Understanding the exfoliation and dispersion of MoS₂ nanosheets in pure water

Abstract

Exfoliation and dispersion of two dimensional (2D) transition metal dichacogenides (TMDCs) such as MoS₂ in water are highly demanded. In this paper, we exfoliate MoS₂ nanosheets in pure water via ultrasonication and attempt to understand the exfoliation process and the dispersion behavior of hydrophobic MoS₂. Cavitation-induced exfoliation leads to thinning and fragmentation of MoS₂ flakes. Formation of mesoporous MoS₂ sheets suggests that the exfoliation initiates from the basal planes. As for the dispersion of MoS₂ nanosheets in water, we find that higher centrifugation rate and thus smaller lateral size, results in improved stability. This can be attributed to the enhanced edge effects. Fragmentation of MoS₂ flakes generates many edges, to which hydrophilic and ionizable groups are attached. Herein, the edge effects include two aspects. On the one hand, edge-attached polar and hydrophilic groups promote interactions of MoS₂ nanosheets with water molecules and result in better wettability, which can be explained by the hemi-wicking model. On the other hand, dissociation of these groups makes MoS₂ sheets negatively charged. It is found that edge charge dominates. As the flake size reduces, attractive van der Waals force and hydrophobic force can be overcome by electrostatic repulsions, which prevent MoS₂ nanosheets from aggregation. This study opens a new vista on exfoliating and dispersing hydrophobic 2D nanomaterials in water.

Introduction

Owing to their intrinsic semiconducting characteristics [1], [2], two dimensional (2D) transition metal dichacogenides (TMDCs) can overcome the drawbacks of zero-gap graphene [3], and therefore receive much attention from both scientific and industrial fields. TMDCs are composed of numerous sandwich-like structures, in which one hexagonal layer of transition metal atoms is inserted into two hexagonal layers of sulfur atoms [4]. While the in-plane atoms are held together through covalent interactions, the sandwich-like structures are kept together via weak van der Waals force [4]. As a typical TMDC, monolayer MoS₂ is of great interest due to its direct bandgap (1.8 eV) [1], large in-plane carrier mobility (200–500 cm² V⁻¹ s⁻¹) [5], high current on/off ratio [6], unique electronic [1], optical and catalytic properties [7], [8], [9]. Possessing diverse and exceptional properties, MoS₂ holds great promise in a variety of applications, including field-effect transistors (FETs) [5], [6], biosensors [10], photodetectors [11] and photocatalysis [9]. Despite these advantages, finding a cost-effective, facile and scalable method for producing high quality MoS₂ sheets still remains a challenge.

Ultrathin MoS₂ nanosheets can be prepared via a number of techniques, such as mechanical exfoliation [12], chemical vapor deposition (CVD) [13] and liquid exfoliation [14]. Compared with other methods, liquid exfoliation offers a promising route for scalable production of MoS₂ nanosheets, due to its low cost, simplicity and relatively high output. As a classical liquid-based approach, chemical exfoliation has been widely used to prepare ultrathin MoS₂ nanosheets [15]. Despite high yield of monolayer nanosheets, chemical exfoliation (i.e. ion intercalation and sonication) suffers from highly flammable and explosive chemicals. Furthermore, the shift from semiconducting state (2H) to metallic state (1T) renders products unsuitable for semiconductor applications [16]. Furthermore, it can also make MoS₂ unstable in aqueous solutions, and even leads to degradation of MoS₂ nanosheets due to their high chemical activity [17].

Direct exfoliation of MoS₂ sheets from bulk materials via ultrasonication in solvents provides another option for liquid exfoliation [14]. During this process, sonication-induced hydrodynamic forces act as the driving force to overcome interlayer van der Waals interactions of 2D materials and eventually peel off atomic-thin layers. Apart from parameters like sonication power and time, solvents also play a vital role in exfoliation yield and efficiency. Properties of different solvents make them behave very differently in terms of exfoliating and stabilizing nanosheets. Coleman et al. use mixing enthalpy theory to predict the energy balance between graphene and solvents [18]. Energetic consumption for exfoliating nanosheets can be expressed by the following equation:

$$\frac{\mathrm{\Delta }{H}_{\text{mix}}}{V_{\text{mix}}}\approx \frac{2}{T_{\text{flake}}}{({\delta }_{\text{G}}-{\delta }_{\text{sol}})}^2\varnothing$$

where $T_{\text{flake}}$ is the thickness of flakes, $\varnothing$ is the volume fraction of graphene, while ${\delta }_{\text{G}}$ and ${\delta }_{\text{sol}}$ denote the square root of surface energy of graphene and solvents, respectively [19]. This prediction can be also employed for other 2D materials. From the expression, one can conclude that if the surface energy of a certain solvent matches that of 2D materials, energy consumption for exfoliation can be minimized. Therefore, exfoliation will occur more easily. Due to their suitable surface energy, some organic solvents (e.g. NMP), are considered to be good solvents for exfoliation [14], [18]. However, since these solvents have toxicity and high boiling point, their usage will bring environmental problems and make flakes easily stacked because of slow solvent evaporation. On the other hand, surfactants can be used for exfoliation in aqueous solutions because their addition can significantly reduce the surface energy of the solvent system and stabilize nanosheets via steric or electrostatic forces [19], [20]. With respect to surfactants, their main drawback is that they are difficult to eliminate completely, resident molecules absorbed on the sheet surface therefore render sheet properties inferior. Hence, finding a green, inexpensive and low-boiling solvent, preferably water, for efficient exfoliation is of great importance. To solve this problem, a mixed-solvent strategy, which adopts mixtures of water and alcohols, has been developed [21]. Recently, Zhao et al. exfoliated MoS₂ nanosheets in the mixtures of water and isopropyl alcohol (IPA) and manifested that MoS₂-based films hold promise in optoelectronic applications [22].

2D materials, e.g. graphene and MoS₂, are conventionally considered to be insoluble in water. However, recent reports demonstrate that the hydrophobicity of graphene and MoS₂ stems from airborne hydrocarbon contamination, whereas clean surfaces of these materials are mildly hydrophilic [23], [24]. This discovery raises the question as to whether the exfoliation and dispersion of nanosheets can be achieved in aqueous solutions in absence of organic solvents and surfactants. Recent studies confirm that MoS₂ nanosheets can be exfoliated in pure water by ultrasonication [9], [25], shear mixing [26], high-pressure homogenizer [27] and freezing expansion [28]. However, few studies pay sufficient attention to interactions of water with MoS₂ flakes and their effects on exfoliation as well as dispersion. Though many efforts have been made, there is still a lack of understanding in terms of exfoliation and dispersion mechanism.

In this paper, we directly exfoliate MoS₂ nanosheets from bulk material in pure water via ultrasonication, and discuss effects and possible mechanism of exfoliation based on characterization results. Furthermore, we find that higher centrifugation rate results in improved colloidal stability in water. This phenomenon can be ascribed to the enhanced edge effects. We believe that our findings give new insights in producing and dispersing hydrophobic 2D nanomaterials in aqueous solutions.