overcoming the Van der Waals attractions between adjacent MoS₂ layers. The shearing mixing occur in areas of localized, turbulent, highly energy-intensive regions, while the shear forces and collision effects might play a crucial role when the shearing mixing process is performed at a high speed of 10 000 rpm. The secondary factors such as viscosity, matching surface energy as well as solubility parameters tend to be tunable on the output as long as the bulk particles can disperse well in the solution over the high speed shear process. Ethanol/water mixtures with different volume fractions have distinctive surface tension, viscosities, Hansen solution constants and Re values.^18,26,28 Meanwhile, the Re values with different ethanol/water ratios at 15 °C (Fig. S5, ESI†) were calculated, and the value of 45 vol% ethanol aqueous solution was about 3 × 10⁴ > 10⁴, corresponding to a full turbulent flow.^55,60 To minimize the energetic cost of dispersion, the enthalpy of mixing process needs to be minimized, while the related calculation studies have estimated the surface energy as high as 250 mJ m⁻² for MoS₂, which is nearly 3.5 times higher than that of graphene (70.5–71 mJ m⁻²).^18,31 It is difficult to find a suitable solvent whose surface energy can match well with the bulk MoS₂ layered materials, that is to say, MoS₂ seems more difficult to be exfoliated than graphene. However, inorganic graphene analogues are distinctively interacted with the solvent occurred at a well-defined surface.³⁴ Successful solvents are not limited to some organic ones, those with Hansen parameters in the correct region also can disperse the nanosheets to some degree.⁶¹ As calculated in previous work, HSP distance of R_a has a minimum value in 45 vol% ethanol/water system.³⁴ It is likely that the synergistic effect of water and ethanol has a great impact on the solubility as well as the exfoliation efficiency. Some Hansen solution constants of solvents and solute reported in previous articles are summarized and provided in the following Table 1.^34,59

Table 1 Hansen solution constants of solvents and solute^34,59

Solvents	Water	Ethanol	MoS2
δD (MPa^1/2)	15.6	18.1	17–19
δP (MPa^1/2)	8.8	17.1	6–12
δH (MPa^1/2)	19.4	16.9	4.5–8.5

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Fig. 5(a) provided the model for the high shear mixer, which could generate high shear speed by using a closely spaced rotor/stator combination (∼100 μm). And the working features were detailedly depicted in Fig. 5(b): the fluid was released from the impeller surface under centrifugal force, accompanying with shear and collisions between MoS₂ particles. A shear-induced interlayer sliding in solvents to make bulk layered materials into nanosheets was caused by two kinds of forces, normal force and lateral force (Fig. 5(c)). Through self-lubricating ability in the lateral direction, it was much easy for lateral force to promote the relative motion between two adjacent layers.^26,33 Meanwhile, rapid and violent fluid movement at such high speed could produce strong physical interactions with bulk materials, which were sufficient to overcome Van der Waals forces between layers. The obtained MoS₂ dispersions are shown in Fig. 5(d), and the dispersed concentration can be further measured by filtration and weighing.

Fig. 5 Schematic process of the simplified model used for fluid dynamics analyses: (a) the model of Silverson Laboratory high-shear mixer with mixing head in a 1 L beaker of MoS₂ dispersion; (b) the amplified model of intrinsic structures, the rotor/stator combination with a gap of 100 μm; (c) the illustration for the possible exfoliation mechanism; (d) MoS₂ dispersion and a typical TEM image of nanosheets produced by direct liquid shear exfoliation.

3.4 Photothermal effects of the as-prepared MoS₂ nanosheets

More recently, MoS₂-based materials as drug loaders have emerged as promising photothermal agents for photothermal therapy due to their high absorption in the near-infrared (NIR) region.^16,17,62 Here, we made preliminary investigation on photothermal performance of pure MoS₂ nanosheets by the NIR laser irradiation technique. By using pure water as the solvent, 5 mL MoS₂ dispersions with various concentrations were irradiated by an 808 nm NIR laser to investigate the photothermal heating effects. In contrast to the pure water and MoS₂ bulk powder sample, aqueous dispersions of MoS₂ nanosheets with various concentrations exhibited a favorable concentration-dependent temperature increase (Fig. 6(a)). MoS₂ nanosheets with suitable smaller sizes presented a good dispersivity in aqueous solution, which would correspond to higher conversion efficiency.⁶² Specially, the sample with the concentration of 500 μg mL⁻¹ exhibited a maximum temperature climb from 22.3 to 40.5 °C within 300 s, which was higher than values reported in other articles.^17,63 Moreover, it was obvious that temperature increased along with the augment of power density, demonstrating a laser-power interrelated photothermal response for irradiated MoS₂ nanosheets dispersions (Fig. 6(b)). To further assess the photothermal transduction performance of the nanosheets, 200 μg mL⁻¹ of the MoS₂ aqueous solution was exposed to NIR laser at 2 W cm⁻² for 300 s, and the laser was subsequently turned off and then rapidly cooled down within 300 s. Fig. 6(c) indicated that the MoS₂ nanosheets dispersion had a favorable thermal conductivity. As shown in Fig. 6(d), little appreciable change was found in the UV-Vis spectrum of MoS₂ dispersion even after being exposed under laser for a certain period of time. These combined results indicated that the synthesized MoS₂ nanosheets exhibited desirable photothermal stability under laser irradiation, which would have potential applications for further photothermal therapy of cancers^16,17,43.

Fig. 6 The photothermal response of the MoS₂ aqueous dispersion with NIR laser irradiation by UV-Vis-NIR absorption spectrum: (a) the temperature elevation of MoS₂ nanosheets with different concentrations under the NIR laser irradiation; (b) photothermal heating curves of MoS₂ dispersion at the concentration of 200 μg mL⁻¹ under various power densities; (c) the photothermal response of the MoS₂ aqueous dispersion at a power density of 2  W cm⁻² and then the laser was turned off after 300 s of irradiation; (d) UV-Vis absorption spectra of the MoS₂ dispersions before and after NIR 808 nm irradiation at the power of 2  W cm⁻².

4. Conclusions

A simple and green strategy of shearing exfoliation with many advantages of ease operating, low cost, free of toxicity and additives, has been successfully developed to produce 2D MoS₂ nanosheets. In addition, when the number of shear cycles reaches 10, the theoretical yield can attain nearly 30%, and the residue and solute can also be recycled for next shearing exfoliation without introducing any impurities. Furthermore, a possible exfoliation mechanism has been proposed: shear forces and collision effects will play the crucial roles when the shearing mixing is performed at a high speed of 10 000 rpm. The secondary factors such as viscosity, matching surface energy as well as the solubility parameters tend to be tunable on the output to a certain extent as long as the bulk particles are well dispersed in the solution over the shear process. Furthermore, the as-prepared MoS₂ nanosheets exhibited desirable photothermal stability under laser irradiation, which made it possible to develop novel MoS₂-based photothermal agents. Overall, this strategy is not only suitable for MoS₂ but also can be extended to a wide range of other layered compounds by means of a reasonable ethanol/water ratio. In other word, this innovative strategy may have a powerful potential for large-scale production of 2D nanomaterials for further applications across different fields in physics, chemistry and materials science.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (Grant Nos 51575507 and 51375474), 863 Plan (Grant No. 2015AA034602), and the “Funds for Young Scientists of Gansu Province (145RJYA280)” scheme.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15310k

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